U-PB EVIDENCE FOR MIDDLE DEVONIAN METAMORPHISM OF THE PETERSEN BAY ASSEMBLAGE ADJACENT TO THE PEARYA TERRANE, ELLESMERE ISLAND

The Pearya terrane occupies northernmost Ellesmere Island, and is separated from the Laurentian margin by (meta)sedimentary and volcanic rocks of the Hazen and Clements Markham fold belts. The Petersen Bay fault zone, on the peninsula between Yelverton Inlet and Kulutingwak Fiord, is the boundary between the crystalline rocks of the Pearya terrane and the Clements Markham fold belt. The Petersen Bay assemblage crops out south of the fault and represents a subduction-related complex comprised of garnet-bearing metasedimentary rocks, amphibolite, serpentinite, phyllite and marble. The metamorphic grade varies from amphibolite facies close to the fault to sub-greenschist facies across a gradient extending 10 km to the south into the Clements Markham Fold Belt (Imina Formation).

Two samples of schist containing the highest-grade assemblages of garnet-staurolite-kyanite-biotite-muscovite-amphibole-plagioclase-quartz-amphibole (sample 17-64) and garnet-kyanite-biotite-muscovite-plagioclase-quartz (17-66) were chosen for in-situ monazite U-Pb dating by sensitive high resolution ion microprobe. Previous workers estimated the peak metamorphic conditions at 600°C and 600 MPa. The monazite shows chemical zonation with distinct core/rim textures. Monazite-I has high-Th and moderate Y content and occurs within garnet rims or forms monazite cores. Monazite-II is characterized by lower Th and moderate Y content and forms rims on monazite-I or occurs as separate grains in the matrix. The textural observations and the presence of allanite suggests progressive monazite growth at the expense of allanite. Monazite-I from 17-66 define a weighted mean ²⁰⁶Pb/²³⁸U age of 396 ± 2 Ma (MSWD=1.6), whereas monazite-II gives an age of 384 ± 2 Ma (MSWD=1.16). Monazite from 17-64 compositionally similar to monazite-II define a weighted mean ²⁰⁶Pb/²³⁸U age of 393 ± 3 (MSWD=1.5).

The monazite results are interpreted as dating metamorphism of the Petersen Bay assemblage; the age is significantly younger than a previously reported hornblende Ar plateau age of 433 ± 3 Ma for the schist and older than muscovite Ar plateau age of 322 ± 2 Ma obtained for pegmatite within the unit. The Middle Devonian monazite age is interpreted to record juxtaposition of the crystalline basement of Pearya terrane and the Clements Markham Fold Belt.