40Ar/39Ar age and geochemistry of the ultrapotassic magmatic rocks from Bugula in Nileke, Xinjiang and its tectonic implication.
新疆尼勒克县布谷拉超钾质岩浆岩的40Ar-39Ar年龄和地球化学特征

The ultrapotassic magmatic rocks with tectonic significance have been confirmed at Bugula in Nileke, West Tianshan. It intruded in the Carboniferous Dahalajunshan Formation outcrop area is about 0.02km2, associated with the copper anomalous or mineralized zones. The ultrapotassic magmatic rocks show typical porphyritic texture, and the dominated phenocrysts are K-feldspar, olivine and pyroxene with some residual leucite or its pseudomorph occasionally. The Bugula ultrapotassic magmatic rocks are defined as the plagioleucitites type of ultrapotassic rocks by means of their rock-forming minerals and geochemical characteristics. Their negative Nb, Ta, Zr, Hf, Ti anomalies and positive Pb anomaly in the trace element spider diagram suggest that the Bugula ultrapotassic magmatic rocks were formed in the orogenic stage. The trace element geochemical study indicaes that the ultrapotassic magma might be the products of partial melting of the phologopite-bearing garnet-spinel lherzolite occurred in the depth of a narrow range of 70km to 80km. A plateau age of 274.2±1.6Ma of a flat 40Ar-39Ar spectrum was obtained for the ultrapotassic magmatic rocks by laser stepwise heating method. Its inverse isochron age of 274.3±4.4Ma is agreed with the plateau age within analytical error. So, we suggested that the eastern Awulale Mountains were within the stage of orogenic evolution in Middle Permian at least on the basis of the geochemical and geochronological studies of the ultrapotassic magmatic rocks.