Geochronology of a Bouguer Gravity Low

Abstract

Dense drill core and subsurface mapping at the Questa molybdenum mine, combined with regional structural tilt, permit detailed 3‐D documentation of plutonic rocks in the Latir magmatic center, New Mexico. Integration of mapping with new and existing U‐Pb zircon geochronology (a total of 136 analyses from 15 intrusions) allows for examination of the space‐time evolution of the Bouguer gravity low associated with the magmatic center. The Questa batholith at Latir was assembled by discrete pulses of magma over a 5.5‐Ma interval (approximately 25.3–19.8 Ma) of generally downward stacking intrusions following the volcanic peak at 25.52 Ma. This contradicts interpretations that the Bouguer gravity lows here and elsewhere are the plutonic crystal cumulate residues left after extraction of large‐volume silicic magmas. Estimates of magma flux based on the volume of plutonic rocks inferred from gravity data combined with the extrapolation of geochronology of exposed surface rocks are questionable because the assumption that unexposed plutonic rocks are the same age as dated surface rocks directly above them likely underestimates the true range of ages.

Rapid Magma Generation or Shared Magmatic Reservoir? Petrology and Geochronology of the Rat Creek and Nelson Mountain Tuffs, CO, USA

“…Finally, the estimates of caldera area used to estimate the original magmatic footprint and the volume of caldera infill rely on the topographic margin of the caldera rather than the hidden structural margin, thereby yielding a slight overestimate. However, even with the error associated with these estimates, the flux estimates are comparable, and far from the orderof-magnitude difference between plutonic and volcanic units reported by other authors (Annen, 2009;Mills and Coleman, 2013;Gaynor et al, 2019a). Average, plutons 3.2 × 10 −3…”

“…These phenomena are integral when considered in the context of calculating magmatic fluxes. A common contention in the field of magmatic petrology is that the rate of magma transport (in km 3 /yr) dictates the behavior of the system, such that low fluxes lead to solidification of magma and production of plutons, while high fluxes lead to the generation of ignimbrites (Coleman et al, 2004;Glazner et al, 2004;Mills and Coleman, 2013;Caricchi et al, 2014;Gaynor et al, 2019a). In this model, the high fluxes necessary for the FIGURE 8 | Proposed model for the eruption of the San Luis ignimbrites, represented as a cross-section through the geological map in Figure 1 (see inset): (A) a laterally extensive zoned (rhyolite-dacite) magma body is the predecessor of the RCT and NMT.…”

“…The Mount Princeton Batholith in the northern SRMVF yields an estimate of 2.4 × 10 −3 using area estimates and vertical exposure (for a cylindrical geometry) in addition to ID-TIMS zircon ages from Mills and Coleman (2013). Meanwhile, estimates from the nearby Questa-Latir region [recently compiled by Gaynor et al (2019a), including data from Tappa et al (2011), Gaynor et al (2019b), and Rosera et al (2013)] yield estimates of 0.8 to 4.1 × 10 −3 km 3 kyr −1 km −2 ( Table 2). On average, plutonic units have fluxes of 3.2 × 10 −3 km 3 kyr −1 km −2 compared with 3.8 × 10 −3 km 3 kyr −1 km −2 for volcanic units mentioned above, consistent with the hypothesis of cogenetic origin.…”

Multiple, Coeval Silicic Magma Storage Domains Beneath the Laguna Del Maule Volcanic Field Inferred From Gravity Investigations

“…Relative to metamorphosed basement rocks, granites commonly yield negative Bouguer anomalies (e.g., L. Cordell et al., 1985; Gaynor et al., 2019) if the Bouguer correction (i.e., surrounding crustal) density is ∼2,600–2,700 kg/m 3 . Metamorphosed basement rocks in the Chilean Central Andes near Laguna del Maule are typically at 2–3 km depth (Godoy et al., 1999) and have densities of ∼2,700 kg/m 3 .…”

The Granite Aqueduct and Autometamorphism of Plutons

“…Spatial patterns of U-Pb zircon dates in the Tuolumne Intrusive Suite [2] and field and geochronologic evidence from other examples (e.g., [19][20][21][22][23]) indicate that large plutons commonly grow by downward stacking of intrusive sheets. In the Tuolumne Intrusive Suite, this pattern applies both to the relations between major map units (Kuna Crest, Half Dome, Cathedral Peak) and to the internal structure of the Half Dome, where crosscutting relations in the field confirm that each deeper compositional cycle is younger than the overlying one [4].…”