Volcanic-Plutonic Transition in the Carboniferous Gobi-Tienshan Intrusive Complex, Southern Mongolia

The Gobi-Tienshan Intrusive Complex is a newly characterized tilted section through the upper 15 km of a continental margin arc in the Gobi desert in southern Mongolia. This continuous section includes a spectacularly exposed transition from hypabyssal plutons to effusive volcanic units. Hypabyssal units cover 100 km2 and represent ~ 2 km of paleo-depth. Volcanic units are exposed over an area of 150 km2; thickness has yet to be determined. Preliminary geochronology indicates that the 1500 km2 complex was intruded over the span of 21 Ma, representing a high-flux magmatic event. Plutons and volcanics are interpreted as related and contemporaneous based on: a) the presence of gradational contacts in some areas between fully crystalline rocks and aphanitic rocks with identical pheoncrysts, b) geochemically identical variation in elemental composition (isotopes in progress), and c) overlapping U/Pb ages from zircons. The nature of the contact between plutons and volcanics is variable along strike, sometimes gradational from fully crystalline groundmass to fully aphanitic groundmass with identical phenocrysts, and sometimes a discordantly intrusive contact. In the hypabyssal system, mafic injection and mingling between mafic and felsic magmas is observed. Similar mingling is seen in the mid-crustal (8-10 km) level of the section, and a large dike swarm seen there is likely feeding mafic injections in the hypabyssal system, but does not intrude into the sub-volcanic level. Conclusions thus far include: 1) mingling occurring in the sub-volcanic system mirrors deeper processes, 2) materials feeding into the sub-volcanic system can be constrained by comparing them to the plutons and dikes exposed at deeper crustal levels, and 3) field relationships identified thus far preclude a simple model in which dikes from deeper plutons feed volcanic eruptions; a more complex model of the interaction between hypabyssal plutons and surface eruptions is necessary in this batholith.