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

LI Xiaoyong~1, GUO Feng~1, FAN Wenming~(1,2), WANG Yuejun~1 and LI Chaowen~1 (1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, GD 510640, China; 2. Resource and environmental Bureau of Chinese Academy of Sciences, Beijing 100864, China)  
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.
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