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Ages and petrogenesis of Jurassic and Cretaceous intrusive rocks in the Matsu Islands: Implications for lower crust modification beneath southeastern China
Abstract
Major and trace element, whole-rock Sr-, Nd- and Hf-isotope, zircon U-Pb age and Hf-O isotope data are reported for the intrusive rocks from the Matsu Islands in the coastal area of southeastern (SE) China, in order to study the ages, sources and petrogenesis of these rocks and evolution of the lower crust. The rocks include gneissic granite, massive granite, brecciated granite and diabase. Secondary ion mass spectrometer (SIMS) zircon U-Pb dating reveals that the rocks in the Matsu Islands were emplaced at ∼160 Ma, ∼130 Ma and ∼94 Ma. The Jurassic granites (∼160 Ma) have high SiO2 (74.1-74.5 wt.%) and K2O+Na2O (8.32-8.33 wt.%) contents and high Rb/Sr ratios of 0.6-1.2 and (La/Yb)CN ratios of 12.6-19.4. Their relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7074-0.7101), variable and negative εNd(t) values (-9.2 to -5.4), and variable zircon εHf(t) (-17.0 to +5.2) and δ¹⁸O (4.7 to 8.1‰) values indicate they were mainly derived from an ancient lower crustal source, but with involvement of high εHf(t) and low δ¹⁸O materials. The Early Cretaceous diabase (∼130 Ma) has SiO2 content of 56.5 wt.%, relatively high MgO concentration, low initial ⁸⁷Sr/⁸⁶Sr ratio and negative εNd(t) value, similar to geochemical features of other Cretaceous mafic rocks in the coastal area of SE China. Zircons from the diabase have high εHf(t) values (-5.5 to +0.2) and relatively low δ¹⁸O values of 4.2 to 5.0‰. These characteristics indicate that the parental magma of the diabase was generated by partial melting of enriched lithospheric mantle, which have been metasomatised by altered oceanic crust-derived low-δ¹⁸O fluids. For the Cretaceous granitoids (∼130 Ma and 94 Ma), they have relatively low SiO2 (68.0-71.3 wt.%) and K2O+Na2O (5.30-7.55 wt.%) contents and low Rb/Sr ratios and (La/Yb)CN ratios of 5.8-7.1. They have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7071 to 0.7082), homogeneous εNd(t) (-4.3 to -4.5) and relatively high zircon εHf(t) values (-3.7 to +1.2) and low δ¹⁸O values (4.6 to 5.9‰). Their isotopic compositions are similar to those of the diabases in this study as well as other Cretaceous mafic rocks in the coastal area of SE China, suggesting that the sources of the Cretaceous granitoids might be the newly formed lower crust related to the underplated mafic rocks. Whole-rock geochemical, Sr-Nd and zircon Hf-O isotopic compositions indicate that the Jurassic granitoids are most likely generated by partial melting of relatively ancient basement rocks, whereas the Cretaceous granitoids were generated by partial melting of relatively young lower crustal rocks with addition of mantle-derived magma. This distinction implies that the pre-existing ancient lower crust beneath the coastal area of SE China has been modified by large-scale mafic magma underplating. Therefore, underplating of mantle-derived mafic magma would result in modification of the pre-existing ancient lower crust and formation of the relatively juvenile lower crust.
... The A-type granitoids in the coastal area of SE China have high SiO 2 content, high Ga/Al ratio, high abundance of Zr, Nb, Ga, and Y, along with low content of Al, Mg, and Ca (Qiu et al., 2004;Chen et al., 2013Chen et al., , 2019. Various earlier studies showed that the I-and A-type granites display juvenile radiogenic isotopic signatures and have mantle-like oxygen isotope compositions (Qiu et al., 1996(Qiu et al., , 1999(Qiu et al., , 2000(Qiu et al., , 2004Xu et al., 1999;Chen et al., 2013Chen et al., , 2014Chen et al., , 2017Zhao et al., 2015Zhao et al., , 2016. Previous studies suggested that the Cretaceous I-and A-type granites in the coastal area of SE China were mainly formed by the magma mixing involving basaltic melts and ancient crustal-derived felsic magmas (e.g., Qiu et al., 2004Qiu et al., , 2012. ...
... The Cretaceous granitic rocks are mostly distributed in the coastal area of the Cathaysia Block ( Fig. 1) (Li, 2000). The Cretaceous intrusive rocks in the coastal area of SE China consist mainly of I-and A-type granites, with minor gabbros, diorites, and syenites (Chen et al., 2017). Chronological studies suggested that the Cretaceous magmatic rocks are mainly emplaced during 125-142 Ma and 87-115 Ma (Liu Q. et al., 2012;Liu L. et al., 2012Liu L. et al., , 2014Cui et al., 2012;Li Z. et al., 2014;Chen et al., 2013Chen et al., , 2014Chen et al., , 2019Yang unpublished data). ...
... The low δ 18 O values of the zircons from the monzogranites pointed to a source that had been modified by earlier interaction with low δ 18 O fluids. The low δ 18 O values in fact resembled those of the zircons from the Early Cretaceous diabases from the Matsu islands (Chen et al., 2017). A slab-derived fluid interacting with juvenile mafic rocks under high temperature could decrease the δ 18 O values of altered mafic rocks (Bindeman et al., 2005). ...
... Previous petrographic and geochemical studies indicate that mantle-derived magmas have played a significant role in the petrogenesis of these A-type granites (e.g., Chen et al., 2013Chen et al., , 2019aChen et al., , 2019bZhao et al., 2015Zhao et al., , 2016Gao et al., 2018). However, despite extensive research, their petrogenesis remains unclear (e.g., Chen et al., 2013Chen et al., , 2017Gao et al., 2018). In this work, we focused on the following five Cretaceous plutons ( Fig. 1): Xiapu; Changchun; Shihu; Ju'an; and Yangjiaxi plutons. ...
... This feature is consistent with the bulk-rock geochemistry of deep negative Eu anomalies and high Rb/Sr ratios resulting from the crystal values were calculated based on the zircon U-Pb age. The reference data of ancient lower crust, juvenile lower crust, and mafic rocks are from Chen et al., (2013Chen et al., ( , 2017. fractionation of feldspar (e.g., Miller and Mittlefehldt, 1984;Halliday et al., 1991). ...
A-type granites are a distinctive group of igneous rocks classified based on chemical and mineralogical criteria. Although numerous theories for the origin of A-type granite have previously been proposed, their genesis remains highly controversial. In the current study, we present bulk-rock geochemistry and zircon Usingle bondPb isotopes, Hf isotopes, and trace elemental data for a suite of quartz monzonite, porphyritic granite, and miarolitic alkali feldspar granite from the coastal area of Fujian, southeastern China, to evaluate the genetic link between these rocks. Zircon Usingle bondPb dating indicates that these rocks were crystallized at ~94–102 Ma. Our data strongly suggests that lithological differences in these rocks are the result of crystal-melt segregation in the shallow crust. The quartz monzonites are enriched in Sr and Ba, with high Zr/Hf and low silica content and a weak negative Eu anomaly, representing the cumulate residue from the crystal-melt segregation of a magma reservoir. A limited fraction of zircon from the quartz monzonite has evolved trace element signatures (high Hf, Nb, Y, U; low Eu/Eu⁎), indicating that the quartz monzonite represents a mixture of accumulated crystals and high silicic melt trapped in cumulate mush. The miarolitic alkali feldspar granites and porphyritic granites are enriched in silica and Rb, and depleted in Sr and Eu. They also display low whole-rock Zr/Hf and Eu/Eu⁎ ratios, and high Rb/Sr ratios, representing highly evolved silicic melts that were segregated from a magma reservoir. The majority of the zircon grains from the miarolitic alkali feldspar granites exhibit the most evolved trace element signatures (high Hf, Nb, Y, U; low Zr/Hf and Eu/Eu⁎) of the entire suite. This reveals that these zircons crystallized from the high silicic melts extracted from a magma reservoir. Magma recharge and the exsolution of volatiles from the interstitial melt have promoted the segregation and upward extraction of high silica magmas from a compacting magma reservoir in the upper crust. Our work demonstrates that the miarolitic alkali feldspar granites with A-type granite features from the coastal area of southeastern China were generated by the crystal-melt segregation process in a shallow crustal magma reservoir.
... Multi-stage Phanerozoic igneous rocks are widely distributed in the Cathaysia block ( Fig. 1A) (Wang et al., 2013). Previous studies indicate that these Zhou et al., 2006;Li and Li, 2007;Wang et al., 2013;Chen et al., 2017). The early Paleozoic, Permian-Triassic, and Jurassic rocks, which mainly consist of granitic rocks, are largely distributed in the inland area of the Cathaysia block (Fig. 1A) (e.g., Zhou et al., 2006;Li and Li, 2007). ...
... Large volumes of felsic magma were generated by partial melting of Paleoproterozoic basement, and the ancient lower crust beneath SE China was replaced by a relatively young lower crust via mafic magma underplating during the Jurassic (Chen et al., 2014). In contrast, Cretaceous granite units (including the HSG) display relatively higher zircon ε Hf (t) values and mantlelike zircon δ 18 O values (ε Hf (t) = ∼0 and δ 18 O < 6.0‰), which is similar to the Cretaceous mafic rocks but different from the above-mentioned Jurassic granite (Li and Li, 2007;Chen et al., 2017;Zhang et al., 2017). Numerous studies have suggested that the Cretaceous granitic plutons in the coastal area of SE China were derived from a juvenile lower crustal source (Chen et al., 2014. ...
It is generally hypothesized that high-silica (SiO2 > 75 wt%) granite (HSG) originates from crystal fractionation in the shallow crust. Yet, identifying the complementary cumulate residue of HSG within plutons remains difficult. In this work, we examine the genetic links between the porphyritic monzogranite and HSG (including porphyritic granite, monzogranite, and alkali feldspar granite) from the coastal area of southeastern China using detailed zircon U-Pb ages, trace elements, Hf-O isotopes, and whole-rock geochemistry and Nd-Hf isotopic compositions. Zircon U-Pb ages indicate that the porphyritic monzogranite and HSG are coeval (ca. 96−99 Ma). The HSG and porphyritic monzogranite have similar formation ages within analytic error, identical mineral assemblages, similar Nd-Hf isotopic compositions, and consistent variations in their zircon compositions (i.e., Eu/Eu*, Zr/Hf, and Sm/Yb), which suggests that their parental magma came from a common silicic magma reservoir and that the lithological differences are the result of melt extraction processes. The porphyritic monzogranite has relatively high SiO2 (70.0−73.4 wt%), Ba (718−1070 ppm), and Sr (493−657 ppm) contents, low K2O and Rb concentrations and low Rb/Sr ratios (0.1−0.2), and it displays weak Eu anomalies (Eu/Eu* = 0.57−0.90). Together with the petrographic features of the porphyritic monzogranite, these geochemical variations indicate that the porphyritic monzongranite is the residual silicic cumulate of the crystal mush column. The HSG (SiO2 = 75.0−78.4) has variable Rb/Sr ratios (2−490) and very low Sr (1−109 ppm) and Ba (9−323 ppm) contents. Zircon from the HSG and porphyritic monzogranite overlap in Eu/Eu*, Zr/Hf, and Sm/Yb ratios and Hf contents; however, some zircon from the HSG show very low Eu/Eu* (<0.1) and Zr/Hf ratios. These features suggest that the HSG represents the high-silica melt that was extracted from a crystal-rich mush. The injection of mantle-derived hotter mafic magma into the mush column and the exsolution of F/Cl−-enriched volatiles (or fluids) from the interstitial melt rejuvenated the pre-existing highly crystalline mush. Subsequent extraction and upward migration of silicic melt resulting from compaction of the mush column formed the HSG at shallow crustal levels, which left the complementary crystal residue solidified as porphyritic monzogranite at the bottom.
