Very-Low-Grade Metamorphism of Graywackes

Abstract

Graywackes are sedimentary clastic rocks which consist of quartz, feldspar, and sheet silicates (micas, chlorite, clay minerals, etc.) and commonly contain rock fragments. If the debris are derived from mafic rocks, metamorphic graywackes show qualitatively the same mineral parageneses as have been discussed in Chapter 12.¹ Quantitatively, however, mafic minerals and Ca-Al minerals are commonly present only in minor amounts, whereas white mica and especially quartz occur in larger amounts. The amount of feldspar in metagraywackes may be very different, depending on the initial quartz-to-feldspar ratio in the clastic sediment; feldspar may be very conspicuous.