PYROXENE MINERAL CHEMISTRY OF THE JINCHUAN INTRUSION, CHINA

Chemical compositions of orthopyroxene and clinopyroxene from the Jinc huan ultramafic intrusion have been obtained by electron microprobe an alysis. The Mg' number (MgO/(MgO + FeO)) for both pyroxenes falls with in narrow ranges, 82-87 for clinopyroxene and 81-85.5 for orthopyroxen e, suggesting limited magma differentiation in regard to the present i gneous body. The Al2O3 content ranges from 2.44 wt.% to 4.43 wt.% and increases with decreasing Mg' of the pyroxenes, i.e., with the more ev olved magma. This is attributed to the relatively greater effects of A l2O3, TiO2, Cr2O3 and Fe2O3 than that of SiO2 on pyroxene crystallizat ion. Negative linear relationships between Ti4+ and Si4+, and Al3+ and Si4+ characterize the pyroxenes. In clinopyroxene, regression of Si4 versus Al3+ results in a straight line with a slope of -1.012, indica ting that the decrease of Si4+ in the crystal structure is matched by an increase only in tetrahedral Al3+; octahedral Al3+ has remained rel atively constant. The negative linear relationship between Ti4+ and Si 4+ in clinopyroxene reflects either a greater tendency of Ti4+ to occu py octahedral sites than Al3+, or that replacement of Al3+ for Si4+ de mands a more efficient charge balance. The scatter in plots of Ti4+ ve rsus Si4+ for orthopyroxene indicates that charge balance is not as cr itical as structure symmetry. The crystallization temperature of pyrox ene is calculated to be 1108-1229-degrees-C using Wood and Banno's (19 73) two pyroxene thermometer, and is within 40-degrees-C of that calcu lated from Wells's (1977) thermometer. The distribution coefficient (K d) for Mg2+ and Fe2+ between clinopyroxene and orthopyroxene is estima ted to be 0.86, which is higher than that of the other intrusions and lower than that of mantle nodules, but still falls within their Kd-1/T trend. This suggests that the Kd value of pyroxene is controlled main ly by temperature.