Petrology and Geochemistry of the Griffis Lake Ultrabasic Sill of the Central Labrador Trough, Quebec

'Alpine-type' peridotite sills occur in the predominantly volcanic sequence along the eastern s ide of the Labrador Trough. Individual sills are more than 2, 000 feet thick, and have been traced for more than 20 miles along strike. One of these, the Griffis Lake sill, is described in detail. The ultrabasic rocks probably were origin ally feldspathic peridotite, with 50 to 60 percent olivine, but were intensely serpentinized and later locally steatitized along structurally controlled zones. Olivine crystals in the sills how a preferred orientation, which, in the absence of evidence of fractional crystallization or of high temperature contact effects, suggests intrusion of olivine crystals in a silicate liquid of gabbroic composition. The lower and upper parts of the s ill s are more gabbroic than the central parts, very likely because of mechanical crowding of olivine crystals towards the central part of the sills during intrusion. Nickel is almost twice as abundant as chromium in the Griffis Lake sill, which is probably the result of extreme differentiation of basic parent magma prior to the in trusion of the ultrabasic material. Most of the ultrabasic sills are overlain by thick gabbro sill s, but chemical data suggest that the ultrabasic rocks and the gabbros were intruded independently and arc not the result of fractional crystallization in place. The ultrabasic rocks may have been intruded prior to or during the folding of the enclosing volcanic rocks.