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STEPWISE METAMORPHISM AND ACCRETION OF UHP ROCKS IN THE NORTHERN CANADIAN CORDILLERA AS INFORMED BY Lu--Hf GARNET GEOCHRONOLOGY AND PHASE EQUILIBRIUM MODELING.

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Title:	STEPWISE METAMORPHISM AND ACCRETION OF UHP ROCKS IN THE NORTHERN CANADIAN CORDILLERA AS INFORMED BY Lu--Hf GARNET GEOCHRONOLOGY AND PHASE EQUILIBRIUM MODELING.
Authors:	Tollefson, Kyle T.¹ kyle.tolleson@ubc.ca, Larson, Kyle P.¹, Dyck, Brendan¹, Cleven, Nathan R.², Vervoort, Jeff D.³
Source:	Geoscience Canada. 2025, Vol. 52 Issue 2, p248-248. 14p.
Subject Terms:	Eclogite, Metamorphism (Geology), Phase equilibrium, Geological time scales, Metamorphic rocks, Neotectonics, *Accretion (Astrophysics)
Abstract:	The St. Cyr klippe (SCK) is one of four eclogite localities in the northern Canadian Cordillera that record evidence of Permian subduction. Limited data from these localities has led to ambiguity in interpreting their tectonic evolution. It has been proposed that the SCK eclogite is hosted within quartzofeldspathic rocks is a single tectonic slice. This interpretation, however, is based on U--Pb zircon geochronology, which may not date metamorphism. Moreover, the pressure (P) and temperature (T) data that were used to infer a single event are derived from spatially disparate eclogite and quartzofelsdspathic rocks. To expand upon the existing but limited data, we carried out multi-fraction garnet Lu--Hf geochronology of eclogites from multiple locations. In addition, we undertook phase equilibria modelling of two eclogites from different structural levels, one with its quartzofeldspathic host rock. P-T paths for the two eclogites have similar peak conditions of 3.4 GPa at 600°C. Si-in-phengite geobarometry on the host rock for the structurally higher eclogite indicates eclogitic peak pressure of -3.0--3.2 GPa prior to final matrix equilibrium at amphibolite conditions of 0.9 GPa at 725°C. The structurally higher eclogite specimen yields a isotope dilution Lu--Hf date of 266 ± 1 Ma (MSWD = 1.7) whereas, the structurally lowest specimen returned an isochron date of 250 ± 1 Ma (MSWD = 2.1). The similarity in peak pressure for the eclogite and host rock indicate the eclogite and host rock were juxtaposed prior to peak-pressure metamorphism and subsequently exhumed as a coherent tectonic slice. The distinct garnet dates at different structural levels are consistent with stepwise metamorphism and accretion of eclogite, similar to the processes observed in metamorphic soles. Such stepwise metamorphism has yet to be accounted for in Permian tectonic models of the northern Canadian Cordillera. [ABSTRACT FROM AUTHOR]
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Abstract:	The St. Cyr klippe (SCK) is one of four eclogite localities in the northern Canadian Cordillera that record evidence of Permian subduction. Limited data from these localities has led to ambiguity in interpreting their tectonic evolution. It has been proposed that the SCK eclogite is hosted within quartzofeldspathic rocks is a single tectonic slice. This interpretation, however, is based on U--Pb zircon geochronology, which may not date metamorphism. Moreover, the pressure (P) and temperature (T) data that were used to infer a single event are derived from spatially disparate eclogite and quartzofelsdspathic rocks. To expand upon the existing but limited data, we carried out multi-fraction garnet Lu--Hf geochronology of eclogites from multiple locations. In addition, we undertook phase equilibria modelling of two eclogites from different structural levels, one with its quartzofeldspathic host rock. P-T paths for the two eclogites have similar peak conditions of 3.4 GPa at 600°C. Si-in-phengite geobarometry on the host rock for the structurally higher eclogite indicates eclogitic peak pressure of -3.0--3.2 GPa prior to final matrix equilibrium at amphibolite conditions of 0.9 GPa at 725°C. The structurally higher eclogite specimen yields a isotope dilution Lu--Hf date of 266 ± 1 Ma (MSWD = 1.7) whereas, the structurally lowest specimen returned an isochron date of 250 ± 1 Ma (MSWD = 2.1). The similarity in peak pressure for the eclogite and host rock indicate the eclogite and host rock were juxtaposed prior to peak-pressure metamorphism and subsequently exhumed as a coherent tectonic slice. The distinct garnet dates at different structural levels are consistent with stepwise metamorphism and accretion of eclogite, similar to the processes observed in metamorphic soles. Such stepwise metamorphism has yet to be accounted for in Permian tectonic models of the northern Canadian Cordillera. [ABSTRACT FROM AUTHOR]
ISSN:	03150941