PRELIMINARY MAPPING, GEOCHEMICAL DATA AND TECTONIC SETTING OF THE GOBI-TIENSHAN TILTED CONTINENTAL ARC SECTION, SOUTHERN MONGOLIA

Initial mapping, major and trace element geochemistry, and Al-in-Hbl barometry detail the upper portion of a newly discovered tilted, continental arc section in southern Mongolia. The tectonic setting of the Carboniferous Gobi-Tienshan intrusive complex (GTIC) (Geomin, 2003) is the Central Asian Orogenic Belt, an arcuate complex of stacked arcs (dominantly juvenile in character) tucked between the Siberian and North China-Tarim blocks. The GTIC is the southernmost magmatic complex in the system and is unusual in its continental and evolved character. Evidence of this continental character includes its calc-alkaline and magnetite-series affinity, and copious inclusion of orthogneissic and paragneissic host materials. 1:100,000 scale reconnaissance mapping of the batholith revealed transitions between 1) sub-areal rhyolitic to andesitic volcanics, 2) extensive hypabyssal intrusions of similar compositions, 3) multi-km scale homogeneous, granitic to granodioritic plutons, and 4) complex magma mixing and mingling zones between granitic to dioritic magmas, including a 4 km diameter enclave megaplume. Portions of the complex hypothesized to represent mid-crust include orthogneissic and migmatized paragneissic complex with a GASP barometric estimation of 5 Kbars (Geomin 2003). Initial T-corrected, Al-in-Hbl barometric results, 2.6 +/- .56 and 2.9 +/- .4 Kbars from granodiorite in the central portion of the section, confirm a depth variation of approximately 10 km in this portion of the field area. The batholith is presently 40 km from north to south, with a projected tilt of ~15o. The complex is uninterrupted by faults with offsets > 7 km, but is bounded to the north and south by major structures. The lack of internal faulting, combined with excellent desert exposures, represents a superb opportunity to study transitions between crustal levels in a continental arc setting and to clarify the tectonic regime during the intrusion and exhumation of the batholith.