Tuesday, September 13, 2011

An Old Habit Kicks In: Looking for Pressure-Temperature Indicators

Granitic intrusions into schist/gneiss

Granite and schist/gneiss cliffs

Typical granite on our traverse

I was recently traversing across an area with intruded sheets of granite into schist and gneiss. Looking at these rocks was not my intended purpose, but I found myself looking for aluminum silicates Al2SiO5 in the schist - andalusite, kyanite or sillimanite. I kept bending down and peering at the schists intently for the the classic cross patern of andalusite. Or better yet, small andalusites psuedomorphed into kyanite and/or sillimanite. These three minerals form under different pressure and temperature regimes with anadlusite being the lower temperature and pressure member, kyanite the high pressure member and sillimanite the high temperature member.

Identifying any of the three can give one a rough idea as to the pressure and temperature of the metamorphosed rock. And if you can identify the shape of one mineral such as the small crosses andalusite forms being replaced by say kyanite you can get an idea of the pressure/temperature path of metamorphism.

I spent two field seasons looking at schists adjacent to plutons and batholiths in the North Cascades and British Columbia Coast Range with the goal of figuring out the depth of the pluton emplacement and the subsequent metamorphic events after emplacement of the plutons. I was struck by how my old metamorphic habits so automatically took over without even thinking about what I was doing.

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