ALBERT

Notes: Abstract  A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction-related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite-pumpellyite and pumpellyite-actinolite. From the apparent interplanar spacing d002 data for carbonaceous material, which range from 3.642 to 3.564 Å, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite-actinolite facies for which the highest temperature may be approximately 300°C. Metamorphic white mica K–Ar ages are reported for magnetic separates and 〈2 µm hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180–130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150–130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach–Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes).

Notes: Abstract A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks. Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (〈0.03 mm diam.). Garnets first appear in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments and metabasalts. 45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser. The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5 mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of garnets. Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular than garnets from lower-grade pelites. The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine.

Notes: Abstract Variations in chemistry and related physical properties of sheet silicates in the Ouégoa district with metamorphic grade are investigated. Weakly metamorphosed rocks prior to the crystallization of lawsonite contain phengite (d 006=3.317–3.323 Å), chlorite and occasionally paragonite while interstratified basaltic sills contain chlorite, minor phengite and stilpnomelane. Pyrophyllite crystallizes before lawsonite in some metamorphosed acid tuffs and is also stable in the lawsonite zone. Paragonite, phengite and chlorite appear to be stable through the sequence from weakly metamorphosed rocks into high-grade “eclogitic” schists and gneisses. Optical, chemical and some X-ray diffraction data is given for representative sheet silicates. Electron probe analyses of 55 phengites, 21 paragonites, 57 chlorites, 12 vermiculites, 2 stilpnomelanes, and 2 chloritoids are presented in graphical form. All K-micas analysed are consistently phengitic (3.29–3.55 Siiv ions per formula unit) and show limited solid solution with paragonite (4 to 13% Pa). The K∶Na ratio of the phengite is strongly dependant on the assemblage in which it occurs; the amount of phengite component and its Fe∶Mg ratio depends on bulk-rock composition. Phengites from acid volcanics have the highest Fe∶Mg ratio, highest phengite component and β refractive indices. Phengites from basic volcanics and metasediments of the epidote zone have the lowest Fe∶Mg ratio. Phengites from lawsonite-zone metasediments have intermediate Fe∶Mg ratios. The phengites show a small decrease in phengite component with increasing metamorphic grade. d 006 for phengites varied from 3.302 to 3.323 Å but at least in the lawsonite and epidote zones appears to reflect composition and had little systematic variation with metamorphic grade; phengites from very low-grade rocks showed the longest values of d 006. Paragonite shows almost no phengite-type substitution and only limited solid solution (4–12%) with muscovite. All paragonites (6) and most phengites (20) which have been examined are 2M1 polymorphs; one Fe2+-phengite appears to be a 1M polymorph. The chemistry of chlorites closely reflects parent-rock chemistry. Chlorites from metasediments have distinctly higher Fe/(Fe+Mg) ratios than chlorites from basic igneous rocks; chlorites from the lawsonite and lawsonite-epidote transitional zone metasediments have the highest Fe/(Fe+Mg) ratios. In metabasalts Fe/(Fe+Mg) ratios appear to reflect individual variations in bulk-rock chemistry and show no direct correlation with metamorphic grade. There is little difference in Al/(Si+Al) ratio between chlorites from sediments and basic igneous rocks although in both lithologies the chlorites from the epidote zone appear to be slightly more aluminous. Fe-rich chlorites of the lawsonite zone metasediments have been altered by a process involving leaching of Fe and Mg and introduction of alkalies to a brown pleochroic Fe-vermiculite. Chemical and physical data for this vermiculite are given. The decrease in Fe/(Fe+Mg) ratio in chlorites and phengites on passing from the lawsonite to the epidote zone can be correlated with the crystallization of Fe-rich epidote and almandine in the epidote zone. Elemental partitioning between coexisting minerals has shown Ti to be partitioned into phengite, while Fe and Mn are strongly partitioned into chlorite. When either stilpnomelane or chloritoid coexists with phengite or chlorite, Fe and Mn are slightly enriched in the stilpnomelane or chloritoid relative to the chlorite.

