Petrogenesis And Geodynamic Processes Of Late Cretaceous-Miocene Magmatic Rocks In The Southern Lhasa Subterrane,Tibet

The continuous collision between the India and the Eurasia plates since the Cenozoic has led to the formation of the youngest and most spectacular collisional orogenic belt of the Tibetan Plateau in the world.The formation and evolution of this collisional orogenic belt has always been one of the hottest issues in Earth-science research.The Lhasa terrane of the southern Eurasia is the closest tectonic unit to the India plate,which was the most affected area during the collisional event.Especially in the southern Lhasa subterrane,the widespread Cretaceous-Miocene Gangdese granitoids and Paleogene-Eocene Linzizong volcanic rocks have always been the focuses of research.These magmatic rocks well recorded the complex processes of the pre-collision,collision,and post-collision between the India and Eurasia.Therefore,they are the key to reveal the evolutional history of New-Tethyan oceanic slab subduction,Indian-Asian collision and Plateau uplift processes.However,the petrogenesis and geodynamic processes of the Gangdese granitoids and the Linzizong volcanic rocks have not been well constrained yet,which would hamper our understanding of their material source and magmatic process resulting from the New-Tethyan oceanic slab subduction and the following continental collision.In this paper,combining with field works and previous research,we chose the Late Cretaceous pre-collisional diorites in Namling area,the Miocene post-collisional adakitic dikes in Xigaze area,and the Paleogene collisional Dianzhong formation volcanic rocks of the Linzizong volcanic rock series in the Linzhou basin as our research objects.For better revealing their petrogenesis and dynamic processes,here we conduct detailed petrologic observation,zircon U-Pb geochronology study,whole-rock geochemical and Sr-Nd-Mo isotopic research for these magmatic rocks and combined with published data.Our main progresses are as following:?1?The formation age of pre-collisional Namling diorites is ca.94.25?92.27 Ma,which is consistent with previous reported U-Pb ages of gabbros-gabbro diorites in same area.These Late Cretaceous gabbros-diorites are normal arc magmatic rocks with almost consistent Sr-Nd isotopic compositions and part of coeval adakitic rocks reported in the southern Lhasa subterrane also show relatively consistent Sr-Nd-Hf isotopic compositions.Our elemental and isotopic analysis suggests that these Late Cretaceous arc magmatic rocks?gabbros-diorites and adakitic rocks?were generated by partial melting of mélanges in different depths of mantle wedge zone,rather than the partial melting of metasomatized mantle wedge.Mélange rocks?including oceanic basalt?MORB?,sediment and mantle wedge peridotite?were fully physical mixed in the subduction channel?the interface between the subducted slab and the mantle wedge?at first.Part of mixture was transferred as diapir or plume into shallower mantle wedge and experienced melting process to generate the Namling Late Cretaceous gabbros-diorites but without adakitic character.While other mélange rock mixture was transfer to deeper subduction channel and melted to produce partial of the coeval adakitic or adakitic-like rocks.The formation of these distinct arc magmatic rocks was likely caused by southward retreat of the Neo-Tethyan oceanic slab during the Late Cretaceous.During this retreating process,the upwelling hot asthenospheic mantle and hot corner flow could provide enough heat source to melt mélanges to produce distinct Late Cretaceous arc magmatic rocks in south Tibet.?2?The post-collisional Xigaze dikes were formed in Miocene with zircon U-Pb ages of ca.14.78?10.31 Ma.These dikes contain enriched Sr-Nd isotope compositions and typical adakitic geochemistry features,and were mainly derived from partial melting of the thickened and juvenile mafic lower-crust of Lhasa terrane.Basing on their Na₂O,K₂O contents and Na₂O/K₂O ratios,the Xigaze Miocene dikes can be divided into K-rich dikes and Na-rich dikes.Enriched Sr-Nd isotope compositions of the two types of dikes demonstrate different degrees of involvement of the ancient India continental crust during their formation process.Besides,the Na-rich dikes contain higher MgO,Cr,Ni and Na₂O contents,which imply the contribution of asthenosphere mantle materials to their magma source.Together with published data,our research suggests that different degrees of crust-mantle involvement led to different geochemical characteristics of Late Oligocene-Miocene adakitic rocks in south Tibet.Because these adakitic rocks were controled by S-N trending fault systems and also combine with previous geophysical evidences,we suggest that the Late Oligocene-Miocene magmatism in southern Lhasa subterrane were triggered by the tearing of subducted India continental slab.?3?The collisional Dianzhong formation volcanic rocks of the Linzizong volcanic rock series exposed in LinZhou basin were all emplaced at ca 62.1?60.9 Ma,which is consistent with the previous reported age data.The petrogenesis of the Dianzhong formation volcanic rocks remains controversial.For example,Mo et al.?2007,2008?suggested the Dianzhong formation volcanic rocks were derived from partial melting of Neo-Tethyan oceanic crust and overlying terrigenous sediments under amphibolite facies,which is the most representative viewpoint at present.However,our elemental and isotopic?Sr-Nd-Mo?evidences support that the Dianzhong formation andesites were more likely derived from diapir melting of mélange rocks.These mélanges experienced fully physical mixing in the subduction channel and then transferred as diapirs or plumes into shallower and hot mantle wedge zone where could melt the mixture and generate the Dianzhong formation andesites.The upwelling hot asthenospheric mantle and hot corner flow were the main thermal sources during this partial melting process.This geodynamic process was controlled by the southward retreat/rotation of subducted Neo-Tethyan oceanic slab during the Paleogene.?4?The three stages of magmatic rocks were produced under different geodynamic processes,and they recorded a complex process from oceanic subduction to continental collision.Different materials and magmatic processes were involved in their formation.