青海南部杂多地区超镁铁质—镁铁质岩石的特征及Ar-Ar定年

Characteristics and Ar-Ar dating of ultramafic-mafic rocks in the Zadoi area,southern Qinghai,China

通过野外地质研究和室内岩相学、岩石地球化学研究,在位于青藏高原中部的青海杂多地区的双湖-澜沧江结合带中发现了一套超镁铁质岩、镁铁质岩。该超镁铁质岩、镁铁质岩沿北西—南东向区域构造线多呈岩脉状侵入在早石炭世杂多群中,局部呈构造透镜体分布在断裂带中或呈包体分布在白垩纪花岗岩中,出露规模不大,局部侵入体具有层状侵入杂岩体的特征。主要岩石类型为辉石橄榄岩、辉长岩、辉长辉绿岩,岩石地球化学具高Ti、Fe、Al、Ga和LREE中等富集的特征。橄榄岩中角闪石的^(40)Ar/^(39)Ar同位素测年得到275.3Ma±1.9Ma的坪年龄,表明这套超镁铁—镁铁质岩石可能为早、中二叠世弧后盆地拉张的产物。

A suite of ultramafic-rnafic rocks was found in the Shuanghu-Lancangjiang junction zone in the Zadoi area, central Qinghai-Tibet Plateau through the field geological study and laboratory petrographic and geochemical studies. The ultramafic- mafic rocks of this suite mostly occur as dikes injected into the Early Carboniferous Zadoi Group along the NW-SE lineament and locally occur as structural lenses in the fault one or as xenoliths in Cretaceous granites. Their outcrops are of small scale and locally the intrusions have the features of stratiform intrusive complexes. The dominant rock types are harzburgite, gabbro and gabbro-diabase and chemically the rocks have higher Ti, Fe, Al and Ga contents and are moderately LREE-enriched. ^40Ar/^39Ar dating of hornblende from peridotite yielded a plateau age of 275.3±1.9 Ma and an isochron age is 275.6±3.1 Ma, which indicates that this suite of ultramafic-mafic rocks is probably the product of Early-Mid PenMan back-arc basin extension.