Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon

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doi: 10.1046/j.1525-1314.2000.00266.x
Authors:Hoskin, P. W. O.; Black, L. P.
Author Affiliations:Primary:
Australian National University, Research School of Earth Sciences, Canberra, Australia
Volume Title:Journal of Metamorphic Geology
Source:Journal of Metamorphic Geology, 18(4), p.423-439. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929
Publication Date:2000
Note:In English. 51 refs.; illus., incl. 3 tables, sketch map
Summary:Protolith zircon in high-grade meta-granitic rocks from Queensland, Australia, were partially recrystallized during granulite-grade metamorphism. Integrated CL, U-Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses were used to describe the zircon in detail. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts that have conformable boundaries with transgressive recrystallized areas, but contrasting CL and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed-system solid-state recrystallization. Recrystallized zircons have flatter LREE patterns, a reduced Ce anomaly and concave HREE patterns. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a 'memory' of the protolith trace element and isotopic composition. This results in the measurement of 'mixed' U-Pb isotope ages. Nonetheless, the age of metamorphism has been determined at 1564 ± 3 m.y. A correlation between apparent age and Th/U ratio is indicative of incomplete re-setting by partial recrystallization. Recrystallization is shown not to affect Lu-Hf ages significantly. Recrystallization has been determined by textural and trace element analysis and EBSP data to have proceeded not by sub-grain rotation or local dissolution/re-precipitation, but by grain-boundary migration and defect diffusion. The formation of metamorphic zircon by solid-state recrystallization is probably common to high-grade terrains worldwide. The recognition of this process of formation is essential for correct interpretation of zircon-derived U-Pb ages and subsequent tectonic models. [P.J.T.]
Sections:Age determination
Subjects:Absolute age; Chemical composition; Dates; Facies; Granulite facies; High-grade metamorphism; Ion probe data; Mass spectra; Metaigneous rocks; Metamorphic rocks; Metamorphism; Nesosilicates; Orthosilicates; Paleoproterozoic; Precambrian; Proterozoic; Recrystallization; Silicates; Spectra; Trace elements; U/Pb; Upper Precambrian; Zircon; Zircon group; Australasia; Australia; Mount Isa Inlier; Queensland Australia; Coen Australia; Georgetown Australia; Yambo Australia
Coordinates:S290000 S100000 E1530000 E1380000
Abstract Numbers:01M/45
Record ID:2000049415
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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