An experimental investigation of the Gabbro-Eclogite transformation and some geophysical implications

Abstract

A detailed experimental investigation of the gabbro-eclogite transformation in several basalts has been carried out. More than 200 runs at temperatures between 900 and 1,250°C and pressures up to 30 kbar were made, and the resultant phase assemblages studied by optical and X-ray techniques. In all the basalts studied, the transformation from the low-pressure gabbroic assemblage (pyroxene + plagioclase) into the high-pressure eclogite assemblage (garnet + pyroxene) was found to proceed via a transitional mineral assemblage characterised by co-existing garnet, pyroxene and plagioclase. This mineral assemblage closely resembles that displayed in nature by basic rocks in the garnet-clinopyroxene-granulite sub-facies. The width of the transitional assemblage (garnet granulite) varies from 3.4 to 12 kbar at 1,100°C in different basalts and the pressures required for incoming of garnet and elimination of plagioclase likewise vary widely according to the particular basaltic composition studied. These variations are caused by small differences in chemical composition, and are interpreted in terms of the principal mineral equilibria which are involved in the transformation. In passing from gabbro to eclogite via the garnet granulite assemblage, the proportion of garnet is observed to increase regularly with pressure, whilst the proportion of plagioclase decreases regularly. Thus the increase in density and seismic velocity from gabbro (ρ ~ 3.0 g/cm 3, V p ~ 6.9 km/sec) to eclogite (ρ ~ 3.5 g/cm 3, V p ~ 8.3 km/sec) is uniformly smeared out over the entire garnet granultte transition interval. The effect of temperature upon the pressures required for the gabbro-eclogite transformation is investigated. Our experimental results, together with the results of other workers on simple systems closely related to the gabbro-eclogite transformation, strongly indicate that eclogite would be stable relative to gabbro and garnet granulite throughout large regions of the normal continental crust. This conclusion has important tectonic implications. The bearing of the experimental results upon the hypothesis that the continental Mohorovičlć discontinuity is caused by an isochemical transformation from gabbro to eclogite is examined. It is concluded that this hypothesis must be rejected on the following grounds: 1. (1) Eclogite, not gabbro, appears to be stable throughout the continental crust. 2. (2) The experimental temperature gradient of the transformation is incompatible with inferred geophysical relationships between the temperature at the base of the crust and thickness of the crust. 3. (3) The transformation cannot explain the seismic velocity distribution in the crust. 4. (4) Minor changes in chemical composition strongly affect the pressure required for eclogite stability. This makes it difficult to understand the uniformity of crustal thickness in stable continental regions. 5. (5) The density of eclogite (3.5 mg/cm 3) is higher than the density of the upper mantle inferred from gravity observations. 6. (6) An upper mantle of eclogitic composition would be gravitationally unstable. The chemical composition of the continental crust is discussed in the light of the widely accepted geophysical model that it consists of a layer of granitic material overlying or passing gradually downwards into a gabbroic layer. Our experimental results are in conflict with the view that the lower crust is generally of gabbroic composition. It is more likely that the lower crust consists on the average of intermediate rocks in the eclogite facies. Although the gabbro-eclogite transformation is not believed to play a significant role in the structure of stable continental regions, it may be of major importance in tectonically active areas where the Mohorovičić discontinuity cannot be clearly recognized e.g., regions of recent orogenesis, continental margins, island arcs and mid-oceanic ridges. It is shown that the basalt-eclogite transformation may provide a tectonic engine of great orogenic significance. Large volumes of basalt, when extruded and intruded at and near the earth's surface may become transformed to eclogite under suitable circumstances on cooling. Because of the high density of eclogite, such large scale transformations would generate gravitational instability. Large blocks of eclogite would sink through the crust, dragging it down initially into a geosyncline, and later causing extensive deformation (folding). Because the density of eclogite (3.5 g/cm 3) is greater than that of the ultramafic rocks which make up most of the mantle (3.3 g/cm 3) blocks of eclogite will sink deep into the mantle, and may undergo partial fusion, leading to generation of andesitic and granodioritic magmas which rise upwards and intrude the folded geosyncline.