Comparison of metamorphic zircons from granulite xenoliths and granulite terrain in northern North China Craton
Abstract
In this research, we conduct a combined study of petrography, geochronology, trace element and Hf isotope compositions on zircons from granulite terrain and granulite xenoliths in the Zhangjiakou region, northern North China Craton.Most zircons from both types of granulite have core-rim structure in their CL images. where the cores commonly show oscillatory growth zoning but the rims are structureless. All the magmatic zircon cores yield essentially identical 2.5 Ga ages which reflect protolith ages, whereas metamorphic zircon rims yield ages of 1.85 Ga and 120-140 Ma, respectively. Most the 1.85 Ga metamorphic zircons which from granulite terrain have Th/U ratios lower than 0.4 with a range of 0.01 to 0.63. However, the range of Th/U ratios of Mesozoic (120-140 Ma) metamorphic zircons (0.12-3.48) overlaps that of the magmatic zircons (0.12-2.44). The Mesozoic and 1.85 Ga metamorphic zircons have REE patterns consistent with magmatic zircons, except that the 1.85 Ga ones show less enrichments of HREE and much less negative Eu anomalies than those of magmatic zircons. Therefore, Th/U ratios and the REE patterns cannot be used to distinguish metamorphic zircons from magmatic ones. Meanwhile, the 1.85 Ga metamorphic zircons have lower formation temperatures (729°), oxygen fugacity (log fO2 with average of -20.9) and 176Lu/177Hf ratios (lower than 0.0002) than the Mesozoic ones (formation temperatures of 750°, log fO2 with average of -9, 176Lu/177Hf ratios larger than 0.0002). Considering all these features, we speculate that the 1.85 Ga metamorphic zircons represent newly grown zircons coexisting with garnets and melt/fluid while the Mesozoic metamorphic zircons were formed quickly without the presence of garnet. The Mesozoic metamorphic zircons vary in their origins; some of these zircons were metamorphic newly grown while the others experienced recrystallization from a U-depleted melt/fluid. Our results suggest that P-T conditions, the melt/fluid and oxygen fugacity play important roles in controlling the trace element distribution and Hf isotope compositions of metamorphic zircons in the granulites.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V51E0407H
- Keywords:
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- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8130 Heat generation and transport;
- TECTONOPHYSICS