Helen McLaughin

MacEachern - Ponsford Memorial Award - 1988

B. Sc. Honours Thesis

Thermobarometry of Meta-Sedimentary Enclaves in the Ten Mile Lake Intrusion, Liscomb, Nova Scotia

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A section of a drill core from the area of the Ten Mile Lake, Liscomb Complex, Nova Scotia, contains many small metasedimentary enclaves in a felsic host. The enclaves investigated are calc-silicate and metapelites and range from having distinct to gradational boundaries and from to being intact to extremely disaggregated within the host.

The enclaves contain both xenoblastic, pre-magma entrainment and idioblastic, syn-magma entrainment minerals. Relict phases in the metapelites are Mn-rich and Mn-poor xenoblastic garnets, kyanite, sillimanite, biotite, plagioclase, zincian hercynite, and staurolite. Syn-entrainment mineral assemblage includes Mn-rich and Mn-poor idioblastic garnets, fibrolite, biotite, plagioclase, and muscovite. Other phases include quartz, sphene, pyrite, chalcopyrite, Fe-Ti oxides, and retrograde chlorite. Calc-silicate enclaves contain grossular, dolomite, zoisite, clinozoisite, epidote, hornblende, apatite, sphene, quartz, plagioclase, pyrite, and chalcopyrite.

Zincian hercynite, present in the source region, reacted to form staurolite during coupled retrograde and metasomatic reactions. Zonation of garnets and the calc-silicate enclaves and carbonate and quartz veinlets are further evidence for chemical exchange between the enclaves and the host. The source region for the enclaves is determined to be pelitic and calc-silicate, locally rich in zinc.

Pressure and temperatures are determined for pelitic enclaves in a tonalite to quartz-diorite intrusion in the Ten Mile Lake gabbro area are based on garnet-biotite-plagioclase - Al₂SiO₅-quartz geothermobarometry in the pelitic enclaves. Temperature and pressures for pre-entrainment metamorphism of the enclaves are 780+50^oC and 5.6+1.5 kb. Syn-entrainment metamorphism is estimated at 560+50^oC and 2.3+1.5kb.

Petrography shows the pre-entrainment P-T estimates may not be coincident, but rather may represent independent maximums. Mineralogy suggests that the source region, at ~17 km, was initially at lower temperatures than indicated by thermometry calculations. The initial intrusion of the magma led to an increased temperature, at a maximum of 780^oC, before entrainment into the magma.

Keywords:
Pages: 79
Supervisor: Rebecca Jamieson

Awards

The MacEachern - Ponsford Memorial Award - 1988

The Effective Writing Award (Thin Thesis Award) - 1990

The Chevron Canada Resources Ltd Scholarship - 1990