Reassessment of Petrogenesis of Carboniferous-Early Permian Rift-Related Volcanic Rocks in the Chinese Tianshan and Its Neighbor

GEOSCIENCE FRONTIERS 3(4) (2012) 445e471

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RESEARCH PAPER Reassessment of petrogenesis of CarboniferouseEarly Permian rift-related volcanic rocks in the Chinese Tianshan and its neighboring areas

Linqi Xia*, Xueyi Xu, Xiangmin Li, Zhongping Ma, Zuchun Xia

Xi’an Institute of Geology and Mineral Resources, China Geological Survey, Xi’an, Shaanxi 710054, China

Received 11 November 2011; accepted 19 December 2011 Available online 30 December 2011

KEYWORDS Abstract The CarboniferousEarly Permian rift-related volcanic successions, covering large areas in CarboniferouseEarly the Chinese Tianshan and its adjacent areas, make up a newly recognized important Phanerozoic large Permian rift-related igneous province in the world, which can be further divided into two sub-provinces: Tianshan and Tarim. volcanism; The regional unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks, the ages TianshaneTarim (360e351 Ma) of the youngest ophiolite and the peak of subduction metamorphism of high pressureelow (central Asia) large temperature metamorphic belt and the occurrence of Ni-Cu-bearing mafic-ultramafic intrusion with age igneous province; of w352 Ma and A-type granite with age of w358 Ma reveal that the final closure of the Paleo-Asian Asthenosphere; Ocean might take place in the Early Mississippian. Our summation shows that at least four criteria, being Mantle plume; normally used to identify ancient asthenosphere upwelling (or mantle plumes), are met for this large Lithosphere contamination igneous province: (1) surface uplift prior to magmatism; (2) being associated with continental rifting and breakup events; (3) chemical characteristics of asthenosphere (or plume) derived basalts; (4) close links to large-scale mineralization and the uncontaminated basalts, being analogous to those of many “ore-bearing” large igneous provinces, display Sr-Nd isotopic variations between plume and EM1 geochemical signatures. These suggest that a Carboniferous asthenosphere upwelling and an Early

* Corresponding author. Xi’an Institute of Geology and Mineral Resources, China Geological Survey, East Youyi Road 438, Xi’an, Shaanxi 710054, China. Tel.: þ86 29 87821934; fax: þ86 29 87821900. E-mail address: [email protected] (L. Xia). 1674-9871 ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Peer-review under responsibility of China University of Geosciences (Beijing). doi:10.1016/j.gsf.2011.12.011

Production and hosting by Elsevier 446 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

Permian plume played the central role in the generation of the TianshaneTarim (central Asia) large igneous province. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction The CarboniferouseEarly Permian rift system comprises eight parts: the Kalpin rift situated in the northwestern margin of the Tarim The CarboniferousEarly Permian rift-related volcanic rocks craton; the western Tianshan Yili rift situated in the Yili Block; the exposed in the Chinese Tianshan and its adjacent areas (Figs. 1 Central Tianshan rift in the central Tianshan Block; the eastern and 2) make up an important recently discovered large igneous Tianshan Bogda-Harlike rift; the eastern Tianshan Jueluotage rift; the province (LIP) in northwestern China (Xia et al., 2003, 2004, Junggar rift located in the central-northern part of the Junggar basin; the 2005c, 2007, 2008a,b). This LIP, which can be further divided into Baishan rift located in the northeastern margin of the Tarim craton, and two sub-provinces: Tianshan and Tarim (Fig. 1a; Bryan and Ernst, the Tarim rift located in the western part of the Tarim craton (Fig. 2). e 2008), has attracted a number of recent studies (Che et al., 1996; The massive Carboniferous Early Permianvolcano-sedimentary Chen et al., 1997b, 2006; Gu et al., 2000; Chen et al., 2001; Wang successions were deposited (in form of an angular unconformity) on et al., 2002; Xia et al., 2002a, 2003, 2004, 2005b,c, 2007, 2008a,b; various types and ages of basement: the Proterozoic Yili Block (in the Zhao et al., 2003, 2004, 2006; Xing et al., 2004; Yang et al., western Tianshan) that consists mainly of metamorphic rocks of 2005b, 2007; Zhu et al., 2005, 2009; Wang et al., 2006, Proterozoic age; the central Tianshan Block that consists of meta- e 2007a,c, 2010, 2011; Zhou et al., 2006a; Pirajno, 2007; Bryan and morphic rocks of Proterozoic and Early Middle Ordovician age; the Ernst, 2008; Pirajno et al., 2008, 2009; Zhou et al., 2009; Zhang Junggar (in the Junggar area) and the eastern Tianshan mobile belt e et al., 2010a,b). The current studies include erosional unconfor- that consists mainly of Early Paleozoic Devonian arc-basin mity between the volcanic rocks and underlying basement, formation; and the Tarim craton with a Precambrian basement radiometric dating, high pressure (HP)elow temperature (LT) formed in Archean and Proterozoic (Figs. 1 and 2). e metamorphic belt, petrologic and geochemical studies of basalts The available isotopic dating data reveal that the Carboniferous and their related rocks and mineralization. This study summarizes Early Permian volcanism may persist for 90 Ma (from 350 to e certain achievements in this domain and reassesses the petrogen- 260 Ma), but the Tianshan Tarim LIP event exhibit two pulses of esis of CarboniferouseEarly Permian rift-related volcanic rocks high volume activity: a first main pulse that occurred at e e and the links between LIP and regional-scale uplift, continental 335 325 Ma and a second main pulse at 285 275 Ma (Fig. 4). rifting and breakup, and large-scale mineralization events. The Carboniferous volcanic successions that consist mainly of basaltic lavas and subordinate amounts of intermediate and silicic lavas, and pyroclastic rocks are mainly exposed in the Tianshan and 2. Geological background Junggar areas, and a small portion of them is buried beneath the Junggar basin (Fig. 2). The Carboniferous rift-related volcanism The CarboniferousEarly Permian rift-related volcanic rocks are includes a bimodal distribution of dominant basic and subordinate 6 2 mainly exposed in a broom-shaped province of w1.7 10 km silicic rocks in Junggar, central and eastern Tianshan, however the within the Chinese Tianshan orogenic belt and its neighboring areas western Tianshan rift rocks display a spectrum of compositions including Junggar, Turpan-Hami and Tarim basins (Fig. 2). This from basic to silicic (Xia et al., 2004, 2005b,c, 2008a,b). The Early area represents a minimum estimate because the volcanic rocks Permian volcanic successions are mainly developed in the Kalpin, extend west and east from China and also have a widespread the Tarim (subcrop) and the Baishan rifts, and they are also distribution in Kazakhstan, Kyrgyzstan and Mongolia, respectively sporadically exposed in the Tianshan area (Fig. 2). These Early (Fig. 1b). Many mafic-ultramafic layered intrusions and widespread Permian volcanic rocks consist mainly of basaltic rocks and minor granitic magmatism also have developed in these areas during this of dacites and syenites that display a compositional bimodality period. This study focuses only on the CarboniferouseEarly (Yang et al., 2005b, 2007; Zhou et al., 2009; Zhang et al., 2010a,b). Permian rift-related volcanic rocks. The thickness of the entire volcanic sequence varies from over The Tianshan orogenic belt is part of a large-scale, composite 13,000 m in the eastern Tianshan, to several hundred meters in the orogenic belt located in northwestern China (central Asia). It was central Tianshan, the Kalpin and the Tarim areas, to over 10,000 m in the product of accretion, subduction, and collision of various the western Tianshan (Xia et al., 2004, 2005b,c, 2008a,b; Zhou et al., continental blocks during the formation, evolution, and disappear- 2009; Zhu et al., 2009; Zhang et al., 2010b). The average lava ance of the Paleo-Asian Ocean between the northern Siberia craton thickness of the TianshaneTarim (central Asia) LIP is estimated to and the southern Tarim and North China (i.e., Sino-Korean) cratons be about 1000 m, thus the entire volume of the Tianshan basalts is (Figs. 1 and 2). By the Early Mississippian, the Paleozoic ocean had w1.7 106 km3. This represents a minimum estimate because: (1) closed along three main sutures, which are, from north to south, the erosion removed a significant portion of the eruptive sequences; and Erqis suture (① in Fig. 3), the Northern Central Tianshan suture (2) the associated intrusives are not taken into account. (② in Fig. 3) and the Southern Central Tianshan suture (③ in Fig. 3). The consequent suture zone became an area of thickened crust characterized by complex tectonic and magmatic activity and 3. The final closure timing of the Paleo-Asian uplift. A major regional upwelling and partial melting event led to Ocean the development of the CarboniferouseEarly Permian rift system and its associated large igneous province (Xia et al., 2002a,b, 2003, There have been a wide variety of inferences on the termination 2004, 2005b,c, 2007, 2008a,b). age of the Paleo-Asian Ocean, such as the Late Devonian L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 447

Figure 1 Distribution of large Igneous Provinces (65e345 Ma) in Asia (a: modiﬁed after Bryan and Ernst, 2008) and sketch map showing the distribution of the CarboniferouseEarly Permian volcanic rocks in the Tianshan and its neighboring areas (b: modiﬁed after Xia et al., 2008b).