... minerals in granites were re-equilibrated at 105-90 Ma (Jahn et al., 1976). Recent zircon U\ \Pb dating results of these igneous rocks from more islets in the vicinity give clearer picture for two major episodes of granitic intrusion at Late Jurassic (162-158 Ma) and Early Cretaceous (140-131 Ma) with coeval volcanism , and minor intrusions of granite and diabase (96-94 Ma) (Chen et al., 2017b;Lee et al., 2015). Up to now there is lack of a detailed comparison of thermal episodes between Matsu Islands and PDMB or SCMB to elucidate the status of these off-shore islets. ...
... Hence the earlier thought suggesting modification of the Late Jurassic crusts by prevailing Early Cretaceous magmatic intrusions as mentioned is possible. Even younger age clusters of 128-121 Ma and 108-97 Ma obtained from both included and interstitial zircons, although only restricted in two samples, may reflect newly crystallized constituent minerals in these granites in response to the influence of later magmatic and metamorphic activities in coastal SE China (Chen et al., 2017b;Cui et al., 2013;Lee et al., 2015;Li et al., 2015). ...
Cathaysia Block in South China had accreted tremendous amounts of Mesozoic magmatic rocks. Recent zircon UPb age results revealed existence of some Early Jurassic gneissic granites (200–190 Ma) in the Pingtan-Dongshan Metamorphic Belt (PDMB), coastal SE China which corresponds one stage of magmatic quiescence in the rest part of East Cathaysia. To better understand Mesozoic thermal evolution of the PDMB, UPb geochronology and Hf isotope composition of zircon separates (LA-ICPMS) and inclusions (NanoSIMS, age only) are examined for two granitic rocks and one meta-sandstone in Pingtan area (northern PDMB) and three granites in Matsu Islands further north. For Pingtan rocks, ages of discrete zircons are close to the Early Jurassic time in two stages (196 Ma and 173 Ma) yet all with predominantly positive ɛHf(t) values (−0.2 to 7.5), whereas zircon inclusions within rock-forming minerals yield a wide age span from Permo-Triassic to Early Cretaceous in six clusters, namely 270–206 Ma, 191–170 Ma, 168–151 Ma, 146–132 Ma, 126–119 Ma and 109–100 Ma that basically coincide the major Mesozoic thermal episodes in Cathaysia except the second cluster. For Matsu granites, discrete zircons exhibit Late Jurassic (162–158 Ma) and Early Cretaceous (137 Ma) ages with ɛHf(t) values of −9.1 to −1.6 and −4.9 to −0.8, respectively. Nevertheless, zircon inclusions show ages corresponding to Pingtan samples in the last four clusters. With reference to Mesozoic magmatic rock units in the neighboring Southeast China Magmatic Belt (SCMB), our data suggest irrelevance of Pingtan samples to the Cathaysia and linkage of off-shore Matsu granites to the PDMB. Noting that there are similarities of zircon ages and ɛHf(t) values between the granite-schist associations in Tananao Metamorphic Belt (TMB) of eastern Taiwan and Pingtan island of northern PDMB, these Early Jurassic granites were probably originated from a same source. Three consistent Early Cretaceous age clusters of zircon inclusions over the studied samples match two well-known magma prevailing episodes in the coastal SE China, with one tectonic episode (128–119 Ma) in the PDMB that is often invisible from the zircon separates remains unexplainable. In response to a plausible tectonic evolution, we support the earlier proposition for a now-concealed microcontinent that had impinged on Cathaysia around 125 Ma, but further suggest that it left Pingtan Island behind as a remnant fragment when drifted away at later time.
... Combining previously published zircon U-Pb ages with the new data obtained during this study and the existing structural, petrological, Zhao et al., 2018). The location of Fig. 2 is shown in Fig. 1. geochemical data of the Cretaceous structures and magmas, this paper suggests a major three-stage magmatic evolutionary history in the Cathaysia Block during Cretaceous: (1) The first-stage magmas (145-137 Ma); (2) The second-stage magmas (136-118 Ma); (3) The third-stage magmas (107-86 Ma) (Chen et al., 2014;Chen et al., 2017;Guo et al., 2012;Li et al., 2014a;Li et al., 2014b;Li et al., 2018a;Zhang et al., 2012). ...
... ε Nd (t) versus ( 87 Sr/ 86 Sr) I diagram for granitoids in Fujian province (modified afterChen et al., 2017). (The new data sources are as follows: JZG, from ...
The geodynamic transformation of the Cathaysia Block and its relationship with the Cretaceous granitic intrusions have been controversial in recent decades. In this study, petrology, zircon U–Pb ages, Lu–Hf isotopic composition, whole-rock geochemistry and Sr–Nd isotopes were carried on Cretaceous granites that were collected from the Fujian province, SE China. Combining previously published ages with the new ages obtained in this study indicates that the magmatism in the study area occurred in three distinct stages at 145–137 Ma, 136–118 Ma and 107–86 Ma. The first stage I-type granites were intruded at 137 ± 1 Ma and 145 ± 1 Ma, which are characterized by low εNd(t) values of −7.3 to −6.8 and εHf(t) values of −12.5 to −2.5. It was formed by partial melting of Mesoproterozoic medium-to-high K basaltic rocks in the thickened crust of ~40 km without significant hybridization of mantle-derived magmas and fractional crystallization. The second stage highly differentiated I-type granites yield zircon U-Pb ages of 125 ± 1 Ma and 128 ± 1 Ma, which contain slightly negative εNd(t) values of −5.4 to −5.3 and εHf(t) values of −8.4 to −3.2. It was generated by partial melting of the Mesoproterozoic basement at the depth of 30–40 km triggered by underplating of a small amount of mantle-derived magma, with primary crustal melts undergoing plagioclase-dominated fractionation crystallization. The third stage I-type granites yield zircon U-Pb ages of 104 ± 1 Ma and 105 ± 1 Ma, which contain εNd(t) values of −1.7 to −7.3 and εHf(t) values of −6.0 to −0.5. It was derived from partial melting of Mesoproterozoic basaltic source at a depth of ~30 km. Besides, a mass of mantle-derived magma was mixed into the crustal melts, accompanied with plagioclase-dominated fractionation crystallization. Combining previously published data and those obtained in this study provides significant information for the tectonic evolution of Paleo-Pacific. In stage 1 (145–137 Ma), the existence of I-type granites and adakitic rocks suggests that the dip-angle subduction of the Paleo-Pacific Plate led to the partial melting of the thickened lower crust under the compressional environment. In stage 2 (136–118 Ma), the generation of I-type and A-type granitic magmas imply the thinning of the lithospheric crust under the extensional environment and the upwelling of the asthenospheric mantle, which are caused by slab rollback of Paleo-Pacific. In stage 3 (107–86 Ma), the appearance of much more I-type and A-type granites indicates that the break-off of the Paleo-Pacific plate led to much stronger crust–mantle interactions under the extensional environment.
... The area of Cathaysia basement is modified fromChen and Jahn (1998). The data for the Late Mesozoic granitoids in the coastal area of SE China are fromYang et al. (2018),Li et al. (2014b),Chen et al. (2017) and references therein. The data for the Triassic granitoids in SE China are fromMao et al. (2013),Li et al. (2012c) andXia et al. (2012). ...
This study presents new zircon U–Pb ages and whole-rock geochemical data for the Fanyindong (FYD) and Jincheng (JC) granitoid intrusions in coastal Zhejiang and Fujian Provinces, respectively, southeast China, to investigate their petrogenesis and provide new constraints on the regional tectonic evolution and crustal growth. Zircon U–Pb ages indicate that the FYD and JC intrusions were emplaced at c. 170 Ma and 193 Ma, respectively. Both granitoids belong to the high-K calc-alkaline series, and display arc-like trace element patterns, with enrichment of large ion lithophile elements and Nb-Ta depletion. The FYD granitoids show adakitic geochemical characteristics, with high radiogenic Sr isotope ratios, and the most enriched bulk-rock Nd and zircon Hf isotopic compositions (crustal Nd–Hf model ages=ca. 2.35–2.75 Ga) ever reported in the Mesozoic granitoids within southeast China. Geochemical characteristics suggest that the FYD granitoids were generated by partial melting of mostly Paleoproterozoic basement rocks in a thickened lower crust. However, whole-rock Sr–Nd and zircon Hf isotopes of the JC granites show depleted source signatures. Geochemical signatures suggest the JC granites originated from the partial melting of a mixed source, consisting of the pre-existing, ancient, lower crust and juvenile, mantle-derived, basaltic rocks. The data from this study, in conjunction with previously published data, indicates that there exists a NE-trending, Early–Middle Jurassic, arc-related, magmatic rock belt along the southeast coast of China, and the Paleo-Pacific Plate subduction most likely started at c. 200 Ma. The early stage subduction of the Paleo-Pacific plate and the induced reactivation and extension of pre-existing E–W striking faults in the Nanling region of southeastern China gave rise to the generation of the JC granites. Ongoing subduction of the Paleo-Pacific plate resulted in a thickening of the continental margin, and subsequent crustal anatexis of basement materials produced the FYD granitoids.
... Multi-stage Phanerozoic magmatic rocks are widespread throughout the CB (e.g., Zhou et al., 2006). Previous studies indicate that these magmatic rocks are mainly formed during Early Paleozoic (420-446 Ma), , Jurassic (155-165 Ma), early Early Cretaceous (125-142 Ma) and late Early Cretaceous to Late Cretaceous (87-115 Ma) (e.g., Chen et al., 2017Chen et al., , 2019aChen et al., , 2019bZhou et al., 2006). The Early Paleozoic, Permian-Triassic, and Jurassic granitoids are widely distributed in the inland area of the CB, whereas the Cretaceous granitic rocks and their erupted analogs are widespread in the eastern area of the CB (Fig. 1a) (Zhou et al., 2006). ...
