Problems of Palinspastic Restorantion
Flysch arenites (graywackes) vary greatly in their framework mineralogy and volatiles-free chemistry. Three distinct groups are recognized on the basis of abundance: (1) quartz-poor graywackes (<15% quartz, average 58% SiO<sub>2</sub>, K<sub>2</sub>O/Na<sub>2</sub>O << 1), (2) quartz-intermediate graywackes (15-65% quartz, average 6874% SiO<sub>2</sub>, K<sub>2</sub>O/Na<sub>2</sub>O < 1), and (3) quartz-rich graywackes (> 65% quartz, average 89% SiO<sub>2</sub>, K<sub>2</sub>O/Na<sub>2</sub>O> 1).
Mineralogical data from modern deep-sea sands, which are regarded as the contemporary analogues of flysch arenites, together with data from sands of rivers that debouch into the deep sea provide the basis for an actualistic hypothesis to explain the compositional variation of ancient graywackes in terms of their geo- tectonic settings. This hypothesis is supported by chemical comparisons.
Quartz-poor graywackes are indicative of magmatic island arcs. Their average chemical composition approximates the average composition of these arcs and that of tholeiitic andesite. Quartz-intermediate graywackes are indicators of Andean type continental margins and approximate the upper levels of continental crust in composition. Quartz-rich graywackes indicate Atlantic-type continental margins. Chemically, they are related to the sand fraction of continental platform cover.