Granite genesis and basin formation in an extensional setting: The magmatic history of the Northernmost New England Orogen*

Abstract

U‐Pb and K‐Ar age determinations on igneous rocks have been used to trace the Late Palaeozoic to Mesozoic magmatic and tectonic history of the northern New England Orogen, northeastern Australia. The oldest igneous rocks in the Connors Arch, tonalites of the Urannah Suite, yield zircon ages of 304.3 ± 5.8 and 308.2 ± 7.1 Ma. One of a swarm of felsic dykes chemically related to the Urannah Suite granites is distinctly younger at 283.9 ± 5.2 Ma. The cross‐cutting Thunderbolt Granite is slightly younger again at 277.9 ± 6.3 Ma. The granites of the Urannah Suite are not the remnants of a Late Devonian ‐ mid‐Carboniferous arc. A zircon age of 294.2 ± 2.8 Ma measured on an ignimbrite from the Carmila beds at Dumbleton Rocks indicates initiation of the adjacent Bowen Basin during, or soon after, the main Late Carboniferous batholith‐forming event. A later generation of more isotopically primitive magmatism is represented by younger plutons and mafic dykes. The Triassic Gloucester Granite east of the Connors Arch has a zircon age of 243.5 ± 4.9 Ma. In the arch itself, the Cretaceous Hecate and Mt Abbot Granites have been dated at 130.8 ± 3.4 and 119.3 ± 2.2 Ma, respectively. The Blackwall Quartz Diorite in the Bowen Basin to the west is of similar age, 132.5 ± 2.4 Ma. K‐Ar mineral ages measured on six chemically distinctive ‘Cretaceous’ granites in the Connors Arch range from 145.3 to 103.3 Ma, substantially extending the known time span of ‘Cretaceous’ magmatism in the region. The entire Late Carboniferous to Triassic magmatic episode occurred during a protracted period of crustal extension that appears to have been terminated by a major compressional event, the Hunter‐Bowen Orogeny. The initiation of Cretaceous magmatism significantly pre‐dates the opening of the Tasman Sea.