Superzoned garnets in the coesite-bearing Brossasco-Isasca Unit, Dora-Maira massif, Western Alps, and the origin of the whiteschists

Rare centimeter-sized superzoned garnets (SZGs) were discovered in two coes ite-bearing whiteschists of the Brossasco-Isasca Unit (BIU), southern Dora- Maira massif (DMM), Western Alps. The superzoned garnet consists of a reddi sh-brown almandine core crowded with inclusions of staurolite, chloritoid, kyanite, chlorite and paragonite, and of a pinkish pyrope rim with sporadic inclusions of kyanite, and magnesian chlorite. The core-rim contact is rel atively sharp and marks the termination of the inclusion-rich portion. The core composition of the superzoned garnet is almost identical to, or slight ly richer in Mg, than that of the rim of porphyroblastic garnet in metapeli tes from the same unit. In the rim of the superzoned garnet, Mg-Fe ratio in creases abruptly towards the outermost rim, whose composition is identical to that of the common pyrope in the whiteschist. At the core-rim boundary, there is no chemical gap. Chloritoid and staurolite are common inclusions i n the core of the superzoned garnet in the whiteschist and in the porphyrob lastic garnet in the metapelite. The staurolite composition (Si = 2.00 and total R2+ < 2.0 for O = 23 basis) and its reverse Fe-Mg distribution with r espect to garnet suggest a HP origin. The Fe-Mg distribution between chlori toid and garnet is reverse in the superzoned garnet, but normal in the garn et of metapelite. Because normal Fe-Mg distribution was reported from other eclogite-facies metapelites, a model petrogenetic grid was constructed in the FMASH model system considering St, Cld, Ky, Chi, Grt, and assuming the following Fe-Mg partitioning of St > Grt > Cld > Chi. The resulting petroge netic grid suggests that the core of the superzoned garnet contains incompa tible assemblages, such as St-Cld-Chl vs. Cld-Chl-Ky. New and literature da ta and results of experiments in the KFASH system suggest that: (1) the sup erzoned garnet was formed under a single prograde high-pressure /ultra high -pressure (HP/UHP) Alpine metamorphism, (2) the almandine inclusion-rich co re of the superzoned garnet crystallized at disequilibrium in a pelitic com position system at around 600 degreesC and less than 16 kbar, probably from a former metapelite xenolith included in a Variscan granitoid, and (3) the chemical environment of the host rock suddenly changed from the normal pel ite to the whiteschist composition by a metasomatic process during the rim growth, i.e., at a stage close to the UHP climax. (C) 2001 Elsevier Science B.V. All rights reserved.