Abstract

The Potato Hill pluton is a nested, composite, Grenvillian intrusion (c. 1020 Ma) consisting of pyroxene-bearing quartz monzodiorite, equigranular biotite + hornblende + garnet granite, and K-feld-spar megacrystic hornblende + biotite quartz monzodiorite. All members are potassic with high Fe/Mg ratios, and have elevated concentrations of trace elements such as Ba, Zr, Nb and Y. Mass balance calculations successfully relate the concentration of major elements between the most primitive (megacrystic quartz monzodiorite) and evolved (equigranular granite) samples to the fractional crystallization of orthopyroxene, clinopyroxene, hornblende, biotite, garnet and plagioclase, and of trace elements (e.g. P, La and Zr) to the removal of accessory phases such as apatite, allanite and zircon. Lithologic zonation of the pluton is therefore attributed to in situ differentiation processes rather than varying degrees of crustal assimilation or the emplacement of discrete magma batches. This is supported by (1) contrasting δ18O values for the Potato Hill pluton (δ18O = +6.8 to +8.5º/) and the gneissic country rock (δ18 O = +10.5 to + 12.1º/), and (2) uniformly low ɛNd values (= -1.6 to + 1.2) for the granitoid rocks.

Tectonomagmatic discrimination diagrams indicate a within-plate, late-orogenic setting for the Potato Hill pluton. Furthermore, trace element signatures closely resemble shoshonitic lamprophyres. Compositional criteria therefore suggest that the Potato Hill pluton is not directly subduction-related, but was formed in a transpressional regime following ocean closure. Younger (post-1 Ga) Grenvillian plutons in the area may record crustal thickening or the subsequent extensional collapse of the eastern Grenville Orogen.

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