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Detrital Zircon U-Pb-Hf Isotopes of Middle Neoproterozoic Sedimentary Rocks in the Altyn Tagh Orogen, Southeastern Tarim: Insights for a Tarim-South China-North India Connection in the Periphery of Rodinia
Lithosphere ( IF 2.4 ) Pub Date : 2020-06-30 , DOI: 10.2113/2020/8895888 Qian Liu 1, 2 , Guochun Zhao 3, 4 , Jianhua Li 5 , Jinlong Yao 4 , Yigui Han 4 , Peng Wang 3 , Toshiaki Tsunogae 2, 6
Lithosphere ( IF 2.4 ) Pub Date : 2020-06-30 , DOI: 10.2113/2020/8895888 Qian Liu 1, 2 , Guochun Zhao 3, 4 , Jianhua Li 5 , Jinlong Yao 4 , Yigui Han 4 , Peng Wang 3 , Toshiaki Tsunogae 2, 6
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The location of the Tarim craton during the assembly and breakup of the Rodinia supercontinent remains enigmatic, with some models advocating a Tarim-Australia connection and others a location at the heart of the unified Rodinia supercontinent between Australia and Laurentia. In this study, our new zircon U-Pb dating results suggest that middle Neoproterozoic sedimentary rocks in the Altyn Tagh orogen of the southeastern Tarim craton were deposited between ca. 880 and 760 Ma in a rifting-related setting slightly prior to the breakup of Rodinia at ca. 750 Ma. A compilation of existing Neoproterozoic geological records also indicates that the Altyn Tagh orogen of the southeastern Tarim craton underwent collision at ca. 1.0-0.9 Ga and rifting at ca. 850-600 Ma related to the assembly and breakup of Rodinia. Furthermore, in order to establish the paleoposition of the Tarim craton with respect to Rodinia, available detrital zircon U-Pb ages and Hf isotopes from Meso- to Neoproterozoic sedimentary rocks were compiled. Comparable detrital zircon ages (at ca. 0.9, 1.3-1.1, and 1.7 Ga) and Hf isotopes indicate a close linkage among rocks of the southeastern Tarim craton, Cathaysia, and North India but exclude a northern or western Australian affinity. In addition, detrital zircons from the northern Tarim craton exhibit a prominent age peak at ca. 830 Ma with minor spectra at ca. 1.9 and 2.5 Ga but lack Mesoproterozoic ages, comparable to the northern and western Yangtze block. Together with comparable geological responses to the assembly and breakup of the Rodinia supercontinent, we offer a new perspective of the location of the Tarim craton between South China and North India in the periphery of Rodinia.Most paleomagnetic studies, in agreement with plume-related magmatic rocks and glacial intervals, support the placement of the Tarim craton to the north or west of Australia in Neoproterozoic reconstructions of Rodinia [1–4]. In contrast, some authors [5–7] argue for a “missing link” configuration of the Tarim craton at the heart of the unified Rodinia supercontinent between Australia and Laurentia, based on early Neoproterozoic to early Ediacaran paleomagnetic and tectonostratigraphic data from the Tarim craton.However, ca. 1.0-0.6 Ga magmatism along the northern margin of the Tarim craton [8, 9] is younger than Grenville-age (ca. 1.35-1.05 Ga) magmatism within the Musgrave and Albany-Fraser blocks of western-central Australia [10, 11], challenging a Tarim-western Australia linkage (e.g., [3]). Furthermore, ca. 1.0-0.6 Ga magmatic rocks from the northern Tarim craton have recently been reinterpreted to correlate with a long-lived subduction event [8, 9]. Such subduction-related magmatism without any Grenville-age unconformity or metamorphism indicates that the Tarim craton was probably located in the periphery of Rodinia with its northern margin facing the circum-Rodinia subduction zones [8, 9], rather than at the heart of Rodinia [6, 7]. In addition, the recognition of ca. 940-900 Ma syncollisional magmatism and