The Saint-Gabriel-de- Brandon area is located about 60 miles NE of Montreal. The area studied covers about 50 sg. mi. It includes the following three division of Precambrian rocks: 1 / A migmatitic complex which constitutes the basement of the area. 2 / A group of supracrustal rocks close to the Grenville Group but with little crystalline limestones. 3/ Several kinds of plutonic rocks. These three "units " can be clearly distinguished by petrographic and structural studies. First the basement complex consist of banded migmatites (alternating quartzo-feldspathic and amphibolite bands of greywacky origin), and some heterogeneous migmatites. Disregarding a few masses and dykes of granite, the parageneses belong to the granulite facies. Bandes leptites and amphibolites of eruptive origin as well as some biotite-amphibole gneisses of volcano-detritic origin, are interstratified into the "paragneisses series", itself composed of aluminous gneisses, of pyroxene gneisses, of quartzites , calc-silicated rocks, and of skarns, all of sedimen tary origin. The parageneses of those para and orthogneisses belong either to the highest amphibolite facies, or to the granulite facies. They appear at a minimum temperature of 670°C and at a Ps comprised between 6 and 10 kb, with PH20 less than Ps. The Saint-Didace adamellitic pluton together with the orthogneissified quartz-mangerites make up, the main plutonics rocks types. They are roughly concordant to the paragneissic formations. Structural study of the area reveals superposition of two major folding phases. The first phase gives rise to isoclinal folds of kilometric size, recumbent to the N or NW, with axes oriented either E-W or NE-SW. Mangerites and most likely the Saint-Didace granite, both appear to be about contemporaneous of this first phase. It was probably at this time that the paragn eisses series were overthrust to the NW over the migmatitic complex which acted as a basement. The second phase features NW -SE to NS trending folds. Being superposed over the first phase, it yieided highly complex interference patterns. It acted upon all of the three above mentioned units , but the plutons behaved as rigid mole (apart from sorne rather small "blades" of mangerites, wich were folded together with the country rock). Geochronologicai study of those three units brings evidence of several superimposed orogeneses. The mobilized part of the migmatitic complex belongs to the Grenville orogenesis (950 M. Y.). The Saint-Didace granite was probably rehomogenised at 1 105 M. Y. The paragnessic series,may have be reset during the Eisonian orogenesis (1 450 M. Y.). The migmatitic basement complex "restite" is of Kenorian orogenesis age (2 300 M. Y. ).