The Cretaceous granitoids in eastern Zhejiang were examined to establish a model for the construction of the highly silicic upper crust in southeastern China. The studied rocks include quartz monzodiorite, quartz monzonite, alkali feldspar granite, and their microgranular enclaves (ME). Zircon UPb dating suggests that the granitic rocks and MEs were emplaced at approximately 102–114 Ma. Geochemical features and isotopic compositions of the quartz monzodiorite, quartz monzonite, and alkali feldspar granite indicate that they are mainly produced by the partial melting of Neoproterozoic to Mesoproterozoic crustal rocks. The quartz monzodiorites, quartz monzonites, and alkali feldspar granite appeared to derive from a similar silicic magma reservoir because they have comparable crystallization age, zircon trace element variation trend, Sr-, Nd-, and Hf-isotopic compositions, and complementary geochemical compositions. The studied quartz monzodiorites, quartz monzonites, and low silica alkali feldspar granites (SiO2 < 75 wt%) are deemed a residual cumulate according to their geochemical compositions and textural features (e.g., aggregates of coarse-grained euhedral plagioclase crystals in “glomeroporphyritic aggregates”), whereas the high silica alkali feldspar granites (SiO2 > 75 wt%) resemble frozen remnants of high silica melts segregated from a crystal mush because they are highly enriched in SiO2 and depleted in Ba, Sr, and Eu. The MEs hosted in the quartz monzodiorites and quartz monzonites have low SiO2 and high MgO contents, high initial ⁸⁷Sr/⁸⁶Sr ratio and negative εNd(t) and εHf(t) values, and variable zircon δ¹⁸O and εHf(t) values. Petrographic observations and geochemical features suggest that they are formed from solidified mafic magmas and generated by the crystal fractionation of mantle-derived hydrous mafic magma with subsequent crustal contamination. Compared with these mafic MEs, the MEs hosted by the alkali feldspar granites have relatively high SiO2 and low K2O contents, indicating that the felsic MEs are generated by crystal fractionation of the parental magma of the mafic MEs. Since abundant MEs occurred in the quartz monzodiorites, quartz monzonites, and alkali feldspar granites, magma recharge may have promoted the extraction and upward percolation of high silicic melts from the compacting crystal-rich mush in the shallow crust. Thus, the chemical and lithological diversity of the granitic plutons in eastern Zhejiang is generated in the upper crust via crystal-melt segregation. Specifically, the quartz monzonites and quartz monzodiorites are formed as residual cumulate, whereas the high silica alkali feldspar granites are generated as frozen remnants of high silicic melts segregated from a compacting crystal mush. More widely, our work suggests that the formation of the highly silicic upper crust in southeastern China is controlled by crystal-melt segregation processes in the shallow crust.
... It is well known that a major magmatic event took place in a vast area of SE China between 165 and 140 Ma (e.g. Sewell et al., 1992;Zhou and Li, 2000;Li et al., 2004Guo et al., 2012;Huang et al., 2013;Zhang et al., 2015;Chen et al., 2017). Among the igneous rocks formed during this time span, strongly metaluminous to weakly-peraluminous biotite monzogranites are predominant (e.g. ...
Six new high precision U–Pb zircon ID-TIMS ages plus thirteen in situ high spatial resolution U–Pb zircon LA-MC-ICPMS ages are reported from Jurassic plutonic (metaluminous to weakly peraluminous biotite granites) and Jurassic to Cretaceous hypabyssal (dacites) rocks from Macao. Despite its relatively small area (∼30 km2), the new ages tightly constrain the Macao granitic magmatism to two periods ranging from 164.5 ± 0.6 Ma to 162.9 ± 0.7 Ma and 156.6 ± 0.2 Ma to 155.5 ± 0.8 Ma, separated by ca. 6 Ma. Inherited zircons point to the existence of a basement with ages up to Paleo-Proterozoic and late Archean in the region. In addition, younger dacitic rocks were dated at 150.6 ± 0.6 Ma and
... They are mostly formed during 165-155 Ma and distribute primarily along the Nanling Ranges and the Wuyishan Mountains (Fig. 1a). Recent studies also identified Jurassic granitoids in the coastal regions of the Cathaysia Block ( Chen et al., 2017). The Cretaceous igneous rocks mostly consist of granitoids, which occur all over SE China, and felsic volcanic rocks, which occur mostly in the coastal regions (Fig. 1a) (e.g., Zhou et al., 2006; Zhou and Li, 2000). ...
In this work, we report the geochemical, whole-rock Sr-, Nd-, and Hf-isotopes, and zircon U–Pb age and Hf–O isotopic compositions for six granitic intrusions in northeastern Fujian, coastal region of southeastern China, to elucidate the petrogenesis of aluminous and peralkaline A-type granites in this region. Zircon U–Pb dating of these rocks (including porphyritic syenites, porphyritic granites, alkali-feldspar granites, and arfvedsonite granites) yielded Late Cretaceous ages of 94–98 Ma. Mineral assemblage and geochemical features suggested that the arfvedsonite granites were peralkaline A-type but the porphyritic syenites, porphyritic granites, and alkali-feldspar granites were aluminous A-type. Geochemical data, whole-rock Nd–Hf isotopes (ε Nd (t) = −5.5 to −4.0, ε Hf (t) = −3.6 to 0), and the zircon Hf–O isotopes (ε Hf (t) = −3.0 to +1.4, δ ¹⁸ O = 5.4‰–6.0‰) indicated that the porphyritic syenites were produced by partial melting of crustal rocks in the lower crust. The porphyritic granites and alkali-feldspar granites had similar geochemical features and zircon Hf–O isotopic composition with the porphyritic syenites, suggesting that they were formed by feldspar-dominant crystal fractionation of deep crust derived syenitic magma under shallow crustal level. Compared with the aluminous A-type granites, the peralkaline A-type granites had lower abundance of Al 2 O 3 , MgO, CaO, Ba, and Sr, higher abundance of SiO 2 and HFSE, as well as higher ratio of total FeO/MgO, Ga/Al, and Rb/Sr. However, the whole-rock Nd-, Hf- and zircon Hf–O isotopic compositions of the peralkaline and aluminous A-type granites were similar. It could be inferred that the peralkaline A-type granites were formed by crystal fractionation of aluminous A-type granitic magmas. Since the studied A-type granitic intrusions were coeval with the bimodal volcanic rocks and the extensional basins in the coastal region of southeastern China, they were likely produced in an extensional setting, possibly during lithospheric thinning that resulted from the subduction of the paleo-Pacific plate beneath southeastern China.
South China Block (SCB) underwent a tectonic switch from compression to extension between Late Jurassic and Early Cretaceous, but the extension process has not been well understood. Here we report the Meiziwo lamprophyres in northern Guangdong formed at Early Cretaceous (ca. 136 Ma; 40Ar/39Ar dating) are characterized by oceanic island basalt (OIB)-like geochemical signatures and Pb enrichment, together with moderately high (87Sr/86Sr)i
(0.70642-0.70766), positive εNd(t) (+4.6 to +5.1), intermediate (206Pb/204Pb)i (18.579-18.602), (207Pb/204Pb)i (15.727-15.733), (208Pb/204Pb)i (38.871-38.909). The lamprophyres were derived from an asthenosphere-dominated source, with recycled oceanic crust and terrigenous sediments contribution. In contrast, the previously reported ca. 154 Ma mafic dyke in eastern Guangdong, and the ca. 165-155 Ma A-type granite belt in southern Hunan and nearby regions, show arc-like signatures and unradiogenic Nd but moderately radiogenic Pb isotopes, suggesting magma derivation from a lithospheric mantle. Hence, the change from lithospheric to asthenospheric sources may be caused by progressive lithospheric extension and thinning
associated with the rollback and tearing of the paleo-Pacific plate. The 136 Ma Meiziwo lamprophyres, together with the 140 Ma asthenosphere-derived OIB-like mafic rocks nearby, signify the petrologic response to regional extension in the South China Block from ca. 140 Ma onward.
The South China Block (SCB) is considered to have undergone extensive reworking of continental crust. However, the mechanisms of this crustal reworking are unclear. Here, we report on Late Jurassic (162–155 Ma) two-mica granites and associated diorites and syenite porphyries from the Huolushan–Longyandong area of Guangzhou City, southeastern China. The two-mica granites have high SiO2 contents (69.6–73.6 wt%) and weakly peraluminous to strongly peraluminous compositions. These rocks have enriched Sr–Nd–Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)i = 0.7094–0.7145; εNd(t) = −10.5 to −9.3; εHf(t) = −12.7 to −4.2). The Huolushan two-mica granites have variable zircon O isotopic compositions (δ¹⁸O = 6.8‰–10.4‰), whereas the Longyandong two-mica granites have homogeneous compositions (δ¹⁸O = 8.6‰–9.9‰). The Huolushan syenite porphyries have moderate SiO2 (59.6–60.1 wt%) and high alkaline (K2O + Na2O = 9.4–9.8 wt%) contents. Their Sr–Nd–Hf–O isotopic compositions ((⁸⁷Sr/⁸⁶Sr)i = 0.7105–0.7119; εNd(t) = −9.9 to −9.8; εHf(t) = −14.1 to −5.9; δ¹⁸O = 6.5‰–10.6‰) are similar to those of the Huolushan two-mica granites. The Huolushan diorites have moderate SiO2 contents (59.9–60.4 wt%) and are high-K calc-alkaline and metaluminous with Mg# values of 43.2–43.8. Their SrNd isotopic compositions ((⁸⁷Sr/⁸⁶Sr)i = 0.7106–0.7109, εNd(t) = −8.8 to −8.6) are less enriched than those of the two-mica granites and syenite porphyries. In addition, the diorites have high zircon δ¹⁸O values (7.8‰–8.9‰) and a wide range of zircon εHf(t) values (−13.4 to −5.6). We propose that the Huolushan two-mica granites were derived from a metagreywacke-dominated hybridized source with minor juvenile crustal rocks and that the Longyandong two-mica granites were generated by partial melting of metagreywackes. The Huolushan syenite porphyries were formed by the mixing of metasedimentary-rock-derived felsic magmas and minor enriched-mantle-derived alkaline magmas. The Huolushan diorites were most likely derived by partial melting of mafic lower crust, with the dioritic magmas undergoing assimilation and fractional crystallization (AFC) during ascent. All of these intrusive rocks were likely formed in an extensional intraplate setting. The mantle-derived magmas, which could have been triggered by lithospheric extension, underplated the middle–lower crust and provided heat for crustal reworking and materials for late Mesozoic crustal growth.
SUMMARY: Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.
In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (≤1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (≤2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by
Petrology, magnetic susceptibilities, zircon U-Pb ages, zircon Hf isotopes and whole-rock geochemical data are used to constrain the evolution of Mesozoic high-potassium granitic rocks that record an Andean-type orogenic cycle in the southeastern China segment of the Western Pacific. Decreasing melting pressures of the granitic magmas from the Late Triassic to the Early Cretaceous, as reflected by decreasing Sm/Yb ratios, point to a general trend of crustal attenuation with time in western Zhejiang Province. Five distinct stages of granitic magmatism are identified: (1) 230 to 215 Ma: high-temperature, high-pressure dehydration melting in a reduced and thickened crust caused by flat-slab subduction of the paleo-Pacific Plate; (2) 170 to 150 Ma: low-temperature, high-pressure water-fluxed melting in an oxidized and thickened crust caused by the foundering of the paleo-Pacific Plate; (3) 140 to 130 Ma: low-temperature, low-pressure dehydration melting of the continental crust caused by extension of the lithosphere; (4) 130 to 125 Ma: high-temperature, low-pressure dehydration melting of the refractory materials in the continental crust caused by further extension of the lithosphere and possibly basaltic underplating; and (5) 115 to 100 Ma: emplacement of fractionation products of hydrous basalts from the enriched continental lithospheric mantle.
