The Study Of Megacrysts In A Basalt Dike Of Luanshiwa, Moate, Xinjiang

Abundant megacrysts of pyroxene, plagioclase and hornblende have been first found in a basalt dike in Moate area in Xinjiang province.This paper focuses on mineralogy and geochemistry of megacrysts by the traditional petrography method of the electron probe and obtainde equilibrium temperatures and pressures of the megacrysts using the mineralogical thermo barometer and carries on the preliminary discussion to the origin model of megacrysts. Besides, quantitative textural analysis method is also be used to get a deeper analysis.The results of the experiments show that the chemical compositions of these megacrysts are inhomogeneous but no compositional zonation structures which may not crystallize in a stable magma chamber. According to the calculations from thermometers and Barometers of Putirka, the conditions to form the clinopyroxene megacrysts are between T/ ℃ :1203.2-1239.8, P/kbar 12.6-15.4Kbar in about 42.4-51.2km below the earth, while T/℃: 1137.0-1183.6℃, P/kbar 10.1-14.9 Kbar in about 34.8-49.3km below the earth for the plagioclase megacrysts. The P-T trajectory indicates the rapid and continuous arise of this magma. The Kcpxd(Fe-Mg)(0.24-0.34) show the clinopyroxene megacrysts were equilibrating to the magma thus they may not belong to the chadacrysts. For these reason, we propose the megacrysts belong to the polybaric-crystallization which crystallize during continue arise in a channel.According to the quantitative textural analysis method, the result shows the basalt dike contains about 30-50% megacrysts and the slopes of the samples are different between the small crystals and big crystals which may hints the basalt magma arise very fast. It is difficult for a small magma to bring such abundant megacrysts to the ground, thus we suggest the influence of fluid drive this magma active.