桐柏地区变质杂岩和侵入岩类Pb同位素组成特征及其地质意义

桐柏地区存在两个铅同位素组成明显不同的基底块体，在桐柏－商城断裂北侧，基底岩石以高放射性成因铅同位素组成为特征，而桐柏－商城断裂南侧，基底岩石以低放射性成因铅同位素组成为特征，它们分别可与南阳盆地以西的北秦岭和南秦岭构造带中基底岩石的铅同位素组成相对比，从而证明桐柏地区是秦岭造山带的构造东延部分．同时，通过桐柏地区侵入岩铅同位素组成特征的分析表明，早古生代侵入岩浆主要来源于北秦岭高放射性成因铅同位素组成的块体，而中生代侵入岩浆则来源于南秦岭低放射性成因铅同位素组成的块体，指示在南阳盆地以东，相当于北秦岭构造块体的地壳深部存在南秦岭的陆壳物质，反映南秦岭的陆壳物质早期向北俯冲叠置于北秦岭块体之下，并作为北秦岭中生代花岗质岩浆的源区

Pb ISOTOPIC COMPOSITIONS OF METAMORPHIC ROCKS AND INTRUSIVE ROCKS IN TONGBAI REGION AND THEIR GEOLOGICAL IMPLICATION

Two basement blocks with distinctively different Pb isotopic compositions are present in Tongbai area. On the northern side of the Tongbai_Shangcheng fault, the basement rock is characterized by the highly radiogenic Pb isotopic composition. However, on the southern side of the Tongbai_Shangcheng fault, the basement rock is characterized by the lowly radiogenic Pb isotopic composition. The Pb isotopic compositions in both of the basement blocks mentioned above resemble those in the basement rocks of the North Qinling and South Qinling structural belts respectively to the west of Nanyang basin. Therefore, the Tongbai region proves the eastward structural extension of the Qinling orogenic belt. At the same time,the analysis of the Pb isotopic composition of the intrusive rocks in the Tongbai region shows that the Early Paleozoic intrusive magma originate from the mass composed of highly radiogenic Pb isotopes in the North Qinling, but the Mesozoic intrusive rocks originate from the mass composed of lowly radiogenic Pb isotopes in the South Qinling. In this sense, the crustal materials of the South Qinling are present in the crust to the east of the Nanyang basin corresponding to the deep crust of the structural mass in the North Qinling. This suggests the northward underthrusting of the continental crust in the South Qinling beneath the North Qinling block in the early period, a source region of the Mesozoic granitic magma in the northern Qinling.