(Xiao et al., 1992; Wang et al., 1994), the earliest Carboniferous et al., 1990; Solomovich and Trifonov, 2002; Konopelko et al., (i.e., Early Mississippian; Xia et al., 2003, 2004, 2005c, 2008a,b; 2007, 2009; Solomovich, 2007; Glorie et al., 2010; Su et al., Wang et al., 2010, 2011; Dong et al., 2011), the EarlyeMiddle 2010, 2011; Xiao et al., 2010; Gao et al., 2011; Li et al., 2011; Carboniferous (Gao and Klemd, 2003; Klemd et al., 2005; Seltmann et al., 2011), the Late CarboniferouseEarly Permian Volkova and Sklyarov, 2007), the Late Carboniferous (Windley (Allen et al., 1992; Chen et al., 1999), and even later (the end of 448 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

Figure 2 Sketch map of geologic tectonic units of the Chinese Tianshan orogenic belt showing the distribution of the CarboniferouseEarly Permian volcanic rocks (modiﬁed after Xia et al., 2008a,b). Subcrop of the Early Permian rift-related volcanic rocks in the Tarim basin is after Yang et al. (2005b) and Chen et al. (2006). WJBS e West Junggar trench-arc-basin system (Early PaleozoiceDevonian); EJBS e East Junggar arc-basin system (early PaleozoiceDevonian); BTB e Bole tectonomagmatic belt (Late Paleozoic); NTOB e North Tianshan ophiolite belt (Carboniferous); BFB e Boluokenu foldbelt (Early Paleozoic); MGFB e Mishigou-Gangou foldbelt (Early Paleozoic); STBS e South Tianshan trench-arc-basin system (Early PaleozoiceDevonian); DBS e Devonian arc-basin system; KCS e Kalpin continental shelf (NeoproterozoicePaleozoic); BBS e Baishan trench-arc-basin system (Early Paleozoic).

Figure 3 Isotopic ages of the major ophiolites, ophiolitic melanges and high pressure metamorphic rocks and contact relationship between ophiolites and their overlying strata in northern Xinjiang, China. From north to south, the main suture zones are: ① Erqis; ② Northern Central Tianshan; ③ Southern Central Tianshan. APACM e Altay active continental margin (Early PaleozoiceDevonian); WJBS e West Junggar trench-arc-basin system (Early PaleozoiceDevonian); EJBS e East Junggar arc-basin system (Early PaleozoiceDevonian); BTB e Bole tectonomagmatic belt (Late Paleozoic); NTOB e North Tianshan ophiolite belt (Carboniferous); BFB e Boluokenu foldbelt (Early Paleozoic); MGFB e Mishigou-Gangou foldbelt (Early Paleozoic); STBS e South Tianshan trench-arc-basin system (Early PaleozoiceDevonian); DBS e Devonian arc-basin system; KCS e Kalpin continental shelf (NeoproterozoicePaleozoic); BBS e Baishan trench-arc-basin system (Early Paleozoic). L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 449

Permian to Mid-Triassic; Brookﬁeld, 2000; Xiao et al., 2008, 2009; Zhang et al., 2007). Recent studies reveal that the widespread EarlyeCarboniferous (i.e., Mississippian) slightly deformed and unmetamorphosed volcanic succession unconformably overlies the strongly deformed and metamorphosed pre-Carboniferous strata in the Chinese Tianshan and its adjacent areas (Fig. 5). We adopt the opinion that this unconformity, which is about the earliest Carboniferous, represents the termination time of the Paleo-Asian Ocean. This inference is consistent with the ages of the youngest ophiolite and the peak of subduction-related metamorphism of the HPeLT metamorphic belt in the Tianshan Orogen. It can be seen from Fig. 3 that the youngest Qinghe ophiolite exposed in the northern Xinjiang of China was formed in Early Mississippian, with a SHRIMP U-Pb zircon age of 352.1 4.4 Ma (Wu et al., 2006) except the Bayangol ophiolite from the North Figure 4 Isotopic age data histogram of the CarboniferousEarly Tianshan ophiolitic belt. Our studies (Xia et al., 2005a; Xu et al., Permian volcanic rocks in the Chinese Tianshan and its neighboring 2006b,c) reveal that the Middle Mississippian Bayangol ophiolite areas (data comes from Yang et al., 1996, 2006, 2007; Chen et al., suite, with ages of 344 Ma (U-Pb LA-ICP-MS zircon age for gab- 1997a, b, 1998; Li et al., 1998; Wang et al., 2002; Liu et al., 2003; bro) to 324.6 Ma (U-Pb SHRIMP zircon age for plagiogranite), was Zhao et al., 2003; Li et al., 2004; Hou et al., 2005; Zhu et al., 2005, emplaced in a Lower Mississippian rift-related volcanic succession 2009; Zhou et al., 2006a; Zhou et al., 2006c; Zhang et al., 2010b). (Basement Unit). We suggested that rifting and fragmentation of the Basement Unit resulted in generation of the North Tianshan oceanic

Figure 5 Columns and outcropping locations of contact relationship between Lower Carboniferous and its underlying strata in the Chinese Tianshan and its neighboring areas (modified after Xia et al., 2008b). 450 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 basin which is a neonatal “Red Sea type” ocean basin after the mafic-ultramafic complex associated with the A-type granite of elimination of the Paleo-Asian Ocean (Xia et al., 2005a). It is 357.7 3.5 Ma in the eastern Tianshan Jueluotage rift, indicating observed that the North Tianshan ophiolite suite is unconformably also that the Paleo-Asian Ocean had already closed in the Early overlain by Pennsylvanian strata and this implies that the living time Mississippian. of the North Tianshan ocean basin was very short. In addition, Wang et al. (2011) recently suggested that the As well known, the timing of the regional HPeLT meta- south Tianshan back-arc basin began to close in the MiddleeLate morphism constitutes an excellent collisional marker for an Devonian and were consumed in the earliest Carboniferous orogeny. At present, it is considered that the termination of the (368e356 Ma) (i.e., Early Mississippian) based on a LA-ICP-MS Paleo-Asian Ocean occurred along the Southern Central Tianshan zircon U-Pb age of 392 5 Ma for gabbro and the Famennian- suture zone (Klemd et al., 2005; Xiao et al., 2009; Wang et al., Visean radiolarian microfossils found in the siliceous matrix of 2010, 2011; Gao et al., 2011), which extends west-east for the Heiyingshan ophiolitic melange from the south Tianshan. about 2500 km from Uzbekistan, Tajikistan, Kyrgyzstan, and In accordance with the evidences presented above, we can infer Kazakhstan to Xinjiang in northwestern China. A HPeLT belt that the final closure of the Paleo-Asian Ocean took place in the extends for at least 1500 km along the Southern Central Tianshan Early Mississippian. suture zone (Gao et al., 1995, 2000; Tagiri et al., 1995; Volkova and Budanov, 1999). So far, there have been published a large number of age data from HPeLT rocks of the above HPeLT belt 4. Regional uplift associated with the (Fig. 3) that range from 422 to 226 Ma (Table 1). As of yet, no TianshaneTarim (central Asia) LIP consensus has been reached on the timing of the regional HPeLT metamorphism, which constitutes a collisional marker for the The mantle-plume theory predicts a considerable lithospheric uplift Tianshan orogeny. An alternative interpretation is suggested as and doming in response to anomalous thermal upwelling (Cox, following: (1) the older ages (422e364 Ma) may indicate 1989; Campbell and Griffiths, 1990; White and McKenzie, 1995; a metamorphism related to an earlier stage of subduction and Pirajno, 2000). This uplift would leave recognizable effects on the accretion (Wang et al., 2010) or were due to the possible presence sedimentation, such as localized shoaling, thinning of strata over the of excess 40Ar within the high pressure metamorphosed minerals uplifted area, and erosional unconformity between the basalts and (Klemd et al., 2005); (2) the ages ranging from 360 to 351 Ma underlying sedimentary sequence (Rainbird and Ernst, 2001). might be the ages of peak HPeLT metamorphism which suggest The sedimentary effects of surface uplift prior to the Tianshan that the Paleozoic ocean most likely closed by Early Mississip- rift-related volcanism have been observed by Xia et al. (2007, pian; (3) the younger ages (346e302 Ma) were regarded as 2008b) in many places from the Tianshan LIP. reflecting the timing of retrograde metamorphism during exhumation of the HPeLT metamorphic rocks (Gao and Klemd, 2003; (1) The basal volcanic units (354e345 Ma; Dahalajunshan Stupakov et al., 2004; Klemd et al., 2005; Volkova and Sklyarov, Formation) of the Carboniferous rift-related volcanic 2007; Simonov et al., 2008; Wang et al., 2010, 2011); (4) the successions unconformably lie upon the Proterozoic youngest ages (291e226 Ma) were interpreted to be due to the dolomite-marble and granite-gneiss in the Tekes and Zhaosu fluid-mediated recrystallization of the zircon grains (de Jong et al., areas of the western Tianshan (Yili Rift; Fig. 2;12inFig. 5). 2009; Su et al., 2010). The regional uplift and erosion prior to volcanism are further Further, in a recent study, Li et al. (2010a) obtained a LA-ICP-MS evidenced in these areas by thick-bedded (80e100 m in zircon U-Pb age of 351.5 1.9 Ma for gabbro of Sidingheishan thickness) boulderstone, conglomerate and gritstone

Table 1 Compiled age data on HPeLT rocks of the Southern Central Tianshan HPeLT belt. Rock/Mineral Ages (Ma) Methods Data sources Blueschists 410 and 350 K-Ar Dobretsov et al., 1987 Phengite 402 K-Ar Yang et al., 2005a Phengite and amphibole 401e364 40Ar/39Ar Gao et al., 2000 Glaucophane 360.7 1.6 40Ar/39Ar Liu and Qian, 2003 Glaucophane 350.89 1.96 40Ar/39Ar Xiao et al., 1992 Phengite 346e311 40Ar/39Ar Gao and Klemd, 2003; Gao et al., 1995, 2006; Klemd et al., 2005 Crossite 344 1 40Ar/39Ar Gao and Klemd, 2003 Garnet and glaucophane 346 3 Sm-Nd isochron Gao and Klemd, 2003 Omphacite-garnet-glaucophane-whole rock 343 44 Sm-Nd isochron Gao and Klemd, 2003 White mica 331e316 40Ar/39Ar Wang et al., 2010 Phengite and glaucophane 327e324 40Ar/39Ar Stupakov et al., 2004; Simonov et al., 2008 Metamorphic zircon from eclogite 319.5e318.7 SIMS U-Pb Su et al., 2010 Rutile from eclogite 318 7 SIMS U-Pb Li et al., 2011 Omphacite-garnet-glaucophane-whole rock 319 4 Sm-Nd isochron Hegner et al., 2010 Mica-whole rock 313e302 Rb-Sr isochron Klemd et al., 2005 Omphacite-garnet-phengite-whole rock 267 5 Rb-Sr isochron Tagiri et al., 1995 Zircon from eclogite, rodingite and metapelite 422e226 SHRIMP U-Pb Zhang et al., 2007; Li et al., 2010b L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 451

