2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 24-6
Presentation Time: 9:20 AM


WARD, William P.1, MCCLELLAND, William C.1, COBLE, Matthew A.2 and MALONE, Shawn J.3, (1)Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA 52242, (2)Department of Geological Sciences, Stanford University, Stanford, CA 94305, (3)Department of Geosciences, Ball State University, Muncie, IN 47306, william-ward@uiowa.edu

New monazite and titanite U-Pb SHRIMP-RG ages help constrain timing of metamorphism in the Pearya terrane basement orthogneiss, northernmost Ellesmere Island. While the geochronology of the igneous components of the Pearya terrane is well understood, the timing of metamorphism is poorly constrained. Age constraints on the Pearya shear zone is limited to a single late Ordovician U-Pb age on a dike that cross-cuts foliation with sinistral kinematic indicators while the timing of amphibolite-facies deformation in the Clements-Markham Fold Belt is defined by a Silurian Ar-Ar hornblende age and K-Ar ages ranging from Ordovician to Carboniferous. Precambrian orthogneiss units near Mitchell Point and Cape Columbia have a penetrative foliations that is interpreted to be related to deformation in the Pearya shear zone. Monazite grains from both Mitchell Point and Cape Columbia samples have well developed apatite-allanite coronas that are consistent with primary monazite replacement during amphibolite-facies metamorphism. In-situ thin section analysis by SIMS yield monazite ages of 963 ± 24 (MSWD=2.54, N=5) Ma for the orthogneiss near Mitchell Point belt and 951 ± 16 Ma (MSWD=2.6, N=5) for a body in the Cape Columbia belt; both ages are consistent with zircon ages previously reported for these samples. In contrast, titanite grains form clusters parallel to the foliation defined by biotite altering to chlorite and yield U-Pb ages that are interpreted to record the timing of metamorphism. SIMS analyses on titanite grains in a sample from near the Petersen Bay fault yield an age of 395 ± 44 Ma (MSWD =1.9, N =19). Titanite in a sample from near Cape Columbia yield an age of 372 ± 21 Ma (MSWD =1.15, N=13). A Devonian age for metamorphism and deformation of the Pearya terrane basement precludes a simple basement-cover relationship between the amphibolite-facies Tonian and older basement and the low grade Neoproterozoic sedimentary sequence of the Pearya terrane.