Geochemistry Of The Cenozoic Bamaoqiongzong Volcanic Rocks In Qiangtang And Its Tectonic Evolution Of Lithosphere

Cenozoic volcanic rocks, which are widely distributed in Qiangtang area, northernTibet, record the tectonic evolution of this area, providing important petrologicalconstraints on uplift mechanisms and geodynamic models for the Tibetan Plateau. Thisstudy is now analysised alkaline potassic-ultrapotassic Cenozoic volcanic rocks fromBamaoqiongzong by data about whole-rock major, trace element and isotopiccompositions. Research into the genesis of olivine, phlogopite, clinopyroxene megacrystsand phenocrysts minerals found in the volcanic rocks employed electronic probe and laserICP-MS techniques. And it also comprehensive analysed the magmatic processes and thenature of the source region of the alkaline potassic-ultrapotassic volcanic rocks fromBamaoqiongzong. Based on the foundations above mentioned, combining with the latestresults of geophysical research, lithospheric tectonic evolution in the northern Tibet wereto be discussed. This study provides new geochemical constraints for the relationshipbetween the northern Tibet Cenozoic alkaline potassic–ultrapotassic volcanic rocks originand tectonic evolution of the lithosphere.These alkaline potassic-ultrapotassic volcanic rocks from Bamaoqiongzong formedduring Paleogene Oligocene (between19~34Ma), including volcanics that are dominatedby basanite–tephrite,phonotephrite,tephriphonolite,phonolite,trachybasalt,alkaline trachyte.They have high alkali content,and Na2O/K2O is0.33~3.33，with an average of0.87.The main rock types are potassic volcanic rocks,belonging to shoshonitic series.The Mg#of ofwhole-rock has large variation range,up to71.08;∑REE=825.84×10-6~1720.11×10-6, withan average of1241.2×10-6, LREE/HREE=20.73~408.72. The Bamaoqiongzong volcanicrocks have chonodrite-normalized trace element variation diagrams that are characterizedby significant light rare earth elements enrichments and fractionation between LREE andHREE is obvious, with chonodrite-normalized values decreasing to the right of thediagrams. They exhibit only weak negative Eu anomalies and positive Eu anomalies,（La/Yb）N=40.9~122.13，(Ce/Yb)N=27.75~81.45，δEu=0.74~1.38.In εNd(t)-87Sr/86Sr illustration, Sr-Nd isotopic composition have a negative εNd(t)values (-0.45~-11.39) and high87Sr/86Sr values (0.7079~0.7094). From basanite tophonolite, εNd(t) values show a trend of gradually increasing.87Sr/86Sr values did notchange significantly, suggesting that magma has a complex evolution.Mineralogical study shows that olivine and phlogopite megacrysts are xenocrysts.The chemical compositions of olivine megacrysts exhibit magmatic genesis, andphlogopite megacrysts are obtained from mantle. The genesis of Clinopyroxenemegacrysts is the crystallization of early magma. Clinopyroxene phenocrysts in basanite-tephrite have a complex zoning patterns, and part of the core shows green. They are called"green-core clinopyroxenes". Composition and structural characteristics of these"green-core clinopyroxenes" reflect the homologous alkaline potassic magma occurredmagma replenishment-mixing process in crustal magmatic chamber,and complex zoningstructure record crystallization process of mixed magma.Sr-Nd isotopic composition variation reveals the replenishment magma in the crustchamber is significantly different with pre-existing magma magma on the Nd isotopiccomposition. The Bamaoqiongzong alkaline potassic-ultrapotassic volcanic rocks containhigh concentrations of Sr and Nd elements, which are some11.5and5.46times greaterthan granulite xenoliths from Cenozoic Qiangtang volcanic field respectively. Thesecharacteristics preclude the possibility of generation of the magmas that formed the Bamaoqiongzong volcanic rocks by assimilation juvenile crust. The value of εNd(t)increase with magmatic evolution, and reflecting OIB-type asthenosphere mantleContributed to the melting process of replenish magma. Basanite and tephrite arecharacterized by high Mg#and Cr content similar to parental magma, the Sr-Nd isotopiccomposition indicates magma originated from ancient enriched lithospheric mantle formedby subduction.Recent seismic tomographic image and temporal and spatial distribution of volcanicrocks in northern Tibet reveal that, cold mantle downwelling in southern Tibet and mantleupwelling flow in west Kunlun-Hoh Xil were the unified mantle convection system, whichwas driven by Neo-Tethys and subduction fragments of Indian continental plate slab off.With the continued shortening of the Asian plate, the front of the Indian continentallithospheric continues to migrate northwards. Fowlling the continuous stream of coldmantle downwelling expand northward, and low-velocity mantle in northern Tibetcontinues to migrated northward. Magmatic activity in Bamaoqiongzong suggests thattoday the west Kunlun-Hoh Xil upper mantle low veolocity body was beneath theQiangtang during34~19Ma.