Chronology,Geochemical Characteristics And Tectonic Significance Of Manita Quartz Diorite In Inner Mongolia

The Manita quartz diorite is located in the northeast of Xiwuzhumuqinqi,Inner Mongolia,and its tectonic position is located in the Hegenshan suture zone in the eastern part of the Xingmeng Orogenic Belt.The tectonic magmatic activities are strong,and ophiolite are discontinuous distributed.In this paper,based on the field geological survey of the Manita quartz diorite,a study is made on its petrological,geochemical and chronology characteristics.This paper summarized its petrological and geochemical characteristics,explored the formation causes and tectonic environment of the Manita quartz diorite.The characteristics and recognition provide petrological and chronologic evidences for the extension time limit of Erenhot-Hegenshan oceanic basin and the process of ocean subduction and extinction of the Paleo-Asian Ocean.The Manita quartz diorite consists mainly of coarse-medium quartz diorite and medium-fine quartz diorite produced as a rock plant,occurred in the northern part of Diyanmiao SSZ ophiolite.The Manita quartz diorite are strongly mylonitized with rhombic grid like strong deformation zones and weak deformation domains,mainly composed of plagioclase(65%±),quartz(15%±),hornblende(15%±)and biotite(<5%).The geochemical characteristics reveal that the rock is characterized with Si O₂(65.56%?75.16%),A1₂O₃(13.54%?16.24%),Mg O(0.33%?1.70%),with rich in Na₂O(4.44%?5.74%)and poor in K₂O(1.60%?2.71%)and high ratio of Na₂O/K₂O(1.64?3.33),belongs to Calc alkaline series.The rock have features of low?REE(36.78×l0^-6?70.10×l0^-6),whereas REE fractionation is clear((La/Yb)_N=5.49?10.11),without obvious Eu anomaly(?Eu=0.89?1.18).The rock is also characterized with relatively enriched large ion lithophile elements(LILE),and depleted high field strength elements(HFSE).All of the geochemical characteristics indicate that the quartz diorite has the characteristics of island arc magmatic rocks.The crystallization age of the Manita quartz diorite obtained by LA-ICP-MS zircon U-Pb dating method is 315.76±0.94 Ma,which reveals that the formation age of rock is Late Carboniferous.In the tectonic environment discrimination diagram,all the samples are in the granite of volcanic arc zone,combined with regional tectonic location and regional tectonic evolution,and Manita quartz diorite,Diyanmiao SSZ ophiolite,fore arc basalt and Baiyinhushu trondhjemite form the intra oceanic subduction rock assemblage.The characteristics and recognition indicate that the eastern part of the Paleo-Asian Ocean was not closed in the Late Carboniferous and was in the process of subduction and extinction.