metamorphism from the Altyn Tagh orogen of the southeastern Tarim craton supports to interpret that this orogen is the suture between the Tarim craton and another block during the amalgamation of Rodinia (e.g., [12–16]). However, the location of the Tarim craton in the periphery of Rodinia remains enigmatic and subsequent dispersal of the Tarim craton from Rodinia is not fully understood.This study first reports on middle Neoproterozoic sedimentary rocks of the Altyn Tagh orogen of the southeastern Tarim craton, which were probably deposited in a rifting-related setting slightly earlier than the breakup of Rodinia due to the opening of the Proto-Tethys Ocean at ca. 750 Ma (e.g., [3, 17]). We suggest the placement of the Tarim craton between South China and North India with respect to Rodinia, based on a comparison of available detrital zircon age spectra and Hf isotope compositions.The Altyn Tagh orogen occupies a crucial junction between the Tarim craton to the northwest, the Qilian orogen to the northeast, and the Qiadam block and the East Kunlun orogen to the southeast (Figure 1(a)). It has generally been divided into four units. From northeast to southwest, namely, the North Altyn Tagh terrane, the North Altyn Tagh subduction-accretion belt, the Central Altyn Tagh terrane, and the South Altyn Tagh subduction-collision belt (Figure 1(b)). The North Altyn Tagh terrane consists mainly of ca. 2.8 Ga and ca. 2.4-1.8 Ga orthogneiss and ca. 1.9 Ga paragneiss [5, 18] comparable to Archean to Paleoproterozoic basement of the Tarim craton [9]. The North Altyn Tagh subduction-accretion belt has been interpreted to represent an early Paleozoic accretionary orogenic system [19]. It consists mainly of ca. 520-450 Ma ophiolitic mélanges [20–22], ca. 512-491 Ma high-pressure (HP)/low-temperature (LT) eclogite and blueschist [23], ca. 514-390 Ma magmatic rocks [24–26], and early Paleozoic volcanosedimentary sequences [27]. In addition, the Suolak Formation containing ca. 760-750 Ma basalt and rhyolite associations are sporadically exposed in the North Altyn Tagh subduction-accretion belt [28, 29]. The Central Altyn Tagh terrane is composed of Meso- to Neoproterozoic metasedimentary rocks, ca. 920 Ma foliated rhyolite, ca. 754 Ma basalt, ca. 703 Ma A-type granite, and ca. 522-433 Ma intermediate to felsic magmatic rocks (Figure 1(c); [16, 25, 28, 30–32]). The South Altyn Tagh subduction-collision belt (Figure 1(b)), regarded as an early Paleozoic collisional orogenic system [19], is dominated by ca. 501 Ma ophiolitic mélange [33], ca. 508-475 Ma (U)HP and ca. 457-436 Ma Barrovian-type metamorphic rocks (e.g., [34–36]), and ca. 517-226 Ma magmatic rocks [25, 31]. Additionally, Meso- to Neoproterozoic metasedimentary rocks, ca. 940-900 Ma granitic rocks with I- and S-type affinities [12, 14], and ca. 763 Ma mafic rocks are recorded in the South Altyn Tagh subduction-collision belt (Figure 1(c)).Sample 17LQ50-1A (GPS: 39°04′40.82″N, 92°15′09.67″E) was collected from an outcrop located ~8 km southwest of the main body of the Lapeiquan ophiolitic mélange in the North Altyn Tagh subduction-accretion belt (Figures 1(a) and 2(a)).The sampled outcrop contains rock associations of phyllite and quartz schist with prevailing cleavage and foliation (Figure 2(b)). It is associated with early Paleozoic sedimentary sequences in the matrix of the Lapeiquan ophiolitic mélange (Figure 2(a)). Due to the poor quality of the outcrop, the relationship of this outcrop with early Paleozoic sedimentary sequences was difficult to establish in the field. Nonetheless, a tectonic contact is inferred (Figure 2(a)), based on (1) the strikingly distinctive rock associations of the studied schists and the conglomerate-sandstone beds of the early