GEOCHEMISTRY OF EARLY CRETACEOUS TRACHYTES OF DONGLINGTAI FORMATION FROM THE XISHAN AREA, BEIJING： CONSTRAINTS ON MELTING OF LOWER MAFIC CRUST

作者：LIXiaoyong,FANWeiming,GUOFeng,WANGYuejun,LIChaowen

摘要：The early Cretaceous trachytes of Donglingtai Formation in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion of Nb-Ta-Ti, Th-U. These trachytes have been highly enriched by Sr and Nd isotopic signatures (87Sr/86Sr(t)= 0.70638～ 0.70672, εNd(t) = -16.3～ -15.7), overlapping Sr-Nd isotopic range of late Mesozoic mafic igneous rocks in the region. Taking into account Nb-Ta fractionation and high Zr/Sm ratio for these trachytes, we consider that the trachytes of Donglingtai Formation are derived from the garnet-bearing amphibolite in the lower crust composed of garnet + plagioclase ± amphibole ± pyroxene residual phases. On the basis of the melting experimental results of crustal materials and regional extensional tectonics during late Mesozoic, it is concluded that the thermal input from underplating basaltic melt results in the partial melting of lower continental crust to generate the trachytes of Donglingtai Formation. And the characteristics of high Sr and Ba for these trachytes suggest that part of underplating basalts should take a share in partial melting at least.

發文機構：GuangzhouInstituteofGeochemistry ZhuzhouInstituteofTechnology ResourceandEnvironmentalBureauofChineseAcademyofSciences

關鍵詞：鐵鎂質殼部分熔融白堊紀北京粗面巖mafic lower crustpartial meltingtrachyteearly CretaceousBeijingXishan

分類號： P534.53[天文地球—古生物學與地層學][天文地球—地質學]P618.31[天文地球—礦床學][天文地球—地質學]