Proterozoic Mantle Heterogeneity: Geochemical Evidence from Contrasting Basic Dykes

Abstract

Summary Two swarms of early Proterozoic basic dykes occur in the Archaean craton of SW Greenland. The MD dykes are tholeiitic dolerites, while the BN dykes are norites. Virtually identical norites occur with tholeiitic dolerites in the Scourie swarm of NW Scotland and in the Laramie, Bighorn and Beartooth ranges of Wyoming, USA. Close geochemical similarities between typical coarse-grained norites and quenched varieties in SW Greenland and Wyoming suggest that the former type has not been strongly modified by crystal accumulation. The mineralogy and geochemistry of the quenched norites closely resemble those of boninites. Most of the norites have high MgO ( c. 13–18 wt%) and correspondingly high Cr and Ni, but they also have relatively high silica, alkali and large ion lithophile (LIL) element abundances. They are all light rare-earth element (REE) enriched (La = 40 times average CI chondrite, La N /Lu N c. 7). In contrast, most of the tholeiitic MD dolerites have fiat, unfractionated REE distributions. The tholeiites were probably derived from a fertile mantle source. However, the norites originated from a depleted harzburgitic mantle which had earlier borne Archaean komatiitic and tholeiitic magmas but which had been metasomatically replenished in LIL elements by fluids derived from Archaean oceanic and lower continental crustal material returned to the mantle during late-Archaean crustal thickening. Some major layered basic igneous intrusions, such as Bushveld and Stillwater, were derived by the mixing of two different basic magmas, which were equivalent to the tholeiitic and noritic (boninitic) dyke magmas of SW Greenland, NW Scotland and Wyoming. The apparently ubiquitous occurrence of tholeiite-norite associations suggests that similar large-scale mantle heterogeneities, due to the voluminous extraction of Archaean komatiitic and tholeiitic magmas, had become well established by the early Proterozoic.