THE GEOMETRIC CONCEPT OF "THE THOMPSON PROJECTION" OF 1957 APPLIED TO METAMORPHOSED CUMULATE GABBROS IN MID-NORWAY

Metamorphosed cumulate gabbros in Ordovician ophiolite fragments in the Støren Nappe of the Mid-Norway Caledonides originally contained predominantly Mg-rich augite and intermediate plagioclase. In the metamorphosed state, they consist of diopside, amphibole zoned from actinolite to hornblende, epidote with Al-rich cores and Fe³⁺-rich rims, and chaotically zoned plagioclase. A problem in visualizing chemographic details of such assemblages and the related reactions in "chemical space" is in obtaining a two-dimensional representation that is faithful in portraying relative phase compositions and tie-line orientations in a system of four or more components.

In 1957 Jim Thompson pointed the way to a practical solution by proposing a "muscovite projection" allowing phases in a quasi-four-component system to be projected onto a two-dimensional surface. This elegant simplification came from a detailed understanding by Jim and students of the mineralogy of metamorphosed shales throughout New England. It served several generations of researchers and students to gain a rapid grasp of the chemography of assemblages and reactions in world-wide occurrences of muscovite-bearing metamorphosed shales. Similar or related strategies were adopted by others to portray relationships in other rock types.

The metamorphosed gabbroic rock considered here consists of four phases, three of which have extensive solid solutions. Our strategy was to project from diopside in the CaO-MgO-AlO_1.5-NaAlO₂ system, ignoring SiO₂. Diopside is midway between CaO and MgO. Actinolite projects at the MgO apex (at the top), whereas CaO-rich phases, such as calcite, project negatively to the same point. Albite plots at the NaAlO₂ apex on the right and anorthite plots below the AlO_1.5 apex on the left, with other plagioclase compositions plotting on a line between. Amphiboles with varied Al and Na substitutions lie in a field extending downward from the MgO apex. The epidote minerals plot below anorthite, and chlorite plots along the AlO_1.5-MgO join. This provides a powerful setting for examining phase relationships in diopside-plagioclase-amphibole rocks. Even where diopside is not stable, its breakdown products must be consistent with the original bulk composition, thus all plotting on the diagram.