SYLVANITE INTRUSIVE COMPLEX, LITTLE HATCHET MOUNTAINS, HIDALGO COUNTY, SOUTHWESTERN NEW MEXICO
The SIC consists mainly of two bulbous intrusions with overlapping compositions. The southern, shallower, bulb is slightly older and in general more mafic with slightly lower incompatible trace element concentrations (SiO2=54-61%, MgO=2-4%, Nb=819 ppm, Zr=217248 ppm, Ba=6641544 ppm, Sr=4051235 ppm, Rb=40151 ppm, Na2O + K2O=69.75%) than the northern, deeper, younger bulb (SiO2=59-63%, MgO=1.752.25%, Nb=1322 ppm, Zr=190252 ppm, Ba=4911361 ppm, Sr=5251179 ppm, Rb=60121 ppm, Na2O + K2O=711.25%). Higher incompatible trace element concentrations at comparable SiO2 in the northern bulb suggests that the major differentiation processes were occurring at greater depth, prior to intrusion at the subvolcanic level.
Modern arc volcanoes have roughly cylindrical, magma chambers that range in size from 7 to 10 km tall and 0.75 to 5 km wide, with the upper surface at depths between 7 and 10 km. By restoring post-emplacement deformation and considering stratigraphic thicknesses, we estimate that the minimum dimensions for the SIC were 1.6 km diameter, 7.2 km tall, with the upper surface at 3 km depth. These data suggest that the SIC was similar to a modern arc subvolcanic magma body.