Mesozoic strata of the Hoh‐Xil‐Songpan‐Ganzi complex in northern Tibet are exposed in a vast (> 370,000 km2) triangle‐shaped orogenic belt bound by the Longmen Shan thrust belt in the east, the Kunlun terrane and North China block in the north, and the Qiangtang terrane and Yidun arc in the south. These strata consist of Middle–Upper Triassic submarine fan and deep marine facies rocks that were deposited in the Paleo‐Tethys Ocean. Late Triassic–Early Jurassic contractional deformation in the eastern Hoh‐Xil‐Songpan‐Ganzi complex marks the demise of the Paleo‐Tethys Ocean basin and the accretion of the Gondwana‐derived Qiangtang terrane to Eurasia. We conducted geological mapping, regional stratigraphic analyses, and U‐Pb geochronology of detrital zircons (n = 4128) on the Mesozoic sequences exposed in the Hoh‐Xil‐Songpan‐Ganzi complex, Kunlun terrane, and Qiangtang terrane. We identify for the first time marine silciclastic sandstone and shale of Jurassic age in the northwestern Hoh‐Xil‐Songpan‐Ganzi complex that unconformably overlie Upper Triassic turbidites. Zircon age data indicate that the Middle–Upper Triassic marine gravity‐flow deposits of the Hoh‐Xil‐Songpan‐Ganzi complex were shed from the North and South China blocks, and Middle–Late Triassic ultrahigh‐pressure Qinling–Dabie orogenic belt, as well as the Kunlun and Qiangtang terranes. In addition, the detrital zircon results suggest vast sediment source to sink distances (>1500 km) for the Middle–Upper Triassic Hoh‐Xil‐Songpan‐Ganzi strata, which is consistent with tectonic models for the Paleo‐Tethys Ocean basin that incorporate significant components of horizontal tectonic transport like opening of large back‐arc basins in response to oceanic slab rollback.
We report geochronologic, whole‐rock geochemical and Sr–Nd isotopic analyses of the granites that are exposed to both the north and the inside of the Bangong–Nujiang Suture (BNS) zone as well as the implications for the Mesozoic history of Tibet. To the north of the BNS, the Larelaxin pluton consists of I‐type quartz diorite and highly fractionated I‐type biotite granite. The Larelaxin pluton is enriched in large‐ion lithophile elements (LILE) but depleted in high‐field‐strength elements (HFSE); therefore, it exhibits the features of volcanic arc rocks. The initial Sr (0.7102 to 0.7215) and negative εNd (t) (−2.91 to −5.20) values imply a mixture of depleted mantle and continental crust. The mean 206Pb/238U zircon age is 168 Ma; we therefore propose that the Bangong–Nujiang Ocean (BNO) had already been subducted beneath the Qiangtang terrain by the middle Jurassic. Inside the BNS, the Rutog granites intruded into the Lagongtang and Duoren formations, which show a continental margin and a forearc basin sedimentary facies, respectively. The mean 206Pb/238U zircon age is 101 Ma. The Rutog granites are monzogranites with a high Na/K ratio (Na2O/K2O > 1) and a high LILE/HFSE ratio, and A/CNK < 1.1. The high Sr/Y ratio (22 to 56) implies that these granites are adakitic. The low initial Sr (0.7044 to 0.7055) and positive εNd (t) (+1.46 to +2.70) values indicate that the protolith of the Rutog granites originated mainly from a depleted source. We attribute the Rutog plutonism to the development of an oceanic arc during the continuing northward subduction of the BNO and propose that the Rutog adakitic granites were formed by melting of the subducted BNO crust with limited crustal contamination. Copyright © 2013 John Wiley & Sons, Ltd.
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