Abstract

The Topsails igneous complex of western Newfoundland is a large (> 6000 km2) intrusive complex of two contrasting geochemical suites and at least four distinct granitoid facies: (i) a fine-to medium-grained biotite-bearing peraluminous granite; (ii) a medium- to coarse-grained metaluminous biotite–hastingsite granite; (iii) a coarse-grained peralkaline aegirine-arfvedsonite granite; and (iv) fine- to medium-grained biotite-hornblende granites and syenites. Peralkaline microgranite dykes cut the other granitoid phases and the country rocks. Peralkaline granites occur within the central parts of the complex, the other granitoid phases being marginal. Volcanic and subvolcanic roof pendants are divisible into peralkaline and non-peralkaline groups. The former are clearly related to the peralkaline granite, and the latter appear to represent the other intrusive phases of the complex. Contemporaneous basalt magmatism is indicated by dykes which cut the biotite granites and are in turn cut by the peralkaline granite. Available Rb–Sr data show ages of 386 ± 9 and 419 ± 5 Ma for the peraluminous and peralkaline granites respectively.The main difference in major elements between the peralkaline and non-peralkaline granites is in their agpaiitic indices, although there is some overlap between them. There are, however, marked dissimilarities in their respective trace element characteristics, e.g., K/Rb, K/Zr, and Rb/Sr, with high Zn concentrations being especially characteristic of the peralkaline rocks.The origin of these rocks is interpreted in terms of a model involving crustal melting due to the interaction of crustal extension, basaltic intrusion, and volatile fluxing, with subsequent high-level intrusion and fractionation, along with metasomatism by magmatically-derived fluids.

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