Year

2015

Degree Name

International Bachelor of Science

ANZSRC / FoR Code

040303 Geochronology

Department

School of Earth & Environmental Sciences

Advisor(s)

Solomon Buckman

Abstract

The Murrawong Creek and overlying Pipeclay Creek formations are a volcaniclastic sedimentary sequence containing some of the oldest fossil assemblages (Middle Cambrian to Ordovician) in the southern New England Orogen (NEO). These ages contrast markedly with the younger Devonian units that comprise most of the neighbouring Gamilaroi (Tamworth Group) and Djungati (Woolomin Group) terranes. Some researchers suggest that the fossils date allochthonous limestone blocks that slumped into younger deep marine basins. This project aims to test these competing hypotheses by utilizing detrital zircon geochronology (U-Pb SHRIMP dating) to establish the maximum depositional age by determining the youngest population of detrital zircons. Previous attempts to extract zircons from pre-Devonian rocks have been unsuccessful, but through targeting Zr-rich sandstone layers using a handheld XRF in the field, six detrital zircons were extracted from the Murrawong Creek Formation. All zircons show minimal rounding indicating minimal residence in sediment systems and are devoid of inherited cores derived from older melted crust. The youngest population of two zircons in the Murrawong Creek Formation have an age of 450 ±10 Ma indicating a maximum depositional age of ca. 460 – 440 (early Late Ordovician). The Pipeclay Creek Formation yielded thousands of zircons with a unimodal detrital zircon population of 443.4 ± 4.3 Ma, indicating a maximum depositional age in the latest Ordovician to earliest Silurian. This is consistent with the slightly older age of the Murrawong Creek Formation. These two formations still represent the oldest sedimentary units in the NEO, albeit younger than the age inferred from biostratigraphy.

In a broader tectonic framework, two tectonic models explaining the Gamilaroi terrane currently exist: 1. the island arc is an exotic terrane which accreted onto the eastern margin of Gondwana via east directed subduction, 2. the island arc developed just outboard of Gondwana and was later merged onto the Gondwanan margin via continuously west directed subduction. The lack of ‘Gondwanan’ Precambrian zircon grains suggest that the sediments are sourced from an island arc receiving no sedimentary influence from Gondwana. Point counting results plotted in QFL diagrams confirm that these quartz-poor sediments were sourced from an undissected arc. Geochemical analysis identified calc-alkaline, tholeiitic, boninitic and MORB-like clasts within the conglomerate. Together with chert clasts, this suggests that sediment deposition occurred within a forearc basin of an intra-oceanic island arc that was eroding detritus derived from old, offscraped accretionary wedge material and the adjacent island arc volcanic edifice. This explains the mixture of Cambrian shallow water fauna from older accreted volcanic seamounts with Ordovician-Silurian zircons in the same formations. It is likely that these represent portions of a forearc basin from the earliest stage of development of the Gamilaroi terrane somewhere in the Panthalassan Ocean. The arc evolved to more felsic composition throughout the Late Ordovician whereby the erosional unroofing of arc plutonic rocks occurred, resulting in the increasing presence of monzonitic clasts and zircons in the Pipeclay Creek Formation. Subsequent arc rifting in the Devonian was followed by collision and accretion onto the Gondwanan margin during the latest Devonian.

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