Date of Degree
MS (Master of Science)
Jane A. Gilotti
Carboniferous sedimentary units unconformably overlie eclogite-facies Paleoproterozoic basement gneisses in Holm Land, North-East Greenland and record change in provenance as a function of tectonic disturbances that happened during mid-Carboniferous time. The Visean Sortebakker Formation contains arkosic sandstone with abundant gneissic lithic grains and a heavy mineral suite dominated by titanite, apatite and rutile but lacking garnet. ICPMS U-Pb ages on detrital zircons define dominant age groups at 1.75 and 1.98 Ga, broad peaks at 1.1-1.5, 2.2-3.0 Ga, and a few Caledonian metamorphic ages between 350 and 400 Ma. In contrast, Moscovian arkosic sandstone of the unconformably overlying Kap Jungersen Formation contains a heavy mineral suite of titanite, apatite, rutile, garnet, and epidote and gives ICPMS U-Pb detrital zircon ages of 390 Ma, 1.75 Ga and 1.98 Ga. CL images demonstrate that low-U rims on some zircons record Caledonian metamorphism.
The dominant peaks in all samples are most compatible with derivation from the underlying gneissic basement of the North-East Greenland eclogite province. The 1.75 and 1.98 Ga peak ages match the protolith age of the Paleoproterozoic calc-alkaline arc related basement. The CL-bright, low-U metamorphic rims yield ages of 335 to 410 Ma that are consistent with metamorphic ages observed in the Paleoproterozoic basement rocks. The large number of discordant grains observed in the detrital populations (23-47%) in all samples is similar to age spectra from basement samples as well. Grains that define broad peaks at 1.1-1.5, 2.2-3.0 Ga in the Sortebakker samples are interpreted to be from metasedimentary units in the structurally higher Caledonian nappes (e.g., Independence Fjord Group). These grains make up 30-35% of the population in the basal units, diminishing to 10% up section. The Caledonian metamorphic grains, appearance of garnet and lack of the 1.1-1.5 and 2.2-3.0 Ga signatures indicate that the basal Kap Jungersen Formation lacked input from sources external to the underlying basement. Reappearance of the 1.1-1.5 and 2.2-3.0 Ga signatures higher in the section probably reflects recycling from the underlying Sortebakker section. Results of this study clearly demonstrate that dating of metamorphic rims of detrital zircons greatly enhances characterization of provenance and depositional history.
Copyright 2011 Thejashwini Ramarao