Journal of Asian Earth Sciences 100 (2015) 60–77 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes Origin of Early Paleozoic garnet peridotite and associated garnet pyroxenite in the south Altyn Tagh, NW China: Constraints from geochemistry, SHRIMP U–Pb zircon dating and Hf isotopes ⇑ Yunshuai Li a,b, Jianxin Zhang b, , Shengyao Yu b, Shengrong Li a, Jianghua Gong c a China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, PR China b State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang, Beijing 100037, PR China c Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang, Beijing 100037, PR China article info abstract Article history: Garnet peridotite, garnet pyroxenite, mafic granulite and dominant felsic granulite constitute a high- Received 31 July 2014 pressure/high-temperature (HP/HT) metamorphic terrane in the south Altyn Tagh, North Tibet (China). Received in revised form 10 December 2014 Garnet peridotite and associated garnet pyroxenite occur as slices or lenses within high pressure felsic Accepted 8 January 2015 granulite. Previous studies indicate that garnet peridotite experienced the peak HP/HT metamorphism, Available online 13 January 2015 followed by a medium pressure granulite-facies overprint. However, the origin of the garnet peridotite and associated garnet pyroxenite remains controversial. In this contribution, we present new zircon Keywords: U–Pb and Hf isotopic data and whole-rock major, trace element and Sr–Nd isotopic data for the represen- U–Pb geochronology tative garnet peridotites and associated garnet pyroxenites. SHRIMP U–Pb dating on the inherited (mag- Sr–Nd isotope Garnet peridotite matic) zircons from the garnet peridotites gave an U–Pb age of ca. 780 Ma, which is interpreted as the age Garnet pyroxenite of the ultramafic magma crystallization. The metamorphic zircons yield an age of ca. 500 Ma, which is South Altyn Tagh consistent with the Sm–Nd isochron age of 493 ± 24 Ma for whole rock, garnet and clinoproxene from Origin of the orogenic peridotites a garnet pyroxenite, representing the timing of peak metamorphism. The Baishiwake garnet peridotites and garnet pyroxenites show relatively high contents in Fe2O3, TiO2,P2O5, Nb, Zr, and Y, but significantly low MgO, Cr, A12O3 and Ni contents. They possess enriched LREE, nearly flat HREE, slightly positive Eu anomaly and weak negative anomalies in Nb and Ti. The relatively high 87Sr/86Sr ratios (0.707–0.710) and low 143Nd/144Nd ratios (0.5123–0.5124) imply a possible crust contamination prior to HP metamor- phism. Integrating these data together with previous studies, we suggest that the Bashiwake garnet per- idotites and associated garnet pyroxenites were derived from mantle-derived magmas which intruded into the continental crust during the Neoproterozoic and then experienced a common HP/HT metamor- phism with felsic crustal rocks in the Early Paleozoic (ca. 500 Ma). Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction volumetrically minor components in the collisional orogens, they are extremely valuable witnesses of geodynamic processes, and Garnet peridotites commonly occur as slices or lenses in conti- could provide crucial information on the orogenesis processes nental collision belts, such as the West Gneiss Region of Norway and crust-mantle interaction. Numerous hypotheses have been (Carswell et al., 1983; Carswell and Cuthbert, 2003; Carswell and proposed for the origin of garnet peridotite and associated garnet Van Roermund, 2005; Medaris and Carswell, 1990; Van peridotite in continental collisional orogenic belt, among which Roermund and Drury, 1998; Brueckner et al., 2010) and the the two end member scenarios are derivation from the mantle- Dabie-Sulu terrane in Eastern China (Yang et al., 1993; Zhang wedge above underthrust continental crust (Brueckner, 1998; et al., 1994, 2000, 2003, 2005c, 2007a,b, 2008; Yang and Jahn, Brueckner and Medaris, 2000); or are the products of HP metamor- 2000; Ye et al., 2009). They are usually associated with garnet phism of mafic/ultramafic complexes previously emplaced in the pyroxenites and granulites, such as in the Bohemian Massif continental crust (Carswell et al., 1983; Zhang et al., 2000). (Medaris et al., 1990, 2006). Although garnet peridotites are just The investigated garnet peridotites and garnet pyroxenites belong to the Bashiwake high-Pressure (HP) granulite-garnet peri- dotite terrane, Altyn Tagh Orogen (North Tibet, China). They gener- ⇑ Corresponding author. Tel.: +86 13661020750. E-mail address: [email protected] (J. Zhang). ally occur as slices or lenses within garnet-bearing felsic gneiss http://dx.doi.org/10.1016/j.jseaes.2015.01.004 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved. Y. Li et al. / Journal of Asian Earth Sciences 100 (2015) 60–77 61 (felsic granulite). Previous geothermobarometric data suggest that margin, comprising high amphibolite to granulite-facies metamor- the garnet peridotite experienced the peak P–T conditions of 