Paleozoic sedimentary sequences [27], (2) the different dip angles of the schistosity of the studied schists and the bedding of the early Paleozoic sedimentary sequences (Figure 2(a)), and (3) the extensive fault activities in the region (Figure 2(a)). Sample 17LQ50-1A is a fine-grained quartz schist. Major mineral compositions include elongated quartz (80%) and tiny muscovite (20%), which delineate well-developed schistosity (Figure 2(c)).Detrital zircons extracted from sample 17LQ50-1A show subhedral, subrounded, and well-rounded forms with aspect ratios of 1-3 (Figure 3). They illustrate predominantly oscillatory zoning and subordinately homogeneous internal structures under cathodoluminescence (CL; Figure 3). Th (2-1926 ppm) and U (12-2765 ppm) contents are conspicuously variable with Th/U ratios of mostly >0.2 (Table DR1 in the GSA Data Repository (GSA Data Repository item 201Xxxx, analytical methods and results (Tables DR1 and DR2) and compiled data (Table DR3) are available online at http://www.geosociety.org/pubs/ft20XX.htm or on request from editing@geosociety.org.)), indicative of a magmatic origin. Only six analytical spots have low Th/U ratios of 0.01-0.10 (Table DR1 in the GSA Data Repository), reflecting their metamorphic origin, as also indicated by their complex, heterogeneous CL images (Figure 3). For <1000 Ma and >1000 Ma zircons, the 206Pb/238U and 207Pb/206Pb ages were adopted, respectively. One analysis with an unreasonable minus error was deleted for discussion (Table DR1 in the GSA Data Repository). Another 140 measurements yielded a large variation of concordant Precambrian ages ranging from ca. 2700 to 888 Ma, characterized by a continuous age distribution of ca. 1.7-0.9 Ga and minor age abundances at ca. 2.7 and 2.0-1.9 Ga (Figure 3). The youngest zircon grain gave a concordant 206Pb/238U age of 888±7 Ma, interpreted as the maximum depositional age of sample 17LQ50-1A. Hf isotope analyses were carried out on 75 dated detrital zircons with a magmatic origin. Most detrital zircons exhibit a large spread of εHft values varying from -10 to +11, with only one grain aged ca. 902 Ma showing a highly negative εHft value of -15 (Figure 4; Table DR2 in the GSA Data Repository).The youngest zircon (888±7 Ma) from sample 17LQ50-1A shows a subhedral, prismatic form with abraded edges (Figure 3), suggesting transportation before deposition to some extent. It indicates that sample 17LQ50-1A might have been deposited at some time posterior to ca. 888 Ma. In addition, the Suolak Formation of the North Altyn Tagh subduction-accretion belt [28, 29] provides a reliable estimate of its minimum age. The Suolak Formation consists of basalt, basaltic breccia and tuff, intermediate to felsic tuff, rhyolite, and chert, in which a SHRIMP zircon U-Pb age of ca. 763 Ma was obtained from a basalt [28] and a LA-ICP-MS zircon U-Pb age of ca. 750 Ma was obtained from a rhyolite [29]. The studied quartz schist is distinct from the volcanic and volcaniclastic sequences of the Suolak Formation (Figure 5) and does not contain any detrital zircons from the ca. 760-750 Ma volcanic rocks of the Suolak Formation [29]. Therefore, the studied sample was probably deposited earlier than the ca. 760-750 Ma Suolak Formation. In conclusion, sample 17LQ50-1A was probably deposited between ca. 880 and 760 Ma.The main body of the Rodinia supercontinent finally assembled along major Grenvillian (ca. 1.3-1.0 Ma) orogenic belts in southern Laurentia, western and northern Australia, Amazonia, and the Maud-Namaqua-Natal Provinces of East Antarctica and Africa [10, 11, 56, 57]. In addition, ca. 1.0-0.9 Ga orogenic belts are documented in southwestern Baltica, the Eastern Ghats belt in India, and the Northern Prince Charles