Petrogenesis of the Permian Luotuoshan sulfide-bearing mafic-ultramafic intrusion, Beishan Orogenic Belt, NW China: evidence from whole-rock Sr–Nd–Pb and zircon Hf isotopic geochemistry

Abstract

The Permian Luotuoshan mafic-ultramafic intrusion is one of the ~280 Ma mafic-ultramafic complexes located in the Beishan Orogenic Belt at the southern margin of the Central Asian Orogenic Belt, NW China. The intrusion is predominantly composed of wehrlite, olivine clinopyroxenite, troctolite, olivine gabbro, and gabbro. Disseminated sulfides occur in wehrlite and olivine clinopyroxenite at the center of the intrusion. SHRIMP zircon U–Pb dating of a gabbro sample yielded a concordia age of 282.6 ± 2.6 Ma. The Luotuoshan mafic-ultramafic rocks are characterized by elevated εNd (282.6Ma) values from +0.42 to +6.10 and εHf (282.6Ma) values from +7.9 to +14.1, relatively low initial 87Sr/86Sr (0.703919–0.705272) and Pb isotope composition, and low ratios of TiO2/Yb, Nb/Yb, etc., suggesting the Luotuoshan magma was derived from a depleted asthenosphere mantle source. Some arc-like geochemical characteristics such as enrichment of light rare earth elements and large ion lithophile elements (Ba, Sr, Rb, etc.), pronounced negative Nb–Ta–Zr–Hf anomalies, and La/Nb, Ce/Pb, and Th/Yb ratios indicate that the mantle source was previously modified by subduction-related metasomatism. Petrographic features and mixing calculations of Sr–Nd–Hf isotopes suggest that the parental magma of the Luotuoshan intrusion underwent a moderate degree of crustal contamination (5–15%) followed by fractional crystallization process. Petrological modeling reveals that both fractional crystallization and crustal contamination may have played the important role in triggering the sulfide saturation in the Luotuoshan magma. The comparable formation age, petrographic and mineralogical characteristics, geochemistry of the parental magma and evolution process compared with intrusions in the Pobei area, indicate that the Luotuoshan intrusion may also have similar mineralization potential. This also suggests that the mineralization prospection of Permian mafic-ultramafic intrusions in the Gansu Beishan area should not be neglected, and it is worth considering that whether further exploration is worthy of being implemented in this region.