19– phic rocks with a protolith age of 2500–2800 Ma and a metamor- 27 kbar and 870–1050 °C, followed by a medium pressure granu- phic age of 1.9–2.0 Ga, and is considered to be the basement of lite-facies conditions related to retrogression or reheating, which the Dunhuang Block and Tarim Craton (BGMX, 1993; Che and is consistent with the metamorphic history of the host felsic gran- Sun, 1996; Yang et al., 2001; Zhang et al., 2007, 2013); (2) the ulites (Zhang et al., 2005a, 2014). At the same time, the garnet peri- North Altyn Tagh subduction-accretion complex, which is discon- dotite has been suggested to have experienced an ultra-high tinuously exposed from Hongliugou to Lapeiquan and character- pressure metamorphic condition due to the reactions of magnesite ized by the occurrence of Early Paleozoic ophiolitic mélange, arc and clinopyroxene and some exsolution features (Liu et al., 2002; volcanic rocks and HP/LT metamorphic rocks (including typical Wang et al., 2011). The origin of the garnet peridotites and their blueschist and eclogite) (BGMX, 1993; Sobel and Arnund, 1999; relationship to the host felsic granulite remains controversial. Pre- Zhang et al., 2005b); (3) the central Altyn Tagh massif, which is vious zircon U–Pb geochronology yielded ages of 493 ± 7 Ma for the main body of the Altyn Tagh and consists of Altun Group the high pressure felsic granulite, 497 ± 11 Ma for the high pres- amphibolite-facies felsic gneisses, marbles and amphibolites and sure mafic granulite, and 501 ± 16 Ma for the garnet peridotite, Jinyanshan Group metasedimentary rocks that are dominated by which were interpreted together as the timing of the HP/HT gran- thick, carbonate-rich, continental marginal sequences with varying ulite facies metamorphism (Zhang et al., 2005a, 2014). A similar amounts of volcanic and clastic rocks. The stromatolites indicate a metamorphic age (498 ± 3 Ma) was also obtained by the zircon Middle to Late Proterozoic age for the Jinyanshan Group (BGMX, U–Pb dating of garnet peridotite (Wang et al., 2011). Recently, 1993). Some recent research suggests that the Altun Group formed the rutile U–Pb datings of the associated felsic granulites and mafic from the Mesoproterozoic to Neoproterozoic, part of which experi- granulites suggested a ca. 450 Ma medium pressure granulite enced an Early Paleozoic tectonic thermal event (Wan et al., 2001; facies overprinting or reheating (Zhang et al., 2014). However, Yu et al., 2013); and (4) the south Altyn Tagh (SAT) subduction- the protolith age of the garnet peridotite has not been well con- collision complex, which consists of blocks of eclogites, garnet- strained, although the Neoproterozoic has been suggested based peridotites and HP granulites within amphibolite-facies gneisses on the dating of a few zircon cores from the garnet peridotite exposed in the southwestern part of the Altyn Tagh. Zircon U–Pb (Wang et al., 2011). geochronology and Sm–Nd isotopic data from the eclogites in the In this contribution, we present new zircon SHRIMP U–Pb dat- western segment of the SAT suggest that the peak HP metamor- ing and whole rock geochemical and Sr–Nd isotopic data for the phism occurred at ca. 500 Ma (Zhang et al., 1999, 2001, 2004; Liu Bashiwake garnet peridotites and garnet pyroxenites. The aims et al., 2012). These four tectonic units could be correlated with are to: (1) further constrain the protolith and metamorphic ages, similar tectonic units in the Qilian and North Qaidam Mountains, (2) evaluate their relations to the associated rocks, and (3) discuss assuming 350–400 km of left-lateral displacement by the Altyn their possible origins. Combining these with the previous data of Tagh fault (Fig. 1)(Zhang et al., 2001; Yang et al., 2001). the garnet peridotites and associated rocks, we propose a specula- A high pressure granulite-garnet peridotite unit in the Bashi- tive scenario for the geodynamic evolution of the Bashiwake garnet wake area within the SAT subduction-collision complex, which peridotites and garnet pyroxenites. occurs as a tectonic block consisting of high pressure felsic granu- lite, mafic granulite and garnet peridotite, separated from amphib- 2. Geological setting and field context olite-facies gneisses by ductile shear zones (Fig. 2). Felsic granulites predominate within this HP unit, whereas and garnet The Altyn Tagh, located at the northern margin of the Qinghai- peridotites and mafic granulites are subordinate. The mafic granu- Tibetan plateau, is bounded to the north by the Tarim Basin and to lites can be subdivided into the following two types: the quartz- the south by the sinistral strike-slip Altyn Tagh fault. It is adjacent bearing mafic granulite, which is generally enclosed directly within to the Qilian Orogen to the east and connected to the Kunlun Oro- felsic granulite and quartz-free mafic granulite (garnet pyroxenite), gen to the southwest (Fig. 1). The Altyn Tagh extends northeast for which occurs as layers in garnet peridotite (Figs.