扬子克拉通北缘早前寒武纪地壳演化——后河杂岩元素和同位素地球化学限制

EARLY PRECAMBRIAN CONTINENTAL CRUST EVOLUTION AT THE NORTHERN MARGIN OF YANGTZE CRATON:CONSTRAIN FROM THE ELEMENTAL AND

陕南汉中碑坝地区是扬子克拉通少数可能存在太古宙结晶基底的地区之一。本文系统地研究了元素和同位素地球化学表明，后河“群”主要岩石类型之一的ＴＴＧ片麻岩形成时代约为２４００Ｍａ，其ＲＥＥ和岩石化学特征与新太古代ＴＴＧ片麻岩类似；混合岩类Ｓｍ－Ｎｄ同位素模式年龄表明其原岩或源岩的形成时代约为２７００Ｍａ的新太古代，在岩石化学成分上与钾质花岗岩或闪长质岩石相当，与太古宙高级区新太古代典型钾质花岗岩相近似；区内斜长角闪岩形成于２２００Ｍａ，与区内混合岩化作用发生的时代稍早或基本一致；局部存在的变流纹岩与火地垭群铁船山组双峰火山岩属同一岩浆作用过程在区内不同区段的体现；后河“群”各类基底岩石有基本同一的Ｒｂ－Ｓｒ等时线年龄（１２０１Ｍａ±２４０Ｍａ），反映了元古宙中晚期构造热事件对早期各类岩石Ｒｂ－Ｓｒ同位素体系有深刻影响。

Beiba area,southern Shanxi province of China,is one of a few localities where the early Precambrian crystal basement may be in existence.This paper reports the recent result of geochemical researches about Houhe complex,the oldest rock suite in the area.The combined geochemical studies of elements and isotopes reveals that the TTG gneiss,one of the major rock types in the Houhe complex,has the gaes of about 2 400 Ma and the characteristics of REE and major elements are similar to those of the Neoarchaean TTG.As an other important rock type in the complex,the migmatite has the Neoarchaean model ages around 2 700 Ma,and shows the properties similar with the potassiumgranolites of Neoarchean.The third rock type of the complex is amphibolite which formed just early or during the period of the regional migmatization.The occasionally exposed metarhyolite in the district is thought to be the result of the same magmatic process with Huodiya bimodal metavolcanic rocks,the Tiechuanshan Formation which locates at the north of the research area.The fact that all the rock types in Houhe Complex almost sharing the same RbSr isochron of 1 201 Ma±240 Ma indicates that the late Protoerozoic tectonothermal event which is popularly recorded in many early Precambriam rock suites in the south Qinling Belt and northern Yangtze craton,had the intense influences on the RbSr isotopic system of Houhe complex.