Tectonic setting and evolution of the Morin anorthosite, Grenville province, Quebec

Tectonic setting and evolution of the Morin anorthosite, Grenville province, Quebec

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The Morin Anorthosite Mass underlies an area of 2 500 square kilometers; it consists of three tectonic units: a dome, a diapir and a nappe.The units have in common a large part of their magmatic history, but their tectonic evolution is different.Under the influence of gravity, the dome and the diapir behaved as buoyant masses, while the nappe and part of the diapir spread laterally.Texture and structure of the anorthosite are correlated with these differences in tectonic evolution.

Jotunitic and mangeritic rocks envelope the dome and part of the diapir, whereas a troctolite sill occurs in the zone of weakness between the dome and the nappe.The jotunites and mangerites shared a large part of their tectonic evolution with that of the dome, but while the dome was emplaced in an advanced stage of crystallization, its envelope was largely liquid.It is probable that anorthosites, jotunites and mangerites are comagmatic, in the widest sense of the word, and it is Noodles possible that troctolite was an early differentiate of the parent magma.The structural relations between the plutonic rocks and the surrounding supracrustal rocks have many characteristics in common with basement-cover relations in polycyclic otogenic zones.

However, the plutonites can be considered as a basement complex only if a rather improbable wholesale remobilization of at least the jotunites and mangerites is postulated.The main phase of penetrative deformation in the supracrustal rocks gave rise to gently inclined and recumbent folds (F2) east and northeast of the Morin Mass, and is correlated with the lateral spreading of the anorthosite nappe.This phase body mist overprinted most older structures, but relicts of the latter (F1) are preserved locally.A set of major open upright folds (F3) is restricted to the area adjacent to the anorthosite nappe and is interpreted as a set of second-order compression folds related to the emplacement of the nappe.

A late phase of deformation gave rise to very gentle major and minor folds (F4) that slightly deform the axial lineation of F2-folds.The regional metamorphism is transitional between the amphibolite and granulite facies; it is roughly contemporaneous with the main phase of deformation, thus with the lateral spreading of the anorthosite nappe.The only well-defined isograd is one that separates orthopyroxene-quartz bearing granulites on the side of the Mass from hornblende-quartz bearing gneisses away from the Mass.Late metamorphism took place in and around the anorthosite dome and produced garnet and clinopyroxene from plagioclase and iron-rich mafic minerals.