Effect of metamorphism and partial melting of host rocks on zircons

Abstract

Abstract. Zircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock-forming processes of migmatites.The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence ofcharacteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial melting.

Melt segregation and magma flow in migmatites: implications for the generation of granite magmas

“…This describes those migmatites that have leucosomes (former melt) with adjacent melanosomes (residuum). Published accounts of leucosomes formed under this condition indicate that the segregation process was particularly effective at separating melt from residuum; typically, the leucosomes contain very few crystals of the major residuum phases (Sawyer 1991;Whitney & Irving 1994), but the entrainment of small accessory phase crystals, such as zircon, is common (Gupta & Johannes 1985;Brouand et al 1990). Thus granite magmas formed where this condition prevailed might be recognised by their very leucocratic nature.…”

Structural and metamorphic controls on the distribution of zircon in an evolving quartzofeldspathic migmatite: an example from the Reynolds Range, central Australia

“…Zircon crystals in metamorphic rocks were initially considered to be largely inert and their morphology was thought to reflect the source rock (Poldervaart, 1955(Poldervaart, , 1956Verspyck, 1961). More recent work has made it clear that zircon is a chemically, petrologically and texturally active phase during high-grade metamorphism, and its morphology is largely shaped by metamorphic processes, especially if migmatization is involved (Kalsbeek & Zwart, 1967;Gupta, 1968;Pupin, 1980;Gupta & Johannes, 1985). Other work has shown that new zircon growth does not necessarily involve a melt, but may occur in hydrous shear zones (Sinha & Glover, 1978;Peterman et al, 1986;Wayne & Sinha, 1988), or even in diagenetic settings (Saxena, 1966).…”