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The discrimination between continental basalt and island arc basalt based on geochemical method

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... Rb/Sr (0.03-0.09) ratios, indicative of negligible contamination by upper crustal rocks (Turner et al., 1996;Xia et al., 2007); (2) the crust has low ε Nd (t) values and Sm/Nd ratios, but a positive correlation between ε Nd (t) values and Sm/Nd ratios is not evident for the studied basaltic rocks ; (3) ...
... and La/Ta (35.8-37.1) ratios of the basalts are similar to those of basalts derived from the lithospheric mantle (La/Nb > 1.5 and La/Ta > 22; Fitton et al., 1988;Xia et al., 2007). The Ce/Pb (5.42-5.59) ...
... The late Miocene basalts are alkaline and exhibit positive Nb and Ta anomalies, similar to OIB-type rocks ( Fig. 3d; Sun and McDonough, 1989;Xia et al., 2007). They have high contents of highly incompatible elements (e.g., Nb, Ta, Ba, and Zr). ...
Article
The timing and mantle processes of the Cenozoic transition from an active continental margin to a trench-arc-basin system in northeast Asia remain controversial. Here we present major and trace element, and Sr-Nd-Pb-Hf isotope data for Cenozoic basaltic rocks from the Russian Far East, which provide new constraints on the deep mantle processes during the transition. The basaltic rocks formed mainly in four stages. The results show that middle Eocene basaltic rocks have arc-like geochemical features, such as depletions in high-field-strength elements, indicating an origin by partial melting of a mantle wedge that had been modified by slab-derived fluids and recycled sediments. Early and middle Miocene basaltic rocks also have arc-like geochem-ical features, and have relatively low contents of fluid-mobile elements and high contents of fluid-immobile elements, as compared with the middle Eocene basaltic rocks. This suggests that these magmas were derived by the melting of previously melted lithospheric mantle that had been modified by slab-derived fluids. In contrast, late Miocene basalts have ocean island basalt-like geochemical features, such as enrichments in high-field-strength elements, indicating their derivation by partial melting of the asthenospheric mantle. The temporal variations in the mantle sources of the Cenozoic basaltic rocks in the Russian Far East, along with an abrupt change in Sr-Nd isotopic compositions at ca. 20 Ma, demonstrate that the transition from an active continental margin to a trench-arc-basin system in northeast Asia occurred at ca. 20 Ma.
... Therefore, the Rb/Nb ratios of the alkaline rocks support crustal contamination to some degree. The alkaline rocks show (Th/Nb) N ratios 1.69 to 11.16, implying that the alkaline rocks have assimilated crustal material ((Th/Nb) N > 1) [46]. In addition, the Nb/Th ratio is considered a critical indicator for crustal contamination processes [36]. ...
... Therefore, the Rb/Nb ratios of the alkaline rocks support crustal contamination to some degree. The alkaline rocks show (Th/Nb)N ratios 1.69 to 11.16, implying that the alkaline rocks have assimilated crustal material ((Th/Nb)N > 1) [46]. In addition, the Nb/Th ratio is considered a critical indicator for crustal contamination processes [36]. ...
... The Nb/Ta ratios (18.1-46.9) of the alkaline rocks are higher than the ratios of average crust (12)(13) [47], which is consistent with the mantle source (15.5-19.5) [46], implying a mantle origin. Seven samples show Y/Nb ratios of lower than 1.2, which is also consistent with a mantle source (<1.2) [48][49][50], implying that the primary alkaline magma derived from the mantle. ...
Article
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The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO2, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe2O3 (Total), MgO, P2O5, TiO2, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.
... In general, island arc volcanic rocks have obvious negative anomalies of Th, Nb, Ta and P, weak negative anomalies of Ti and Zr, and depleted HFSE and enriched LILE (Li and Lu, 1999). In addition, low Zr content (< 145 ppm) and Zr/Y ratios (< 6.5) are significant characteristics of island arc volcanic rocks (Wilson, 1989;Li and Lu 1999;Condie, 2001;Xia et al., 2007). Compared to island arc volcanic rocks, continental basalts are characterized by a lack of negative anomalies of Nb, Ta and Ti, and have flat REE patterns (Xia et al., 2007). ...
... In addition, low Zr content (< 145 ppm) and Zr/Y ratios (< 6.5) are significant characteristics of island arc volcanic rocks (Wilson, 1989;Li and Lu 1999;Condie, 2001;Xia et al., 2007). Compared to island arc volcanic rocks, continental basalts are characterized by a lack of negative anomalies of Nb, Ta and Ti, and have flat REE patterns (Xia et al., 2007). Continental basalts also have high Zr content and Zr/Y ratios (Xia et al., 2007). ...
... Compared to island arc volcanic rocks, continental basalts are characterized by a lack of negative anomalies of Nb, Ta and Ti, and have flat REE patterns (Xia et al., 2007). Continental basalts also have high Zr content and Zr/Y ratios (Xia et al., 2007). Ratios of Nb/La are ≥1 and (Th/Nb) N are < 1 in Continental basalts (Kieffer et al., 2004;Xia et al., 2007). ...
Article
Epidosites are suggested to form in up-flow zones located at the base of ore-forming oceanic hydrothermal systems, and are commonly preserved in ophiolitic sequences. However, occurrences of epidosites in Archean tectonic mélanges are rarely documented. In this study, we report the presence of late Neoarchean epidosites in the Zanhuang ophiolitic mélange in the North China Craton using meter-scale litho-structural and petrologic analysis, zircon U-Pb dating, major and trace element, and oxygen and strontium isotopic geochemistry. Field studies show that the epidosites are mainly preserved as the cores of meta-pillow lavas, and are distributed as blocks in different matrices within the tectonic mélange. The pillow rims are used as protolith to estimate the element mobility during the alteration process. The epidosites display strong depletion in Na2O (0.01–0.27 wt.%), MgO (0.43–3.01 wt.%), depletion in large ion lithophile elements (LILE) (e.g. Rb, Ba, Cs, U) and transition metals (e.g., Co, Sc, Cu, Ni, Zn), and show enrichment of CaO (13.35–21.0 wt.%), Sr and Pb. The high field strength elements (HFSE) (e.g. Zr, Hf, Ta, Th) of epidosites are varied but rare earth elements (REE) are unchanged. The amphibolitic pillow rims are characterized by enrichment of LILE, flat heavy rare earth elements (HREE) patterns, and depletion in HFSE with negative Ti anomalies and positive Pb and Ba anomalies. Oxygen and strontium isotopic ratios of epidosites are 5.0–8.6‰ and 0.70284–0.70551, respectively, both of which are greater than those of fresh basalts. The major and trace elements, oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence. Magmatic zircons from two samples of epidosites yield ²⁰⁷Pb/²⁰⁶Pb ages of 2512 ± 13 Ma (MSWD = 0.51) and 2542 ± 13 Ma (MSWD = 1.4), confirming that the epidosites were generated at ca. 2.51-2.54 Ga. Based on geochemical data and field observations, we suggest that the protolith of the epidosites formed in an arc-related tectonic setting during the late Neoarchean. This study shows that hydrothermal activity operating in the late Neoarchean was similar to that of the modern ocean floor.
... suggesting that they originated in a subduction zone setting (e.g., Pearce, 2014;Dilek and Furnes, 2014;Zheng et al., 2015). However, the subduction zone signature can be inherited from several mechanisms, such as the Early Paleozoic juvenile subduction, crustal contamination of intraplate basaltic rocks, or melting of an old subarc mantle wedge without juvenile subduction (e.g., Pearce, 2014; Zheng et al., 2015;Xia et al., 2007. ...
... On a Th/Yb versus Nb/Yb diagram (Fig. 18d), they fall in the subduction-related field above the E-MORB array, with relatively high Th/Yb and Nb/Yb ratios. The Guizi mafic rocks possess (Th/Nb) N > 1 and Nb/La < 1 values showing that they could not form in a continental setting (Xia et al., 2007. In Figs. ...
... Moreover, on the Th/Yb versus Nb/Yb diagram (Fig. 18d), they are displaced above the N-MORB array, within subduction-related arc to forearc field. The mafic samples also possess (Th/Nb) N > 1 and Nb/La < 1, suggesting that they could not form in a continental setting (Xia et al., 2007 ...
Article
The geological evolution of the Early Paleozoic Wuyi-Yunkai orogen in South China is a scientific question of a long-standing debate. We document the presence of a NE-NEE-striking Early Paleozoic subduction-accretion shear zone, a possible mélange belt, in the northwestern Yunkai Domain. The northwestern Yunkai shear zone consists predominantly of tectonically juxtaposed fragments of Early Paleozoic flysch, arc volcanic rocks, and a forearc ophiolite. The Yunkai shear zone displays typical mélange structures in several locations; however, these structures are not continuous throughout the shear zone. The shear zone provides evidence for greenschist to amphibolite facies metamorphism and intense deformation resulting from Early Paleozoic to Early Mesozoic tectonic events. The flysch fragments are characterized by northwestward younging, northwestward thrusting, and northwestward migration of deformation and metamorphism. The arc volcanic rocks consist of basaltic-andesite, andesite, and dacite with a mainly sanukitic composition and also include Nb-enriched basalts. They are characterized by enrichment of LREEs, LILEs, Pb and depletion of HFSEs, suggesting a continental forearc or a continental arc tectonic setting. The ophiolitic fragments consist of MORB-like basalt and dolerite/gabbro. They have slightly depleted to flat LREE patterns and are characterized by enrichment of LILEs and Pb and depletion of HFSEs, indicating a continental forearc setting. Zircon U-Pb analyses yield 460–443 Ma and 455–437 Ma ages for the sanukitic volcanic rocks and ophiolitic fragments, respectively, suggesting that they formed in the Late Ordovician to Early Silurian. Both the sanukitic volcanic rocks and ophiolitic fragments possess negative to positive zircon εHf (t) values (–11.0 to +2.3), indicating that they may have been generated by partial melting of an old subarc mantle wedge source metasomatized by slab-derived fluids and/or melts. Recognition of the Early Paleozoic subduction-related magmatism and subduction-accretion structures in the northwestern Yunkai Domain has important implications for the tectonic history of the Wuyi-Yunkai orogen, South China. Collectively, combined with previous studies, we propose that the Huanan oceanic lithosphere began to subduct southeastward beneath the Yunkai terrane (arc) as early as 460 Ma, and the subduction continued between 460 and 440 Ma.
