Mineralogy and Geothermometry of the mid-Jurassic Trail Pluton, Southeastern British Columbia

English Abstract:

The Trail pluton, located in southeastern British Columbia, is part of a suite of mid-Jurassic granitic rocks of probable continental arc affinity that were transported eastwards as a result of Cordilleran tectonics. Previous work on the nearby Nelson Batholith has suggested that magmatic rocks in this area were subjected to substantial post-equilibration rotation and tilting, possibly as a result of movement on listric faults. Twenty-five samples were collected from the Trail pluton for the purpose of mineralogical description and amphibole-plagioclase thermometry using the HB-PLAG program by Holland and Blundy (1994). The average modal mineralogy of the samples is: quartz 13%, plagioclase 33%, potassium feldspar 20% (late in the magmatic crystallization history), amphibole 14%, biotite 10%, titanite 1%, and a minor amount of alteration minerals such as chlorite and epidote. Many samples show evidence of localized strain, including comminution of grain size in quartz and feldspars and folding of biotite. Most plagioclase crystals fall within the composition range An₃₉ to An₃₁. The potassium feldspar is low in Na, suggesting a lower crystallization temperature. All amphiboles are calcic, ranging from magnesiohornblende to tschermakite to pargasite. Nearly all amphiboles have Al^vi > 1.0, giving them the prefix alumino. Preliminary thermometry calculations using the edenite-tremolite end-member system give an average result of 745°C for amphibole-plagioclase equilibration at P = 5 kbar. This result agrees well with calculations by Ghent et al. (1991) for both the Nelson Batholith and Bonnington pluton. Further work on this project will include barometric calculations as well as an examination of possible differential uplift and tilting of the Trail pluton.