Igneous Petrology of the Clarence and Awatere Valleys, Marlborough

Abstract

The Central Marlborough Igneous Province (lower Upper Cretaceous) includes intrusive centres at Mandamus, Tapuaenuku and Blue Mountain, extensive basic and intermediate volcanics in the Clarence and Awatere Valleys, and numerous dikes including a major swarm in the Inland Kaikoura Range. Comagmatic dikes are present in the lower North Island at Cape Palliser (Challis, 1960). Volcanics preserved in the Clarence Valley (Gridiron volcanics) comprise an alkali olivine basalt – transitional trachybasalt series, and contain varying amounts of olivine, clinopyroxene, plagioclase and opaque phenocrysts. The Lookout volcanics of the Awatere Valley form a more extensive ankaramite - trachybasalt - transitional trachybasalt - trachyandesite series. There are many petrographic similarities between the two suites, but alkali basalts with groundmass olivine are rare in the Lookout volcanics, and ankaramites do not occur in the Gridiron volcanics. Ankaramites similar to those in the Awatere Valley outcrop at Hungry Hill and the Korari Volcanics of the Mandamus centre (Reid, 1972) also show affinities with the Gridiron and Lookout sequences. A major intrusive centre at Tapuaenuku consists of a sequence of pyroxenites, gabbros, monzogabbros and more felsic rocks. The layered pyroxenites and gabbros exposed in this complex are in many respects identical to the Blue Mountain layered rocks (Grapes, 1972, 1975). Cumulus minerals in these intrusions (olivine+clinopyroxene+plagioclase+opaque minerals) are similar to the phenocrysts of the Gridiron and Lookout volcanics, providing evidence that the Blue Mountain and Tapuaenuku layered rocks and the Gridiron and Lookout volcanics crystallised from comparable parental magmas. The basic volcanics show many geochemical features typical of 'alkaline' suites, such as high TiO2, Na2O+K2O, P2O5, Zr and low SiO2, but are usually hy-normative with oversaturated differentiates. The Gridiron volcanics have higher P2O5, Sr and Ba but lower TiO2, Rb and Zr than comparable Lookout rocks, but in most respects the two volcanic lineages are similar. Many of the dikes in the province are chemically related to the volcanics, but a variety of strongly alkaline dikes associated with the Tapuaenuku Plutonic Complex appear to define an alkali basalt - phonolite (tinguaite) lineage. Variation in the volcanics is generally consistent with gravitational differentiation of an alkali basalt parental magma in shallow subvolcanic magma chambers. Most of the major and trace element distributions may be explained by removal of the main phenocryst minerals present in the volcanics, but variation of Cr and Ni suggests some mixing of differentiated and relatively unmodified parental magma also took place, probably due to periodic refilling of the magma reservoir. Fractionation is believed to have occurred under relatively high PH2O, and fO2 in the Lookout series, resulting in considerable removal of Fe-Ti oxides, but suppression of early plagioclase crystallisation. The Gridiron volcanics evolved under rather lower PH2O and fO2. The Marlborough volcanics may be compared with transitional basalt suites from areas such as Afar, Pantellaria, Socorro Island and N.S.W., although the peralkaline oversaturated rocks frequently associated with transitional basalts have not been found in Marlborough. The volcanics were erupted onto a Low-lying continental platform, under terrestrial or shallow marine conditions. Structural analysis of the Tapuaenuku Dike Swarm suggests tensional conditions associated with doming due to intrusion, and magma may have ascended along approximately NE-SW oriented, faults or rifts. However there is no evidence that magmatism relates to the currently active faults in this area.