2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 88
Presentation Time: 8:00 AM-12:00 PM


ECONOMOS, Rita C., Earth Sciences, Univ of Southern California, 3651 Trousdale Parkway, ZMB 117, Los Angeles, CA 90089, SIAD, Lkhagua Ochir, School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, P.O. Box 46/520, Ulaanbaatar, 210646, Mongolia and PATERSON, Scott R., Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, Los Angeles, CA 90089, economos@usc.edu

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.