EXAMINATION OF A VOLCANIC-PLUTONIC CONTACT IN A TILTED MAGMATIC ARC CRUSTAL SECTION IN SOUTHWESTERN MONGOLIA

Much of Mongolian geology consists of a large collage of collided crustal and oceanic fragments, called the Central Asian Orogenic Belt. In the remote southwestern corner of Mongolia, the Gobi Tienshan Intrusive Complex (GTIC), one of these collided fragments, represents a tilted magmatic arc crustal section from the paleo-surface to 12 km depth. Our work included mapping the surface volcanic sequence and sub-volcanic plutons and examining samples collected along transects across their contact. Mapping revealed a discordant intrusive relationship along the eastern contact and a western contact concordant with overlying volcanic rocks that demonstrated a gradational reduction grain size approaching the contact from fully crystalline to aphanitic groundmass over 50-100 m. This result suggests a very shallow, cool intrusive environment. The two main observed subvolcanic rock types are rapikivi porphyries and K-feldspar rich syenogranites. Meanwhile, volcanics display compositions from rhyolites and dacites to rarer basaltic andesites. This mis-match suggests that the presently exposed subvolcanic chamber is one of many subvolcanic systems that existed in the GTIC, but is the only preserved due to either its late cooling, or its position in the plane of the currently exposed section.

Whole rock geochemical analyses confirm that volcanic rocks have multiple sources over time, including, but not limited to, the currently exposed subvolcanic chamber. Rare earth element patterns for volcanics across a wide compositional spectrum and granitic subvolcanic units are “sea-gull” shaped with Ce/Yb ratios falling between 2.8 and 8 for all units excepting one outlier. Meanwhile, plutonic units of granitic and granodioritic compositions display much more complex REE patterns. The cause for homogeneity in REE patterns is unclear, but a single source for these units is ruled out by Sr and Nd isotopic diversity that generally matches that of the deeper batholith. We hypothesize that this result implicates some accessory phase related process in GTIC subvolcanic chambers, including that presently exposed. Isotopic data also shows us that crustal sources were tapped throughout the complex, including the subvolcanics and volcanics, providing evidence for a continental margin tectonic setting for the GTIC.