Jane Gilotti and Ph.D. student, Claudia Sartini-Rideout, discuss a vertical garnet lineation in eclogites near Danmarkshavn, Greenland Caledonides.

HP and UHP Metamorphism in the Greenland Caledonides

Most tectonic models for Caledonian collision in the North Atlantic region postulate that Laurentia (Greenland) was thrust over Baltica (Scandinavia) in order to produce the well-known high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism in western Norway. However, our 1990 discovery of similar HP eclogites in Greenland and our more recent discovery of UHP metamorphism require revision of these models. Taken together, these two coherent continental eclogite terranes comprise the most extensive exposure of a deep, ancient mountain root on the planet - giving us a view of mountain belt anatomy that is generally hidden at the base of active orogens like the Himalayas. The ultimate goal of this project is to understand how this thick or deep orogenic root formed and how it was brought back up to the earth's surface. Current research seeks to improve our reconnaissance level understanding of the Greenland eclogite province by documenting the extent, structural setting, age, metamorphic evolution, and geochemical signatures of HP and UHP metamorphism. Previous research on the Greenland eclogites has been funded by GEUS, NSF EAR-95089218, the National Geographic Society, and the CIFRE fund at The University of Iowa.

Radial fractures surround a polycrystalline, palisade quartz inclusion in garnet in a kyanite eclogite from North-East Greenland. The quartz is a pseudomorph after coesite and textural evidence for UHP metamorphism.

Current research in Greenland is underway thanks to a new NSF project entitled, "Formation and exhumation of UHP and HP terranes in the Greenland Caledonides - understanding processes at the base of continental collisions." This project is a collaborative effort with Prof. Bill McClelland at the University of Idaho. The goals are: 1) to identify the extent and timing of UHP metamorphism; 2) to test the robustness of our previous ages for HP metamorphism by comparing zircon populations from the retrograde margins of eclogite pods to zircon populations from the well-preserved centers; 3) to establish the pressure-temperature-time-deformation history of this slab of eclogite-bearing crust as it returns to the earth's surface; and 4) to determine the geometry and timing of contractional and strike-slip fabrics that might be possible exhumation structures.

Two-mica leucogranites contain huge rafts of brown-weathering metasedimentary rocks. Cliff on north wall of Nordenskjöld Glacier, Greenland Caledonides.

Crustal Melting and Extension

Migmatite complexes and leucogranites make up a large component of the middle crust now exposed in the southern half of the Greenland Caledonides. A popular view is that this melting is contemporaneous with extensional detachment faults that unroof the migmatite complexes. The leucogranites are 430-424 million years old, and they intrude Neoproterozoic to Paleozoic sedimentary sequences in the upper plate of the detachments (above the migmatite complexes). Our work has focused on the Payer Land detachment, which is unique in that it exhumes HP/HT rocks from 50-60 km depth. McClelland and I have recently dated zircons from the HP kyanite + K-feldspar + plagioclase + quartz ± garnet melts using the USGS/Stanford University Sensitive High-Resolution Ion Microprobe (SHRIMP) Lab and found that anatexsis occurred 405 million years ago. Extensional exhumation in Payer Land must be younger than 405 Ma and, therefore, unrelated to the leucogranites. These results raise the question of whether there is a genetic link between the leuocogranites and extension in East Greenland? - a question we plan to pursue.

Lobate symplectites of diopside + plagioclase exsolving from omphacite signal isothermal decompression of eclogites in North-East Greenland.

Microstructures
Observing microstructures in deformed and metamorphosed rocks is the starting point for understanding grain scale processes like plastic flow, solid state recrystallization, growth or resorption of minerals, melting, and fluid - rock interactions to name a few. My colleagues and I have used microstructures in our recent research to characterize the deformation mechanisms and kinematics of mylonite zones, recognize UHP metamorphism, study prograde corona textures in eclogites and decompression reactions frozen in symplectites. Microstructural observations continue to be an important aspect of our research. For example, M.S. student Max Ostby is currently describing the microstructures of mylonites along the Payer Land detachment, while Ph.D. candidate Claudia Sartini-Rideout is using corona textures in partially eclogitized leucogabbros to forward model prograde metamorphism in eclogites.

Microstructures can be varied as well as aesthetically pleasing. In my former life at the New York State Geological Survey, Phil Whitney and I curated an exhibit for the New York State Museum called Splendor in Stone that showcases spectacular images of rocks viewed through the microscope.

Recent Publications:

• Sartini-Rideout, C., Gilotti, J.A. and McClelland, W.C. in press. Geology and timing of dextral strike-slip shear zones in Danmarkshavn, North-East Greenland Caledonides.  Geological Magazine.
• McClelland, W.C., Power, S.E., Gilotti, J.A., Mazdab, F.K., and Wopenka, B., in press.  U-Pb SHRIMP geochronology and trace element geochemistry of coesite-bearing zircons, North-East Greenland Caledonides.  In:  Hacker, B., McClelland, W.C., and Liou, J.G. (eds.), Ultrahigh-Pressure Metamorphism: Deep Continental Subduction, Geological Society of America Special Paper.
• Gilotti, J.A. and McClelland, W.C. 2005. Leucogranites and the time of extension in the East Greenland Caledonides. Journal of Geology 113, 399-417.
• Gilotti, J.A., Nutman, A.P. and Brueckner, H.K. Devonian to Carboniferous collision in the Greenland Caledonides: U-Pb zircon and Sm-Nd ages of high-pressure and ultrahigh-pressure metamorphism.  Contributions to Mineralogy and Petrology  (in press).
• Higgins, A.K., Elvevold, S., Escher, J.C., Frederiksen, K.S., Gilotti, J.A., Henriksen, N., Jepsen, H.F., Jones, K.A., Kalsbeek, F., Kinny, P.D., Leslie, A.G., Smith, M.P., Thrane, K. and Watt, G.R. 2004. The foreland-propagating thrust architecture of the East Greenland Caledonides 72°-75°N. Journal of the Geological Society London. (in press).
• McClelland, W.C. and Gilotti, J.A. 2003. Late stage extensional exhumation of high-pressure granulites in the Greenland Caledonides. Geology 31, 259-262.
• Elvevold, S., Thrane, K. and Gilotti, J. A. 2003.Metamorphic history of high-pressure granulites in Payer Land, Greenland Caledonides.  Journal of Metamorphic Geology 21, 49-63.
• Gilotti, J.A. & Elvevold, S. 2002. Extensional exhumation of a high-pressure granulite terrane in Payer Land, North-East Greenland Caledonides: structural, petrologic and geochronologic evidence from metapelites. Canadian Journal of Earth Science 39, 1169-1187.
  
• Gilotti, J.A. & Ravna, E.J.K. 2002. First evidence for ultrahigh-pressure metamorphism in the North-East Greenland Caledonides. Geology 30, 551-554.
  
• Lang, H.M. & Gilotti, J.A. 2001. Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland Caledonides. Journal of Metamorphic Geology 19, 495-515.
  
• Elvevold, S. & Gilotti, J. A. 2000. Pressure-temperature evolution of retrogressed kyanite eclogites, Weinschenk Island, North-East Greenland Caledonides. Lithos 53, 127-147.
  

Current Graduate Students:

Claudia Sartini-Rideout Ph.D.
Emily Miller, M.S.