The Research Of Geodynamic For Ultrabasic-Basic Xenoliths In Emeishan Basalt From Zhongdian, Yunnan

Emeishan Basalt is the volcanic which widely distributed in southwest of China atPermian. The previous studies show that it is the product of mantle plume activity. Wefound a Series of ultrabasic-basic xenoliths that have variation ofduniteâ†'lherzoliteâ†'olivine pyroxeniteâ†'picritesâ†'gabbroâ†'diabase-gabbroâ†'diabase,provides a natural probe for the research of geodynamic of Emeishan plume.Based on the research of mineralogy and petrology using EMPA, SEM and EnergySpectra, we found a kind of Black Opaque Material (BOM) which is composed ofmicrocrystalline silicate and spinel group minerals from ultrabasic-basic xenoliths inEmeishan Basalt. From the crystal chemistry and element geochemistry of thesemicrocrystalline minerals, we draw that they are the products and relics of mantle fluidwhich is not immixed with the host magma (e.g. basalt magma). During the evolutionof the mantle fluid, differentiation by liquation is expressed assilicateâ†'spinelâ†'magnetiteâ†'chromiteâ†'ilmenite. The mantle fluid can also lead tothe alternation and mineralization for host rocks and xenoliths, composing to be animportant dynamic and substance source, while the mantle plume in western Yunnan isfavorable to the endogenous metal mineralization after Permian.The major elements of the xenoliths have the characteristics of alkaline basalts,which is consistent with other Emeishan basalt in this area. In the Harker diagram,xenoliths mainly experienced fractional crystallization of olivine-based. Traceelements have the characteristics of ocean island basalt. Not easily altered elementschanges, the description of rocks experienced strong alteration. The rare earth elementsanalysis showed that all the samples have negative Sr anomalies, but does not have a negative Eu anomaly, indicating that does not exist in the fractional crystallization ofplagioclase. La/Ta value concentrated in (13.02~14.45) showed that ultrabasic magmadid not experience the lithospheric mantle contamination.In the study of the olivine and spine mineral geochemistry, elementalgeochemistry in the ultrabasic-basic xenoliths, Draw conclusions that ultrabasicmagma source region has the characteristics of the pomegranate lherzolite zonetransition to the harzburgite area. Its formation temperature1430℃and pressure2.4GPa depth85km are relatively high, and accompanied by the crystallization of theultrabasic magma, And there is no strong assimilation effect. Ultrabasic magma derivedfrom the mantle plume activity, has not been affected by lithospheric mantle and crustalmaterial contamination. We further believe when the later basaltic magma upwellingcapture ultrabasic magmatic enclaves, Previous ultrabasic magma already due to themantle plume activity rose to the shallow crustal magma chambers and layeredfractional crystallization. And combine the characteristics of the Lijiang picrites, wedraw that the upper mantle of this area exist Emeishan large igneous provinces mantleplume activity, and may represent the early stages of the mantle plume activity.