Evolution in the understanding of metamorphism of basaltic rocks and their tectonic settings—a personal perspective

Abstract

In the mid 1950s, the qualitative relationships among metamorphic mineral fades developed in rocks of basaltic composition were relatively well understood, but values for the attending state variables were only guessed at, and the thermodynamic basis underlying metamorphic reactions had not yet begun to exert a strong influence on petrology. The relationships of subsolidus recrystallisation to the structure of the Earth at several scales, to tectonic processes, and to planetary evolution were virtually unknown. Rapid strides were about to be made, thanks to substantial increases in governmental support of research, and to the resultant technological advances in mineralogy, petrology, and geochemistry; the heightened activity went hand‐in‐hand with larger scale revolutions in thinking due to the advent of plate tectonic concepts, planetological information provided by space exploration, and comparative studies of metamorphic terranes around the world.

Experimental phase equilibrium and calorimetric studies, laboratory‐calibrated stable isotope fractionations, and synthetically determined exchange reactions subsequently allowed thermobarometric methods to be employed for a broad range of metabasaltic parageneses, and the erection of a quantitative petrogenetic grid. Coupled with computed thermal models for divergent and convergent lithospheric plate boundaries as well as stable plate interiors, it became possible to estimate the three‐dimensional spatial arrangement of metamorphic assemblages for both oceanic and continental crustal regimes. Mineral parageneses in ophiolites, oceanic spreading centres, back‐arc basins, and geothermal districts are now reasonably well known. So are phase assemblages in subducted metabasalts. However, inclusions of coesite and diamond in garnet at four disparate Eurasian sites, indicate that initially shallow portions of the continental crust have been taken to depths exceeding 100 km, then returned to the surface by as yet poorly understood processes. The deepest portions of continental interiors are still not well characterised.

Continuing advances in instrumentation now allow the quantitative documentation of mineral paragenetic, thermobarometric, and age relationships in recrystallised mafic volcanics and their intrusive equivalents. Investigations of reaction rates, transformation mechanisms, and clay‐mineral generation are in their infancy, as is the study of fluid‐basalt interaction. All of these lines of research assume increasing importance as nations and institutions begin to focus their intellectual resources on global habitability, environmental change, and technological capability.

Key words:

• metamorphism
• parageneses
• thermobarometry
• metabasalt