Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps

Abstract

Petrographical and mineral chemical data are given for the eclogites which occur in the garnet‐kyanite micaschists of the Penninic Dora‐Maira Massif between Brossasco, Isasca and Martiniana (Italian Western Alps) and for a sodic whiteschist associated with the pyrope‐coesite whiteschists of Martiniana. The Brossasco‐Isasca (BI) eclogites are fine grained, foliated and often mica‐rich rocks with a strong preferred orientation of omphacite crystals and white micas. Porphyroblasts of hornblende are common in some varieties, whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz, jadeite and garnet. These features differentiate the BI eclogites from the eclogites that occur in other continental units of the Western Alps, which all belong to type C. Garnet, sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist. Sodic pyroxene in the eclogites is an omphacite often close to Jd50Di50, with very little acmite and virtually no AlIV, and impure jadeite in the leucocratic layers and in the sodic whiteschist. Garnet is almandine with 20–30 mol. % for each of the pyrope and grossular components in the eclogites and a pyrope‐rich variety in the sodic whiteschist. White mica is a variably substituted phengite, and paragonite apparently only occurs as a replacement product of kyanite. Amphibole is hornblende in the eclogites, but the most magnesian glaucophane yet described in the sodic whiteschist. Quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets. The P‐T conditions during the VHP event are constrained in the eclogites by reactions which define a field ranging from 27–28 kbar to 35 kbar and from 680 to 750° C. These temperatures are consistent with the results of garnet‐pyroxene and garnet‐phengite geothermometry which suggest that the eclogites may have equilibrated at around 700° C. In the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite‐pyrope garnet‐glaucophane compatibility. As in the eclogites water activity must have been low. Such conditions are close to the P‐T values estimated for the early Alpine recrystallization of the pyrope‐coesite rock and, like petrographical and mineralogical features, set aside the BI eclogites from the other eclogites of the Western Alps, instead indicating a close similarity to some of the eclogite bodies occurring in the Adula nappe of the Central Alps. An important corollary is that glaucophane stability, at least in Na‐ and Mg‐rich compositions and under very high pressures, may extend up to 700° C, in agreement with the HT stability limit suggested by experimental studies.

Microstructures and crystallographic preferred orientations of omphacite in Alpine eclogites: implications for the exhumation of (ultra-) high-pressure units

“…The metamorphic conditions are 28-35 kbar and 700-750 C (Chopin et al, 1991;Avigad, 1992;van der Klauw et al, 1997;Nowlan et al, 2000;Rubatto and Hermann, 2001). The upper parts (units II and III) have only been affected by ''normal'' HP (eclogite-facies) metamorphism at 15-20 kbar and 500-600 C (Chopin et al, 1991;Kienast et al, 1991;Henry et al, 1993;Nowlan et al, 2000). The Brossasco-Isasca Unit is underlain by the Pinerolo unit, which is supposed to originate from the Brianconnais (e.g., Philippot, 1990;Chopin et al, 1991;Henry et al, 1993), and has been affected by blueschist-facies metamorphism, and overlain by the Dronero unit, lithologically similar to the Brossasco-Isasca Unit.…”

Petrology of the nyböite-bearing eclogite in the Donghai area, Jiangsu Province, eastern China

“…These natural occurrenccs of amphiboles suggest that the nyb6ite is a stable phase under high-temperature and very high-pressure conditions, where coesite is stable, although there still remains the question of the exact timing of the amphibole growth. Kienast et al (1991) found a primary glaucophane from the pyrope-bearing mica schist in the southern part of the Dora Maira, western Alps. They estimated the maximum P-T conditions as 680-750 ~ and more than 28 kbar, which is very similar to those in the coesite eclogite in the Norway and Jianchang eclogites.…”

Blueschists and Eclogites of the World and Their Exhumation

“…It consists of Hercynian and old crystalline continental basement rocks metamorphosed under UHP and HP conditions as a result of the MesozoicCenozoic convergence of Europe and Adria. The HP and UHP belt is a stack of four major units separated by four low-or high-angle faults; all have been affected by Alpine HP metamorphism and subsequent re-equilibration at low pressures Kienast et al, 1991). Peak P/T conditions differ greatly from unit to unit, indicating that they have risen from different depths within the Eoalpine orogenic wedge subsequent to recrystallization (Avigad, 1992).…”