E&ES Dept  RPI Info

Metamorphic Petrology Lab at Rensselaer Polytechnic Institute


Rocks tell a story. It is the goal of the geologist to read the story of the history of the earth through the record left in the rocks. In particular, in order to understand the processes of mountain building and collisional orogenesis, we go to the crystalline metamorphic rocks found in the cores of these mountain belts. At least in part, the story of the collision of continents is contained in the chemistry and textures of metamorphic minerals. For example, subduction and continental collision cause lithosphere plates to be deeply buried.

The rocks within them experience increases in pressure (sometimes enormous increases) and temperature. All the while, the rock is reacting, forming new minerals, the chemistry of which is strongly controlled by the pressure and temperature. Some minerals, such as garnet and plagioclase, quite faithfully retain their composition from the time at which they were produced. These minerals are a type of "chemical tape recorder" that can be used to decipher the pressure-temperature history, or P-T path, of the rock during its evolution.

Reading the history of the Earth through metamorphic rocks requires a multi-faceted approach. My research group has been working on developing new techniques for the interpretation of metamorphic petrogenesis and especially mineral reactions and chemical zonation. The mineral garnet has proved to be an exceptionally good recorder of reaction histories. Traditionally, we have always used major element zoning in garnet (Fe, Mg, Ca, and Mn) and recently we have been exploring trace element zoning (especially Y).

Some accessory minerals also record a rock's history. Monazite is especially useful because it can also be dated, to provide not only a temperature but a time of formation.

The ultimate goal of this work is to construct pressure-temperature-time histories that can be used to interpret the tectonic evolution of an orogenic belt. We have applied these techniques to areas in the Alps, New England, southern Chile, British Columbia, the Mojave desert, the Adirondacks of New York, the Limpopo belt of South Africa and the Caledonides of Norway.

About these figures
The two images are x-ray composition maps showing the distribution of Ca/Na and Fe/Mg in a meta–anorthosite from the Adirondack Mountains, New York. Warm colors are high concentration, cool colors are low concentration. The Ca/Na map shows zoning in plagioclase which is high in the cores (red = An48) and low towards pyroxene (blue = An43). The Fe/Mg map shows a large relict igneous orthopyroxene that has reacted to metamorphic orthopyroxene (green) plus clinopyroxene (blue). The red necklaces are garnet that has formed during cooling of the Adirondacks from approximately 800° C to 735° C at a depth of 30-35 km. The reaction history and zoning observed in minerals confirms a counterclockwise P-T path and nearly isobaric cooling for these rocks.