Geochronology of a Bouguer Gravity Low

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.