Geochemistry And Provenance Of Lower Permian Sedimentary Rocks, Nambucca Block, Southern New England Orogen

It is well documented that sediments record geochemical signatures that reflect the composition and tectonic setting of the rocks providing the detritus for them. This is true of the turbidites in the Nambucca Block that record compositions that vary from basaltic to rhyodacitic but are dominantly andesitic to dacitic. They show chondrite and N-MORB patterns, characteristic of calc-alkaline, continental arc rocks, namely light REE (av. La/Yb) (N) = 6.3) and Th enrichment, Ti, Nb and Ta depletion, and substantial within plate components (Nb/Yb; 0.65-5.16; x = 3.15). Rare occurrences of rocks with intra-oceanic island arc affinities and felsic volcanic rocks showing tectonic fabrics occur, indicating more distal sources contributed detritus. Most detritus in the turbidites has been derived from the calc-alkaline, continental rocks of the Keepit arc, some from the subduction-accretion sequences of the Tablelands Complex. CIA [Al2O3/(Al2O3+Na2O + K2O + CaO)x100] values (48-83; x = 68) indicate that erosion of these arc sequences occurred during glacial, temperate and humid conditions at different times. Further, index of chemical variability values (0.3-1.85; x = 0.85) reveal a compositional maturity and a high percentage of clays in the source rocks suggesting a passive tectonic setting in which minimal uplift and extensive weathering took place.