Barry Roser

Whole-rock geochemical variations and evolution of the arc-derived Murihiku Terrane, New Zealand

Arc-flank volcaniclastic sedimentation in the Murihiku Terrane of New Zealand lasted about 120 million years from Late Permian to Early Cretaceous time. Despite the effects of pervasive zeolite-facies alteration, whole-rock geochemical parameters for sandstones, siltstones and tuffs record changes in source-rock composition, both in time and along the length of the depositional basin. Sandstones are considered to give a more reliable indication of the state of evolution of the source volcanic arc than do the siltstones. The siltstones commonly contain detrital white mica flakes that are generally lacking in the sandstones, and are possibly of distal continental origin. Some also contain very fine felsic ash particles. Average abundances and normalized multi-element diagrams are used to estimate proportions of three model end-member source constituents, average upper-continen- tal crust (UCC), high-K rhyolite (RHY) and basaltic andesite (AND). Sandstone provenance for the Southland Syncline sector changed from a predominantly basaltic-andesite source in Late Permian to early Middle Triassic time, for example, UCC:RHY:AND = 0:17:83 in the Early to early Middle Triassic, to highly felsic in the Middle to Late Triassic, reaching UCC:RHY:AND = 2:74:24 in the Late Triassic Oretian Stage. A UCC component became increasing significant from latest Triassic upward and the proportion of mafic to felsic volcanism increased again, with UCC:RHY:AND = 15:30:35 in the Middle Jurassic Temaikan Stage. Mix modelling suggests that along-arc source propor- tions varied, with greater mafic and upper continental crust contributions in the northern Kawhia seg- ment than in the Southland segment. These patterns may be explained by deposition at an oceanic Aleutian-type arc margin, with transition to a continental oceanic arc character induced either by arc evolution and dissection, forearc sliver translation, or underplating of rafted microcontinental fragments.

Geochronology and geochemistry of the Dunedin Volcanic Group, eastern Otago, New Zealand

Abstract Fifty‐six previously unpublished K‐Ar ages for the Dunedin Volcanic Group and previously published K‐Ar and 40Ar/39Ar ages demonstrate that activity in the centrally situated Dunedin Volcano (here given formal lithostrati‐graphic status) lasted from 16.0 ± 0.4 to c. 10.1 Ma, and that of the surrounding Waipiata Volcanics lasted from 24.8 ± 0.6 to 8.9 ± 0.9 Ma. Apart from a gap at c. 20 Ma, recorded Waipiata activity climaxed at c. 16–14 Ma when activity of the Dunedin Volcano was beginning; it outlasted that of the Dunedin Volcano by c. 1 m.y. The total volume erupted by the Dunedin Volcano may have exceeded that of the largely monogenetic Waipiata Volcanics by an order of magnitude. New major‐ and trace‐element analyses are given for 87 whole‐rock samples and kaersutite. The whole‐rock data demonstrate the exclusively alkalic nature of the group, the Waipiata Volcanics being more strongly alkalic than most of the mafic members of the central volcano. This fractionated to give a much greater volume of phonolitic differentiates than the Waipiata Volcanics. As for other intraplate Cenozoic volcanism in the New Zealand region, ranging overall from tholeiitic to highly alkalic, major‐ and trace‐element patterns support an origin from a garnet‐bearing ocean island basalt source region with high U/Pb mantle characteristics.

The Chrystalls Beach‐Brighton block, southeast Otago, New Zealand: Petrography, geochemistry, and terrane correlation

Abstract The Chrystalls Beach‐Brighton coastal block in southeast Otago has commonly has been placed in Caples Terrane, but has recently been described as a geochemically anomalous area of uncertain terrane affinity. Data points on discriminant diagrams occupy fields centred between those for type Caples Group and Torlesse Terrane, overlapping both. The psammites average 71.9% SiO2, closely comparable to Torlesse Terrane psammites, in contrast to the majority of type Caples Group psammites (av. 64.3%) and Waipapa Terrane psammites (64.4%). QFL plots show the Chrystalls Beach psammites as a petrofacies distinct from those described hitherto for Torlesse Terrane (lithic feldsarenites) and Caples Group and Murihiku Terrane (volcarenites).

