MAJOR ELEMENT, REE, AND PB, ND AND SR ISOTOPIC GEOCHEMISTRY OF CENOZOIC VOLCANIC-ROCKS OF EASTERN CHINA - IMPLICATIONS FOR THEIR ORIGIN FROM SUBOCEANIC-TYPE MANTLE RESERVOIRS

Major- and rare-earth-element (REE) concentrations and U-Th-Pb, Sm-Nd, and Rb-Sr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) Nd-Sr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) Sr-Pb also shows an anticorrelation, similar to that of Hawaiian and Walvis Ridge basalts, (3) Nd-Pb shows a positive correlation, and (4) the Pb-207/Pb-204 vs Pb-206/Pb-204 plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the Pb-208/Pb-204 vs Pb-206/Pb-204 plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components-a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.