The role of fluids in the formation and subsequent development of early continental crust |
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| Authors: | Kenneth D Collerson Brian J Fryer |
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| Institution: | 1. Department of Geology, Memorial University of Newfoundland, A1B 3X5, St. John's, Newfoundland, Canada
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| Abstract: | A petrogenetic model is developed to explain the evolution and geochemical character of granitic rocks in early Archean (pre 3.6 Gyr) continental crust taking into account the following important geological constraints, viz.: - High geothermal gradients (probably in excess of 90 ° C/km) and resulting widespread granulite facies metamorphism even at relatively shallow depths
- The fractionation of certain major and trace elements under granulite facies conditions
- The composition and geochemical behaviour of fluids which emanate from or pass through terrains undergoing granulite facies metamorphism viz. carbonic fluids containing significant amounts of SO2 and halogens.
In this model tonalitic and trondhjemitic intrusives are regarded as being derived dominantly by partial melting of mafic granulite. The ubiquitous potassic granites, which typical post-date sodic plutonic activity are interpreted to be anatectic melts generated under granulite or amphibolite facies conditions from the previously formed ‘plagiogranites’. The presence of a postulated granulite facies source area for Archean tonalitic rocks, and the geochemical character of fluids which accompany metamorphism under such conditions explains the HREE geochemistry of these suites and casts doubt on the validity of applying currently used trace element fractional melting or crystallization models to these terrains. Similarly it suggests that petrogenetic interpretations based on Sr and Pb isotopic systems must be reevaluated because of the extreme mobility of both parent and daughter elements under granulite facies conditions. |
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