HR: 1340h
AN: V13E-0577    [Abstracts]
TI: Vestiges of a 3.2 Ga Subduction Zone, as Evidenced by High-Pressure, Low-Temperature Metamorphism in the Barberton Greenstone Belt
AU: * Stevens, G
EM: gs@sun.ac.za
AF: Department of Geology, Stellenbosch University Private Bag X1, Matieland, 7602 South Africa
AU: Moyen, J
EM: moyen@sun.ac.za
AF: Department of Geology, Stellenbosch University Private Bag X1, Matieland, 7602 South Africa
AU: Kisters, A
EM: akisters@sun.ac.za
AF: Department of Geology, Stellenbosch University Private Bag X1, Matieland, 7602 South Africa
AB: The lack of high-pressure, low-temperature metamorphism in the Archaean rock record, one of the most diagnostic fingerprints of subduction-driven tectonics, is commonly considered to indicate that plate tectonics was absent during the Archean, or at least very different in form and function from what it is now. Here we report evidence for high-pressure, low-temperature metamorphism in supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain in South Africa that challenges this paradigm. In the Inyoni shear zone, located between the 3.45 Ga Stolzburg homogeneous tronhjemite pluton and the ca. 3.2 Ga Badplaas granodioritic to trondhjemitic gneissic complex, garnet-bearing amphibolites are preserved within the 3.22 Ga greenschist to amphibolite facies high-strain structure. Peak temperature conditions for these amphibolites are recorded by epidote-amphibole-plagioclase (An30) ± clinopyroxene assemblages, which yield PT conditions via conventional thermobarometry of 750 to 850 °C and 0.7 to1.0 GPa. However, the cores of garnet crystals, that are of more calcic and less magnesian composition than the rims, contain inclusions of quartz, clinopyroxene, amphibole, epidote and plagioclase (An10), which also differ in composition from the matrix generations of the same minerals. This earlier assemblage constrains peak pressure conditions to between 1.2 and 1.5 GPa at temperatures below 650-700 °C. This points to very low apparent geothermal gradients of 12-15 °C. These values are similar to those found in recent subduction zone settings. These highest-pressure metamorphic rocks occur on the eastern margin of a coherent granitoid terrain, at least some 500 km² in extent, which has experienced peak metamorphic pressures of 0.8 to 1.1 GPa. This clearly demonstrates that the ca 3.23 crust in this area was cold enough and stiff enough to allow for tectonic doubling of crustal thickness. The timing of high-pressure metamorphism coincides with the proposed main phase of terrane accretion in the structurally overlying Barberton greenstone belt. We suggest that these high-pressure, low-temperature conditions represent the first metamorphic evidence for subduction-driven burial of continental crust in the mid-Archaean and are the deep crustal manifestation of the terrane accretion proposed on the grounds of structural, geochronological and sedimentological evidence from the fold and thrust belt.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3652 Pressure-temperature-time paths
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005