4d Architectural Variation in the Gobi-Tienshan Intrusive Complex, Southern Mongolia

The Gobi-Tienshan Intrusive Complex is a recently characterized tilted section through the upper 15 km of a Carboniferous aged continental margin arc in the Gobi desert, southern Mongolia. Preliminary geochronology indicates that the GTIC, a 2,500 km2 batholith, intruded over a 21 m.y. timespan. This high- flux magmatic event is reflected in field relationships identified through detailed mapping of a depth-transect through the crustal section, which revealed dramatic changes in magmatic architecture at different crustal levels. The deepest exposed levels (12-15 km) are complex mingling zones comprised of 10s to 100s of meters scale intrusions of granite to quartz diorite intruded by mafic dikes that display ubiquitous mixing and mingling structures. These dikes disaggregate into enclaves and hybridize with host magmas to generate magmas that re-intrude and re-mix with the host magmas. Even though these dikes are observed over a huge area, as well as intruding a huge range in host magma compositions, their mechanical behavior when intruding host crystal mushes is identical. At shallower levels (6-10 km), multi-km scale granodioritic plutons host literally hundreds of millions of microgranitoid enclaves, which can be texturally and geochemically linked to mafic source materials deeper in the complex. At this crustal level, large granodioritic plutons host granitic intrusions that are organized into sets of 100 meter wide sheets. These sheets contain granites with a wide variety of textural and geochemical characteristics, including many high K granites. Contact relationships indicate that granodiorites were partially molten at the time of granitic intrusion, but major mixing and mingling between these units did not occur. The shallowest levels of the complex show more evolved granites and syenogranites that contain sheets of quartz dioritic magma that is observed to have hybridized with units in the subvolcanic level. Finally, all units are cross-cut by an additional dike swarm of basaltic to rhyolitic composition, which does not cross into the youngest subvolcanic intrusions. The injection of mafic magmas and their hybridization with host magmas are a ubiquitous and time-transgressive feature in the batholith. Meanwhile, the magmatic architecture of the GTIC transitions with depth from complex mingling and mixing to large homogeneous bodies to subvolcanic intrusions with mingling structures. We also observe architectural transitions through time, from the building of magma bodies, internal processes occurring in plutons, and the final cooling stages of the magmatic system that may provide clues to the evolution of rheological behavior during these stages and with depth in the arc.