Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.

Geochemistry and petrology of Emeishan basalts and subcontinental mantle evolution in southwestern China

Three major volcanic rock sequences in the P_2β formation (Emeishan basalts) were sampled during a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China. Two of the three sections—Emei and Tangfang are composed of continental flood basalts (CFB) while the third—Ertan is an alkalic center. Multi-element chemical analyses indicate a predominance of low-MgO transitional quartz tholeiites at Emei and Tangfang, whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites. Consanguineity of the rocks from the three sections is suggested by consistently high TiC₂, K₂O, incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions, but antypical of CFB. Sr isotope data for ten Emei basalt samples (⁸⁷Sr/⁸⁶Sr=0.7066–0.7082) which show no correlation with Rb/Sr ratios (0.02–0.12) and Nd isotopes for two of the samples (^l43Nd/¹⁴⁴Nd= 0.51171–0.51174) are interpreted as being related to the mantle evolution. The primary magmas responsible for all the three sequences have been modeled in terms of a uniformly metasomatized mantle source. Trace element models support the derivation of the Emei and Tangfang primary magmas from 10–15 percent partial melting of spinel lherzolite. followed by fractional crystallization of olivine and clinopyroxene. The primary alkaline olivine basalts at Ertan are generated by 7–10 percent partial melting of a chemically equivalent source in the garnet-peridodite stability region. The assumed mantle composition is characterized by Rb=3.8–5.5 ppm. Sr=62–83 ppm, Ba=45–64 ppm, La=3.8–5.6 ppm. and Yb=0.46–0.57 ppm. The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par tial melting. Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diapir acts as the agent for upward enrichment as well as uplift and extension of the crust.