PROVENANCE & TECTONIC SETTING OF NORTHLAND ALLOCHTHON ROCKS, NORTH ISLAND, NEW ZEALAND USING GEOCHEMICAL ANALYSIS

Abstract

The Northland Allochthon is a displaced rock unit in New Zealand. It overlies the autochthonous sediments of non-marine to marine transgressive sediments and unconformably overlain by the Waitemata and correlative groups of Upper Oligocene-Lower Miocene age. Lithologically, the allochthonous unit is mostly composed of sandstone, siltstone, greensand, siliceous mudstone, argillaceous micritic limestone, and rare coal measures, along with dispersed organic matter. In this paper, the provenance & tectonic setting of different lithofacies of Northland Allochthon rocks is inferred using geochemical analysis. Major and trace element composition of these lithofacies are analyzed by X-ray spectrometer using standard analytical techniques. The results indicate that SiO2 and Al2O3 are most abundant with dominance of quartz, feldspar and clay. The SiO2 correlate negatively with all other oxides, however, Al2O3 correlate positively. Variation of CaO, K2O and Na2O content appears to be independent of Al2O3 content. This indicates that major element variation could result from processes other than simple dilution by Si2O2 and Al2O3. Trace elements exhibit a systematic relationship with Al2O3 and its concentration increase as grain size and SiO2 content decreases. K2O/Na2O vs SiO2 discriminant plot indicates the tectonic setting of Northland Allochthon rocks, New Zealand belong to active continental margin. Immobility of elements is indicated by Zr and Yttrium concentration. Depletion of Al is indicated by the replacement of plagioclase and matrix by calcite tends to behave like Ti as suggested by a positive correlation between these elements.