Geochemistry and petrogenesis of Cenozoic basaltic rocks from Shandong, Anhui and Jiangsu provinces, eastern China

Abstract
Seventy nine Cenozoic basaltic rocks from Shandong, Anhui and Jiangsu provinces and twelve ultramafic xenoliths from Jiangsu province have been analyzed for major and trace element contents and Sr-Nd isotopic compositions as well as 40Ar-39Ar dating, in order to (1) investigate the temporal and spacial variation of these basaltic rocks, and (2) discuss the origin and fractionation of the parental magma, and (3) identify the geological processes beneath the study area, and (4) evaluate the relationship between the basaltic rocks and geological setting.
Based on 40Ar-39Ar and K-Ar dating, the basaltic volcanism in the study area may be divided into three eruptive episodes: Paleocene, Oligocene to Miocene and Pleistocene. The rock type of analyzed basaltic rocks vary from olivine tholeiite to alkali basalts with time. The eruptive episode of Oligocene to Miocene is a transitional one and tholeiite coexists with olivine tholeiite during that episode. In addition, the age of the analyzed basaltic rocks tends to become younger from south (about 13.7 Ma) to north (about 0.82 Ma).
The common rock types of basaltic rocks from Shandong, Anhui and Jiangsu provinces include quartz tholeiite, olivine tholeiite and alkali basalt. The phenocrysts of basalts from Shandong province consist mainly of olivine (euhedral to subhedral) with minor amount of zoned titanaugite. The phenocrysts of tholeiite from Anhui province consist predominantly of plagioclase and clinopyroxene and the groundmass is mainly composed of plagioclase, pyroxene and magnetite. The major phenocrysts are olivine and titanaugite in the alkali basalt. The phenocrysts of alkali basalt from Jiangsu provinces consist mainly of olivine with minor titanaugite and the groundmass is mainly composed of olivine, clinopyroxene, plagioclase and Ti-Fe oxide minerals. The olivine tholeiite are mostly porphyritic with olivine and pyroxene as the major phenocrysts. The groundmass is mainly composed of pyroxene, plagioclase and magnetite.
The average (La/Yb)N ratios of basaltic rocks from Changlo area are 38.09 and 19.20 for alkali basalt and olivine tholeiite respectively. The average (La/Yb)N ratios of basaltic rocks from Penglai area are 52.85, 42.73 and 40.79 for alkali basalt, olivine tholeiite and tholeiite respectively. The (La/Yb)N ratios of basalts from Shandong provinces increase with decreasing SiO2 contents and LREEs in basalts from Penglai area are higher than those from Changlo area. The rare earth element contents in alkali basalts are generally higher than those in tholeiites expect for Yb and Lu. In addition, the compatible elements (Ni, Co, Cr, Sc) contents in tholeiites are slightly higher than those in alkali basalts. In Jiangsu basaltic rocks the incompatible elements vs. MgO/ΣFeO plots show negative trends indicating that these elements are relatively lower in the early stage of magmatic evolution. The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks of this study are similar to those of OIB.
The 87Sr/86Sr (0.702907~0.704349) and 143Nd/144Nd (0.512659~0.513371) ratios show wider variations, ranging from depleted mantle to EMI-type mantle, indicating upper mantle heterogeneity and some parts of the upper mantle may have undergone the metasomatism event. Calculation based on olivine-spinel geothermometries and orthopyroxene-clinopyroxene geothermo- barometry indicate that the equilibrium conditions of the spinel lherzolite xenoliths and garnet lherzolite xenoliths in the study area are: T= 913-1098℃ , P= 13-24.2kb and T= 1112 ℃-1171℃, P= 18.6-22.3 kb corresponding to depths of 45-83 km. The geothermometer suggests that the geothermal gradient of the upper mantle beneath the study area is approximately between MORB and oceanic geotherm indicating that the mantle may be under a “ hotter” condition. We suggest that lithospheric mantle thinning accompanied by asthenosphere upwelling (or diaprism) caused decompressed partial melting in the study area.
Based on geochemical data of the basalts and ultramafic xenoliths we suggest that the lithospheric mantle may have undergone partial melting which caused the mantle depletion. Part of the upper mantle in the study area may have experienced fluid (enriched in CO2 and H2O) metasomatism resulted in chemical heterogeneity. During Cenozoic era, the extension process may cause asthenosphere upwelling (or diaprism) to heat the bottom of the lithospheric mantle which gave rise to different degree of partial melting and parts of the melts may move upward to mix with the lithospheric mantle and produce different parental magmas in the study area.