Gneiss age dating
We will need to integrate metamorphic geology - which charts the evolution of pressure and temperature of the once deeply buried rocks - with regional geology and, specifically, information on timing.
In addition to the diagrams, photographs and tables of data provided, there are rock samples that you will need (check the module discussion room to locate these).
Consequently isotopic ages from metamorphic rocks will at best only date a specific event in the rock's history (or at worst will be some meaningless amalgamation of events with some mixture of ages).
Notwithstanding these general problems, the following time constraints are the best currently available for the WGR. Contraction, extension and timing in the south Norwegian Caledonides: the Sognefjord transect.
The field data and photographs presented here were collected by Rob Butler on a short visit to the area.
for the suite as the best estimate for the time of crystallizationmore » of the Oliverian gneisses.
Possibly, the Whitefield, Gneiss in the Jefferson dome represents a 10 to 15 m.y.
Available radiometric and fossil evidence suggests that the Ammonoosuc Volcanics have a Middle Ordovician age but are somewhat older than the Oliverian gneisses.
New U-Pb zircon data from Oliverian gneisses of six domes plot on a concordia diagram as an almost colinear array that yields an upper intercept age of about 444 m.y.
1700 Ma, while others have no zircon grains younger than ca. Structurally above the Archean gneiss is a heterogeneous paragneiss that contains calc-silicate and quartzitic rocks with detrital zircons as young as ca. Amphibolite in this unit contains zircons dated at ca.