The origin and tectonic significance of high-K granites (>3 wt% K2O at 70 wt% SiO2), calc-alkaline I-type granites in particular, remain controversial. This paper takes granitic plutons distributed in the coastal region of the Guangdong Province of southeastern China as examples to explore the genesis of such rocks. SIMS zircon U–Pb geochronological data show that the granites were emplaced at 166–159 Ma. These granites can be subdivided generally into two groups on the basis of integrated mineralogical, geochemical, whole-rock Sr–Nd isotopic and in situ zircon Hf–O isotopic studies. The group A granites (SiO2 = 64–72 wt%) are characterised by their common occurrence of amphibole (±titanite) and dominantly metaluminous feature (A/CNK = 0.85–1.03). They are high in K2O (3.5–7.0 wt%) and K2O/Na2O (>1), and have trace element concentrations (e.g., Nb, Y, Zr and Ga) similar to typical I-type granites in the Lachlan Fold Belt, southeastern Australia. Their whole-rock ISr (0.7057–0.7077) and εNd(t) (−6.46 to −3.13) are less evolved than many coeval granites in this region. As in situ zircon Hf–O isotopes show little evidence of magma mixing, these granites with low zircon δ18O (6.3−7.9‰) and high εHf(t) (−5.9 to −0.2) could have been generated from melting of oxidised high-K basaltic rocks. The group B granites, emplaced to the east of group A granites, are dominantly weakly peraluminous (A/CNK = 1.00–1.05). They have higher SiO2 (70–76 wt%), less common or absence of amphibole, higher zircon δ18O (6.6−9.0‰) and lower εHf(t) (−11.4 to −5.9) than the group A granites. Zircon Hf–O isotope data reveal that the group B granites contain higher percentage of supracrustal materials than those of the group A, but the variations of major and trace elements do not support an assimilation and fractional crystallization (AFC) model. Instead, the group B granites, with features transitional between typical I-type and S-type granites, were most likely formed in a region where there was physical juxtaposition between infracrustal metaluminous and supracrustal peraluminous source rocks. Thus, granites of both groups represent products of crustal reworking likely due to asthenosphere upwelling and/or underplating and intrusion of mafic magmas. The close association in time and space of these granites with OIB-like basaltic rocks and the secular compositional change of Jurassic basaltic rocks in the region suggest that these rocks probably formed in an intraplate extensional setting resulted from the delamination of a flat-subducted oceanic slab.
Zircon U-Pb ages and in-situ trace elements and Hf-O isotope compositions, together with whole rock geochemical and Sr-Nd-Hf isotopic data, are presented for Cretaceous granitoids in southeastern (SE) China in order to establish their origin and the evolution of the underlying lithosphere during the Late Mesozoic. Two stages of Cretaceous magmatism, with contrasting geochemical features, have been identified: an earlier adakite-like biotite granite as represented by the Shangying pluton and a later enclave-bearing monzogranite as represented by the Zaoshan pluton. The Shangying biotite granites have a zircon U-Pb age of 99 ± 1 Ma. They have relatively low Y and Yb contents, with high La/Yb and Sr/Y ratios, showing geochemical features of adakite. Their Sr-Nd-Hf isotope compositions are similar to those of Early Cretaceous mafic rocks in the same area, indicating that they were generated by partial melting of juvenile granulitic crust at a depth of about 40 Km. The source was formed by underplating of enriched lithosphere mantle-derived magmas. In contrast, the Zaoshan calc-alkaline monzogranites, their enclaves and associated dolerite dykes from the Zaoshan pluton have an emplacement age of ~ 88 ± 1 Ma. The dolerites have high MgO contents, relatively low SiO2 concentrations and low La/Yb ratios, and depleted Hf isotope compositions. All these geochemical features suggest that they were derived from a depleted spinel Iherzolite mantle source. The enclaves have high SiO2 contents, indicating that they were derived from a crustal source. They have variable zircon Hf and O isotope compositions, suggesting that two components, i.e., a high εHf(t) and a low δ18O component and a low εHf(t) and high δ18O component, were involved in their origin. The high zircon εHf(t) values and low δ18O values are similar to those of the dolerites, indicating a common source. Thus, we suggest that the enclaves were generated by partial melting of newly underplated depleted mantle-derived materials. The monzogranites have distinctly different zircon Hf and O isotope compositions from the enclaves, indicating that the parental magmas were mainly derived from ancient crust that interacted with underplated depleted mantle-derived magmas. The monzogranites have relatively high HREE contents, suggesting a garnet-free source (< 32 km), distinct from the Early Cretaceous adakite-like granites that were generated from a garnet-bearing source. Combined with previously published data, it is evident that Early Cretaceous adakite-like magmatic rocks (107-99 Ma) and associated mafic rocks (100-107 Ma) were widespread in SE China, indicating a crustal thickening event that was possibly induced by underplating of mantle-derived magma. Subsequently, between 99 to 87 Ma, crustal extension and lithospheric thinning induced the later widespread I-type granites with similar geochemical features to the Zaoshan pluton. A-type granites and syenites of this age are also present in SE China. The transition in geochemical and isotopic data, from enriched to depleted Hf isotope compositions as seen in the monzogranites, their enclaves and intrusive dolerites, suggest that depleted mantle-derived materials were involved in the generation of the monzogranites, further indicating continental crustal reworking and later crustal growth in SE China during the early Late Cretaceous.
Whole-rock geochemical and Sr–Nd–Hf isotopic data and in
situ zircon U–Pb and Hf–O isotopes have been determined for
mafic (gabbro and diorite) and felsic (I- and A-type granites) rocks
from the Zhangzhou batholith in southeastern (SE) China, in order to
constrain their source and petrogenesis. The batholith consists of
gabbro, diorite, granodiorite, monzogranite and alkali feldspar granite,
with mafic microgranular enclaves in the monzogranite. Zircon SIMS and
LA–ICP–MS U–Pb dating gives consistent emplacement
ages of 107–97 Ma for these rocks, establishing that the mafic and
felsic magmas were coeval. The gabbros and diorites have relatively high
MgO contents (up to 5.2 wt.%) at low silica concentrations (up to 49.9
wt.%), with relatively homogeneous whole-rock initial
87Sr/86Sr ratios (~ 0.706), negative
ɛNd(t) values of ‑ 3.4 to ‑ 2.7,
chondrite-like ɛHf(t) values of ‑ 0.3 to + 0.5,
zircon ɛHf(t) values of ‑ 0.8 to + 3.2 and
δ18O values of + 5.0 to + 6.1‰, indicating that
they were derived by partial melting of an enriched subcontinental
lithospheric mantle, coupled with olivine and pyroxene fractionation.
The calc-alkaline granodiorites and monzogranites are metaluminous and
have relatively high SiO2 and low MgO contents. They have
whole-rock initial 87Sr/86Sr ratios of 0.706,
ɛNd(t) ratio of ‑ 3.0 to ‑ 4.0,
ɛHf(t) values of ‑ 2.0 to + 0.3 and zircon
ɛHf(t) values of ‑ 4.4 to + 0.3, with Nd and Hf
model ages of about 1.3 Ga, indicating that they were mainly derived by
partial melting of old continental crustal materials. The alkali
feldspar granites have geochemical features similar to A-type granite,
with relatively high K2O + Na2O and Nb contents
and FeO/MgO and Ga/Al ratios. They have distinct isotopic compositions
from the associated mafic rocks, with ɛNd(t) values
of ‑ 4.9, ɛHf(t) values of ‑ 3.2 to
‑ 3.5, zircon ɛHf(t) values of ‑ 5.1 to +
1.0 and δ18O values of + 5.1‰ to + 6.3‰.
They are depleted in Sr, Ba and Eu, indicating that they were mainly
derived from partial melting of crustal materials with plagioclase in
the residue. Field observations, as well as the petrographic and
geochemical data, suggest that the mafic and felsic rocks in the
Zhangzhou batholith- were the result of mixing between lithospheric
mantle-derived and crustally-derived magmas, coupled with crystal
fractionation, in an extensional setting related to Cretaceous
subduction of the Paleo-Pacific Plate.
Zircon U-Pb dating and Hf-isotope compositions and bulk
geochemical analyses of granites and metasedimentary rocks from the
NE-striking Pingtan-Dongshan Metamorphic Belt (PDMB), coastal
South China, are used to constrain their formation ages and tectonic
settings. LA-ICPMS zircon U-Pb dating results reveal four periods
of magmatism/metamorphism along the PDMB, including ~ 187 Ma,
133-130 Ma and 108-100 Ma granitic magmatism in the northern
PDMB and 147-140 Ma granites and metamorphic rocks in the southern
belt. A low P/T metamorphic event at 144-140 Ma and two
deformation phases at 130-125 Ma and 108-100 Ma are also
recognized. The Early Yanshanian high-K calc-alkaline granites
(187-140 Ma) are characterized by high contents of
SiO2, alkali and low light rare earth element (LREE)/ heavy
rare earth element (HREE) fractionation (LREE/HREE = 4.82-11.54,
(La/Yb)N = 3.99-13.53) and weak Eu depletion
(0.54-0.61), whereas the Late Yanshanian calc-alkaline granites
(140-100 Ma) have low A/CNK, middle LREE/HREE ratios
(10.13-19.18) and positive to inconspicuous Eu anomalies
(0.79-1.95). Granites in both periods are depleted in high field
strength elements (HFSE, e.g. Nb, Ta, Ti) but enriched in LREE and large
ionic lithophile elements (LILE), with low Rb/Ba and Rb/Sr ratios and
A/CNK values, indicating that they are typical I-type granites in the
arc-related tectonic setting due to subduction of the Paleo-Pacific
plate. However, both the Early and Late Yanshanian granites have
distinct Sr, Nb and K2O contents and La/Yb, Sr/Y and Eu/Eu*
ratios, suggesting different P-T conditions. Zircon grains from
the granites in the northern PDMB have higher Hf-isotope compositions
with ɛHf(t) = + 0.01 to + 6.47 and Hf model ages
(TCDM) = 0.82-1.16 Ga, whereas those from
the granites in the southern PDMB show slightly lower Hf-isotope
compositions with ɛHf(t) = - 6.63 to + 0.07 and
older Hf model ages (TCDM) = 1.19-1.61 Ga.
Compared with the metamorphic basement near the study area, the granites
in the northern PDMB were probably derived from a Neoproterozoic crust,
whereas those in the southern PDMB may have originated from a mixed
source of Neoproterozoic and minor older crustal materials. We propose
that the Early Yanshanian granites in the PDMB are the eastern extension
of the E-W-striking granitoid-volcanic belt in the Nanling
Range, whereas the Late Yanshanian igneous rocks mostly distribute in
coastal Southeast China and strike NE. The distinct distribution and
geochemical features between the Early and Late Yanshanian magmatic
events can be related to the change of direction and dip angle of the
Paleo-Pacific plate that was subducted underneath the Eastern Eurasian
continent in the two episodes.
