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Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the Strangways Metamorphic Complex (central Australia)
Author(s) -
Ballèvre M.,
Möller A.,
Hensen B. J.
Publication year - 2000
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.2000.00289.x
Subject(s) - geology , metamorphism , granulite , metamorphic facies , shear zone , hornblende , geochemistry , metamorphic rock , plagioclase , orogeny , petrology , crust , greenschist , staurolite , kyanite , geomorphology , facies , quartz , tectonics , biotite , seismology , paleontology , structural basin
Foliated garnet‐bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine‐grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well‐developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn‐ to late kinematic growth of the amphibolite facies mineral assemblages. All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar. An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along‐strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny.