ABSTRACT

Thin-sections of 119 igneous and metamorphic rocks and 44 clastic rocks were examined in an attempt to evaluate the use of undulatory extinction and polycrystallinity in detrital quartz grains for the determination of provenance of sediments.

Non-undulatory quartz is uncommon in plutonic igneous rocks, schists, and gneisses. For 101 rocks of these types examined, the average amount of total quartz which is of the non-undulatory type is only 14.9 percent. Differences in the percentages of non-undulatory quartz among these three groups of rocks are not statistically significant. The quartz in extrusive igneous rocks, however, is almost entirely (91%) of the non-undulatory type; it is probable, however, that quartz-bearing extrusive rocks supply only very small quantities of sand-sized quartz grains into the sedimentary cycle.

Mineralogically immature clastic sedimentary rocks contain low percentages of non-undulatory quartz, similar to the potential source rocks examined (excluding extrusive igneous rocks). Mineralogically mature rocks, however, are characterized by high percentages of non-undulatory quartz; the average amount of total quartz which is of the non-undulatory type in the 20 orthoquartzites examined was 43.1 percent; values ranged from 14 to 80 percent. The undulatory grains appear to be destroyed selectively by mechanical and chemical agencies during successive sedimentary cycles; this might be expected on thermodynamic grounds, since strained (undulatory) grains have higher free energies than strain-free ones. Consequently, and abundance of non-undulatory quartz grains in a sedimentary rock robably indicates that the assemblage of grains has passed through several sedimentary cycles, rather than derivation from either plutonic igneous or metamorphic source rocks.

It appears likely that large quantities of polycrystalline quartz grains are derived from both plutonic igneous and metamorphic rocks. Large amounts of polycrystalline quartz grains in the total quartz of a sedimentary rock probably indicate that a significant proportion of the quartz grains have been derived from primary source rocks relatively late in the history of the assemblage of quartz grains. Detrital polycrystalline quartz grains are less stable than either the undulatory or non-undulatory varieties, and are extremely rare in mineralogically mature sandstones.

It is concluded that the presence or absence of undulatory extinction and polycrystallinity in clastic quartz grains is of very limited usefulness in determining the provenance of sediments.