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Chemical Compositions and Tectonic Significance of Chrome-Rich

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Chemical Compositions and Tectonic Significance of Chrome-Rich Chemical Compositions and Tectonic Significance of Chrome-Rich Spinels in the Tianba Flysch, Southern Tibet Bin Zhu, William S. F. Kidd, David B. Rowley,1 and Brian S. Currie2 Earth and Atmospheric Sciences, State University of New York, Albany, New York 12222, U.S.A. (e-mail: [email protected]) A B S T R A C T Turbiditic sandstones from the mid–Late Cretaceous Tianba Flysch sequence are well exposed near Tianba in the north Nieru Valley, southern Tibet, and contain significant amounts of chrome-rich spinels. Microprobe results show 2ϩ that the spinels have a well-developed Fe-Ti trend and have Cr/(Cr ϩ Al) 0.4–0.65, Mg/(Mg ϩ Fe ) 0.3–0.9, and TiO2 11%. The occurrence of melt inclusions, and well above 0.2% TiO2 contents, shows that a volcanic suite of rocks was the source of these Cr-rich spinels. Specifically, these spinels compositionally resemble spinels from intraplate basalts (e.g., Hawaii and Disko Island, western Greenland) rather than from ophiolitic ultramafic or gabbroic material expected in an intraoceanic subduction system associated with the closing of the Neo-Tethys. On the basis of pa- leotectonic reconstruction, the presence of mid–Late Cretaceous fossils in the strata, and the limited range of chemical compositions, we suggest that 117-Ma volcanics of the Rajmahal-Kerguelen hot spot were the source for the chrome- rich spinels in the Tianba Flysch. Introduction The petrologic, mineralogic, and geochemical char- provenance studies for a variety of reasons: (1) spi- acteristics of sedimentary basin deposits can nels crystallize from mafic and ultramafic magmas greatly aid in reconstructing the tectonic evolution over a wide range of conditions and therefore are a of adjacent orogenic belts (Dickinson and Suczek sensitive indicator of the original host magmatic 1979; Zuffa 1980; Ingersoll et al. 1984; Dickinson rock composition (Irvine 1967; Allan et al. 1988; 1985; Garzanti et al. 1996; Dewey and Mange Roeder 1994). (2) The unusual chemical durability 1999). The original composition of sediment source of chromian spinel makes its original composition lithologies, however, may be obscured by many fac- more likely to be preserved against processes of tors, including climate, relief, transport mecha- weathering and diagenesis. (3) The hardness and nism, postburial diagenesis, and regional meta- lack of cleavage make spinel physically resistant to morphism (Morton 1985, 1991). Since chemically mechanical breakdown. As such, detrital spinel has and mechanically unstable minerals are altered or the potential to survive weathering and transport eliminated by weathering and diagenesis, studies processes and exist as a heavy-mineral component of resistant minerals in sedimentary rocks are ap- of clastic sedimentary rocks (Pober and Faupl 1988; plied in paleotectonic reconstructions with increas- Cookenboo et al. 1997; Ganssloser 1999; Lenaz et ing frequency (Morton 1991; Caironi et al. 1996; al. 2000). (4) Since compositional analysis of spinels Lihou and Mange-Rajetzky 1996; Sciunnach and is routinely applied in petrologic studies of spinel- Garzanti 1996; Cookenboo et al. 1997; Dewey and bearing igneous rocks, a large volume of micro- Mange 1999). Of these resistant minerals, chromian probe data is available in the literature (Dick and spinel is of particular significance to sedimentary Bullen 1984; Arai and Okada 1991; Arai 1992; Lee 1999; Barnes and Roeder 2001; Kamenetsky et al. Manuscript received June 10, 2003; accepted January 15, 2001). 2004. Recently, there has been a great deal of interest 1 Department of Geophysical Sciences, University of Chi- cago, Chicago, Illinois 60637, U.S.A. in the early tectonic history of the Himalaya, the 2 Department of Geology, Miami University, Oxford, Ohio orogenic product of continent-continent collision 45056, U.S.A. between Asia and India (Tapponnier et al. 1981; [The Journal of Geology, 2004, volume 112, p. 417–434] ᭧ 2004 by The University of Chicago. All rights reserved. 0022-1376/2004/11204-0003$15.00 417 418 B . Z H U E T A L . Figure 1. Regional geological map of Tethyan Himalaya (after Rowley 1996). BNS p Banggong-Nujiang suture; IYZS p Indus-Yarlung-Zangbo suture; MCT p Main Central Thrust; STDS p Southern Tibet Detachment System. Allegre et al. 1984; Wang et al. 2000; Yin and Har- Geologic Overview rison 2000; Wan et al. 2001; Ziabrev et al. 2001; Davis et al. 2002). Much of the pre–middle Tertiary The Tethyan Himalaya, which lie between the record of this tectonism is recorded in the sedi- High Himalayan Crystalline belt to the south and mentary rocks situated south of the Indus-Yarlung- the Indus-Zangbo suture and the Lhasa block to the Zangbo suture in southern Tibet (Garzanti et al. north (fig. 1), consist primarily of Late Paleozoic 1987, 1996; Garzanti 1993, 1999; Beck et al. 1995; to Early Eocene sedimentary rocks, originally de- Rowley 1996, 1998). Although the initial collision