Philippa M. Black

Coalification and graphitization in high-pressure schists in New Caledonia

The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.

Northland ophiolite, New Zealand, and implications for plate-tectonic evolution of the southwest Pacific

The Northland ophiolite and coeval ocean-floor sedimentary rock form the Northland allochthon obducted from the northeast onto New Zealand in the late Oligocene. The ophiolite was probably emplaced as a single sheet and separated into individual massifs during subse quent movement of the allochthon. Chemically, the bulk of the igneous rocks are normal mid-ocean-ridge basalts, but the ophiolite also includes a younger suite of hornblende modal within-plate alkalic rocks believed to represent seamounts. Two alternative models, one involving a subduction flip, the other involving continuous westward subduction, are proposed to account for the obduction of the upper part of the Northland ophiolite. The oceanic crust from which the ophiolite originated was formed simultaneously with Tasman Sea spreading on the western rim of a once much larger South Fiji plate assemblage.

K‐Ar ages of early Miocene arc‐type volcanoes in northern New Zealand

Abstract Understanding the temporal and spatial development of the early Miocene Northland Volcanic Arc is critical to interpreting the patterns of volcanic activity in northern New Zealand through the late Cenozoic. The northwesterly trending arc is considered to have developed above a southwest‐dipping subduction system. The distribution of its constituent eruptive centres is described in terms of an eastern belt that extends along the eastern side of Northland and a complementary broad western belt which includes subaerial and submarine volcanic edifices. Critical examination of all 216 K‐Ar ages available, including 180 previously unpublished ages, and their assessment against tectonic, lithostratigraphic, seismic stratigraphic, and biostratigraphic constraints, leads us to deduce a detailed chronology of periods of activity for the various early (and middle) Miocene arc‐type volcanic complexes and centres of northern New Zealand: Waipoua Shield Volcano Complex (19–18 Ma, Altonian); Kaipara Volcanic Complex (23–16 Ma, Waitakian‐Altonian); Manukau Volcanic Complex (c. 23–15.5 Ma, Waitakian‐Clifdenian); North Cape Volcanic Centre (23–18 Ma, Waitakian‐Altonian); Whangaroa Volcanic Complex (22.5–17.5 Ma, Waitakian‐Altonian); Taurikura Volcanic Complex (22–15.5 Ma, Otaian‐Clifdenian); Parahaki Dacites (22.5–18 Ma, Waitakian‐Altonian); Kuaotunu Volcanic Complex (18.5–11 Ma, Altonian‐Waiauan). In general, volcanic activity does not show geographic migration with time, and the western (25–15.5 Ma) and eastern (23–11 Ma) belts appear to have developed concurrently.

Regional high-pressure metamorphism in new caledonia: Phase equilibria in the ouégoa district

A sequence of isograds based on the appearance or disappearance of pumpellyite, lawsonite, Na-amphibole, omphacite, fully-ordered graphite, epidote, almandine, and barroisitic hornblende have been mapped in the Ouegoa district. Phase relations between minerals within the progressive metamorphic sequence which passes from lawsonite—albite schists through glaucophanic albite—epidote—almandine schists to eclogitic graphitic quartzo-feldspathic gneisses are described. Individual mineral compositions and relative abundances of minerals change subtly throughout the metamorphic sequence but at any one level in the metamorphic progression, mineral composition depends only on parent-rock chemistry. Sodic amphiboles are consistently more aluminous than chlorites, irrespective of parent-rock composition or metamorphic grade and are stable in paraschists with assemblages such as lawsonite—albite—chlorite in the lawsonite zone or epidote—albite—chlorite in the epidote zone. In the lawsonite zone paragonite is confined to aluminous assemblages but the establishment of the omphacite—paragonite tie line in the epidote zone increases the stability field of paragonite into less aluminous compositional fields. The composition of the calcic amphibole changes progressively from actinolite in low-grade rocks to barroisitic hornblende in the higher-grade part of the epidote zone. The attainment of a barroisite hornblende composition allows the establishment of a paragonite—hornblende tie line which breaks the omphacite—albite tie line; paragonite then becomes stable in basic assemblages and hornblende in pelitic lithologies. All reactions involving the appearance or disappearance of minerals are complex and involve changes in compositions of coexisting minerals. Both mineral composition and stable isotopic data (Black, 1974b) suggest that the Ouegoa rocks recrystallized in a system which can be regarded as open with respect to the fluid phase. The Ouegoa paraschists are graphitic and the ‘buffer assemblage’ calcite—quartz—plagioclase (An5–7)—paragonite—graphite is common in the epidote zone and lawsonite—quartz—calcite—grossularitic garnet—graphite occurs in the lawsonite zone. Using equilibrium constants and equations involving gas species in the system C—H—O in equilibrium with graphite and the two buffer assemblages, and temperatures and pressures (Ptotal) estimated for the Ouegoa rocks by oxygen isotope geothermometry and geological considerations, the composition of the fluid phase was estimated. Although H2O is always the major constituent of the fluid phase PH2O = Ptotal only over a very narrow temperature range (350–400°C at Ouegoa). At higher temperatures CO2 becomes increasingly important while at lower temperatures CH4 may be important i.e. over a restricted range metamorphism becomes progressively ‘wetter’ with increasing temperature in contrast to still higher grades where the rocks become progressively ‘dryer’. The crystallization of the eclogitic paragneisses is effected in high grades of metamorphism by ‘drying’ of the fluid phase by dilution with CO2. A Schreinemaker analysis of the system Na2O + CaO + total Fe as FeO + MgO + A12O3+ H2O + excess SiO2 using compositions of the natural mineral phases (omphacite, hornblende, glaucophane, epidote, paragonite, albite, chlorite, almandine) from highgrade epidote-zone metasediments and an isothermal Ps—μHH2O projection shows that almandine—omphacite and glaucophane—paragonite assemblages are clearly confined to low μHH2O fields, and chlorite—albite and glaucophane—epidote assemblages to high μHH2O fields.

