A crystal mush origin for the high Sr/Y plutons: Insights from the Nyemo Igneous Complex in the Gangdese arc, southern Tibet

High Sr/Y plutons generated in continental arc resemble geochemically the Archean trondhjemite-tonalite-granodiorite suite and are widely used to interpret processes of intermediate-felsic crust generation during the Archean. However, this geochemical signature could be overinterpreted without detailed petrographic obser-vation. Here we carry out a study of detailed field and petrographic observations, geochronology, and geochemistry for the Nyemo Igneous Complex (NIC) in the central Gangdese arc, southern Tibet. Our results show that the NIC is composed of gabbros, monzogabbros, monzodiorites, monzonites, and monzogranites that were formed at 91-89 Ma. Petrographic observations and geochemical data suggest that the diverse lithologies of the NIC can be ascribed to a cogenetic magma fractionation sequence defined by olivine -> orthopyroxene + clinopyroxene + apatite + Fe-Ti oxides -> plagioclase + biotite -> K-feldspar -> amphibole. The early crystallization of biotite and relatively late crystallization of amphibole indicates that the primary melts of the NIC have low water contents. The monzodiorites and monzogranites from the NIC have high Sr (412-1026 ppm), low Y (4.30-25.4 ppm), and thus high Sr/Y ratios (16-140). The orthocumulate texture defined by euhedral pheno-crysts of normally zoned plagioclase and subhedral biotite indicates these high Sr/Y rocks are unlikely pure melts and instead represent fossil plagioclase-rich magma mush. We emphasize that the high Sr/Y signature does not necessarily reflect the pressures of magma generation involved of garnet/amphibole as residual or fractional phases. Late Cretaceous arc magmatic rocks in the eastern and central Gangdese arc show different crystallization sequences, requiring the presence of contemporaneous primary melts with distinct H2O contents along the Gangdese arc, which probably were caused by variable water input from the Neo-Tethyan oceanic lithosphere.

Automated summary

This study investigates the Nyemo Igneous Complex (NIC) in the central Gangdese arc, southern Tibet through detailed field and petrographic observations, geochronology, and geochemistry. The results show that the NIC is composed of various rock types formed at a specific time period and has a high Sr/Y ratio. Petrographic observations and geochemical data indicate that the diverse lithologies of the NIC can be explained by a co-genetic magma fractionation sequence.