MODAL ANALYSIS OF SANDSTONE AND SILTSTONE LITHOFACIES FROM NORTHLAND ALLOCHTHON ROCKS, NEW ZEALAND

Abstract

The Northland Allochthon is a structurally displaced rock unit located between Three Kings Island and Mt. Camel in northern North Island, New Zealand. Lithologically, the allochthonous unit is composed of different lithofacies comprising sandstone, siltstone, greensand, siliceous mudstone, argillaceous micritic limestone and rare coal measures along with dispersed organic matter. The objective of this paper is to study modal analysis of different lithofacies of Northland Allochthon to infer relationship between sandstone composition and tectonic setting of displaced allochthonous unit. For this purpose, petrographic technique is used following Point Counting method for classification of grains. Results indicate that facies of Tokerau Clastics and Mangakahia complex of the Northland Allocution rocks plot within the recycled orogenic fields while samples from Omahutta facies of the Motatau Complex fall in the transitional and Craton interior zones. QmPK plot reflects increasing maturing of the detritus with time. The relative concentration of quartz over feldspar indicates that feldspar has either been destroyed by abrasion/chemical weathering  or has been diluted by cycle quartz.