... The basanites are enriched in LREEs and have no negative anomalies of Nb and Ti elements, indicating that they may come from asthenosphere mantle (Condie 2001;Ernst and Buchan 2003;Xia et al. 2007). In addition, high Nb/La ratios (1.28-1.31) ...
... La/Ta , indicating that the andesitic basalts and diabase porphyries belong to IAB and N-MORB (Condie 1989). The Island arc basalts have low Zr content (< 130 × 10 −6 ) and Zr/Y ratio (< 4) (Xia et al. 2007). The andesitic basalts and diabase porphyries have low Zr content (7.17 × 10 −6 -26.4 × 10 −6 ) and Zr/Y ratio (0.27-1.11), indicating that they formed in Island arc setting. ...
Article
The precise age of slab rollback of the Paleo-Pacific Plate during Early Cretaceous has been contentious. In this study, we have conducted LA-ICP-MS zircon U-Pb dating, major and trace elements, and Sr-Nd isotopes analyses on the late Early Cretaceous diabase porphyries, andesitic basalts and basanites in the Yanbian area (NE China). Zircon U-Pb dating of basanites, andesitic basalts, and diabase porphyries yielded weighted average ages of 104 ± 1.9 Ma (MSWD = 0.55, n = 5), 106 ± 1.6 Ma (MSWD = 0.68, n = 6), and 111 ± 2 Ma (MSWD = 16, n = 3), respectively. The whole-rock geochemical results indicate that the late Early Cretaceous rocks predominantly belong to the low-K tholeiitic and medium-K calc-alkaline series. The diabase porphyries and andesitic basalts are characterized by large ion lithophile elements (e.g. Ba, K, and Sr) enrichment and high field strength lements (e.g. Nb and Zr) depletion with weak positive Eu anomalies, low initial 87Sr/86Sr ratios (0.70361–0.70394) and high εNd (t) values (5.96–7.87). The tectonic iscrimination diagrams show that the diabase porphyries and andesitic basalts formed in the continental Island arc environment related to subduction of the Paleo-Pacific Plate in the late Early Cretaceous. In contrast, the basanites are enriched in Ba, Th, and U and depleted in K, Sr, and Ti, with weak negative Eu anomalies. The basanites display high magnesium values (Mg# = 0.79), which are approaching the primary magma. They have low Ti values (TiO2 = 1.45%) and Al values (A12O3 = 9.98–10.15%) and high Co values (78.1–80.2 ppm) and Ni values (549.5–585.5 ppm). The basanites formed in the intraplate extensional tectonic setting and were the products of lithospheric thinning and mantle upwelling during the late Early Cretaceous. The spatial-temporal relationship of late Early Cretaceous magmatic rocks in NE China suggests a slab rollback from high-angle subduction to low-angle subduction of the Paleo-Pacific Plate at ca. 106 Ma.
... and high (Th/Nb) N (2.79-6.43) of two samples also demonstrate a significant role of crustal contamination in the petrogenesis of these rocks (Fig. 11a). The crustal contamination has no significant effect on the initial concentration of Zr and Y, which can use to discuss the petrogenesis (Fig. 11b; Winchester and Floyd, 1976;Xia et al., 2007). On the contrary, crustal contamination A. -B. ...
... Zeng et al. Gondwana Research 89 (2021) 66-87 will reduce the concentration of Ti, Nb and Ta, and their projection position will move towards the direction of the decrease of Ti, Nb, Ta in the diagrams, thus the characteristics of island arc or active continental margin basalts appear ( Fig. 11c; Xia et al., 2007). The sample N18T6 has high Cr (358-431 ppm) similar to the composition of the primary mantle-derived magmas (Cr = 300-500 ppm; Frey et al., 1978;Hess, 1992) and variable Ni (71.8-117 ppm) contents, which show fractional crystallization trend of olivine (Fig. 11f, g). ...
Article
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Large-scale geodynamic processes not only result in magmatism, but also generate corresponding tectonic and sedimentary evidence. Therefore, magmatic, sedimentary, and tectonic evidences can be used to investigate lith-ospheric geodynamics. This study reports Late Cretaceous magmatic-sedimentary-tectonic evidence from the Tari Co area in central Tibet of post-collisional extensional collapse of the lithosphere. Zircon U\ \Pb dating shows that the Tari Co Late Cretaceous igneous rocks formed during ca. 90-75 Ma, including (1) thickened lower crust-derived adakitic granodioritic porphyry (90.4 ± 1.0 Ma and 90.7 ± 1.0 Ma) with high Sr/Y (34.4-53.8) ratios but low Y (5.48-9.14 ppm), Cr (10.2-14.4 ppm), and Ni (4.33-6.63 ppm) contents; (2) delaminated lower crust-derived dioritic porphyry dike (80.5 ± 1.6 Ma) with high Mg# (48-53) and moderate SiO 2 (57.9-63.6 wt%), Cr (40.3-54.1 ppm) and Ni (43.5-56.4 ppm) contents; and (3) mafic dikes (74.8 ± 1.0 Ma) derived from different magma sources. According to sedimentary facies, gravel composition, and depositional age, the Upper Cretaceous Jingzhushan Formation in study area is divided into the lower subunit and upper subunit. Different from the typical lower subunit of the Jingzhushan Formation with sub-rounded limestone gravels as the main components, the upper subunit of the Jingzhushan Formation comprises slump breccias and coarse conglomerates. The gravel clasts in the upper subunit are mostly adakitic granodioritic por-phyry (90.1 ± 1.4 Ma), indicating a sedimentary response to the crustal uplift associated with lithospheric delamination. In terms of structural deformation, the study area gradually entered the post-collisional setting in the late Late Cretaceous time (ca. 79-75 Ma), forming an east-west trending extensional tectonic system comprising extensional and step faults, grabens and horsts, and mafic dikes. Based on the Late Cretaceous geological records and previous studies, we suggest a three-stage post-collisional model including melting of thickened crust, lithospheric delamination, and gravitationally driven ductile collapse. Our results provide new insights into the evolution of continental orogens.
... As revealed by previous studies, magmatic rocks with volcanic arc features do not necessarily indicate subduction environment, and they may indicate that magma sources were affected by the contamination of crustal materials or early subduction events (Aldanmaz et al., 2000;Pearce et al., 1990;Peccerillo, 1998;L. Q. Xia, 2014;L. Q. Xia, Xia, Xu, Li, & Ma, 2007). As mentioned above, the magmas of the bojites in Yajiangqiao area were derived from the lithospheric mantle, and limited crustal contamination was occurred in the magma evolution. ...
... L. Q. Xia et al. (2007) and L. Q. Xia (2014) (Ames, Tilton, & Zhou, 1993; W. D. Sun, Li, & Chen, 2002;Zheng, Fu, Gong, & Li, 2003 (Xiao, Deng, Ma, et al., 2002), and it generally occurred between 15 and 20 Ma after the collision (Cvetkovi c et al., 2004). Therefore, it can be inferred from the above-mentioned discussion that the SCB experienced collision and compression during the early-middle Triassic and were transformed to post-orogenic extension in the late Triassic (Y. ...
Article
The mafic igneous activities in the late Triassic play an important role in the tectonic evolution of the Hornblende South China Block (SCB). In this paper, relevant data of the bojites in Yajiangqiao area, eastern Hunan Province, South China were provided, including zircon U–Pb ages, whole‐rock geochemical data, Sr–Nd–Pb isotopic data, and zircon Hf isotope data. These data indicate that the bojites were crystallized at ~ 215.7 Ma, and thus are coeval with biotite monzonitic granites of Yajiangqiao pluton. The bojite samples feature a low SiO2 content of 48.84–49.94 wt%, a low ALK (K2O + Na2O) content of 5.13–6.13 wt%, a relatively high Al2O3 content of 17.64–21.22 wt%, a moderate MgO content of 4.32–7.07 wt%, and a moderate FeOT content of 5.66–7.33 wt%. In addition, the samples have a total rare earth element (REE) content of 108–163 ppm. They display right‐declined REE patterns, enriched in light rare earth elements and large‐ion lithophile elements (such as Rb, K, Th, and U) and depleted in high field strength elements (such as Nb, Ta, P, and Ti). All the samples have homogeneous Sr–Nd–Pb and zircon Hf isotopic compositions, with the (87Sr/86Sr)i ratio varying from 0.708 202 to 0.709 515, (143Nd/144Nd)i ratio from 0.512 156 to 0.512 229, (206Pb/204Pb)t ratio from 18.185 to 18.264, (207Pb/204Pb)t ratio from 15.667 to 15.672, (208Pb/204Pb)t ratio from 38.510 to 38.587, and initial 176Hf/177Hf ratio ranging from 0.282 428 to 0.282 530. Meanwhile, the calculated εNd(t) and εHf(t) values range from −4.0 to −2.6 and from −7.9 to −4.1, respectively, with two‐stage Nd model ages of 1.20–1.32 Ga and two‐stage Hf model ages of 1.35–1.56 Ga, respectively. As indicated by geochemical data, the primitive magmas of the bojites in Yajiangqiao area were derived from lithospheric mantle in post‐collisional extensional environment, which underwent low‐degree partial melting (1–5 %) of spinel‐garnet lherzolites (spinel > garnet) and was subjected to metasomatism of slab‐derived fluids. Furthermore, it can be concluded from these data and previous data that the SCB was transformed from syn‐collisional compression to post‐collisional extension in the late Triassic (~ 234 Ma) and the post‐collisional extension lasted until 215 Ma.
