Geochronology of a Bouguer Gravity Low. Issue 3 (8th March 2019)

• Record Type: Journal Article
• Authors: Gaynor, Sean P.; Coleman, Drew S.; Rosera, Joshua M.; Tappa, Michael J.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 3(2019)
• Page Start: 2457
• Page End: 2468
• Publication Date: 2019-03-08
• Subjects: batholith formation -- radioisotope geochronology -- pluton emplacement -- thermochronology; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2018JB015923
• Languages: English
• ISSNs: 2169-9313

Abstract

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. Plain Language Summary: Long‐lived magmatic centers produce supervolcano eruptions and crust‐building granites; however, the relationships between the two are controversial. The Questa‐Latir region of northern New Mexico exposes an approximately 6‐km vertical section through a 500‐km 3 magmatic center. Early volcanism peaked with a supervolcano scale eruption at 25.52 million years ago. No magmas associated with this event remained in the shallow subsurface. Over the following 6 million years, pulses of magma intruded into the subsurface and rapidly crystallized, forming a series of downward stacking sheets. Despite the presence of a large volume of fossil magmas in the subsurface, there are no data in the Latir to indicate that the preserved volume represents a large magma chamber capable of generating a supervolcano eruption. Instead, it is likely that supervolcano eruptions and granite formation are discrete events within the lifespan of a long‐lived magmatic center. Key Points: Zircon geochronology of the Questa batholith indicates that plutonic assembly was accomplished through discrete pulses of magma Intrusive sheets were generally downward stacking over 8 Ma and accumulated to approximately 500 km 3, in agreement with gravity data Estimating magma flux by extrapolating igneous volumes using unexposed plutons is likely inaccurate without also extrapolating age range