preserved between the Proterozoic basement and the over- 5. Classification of basaltic rock types lying volcanic succession; these conglomerates contain sub- rounded to sub-angular clasts of the underlying basement The TianshaneTarim LIP CarboniferouseEarly Permian rift- rocks. related volcanic successions comprise thick piles of basaltic (2) In the Luotuogou and Maanqiao areas of the central Tianshan lavas and subordinate intermediate and silicic lavas and pyro- Rift (Fig. 2; 17 and 22 in Fig. 5), the Lower Mississippian clastics. We (Xia et al., 2005b, 2007) have demonstrated that Maanqiao Formation consists of (from bottom to top) thick- intermediate and silicic rocks resulted from continuous fractional bedded boulderstone, conglomerate (of which the gravels crystallization starting from basaltic parents, possibly with a role consist of basement rocks), gritstone, sandstone and siltstone for crustal contamination. This study focuses only on the basaltic interbedded with shale and intercalated with limestone, lavas that dominate (>80 Vol. %) the CarboniferouseEarly basalts, and a few rhyolites. It has an angular unconformity or Permian rift system. faulted contact with the Proterozoic granite-gneiss and the The TianshaneTarim CarboniferouseEarly Permian basaltic e metamorphosed Middle Lower Ordovician volcanic rocks in rocks can be divided into two major magma-type basalts: high- the central Tianshan Block. The basement rocks are capped Ti/Y (HT, Ti/Y > 500) and low-Ti/Y (LT, Ti/Y < 500) basalts on by a subaerial unconformity, which are manifested by paleo the basis of Ti/Y ratios (Fig. 6). Ti/Y, rather TiO2, is used as weathering zone in many cases, and usually by relict gravels a discriminator of magma types, because TiO2 contents generally and basal conglomerates. Provenance analysis suggests that increase during fractional crystallization, but Ti/Y does not vary the gravels were mainly from the basement rocks. The much (Peate et al., 1992). According to Nb/La ratios (index of aforementioned Mississippian volcanic-sedimentary forma- crustal contamination, Kieffer et al., 2004), the HT and LT lavas tion is characterized by a retrogradational sequence of tran- can be further divided into HT1 (Nb/La > 0.85) and HT2 sition from continental to marine facies and reflects (Nb/La < 0.85), and LT1 (Nb/La > 0.85) and LT2 (Nb/La < 0.85) a progressive rift extension. lavas, respectively (Fig. 6). Fig. 7a and b shows that most of the (3) In the eastern Tianshan Bogda-Harlike Rift basin (Fig. 2), the HT lavas, consisting of alkali basalt and minor basalt, belong to e basement volcanic succession contact is well exposed in the the alkaline series except one HT1 and five HT2 samples that hills around Qijiaojin and Balikun (25 in Fig. 5). Here, belong to the tholeiitic series and nearly all of LT lavas, consisting basement lithologies, consisting of deformed and meta- of basalt, basaltic andesite, and minor alkali basalt and basaltic morphosed Devonian volcanic successions, are covered with trachyandesite, belong to the tholeiitic series except five LT2 variable thicknesses of paleo weathering horizons, basal samples that belong to the alkaline series. conglomerates and gritstones. At Qijiaojin and Balikun areas, this terrigenous sedimentary succession is overlain by the volcanic units of Tournaisian Qijiaojin Formation (Fig. 2;25 6. Discrimination of tectonic setting for Tianshan in Fig. 5), indicating that the region had already undergone Carboniferous rift-related basaltic rocks erosion and incision prior to the Carboniferous rift-related volcanism in this eastern region of the Tianshan province. There have been many different opinions concerning the geolog- (4) In the Xiaocaohu, Saimashan and Hazanbulak areas (23 and ical setting of the Tianshan Carboniferous volcanic rocks. Some 24 in Fig. 5) of the eastern Tianshan Jueluotage Rift (Fig. 2), have considered them to present island arcs or continental arcs the Silurian and Devonian rocks are mostly overprinted by an intense deformation and greenschist-facies metamorphism, and unconformably overlain by the Mississippian thin-layered limestone which does not record this deformation and metamorphism. It can be observed that the paleo weathering horizons, basal conglomerates and gritstones with variable thicknesses distribute between the Mississippian thin-layered limestone and the SilurianeDevonian schists. (5) Elsewhere, in the Junggar area and the northern Tarim craton, the Lower Mississippian volcanic series have widely angular unconformity contact with the pre-Carboniferous strata (Fig. 5).

It should be noted that alternative interpretations exist for the “surface uplift prior to the Tianshan rift-related magmatism” (e.g., lithospheric uplift resulted from asthenospheric mantle upwelling caused by detachment and sinking of the thickened subcontinental mantle root after collision, Xia et al., 2003, 2004). In addition, if the uplift of crust/lithosphere is due to ascent of plume, we must observe a systemic variation of elevation from center of plume to Figure 6 Classiﬁcation of the CarboniferouseEarly Permian rift- its peripheries, resulting in systemic change of erosion degree and related basaltic lavas from the Chinese Tianshan and its neighboring most importantly, eroded materials should have been re-deposited areas in terms of Ti/Y versus Nb/La. Data sources include: Che et al. in vicinity basins. Unfortunately, we do not yet obtain, at present, (1996); Chen et al. (1997b); Chen et al. (2001); Xia et al. (2004, these observations. Thus, the veritable mechanism inducing the 2005c, 2007, 2008a,b); Xing et al. (2004); Zhao et al. (2004); Yang uplift of crust/lithosphere prior to the TianshaneTarim LIP mag- et al. (2005b); Zhu et al., (2005, 2009); Zhou et al. (2009); Zhang matism needs further studies. et al. (2010b). 452 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

stage accompanied by extensive continental rift-type volcanism. Debates essentially concern two fundamental questions: (1) the final closure timing of the Paleo-Asian Ocean and (2) nature and geological setting of the Carboniferous volcanic rocks. The Permian magmatic rocks have been considered to be the products of an intraplate magmatism by most researchers (e.g., Chen et al., 1997a,b, 1998, 2006; Jiang et al., 2000, 2006; Pirajno, 2000, 2007; Xia et al., 2002a,b, 2003, 2004, 2005c, 2007, 2008b; Xing et al., 2004; Zhao et al., 2004; Zhou et al., 2004, 2006a, 2009; Yang et al., 2005b, 2006, 2007; Li et al., 2006b,c; Pirajno et al., 2008, 2009; Zhang et al., 2008b, 2010a,b; Tang et al., 2010; Xiao et al., 2010). In the previous section, we pointed out that the Mississippian volcanic-sedimentary formation in the Tianshan is characterized by a retrogradational sequence that forms a transition from continental to marine facies and that reflects a progressive extension of the rift. In addition, regional geological investigation reveals that the Lower Mississippian volcanic series, which are slightly deformed and non-metamorphosed, have widely angular unconformity contact with the pre-Carboniferous strata, which are strongly deformed and metamorphosed, in Tianshan and its neighboring areas (Fig. 5). These all imply that in the Carbonif- erous, the Tianshan orogenic belt entered a postorogenic rift (extensional) stage. It must be pointed out that the crucial evidences used for arc/active margin models are the variable arc-like geochemical signatures in some basaltic rocks and granitoids. However, it has been attested that geochemical characters of granitoids should not be simplistically used for discriminating tectonic regimes because their geochemistry is reflective of their sources as well as the melting and crystallization histories, not their tectonic environ- ments in which they intrude (Forst and Forst, 2008; Forst et al., 2001). Although basaltic magmas are generally believed to be more reliable for constraining their tectonic settings, the practice Figure 7 (a) SiO2 versus Nb/Y diagram (after Winchester and Floyd, of simplistically using some arc-like geochemical signatures to 1977) and (b) FeOT/MgO versus SiO2 diagram (after Miyashiro, 1975) discriminate between continental intraplate basalts and arc basalts for CarboniferouseEarly Permian basaltic lavas in the Chinese should also be cautioned. This is because that “contamination by Tianshan and its neighboring areas. (b) is for the sub-alkaline volcanic continental crust or lithosphere can impart subduction-type rocks as plotted in (a). Data sources are as in Fig. 6. signatures (e.g., low Nb, low Ta and low Ti) and lead to the misidentification of contaminated continental intraplate basalts as arc related” (Ernst et al., 2005; Xia et al., 2007, 2008a,b). (Ma et al., 1997; Zhu et al., 2005, 2009; Li et al., 2006b,c; Wang In geochemical studies, some geochemical diagrams can be et al., 2006, 2007a,c; Zhao et al., 2006; Xiao et al., 2008, 2009; used to discriminate the tectonic settings for basaltic rocks. When Tang et al., 2010). The results from a geochemical and geochro- we utilize Zr/Y-Zr diagram, it can be seen that most of the nological study on granitoids in Uzbek and Kyrgyz Tianshan also TianshaneTarim CarboniferouseEarly Permian basaltic lavas plot argued that Carboniferous magmatism is continental arc-related in the within-plate basalts (WPB) field (Fig. 8a). In contrast, when and Permian one is post-collisional (Solomovich and Trifonov, several geochemical diagrams (Fig. 8bed) using Nb, Ta or Ti as 2002; Konopelko et al., 2007, 2009; Solomovich, 2007; discriminating factors are utilized, it can be observed that Seltmann et al., 2011). In addition, Tang et al. (2010) also sug- uncontaminated and slightly contaminated samples (i.e., HT1 and gested that the DabateeLamasu area (northwestern Chinese LT1 lavas with Nb/La >0.85) are plotted still in the WPB field, Tianshan) was a continental arc during the Late Devoniane but the plots of all other lithospherically contaminated samples Carboniferous but had entered a post-collisional stage by the Early (i.e., HT2 and LT2 lavas with Nb/La <0.85) displace toward lower Permian (w290 Ma). Others have proposed that the western Nb, Ta or Ti and into the island arc (or active continental margin) Tianshan Carboniferous volcanic basin, located on the Yili Block, field. In this case, we cannot regard the lithospherically contam- was developed in a continental rift (Xiao et al., 1992; Che et al., inated basaltic lavas as arc related. Besides, although the HT2 and 1996); the Bogda area of eastern Tianshan was also a Carbonif- LT2 lavas, which generally belong to the tholeiitic series (Fig. 7), erous continental rift (Gu et al., 2000); there have been occurred are characterized by low Nb/La ratios ( <0.85) and depletions in subcropped Carboniferous bimodal rift-related volcanic rocks in Nb, Ta and Ti (Figs. 6 and 11b), their concentrations of incom- the Junggar area (Wang et al., 2002). Xia et al. (2002a,b, 2003, patible trace elements are conspicuously higher than those of 2004) have suggested that in the Early Mississippian, the Paleo- subduction-zone basalts (Fig. 11b). The HT2 and LT2 lavas cannot zoic ocean in what is now the Tianshan area had closed and the be regarded, therefore, as volcanic arc basalts, and they are Tianshan orogenic belt entered a postorogenic rift (extensional) lithospherically contaminated continental basalts. Thus, the L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 453

