THE GEOCHEMISTRY OF CENOZOIC BASALTS AND PERIDOTITE XENOLITHS FROM HANNUOBA REGION, EASTERN CHINA: IMPLICATION FOR THEIR PETROGENESIS AND SUBCONTINENTAL MANTLE HETEROGENEITY by

The Cenozoic Hannuoba basaltic province, 200 km northwest of Beijing, China, is composed of intercalated tholeiitic, transitional and alkalic basalts. Although the alkalic basalts contain abundant upper mantle xenoliths, they range in MgO content from 5.7 to 10.6% MgO. Most of the major and trace element abundance variations in the alkalic basalts can be explained by segregation of a clinopyroxenerich assemblage containing significant amounts of garnet and minor Fe-Ti oxides. Incompatible element abundances and SrNd-Pb isotope compositions in these alkalic basalts are remarkably similar to those of alkalic basalts from oceanic islands. Obviously, there is a world-wide source for alkalic basalts that can be tapped in oceanic and continental settings. Compared with Hannuoba alkalic basalts of similar MgO content Hannuoba tholeiitic lavas have lower incompatible element abundances, and different incompatible element abundance ratios (e.g., Ba/La, La/Sm, Sm/Nd, Zr/Nb and Zr/Hf). However, these tholeiitic lavas are not anomalously depleted in Nb and Ta; therefore, they are not similar to arc basalts and many continental flood basalts. Although radiogenic isotope data (Sr, Nd and Pb) show that the alkalic and tholeiitic lavas are isotopically distinct, variations in incompatible element abundances and abundance ratios in all lava types define coherent trends suggestive of a mantle mixing process. The alkalic and tholeiitic lavas from the Hannuoba region formed from different mantle sources. The alkali basalts were generated by small degrees of partial melting of asthenosphere, perhaps a PREMA component, whereas the tholeiites were generated by partial melting of an enriched component (EMI-like) in the oceanic mantle. Subsequent contamination in the lower crust may have occurred. The enriched component may have resided in the upwelling asthenosphere or subcontinental crust lithosphere. Mixing between the PREMA and EMI-like mantle components created the linear trends of radiogenic isotopic ratios defined by Hannuoba lavas. This mixing between mantle components may have occurred during the rifting event that caused extension in northeast China -40my ago; that is, uprising asthenosphere may have caused rifting and the partial melting of continental lithosphere. The proximity of upwelling asthenosphere and heated, former continental lithosphere enabled mixing of isotopically diverse components. This proposal is consistent with the transition from dominantly alkalic to dominantly tholeiitic volcanism at Hannuoba. Based on geochemical studies of six anhydrous spinel peridotite xenoliths in basanites, the upper mantle beneath Hannuoba, eastern China is compositionally heterogeneous. These samples range in Sr and Nd isotopic ratig fr~o& MORBlike to neg 4 bulk-earth estimates. The low igSr/ Sr and high 143Nd/ Nd xenoliths contain the largest amount of a basaltic component (e.g., CaO and A1203 ), but they are relatively depleted in light rare earth elements compared to chondrites. Other xenoliths have U-shaped chondritenormalized REE patterns. Trace element and radiogenic isotopic data for the peridotite require enrichment processes acting on depleted mantle. Constraints on these processes are: (a) inverse correlations between basaltic constituents, such as CaO and A1203 , and La/Sm, and (b) sam es most depleted jn CgO and Al203 have the highest Sr/ Sr and lowest Nd/ Nd. These trends can be explained by a model whereby garnet peridotite zoned in isotopic composition undergoes partial melting. Because of a gradient in degree of melting, e.g., from the wall-rock contact to hotter interior, or as a function of depth in a diapir, melts initially segregate from regions where the degree of melting is high. Subsequently the recently created residues are infiltrated by slower segregating incipient melts. Preferential mixing of these incipient melts with residues from high degrees of melting can explain the observed complex geochemical trends seen in Hannuoba and many other peridotite xenolith suites. Clinopyroxene-rich veins in some of the peridotites may reflect pathways of ascending melt. Thesis Supervisor: F.A. Frey, Professor of Geochemistry