Abstract
Sagsai Pluton intruded the low-grade metavolcanosedimentary complex of the Cambrian Tugrug Fm. in the Sagsai river valley SW of Tsogt soum, Gobi-Altai aimag (SE Mongolian Altai). This oval body (similar to 9 x 12 km) is composed mainly of amphibole monzogabbros to quartz diorites, biotite to amphibole-biotite syenites to quartz monzonites.
It is intruded by anatectic two-mica granites along the W rim. The abundant mafic microgranular enclaves enclosed by syenites to quartz monzonites are interpreted as an evidence of vigorous magma mingling and mixing.
The newly obtained LA ICP-MS zircon ages for two samples of quartz monzonites gave emplacement ages of 307 +/- 1 Ma and 309 +/- 2 Ma (2 sigma), respectively. The zircons highly have epsilon(t)(Hf) values varying broadly from +2.4 to +11.6 ruling out a participation of an old, mature crust.
One gabbro body gave zircon U-Pb age of 322 +/- 2 Ma with positive epsilon(t)(Hf) values of +7.4 to +11.5. The latter suggest an exclusively juvenile, depleted mantle-derived magma source.
According to calculated P-T conditions, we propose three main stages in the Sagsai Pluton evolution. The gabbro started to grow at >1000 degrees C and 0.8-0.9 GPa.
The ensuing magma mixing with crustal melts and final crystallization of gabbro occurred at similar P-T conditions (740-830 degrees C and 0.5-0.6 GPa) as partial melting of migmatite xenoliths in the Sagsai Pluton. The P-T conditions of the Sagsai Pluton final emplacement calculated from quartz syenite (c. 600 to 670 degrees C and 0.4-0.5 GPa) are consistent with the mineral assemblage in the contact aureole (cordierite-biotite and biotite-chlorite schists).
The magmatic evolution of Sagsai Pluton confirms a model of deep lithosphere thermal anomaly associated with heterogeneous melting of crust in a post-subduction extensional regime, perhaps shortly after the slab break-off.