Figure 8 Tectonic setting of the CarboniferouseEarly Permian basaltic lavas from the Chinese Tianshan and its neighboring areas. a: Zr/Y versus Zr diagram (after Pearce, 1982); b: Th/Yb versus Ta/Yb diagram (after Pearce, 1982); c: Hf/3-Th-Ta diagram (after Wood, 1980); d: Hf/3- Th-Nb/16 diagram (after Wood, 1980). Data sources are as in Fig. 6.

TianshaneTarim CarboniferouseEarly Permian basaltic lavas did the contamination of an older continental crust or lithosphere. In indeed erupt in an intracontinental rift setting based on contrast, in the central and the eastern Tianshan and the Junggar, geochemical discrimination. This is consistent with the geological the high 3 Nd(t) values of LT2 lavas may be related to the evidence for an intracontinental rift setting. contamination of upper crust containing early Paleozoic and In addition, the variations of 3 Nd(t) values of the Tianshane Devonian arc-basin volcanic rocks and/or to a pre-Carboniferous Tarim CarboniferouseEarly Permian basaltic lavas are closely subduction enrichment of the lithospheric mantle source region. related to both of the degrees of contamination by continental The above characteristics of variations of 3 Nd(t) values also crust or lithosphere and the nature of the basement. It can be seen demonstrate that the TianshaneTarim CarboniferouseEarly from Fig. 9 that: (1) uncontaminated and slightly contaminated Permian basaltic lavas were formed in an intracontinental rift samples (i.e., HT1 and LT1 lavas) constantly exhibit moderate setting rather than in an island arc (or active continental margin) positive 3 Nd(t) values (þ2toþ8); (2) strongly contaminated setting. samples (i.e., HT2 and LT2 lavas) from the Yili, Kalpin, Tarim and In addition, recent data show that there are not only the Early Baishan rifts with Precambrian basement are characterized by Permian but also the EarlyeCarboniferous Ni-Cu-bearing maﬁc- lower 3 Nd(t) values (10 to þ6); (3) whereas the strongly ultramaﬁc intrusions occurring in the Tianshan and its adjacent contaminated LT2 lavas from the central Tianshan, the eastern areas. These comprise the Sidingheishan intrusion with LA-ICP- Tianshan and the Junggar with younger (Early Paleozoic and MS zircon U-Pb age of 351.5 1.9 Ma in the Jueluotage rift (Li Devonian) arc-basin-crust basement are characterized by high et al., 2010a), the Kalatongke intrusion with SHRIMP zircon 3 Nd(t) values (þ4toþ10). The difference between the above later U-Pb age of 287 5 Ma in the Altay (Han et al., 2004), the two groups of lava may be related to the nature of the lithosphere Huangshandong complex with SHRIMP zircon U-Pb age of through which the asthenosphere- or plume-derived melts have 274 3 Ma in the Jueluotage rift (Han et al., 2004), the Xiangshan erupted. The lower to negative 3 Nd(t) values of HT2 and LT2 lavas intrusion with SHRIMP zircon U-Pb age of 285 1.2 Ma in the from the Yili, Kalpin, Tarim and Baishan rifts may be related to Jueluotage rift (Qin et al., 2003), the Huangshan intrusion with 454 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

Nb/La ratio is a reliable trace element index of crustal contamination (Kieffer et al., 2004). The HT1 and LT1lavas have high-Nb/La ratios (>0.85) indicating their uncontaminated and slightly contaminated nature (Fig. 6). The majority of HT1 and LT1 lavas exhibit element ratios that overlap with the ﬁeld of oceanic-island basalts (OIB) (Fig. 10). They show “humped” trace element patterns (i.e., absence of negative Nb, Ta, Zr, Hf and Ti anomalies) that are very similar to OIB (Fig. 11a). These suggest that the HT1 and LT1 lavas may have some common source and/or process with the OIB. One distinctive feature of TianshaneTarim (central Asia) LIP is that correlations between Sr-Nd isotopic ratios appear to converge toward a common end-member, FOZO representing a “common” depleted lower mantle reservoir for mantle plumes (Hart et al., 1992), despite the considerable scatter of the data (Fig. 12). Another is that certain samples with high-Nb/La ratios 87 86 also show high 3 Nd(t)(2e6) and low Sr/ Sr(t) values (0.704e0.705) (Fig. 12), thus likely reﬂecting the isotopic signature of the least-contaminated asthenosphere (or plume) Figure 9 Nb/La versus 3 Nd(t) diagram for CarboniferouseEarly Permian basaltic lavas in the Chinese Tianshan and its neighboring component. Some Carboniferous LT1 pillow lavas from the 87 86 areas. Data sources are as in Fig. 6. central Tianshan rift show a trend of increasing Sr/ Sr(t) ratios with relatively constant 3 Nd(t) values (Fig. 12). This may suggest interaction with seawater or contamination with carbonate crust during subaqueous eruption (Yogodzinski et al., 1996). SHRIMP zircon U-Pb age of 269 2 Ma in the Jueluotage rift In addition, if we observe only the variations of Sr-Nd isotopic (Zhou et al., 2004), the Baishiquan complex with SHRIMP zircon ratios for the uncontaminated LT1 and HT1 lavas, except for the U-Pb age of 285e284 Ma in the central Tianshan rift (Wu et al., above LT1 pillow lavas, from the TianshaneTarim LIP to better 2005), the Poyi complex with SHRIMP zircon U-Pb age of illustrate that the observed isotopic trends are mantle-derived, 278 2 Ma in the Baishan rift (Li et al., 2006a) and the Poshi rather than resulted from lithosphere contamination, it can be seen complex with SHRIMP zircon U-Pb age of 274 4 Ma in the that the uncontaminated LT1 and HT1 lavas show a distinct trend Baishan rift (Jiang et al., 2006). These evidences also suggest that (Dashed arrow in Fig. 12) with one end-member pointing to the the Tianshan and its neighboring areas were in an intracontinental FOZO and another end-member pointing to the EM1 component, rifting setting during EarlyeCarboniferous to Early Permian. representing either ancient pelagic sediments or old cratonic lithospheric mantle (Hart et al., 1992; Hofmann, 1997; Zhang 7. Temporal and spatial distribution of the et al., 2008a). This implies that the TianshaneTarim LIP has TianshaneTarim (central Asia) geochemical signatures for its uncontaminated primitive magmas e that indicate a dominantly primitive mantle-plume (or astheno- Carboniferous Early Permian basaltic lavas sphere) origin and their parental magmas have undergone, during ascent, interaction with deep mantle lithospheric domains carrying 7.1. Asthenosphere (or plume) signature in the TianshaneTarim (central Asia) LT1 and HT1 lavas

Two things, such as occurrence of picritic basalts or picrites and short duration large volume events in an intraplate setting, are usually regarded as the criteria for identification of ancient plumes (Campbell, 2001). Thus, the absence of picritic rocks and the large age range of the TianshaneTarim CarboniferouseEarly Permian volcanic rocks seem to be not favorable for plume model. However, as indicated by Ernst et al. (2005), although most LIPs are emplaced within <10 Ma, the magmatic activity of some LIPs may still persist for tens of millions of years and even to 100 Ma or more. Short duration of magmatic events, therefore, is not an absolute criterion for identification of ancient LIPs or plumes. Furthermore, we also cannot exclude the possibility to find the picritic rocks in the Tianshan (central Asia) LIP in future. Although it is difficult to recognize plume involvement based on geochemistry, some generalizations are still possible. Uncontami- nated asthenosphere- (or plume-) generated basaltic rocks will Figure 10 La/Ba versus La/Nb plots for CarboniferouseEarly normally have flat REE patterns or LREE-enriched patterns and Permian basaltic lavas in the Chinese Tianshan and its neighboring lack negative Nb, Ta and Ti anomalies (Campbell, 2001; Ernst et al., areas. The dispersion to higher La/Nb and lower La/Ba ratios may 2005). Saunders et al. (1992) also suggested that asthenosphere (or represent the effects of lithosphere contamination. Field for oceanic- plume) components are characterized by high-Nb/La ratios of island basalts (OIB) is after Fitton et al. (1991) and Fitton (1995). 87 86 higher than or near one, low Sr/ Sr ratios and high 3 Nd values. Data sources are as in Fig. 6. L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 455

Figure 11 Primitive mantle (after Sun and McDonough, 1989) normalized incompatible trace element spider diagrams for Carboniferouse Early Permian basaltic lavas in the Chinese Tianshan and its neighboring areas. Patterns for oceanic-island basalts (OIB) are from Sun and McDonough (1989). The shaded area shows the range for subduction-zone basalts, with the lower and upper limits being deﬁned by “average” low-K and high-K basalts, respectively (Tatsumi and Eggins, 1995). Data sources are as in Fig. 6. the geochemical signatures characteristic of old cratonic litho- reﬂect fundamental differences in their mantle sources. They infer spheric mantle (EM1-type source). Recent studies of Zhang et al. that the presence of an old, stable, deep continental lithospheric (2008a) reveal that there are systematic differences in geochem- root associated with plume-derived magmatism is a favorable ical signatures between the ore-bearing and ore-free LIPs that condition for the mineralization. Although the ore-bearing and 456 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

