Chemical and isotopic effects of late Archaean high-grade metamorphism and granite injection on early Archaean gneisses, Saglek-Hebron, northern Labrador

Abstract

Summary Early Archaean (3700–3800 Ma) layered tonalitic to granodioritic Uivak gneisses form approximately 60–70% of the Archaean gneiss complex between Saglek and Hebron fiords, northern Labrador. Effects of late Archaean metamorphic and igneous activity can be recognized on the geochemistry and isotope chemistry of the Uivak gneisses: depletion in rocks affected by 2900 Ma granulite facies metamorphism, with a loss of K, Rb and redistribution of REE and a corresponding enrichment of LIL elements in the amphibolite facies area. LIL enrichment is at least in part by addition from intrusive late Archaean granitoid sheets. The depleted granulites (K/Rb 500–2000) show a scatter of data points on a Rb/Sr- 87 Sr/ 86 Sr diagram to the left of a 3560 Ma reference isochron (ISr 0.700) consistent with marked Rb loss and some homogenization of Sr. Amphibolite facies gneisses from a broad transitional zone into granulite facies have K/Rb values of 200–300 and plot on a c. 3560 Ma isochron with a high ISr (0.7038) consistent with a slight loss of Rb at 2900 Ma but without marked Sr homogenization. Amphibolite facies gneisses away from the transition zone show marked enrichment in LIL elements with unusually low K/Rb ratios (100–200). Rb/Sr data plot close to a 3560 Ma reference isochron, ɛSr 3560 values are strongly negative, interpreted as due to Rb addition at 2900 Ma rather than derivation from an unusually depleted mantle source. Pb-Pb isotope data from the depleted granulite show a cluster of unradiogenic data points. Samples from a monolith of gneisses in a transitional zone between amphibolite and granulite facies yields a 3808 Ma isochron (μ 1 8.5) suggesting there was locally little disturbance of the Pb/U system at 2900 Ma. Pb-Pb data from the Rb-enriched amphibolite facies area show a markedly higher proportion of radiogenic Pb compared to the granulite facies rocks which cannot be explained by U movement alone. Local galena mineralization suggests that Pb as well as U was mobile during the late Archaean and we interpret the results as mixtures of > 3600 crustal Pb with younger material. Sm-Nd data points (Collerson & McCulloch 1982) show a scatter about a 3665 Ma isochron with an average ɛNd 3665 of +2.48. Marked LIL + LREE enrichment is suggested and we interpret the scatter of ɛNd 3665 values as being due to late movement of LREE. The late Archaean granitoid sheets have a chemistry and petrology suggesting derivation largely by partial melting of earlier sial with modifications by fluid redistribution of alkalis. New minerals such as allanite and apatite grow in older rocks adjacent to young veins. Sr, Nd and Pb data all suggest mixing between early Archaean crust and a component with mantle-like isotopic character at c. 2900 Ma. The mixing process was complex and took place by ingestion of older rocks in younger granite, mixing of partial melts of different origin and fluid transport of components from younger intrusions into older gneisses.