The distribution of mineral dates in time and space

Abstract

Most igneous and metamorphic mineral dates indicate times of rock cooling, the terminal events of crustal adjustments. Accordingly, the distribution of mineral dates in time indicates the periodicity of such events, and the distribution in space indicates the geometry of their occurrence. By this we may test the validity of such concepts as "orogenic periodicity," "cyclic orogeny," and "continental accretion." A plot of mineral date abundance against age shows that crustal adjustments are periodic and roughly cyclic. Intervals for which abundant mineral dates have been preserved are about 175 to 250 million years in length, with cycles of about 350 to 500 million years. Intervals of date abundance fall in the ranges: -2710 to -2490 m.y., -2220 to -2060 m.y., -1860 to -1650 m.y., -1480 to -1300 m.y., -1100 to -930 m.y., -620 to -280 m.y., and -120 to the present. Mineralogenic events of very different ages are commonly recorded in the same area, not infrequently in the same rock. Large areas, however, can be characterized by the mineral dates of the last interval of mineralogenic activity by which they were seriously affected. In North America these mineralogenic provinces show a crudely concentric pattern, younger dates outward. Similar arrangements have not been found in other continents.