Title: Geochemical constraints on petrogenesis of Hashatubei ultramafic complex, Inner Mongolia
Abstract: Hashatubei ultramafic complex, which located in the northwestern margin of North China craton, is characterized by a small exposed range and various petrographical faces, and dominated by serpentinizated peridotite, olivine websterite, diallage pyroxenolite, hornblende pyroxenolite, and less grabbro and quartz-diorite-porphyrite. It is similar to Alaska ultramafic complex in petrographic characteristics, serpentinizated peridotite was placed in the center of the complex, and pyroxenolite in the margin, petrochemical features show the complex belong to ferreous ultramafic rocks with higher contents of Al2O3 and CaO etc.. Based on petrographical and geochemical signatures, ultramafic Pocks can be classified into two series. First series with coarse grain minerals and flat-type chondrite normalized diagram for REE, is rich in U and Ph, depleted in LILEs (Rb, Cs and Be) and HFSE (Nb, Ta and Ti), and varies largely in Zr, Hf and HREE etc.; which imply series 1 was formed in the process of in situ cumulus crystallization in magma chamber by magma which derived from higher degree of enriched lithospheric mantle melting and intruded by stage after experienced slight fractionation ill continent crustal extension. Second series with compact massive texture, brown weathering surface caused by abundant magnetite oxidization, is rich in Nb , Ta, Ph and LREE, depleted in Rb, Ca, Ii and Ti, and has highest Sigma REE, which imply that this series was formed by magma originated from small degree of enriched mantle melting in initial ocean crustal setting. The ratios of trace element with same distribution coefficient (K-D) of two series distributed clearly in two different fields respectively, Nd isotopes vary in a very small range (Nd-143/Nd-144 = 0.5122 similar to0.5124), which indicated the magma source was EMI mantle domain. Petrographical and geochemical results lead to the following conclusions, The complex was a long-lived magma chamber, different type of ultramafic rocks were crystallized br magmas from two different source in different tectonic settings, the northwestern margin of North China craton experienced a long-term crustal accreting event, and developed from extension and thinning of continental crust to initial oceanic crust in Proterozoic era. Metamorphic intermediate-basic volcanics in southern country rock (Pt1-2 muscovite-quartz-schist) have similar geochemical features and couple in tectonic settings evolution with the complex.
Publication Year: 2001
Publication Date: 2001-04-01
Language: en
Type: article
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