No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Heterogeneity of the lower crust: Evidence from geochemistry of the Hannuoba granulite xenoliths, Hebei province
... Abundant lower crustal and upper mantle xenoliths are found in these basalts. The xenoliths have been studied to varying extents [31,[35][36][37][38][39][40][41][42][43][44][45][46]. The samples used in this study are summarized in Fig. 2 and described below. ...
... Archean terrain granulite data from [69]. The Hannuoba granulite xenoliths include data of [40][41][42]. Worldwide granulite xenoliths data are from GERM (http://www.EarthRef.org). aluminous phases such as garnet, spinel, and plagioclase. ...
... Lherzolite wall of DMP-314 is thought to be high pressure cumulate. Literature data [40,42] for intermediate-mafic and granulites with SiO 2 N 50 wt.% and without convex-upward REE patterns are also included. ...
Abundant lherzolite, garnet pyroxenite and granulite xenoliths are found in the Neogene Hannuoba basalt of the North China craton. Garnet pyroxenites generally occur as veins/layers in spinel lherzolites. There is a gradual decrease in olivine and an increase in orthopyroxene mode going from the lherzolite to the pyroxenite, suggesting that orthopyroxene may be forming at the expense of olivine. Garnet pyroxenites are enriched in the highly incompatible elements (e.g., Rb, K, Na, Sr, Ba, Nb and Ta) but have high and uniform Ni contents and Mg#s (83–90). This set of geochemical observations is paradoxical because the enrichments in highly incompatible elements signify derivation from a melt having either an evolved character or a significant fluid component, but the high Ni contents and high Mg#s suggest a much more primitive origin. A somewhat similar paradox is observed in the granulite xenoliths. Many of the granulite xenoliths have intermediate compositions, characterized by SiO2 > 50 wt.%, high Al2O3, Na2O, and Sr contents, low Y and heavy rare-earth contents, and high Sr/Y, La/Yb and Na2O/K2O ratios. However, these intermediate granulites have unusually high Mg#s (54–71) and high Ni (21–147 ppm) contents for their SiO2 contents and would otherwise suggest that these granulites are more primitive than their SiO2 contents indicate.
... Although there are no apparent mantle and lower-crust xenoliths in the Jining basalts, the alkali-rich lavas of the Hannuoba area some 100 km to the east host abundant deep-seated xenoliths. The xenoliths vary from ultramafic to mafic to felsic with peridotites being dominant, pyroxenites and mafic granulites subordinate, and felsic granulites rare (Zhang et al. 1998a). Petrographically, the granulite xenoliths have diverse mineral constituents and can be subdivided into three types: felsic, noritic, and TPG. ...
... Petrographically, the granulite xenoliths have diverse mineral constituents and can be subdivided into three types: felsic, noritic, and TPG. TPG is composed of clinopyroxene + orthopyroxene + plagioclase ± biotite ± K-feldspar, which may be widely distributed in the lower crust beneath the Hannuoba region (Zhang et al. 1998a). On the basis of detailed isotopic studies of TPG xenoliths, Zhang et al. (1998a, b) (Fig. 12), the alkali olivine basalts were plotted between basanites and the average compositions of TPG, indicating that at least three end-members would have been involved in the genesis of alkali olivine basalts: i.e., EM1, DMM, and TPG. ...
The Jining volcanic field located in the southern margin of the Mongolian plateau and the western North China Block consists of four rock types: quartz tholeiite, alkaloolivine tholeiite, alkali olivine basalt and basanite. These rocks have a wide range of K-Ar ages from ∼36 to < 0.2 Ma. The early volcanism was voluminous and dominated by flood-type fissure eruptions of tholeiites, whereas the later phase was represented by sparse eruptions of basanitic lavas. Thirty-six samples analyzed in this study show a wide range in SiO2 contents from 44% - 54%. They all are sodium-rich and high-Ti basalts that, however, show marked isotopic variations between two end-members: (1) tholeiites that have higher 87Sr/86Sr of 0.7048 - 0.7052, and lower eNd of-0.8 to -2.4 and Pb isotope ratios (206Pb/ 204Pb of 16.9 - 17.2, 207Pb/204Pb of 15.3 - 15.4 and 208Pb/204Pb of 37.1 - 37.7); and (2) basanites that have lower 87Sr/86Sr of 0.7035 - 0.7044, and higher €Nd of+ 1.3 to +4.9 and Pb isotope ratios (206Pb/ 204Pb of 17.7 - 18.0, 207Pb/204Pb of 15.4 - 15.5 and 208Pb/204Pb of 37.8 - 38.2). Alkali olivine basalt that occurs as a subordinate rock type is geochemically similar to the basanites, but isotopically similar to the tholeiites, characterized by the highest 87Sr/86Sr ratio among the three basaltic suites, coupled with a low Nb/U value (∼33). In Sr-Nd-Pb isotopic plots, the tholeiites extend toward the EM1 (i.e., enriched mantle type 1) component, whereas the basanites trend toward the Indian Ocean mid-ocean ridge basalt (MORB) field. Adopting the 'plum-pudding' model by Morris and Hart (1983), we suggest that the Oligocene tholeiites were generated by high degree melting of an ascended asthenospheric mantle that was contaminated with a large amount of EM1-type continental lithospheric material during the early Cenozoic. On the other hand, the late Tertiary and Quaternary basanites may have originated predominantly from a depleted asthenosphere component with small but variable degrees of contribution by the continental lithospheric mantle. The generation of alkali olivine basalt requires addition of a two-pyroxene granulite component, thus we suggest it was derived from a basanitic parental magma that underwent minor amounts of lower crustal contamination during ascent to the surface.
... Direct crystallization from the host magma for the rim of some xenocrysts is also possible, producing euhedral crystals Fan and Hooper (1989), Zheng et al. (1998, 2001), Chen et al. (2001). Lower crust granulite is granulites from both the Archean terrain (Zhang et al., 1998) and xenoliths in Cenozoic Nushan basalts (Huang et al., 2004). Underplated granulite represents granulite xenoliths in Cenozoic Hannuoba basalts, which were formed by Mesozoic basaltic underplating and subsequent metamorphism (Chen et al., 1998Chen et al., , 2001 Fan and Liu, 1996; Fan et al., 2001; Liu et al., 2001a,b; Zheng et al., 2003). ...
... Formation of complexly-zoned clinopyroxenes: For sample FC-10, this clinopyroxene records a very complex history of the crystal growth. The core possesses major elemental chemistry similar to that of clinopyroxenes from Archean lower crust granulites exposed on the surface (Zhang et al., 1998 ) and granulite xenoliths hosted in the Cenozoic basalts (Huang et al., 2004) (Fig. 9B). For example, clinopyroxene cores plot within the field for the old lower crust granulites in a Na 2 O–Mg# diagram (Table 1 andFig. ...
Mesozoic Fangcheng basalts from the North China Craton contain many clinopyroxene xenocrysts and pyroxenite xenoliths, which provide important information about melt circulation and crust-mantle interaction in the evolution of the sub-continental lithosphere beneath the region. All the xenocrysts show textural and chemical zoning. The zoning is mostly simple (simply-zoned), but some show complex patterns (complexly-zoned). In-situ major and trace element analyses suggest that all the simply-zoned xenocrysts may have been disaggregated from pyroxenite veins at mantle depth. The zoning is interpreted to result from chemical exchange with the host magma after their entrainment. The complexly-zoned xenocrysts record a complicated history of the lithospheric evolution: their cores could preserve information of high-temperature granulite-facies metamorphism in the lower crust and their intermediary zones might record metamorphic overgrowth in the mantle spinel-facies stability field. Therefore, the cores of the complexly-zoned xenocrysts have probably been derived from the Archean lower crust or newly-accreted lower crust and the intermediary zones could be formed through crust–mantle interaction. All the pyroxenite xenoliths are cumulates and were crystallized from the LREE enriched melts at mantle depth or crust–mantle transitional zone. Thus the clinopyroxene xenocrysts and pyroxenite xenoliths provide evidence for the existence of considerable crust–mantle interaction and melt circulation in the lithospheric mantle, which led to rapid lithospheric enrichment.
... The Hannuoba mafic granulite xenoliths are dominated by pyroxenes with an assemblage of mainly Cpx þ Opx þ Pl AE Grt (Zhang et al., 1998;Chen et al., 2001;Liu et al., 2001), contrasting with the assemblages of the experimental restites, which consist of Hbl þ Pl þ Grt AE Cpx. Chemically, the mafic granulite xenoliths have higher MgO and CaO but lower Al 2 O 3 than the experimental restites. ...
Mesozoic intermediate-felsic magmatic rocks in the eastern North China Craton commonly show geochemical similarity to adakites. However, the lack of direct constraints from partial melting experiments at high pressures and temperatures fuels a debate over the origin of these rocks. In this work, we performed partial melting experiments at 1.5 GPa and 800–950 °C on amphibolite samples collected from the vicinity of the Mesozoic potassium-rich adakitic rocks in the Zhangjiakou area, northern margin of the North China Craton. The experimental melts range from granitic to granodioritic compositions, with SiO2 = 56.4–72.6 wt.%, Al2O3 = 16.1–19.3 wt.%, FeO∗ = 2.4–9.6 wt.%, MgO = 0.3–2.0 wt.%, CaO = 0.6–3.8 wt.%, Na2O = 4.7–5.3 wt.%, and K2O = 2.6–3.9 wt.%, which are in the ranges of the surrounding Mesozoic potassium-rich adakitic rocks, except for the higher Al2O3 contents and the data point at 1.5 GPa and 800 °C. Trace element compositions of the melts measured by LA-ICP-MS are rich in Sr (849–1067 ppm) and light rare earth elements (LREEs) and poor in Y (
... Histograms of SiO 2 (a) and Mg # (b) distributions of the Hannuoba granulite/metasedimentary xenoliths (including data fromZhang et al., 1998a) to illustrate the difference in dominant rock types from worldwide granulite xenoliths (c, d) (GERM available online at http://EarthRef.org). ...
Thirty granulite and pyroxenite xenoliths from the Neogene Hannuoba basalt of the North China craton have been analyzed for major and trace element compositions. The granulites range in composition from mafic to felsic with SiO2 = 45.7 to 73.0% and also contain metasediments. The compositions of mafic and intermediate granulites can be explained by fractional crystallization of a magma chamber in the lower crust. The magmatic granulite xenoliths are interpreted as product of basaltic underplating and subsequent fractional crystallization at the base of the crust. Thermobarometric studies and correlation of calculated P-wave velocities with regional seismic refraction results suggest that the upper part of the lower crust, which accounts for two thirds of the entire lower crust in the North China craton at a 24- to 38-km depth, is dominated by intermediate and felsic compositions. Only the lowermost crust (38–42 km) has a mafic composition. This is also supported by the abundance of intermediate and felsic granulite xenoliths, which account for 45% of the granulite population collected. The calculated bulk lower crust in the Hannuoba area has an intermediate composition with SiO2 = 58%. Because other parts of North China also show a similar velocity structure, with the high-velocity layer confined to the lowermost 3 to 5 km of crust, the results from the Hannuoba area are considered to be representative of the reactivated North China craton as a whole.
... The Hannuoba basalts occur along the northern margin of the Trans-North China Orogen and have been dated at 14^27 Ma by the K^Ar method (Zhu, 1998). Abundant lower crustal and upper mantle xenoliths are found in the basalts and have been studied to varying extents (Song & Frey, 1989; Tatsumoto et al., 1992; Fan et al., 1998 Fan et al., , 2001 Zhang et al., 1998; Chen et al., 2001; Liu et al., 2001 Liu et al., , 2004 Liu et al., , 2005 Xu, 2002; Zhou et al., 2002; Wilde et al., 2003; Rudnick et al., 2004; Zheng et al., 2009). ...
We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism. The $2·5 Ga zircons have positive e Hf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative e Hf(t) values, indi-cating an ancient continental crustal origin.These observations sug-gest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 AE 3 Ma (2), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive e Hf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occur-rence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies. The e Hf(t) values of these zircons vary greatly from ^47·6 to 24·6. The 170^110 Ma zircons are generally characterized by negative e Hf(t) values, whereas the 110^100 Ma zircons have positive e Hf(t) values. These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive e Hf(t) values. These features imply that the melt^peridotite interac-tions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.
... JSB-01 is very small (~2 cm in diameter), in which fresh garnet cores are rimed by dark kelyphite. DMP-134 shows an equigranular texture, in which orthopyroxene (En 86 Fs 14 ) is rare, and many fine spinels are found around coarse garnets. In DMP-254, orthopyroxene is En 90-92 Fs 10-8 , and only one idiomorphic spinel enwrapped partially by garnet is found. ...
