Petrogenesis And Magma Formation Dynamics Of K-Feldspar Megacrysts Granites In Gangdese Belt, South Tibet

The formation,storage,and evolution of magmas in the subduction zones and continental collision belts are essential for understanding the construction of continental crust.The classic magma chamber was thought to be a pool with high melt volumes.Recent model about crystal-rich magma mush evolution has been proposed but it is still hotly debated.Further study of crystals can help us understand the dynamics of mush rejuvenation.The formation of the Tibet Plateau had experienced complex tectonic evolution processes including Proto-Tethys,Paleo-Tethys,Neo-Tethys and the continent collision between India and Eurasia plates.The widely exposed granites and volcanic rocks with crystallization age of 100-80 Ma and 65-40 Ma in the Gangdese belt,representing the subduction of the New-Tethys mid ocean ridge and the subsequent continent collision between India and Eurasia plates.For the past decades,researchers have had clarified the tectonic evolution of the Tibet Plateau.These studies are mainly about whole-rock geochemistry,U-Pb geochronology and Sr-Nd-Hf isotope analyses.Many studies about progenesis,magma source and magmatism dynamic have been published but few are about the assembly of magma and evolution of magma reservoir.Thus,the widely spread igneous rocks in the Gangdese belt provides us a good natural laboratory to explore the nature and evolution of magma reservoirs.In this study,we carry out an integrated study of field geology,petrography,LAICP-MS U-Pb zircon geochronology,geochemistry,and mineral composition,together with quantitative textural analysis,in-situ trace element and Pb isotope analysis for Kfeldspar megacrysts from the monzogranites and mafic microgranular enclaves(MMEs)of the Quxu batholith,Nyemo bathlith and Chabu batholith,in the Gangdese belt,southern Tibet.We aim to investigate the petrogenesis of the these monzogranites,the origin of the K-feldspar megacrysts and the implications for the evolution of magma reservoir anddynamic of mush rejuvenation.We will further discuss the geodynamic implications.The main results related to this study are presented below:1.The Eocene Quxu monzogranites have a magma crystallization age of ~50 Ma.The monzogranite is I-type granite with arc magma characteristics.The Kfeldspar/plagioclase in monzogranite and MME have complex zoning and their Pb isotope show large variation(206Pb/204 Pb = 18.41-18.80,207Pb/204 Pb = 15.42-15.75 and 208Pb/204 Pb = 38.29-39.06).The magma source was formed by partial melting of newly formed crustal material.The contribution of India magma could exist.The Eocene Nyemo monzogranites have a magma crystallization age of ~50 Ma is I-type granite with arc magma characteristics.The feldspar megacrysts from the Nyemo monzogranites and MMEs usually have reverse zoning and oscillatory/no zoning with middle Pb isotope ratios.The Nyemo monzogranites were formed by partial melting of newly formed crustal material with no contribution of India plate crust.The Miocene Chabu granites(~15Ma)have northern and southern units.The northern monzogranites have middle Sr/Y while the southern granites have high Sr/Y.The K-feldspar megacrysts from the Chabu granites show oscillatory zoning or no zoning with high Pb isotopic composition.The magma was formed by partial melting of newly formed crustal material which are modified by the ancient Indian crust.2.The K-feldspar/plagioclase megacrysts from the Quxu monzogranites and MMEs can be divided into three types: normal zoning,reverse zoning and oscillatory zoning or no zoning.The Pb isotopes variation usually couples with the An/Or values of the feldspars.The feldspar megacrysts from the Nyemo monzogranites and MMEs usually have reverse zoning and oscillatory/no zoning with middle Pb isotope ratios.Most feldspar megacrysts from the monzogranites have middle Pb isotopic composition,feldspar megacrysts from the MMEs have relatively lower Pb isotopic composition.The K-feldspar megacrysts from the Chabu granites show oscillatory zoning or no zoning with high Pb isotopic composition.All these features indicate that the formed in a melt-present environment.The CSD diagram suggest that these K-feldspar have large character length.The duration time of the Quxu megacrysts,Nyemo megacrysts and Chabu megacrysts are 0.14-0.33 to 4.1-52 k.yr,0.26-0.3 to 7.5-62 k.yr and 0.15-0.52 to 4.2-82 ky.r,suggesting that megacrysts are prolonged.The Quxu and chabu K-feldspar formed in a batch system,the Nyemo K-feldspar formed by crystals accumulates.3.In the magma reservoir,crystals could be transported or injected into different batches by the magma pulse and the magma mush is rejuvenated.During the assembly of multiple magma pulses,K-feldspars are prolonged and finally grow to megacrysts.The injection of magma pulses could be an important mechanism for the formation of Kfeldspar megacrysts.Thermal cycling and magma convection caused by the repeatedly injection of hot magma pulse could be the main factor leading to the rejuvenation of the solidifying mush.With the variation of magma flux,the magma processes like crystal accumulation and growth,rheological locking and rejuvenation,magma injection and extraction could occur repeatedly.4.Combined with previous research.The subduction of the Indian plate oceanic crust and subsequent continent collision between India and Eurasia plates occurred at 65-40 Ma.With the break-off of the subducting India plate oceanic crust,a series of syncollisional rocks were produced.Subsequently,magmatic activity weakened,and the lithosphere of the Tibet Plateau was gradually transformed by Indian crustal materials.At about 30-10 Ma,with the delamination of the thickened lower crust of the Tibet Plateau,a series of high Sr/Y magmatic rocks were formed.