... These geochemical characteristics provide evidence that the early-Permian gabbro is impacted by the subduction related fluid or melt effect in the mantle source area, so it should be a subduction phase. For basic lava with subduction belt characteristics, we used the Cr-Y and Y/15-La/10- Nb/8 diagram to separate plate basalts from the island arc and active continental margin basalts Xia et al., 2007). The gabbro and diorite collected in this study fall into the volcanic arc tholeiite basalt zone or its neighbor (Figs. ...
Article
The Early Permian Yuejin Sumu intrusive rocks are located approximately 40 km northwest of Xilinhot City, Inner Mongolia. The magmatic rocks primarily composed of gabbro, diorite, and syenogranite. Through the study of zircon LA-ICP-MS U–Pb geochronology and rock geochemistry, age dates of the three lithologies show that the three formed simultaneously in the late Early Permian (gabbro formation age is 276.7 ± 1.3 Ma; diorite is 276.9 ± 0.8 Ma; syenogranite is 275.0± 1.4 Ma). The geochemical characteristics of the three lithologies are primarily calc-alkaline, and some samples, such as gabbro and diorite, show a transition to the alkaline series. The oxides in the gabbro and diorite are linearly related to SiO2. With magma MgO content, Al2O3 content increases first and then decreases, while CaO content decreases sharply, suggesting that the magma for the gabbro and diorite is a mantle-derived magma that undergoes separation and crystallization. The crystalline minerals include magnesia-rich pyroxene in the early stage and the plagioclase and pyroxene in the late stage. The syenogranite has high aluminum content (12.8–14.6%), and the ratio of FeOT/Mg# is high. Rare earth elements are characterized by a mid-range of the light and heavy rare earth elements. Eu shows significant negative anomalies, and the whole rare earth elements (REEs) are characterized by V-shaped distribution. The syenogranite are enriched in K, Rb, Th, U, Zr, and depleted in Eu, Ba, Sr, Ti, P, Nb, Ta, indicating that the magma may derived from crust in a typical post-orogenic environment. Petrology and geochemical characteristics of the Early Permian intrusive rocks indicate that in the Early Permian (~274–278 Ma) in the Xilinhot area was back-arc extensional environment at the active continental margin. Mantle-derived magma caused partial melting of the crustal material. The mantle-derived magma and the melt of the crustal material formed the Early Permian bimodal-like Yuejin Sumu intrusive rocks.
... The protoliths of the basic volcanic rocks are basalt and andesite with high SiO 2 , Al 2 O 3 and K 2 O, combined with low TiO 2 and P 2 O 5 , which defines them as examples of the medium-K calc-alkaline series. As for the trace elements, the LILE (Rb, Ba, K and Sr) are relatively more enriched than the HFSE (Nb, Ta, Ti, Zr and Hf) and REE, while the LREE are relatively more enriched than the HREE, and (Th/Nb) N is far larger than 1, Nb/La is far <1, relatively low Zr contents and Zr/Y ratios, the trace element signatures indicate that these rocks are the product of subduction-related magmatism at a convergent margin (Meschede, 1986;Xia et al., 2007). The closely-spaced oscillatory zones of the inherited zircons in the basic metavolcanic rocks, Sm/Nd and ε Nd (t) close to crustal values, LILE enrichment and HFSE depletion, strong negative anomalies of Nb, Zr and Hf, low Nb/U ratio (5.48-15.93, ...
Article
Paleoproterozoic (~2.3) Ga metavolcanic rocks occur in the Lengkou area, northern Zhongtiao Mountains of the North China Craton (NCC). The area is dominated by metabasic volcanic rocks, intercalated with metamorphosed intermediate to acid volcanic rocks. The Lengkou metavolcanic rocks have a magmatic zircon U-Pb age of 2317 ± 13 Ma, but also contain some 2508 Ma inherited zircons. The Lengkou metabasic and intermediate volcanic rocks are calc-alkaline with high SiO2 and K2O contents. They display fractionation of the HREE relative to the LREE, relative enrichments in large ion lithophile elements and depletion in high field strength elements resulting in negative Nb-Ta-Ti and Zr-Hf anomalies. Therefore, they are inferred to have formed in a volcanic arc setting. The meta-acid volcanic rocks of the Lengkou series have geochemical features similar to the surrounding Sushui Complex (Neoarchaean tonalitic gneiss), resulting from partial melting of ancient crust. The Lengkou metabasic volcanic rocks have a low ¹⁴³Nd/¹⁴⁴Nd ratio and εNd(t) value of 0.8–1.5, with TDM of 2.57–2.63 Ga, indicating the mantle-derived mafic rocks were variably contaminated by older continental material. The integrated data from the Lengkou metavolcanic rocks indicate a convergent subduction-related regime in the early Paleoproterozoic. The isotopic signatures and the xenocrystic zircons demonstrate that subduction occurred at a margin of continental crust, rather than within intra-ocean, exploring an Andean-like setting in the NCC at that time.
... implying that it does not belong to the product from a partly fused mantle peridotite(Cox, 1980). Both the gabbro and quartz diorite reveal an enrichment of LREE, loss of HREE, and depleted HFS elements like Nb, Ta, and Hf features that imply two kinds of possible formation mechanisms, (a) strong crustal contamination and (b) effected by the fluid/melting's metasomatism in the subduction zone.The ratio of (Th/Nb) N and Nb/La can be used to identify whether there was contamination from the crust or not(Xia, Xia, Xu, Li, & Ma, 2007). The Hunchun gabbro and quartz diorite both contain high (Th/Nb) N (gabbro at 1.91-5.46; ...
... The REE patterns and the multi-element spider diagrams of the analyzed samples are almost uniform to those of typical IAB from the Tonga-Kermadec, New Zealand ( Fig. 5; Ewart et al., 1998), which is as well construable with an island-arc setting. In addition, the Wulong diorite porphyry has low Zr contents (<130 ppm) and Zr/Y ratios (<4), which is characterized by island arc basalt and distinct from the continental arc basalt (Xia et al., 2007). Several contemporaneous gabbro intrusions and extrusive equivalents with island-arc features are distributed along the NQB, e.g., at Guanzizhen (489 Ma, Pei et al., 2007a), Yanwan and Xieyuguan basalts (483 and 472 Ma, respectively; Chen et , 2008b;Yan et al., 2007), Luohansi (475 Ma, Liu et al., 2007); Sifangtai (460 Ma, Liu et al., 2012), and Fushui (488 Ma, Wang H et al., 2014) gabbros. ...
Article
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The Wulong diorite porphyry displays a very unusual flower-like glomerophyric texture and is thus well-known as a gemstone type of “Luoyang peony stone” for non-geologists. Hereby, we present a comprehensive study in terms of petrography, zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope. The rock exposed as small-scale dykes in the field, intruding in the Archean migmatized biotiteplagioclase genesis at the southern margin of the North China Craton. It exhibits typical porphyrite texture with plagioclase and amphibole as dominant and subdominant phenocrysts, respectively. The diorite porphyry rocks contain 51 wt.%–59 wt.% SiO2, belonging to medium-K calc-alkaline rock series. They are characterized by enrichment of large ion lithophile elements (LILEs, e.g., Ba) and depletion of high field-strength elements (HFSEs, e.g., Nb, Ta, Zr and Hf), resembling island arc magmatic rocks. They show high initial Sr isotopic ratios (0.710 7–0.715 8), low ɛNd(t) values (-9.9 to -13.3) and variable initial Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb vary from 17.3 to 19.3), similar to the coeval intermediate and mafic magmatic rocks of adjacent region. These geochemical features indicate that the Wulong diorite porphyry is likely to originate from an enriched mantle with contamination of crustal materials. Zircon U-Pb dating results reveal a magma crystallization age of 480±3 Ma, consistent with the summit of the magmatic events of the Qinling orogenic belt and reflecting the northernmost Paleozoic magmatic activity of the belt. This may indicate the influence of northward subduction of Paleotethyan oceanic slab can extend a distance of ca. 200 km to the north and reach Luoyang-Songshan region in the East Qinling Orogen.
... Hence magma contamination associated with continental crust can significantly dilute the concentration of certain elements (e.g. Nb, Ti) which may lead to misinterpreting the rock tectonic setting i.e. contaminated intra-plate basalts can be mistaken for subduction related basalts (having low Nb and low Ti) (Ernst et al. 2005;Xia et al. 2007). ...
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Rift related magmatism during Permian time in the northern margin of Indian plate is represented by basic dykes in several Himalayan terranes including north western Pakistan. The field relations, mineralogy and whole rock geochemistry of these basic dykes reveal significant textural, mineralogical and chemical variation between two major types (a) dolerite and (b) amphibolite. Intra-plate tectonic settings for both rock types have been interpreted on the basis of low Zr/Nb ratios (< 10), K/Ba ratios (20–40) and Hf-Ta-Th and FeO-MgO-Al2O3 discrimination diagrams. The compositional zoning in plagioclase and clinopyroxene, variation in olivine compositions and major elements oxide trends indicate a vital role of fractional crystallization in the evolution of dolerites, which also show depletion in rare earth elements (REEs) and other incompatible elements compared to the amphibolites. The equilibrium partial melting models from primitive mantle using Dy/Yb, La/Yb, Sm/Yb and La/Sm ratios show that amphibolite formed by smaller degrees (< 5%) of partial melting than the dolerites (< 10%). The trace elements ratios suggest the origination of dolerites from the subcontinental lithospheric mantle with some crustal contamination. This is consistent with a petrogenetic relationship with Panjal trap magmatism, reported from Kashmir and other parts of north western India. The amphibolites, in contrast, show affinity towards Ocean Island basalts (OIB) with a relatively deep asthenospheric mantle source and minimal crustal contribution and are geochemically similar to the High-Ti mafic dykes of southern Qiangtang, Tibet. These similarities combined with Permian tectonic restoration of Gondwana indicate the coeval origin for both dykes from distinct mantle source during continental rifting related to formation of the Neotethys Ocean.