Zircon U–Pb and Hf isotope compositions have been systematically measured to constrain the timing and petrogenesis of the Late Mesozoic volcanic sequences in southeastern Zhejiang, SE China. The Early Yanshanian (Jurassic) volcanism took place at 177 Ma and formed scattered volcanic rocks in the region. In contrast, the large-scale Late Yanshanian (Cretaceous) volcanism, with multiple eruptive stages between 140 and 88 Ma, formed the widespread lower and upper volcanic series. Two volcanic hiatuses were clearly identified at 120–110 Ma between the lower and upper volcanic series and at 128–122 Ma between the two subdivided cycles within the lower series. In addition, the magnitude of the two volcanic cycles in the lower series is significantly different. The cycle I of the lower series (140–128 Ma) occurred in the largest scale, which is much larger than the cycle II (122–120 Ma) as well as the whole upper series (110–88 Ma). Petrogenetically, the widespread Late Mesozoic volcanic rocks were mainly derived from the partial melting of the Paleoproterozoic crustal basement materials, with obvious addition of juvenile materials after 122 Ma. The analyzed zircons of the Early Yanshanian volcanic rocks show crustal model ages (TDM2) mainly of 2.00–2.16 Ga, and weighted mean εHf(t) value of − 13.4. The εHf(t) and TDM2 values are ca. − 10 and 1.67–2.00 Ga for zircons from the cycle I volcanic rocks, − 11.4 to − 1.4 and 1.27–1.90 Ga for those from the cycle II volcanic rocks, − 9.3 to − 2.7 and 1.33–1.78 Ga for those from the acidic rocks of the upper series, respectively. The characteristics of the Late Mesozoic volcanic sequences indicate the varying influence of the subduction of the paleo-Pacific plate. Our study supports that the subduction style changed from low-angle to intermediate-angle forward subduction at the first hiatus of 128–122 Ma and then dramatically changed to rapid high-angle rollback subduction at the second hiatus of 120–110 Ma.
This paper reports on a rare magmatic suite of adakitic rocks and associated magnesian and potassium-rich magmatic enclaves and dikes, which occur in the Tunchang–Fengmu area, Hainan Island (Southeast China). LA-ICP‐MS zircon U–Pb age data show that they were generated in the late Early Cretaceous (~ 107 Ma). The adakitic rocks, consisting mainly of granodiorites and biotite granites, are high-K calc-alkaline and have low Mg# values (0.27–0.50). They are geochemically similar to slab-derived adakites, e.g., with high SiO2, Al2O3, Sr, Sr/Y and La/Yb values, low Y and Yb contents, and negligible Eu and positive Sr anomalies. They also have relatively uniform (87Sr/86Sr)i (0.7086–0.7096), (206Pb/204Pb)i (18.50–18.61), (207Pb/204Pb)i (15.56–15.64) and (208Pb/204Pb)i (38.17–38.44) isotope ratios, with slightly variable εNd(t) (− 3.85 to − 6.55) and zircon in situ εHf(t) (− 4.7 to + 1.7) values. The mafic enclaves and dikes display disequilibrium textures (e.g., multiple-zoned clinopyroxene with low-MgO rims in contact with perthite and quartz microcrystals). They are high-K calc-alkaline and shoshonitic, and all but one sample have high Mg# (0.63–0.72) values. These mafic rocks are characterized by light rare earth element enrichment and heavy rare earth element (REE) depletion, negligible Eu and Sr and positive Pb anomalies, and Nb and Ta depletion. They have slightly more variable initial 87Sr/86Sr isotope ratios (0.7064–0.7086), εNd(t) (− 5.1 to + 0.1) values, and (206Pb/204Pb)i (18.35–18.50), (207Pb/204Pb)i (15.45–15.59) and (208Pb/204Pb)i (38.18–38.70) ratios. One mafic dike sample has zircon in situ εHf(t) values (− 4.94 to − 2.42) similar to those of adakitic rocks (− 4.7 to + 1.7) in the area. We suggest that the adakitic rocks were most likely generated by partial melting of newly underplated basaltic lower crust with arc-like geochemical characteristics, and the primitive compositions of the mafic enclaves and dikes likely originated from lithospheric + asthenospheric mantle sources metasomatized by subducted oceanic sediments or a relatively juvenile lithospheric mantle source. Mantle-derived primitive magmas likely underwent mixing at depth with minor crustally-derived felsic magmas before being injected into the adakitic magma chamber. Such injections may have broken up the magma into discrete globules and convective motion distributed the enclaves through the adakitic host. Asthenosphere upwelling due to the roll-back of the subducted Paleo-Pacific plate likely triggered the coeval late Early Cretaceous crust- and mantle-derived magmatism, resulting in the magma hybridization observed on Hainan Island.
In this work, we established a highly reproducible analysis of Sm, Nd concentrations and Nd isotopic compositions in geological samples by isotope dilution analysis with MC-ICP-MS. This technique is superior in terms of the analytical reproducibility or rapidity of analysis compared with quadrupole ICP-MS or with thermal ionization mass spectrometry (TIMS) isotope dilution techniques. Samples were spiked with 149Sm–150Nd enriched tracer and then digested by a commonly used HF, HNO3 and HClO4 acid protocol. The bulk rare earth elements (REEs) were separated from the sample on a standard cation exchange resin, and further purified on Eichrom Technologies Ln Resin, to obtain Sm and Nd fractions prior to mass spectrometric measurements. Replicate analyses of international certified reference materials (CRMs) demonstrate that our obtained 147Sm/144Nd and 143Nd/144Nd isotopic ratios are in good agreement with previously published values from isotope dilution methods. In addition to determining the concentrations of Sm and Nd, the Nd isotopic composition can be measured simultaneously during Nd isotope dilution run. Additionally, a mineralSm-Nd isochronal age that is identical to, within error, a U-Th-Pb zircon age for the same rock is further measured and validates the robustness of the present protocol. Therefore, the high actual sample throughput inherent to the MC-ICP-MS can be fully exploited for the determination of Sm and Nd concentrations and Nd isotopic compositions.
Experimental petrology can be used in forward and inverse approaches. The forward approach defines the compositions of liquids generated by partial melting of possible source rocks at various depths. The inverse approach determines conditions for multiple-mineral saturation at the liquidus of primitive magmas, correlates them with residual minerals of possible source rocks, and thus provides estimates of depths and temperatures required for their derivation. Review of a selection of forward and inverse results is followed by evaluation of petrological and geophysical processes in layered mantle and in subduction zones. Physical constraints imposed by solidus curves and geotherms present problems for models that derive basalts from deep mantle reservoirs, separated from overlying convecting layers. Magmas from mantle are limited to compositions less siliceous than basaltic andesite, with rare exceptions. Granite liquids cannot be generated from normal peridotite, nor from oceanic crust at mantle pressures in subduction zones. In continental crust, hydrous granite liquid is generated at depths of less than 30 km. Basaltic andesite and picritic basalt are parental magmas for the calc-alkaline series. Andesite is not primary from subcontinental depths, and can be generated as liquid in continental crust only if temperatures exceed about 1100 degrees C. Calc-alkaline magmas may contain components from mantle peridotite, subducted oceanic crust, and continental crust.
The nonuniqueness in determining the composition of the crust can be reduced by comparing the data for P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs ratio, mass density (ρ) and Lamé impedances (ρλ, ρμ) to data determined from laboratory measurements of these physical variables on a variety of crustal rock samples. The composition of the crust along the Tunxi-Wenzhou transect, southeastern China, is presented as a model based on a complete set of geophysical data involving seismic P-wave and S-wave velocity, density, gravity, heat flow, and temperature surveys, which allow us to place tighter constraints on possible crustal models. The Yangtze and the Cathaysia blocks have a crustal structure characterized by remarkable differences between the upper, middle, and lower crust. Integrated geophysics data are here interpreted to indicate: (1) an average composition of granite gneiss for the upper crust, with presence of mica quartz schist, felsic granulite, paragranulite, and granite-granodiorite beneath the Yangtze block, and basalt down to the upper part of the middle crust just over the Jiangshan-Shaoxing fault, followed laterally by granite gneiss beneath the Songyang, Qintiang, and Wenping depressions; (2) granite-granodiorite and biotite gneiss for the upper half of the middle crust, with presence of mica quartz schist beneath the Cathaysia block; (3) gabbro and in less proportion basalt for the lower part of the middle crust; and (4) amphibolite for the lower crust, with presence of mafic garnet granulite just over the Jiangshan-Shaoxing fault and beneath the Wenping depression.
To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma Talun metagranite, the oldest granitic intrusion ever reported in the Taiwan region and along the eastern coast area of South China. In spite of a large age uncertainty, the occurrence of this metagranite is not consistent with the apparent younging trend of Jurassic‐Cretaceous igneous activity toward the coastline in South China, and should be taken into consideration by future studies.
[1] Abstract: The application of multiple collector inductivelycoupled plasma source mass spectrometry(MC-ICPMS) to 176 Lu- 176 Hf and 92 Nb- 92 Zr chronometryhas been hampered bycomplex Zr-Hf purification procedures that involve multiple ion exchange column steps. This studypresents a single-column separation procedure for purification of Hf and Lu byion exchange using Eichrom 1 LnSpec resin. The sample is loaded in pure HCl, and element yields are not dependent on the sample matrix. For 92 Nb- 92 Zr chronometry, a one-column procedure for purification of Zr using Biorad 1 AG1- 8 resin is described. Titanium and Mo are completelyremoved from the Zr, thus enabling accurate 92 Zr measurements. Zirconium and Nb are quantitativelyseparated from rock samples using Eichrom Ln-Spec resin, allowing measurements of Zr/Nb with a precision of better than ±5% (2s). The Ln-Spec and anion resin procedures maybe combined into a three-column method for separation of Zr-Nb, Hf, Ta, and Lu from rock samples. For the first time, this procedure permits combined isotope dilution measurements of Nb/Ta, Zr/Hf, and Lu/Hf using a mixed 94 Zr- 176 Lu- 180 Hf- 180 Ta tracer. Analytical protocols for Zr and Hf isotope measurements using the Micromass Isoprobe, a second generation,
A 400km-long wide-angle seismic profile in the South China crosses three tectonic domains: southeast continental margin of Yangtze block, northwest continental margin of Cathaysia block, and South China Suture Zone separating these two blocks. The crustal velocity model constructed from traveltime fitting shows that the crust thickness thins from the northwest to the southeast along the profile. It also reveals the crustal contact relationship between the Yangtze and Cathaysia blocks, where the Wuchuan-Sihui fault is the boundary between them. Combining this profile with two more profiles in the continental South China and three profiles in the northern margin of South China Sea, constructs a continent–ocean transition section. This cross-section reveals that the Moho depth shallows gradually along the cross-section from the Yangtze block to the Cathaysia block and the northern margin of South China Sea, but shallows abruptly in the continent–ocean transition to the South China Sea.
South China hosts the most abundant and largest tungsten (W) deposits in the world, being a famous W metallogenic region. Located at the eastern part of the South China Block, which was formed by amalgamation of the Yangtze and Cathaysia Blocks during the Neoproterozoic, these W deposits were mainly formed during the Mesozoic. The W mineralization is dominanted by greisen, quartz-vein, skarn, and porphyry types, all of which are genetically related to the evolution of highly fractionated granitoids. Four episodes of W mineralization are recognized: 1) Late Triassic (230 to 210 Ma) in the central and western parts of South China; 2) Middle Jurassic (ca. 170 Ma) to Early Cretaceous (ca. 140 Ma) in the interior of South China, with the mineralization being concentrated in southern Jiangxi Province between 165 and 150 Ma; 3) Early Cretaceous (136 to 120 Ma) with deposits across South China; and 4) Late Cretaceous (100 to 80 Ma) mainly in the southwestern parts of South China. These four periods of mineralization are closely related to the closure of paleo-Tethys and subduction of the paleo-Pacific plate. In the Late Triassic, these two events caused local extensional environments, facilitating emplacement of the peraluminous granitoids, and formation of the W deposits. In the Middle Jurassic, break-off of the subducting oceanic plate resulted in emplacement of highly fractionated granites in the Nanling region. Later anticlockwise rotation of the paleo-Pacific plate created widespread S-type granitoids and associated Middle Jurassic to Early Cretaceous W mineralization in the interior of South China. Since 136 Ma, rollback of the subducting Pacific plate resulted in weak W mineralization across South China. Finally, a change of direction in the retreating plate from SE to ESE resulted in intensive mineralization of the southwestern part of South China.
