Continuous zircon growth during long-lived granulite facies metamorphism: a microtextural, U–Pb, Lu–Hf and trace element study of Caledonian rocks from the Arctic

• Published in: Contributions to mineralogy and petrology Vol. 168; no. 4; p. 1
• Main Authors: O’Brien, Tim M., Miller, Elizabeth L.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-10-2014; Springer; Springer Nature B.V
• Subjects: Cambrian; Crystallization; Earth and Environmental Science; Earth Sciences; Geology; Granite; Metamorphic rocks; Metamorphism; Metamorphism (Geology); Mineral Resources; Mineralogy; Ordovician; Original Paper; Petrology; Precambrian; Precipitation (Meteorology); Rocks; Rocks, Metamorphic; Silurian; Trace elements; Veins (geology); Zirconium; Canada; Arctic region; Chukchi Borderland; Caledonian orogeny; Lu–Hf; Rare earth elements; Zircon microtextures; U–Pb
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-014-1071-x

Microtextural, U–Pb, trace element and Lu–Hf analyses of zircons from gneisses dredged from the Chukchi Borderland indicate a long-lived, Cambrian–Ordovician, granulite facies metamorphism. These results reveal a complete prograde, peak and cooling history of zircon growth during anatexis. Early increasing temperatures caused modification and Pb-loss of Precambrian zircons by recrystallization and dissolution/re-precipitation of existing grains. Small variations in initial
176
Hf/
177
Hf results (0.282325–0.282042) and flat HREE patterns of these zircons indicate that they grew by dissolution/re-precipitation in the presence of garnet. Zircons subsequently crystallized from a partial melt during peak to post-peak metamorphism from 530 to 485 Ma. A broad range of initial
176
Hf/
177
Hf ratios (0.282693–0.282050) and mineral inclusions within zircons suggest that this phase of growth incorporated Zr and Hf obtained from the breakdown of Zr-enriched phases. Microtextural evidence along with trace element and isotopic data suggests that final growth of metamorphic rims on zircon occurred during slow cooling and crystallization of residual partial melts during the early Ordovician (485–470 Ma). Younger, late Ordovician–Silurian (420–450 Ma) euhedral, oscillatory-zoned, trace element-enriched zircons crystallized within leucocratic veins that intrude the gneisses. Their age corresponds to granitoids dated from this same dredge. The intrusives and veins provide evidence that the Chukchi Borderland rifted from a position near Pearya and northwest Svalbard, which represent the northern continuation of the Caledonian orogen. Evidence for earlier Cambrian metamorphism has not been reported from this region. The age of granulite facies metamorphism reported here represents the earliest phase of deformation in the Arctic Caledonides.