Age of the Khangai batholith and challenge of polychronic batholith formation in Central Asia

Abstract

The Central Asian fold belt (CAFB) exhibits many manifestations of granitoid magmatism. The granite formation is largely determined by the convergence of the Siberian continent with structures of the Paleo� Asian ocean, which was practically continuous from the Late Riphean until the Early Mesozoic. This pro� cess was responsible for development of differentage folded domains (Early and Late Baikalian, Cale� donian, Hercynian, and IndoSinian) and conjugate subductionrelated magmatic (including granitoid) belts. An important role in granite formation also belonged to mantle plumes in addition to conver� gence. Their influence on the lithosphere resulted in the formation of giant batholiths represented by accu� mulations of closely spaced large granite plutons (1). Such batholiths were formed in the Late Cambrian- Early Ordovician (Altai-Sayany), Early Permian (Angara-Vitim), Late Permian-Early Triassic (Khan� gai), and Late Triassic-Early Jurassic (Khentei). The individual batholiths are >120 000-150 000 km 2 in size. The geological position of batholiths is independent from basement structures, and they usually comprise several terranes differing in their geological, including magmatic, history. Therefore, batholiths frequently include granitoids independent of them with respect to their formation time and formation conditions. Nevertheless, such a cooccurrence of differentage granites within a single region provides grounds for assumption of the polychronic nature of batholiths and their very long development history (2). This problem is of principal significance for interpreting the geodynamic evolution of the Central Asian fold belt and understanding the nature of granitoid magma� tism. One of the possible approaches to its solution consists in defining differences in the distribution of igneous rocks of different age groups and different geodynamic origin. For example, igneous rocks of the convergence zone are usually characterized by a distri� bution along belts, while products of plume magma� tism are localized above their sources. Precisely such an approach was used for consider� ing the problem of polychronic granite formation in the Khangai batholith. Recent geochronological investigations have revealed the relatively significant scatter of granitoid ages ranging from 300 to 220 Ma with dates of 269-242 Ma being prevalent (3-5) (Fig. 1). At the same time, these data are insufficient for solving the abovementioned problem, since no geochrono� logical dates are available for several key massifs in the southern, central, and southwestern parts of the batholith. Therefore, geochronological investigations of granitoids were conducted in the Tatsin Gol, Erdene Tsogt, Eginaba, Buyant Gol, and Yarugin Gol massifs located in these areas of the batholith. The Tatsin Gol massif is the largest one in the south� eastern part of the Khangai batholith. It is a typical representative of the granodiorite-granite formation, or the Khangai intrusive complex. The massif is com� posed of hornblende-biotite and biotite granodior� ites, monzogranites, and granites. They were formed during two intrusion phases. The first phase is largely represented by granodiorites accompanied by quartz diorites and tonalities. The rocks of the second phase include mediumto coarsegrained monzogranites and granites usually with a distinct porphyroid texture. For geochronological investigations, a sample of light gray porphyroid biotite-hornblende granites (Sample KHAN�09/46) was taken from the central part of the