关键词: 同位素年代学, 地球化学, 岩石成因, 笔架山火山岩带, 新疆北山地区

Abstract:

The Permian Bijiashan volcanic belt is studied. Based on geochronological petrologic geochemical data of the volcanic rock, the authors discuss the genetic relationship between the volcanic rocks and intrusive rocks, magmatic evolution as well as source character. The Bijiashan volcanic rocks located at the north  Bijiashan mafic-ultramafic belts are composed of basalt, basaltic andesite, andesite, dacite and rhyolite. LA-ICP-MS zircon U-Pb dating of rhyolite suggests an isotopic age of (285.0±2.9) Ma, belong to the Wulaer period of the Early Permian. The volcanic rocks mainly belong to tholeiitic series. The petrological and geochemical characters of basalt, basaltic andesite, andesite in this belt indicate that they are derived from tholeiitic basaltic magma by fractional crystallization, and the basaltic magma have assimilated during upwelling. The basalt has high TiO₂ (2.35%-3.81%), TFeO (10.59%-13.87%) and P₂O₅(0.35%-0.50%), and belongs to high-Ti basalt series, which is rich in LREE and large ion lithophile elements, Nd, Sr isotopic composition with OIB affinity. It is believed that these basalts were derived from spinel-region. The temporal relationships, geochemical characteristics and magmatic evolution of Bijiashan intrusive rocks and volcanic rocks indicate that the Bijiashan mafic-ultramafic intrusive rocks and the volcanics are derived from the same magma, the numerous cumulations form intrusive rocks after magma entered present chamber, whereas the evolved magma overflowed along the faulted zones or the upper boundary of magma chamber and formed the basic-intermediate volcanic rocks, and acidic volcanic rocks are caused by lower crust re-melting caused by the heat of the mantle-derived magma.

Key words: isotopic geochronology, geochemistry, petrogenesis, Bijiashan volcanic rocks, Beishan region in Xinjiang