Clinopyroxene-amphibole-plagioclase symplectites in Norwegian eclogites: microstructures, chemistry and the exhumation P-T path
Department of Earth Sciences, University of Oxford
Eclogites from the Western Gneiss Complex of western Norway commonly contain abundant symplectite formed by partial breakdown of omphacite. Samples from the Stadlandet and Nordfjord areas, encompassing the UHP, Transition and HP zones of Wain (1997, Geology, 25, 927-930), contain Cpx-Pl-Hbl symplectites of similar microstructure and chemical character.
Symplectite colonies nucleated at omphacite-omphacite grain boundaries, and advanced into one of the grains, maintaining structural continuity with the other. Narrow, discontinuous hornblende lamellae are locally interleaved with clinopyroxene and plagioclase. The scale of the intergrowth becomes finer away from the original grain boundary, with lamellar densities varying from 25 to 200 per mm. Both Cpx and Hbl show systematic compositional variation as a function of lamellar density. The jadeite content of Cpx drops abruptly from ca. 47% in omphacite to 30% in adjacent symplectite Cpx, then decreases steadily across the colony to ca. 10% at the final growth front. Hornblendes show decreasing Na and octahedral Al across the same interval. Plagioclase is a relatively uniform oligoclase.
The systematic trends indicate that phase compositions have not been modified significantly since growth, and suggest that local equilibrium was achieved among the products of omphacite breakdown. P-T sensitive equilibria among the three intergrown phases should, therefore, constrain the exhumation path. Hornblende-plagioclase thermometry indicates that symplectite developed over the T range 700 - 600 °C. Pressure-sensitive equilibria imply growth from 14 to 7 kbar, values within error of those calculated from jadeite + SiO2 = albite on the assumption of unit activity of SiO2.
The overall exhumation P-T path is constrained to be convex to the T axis. Temperatures at 10-12 kbar are lower than predicted for erosion-dominated exhumation, but higher than expected for some mechanisms of extensional unroofing. The similarity of symplectite chemistry and microstructure across the regional eclogite zones suggests that at this stage of exhumation the Western Gneiss Complex acted as a coherent crustal block. Thus, any tectonic juxtaposition of UHP and HP terrains is likely to have occurred earlier, under eclogite-facies conditions.
(Mineralogical Society, Winter Conference, Derby, January 2002)
Created/modified 14 January 2002