... The enrichment of LREE and inclined-LREE with flat-HREE patterns are comparable with features of continental margin volcanic arc basalt, which are distinct from the normal mid-ocean ridge basalt and oceanic island basalt. Some elemental anomalies (such as K*, Nb* and Sr*) can be used to determine tectonic setting and related evolutionary processes of magma (Xia et al. 2007). Basalts with K* values higher than 1 could have originated from the island arc region and have close relationship with subduction (Nakamura et al. 1985). ...
Article
The Taipusi area in the Bainaimiao Arc Belt is located in the northern margin of the North China Craton, at the southern margin of the middle Central Asian Orogenic Belt. It is characterized by large exposures of mafic dikes. In this contribution, we present first-hand whole-rock major and trace elements, zircon U–Pb geochronology and in situ trace element geochemistry data for these mafic rocks, which reveal their petrogenesis and tectonic evolution. These mafic dikes display varied compositions of \(\hbox {SiO}_{2}\) (49.42–54.29%), \(\hbox {TiO}_{2}\) (0.63–1.08%), \(\hbox {Al}_{2}\hbox {O}_{3}\) (13.94–17.60%), MgO (4.66–10.51%), \(\hbox {Fe}_{2}\hbox {O}_{3}\) (1.59–3.07%), FeO (4.60–6.90%), CaO (4.57–8.91%), \(\hbox {Na}_{2}\hbox {O}\) (1.61–4.26%), \(\hbox {K}_{2}\hbox {O}\) (0.92–2.54%) and \(\hbox {P}_{2}\hbox {O}_{5}\) (0.11–0.29%). They are mainly of high-K calc-alkaline series with indistinct Eu anomalies, enriched in large ion lithophile elements (e.g., Rb, Ba, K and Sr) but depleted in high field strength elements (e.g., Nb, P and Ti). These suggest that the crystallizing magma was derived from enriched mantle altered by metasomatic fluids in a subduction setting with imprints of active continental margin features. The high concentrations of Hf, U, Th, Pb and Y, pronounced positive Ce but slightly negative Eu anomalies in zircons indicating that the magma underwent a fractional crystallization and crustal contamination process, with medium to high \(f\hbox {O}_{2}\). Zircon LA–ICP–MS U–Pb dating yielded concordant ages of 437–442 Ma for these mafic dikes, which is consistent with the early Paleozoic volcanic arc magmatic activity in the Bainaimiao area. Hence, we conclude that the Bainaimiao Arc Belt is a continental arc formed by the southward subduction of the Paleo-Asian ocean during early Paleozoic.
... The Zr/Sm value is also a distinctive indicator for discriminating tectonic settings for basalts, with arc basalts usually having Zr/Sm values lower than the chondritic value (∼25), such as the Aleutian arc basalts with an average Zr/Sm of ∼20 (Mcculloch and Gamble, 1991). In contrast, intraplate basalts have Zr/Sm values higher than 25 (Rudnick and Gao, 2003), together with high Zr contents (> 70 ppm) and Zr/Y ratios (> 3) whether contaminated (Xia et al., 2007). The Yingfeng gabbros have Zr/Sm values of 19.66-24.59, ...
Article
The role of the Tarim Block in the reconstruction of the Neoproterozoic supercontinent Rodinia remains contentious. Here we report a suite of high–Mg gabbroic dykes from the Yingfeng area in northwestern Quanji Massif, which is considered as a fragment of the Tarim Block in NW China. Magmatic zircons from these dykes yield to have a weighted mean ²⁰⁶Pb/²³⁸U age of 822.2 ± 5.3 Ma, recording the timing of their emplacement. The gabbros have high MgO (9.91–13.09 wt%), Mg numbers (69.89–75.73) and CaO (8.41–13.55 wt%), medium FeOt (8.50–9.67 wt%) and TiO2 (0.67–0.93 wt%), variable Al2O3 (13.04–16.07 wt%), and high Cr (346.14–675.25 ppm), but relatively low Ni (138.72–212.94 ppm), suggestive of derivation from a primary magma. The rocks display chondrite-normalized LREE patterns with weak fractionation but flat HREE patterns relative to those of the N-MORB. Their primitive mantle normalized trace elemental patterns show positive Rb, Ba and U but negative Th, Nb, Ti and Zr anomalies, carrying characteristics of both mid-ocean ridge basalts and arc basalts. The εHf(t) values of the zircons from these rocks vary from +4.7 to +13.5 with depleted mantle model ages (TDM) of 1.23–0.85 Ga, and the youngest value nearly approaching that for the coeval depleted mantle, suggesting significant addition of juvenile materials. Our data suggest that the strongly depleted basaltic magma was probably sourced from a depleted mantle source that had undergone metasomatism by subduction–related components in a back-arc setting. Accordingly we postulate that a subduction–related tectonic regime possibly prevailed at ∼0.8 Ga along the southeastern margin of the Tarim Block. Combining with available information from the northern Tarim Block, we propose an opposite verging double–sided subduction model for coeval subduction of the oceanic crust beneath both the southern and northern margins of the Tarim Block during early Neoproterozoic.
... The longer primitive melt stays in a crustal magma chamber before eruption, the greater the opportunity for the assimilation of crustal material. To demonstrate the effects of crustal contamination on these within-plate basalts, the reference data from CFBs have been divided into uncontaminated (primitive mantle-normalized (Th/Nb) N b 1) and contaminated ((Th/Nb) N N 1) groups as suggested by Xia et al. (2007). The (Th/Nb) N ratios of the Tengchong volcanic rocks range from 3.72 to 20.72 which is strong evidence for crustal contamination. ...
Article
SHRIMP zircon Pb/U dating of Cenozoic volcanic rocks in the Tengchong area, western Yunnan Province, China, shows that the dacite and andesitic breccia lavas from Qushi village were intruded at 480 ± 10 ka and 800 ± 40 ka, respectively. Moreover, Pb/U dating of trachyandesite from Tuantian village and olivine basalt from Wuhe village give weighted mean ²⁰⁶Pb/²³⁸U ages of 2.82 ± 0.08 Ma and 12.28 ± 0.30 Ma. Corrections for initial ²³⁰Th disequilibrium of zircon were used for the former two younger ages.
... However, those authors overlooked the contact relationships between the post-orogenic rocks and the molasse deposits. In addition, samples derived from juvenile continental crust and contaminated with mantle material can have an arc-like signal, which could be misinterpreted as indicative of formation in a subduction setting (Xia et al. 2007). Some of the mafic-intermediate rocks collected in this study have been shown to have some degree of crustal contamination. ...
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We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304Main the Carboniferous. The ages also indicate that the Palaeo-KaramailiOcean had already closed by 342Ma.Moreover, the volcanic rocks also contained 10 inherited zirconswith ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial 87Sr/86Sr values (0.7034–0.7042) and positive εNd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite withminor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive εNd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).
... All of these samples plot in the volcanic arc field of this discrimination diagram (Fig. 15), indicating that although these intrusions cannot directly have formed in a volcanic island arc environment they were derived from sources that were influenced by the subduction of the paleo-Pacific Plate, most likely as a result of source contamination or the assimilation of subduction-related material. These processes can lead to the misidentification of contaminated continental basalts as arc related basalts (e.g., Xia et al., 2007;Wallace et al., 2015). Uncontaminated asthenospheric (or plume)-generated continental basaltic rocks are normally characterized by (Th/Nb) N values <1, Nb/La values P 1, low 87 Sr/ 86 Sr (t) ratios, high eNd (t) values, La/Nb and La/Ba ratios that are similar to typical OIB compositions, and hump-shaped mantlenormalized multi-element variation diagram patterns that are free of negative Nb, Ta and Ti anomalies (e.g. ...
Article
The Zhonggu ore field is located in the southern Ningwu volcanic basin, the largest iron ore district within the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) of eastern China. This area has produced more than 2000 Mt of iron ore although comparatively little research has been done on the ore-bearing intrusions in this region despite the significance of the mineralization in this area. This in turn means that the petrogenesis and the links between these intrusions and the metallogenesis in this area remain unclear. This study focuses on the six intrusions that are associated with almost all of the iron deposits within the Zhonggu ore field, namely the Gushan porphyritic gabbrodiorite, the Longshan gabbrodiorite, the Hemushan diorite, the Baixingshan diorite, the Zhongjiu diorite and the Taipingshan monzonite. All of these rocks are either high- to medium-K and calc-alkaline or are shoshonitic, although the former dominates over the latter. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating yielded ages of 132.6±1.6 to 131.6±1.6, 130.7±2.2 to 129.4±1.9, 131±2.0, and 132.3±2.1 Ma for intrusions associated with the Longshan, Taipingshan, Baixiangshan and Zhongjiu deposits, respectively, indicating that all of these intrusions were emplaced in the Early Cretaceous (132.6 to 129.4 Ma). This magmatism and mineralization was contemporaneous with the second stage (135 to 127 Ma) of magmatic activity that has been documented elsewhere within the MLYRMB. The mineralized intrusions associated with the Zhonggu iron ore field have similar chondrite-normalized rare earth element (REE) and primitive mantle-normalized multi-element variation patterns that are characterized by light REE (LREE) and light ion lithophile element (LILE; Rb, Ba, K, and Sr) enrichments and heavy REE (HREE) and high field strength element (HFSE; Nb, Ta, Zr, and Hf) depletions. These intrusions also have total REE (ΣREE) concentrations that are consistent with the evolution of the magma from mafic-intermediate to felsic. The magmas that formed these intrusions were derived from an enriched region of the mantle and underwent contamination by the assimilation of continental crustal or lithospheric material. The magmatism within the Zhonggu iron ore field formed during an Early Cretaceous period of lithospheric thinning and extension. In addition, the arc affinities of the intrusions in this region indicative of a relationship to the subduction of the Paleo-Pacific Plate, a process that was most likely the main factor in the generation of the mineralization within the MLYRMB. The Fe mineralization-related intrusions in the study area are also distinctly different from intrusions in this region associated with Cu–Au mineralization, most likely as a result of the heterogeneous distribution of slab-derived fluids in this period over time. This research also highlights the exploration potential of this region, indicating that the 132–129 Ma monzonites and diorites in this area and beyond should be considered prime targets for Fe exploration but are also unlikely to be related to Cu–Au mineralization.
