Hydromagmatic amphibole in komatiitic, tholeiitic and ferropicritic units, Abitibi greenstone belt, Ontario and Quebec: evidence for Archaean wet basic and ultrabasic melts

Detailed petrographic, electron microprobe and ion probe studies of Archaean hydromagmatic amphiboles from the Abitibi greenstone belt, Canada, yield new insights into the origin of Al-undepleted komatiitic and Al-depleted tholeiitic and ferropicritic melts. The amphiboles are present in peridotite layers and basal chill zones of thick differentiated basic and ultrabasic sills and flows, and are titanian pargasite-hastingsite in composition. They can be grouped into two petrographic types: (1) amphibole in the groundmass; and (2) amphibole-bearing melt inclusions. The groundmass amphiboles are oikocrysts, rims and interstitial grains, present in minor to major amounts. The oikocrysts host cumulus olivines (Fo(83-84)) that are rounded in shape, embayed, and smaller in size. The amphibole-bearing melt inclusions are hosted in cumulus olivines (Fo(83)-(84) in komatiitic rocks and Fo(79) in tholeiitic rocks), spherical to ovoid in shape, 50-500 mum in size, and dominated modally by amphibole. The melt inclusions also contain euhedral chromite and aluminous spinel and micrometric clinopyroxene and glass, and sub-micrometric iron-nickel sulphide, chloro-apatite and ilmenite. In-situ ion probe analyses indicate the amphibole is: (1) enriched in Nb, LREE and Zr and depleted in Sr and HREE relative to primitive mantle; (2) contains up to 1-3 wt% H2O; and (3) overall displays deltaD values from 50parts per thousand to - 140parts per thousand, including many values in the accepted magmatic range of - 60parts per thousand to - 90parts per thousand.