Giant Zircon Crystals in Ultramafic Rocks Within the Lewisian Complex: Occurrence and Implications

• Main Authors: Faithfull, John, Dempster, Tim, Reilly, Monica
• Format: Conference Object
• Language: unknown
• Published: 2016
• Subjects: QE Geology; Greenland
• Online Access: http://eprints.gla.ac.uk/118578/

Giant zircons (mm to 1 cm+) occur in orthopyroxenite veins cutting a ca. 50m meta-peridotite body containing fresh olivine, hosted by typical Scourian garnet-plagioclase-2-pyroxene gneisses at Loch an Daimh Mor, near Badcall in the Assynt Lewisian terrane. One sample from here has been dated (GST15: 2470+-30Ma; Kinney and Friend, 1997) and also been the subject of additional detailed microchemical, isotopic and microstructural studies (Timms et al, 2006, Timms and Reddy, 2009), but no detailed or accurate account of the occurrence has been published. The whole area has experienced one or more granulite facies events, with one or more later amphibolite-facies episodes during the Mesoarchaean-Palaeoproterozoic. While the host gneisses mostly preserve granulite assemblages, the meta-peridotite and orthopyroxenite veins display some retrogression to serpentinite, and locally, carbonate-amphibole assemblages. The orthopyroxenite veins are abundant throughout the meta-peridotite, and vary in thickness from ca. 1cm, up to 30cm or more. The thicker veins are often zoned, with inward enrichment in phlogopite, and locally inner zones of clinopyroxene+-pargasite, and in one case, a central rib of sulphate-rich scapolite and K-feldspar. The zircon is mostly associated with the early-formed orthopyroxene and phlogopite, and may form up to 10% modally, on a hand-specimen scale. The abundance of orthopyroxene, and the sulphate-rich scapolite strongly support initial zircon crystallization during granulite-facies conditions. Although the zircon crystals in the sample GST15 studied by Timms and others (op. cit.) showed evidence of strong plastic deformation, and short-length U-Th-Pb diffusion, more detailed sampling and examination using BSE and CL reveals a very diverse range of zircon textures from the locality, with local excellent preservation of early sector-oscillatory zoned zircon, and a complex stratigraphy of later zircon-modifying and zircon-forming episodes. Because of the large crystal size, many of these features are on a scale much larger than the likely diffusion length of U and Pb in zircons, and larger than ion beam diameters, and hence this stratigraphy has great potential in investigating some of the possible isotopic resetting effects which have been suggested to effect the typical small 50 to 200 micron zircons in TTG and granite gneisses within the Scourie area. We suggest that these remarkable zircon-rich rocks do not necessarily imply the existence of extraordinary Zr-enriched parental melts. Rather they may have been formed by the reaction of “normal” high-grade melts of TTG gneisses, or possibly, metagabbros, which have migrated into olivine-rich host rocks. Desilication of these melts by reaction with olivine to produce opx reduces silica activity, and suppresses zircon nucleation, allowing Zr enrichment in the melt, and the formation of small numbers of large crystals, rather than the typical large numbers of small crystals seen in silica-saturated intermediate-granitic melts. If this hypothesis is correct, we might expect that assemblages rich in large zircons might be commonly developed where olivine-rich rocks meet high-grade silica-rich melts in the lower crust. We record two other giant-zircon occurrences within the Scottish Lewisian which are associated with high-grade ultramafics close to TTG gneisses, plus occurrences in the Alps and Greenland, and suggest that this process may indeed be widespread. Such occurrences may offer a valuable high-resolution geochronological targets in high-grade terranes with complex histories, and reaction of silicic melts with deep crustal olivine-bearing rocks may be a widespread process which may result in distinctive geochemical signatures.