Geology of the southern part of the Longwood complex, Southland coast and Centre Island, New Zealand

Abstract

This thesis combines the results of field work, petrographic studies and mineralogical and geochemical data from plutonic rocks of the southern part of the Longwood Complex, Southland, New Zealand.

Data presented here are consistent with the Longwood Complex, part of the Brook Street Terrane, representing the remnants of the root of a volcanic arc active during Permian to Mesozoic times.

The Pahia Intrusives comprise a wide range of rock types ranging from peridotite, gabbro, norite to pyroxene diorites. Primary igneous layering and mineral layering is evident, although the internal structure of the Pahia Intrusives thoughout the area is not known. Disruption of layering is common and mingling and mixing of felsic and mafic magmas is also evident. Rock compositions are compatible with cumulate formation and fractional crystallisation from a high-AI calc-alkaline to tholeiitic parent. Two-pyroxene geothermometry suggest temperatures of crystallisation between 850°C and 1030°C.

The Pahia Intrusives are intruded by a series of similar but petrographically and geochemically distinct granitoid bodies, the Boat Harbour Diorite at Pahia, the Austin Quartz Monzodiorite at Wakaputa and the Oraka Diorite at Oraka. Intrusive relationships are frequently complex, a consequence of magma mixing and mingling, wall rock interaction and multiple injection of magma. Geochemical trends are consistent with crystal fractionation from a similar or related magma from which the Pahia Intrusives are derived. Enclaves occur in the granitoid rocks. The igneous textured xenoliths are indicative of mixing and mingling. Foliated xenoliths are orientated NNW-SSE in the Austin Quartz Monzodiorite which also reflects the orientation of basaltic and hornblende microdiorite dykes in the granitoid rocks. The possibility exists that the core of the Austin Quartz Monzodiorite and other granitoid rocks may be centred in the Longwood Range, north of the study area and that the quartz monzodiorite may represent a "mega-dyke" intrusion. Orientation of mafic and granite dykes in the granitoid rocks may indicate E-W extension.

Three minor intrusions have been identified. The Colac Granite, a hornblende-biotite granite intruding the Oraka Diorite at Colac Bay, the Wakaputa Granite which intrudes the Austin Quartz Monzodiorite at Kawakaputa Bay and the high-potassic Ruahine Granite which intrudes olivine-bearing cumulate gabbro of the Pahia Intrusives at Ruahine Bay.

Geochronological studies indicate that the U-Pb zircon dated Colac Granite, 260 Ma. is the oldest unit yet found in the Longwood Complex. Field relationships indicate that the Oraka Diorite and Pahia Intrusives at Oraka exceed 260 Ma., in excess of K-Ar and U-Pb zircon dates from elsewhere in the Pahia Intrusives. It is inferred that the occurences of cumulate and non-cumulate ultrabasic, gabbro and pyroxene diorite at Pahia (dated at 207 Ma. by U-Pb zircon methods), Wakaputa-Ruahine (dated at 230 Ma. by U-Pb zircon methods) and Centre Island (no ages available) may reflect several intrusions of the Pahia Intrusives.

Sediments of Quaternary age occur sporadically along the Southland coast and on Centre Island and comprise gravels-cobbles and sand in raised beach deposits, together with siltstone containing carbonaceous material.