The Longwo pluton is a typical example of the Early Yanshanian granites with mantle components in Nanling region. Lithologically, this pluton consists mainly of granodiorite, and contains melanocratic dioritic enclaves. Zircon ELA-ICP-MS dating for the granodiorites yields an age of 169. 1 ± 2. 5 Ma, indicating that they were formed in Middle Jurassic. Geochemically, the granodiorites are weakly peraluminous ( A/ NKC = 1.0 ∼ 1.1), and are relatively poor in alkalis and rich in potassium ( K2 O/Na2 O = 1.15 ∼ 1.45). The rocks also have enriched LREE and LILE (e. g., Rb, Cs, Th, U) concentrations, and low HFSE (e. g., Nb, Ti) abundances. Dioritic enclaves have similar mineral assemblages to those of the host rocks, but contain more mafic minerals and higher contents of siderophile elements (e. g., V, Cr, Co, Ni). The variation trends of major and trace element abundances between the enclaves and the host rocks suggest a mixing process in their petrogenesis. They also display close Sr, Nd isotopic compositions, with ISr and εNd (t) values of 0.70843 - 0.70995, -6.53 - -8.89 for the host granodiorites, and of 0.70797 - 0.70882, -4.71 - -9.24 for the dioritic enclaves, which are all indicative of mixing of crustal materials with mantle components. A simple binary mixing calculation shows that the ratios of mantle components involved in the genesis ranged from 32.9% to 40.4% in the host granodiorites, and from 31.8% to 46.4% in the dioritic enclaves. Integrated geological and geochemical data suggested that the Longwo granodiorites and their dioritic enclaves were generated by mixing of mantle-derived basic magma and its induced crustal felsic magma under a tensile environment.
We report analyses of the 176Hf/ 177Hf ratio for 25 chondrites from different classes of meteorites (C, O and E) and the 176Lu/ 177Hf ratio for 23 of these as measured by plasma source mass spectrometry. We have obtained a new set of present-day mean values in chondrites of 176Hf/ 177Hf = 0.282772 ± 29 and 176Lu/ 177Hf = 0.0332 ± 2. The 176Hf/ 177Hf ratio of the Solar System material 4.56 Ga ago was 0.279742 ± 29. Because the mantle array lies above the Bulk Silicate Earth in a 143Nd/ 144Nd versus 176Hf/ 177Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high 3He/ 4He ratios. Comparison of observed Hf/Nd ratios with those inferred from isotopic plots indicates that, in addition to the two most prominent components at the surface of the Earth, the depleted mantle and the continental crust, at least one more reservoir, which is not a significant component in the source of oceanic basalts, is needed to account for the Bulk Silicate Earth Hf-Nd geochemistry. This unaccounted for component probably consists of subducted basalts, representing ancient oceanic crust and plateaus. The lower continental crust and subducted pelagic sediments are found to be unsuitable candidates. Although it would explain the Lu-Hf systematics of oceanic basalts, perovskite fractionation from an early magma ocean does not account for the associated Nd isotopic signature. Most basalts forming the mantle array tap a mantle source which corresponds to residues left by ancient melting events with garnet at the liquidus.
We performed vapor-absent melting and crystallization experiments on two bulk compositions that model metamorphic rocks containing
a single hydrous phase: a biotite gneiss [37% bio (mg-number 55), 34% qtz, 27% plg (An38), 2% ilm] and a quartz amphibolite [54% hbl (mg-number 60), 24% qtz, 20% plg (An38), 2% ilm]. Experiments were performed at 3 and 5 kbar in internally heated pressure vessels (IHPV), and at 7, 10, 12·5 and
15 kbar in piston cylinder apparatus (PC), from the vapor-absent solidi to (at least) the temperature at which the hydrous
mineral disappeared. Dehydration-melting begins at similar temperatures in both bulk compositions, ranging from T∼850°C at
P = 3 kbar T∼930°C at P = 15 kbar. The hydrous mineral disappears ∼50°C above the solidus in both systems, except in IHPV experiments at f(O2) above Ni–NiO, in which biotite stability extends up to atleast 80°C above the solidus. At the T at which the hydrous minerals disappear the biotite gneiss produces 2–3 times more melt than the quartz amphibolite (50–60
wt% vs 20–30 wt%). In both systems, variations in melt productivity with P are controlled by three competing factors: (1) the positive d P/dT slopes of the solidi, (2) decreasing H2O activity with increasing P at constant H2O content, and (3) Na2O activity, which increases with P concomitantly with breakdown of plagioclase. Melt productivities at T = 920–950°C are maximized at intermediate pressures (∼7 kbar). The biotite gneiss produces strongly peraluminous granitic
melts (SiO2>70 wt%) and residual assemblages of quartz norite (P>12·5 kbar) or garnet pyroxenite (P>12·5 kbar). The quartz amphibolite produces strongly peraluminous granodioritic melts (SiO2>70 wt%) that coexist with clinopyroxene + orthopyroxene + plagioclase + quartz ± at P>10 kbar)garnet. The results of coupled melting and crystallization experiments on the quartz amphibolite suggest that strongly
peraluminous granitoid rocks (e.g. cordierite-bearing and two-mica granites) can be derived from melting of Al-poor protoliths.
The geochemistry of granitic rocks appears to contain information about some aspects of the preanatectic textures of the protoliths. There are marked contrasts in the covariations between Ba (a proxy for melting proportion in the protolith) and the P2O5, TiO2, Y, Zr and Ce concentrations in I-and S-type granitic magmas (rocks). These contrasts are interpreted as features resulting from the interplay between differences in the locations of accessory mineral grains in the protoliths, their solubilities in the granitic melts and the degree of foliation developed in the protoliths. It is inferred that S-type protoliths are more highly foliated, with accessory minerals concentrated into the biotite-rich layers that form the foci for partial melting. I-type protoliths appear to have less pronounced foliation and a more random distribution of both hydrous mafic silicates and accessory grains.
Field observations from an exposed section of deep continental crust, the Athabasca granulite terrane (AGT), Saskatchewan, Canada, provide a view of granite genesis and a mechanism for deep-seated contamination of felsic and mafi c magmas. The 1.9 Ga Chipman mafi c dike swarm was emplaced into the Chipman Tonalite (ca. 3.3 Ga) and the megacrystic Fehr granite (ca. 2.6 Ga) at a crustal depth of similar to 40 km. The Fehr granite shows evidence for extensive partial melting and generation of granitic leucosome. Mafi c dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures. The AGT provides a view of a dynamic, heterogeneous, and locally fertile deep crust. Mantle-derived mafi c magma promotes extensive partial melting of fertile granitoids, which in turn fi lter and entrap later mafi c dikes and sills. The result is almost inevitable mingling and hybridization (i.e., contamination) of mafi c and felsic end members. This interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa.
Late Cretaceous granitic rocks constitute an essential part of the pre-Tertiary Tananao metamorphic basement complex of Taiwan. They are dominantly of granodiorite to quartz monzonite composition. Most granitic rocks are peraluminous (A/CNK > 1.0 and normative corundum > 1%) and display moderately fractionated LREE and relatively unfractionated HREE patterns with negative Eu anomalies. On a primitive mantle-normalized trace-element diagram, they show a significant Nb depletion which is typical of the calc-alkaline magmatism from the subduction-zone environment. They fall within the volcanic arc field on the discrimination diagram of Pearce, Harris and Tindle (1984). The lack of systematic inter-element relationships suggests that the role of fractional crystallization is not significant and that these granitic rocks were derived from heterogeneous protoliths. Geochemical data suggest Taiwan granitic rocks are contaminated I-type and I-type granites related to the subduction of the Paleo-Pacific plate beneath the eastern margin of the Eurasia plate during late Mesozoic time.
We report in the paper integrated analyses of in situ zircon U–Pb ages, Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes for the Longlou granite in northern Hainan Island, southeast China. SIMS zircon U–Pb dating results yield a crystallization age of ∼73 Ma for the Longlou granite, which is the youngest granite recognized in southeast China. The granite rocks are characterized by high SiO2 and K2O, weakly peraluminous (A/CNK = 1.04–1.10), depletion in Sr, Ba and high field strength elements (HFSE) and enrichment in LREE and large ion lithophile elements (LILE). Chemical variations of the granite are dominated by fractional crystallization of feldspar, biotite, Ti–Fe oxides and apatite. Their whole-rock initial 87Sr/86Sr ratios (0.7073–0.7107) and εNd(t) (−4.6 to −6.6) and zircon εHf(t) (−5.0 to 0.8) values are broadly consistent with those of the Late Mesozoic granites in southeast China coast. Zircon δ18O values of 6.9–8.3‰ suggest insignificant involvement of supracrustal materials in the granites. These granites are likely generated by partial melting of medium- to high-K basaltic rocks in an active continental margin related to subduction of the Pacific plate. The ca. 73 Ma Longlou granite is broadly coeval with the Campanian (ca. 80–70 Ma) granitoid rocks in southwest Japan and South Korea, indicating that they might be formed along a common Andean-type active continental margin of east–southeast Asia. Tectonic transition from the Andean-type to the West Pacific-type continental margin of southeast China likely took place at ca.70 Ma, rather than ca. 90–85 Ma as previously thought.
The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U-Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89-1.83 Ga, ~ 1.43 Ga, ~ 1.0-0.98 Ga and ~ 0.82-0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (TDMC) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9-1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga.Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia at ~ 1.9-1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0-0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6-0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high crustal incubation time of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China-Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen.
The Quanzhou (QZ) and Huacuo (HC) gabbro–granite complexes on the southeast coast of Fujian, South China, are important components of a Late Mesozoic calc-alkaline volcanic–plutonic belt in the region. The complexes provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas, and their interactions within the intrusive environment. The complexes are composed mainly of monzogranite and biotite granodiorite in the QZ complex, and biotite granite in the HC complex, with lesser amounts of hornblende gabbro. Zircon U–Pb dating provides consistent crystallization ages of 109 ± 1 Ma and 108 ± 1 Ma for the QZ gabbros and monzogranites, and an age of 111 ± 1 Ma for the HC gabbro, which is contemporaneous with the spatially associated HC granites. Both the mafic and felsic intrusions in these complexes are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and are depleted in high-field-strength elements (HFSEs; e.g. Nb and Ta). They show similarly homogeneous Sr–Nd isotopic compositions. All these factors indicate a close genetic relationship between the gabbroic and granitic rocks in the QZ and HC complexes. Although the enriched Sr–Nd isotopic signatures of the QZ and HC gabbros seemingly point to an enriched mantle source (EM-1), they have highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from negative to positive (specifically –4.6 to +6.1 for the QZ gabbros and –4.8 to +11.6 for the HC gabbros). We interpret the parental basic magmas of these gabbros to have received contributions from a depleted mantle source and crustal components. Contributions from such a depleted mantle source resulted in the growth of juvenile basaltic lower crust, the partial melting of which generated the parental felsic magmas of the QZ and HC complexes. Furthermore, based on a synthesis of petrography, geochronology, elemental and isotopic geochemistry and tectonics, we propose that break-off and rollback of the Late Mesozoic subducted Palaeo-Pacific Plate triggered the upwelling of asthenospheric mantle below the coastal area of the South China Block, which induced extension of the overlying continental lithosphere, and finally initiated the large-scale Late Yanshanian magmatism in the study area.
