Age, geochemical characteristics and petrogenesis of andesites in Maquan basin, Ganhang uranium metallogenic belt

The Ganhang uranium metallogenic belt is located at the junction of the Yangzi plate and the Huaxia plate. The Maquan basin is a uranium-bearing volcanic basin in the middle of this belt, and the uranium ore body is hosted in andesite. However, there is controversy over the rock types and lithological affiliation, and there is a lack of precise rock ages. Therefore, this paper focuses on the study of neutral volcanic rocks in Maquan basin, with the aim of investigating their chronology, petrology, and geochemical characteristics. The results show that the SHRIMP zircon U-Pb age of neutral volcanic rocks is 137.1±1.1 Ma (n=15, MSWD=1.5), which is a product of Early cretaceous magmatism. The neutral volcanic rocks in Maquan basin have a massive and porphyritic texture, with plagioclase as the main phenocryst, and contain a small amount of pyroxene, hornblende, and biotite. They are rich in alkali, high in potassium, low in iron, and poor in titanium, and they are enriched in light rare earth elements and large ion lithophile elements (Rb, Ba, K), but lack high field strength elements (Nb, Ta, Ti). These geochemical characteristics indicate that they are andesites, belonging to the typical high-potassium calc-alkaline volcanic rock series. The andesites have (⁸⁷Sr/⁸⁶Sr)_i=0.70855~0.70882, ε_Nd(t)=-9.77~-7.02, (²⁰⁶Pb/²⁰⁴Pb)_i=18.17~18.27, (²⁰⁷Pb/²⁰⁴Pb)_i=15.56~15.62, (²⁰⁸Pb/²⁰⁴Pb)_i=38.43~38.61. The (⁸⁷Sr/⁸⁶Sr)_i-(²⁰⁶Pb/²⁰⁴Pb)_i and (¹⁴³Nd/¹⁴⁴Nd)_i-(²⁰⁶Pb/²⁰⁴Pb)_i as well as (¹⁴³Nd/¹⁴⁴Nd)_i-(⁸⁷Sr/⁸⁶Sr)_i diagrams indicate that the andesite source region in Maquan basin has the characteristics of enriched mantle EM Ⅱ. The tectonic discrimination diagrams indicate that the andesites in Maquan basin were formed in an intraplate extensional environment. The comprehensive study shows that the andesites in Maquan basin were produced by partial melting of enriched mantle EM Ⅱ mobilized by mantle fluids in an early cretaceous extensional tectonic setting.