PETROLOGICAL CONSTRAINTS ON THE SOURCES OF GRANITES FROM THE HANGAY MOUNTAINS, CENTRAL MONGOLIA

The Hangay Mountains of central Mongolia are the second largest mountain range in Mongolia, with mountains up to 4,000 meters in elevation. They are a domal uplift of Archean to Proterozoic granitic and gneissic basement overlain by Cambrian to Devonian metasediments. This block was deformed during a late Paleozoic orogenic event and intruded by a Permian to Jurassic batholith complex that is over 100,000 km² in area. This study investigates a region in the southern portion of this batholith complex.

Mongolia is located in the Central Asian Orogenic Belt and its modern tectonic framework is influenced by both Himalayan crustal compression to the south and the Baikal rift to the north. The country has a complex geology formed during a series of subduction events, and is comprised of island arc, accretionary wedge, and other associated terranes. The granitoids intruded in the Hangay region both during and after the subduction events influencing the region have been shown to either originate from an enriched mantle source or from the underlying Precambrian craton. (Jahn, 2004)

Twenty-eight samples were collected for analysis. Most of the samples were a homogenous coarse-grained biotite granitoid containing two feldspars. This phase has a published age of 250 Ma (Jahn, 2004). Some biotite-hornblende granitoids were also present, and were collected along with samples of the surrounding metasedimentary rocks. Garnet-bearing pegmatite and aplite dikes frequently crosscut the main phase biotite granite. Xenoliths within the biotite granite represent both the surrounding metasedimentary rocks and a more mafic igneous phase. While most of the igneous samples were massive, some in the northern end of the study area showed a distinct foliation. Geochemical and petrographic characteristics of the samples will be determined in order to evaluate the potential sources of the granitoids.

The highest peaks of the Hangay Mountains preserve an uplifted pre-Oligocene erosion surface upon which Oligocene volcanic and sedimentary rocks are deposited. In the field, samples were collected along an elevation transect ranging from 3223 meters to 2351 meters. (U-Th)/He ages using five of the samples will be measured in order to provide constraints on the timing and mechanism of doming and uplift of the Hangay.