GEOCHEMISTRY, ISOTOPIC GEOCHRONOLOGY, AND GENESIS OF THE VERKHNYAYA UNDA GRANITOID BATHOLITH

Abstract

The Verkhnyaya Unda granitoid batholith is the largest pluton of the Later Paleozoic Unda complex prevailing among the granitoids of Eastern Transbaikalia. The geology and geochemistry of this batholith is examined in detail, and the representative petrochemical and expansed minor-element characteristics of its granitoids are presented. The granitoids are calc-alkalic rocks of elevated alkalinity with an over-clarke sodium concentration and a clarke potassium concentration. The amphibole-biotite granitoids (subalkaline quartz diorites and granodiorites) related to the main intrusive phase are dominant in the batholith. Its other rocks are subalkaline granitoids. According to K-Ar and Rb-Sr data, the granitoids of the main intrusive phase have ages of 275-250 Ma (Medium-Later Permian), while the youngest granites and leucogranites of the pluton were formed in the Later Jurassic (∼200 Ma). The initial 87Sr/86Sr ratio of the granitoids ranges from 0.7077 to 0.7098, which shows that the crustal component prevailed in their parental magma. The rocks are characterized by belowclarke concentrations of coherent siderophile, incoherent granitophile, and rare earth elements, with especially deficient heavy lanthanides. Their elevated Eu/Eu* ratio and the absence of an Eu minimum in their REE patterns indicate poor magmatic differentiation. The Gazimur quartz diorites and tonalites are shown to belong to the batholith periphery rather than to the Earlier Paleozoic granitoids. The batholith granitoids are comparable with the tonalite-granodiorite-trondhjemite association of batholiths typical of convergent geologic boundaries. Parental magmas of the batholith were probably generated during the differential melting of metabasites in the lower crust.