Emplacement-related microstructures in the margin of a deformed pluton: the San José tonalite, Baja California, México

Abstract

Abstract In the San Jose tonalite pluton, Baja California, Mexico, interior magmatic fabrics give way progressively to subparallel, solid-state fabrics with S–C structures near the margins. The outer rim (‘marginal northern unit’), approximately 1 km thick, is inferred to have been deformed during the emplacement of the inner parts of the magma chamber. The rocks show a transition from strongly deformed at the outer edge to progressively less deformed towards the inner edge of the marginal northern unit. A small amount of melt appears to have been present during the deformation, as indicated by (1) apparent contact melting between some grains of plagioclase, and (2) coarse-grained aggregates and single-grains of quartz, K-feldspar and/or more sodic plagioclase in fracture-controlled openings in primary plagioclase crystals. The more sodic plagioclase typically grew as rims on the walls of the openings in twin continuity with the primary plagioclase. The deformed rocks have a prominent foliation marked mainly by aggregates of biotite, with contributions by hornblende and titanite grains, as well as apparently recrystallized aggregates of quartz and/or feldspar, at least some of which may have originated as zones of fragmentation. Many primary grains of plagioclase, hornblende, biotite and titanite show evidence of brittle deformation and fragmentation. The shapes of aggregates in the folia, relationships between incipient folia and residual primary grains, and occurrences of many folia in narrow zones transecting strong minerals (plagioclase and hornblende), suggest that brittle processes may have controlled at least some of the foliation development. However, evidence of strain-induced grain-boundary migration and bulge nucleation of new grains of quartz and plagioclase indicates that dislocation creep also played a part in the deformation. Recrystallization of plagioclase, together with intrusion of the pluton at greenschist facies ambient temperatures, confirms high-temperature deformation, with heat derived from the cooling magma.