Derek Nicol MSc thesis abstract

Thesis Title: 
Assimilation of Basic Xenoliths within Centre 3 Syenites of the Coldwell Alkaline Complex, Ontario

This thesis describes the occurrence, mineralogy and assimilation of basic xenoliths hosted by Centre 3 syenites.  Field work was carried out in two locations, one in the vicinity of Neys/Ashburton and the other a large megaxenolith hosted by Centre 1 syenites in the vicinity of Wolf Camp Lake. 

Least altered xenoliths consist of plagioclase, pyroxene amphibole, biotite, apatite and opaque phases.  With increasing assimilation this changes to a combination of plagioclase, amphibole, biotite, apatite, opaque phases, alkali feldspar, calcite, fluorite, sphene, zircon, REE phases and quartz.

Plagioclase is replaced by alkali feldspar in the form of porphyroblasts and crystals in the groundmass.  Plagioclase is also decalcified to more albitic compositions along with recrystallization.  Amphibole compositions extend over the same range of amphibole compositions in the host ferro-edenite syenite.  The general effect of xenolith assimilation is the equilibrium of a xenolith's mineral assemblage to that of the host syenite.  Assimilation processes seen at Wolf Camp Lake are similar to those seen at Neys/Ashburton.

Bulk rock data along with mineralogical compositional variation in clinopyroxenes, suggest a tholeiitic basalt parentage for xenoliths in both areas.  Cr and Ni contents indicate an evolved nature to the parent volcanics.  Data also suggest the possible existence of a second undersaturated type of volcanic xenolith present at Neys/Ashburton.  Parental basalts are postulated to be coeval volcanics related to the formation of the Coldwell Complex.

Modelling by mass balance mixing calculations of contamination of host syenites indicates that contaminated ferro-edenite syenites are the result of direct assimilation of volcanic xenoliths by ferro-edenite syenite.  Quartz syenites are found to be unsuitable parents to contaminated ferro-edenite syenites.

A copy of the thesis can be downloaded here