Mesozoic crustal thickening of the eastern North ... - GeoScienceWorld

Mesozoic crustal thickening of the eastern North China craton: Evidence from eclogite xenoliths and petrologic implications Wenliang Xu College of Earth Sciences, Jilin University, Changchun 130061, China Shan Gao State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China, and State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China

Qinghai Wang Dongyan Wang Yongsheng Liu

⎤ College of Earth Sciences, Jilin University, Changchun 130061, China ⎥ ⎦ State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

ABSTRACT A suite of xenoliths of eclogite, garnet clinopyroxenite, and felsic gneiss is found in Early Cretaceous high-Mg [Mg# ⬎45, where Mg# ⴝ molar 100 ⴛ Mg/(Mg ⴙ Fetotal)] adakitic intrusions from the Xuzhou-Huaibei (Xu-Huai) region along the southeastern margin of the North China craton. The primary mineral assemblage of garnet ⴙ omphacite/augite ⴙ quartz ⴙ rutile ⴞ pargasite of the eclogite and garnet clinopyroxenite xenoliths defines a minimum pressure of ⬎1.5 GPa, while the estimated peak metamorphic temperatures range from 800 to 1060 ⴗC. An Sm-Nd whole-rock–garnet isochron and zircon U-Pb dates show that timing of the eclogite facies metamorphism took place ca. 220 Ma. This Triassic age agrees with the age of eclogites from the Dabie-Sulu ultrahigh-pressure metamorphic (UHPM) belt. The ages of abundant Late Archean to early Paleoproterozoic (2.3–2.6Ga) inherited zircons correspond to the most prominent crustal growth event in the North China craton. In addition, these xenoliths and their host high-Mg adakitic intrusions have complementary major and trace element compositions, suggesting that the adakites formed by partial melting of Archean metabasalts that were the protoliths of the Xu-Huai eclogite and garnet clinopyroxenite xenoliths. Trace element and Sr-Nd isotopic modeling shows that the high-Mg adakitic intrusions can be modeled as melts from ⬃40% partial melting of the metabasalts in the eclogite facies, followed by interaction with the convecting mantle and variable degrees of crustal assimilation. Together with the similar zircon age populations between the xenoliths and the host rocks, these lines of evidence strongly suggest their genetic link via thickening, foundering, and partial melting of the Archean North China craton mafic lower crust, followed by adakitic melt-mantle interaction. The crustal thickening resulted from Triassic collision between the Yangtze craton and the North China craton, which produced the Dabie-Sulu UHPM belt in the subducting Yangtze plate and eclogitization of the basaltic crustal root of the overriding North China craton plate. Such processes may have played an important role in generating the highMg character of the continental crust. Keywords: eclogite, adakite, tonalite-trondhjemite-granodiorite, continental crust, partial melting, North China craton. INTRODUCTION Sodium-rich granitoids of tonalitetrondhjemite-granodiorite (TTG) suites are the most voluminous rock type in the Archean continental crust, as well as important juvenile components added to the continental crust in the Phanerozoic (e.g., Andes). The average continental crust has a granodioritic bulk composition and an unusually high Mg# (Kelemen, 1995; Rudnick and Gao, 2003; Liu et al., 2005). It is thus critical to understand the source and origin of TTGs and in particular their high-magnesium (Mg) members to better understand the origin and evolution of continents (Foley et al., 2002; Rapp et al., 2003; Condie, 2005; Xiong et al., 2005). Because of geochemical similarities between Archean TTGs and modern adakites, they are assumed to share a common origin by partial melting

of subducted oceanic crust (e.g., Martin et al., 2005) or partial melting of thickened mafic lower continental crust (Condie, 2005). In either case, TTGs and/or adakite appear to form by melting of eclogite or amphibole eclogite; partial melting of amphibolite to form TTGs, as proposed by Foley et al. (2002), is not favored by recent experimental work (Rapp et al., 2003; Xiong et al., 2005). Once eclogite is formed at the base of the thickened continental crust, its density will exceed that of peridotite (Rudnick and Fountain, 1995). This density contrast will drive the mafic lower crust to be recycled into the asthenosphere together with the underlying lithospheric mantle (Kay and Kay, 1993; Rudnick, 1995; Jull and Kelemen, 2001; Gao et al., 2004). This process has been used to explain the unusually evolved crustal composition, the

