Zircon Hf-O and apatite Sr-O isotopes jointly reveal the origin and mineralization of the Miocene adakitic porphyries in the Gangdese belt

Abstract

The Gangdese belt is a significant belt for exploring the dynamic processes of continental collision orogeny and mineralization. The impact of petrogenesis and chemical heterogeneity of the E-trending Miocene adakitic rocks in Gangdese porphyry deposit belt on mineralization mechanisms has always been controversial. New whole-rock Sr-Nd isotope, zircon U-Pb dating and Hf-O isotope data, especially integrating apatite volatile (Cl and S) and Sr-O isotope compositions, of the Miocene mineralized porphyries from Lakange deposit can provide the development of metallogenic system origin, as well as magmatic and volatile process. The Lakange granites show adakitic geochemical composition affinity with high potassic calc-alkaline. They have high SiO2 content, low MgO and Ni contents, positive εNd(t) value (0.1 ∼ 0.8), and high (87Sr/86Sr)i ratios (0.705088 ∼ 0.705269), positive zircon εHf(t) (7.6 ∼ 11.2), medium δ18O (4.33‰∼7.27‰) values, combining with apatite Sr-O isotopes (87Sr/86Sr = 0.7040 ∼ 0.7169; δ18O = 7.0‰∼8.2‰), implying that the granites are originated from the partial melting of the juvenile lower crust beneath the southern Lhasa subterrane. The Miocene adakitic rocks of the Gangdese metallogenic belt exhibit spatially isotopically heterogeneity in a west to east direction (87°E-93°E). These variations are attributed to the lower crust modification by the oxidized mantle-derived melts/fluids from metasomatic subcontinental lithospheric mantle in Miocene, resulting in the formation of juvenile lower crust, that thickened the crust and shortened the lithospheric mantle. In-situ Sr-O isotope of apatites identified the oceanic components in the volatile sources, indicating that volatiles existed beneath the Lhasa terrane during Neo-Tethyan oceanic slab subduction prior to the Miocene partial melting of the lower crust. The high volatile concentrations (Cl and S) aid in the transport of metals that produce the Gangdese Miocene porphyry deposits.