Mantle source, magma differentiation and sulfide saturation of the ∼637 Ma Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block, Central China

Abstract

The sill-like Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block formed at 637±4Ma and consists of a lherzolite unit and an olivine gabbronorite unit, with a marginal zone between the lower margin of the olivine gabbronorite unit and the underlying country rocks composed of marble and schist of the Meso- to Neoproterozoic Zhujiashan Formation.Rocks from the lherzolite unit have 41.4–44.0wt.% SiO2 and 29.0–38.4wt.% MgO and contain olivine with Fo values ranging from 80 to 85mole%, indicating crystallization from a moderately evolved magma. They commonly have <0.1wt.% S and low, variable PGE (2–40ppb) and display flat primitive mantle-normalized chalcophile element patterns with relatively low Cu/Pd ratios (3900–427,415), indicating derivation possibly from S-undersaturated magmas. Rocks from the olivine gabbronorite unit contain 44.0–47.1wt.% SiO2 and 22.5–28.9wt.% MgO and contain olivine with relatively low Fo values (67–68mole%), indicating crystallization from a more evolved magma. They contain 0.03–0.9wt.% S and <15ppb PGE and have Cu/Pd ratios (462,500–10,915,500) much higher than those for the rocks from the lherzolite unit. Rocks from the olivine gabbronorite unit show relatively steep, trough-like patterns depleted in PGE relative to Ni and Cu, indicating that they formed from sulfide-saturated magmas. Rocks from the marginal zone have relatively high SiO2 (45.1–53.1wt.%) and low MgO (7.4–19.6wt.%) and have chemical composition varying between the olivine gabbronorite and the country rocks. They contain 0.57wt.% S and <10ppb PGE, with high Cu/Pd ratios (404,300) and chalcophile element patterns similar to the olivine gabbronorite unit. Rocks from the lherzolite unit have (Th/Yb)PM ratios (3.7–9.1) much lower than the upper crust (28.2) and the country rocks (41.3), indicating small degrees of crustal contamination. Their ɛNd(t) (−4.4 to +1.1) and γOs(t) (8–36) values indicate an EMI mantle source. Primary magma was PGE-poor because sulfide retained in the mantle source due to small degrees of partial melting of the enriched mantle source.We propose that moderately evolved magmas may have reached sulfide saturation due to fractionation of olivine in a deep-seated staging magma chamber, forming an olivine-laden crystal mush with minor sulfide droplets in the lower part and more evolved, PGE-depleted magmas in the upper part of the chamber. The evolved, PGE-depleted magmas were forced out of the staging chamber and entered the shallow magma chamber to form the olivine gabbronorite unit. The lherzolite unit formed from the emplacement of the olivine-laden crystal mush into the same chamber. Because of paucity of extensive crustal contamination, only minor amounts of sulfide segregation occurred in the olivine gabbronorite unit and the marginal zone. The Zhouan intrusion thus may not be a good target for economic Ni–Cu sulfide deposit.