Discovery of two stages of the Early Paleozoic adakitic intrusive rocks in the Kunlun River area, East Kunlun: Implications for collisional orogenic processes

The Early Paleozoic (Silurian to Devonian interval) is the main stage for the closure of the Proto-Tethys Ocean and collisional orogenesis in the East Kunlun. Abundant rocks, including eclogites, A-type granites, mafic-ultramafic rocks, as well as the associated mineral deposits, such as the Xiarihamu Ni deposit and Baiganhu W-Sn deposit, were formed during this period. However, the detail collisional orogenic process and exhumation mechanism of eclogites in the Early Paleozoic remain unclear. The discovery of the Early Paleozoic adakitic intrusive rocks in the Kunlun River area may provide the important evidence for the solution to these questions. LA-ICP-MS zircon U-Pb dating indicates that there are Late Ordovician (446Ma) and Late Silurian (427 similar to 425Ma) adakitic intrusive rocks in the Kunlun River area. The Late Ordovician adakitic rocks are granodiorites and exhibit high SiO2 (67.55% similar to 68.21%), Al2O3 (14.59% similar to 15.89%), and Na2O (4.91% similar to 5.15%), and low (K)2O (1.54% similar to 1.64%). They are depleted in heavy rare earth elements with low Y (7.76x10(-6) similar to 8.61x10(-6)) and Yb (0.67x10(-6) similar to 0.93x10(-6)) and high Sr (484x10(-6) similar to 621x10(-6)), Sr/Y ratios (56 similar to 80), Cr (31.2x10(-6) similar to 38.3x10(-6)), and Ni (19.0x10(-6) similar to 22.5x10(-6)). They have relatively depleted whole-rock Nd isotopic compositions with positive epsilon(Nd)(t) values (+1.1 similar to +1.2). The magmatic zircons of the Late Ordovician (446Ma) adakitic granodiorites have two groups of eHf(t) values: +2.2 similar to +10.6 for most of magmatic zircons and -11.5 similar to -1.3 for some of them. The Late Silurian adakitic intrusive rocks consist of granodiorites and granites. They contain high SiO2 (66.36% similar to 71.14%), Al2O3 (15.5% similar to 19.65%) and Na2O (5.65% similar to 7.68%), but low K2O (0.68% similar to 1.47%). They are strongly depleted in heavy rare earth elements with low Y (1.90x10(-6) similar to 5.02x10(-6)), Yb content (0.16x10(-6) similar to 0.51x10(-6)), high Sr (362x10(-6) similar to 1100x10(-6)), and Sr/Y ratios (130 similar to 514). They have relatively enriched whole-rock Nd isotopic compositions with negative epsilon(Nd)(t) values (-1.45 similar to -1.24). The magmatic zircons from Late Silurian (427 similar to 425Ma) adakitic intrusive rocks have variable epsilon(Hf)(t) values ranging from -10.2 to +9.3. We suggest that the Late Ordovician adakitic intrusive rocks were most likely formed by the mixing between coeval subducted ocean crust and ancient continental crust-derived magams during ascent, whereas the Late Silurian adakitic intrusive rocks were generated by partial melting of the thickened lower crust. Taking into account regional geological data, we suggest that, in the East Kunlun, the subduction and partial melting of oceanic crust happened during Late Ordovician (446Ma), and several other processes including the continental collision, slab break-off, exhumation of the high pressure-ultra high pressure (HP-UHP) metamorphic rocks occurred simultaneously during 436 similar to 425Ma, which triggered the generation of adakitic, mafic-ultramafic, and A-type granitic magmas