Blueschists of the New England Orogen: Structural development of the Rocksberg Greenstone and associated units near Mt Mee, southeastern Queensland
Titel:
Blueschists of the New England Orogen: Structural development of the Rocksberg Greenstone and associated units near Mt Mee, southeastern Queensland
Auteur:
Holcombe, R. J. Little, T. A.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 41 (1994) nr. 2 pagina's 115-130
Jaar:
1994-04
Inhoud:
A coherent terrane of blueschist facies rocks exposed near Mt Mee in the Late Palaeozoic New England Orogen in southeastern Queensland contains metamorphic structures and fabrics related to both subduction and uplift. Protoliths of the Rocksberg Greenstone were mafic volcaniclastic units containing coarse detritus derived mainly from a compositionally restricted, metabasaltic source and mixed with serpentinite detritus. The Rocksberg Greenstone is interpreted as the remnants of the volcaniclastic apron of a seamount constructed on oceanic lithosphere, possibly within a fracture zone where ultramafic rocks were exposed and eroded. The seamount was dismembered by imbricate faulting, and juxtaposed against mylonitized, siliceous hemipelagic sediments during subduction (D1) in middle Carboniferous times. Blue amphibole-rich assemblages in the Rocksberg Greenstone are restricted to high strain shear zones that focused fluid movement, which affected both bulk composition and reaction kinetics. Subsequent exhumation of these rocks from depths of at least 18 km began in the Late Carboniferous. Shallowly dipping D2 foliation fabrics, synkinematic with a greenschist facies thermal overprint of the blueschist facies rocks, are correlated with similar fabrics occurring within an extensional core complex at the northern end of the North D'Aguilar Block and dated at about 307 Ma. During D2 the strain history changed progressively from a coaxial to a non-coaxial regime with a concurrent change in the orientation of the principal stretch due either to progressive orientation changes during differential uplift, or to a transition into a transpressional dextral wrench regime. Subsequent folding, perhaps within a dextral wrench regime in the Early to mid-Permian, accompanied west-block-down movement on the adjacent North Pine Fault, juxtaposing the Rocksberg Greenstone against still higher levels of the accretionary complex. The transition from blueschist facies to greenschist facies conditions that accompanied the change from subduction-related to exhumation-related deformation differs from that for blueschist terranes in which the geothermal gradient during both deep burial and exhumation is uniformly low. It is likely that in those terranes, such as the Franciscan subduction complex, a clear distinction cannot be made between subduction and exhumation fabrics and not all metamorphic fabrics may be related solely to the subduction process as is the common interpretation.
Tijdschrift beschrijving