HR: 17:30h
AN: V54B-07    [Abstracts]
TI: UHP Metamorphism in the Over-riding Plate of a Collisional Orogen, NE Greenland Caledonides
AU: * McClelland, W C
EM: wmcclell@uidaho.edu
AF: Univ of Idaho, Geological Sciences, Moscow, ID 83844 United States
AU: Gilotti, J A
EM: jane-gilotti@uiowa.edu
AF: Univ of Iowa, Geoscience, Iowa City, IA 52242 United States
AB: Ultrahigh-pressure (UHP) metamorphic terranes are generally modelled as subducted slabs in collisional orogens. Residence times at UHP conditions are thought to be short and exhumation processes interpreted to be fast. UHP metamorphic processes in the over-riding plate of collisional orogens are less well known. Eclogite-bearing Laurentian continental basement exposed in NE Greenland represents the upper, over-riding plate in the Caledonian collision. U-Pb SHRIMP ages and trace-element SHRIMP data (collected at the USGS-Stanford Ion Probe Lab) from coesite-bearing zircon suites formed in the Greenland Caledonides indicate that zircon growth records a protracted history of UHP and subsequent retrograde metamorphism. Kyanite eclogite and quartzofeldspathic host gneiss samples from an island in Jokelbugt (78° 00 N, 18° 04 W) contained zircons with well defined domains in CL images. Components of the eclogite facies inclusion suite include coesite, kyanite, omphacite, garnet and rutile. Core domain trace-element signatures reflect modification of igneous protolith zircon. Rim signatures show flat HREE patterns characteristic of eclogite facies zircon. The ²⁰⁷Pb-corrected ²⁰⁶Pb/²³⁸U ages range from 330 to 390 Ma for host gneiss and 330 to 370 Ma for kyanite eclogite. Weighted mean ²⁰⁶Pb/²³⁸U ages for coesite-bearing domains vary from 364±8 Ma for the host gneiss to 350±4 Ma for kyanite eclogite. Metamorphic zircon formed in all samples by both new zircon growth and recrystallization. The span of 350 to 370 Ma ages from kyanite eclogite is inferred to record continuous growth and thus relatively long (>10 My) residence time at eclogite facies to UHP conditions. Rim domains with ages as young as 344±3 Ma record zircon growth during exhumation. The older age span observed for the host gneiss indicates that new zircon growth initiated at least 10 million years earlier in the host gneisses than in kyanite eclogites. Age variations observed in UHP terranes may, therefore, reflect differences in bulk composition rather than differences in timing of metamorphism. Long residence times indicated for the NE Greenland eclogites are interpreted to be characteristic of UHP metamorphism in the over-riding plate of collisional orogens.
DE: 1115 Radioisotope geochronology
DE: 3654 Ultra-high pressure metamorphism
DE: 4207 Arctic and Antarctic oceanography (9310, 9315)
DE: 8108 Continental tectonics: compressional
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005