collapse of mountains, basin formation, and associated magmatism (Kay and Kay, 1993; Rudnick, 1995; Gao et al., 2004). Although a genetic link between TTG and/or adakite and eclogite has been inferred on geochemical and experimental grounds (Rollinson, 1997; Barth et al., 2001; Rapp et al., 2003; Xiong et al., 2005), physical evidence is scarce (Ducea and Saleeby, 1998). In this study we focus on a rare suite of eclogite and garnet clinopyroxenite xenoliths entrained by Early Cretaceous high-Mg adakitic porphyries within the southeastern margin of the North China craton. Age, geochemical, and isotopic data provide compelling evidence for a genetic link between the eclogite and/or garnet clinopyroxenite xenoliths and the high-Mg adakitic magmas. GEOLOGIC BACKGROUND Recent studies of high-Mg adakites, andesites, and dacites in the North China craton have indicated that removal of the Archean lithospheric keel was possibly accompanied by foundering of the eclogitic lower crust, although other models (e.g., thermal and/or chemical erosion and plume) have been proposed (Gao et al., 2004, and references therein). Jurassic–Cretaceous intermediate to felsic volcanic and intrusive rocks with trace element features characteristic of adakites are widespread in eastern China, including the North China craton (Fig. 1; Zhang et al., 2001; J.-F. Xu et al., 2002; Gao et al., 2004). Because high Sr/Y and La/Y ratios require the presence of garnet in the crust at pressures ⬎1.0 GPa (Rapp et al., 2003; Xiong et al., 2005), these adakitic rocks have been used to suggest the presence of a Tibet-like plateau in eastern China during the early Mesozoic that provided the necessary crustal thickness for the stability of garnet (Zhang et al., 2001; J.-F. Xu et al., 2002). A variety of rare lower crustal xenoliths of eclogite, garnet clinopyroxenite, and felsic gneiss occur in several small adakitic porphyries in the Xuzhou-Huaibei (Xu-Huai) region along the southeastern margin of the North China craton (W.L. Xu et al., 2002). From north to south, they include the Liguo, Banj-

䉷 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; September 2006; v. 34; no. 9; p. 721–724; doi: 10.1130/G22551.1; 3 figures; Data Repository item 2006151.

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Figure 1. Regional tectonic map of Qinling-Dabie-Sulu collision zone and adjacent parts of North China craton showing distribution of Early Cretaceous high-Mg adakitic intrusions that contain eclogite and garnet clinopyroxenite xenoliths in Xuzhou-Huaibei (Xu-Huai) region. Inset A shows major tectonic divisions of China and locations of study area, where YZ and SC denote Yangtze craton and South China orogen, respectively. Also shown are subdivisions of North China craton (Zhao et al., 2001), where WB, TNCO, and EB denote western block, trans-North China orogen, and eastern block, respectively. Inset B shows distribution of Mesozoic adakitic volcanic (filled circles) and intrusive (filled squares) rocks in eastern China (Zhang et al., 2001).

