HORNBLENDE GABBRO BLOCK IN SERPENTINITE MÉLANGE, PEEL-MANNING FAULT SYSTEM, NEW SOUTH WALES, AUSTRALIA: LU-HF AND U-PB ISOTOPIC EVIDENCE FOR MANTLE-DERIVED, LATE ORDOVICIAN IGNEOUS ACTIVITY

Abstract

In the southern New England Fold Belt, the Peel-Manning Fault System (PMFS) is a major structural element characterized by tectonic blocks in a serpentinite mélange. We document a large (>I km), intensely deformed block of noncumulate and cumulate homblende gabbro from the PMFS. Cumulate layering is preserved in some outcrops, and relicts of magmatic plagioclase, magnesio-hornblende, and minor clinopyroxene are recognizable microscopically despite the gabbro having undergone amphibolite facies and subsequently prehnite-pumpellyite facies metamorphism. The composition of the magmatic clinopyroxenes and amphiboles indicates low-pressure crystallization, suggesting formation in a shallow crustal reservoir. Zr/TiO2, ratios, chondrite-normalized rare earth elements ([La/Yb]N, = 6.4-2.7), and rock/MORB (mid-ocean ridge basalt) patterns indicate that the gabbros have a calc-alkaline, basaltic magmatic affinity and a composition transitional between oceanic and continental arc rocks. The mafic rocks intermingled with and intruding the gabbro have a cale-alkaline arc to tholeiitic back-arc basin affinity ([La/Yb]N = 2.8-1.2). Y/15-La/10-Nb/8 plots confirm the arc affinity of these rocks. Peak metamorphic assemblages formed at temperatures of ~590°C and pressures of 550 MPa, suggesting that a geothermal gradient of ~29°C/km operated during deformation. Single-zircon U-Pb analyses from two rock samples by laser ablation quadrupole inductively coupled plasma mass spectrometry (ICPMS) yielded a combined age of 444.7 ± 2.4 Ma (2aσ) Further, Lu-Hf TDM model ages obtained from single zircons from the same samples analyzed by laser ablation multicollector ICPMS were calculated as 447 ± 25 Ma (2σ). The similarity of zircon crystallization ages (U-Pb) and TDM model ages (Lu-Hf) suggests little if any crustal residence between the time of partial melting of depleted mantle to produce the gabbro magma and the time of zircon crystallization. This study provides evidence for Late Ordovician arc magmatism and links all the early Paleozoic arc-related events recognized previously. © 2006 by The University of Chicago. All rights reserved.