GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE LATE ARCHEAN SKJOLDUNGEN ALKALINE IGNEOUS PROVINCE, SE GREENLAND

The present work reports the first broad geochemical investigation of the recently discovered late Archean (2700 Ma) Skjoldungen alkaline igneous province (SAP) in southeast Greenland. The rocks studied range in composition from ultramafic to felsic and comprise pyroxenites, hornblendites, hornblende norites and diorites, monzonites, syenites, and nephelinitic rocks and carbonatites. Various lithologic units from the host Archean gneissic basement are also investigated. The magmatic rocks show remarkably coherent major element, trace element, rare earth element (REE) and Sr and Nd isotope Systematics, suggesting a petrogenetic relationship. The most important geochemical features are high normative proportions of nepheline, forsterite and albite, low TiO2 (<1.5 wt %) and moderate FeO (total) (<12 wt %) contents, enrichments in large ion lithophile elements (LILE) and light rare earth elements both absolute and relative to high field strength elements (HFSE) that display large negative anomalies, and generally low to moderate abundances of compatible elements. Field relations and REE and compatible element systematics among Skjoldungen rocks suggest that mafic and ultramafic hornblende-rich samples may represent cumulate lithologies of the regional parental magma. On the basis of mineral data, this is deduced to have had mg-number of 0.64, shoshonitic affinities (K2O similar to 1.5 wt %), been close to silica saturation and volatile rich. Major element, trace element and REE Systematics further suggest that felsic intrusions are related to the mafic regional parental magma through extensive olivine, hypersthene and hornblende fractionation. Lack of correlation between La/Yb and other critical trace and REE ratios indicates that apatite, zircon and titaniferous minerals were not important cumulus phases at advanced stages of evolution. The measured Sm-Nd whole-rock isochron age is 2716 +/- 23 Ma (2 sigma error) [mean square of weighted deviates (MSWD) = 1.4]; whereas linear regression of the Sr isotope data yields an age of 2604 +/- 7 Ma (2 sigma error) (MSWD = 22.2). The age obtained by Nd isotopes is corroborated by U-PB zircon results (2698 +/- 7 Ma), suggesting that the Sm-Nd system remained closed since crystallization. By contrast, the 100 Ma younger age obtained by Sr isotopes suggests that the Rb-Sr system has been disturbed Initial Nd-143/Nd-144 ratios span a narrow range corresponding to E(Nd)(2.7 Ga) = +0.74 to -1.09, whereas initial E(Sr) values at 2.7 Ga cover a comparatively larger interval from -10 to +20. Neither initial E(Nd) nor initial E(Sr) values conform to previously suggested mantle depletion curves and no meaningful correlation exists between Nd and Sr isotopes for the Skjoldungen magmatic rocks as a whole. Although compositionally, heterogeneous, the analyzed suite of samples from the host agmatitic basement is extremely homogeneous with respect to age, with T-CHUR crustal residence times around 2700-2800 Ma confirming limited available isotopic evidence. Large-scale assimilation of Archean crust or recycling of sediments derived from the local basement into the mantle source fails to explain adequately negative Nb anomalies and low E(Nd) signatures characteristic of the Skjoldungen intrusions. Rather, the near-chondritic isotopic composition of Nd in the Skjoldungen samples together with the decoupled LILE and HFSE enrichment and slightly positive E(Sr) values are considered to reflect characteristics of the mantle source in a subduction zone environment. The geodynamic site hosting the Skjoldungen province thus may be an early manifestation of modern-style plate tectonics.