2 CALC-ALKALINE VOLCANIC TRENDS IN THE ARCHEAN - TRACE-ELEMENT EVIDENCE

Two differentiation trends of intermediate to felsic volcanics, both of calc-alkaline major-element chemistry, exist in the Archean 'greenstone belts' of the world. New data from Hackett River, N.W.T. indicates that an Archean undepleted trend with trace-element contents similar to modern arc and/or continental calc-alkaline volcanics, exhibits a continuous variation from 55% to over 70% silica; a continuum of melting and crystallization processes similar to modern environments is inferred. A distinct Archean depleted trend, with large depletion in heavy rare earths and yttrium, and higher CaO and Al2O3 than the undepleted trend, has representatives from 62% to over 70% silica. The two trends can be separated by the use of Ce vs. Yb and La/Sm vs. Sm/Yb diagrams. The depleted-trend volcanics and related intrusive bodies appear to lack modern analogues; they represent equilibration with an eclogite residuum. The limited information available allows the possibility that the trend marks the ascent of little-modified melts from a descending oceanic slab. The interleaving of both trends at Hackett River and elsewhere indicates that two separate sequences of magmatic processes were contemporaneously and cospatially active during volcanism. The importance of garnet residua in Archean magmas suggests that geothermal gradients were not vastly higher than at present, in agreement with metamorphic evidence. © 1979 Springer-Verlag.