ing, and Jiagou intrusions (Fig. 1). These intrusions have SiO2 ⫽ 55–67 wt%, Na2O ⫽ 3.7–5.4 wt%, NaO2/K2O ⫽ 1.7–5.6, Sr ⫽ 452–1180 ppm, Y ⫽ 7–17.6 ppm, Yb ⫽ 0.5– 1.65 ppm, and Sr/Y ⫽ 36–123, and most of them have LaN/YbN ⫽ 10–17 (Table DR11). They are either adakitic or adakite-like in terms of the geochemical characteristics. In addition, these rocks are high in Mg# (49–61) and Cr (59–228 ppm). These characteristics resemble the Late Jurassic Xinglonggou lavas of the North China craton from western Liaoning (Gao et al., 2004). Oscillatory magmatic zircons from the Liguo and Jiagou porphyries were dated as 130–132 Ma by the sensitive high-resolution ion-microprobe (SHRIMP) UPb method (Xu et al., 2004; Fig. DR1; Table DR2). 1GSA Data Repository item 2006151, Appendices 1 (morphology of zircons) and 2 (estimates of pressures and temperatures), Tables DR1–DR8 (xenolith data), and Figures DR1–DR7, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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PETROGRAPHY AND PRESSURETEMPERATURE CONDITIONS OF ECLOGITE AND GARNET CLINOPYROXENITE XENOLITHS Eclogite and garnet clinopyroxenite xenoliths are rounded or irregular in shape and ⬃10 ⫻ 6 ⫻ 4 cm3 to 4 ⫻ 3 ⫻ 2 cm3 in size (Fig. DR2A; see footnote 1). Although both rock types consist of a similar primary mineral assemblage of garnet ⫹ clinopyroxene ⫹ quartz ⫹ rutile ⫾ pargasite with accessory apatite, zircon, and pyrite, clinopyroxene in eclogite contains 2.69%–3.90% Na2O and 19– 29 mol% of jadeite component and is thus omphacite (Tables DR3, DR4). By contrast, clinopyroxene in garnet clinopyroxenite contains ⬍1.96% Na2O and ⬍13 mol% of jadeite component and is therefore augite (Table DR4). Clinopyroxenes from the eclogite and garnet clinopyroxenite xenoliths frequently show garnet and amphibole exsolutions (Fig. DR2D). The presence of retrograde amphibole, capped titanite, and plagioclase ⫹ amphibole symplectites in some of the xenoliths suggests an amphibolite facies retrograde metamorphism. Garnet clinopyroxenite xeno-

liths are far more abundant than eclogite xenoliths; only 3 eclogite xenoliths have been identified compared to more than 30 garnet clinopyroxenite xenoliths in our collection. Various approaches have been used to determine the pressure and temperature of formation of eclogite and garnet clinopyroxenite xenoliths (Appendix 2; see footnote 1). The 19–29 mol% jadeite component in omphacite (Table DR4) implies that the eclogite formed at pressures ⬎1.5GPa, corresponding to crustal depth of ⬎45–50 km. The observed spinel to garnet transition in garnet clinopyroxenite xenoliths (Fig. DR2B) and ubiquitous presence of rutile in both eclogite and garnet clinopyroxenite xenoliths also suggest a minimum pressure of ⬎1.5GPa (Xiong et al., 2005). In addition, the observed quartz-rod exsolutions in omphacite, rutile exsolution in garnet, and aluminum-rich titanite exsolution in clinopyroxene as well as the local presence of corundum (Fig. DR2C) in garnet clinopyroxenite xenoliths imply that their protoliths had been subjected to high- to ultrahighpressure metamorphism (⬃2.0–3.1 GPa). It is concluded that both the eclogite and garnet clinopyroxenite xenoliths formed during eclogite facies metamorphism. Temperatures were estimated based on the garnet-clinopyroxene Fe-Mg geothermometer. Garnet and clinopyroxene compositions vary widely depending upon their occurrences. Those from samples B1–10 and L–4 (Tables DR4 and DR5) show negligible textural evidence of retrogression and yield temperatures ranging from 800 to 1060 ⬚C. In contrast, garnet and clinopyroxene from retrograde samples give temperatures varying from 760 to 650 ⬚C. We interpret that the high temperatures of ⬎800 ⬚C represent peak temperatures of eclogite facies metamorphism, leading to partial melting, while the low temperatures of ⬍750 ⬚C correspond to retrogression to amphibolite facies metamorphism during ascent and associated cooling of the host magma. AGES OF ECLOGITE FACIES METAMORPHISM Because omphacite has an Sm-Nd isotopic composition almost identical to that of the whole rock, the garnet–whole-rock isochron can be used to obtain an age (Chavagnac and Jahn, 1996). Only one of the three eclogites was large enough to separate garnet and zircon. This xenolith yielded a garnet–wholerock Sm-Nd isochron of 219 ⫾ 5 (2␴) Ma (W.L. Xu et al., 2002; Table DR6; Fig. DR3 [see footnote 1]). To further constrain the age, we separated zircons from the same eclogite and from six garnet clinopyroxenite xenoliths. Due to the generally small size of the xenoliths, fewer than 12 zircon grains from each of the samples were separated and dated on the SHRIMP II at the Beijing SHRIMP Center

