HOW ARE SEDIMENTS ADDED TO THE LOWER CONTINENTAL CRUST?

Sediments metamorphosed under high or ultra-high pressure conditions are found both within metamorphic terranes and as xenoliths from the deep crust carried to the surface in fast-erupting magmas (e.g., alkali basalts, kimberlites). The latter are emplaced into the lower crust where they remain until entrained by the host magmas. We seek to understand how sediments can be transported to the base of the crust and stay there beyond a single orogenic cycle. Several processes could be important, including thrust faulting in back- or fore-arc settings or relamination in subduction zones (Hacker et al., 2011, EPSL). These processes should yield distinctive P-T-t paths.

High-grade (gnt-sill) metapelites from Kilbourne Hole, New Mexico, represent such lower crustal lithologies. This young maar erupted within the Rio Grande rift also hosts mafic garnet-pyroxene granulites. Preliminary pseudosection modeling (Perple_X) shows that while the mafic granulites equilibrated at pressures of 1.7-1.9 GPa and 870-960°C, the metapelites equilibrated at shallower levels, but hotter temperatures: 0.6-1.3 GPa and 910-980 °C (temperature also constrained via Zr-in-rutile). While the major mineral assemblage of the metapelites shows no evidence of ultra-high pressure metamorphism, as might be expected if they formed by relamination, it is possible that heating associated with the Rio Grande Rift has obliterated much of their prograde path. Future work will seek to constrain the prograde path based on inclusions in garnets (quartz barometry, aluminosilicates, etc.), as well as thermochronology to determine depths of entrainment and the timing of their emplacement into the lower crust.