An experimental investigation of the gabbro to eclogite transformation and its petrological applications

Abstract

The mineral assemblages of a variety of basaltic compositions have been studied experimentally in the pressure range from 1 bar to 30 kb at temperatures above 1000°C and below the basalt solidus. At low pressures, less than 10 kb at 1100°C, the mineral assemblages match those of gabbros and pyroxene granulites but at pressures above 21 kb at 1100°C the major phases are pyrope-almandine garnet and clinopyroxene and the mineral assemblages match those of eclogites. At intermediate pressures the mineral assemblages are characterized by co-existence of garnet, clinopyroxene, plagioclase and, commonly, quartz. The transition interval between the gabbroic and eclogitic assemblages is a broad one characterized by gradual increase in garnet and in the pyrope content of the garnet, and decrease in plagioclase and in the anorthite content of the plagioclase. The roles of variable SiO 2-saturation, Na 2O content, albite: anorthite ratio of plagioclase, oxidation state, and of variations in the Mg/(Mg + Fe 2+) (atomic) ratio have been studied to determine the influence of particular chemical parameters on the pressure at which a given phase (e.g. garnet) appears or disappears. Low values of the Mg/(Mg + Fe 2+) ratio cause garnet to appear at lower pressures. In undersaturated compositions (olivine-normative), garnet appears at lower pressures than in quartz-normative compositions and in addition there is an intermediate assemblage of aluminous pyroxenes + plagioclase + spinel present, particularly in magnesian basalts, between the low pressure olivine-bearing and higher pressure garnet-bearing assemblages. The pressure required for elimination of plagioclase varies from 15 to 20 kb at 1100°C in the spectrum of basaltic compositions studied. The pressure required for the appearance of garnet and disappearance of plagioclase in a given composition is strongly dependent on temperature. P-T gradients for these boundary reactions in a quartz tholeiite composition have been established in the 1000–1250°C temperature range. When extrapolated to lower temperatures, these gradients suggest that eclogite mineralogy is stable in dry basaltic rocks along normal geothermal gradients throughout the entire crust. The observed mineral assemblages at various pressures and the effects of chemical parameters on mineralogy in a given P-T field are closely matched with natural pyroxene granulite and eclogite occurrences and with experimental work in simple systems. The experimental work provides some quantitative data on dry solid-solid reactions which are strongly pressure and temperature dependent and which, in natural rocks, provide criteria for subdivision of granulite facies metamorphic rocks into high pressure, intermediate pressure and low pressure types. By comparison of the experimental data with estimates of the P H 2O , T conditions of other metamorphic facies, based in part on dehydration reactions, it is argued that eclogite mineralogy may be stable in dry basaltic rocks within the almandine amphibolite facies, the glaucophane schist facies and part of the greenschist facies of regional metamorphism.