... The Zr/Sm value is also a distinctive indicator for discriminating tectonic settings for basalts, with arc basalts usually having Zr/Sm values lower than the chondritic value (∼25), such as the Aleutian arc basalts with an average Zr/Sm of ∼20 (Mcculloch and Gamble, 1991). In contrast, intraplate basalts have Zr/Sm values higher than 25 (Rudnick and Gao, 2003), together with high Zr contents (> 70 ppm) and Zr/Y ratios (> 3) whether contaminated (Xia et al., 2007). The Yingfeng gabbros have Zr/Sm values of 19.66-24.59, ...
... However, bimodal volcanic These results are consistent with the continental-marine sedimentary environment as inferred from the field relations. Therefore, the Dahalajunshan Formation is inferred to have formed in a continental margin arc setting (Shao et al. 2006;Shu et al. 2007;Xia et al. 2007;Dong et al. 2010;Han et al. 2010;Li et al. 2010b;Liu et al. 2013). ...
Article
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The widespread Late Devonian–Middle Carboniferous volcanic rocks in the Chinese Western Tianshan provide important constraints on the subduction history of the South Tianshan oceanic lithosphere. Here, we investigate the basalt, basaltic andesite, andesite, trachyandesite, and rhyolite from the Dahalajunshan Formation from Western Tianshan. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon geochronology constrains their age of formation to between 376 and 333 Ma (i.e., Late Devonian–Middle Carboniferous) with distinct variation in space (from west to east) and time (from early to late). Based on geochemical, zircon geochronological, and Sr–Nd isotopic data, we demonstrate that the Dahalajunshan volcanic was generated in a continental arc setting associated with the subduction of the south Tianshan Ocean during Late Devonian to Middle Carboniferous. The volcanic rocks belonging to Dahalajunshan Formation in the northwestern part of the Yili Block suggest that the northward subduction of the south Tianshan Ocean was initiated in the Early Devonian; those in the southern and eastern part of the Yili Block were probably produced by a northward subduction of South Tianshan Ocean during Late Devonian to Middle Carboniferous.
... For example, both units display similar geochemical features, such as low SiO 2 , high TFeO, and identical REEs and trace elements patterns (Figs. 7 and 8), suggesting similar sources. Incompatible element enrichments combined with depletions of Nb, Ta and Ti indicate that they likely originated from the fertile subcontinental lithosphere mantle (Shirey et al., 1994;Fitton, 1995;Xia et al., 2007;Peng et al., 2012;Wang et al., 2014) that had been metasomatized by an ancient subduction event (Fitton, 1995;Kerrich et al., 1999). This is further supported by the ∼2.5 Ga occurrence of subduction in Wutai area (Wang et al., 2004). ...
Article
Mid-Neoproterozoic low-δ¹⁸O metamafic rocks from the Leeuwin Complex, southwestern Australia, are reported for the first time. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating of these upper amphibolite- to granulite-facies mafic rocks yields igneous protolith ages of 674–660 Ma. The metamafic rocks are generally classified as subalkaline tholeiitic rocks with an ocean island basalt (OIB) affinity. They have low Mg# values (22–50) and Cr (0.19–105 ppm) and Ni (0.62–115 ppm) contents, with whole-rock εNd(t) values of − 1.4 to + 1.5 and zircon εHf(t) values of − 0.3 to + 3.5. Using these data in combination with the incompatible trace element characteristics, it is inferred that the protoliths of the rocks were derived from low-degree partial melting of relatively depleted asthenospheric mantle in a continental rift environment, and the magmas underwent some crustal contamination and fractional crystallization of mafic minerals. Zircon cores from the metamafic rocks yield δ¹⁸O values of 0.89 to 4.10‰, which are lower than normal mantle values (5.3 ± 0.3‰). These cores preserve oscillatory zoning or banding in cathodoluminescence images, and individual samples have concordant ages and preserve a narrow range of δ¹⁸O values, suggesting that the low-δ¹⁸O signatures are of primary magmatic origin. It is inferred that these low-δ¹⁸O metamafic rocks were generated by contamination by low-δ¹⁸O felsic crustal wall rocks and interaction of the magma with surface water at shallow depths in an extensional regime during the mid-Neoproterozoic.
Article
The Hudian gneissic monzonitic granite with lentoid dark enclaves and foliated mafic dykes was exposed in the western segment of the North Qinling Orogenic Belt (NQOB). It has long been considered as an intrusion into the Kuanping Complex, the northernmost component of the NQOB. LA–ICP–MS zircon dating yields consistent formation age of ca. 1.77 Ga for the granite and dark enclaves and the same metamorphic age of ca. 400 Ma for the granite and the mafic dykes. The formation age of ca. 1.77 Ga of the Hudian pluton is much older than the deposition age (<530 Ma) of its wall-rock Kuanping clastic rocks and no detrital zircon age of ca. 1.77 Ga was found in the Kuanping clastic rocks, suggesting that the Hudian monzonitic granite is not an intrusion in the clastic rocks, and the pluton was far apart when the Kuanping clastic rocks were deposited. The granite is low in Al2O3, CaO, MgO and Mg# high in SiO2, Na2O + K2O, Fe2O3T, Zr and 10,000*Ga/Al, and displays slightly right dipping “gull-wing” shaped rare earth element patterns, agreeing well with the features of A-type granites; the dark enclaves are basalt-andesitic rocks and show characteristics of crust-contaminated continental within-plate basaltic rocks, all suggesting a tectonic setting of extension. The major and trace element compositions of the Hudian monzonitic granite and its dark enclaves and the zircon εHf (t) values (−12.1 to −7.1) of the Hudian monzonitic granite all show strong comparability to the 1.75–1.8 Ga A-type granites and coeval intermediate-basic rocks of the Xiong’er Group in the southern North China Craton (NCC), indicating that the Hudian pluton maybe initially formed in the southern NCC. Considering the absence of obvious 1.75–1.8 Ga detrital zircon records in the Kuanping Complex, and the common metamorphic age of ca. 400 Ma in the Kuanping Complex and Hudian pluton as well as other units of the NQOB, we propose that the NCC has no affinity to the NQOB and the Hudian pluton was involved in the NQOB during the collision between the southern NCC and the NQOB at ca. 400 Ma.
Article
Situated between the North China Craton to the east and the Tarim Craton to the west, the northern Alxa area in westernmost Inner Mongolia in China occupies a key location for interpreting the late-stage tectonic evolution of the southern Central Asian Orogenic Belt. New LA-ICP-MS zircon U–Pb dating results reveal 282.2 ± 3.9 Ma gabbros and 216.3 ± 3.2 Ma granites from the Yagan metamorphic core complex in northern Alxa, NW China. The gabbros are characterized by low contents of Si, Na, K, Ti and P and high contents of Mg, Ca, Al and Fe. These gabbros have arc geochemical signatures with relative enrichments in large ion lithophile elements and depletions in high field strength elements, as well as negative εNd(t) (−0.91 to −0.54) and positive εHf(t) (2.59 to 6.37) values. These features indicate that a depleted mantle magma source metasomatized by subduction fluids/melts and contaminated by crustal materials was involved in the processes of magma migration and emplacement. The granites show high-K calc-alkaline and metaluminous to weakly peraluminous affinities, similar to A-type granites. They have positive εNd(t) (1.55 to 1.99) and εHf(t) (5.03 to 7.64) values. These features suggest that the granites were derived from the mixing of mantle and crustal sources and formed in a postcollisional tectonic setting. Considering previous studies, we infer that the final closure of the Palaeo-Asian Ocean in the central part of the southern Central Asian Orogenic Belt occurred in late Permian to Early–Middle Triassic times.
Article
The Neoarchaean crustal evolutionary processes of the North China Craton (NCC) provide a window to understanding the crust–mantle interaction in the Early Earth. The Jiefangyingzi amphibolites are located in the Bainaimiao arc belt, along the northern margin of the NCC. LA–ICP–MS U–Pb isotopic data reveal that the Type 1 amphibolites were formed at ~2562 Ma, and the Type 2 amphibolites were formed at ~2504 Ma, but were subsequently subjected to metamorphism at ~1889 Ma. Lu–Hf isotopic data of the Type 1 amphibolite show positive εHf(t) values (+1.2–+6.0) and TDM from 2612 Ma to 2791 Ma, suggesting a significant episode of 2.6–2.8 Ga crustal growth within the study area. Geochemically, the Type 1 amphibolites show high SiO2 contents (53.37%–55.19%), moderate LREE–enriched patterns (La/Sm)N = 2.42–3.15 and HREE–depleted patterns (Gd/Yb)N = 1.86–2.64, with negative Nb anomalies, which are similar to IAB–like rocks, suggesting that they were derived from partial melting of arc–related depleted mantle with intense subduction–related fluids metasomatism. The Type 2 amphibolites have lower SiO2 contents from 44.28% to 50.77%, and show LREE–depleted REE patterns ((La/Sm)N = 0.67–1.02), nearly unfractionated HREE patterns ((Gd/Yb)N = 0.89–1.77), with negative Nb, Th and positive Sr, Zr anomalies, which are close to N–MORB–like rocks, indicating that they were generated from partial melting of depleted mantle with slight fluid metasomatism. The geochronological, Lu–Hf isotopic and geochemical features show that the IAB–like rocks were formed in an intra–oceanic arc environment, coincide with the significant ~2.5–2.6 Ga episode of crustal growth in the NCC; The slightly younger MORB–like rocks reflect the spreading of the oceanic crust might last until ~2.5 Ga in the central zone.
