ZIRCON GEOCHRONOLOGY OF THE LAC PICHÉ ANORTHOSITE, FORÊT MONTMORENCY COMPLEX, GRENVILLE PROVINCE, QUEBEC: IMPLICATIONS FOR MASSIF ANORTHOSITE DEVELOPMENT IN THE CRUML BELT

The "CRUML belt" comprises a linear series of relatively small, unmetamorphosed, late- to post-tectonic, Grenville-aged andesine anorthosite plutons that extends from Quebec City to north of Chicoutimi, a distance of >400 km. The two northern plutons are more evolved chemically than the five southern ones (e.g., An₃₀Or₁₀ vs. An₄₀Or₅) and appear younger, with Labrieville and St. Urbain having yielded U-Pb zircon dates of ~1010 and ~1055 Ma, respectively. However, with only two ages available, the questions arise as to whether the southern and northern plutons were emplaced in two separate magmatic pulses or whether they form a northward-younging sequence. To test these possibilities, jotunites and mangerites associated with the newly recognized Lac Piché massif, a member of the southern group, were investigated via U-Pb zircon geochronology. Zircon in the investigated samples are dominantly elongate (100–500 μm), euhedral, and prismatic, with terminations locally showing some partial rounding. Cathodoluminescence and back-scattered electron imaging shows that the zircons contain irregular to vaguely prismatic cores surrounded by thick, structureless overgrowths. Both domains exhibit concentric internal zoning that conforms to grain boundaries, or display “feathery” oscillatory growth structures, which are consistent with igneous crystallization. The images indicate that zircon rims are consistently, albeit slightly, richer in U than are the cores (Avg U~50-60 ppm for mangerite zircons and ~20 ppm for jotunite zircons, with Th/U~0.4). Such low U concentrations and “magmatic” Th/U ratios are similar to those for zircons from St. Urbain and Labrieville. The zircons proved difficult to analyze with the desired precision, generally due to their low Pb concentrations (<15 ppm), and we have retained only those results showing less than 5% discordance. Two mangerite samples yielded ages of ~1085 and ~1052 Ma respectively, whereas a jotunite yielded an age of ~1067 Ma. Within error, these new ages are similar to or slightly older than those for St. Urbain. Looking at the special distribution of the CRUML-belt plutons, the new ages permit a northward age progression similar to a modern hot spot track. To understand and to confirm further this interpretation, additional field work and age determinations are needed.