THERMAL HISTORY AND METASOMATIC PROCESSES IN THE UPPER MANTLE BENEATH WANGQING, NE CHINA──IMPLICATION FROM MINERAL CHEMISTRY OF A SPINEL PERIDOTITE XENOLITH
Xu Yigang; Lin Chuanyong; Martin A. Menzies Jean-Claude C. Mercier(Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640)(Department of Geology, RHBNC, University of London, Egham Hill, Egham, Surrey TW20OEX, UK)(Institute of
A spinel harzburgite(WQ91-28) from Wangqing, northeastern China contains large exsolved orthopyroxene(Opx) porphyroclasts (1cm) and metasomatic assemblages around primary orthopyroxene and spinels. In order to investigate the themial history experienced by this sample and to determine the nature of metasomatism, we have analysed its constituent minerals using electron microprobe for major elements and ion microprobe for trace elements (Sr, Ti, Zr, Y and REE). The primary minerals are strongly depleted in basaltic components, consistent with its petrographic characteristics. The Opx popohyroclasts are chemically zoned with Cr, Al, and Ca decreasing from core to rim. Opx neoblasts have compositions similar to porphyroclast rim. Themiometric calculation indicates that the uppermantle beneath Wangqing experienced a cooling process from 1200 to 850℃ . A possible tectonic phenomenon connected to this proeess is a diapiric uplift. Major element and trace element abundances in primary and metasomatic clinopyroxenes(Cpx) suggest that the WQ91-28 recorded at least two episodes of metasomatism. The first metasomatism is cryptic in nature and the metasomatic agent is characterised by high LREE and low HREE contents. The second episode of metasomatism happened shortly before the basaltic eruption. The metasomatic agent which is responsible for development of Opx epolacement rim and Cpx-Ol-feldspar-glass assemblage around spinel cannot be the host basalts, but is related to carbonatite melts.
【CateGory Index】： P574.2