Zircon growth in UHT leucosome: constraints from zircon-garnet rare earth elements (REE) relations in Napier Complex, East Antarctica
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Accession number;04A0874347
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| Title;Zircon growth in UHT leucosome: constraints from zircon-garnet rare earth elements (REE) relations in Napier Complex, East Antarctica |
| Author;
HOKADA T
(National Inst. Polar Res., Tokyo, Jpn)
HARLEY S L
(Univ. Edinburgh, Edinburgh, Gbr)
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Journal Title;J Mineral Petrol Sci
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Journal Code:G0150B
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ISSN:1345-6296
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VOL.99;NO.4;PAGE.180-190(2004)
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| Figure&Table&Reference;FIG.6, TBL.5, REF.18 |
| Pub. Country;Japan |
| Language;English |
| Abstract;Feldspathic leucosomes occur in an ultrahigh-temperature (UHT) metamorphosed garnet-bearing paragneiss/quartzite in the Napier Complex, East Antarctica. Coarse (200-400.MU.m) zircon grains occurring in the leucosomes display 3 textural domains: (I) dark-CL (cathodoluminescence) structured inner-core, (II) bright-CL structured outer-core, and (III) dark-CL structureless rim. Chemical compositions, especially chondrite-normalized REE patterns obtained by SIMS analysis, correlate with these three domains: the inner-core (domain-I) shows HREE-enrichment with Yb(n)/Gd(n)=3.3, whereas the outer-core (II) and rim (III) have flat to relatively depleted HREE patterns with Yb(n)/Gd(n)=0.7-0.8. Th/U ratios decrease from 3.2 in the zircon inner-core (I) to 1.2 in outer-core (II) and to 0.3 in the rim (III). Garnets near such zircon grains display two trace element compositional features. Firstly, high core Zr contents (300ppm) decrease to 100ppm within 50-100.MU.m of grain rims. Secondly, the HREE distribution between zircon inner-core (I) and garnet core is 2 at Gd (GdDZrn/Grt=2), rising to 8 at Lu (LuDZrn/Grt=8), whereas those defined from zircon outer-core (II) or rim (III) and garnet core or rim are much lower and generally below I for Gd through to Lu (GdDZrn/Grt=0.8-1.2; LuDZrn/Grt=0.6-0.7). This marked change in the HREE distribution between zircon and garnet must reflect either a change in the minerals with which the zircon was growing or being modified, a change in the physical and chemical conditions of zircon growth, or a combination of the two. Based on comparisons with recent estimates of equilibrium zircon/garnet HREE distribution coefficients we infer that the inner-core (I) did not grow with the garnet that occurs in the paragneiss but grew within a garnet-absent melt that was then injected into the gneiss.... (author abst.) |
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