| The Permian Emeishan large igneous province (ELIP) is only internationally admitted large igneous province in China. There are the many achievements about ELIP, but many scholars hold different opinions, such as the classification and the cause of the high-Ti and low-Ti basalts, the relationship between ELIP and the mantle plume, the place of the mantle plume axis position. This paper studied the petrology, mineralogy and geochemistry of Hutiaoxia and jin’an basalt, which are located in the Northwest and southeast of lijiang county, respectively.The rock association characteristics of Hutiaoxia lavas are bimodal volcanic rocks, which are consist of a little picrates and baesites. The olivine and a little of clinopyroxene in the Hutiaoxia’s lavas are products of the melts’ fractional crystallization. Most of the Olive phenocryst in the picrate had been iddingsited without kinking. The Mg#value of low calcium augite is84.92%in the augitophyric basalt, and its initial melt temperature is1441℃. There are mainly composed of basic basalts in the Jin’an. Its Mg#value of the clinopyroxene phenocryst is66.21%-83.21%, and its initial melt temperature and initial melt pressure are1021-1207℃and2.2-9.2Kb, respectively. So, the olivine, clinopyroxene and a little Monoclinic feldspar in the Jinan are products of the melts’ fractional crystallization. The results showed that Hutiaoxia lava has deeper magma source area, higher melt pressure and melt temperature than jinan basalt. The picrite distribution in Lijiang, which is the center of ELIP, reaches to highest point and rimland lower.The paper mainly studies the mineral compositions, major element, trace element, rare earth element, Sr-Nd-Pb isotopic as well as U-Pb ages of the Permian basalts at the Hutiaoxia and Jinan areas. In the Hutiaoxia, the Mg#value of its picrates and basalts is57.57%-75.17%， its CaO/Al2O3ratio is0.80%-1.34%, and its age-corrected Nd-Sr-Pb isotope ratios and εNd(t) value, which cover a rather small range, are0.70508-0.70526and0.43-0.54, respectively. In the Jinan, the Mg#value, CaO/Al2O3ratio, εNd(t) value and (87Sr/86Sr)t ratio of its basalts are30.41%-51.89%,0.49%-0.79%,0.43-0.54and0.70508-0.70526, respectively. The higher-εNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have ocean-island-like patterns of alteration-resistant in compatible elements. Heavy rare-earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by rather small amounts of partial melting, Rough correlations of isltope ratios with ratios of alteration-ersistant highly incompatible elements (e.g., Nb/La、(Th/Ta)、(La/Nb)、(La/Ta)、La/Sm and Nb/U) suggest minor amounts of contamination involving continental material or continental lithospheric mantle with a relatively low-εNd component in the source. Overall, our results are consistent with other evidence suggesting a plume-head origin for the Emeishan province.Basalts can be used as melting temperature and pressure probes of the mantle. We calculated primary magmas using PRIMELT2software. The results show that the primary magmas of Emeishan Large igneous could be generated in the pressure range21.2-44.9Kb at an approximate depth of148km from Garnet stable phase sources. And the generated temperatures in range1401-1726℃, which excess normal asthenosphere temperature121-376℃, provide further evidence of Emeishan mantle plume. Furthermore, the distributions of P-T also strengthen the existence of Emeishan mantle plume. The calculated melt temperature, melt pressures and formation depths of the original magma is characteristic of gradual reduction from Lijiang area to all directions of ELIP.Based on the present studies, the paper shows that the axis of the Emeishan mantle plume should be located beneath the Lijiang County Town, Yunnan Province. |
PDF Full Text Request