Μεταμόρφωση Χαλαζιοαστριούχων Πετρωμάτων = Metamorphism of Quartzofeldspathic Rocks.

Περίληψη

The quartzofeldspathic metamorphic rocks constitute the largest part of the continental crust deriving from graywacke sandstone, siltstone and granitoid protoliths. Specifically, metagraywackes originate from clastic sediments (turbidites) containing clasts of volcanic, igneous and metamorphic rocks. The most abundant clastic minerals are quartz and feldspar. The metagraywackes, apart from quartz and feldspar, also contain clay minerals. Metagraywackes are distinguished into medium and high pressure. Under medium-pressure and in amphibolite metamorphic conditions metagraywackes contain the non-diagnostic paragenesis Qtz + Pl + Ms + Bt + Grt. High-pressure metagraywackes are distinguished in those of moderate and high temperature. In the first case, metamorphism takes place under blueschist conditions and leads to the assemblage Lws + Phe + Ab + Chl + Ttn + Gln + Arg + Pmp. At high temperatures garnet and K-feldspar are formed. Under low pressure-high temperature conditions, metagraywacke is characterized by the presence of K-feldspar, cordierite, sillimanite and orthopyroxene, the disappearance of mica and the occurrence of partial melting that results in the formation of migmatites.
      
Unlike metagraywackes, granitoid rocks are in anhydrous state when they are about to be transformed. However, water is a key factor in metamorphism and a significant amount may be necessary for metamorphic reactions to begin. Thus, deformation is another key factor in metamorphism of granitoid rocks. In the absence of deformation, textural characteristics of the parental rock can be preserved even in ultra-temperature metamorhism. Characteristic minerals of metagranitoids are prenite and pumpellyite, where their presence is associated with the existence of hydrothermal solutions, stilpnomelane, which is an index mineral of lower greenschist and blueschist facies.