P–T evolution in greenstone‐belt mafic amphibolites: an example from Plutonic Gold Mine,Marymia Inlier,Western Australia |
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| Authors: | M F GAZLEY J K VRY J C BOORMAN |
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| Institution: | 1. School of Geography, Environment and Earth Science, Victoria University of Wellington, PO Box 600, Wellington, New Zealand (m.gazley@gmail.com);2. Barrick (Australia Pacific) Limited, Locked Bag 12, Cloisters Square, Perth 6850, WA, Australia |
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| Abstract: | The metamorphosed mafic rocks of Archean greenstone belts host major orogenic gold deposits, and may record information about changing pressure–temperature (P–T) conditions that could contribute to understanding of Archean geodynamic processes. Until recently, it was difficult to obtain good constraints on pressure and temperature from these rocks. Here we present results of P–T pseudosection calculations in the NCFMASHTOS (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O–SO2) system, using as an example typical amphibolite facies metabasaltic rocks from the Plutonic Gold Mine in the Neoarchean Plutonic Well Greenstone Belt (PWGB), Marymia Inlier, Western Australia. The pseudosections together with observed mineral compositions and mineral assemblages in the rocks are used to argue that a previously unrecognized steep pressure increase (from ～3–4 kbar at ～500 °C to ≥8 kbar at ～600 °C) accompanied metamorphism to peak temperatures. The P–T data presented here could be the result of either horizontal or vertical tectonics. Existing models for the early evolution of the PWGB involve nappe stacking supported by relatively cold strong crust, with little overall change in thickness. While the available evidence from the study area and the wider region is not yet sufficient to confirm whether the peak metamorphic conditions were attained by horizontal or vertical tectonic means, the P–T data presented here can provide region‐specific constraints for computer modelling that may provide a more definite answer in the future. |
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| Keywords: | amphibolite facies mafic rocks Archean geodynamics Plutonic Gold Mine pressure– temperature conditions thermocalc |
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