Metamorphism of Quartzofeldspathic Rocks

Abstract

Metamorphosed quartzofeldspathic rocks are derived from graywacke sandstone and siltstone (clastic sediments; Table 2.3), and granitoid protoliths such as granite, alkali-feldspar granite, granodiorite and tonalite (Table 2.3). They constitute the largest portion of the continental crust. For instance, quartzofeldspathic gneisses (metasedimentary paragneisses and granitic orthogneisses), commonly migmatitic, are the dominant rock type of the continents, forming extensive terranes. Because the main metamorphic constituents – Qtz, Kfs, Pl, Bt, Ms, Hbl – occur over a wide range of P–T conditions, and because Al-silicates are typically absent due to a relatively high silica content (Table 2.3), quartzofeldspathic rocks may not be a particularly useful indicator of metamorphic grade and are often neglected in textbooks. However, in this chapter we examine the progressive metamorphism of metagraywacke-type rocks and describe selected mineralogical features that can be used to determine metamorphic grade in metagranitoid rocks.