Age relationships and tectonic implications of late Cenozoic basaltic volcanism in Northland, New Zealand

An episode of late Miocene ‐ Recent essentially basaltic volcanism is the latest in a sequence of magmatic events recognised in the tectonically complex geological development of the Northland Peni...

Melt generation models for the Auckland volcanic field, New Zealand: constraints from UTh isotopes

Abstract New major and trace element, Sr, Nd, Pb, and U Th isotope results are presented for Quaternary volcanic rocks from the Auckland volcanic field, North Island, New Zealand. The rocks are predominantly small volume basanites and alkali basalts, with just 4.1 km 3 having been erupted from 49 centres in the last 150 ka. They have many of the characteristics of HIMU Ocean Island Basalts (OIB), including high Ce/Pb, Nb/Ba, Nb/Ce, U/Pb and low Nb/U, and Sr and Nd isotope ratios (0.70287–0.70327 and 0.51315-0.51289) that plot on the low side of the mantle array. Their initial Pb isotope ratios ( 206 Pb/ 204 Pb= ∼ 19.2) are unradiogenic relative to those in HIMU OIB, suggesting that the HIMU signature in the source of the Auckland rocks is young ( unusually primitive (MgO > 11%; mg * = 0.65–0.70), and they exhibit some of the most marked ( 230 Th/ 238 U) disequilibrium (28–48%) known from intraplate basalts. Olivine fractionation-corrected trace element arrays are used to assess the degrees of partial melting (∼ 1.5–3.5%), the relative partition coefficients, and the source trace element ratios. The initial ( 230 Th/ 232 Th) values are much higher than those inferred for their source regions, and so some form of dynamic melting model is required, and the matrix peridotite must have been in the melt zone for relatively long periods of time, typically 0.5–1.0 Ma. Using dynamic and melt percolation models (after [1,2]) the calculated melt rates are low (2 × 10 −5 –2 × 10 −4 kg m −3 yr −1 ), consistent with the low eruption rate of 1–2 km 3 in the last 50,000 yr. It is inferred that these reflect very low upwelling rates beneath the Auckland volcanic field, and that the source characteristics (HIMU) of the Auckland volcanics are from relatively shallow levels in the upper mantle (80–140 km), rather than a deep-seated mantle plume.

Oxygen isotope study of metamorphic rocks from the Ouégoa district, New Caledonia

Abstract48 minerals from 18 in situ metamorphic rocks (mostly metasediments) from the Ouégoa district have been studied. Particular emphasis was placed on obtaining isotopic data for quartz, calcite and muscovite but some pyroxenes and amphiboles were also examined. Data for Ouégoa rocks show they have tended to be isotopically homogenized by metamorphism and that the effect of increasing metamorphism is to progressively deplete the rocks of heavy C and O isotopes. These results indicate that during metamorphism the rocks isotopically exchanged through the medium of a widespread oxygen-carrying fluid phase. Tentatively assigned temperatures obtained from isotopic data for quartz-calcite and quartz-muscovite pairs, using the calibration curves of Epstein and Taylor (1967), indicate lawsonite in the Ouégoa schists to be stable over a temperature range of 250 to 400° C and epidote from ca. 380° to at least 550° C. Temperatures for metamorphic zones in Ouégoa blueschists closely parallel those obtained for Type III and IV glaucophane-bearing rocks from Ward Creek, California (Taylor and Coleman, 1968). The measured tectonic thickness of lawsonite-bearing schists has been used to calculate a lithostatic pressure increment of 2 Kb and geothermal gradient of 20° C per km for the lawsonite zone. Comparison of lithostatic pressure increment with total pressure increment estimated from the stability relations of lawsonite over the temperature range 250–400° C (3.5 Kb Nitsch, 1972) suggest Ptotal ≠ Plithostatic and that that the pressure of the fluid phase may have exceeded lithostatic pressure.

