Zircon U Pb geochronology and Sr-Nd-Pb-Hf isotope geochemistry of the Dahuabei pluton from the northern margin of North China Craton: Implications for petrogenesis, tectonic setting and Au Mo mineralization

Abstract

The Late Paleozoic Dahuabei Pluton is located in the Wulashan area at the northern margin of the North China Craton (NCC), adjacent to the Hadamengou gold deposit, the largest gold deposit in Inner Mongolia, China. However, its petrogenesis and its relationship with AuMo mineralization remain debatable. In this contribution, a geochemical, isotopic, and geochronological study was carried out on the Dahuabei pluton to constrain its magma sources, petrogenesis, and tectonic setting, and further discuss the linkage between the magmatism and AuMo mineralization. The Dahuabei pluton mainly consists of monzogranite and syenogranite, with abundant mafic microgranular enclaves (MMEs) in the monzogranite. Zircon UPb dating shows that the ages of the Dahuabei pluton range from 357 to 353 Ma, suggesting an Early Carboniferous emplacement. The crystallization age of the MMEs (355.4 ± 3.2 Ma) is nearly simultaneous to that of host monzogranite (356.5 ± 2.0 Ma). Geochemically, the granitoids show high-K calc-alkaline and weakly peraluminous affinities and are classified as highly to moderately fractionated I-type granites. Moreover, they are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, resembling typical continental arc magmatic rocks. Isotopically, the granitoids have negative εNd(t) (−10.0 to −7.3) and εHf(t) values (−10.0 to −4.4), as well as old Nd (1923–1704 Ma) and Hf (1986–1697 Ma) model ages, suggest that the granitoids were mainly derived from partial melting of ancient mafic lower continental crust, with involvement of mantle-derived components. The sharp contacts, fine-grained igneous textures, numerous xenocrysts from host granites, and similar crystallization ages with host granites of the MMEs suggest a magma mingling process, in which the MMEs represent discrete mafic to hybrid magma globules formed by undercooling after the injection of hotter mafic magma into cooler felsic magma. Geochemical and isotopic data (εNd(t) of −6.2 to −5.4 and εHf(t) of −10.2 to −2.8) indicate that the parental magma of MMEs was derived from partial melting of the enriched subcontinental lithospheric mantle metasomatized by slab-derived fluids and had experienced significant fractional crystallization. The indistinctive chemical and isotopic contrast between MMEs and host monzogranite implies their chemical and isotopic exchange during various degrees of magma mingling. Integrating the geochronological, geochemical, and Pb isotope data of the Hadamengou AuMo deposit and the Dahuebei pluton, we propose that the magmatic-hydrothermal system emanated from the magma mingling event during the emplacement of the Dahuabei pluton might be favorable for the mineralization. The Late Devonian to Early Carboniferous magmatism may have been a trigger for the formation of the AuMo deposits in the northern margin of the NCC.