Metamorphic evolution of a subduction complex, South Shetland Islands, Antarctica |
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| Authors: | R A J TROUW L S A SIMÕES & C S VALLADARES |
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| Institution: | Instituto de Geociências, Universidade Federal do Rio de Janeiro, CEP 21910–900, Rio de Janeiro, Brazil (e-mail:;.br), Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, CEP 13500–230, Rio Claro, SP, Brazil,;DGEL, Universidade do Estado do Rio de Janeiro, CEP 20559–900, Rio de Janeiro, Brazil |
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| Abstract: | A subduction complex composed of ocean floor material mixed with arc-derived metasediments crops out in the Elephant Island group and at Smith Island, South Shetland Islands, Antarctica, with metamorphic ages of 120–80 Ma and 58–47 Ma, respectively. Seven metamorphic zones (I–VII) mapped on Elephant Island delineate a gradual increase in metamorphic grade from the pumpellyite–actinolite facies, through the crossite–epidote blueschist facies, to the lower amphibolite facies. Geothermometry in garnet–amphibole and garnet–biotite pairs yields temperatures of about 350 °C in zone III to about 525 °C in zone VII. Pressures were estimated on the basis of Si content in white mica, Al2O3 content in alkali amphibole, NaM4/AlIV in sodic-calcic and calcic amphibole, AlVI/Si in calcic amphibole, and jadeite content in clinopyroxene. Mean values vary from about 6–7.5 kbar in zone II to about 5 kbar in zone VII. Results from the other islands of the Elephant Island group are comparable to those from the main island; Smith Island yielded slightly higher pressures, up to 8 kbar, with temperatures estimated between 300 and 350 °C. Zoned minerals and other textural indications locally enable inference of P–T –t trajectories, all with a clockwise evolution. A reconstruction in space and time of these P–T –t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases (D1 & D2) that affected the area. This thermal structure is in good agreement with the one expected for a subduction zone. The arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex. In this model, D1 represents the subduction movements expressed by the first vector of the clockwise P–T–t path, D2 reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure, and D3 corresponds to isostatic uplift accompanied by erosion, under circumstances of decreasing temperature and pressure. |
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| Keywords: | accretionary wedge geothermobarometry P–T–t paths Sanbagawa type metamorphism |
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