Major element compositions of garnet, clinopyroxene, orthopyroxene and spinel in the garnet-bearing lower crust and upper
mantle xenoliths from Hannuoba, North China craton are analyzed by the electron microprobe (EMP). The pressure-temperature
estimates reveal the increasing temperature and pressure from core to rim for granulites. In contrast, mantle xenoliths with
core temperature > 930°C recorded a history of decrease in temperature and pressure. However, those with core temperature
< 930°C show a negligible change. The final pressures recorded by these xenoliths cluster at 0.9–1.5 GPa. The presence of
highNa 2O cpx in granulite xenoliths suggests that they are products of the transition from granulite to eclogite metamorphism corresponding
to the increasing temperature and pressure. Together with previous studies, it is suggested that the P-T changes preserved in the xenoliths are related to lithospheric thickening and subsequent thinning prior to their eruption
in the Cenozoic.
... a Al O versus MgO; b CaO versus MgO; c SiO versus MgO; d Na O versus MgO. Pyroxenite data ofChen et al. 1997 andZhang et al. 1998a are included too. TheŽ. ...
Spinel and garnet pyroxenite xenoliths in Cenozoic basalts from Hannuoba, North China show extremely heterogeneous chemical and isotopic compositions (εNd=−27 to +34). Most of these pyroxenites are relatively young, probably late Mesozoic in age, although a few Al-pyroxenites could be very old (∼2 Ga). While their texture and major element compositions suggest an origin of high pressure cumulates, the trace element and isotopic compositions of the Hannuoba pyroxenites require multiple segregation processes from different parental magmas. Strong LREE enrichment, ubiquitous HFSE depletion and some Eu anomalies of the Al- and Cr-pyroxenites indicate the involvement of crust components in their source. Their Sr–Nd isotopic ratios are negatively correlated and plot below the MORB–OIB–IAB–sediment trend, suggesting that the parental melts of the Cr- and Al-pyroxenites may have been derived from a mixture of asthenospheric melts and a long-term evolved continental crust. The garnet pyroxenites significantly deviate from the isotopic array defined by the Al-pyroxenites, due to their relatively high 87Sr/86Sr at given εNd. They thus more likely represent segregates from melts derived from partial melting of hydrothermally altered oceanic crust (basalts+marine sediment). If the crustal component involved in the Al-pyroxenites is subducted terrigenous sediments or other continental materials from the Archean Sino-Korean Craton, the Al-pyroxenites and garnet pyroxenites may have formed contemporaneously at a palaeo-convergent plate margin. This may be related to the subduction of the Mongol–Okhotsk plate beneath North China during the late Jurassic. Alternatively, if the delaminated lower crust was involved, it implies that most of the Al-pyroxenites are younger than the garnet pyroxenites, and their formation may be temporally correlated with lithospheric thinning during the Cretaceous. This model is attractive because the inferred tectonic evolution from a convergent setting to an extensional environment is consistent with the geologic record in the area.
Widespread Mesozoic granitoids in the Cathaysia Block of South China are associated with intensive reworking of the lower crust as a result of magma underplating. This inference is based mainly on studies of mafic igneous rocks, whereas there is little evidence from lower-crustal rocks. Lower-crustal xenoliths in Mesozoic basalts in the Daoxian region within the Cathaysia Block might record information on the relationship between magma underplating and remelting of the pre-existing crust beneath the block. The xenoliths are mainly mafic granulites, with minor felsic granulites. The mafic granulites have low SiO2 contents (47.22–49.46 wt%) and high Mg# values (77.8–82.4), suggesting that their protoliths were derived from a mantle source. The felsic granulite xenoliths have high SiO2 (69.56–70.27 wt%) and low MgO (1.63–2.50 wt%) contents, and zircons in these xenoliths yield negative εHf(t) values (−6.1 to −12.6) and high δ18O values (6.8–7.6‰), consistent with a crustal source. Both mafic and felsic granulite xenoliths record magmatic (226–218 Ma) and metamorphic (218–193 Ma) events, suggesting a genetic link between mafic and felsic rocks in the lower crust. We propose that the magma underplating was responsible for the origin of the mafic granulites and partial melting of pre-existing lower crust, as recorded by the felsic xenoliths. The granulite xenoliths also contain abundant inherited zircons that formed during the Archean–Neoproterozoic (2584–659 Ma), early Paleozoic (peaking at ca. 425 Ma), and late Paleozoic (peaking at ca. 261 Ma). Zircons from the Daoxian granulite xenoliths have distinct HfO isotopic compositions that record the multistage evolution of the lower crust beneath the Cathaysia Block. This evolution involved the accretion of juvenile crust during the late Archean (εHf(t) = +4.2 to +4.6) and late Paleozoic (εHf(t) = +1.3 to +5.3; δ18O = 5.8–6.2‰), crustal reworking during the Neoproterozoic (εHf(t) = −7.5 to −11.8; δ18O = 5.1–9.5‰) and early Paleozoic (εHf(t) = −0.5 to −2.2; δ18O = 7.3–7.5‰), and crustal accretion with significant reworking during the early Mesozoic (εHf(t) = −19.2 to +5.9; δ18O = 6.8–7.6‰). The UPb ages and HfO isotopic data of zircons from the Daoxian granulite xenoliths are consistent with the distribution of Phanerozoic igneous rocks in the Cathaysia Block. These data support the inference that Mesozoic magma underplating triggered the remelting of pre-existing crustal materials and produced extensive granitoid magmatism.