87 86 Figure 12 Sr/ (t) versus 3 Nd(t) diagrams for Carboniferouse Early Permian basaltic lavas in the Chinese Tianshan and its neighboring areas. Upper crust (Weaver, 1991) and a theoretical lower crustal composition (Ben Othman et al., 1984) are indicated for reference. The mantle sources of MORB and OIB and the mantle end- members DMM, PREMA, FOZO, HIMU, BSE, EMI and EMII are from Zindler and Hart (1986) and Hart et al. (1992). Data sources are as in Fig. 6. ore-free LIPs are equally derived from primitive plume sources, the parental magmas of ore-bearing LIPs have undergone, during ascent, interaction with a source geochemically identical to ancient cratonic lithospheric mantle (EM1-type source) whereas ore-free LIPs do not show strong interaction with a geochemical reservoir equivalent to old lithospheric roots and the uncontami- Figure 13 Distribution of the CarboniferouseEarly Permian nated primitive basalts from ore-free LIPs display isotopic varia- basaltic lavas from the Chinese Tianshan and its neighboring areas on, tions between plume and EM2 geochemical signatures. Therefore, (a) Nb/Th-Zr/Nb, and (b) Zr/Y-Nb/Y diagrams (diagrams after Zhang et al. (2008a) suggest that the ore-bearing LIPs are pre- Condie, 2005). Abbreviations: UC e upper continental crust; CB e disposed to be favorable for large-scale Ni-Cu-PGE mineraliza- arc-related basalts or contaminated (by continental crust or/and tion as a consequence of scavenging Ni and PGE from mantle subcontinental lithosphere) continental basalts; PM e primitive sulfides in the old cratonic lithospheric mantle settings. The results mantle; DM e shallow depleted mantle; HIMU e high-m (U/Pb) of this study support this opinion. The TianshaneTarim LIP is an source; EM1 and EM2 e enriched mantle sources; OIB e oceanic- ore-bearing LIP and associated with magmatic and hydrothermal island basalt; DEP e deep depleted mantle; EN e enriched compo- ore deposits and mineralization of various types, size and grade. nent; REC e recycled component. Data sources are as in Fig. 6. Condie (2003, 2005) suggests that it is possible to characterize some of the isotopic mantle domains with four immobile incompatible element ratios: Nb/Th, Zr/Nb, Zr/Y and Nb/Y. These high field strength element ratios have the advantage that they do not change component (PM). This is consistent with the aforementioned major with time as isotopic ratios do, and they neither are affected by and trace element and isotope geochemical evidence for plume- secondary alteration. As Condie (2005) suggested that the DNb line derived basalts of LT1 lavas. Another feature suggested by Fig. 13 can separate plume from non-plume basaltic sources on a Zr/Y-Nb/Y is the presence of a significant contribution of a recycled mantle plot (Fig. 13b). On this diagram, basalts plotting below the DNb line component (REC) for the uncontaminated and slightly contami- come from either a shallow depleted source (DM) or from subduction nated HT1 lavas from the TianshaneTarim (central Asia) LIP. zones, or they represent plume-derived basalts that have been contaminated by continental crust or/and subcontinental lithosphere. 7.2. Asthenosphere (or plume)elithosphere interaction In this case, the only major problem is that subduction-related basalts overlap the contaminated intracontinental ones on these diagrams The HT2 and LT2 lavas are characterized by low Nb/La (<0.85; (Fig. 13), and hence it is essential to use the geochemical methods to Fig. 6), high large ion lithophile element concentrations and distinguish intracontinental basalts from subduction-related ones as pronounced negative Nb, Ta, Zr, Hf and Ti anomalies (Fig. 11b) that we have done in Section 6. suggesting that components other than asthenosphere (or plume) Fig. 13 clearly shows that most uncontaminated and slightly must have been involved in the generation and evolution of the contaminated LT1 basalts from the TianshaneTarim (central Asia) TianshaneTarim basaltic rocks. The most likely components are LIP plot above the DNb line in the mantle-plume field defined by the from the lithosphere. There are still debates regarding the method deep depleted plume component (DEP) and the primitive mantle by which the lithosphere contributes to magma generation. Either L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 457 contamination of asthenosphere- (or plume-) derived magmas by The basalt data from the TianshaneTarim (central Asia) LIP lithosphere-derived melts (Arndt et al., 1993) or whole-scale form a positive array on the diagrams of La/Yb versus Gd/Yb melting of the subcontinental lithospheric mantle (CLM, (Fig. 14). The Gd/Yb(BSE) ratio in the TianshaneTarim basalts Gallagher and Hawkesworth, 1992; Hawkesworth et al., 1995; varies only slightly (1.2e4), whereas La/Yb(BSE) displays much Hooper et al., 1995; Rogers et al., 1995) or melterock reaction higher variation (1.5e20). Large variations in La/Yb can be resulting from the infiltration of asthenosphere- (or plume-) related to changes in the degree of partial melting, fractional derived magmas into the lithosphere in a process related to the crystallization and to crustal contamination, or a combination of thermomechanical erosion of the lithosphere mantle (Macdonald these. Melts parental to the basalts of the TianshaneTarim LIP et al., 2001) has been proposed. We adopt the third opinion of were contaminated by continental lithosphere, which has a strong Macdonald et al. (2001) in this study. influence on the La/Yb ratio but minor effects on Gd/Yb. There- Incompatible elements such as La or Ba should increase fore, the model can only be regarded as semi-quantitative and use relative Nb if basaltic magma is contaminated by lithospheric for guidelines only. Nevertheless, the partial melting model may material, which usually has high La/Nb, Ba/Nb and low La/Ba still be of use to identify the most likely source and degree of (Weaver and Tarney, 1984; Wedepohl, 1995). Fig. 10 displays the melting responsible for the generation of the TianshaneTarim variations of La/Nb against La/Ba for the Tianshan basaltic lavas, CarboniferouseEarly Permian rift-related basaltic lavas. in comparison with the field of OIB (Fitton, 1995; Fitton et al., Fig. 14 shows that the Carboniferous LT (including LT1 and 1991). Majority of HT2 and LT2 lavas have higher La/Nb and LT2) and HT2 lavas lie within the garnet-spinel transition zone or lower La/Ba ratios indicative of the influence of crustal or/and are superimposed on the modeled partial melting trend defined by subcontinental lithospheric component. garnet peridotite with an initial BSE source composition at 3 GPa HT2 and LT2 lavas from the TianshaneTarim (central Asia) LIP and degrees of melting being in the range of 1%e30%, and the 87 86 are characterized by variable 3 Nd(t)(10 to þ10) and Sr/ Sr(t) plots of the Early Permian HT (including HT1 and HT2) and LT (0.704e0.710) (Fig. 12). The Sr-Nd isotopic compositions of the (including LT1 and LT2) lavas lie within the garnet stability field HT2 and LT2 lavas plot in a restricted field between the two mixing at 3e4 GPa or are superimposed on the modeled partial melting lines of “FOZOeLower Crust” and “FOZOeUpper Crust” trend defined by garnet peridotite with BSE source composition at (Fig. 12). These may be related to the contamination of an older 4 GPa and degrees of melting <17%. continental lithosphere and the HT2 and LT2 lavas are All of these indicate that the Carboniferous lavas were lithospherically-contaminated continental basalts derived from the generated at shallower levels (w3 GPa) by larger degrees of asthenosphere (or plume). partial melting from the asthenosphere (or mantle plume), and the It can be seen from Fig. 13 that most HT2 and LT2 lavas fall in the Early Permian lavas were, in contrast, derived by smaller degrees field defined by the enriched component (EN), and they represent of partial melting at a higher depth (w4 GPa). asthenosphere- (or plume-) derived basalts that have been contaminated by continental crust or/and subcontinental lithosphere. This is also consistent with the aforesaid conclusions obtained on major and trace element and isotope geochemical studies. Some chemical and isotopic composition of the HT2 and LT2 lavas may inherit that of the CLM. However, as above-mentioned, the estimated volume of the TianshaneTarim basalts is about 1.7 106 km3. It is thus difficult to imagine that such large volume magma was generated by melting of lithospheric mantle which is stable for a long time period in a non-convective state. The thermomechanic model suggests that only a small amount of melts can be produced from the lithospheric mantle by conduction of heat from mantle plume (McKenzie and Bickle, 1988; Arndt and Christensen, 1992). Generation of the TianshaneTarim basalts due to melting of the CLM is considered as unlikely although a contribution from the CLM cannot be excluded. In summary, the generation of large amount of Tianshane Tarim basalts is likely confined to the convective asthenosphere or plume. The geochemical variation of the HT2 and LT2 lavas can therefore be accounted for by lithospheric contamination of asthenosphere- (or plume-) derived magmas. Figure 14 La/Yb versus Gd/Yb [normalized to bulk silicate earth (BSE): McDonough and Sun, 1995] plots of the CarboniferouseEarly 7.3. Melting conditions and source characteristics Permian basaltic lavas from the Tianshan and its neighboring areas. The trend lines are non-modal batch melting curves calculated by REE such as La, Gd and Yb are particularly useful, because their Reichow et al. (2005) for an initial BSE (bulk silicate earth) source. relative abundances are strongly dependent on the degree of partial Bulk distribution coefficients are calculated by Reichow et al. (2005) melting and nature of aluminous phase (spinel or garnet) in the using mineral proportions for garnet peridotite (GP) at 3 and 4 GPa mantle source. Here, we adopt the REE modeling of Reichow et al. after Walter (1998) and spinel peridotite (SP) given by McKenzie and (2005) (Fig. 14), to illustrate the depth and degree of melting O’Nions (1991). Distribution coefficients used for the REE are from necessary to account for the variation in REE ratios of the Tianshane Hanson (1980) and Hart and Dunn (1993). Partial melting of the BSE Tarim basalts, and to constrain whether spinel or garnet was present source for garnet peridotite and spinel peridotite is indicated by trend in the source. line (GP) and (SP), respectively. Data sources are as in Fig. 6. 458 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

7.4. Geodynamic implication of temporal and spatial Thus, it may be inferred that a Carboniferous upwelling distribution of two rock types asthenosphere impacts on the base of the lithosphere and thins laterally, and then the maximum transient uplift preceding the The temporal and spatial relationship between the LT and HT lavas Carboniferous Tianshan province eruptive episode should have in the TianshaneTarim (central Asia) LIP is critically important in been in the central Tianshan rift region, with broadening but understanding the thermal structure of the TianshaneTarim mantle diminishing uplift further to the circumjacent regions. However, melting anomalies (or plumes) and the nature of the asthenosphere- the Early Permian Tarim province eruptive episode would have (or plume-) lithosphere interaction. The following generalization been related to an Early Permian plume, the axial area of which can be made in terms of data available up to date. was beneath the Tarim and the Baishan.

