Origin Of Water-bearing Igneous Complex In The Comei Large Igneous Province,SE Tibet

Large igneous provinces(LIPs)are dominated by extensive synchronous basaltic rocks with minor silicic rocks,and have high mantle potential temperature and lower water content.But the Comei LIP which exposed in the Eastern Himalaya,SE Tibet,display diverse lithologies,including basalts,diabase dikes/sills,gabbroic intrusions and minor picritic porphyrites,pyroxenites,diorite dikes/sills and silicic volcanic rocks.Such diverse lithologies in the Comei LIP provide a good example to investigate the origin of compositional diversity and processes of magamtic evolution.Previous studies mainly focused on distribution of igneous rocks,regional geochronological frame and petrogenesis and tectonic setting of the basaltic rocks,while the petrogenesis of picritc porphyrites,pyroxenites and intermediate rocks,the composition and melting conditions of primitive magma,and processes of magmatic evolution were less studied.Therefore,this study selected picritic porphyrites and associated basalts in Cona area and pyroxenites,gabbros,and diorites in Chigu Tso region in order to reveal their petrogenesis,the primitive composition and melting conditions,the processes of magmatic evolution,the relationship between different rock types and the origin of compositional diveristy.The main conclusions of this thesis are as follows:(1)By deducting the accumulated olivines,we determined a parental magma MgO content for the picritic porphyrites of approximately 20% MgO,corresponding to a mantle potential temperature of >1550 °C for the parental magma.Such a high temperature provides solid evidence for a mantle-plume origin of the Comei LIP.Similar Nd and Os isotopic characteristics between the picritic porphyrites and associated basalts suggest a common hot mantle source region for their generation.(2)The Chigu Tso intrusive suite from the middle part of Comei LIP displays highly diverse lithologies,including pyroxenite,gabbro,amphibole gabbro,diabase,diorite,quartz diorite and tonalite.Zircon U-Pb dating of two dioritic samples give ages of approximately 133 Ma which is identical to the age of Comei LIP(132 ± 2Ma).According to their ?Nd(t)value,this suite could be divided into two series: Pyroxenites and Group I gabbros and Group II gabbroic rocks and Diorites.Pyroxenites and group I gabbros exhibit negative ?Nd(t)value,trace-element and isotopic modelings sugget that they originated from primitive magma by crustal contamination and crystal fractionation.Group II gabbroic rocks and diorites have uniform Nd isotopic features which are similar to that of basaltic rocks in Comei LIP which represent a completed basic to intermediate fractional crystllization sequence in Comei LIP.(3)The caculations of clinopyroxene-melt thermobarometer and hornblende thermobarometer and hygrometer of the basaltic and intermediate rocks in Cona and Chigu Tso area indicate that there are at least two staging magma chambers in the crust.The lower crustal magma chamber(20 ~ 30 km)provides room for primitive magma conducting olivine-pyroxene fractional crystallization and/or crustal contamination,and formed the Cona picritic porphyrites,evolved basalts and the Chigu Tso pyroxenites and group I gabbros.The upper crustal magma chamber(approximately 6 km)facilitates clinopyroxene-hornblende-plagioclase-titanite fractional crystallization of water-rich evolved basalts,and formed the group II gabbroic rocks and diorites.Water content is one of the key factors which controlling the basatic rocks fractional crystalllization to intermediate rocks.