Deposition of Malene supracrustal rocks on an Amîtsoq basement in outer Ameralik, southern West Greenland

Abstract

Contact relations between Amîtsoq gneisses and Malene supracrustal rocks are described from Simuitâ and Qilángârssuit. It is concluded that primary depositional contacts are still preserved at some localities and that local apparent gradational features may represent a regolith. The sequence on Simuitâ is: Amîtsoq gneiss, banded white quartzite, mica schist with 'chert' and amphibolite. Primary layering in the lower unit is continuous and not truncated at the contact with the gneisses. The suggested primary contact and composition of the quartzitic unit are consistent with derivation by rapid weathering of older sial to form a quartz-rich arkose. The overlying mica schists are highly ferromagnesian and have high Cr and Ni contents suggesting a possibie basic source. The amphibo!ite is interpreted as being derived from originallayered basic igneous parent probably tuffs interlayered with differentiated sills. The succession on Qilángârssuit consists of basal mica schists and 'cherts', amphibolites, and an upper AI-rich metasediment unit. The lower units are interpreted as deposited on an Amîtsoq gneiss basement. The upper unit is progressively cut off to the north by a thrust contact with an overlying Amîtsoq gneiss unit. The differences between the basal Malene sequences over 12-15 km suggest they represent local, near-shore depositional environments rather than more extensive ocean basins.