Experimental petrology and global tectonics — A preview

Abstract

The new global tectonics provides a convenient tectonic framework for igneous and metamorphic petrology in the five environments: oceanic plates, continental plates, divergent plate boundaries, convergent plate boundaries, and transform plate boundaries. Models have been developed involving: 1. (1)the generation of oceanic crust from basalt and serpentinite at mid-ocean ridges; 2. (2) metamorphism and hydration of the crust to yield greenschist, hornblende-gabbro, or amphibolite; 3. (3) lateral transport of the crust by sea-floor spreading; 4. (4) subduction of the crust at convergent plate boundaries, with 5. (5) blueschist metamorphism associated with oceanic trenches; 6. (6) transformation of the subducted crust into quartz-eclogite; 7. (7) partial fusion of amphibolite or quartz eclogite and subducted oceanic sediments to yield the calc-alkaline magma series; and 8. (8) the emplacement of batholiths in high-temperature metamorphic rocks at the base of the island or continental margin arc complexes. We have assumed starting materials, beginning with mantle peridotite beneath the oceanic rises, and a series of processes involving partial fusion of mantle peridotite, crystallization, hydration, solid-solid phase transformations, dehydration, and partial fusion of the diverse materials in the subducted oceanic crust. Experimental petrology makes possible the testing of models and determination of the pressures and temperatures for the operation of these processes. Water is an important component. For processes at the oceanic ridges we do not know how much water is juvenile and how much is introduced from the ocean. The hydrated oceanic rocks are subducted and then dehydrated, but there remains some uncertainty about whether this water is carried deep enough to participate in the generation of calc-alkaline magmas, and possibly juvenile water is involved. The combined approaches of geophysics, geochemistry, geology, and experimental petrology within the framework of the new global tectonics are providing new insights into the differentiation of the earth.