Geochemical characteristics of Early Cretaceous gabbro in Muzidian area,North Dabie and its indicative significance for tectonic environment
In order to reveal the genesis and tectonic background of basic rocks in Dabie orogenic belt,LA-ICP-MS zircon dating and geochemical analysis were carried out to discuss the diagenetic age and geochemical characteristics of Leijiawan gabbro in Muzidian area,North Dabie,and by comparing with the basic-ultrabasic rocks in Anhui section,the genesis and tectonic environment of Dabie orogenic basic-ultrabasic rocks are revealed.The results show that the diagenetic age of the Leijiawan gabbro in the Muzidian area of North Dabie is 122.3±1.0 Ma.The SiO2 content of the Leijiawan gabbro is between 45.00%and 50.69%,with an average of 48.02%.The Mg#value ranged from 45.62 to 53.49,with an average of 47.86.The content of K2O+Na2O varied from 4.68%to 7.03%,with an average of 6.31%,belonging to the alkaline series.It is enriched in light rare earth elements and large ion lithophile elements(LILE)such as Rb,Ba,K and Sr,and depleted in heavy elements and high field strength elements(HFSE)such as Ta,Nb,Zr and Ti.It has similar field and geochemical characteristics to the mafic-ultramafic rocks of Anhui.The petrogenesis study shows that the mantle-derived magma of Leijiawan gabbro was added with crustal materials,and the mode of action is the mixing in magma source.The addition of crustal material may be related to the delamination of thickened lithosphere after subduction of Yangtze plate to North China plate since Triassic.The occurrence of mafic-ultramafic rocks at ca.130 Ma in Dabie orogenic belt may imply the structural system transforms from compression to extension.
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