Chronology,Geochemistry And Tectonic Setting Of Neoproterozoic Intrusive Rocks In The Yuanbazi Area Of Lueyang,Southern Shaanxi

The Neoproterozoic magmatic rocks widely developed in the northwestern margin of the Yangtze plate are considered to be closely related to the aggregation and disintegration of Rodinia supercontinent.Further study on the formation age and tectonic setting of the Neoproterozoic magmatic rocks in this area is of great significance for restoring the Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze plate and providing new effective evidence for the Rodinia supercontinent amalgamation and breakup mechanism.Therefore,the Erliba granodiorite and Yuanbazi pluton exposed in Lueyang area of Eastern Bikou micro block are taken as the research objects in this paper.The formation age and tectonic environment of Erliba granodiorite and Yuanbazi pluton are studied through detailed field geological survey and systematic petrography,whole rock geochemistry,isotope chronology and zircon Hf isotope analysis comprehensive research.Combine existing research results,the Neoproterozoic tectonic evolution of the northwest margin of the Yangtze plate is restored,which provides new effective constraints for the amalgamation and breakup mechanism of Rodinia supercontinent.The main achievements and understandings are as follows:1.The Erliba pluton is mainly composed of granodiorite,A/CNK is 1.05?1.53,Moreover,it is enriched in LREEs and LILEs,depleted HFSEs(e.g.Ti,Nb,and Ta),indicating calcalkaline I-type granite affinity;the Yuanbazi pluton is mainly composed of gabbro diorite and granodiorite,gabbro diorite it has lower SiO2,higher Fe2O3T and MgO,and the granodiorite is quasi aluminum-weak peraluminous,with the characteristics of I-type granite.Different rock types are characterized by enrichment of LREEs and LILEs and depletion of HFSEs.2.Isotopic geochronology results show that the average ages of Erliba granodiorite samples are 821.3±5.5Ma and 822.6±5.8Ma,respectively;the crystallization ages of Yuanbazi gabbro diorite and granodiorite samples are 825.9±5.5Ma and 822.2±4.5Ma,respectively,indicating that the emplacement age of Erliba and Yuanbazi plutons is Neoproterozoic.Zircon Lu-Hf results show that the ?Hf(t)of Erliba granodiorite ranges from-14.06 to-7.38,with an average of-9.83,indicating that the magma is derived from ancient crustal materials,and the corresponding two-stage model age is 2247-1909Ma.3.Erliba granodiorite is mainly formed by partial melting of ancient crustal metabasic rocks;Yuanbazi gabbro diorite is the product of fractional crystallization of mantle derived basaltic magma and a certain degree of crustal contamination,and granodiorite is the mixture of evolved mantle-derived basaltic magma and crust derived intermediate acid magma.Comprehensive analysis shows that in the early and middle Neoproterozoic,under the background of continuous oceanic subduction,the subducted melting/fluid metasomatized lithospheric mantle produced basaltic magma underplating the lower crust,which triggered partial melting of metabasic rocks in the lower crust to produce acid melt and forming Erliba pluton.The underplating mantle derived basaltic underwent separation crystallization in the magma chamber,and then crustal contamination occurred in the ascending process to form Yuanbazi gabbro diorite.The evolved mantle derived basaltic magma and crust derived magma mixed to form Yuanbazi granodiorite in the ascending emplacement process.4.Based on the existing research data,this paper divides the Neoproterozoic tectonic evolution of the study area into:(1)Passive continental margin stage(1200-1000Ma);(2)Continuous oceanic crust subduction stage(950-720Ma);(3)Arc-continent or continentalcontinent collision stage(720-700Ma);(4)Extensional cracking-stable deposition stage(700541Ma).