COMPOSITION AND AGE OF PLAGIOGRANITOIDS IN THE SOUTH OF THE LAKE ZONE (Western Mongolia)
We consider the geologic structure, composition, and age of early Paleozoic intrusive rock associations of the Tugrug, Udzur-Hunga, Hatan-Hunga, Mandalt, Bayasgalant, and Dut Uul plutons. The plutons are located among late Neoproterozoic-early Cambrian volcanic and volcanosedimentary strata in the southern part of the island-arc system of the Lake Zone in Western Mongolia. We have recognized plagiogranitoid and gabbroid associations of different petrochemical compositions, ages, and geodynamic settings of formation in these plutons. The results of geochronological studies show that the plagiogranitoid associations in the south of the Lake Zone formed in the period from 531 to 481 Ma. Two major stages of intrusive magmatism in the zone have been established in this time interval: island-arc, 531-517 Ma, and accretion-collision, 504-481 Ma. The plagiogranitoid associations that formed at the island-arc stage are the most widespread, whereas those formed at the accretion-collision stage are much scarcer. The intrusive plutons of all ages are composed predominantly of granitoid associations of the tonalite-trondhjemite series, which are calc-alkalic rocks. Study of xenogenic and inherited zircons from plagiogranitoid associations formed at the island-arc and accretion-collision stages of the regional evolution has shown their age range 664-520 Ma. Four age groups of inherited zircon have been recognized (~664, 570-560, 545-531, and 530-520 Ma), which generally correspond to the stages of island-arc (volcanic and intrusive) and ophiolite magmatism and, most likely, reflect the additional magma-generating sources of parental plagiogranitic melts.