Vertical changes in magmatic architecture, hybridization, and geochemistry in a tilted arc crustal section of the Gobi-Tienshan Intrusive Complex, Mongolia.

摘要

The Gobi-Tienshan Intrusive Complex (GTIC) in southwestern Mongolia is a late Carboniferous (299-292 Ma) magmatic complex in the southernmost region of the Central Asian Orogenic Belt, a massive tectonic collage situated between the Siberian, North China, and Tarim blocks. The GTIC is tectonically important for its position in the complex late Carboniferous to early Permian subduction-collision zone that represents the final amalgamation of the Central Asian Orogenic Belt. The GTIC is also important from the standpoint of magmatic studies because it exposes an uninterrupted depth-transect from plutons emplaced at 12-15 km depth (confirmed by Al-in-hornblende thermobarometry) to the correlative overlying volcanic pile.;Detailed mapping revealed a short-lived magmatic system with depleted mantle, lower crustal, and upper crustal sources. Mafic and felsic magmas are observed interacting in a variety of physical settings at variable emplacement depths. In the deepest portions of the section, small scale intrusions of a spectrum of compositions are injected with mafic dikes that disaggregate into enclave-shaped bodies and generate hybrid magmas with their hosts. This area is the site of extensive magma hybridization. In the intermediate emplacement levels of the complex, large granodioritic plutons contain copious mafic microgranitoid enclaves, including one 13 km2 area studied in detail that is composed of 30-50% enclaves. Thermobarometry in this area revealed isobaric crystallization for granodiorite and enclave magmas, suggesting emplacement by a rising diapir thermally driven by mafic magma injection. Finally, the shallowest plutons are observed in contact with volcanic roof rocks. This subvolcanic system is also a site of local hybridization between granitic magmas and injected mafic dikes, and may be the site of alteration of the volcanic geochemical signal, generating discrepancies between trace element compositions of plutons and volcanics. Thus, intrusive style and pluton size vary significantly with depth in the GTIC and magma mingling and some hybridization are observed at all crustal levels.;Our tectonic interpretation of the GTIC is that it represents one portion of a broader subduction event in the late Carboniferous that generated a flare-up of subduction related magmatism immediately pre-dating the continental collision and the final amalgamation of the Central Asian Orogenic Belt.