FeO and MgO contents (in weight percent) vary inversely in accordance with MgO substituting for FeO. The almandine to pyrope-rich almandine compositions (Table T2; Fig. F1) are typical for pelitic materials metamorphosed under relatively high-temperature/low-pressure conditions. The pelitic nature of the source area is inferred from the high Al2O3 and low CaO contents (Deer et al., 1966), indirect evidence for a past weathering history that leached soluble components and concentrated insoluble residues. The higher MgO compositions and lack of evidence for zoning (Table T1) suggest relatively high temperatures. The grossular content is only 2 wt% in the Mg-rich end-member compared to 5 wt% in the Fe-rich end-member, consistent with (FeO + MgO) substituting for (CaO + MnO) at higher grades (Deer et al., 1966).
These garnet data complement the provenance information provided by a small number of gneiss pebbles (Nicolaysen et al., 2001). Specifically, the garnets apparently sample a much wider range of metamorphic materials, with pyrope contents ranging from 17 to 38 wt%. This suggests that the source area was extensive enough to expose a modestly wide range of metamorphic facies and/or a variety of bulk compositions.