Trace element evidence for plagioclase recycling in calc-alkaline magmas

Abstract

Trace element zoning in plutonic plagioclase feldspars from the Val Fredda Complex, southern Adamello Massif, Italy has been measured using secondary-ion mass spectrometry (SIMS). The studied samples range from hornblende-gabbro through diorite to granitoid with an overall range in plagioclase composition from An 96 to An 15. Individual plagioclases display extensive zonation in anorthite through a considerable variety of zoning textures. There is equally extensive zoning in trace elements, notably Sr and Ba. Broadly similar trace element zoning patterns and concentrations are displayed by all plagioclases irrespective of host rock composition. Using a relationship between X An and the distribution coefficients D Sr and D Ba trace element concentrations in plagioclase are converted to those of the melt which precipitated each crystal zone. The calculated variations in Ba and Sr are greater than can be attributed to changes in pressure or temperature alone, indicating that compositional variation in the coexisting melt was the principal cause of zoning. The Ba Sr melt trend for each crystal describes a two-stage crystallisation history. The calcic cores of all crystals show melt trends of increasing Sr and Ba, which are attributed to mixing and fractionation in parental hydrous picrobasalt magmas. In contrast the melt trends recorded by the rims vary from sample to sample, but essentially show decreasing Sr and Ba consistent with fractionation of sodic plagioclase, alkali feldspar and/or biotite. The plagioclase cores are thought to have crystallised in a common magma chamber at 6–10 kbar pressure, while the rims crystallised at the emplacement level (1.5 kbar) from interstitial melt whose composition is determined by that of the host magma. The data suggest that early-formed calcic plagioclase was “recycled” by the magma during fractionation. Vigorous convection is invoked to both retain relatively dense calcic plagioclase within the evolving magma and to periodically re-entrain cumulus crystals into more evolved magmas. The occurrence of calcic plagioclase cores in felsic derivative magmas indicates that fractionation from picrobasalt to tonalite occurred on a relatively short time scale.