HYBRID IGNEOUS ROCKS AND THE PRESERVATION OF EVIDENCE OF DEEP-CRUSTAL UHT METAMORPHISM

The production of some intermediate arc magmas and hybrid igneous rocks (i.e. norites, diorites, tonalities) has been attributed to, amongst other theories, contamination of mafic magma by partially melted crust. Studies on the partial melting of aluminous country rocks and the assimilation of that melt into a primary mafic magma focus principally on the melted products, and assume that any entrained source material will be partially or wholly dissolved. Monzonorites from the Cortlandt Complex, NY formed via such ultra-high temperature (UHT) melt mixing processes, however some are chemically and texturally related to entrained aluminous xenoliths (consisting of a residual mineral assemblage of spl-mt-ilm-hem-sil-spr-crn). Recent quantitative thermodynamic modeling suggests that only a fraction of the partial crustal melt produced during the first few hundred degrees of heating can chemically mix with a mafic melt to produce a hybrid rock; the remaining fraction of the modeled unmelted crustal material predicts the same mineral assemblages as seen in the entrained xenoliths. Therefore, although the monzonorites at the Cortlandt formed via generally accepted magmatic processes, the aluminous xenoliths represent preserved residual crustal material that endured peak temperature conditions representative of magma emplacement (~1150 ˚C). Hybrid igneous rocks are commonly found in various igneous and metamorphic terranes around the world, yet residual pelitic material is much less ubiquitous. Density calculations estimate that at peak temperature the residual xenolith has a specific density of ~3.8 g-cm^-3, which are comparable to the density of the upper mantle. These results, combined with geophysical studies and observations of xenolith placement within the Cortlandt Complex, suggest that the occurrence of the xenoliths is due to erosion to the deepest portions of the magma chamber. Therefore, hybrid igneous rocks in shallower crustal levels may be tracers for deep UHT pelitic residuum but, due to the density of the residual material, the (possibly very common) residuum sinks to unexposed depths possibly as low as the crust–mantle boundary.