Bachelor of Science (Honours)
School of Earth & Environmental Sciences
Becker, Alexandra, The age and provenance of Carboniferous accretionary complex at Crescent Head and Point Plomer, southern New England Orogen, NSW., Bachelor of Science (Honours), School of Earth & Environmental Sciences, University of Wollongong, 2014.
The Deep-marine turbidites at Crescent Head and Point Plomer on the mid north coast, New South Wales form part of the Carboniferous-Permian Tablelands complex of the southern New England Orogen. To date, sedimentary sequences have received scant attention and no reliable age dating. Standard petrographic, geochronological and geochemical techniques (field mapping, stratigraphic logging, petrography, point counting, detrital zircon age dating and whole rock geochemistry) were undertaken to provide the first detailed descriptions and age constraints for these turbidite sequences. Stratigraphic data shows a maturation trend from juvenile, mafic-derived sediment of the Point Plomer sequence to mature, quartz-rich sediment of the Crescent Head sequence. Units at Point Plomer strike NW-SW while at Crescent Head they strike more NE – SW. U/Pb (SHRIMP) dating of detrital zircons extracted from turbidites at Point Plomer and Crescent Head reveal a youngest zircon population age of the latest Permian; 253.7 ± 4.6 Ma for Point Plomer and 256.9 ± 7.6 Ma for Crescent Head. Both samples also contain an older Carboniferous peak, but only the more mature, quartz-rich sandstones from Crescent Head have older (Palaeoproterozoic to Devonian) peaks typical of a Gondwanan continental source. The less mature greywackes from Point Plomer had a low yield of volcanic zircons with a narrow age range (Permian – Carboniferous), more typical of a juvenile island arc source. Petrographic point-counting data showed that the Crescent Head sequence is a mature feldspathic litharenite sandstones, derived from a mixed-recycled continental tectonic setting. In contrast, the Point Plomer sequence is dominated by immature, lithic arkose sandstone and shales, derived from a mafic-intermediate transitional-dissected magmatic arc provenances. Geochemical data revealed Nb and Tb negative anomlies and the enrichment of Hf and Zr in REE analysis, confirming both the Point Plomer and Crescent Head sequences are derived from subduction settings. Trace-element geochemical data revealed both sequence are depleted mantle sources, but the Crescent Head sequence is derived from an evolving arc and has undergone more weathering than the Point Plomer sequence. Major elemental analysis has revealed the Crescent Head sequence has high SiO 2 and low K2O typical of a continental arc provenance setting, while the Point Plomer sequence has the high K 2O and low SiO2 typical of an oceanic arc provenance setting. The differences between the Crescent Head and Point Plomer sequences, suggest that the Point Plomer and Crescent Head sequences are derived from two distinct and possibly separate tectonic settings during the Carboniferous to latest Permian age. The abundance of Permian granites in the southern New England Orogen confirms it was an active continental margin and the most likely source for the silica-rich sediments of Crescent Head. However, the immature, mafic-derived sediments of the Point Plomer sequence hint at a distinctly different island-arc provenance. The only known rocks of similar age and composition are those of the Gympie terrane, which arrived and collided with the active continental margin of Gondwana during the Permo-Triassic resulting in a period of mixed provenance mixing into deep marine, trend-fill environment before collision and onset of the Hunter-Bowen Orogeny.