Subduction of arc basaltic andesite: Implications for the tectonic history of the southern New England Fold Belt

Serpentinite mélange in the Peel-Manning Fault System, southern New England Fold Belt hosts tectonic blocks of varying size and lithology. One of the largest occurs at Woodsreef, northern New South Wales, as multiply deformed amphibole schists recording at least three phases of deformation (D₁-D₃). They have a peralkaline, composition (molecular Na₂O+K₂O = 0.083-0.189; Al₂O₃ = 0.046-0.130) and contain aegirine, quartz, albite, arfvedsonite and less common actinolite, neptunite, aenigmatite and bafertisite, as well as relict pre-D1 aegirine. The Zr/ TiO₂ and Nb/Y ratios suggest that the protoliths had originally basaltic to basaltic andesite composition, indicating that they have been altered to a rock of peralkaline composition. Chondrite normalised patterns ((Ce/Yb)_N = 4.98-9.36), Hf/3-Th-Ta, Hf/3-Th-Nb/16 and Y/15-La/ 10-Nb/8 discrimination diagrams, and rock/MORB plots indicate that the protoliths had a calc-alkaline, arc composition. ¹⁴³Nd/¹⁴⁴Nd data (ε_Nd(500) = 2.8-3.3; -4.8; ε_Nd(400) = 2.0-2.5; -5.8) imply that they have been derived from isotopically mildly depleted and enriched sources. Single-grain ⁴⁰Ar-³⁹Ar dating of felsic and mafic fractions from one sample, produces ages of 1200-1100 Ma (albite-quartz vein) and 335.2 ± 2.1 Ma, 332 ± 3.7 Ma and 291 ± 3.7 Ma (arfvedsonite-rich matrix). The Neoproterozoic age obtained from the felsic fraction is attributed to the presence of excess ⁴⁰Ar in quartz and albite, and the 291 Ma age to partial resetting. The 335-332 Ma ages are interpreted as cooling ages related to uplift after subduction. The basalts and basaltic andesites underwent alteration either within a volcanic pile or during subduction after collision with a ?west-dipping subduction zone at the proto-Gondwana margin. With continuing subduction, they were recrystallised and deformed in the deeper levels of the subduction zone, producing aegirine with up to 38% jadelte in solid solution. Replacement of the high-P assemblages by lower P/high-T arfvedsonite-bearing assemblages then took place during uplift at ca 334 Ma.