Article
The Carboniferous–Early Permian rift-related volcanic successions, covering large areas in the Chinese Tianshan and its adjacent areas, make up an important phanerozoic large igneous province in the word, which can be further divided into two sub-provinces: Tianshan and Tarim. The Early Permian volcanic rocks have been considered to be the products of an intraplate volcanism by most researchers. However, there is still strong controversy about the nature and geological setting of the Carboniferous volcanic rocks. The regional angular unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks, the ages (360–351 Ma) of the youngest ophiolite and the peak of subduction metamorphism of high pressure-low temperature metamorphic belt and the occurrence of Ni-Cu-bearing mafic-ultramafic intrusion with age of ~352 Ma and A-type granite with age of ~358 Ma reveal that the final closure of the Paleo-Asian Ocean might take place in the Early Mississippian. Our studies reveal that although contamination by continental crust or lithosphere can impart subduction-like signature (e.g., low Nb, low Ta and low Ti) and lead to misidentification of contaminated Carboniferous basaltic lavas from the Chinese Tianshan and its neighboring areas as arc related, there are still some essential differences between the Carboniferous basaltic lavas and arc related ones; such as: uncontaminated Carboniferous basaltic lavas have higher Nb concentrations (9–22 ppm), Nb/La > 1, “hump-shaped” OIB-like trace element patterns and moderate positive ɛNd values that distinguish them from the arc related ones; whereas, the contaminated Carboniferous basaltic lavas are characterized by pronounced negative Nb, Ta and Ti anomalies, but, their concentrations of incompatible trace elements are conspicuously higher than those of subduction-zone basalts that also distinguishes them from the arc related ones. Our summation suggests that the Carboniferous volcanic successions did indeed erupt in an intracontinental rift setting and their generation is likely confined to mantle plume.
Article
Carboniferous magmatism in the Darbut region is critical for understanding the evolutionary history of the accretionary orogenesis of the West Junggar area in the Central Asian Orogenic Belt. In this paper, zircon U–Pb geochronological, whole‐rock geochemical and Sr–Nd–Pb–Hf isotopic analyses of the basalts, basaltic andesite, and dacite from the Carboniferous Tailegula Formation, Darbut region, have been performed. The magmatic zircons from basalt, basaltic andesite, and dacite yield concordia U–Pb isotope ages of 344 ± 4, 347 ± 3, and 340 ± 5 Ma, respectively, which are interpreted as the crystallization ages of these rocks. The basalts and basaltic andesite are characterized by an alkali composition; high Nb/Yb, Th/Yb, Ta/Yb, La/Sm, and Sm/Yb trace element ratios and Pb isotopic ratios; moderate (87Sr/86Sr)i ratios; low (143Nb/144Nb)i ratios; and positive εNd(t) (+0.66 − +4.73) and εHf(t) (+1.7 − +15.2) values. These characteristics indicate that the magmas were derived from mixed mantle sources, that is, enriched mantle sources and high U/Pb ratio mantle (HIMU) component sources with ≤10% partial melting of garnet and spinel lherzolite. The samples display pronounced light rare earth elements (LREEs) and incompatible element enrichment patterns with positive Nb–Ta anomalies and a high Pb content and Nb/La ratios but low (Th/Nb)N ratios and a lower degree of partial melting, implying that these rocks were formed in a continental extensional setting. Based on our results and that of previous studies, we conclude that the early Carboniferous volcanic rocks in the Darbut region formed by mantle plume or regional‐scale mantle upwelling in a continental extensional setting after closure of the Junggar Ocean.
Article
The subduction history of the Neo-Tethys is critical for understanding the early-stage tectonic evolution of the Qinghai-Tibetan Plateau. Arc volcanics, which usually record the subduction process, thus can help trace the evolution of the Neo-Tethys. In this paper, we report for the first time zircon LA-ICP-MS U-Pb ages, whole-rock major and trace element geochemistry, as well as Sr-Nd-Hf isotopic data for the volcanic rocks from the Danshiting Formation at the southern edge of the Lhasa Terrane. Two samples collected from the Danshiting Formation volcanic rocks yield zircon U-Pb ages of 97.1 ± 0.8 Ma and 90.1 ± 0.7 Ma, respectively. The volcanic rocks of the Danshiting Formation are composed dominantly of basalts and andesites characterized by depletion of high-field-strength elements, low (⁸⁷Sr/⁸⁶Sr)i ratios (0.7042-0.7045), high εNd(t) (+2.1 to +2.7) and high εHf(t) values (mean + 11.6). These rocks show geochemical affinity of arc volcanic rocks. The volcanic rocks of the Danshiting Formation may have formed during the northward subduction of the Neo-Tethys. This paper, combined with the previous data, briefly divide the northward subduction of the Neo-Tethys into four periods: the Jurassic (195-150 Ma), when the magmatic activity was controlled by the northward high-angle subduction of the Neo-Tethys that produced volcanics of the Yeba and Bima formations; the Late Jurassic to Early Cretaceous (150-110 Ma), when the Neo-Tethys subducted at low-angles; the Late Cretaceous (110-80 Ma), when subduction was of the Andean-type and produced volcanic rocks of the Danshiting Formation; the Paleogene (70-40 Ma), when there was transition from ocean-to-continent subduction to continent-continent collision that produced volcanic rocks of the Linzizong Group.
Article
Amphibolite is distributed sporadically in Paxialayidang area on the southern margin of Altun terrain. The amphibolite is ellipsoidal or lenticular in shape and is wrapped by marble of Altun terrain. In this paper, the authors studied petrology, geochronology and geochemistry of the amphibolites. LA-ICPMS zircon U-Pb dating yielded ages of (432.5±5) Ma and (857.4± 7.1) Ma for metamorphic event and protolith respectively, and shows that the protolith was formed during Neoproterozoic Qingbaikou period. Amphibolite is slightly enriched in REE, and shows unnoticeable fractionation between light rare earth elements (LREE) and heavy rare earth elements (LREE) and the signatures of subduction zone basalt. The primary magma originated from an enriched mantle source, which was formed by the recirculation of subducted slab, and was modified by fluid or melt subsequently. Combined with the evolution of regional tectonic, the authors hold that the protolith of amphibolite was formed in an incipient continental rift and was a transitional type formed during the transition from continental extension to continental rift. The younger age of (432.5 ±5) Ma represents the time of metamorphic event, which was caused by the detachment of continental lower crust during Early Silurian. In this paper, the authors provide new geochronological and geochemical evidence about basic magmatic activity during middle-late stage of Neoproterozoicin on the southern margin of Altun terrain. The basic magmatic activity was the response to the Rodinia supercontinent breakup on the southern margin of Altun terrain.
Article
The Urad Zhongqi area of western Inner Mongolia crosses the North China Plate margin and Xing'an-Mongolia Orogenic Belt, which distributed regional basic rock massif occurred as small stocks, and a lack of reliable data of intrusion age. This paper reports LA-ICP-MS zircon U-Pb dating, geochemical and Hf isotopic data of Hadahushu mafic intrusion in Urad Zhongqi area, with the aim of constraining its petrogenesis and the beginning time of subduction of the Paleo-Asian oceanic plate. LA-ICP-MS U-Pb dating results of zircons from the mafic intrusion indicate that the intrusion formed at the Late Cambrian (513±2 Ma). The geochemical characteristics of these rocks indicate that they are the calc-alkaline series, and are characterized by enrichment in LILE (such as K, Ba, Rb), relatively depleted in HFSE (such as Nb, Ta, Zr, Ti) and LREE. The εHf(t) values and one-stage model ages (TDM1) of zircons from the rock vary from 14.15 to 15.03 and from 518 to 556 Ma, respectively. These characteristics suggest that the primary magma was derived from the depleted lithospheric mantle source. Based on the previous study, we conclude that the formation of mafic intrusion was related to the subduction of the Paleo-Asian oceanic plate. © 2016, Editorial Department of Earth Science. All right reserved.
Article
The volcanic rocks from the Liushugou formation are widely distributed in Balikuntage area, Xinjiang. Study on the basalt, their petrology, geochemistry and isotope geochemistry indicate that, the basalts in this area belong to tholeiite series and are characterized by LREE, LILE (Sr, K, Rb, Ba, Th) enrichments and HFSE (Nb, Ta, Ti), HREE depletion, low Sr-Nd-Pb isotope (⁸⁷Sr/⁸⁶Sr) t = 0.70382-0.70667; (²⁰⁶Pb/Z04Pb) t = 17.857-18.054; (²⁰⁷Pb/²⁰⁴Pb) t = 15.494-15.545; (²⁰⁸ Pb/²⁰⁴ Pb) t = 37.746-37.798 and high ϵNd (ϵNd(t) = + 4-+ 8.3), which show its depleted mantle origin and contamination of crust materials. Through comparing with the basalts of the eastern Bogda in the same period, the results show that Balikuntage has the similar structure attributes with the Bogda area, which is the continental rifting in the late Carboniferous. But they belong to different stages of rifting evolution, thus, their geochemistry exhibits some disparities.
Article
Mid-Late Neoproterozoic to Early Paleozoic volcanism in the Qilianshan area, which shows systematic variations in space and time, is the volcanic response to the tectonic evolution of the Qilianshan. The volcanism gradually changed from continental rift-related and continental flood basaltic through MORB-type, island-arc and back-arc to post-collisional rift-related eruptions along with the tectonic evolution of the Qilianshan shifting from rifting and break-up of Rodinia through opening, spreading, subducting and closing of the Early Paleozoic oceans to arc-continent and continent-continent collision. The continental rift-related and flood lavas with ages of 850-604. Ma distribute mainly on the Qilian and Qiadam Blocks. The widespread MORB-type and "island-arc-backarc"-type lavas were generated from ~550 to 446. Ma in both the North Qilian and the South Qilian ocean-trench-arc-basin systems. In the meantime the intracontinental rift-related volcanism occurred in the central Qilian Block between ~522 and 442. Ma. The Early Paleozoic oceanic basins were closed at the end of Ordovician (~446. Ma). Subsequent post-collisional vocanism occurred on the northern margin of the Qilian Block from ~445 to ~428. Ma. Such spatial-temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Qilianshan. These processes involve: (1) upwelling of mantle plumes or a mantle superplume and subsequent rifting and break-up of Rodinia and subsequent opening, spreading and subduction of Early Paleozoic oceans followed by island-arc formation, (2) roll-back of the subducted oceanic slabs followed by back-arc extension and back-arc basin formation, (3) ocean closure and slab break-off followed by upwelling of asthenosphere and post-collisional volcanism. Intensive orogenic activities occurred in the Late Silurian and Early Devonian (~420 - ~400. Ma) in response to the exhumation of the subducted crustal materials. Mountain collapse and lithosphere extension happened in the priod of ~400-360. Ma and formed post-collisional granitic intrusions, which marked the end of a complete orogenic cycle.
