Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Autor: Wenjiao Xiao, Hanlin Chen, Shufeng Yang, Zilong Li, Chuan-Wan Dong, Yoshiaki Tainosho
Rok vydání: 2006
Předmět:
Zdroj: The Island Arc. 15:210-222
ISSN: 1440-1738
1038-4871
DOI: 10.1111/j.1440-1738.2006.00510.x
Popis: A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse-grained and fine-grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high XMg and low Al2O3 contents, whereas clinopyroxenes have low TiO2 and Al2O3 contents. Amphiboles and biotites have a high Mg/(Mg + Fe2+) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al2O3, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid-oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc-alkaline basalt and andesite with an island-arc affinity. The rock has a high 143Nd/144Nd ratio with ɛNd(0) > 0, indicating derivation from a mantle-depleted source. In the present study, a two-stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc-alkaline basalt and andesite with island-arc affinity were subducted into a deeper level of the crust and subjected to granulite-facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.
Databáze: OpenAIRE
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