Geochemistry and Chronology of Tectonic Blocks in Serpentinite Mélange of the Southern New England Fold Belt, NSW, Australia

Abstract

Tectonic blocks are widely distributed throughout the serpentinite mélange of the Peel-Manning Fault System, southern New England Fold Belt, Australia. Two meta-diorite blocks from the Pigna Barney area have distinctly differing chemical signatures. One resembles boninite, containing high Ni and Cr, low rare earth element (REEs), and high field strength element (HFSEs) contents. Further, it exhibits an enriched chondrite normalized LREE pattern and an atypical eNd 536Ma value of –4. Well-defined isochrons for this sample gave Sm–Nd and Rb–Sr ages of 536±177;38 Ma and 426±177;38 Ma respectively. The former represents the emplacement age and the latter resetting of the Rb–Sr system during a later metamorphic event. The initial Nd isotopic composition suggests an enriched source. The other meta-diorite block has a flat REE pattern, a Ti/V ratio >10 and Y, La and Nb values typical of tholeiitic magmas erupted in an island arc setting. By contrast, blueschists from this locality and from the Glenrock Station area show depleted REE patterns, Ti/V = 20–50, and eNd 480Ma = +8, features characteristic of normal mid-ocean ridge basalts (N-MORB). Meta-diorites from the Glenrock Station area have moderately depleted signatures (eNd 400Ma = +4.5) and Ti/V ratios and REE patterns typical of calc-alkaline and tholeiitic island arc magmas. These contrast with hornblende cumulate rocks from this location, which have very high Cr and Ni, low Zr and Y, and show slight LREE-enriched chondrite normalized patterns. Chromites (100 Cr/Cr+Al = 85) and clinopyroxenes (Wo 93En 42. 3Fs 7.7 – Wo 48En 43.7Fs 8.3) in these rocks reveal an island arc affinity. The eNd 420Ma values vary from +2 to +4 and Rb–Sr ages from 425±177;44 Ma to 411±177;15 Ma. The core of a single grain of cumulus hornblende gave a poorly defined 40Ar/ 39Ar age of 394 Ma in contrast to younger ages (235–263 Ma) obtained from other grains. The younger ages are attributed to resetting of the K-Ar system during a metamorphic event. This study has provided further evidence for an Early Paleozoic arc-trench system along the eastern Gondwana margin.