orogenic belt in East Antarctica [11, 56]. Such early Neoproterozoic tectonic events are also preserved in the Altyn Tagh orogen of the southeastern Tarim craton, as manifested by extensive ca. 940-900 Ma felsic magmatism in Central and South Altyn Tagh (Figure 1(c)). Geochemical studies revealed high-K calc-alkaline I-type and S-type affinities for the ca. 940-900 Ma granitic rocks and proposed an active continental margin [14] or a syncollisional setting [12]. A syncollisional regime is preferred based on ca. 910 Ma metamorphic zircon overgrowths on Mesoproterozoic zircon cores documenting in Mesoproterozoic metasedimentary rocks of the South Altyn Tagh subduction-collision belt [13]. It reflects metamorphism of Mesoproterozoic supracrustal material at ca. 910 Ma [12], which was more likely related to syncollision. Recycling of Mesoproterozoic supracrustal material is also supported by ca. 1.9-1.4 Ga zircon Hf isotope model ages and coeval inherited zircon cores in some ca. 940-900 Ma granitic rocks [14, 15]. On the other hand, an active continental margin is also difficult to reconcile with the lack of typical arc-like products in the Altyn Tagh orogen, such as mafic-intermediate magmatic rocks and calc-alkaline granitoids. In addition, comparable ca. 1.0-0.9 Ga magmatic rocks are also distributed throughout the adjacent Qilian and Qaidam regions [14, 58]. Therefore, an early Neoproterozoic (ca. 1.0-0.9 Ga) collisional orogen characterizes the southeastern Tarim craton, which is coincident with the late assembly of the Rodinia supercontinent.Following early Neoproterozoic final assembly, the interior of Rodinia underwent rifting-related extension at ca. 825-750 Ma, leading to the development of anorogenic magmatic rocks in most Rodinia terranes but not forming new oceans (e.g., [3]). The Rodinia supercontinent had not broken up until ca. 750-600 Ma due to the diachronous opening of relevant ocean domains [3, 17, 57]. Among these oceans, the Proto-Tethys Ocean separating Tarim, South China, and North China from other East Asian blocks was opened as early as ca. 750 Ma [3, 17]. Ca. 760-750 Ma basalt and rhyolite associations in the North Altyn Tagh subduction-accretion belt (Figure 1(c); [28, 29]) are coincident with the opening of the Proto-Tethys Ocean leading to the separation of the Tarim craton from Rodinia. The studied sample 17LQ50-1A deposited between ca. 880 and 760 Ma probably correlates with a rifting-related extensional setting prior to the separation of the Tarim craton from Rodinia. All these Neoproterozoic sedimentary and volcanic rocks of the North Altyn Tagh subduction-accretion belt were probably preserved in response to a well-preserved ca. 0.8-0.6 Ga rift depression throughout the Tarim basin [59]. Other geological records indicating middle-late Neoproterozoic extension in the Altyn Tagh orogen (Figure 1(c)) include ca. 763 Ma mafic rocks and ca. 703 Ma A-type granite in Central and South Altyn Tagh [15, 28, 32] and ca. 820-750 Ma MORB-like protoliths of the early Paleozoic eclogite and garnet peridotite in South Altyn Tagh [34, 35, 48]. Comparable ca. 800-600 Ma rifting-related mafic to felsic magmatism also operated throughout Qilian and Qaidam [60, 61]. Therefore, a middle-late Neoproterozoic (ca. 850-600 Ma) rifting regime was operating in the southeastern Tarim craton, associated with the extension and breakup of Rodinia.In conclusion, the southeastern Tarim craton underwent ca. 1.0-0.9 Ga collision and ca. 850-600 Ma rifting in response to the assembly and breakup of Rodinia, respectively.The ca. 2.7 Ga and ca. 2.0-1.9 Ga zircons from sample 17LQ50-1A are coincident with two episodes of granitic magmatism at ca. 2.8-2.3 Ga and ca. 2.0-1.8 Ga in the North Altyn