Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
The Shuteen Cu-Au prospect, ca. 450km SSE of Ulaanbaatar (South Mongolia) is associated with Carboniferous subduction-related magmatic activity. We initiated a detailed geochemical study to decipher the genetic relationship between subduction-related volcano-plutonic rocks hosting the intrusion-related mineralization.
In the Paleozoic, south Mongolia experienced island arc accretion and Andean-type magmatism. Extensive rifting and formation of the South Mongolian paleo-ocean (SMPO) characterize Ordovician to Silurian times, with migration of the South Mongolian microcontinent in the Devonian closing the SMPO. The 321 Ma Shuteen Pluton and Dusiin Ovoo Volcanic Formation comprise the 200km2 Shuteen Complex, which represents the root of a magmatic arc associated with subduction of SMPO beneath the South Mongolian microcontinent.
Major/trace element data indicate the Shuteen Complex comprises well-differentiated, high-K series rocks, with high Al2O3 compositions and <1 ASI molar ratios, whilst Na2O contents and occurrence of accessory titanite indicate plutonic rocks to be "I-type". The Shuteen Complex has chemical characteristics typical of (calc-alkaline series) volcanic arc rocks. Most Shuteen Complex rocks have high Sr/Y and La/Yb ratios, and low Y concentrations indicative of (Archean) high-Al TTD type (high Al2O 3 trondhjmite-tonalite-dacite) magma, whilst low Sr samples are typical of island arc ADR type (andesite-dacite-rhyolite) magmas. Isotopic data indicate fractional crystallization and partial melting of altered MORB produced Shuteen Complex magma and residual eclogite facies. N-MORB normalized plots of Shuteen andesite and granitoids reveal subparallel enriched LREE, depleted HREE patterns. Initial 87Sr/86 Sr and 143 Nd/144 Nd isotope ratios for Shuteen plutonic rocks indicate a N-MORB source.
High pressure partial melting of subducted slab, including oceanic sediment, produced residual eclogite and source magma for the Shuteen Complex, although our model also requires contamination by crustal material. During its ascent through the mantle wedge, slab-derived magma can assimilate mantle material - however, Shuteen rocks have a Cr:Ni ratio of 2:1, and average Mg-number ∼34, which indicate magma genesis involved negligible mantle contribution. Melting of lower crust was unlikely to produce adakitic-type rocks at Shuteen, as the crust beneath South Mongolia in the Trans-Altai suture zone
Preliminary fractional crystallization modeling, involving hornblende and plagioclase crystallization, and 30% fractional crystallization of a magma with 10% contaminated crustal material, is consistent with geochemical characteristics in plutonic rocks exposed in the Shuteen area.
