Magmatic characteristics and geochronology of Ordovician igneous rocks from the Cadia – Neville region, New South Wales: implications for tectonic evolution

Abstract

The Ordovician volcanic and intrusive rocks of the Cadia -Neville region, in the southern Molong Volcanic Belt section of the Macquarie Arc in central-western New South Wales, display a temporal progression from shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornian (ca 460 -453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-type dacite) in the late Eastonian to early Bolindian (ca 450 -445 Ma) to large-volume, high-K calc-alkaline to shoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia Intrusive Complex) at about 445 -440 Ma. The two episodes of shoshonitic magmatism are separated by at least 15 (but up to *20) million years, during which time limestones, associated with regional uplift, were deposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions. Despite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism have strikingly similar geochemistry, indicating that a similar mantle source and partial melting processes were involved over this time interval. The arc-type geochemistry of the volcanic and intrusive rocks in the Cadia -Neville region is very similar to the lavas and intrusions forming the Pliocene Tavua Caldera in Viti Levu, Fiji. The Pliocene shoshonitic rocks at Tavua Caldera were emplaced following cessation of arc magmatism when the arc moved into an extensional phase. By analogy with the Pliocene tectonic setting in Fiji, the arc-like signature of the Ordovician magmas of the Macquarie Arc may be due to preprocessing of the mantle above a subduction zone prior to extension and fragmentation of arc lithosphere that led to generation of the subduction-modified shoshonite-dominated magmas.

Strain localization in brittle–ductile shear zones: fluid-abundant vs. fluid-limited conditions (an example from Wyangala area, Australia)

“…Narrow contact aureoles in the host rock and undeformed margins of the plutons are considered to support a passive emplacement along pre-existing fault planes (Paterson and Tobisch, 1992;Paterson et al, 1990). The crystallization ages of the plutons in the Eastern Lachlan province are estimated from geochronological studies and show a range between 435 and 425 Ma (Lennox et al, 2005;Squire and Crawford, 2007). The recent zircon Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb study by Lennox et al (2014) dates the crystallization of the Wyangala granite as late Silurian (425.2 ± 3.5 Ma).…”

Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation

“…It was clearly part of the Ordovician mega-fan but it is not known if it was either (1) part of the Bega Tract in the backarc basin east of the Macquarie Arc, (2) part of the Wagga-Omeo and Girilambone Zones in the shrinking Wagga Marginal Sea that was being subducted, or (3) developed through the opening in the arc formed by the hypothetical strike-slip fault that enabled a new fan lobe to form east of and behind the Goonumbla-Trangie Volcanic Belt. Renewed widespread volcanism, mainly of shoshonitic affinity, followed in the Eastonian-Bolindian stages as the arc was undergoing further strike-slip faulting Squire & Crawford 2007). This was accompanied by vast growth of the subduction complex to the west that continued into the early Silurian (Figure 3).…”

“…It is clear from the correlation charts in Crawford et al (2007b) that regional variation in geochemical character occurs across the Macquarie Arc, and therefore not too much emphasis should be placed on the geochemical significance of these so-called phases of magmatic activity (cf. Squire & Crawford 2007). …”

Sedimentary Crust and Metallogeny of Granitoid Affinity: Implications from the Geotectonic Histories of the Circum‐Japan Sea Region, Central Andes and Southeastern Australia

“…Glen et al, 2007;Fergusson, 2009). The porphyry Cu deposits may have formed in relation to this arc magmatism in the Late Ordovician to earliest Silurian time, of mainly 460-440 Ma (Squire & Crawford, 2007). Detailed data for the redox state of the arc magmatism are not available, but remarkable aeromagnetic anomalies on the ), respectively (Tainosho et al, 1988).…”