Low grade metamorphism and hydrothermal alteration in the basement greywacke terranes of the northern and central North Island, New Zealand: Reconnaissance study

Abstract

Combined field, petrographic, clay mineral (crystallinity and chemistry), stable isotope and whole rock minor and major chemical studies were made of the pre-Tertiary greywacke rocks of the North Island, New Zealand. The aim was to evaluate the affect of alteration processes on these rocks. The study examined samples from many greywacke outcrops in the northern half of the North Island representing the Murihiku, the Manaia Hill, the Bay of islands and the Omahuta terranes. Greywacke drillcores from the Rotokawa, Broadlands, Kawerau and Ngawha geothermal reservoirs were also examined. Results from the field, petrographic and whole rock minor and major chemical studies confirm previous interpretations that the greywackes of the North Island comprise incomplete remnants of a forearc basin (Murihiku terrane), a trench slope basin (Manaia Hill terrane) and accretionary complexes (Bay of Islands and Omahuta terranes), developed during the late Permian to mid Cretaceous, in a subduction system that extended along the eastern margin of Australian- Antarctic Gondwanaland. The North Island greywackes mainly consist of poorly sorted, angular to subrounded, clastic grains of feldspars, quartz and different types of lithic debris ranging in particle size from fine silt to coarse sand. The relative proportions of these constituents, especially quartz and rock fragments, and as well as the whole rock minor and major chemical compositions of the graywackes vary between terranes reflecting the distinct tectonic settings of their sedimentary basin. The North Island grewywackes contain different metamorphic mineral assemblages which reflect the differences in physical conditions of very low-grade metamorphism that altered these rocks. The Murihiku terrane contains the assemblage Lu+Cg+Ab+Hu+Qz+Ca+Ti which correlates with zeolite facies. The Manaia Hill and the Bay of Islands terranes contain the assemblage Pr+Ch+Qz+Pm+Lu+Ca or compatible sub-assemblages which have been recognised as belonging to the zeolite to prehnite-pumpellylite transition zone. The Omahuta terrane contains the assemblage Pm+St+Ac+Ep+Qz+Al+Ch+Ca which correlates with pumpellyite-actinolite facies. A similar pumpellyite-actinolite facies assemblage is also present in the Melange zone between the Manaia Hill and the Bay of Islands terranes. Illite (IC) and chlorite (ChC) crystallinity indices (X-ray peak width) were determined. The IC values show a systematic variation with regional zonation which defines the very low-grade metamorphic conditions of these rocks. The chlorite crystallinity also shows a very similar zonation confirming that ChC is a reliable indicator of increasing metamorphic grade. The results also demonstrate that ChC is a useful complementary tool, especially where the interpretation of IC is hindered by the presence of mixed-layer smectite or detrital mica. The metamorphic conditions indicated by the clay crystallinity zones closely agree with the metamorphic conditions inferred from the mineral facies. The general correlations are: diagenetic crystallinity w ith zeolite facies; lower anchizone crystallinity with the prehnite-pumpellyite zone and upper anchizone crystallinity with the pumpellyite-actinolite zone. The results also show that greywacke rocks from the geothermal environment exhibit enhanced chlorite crystallinities and occupy a distinct position on an IC versus ChC plot. The chemical compositions of chlorite in selected greywacke outcrops and drillcores show that they are trioctahedral and have homogenous compositions which lie in the magnesium-rich chamosite field on both thin section and the regional scales. Illites are dioctahedral but show variations in both thin section and the regional scales, reflecting real compositional heterogeneities within each illite population. Illite and chlorite in greywackes from geothermal and non-geothermal environments show clear compositional differences indicating that different factors controlled their compositions in the two environments. Comparison between illite and chlorite crystallinities and chemical composition show a clear relation for chlorite. The 18O values of 128 North Island greywackes show variations with a restricted composition range in each terrane. The 18O values of the Murihiku terrane greywackes range between 11.2 and 14.1 %, with one value of 16.8%. The 18O values of most greywackes from the Manaia Hill terrane show a range between 6.5 and 12.5%, with a few values up to 13%. Greywackes from the Bay of Islands terrane are relatively high in 18O (11.4 to 13.5%), whereas the Omahuta terrane greywackes have 18O values ranging from 7.5 to 9.5%. Greywackes from the Melange zone have 18O values (11.2 – 14.1%) very similar to those from the Bay of island terrane. These variations mainly reflect the primary sedimentary differences which were little affected by the regional very-low grad metamorphism. The hydrogen isotope compositions of 29 greywackes and 7 chlorites range from -81 to -52%, with a value of -103%. These values are interpreted to reflect the general D trend in the North Island greywackes. The stable oxygen and carbon isotope compositions of 29 calcites from the greywackes were measure. Their 18O values (7.9 to 26.6%) suggest that they were deposited in isotopic equilibrium with 18O-rich metamorphic fluids (18OH2O, 0+4 to 18+2 %), which are quite different from those of the present day surface waters of New Zealand. These results suggest that the pore fluid had evolved through water/rock interactions by oxygen isotope exchange with the host rocks at temperatures very similar to those estimated from the fluid inclusion Ths and metamorphic mineral assemblages. The 13C values of calcite (-24.0 to -27.0%) suggest that they were deposited from, or are in isotope equilibrium with, fluid which had 13Cco2 values very similar to CO2 that derived from a low-13C source. Both the 13C values of calcite and their inferred 13Cco2 values indicate that direct oxidation of organic matter in the greywackes was the mechanism that produced the carbon in their calcites.