Basaltic composition xenoliths and the formation, modification and preservation of lower crust

Abstract

Due to "flake tectonics' the deepest levels of the crust are infrequently exposed and these exposures yield scanty information on lower crustal composition. Evidence based on xenoliths is direct, but is also circumstantial as the xenolith populations are non-representative. The xenoliths, and magmas derived from the lower crust, may be more instructive in indicating mechanisms for the evolution of the lower crust, than in directly revealing lower crustal composition. Both xenoliths and lower crustally-derived silicic magmas are used as evidence to support formation of the dense garnet-bearing granulite and eclogite required for lower crustal delamination, a process that leaves andesitic composition upper crust. The removal of more mafic lower crust, leaving less mafic upper crust is a possible mechanism for creating an andesitic crust without deriving large amounts of andesite directly from the mantle. -from Authors