Petrogenesis Of The Cenozoic Alkaline Potassic-ultrapotassic Volcanic Rocks From Qiangtang,Northern Tibet

With the continuous deepening of multidisciplinary research on the Tibetan Plateau and the progress of global continental dynamics research,the lithosphere tectonic evolution and deep dynamics of plateau uplift have become the focus of theoretical studies on the collision dynamics of the Tibetan Plateau.The volcanism that plays key role during the lithosphere tectonic evolution and plateau uplift is receiving widespread attention in the academic community.The study area of the Qiangtang area in northern Tibet is the volcanic belt with the earliest occurrence of volcanic activity in the Cenozoic.In addition,it develops most complete volcanic series,which records the important mechanisms of post-collisional volcanic rock formation and tectonic evolution.In this paper,the alkaline potassic-ultrapotassic volcanic rocks since 30Ma and high potassium calc-alkaline basalt?36-44Ma?in the Qiangtang area are highlighted.Precisely,Series volcanic rocks of basanite,tephrite,tephri phonolite,phonolite,alkaline trachyte were yielded in the Huochetu o area.Whereas to the Yulinshan area,basanite and basic tephrite were missing,comparing to Huochetuo;and the Gemucuo area miss the phonolitic basanite and tephri phonolite.Technically,it refers to the major,trace,isotopic geochemistry and petrologic study,as well as experimental petrologic study.The major and trace geochemical data reveals that the Cenozoic volcanic activity in Qiangtang area not only has the general rule of the migration from south to north with time,but also the chemical composition shows the regular patterns of magmatic changes:from sodium alkaline basalt to high K calc-alkaline series,and then the basanite.Xeno crystals of olivine and clinopyroxene,with small amount of phlogopite,are observed in these Cenozoic alkaline potassic-ultrapotassic volcanic rocks.Electron microprobe and X-ray energy spectrum techniques were used to systematically detect the mineralogical and geochemical characteristics of olivine and near-rhombic quartz.It is noteworthy that these quartz was yielded in the olivine xeno.It shows that the near-rhombic quartz,fine quartz vein and vein of Fe-Mg carbonate are all related to peridotite alteration by CO₂ fluids.In this dissertation,the high temperature and high pressure rock dehydration melting experiment was conducted to focus on leucite-bearing basanite near-liquidus phase of 1.0-3.0GPa pressure,which hold characteristics of parent magma in the Cenozoic alkaline potassium-ultra-basic volcanic rocks.The result shows that:?1?when the pressure is less than 1.6 GPa,olivine is the first mineral that emerges near liquidus,followed by a combination of monoclinic pyrite,calcite,biotite,and albite with decreasing temperature.?2?the association of olivine+monoclinic pyroxene near liquidus would shift into clinopyroxene when pressure is in the range of 1.6¹.9Gpa.This process is similar to those of the dehydration melting experiment conducted on meliliteolibine nephelinite and potassic basanite,respectively.However,the shift temperature seems vary in small range according to chemical composition.In the case of H₂O=0.57,CO₂ supersaturation,the shift pressure is 1.6 GPa and 2.0 Gpa respectively for the melilite olibine nephelinite and potassic basanite.The melting experiment result display that leucite-bearing basanite in Qiangtang would originated from different magma champs in different temperature and pressure via partial melting as following:?1?phlogopite-bearing peridotite magmatic source under a pressure of less than 2.0 Gpa;?2?phlogopite-pyroxenite magmatic source partially melted at a pressure greater than 2.0 Gpa;?3?Orla ndo's experimental results on potassic basanite showed that garnet and pyroxene arise near the liquidus at a pressure greater than 3.4 Gpa,it suggest that partial melting of the source region of phlogopite-containing garnet pyroxenite would be a possible magmatic source at a pressure greater than 3.4 GPa.Comprehensive geologic and geochemical theoretical calculations have put forward two constraints on the selection of magma source composition:?1?the current lithosphere thickness limits the burial depth of leucite-bearing basanite magma produced in the Quaternary of the western Kunlun;?2?Constraints would also from REE composition of leucite-bearing basanite.Finally,protoliths of leucite-bearing basanite probably the phlogopite-hornblende pyroxenite under pressure greater than 3GPa.This result provides important experimental petrology and geochemistry theoretical basis for the study of the genesis of Cenozoic potassic-ultrapotassic volcanic rocks.Geochemical and Sr-Nd isotopic studies has been determined that the Cenozoic alkaline potassic-ultrapotassic volcanic rocks would originated from enriched mantle with EM2 properties,indicating the magma source in Qiantang area experienced a shift from depleted asthenosphere mantle to EM2.This dissertation synthesizes data of experimental geology,geochemistry,and geophysics.It also involve creep rate of lithospheric mantle,aiming to determine the transformation relations and conditions of asthenosphere and lithosphere mantle.In addition,the relationship between the origin of alkaline potassic-ultrapotassic magma and the lithospheric tectonic evolution in Qiantang was discussed.It comes to that the post-collision alkaline potassic-ultrapotassic volcanic rocks in northern Tibet was triggered by deep-buried mantle materials upwelling and lithospheric mantle pulsation thickening-thinning melting,which were induced by lithospheric subduction of the Indian continent and the subsidence of the early plate-breaking.This conclusion provided a new idea for the study of the deep dynamic mechanism of lithosphere tectonic evolution during the post-collision of the Tibetan Plateau.