posited along the northern edge of the Indian con- of India and Asia is believed to have begun during tinent. Deposition began with a Late Paleozoic- the mid-Tertiary (Garzanti et al. 1987; Rowley Triassic rifting (Sengor et al. 1988; Sciunnach and 1996, 1998; Wan et al. 2001), less is known about Garzanti 1996; Garzanti 1999) during the initial de- the earlier history of Himalayan orogenesis. In this velopment of the Neo-Tethyan Ocean. During the article, we report provenance data from the Cre- Mesozoic, a relatively wide passive continental taceous Tianba Flysch of the Nieru Valley in the margin subsequently developed along the north rim Tethyan Himalaya to better understand the tec- of the Indian continent. In mid-Cretaceous time, tonic history of the region. The Tianba Flysch con- northward-directed subduction of the Neo-Tethyan sists of a section of lithic-rich turbidite sandstones oceanic crust beneath the southern margin of Asia and interbedded shales, which in terms of its li- resulted in the development of a magmatic arc and thology and bedding characteristics appears in out- a fore-arc-related basin (Xigaze fore-arc basin) along crop to be a typical collision-related flysch deposit the southern margin of the Lhasa block (Einsele et (Rowley and Kidd 1981; Garzanti et al. 1987) and al. 1994; Durr 1996). With continued subduction, might be presumed to record an arc-continent col- the India-Asia collision initiated in the early Ter- lision or ophiolite-obduction event in the mid–Late tiary and produced the Indus-Yarlung-Zangbo su- Cretaceous (Searle 1983). Our data provide insight ture. Therefore, the strata of the Tethyan Himalaya into the early evolution of Himalayan orogensis record the entire depositional history of the north- and allow a clear test of the hypothesis that there ern Indian passive margin. was a mid–Late Cretaceous ophiolite-obduction In southern Tibet, the Tethyan Himalaya can be event in the eastern Tethyan Himalaya during the divided into two subzones of different lithologic mid–Late Cretaceous. assemblages (fig. 2) that are separated by the east- Journal of Geology T I A N B A F L Y S C H 419 upward into siltstones and shales and contain abun- dant sedimentary structures including sole marks, horizontal laminations, and ripple cross-lamina- tions. These rocks are interpreted as Bouma-type turbidites and indicate deep water, continental rise, or perhaps slope environment. The Tianba Flysch is conformably overlain by greenish-gray burrowed shales that contain a few thin sideritic sandstone beds, and an interval con- taining some large (up to 1-m diameter) calcareous nodules, one of which yielded an ammonite. Bel- emnites are also found in the shales above the flysch, and preliminary investigation of radiolarian fossils in the sideritic sandstones points to a Late Cretaceous age of deposition (N. Shafique, pers. comm., 2002). Figure 2. Simplified tectonic map of the study area Above the concretionary shales, a significant (after Willems et al. 1996). fault places me´lange, including blocks of pink lime- stones (Chuangde “formation”; Wang et al. 2000), over the dipping and folded Cretaceous rocks de- west trending Gyirong-Kangmar thrust (Burg and scribed previously (fig. 4). The pink limestone Chen 1984; Liu 1992). The northern zone is dom- blocks in the hanging wall of this fault contain inated by slightly metamorphosed deposits of outer abundant foraminifera (Globotruncana linneiana, shelf, continental slope, and rise environments, G. elevata, Marginotruncana stuarti, M. stuarti- whereas the southern zone is characterized by non- formis, Heterohelix sp) that indicate a Campanian metamorphic, shallow-water shelf calcareous and depositional age (Willems et al. 1996; Wang et al. terrigenous deposits ranging from Late Paleozoic to Eocene (Wen 1987; Xizang BGMR 1992; Willems et al. 1996). This study concentrates on the well-exposed lithic-rich arenites at the northern end of the Nieru Valley (fig. 3), which belong to the northern zone of the Tethyan Himalaya. Near Tianba, the lower part of the section consists of thin-bedded gray shales, silty shales, and cherts of the Jiabula Group (Liu 1992; Xizang BGMR 1992). These rocks con- tain abundant fossils of nektonic organisms in- cluding ammonites and belemnites, which indicate a Berriasian-Aptian age (Wang et al. 2000; Z. Bing- gao, pers. comm., 2000). The Jiabula Group con- tains pyrite concretions, dark shales, and laminated grayish-green cherts indicating deposition within deep-water, sediment-starved basin (Liu 1992; Wang et al. 2000). The upper part of the Jiabula Group in the north- ern Nieru Valley is made up of the Tianba Flysch. In this area, the Tianba Flysch is about 220 m thick (fig. 4) and consists primarily of well-bedded sand- stone, siltstones, and shales. The contact between the Tianba Flysch and the underlying dark shales and cherts is conformable. The basal interval of the flysch is characterized by a transition from black cherts to olive-colored argillites and mica-rich silt- stones that rapidly coarsen upward into the graded Figure 3.
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