Exhumation of an Ultrahigh-Pressure Slice From the Upper Plate of the Caledonian Orogen—A Record From Titanite in North-East Greenland

Abstract

Ultrahigh-pressure (UHP) rocks in North-East Greenland lie within a larger region of high-pressure Laurentian crust formed in the overthickened upper plate of the collision with Baltica. Coesite-bearing zircon dates UHP metamorphism to 365–350 Ma, which formed at the end of the Caledonian collision as a result of intracontinental subduction facilitated by strike-slip faults that broke the lithosphere. Rutile is the stable Ti-bearing phase at UHP, while titanite forms on the retrograde path. Trace elements and U-Pb in titanite were analyzed for six UHP gneisses. Zr-in-titanite temperatures range from 764 to 803°C and lie on the isobaric part of the pressure-temperature path at 1.2 GPa, which fits Ti-phase stability determined by thermodynamic modeling. Large (>600 μm), zoned titanite preserves three distinct trace element patterns that are due to metamorphism, melting and garnet breakdown. Weighted mean ²⁰⁶Pb/²³⁸U ages range from 347 ± 5 Ma to 320 ± 11 Ma, but age variation as a function of trace element domain for individual samples is not resolvable within uncertainty. Titanite records a prolonged period of exhumation that is also seen in the zircon record, where phengite decompression melting started at ca. 347 Ma, leucosome emplacement accompanied retrograde metamorphism from 350 to 330 Ma; and titanite grew during isobaric cooling from 345 to 320 Ma when the UHP rocks stalled at lower crustal levels. The same transforms that originally break the lithosphere play a significant role in channeling the UHP rocks back to the lower crust via buoyancy driven exhumation, after which time titanite formed.