The Mesozoic geology of southeastern (SE) China was characterized by
emplacement of voluminous felsic magmas. We present zircon
U-Pb-Hf isotopic compositions and whole-rock major and trace
elemental compositions of a series of felsic volcanic rocks from the
eastern Guangdong and Fujian provinces. The volcanic rocks have been
divided into three formations (the Douling, Nanyuan and Shimaoshan
Fms.). Zircon U-Pb dating defines three eruption cycles:
168-145 Ma for the Douling Fm., 143-130 Ma for the Nanyuan
Fm., and 104-95 Ma for the Shimaoshan Fm. In situ Hf isotope
analyses on dated zircon yield an ɛHf(t) range of
- 13.8 to + 6.9 and a TDM(Hf) range from 1400 to 486 Ma
with a peak at ca. 930 Ma. The majority of the Mesozoic felsic lavas
have more radiogenic Hf than that of metamorphic basement rocks and
related igneous rocks, requiring the involvement of a juvenile component
in their origin. The Mesozoic felsic magmas are predominantly dacites
and rhyolites - with only minor basalt and andesite - and
show relatively homogeneous Hf isotopic compositions of zircon in single
sample. Combined these data indicated that neither differentiation of a
mantle-derived magma, nor mixing of magmas from different sources, was a
dominant petrogenetic process. Both the ɛHf(t) of
zircon and the magma temperature increase from the Douling and Nanyuan
Fms. to the Shimaoshan Fm. All these facts imply an increasing
contribution of contemporaneous underplated hot and dry mantle-derived
magmas, as both a heat source inducing crustal melting and as a source
of material (melt) that variably mixed with the local crustal melts. The
felsic crustal melt component was generated at amphibolite-facies
conditions with a residual assemblage of plagioclase +/-
hornblende +/- garnet +/- zircon. Our results suggest a
progressive role of crust-mantle interaction in generating the
episodic felsic volcanic eruption during the Mesozoic.
The coastal Changle-Nan’ao tectonic zone of SE China contains important geological records of the Late Mesozoic orogeny and post-orogenic extension in this part of the Asian continent. The folded and metamorphosed T3–J1 sedimentary rocks are unconformably overlain by Early Cretaceous volcanic rocks or occur as amphibolite facies enclaves in late Jurassic to early Cretaceous gneissic granites. Moreover, all the metamorphic and/or deformed rocks are intruded by Cretaceous fine-grained granitic plutons or dykes. In order to understand the orogenic development, we undertook a comprehensive zircon U–Pb geochronology on a variety of rock types, including paragneiss, migmatitic gneiss, gneissic granite, leucogranite, and fine-grained granitoids. Zircon U–Pb dating on gneissic granites, migmatitic gneisses, and leucogranite dyke yielded a similar age range of 147–135 Ma. Meanwhile, protoliths of some gneissic granites and migmatitic gneisses are found to be late Jurassic magmatic rocks (ca. 165–150 Ma). The little deformed and unmetamorphosed Cretaceous plutons or dykes were dated at 132–117 Ma. These new age data indicate that the orogeny lasted from late Jurassic (ca. 165 Ma) to early Cretaceous (ca. 135 Ma). The tectonic transition from the syn-kinematic magmatism and migmatization (147–136 Ma) to the post-kinematic plutonism (132–117 Ma) occurred at 136–132 Ma.
The Phanerozoic tectonic regimes of the South China Block (SCB) hold a key to understanding of its geodynamic evolution with respect to formation of numerous mineral resources. Despite long-time debates in the past three decades, there is still no consensus on the two key points whether the Phanerozoic tectonothermal events were due to subduction of the Pacific plate or intracontinental reworking and whether the three periods of tectonothermal events in the middle Paleozoic (Kwangsian), Triassic (Indosinian) and Jurassic–Cretaceous (Yanshanian) are mainly driven by tectonic transition in subduction of the oceanic crust from Paleotethyan in the west to Pacific in the east. This paper presents an overview of key geological observations in the SCB with respect to its Phanerozoic tectonics. Available data show that there are distinctive sedimentary, magmatic, structural and metamorphic records across the Xuefeng-Jiangnan Domain in the SCB. The geological signatures associated with the Kwangsian and Indosinian tectonothermal events are predominantly preserved in the eastern SCB, including the eastern Yangtze and Cathaysia Blocks to the east of the Xuefeng-Jiangnan Domain. They are characterized by strong thrusting/transpression, anatexic granitic magmatism, high-grade metamorphism and the poor involvement of the juvenile mantle-derived rocks. The two events were dated at ca. 400–460 Ma and ca. 200–250 Ma, respectively. The Yanshanian tectonothermal event is dominantly represented by the development of a wide magmatic belt of exceeding 1300 km (from the coastal province to the Xuefeng-Jiangnan Domain) and a broad deformational belt of more than 2000 km (from the coastal province to the Sichuan basin). The Yanshanian I-, S- and A-type granites, syenite and volcanic rocks display two arrays, which are oblique and parallel to the coastal provinces of the southeast China, respectively. They were mainly formed at the three age-spans of 152–180 Ma, 120–130 and 87–107 Ma with the peak of 158 Ma, 125 Ma and 93 Ma, respectively. The stillstand time of the Yanshanian magmatism was temporally overlapped by the deformation time of the top-to-the-NW progressive transpression or sinistral strike-slip at 132–142 Ma and 95–112 Ma, respectively. In conjunction with the observations and controversies, a geodynamic model is proposed for the Mesozoic tectonic evolution of the SCB.
Detrital zircon provenance data for the Tananao schist in eastern Taiwan is consistent with its protolith being deposited on the South China continental margin at around, or soon after, 150 Ma, rather than being of an exotic origin and much older as previously suggested. The absence of ca. 200 Ma zircons agrees with the presence of a magmatic gap in the region after the orogenic and magmatic front migrated to central South China, due to a flat-slab subduction. The characteristic lack of input from interior South China (i.e., the lack of 1100–750 Ma and 470–420 Ma populations), and the immature nature of some of the schist units, suggest that they were sourced from the nearby coastal regions. On the other hand, they exhibit a dominant 190–150 Ma magmatic zircon population, suggesting the presence of abundant magmatic rocks of that age along the coastal regions. This, along with our newly discovered ca. 180 Ma I-type granites from eastern Zhejiang and other ca. 190–180 Ma magmatic rocks recently reported from the coastal regions, led us to propose that a new continental arc was initiated after ca. 190 Ma along the coastal region after a magmatic gap due to flat-slab subduction. This newly initiated arc likely persisted until ca. 90 Ma, and is represented by the I-type granitic rocks in eastern Taiwan. Slab roll-back likely caused the arc system to retreat towards the Pacific Ocean after 90 Ma, and ca. 60–17 Ma bimodal magmatism adjacent to the South China Sea signifies continental margin extension in the lead-up to, and during, the opening of the South China Sea. We thus argue that the continental margin of East Asia was transformed from an Andean-type plate margin at 280–90 Ma, to the present-day Western Pacific-type plate margin soon after 90 Ma.
UPb zircon/monazite dating and Sr-isotope results for granitoids from Taiwan and Chinmen-Lieyü, SE China place important constraints for the tectonic evolution and the timing of felsic magmatism associated with mid-Cretaceous subduction process. The Chinmen gneiss (139.4 Ma) and granite (100.9 Ma) represent the oldest granitic units and belong to the previously recognized late Yanshanian phase of magmatism. A younger suite of granitoids occurs in the Tananao Metamorphic Complex of Taiwan and includes the Yuantoushan and Tachoshui granites (87.3 Ma) and the Kanagan gneiss (90.2 Ma). These mid-Cretaceous ages for the younger granitoid suite mitigate against the existence of the hypothetical late Paleozoic “Dongnanya” microcontinent in the study area as proposed by other investigators. UPb zircon inheritance ages, ranging between 1.67 and 2.09 Ga indicate the presence of a Precambrian basement beneath much of coastal SE China and Taiwan, however, there is no evidence of Archean crust as indicated by zircon inheritance patterns for rocks west of the present study area.
Zircon megacrysts represent a late stage in the crystallisation of the magmas that produced the low-Cr megacryst suite (Ol+Opx+Cpx+Gnt+Ilm+Phl+Zir) found in many kimberlites, and may carry information on the sources of the parent magmas and the interaction of these magmas with the cratonic lithosphere. The isotopic composition of Hf has been measured in 124 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites, using a laser-ablation microprobe (LAM) and a multi-collector (MC) ICPMS. The zircons range in age from 90 Ma to ca 2500 Ma, allowing indirect analysis of mantle-derived Hf over a long time span. Most values of εHf fall between 0 and +10, but zircon suites from several kimberlites range down to εHf = −16. Combined with published Nd data on the silicate members of the low-Cr megacryst suite, these data indicate crystallisation of zircon from magmas lying well below the terrestrial εHf-εNd array. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived peridotite xenoliths suggest that cratonic lithospheric mantle has Hf/Nd (0.3–0.5) greater than estimated Bulk Silicate Earth. The depleted and metasomatised lherzolites and harzburgites that make up much of the Archean lithospheric mantle have Lu/Hf ratios (≤0.15) low enough to account for the lowest εHf observed in the zircons, over time spans of 1–3.5 Ga. We therefore suggest that the magmas from which the kimberlitic zircons crystallised were derived from Depleted Mantle or OIB-type sources, and developed negative εHf through reaction with the subcontinental lithospheric mantle.
We describe a precise and accurate method for the direct determination of the Sr-87/Sr-86 isotope ratio of bottled Sr-rich natural mineral drinking water using multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The method is validated by the comparative analysis of the same water with and without cation-exchange resin purification. The work indicates that isobarically interfering elements can be corrected for when Rb-87/Sr-86 < 0.05 (Rb/Sr < 0.015), and that the matrix elements (Ca, Mg, K and Na) have no significant effect on the accuracy of the Sr isotope data. The method is simple, rapid, eliminates sample preparation time, and avoids potential contamination during complicated sample-preparation procedures. Therefore, the high sample throughput inherent to the MC-ICP-MS can be fully exploited.
Several Late Yanshanian syenitic and gabbroic rocks from the coastal area of southeastern (SE) China have been studied to determine zircon U–Pb ages and Hf isotopic compositions as well as whole-rock elemental and Sr–Nd isotopic compositions to constrain their sources and tectonic settings. The systematic LA-ICP-MS zircon U–Pb dating results indicate that they can be divided into two main stages: the early stage (141–118Ma) and the late stage (98–86Ma), which temporally correspond to the regional lower and upper volcanic series, respectively. The early stage syenitic and gabbroic rocks are accompanied by vast rhyolitic and dacitic volcanic rocks with minor granites. Their parental magma was derived from an enriched mantle metasomatized by subducted sedimentary materials. The late stage syenitic rocks mainly occur as the central intrusion of the caldera with ages close to the accompanied volcanics, forming a caldera-related volcanic-intrusive ring complex. They were produced by magma mixing between depleted asthenosphere melts and subduction-related enriched mantle melts. It is suggested that the Late Yanshanian tectonic settings of SE China transformed from a compressional environment to an extensional environment, corresponding to the transformation from forward to rollback subduction of the paleo-Pacific plate at approximately 110Ma.