How has the Earth’s deep continental crust evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in igneous rocks can be an important source of information. The North China Craton (NCC) is one of the oldest cratons in the world and Phanerozoic igneous rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continental crust. Abundant data of U–Pb ages and Hf isotopes in zircons were obtained for lower crustal xenoliths from over ten localities to constrain the history beneath the craton. The oldest components of the NCC may be ~4.0 Ga. The craton experienced complex accretion and reworking processes in its deep crust, accompanied by the formation and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. In the lower crust, the ancient components of the craton were reworked in Paleoarchean (3.80–3.65 Ga) time. The craton also experienced two important accretionary episodes, in the Neoarchean (2.8–2.5 Ga) and the Paleoproterozoic (2.3–1.8 Ga). Asthenospheric upwelling in Neoproterozoic time (0.6 Ga) locally modified the lower crust. Subduction and collision of the surrounding blocks, such as the Yangtze Craton, in Paleozoic and in early Mesozoic time also strongly modified the lower crust, especially along the cratonic margins. Accretion and modification of the lower crust during late Mesozoic–Paleogene were obvious due to the addition of depleted-mantle materials (underplating).
Clinopyroxenes of pyroxenite xenoliths from the Hannuoba basalt were in situ analyzed by LA-ICP-MS. Clinopyroxenes of spinel phlogopite clinopyroxenites have considerably low Cr, Co and Ni contents and Mg# and high CaO, Zr, Hf, Ta and LREE contents, which suggests that the spinel phlogopite clinopyroxenites could be cumulates modified by subsequent metasomatism. Clinopyroxenes of olivine garnet pyroxenite vein hosted in spinel lherzolite have relatively high Na2 O and low CaO contents, and are characterized by higher Ti, Zr, Hf and Y contents compared with the other garnet-bearing pyroxenites. These features indicate a possible Na-rich meltmantle peridotite interaction responsible for the formation of these garnet pyroxenite veins. La-enrichment and/or trace elemental zonation demonstrated by clinopyroxene of some garnet-bearing pyroxenites indicate multi-episode partial melting and mantle metasomatism in the north of North China craton. On the other hand, the in situ analyses for fluid inclusion-bearing area of clinopyroxene suggest that fluid inclusion is not only enriched in light rare earth elements and large ion lithophile elements, but also enriched in some high field strength elements (e.g., Nb, Ta, Zr and P). These observations imply that the fluid involved in mantle metasomatism could contain certain amount of anion (e.g., F- and Cl-), and does not fractionate LREE from HFSE.
A ultramafic pyroxenite pluton has been discovered in Zhongzaohuo area in the East Kunlun orogen Recently. This paper reports the results of petrological, geochemical and genetic mineralogy research on the pyroxenite pluton. The rock is mainly composed of clinopyroxene, orthopyroxene and amphibole, and minor plagioclase, quartz, biotite and iron opaque minerals. Amphibole and biotite were formed during retrograde metamorphism. The discriminant analysis results suggest that the Opx are magmatogenic, thus the rock should be named pyroxenite rather than granulite. The rock has high MgO, CaO and low Al2O3 and enriched in Rb and Th and depleted in Nb and Ti, showing clear evidence for an enriched mantle source. Field occurrence of the pyroxenite pluton suggests that the pyroxenite pluton was formed after the mylonization of the surrounding rocks. Combined with the tectonic evolution of East Kunlun, we come to the conclusion that the subduction of an Paleo-Tethys(A'nyemaqen) oceanic slab at the Middle Permian led to fluid and Si-rich? melt metasomatism, inducing partial melting of an enriched lithospheric mantle(peridotites) to form the ultramafic pyroxenite magma. The pyroxenite magma underplated the overlying lower crust, captured the metamorphic zircons of the granulite and exchanged some trace elements, but didn't result in the lower crust partial melting to form any felsic magma. The pyroxenite magma emplaced alone eventually.
