| The origin of enriched components in time and space of magmatism in collision zones provides critical insights into the mantle dynamic processes associated with subduction-and collision-related events.However,determining the origin of such enriched components remains a significant challenge.Here,we report a comprehensive dataset of the Linzizong volcanic succession(LVS)from four locations with discernible distance different to the Indus-Yarlung suture in the eastern Lhasa Terrane,and one location from the western Lhasa Terrane,southern Tibet.In the eastern Lhasa Terrane,the LVS rocks in this study can be divided into two groups:a calc-alkaline Group 1(69-55 Ma),mainly including basaltic-andesitic varieties,and a shoshonitic Group 2(52-50 Ma),consisting predominantly of silicic rocks with minor mafic compositions.Group 1 samples are likely derived from the fractional crystallization of primitive basaltic melts as a result of the partial melting of a metasomatized mantle wedge.The composition of such a mantle source was strongly influenced by slab-derived fluids and ancient lithospheric mantle-derived melts as a function of the distance to the suture.For example,samples located near the suture display depleted Sr,Nd,Pb,and Hf isotopic compositions,heavy ?26Mg values,and high Ba/Th and Sr/Th ratios,which are mainly affected by the dehydration of the subducted Neo-Tethyan slab.In contrast,samples located further north to the suture have enriched Sr,Nd,Pb,and Hf isotopic compositions,as well as high Th/La and La/Sm ratios,which can be attributed to the involvement of the ancient central Lhasa lithospheric mantle-derived melts.Group 2 silicic samples are isotopically enriched both near and far away from the suture.This is likely due to the input of crustal materials from the Indian continent and ancient lower crust-derived material in the central Lhasa subterrane,respectively.Group 2 mafic samples have depleted isotopic compositions and are characterized by elevated(La/Yb)N and Dy/Yb ratios as well as low Zr/Nb ratios,indicating an origin of enriched garnet-bearing lithospheric mantle with contributions from asthenosphere-derived materials.Our results show that the tectonic interpretations for the variations in the whole-rock Nd and zircon Hf isotopic compositions towards negative values at-51 Ma should be treated with caution,depending on the distance between the samples and the suture,In the western Lhasa Terrane,the LVS rocks in Yare area were erupted at 65-51 Ma(i.e.,the Dianzhong Formation:64.5-55.7 Ma,the Nianbo Formation:54 Ma,and the Pana Formation:51.4-51.1 Ma).The highly fractionation rhyolites in the Dianzhong Formation show positive zircon vHf(t)and most likely derived from partial melting of juvenile crust.Plagioclase-,K-feldspar-,apatite-and zircon-conctolled fractionation results in the variation of geochemical composition from andesitic to rhyolitic magmas,The Yare basalts and andesitic basalts in the Nianbo Formation most likely derived from the partial melting of an enriched lithospheric mantle,with input from asthenosphere.The Pana Formation dacites can be incorporation of partial melting of potassic-rich lower crust,with input of sedimentsBased on the above results,we suggest that the Dianzhong Formation(68-55 Ma)correspond to subduction-related magmatism,the Nianbo Formation(?55 Ma)are likely consequences of the initial Indian-Asian collision,and the Pana Formation can be interpreted as post-collisional magmatism in response to slab breakoff of the Neo-Tethyan oceanic lithosphere.The tectonic interpretations for the variations in the compositions towards negative values at?55 Ma should be treated with caution,depending on the distance between the samples and the suture.By considering that the transport of Indian continental crust-derived materials from the trench to the magma source region would require time,the continental collision of India and Asia should have initiated early than 50 Ma. |
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