7.4.1. Carboniferous 8. Causal relation between Tianshan (central Asia) The Carboniferous rift-related basaltic lavas display a spatial LIP and continental breakup chemical variation such that LT1 lavas predominantly occur in the central Tianshan rift and the LT2 and HT2 lavas in the circum- Several recent articles (e.g., Zhu et al., 2005, 2009; Wang et al., jacent regions of the central Tianshan rift. In the Tianshan LIP, the 2007a,c) proposed that the Carboniferous volcanic rocks from the volume of the LT2 and HT2 lavas with lithospheric signature is western Tianshan are arc related, and these authors even considered much greater than that of the LT1 lavas that are dominated by that the Carboniferous volcanism in the western Tianshan Yili basin asthenosphere (or plume) signature. As such, lithospheric signa- was related to the subduction of the North Tianshan oceanic basin. ture observed in the Carboniferous basaltic lavas may be derived As mentioned above, we have demonstrated that the Earlye from crust and/or CLM (continental lithosphere mantle) contam- Carboniferous basaltic lavas from Yili rift are lithospherically- ination rather than source characteristics. contaminated LT2 lavas which are easily misidentified as arc At present, there are not effective means to judge exactly related. Secondly, our geochronological studies (Xia et al., 2005a; where the ancient Tianshan mantle melting anomaly center was. Xu et al., 2006b,c) reveal that the formation ages of the North We have no choice but to assume that in Carboniferous the axial Tianshan ophiolite are 344 Ma (U-Pb LA-ICP-MS zircon age for area of the mantle melting anomaly was likely in the central gabbro from the ophiolite) to 324.8 Ma (U-Pb SHRIMP zircon age section (i.e., the central Tianshan rift) of the Tianshan LIP. In this for plagiogranite from the ophiolite) and the eruption ages of the assumed axial area of the mantle melting anomaly, the mantle Carboniferous volcanic rocks from the western Tianshan are in temperature was high enough to allow melting to begin at rela- range from 354 to 306 Ma. It cannot be considered, therefore, that tively great depth (garnet stability field) with a higher degree of the Northern Tianshan ophiolites and the Carboniferous volcanic e melting (10% 30%, Fig. 14). The melts derived from such a large rocks from the western Tianshan were emplaced at same time. degree of melting of mantle peridotite in the presence of garnet We (Xia et al., 2005a) described that the Middle Mississippian evolved to form the LT1 lavas from the central Tianshan rift. In North Tianshan ophiolite suite was emplaced in a Lower Missis- the meantime, the melting in the axial area of the mantle melting sippian rift-related volcanic-sedimentary succession of continental anomaly extended upwards to the shallow level (garnet-spinel to shallow-marine facies (Basement Unit) and the rift-related mafic transition zone) of west, east and north sides (i.e., the western lavas from the Basement Unit appear an intracontinental geochem- Tianshan, the eastern Tianshan and the Junggar) of the axial area ical characteristics, whereas, younger mafic lavas from the Ophiolite of the mantle melting anomaly. In the periphery of the Tianshan Unit exhibit typical depleted MORB features. We suggested that the LIP, the geotherm was lower than those underneath the axial area. Middle Mississippian North Tianshan oceanic basin was not a part of < This led a relatively low degree of melting ( 10%, Fig. 14). The the Tianshan Paleozoic ocean basin and after closure of the Tianshan low-degree melts derived from garnet-spinel transition zone of Paleozoic ocean basin the rifting and fragmentation of the Basement mantle evolved during ascent to form the LT2 and HT2 basaltic Unit resulted in generation of the North Tianshan oceanic basin lavas from the eastern and western Tianshan, and the Junggar. which is a neonatal “Red Sea type” ocean basin (Xia et al., 2005a) The spatial variation in rock types, on the other hand, is likely produced by continental breakup. It is observed that the North controlled by the difference of mantle geothermal structure. Tianshan ophiolite suite is unconformably overlain by Pennsylva- Consequently, the LT1 lavas were generated probably in the nian strata and this implies that the living time of the North Tianshan Carboniferous mantle melting anomaly center region where the ocean basin was very short. Thus, it is unreasonable to invoke the geotherm was higher, the magmas, therefore, might have arisen subduction model to interpret the generation of the Carboniferous more rapidly without pronounced contamination by lithospheric volcanic rocks from the western Tianshan. materials. In contrast, the LT2 and HT2 lavas resulted from melting of the mantle at the Carboniferous mantle melting anomaly periphery. Due to the relatively cooler geothermal 9. There is not an assemblage of Nb-enriched arc structure, the magmas might have ascended more slowly with basalt, adakite and high-Mg andesite existed in the marked contamination by lithospheric materials. Tianshan Carboniferous volcanic successions

7.4.2. Early Permian As mentioned above, there have been still two different opinions The uncontaminated and slightly contaminated HT1 and LT1 concerning the geological setting of the Tianshan Carboniferous lavas with Early Permian age predominantly occur in the Tarim volcanic rocks. Some have considered them to present continental and Baishan rifts, and the Early Permian contaminated HT2 and rifts and others have interpreted them as the product of an island LT2 lavas mainly occur in the Kalpin rift of west side and the arc or an active continental margin setting. Especially, Zhao et al. Tianshan area of north side. These imply that the Early Permian (2006) and Wang et al. (2006, 2007c) have proposed that an Tarim plume center was beneath the Tarim and the Baishan rifts. assemblage of Nb-enriched arc basalt, adakite and high-Mg L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 459 andesite occurred in the Tianshan Carboniferous volcanic 9.1. Comparison of fundamental geochemical successions and they have regarded this rock assemblage as an characteristics of some typical Nb-enriched arc basalts important petrological evidence for the Tianshan Carboniferous with those of continental basalts from several LIPs arc regime. We will compare these Tianshan rocks with the typical Nb-enriched arc basalts, adakites, high-Mg andesites and conti- 9.1.1. Nb-enriched island arc basalts nental basalts in this section. It is the aim of this section to Some volcanic arc maﬁc lavas exhibit enrichment in HFSE and determine whether or not an assemblage of Nb-enriched arc thus possess low LILE/HFSE and LREE/HFSE ratios, weakly basalt, adakite and high-Mg andesite exists indeed in the Tianshan negative or even positive Nb anomalies, and comparatively high Carboniferous volcanic successions. TiO2 contents relative to other arc maﬁc magmas. Due to their

Figure 15 w(SiO2) (%) versus Nb/Y (after Winchester and Floyd, 1977) and w(SiO2) (%) versus FeOT/MgO classification diagrams for (a and b) Nb-enriched arc basalts from the Sulu Arc (southern Philippines) and the Kamchatka Arc (Russia) and (cef) lithospherically uncontaminated and less-contaminated continental basalts from the Emeishan (China), the Tianshan (China), the Deccan (India) and the Siberia (Russia). Data sources: Sulu Arc (Sajona et al., 1994, 1996, 2000; Castillo et al., 2007); Kamchatka Arc (Kepezhinskas et al., 1996, 1997); Emeishan LIP (Chung and Jahn, 1995; Xu et al., 2001; Xiao et al., 2003, 2004; Zhang et al., 2006; Zhou et al., 2006a,b,c; Wang et al., 2007a,b,c; Fan et al., 2008); Tianshan Carboniferous basalts (Che et al., 1996; Chen et al., 2001; Xia et al., 2004, 2005c, 2007, 2008a,b; Zhu et al., 2005, 2009); Deccan LIP (Lightfoot and Hawkesworth, 1988; Peng et al., 1994; Chandrasekharam et al., 1999; Mahoney et al., 2000; Melluso et al., 2006); Siberian LIP (Lightfoot et al., 1990, 1993; Wooden et al., 1993). 460 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 unusually high-Nb concentrations (>8 ppm versus <4 ppm for incompatible trace element patterns that exhibit a range of Nb typical arc lavas) these mafic lavas have been labeled high-Nb arc anomalies from positive to only weakly negative and steep slopes, basalts (HNB) or Nb-enriched arc basalts (NEB). They have been with Nb/Yb(PM) Z 3.6e21.8 (Fig. 17a and b). There is a relatively reported to occur in several modern arc systems, such as Panama limited variation in Sr and Nd isotopic compositions among the and Costa Rica (Reagan and Gill, 1989; Defant et al., 1992), the Sulu and Kamchatka Arcs samples (Fig. 18a and b). Values of Cascades (Leeman et al., 1990; Defant and Drummond, 1993), 87Sr/86Sr range from 0.70341 to 0.70373 for the Sulu arc NEB and Baja California (Aguillon-Robles et al., 2001), Kamchatka overlap with the Sulu arc basalts (87Sr/86Sr Z 0.70349e0.70367) (Kepezhinskas et al., 1996, 1997), and Sulu Arcs (Castillo et al., (Sajona et al., 2000; Castillo et al., 2007). Ratios of 87Sr/86Sr of 2002, 2007; Sajona et al., 1993, 1994, 1996, 2000). the Kamchatka arc NEB (87Sr/86Sr Z 0.70294e0.70302) are For example, the Nb-enriched arc mafic lavas from the Sulu slightly lower than the arc basalts (87Sr/86Sr Z 0.70310e0.70345) Arc and the Kamchatka Arc range from basalt to basaltic andesite (Kepezhinskas et al., 1996, 1997). The values of 3 Nd of the Sulu belonging to the tholeiitic to alkaline series (Fig. 15a and b). arc basalts (7.26e7.51) fall within the Sulu arc NEB range These Nb-enriched arc basalts have higher absolute Nb contents ( 3 Nd Z 5.48e8.35) (Sajona et al., 2000; Castillo et al., 2007), also (8e48 ppm) and Nb/La ratios (> 0.9) relative to typical island arc the Kamchatka arc basalts have the similar 3 Nd ratios basalts (Nb < 6 ppm, Nb/La < 0.8) (Fig. 16a and b). They are (6.67e10.18) with the Kamchatka arc NEB ( 3 Nd Z 8.84e9.21) characterized by an intraplate-type primitive mantle-normalized (Kepezhinskas et al., 1996, 1997). As a whole, the Sulu and the

