Steven R. McCutcheon

The Mount Pleasant caldera complex comprises part of the Piskahegan Group that is divisible into exocaldera, intracaldera and late caldera-fill sequences. Each sequence consists of a number of formally defined rock units.

Intracaldera rocks (caldera complex sensu stricto) underlie a 110 km2 triangular area that is characterized by 1) voluminous, high silica, ash-flow tuffs with no observed outside, correlatives; 2) sedimentary breccias (mesobreccias) and a concentration of small, high-level intrusions along two sides of the triangle and 3) regional gravity and magmetic anomalies. One spore locality in the upper part of the exocaldera sequence shows that the caldera formed in the Late Devonian time, coeval with post-tectonic granites.

Textural evidence from Piskahegan Group rocks indicates that eutaxitic foliation can form by mechanical compaction of cold pumice. However, compaction-type foliation can be distinguished from that produced by welding if "fossilized" pumice is present. Textural evidence also suggests that embayed quartz and sieve-textured plagioclase can form as a result of "cellular-growth".

Mineral chemistry shows that remnant clinopyroxene is all non-orogenic and tholeiitic. Also, apatite and zircon chemistries cannot be used to distinguish rock units because there is as much compositional variation within a single sample as there is between samples or among rock units. Similarly, the compositions of glass inclusions in quartz phenocrysts cannot be used to distinguish rock units. Furthermore, they do not reflect original bulk-magma composition.

Age relationships, compositional variations and the relative abundance of igneous units in the Piskahegan Group suggest that the rocks were derived from a large, anorogenic, compositionally-zoned magma chamber underplated rhyolitic units can be largely explained by double-diffusive fractional crystallization (DDFC) and crystal resorption. Fractional crystallization, assimilation and magma mixing are required to explain variations among the metaluminous basaltic and andesitic rocks. A late-stage porphyritic microgranite represents phenocryst-rich hybrid magma that accumulated near the mafic-felsic interface in the chamber.

The Mount Pleasant Sn-W deposit post-dates the intracaldera rocks and formed by in situ cooling and de-volatization of granitic magma that was emplaced at the caldera margin. REE chemistry suggests that the magma evolved by DDFC and mass-balance calculations show that the magma initially could have contained less than 1% H₂O and as little as 1-3 ppm Sn and W.

Keywords:
Pages:
Supervisor: Paul Robinson