The influence of erosion upon the mineral fades of rocks from different metamorphic environments

TLDR

Metamorphic rocks exposed at the surface record a range of pressure–temperature conditions along a metamorphic geotherm that can be concave or convex, reflecting conductive temperature increase and erosion‑driven pressure decrease, with magma addition producing a convex geotherm. The authors propose that identifying the curvature of metamorphic geotherms can reveal the influence of magmatic activity in older metamorphic terrains. They find that the metamorphic geotherm exhibits lower maximum pressures than early recrystallization, a polychrome progression from low to high temperatures, significant crustal melt formation after low‑medium temperature metamorphism, and that blueschists form at the low‑temperature end and are quickly replaced by greenschist‑ or amphibolite‑facies rocks, accounting for the scarcity of Precambrian blueschists.

Abstract

Metamorphism of tectonically thickened continental crust or subducted sediment wedges is likely to take place in a thermal regime where temperature increases by conductive relaxation whilst concurrently pressure decreases by erosion of the pile. The mineral facies of rocks reaching the surface do not reflect any one geotherm through the pile but lie on a locus of P–T conditions, the metamorphic geotherm, which will generally be concave towards the temperature axis. Maximum pressures on the metamorphic geotherm are significantly less than maximum pressures experienced by rocks during the early stages of recrystallization. The metamorphic geotherm is polychrome, points at lower temperatures reflecting conditions earlier in the development than those at higher temperature; crustal melts are developed after low-medium temperature metamorphism and the amount of such melts could be significant. Blueschists develop on the low temperature end of the metamorphic geotherm and are succeeded in exposure at the surface by greenschist- or amphibolite-facies rocks; the time-scale for this process is consistent with the virtual absence of Precambrian blueschists. Crust thickened by addition of hot magma is likely to yield a metamorphic geotherm convex towards the temperature axis. Recognition of differently curving metamorphic geotherms can be used to assess the part played by magmatic activity in older metamorphic terrains.