Dimensions and characteristics of intrusive complements to silicic ignimbrites: insights from the Wyoming batholith, USA

Abstract:

Large volumes of silicic ignimbrite are preserved in the geologic record, including Cenozoic examples such as the Altiplano-Puna plateau of the Andes and the volcanic provinces of the western United States. The magma chambers supplying these eruptions are inferred to be silicic, but the structural, petrologic, and geochemical details are unknown because the batholiths are not exposed. We propose that the Neoarchean Wyoming batholith represents an analog for the plutonic complex underlying these ignimbrite systems and provides an opportunity to examine the shallow magma chamber directly. This 2625 Ma batholith extends for more than 200 km across central Wyoming in the Granite and Laramie Mountains. The magnesian high-K granite batholith exhibits continental arc chemical and isotopic characteristics. Aside from more leucocratic margins, the batholith is petrologically and geochemically homogeneous biotite granite with no macro-scale zoning or heterogeneity despite being exposed at different structural levels. These characteristics are consistent with a sill-shaped magma chamber, where the rise of buoyant evolved liquids and sinking of dense cumulates promotes effective mixing by vertical convection. Nd isotopic variations across the batholith indicate that horizontal homogenization is incomplete, preserving information about the feeder system to the batholith and variations in magma sources. Excellent exposure of the batholith provides the opportunity for future detailed work to investigate magma chamber processes, including chamber architecture, evidence of eruption, and detailed time-scale over which the batholith was constructed. In addition, parts of the batholith have been affected by late-stage hydrothermal fluids, which provide an opportunity to investigate the effects of alteration on geophysical measurements of subsurface granite intrusions.

Photo showing incorporation of marginal leucocratic granite within biotite granite of the Wyoming batholith. The large, folded enclave of leucogranite is interpreted to have detached from the batholith roof and been deformed during magmatic flow along with other smaller and wispy inclusions of the leucocratic granite.