High-pressure Granulite Metamorphism Research Articles

Thermal neutron dosimetry for fission track dating with metal activation monitors: A reinvestigation

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high dT/dP type and the other low dT/dP type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high dT/dP metamorphic belt is juxtaposed against an outboard low dT/dP metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate dT/dP environment in the subduction zone and a high dT/dP environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high dT/dP and low dT/dP metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate dT/dP and the upper plate will register penecontemporaneous high dT/dP metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

A geochemical study of Lewisian metasedimentary granulites and gneisses in the Scourie-Laxford area of the north-west Scotland

AbstractMajor element, trace element, and REE data for metasedimentary granulites and their retrogressed derivatives formed from Archaean parent-rocks at two localities in the Lewisian complex of north-west Scotland are presented.The metasedimentary rocks are enclosed in and intruded by metatonalites and related calc-alkaline rocks and have been highly deformed and metamorphosed along with these rocks. They are geochemically distinct from this meta-igneous suite and range from highly aluminous to highly siliceous types. Comparisons of major and trace element data with those for unmetamorphosed Archaean and post-Archaean sediments suggest that they are derived from a detrital shale-greywacke assemblage, which may have included a volcaniclastic component. High ΣREE and ΣLREE suggest that the source-rocks included fractionated felsic igneous rocks.Evidence that depletion in the large ion lithophile elements K, Rb, Th accompanied high-pressure granulite metamorphism supports the view, based on data derived from the associated meta-igneous rocks, that depletion was effected by active fluids rather than by abstraction of a partial melt and suggests that removal of K and Th began only when a considerable reduction of Rb had taken place.