We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism. The $2·5 Ga zircons have positive e Hf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative e Hf(t) values, indi-cating an ancient continental crustal origin.These observations sug-gest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 AE 3 Ma (2), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive e Hf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occur-rence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies. The e Hf(t) values of these zircons vary greatly from ^47·6 to 24·6. The 170^110 Ma zircons are generally characterized by negative e Hf(t) values, whereas the 110^100 Ma zircons have positive e Hf(t) values. These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive e Hf(t) values. These features imply that the melt^peridotite interac-tions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.
Oxygen isotope analyses were carried out using SIMS technique (Cameca-1280) on zircons from well-studied lower crust xenoliths/xenocrysts from the North China Craton (NCC). The results show that magmatic zircons with Archean-Paleoproterozoic (207)pb/Pb-206 ages have an extremely large variation in delta O-18 values that range from less than normal mantle value (4.74 +/- 0.28%. (2 sigma)) to O-18-enriched supracrustal value (8.88 +/- 0.40%. (2 sigma)) and differ in localities of the NCC (delta O-18(Qjndao) from 5.27 +/- 0.45%. to 8.88 +/- 0.40 parts per thousand; delta O-18(Hannuoba) from 6.78 +/- 0.23%. to 7.69 +/- 0.41 parts per thousand; delta O-18(Xruyan), from 4.74 +/- 0.28%. to 6.57 +/- 0.39 parts per thousand). Such a variation demonstrates that the Neoarchean lower crust beneath the NCC was highly heterogeneous in oxygen isotope compositions, even in an individual region. The fact that the predominant zircons have high delta O-18 values suggests that the recycling of O-18-enriched supracrustal materials into the Neoarchean lower crust indeed happened during zircon crystallization. Some zircons with core-rim texture have nearly identical delta O-18 both in core and its rim. On the other hand, others have a distinctive oxygen isotope ratio between the core and its rim, possessing slightly higher delta O-18 in the rim than its core or vice versa. Such an oxygen isotope feature in zoned zircons suggests a complicated evolutional history of ancient lower crust in the NCC. This study provides convincing evidence that a large amounts of the supracrustal materials have been recycled into the Neoarchean lower crust beneath the NCC, a process resulting in the highly heterogeneity in oxygen isotopes.
We report U–Pb ages and Hf isotopes of zircons in granulites from two continental suture zones in the North China Craton (NCC)—from granulite xenoliths entrained in the Cenozoic Hannuoba basalts within the Trans-North China Orogen (TNCO) that amalgamates the Western and Eastern Blocks, and from a mafic granulite associated with ultrahigh-temperature (UHT) granulites within the Inner Mongolia Suture Zone (IMSZ) that welds the Yinshan and Ordos Blocks. The zircons in these rocks, together with the inherited zircons from Mesozoic subalkalic intrusives of these regions, show several distinct U–Pb age populations and provide constraints on the evolution of the lower crust beneath the NCC. Parts of the older zircons from the TNCO yield discordant data that project to ages of about 2.5 to 2.7Ga. These old zircons show Hf TDM model ages of 2.6–2.9Ga suggesting the existence of Archean lower crust beneath the TNCO, which is coupled with the Neoarchean metamorphic basement and greenstone belt. A major zircon population grew at 1.75–2.0Ga, consistent with the timing of the Paleoproterozoic collision between the various crustal blocks within the NCC, suggesting that this event had a significant effect on the Archean lower crust. The older inherited zircons from the Western Block also yield discordant data that project to younger ages of about 1.8 to 1.9Ga. This age is consistent with the timing of intrusion of the mafic magmatic protoliths of the granulites associated with the UHT rocks in the khondalite belt within the IMSZ. These older zircons yield Hf TDM model ages of 2.0–2.5Ga suggesting that the lower crust beneath the Western Block in the NCC to be dominantly Paleoproterozoic, and decoupled with the Archean metamorphic basement of the region. Yet another population of zircons from the Hannuoba banded granulite xenoliths, garnet-bearing pyroxenite cumulate and dark pyroxenite xenoliths yield a large spread of Phanerozoic concordant ages ranging from 470Ma to 40Ma with peaks at 315Ma, 220–230Ma, 120Ma and 46Ma. The Late Paleozoic zircons predominantly occur in the garnet-bearing pyroxenite xenoliths and yield variable εHf(t) values of −24 to +18. Triassic–Cretaceous zircons mainly occur in the banded granulite xenoliths and yield highly variable εHf(t) values, ranging from −48 to +16. The Tertiary zircons occur only in the dark pyroxenite xenoliths and show a restricted εHf(t) with a dominant range from 0 to +18. These data suggest episodic magmatic underplating in the ancient lower crust of the NCC, lasting continuously throughout Phanerozoic, producing zircons from the underplated magmas or providing the heat source for the recrystallization of zircons from the ancient crust. Our data clearly demonstrate the important events of Phanerozoic magmatic underplating beneath the Neoarchean–Paleoproterozoic crust of the NCC.
Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton (NCC), were studied to reveal
the Mesozoic crust-mantle interaction. These xenoliths are characterized by low SiO2 (37.7 wt.%–46.0 wt.%) and high Al2O3 (10.8 wt.%–17.9 wt.%) contents. Their Mg# (60–75, Mg#=100×Mg/(Mg+Fe), atomic number) are relatively low for their low SiO2 contents. They have low rare-earth element (REE) contents and LREE-rich REE patterns, and show remarkable enrichments in
Sr relative to the adjacent REE. Some of them exhibit convex REE patterns with a maximum at Nd and remarkably positive Eu
anomalies. Taking into account their high garnet mode (generally >30%), these features suggest that they are high-pressure
metamorphic products of low-pressure cumulates (e.g., gabbro) after it had been depressed into the garnet stability field.
They have evolved Nd and Sr isotopic compositions (143Nd/144Nd=0.511 763–0.512 173, 87Sr/86Sr=0.705 34–0.706 99) and fall in the trend defined by the >110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster
in 83–134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct
evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue
model of ancient lower crust after lithospheric thinning alone can reasonably explain the above features of the garnet-rich
granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba
basalts are products of the Mesozoic magmatic underplating and mixing with the pre-existing lower crust (i.e., AFC process).
However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted (a fossil
oceanic slab) or underplated into the base of the lithospheric mantle, or from partial melting of Archean lithospheric mantle
that was variably hybridised by melts derived from foundered lower crustal eclogite, although it cannot be excluded that some
of the melts were derived from depleted mantle peridotite. In other words, parent melts of most granulite xenoliths could
share the same petrogenesis as the >110 Ma Mesozoic basalts from the NCC.
Key Wordsgarnet-rich granulite xenolith-trace element-zircon U-Pb age-Nd and Sr isotopes-crust-mantle interaction-North China craton
U-Pb zircon dating by LA-ICP-MS and SHRIMP for one olivine pyroxenite yields complex age populations including Mesozoic ages
of 97–158 Ma and 228 ± 8.7 Ma, Early Paleozoic ages of 418–427 Ma, Paleoproterozoic age of 1844±13 Ma, Neoarchean age of 2541
± 54 Ma and middle Archean age of 3123 ± 4.4 Ma. The 97–158 Ma and 228 ± 8.7 Ma zircons show typical igneous oscillatory zonation
in CL images, suggesting two episodes of magmatic events. Overlapping of the 97–158 Ma ages with that of granulite xenoliths
indicates that the Mesozoic granulite-facies metamorphism was induced by heating from the basaltic underplating at the base
of the lower crust. Both processes lasted at least from about 158 to 97 Ma. Ages of 418 –427 Ma could be records of the subduction
of Mongolia oceanic crust under the North China craton. Ages of 1.84 Ga, 2.54 Ga and 3.12 Ga correspond to the three important
crust-mantle evolutionary events in the North China craton, and imply preservation of Precambrian lower crust in the present-day
lower crust.
Keywordsolivine pyroxenite xenolith-zircon U-Pb age-basaltic underplating
ResearchGate has not been able to resolve any references for this publication.