Amphibolite facies metamorphism in the Schapenburg schist belt: A record of the mid-crustal response to 3.23 Ga terrane accretion in the Barberton greenstone belt

Abstract

The Schapenburg Schist Belt is one of several large greenstone remnants exposed in the granitoid-dominated terrane to the south of the Barberton greenstone belt and is unique in that it contains a well-developed metasedimetary sequence in addition to the typical mafic-ultramafic volcanic rocks. Detrital zircons within the metasediments have ages as young as ~3.24 Ga and consequently, these rocks correlate with Fig Tree Group sediments exposed in the central portions of the Barberton greenstone belt some 60km to the north, where they are metamorphosed to greenschist facies conditions. The Schapenburg metasediments are relatively K2O-poor, and are commonly characterised by the peak metamorphic assemblage garnet + cordierite + gedrite + biotite + quartz ± plagioclase. Other assemblages recorded are garnet + cummingtonite + biotite + quartz, cordierite + biotite + sillimanite + quartz and cordierite + biotite + anthophyllite. In all cases the peak assemblages are texturally very well equilibrated and the predominantly almandine garnets from all rock types show almost flat zonation patterns for Fe, Mg, Mn and Ca. Analysis using FMASH reaction relations, as well as a variety of geothermometers and barometers, has constrained the peak metamorphic pressure-temperature conditions to 640 ± 40°C and 4.8 ± 1.0 kbar. The maximum age of metamorphism is defined by the ~3.23 Ga age of a syntectonic tonalite intrusion into the central portion of the schist belt. Thus, sedimentation, burial to mid-crustal depths, and amphibolite facies equilibration were achieved in a time span similar to ~15 Ma. The strong bedding-parallel foliation in the metasediments dips to the east at an angle of 75 to 85° In this foliation plane elongated cordierite rods produced during prograde metamorphism define a close to vertical mineral lineation. The metasedimentary succession youngs to the east and in this direction is overlain by older Onverwacht Group rocks. Thus, the cordierite lineation might represent a transposed lineation developed during thrust stacking. Post-kinematic static recrystallization has largely obscured the prograde history of the rocks. However, the pressure-temperature conditions of peak metamorphism, the age of metamorphism, the rapidity of metamorphism following sedimentation, the presence of cordierite on the prograde path, as well as the timing of metamorphism relative to deformation strongly suggest that metamorphism occurred in an arc-style collisional setting. Thus, the rocks of this study most likely represent an exhumed mid-crustal terrane equilibrated during the proposed ~3230 Ma terrane accretion event in the Barberton Greenstone Belt, and record an apparent geothermal gradient typical for portions of modern orogenic belts of this type.