Article
The early Permian Bogda orogenic basalts and acidic volcanic rocks, developed in the Jijitaizi area of the eastern section of Bogda orogenic belt, are bimodal volcanic rocks, which are composed by sub-alkaline basalt and rhyolite, mainly belonging to the calc-alkaline series. The basalts have high values of Al (Al2O3 = 14.06%-20.93%), Na2O/K2O (2.34-28.36) and TiO2 (1.26%-l. 52%), which are slightly higher than the ones of N - MORB, and they have low contents of Mg (MgO=2.99%-8.62%, Mg# =27-52). These major element characteristics indicate that these basalts had experienced an obvious fractional crystallization of olive and pyroxene during basaltic magmatism, which were the evolution products of original magma's fractional crystallization. The basalts are slightly enriched in trace elements (such as Rb, Ba and P), with no Eu anormalies (δEu=0.85-1.02), but are relatively depleted in Nb, Ta, Sr and Ti. These trace element characteristics of volcanic rocks suggest that, these basalts were originated from the partial melting of the depleted lithospheric peridotite mantle and had suffered a certain degree of crustal contamination, showing the geochemical features of intraplate basalts formed within the intracontinental extensional environment. The rhyolites of bimodal volcanic rocks have high values of SiO2 (73.58%-75.45%) and ALK (Na2O + K2O=8.56%-8.79%), but low contents of TiO2 (0.12%-0.18%), A12O3 (12.77%-13.24%) and MgO (average about 0.21%). Their trace elements generally have the enrichment features of Rb, Th and Ba, obvious depletion of Nb, Ta, Sr, P and Ti, and the REE distribution patterns are characterized by LREE enrichment, right-deviation type and obvious depletion of Eu (δEu = 0.52-0.70). The REE characteristics indicate that, the rhyolites have similar geochemical characteristics of A-type granite, suggesting that the rhyolites were originated from the fractionation of underplating basaltic magma. The geochemical characteristic of the bimodal volcanic rocks in Jijitaizi area of the eastern section of Bogda orogenic belt indicate that, these volcanic rocks were formed in a post-collisional extensional environment. The LA-ICP-MS zircon U-Pb ages of rhyolites are (294.2±1.3) Ma, suggesting that this set of volcanic rocks was formed in the beginning of early Permian. Our study of the bimodal volcanic rocks in Jijitaizi area indicate that, the research area had witnessed the transitional period of geodynamic environment from Carboniferous collision and compression to Permian stretching and extension, and it could provide an evidence for understanding the tectonic evolution and plate system of the Bogda orogenic belt of the late Paleozoic.
Article
The Early Permian volcanic rocks in Haerjiawu Formation are distributed in eastern Tangbale ophiolite belt on the southwest margin of Junggar basin. The volcanic rocks are mainly composed of greyish-green or purplish-red (vesicular) (amygdaloidal) basalt, basaltic andesite, and andesite. The high-precision LA-ICP-MS zircon U-Pb dating reveals that they were formed in Early Permian at the age of (296.6±8.1) Ma, MSWD=7.7 (93% conf.). Geochemically the volcanic rocks belong to sub-alkaline series. They have relatively high content of alkali (w(Na2O+K2O)=3.95%-6.39%), and enriched in sodium. The LREE are relatively enriched ((La/Yb)N=3.49-14.66) and well fractionated ((La/Sm)N=1.68-3.51); while the HREE fractionation is weak ((Gd/Yb)N=1.38-2.56). Only some of the samples have weak negative Eu anomalies (δEu=0.88-1.15). All samples are enriched in LILE; while HFSE are relatively depleted in Nb and Ta but slightly enriched in Zr, Hf. These characteristics, along with element correlation diagram, indicate that the volcanic rocks in Qiada area mainly generated in intra-plate continent with partial island arc environments. These volcanic rocks were from extensional setting of post-collisional period. These island-arc characteristics are inherited from pre-collisional arc component.
Article
There exposed part of the Ophiolitic Melange in Baitutang, the norht of the South Tianshan in Xinjiang. To understand the ultrabasic rocks forming environment, this paper focuses on the 9 samples of this area are tested by the study of geochemical method. Research and analysis of the geochemical results that the serpentinite in the area with low SiO2, TiO2, CaO, K2O, Na2O, MgO, Al2O3 for the characteristics, with high concentration of compatible elements Cr, Co, and Ni, depleted in incompatible elements, and large ion lithophile element content is low, the overall features similar to the Ocean lherzolite, features representing the mantleresidue. The basalts possess high TiO2 and MgO contents, not only low Al2O3 and K2O, but also Na 2O>K2O. Compared with MORB, the elements of Nb, Ta apparent are loss. Compared with the primitive mantle, with slow features similar to the N-MORB's losses, The geochemical characteristics of the main elements and trace elements show that rock should be formed in the tectonic environment similar to mid-ocean ridge, and the magma derived from depleted mantle zone.
Article
The Kalagailei volcanics-hosted hydrothermal copper-gold deposit is located in the western part of Nalati tectonic belt of Western Tianshan Mountains, Xinjiang. The ore-controlling strata are basaltic tuffs, epimetamorphic rocks and crypto-explosive breccia of Lower Carboniferous Dahalajunshan Formation. The copper ore bodies occur in crypto-explosive breccia pipes and fissures of the structural fracture zone, or in the micro-fractures of schistosity and cleavage. Silicification, chloritization, tourmalinization are the alteration indications of high-grade ores. Geochemical data show that volcanic rocks of Dahalajunshan Formation consist mainly of calc-alkaline series with partial tholeiite series, which are enriched in LREE and large-ion-lithophile elements (Rb, Ba, Th, K) and comparatively depleted in HREE and high field strength elements (Nb, Ta, Ti, P), possessing continental volcanic arc affinity. In combination with the tectonic evolution characteristics of the south margin of Yili-Central Tianshan plate, the authors consider that the Dahalajunshan Formation of the ore district occurred in a comparatively mature continental volcanic arc environment of plate subduction-collision in the later period.
Article
Carboniferous volcanic rocks are widely distributed in northern Alashan League and its neighboring areas, and their genesis and tectonic setting have always been an issue of controversy. Based on analysis of petrology and petrogeochemistry characteristics of Carboniferous volcanic rocks in the study area, this paper attempts to clarify the palaeo-tectonic setting and provides independent evidence for understanding the properties of Carboniferous basin. The volcanic rocks are dominated by intermediate-acidic volcanic rock. The majority of basalts, basaltic-andesite and andesite fall into sub-alkaline series. Mg# number of this suit ranges from 0.29 to 0.69. These volcanic rocks samples exhibit strong depletion in the high field strength elements Nb, Ta and Ti, and minor enrichment in the LREE. In terms of REE, (La/Yb)N number ranges from 2.19 to 10.10, showing less obvious Eu depletion (δEu=0.81 to 1.08), and slightly right-inclined REE distribution patterns, with higher εNd(t) value (+1.10 to +6.35). The samples show not only the marks of those within plate setting as a whole, but also those of subduction zone. Combining with the regional geological characteristic, we conclude that the Carboniferous magmatic activity in the study area generated in a within-plate setting, with variable degrees of contamination of crust during magma ascending, and probably related to mantle plume event.
Article
The Nb-enriched basalts-basaltic andésites in Heishantou Formation, which are associated with high magnesium andesite and adakite, is main composition of the Early Carboniferous marine volcano-sedimentary in West Junggar. All analyzed samples are basaltic in composition with SiO2 contents ranging from 49. 62% to 55. 68%, and the average value is 52. 70%: Moreover, the samples have high TiO2 (1. 16% ∼ 1. 99%) with average 1. 56%: Also Na2O enriched but K2O depleted, with Na2O from 3. 17% o 6. 35%, the average is 4. 90%. The ratios of Na2O/K2O in samples ranging from 1. 19 to 26. 08, and most ratios > 4 (average s 8. 93). The Nb-ennched basalts are characterized by obvious Nb, Sr and Zr enrichment, with the contents Nb > 7 × 10-6 (7. 29 × 10-6 -12. 32 × 10-6, average is 9.48 × 10-6), the contents Sr >400 × 10-6 (618 × 10-6 ∼ 1107 × 10-6, average is 825 × KT-6), the contents Zr > 158 × 10-6(159×10-6 -217 × 10-6, average is 182. 58 × 10-6), the ratios of Zr/Y > 4 (6.62-11.09, averageis 8.62). This all features distinguished differ from island arc basaltic rocks. Moreover, the samples show ight rare earth elements enrichment and heavy rare earth elements depletion [(La/Nb)PM > 0. 7 (1.23 ∼ 2. 26), average s 1. 94], very weak Eu positive anomalies. Isotopically, the studied basalts all display consistently Sr-Nd isotopic compositions [(87Sr/86Sr)i =0.7035∼0.7039, 143Nd/144Nd =0. 5128∼0. 5129, εNd(t) =5. 49 ∼ 7. 13]. This geochemical characteristics are consistent with the Nb-enriched basalts defined by Sajona et al. (1993, 1994, 1996), but obviously different from island arc basalts. The discovery is geochemical evidence for the Nb-enriched basalts occurring in Early Carboniferous, West Junggar. This is also abundant in ocation and horizon of the Nb-enriched basalts in Xinjiang and even China. Furthermore, this is more mportant proof for confirming sland arc settings in study area at Early Carboniferous.
