ISOTOPIC AND TRACE-ELEMENT STUDIES FOR CENOZOIC VOLCANIC-ROCKS FROM WESTERN CHINA - IMPLICATION FOR A CRUST-LIKE ENRICHED COMPONENT IN THE MANTLE

Sr, Nd, Ce and O isotopic ratios and trace element abundances were measured for Cenozoic SiO2- and (Na2O+K2O)-rich volcanic rocks from Yutian of Xinjiang, Yumen of Gansu and Tengchong of Yunnan, Western China. All samples investigated, especially the samples from Yutian of Xinjiang, are highly enriched in LREE and other incompatible elements, and are characterized by higher Sr-87/Sr-86 (0.706-0.709) and lower 144Nd/144Nd (0.5121-0.5126) ratios as compared with MORB and OIB. High Ce-138/Ce-142 ratios (0.0225767-0.0225810) were also found for the volcanic rocks from Yutian of Xinjiang and Yumen of Gansu provinces. The volcanic rocks from the three areas are concluded to have been derived from LILE (large ion lithophile element)-enriched mantle source regions without significant crustal contamination. Their high delta-O-18, and low HFSE (high field strength element)/LILE ratio characters imply a crust-like component existing in their mantle source regions, and hence the isotope variations of the volcanic rocks are modeled with mixing between two end members; depleted mantle and crust-like component. The mantle source region below western China, which indicates integration of subduction-related mantle, is disclosed to be clearly different from that below eastern China.