Origin of high An plagioclase in the early Permian ~280 Ma) Xiaohaizi wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
ABSTRACT The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5 48.7 wt. and Na2O K2O (2.4 3.8 wt. contents, displaying sub alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9 4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5 23.0 wt. and very low Na2O (0.22 0.34 wt. contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000degC) and assumed H2O ~1.5 wt. conditions, the Ca/Na ratio of the parental magma cannot generate high An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon d18O (VSMOW) values (2.99 3.71%0) are significantly lower than that of mantle derived zircon (5.3 0.6%0) Such low zircon d18O values may be due to incorporation of large amounts of low d18O, hydrothermally altered oceanic crust. However, geochemical and Sr Nd Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high An plagioclases.