Sedimentary provenance study of the post-Early Permian to pre-Early Cretaceous metasedimentary Duque de York Complex, Chile

El Complejo Duque de York constituye una sucesion metasedimentaria del post Permico temprano a pre Cretacico temprano que aflora en los archipielagos de Madre de Dios y Diego de Almagro a lo largo de los Andes Patagonicos de Chile. La petrografia y geoquimica de las areniscas y limolitas de este complejo han sido analizadas para caracterizar su fuente y regimen tectonico depositacional. La composicion modal de las areniscas esta dominada por feldespato y, en una proporcion similar pero menor, por cuarzo. La composicion mineralogica de las areniscas y limolitas es compatible con un metamorfismo de bajo grado de la facies sub-esquistos verdes, el cual no afecto significativamente a la composicion geoquimica de estas rocas. A pesar de lo anterior, los analisis geoquimicos revelan distintos grados de enriquecimiento en K+, especialmente en las limolitas. El valor del indice de Alteracion Quimica de las areniscas y limolitas fluctua entre 58 y 71, lo que indica que el sedimento sufrio una alteracion quimica moderada en su fuente o durante el transporte. Las composiciones modales de las areniscas son concordantes con la erosion de las raices plutonicas de un arco magmatico. Los indices geoquimicos de proveniencia sugieren una fuente relativamente evolucionada, de composicion similar a la composicion tipica de una granodiorita de arco magmatico. Los datos indican que la acumulacion del material detritico ocurrio en un margen continental activo. Similitudes geocronologicas, petrograficas y geoquimicas entre los metasedimentos de Duque de York, del Grupo LeMay (Antartica Occidental) y del terreno Rakaia (Permico-Triasico Superior; Nueva Zelanda) sugieren un regimen geodinamico comun para estas tres sucesiones. Tal regimen probablemente constituyo un extenso margen continental activo durante el Paleozoico superior al Mesozoico temprano, posiblemente a lo largo del segmento Antartico de Gondwana.

Composition of Monocrystalline Detrital and Authigenic Minerals, Metamorphic Grade, and Provenance of Torlesse and Waipapa Graywacke, Central North Island, New Zealand

Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.

Geochemistry of sediments in three sectors of Trincomalee Bay, Sri Lanka: provenance, modifying factors and present environmental status

PurposeThe geochemical compositions of sediments from three sectors in Trincomalee Bay (Koddiyar Bay, Thambalagam Bay and the Inner Harbour) in Sri Lanka were examined to determine fluvial and marine contributions and the effects of sorting and heavy mineral concentration. The present environmental status of the bay was also assessed.Materials and methodsForty-nine sediment samples were collected from Trincomalee Bay and analysed by X-ray fluorescence, yielding data for the major elements and 17 trace elements. Mean grain size and sorting were also measured. Data were compared with the compositions of sediments from the lower Mahaweli River, which supplies most of the clastic detritus to Trincomalee Bay.Results and discussionSediments in the three sectors differ significantly in chemical composition, according to position relative to the Mahaweli River delta source, depositional environment, heavy mineral concentration and marine influences. According to accepted sediment quality guidelines, some As contamination may have occurred in the Inner Harbour and Thambalagam Bay and Cr contamination in all three sectors.ConclusionsProximal Koddiyar Bay sediments compare closely with Mahaweli River bedload. Although the clastic component in the more distal Thambalagam Bay and the Inner Harbour is also derived from the Mahaweli River, compositions are modified significantly by marine contributions. High concentrations of elements including Ti, Zr, Ce, Nb and Y in NW Koddiyar Bay are consistent with heavy mineral concentration by winnowing in high-energy zones. Some decoupling of Fe–Ti- and Zr-bearing heavy mineral assemblages may occur within the bay. Al-normalized metal enrichment factors and contour maps show that apparent contamination by As and Cr is spurious and is caused by locally high background levels from Mahaweli River detritus. This illustrates the importance of establishing local background levels of elements during environmental studies.

Geochemistry of the Willsher Group, southeast Otago, New Zealand, and comparison with the Murihiku and Dun Mountain‐Maitai Terranes

Abstract Triassic Willsher Group sandstones and siltstones in the Kaka Point Structural Belt of southeast Otago have geochemical provenance signatures and petrographic characteristics indicative of derivation from a moderately evolved, partially dissected, medium‐K volcanic arc, probably of continental island arc nature. Pre‐Etalian and Etalian sandstones are andesitic to dacitic in composition, whereas those in the Kaihikuan Stage are more felsic. These features are demonstrated by increased silica contents, immobile element ratios, and rare earth element parameters including LaN/YbNand LaN/SmN ratios. Significant negative Eu anomalies appear at the top of the Willsher succession. Comparison with the chemistry of coeval and older sandstones from the adjoining Dun Mountain‐Maitai Terrane shows the Willsher Group sediments are substantially more felsic, and close linkage between the two terranes can be ruled out. Comparison with coeval intervals in the adjacent Murihiku Terrane, including new rare earth element data presented and discussed here, shows that pre‐Etalian Willsher sandstones are more felsic, but that Etalian and Kaihikuan equivalents are similar in composition. In both the Willsher Group and the Murihiku Terrane, sandstones become more felsic stratigraphically upward, although the Willsher Group shows more variability within age groups, probably due to local variations in source proportions. These features and others previously described suggest the Kaka Point Structural Belt should be regarded as a separate subterrane of a greater Murihiku Terrane.