Replenishment episodes and crustal assimilation in the development of an early Tertiary magma chamber, East Greenland: evidence from layered cumulates of the Kælvegletscher ultramafic complex, Kangerlussuaq

Abstract

From the early Tertiary Kaelvegletscher ultramafic cumulate complex, emplaced into the Archaean basement on the west side of the Kangerlussuaq Fjord, East Greenland, we present geochemical and isotopic data from an outcrop of finely layered dunitic adcumulates. The dunites were deposited in a trough structure, interpreted to be a fossilized feeder channel to the Kaelvegletscher magma chamber. Detailed geochemical sampling of the trough reveals subtle cryptic compositional variations of olivines (Fo = 86.5–89.3; Ni = 2000–2700 ppm) and chromites (Cr# = 66–80) in a stratigraphical profile perpendicular to the layering as well as relatively large simultaneous variations of whole-rock 87Sr/86Sr ratios. The dunites are separable into sequences of normal and reverse cryptic zonations which are interpreted as resulting from fractional crystallisation and magma chamber recharge, respectively. Up to 20% of magma chamber replenishment by high-Mg melts is suggested. Sr-isotope compositions of the cumulates correlate with olivine compositions and suggest mixing of fractionated and unfractionated parental melts assimilating up to 8% local basement. We propose the existence of a vigorous volcanic system at the Kaelvegletscher site in early Tertiary times, where plume-derived magmas incorporated minor amounts of local basement and underwent fractionation and mixing in crustal chambers. Estimates of processed magma volumes during deposition of the trough cumulates are in the range of 100–130 km3.