Effects of shallow basaltic intrusion into pyroclastic deposits, Grants Ridge, New Mexico, USA

A localized aureole up to 10 m wide developed around a 150-m-wide, 2.6 Ma b asaltic plug at Grants Ridge, New Mexico. The plug intruded into nonwelded, pumice-rich compositionally homogenous tuff and volcaniclastic sediments o f similar age (3.3 Ma). Color variation (pinkish to orange), strong local c ontact welding, brecciation, partial melting, and stoping characterize the host rock within the contact zone. Despite the high-temperature basaltic in trusion, there is no indication of extensive fluid-driven convective heat t ransfer and pervasive hydrothermal circulation and alteration of the countr y rock. The proportion of volcanic glass, loss on ignition (LOI), fluorine, iron, and some trace and rare earth element contents in the host rocks are somewhat depleted at the contact of the intrusion. Conversely, the degree of devitrification and the potassium content are higher along the contact. Vapor-phase expulsion of elemental species as complexes of fluoride, chlori de, hydroxide, sulfide, and carbon dioxide may have been responsible for th e minor depletion of the elements during the devitrification of silicic gla ss at near-solidus temperature related, to the basaltic intrusion. The resu lts of finite-difference numerical modeling of the intrusion as a dry, cond uction-dominated system agree well with geochemical and mineralogical data. Contact welding of the host rocks apparently occurred at temperatures > 70 0 degrees C under a density-driven lateral load of approximately 1 MPa, cor responding to the observed depth below the former ground surface of similar to 100 m. Other physical changes in the first 10 m of host rock, represent ed by partial devitrification and color changes, apparently occurred at tem peratures of 500-600 degrees C, which probably persisted for up to 55 years after the emplacement of the basaltic plug. Devitrification is generally e nhanced by the presence of aqueous fluids; however, the abundance of volcan ic glass within a short distance (similar to 10 m) from the plug is consist ent with our inference that the plug intruded into a dry (unsaturated) envi ronment. (C) 1999 Elsevier Science B.V. All rights reserved.