Tagh terrane and the Tarim craton (e.g., [5, 18]), indicating a derivation from the Tarim basement. Similarly, the ca. 940-900 Ma felsic magmatic rocks in Central and South Altyn Tagh (Figure 1(c)) could be a potential source for the ca. 910 Ma detrital zircons in the studied sample. Some ca. 1497-1470 Ma diabase dikes were documented in the Kuluketage area of the northeastern Tarim craton [62, 63], but these mafic rocks might not have been the primary source material for the ca. 1.4 Ga detrital zircons in our studied sample because zircons typically crystallize from magmas with greater than 60% SiO2 with much lesser abundance in lower silica magmas (e.g., [64]). Nonetheless, other source terranes that once connected with the southeastern Tarim craton during depositional time but subsequently drifted away were also required because ca. 1.7-1.1 Ga source rocks are lacking in the region. This highlights the importance of this study, in which we established the paleoposition of the Tarim craton within Rodinia by comparing available ages and Hf isotopes of detrital zircons from Meso- to Neoproterozoic sedimentary rocks in possible Rodinia terranes (Figures 4 and 6; Table DR3 in the GSA Data Repository).A linkage between the southeastern Tarim craton and the Cathaysia block is suggested by comparable detrital zircon age populations at ca. 0.9, 1.3-1.1, and 1.7 Ga (Figure 6). The Cathaysia block exhibits variably negative to positive zircon εHft values at ca. 1.7 Ga, consistent with those of the coeval detrital zircons in the studied sample (Figure 4). Mostly positive εHft values of the ca. 1.3-0.9 Ga detrital zircons in this study also agree with a large portion of the positive zircon εHft values at ca. 1.3-0.9 Ga in Cathaysia (Figure 4). Notably, a zircon age peak at ca. 758 Ma in Cathaysia is absent in the southeastern Tarim craton because the studied sample was deposited prior to 760 Ma and unlikely contains such a young age peak at ca. 758 Ma. In addition, ca. 940-900 Ma collision operating within the southeastern Tarim craton is consistent with ca. 1.0-0.9 Ga collision recording in the Wuyi-Yunkai domains along southeastern Cathaysia (Figure 7; [52, 65]). The ca. 820-710 Ma sedimentation and bimodal magmatism due to the Nanhua rifting across South China [66–68] also matches well with the rifting-related records of the southeastern Tarim craton (see the above section).A connection between the northern Tarim craton and the northern and western Yangtze block is indicated by the same prominent age spectrum at ca. 830 Ma and minor spectra at ca. 1.9 and 2.5 Ga of detrital zircons (Figure 6). Another age peak at ca. 2.6-2.7 Ga in the northern and western Yangtze craton probably reflects the importance of ca. 2.6-2.7 Ga crystalline basement in the Yangtze craton [69, 70], which is relatively rare in the Tarim craton. Moreover, a ca. 1.0-0.6 Ga active continental margin along the northern Tarim craton [8, 9] is comparable to the ca. 1.0-0.7 Ga Panxi-Hannan belt along the northern and western peripheries of the Yangtze block [71, 72]. Together they probably belong to the Neoproterozoic accretionary orogen along the northern margin of Rodinia [9, 71, 73].Collectively, a Tarim-South China linkage can be established in the periphery of Rodinia (Figure 7). South China was assembled by the connection of the Yangtze and Cathaysia blocks at ca. 820-800 Ma [68] and occupied a position adjacent to North India and western Australia with respect to Rodinia (Figure 7), based on comprehensive geologic, geochemical, geochronological, paleomagnetic, and faunal data [73]. We further suggest a location of the Tarim craton between South China and North India (Figure 7), according to the following lines of evidence.First, a