Article
The age of basalts in the Youlebasi of Qipan Country in the Southwest of Tarim Basin by zircon LA-ICP-MS U-Pb dating is 298.3±2.8 Ma(MSWD=2.6), which represents the crystallization age of the volcanic rocks erupted in Early Permian. The basalts have the characteristics of high Ti(Ti/Y=513.86~577.35), high Nb(28×10-6~35.7×10-6) and low Zr/Nb ratios(10~10.86), which indicates that the basalts were derivate from enrich mantle sources. While the slightly low Nb/U(~30)and Ce/Pb(~15)indicated that the basalts derivate from the continental lithosphere or subjected to some degree of crustal contamination. The Youlebasi basalts have higher TiO2 and P2O5 contents, enrich LREE, Rb and Ba, which indicates that basalts have geochemical characteristic of mantle plume. Based on large-scale volcanic eruption Tarim area and the characteristics of enrichment incompatible elements and facies paleogeography, the basalts in the Southwest of Tarim may be caused by volcanic of mantle plume or enrichment lithospheric mantle partial melting caused by rise and heat of mantle plume.
Article
SHRIMP U-Pb zircon ages are reported for rhyolites in the bimodal volcanic rocks from the Phlaythong area at the Southern of Laos. The zircons with well-defined oscillatory zoning from rhyolite sample yielded mean 206Pb/238U age of (229.0±2.0) Ma, which represents the eruption age of the volcanic rocks during Middle to Upper Triassic (T2-3). Geochemically, the basalt shows an average SiO2 content of 50. 70%, enrichment of TiO2, MgO, CaO, TFeO, LILE(Sr, Rb, Ba, Th) and LREE, and depletion of alkali, P2O5 and HFSE. Rhyolites are generally characterized by rich SiO2(76.33%), alkali(w(K2O)>w(Na2O)), and poor TiO2, MgO, CaO and TFeO, they have relatively high Th, U and Zr and rather low Ta, Nb, Ba, Sr and Ti, and display obvious negative anomaly of Ba, Sr and Ti, obvious enrichment of LREE, noticeable negative Eu anomalies (δEu=0.31-0.82). According to this research, the basalt was originated from depleted asthenosphere mantle (DM) material upwelling and contaminating a little part of crustal materials, and rhyolite resulted from melting crustal materials heated by basaltic magma and felsic magma. The bimodal volcanic rocks are the products of partial extension in an intracontinental rift.
Article
The Bikou Group basalt lenticular or banded in form is distributed in tuffs and tuffaceous phyllites of Huachanggou area. Based on detailed field geological survey and systematic laboratory studies including microscope observation, major and trace element analysis and radioactive isotope analysis, the authors investigated the petrological, geochemical and chronological characteristics of the basalt. The result shows that the basalt in Bikou Group has been subjected to metamorphism of greenschist facies, and mainly consists of plagioclase, chlorite and epidote. The basalt exhibits low SiO2 (44.67%-49.76%), TiO2 (1.14%-1.34%) and REE, high TFe2O3(12.03%-15.47%), MgO (7.57%-9.3%) and CaO (7.29%-10.54%), with LREE depletion and HREE enrichment. It originated from asthenosphere, occurred in N-MORB setting and was mixed with the old crust. The basalt sample yielded zircon LA-ICP-MS U-Pb age of (316.3±6.0) Ma (MSWD=0.78; n=7), which indicates that the Bikou Group volcanic rocks are products of at least two periods of volcanic activities, i.e., Neoproterozoic and late Paleozoic. They were formed massively in Neoproterozoic and reformed by the volcanic activity in late Paleozoic.
Article
A 500km-long high-pressure metamorphic belt occurs in the Qiangtang Block of northern Tibet. Due to little geochemical investigation, the primary rock formation and original geodynamic setting of the high-pressure metamorphic rocks (eclogite and blueschist) remain unconstrained so far. In this paper we present the results of a geochemical investigation on eclogite and blueschist from this belt, and discuss their geodynamic implications. The eclogites collected from the Guoganjianian localities occurs as blocks and lenses in Grt-Phn schist and marble. Geochemically, the Guoganjianian eclogite is tholeiitic and has low rare earth element abundances (ΣREE=51. 19 × 106 ∼ 59.43 × 10-6) and display light REE (LREE) depleted patterns with (La/Yb)N values of 0. 59 ∼ 0.70, without negative Nb, Ta and Ti anomalies. These features are comparable with those of normal mid-ocean ridge (N-MORB). The blueschist collected from the Hongjishan localities occur as blocks and lenses in metasandstone and Phn-Qtz schist. These metamafic rocks are alkaline-tholeiitic and have high TiO2(2. 97% ∼ 4. 14%), high P2O5(0. 29% ∼ 0. 48%) contents, and are enriched in light rare earth elements (LREEs) with (La/Yb)N values of 6. 10 ∼ 11. 6 and high field strength elements (HFSEs), similar geochemically to Ocean Island basalt (OIB). Geochemical signatures also indicate that upper continental crust contamination played insignificant role in the generation of protolith of the Hongjishan blueschist. The features of field relations and geochemistry are comparable with those of Permian mafic dyke swarms in south Qiangtang terrane. These rocks, altogether, record a complete history from ocean crust subduction, to continental subduction, and to continental collision.
Article
Neoproterozoic magmatic rocks are widespread on the western margin of the Yangtze block, and their genesis and tectonic setting have long been problems of hot debate. Petrological, geochemical and Sm Nd isotopic data are reported for Neoproterozoic Kangding-Danba mafic dykes in the Kangdian rift on the western margin of the Yangtze block. This paper deals with petrogenesis and magma source characteristics of these rocks. The rock samples are of tholeiite series, and their geochemical and Nd isotopic characteristics suggest that they were generated in an intraplate setting with variable degrees of contamination of young island arc crust during magma ascending. The magma was generated by partial melting of DM mantle source region within the spinel stability field, and was probably related to Neoproterozoic plume event related to the breakup of Rodinia supercontinent.
Article
By studying geochemical characteristics of Ordovician volcanic rocks in the Beishan area of Gansu and Inner Mongolia, the authors consider that the volcanic rocks in this area mainly belong to the andesitic basalt predominated by tholeiite series. The samples collected from Huaniushan and Xichangjing are presented nearly flat-depleted of LREE and the slightly enriched HREE in the chondrite-normalized REE pattern, and their HFSE characteristic is similar to that of the N-MORB ophiolite. The samples from Yihatuoli and Xirehada are enriched in LREE and depleted in HFSE (Nb, Ta, Ti), which are similar with those of the island-arc volcanic rock. however, they have ratio values of (Th/Nb)N≫1 and Nb/La<1, as well as Zr/Nb similar to the N-MORB, which suggested that the volcanic rocks were contaminated by the crust. The features of Zr>70.00×10-6 and Zr/Y>3.00 of the samples are accordant with those of intra-plate basalts. Combined with the research of tectonic evolution in the study area, the authors concluded that Ordovician volcanic rocks in Huaniushan and Xichangjing formed at mid ocean ridge settings, and Ordovician volcanic rocks in Yihatuoli and Xirehada formed at a extensional rifting setting within the plate.
Article
Ophiolites that discontinuously crop out along the Yarlung-Zangbo suture zone (YZSZ) are remnants of Neo-Tethyan ocean lithosphere emplaced during subduction of the Indian plate beneath Eurasia. The Purang massif is one of the largest ophiolites in the western part of the suture with exposed area of about 650 km2 and consists dominandy of harzburgites, minor Cpx- harzburgites and dunites as well as a series of uppermost crustal rocks (siliceous limestone, siliceous shale, chert, basaltic lavas, and pyroclastic rocks), without cumulate rocks and pillow lava. In the northwestern part of the massif, peridotites were intruded by diabase dykes and sills. The diabases show N-MORB type REE patterns with (143Nd/144Nd)t being 0.512904∼ 0.512909 and ε Nd(t) +8.6∼+ 8.7, and are characterized by LILE (mainly K, Sr, Rb) enrichment and noticeable Nb, Th and U negative anomalies. They have a supra-subduction affinity and were formed in a back-arc basin setting. A comparison with other YZSZ ophiolites suggests that various intra-oceanic supra-subduction events within the Neo-Tethys began in different periods. The oceanic crust of the eastern part of YZSZ was formed in about Middle Jurassic in the back-arc basin, while those in central and western segments were formed in about Early Cretaceous above the SSZ setting.
Article
The Wuluate Formation of Lower Carboniferous is distributed in the northern part of Western Kunlun Mountains. Associated with thick carbonate rock and thin marble, it is a set of volcano-sedimentary rocks mainly composed of grayish green pillow basalts and grayish white rhyolite. Based on the study of the chemistry of lavas, the authors hold that the major element values of the basalts are similar to the average values of tholeiite: SiO2 values vary between 52.10% and 54.42%, (Na2O/K2O)>1, TiO2(0.50%∼2.03%), and Mg# values are in the range of 52∼70. The ΣREE values of the samples range from 14.46×10-6 to 91.83× 10-6, whereas (La/Yb)N values vary between 0.46 and 1.69, respectively. All these characteristics are extremely similar to things of the E-BABB and different from features of OIB and N-MORB. The basalts show depletion of Rb, suggesting that the lower crust contamination existed in the magma processing. Rhyolite belongs to both calc-alkaline and high-K calc-alkaline volcanic rock series. Concentrations of REE are higher than those of basalts, the (La/Yb)N radios are between 1.30 and 11.99, and (La/Sm)N radios are from 1.15 to 4.07. All these features are similar to features of the lower crust. Different REE patterns and trace element features between the two rock types and the absence of trend from the basalts to the rhyolite imply that the rhyolite and basalt had different sources and origins. Trace element diagrams and regional geological characteristics indicate that the Wuluate Formation formed in an extension environment of the back-arc pull-apart condition. The study of the genetic characteristics of rocks suggest that the basalts resulted from 5% - 10% partial melting of spinel lherzolite in the depleted mantle, and were contaminated by the lower crust later. The heat carried by the mantle material led to the partial melting of the lower crust, and then the rhyolite was produced. Comprehensive studies show that the northern part of Western Kunlun Mountains was in a stretch tension environment (back - arc extensional environment) in the early Carboniferous, and the Wuluate Formation was the magmatic product of the tectonic event.
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