CO-OCCURRENCE OF ANDESINE AND LABRADORITE ANORTHOSITES IN THE LAC CHAUDIÈRE PLUTON, GRENVILLE PROVINCE, QUEBEC

The Lac Chaudière pluton (LCD), a member of the CRUML belt of massif anorthosites, is located ~150 km NE of Quebec City. LCD lies between the St. Urbain and Lac à Jack (LAJ) plutons and all three anorthosites coincide with regions of very low intensity on the regional aeromagnetic map. The aeromagnetic map suggests that LCD and LAJ are connected via a narrow isthmus.

Detailed mapping of LCD, using a 1:20K base, has established its size (~5 × 7 km), shape (sub-circular), and form (domical). Mapping has also shown that only anorthositic rocks occur within the boundaries of the LCD, and separate andesine and labradorite suites (AS and LS) have been recognized. These suites can be distinguished on the basis of color, texture, and extent of weathering, as well as composition. Moreover, mapping has revealed that LCD is surrounded entirely by jotunite. Thus, any physical continuity between LCD and LAJ must be in the subsurface.

Rocks of the AS (~An₃₉Or₆, ~1150 ppm Sr, ~250 ppm Ba ) make up most of the massif, with pure anorthosite, typically >95% plag, being most common. Leuconorite, which contains accessory biotite and hemoilmenite, occurs in layered sequences but is found mostly as elongate lenses (~10 × 20 cm) within anorthosite. Such lenses locally have been flattened into disc-shaped bodies that define a foliation. All rocks of the LS (~An₅₅Or₃) are strongly foliated, in addition to being compositionally layered. The most distinctive unit (referred to as "tiger-striped norite") is characterized by wispy, cm-scale opx layers. Other LS lithologies include layered sequences of leuconorite, melanorite, and pyroxenite, in which layers range in thickness from cm to m. In addition to plag, opx, and hemoilmenite, LS rocks contain cpx and mgt (Fo₆₅ olivine occurs in one sample - the only known occurrence in the CRUML belt).

The LS forms a discontinuous ring ~100 m thick around the dome, with AS on its inside and outside. The contact between the LS and AS is concordant, and neither inclusions nor cross-cutting features have been observed - unlike similar occurrences in other CRUML-belt anorthosites, where discordances are common. Nevertheless, we infer that the LS is older because it is entirely enclosed within the AS. Mineralogically and structurally, the LS and AS are quite different and we infer a separate petrogenesis for each.