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Evidence of metamorphism at ultrahigh‐pressure (UHP) conditions is documented by the presence of coesite, diamond and/or majoritic garnet. However, the growth of UHP‐stable phases such as majoritic garnet is often volumetrically low, and overprinting during exhumation can obscure evidence of UHP growth, making it difficult to positively identify UHP rocks. In this study, we selected garnet‐kyanite schists from three microdiamond‐bearing localities within the Rhodope Metamorphic Complex, located in eastern Greece. Samples from Xanthi, Sidironero, and Kimi have similar bulk rock compositions, but the pressure–temperature (P–T) paths differ. Because the major phases record vanishingly little evidence of metamorphism at UHP conditions, we analyzed zircon grains with complex textures to evaluate if zircon preserves a record of UHP metamorphism. Zircon grains from all localities have cores and rims separated by a characteristic interface domain, as revealed by cathodoluminescence (CL) imaging. The detrital igneous cores range in age from c. 2.5 Ga to 220 Ma and exhibit a negative Eu* anomaly, a Yb/Gd of 10–100, and variable Th/U (0–1.2). Rims yield dates of 150–125 Ma with Yb/Gd of 0.1–10 and Th/U of 0–0.2. Interface domains yield dates 165–145 Ma with Yb/Gd ranging between 0–1000 and Th/U < 0.2. We interpret the distinctive CL textures and Yb/Gd of the interface domains as evidence of zircon that reacted at UHP. The interface domain in zircon from all petrographic contexts yields variable Yb/Gd ratios that are significantly higher than both cores and rims. We therefore interpret that zircon recrystallized via interface‐coupled dissolution–reprecipitation reaction; this process preferentially partitioned heavy rare earth elements within the interface domain, which explains the higher Yb/Gd ratios. The rim domains equilibrated with the matrix, producing a relatively homogeneous and low Yb/Gd ratio in these domains. The spatial extent and degree of preservation of interface domains are interpreted as a function of the P–T path and minor variations in bulk composition. Interface domains are best preserved in rocks from Xanthi and Sidironero; in these samples, thin, homogeneous, garnet‐stable rims only partially overprint and crosscut the interface domain. In contrast, rocks from Kimi followed a higher‐temperature trajectory and the zircon grains grew large rim domains that overprinted much of the interface domain and the detrital core. Zircon grains from plagioclase‐rich versus quartz‐rich domains within samples from Sidironero show differences in texture, which indicates that local bulk composition can affect what evidence of UHP metamorphism is preserved. Collectively, these samples provide a new, durable marker of metamorphism in UHP rocks and yield new insight about which factors affect the preservation of UHP textures.
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Kyanite preserves prograde and retrograde metamorphic events as revealed by cathodoluminescence, geochemistry, and crystallographic orientation
Abstract Cathodoluminescence (CL) images of kyanite reveal several internal textures, including sector zoning, oscillatory zoning, and cross‐cutting relationships among different domains. Many textures observed in kyanite correspond to discrete events, thereby connecting kyanite textures to the pressure–temperature (P–T) history of the rock. To evaluate the record of metamorphism preserved by kyanite, metapelites were selected from three different orogens that reflectP–Tconditions ranging from amphibolite to ultrahigh‐pressure (UHP) facies. Cross‐correlation of variations in CL intensity, chemistry, and crystal orientation within kyanite indicate the following findings. First, the preservation of original growth zones in kyanite from poly‐metamorphic rocks demonstrates that growth zoning in kyanite persists through metamorphic events and is not erased by diffusion or complete recrystallization. In some samples, kyanite retains evidence of its reaction history during growth. Second, measured changes in absolute crystallographic orientation do not correspond with changes in CL intensity in any of the measured samples, including kyanite twins. Third, both kink banding and undulatory extinction are present across all samples, consistent with rotation about <010> in the (100)[001] slip system. Kyanite from (U)HP samples exhibits higher amplitude undulations than kyanite from lower‐grade lithologies, suggesting that crystallographic orientation data may provide complementary insight about deformation along theP–Tpath. Fourth, specific CL and trace element signatures in kyanite can be correlated with discrete metamorphic histories; yet, CL intensity and colour are affected by multiple elements, not a single controlling element. In sum, multiple generations of kyanite can be identified by careful cross‐correlation of CL and geochemical data, and when combined with crystal orientation data, kyanite provides a robust record of a rock'sP–Tevolution.
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- PAR ID:
- 10449558
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Metamorphic Geology
- Volume:
- 39
- Issue:
- 7
- ISSN:
- 0263-4929
- Page Range / eLocation ID:
- p. 843-866
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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