Displacement of relict zircons during growth of feldspathic porphyroblasts
Relict zircon crystals are almost invariably present in feldspathic-porphyroblastic schist and gneiss. Rounded sedimentary zircons acquire overgrowths and begin to refacet in higher-grade metamorphic events; however, new euhedral zircons composed of reconstituted material from dissolved residual crystals are thought not to form within or below the amphibolite-granulite transitional facies. Lead-loss and other alteration effects do not destroy or remove traces of the original zircon grains. Zircon distribution within even-grained igneous rock is essentially random, and clastic sedimentary rocks have grains concentrated as heavy minerals in lenses or sheets related to bedding. Variations in these patterns of distribution are found in porphyroblastic schists and gneisses, and they suggest that relict zircons have been physically displaced outward during the growth of feldspathic porphyroblasts. Zircons in all rocks examined increase in concentration within the wrapped-around biotite-rich shells enclosing feldspathic augen and porphyroblasts. At the same time, data show that relict zircons have been systematically removed from the region of porphyroblast growth. Explanations suggesting compaction of matrix and (or) replacement coupled with ionic migration appear inadequate to fully describe the growth of these porphyroblasts and the creation of their wrapped-around shells. Physical displacement due to “force of crystallization” is suggested as a required mechanism during the formation of these porphyroblasts.