Conflicting mineral and whole-rock isochron ages from the Late-Archaean Lewisian complex of northwestern Scotland: Implications for geochronology in polymetamorphic high-grade terrains

Abstract

We report major and trace element (including rare-earth element, REE) geochemistry and whole-rock Sm Nd geochronology on two suites of mafic/ultramafic inclusions in an agmatite complex within a trondhjemitic host at Gruinard Bay, in the Late Archaean Lewisian Complex of northwest Scotland. A suite of medium-grained equigranular amphibolites (main amphibolite suite, MAS) has broadly chondritic high field strength element (HFSE) ratios (Ti/Zr ca. 115, Ti/Y ca. 318, Y/Cr ca. 0.39), flat heavy REE patterns at five to fifteen times chondrites, usually with mild light REE enrichment (La N < 50) and small Eu anomalies. This is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite-trondhjemite-granodiorite (TTG) suite magmas, as argued previously by Rollinson and Fowler (1987). A Sm Nd regression line (MSWD = 3.4) obtained from the MAS yields an age of 2.943 ± 0.091 Ga with an initial ε Nd value of +2.69 ± 0.62. A suite of coarser grained hornblendites and metagabbros (hornblendite-metagabbro suite, HMS) displays significant geochemical differences from the MAS, exemplified by significantly light REE-enriched patterns in which the hornblendites have the highest total REE with La N/Nd N< 1 and large negative Eu anomalies, and the most felsic metagabbros have low total REE and large positive Eu anomalies. Samples from the HMS yield a Sm Nd isochron (MSWD = 0.52) age of 2.846 ± 0.073 Ga with an initial ε Nd value of +1.10 ± 0.72. The HMS geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local TTG magmas. Combining the new data with previously published data from the host trondhjemites at Gruinard Bay (Whitehouse, 1989a) yields a Sm Nd isochron (MSWD = 1.09) age of 2.795 ± 0.028 Ga with an initial ε Nd value of +0.66 ± 0.38. The relatively low initial ε Nd values for the HMS and combined HMS-trondhjemite isochrons compared to contemporaneous depleted mantle (ε Nd(2.8) ca. + 2.1 to + 3.3) is consistent with derivation of a parental magma from light-REE-enriched components of a mafic (basaltic) precursor which was itself derived from a depleted mantle source approximately 100–200 Ma previously. The MAS suite at Gruinard Bay may be representative of such a mafic precursor. Our new geochronological data differ significantly from the ca. 3.3 Ga mineral Sm Nd and Pb Pb regression ages presented recently for two members of the HMS by Burton et al. (1994) which, if correct, require radical revision of the early geological history of the Lewisian Complex. Comparison of the relatively old mineral ages with our new whole-rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole-rock systematics. However, we argue that extensive retrogression, evident in all lithologies at Gruinard Bay, makes them less than ideal samples for a mineral isotopic study and it is likely that the ca. 3.3 Ga ages may be spurious, the product of disturbance of isotope systematics during the Laxfordian. On the basis of the geochronology presented here, evolution of the Lewisian closely follows generally accepted models for late-Archaean crust formation.