The Petrology, Geochemistry and Petrogenesis of Igneous and Metamorphic Rocks from the Paparoa Range, Southwest Nelson

Abstract

The Paparoa Range is a horst of basement granite and gneiss near the west coast of the South Island, New Zealand. Mapping in the central Paparoa Range has led to the recognition of four formations within the Charleston Metamorphic Group (where previously there were two), and three post-metamorphic granitoids (previously one). Pecksniff Metasedimentary Gneiss consists of banded psammitic and pelitic gneisses and migmatites, with rare calc-silicate lenses. It appears to represent a metamorphosed greywacke-argillite sequence, and is similar (but not identical) in composition to nearby quartz-rich sandstones and argillites of the Greenland Group. It contains rare calcareous concretions (calc-silicates). Awakiri Tonalite Gneiss (new) is a medium-grained foliated biotite tonalite gneiss, and has similar chemistry and mineralogy to the I-type granitoids of magmatic arcs. Okari Granite Gneiss comprises coarse-grained foliated K-feldspar - megacrystic muscovite biotite granite gneiss, and occurs as several elongate plutons. It has a strong S-type character, and is similar in composition to granitoids of the Karamea Suite. Faraday Microgranodiorite (new) is a fine-grained foliated muscovite biotite granodiorite which forms three small plutons in the study area. It is believed that these rocks, which have very similar composition to the migmatitic leucosomes, formed by partial melting of Pecksniff Metasedimentary Gneiss during metamorphism. Metamorphic assemblages and thermobarometric calculations indicate that the gneisses were metamorphosed at temperatures of 600 ± 50°C and pressures of 4 ± 1kbar. Therefore, conditions were such that if the system was sufficiently hydrous, partial melting may have occurred. The metamorphic grade increases slightly to the southwest of the area, where a northwest-trending K-feldspar - sillimanite isograd has been mapped, and is higher than in the Victoria Paragneiss, 30 km to the southeast. Pegmatitic dykes, which are common in the gneisses, are believed to have formed from locally-derived hydrothermal fluids during metamorphism, whereas leucosomes, which are abundant in the metasedimentary gneisses, apparently reflect partial melting during metamorphism. It is considered that metamorphism occurred in the late Precambrian (about 680 Ma.). After metamorphism, the gneisses were intruded in the east of the study area by Steele Granodiorite, Blackwater Granite, and Buckland Granite, in that order. Steele Granodiorite is a dark grey coarse-grained biotite granodiorite which contains K-feldspar megacrysts. Blackwater Granite consists of light grey or pink medium-grained muscovite biotite granite, and Buckland Granite is a white medium-grained biotite muscovite ± garnet granite. Mineralogical and chemical correlation suggests that all three granitoids belong to the Rahu Suite, and are thus of early Cretaceous age. Rb-Sr geochronology indicates an age of 109 ± 6 Ma. for Buckland Granite, which provides a minimum age for the other two. The chemical variation within Steele Granodiorite and Buckland Granite can be largely attributed to in situ fractionation, whereas much of the chemical and isotopic range within Blackwater Granite is apparently due to alteration. Buckland Granite covers a large area, and field evidence suggests that it intruded in several batches. There is a wide range of initial 87Sr/86Sr ratios within Buckland Granite (0.7058 - 0.7095), which apparently correlates with the different batches, and indicates that there were at least two isotopically distinct source components: one igneous (I-type) and one sedimentary (S-type). The Rahu Suite granitoids are intermediate in terms of the I/S classification, and are best regarded as I-(Caledonian) types. Further igneous activity in the late Cretaceous resulted in the intrusion of lamprophyre, diorite and trachyte dykes, and a breccia pipe. Rare olivine, clinopyroxene, hornblende and biotite megacrysts in some of the lamprophyres are possibly of cognate origin.