Early Oligocene anatexis in the Yardoi gneiss dome, southern Tibet and geological implications

Abstract

The Yardoi gneiss dome is located to the easternmost of the North Himalayan Gneiss Dome (NHGD), southern Tibet. It consists of metapelite, garnet amphibolite, granite and leucogranite, and is a key subject to constrain the formation and tectonic evolution of NHGD. SHRIMP zircon U/Pb data on the leucogranite yield an age of 35.3±1.1 Ma, which is substantially older than that of the similar leucogranites to the west. Sr and Nd isotope systematics indicate that this leucogranite was derived from partial melting of the mixed garnet amphibolite and metapelite. Our data suggest that (1) during the early stage of Himalayan magmatism, amphibolite dehydration melting overwhelmed that of the metapelite; and (2) such a melting at middle-lower crust might be a major factor that initiated the movement along the Southern Tibetan Detachment System (STDS). leucogranite, metapelite, crustal anatexis, Himalayan Orogenic Belt, North Himalayan Gneiss DomeConvergence between the Indian and Eurasian plates resulted in intensive structural deformation, high-grade metamorphism and crustal anatexis in the Himalayan Orogenic Belt. In southern Tibet, the post-collisional tectonics is represented by the development of concurrent N-S compression and extension [1][2][3][4][5] , and intensive crustal anatexis [6][7][8][9][10] which formed two subparallel granitic belts separated by the Southern Tibetan Detachment System (STDS), e.g. the High Himalayan Granitic belt (HHG) to the south, and the North Himalayan Gneiss Dome (NHGD) to the north (Figure 1(a)). They have preserved the records on the nature of deformation, high-grade metamorphism, crustal anatexis and near-surface processes associated with the active continental collision between the Indian and Eurasian plates. Documenting the timing and nature of these processes is essential to addressing the way of interplay and degree of coupling between these processes. A large number of studies have been directed toward better understanding different aspects of the HHG and yielded abundant meaningful information with regard to how the crust behaved during the collision [1][2][3][4][5][6][7][8][9][10][11][12] . However, much less effort has been made on the NHGD.Gneiss and amphibolites within the NHGD commonly experienced amphibolite to granulite facies metamorphism at T > 800℃, and P > 800 MPa. Partial melting at these conditions is widespread and represented by the occurrence of migmatite and leucogranites [13][14][15][16] . Available data show that these high-grade metamorphic rocks and leucogranites have similar mineral and isotope compositions to those in the HHG, and experienced similar deformation. They were considered as the northern extension of High Himalaya Crystalline Sequence (HHCS) [16] . Both suites of rock from NHGD and HHG have been regarded as a mid-lower crustal channel [17,18] . Metamorphic rocks and granites in the NHGD are structurally lower than STDS. Knowledge of the relationship between NHGD and STDS is critical to test tectonic models for the evolution of southern Tibet

Episodic crustal anatexis and the formation of Paiku composite leucogranitic pluton in the Malashan Gneiss Dome, Southern Tibet

“…The co-existence of these zircon overgrowth textures in the mantle domains at 33.6±0.6 Ma indicates that the source regime for the tourmaline leucogranite had experienced metamorphism under P-T conditions higher enough to induce partial melting at the same time. This event could correspond to crustal anatexis at ~35.3 Ma during the tectonic transition from compressive shortening to extension in the Yardoi dome at the eastmost of NHGD [12], migmatization at 35.0±0.8 Ma in the Mabja Dome [19], and the earliest phase of partial melting at 36.5±2.2 Ma recorded in the deformed granites within the STDS in the Gyirong areas [20]. Similar to zircon grains from leucogranites along the Himalayan belt, zircons from sample T0659-A also contain up to 2000 ppm U.…”

“…All these rock units are separated by ductile detachment fault. Except for the Kangmar Dome, all granites within the NHGD are younger than ~44 Ma [3,4,8,12,13,16,19,26,30]; whereas the granites within the HHCS formed at 37-10 Ma and are characterized by apparently lower melt temperature [3].…”

“…Experimental results have demonstrated that partial melting of Formation-I kyanite-bearing metapelite indeed can produce melts with elemental and isotopic compositions resembling the Himalayan Cenozoic leucogranites [6,7], however, increasing number of updated studies have documented that episodic anatexis occurred in the Northern Himalayan Gneiss Domes (NHGD) as well as in the High Himalayan Crystalline Sequence (HHCS) since the continental collision between India and Eurasia. These anatectic episodes include (1) dehydration melting of a source consisting dominantly of amphibolite with subordinate pelitic gneiss at thickened crustal conditions [8][9][10][11][12][13][14]. These melting events are represented by older than 35 Ma peraluminous granitoids with relatively high Na/K and Sr/Y ratios; (2) fluid-present melting of metapelite since ~38 Ma to produce granitic melts with high CaO and Sr contents and low Rb/Sr ratios [15][16][17][18]; and (3) late Eocene to early Oligocene anatexis recorded in syn-collision leucogranites and migmatites in the Gyirong area and the Mabja Gneiss Dome [19][20][21].…”

“…This pluton consists of tourmaline leucogranite, two-mica leucogranite, and garnet-bearing granite. In order to narrow down the formation age and characterize the mineral and geochemical composition of this leucogranitic pluton, we have sampled along traversals across this pluton as shown in Figure 1(b) and conducted bulk-rock major and trace element and radiogenic isotope (Sr and Nd), as well as Figure 1 (a) Simplified geologic map of the Himalayan orogenic belt, southern Tibet (after Zeng et al [12]); (b) simplified geological map of the Malashan Gneiss Dome (after Aoya et al [24]). YTS, Yarlung-Tsangpo suture; STDS, Southern Tibet Detachment System; MCT, Main Center Thrust; MBT, Main Boundary Thrust; LH, Lower Himalayan Crystalline Sequence.…”

Early Cretaceous high-Ti and low-Ti mafic magmatism in Southeastern Tibet: Insights into magmatic evolution of the Comei Large Igneous Province

“…Since the onset of collision between the Indian and Eurasian plates, these sediments have undergone multiple phases of structural deformation, sub-greenschist facies metamorphism, and intrusion by magmas of basaltic to granitic composition (Aikman et al, 2008;Qi et al, 2008； Zeng et al, 2009, 2011a& b, 2012a, 2015bHou et al, 2012;Gao et al, 2009Gao et al, & 2012Xie et al, 2010;Webb et al, 2013;Diedesch et al, 2016). Around the Dala area, cross-cutting relationships show that there are at least four episodes of magmatism (Figs.…”