Chemical changes during dyke metamorphism in high-grade basement terrains

Abstract

Trace element geochemistry is an important tool in deciphering the petrogenesis of igneous, and particularly basaltic, igneous rocks, and in assigning a possible tectonic setting to these rocks1,2. Studies of the trace element geochemistry of metamorphosed igneous rocks often implicitly assume element immobility during metamorphism even though recent studies3–7 have demonstrated the sometimes quite extreme mobility of certain trace elements, and especially of the petrogenetically important rare-earth elements (REE), during low-grade (zeolite–greenschist facies) metamorphism. The mobility of trace elements at higher grades of metamorphism is poorly known, although there is some evidence8 for the mobility of large ion lithophile (LIL—Rb, K, Th, Ba, Sr) elements, but not the REE, during amphibolite fades metamorphism. The REE also seem to be stable through the amphibolite to granulite facies transition9,10. We have studied a suite of ultramafic to mafic dykes (the Scourie dykes) cutting regrogressed Lewisian granulites in the Assynt region, north-west Scotland11 and conclude that the mobility of trace elements is not controlled solely by the degree or grade of metamorphism, but more by the availability of a fluid phase for element transfer.