Abstract
Petrographic and mineral chemical data are presented for orthopyroxene-Fe-Ti oxide symplectites in three intrusive bodies from southwestern Norway: the Bjerkreim-Sokndal lopolith, the Hidra anorthosite, and the Lyngdal hyperite. Detailed studies of the Bjerkreim-Sokndal lopolith indicate that symplectites are not common, occurring in only six thin sections out of 1037 studied. Only one of these samples contains olivine, and the symplectites clearly do not replace or form pseudomorphs after olivine. The symplectites in the Hidra anorthosite and Lyngdal hyperite do appear to replace olivine. The proportions of orthopyroxene and Fe-Ti oxide in the symplectites are fairly constant and range from 73:27 to 66:34 (vol%). Microprobe data demonstrate that the orthopyroxenes do not have compositions appropriate for crystallization at or above the solidus. Many of the pyroxenes in these intrusives have re-equilibrated during subsolidus cooling, but the orthopyroxenes in the symplectites show no exsolution and thus must have formed subsolidus. Pyroxene thermometry indicates temperatures of formation of 700-800 °C at estimated pressures of 5 kbar or less (upper amphibolite-granulite facies conditions). The Fe-Ti oxide in most of the symplectites is titaniferous magnetite that shows complex exsolution phenomena indicative of extensive re-equilibration during subsolidus cooling. It is virtually impossible to reconstruct the original compositions or to apply oxide thermometry.
The symplectites do not result from discontinuous precipitation from a supersaturated solid solution, eutectoidal breakdown of a pre-existing phase or isochemical replacement of olivine. It is suggested that the occurrence of symplectites reflects the overall re-equilibration of the original magmatic mineral assemblage during high-grade retrogressive metamorphism. They form only in the vicinity of orthomagmatic Fe-Ti oxide. The precise mechanism of formation is obscure, but appears to involve co-operative nucleation, predominantly at grain boundaries, and sympathetic growth via a bridging mechanism. The development of oriented intergrowths is governed by the requirements of minimal interfacial and strain energy. Diffusion mainly occurs along grain boundaries or advancing interfaces and is probably aided by a thin film of intergranular fluid. The supercooling necessary for symplectites to form probably reflects the high energy barriers to solid-state nucleation and growth.
