GEOCHEMICAL CONSTRAINTS ON THE DEPOSITIONAL SETTING OF PALEOZOIC CHERTS FROM THE NEW-ENGLAND OROGEN, NSW, EASTERN AUSTRALIA

Despite widespread occurrences of radiolarian cherts within the New England orogen (NEO), their age, geochemistry and sedimentology have not been studied. Recent radiolarian age determinations of these cherts have markedly changed previous conceptions as to their age. Geochemical analyses constrain the depositional setting of two distinctive types of chert facies within two terranes of the orogen. Type I are ribbon-bedded cherts of the Djungati terrane and varicoloured ribbon-bedded cherts of the Anaiwan terrane. Low values of Al (Al + Fe + Mn) and corresponding high values of Fe2O3 indicate a possible hydrothermal contribution, while MnO TiO2 ratios indicate accumulation far from terrigenous sedimentary influences. High Si (Si + Al + Fe) ratios indicate a high biogenic content. Type II are tuffaceous cherts which are common in both terranes. High values of Al (Al + Fe + Mn) and low Si (Si + Al + Fe) and MnO TiO2 values indicate deposition in an environment which is more proximal to a terrigenous source than that of Type I cherts. The abundance of both Na2O and Al2O3 increases relative to silica in Type II cherts and indicates dilution by the input of volcanic ash. This is further substantiated by the ternary K-Ba-V × 10 plot. An Al/1bFe/1bMn diagram indicates that Type II cherts are non-hydrothermal in origin. Both chert types were conveyed into a trench by subduction processes. As subduction continued, hemipelagic sedimentation was swamped by the influx of coarse-grained volcaniclastic sediments deposited from turbidity currents reaching the trench. The diverse sedimentary types were tectonically intercalated to form sequences of imbricate thrust slices within two distinctly different subduction complexes which have subsequently been tectonically juxtaposed. © 1990.