Make a P–T-t diagram based on this information from the eclogite province of the Bergen Arcs (for references, see Bingen et al, 2004): (1) Granulite facies: almost 1 GPa and 800–850 °C at ~930 Ma (U–Pb ages of zircon). (2) Eclogite facies: 1.8–2.1 GPa and ~700 °C at 423 ± 4 Ma (U-Pb zircon rim crystallization age). (3) Amphibolite facies retrogressive shearing: 0.8–1.2 GPa and ~690 °C at 409 ± 8 Ma (Rb/Sr). Other ages: Dikes predating amphibolite facies shearing: 422 ± 6 Ma to 428 ± 6 Ma (Rb/Sr) and 418 ± 9 (U-Pb). Apatite fission-track ages around 180 Ma indicate time of cooling through 150 °C. to

Question

What will be an ideal response?

Answer 1

Answer: The data and path are shown in the figure. The old (930 Ma) granulite fabric is not directly related to the Caledonian ages of eclogite and amphibolite facies metamorphism and must therefore be considered separately (it may or may not be that the rocks were at or close to the surface between the granulite and eclogite facies tectonometamorphic events). A path can be drawn between the 423 ± 4 Ma eclogite-facies event, the 409 ± 8 Ma amphibolite facies event and the AFT date. The latter date indicates burial depths of around 6 km and thus low pressures. The path seems to represent the retrograde part of a clockwise P – T path, consistent with a subduction zone setting.