Re-Os Isotopic Evidence for a Subcontinental Lithospheric Mantle Source for the Scourie Dykes

Louise R. Frick VIEPS Department of Earth Sciences, Monash University, Clayton, VIC 3168, Australia

David D. Lambert VIEPS Department of Earth Sciences, Monash University, Clayton, VIC 3168, Australia

Ian Cartwright VIEPS Department of Earth Sciences, Monash University, Clayton, VIC 3168, Australia

Shane J. Reeves VIEPS School of Earth Sciences, University of Melbourne, Parkville, VIC 3052, Australia


The Scourie dyke suite of NW Scotland is an extensive swarm of early Proterozoic picritic to tholeiitic magmas that intruded the Lewisian granulite terrain. The picritic members of the suite (bronzite picrites and olivine gabbros) have
elevated PGE concentrations (Frick et al., 1994) and are
geochemically similar to 'U-type' magmas or siliceous high-magnesian basalts (SHMB) associated with platinum-group element (PGE) mineralised mafic layered intrusions. It has been proposed these 'U-type' magmas are the product of 'second-stage' partial melting of subcontinental lithospheric mantle (SCLM) (Sun et al., 1989). We have evaluated this petrogenetic model using trace element, Re-Os and Sm-Nd isotopic data for the four different members of the Scourie dyke swarm in northwest Scotland. This suite of dykes have been previously interpreted to contain a significant enriched SCLM component (Waters et al., 1990). These Proterozoic dykes intruded the Archaean (2.9 Ga) Lewisian lower crustal gneiss complex in two episodes and include comagmatic (2.4 Ga) high-MgO bronzite picrites and norites as well as (2.0 Ga) olivine gabbros and volumetrically-dominant (2.0 Ga) quartz tholeiites.

Results and Discussion

Concentrations of Os within the bronzite picrites and norites range from 0.50 to 1.79 ppb, whereas the olivine abbros range from 2.78 to 5.25 ppb. As expected in fractionated basaltic rocks, the Os concentration within the single quartz tholeiite sample analysed was low (0.085 ppb) whereas its Re concentration was high (2.59 ppb). Re-Os data for the bronzite picrites, olivine gabbros and norites fall within the fields for SCLM as defined by analyses of xenoliths from the Kaapvaal (Walker et al., 1989), Siberian (Pearson et al., 1995) and Wyoming (Carlson and Irving, 1994) Cratons. Re-Os isotopic data for bronzite picrite and norite samples define an isochron with an age of 2381 ± 55 Ma (MSWD = 4.0) and a near-chondritic initial Os isotopic composition (*Os = +1.5 ± 0.9). This age overlaps a precise U-Pb baddeleyite age for the bronzite picrite dyke (Heaman and Tarney, 1989) and is consistent with the contemporaneous emplacement of bronzite picrite and norite dykes, and the consanguineous nature of their parental magmas. One olivine gabbro dyke sample has a very high Os concentration (5 ppb) and yields a sub-chondritic initial Os isotopic composition (*Os = -4). The unique combination of high PGE concentrations, low Re/Os ratios and low initial Os isotopic compositions (*Os = -4 to +2) are most consistent with second-stage melting of a refractory (melt-depleted) SCLM source for the Scourie bronzite picrite, norite and olivine gabbro dykes. The Os isotopic data permit initial melt depletion at > 2.7 Ga, consistent with previous estimates of lithosphere formation based on dyke whole rock Sm-Nd and Pb isotopic data (Waters et al., 1989). However, high (La/Yb)n ratios and low initial Nd isotopic compositions (*Nd = -3 to +3) for these same dykes require re-enrichment of the SCLM source in incompatible trace elements. The significantly different initial Os isotopic compositions of the olivine gabbro (*Os = -4) and bronzite picrites (*Os = +1.5) coupled with the different degrees of trace element enrichments require different mechanisms for the enrichment of their respective parental magma sources. Re-Os isotopic data for a 2.0 Ga quartz tholeiite dyke from Scourie suggest that the mantle source was distinctly different from that of the olivine gabbro dykes of similar age. The extremely radiogenic initial Os isotopic composition (*Os = 2470) requires isolation of the magma source some 400 Ma prior to partial melting, and may therefore be related to emplacement of melts into the Lewisian continental lithosphere at 2400 Ma.


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