OPEN-SYSTEM MAGMA CHAMBER: AN EXAMPLE FROM THE XINJIE MAFIC-ULTRAMAFIC LAYERED INTRUSION IN PANXI REGION, SW CHINA

Abstract

The Xinjie mafic-ultramafic layered intrusion is one of those that are associated with the Emeishan flood basalts. There are three igneous cycles, including six petrofacies. The lower part of each cycle consists of ultramafic rocks with a cumulus texture, and the upper part is composed of gabbro and troctolite. The major elements, trace elements and isotope analyses of the first cycle and the lower part of the second cycle show that their geochemistry resembles the Emeishan basalts, and their 87Sr/86Sr, εNd (t), 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values range from 0.70610 to 0.70636, 1.01 to 1.75, 17.895 to 18.935, 15.563 to 15.639, 38.345 to 39.037 respectively. This similarity implies the genetic relation between the Xinjie intrusion and the Emeishan basalts, whereas the similar isotope ratios and primitive mantle-normalized high strength element patterns with oceanic island basalts (OIB) suggest that the magmas were derived from a mantle plume. The correlation between MgO contents and major elements reflect the fractional crystallization of olivine, orthopyroxene and minor magnetite. In addition, some samples with (La/Nb)PM and (Th/Ta)PM > 1 indicate the contamination of crustal materials or lithospheric mantle. The O isotope data show that some samples of the upper part of the first cycle and the second cycle have high δ18O (> 6‰, suggesting that crustal contamination occurred at the upper part of the magma chamber, not at the lower part. No correlation between δ 18O and SiO2, implies that the contaminants are not upper crust but lower crust with lower SiO2, contents. The modeling calculation of Nd isotope and O isotope indicates that the mean contaminated degree is about 12% ~ 15%. It is likely that the contamination of lower crust caused decreasing FeO contents of contaminated magams, which gave rise to immiscibile sulfide liquid, thus enrichment of platinum-group elements at the bottom of the Xinjie intrusion.