Yarrol terrane of the northern New England Fold Belt: forearc or backarc?

The Upper Devonian to Lower Carboniferous volcanosedimentary rocks of the Y arrol terrane of the northern New England Fold Belt have previously been as cribed to a forearc basin setting. New data presented here, however, sugges t that the Yarrol terrane developed as a backarc basin during the Middle to early Late Devonian. Based on field studies, we recognise four regionally applicable strati graphic units: (i) a basal, ?Middle to Upper Devonian sub marine mafic volcanic suite (Monal volcanic facies association); (ii) the l ower Frasnian Lochenbar beds that locally unconformably overlie the Monal v olcanic facies association: (iii) the Three Moon Conglomerate (Upper Devoni an - Lower Carboniferous): and (iv) the Lower Carboniferous Rockhampton Gro up characterised by the presence of oolitic limestone. Stratigraphic and co mpositional differences suggest the Monal volcanic facies association post- dates Middle Devonian silicic-dominated magmatism that was coeval with gold -copper mineralisation at Mt Morgan. The Lochenbar beds, Three Moon Conglom erate and Rockhampton Group represent a near-continuous sedimentary record of volcanism that changed in composition and style from mafic effusive (Lat e Devonian) to silicic explosive volcanism (Early Carboniferous). Palaeocur rent data from the Three Moon Conglomerate and Rockhampton Group indicate d ispersal of sediment to the west and northwest, and are inconsistent with d erivation from a volcanic-are source situated to the west (Connors-Auburn A rch). Geochemical data show that the Monal volcanic facies association rang es from tholeiitic subalkaline basalts to calc-alkaline basaltic andesite. Trace and rare-earth element abundances are distinctly MORE-like (e.g, ligh t rare earth element depletion), with only moderate enrichment of the large -ion lithophile elements in some units, and negative Nb anomalies, suggesti ng a subduction-related signature. Basalts of the Monal volcanic facies ass ociation are best described as transitional between calc-alkali basalts and N-MORB. The elevated high field strength element contents (e.g. Zr, Y, Ti) are higher than modern island-are basalts, but comparable to basalts that floor modern backarc basins. This geochemical study, coupled with stratigra phic relationships, suggest that the eruption of backarc basin basalts foll owed widespread Middle Devonian, extension-related silicic magmatism (e.g. Retreat Batholith, Mt Morgan), and floored the Yarrol terrane. The Monal vo lcanic facies association thus shows similarities in its tectonic environme nt to the Lower Permian successions (e.g. Rookwood Volcanics) of the northe rn New England Fold Belt. These mafic volcanic sequences are interpreted to record two backarc basin-forming periods (Middle - Late Devonian and Late Carboniferous - Early Permian) during the Late Palaeozoic history of the Ne w England Orogen. Silicic-dominated explosive volcanism, occurring extensiv ely across the northern New England Fold Belt in the Early Carboniferous (V arrol terrane, Campwyn Volcanics, Drummond and Burdekin Basins), reflects a nother period of crustal melting and extension, most likely related to the opening of the Drummond Basin.