Figure 16 Nb/La versus Nb (ppm) diagrams for (a and b) Nb-enriched arc basalts and island arc basalts from the Sulu Arc (southern Philippines) and the Kamchatka Arc (Russia) and (cef) continental basalts from the Tianshan LIP (China), the Emeishan LIP (China), the Siberia LIP (Russia), and the Deccan LIP (India). Data sources are as in Fig. 15. L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 461

87 86 Kamchatka Arcs samples have low Sr/ Sr and high 3 Nd ratios during their ascending en route to surface. It can be seen from (Fig. 18a and b). Fig. 16 that the uncontaminated and less-contaminated basalts from several LIPs (e.g., Deccan, Emeishan, Siberia and Tianshan) 9.1.2. Continental basalts have high-Nb concentrations (4e49 ppm) and Nb/La ratios Continental basalts generally include ﬂood basalts and rift-related (>0.9). The strongly contaminated continental basalts exhibit low basalts and they can be generated by partial melting of the Nb/La ratios (<0.9) that distinguish them from the uncontami- asthenospheric mantle or mantle plumes. The mantle-derived nated and less-contaminated basalts, but their Nb concentrations magmas are usually contaminated by the lithospheric materials (3e44 ppm) are similar.

Figure 17 Comparison of primitive mantle-normalized incompatible trace element patterns for (a and b) Nb-enriched arc basalts from the Sulu Arc (southern Philippines) and the Kamchatka Arc (Russia) and (cef) lithospherically uncontaminated and less-contaminated continental basalts from the Tianshan (China), the Emeishan (China), the Siberia (Russia), and the Deccan (India) with that of oceanic-island basalts (OIB, after Sun and McDonough, 1989). The normalization values are from Sun and McDonough (1989). Data sources are as in Fig. 15. 462 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

87 86 Figure 18 (a and b) Plots of 3 Nd and Sr/ Sr for Nb-enriched arc basalts and island arc basalts from the Sulu Arc (southern Philippines) and the 87 86 Kamchatka Arc (Russia). (cef) Plots of 3 Nd(t) and Sr/ Sr(t) for continental basalts from the Tianshan (China), the Emeishan (China), the Siberia (Russia), and the Deccan (India). Diagrams after Zindler and Hart (1986).UCe upper crust; LC e lower crust; EM I and EM II e enriched mantle I and II sources; HIMU e high-m mantle source; DM e depleted mantle source. Data sources are as in Fig. 15.

The uncontaminated and less-contaminated continental mafic 3 Nd(t)(4 to 8.4), whereas the strongly contaminated basalts have 87 86 lavas consist of basalt and minor basaltic andesite; most belong to higher Sr/ Sr(t) (0.7035e0.712) and lower to negative 3 Nd(t) the tholeiitic series and a part of them belongs to alkaline series (11 to 9) (Data sources are as in Fig. 15). It is must be mentioned (Fig. 15cef). They show moderate to strong enrichment of that at the condition of LIP with younger arc-basin-crust basement incompatible trace elements and possess weakly negative to the lithospherically-contaminated basalts exhibit low 87Sr/86Sr(t) positive Nb, Ta and Ti anomalies, and P is strongly to slightly and high 3 Nd(t), that are similar to the characteristics of the island deplete compared with elements of similar compatibility arc basalts. This may be related to the contamination of crust (Fig. 17cef). Apart from these anomalies, mantle-normalized containing younger arc-basin volcanic rocks and/or to previous trace element concentrations increase regularly with increasing subduction enrichment of the lithospheric mantle source region incompatibility and show typical intraplate patterns. Their primi- (Xia et al., 2004, 2005c, 2007, 2008a,b). tive mantle-normalized incompatible trace element patterns have shallower slopes, with Nb/Yb(PM) Z 1.8e10.4 (Fig. 17cef), than 9.1.3. Difference between the Nb-enriched arc basalts and the the Nb-enriched arc basalts. Nb-enriched continental basalts There is considerable variation in Sr and Nd isotopic compo- Although the Nb-enriched arc basalts and the Nb-enriched sitions among the continental basalts of LIPs (Fig. 18cef). The continental basalts are all characterized by enrichment of HFSE, uncontaminated and less-contaminated basalts are characterized they have still lots of differences. For example: (1) the absolute Nb by loweremoderate 87Sr/86Sr(t) (0.7033e0.7075) and moderate contents (>8 ppm) of the NEB are obviously higher than those of L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 463 normal arc basalts (Nb < 4 ppm), but the uncontaminated and Another significant question is that the contamination by less-contaminated continental basalts have similar Nb concentra- continental crust or lithosphere can impart subduction-type tions (4e49 ppm) to the strongly contaminated continental basalts signatures (e.g., low Nb, La and Ti) and lead to the misidentifi- (Nb Z 3e44 ppm) (Fig. 16); (2) despite the similar intraplate cation of continental basalts as arc related (Ernst et al., 2005). mantle-normalized incompatible trace element patterns the Fig. 11 shows that the Tianshan Carboniferous lithospherically- patterns of the uncontaminated and less-contaminated continental contaminated basaltic rocks have LREE-enriched patterns with basalts possess shallower slopes (Nb/Yb(PM) Z 1.8e10.4) than the noticeable negative Nb, Ta and Ti anomalies which are easily NEB (Nb/Yb(PM) Z 3.6e21.8) (Fig. 17); (3) the NEB and the misidentified as arc related. However, their concentrations of 87 86 normal arc basalts have similar low Sr/ Sr and high 3 Nd ratios, incompatible trace elements are conspicuously higher than those while the Sr and Nd isotopic compositions of the uncontaminated of subduction-zone basalts. They cannot be regarded, therefore, as and less-contaminated continental basalts (with loweremoderate volcanic arc basalts. 87 86 Sr/ Sr(t) and moderate 3 Nd(t)) are quite different from those of In summary, comparison with typical Nb-enriched arc basaltic the strongly contaminated continental basalts (with higher rocks and continental basaltic rocks of LIPs reveals that the 87 86 Sr/ Sr(t) and lower to negative 3 Nd(t)) (Fig. 18). Tianshan Carboniferous Nb-enriched basaltic rocks are not the There is still hot debate about the origin of the NEB. One NEB, and are continental rift-related asthenosphere- (or plume-) model postulated that the enrichment in HFSE of the NEB arises generated basaltic rocks. from metasomatism of the mantle wedge peridotites by melts derived from the subducting oceanic lithosphere, termed 9.2. Are there any adakites occurred in the Tianshan “adakites” (Kepezhinskas et al., 1996; Sajona et al., 1993, 1994, Carboniferous volcanic successions? 1996; Defant and Kepezhinskas, 2001). An alternate model, proposed by Castillo et al. (2002, 2007), suggested that the magma The Tianshan Carboniferous rift-related volcanic successions source beneath the island arc is a “marble cake” type or veined comprise thick piles of basaltic lavas and subordinate intermediate mantle, with OIB-like enriched components of varying size and silicic lavas and pyroclastics (Xia et al., 2003, 2004, 2005b,c, embedded in a MORB-like depleted matrix, and the varying 2007, 2008a,b). We have demonstrated that intermediate and enrichment in Nb and other HFSE among the Nb-enriched arc silicic rocks resulted from continuous fractional crystallization lavas results from different degrees of mixing between above two starting from basaltic parents, possibly with a role for crustal source components. The second model is more convictive. contamination (Xia et al., 2005b, 2007). However, Wang et al. As regards the continental basalts, their origin can be linked to the (2006, 2007c) and Zhao et al. (2006) deemed that some Carbon- asthenospheric mantle (or mantle plumes). However, all of Earth’s iferous andesites, dacites and rhyolites in the northern Tianshan continental basalts show compositional evidence for the involvement with high Sr/Y ratios (19e100) ought to be classified as adakites. of continental lithosphere, including crust and continental litho- Adakite is a petrologic term that Defant and Drummond (1990) spheric mantle, in at least part of their eruptive sequences. For first introduced w19 years ago to refer to “volcanic or intrusive example, the lithospherically uncontaminated and less-contaminated rocks in Cenozoic arcs associated with subduction of young continental basalts have higher Nb/La ratios (>0.9) than the strongly (25 Ma) oceanic lithosphere”; these rocks are characterized by contaminated continental basalts (Nb/La < 0.9) (Fig. 16). w(SiO ) 56%, w(Al O ) 15% (rarely lower), w(MgO) usually It must be pointed out that although the lithospherically 2 2 3 <3% (rarely above 6%), Sr > 300 ppm, Y < 15 ppm, Sr/Y > 20, uncontaminated and less-contaminated continental basalts exhibit Yb < 1.9 ppm, La/Yb > 20, low HFSEs and 87Sr/86Sr usually similar Nb/La ratios to the NEB, there are some essential differ- <0.7040 (Castillo, 2006). Thus, adakite is identified almost ences between them. Besides aforementioned three differences, Th exclusively by its chemistry, primarily using its high Sr/Yand La/Yb positively correlates with Th/Nb for the uncontaminated and less- ratios and low Yand Yb contents. Hence the two almost universally contaminated continental basalts, but the Nb-enriched arc basalts used diagnostic tools to define adakite are the Sr/Y versus Y and show general negative correlation between Th and Th/Nb (Fig. 19). La/Yb versus Yb diagrams (Castillo et al., 1999; Fig. 20). This difference may be related to their different tectonic settings. It can be seen from Fig. 20 that although the Tianshan inter- In the intracontinental setting, crustal rocks and their partial melts mediate and silicic lavas regarded by Wang et al. (2006, 2007c) as are generally characterized by very low TiO contents (Wilson, 2 adakites have adakite-like Sr/Y ratios (19e100) and Y 1989) and fusible crustal rock types are generally much richer (6.17e16.6 ppm), Yb (0.59e1.83 ppm) contents, their La/Yb ratios than OIB in Ba, Rb, Th, K, and light REEs, but have similar or (5e19.3) are markedly lower than those of adakite (La/Yb > 20). lower contents of Nb, Ta, P, Zr, Hf, Y, and middle REEs All of them plot outside of the adakites field in the La/Yb versus Yb (Thompson et al., 1984). Thus lithospheric contamination of diagram (Fig. 20b), and only six samples (TW208, DXTW1, XT27, a magma derived from an asthenospheric OIB-like mantle source XT29, XT30, WXT744; Wang et al., 2006, 2007c) plot within the (or mantle plumes) could produce that Th/Nb increases with adakites field of the Sr/Y versus Y diagram (Fig. 20a). Hence, these increasing Th shown by the continental basalts of LIPs intermediate and silicic rocks are not the adakites. (Fig. 19bee). In contrast, in the subduction-zone setting, the subduction-related fluids carry La in preference to Nb (Keppler, 1996; You et al., 1996). Thus, under the influence of subduction- 9.3. Are there any high-Mg andesites occurred in the related fluids, Th/Nb in the Nb-enriched arc basalts derived from Tianshan Carboniferous volcanic successions? an OIB-like enriched source components increases with decreasing Nb, whereas Th keeps in constant or slightly lowering (Fig. 19a). High-Mg andesites (also called Boninites) are high w(SiO2)(>55%), It appears, therefore, that the Tianshan Carboniferous lith- high w(MgO) (>9%) arc lavas characterized by high compatible ospherically uncontaminated and less-contaminated continental trace element (Ni Z 70e450 ppm, Cr Z 200e1800 ppm) and very basalts with high Nb/La should not be regarded as Nb-enriched arc low w(TiO2) contents (<0.3%) (Crawford et al., 1981; Hickey and basalts. Frey, 1982). These rocks display unusual broad ‘V’-shaped 464 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