A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantle- derived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.
The origin of different kinds of granitic rocks is examined within the framework of experimental studies of melting of metamorphic rocks, and of reaction between basaltic magmas and metamorphic rocks. Among the types of granitic rocks considered in this chapter, only peraluminous leucogranites represent pure crustal melts. They form by dehydration-melting of muscovite-rich metasediments, most likely during the fast adiabatic decompression that results from tectonic collapse of thickened intracontinental orogenic belts. All other granitic rocks discussed here represent hybrid magmas, formed by reaction of basaltic melts with metamorphic rocks of supracrustal origin. These hybrid rocks include Cordilleran granites, formed at or near convergent continental margins, strongly peraluminous 'S-type' granites, alumina-deficient 'A-type' granites, and rhyolites associated with continental flood basalts. The differences among these types of granites reflect differences both in their source materials and in the pressures at which mantle-crust interactions take place. In turn, these variables are correlated with the tectonic settings in which the magmas form. Hybrid mafic cumulates are also produced by mantle-crust interactions, simultaneously with the granitic melts. These cumulates range from orthopyroxene + plagioclase-rich assemblages at low pressure to clinopyroxene + garnet-rich assemblages at high pressure, and are known to be important constituents of the lower continental crust. With the exception of peraluminous leucogranites, generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.
Rb-Sr age data and initial Sr87/Sr86 ratios (I values) are reported for the Mesozoic granitic batholiths along the coast of southeastern China. Two major thermal episodes are recognized: 90 to 120 m.y. B.P., derived from the mineral isochrons, and 165 ± 13 (2σ) m.y. B.P., derived from a whole-rock isochron. These episodes appear to be correlative with periods of rapid spreading of the Mesozoic Pacific Ocean floor and embrace two orogenic phases of the Yenshan orogeny. The I values of granitic gneisses (0.7060 to 0.7159) vary systematically with the Rb/Sr ratios of their whole-rock samples. The high I value (0.7112) of a pegmatite dike cutting a granitic gneiss in Chinmen suggests that the pegmatite is derived from remelting of upper crustal rocks. The narrow range of I values for more mafic intrusive bodies in Matsu (0.7065 to 0.70695) suggests that they are genetically related and are probably derived from an upper mantle source and have some crustal contamination. An un-metamorphosed granitic sample from Changlo has a mineral isochron age of about 120 m.y., which is interpreted as the time of intrusion. Its rather low I value (0.70546) suggests that the magma source is likely in the upper mantle. The Yenshan orogenic belt in southeastern China may belong to the inner belt of the Pacific-type orogeny; it is characterized by polycyclic deformation, magmatism, and regional metamorphism. Unlike a classical geosynclinal regime, the magmatic activities in this region were not immediately preceded by a major marine geosynclinal subsidence. The apparent absence of marine deposits after Triassic time suggests that marine transgression is not a universal result of rapid sea-floor spreading. The extensive development of magmatic belts along the eastern margin of the Asian continent appears to require large-scale consumption of oceanic lithosphere at the edge of the continental plate. The age distribution and structural trend data of the magmatic rocks suggest that the presumed subduction zone may have dipped westward or northwestward in the western Pacific region during late Mesozoic time.
It has been long recognized from Nd and Sr isotopes that depleted mantle sources consist of recycled oceanic materials, but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrothermal alteration. Zircon is expected to preserve this signature because it is resistant to high- temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and oxygen isotope study of whole-rock and mineral samples from a Mesozoic A-type granite at Nianzishan in northeastern China. The Sm-Nd isotope results show positive εNd(t) values of +0.86 to +4.27 with young Nd model ages of 569 846 Ma, manifesting a significant input of newly mantle derived material. The zircon delta18O values of 3.120/00 4.190/00 are significantly lower than the delta18O value of 5.30/00 ± 0.30/00 for the normal mantle zircon and thus appear to require remelting of hydrothermally altered oceanic crust. The combined Nd-O isotope studies not only provide compelling evidence for geochemical recycling of young juvenile crust by plate subduction, but also demonstrate that the granitic magmas can result from partial melting of mantle-derived rocks that were subjected to seawater- hydrothermal alteration before magma generation. Disequilibrium oxygen isotope fractionations are observed between common rock-forming minerals with significantly lower delta18O values for alkali feldspar than seawater, corresponding to meteoric-hydrothermal alteration after magma crystallization.
We propose a flat-slab subduction model for Mesozoic South China based on both new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon data and a synthesis of existing structural, geochronological, and sedimentary facies results. This model not only explains the development of a broad (˜1300-km-wide) intracontinental orogen that migrated from the coastal region into the continental interior between ca. 250 Ma and 190 Ma, but can also account for the puzzling chain of events that followed: the formation of a shallow-marine basin in the wake of the migrating foreland fold-and-thrust belt, and the development of one of the world's largest Basin and Range style magmatic provinces after the orogeny. The South China record may serve as an example of the multiple effects of flat-slab subduction, including migrating orogenesis and foreland flexure, synorogenic sagging behind the active orogen, postdelamination lithospheric rebound, and the development of a Basin and Range style broad magmatic province.
Stratigraphic correlations and tectonic analysis suggest that the Yangtze block of South China could have been a continental fragment caught between the Australian craton and Laurentia during the late mesoproterozoic assembly of the supercontinent Rodinia. The Cathaysia block of southeast China may have been part of a 1.9 1.4 Ga continental strip adjoining western Laurentia before it became attached to the Yangtze block around 1 Ga. This configuration provides a western source region for the clastic wedges in the Belt Supergroup of western North America which contain detrital grains of 1.8 1.6 Ga and 1.22 1.07 Ga. The breakup of Rodinia around 0.7 Ga separated South China (Yangtze plus Cathaysia blocks) from the other continents.
There is an ongoing debate concerning whether the vast Nanling Mountain granitoids of South China are the result of Jurassic paleo-Pacific subduction. We address this question by examining two Mesozoic basalt successions in South China, one parallel and the other oblique to the convergence boundary between the paleo-Pacific plate and South China continent. The geochemical characteristics of these basalts are used to constrain the influence of this presumed subduction system on mantle composition. 40Ar/39Ar age, major element and trace element abundances, and Sr, Nd and Pb isotope data indicate that the northeast-southwest trending basalts in the Southeast Coast Magmatic Belt that formed during 101–76 Ma exhibit arc-like signatures and are derived from subduction-modified mantle. However, the east–west trending basalt array extending from the Cathaysia Interior to the Cathaysia Folded Belt, which decreases in age from 175 to 98 Ma in an eastward direction, is characterized by lithosphere-modified OIB-like asthenosphere composition, except for a few younger rocks from Huichang which resemble Southeast Coast Magmatic Belt basalts. These results suggest that the Jurassic mantle beneath the Cathaysia Interior and the Cathaysia Folded Belt was rather homogeneous and undisturbed, and was not affected by the paleo-Pacific subduction system. Alternatively, post-orogenic (Indosinian) extension accompanied by mafic underplating may have been the cause of vast Jurassic granitic magmatism in South China.
The Pingtan and Daiqianshan igneous complexes, southeastern China, are typical of calc-alkaline series developed at active continental margins. These two complexes are dominated by granites or gabbros, with minor diorites and granodiorites, and these rock types closely coexisted both temporally and spatially. The major and trace element signatures of gabbros indicate they were crystallised from high aluminium basaltic magmas, which were generated from metasomatised upper mantle. The typical metaluminous calc-alkaline composition, trace element patterns and Sr–Nd isotopes indicate the granites in the complexes are I-type granites, which may have been generated from the deeper crust by a thermal contribution from the mantle. Field relationships, petrography and geochemistry favour a mechanical mingling together with a limited chemical mixing between high aluminium basaltic magma and granitic magma, resulting in the formation of diorites or granodiorites. The essentially bimodal character of these complexes reflects the general character of back-arc extensional magmatism, which was induced by the subduction of the Pacific plate beneath the Eurasian continent in late Mesozoic time.
Zircon and baddeleyite U–Pb geochronological dating is widely used in solid Earth sciences and the advent of rapid in-situ methods of analysis, such SIMS and ICP-MS, has re-emphasized the importance of having uniform standards. Recently, it has been shown that Hf isotopic data can provide important information on these minerals since they contain high concentrations of Hf, but have low Lu/Hf ratios, which results in negligible age correction. However, the complex internal structures that result from multiple thermal events, such as inherited cores and metamorphic overgrowths, require that the Hf isotopic data be measured with high spatial resolution. However, the isobaric interferences of 176Yb and 176Lu on 176Hf hamper the precise determination of the 176Hf/177Hf ratio during in-situ laser ablation MC-ICPMS analysis. It is shown here that mass biases of Yb (βYb) and Hf (βHf) change with time during analyses and behave differently for solutions and solid material. Therefore, it is suggested that the mean βYb value of the individual spot be used to obtain the precise isotopic composition for in-situ zircon and baddeleyite Hf isotopic analyses. For low Yb/Hf (176Yb/177Hf0.001) zircons and baddeleyites, since the interference of 176Yb on 176Hf is significant. Using the mean βYb value of the individual spot and newly published Yb isotopic abundance data, six standard zircons and two standard baddeleyites, have been investigated using a Neptune MC-ICPMS, with 193 nm laser. For zircons, the obtained 176Hf/177Hf ratios are 0.282307±31 (2SD) for 91500, 0.282680±31 (2SD) for TEMORA, 0.281729±21 (2SD) for CZ3, 282177±17 (2SD) for CN92-1, 0.282983±17 (2SD) for FM0411, and 281234±11 (2SD) for Phalaborwa. The baddeleyites from Phalaborwa and SK10-2 have 176Hf/177Hf ratios of 0.281238±12 and 0.282738±13 (2SD). These results agree well with the values obtained by the solution method and indicate that these standards have different Hf isotopic compositions, in which the extremely low 176Lu/177Hf and 176Yb/177Hf values of CZ3 zircon and Phalaborwa baddeleyite make them excellent standards for machine calibration during in-situ zircon Hf isotopic measurement, with the other standards being more suitable for the development of the correction method.
The NE-trending Changle-Nanao shear zone has affected several granitoid plutons in the Dongshan area, southeast China. Microstructures in the granitoids range from undeformed to solid state lineations and foliation. Migmatitic textures are common in some units. Microprobe analysis of total Al has been carried out on hornblende rims in four samples of gneiss from various points across the shear zone. Confining pressures calculated fall into range of: 4.2–4.9 kb. The average pressure (∼ 4.5 kb) suggests that the shear zone may have reached at least 16 km depth. Fluid inclusion studies referred from the correspondent PT isochores indicate that the temperature of ductile deformation in the shear zone was ∼ 740°C. Observations on intrusive relations between the various granitic phases and their relative degrees of deformation suggest that emplacement of the different magma phases may be closely related in time and to movement along the Changle-Nanao shear zone. Zircon from one deformed pluton yielded a crystallization UPb age of 121.5 ± 2.8 Ma. Field and microstructural data suggest the dated pluton is syntectonic, which constrains the upper age of high temperature deformation in the orthogneiss along the Changle-Nanao shear zone at that locality.