40Ar/39Ar thermochronology of epidote blueschists from the North D'Aguilar block, Queensland, Australia: Timing and kinematics of subduction complex unroofing
Mechanisms proposed for exhumation of blueschists include whole-crust uplift and erosion, denudation by crustal-scale extensional faulting or ductile strain, and transport of blocks entrained in rising diapiric masses. Insight into the unroofing process, and the role of granitic plutonism, can be gleaned from study of North D9Aguilar block in the New England orogen of southeastern Queensland, Australia. Epidote blueschist facies rocks occur as structurally coherent schists and as blocks in serpentinite matrix melange. These subduction-underplated rocks formed at depths >18 km and occur in the lower plate of a metamorphic core complex. A slate from the upper plate has a whole-rock total-gas age of 315 Ma, interpreted as a minimum age for subduction along this part of the paleo-Pacific margin. Exhumation of lower plate schists was coeval with overprinting of early high-pressure (M 1 ) metamorphic fabrics by a greenschist facies fabric (M 2 ) and was accomplished in part by ductile stretching and normal faulting. Phengites from most lower plate schists yield 40 Ar/ 39 Ar plateau ages of ca. 299–296 Ma, recording the time elapsed since the schists cooled below ∼350 ± 50 °C following M 2 . Plateau-shaped age spectra and small structural depth-related age gradients indicate rapid cooling of schists in the footwall of a present-day low-angle normal fault. Samples from deeper levels of the lower plate remained at >350 °C for 35 m.y. longer than the other samples and contain extraneous Ar possibly absorbed from fluids originating at depth. At ca. 307 Ma a granodiorite pluton was intruded into the lower plate near the peak of M 2 . A synkinematically intruded, in part mylonitic pluton increased the regional M 2 fabric to amphibolite facies in its aureole. Heat from the granitic pluton, and perhaps others like it at depth, may have played a strain-softening role in formation of the core complex. Similar 40 Ar/ 39 Ar cooling ages for different blueschist and greenschist blocks in serpentinite matrix melange support the view that the Australian melange was uplifted by extensional tectonic processes unrelated to serpentinite diapirism.