Figure 19 Th/Nb versus Th (ppm) diagrams for (a) Nb-enriched arc basalts from the Sulu Arc (southern Philippines) and the Kamchatka Arc (Russia) and (bee) lithospherically uncontaminated and less-contaminated continental basalts from the Tianshan LIP (China), the Emeishan LIP (China), the Siberia LIP (Russia) and the Deccan LIP (India). Data sources are as in Fig. 15.

chondrite-normalized REE patterns (Fig. 21a), which are rarely contents (all samples have higher w(TiO2) ranging from 0.38% to observed in other types of volcanic rock. The unusual shape of the 1.08%). Particularly, these rocks do not possess the distinctive REE patterns may be a consequence of the metasomatism of ‘V’-shaped chondrite-normalized REE patterns of high-Mg a strongly light-REE depleted mantle peridotite source by a light- andesites (Fig. 21bed). It seems, consequently, that these REE enriched fluid or slab melt (Wilson, 1989; Rapp et al., 1999; Carboniferous andesitic rocks do not belong to the high-Mg Martin et al., 2005). andesites (or diorites). Some Tianshan Carboniferous Mg-enriched andesitic rocks, including one basalt sample, five basaltic andesite samples, eleven 9.4. Summary and conclusions andesite samples and three diorite samples, were treated by Wang et al. (2006) as the high-Mg andesites (or diorites). Are these rocks The Carboniferous basaltic lavas in the Tianshan LIP can be indeed the high-Mg andesites (or diorites)? divided into two subtypes: high Nb/La (Nb/La > 0.9) and low The chemical compositions (Wang et al., 2006) show that these Nb/La (Nb/La < 0.9). The former is similar geochemically to the intermediate rocks are not characterized by high w(MgO) lithospherically uncontaminated and less-contaminated conti- (15 samples Z (3.12e6.80)%; five samples Z (7.16e9.72)%), nental basalts from several typical LIPs such as Deccan, Siberia high compatible trace element (six samples have higher Ni and Emeishan, and is obviously different from the Nb-enriched arc (76.2e276 ppm) and Cr (253e1132 ppm), the rest are low Ni basalts. The latter is lithospherically strong-contaminated conti- (2.28e45.6 ppm) and Cr (13.8e166 ppm)) and very low w(TiO2) nental basalts and has distinct trace element and isotope features L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471 465

composed of dominant basaltic lavas and subordinate amounts of intermediate and silicic lavas, and pyroclastic rocks, and associated shallow mafic-ultramafic and granitic intrusive rocks. (2) The regional unconformity of Lower Carboniferous upon basement or pre-Carboniferous rocks and the ages (360e351 Ma) of the youngest ophiolite and the peak of subduction metamorphism of high pressureelow temperature metamorphic belt and the occurrence of Ni-Cu-bearing maficeultramafic intrusion with age of w352 Ma and A-type granite with age of w358 Ma reveal that the final closure of the Paleo-Asian Ocean might occur in the Early Mississippian. (3) The TianshaneTarim (central Asia) LIP was associated with surface uplift that is inferred mainly from record of erosion and the onset of regional surface uplift preceded the TianshaneTarim (central Asia) LIP magmatism. Is the “surface uplift prior to magmatism” due to a result of orogenic event or related to activity of mantle plume? This issue needs further studies. (4) Based on geochemical variation, the TianshaneTarim (central Asia) LIP basaltic lavas can be classified into two major magma types: HT (Ti/Y > 500) and LT (Ti/Y < 500). According to Nb/La ratios, the HT and LT lavas can be further divided into HT1 (Nb/La > 0.85) and HT2 (Nb/La < 0.85), and LT1 (Nb/La > 0.85) and LT2 (Nb/La < 0.85) lavas, respectively. The predominant geochemical signatures of this studied LIP magmas are inferred to be derived from asthenosphere or deep-seated mantle plumes. The available elemental and Sr-Nd isotope data suggest that geochemical variation of the HT2 and LT2 lavas can be accounted for by lithospheric contamination of asthenosphere- (or plume-) derived magmas whereas the parental magmas of the HT1 and LT1 lavas have not undergone, during their ascent, pronounced lithospheric contamination. Figure 20 Plots of: (a) Sr/Y versus Y (ppm); (b) La/Yb versus Yb (5) The TianshaneTarim (central Asia) CarboniferouseEarly (ppm) for the Tianshan Carboniferous high Sr/Y andesites, dacites and Permian rift-related basaltic lavas display a temporal and rhyolites that are regarded by Wang et al. (2006, 2007c) as the spatial chemical variation. During Carboniferous predomi- adakites. These two diagrams (after Castillo et al., 1999) are nantly uncontaminated and less-contaminated LT1 lavas commonly used to show “distinct” adakite and island arc andesite, erupted in central Tianshan rift and predominantly the strongly dacite and rhyolite compositional fields. contaminated LT2 and HT2 lavas erupted in the circumjacent regions of the central Tianshan rift. The LT1 lavas were generated by a higher degree (10%e30%) of partial melting in being sharply distinguished from the volcanic arc basaltic rocks. the garnet stability field of the Carboniferous upwelling Our studies reveal that although some of the Carboniferous asthenosphere compared to the LT2 and HT2 lavas. The lower intermediate and silicic lavas in the Tianshan LIP have adakite- degree (<10%) of partial melting in the spinel-garnet transition like Sr/Y ratios and Y, Yb contents, their La/Yb ratios zone of the upwelling asthenosphere, as is characteristic of the < (La/Yb 19.3) are evidently lower than those of adakite HT2 and LT2 lavas, may be the result of a relatively lower > (La/Yb 20). These rocks cannot be thus classified as adakites. geotherm. During Early Permian predominantly uncontami- Besides, the Carboniferous intermediate Mg-enriched rocks in the nated and less-contaminated HT1 and LT1 lavas erupted in the Tianshan LIP ought not to be treated as the high-Mg andesites (or Tarim and Baishan rifts and predominantly the strongly > diorites), owing to higher w(TiO2) contents ( 0.38%) and contaminated LT2 and HT2 lavas erupted in the Kalpin rift of moderately-strongly light-REE enriched chondrite-normalized west side and the Tianshan area of north side. These imply that REE patterns. To sum up, there is not an assemblage of the Carboniferous mantle melting anomaly center was likely Nb-enriched arc basalt, adakite and high-Mg andesite existed in beneath the central Tianshan and the axial area of the Early the Tianshan Carboniferous volcanic successions. Permian plume was beneath the Tarim and the Baishan. (6) Our studies show that there is not an assemblage of 10. Summary Nb-enriched arc basalt, adakite and high-Mg andesite existed in the Tianshan Carboniferous volcanic successions. (1) The TianshaneTarim (central Asia) CarboniferousEarly (7) Temporal coincidence between continental LIPs and conti- Permian rift-related volcanic successions, covering large nental breakup has been noted for almost four decades. areas in northwestern China, make up one of the oldest Similar relationship has been found for the Tianshan LIP and Phanerozoic LIP in the world, which can be further divided opening of the Middle Mississippian (344e325 Ma) North into two sub-provinces: Tianshan and Tarim. The province is Tianshan Ocean. 466 L. Xia et al. / Geoscience Frontiers 3(4) (2012) 445e471

Figure 21 Chondrite-normalized rare earth element (REE) patterns for (a) typical high-Mg andesites (i.e., Boninites, Crawford et al., 1981; Hickey and Frey, 1982) and (bed) the Tianshan Carboniferous Mg-enriched andesites, basalts, basaltic andesites and diorites that are regarded by Wang et al. (2006) as the high-Mg andesites and high-Mg diorites. The normalization values are from Sun and McDonough (1989).

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