GENESIS OF THE SIDERITE IN JURASSIC OLIVINE BASALT, KARAMAY, XINJIANG, NW CHINA

Abstract

The Jurassic basalts in Karamay are composed of amygdaloidal basalt, olivine basalt, and sidertite-bearing olivine basalt. These rocks are mainly composed of olivine, clinopyrxene, plagioclase (mostly andesine-labradorite), glass, ilmenite, and apatite. Primary siderite (≈ 10%) and alkali feldspar are observed in siderite-bearing olivine basalt samples. Siderites usually enclose rounded plagioclases as inclusions. The boundaries between siderite and plagioclase crystals are serrated in some cases. Siderite coexists with glass, plagioclase, clinopyroxene and ilmenite. Some siderites and glasses filled in the trellises constructed by plagioclase crystals. The glasses in siderite-bearing olivine basalt characterize with natures of Fe-rich (FeO up to 29.2%) and low-Ca (CaO < 1.5%). The carbonates crystallized at earlier stage belong to the solid solution of siderite-dolomite (dominate in siderite) with small amounts of dialogite. Crystallization of plagioclase would decrease the CaO contents in magma. Thus, siderite component becomes relatively more important at late stage. The compositional zoning of siderite is clear on BSE image with drastic changes in FeO and CaO contents. The Mg# of olivine and clinopyroxene is merely about ≈ 70, clinopyroxene is mostly of Ti-rich augite. The Mg# of glass is low but has a wide range of variation, from 23 (for siderite-bearing olivine basalt) to 64 (for olivine basalt). Thermodynamic calculations reveal that these basalts formed at 1300°C-1340°C and 3.0-4.0GPa. Thus, their parent magma might be derived directly from the asthenosphere with low partial melting of CO2-bearing peridotite. Siderite crystallized via fractional crystallization during magma ascending process.