THE MONTAUBAN GROUP: A TESTIMONY OF EARLY MESOPROTEROZOIC OCEANIC VOLCANISM ALONG THE LAURENTIAN MARGIN

The origin of large tracts of medium- to high-grade quartzofeldspathic gneiss in the Grenville Province remains obscure, thus hindering lithostratigraphic and tectonic reconstruction. Rocks of the Montauban region, south-central Québec, are no exception. They are well known however for hosting small, stratiform, polymetallic Au-Ag-Zn-Pb ore deposits. The amphibolite-facies orebodies and hydrothermally altered wall rocks occur within the ca. 1.45 Ga Montauban Group, a sequence dominated by quartzofeldspathic gneiss with minor quartzite and amphibolite, locally pillowed metabasalt, and sporadic outcrops of marble and calc-silicate rocks. In the absence of geochemical fingerprinting, the recognition of relict primary features provides essential clues for the proper interpretation of these rocks.

Proximal lapilli tuff and vesicular basaltic pillow lava occur associated with thinly bedded intermediate to felsic rocks interpreted as distal pyroclastic subaqueous fallout deposits. These volcaniclastic rocks are laterally intercalated with epiclastic sediments. Such volcanic rocks are consistent with deposition of the Montauban Group in a shallow submarine environment at the late stage of an andesitic to felsic volcanic cycle, a common occurrence in mature island-arc or backarc settings where VMS deposits are commonly formed. The trace element, REE and Nd isotopic geochemical signature of the metabasalts and associated gabbroic sills is conform to that of tholeiitic, juvenile oceanic suites. The signature is transitional between that of volcanic arc basalts and mid-ocean ridge basalts, suggesting a backarc setting.

The Montauban Group is intruded by the 1.4 Ga La Bostonnais Complex, a calc-alkaline plutonic suite ranging from gabbro to granite but dominated by pyroxene-hornblende diorite, hornblende quartz-diorite and granodiorite. Taken together, the Montauban Group and La Bostonnais Complex demonstrate the importance of early Mesoproterozoic oceanic volcanism and arc plutonism in the Laurentian growth history.