Petrological and geochemical studies on jadeite-bearing rocks and their blue-schist facies (Queyras, French Western Alps)

H.-P. Schertl, M. Dreizler (Bochum), C. Chopin (Paris)
 

Investigations of jadeitites and jadeite-rich rocks, which are typically found in subduction zone-related serpentinites, have already been a tradition in the field of petrology at the RUB's Institute of Geology, Mineralogy & Geophysics. In general, today two "end-member formation hypotheses" are accepted for such rocks. They can form, on the one hand, the metasomatosis of a suitable protolithe (for example, so-called "plagiogranites"), and, on the other hand, the result of circulating aqueous fluids in the subduction channel of a subduction zone.

Therefore, these jadeitite-rich rocks are important indicators of fluid compositions within such subduction zones. The investigations are thus the products of such fluid-rock interactions. There are approximately 25 localities around the world, often characterized by the occurrence of jadeite-rich blocks in serpentinites. However, there are only 3-4 occurrences of jadeite-rich rocks in blueschists and eclogitic side rocks, e.g. from the Dominican Republic or the Cyclades. The occurrence of Queyras belongs to this group and thus offers the unique possibility to study the direct contact conditions. The blueschists associated with the jadeite-rich rocks are of particular interest since they still contain relicts of the magmatic protolith.

Within the framework of the planned work, careful petrographic investigations are planned (polarization microscopy, cathodoluminescence microscopy), and, based on this, mineral-analytical investigations with the electron beam microscope as well as geochemical studies. Except for short descriptions from the 70s of the last century, there are no detailed data on this specimen, so the following general questions are of importance:

• What maximum pressure-temperature conditions have the jadeite-rich rocks experienced?
• Can the jadeite rocks and the neighboring blueschists reconstruct PT paths, and if so, are there indications for an in-situ metamorphosis?
• Are there any indications for a direct formation of the jadeitites from aqueous fluids (for example, fluid inclusions) with regard to possible genotypes of origin? Is there evidence of metasomatic transformations (a suitable protolith)?
• Can geochemical investigations about the magmatic starting rock of the blueschist be made?
• What conclusions can be drawn with respect to the local geotectonic environment?