Petrography and mineral compositions of gabbros recovered in Deep Sea Drilling Project Hole 453 on the western side of the Mariana trough.

Abstract

The compositions, mineralogies, and textures of gabbros recovered in polymict breccias in Hole 453 indicate that they are the cumulus assemblages of calc-alkalic crystal fractionation that occurred beneath the W Mariana Ridge. They are among a class of gabbros known only from other calc-alkalic associations and differ from gabbros of stratiform complexes, ophiolites and the ocean crust. Particularly abundant in the Hole 453 breccias are olivine-bearing gabbros with extremely calcic plagioclase but with fairly iron-rich olivines. Other gabbros contain biotite and amphibole and occur in breccias with fairly high-grade greenschist facies metabasalts. One unusual gabbro has experienced almost complete subsolidus recrystallization to an assemblage of aluminous magnesio-hornblende, anorthite, and green hercynitic spinel. This reaction, the extremely calcic plagioclase, the occurrence of biotite and amphibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros occurred at elevated P(H2O). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply that considerable uplift may have attended rifting. They were also subjected to hydrothermal alteration after breccia formation. Temperatures of at least 200oC - and probably 350oC - were reached, and most likely could not have been attained without extrusion or intrusion or magmas nearby, even though no such rocks were cored.-Author