Pre- or Synmetamorphic Metasomatism in Peraluminous Metamorphic Rocks?

Abstract

The geology, petrography, mineralogy, geochemistry, and, especially, textural relationships of four occurrences of peraluminous metamorphic rocks from two continents are discussed and compared in order to answer the question put in the title: 1. Corundum-fuchsite rocks from Archaean greenstone belts in Southern Africa with Al₂O₃- contents up to 85 % and high Cr₂O₃ occurring within ultramafic bodies as transgressive masses are found to be metamorphosed former alunite deposits that, in turn, had originated during postvolcanic hydrothermal to solfataric alteration or metasomatism of the ultrabasic igneous rocks. Their extremely low Ga/Al ratios indicate a very potent Ga depletion for which, however, the crystal chemical properties of the pre-existing alunite cannot be made responsible. — 2. Stratabound corundum-sillimanite rocks from the Proterozoic Namaqualand Belt of South Africa with Al₂O₃ up to 77 %, and associated topazites, tourmaline, and dumortierite rocks may be best explained by near-surface, postvolcanic rock alteration of a thick, slowly cooling ignimbrite unit, combined with subsequent deposition of Al-rich products in an evaporitic environment. However, there is also textural evidence for later synmetamorphic metasomatism in the presence of acid, fluorine-rich fluids leading to still higher enrichment of Al. — 3. The peraluminous quartzites of the Carolina Slate Belt, USA, carrying andalusite, pyrophyllite, alunite, topaz, lazulite, and Al-phosphate minerals of the florencite-woodhouseite- svanbergite group have experienced only low-grade regional metamorphism within the stability field of pyrophyllite. Alunite, Alphosphates, and even andalusite were found as initial, premetamorphic products of the postvolcanic hydrothermal systems that caused hydrogen metasomatism. However, while andalusite was partly hydrated during regional metamorphism to pyrophyllite, alunite remained stable and recrystallized to a well-oriented fabric. More Fe-rich rocks carrying rosettes of chloritoid also date back to the time of the hydrothermal system, in which they were probably formed from basaltic protoliths. — 4. With this knowledge, the controversial staurolite- and kyanite-quartzites of Big Rock, New Mexico, are reinterpreted as products of a former hydrothermal system as well, that, however, was subsequently metamorphosed under higher grades of regional metamorphism than in Case 3. — Ga/Al ratios of the rocks of occurrences 2.–4. are generally normal except for those containing abundant topaz, which may have a capacity for Ga depletion.

Thus, although synmetamorphic metasomatism may locally play a role, the origin of the unusual chemistry of many peraluminous rocks is mainly due to premetamorphic, postvolcanic, metasomatic events that led to “advanced argillic alteration” as in the wall rocks of ore deposits. Peraluminous metamorphic rocks may, therefore, be important indicators for the proximity of gold and other rare metal deposits.