Petrochemistry and Metamorphism of the Tallan Lake Sill, Bancroft Area, Ontario

Abstract

<p>The Precambrian Tallan Lake Sill is an elongate, stratiform amphibolite body, belonging to a group of differentiated gabbros emplaced around 1250±25 m.y. ago in the Bancroft area of Ontario.</p> <p>Although metamorphism has obliterated all but a few primary igneous textures, it is clear from major element chemistry that the sill is a differentiated ferrogabbro of tholeiitic affinity. Some critical field observations combined with model calculations make it highly probable that the syenite underlying the metagabbros is a differentiate of the latter, indicating that the stratigraphic succession in this part of the Grenville Group is overturned.</p> <p>Alkali values can not be relied upon in these model calculations, and it is shown that the chemistry of many rocks in the Bancroft area shows the effects of spilitisation and other chemical alteration, presumably due to an early episode of burial metamorphism. Metamorphism in the area is of the low-pressure intermediate type and is reflected in the coexistence of garnet, cummingtonite and hornblende in many Tallan Lake samples.</p> <p>A qualitative petrogenetic grid based on the rules of Schreinemakers' has been derived for the hornblende-cummingtonite-garnet paragenesis and the importance of the coexistence of cummingtonite and garnet as a possible geobarometer is pointed out.</p> <p>Assemblages in marbles surrounding the Tallan Lake Sill point to a metamorphic maximum P and T of around 4.0-6.5 kb and 625-650ºC, respectively, but retrograde reactions and extensive exsolution of coexisting amphiboles suggest a gradual decline in temperature, possibly reflecting the slow cooling and unroofing of neighbouring mantled gneiss domes.</p> <p>The differentiation mechanism envisaged for the Tallan Lake Sill, involving mechanical separation of a residual, interstitial syenite liquid may have wider application, and it is suggested that the chemical discontinuity known as the Daly Gap might be so explained.</p>