Douglas S. Coombs

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Recommended nomenclature for zeolite minerals: report of the subcommittee on zeolites of the International Mineralogical Association, Commission on New Minerals and Mineral Names

Abstract This report embodies recommendations on zeolite nomenclature approved by the International Mineralogical Association Commission on New Minerals and Mineral Names. In a working definition of a zeolite mineral used for this review, interrupted tetrahedral framework structures are accepted where other zeolitic properties prevail, and complete substitution by elements other than Si and Al is allowed. Separate species are recognized in topologically distinctive compositional series in which different extra-framework cations are the most abundant in atomic proportions. To name these, the appropriate chemical symbol is attached by a hyphen to the series name as a suffix except for the names harmotome, pollucite and wairakite in the phillipsite and analcime series. Differences in space- group symmetry and in order-disorder relationships in zeolites having the same topologically distinctive framework do not in general provide adequate grounds for recognition of separate species. Zeolite species are not to be distinguished solely on Si : Al ratio except for heulandite (Si : Al < 4.0) and clinoptilolite (Si : Al ≥ 4.0). Dehydration, partial hydration, and over-hydration are not sufficient grounds for the recognition of separate species of zeolites. Use of the term ‘ideal formula’ should be avoided in referring to a simplified or averaged formula of a zeolite. Newly recognized species in compositional series are as follows: brewsterite-Sr, -Ba; chabazite-Ca, - Na, -K; clinoptilolite-K, -Na, -Ca; dachiardite-Ca, -Na; erionite-Na, -K, -Ca; faujasite-Na, -Ca, -Mg; ferrierite-Mg, -K5 -Na; gmelinite-Na, -Ca, -K; heulandite-Ca, -Na, -K5 -Sr; levyne-Ca, -Na; paulingite- K, -Ca; phillipsite-Na, -Ca, -K; stilbite-Ca, -Na. Key references, type locality, origin of name, chemical data, IZA structure-type symbols, space-group symmetry, unit-cell dimensions, and comments on structure are listed for 13 compositional series, 82 accepted zeolite mineral species, and three of doubtful status. Herschelite, leonhardite, svetlozarite, and wellsite are discredited as mineral species names. Obsolete and discredited names are listed.

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.

Mineral Reactions in Zeolitic Triassic Tuff, Hokonui Hills, New Zealand

Textural evidence and other considerations indicate the following paragenetic sequence of reactions in marine Triassic tuff beds of rhyolitic to andesitic parentage that are scattered through a 4.8 to 8.5 km thickness of the Murihiku Supergroup, Hokonui Hills, Southland, New Zealand: (1) Glass → montmorillonite ± illite; (2) glass →heulandite + chlorite and celadonite; and (3) heulandite → laumontite, or prehnite, or calcite or analcime, or albite. Chemical analyses of the altered tuff indicate that Ca and Na ions have been relatively mobile. Heulandite and laumontite altered tuffs are Ca-enriched, whereas analcime tuff is Na-enriched relative to unaltered volcanic rocks. Heulandite, chlorite, and celadonite have been analyzed by electron microprobe. Heulandite with high Si/Al ratio, sometimes in the clinoptilolite range, is associated with calcium-poor pyroclastic feldspar, whereas heulandite with low Si/Al ratio is associated with calcium-rich pyroclastic plagioclase. Such data indicate that the Si/Al ratio in the heulandite was controlled by the Si/Al ratio of the glass precursor. Chlorite and celadonite have high Fe/Mg ratios and variable Al contents. Some celadonite appears to form interlayered structures with chlorite. Distribution patterns and stability relations of analcime with quartz and of laumontite show that average temperature gradients did not exceed about 25°C/km. The breakdown of heulandite to Na-aluminosilicates (analcime or albite) or to Ca-aluminosilicates (laumontite or prehnite) over a wide stratigraphic interval suggests that such factors as P H 2 o and activity of various ions in stratal waters played a more significant role than depth of burial in controlling distribution of the diagenetic and very low grade metamorphic phases in the Hokonui Hills.

Murihiku Supergroup (Triassic—Jurassic) of Southland and South Otago

Abstract The name “Murihiku Supergroup” is proposed for those rocks, in general of older Mesozoic age, which overlie the Permian Bryneira, Productus Creek, Arthurton, and Kuriwao Groups in the Southland Syncline and underlie the erosion surface on which Cretaceous and Tertiary beds were deposited with major unconformity. North Etal and Taringatura Groups are introduced; North Range and Wairuna Peak Groups are given formal status. All are included in the Murihiku Supergroup.

