Beneath the Perth Basin : new U-Pb SHRIMP zircon ages from the Pinjarra Orogen, Western Australia, 2016

Abstract

This Record contains new zircon U Pb geochronological data, obtained via Sensitive High-Resolution Ion Micro Probe (SHRIMP), from two samples of metamorphosed felsic igneous rocks from the Proterozoic Pinjarra Orogen (Western Australia), intersected in diamond drillcore at the base of deep petroleum exploration wells penetrating the Paleozoic sedimentary successions of the Perth Basin. In the southern Perth Basin, petroleum exploration well Sue 1 was terminated at depth 3074.2 m, in crystalline basement rocks of the southern Pinjarra Orogen. Abundant zircon from a biotite-bearing felsic orthogneiss at depth 3073.2–3073.7 m yielded a complex array of U–Pb isotopic data, indicative of significant post-crystallisation disturbance of the isotopic system. A Discordia regression fitted to the array yielded an upper intercept date of 1076 ± 35 Ma (all quoted uncertainties are 95% confidence intervals unless specified otherwise) interpreted to represent magmatic crystallisation of the igneous precursor to the orthogneiss, and a lower intercept date of 680 ± 110 Ma which is our best estimate of the age of the tectonothermal event responsible for post-crystallisation disturbance of the U–Pb system. Crust of known Mesoproterozoic age is rare in the southern Pinjarra Orogen: pre-1000 Ma igneous crystallisation ages in the Leeuwin Complex were previously known only from two c. 1090 Ma garnet-bearing orthogneisses at Redgate Beach (Nelson, 1999), 30 km west of Sue 1. All other dated outcrops have revealed Neoproterozoic (780–680 Ma) granitic protoliths reworked by Early Cambrian (540–520 Ma) magmatism, deformation and metamorphism (Nelson, 1996, 2002; Collins, 2003). In the northern Perth Basin, petroleum exploration well Beagle Ridge 10A was terminated at depth 1482 m, in crystalline rocks of the northern Pinjarra Orogen. A leucocratic orthogneiss sampled within the interval 1464.0–1467.0 m yielded only sparse zircon, but four of the seven grains analysed yielded a weighted mean 207Pb/206Pb date of 1092 ± 27 Ma, interpreted to represent magmatic crystallisation of the precursor to the orthogneiss. Our data show no evidence for Neoproterozoic U–Pb resetting of the c. 1090 Ma zircons: where present, isotopic disturbance is predominantly geologically recent. The two newly dated samples are located at opposite ends of the Perth Basin (about 470 km apart), and although the two magmatic crystallisation ages are imprecise, the date of 1092 ± 27 Ma from the Beagle Ridge 10A leucocratic orthogneiss is indistinguishable from the date of 1076 ± 35 Ma from the Sue 1 felsic orthogneiss. Furthermore, both rocks contain inherited zircon (1620–1180 Ma in Sue 1; 1290–1210 Ma in Beagle Ridge 10A), indicating the presence of pre-1100 Ma crustal components in their parent magmas. The possibility of an extended crustal prehistory is supported by Sm–Nd analyses of buried Pinjarra Orogen orthogneisses (Fletcher et al., 1985; Fletcher and Libby, 1993), which yielded depleted-mantle model ages spanning 2234–2078 Ma in the north (near Beagle Ridge 10A) and 2040–1996 Ma in the south (including one analysis of granitic gneiss obtained from Sue 1). Our new U–Pb zircon data expand the known extent of 1100–1050 Ma felsic magmatism, and in Sue 1, we have established the first direct link between 1100–1050 Ma protolith ages and Sm–Nd depleted-mantle model ages of 2200–2000 Ma in the southern Pinjarra Orogen. This Sm–Nd signature appears to be characteristic of (1) granitic basement beneath the Perth Basin, and (2) granitic and metasedimentary outcrop in the Northampton and Mullingarra complexes of the northern Pinjarra Orogen, where 1100–1050 Ma protoliths are widespread. It is therefore possible that a belt of isotopically coherent Paleoproterozoic crust spans the full strike length of the orogen (Fletcher and Libby, 1993). This belt is distinct from more juvenile components of the Leeuwin Complex to the west, where 780–680 Ma protoliths are paired with Sm–Nd depleted-mantle model ages of 1600–1100 Ma.