FELDSPAR THERMOMETRY OF GRENVILLIAN-AGE UHT MIGMATITES, MOLLENDO-CAMANA BLOCK, SOUTHERN PERU

Abstract

Feldspar thermometry was applied to the leucosome fraction of the 4000 km 2 Mollendo–Camana UHT migmatite complex, a part of the Grenvillian basement of the Peruvian Andes. The Mollendo–Camana stromatic migmatites consist of a quartz-bearing, K-feldspar-rich and plagioclase-poor anhydrous leucosome interlayered with a melanosome containing Al-rich orthopyroxene + sillimanite. In most domains of melanosome, the high Al content of orthopyroxene attests to UHT metamorphic conditions. The composition of the leucosome is far from the minimum melting of H 2 O-undersaturated granite at high temperature. Moreover, the absence of back reactions at the leucosome–melanosome interface shows that saturation in H 2 O was not attained during cooling. This indicates that the leucosome, although likely the locus of partial melting, is a K-rich cumulate left over after extraction of H 2 O-bearing, probably Na-bearing melts, most of which probably resulted from vapor-absent melting of micas. The thermal history of the leucosome fraction is better understood thanks to improved versions of the feldspar ternary solvus and correction routines for post-crystallization diffusion of elements: after re-integration of exsolved phases and correction for Na–K exchange between K-feldspar and plagioclase, equilibrium compositions (and hence temperatures) of feldspar pairs can be obtained. Temperatures for K-feldspar – plagioclase pairs (42 samples) range from 963° to 717°C, with a large population around 860°C. These temperatures are systematically lower and have a wider spread than those recorded by Al-orthopyroxene thermometry ( ca. 1000°C) in adjacent bodies of melanosome. The highest two-feldspar temperatures straddle the liquidus of quasi-anhydrous haplogranites, suggesting the coexistence under UHT conditions of solid melanosome, largely consolidated leucosome and eventually minor amounts of highly viscous haplogranitic (and syenitic) melts. The lowest feldspar-based temperatures are from strained rocks that otherwise preserved Al-rich (high-T) orthopyroxene. This suggests deformation-induced redistribution of Al–Ca in the feldspar structure by dislocation creep and recrystallization of K-feldspar and plagioclase crystals. Consequently, inasmuch as they were involved in high-T to UHT deformation, ternary feldspars did not survive, and recrystallized as feldspar pairs that record an array of temperatures that can be more than 200°C lower than peak conditions.