Continuous metamorphic gradient documented by graphitization and K-Ar age, southeast Otago, New Zealand

Abstract The Chrystalls Beach-Brighton coastal section, southwest of Dunedin, New Zealand, exposes Otago Schist, including the weakly metamorphosed Triassic accretionary melange of the Chrystalls Beach Complex and higher-grade rocks of a largely felsic, arc-derived provenance. Melange zones contain pelagic sediments, some of which are manganiferous, and metabasites. The grade of metamorphism increases progressively from southwest to northeast. Four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks: pumpellyite-chlorite, pumpellyite-actinolite, epidote-actinolite, and biotite. d002 data for carbonaceous material, b0 values for phengite, and metamorphic phengite K-Ar ages are reported for 32 pelitic and semipelitic samples. Remarkably tight correlations are revealed between d002 for progressively graphitized carbonaceous material, K-Ar ages that range from 184.6 to 138.7 Ma, and linear distance along the section. Both the d002 values and the K-Ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes. All rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements. A convex trend for phengite b0 values plotted against advancing graphitization suggests P/T conditions during metamorphism as in the high-pres sure intermediate facies series. The age of the metamorphic peak is inferred to be 175-155 Ma (Middle to Late Jurassic), with cooling to the mica closure temperature at 155-135 Ma (Late Jurassic to Early Cretaceous). Unloading, denudation, and cooling were accompanied by localized hydrothermal events. The oldest covering strata were deposited at ca. 100 Ma and denudation was completed by ca. 75-80 Ma. Middle Triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 Ma. Associated turbidites contain suspected late Middle to early Late Triassic tube fossils

Composition of Analcime from Sedimentary and Burial Metamorphic Rocks

Data for correlation of unit cell size with composition of natural and synthetic analcimes are reviewed and are plotted on a determinative graph. Unit cell sizes are given for about 40 analcime samples from sedimentary and burial metamorphic rocks from 13areas. The range in cell sizes i s found to be from a o = 13.721A to 13.679A, corresponding to a range in composition of approximately Na 16 Al 16 Si 32 O 96 · 16H 2 O to Na 13 Al 13 Si 35 O 96 · n H 2 O. Silica-rich analcimes apparently form by reaction of acid volcanic ash with saline water. Analcimes formed in the absence of quartz or other silica minerals are less siliceous than about Na 14.5 Al 14.5 Si 33.5 O 96 · n H 2 O. Irrespective of this, relatively silica-poor analcimes form either by direct precipitation or by reaction of clay minerals and perhaps other materials with waters of high alkalinity. Dolomite is often present in the resulting assemblage. Analcime formed during burial metamorphism tends to be of intermediate composition close to Na 14 Al 14 Si 34 O 96 n H 2 O and may represent a closer approach to the quartz-analcime equilibrium over an appreciable range of temperatures and pressures. Cell dimensions of analcime from the Murihiku Supergroup of New Zealand are very much alike, even in samples separated stratigraphically by as much as 4600 m. In other suites, there are wide variations in composition between analcimes from closely spaced horizons. Factors which affect analcime composition evidently include the composition of the parent material and the chemical environment in which the analcime crystallizes. The silica content of natural analcime is not solely a function of temperature and has little significance as a geothermometer.

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.

Andradite and andradite-grossular solid solutions in very low-grade regionally metamorphosed rocks in Southern New Zealand

AbstractAlmost pure andradite and intermediate members of the andradite-grossular series (gros40–49, and 47–54, py0–3, alm0–3, spess0–2, hydrogarnet0–3), often framboidal in habit, are widespread in metabasites including lavas, minor intrusions, and volcanic sandstones and breccias metamorphosed under prehnite-pumpellyite and pumpellyite-actinolite facies conditions, possibly extending into the zeolite facies. Coexisting phases include iron-rich epidotes (100 Fe*/Fe*+Al=22–34), pumpellyite, prehnite, actinolite, and chlorite, electron microprobe analyses of which are given, as well as quartz, albite, and calcite. Zoisite (100 Fe*/Fe*+Al=1–5) and iron-poor epidote (100 Fe*/Fe*+Al=11–18) occur in 2 rocks in pseudomorphs after plagioclase together with more iron-rich epidote, but not in close association with the garnets. Coexisting pumpellyite is iron-rich (FeO* 9–14%) in the prehnite-pumpellyite facies and iron-poor (FeO* 5%) in the pumpellyiteactinolite facies. Chlorites and actinolites vary widely and sympathetically in FeO/MgO+FeO ratio. Andradite is also described from a stilpnomelane-actinolite-hematite-bearing andradite quartzite of the pumpellyite-actinolite facies.Conditions of formation involved temperatures of 300 to 400 ° or less, at pressures up to a few kilobars. A wide range of oxygen fugacities is possible, but