GEOLOGY, September 2006

Figure 2. Comparison of zircon age distribution between eclogite and garnet clinopyroxenite xenoliths (Table DR7; see footnote 1), their host high-Mg adakitic intrusions (Table DR2; see footnote 1), and North China Archean and Paleoproterozoic basement (see Supplementary Data of Gao et al., 2004). 207Pb/206Pb age is used for zircons older than 1000 Ma, while 206Pb/238U age is used for younger ones.

(Table DR7). Although only four zircons from the eclogite were available, they show an age pattern similar to the more abundant zircons from the garnet clinopyroxenites, which is characterized by three major age peaks at 2350–2550, 210–260, and 125–136 Ma (Fig. 2; Table DR7; Fig. DR4). The Late Archean– early Paleoproterozoic ages overlap with those of the felsic gneiss xenoliths (Xu et al., 2004; Table DR8) and with the age of the North China craton basement. The Early Cretaceous ages (125–136 Ma) of the eclogite and garnet clinopyroxenite xenoliths are from the rims of older zircons and overlap with the age of the high-Mg adakitic intrusions (Fig. 2). They show oscillatory zoning and Th/U ratios of 0.25–0.73 (Appendix 1; Table DR7), characteristic of magmatic zircon. They clearly represent magmatic overgrowth formed during entrainment by the high-Mg adakitic magma. The Triassic ages are characterized by very low U-Th concentrations (1–23 ppm) and therefore the 206Pb/238U ages have large analytical uncertainties, and 207Pb concentrations are below detection limits (Table DR7). The five Triassic zircons from the same garnet clinopyroxenite xenolith (JG1–36, Table DR7) yield a weighted 206Pb/238U age of 222 ⫾ 37Ma (1␴; mean square of weighted deviates ⫽ 0.27), which agrees with the age [206 ⫾ 15 Ma (1␴)] of the Triassic zircon from the eclogite. All of these zircons lack internal zoning and typically have Th/U ratios of 0.01– 0.24, characteristic of metamorphic zircons (Appendix 1; Table DR7). Although less precise, the zircon ages agree with the Sm-Nd isochron within error. We therefore conclude that the timing of the eclogite facies metamorphism is Triassic and coincides with the ultrahigh-pressure metamorphic (UHPM) ages


Figure 3. Comparison of selected major and trace element compositions of Xuzhou-Huaibei (Xu-Huai) eclogite (including garnet clinopyroxenite) xenoliths, their host high-Mg adakitic intrusions (ADI), and Archean metabasalts from North China craton (Gao et al., 1998). See Figures DR5 and DR6 (see footnote 1) for complete comparison of rare and trace element compositions.

of the Dabie-Sulu belt (220–240 Ma; Hacker et al., 1998; Ayers et al., 2002). CRUSTAL THICKENING, PARTIAL MELTING, AND PETROLOGIC IMPLICATIONS Complementary major and trace element compositions between eclogite xenoliths and Archean granitoids have been used to document their genetic link (Rollinson, 1997; Barth et al., 2001). Assuming that the Archean metabasalts from the North China craton (Gao et al., 1998) represent the precursor to the XuHuai eclogite and garnet clinopyroxenite xenoliths, these rocks show excellent complementary major, trace, and rare earth element compositions (Fig. 3; Figs. DR5, DR6 [see footnote 1]). Trace element modeling using the batch melting model indicates that the average high-Mg adakitic intrusion can be interpreted as the product of ⬃40% melting of the average metabasalt (Fig. DR6). Our studies of the Late Jurassic high-Mg Xinglonggou lavas from western Liaoning suggest that they were derived from ancient

