School of Earth & Environmental Sciences
Stovin-Bradford, Ruby R., Mineralogy and geochronology of high-pressure, low-temperature metamorphic rocks using a combination of field studies, petrography and zircon U-Pb dating, IBSci Hons, School of Earth & Environmental Sciences, University of Wollongong, 2016.
The serpentinite mélange at Port Macquarie hosts an assortment of island-arc and accretionary complex rocks including chert, shale, felsic to ultramafic dykes and high- pressure, low-temperature (HP/LT) blueschist facies rocks that together represent a dismembered ophiolitic sequence. Rare blocks of blueschist conglomerate from Rocky Beach reported for the first time here contain felsic, basaltic and pelitic clasts, within an arkose matrix. Dominant assemblages in both the clasts and matrix established through petrography and geochemistry are glaucophane + phengite + alkali feldspar ± lawsonite ± chlorite ± pyroxenes ± hornblende. The coexistence of glaucophane and lawsonite confirms that this conglomerate underwent HP/LT blueschist facies metamorphism.
Due to the low metamorphic temperature (<400˚C) overgrowths did not develop on the protolith zircons therefore a direct age of metamorphism was not obtained. Nonetheless the U-Pb zircon dates presented in this thesis help to resolve an ongoing controversy over an Ordovician or younger Palaeozoic age of metamorphism. The sample of blueschist conglomerate gave a small yield of zircons with variable rounding and pitting associated with abrasion within sedimentary systems. This morphology highlights the detrital nature of the zircons, therefore allowing the maximum age of metamorphism to be established by the youngest detrital grain. U-Pb zircon ages range from Archean (2936 ± 14 Ma) to Carboniferous (343 ± 8 Ma). These results constrain the maximum age of metamorphism to be less than 343 ± 8 Ma, revealing a much younger HP metamorphic event than the Ordovician 470 Ma age obtained through K-Ar dating of phengite micas, a technique prone to giving excessive ages due to retention of radiogenic argon in HP/LT metamorphic rocks.
These results are compatible with a tectonic model for the eastern margin of Gondwana which incorporates periods of accretion and “Andean-type” magmatism with episodic events of island-arc collisions (Gympie terrane). A possible interpretation using this model is that this Carboniferous detrital zircon came from the erosion of material from a Carboniferous continental arc to the west. Following which it was deposited into the fore-arc basin and trench as a deep marine, mass-flow deposit before being subducted and subjected to blueschist facies metamorphism. These blueschist conglomerates were exhumed rapidly as serpentinite diapirs prior to or during the docking of the Gympie terrane onto Gondwana at the Permo-Triassic transition.