North Indian affinity for the southeastern Tarim craton is indicated by the similar age spectra (at ca. 1.2-0.9 and 1.7 Ga) and εHft values for detrital zircons (Figures 4 and 6). This inference is also supported by the ca. 940-900 Ma collisional orogen in the southeastern Tarim craton, which is comparable to the ca. 990-900 Ma Eastern Ghats belt in India (Figure 7). In addition, the late Neoproterozoic strata in the Greater and Lesser Himalaya terranes were accumulated along the passive margin of North India [74, 75]. Their age spectra at ca. 1.2-0.9 and 1.7 Ga are consistent with our results (Figure 6), further demonstrating the close linkage between the southeastern Tarim craton and Great India (Figure 7). Furthermore, the ca. 1.4 Ga detrital zircons with positive εHft values in the studied sample were probably derived from the Northern Prince Charles Mountains of East Antarctica [11, 56] that exhibit a number of positive zircon εHft values at ca. 1.4 Ga (Figure 4).Second, the ca. 1.0-0.6 Ga arc magmatic rocks along the northern Tarim craton are coeval with the ca. 1.0-0.8 Ga arc-related magmatic rocks of northwestern India and the ca. 800-720 Ma Andean-type arcs in the Seychelles and Madagascar [71, 76, 77], occupying the circum-Rodinia subduction-accretion system (Figure 7).Third, we argue against an Australian affinity for the Tarim craton in the reconstruction of Rodinia [1, 2, 4]. The age spectra in this study are different from those in northern or southern Australia, which are characterized by a predominant age population at ca. 1.7-1.6 Ga but devoid of 0.9 and 1.4-1.3 Ga ages (Figure 6). A western Australian affinity is also questionable because western Australia lacks ca. 0.9 and 1.4 Ga detrital zircons from Meso- to Neoproterozoic sedimentary rocks (Figure 6). Importantly, even younger Permian sedimentary rocks in western Australia also lack ca. 0.9 and 1.4 Ga detrital zircons (e.g., [78]), implying coherent absence of ca. 0.9 and 1.4 Ga events in western Australia. In addition, the Grenvillian (ca. 1.3-1.0 Ga) orogens extending from the Albany-Fraser belt to the Musgrave block in western to central Australia (Figure 7) are significantly older than the Neoproterozoic orogens in the peripheries of Tarim, challenging a western Australian affinity for the Tarim craton.This study first reported middle Neoproterozoic sedimentary rocks in the Altyn Tagh orogen of the southeastern Tarim craton related to a rifting-related setting slightly earlier than the breakup of Rodinia at ca. 750 Ma. A new perspective on the location of the Tarim craton between South China and North India in the periphery of Rodinia is advocated based on Neoproterozoic geological records and detrital zircon U-Pb-Hf isotopes.The authors declare that they have no conflicts of interest.This study was financially supported by a NSFC Project (41730213), Grant-in-Aids for Scientific Research from Japan Society for the Promotion of Science (JSPS) to Prof. Toshiaki Tsunogae (18H01300) and Dr. Qian Liu (No. 19F19020), and an open project from the State Key Laboratory for Mineral Deposits Research, Nanjing University (No. 21-16-03). JSPS International Research Fellowship is much appreciated. We appreciate the great help from Lei Wu, Jianfeng Gao, and Liang Li in experimental analyses.Table DR1: U-Pb dating results for detrital zircons from sample 17LQ50-1A and zircon standards GJ-1 and Plešovice. Table DR2: Hf isotope compositions of detrital zircons from sample 17LQ50-1A and zircon standards GJ-1, 91500, Plešovice, Mud Tank, and Penglai. Table DR3: compilation of detrital zircon ages from northern Tarim and relevant Rodinia blocks. Analytical methods.
更新日期：2020-06-30
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