Wednesday, December 30, 2015

More Evidence for Active Plate Tectonics and Subduction From NeoArchean/PaleoProterozoic North China

Petrology and geochemistry of the Guyang hornblendite complex in the Yinshan block, North China Craton: Implications for the melting of subduction-modified mantle


Ma et al


The hornblendite complexes hosted in the Guyang granite-greenstone terrane form part of the Neoarchean basement in the Western Block of the North China Craton. In this study, we focus on the largest one from this block, previously named the Guyang komatiite, and present results from lithological, geochronological and geochemical studies. The dominant lithology in the Guyang hornblendite complex is greenschist facies hornblendite, and can be divided into clinopyroxene hornblendite (∼75%) and olivine-orthopyroxene hornblendite (∼25%). The oldest calculated Re depletion model ages(TRD) of these hornblendites is of 2454 Ma, and the zircon U-Pb age of the wallrock is 2480 Ma, with single-stage depleted mantle Nd model ages TDM1(Nd) varying from 2.61 to 2.88 Ga. These data suggest that the Guyang hornblendite complexes formed during Neoarchean-Paleoproterozoic time. The hornblendites show low SiO2, high MgO contents and are enriched in Cr, Ni, and LREE with negative Ce anomalies and depleted in Ti, Nb and Ta. Their 187Os/188Os ratios range from 0.11145 to 0.11279, with γOs(2.5Ga) varying from −3.9 to +1.4. Geochemically, the olivine-orthopyroxene hornblendite shows little variation with typical cumulate feature. In contrast, the clinopyroxene hornblendite shows a large range of chemical variation. In the CaO vs. MgO and CaO/Al2O3 vs. MgO diagrams, the clinopyroxene hornblendite shows Opx fractionation trend. Combined with lithological information, we infer that the hornblendite magma was emplaced as a crystal mush, with orthopyroxene as cumulus phase. The Os and Nd isotope composition, negative Nb, Ta, Ce anomalies, and the relationships in Th/Yb-Nb/Yb diagram suggest that the Guyang hornblendites formed from a source mantle that was modified by subduction-related melts/fluids derived from a seawater-altered basaltic slab. Compared with typical Archean komatiites, the rocks of present study show significant differences in lithological and geochemical characteristics, and thus it cannot be named as komatiite. To explain these characteristics, we propose a geodynamic model involving ridge subduction and slab window mechanism to account for the formation of the Guyang hornblendite complex.

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