Plumbing the Depths: Pb Isotope Constraints on the Origin of the 3.65 Ga Amîtsoq Gneiss

Abstract

Archaean crust largely consists of tonalite-trondhjemite-granodiorite (TTG) gneiss provinces with associated greenstone belts. Understanding their genesis is a fundamental task in deciphering crustal evolution through time. The oldest well-exposed major TTG-greenstone belt terrain is the Itsaq gneiss complex of southern West Greenland. This complex consists of the TTG-type Amhsoq gneiss with its highly metamorphosed metasedimentary and mafic inclusions (the Akilia association), and the Isua greenstone belt. The isotope-geochemical signatures of these units, and the conclusions drawn from them, are highly debated issues with far-reaching implications for crustal and mantle evolution. The main controversy centres around the significance of ionprobe U-Pb zircon dates. Zircons from these rocks typically show a wide range (c. 3.60 to 3.87 Ga) of near-concordant U-Pb dates. Initial Nd isotope signatures of individual samples, when corrected for ~47Sm decay by assuming that the oldest U-Pb zircon dates represent the age of rock formation, span a wide range between +5 and 5 epsilon units (Bennett et al., 1993). These Nd isotope data have been interpreted to reflect both extreme mantle isotope heterogeneity and extreme LREE-depletion of early Archaean mantle, implying the existence and persistence of a voluminous crust during early Archaean times (McCulloch and Bennett et al., 1994). Such an interpretation of the Nd isotope data of the Itsaq gneiss complex would require a substantial (irreversible?) change in processes of crustal evolution between midand early Archaean times. Here we challenge the above scenario based on new, previously unpublished and published Pb isotope data for the Amitsoq gneiss. The ancient Amksoq gneiss is characterised by extremely low I.t (= 238U/2~ values and contains the earth's least radiogenic silicate Pb. In addition to previously published and unpublished whole-rock and feldspar Pb isotope data of Am]tsoq gneiss samples, we have specifically included samples in this study which contain zircons with some ion-probe U-Pb dates extending back to >3.8 Ga. Plotting all 84 samples (whole rocks, feldspars, leachates) in a common Pb diagram reveals two major features. Firstly, in spite of the wide area over which the samples were collected, a tight regression line (MSWD = 17.6) corresponding to an age of 3654_+73 Ma is obtained. The second, far more important, feature is that independent of the exact slope of the regression, a very well-defined intercept with plausible terrestrial Pb-isotope evolution curves is obtained. Thus, relative to the depleted mantle evolution curve of Kramers and Tolstikhin (1997), intercept ages of a family of regression lines calculated with sub-sets of the data range only between 3.64 and 3.68 Ga. The scatter around the Pb/Pb regression line is most probably the result of minor initial Pb-isotopic heterogeneity and/or some post-formational metamorphic disturbance. Whilst the slope of the Pb/Pb regression line could, in theory, be spurious if not all samples were genetically related, the really important intercept constraint from the least radiogenic Pb data cannot be discounted by arguing that unrelated rocks have been studied. The only straightforward interpretation of the Amhsoq Pb isotope data is that the magmatic precursors of the Amltsoq gneisses were formed over a short period of time (< 100 Ma), most likely c. 3650 Ma ago. The Pb-isotopic constraints are, and have always been, in obvious conflict with the extended (3.87 to 3.6 Ga) crustal growth history proposed by Nutman et al. (1996) and with the Nd isotope interpretation of Bennett et al. (1993). These authors summarily dismissed the Pb-isotope evidence by stating that these might reflect post-igneous disturbance. However, the only scenario under which the original Pb isotope ratios of 3.87 to 3.6 Ga old rocks could evolve into the observed Pb-isotope array would involve three stages. Firstly, a geological event must be proposed at 3.65 Ga capable of completely