Zircon as a metamorphic timekeeper: A case study from the Caledonides of central Norway

Abstract

During metamorphism, inherited or detrital zircon grains commonly serve as a template onto which new generations of metamorphic zircon precipitates, charting successive episodes in the thermal history of rocks. In addition, chemical modification of pre-metamorphic zircon can leave an imprint in the U–Pb systematics. We present a case study from a sample of migmatitic amphibole–biotite gneiss from central Norway that contains zircon grains that chart much of the history of an entire orogenic episode, not only through generation of new zircon, but also resetting of the U–Pb system via radiogenic-Pb loss. The investigated gneiss, from the Helgeland Nappe Complex, within the Uppermost Allochthon of the Scandinavian Caledonides records a prolonged metamorphic evolution. Phase equilibria modelling implies peak metamorphic conditions of 7.5–8.0 kbar and 650–675 °C, consistent with low-degree partial melting and the presence of small volumes of leucosome. Laser ablation split stream analysis that simultaneously measures U–Pb, REE and Lu–Hf in the same volume of zircon, yields a range of analyses that spread along concordia. A zircon population between 485 and 443 Ma is characterized by faded and blurred oscillatory-zoned domains that have gained nonformula elements. These analyses are interpreted to indicate the presence of reactive solutions during this time period that led to leaching and mobility of Pb. Another texturally distinct zircon component yields an age of 478 ± 4 Ma, has typical igneous REE zircon profiles, and records low apparent alpha doses, interpreted to reflect new zircon growth during migmitization. A further population of zircon rims and discrete grains yields an age of 453 ± 2 Ma, interpreted as a later episode of zircon growth during high-grade metamorphism. A final set of zircon overgrowths yield an age of 439 ± 3 Ma, also interpreted to reflect new metamorphic growth. Many zircon growth phases in this rock have similar REE patterns, implying broadly isochemical metamorphism, consistent with the evidence for only limited partial melting. Furthermore, the Hf isotopic signature in the analyzed zircon grains suggests that there was no release of radiogenic Hf during metamorphism. The metamorphic zircon in the studied rock records several episodes of growth and/or alteration over a period of >40 Ma that was related to protracted destruction of the Iapetus Ocean. In the context of regional data, the new results suggest that the Helgeland Nappe Complex, which is generally considered to be exotic to Baltica, underwent geographically widespread, long-lived Ordovician high-temperature metamorphism. This record of metamorphism is distinct from that recorded by tectonostratigraphically lower units, which are conventionally regarded to have been derived from Baltica. The results shed light on the assembly of the Scandinavian Caledonides and show that zircon from individual samples may be used to unravel complex orogenic histories.