IDENTIFYING THE PRIMARY MAFIC PHASE IN ARCHAEAN ANORTHOSITES: EVIDENCE FROM THE AKULLEQ TERRANE OF WEST GREENLAND

A suite of 39 samples from 6 Archaean anorthosite bodies of the Akulleq Terrane of West Greenland were analyzed using XRF, INAA, and EMP methods. The suite comprises anorthosite, leucogabbro, and gabbro with normative feldspar compositions of ~An_63-86, An_57-74, and An_62-71, respectively. The samples have undergone high-grade amphibolite-facies metamorphism and consist of a metamorphic assemblage of white to gray plagioclase and black hornblende, with minor biotite. Four samples from one locality (Naujanguit) contain pale-green clinopyroxene, but these samples have the highest mg values (0.70-0.73 vs. 0.37-0.68). Approximately 5% magnetite and ilmenite are found in four samples from Itivdleq that have the highest TiO₂ concentrations.

MgO concentration effectively separates anorthosites (<1 wt% MgO, 12.7-17.3% CaO, 1.5-3.7% Na₂O, 0.06-0.33% K₂O), leucogabbros (1-6 wt% MgO, 11.6-14.3% CaO, 2.0-3.7% Na₂O, 0.22-0.69% K₂O), and gabbros (9-11 wt% MgO, 11.0-14.9 CaO, 1.2-2.1% Na₂O, 0.27-0.70% K₂O). Ni, Co, Zn, V, and Cr concentrations are positively correlated with MgO, forming linear arrays. The cpx-bearing samples show increased Ni/Mg and Cr/Mg and decreased Co/Mg whereas the oxide-bearing samples have increased V/Mg, Zn/Mg, and Co/Mg. The three lithologies have overlapping Ba (14-69, 30-102, and 22-73 ppm, respectively) and Sr (106-221, 116-308, and 78-127 ppm). The three lithologies have low (La_N=0.5-22.6, Yb_N=0.2-12.3) overlapping REE concentrations. The anorthosites and leucogabbros have fractionated LREE, flat HREE, and positive Eu anomalies whereas the gabbros have flat patterns. Samples with the lowest REE concentrations display the largest Eu anomalies.

The major-element data are consistent with original assembly of these rocks as mixtures of plag and a single mafic component. Na₂O data suggest that this mafic component was Na-bearing and possibly amphibole. Trace elements are expected to show cryptic primary phases since erased by metamorphism, but these data are consistent with the current mineralogy (plag + hbl ± cpx ± ox) lending further weight to the idea of a primary amphibole. The chemical and mineralogical data for these 6 anorthosite bodies are very similar and are consistent with the idea that these discontinuous bodies were a single continuous sheet prior to deformation.