Lecture, Monday June 20, High-pressure metamorphism in The Nordre Strømfjord Shear Zone, Central West Greenland

06-06-2011

Open lecture by dr. Bill Glassley, John Korstgård and Kai Sørensen

Monday 20 June from 9:00 to 10:00 a.m.
The lecture will take place in the "Tove Birkelund" meeting room at GEUS - will be open for everyone interested. Entrance from staircase N from 9:45 a.m. (manned from 8:45 to 9:00).

Dr. Bill Glassley from University of California works together with John Korstgård and Kai Sørensen in the shear-zone of Nordre Strømfjord. Together they will do a talk on a new important aspect of their work in Nordre Strømfjord, namely the finding of a high pressure paragenesis in a ultramafic lens in the Ataneq fjord.

Abstract:
In the eastern reach of Ataneq fjord there occurs a meter-scale tectonic unit that records evidence of high-pressure metamorphism. The unit is a complex assemblage of lithologies, with compositions ranging from ultramafic to calcic. Field and petrographic evidence suggests the original unit consisted of an ultramafic body in contact with a calcium-rich rock unit. The strong compositional contrast between these units lead to the development of a 2 m thick metasomatic reaction zone with multiple cm-scale compositional bands. Metamorphism of the reaction zone resulted in growth of variety of mineral assemblages, including a narrow, 10-cm thick band of garnet-olivine-orthopyroxene rock. Geothermobarometric analysis of this and associated rocks requires that this rock experienced minimum P-T of 18 kb and 750 C. The compositional characteristics of these minerals exactly match that of similar rocks in the Alpine system that were metamorphosed at depths exceeding 75 km. We interpret this and other exotic lithologies that occur along a belt at the northern margin of the Nordre Strømfjord Shear Zone as remnants of a subduction channel deformed and metamorphosed during the collisional Nagsuggtoqidian Orogeny. The rarity of preserved high-pressure metamorphic rocks in this belt is due to the relatively slow uplift on this segment of the collision zone, as demonstrated by Ar-Ar dating.

For mere information om foredraget kontakt Bo Møller Stensgaard (bmst@geus.dk) eller Jochen Kolb (jkol@geus.dk)

Vertical gradient of the total magnetic intensity field Aeromag from the coast to the Inland ice in central West Greenland. Covering the entire strike of the ice-free parts of the Nordre Strømfjord Shear Zone. Enlarge figure

Landsat satellite image mosaic between Ataneq and the Inland Ice in central West Greenland. The shear zone is seen as a distinct E-W trending feature (the dashed area bounded by solid white lines) which in segmented by a number of WNW-ESEE and ENEWSW striking faults clearly expressed in the topography. Across this distance the shear zone is also seen to increase in width from app. 5 kilometres in the east to 12 kilometres in the west. Enlarge figure