Geochemistry and tectonic significance of the Tangihua Ophiolite Complex, New Zealand

Abstract The ophiolitic igneous complexes of the Tangihua Complex of Northland, New Zealand consist of massive and pillowed basaltic lava sequences with intercalated sediments and lesser gabbro, microgabbro and basaltic intrusions. The complex also includes minor felsic derivatives, younger alkalic intrusions (which exhibit within-plate characteristics) and rare ultramafic rocks. The lavas are relatively homogeneous and are dominantly tholeiitic basalts with minor calc-alkaline and alkaline affinities. The complex nature of the chemistry, in particular those lavas with transitional arc signatures, and the presence of non-arc lavas, is clear evidence that these volcanics formed in a suprasubduction zone environment. The combined geochemical and tectonic constraints suggest that the Tangihua Complex formed in either a transitional zone between an arc and a back-arc setting, or in a zone of migration from arc to back-arc volcanism. As such, the field, age and geochemical data support the model in which these rocks formed suprasubduction zone crust and are fragments recording some or all stages of arc initiation, arc rifting and back-arc basin formation. The orientation of the arc and back-arc volcanic rocks in relation to the over tectonic regime suggests subduction towards the west. Recent work in the SW Pacific shows that Late Cretaceous ophiolites are present in Papua New Guinea, New Caledonia and New Zealand. The formation of these ophiolitic systems supports the presence of a large-scale tectonic regime convergence and consequent westward subduction stretching from Papua New Guinea to New Zealand during the late Cretaceous to early Palaeocene.

Geochemistry of late Cenozoic basaltic volcanism in Northland and Coromandel, New Zealand: implications for mantle enrichment processes

New major and trace element, and Sr, Nd and Pb isotope results are presented for selected basaltic rocks from young volcanic fields (<10 Ma) in the Northland province of the Northland–Auckland Peninsula, and Mercury Island on the eastern side of the Coromandel Peninsula, North Island, New Zealand. The rocks are mainly alkali basalts and tholeiites which are different from those in the adjacent Auckland volcanic field. Samples from Kaikohe–Bay of Islands (KBI) tend to have low silica contents and high Nb/La ratios (1.05–1.62), low Sr (0.7027–0.7030) and high Nd (0.51299–0.51304) isotope ratios and relatively high /, but low / and /, and so low Δ/ (4.1±2.2) and Δ/ (10.7±6.7). Ti Point and Stony Batter samples (TPSB) and Mercury Island basalts (MIB) have similar compositions, relatively high silica contents and low Nb/La ratios (0.23–0.58), high Sr (0.7039–0.7048) and low Nd (0.51278–0.51293) isotope ratios, with higher Δ/ and Δ/ (8.1±1.6 and 30.9±5.0, respectively). The rocks from Whangarei–Puhipuhi field (WHP) have intermediate compositions, in that their Sr and Nd isotope ratios are similar to KBI, but their Pb isotope ratios are more similar to the TPSB and MIB. Furthermore, the WHP rocks have similar large ion lithophile element (LILE) abundances to KBI but with negative Nb anomalies which are a feature of TPSB and MIB. Although AFC processes could explain co-variations of isotope ratios in the samples from the different fields they are difficult to reconcile with the observed relationships between isotope ratios, major elements and Nb/La. Thus, the major differences in the geochemical features of the Northland volcanic rocks are more likely to represent source variations in the underlying mantle. The KBI samples have isotope and trace element compositions similar to the Auckland basalts, and to many ocean island basalts (OIB). In contrast, negative Nb anomalies and high Δ/ and Δ/, high Sr and low Nd isotope ratios are features of subduction-related volcanism. Therefore, two very different mantle enrichment processes have been identified in the mantle beneath the Northland–Auckland peninsula, reflecting changes in the tectonic setting from plate convergence to intraplate over the last 30 Ma. The KBI, WHP, TPSB and MIB have similar MgO and FeOt (total iron as FeO) contents at mg# ∼0.70 indicating that they were generated at similar depths and temperatures. However, the Auckland basalts have significantly higher FeOt and MgO contents, and they are therefore considered to have been derived from greater depths and higher temperatures. This constrains the spatial relationships between the two mantle domains with intraplate sources dominant at depth and the subduction related enrichment processes at shallower levels in the mantle underlying the Northland Peninsula.

Blueschist ophiolites in the melange zone, northern New Caledonia

A regional melange zone, 150 km long and 30 km wide, forms the southern boundary and structural capping to a high-pressure blueschist belt in northern New Caledonia. The disrupted country rocks in the melange zone are Mesozoic metagrey-wackes and Eocene chert-limestone sequences which have been penetrated from below by tectonically-injected ophiolite slivers containing metamorphosed serpentinite, gabbro, dolerite, basalt, tuff, chert and shale. An ocean crust origin for these rocks is indicated by chemical, mineralogical and radiometric data from coastal outcrops at Anse Ponandou on the northeast coast. The age (41 m.y.), metamorphic environment (350 ° C at 7 kb), and mineral association (acmitic jadeite-riebeckite-pyropic spessartine-pistacitic epidote-lawsonite-high Si phengite) are significantly different from those of the adjacent regional high-pressure schist belt, indicating a separate structural site for blueschist metamorphism of buried ophiolitic ocean crust during early Tertiary orogenesis.