Abstract
Concentrations of potassium and rubidium are reported for 62 Precambrian shield whole-rock samples belonging to the pyroxene granulite facies of world-wide distribution. Compared with the Main Trend established by Shaw (1968) for igneous and quasi-igneous rocks of the upper crust, the high-grade metamorphic granulites exhibit a similar correlation but with a measurable depletion in Rb for a given K content. The Metamorphic Trend may be described by the equations: log 10 (ppm Rb) = 1.136 log 10 (per cent K) + 1.495 or ppm Rb = 31.28 (per cent K) 1.136. The square of the product moment correlation coefficient shows that 80 per cent of the variation in Rb is associated with variation in K. All suites studied have initial 87Sr 86Sr ratios less than 0.707. This low value argues against repeated extraction of Rb and K through time on a regional scale. Rather, the major depletion in these lithophile elements occurred in the early stages of crustal evolution.
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