内蒙古东乌旗角闪辉长岩年龄、地球化学特征及其对晚古生代构造演化的约束

Zircon U-Pb age and geochemistry of the bojite in Dong Ujimqi,Inner Mongolia and its constrains on the Late Paleozoic tectonic evolution

兴蒙造山带东部内蒙古东乌旗地区晚古生代构造岩浆活动复杂,是解决兴蒙造山带晚古生代构造演化的关键地区。对东乌旗地区新识别出的角闪辉长岩进行了系统的地质特征、岩相学、LA-ICP-MS锆石U-Pb年代学、Hf同位素和岩石地球化学研究。东乌旗角闪辉长岩主体呈包体状产出,主要由斜长石(55%~60%)、角闪石(15%~20%)、辉石(10%~15%)和少量磁铁矿等组成。获得角闪辉长岩样品的锆石U-Pb年龄为299±3 Ma,表明岩体形成于晚石炭世。6件样品的地球化学数据显示,SiO_(2)含量为49.88%~51.98%,TiO_(2)含量为1.2%~1.86%,MgO含量4.42%~7.41%,Al_(2)O_(3)含量为15.55%~16.84%,MnO含量为0.12%~0.17%,CaO含量5.67%~6.52%,Na_(2) O+K_(2)O含量3.35%~4.63%,m/f值为0.41~0.49,Mg^(#)值为52.56~60.82,TFeO值为7.12~9.18;稀土元素球粒陨石标准化配分模式表现为轻稀土元素(LREE)相对于重稀土元素(HREE)富集的平坦右倾型,轻、重稀土元素比值(LREE/HREE)为4.96~6.92,岩石富集大离子亲石元素Rb、Ba、K等,相对亏损高场强元素Nb、Ta、Ti,LREE/HREE值较稳定,δEu值为0.87~0.98,显示具有弱的正Eu异常。角闪辉长岩中锆石εHf(t)值为5.1~11.25,单阶段地壳模式年龄(T DM 1)为1039~604 Ma。结合区域地质演化,认为东乌旗角闪辉长岩可能是由于受到洋壳的俯冲,上覆亏损地幔发生部分熔融,产生钙碱性岩浆,且在上升侵位的过程中受到地壳物质较弱程度的混染,形成于古亚洲洋俯冲环境。东乌旗地区在晚石炭世—早二叠世存在少量的基性岩浆活动,标志着构造背景由俯冲造山向造山后伸展转变。

The research area is located in the east of the Xing’anling-Mongolian Orogenic Belt,Dong Ujimqi of Inner Mongolia.The complex tectonomagmatic activity in this area is the key to solve the tectonic evolution of Xing’anling-Mongolian Orogenic Belt.This study carried out zircon U-Pb dating analyses and petrographical,geochemical for the bojites firstly recognized in the Dong Ujimqi,so as to understand the characteristics of the mantle nature and structure.The bojite generally extends north-east and presents like rock plant.In these rocks,main rock forming minerals are plagioclase,hornblende and pyroxene.The dating results indicate that the bojite was formed in the Late Carboniferous(299±3 Ma).The geochemical data of the samples show that:SiO_(2)(49.88%~51.98%),TiO_(2)(1.2%~1.86%),MgO(4.42%~7.41%),Al_(2)O_(3)(15.55%~16.84%),MnO(0.12%~0.17%),CaO(5.67%~6.52%),Na_(2) O+K_(2)O(3.35%~4.63%),m/f(0.41~0.49),Mg^(#)(52.56~60.82),TFeO(7.12~9.18).The standardized distribution pattern of REE chondrites is right-leaning with relatively enriched LREE.The rocks are enriched in LILE(Rb,Ba,K),and depleted in HFSE(Nb,Ta,Ti),LREE/HREE is stable,with negative Eu anomalies(δEu=0.87~0.98).TheεHf(t)values of zircons from the bojite vary between 5.1~11.25,and their Hf one-stage model ages vary from 1039 Ma to 604 Ma.Combined with regional tectonic evolution,it may be due to the subduction of oceanic crust,the thermal dehydration of subducted plate resulted in the partial melting of overlying depleted mantle and the generation of calc alkaline magma.In the process of ascending and emplacement,it was contaminated by crustal materials in a weak degree,which formed in the subduction environment of Paleo Asian Ocean.There was a small amount of basic magmatism in Dong Ujimqi in the Late Carboniferous—Early Permian,which indicated that the tectonic setting changed from subduction orogeny to post orogenic extension.