Picking Apart a Bouguer Anomaly Using Space and Time

Abstract:

Bouguer gravity data in the Southern Rocky Mountain Volcanic Field (SRMVF) have been used to interpret the architecture of the batholithic roots of the volcanic system (e.g., Lipman, Geosphere v. 3). When combined with geochronology from surface exposures, the data are interpreted as defining rapidly assembled batholiths up to 1000’s of km³ that were once vertically extensive, long-lived, high-melt fraction magma chambers. This provides the foundation for interpreting the evolution of large-volume silicic eruptions, links between plutonic and volcanic rocks, and economic mineralization in the SRMVF and elsewhere. We used surface exposures, mine access and dense drilling in the Questa caldera to develop a detailed 4-D image of the magma system to test inferences about the origin of the batholith.

The bulk of the Questa magma system accumulated between ~27 and 19 Ma, straddling the eruption of the ~500 km³ Amalia tuff at 25.52 Ma. Key observations at Questa are: 1) ages agree with cross-cutting relationships; 2) there is overlap in zircon age spectra between samples interpreted to differ in ages by 10’s of ka, though samples that differ in age by 100’s of ka have no overlap; 3) the only intrusive rocks that are the same age as the Amalia tuff are small ring dikes; 4) economic mineralization (MoS₂) is associated with small bodies, intruded episodically over 400 ka, quickly following eruption of the tuff; 5) zircon ages overlap with low-T chronometers from individual samples; 6) ages of intrusive rocks decrease with structural depth; 7) temporal changes in the isotope geochemistry indicate melts were derived from multiple sources and had minimal interaction, consistent with zircon age populations.

None of the data support the existence of a large-volume, high-melt fraction body during the system’s 8 Ma magmatic history. Rather, they indicate that only small volumes in the system were crystallizing zircon at any time. There is no evidence for a large remnant crystal mush associated with accumulation, evolution and eruption of the Amalia Tuff. Mo-mineralization did not occur via extreme differentiation of a large magma body. Granted, sample availability limits our observations to the upper ~10% of the modeled gravity anomaly; however, speculating that system-wide changes occur just beyond the reach of the data is not justifiable.