贺根山洋闭合与后造山伸展——来自科右中旗构造混杂岩带内晚三叠世花岗岩的证据

Hegenshan ocean basin closure and post-orogenic extension——Evidences from Late Triassic granite in Horqin Right Middle Banner tectonic melange belt

内蒙古科尔沁右翼中旗巨日合A2型二长花岗岩位于贺根山缝合带东段,侵位于早石炭世杜尔基构造混杂岩(带)中。笔者等通过对其进行系统的野外地质特征、岩石学、地球化学和锆石U-Pb年代学研究,旨在厘定岩体成因和构造背景,为古亚洲洋分支洋盆—贺根山洋闭合时限提供证据。巨日合二长花岗岩主要由细粒二长花岗岩和中粒二长花岗岩组成。LA-ICP-MS锆石U-Pb测年结果显示巨日合二长花岗岩侵位于晚三叠世(203.7±1.6 Ma)。地球化学研究显示,该岩体高硅(SiO_(2)=71.08%~76.62%)、富碱(Na_(2)O+K_(2)O=7.99%~9.28%),贫钙(CaO=0.25%~1.67%)和镁(MgO=0.05%~0.63%);负Eu异常显著(δEu=0.14~0.50),Rb、Th、U、K、Ga元素较富集,而Ba、Sr、P、Ti元素相对亏损,且10000Ga/Al(3.62~4.90)、TFeO/MgO(3.40~14.80)、(Na_(2)O+K_(2)O)/CaO(5.03~31.96)值较高,显示出A2型花岗岩特征,代表其形成于后造山伸展拉张环境。同时结合岩浆起源温压条件研究,认为岩体可能为壳源含斜长石的角闪石质岩石在脱水条件下部分熔融及其后的结晶分异作用成因。结合中亚造山带东段早石炭—晚三叠世蛇绿岩、洋内弧和后造山型岩浆岩的时空演化关系,揭示贺根山洋可能在二叠纪末期闭合,并在三叠纪进入后造山伸展拉张阶段。

Objectives:Lying along the eastern Hegenshan suture zone in Horqin Right Middle Banner of Inner Mongolia,the Jurihe A2-type monzogranite intruded into Early Carboniferous Duerji tectonic melange belt.Based on systematic field geological characteristics,petrology,geochemistry,zircon U-Pb chronology of it,the purpose of study is to determine genetic type of pluton,tectonic setting,and provide evidence for the closure time of Hegenshan Ocean,a branch of the Paleo-Asian Ocean.The Jurihe monzogranite is mainly composed of fine-grained and medium-grained monzogranite.LA-ICP-MS zircon U-Pb dating shows that the age of granite is 203.7±1.6 Ma,indicating that formation age is Late Triassic.The granite is geochemically characterized by high SiO_(2)(71.08%~76.62%)and alkali(Na_(2)O+K_(2)O=7.99%~9.28%),but low CaO(0.25%~1.67%)and MgO(0.05%~0.63%).The obvious negative Eu anomaly(Eu=0.14~0.50),relative enrichment of Rb,Th,U,K and Ga,depletion of Ba,Sr,P and Ti,as well as high 10000Ga/Al(3.62~4.90),TFeO/MgO(3.40~14.80),(Na_(2)O+K_(2)O)/CaO(5.03~31.96)values show characteristics of A2-type granite,representing that it was formed in post-orogenic extensional environment.At the same time,combining with the temperature and pressure conditions of magma origin,it is believed that the pluton may be caused by partial melting of crustal hornblende rocks containing plagioclase under the condition of dehydration and subsequent crystallization differentiation in postorogenic stage.Combined with the spatio—temporal evolution of Early Carboniferous—Late Triassic ophiolites,intra oceanic arcs and post-collisional granitoids in the eastern part of Central Asia Orogenic Belt,suggesting that the Hegenshan Ocean was probably closed at the end-Permian and entered post-orogenic extension stage in Triassic.Methods:Based on field geological surveying,petrology,geochemistry and LA-ICP-MS zircon U-Pb geochronology of the Jurihe monzogranite,this paper discusses the petrogenesis and tectonic setting of the monzogranite,and the final closure time and subduction process of the Hegenshan Ocean.Results:(1)The Jurihe monzogranite was intruded into the Early Carboniferous Duerji tectonic melange belt in the Horqin Right Middle Banner.The LA-ICP-MS zircon U-Pb age is 203.7±1.6 Ma,indicating that its emplacement occurred during the Late Triassic.(2)The Jurihe monzogranite is characterized by high SiO_(2),rich alkali,and poor CaO and MgO,with A/CNK=0.93~1.14 and A/NK=1.09~1.26.The negative Eu anomaly is significant,and Rb,Th,U,K,and Ga elements are relatively enriched,while Ba,Sr,P,and Ti elements are relatively depleted.Additionally,the 10000Ga/Al,TFeO/MgO,(Na_(2)O+K_(2)O)/CaO values are high,indicating a metaluminous-weakly peraluminous A2-type granite.(3)The Jurihe monzogranite is the product of extension after closure of the eastern Hegenshan suture zone,which provides petrological evidence for the Hegenshan Ocean closing at end-Permian and entering post-orogenic extension stage in Triassic.Partial melting and subsequent crystallization differentiation of shallow crustal materials under low-pressure and high-temperature may be its genesis,which records information on the evolution of post orogenic plate break-off and mantle derived basaltic magma underplating in the eastern Hegenshan suture zone during Late Triassic.Conclusions:This study determines the Late Triassic(203.7±1.6 Ma)Jurihe monzogranite,which was probably formed in post-orogenic extension setting and is the A2-type granite.The Jurihe A2-type monzogranite might have been derived from underplating of mantle derived basaltic magma caused by the separation of previously subducted plate fragments in the shallow low-pressure environment and subsequent crystallization differentiation.The discovery and confirmation of the Jurihe A2-type monzogranite in the Late Triassic provide new evidence for Hegenshan Ocean closure and subsequent post-orogenic extension setting.