Geochemistry and Possible Protoliths of Coesite-bearing Pyrope Quartzites and Related Rocks of the Southern
Dora-Maira Massif, Western Alps

Ch. Grevel Institut für Mineralogie, Universität Münster, Corrensstr. 24, D-48149 Münster

and Institut für Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

H.-P. Schertl Research Group on High Pressure Metamorphism, Institut für Mineralogie,

Ruhr-Universität Bochum, D-44780 Bochum, Germany

In the southern portion of the Dora-Maira Massif near Parigi, Chopin (1984) described a pyrope plus coesite bearing metamorphic rock exclusively forming lenses within a rather homogeneous biotite-phengite gneiss country rock. While Chopin (1984) had suggested the pyrope quartzites which underwent peak metamorphic conditions of about 37 kbar and 800 °C (Schertl et al., 1991) to be of sedimentary origin, e.g. evaporitic clays, Compagnoni et al. (1995) consider these rocks to be products of Mg-metasomatism developed at the expense of their country rocks. Because of the controversial discussion concerning the genesis of these rocks, chemical data of biotite-phengite gneisses, pyrope-quartzites, and related rocks are presented.

The pyrope quartzite typically containing high amounts of MgO is strongly depleted compared to the surrounding biotite-phengite gneiss in Na, Ca, and Fe. As this behaviour is characteristic for metasomatic alterations of granitic gneisses elsewhere in the Alps along shear zones forming magnesiochlorite-quartz-muscovite rocks or "leucophyllites" (Modjtahedi and Wieseneder, 1974) it seems possible that the pyrope-quartzites -or a lower grade equivalent- had also been subjected to such type of metasomatism. On the other hand, the mineral assemblage of the pyrope quartzites includes boron-bearing minerals like dravite and magnesiodumortierite which seems to support an evaporitic participation.

Rare earth element (REE) studies on pyrope-quartzites and their adjacent country rocks yield, normalized to post-Archean Australian shales (PAAS), flat parallel patterns except for strong negative Eu-anomalies. Whereas the spectra of all the country rocks are nearly identically, those of the pyrope-quartzites show a distinct variation in their REE-concentrations but are strictly parallel to each other. Modjtahedi and Wieseneder (1974) as well as Kiesl et al. (1983) concluded that during "leucophyllite" formation there exists no overall gain or loss of REE. Although the pyrope quartzites show either enrichment or depletion of REE in comparison to their adjacent country rocks, a metasomatic origin of pyrope quartzites at the expense of their country rocks seems possible in principle. However, the REE-distribution of whiteschists from Sar e Sang/Afghanistan (Kulke and Schreyer, 1973), representing former evaporites, clearly fall within the range of REE-variations of the pyrope quartzites as well.

Thus, based on the geochemical investigations alone it is impossible to decide conclusively between the sedimentary or the metasomatic alternative. These two seemingly contradictory hypotheses can be combined considering Alpine geology on a larger scale. It is well known that the Hercynian basement rocks were covered unconformably during Permotriassic times by sediments involving evaporites. The prograde subduction-related Alpine metamorphism could have been responsible for mobilisation of highly saline fluids from those formations through interaction of their evaporites with H2O released by dehydration reactions. These fluids would thus penetrate not only neighbouring sediments but also the underlying brittle basement along zones of weakness (shear zones). Here they can interact, perhaps preferably with granitic compositions, to cause the Mg-metasomatism inferred.


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