北京西山东岭台组粗面质火山岩地球化学:下地壳熔融成因

北京西山东岭台组粗面岩表现出LREE富集和HREE平坦的稀土配分型式 ,富集LILE (Ba、K和Sr)、和亏损Nb Ta、Th U和Ti,Sr Nd同位素组成富集 (87Sr/ 86Sr(t) =0 .70 6 38～ 0 .70 6 72 ,Nd(t) = 1 6 .3～ - 1 5 .7)。结合所表现出的明显Nb/Ta分馏和高的Zr/Sm比值 ,认为它们来源于大陆下地壳含石榴子石的角闪岩部分熔融作用而成 ,主要的残留相为石榴子石 +斜长石±角闪石±辉石。根据地壳岩石熔融实验研究结果 ,西山地区东岭台组粗面岩的熔融要求有外来的热源参与 ,与区域上同时代发生的玄武岩浆底侵作用具时空上的密切联系 ;这些岩石所显示的高Sr和Ba特征也暗示至少有部分底侵的玄武质岩石参与了部分熔融作用

EARLY CRETACEOUS TRACHYTES OF DONGLINGTAI FORMATION FROM THE XISHAN AREA IN THE NORTHERN NORTH CHINA BLOCK:CONSTRAINTS ON MELTING OF LOWER MAFIC CRUST

The trachytes of Donglingtai Formation of Early Cretaceous in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion in Nb-Ta-Ti, Th-U. These trachytes have highly enriched Sr and Nd isotopic signatures overlapping Sr-Nd isotopic range of Late Mesozoic mafic igneous rocks in the region. Taking into account of Nb-Ta fractionation and high Zr/Sm ratio for these trachytes, we consider that the trachytes of Donglingtai Formation were derived from the garnet-bearing amphibolite in the lower crust composed of garnet+plagioclase(amphibole) pyroxene residual phases. Combined with the melting experimental results of crustal materials and regional extensional tectonics during late Mesozoic, it is concluded that thermal input from underplating basaltic melt resulted in the partial melting of lower continental crust to generate the trachytes of Donglingtai Formation.