Bachelor of Science (Honours)
School of Earth & Environmental Sciences
Associate Professor Allen Nutman
Richardson, Joshua, Geochemistry and Geochronology of the Igneous Rocks within the Lorne Basin, NSW., Bachelor of Science (Honours), School of Earth & Environmental Sciences, University of Wollongong, 2013.
The circular Lorne Basin, located in north-eastern New South Wales, Australia, contains plutonic and volcanic rocks that appear to be linked with the basin’s evolution. This thesis presents a detailed petrographic, geochemical and geochronological investigation of the most prominent units of igneous rocks within the basin to establish their origin. SHRIMP U-Pb zircon geochronology has been undertaken on several igneous rocks to constrain basin development. A felsic volcanic clast from the Jolly Nose conglomerate in the lower part of the basin’s stratigraphy indicates an eruption age of 217±10 Ma (all ages are 206Pb/238U weighted means at 95% confidence and MSWD≤1). This indicates basin initiation in the Triassic. Inherited zircon in this felsic volcanic rock plus detrital zircons in a sedimentary clast in the same horizon show derivation from mostly Carboniferous (c. 320 Ma) sources in the hinterland of the basin, with older Gondwanan-aged grains (particularly latest Neoproterozoic and c. 1 Ga) also present. The Diamond Head rhyolite from higher in the basin’s stratigraphy has a zircon eruption age of 217±10 Ma, indistinguishable from the age of the felsic volcanic clast in the Jolly Nose conglomerate. Plutonic rocks that cut the felsic volcanic units have yielded marginally younger ages. Thus a quartz-diorite from the North Brother intrusion yields an age of 212±4.4 Ma, whereas a similar rock from the Middle Brother intrusion has been dated by Geoscience Australia at an indistinguishable age of 212±4.4 Ma. These results indicate the rapid evolution of the Lorne Basin for less than 10 million years in the late Triassic.
The plutonic rocks range from diorite to quartz-rich granitic composition, whereas the Diamond Head volcanic is rhyolitic. The majority of the igneous rocks are highly fractionated, with enrichment in SiO2, Al2O3, Na2O, K2O, FeOt, Zr, Ba, Pb and Sr with low abundances of TiO2, P2O5, MgO, Cr and Ni. Harker diagrams indicate linear trends for many elements, with hornblende and plagioclase control being important. Aberrant data such as very low Na2O and Sr in some volcanic rocks is attributed to weathering or alteration, with the breakdown of plagioclase. The geochemical signatures of the rocks such as low Ni and Cr are counter those expected in impact generated melts. Therefore the Lorne Basin is interpreted as a caldera, not an impact structure. In discrimination plots, the Lorne Basin igneous rocks straddle the divide between I- and A-type rocks. Primitive mantle normalised trace element spectra (‘spider diagrams’) indicate enrichment of the light REE (La and Ce) compared with Y, being used as a proxy for the middle-heavy REE. They also show depletion of Nb and Ti, and enrichment of Pb. All these are classic traits of rocks whose compositions are governed by fluid fluxing melting of a mantle wedge, above a subduction zone. Coupled with these signatures is evidence of crustal assimilation, as shown by the overall evolved compositions and the presence of pre-Triassic xenocrystic cores in igneous zircon in the Lorne Basin rhyolites.
The Lorne Basin caldera likely formed within a foreland basin associated with subduction off the east coast of Gondwana. The Coastal Suite of the New England Batholith and the Median Batholith within New Zealand were compared with the Lorne basin Granitoids. Both suites have geochemical signatures compatible with the coeval Lorne Basin igneous rocks. The Coastal Suite shows a similar tectonic setting to the Lorne Basin with transitional compositions between I- and A-type granites. On the other hand the Median Batholith is more I-type in character, and probably represents the volcanic arc closer to the subduction zone. Determining the relationship between the Lorne Basin, Coastal Suite and the Median Batholith helps to establish the Triassic evolution of the eastern Gondwanan active margin.