eclogitic lower crust (with Sr-Nd isotopic compositions similar to the Xu-Huai eclogite and garnet clinopyroxenite xenoliths), which foundered into the convecting mantle and was subsequently melted. The resulting adakitic melts interacted with peridotite during ascent, increasing their Mg, Cr, and Ni contents (Gao et al., 2004). The Xu-Huai adakitic intrusions show Sr-Nd isotopic compositions that either agree with those of the Xu-Huai eclogite and garnet clinopyroxenite xenoliths within error or with those of the Xinglonggou lavas, followed by an assimilation and fractional crystallization process within the North China craton crust (Fig. DR7). The estimated peak metamorphic conditions of 800–1060 ⬚C and ⬎1.5GPa are also consistent with partial melting of metabasalt to produce an eclogite residue and adakitic and/ or TTG melt (Sen and Dumm, 1994; Rapp and Watson, 1995). Together with similar zircon age populations of the xenoliths and the highMg adakitic intrusions (Fig. 2), these lines of evidence provide strong arguments for their genetic link via partial melting.

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Eclogites and host gneisses from the DabieSulu belt frequently show inherited magmatic zircon cores of 700–800 Ma, characteristic of the Yangtze Precambrian basement, with Archean zircons being absent (e.g., Hacker et al., 1998). In contrast, inherited zircons from the Xu-Huai eclogite, garnet clinopyroxenite, and felsic gneiss xenoliths are dominantly Late Archean and early Paleoproterozoic, and coincide with the most important period of crustal growth of the North China craton (Fig. 2). Lower crustal xenoliths of eclogite, highpressure mafic granulite, and pyroxenite were also reported from a Mesozoic volcaniclastic diatreme in Xinyang, near the southern margin of the North China craton (Zheng et al., 2003, 2004) (Fig. 1). The protolith of felsic granulite was dated as older than 3.6 Ga, one of the oldest known ages of the North China craton (Zheng et al., 2004). From studies of mafic xenoliths, Zheng et al. (2003) concluded that the depth of the Mesozoic lower crust there was 41–56 km. We propose that a lower continental root consisting of eclogite and garnet clinopyroxenite existed along the southern and eastern margins of the North China craton during the Triassic. It formed by thickening of the Archean North China craton crust in response to subduction of the Yangtze craton, which produced the world’s largest UHPM belt (the Dabie-Sulu). This led to transformation of the mafic lower crust into eclogite and garnet clinopyroxenite and its subsequent delamination into the convecting mantle, followed by partial melting and production of widespread adakitic magmas in the North China craton and possibly in eastern China (Fig. 1). Adakitic rocks with low-Mg character may have been formed by melting of the thickened lower crust without interaction with the mantle (Stern and Hanson, 1991), whereas those of high-Mg character may have been formed by melting of foundered lower crust in the convecting mantle followed by interaction with the mantle. The age of the Xu-Huai intrusions indicates that the foundered eclogitic lower crust had been preserved within the North China craton mantle at least until 125–130 Ma. Such a foundering process and subsequent production of TTG and/or adakitic melt and meltperidotite interaction may have played an important role in generating the high-Mg character of the continental crust (Kelemen, 1995; Liu et al., 2005). ACKNOWLEDGMENTS This study was financially supported by the National Nature Science Foundation of China (grants 40521001, 40472033, 40133020) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0441). We thank B. Song and D.Y. Liu for their technical support during the SHRIMP II analysis, and

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Xiao-Rui Wang for analysis of geochemical and isotopic compositions of some of the samples. We also thank J.C. Ayers, M. Menzies, R.P. Rapp, and an anonymous reviewer for constructive criticism.

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