The Re-Os isotopic system has been used in Archaean terrains to provide geochronological constraints on the
formation of ore deposits (Lambert et al., 1994; Foster et al., subm.) and to investigate the petrogenesis of Archaean greenschist-facies komatiites (Foster et al., subm.). It has been suggested that the Re-Os geochronometer may be able to 'see through' crustal metamorphic events due to the refractory nature of osmium and its low abundance in crustal rocks and fluids. This is supported by the obtainment of an 2476 ± 14 Ma isochron for the bronzite picrite Beannach Dyke from Scourie (Frick et al., subm.) in agreement with published ages (Heaman and Tarney, 1989). In order to test the usefulness of the Re-Os isotopic system in high grade metamorphic terrains, we have obtained high-quality Carius tube/N-TIMS Re-Os isotopic data for a suite of Archaean tonalitic to basic gneisses from the central granulite region.
The Central Region of the Lewisian complex of northwest Scotland is an example of a well-preserved portion of lower crust that has a history extending back to the middle Archaean. Sm-Nd and Pb-Pb isotopic data for the gneisses from the central Lewisian region suggest their formation from supracrustal sediments and mafic and ultramafic rocks which were incorporated at around 2.92 Ga into a developing tonalitic plutonic complex. In the Central Region, textures attained during the extended granulite facies metamorphic event (Badcallian), whose peak at 2.66 Ga is recorded in zircons, are well preserved. The Central Region was little affected subsequent metamorphic events i.e. the Inverian retrogression, which predates the earliest intrusion of the Scourie Dyke Suite at ~2.4 Ga, and the Laxfordian deformation which postdates the latest intrusion of Scourie Dykes at ~2.0 Ga and predates the intrusion of post tectonic pegmatite (ca 1.8 Ga) at Laxford. These events pervasively affected both the Northern and Southern Regions of the Lewisian Complex.
Concentrations for the gneisses range from 8.5 to 87 ppt common Os and 0.082 to 1.2 ppb Re. The isotopic data define a five-point isochron with a Model 1 McIntyre regression age of 2053 ± 150 Ma (MSWD = 5), and a radiogenic initial isotopic composition of *Os = 536 ± 124.
The depletion of gold in the Lewisian granulites,
compared with crustal estimates has been recently linked to the oxidation of primary sulphide (pyrrhotite) to pyrite and magnetite (Cameron, 1994). The timing of this depletion was assumed to be Inverian, after the peak Badcallian metamorphism, as the oxidation of pyrrhotite to pyrite cannot be accomplished at the low ¶O2 values of the granulite facies metamorphism. The oxidation event is likewise presumed to predate the intrusion of the quartz dolerite dykes (2.0 Ga) in that they exhibit primary magmatic sulphide mineralogy and textures. As the sulphides within the gneisses are host to the chalcophile elements osmium and rhenium, it is likely that this recrystallisation effectively 'reset' the Re-Os geochronometer, with the mobilisation of Au and other
precious metals from primary sulphide hosts by grain boundary fluids. The isochron age therefore is likely to represent the closure of the sulphide host to rhenium and osmium, after recrystallisation. The inclusion of tonalitic, intermediate and mafic lithologies in the regression suggests a regional rather than a local scale for the event. The isochron age obtained here suggests that the intrusion of the volumetrically significant quartz dolerite dyke suite at ca 2.0 Ga may have provided the thermal trigger for the sulphide recrystallisation in the surrounding gneisses. Alternatively, given the large uncertainty in the age, it may be an expression of late Inverian metamorphic effects which were not pronounced in the Central Region due to the lack of externally derived fluids (Cartwright and Valley, 1992). This study illustrates the potential uses of the Re-Os geochronometer as a tool in the dating of metamorphic events in which sulphide phases experience recrystallisation.
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Cartwright, I & Valley, J.W., J. Geol. Soc. 149, 115-125 (1992).
Foster, J.G. et al., submitted to Nature.
Frick, L.R. et al., submitted to Geochim. cosmochim. Acta.
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Lambert, D.D. et al., J. Petrol. 35, 1717-1753 (1994).