Notes: Abstract The chemistry and phase relations of calcic and sodic amphiboles in the Ouégoa blueschists are investigated. The first appearance of sodic amphiboles is controlled by bulkrock chemistry. Sodic amphibole appears first in weakly-metamorphosed pumpellyite metabasalts prior to the crystallization of lawsonite but does not crystallize in pelitic schists until the middle of the lawsonite zone; sodic amphibole continues as an apparently stable phase in rocks of all bulk compositions into, and throughout, the highest-grade rocks in the district. Calcic amphibole is widespread in metabasalts of the lawsonite and epidote zones and also occurs in metasediments of appropriate composition. Coexisting pairs of calcic and sodic amphiboles are common in metabasalts but they have also been found in some metasediments. A grunerite-riebeckite pair is described. Electron-probe analyses of 120 amphiboles from representative rock-types are presented in graphical form. Sodic amphiboles show an increased Mg/(Mg+Fe) ratio with increasing metamorphic grade. Sodic amphiboles in pelitic schists are ferroglaucophane in the lawsonite zone and crossite and glaucophane in the epidote zone. Sodic amphiboles in metabasalts are iron-rich crossites in weakly-metamorphosed rocks and more-magnesian crossites and glaucophanes in the lawsonite and epidote zones. The abrupt increase in Mg/(Mg+Fe) ratio in sodic amphiboles at the epidote isograd is attributed to the crystallization of epidote and almandine which take the place of lawsonite and spessartine of the lawsonite zone. Calcic amphiboles are fibrous actinolites in the lawsonite zone and grade with increasing Al and Na/Ca ratio into prismatic blue-green hornblendes (barroisites) in the upper epidote zone. In calcic amphiboles, increasing metamorphic grade effects the coupled substitution of (Na+Al) for (Ca+Mg) and a small increases in Fe/Mg ratio; octahedrally and tetrahedrally coordinated Al increases in an approximately 1∶1 ratio. Both the calcic and the sodic amphiboles show an increase in A-site occupancy with increasing metamorphic grade. In two-amphibole assemblages Ti, Mn and K are concentrated in the calcic amphibole. The textural and chemical relations between coexisting calcic and sodic amphiboles are discussed. If the calcic and sodic amphiboles are an equilibrium pair then the data collected from the Ouégoa amphiboles gives a picture of a very asymmetric solvus in the system glaucophane-actinolite-hornblende, i.e. steep-sided to glaucophane and with a gentle slope to the calcic amphibole field; there is no indication of any termination of the solvus under the pressure-temperature conditions of crystallization of the Ouégoa schists.

Notes: Abstract The compositions of metamorphic pyroxenes from blueschists in northern New Caledonia are investigated. Aegerine-augite occurs in siliceous metasediments and aegerine in some low-grade sodic basic schists. Calcic metamorphic pyroxene (omphacite and chloromelanite) appears first in metabasalts in higher grades of the lawsonite zone and is widespread in metamorphosed igneous rocks and quartzofeldspathic gneisses of the epidote zone. Omphacites in basic rocks have higher Mg∶Fe ratios and are less jadeitic than omphacites from adjacent interbedded quartzofeldspathic gneisses. With increasing metamorphic grade pyroxenes become more jadeitic and diopsidic at the expense of their acmite component. Elemental partitioning between coexisting pyroxenes, garnets and amphiboles from in situ regional metamorphic rocks is generally regular, suggesting equilibrium crystallization. Omphacite appears to be a stable phase within blueschist facies over a temperature range of at least 350° to 550° C. The “eclogitic” assemblage almandine-omphacite is stable within the earth's crust in metamorphosed sediments and igneous rocks over a temperature range of 400° to at least 550° C. No estimate of absolute pressures involved in metamorphism in the Ouégoa district can yet be made.

Notes: Abstract 48 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 P total ≠ P lithostatic and that that the pressure of the fluid phase may have exceeded lithostatic pressure.

Notes: Abstract A network of 5 cm wide subplanar zones of garnet-granolite with accessory apatite as the dominant hydrous mineral, is overprinted on basic hornblende-granolites in an area of present and past tectonic uplift. Fracturing and the garnet forming reactions appear to be caused by destabilisation of hornblende, as a hydrous phase, through a critical drop in the P∶T ratio. Whereas the apatites from the hornblende-granolite have normal compositions, apatites from the garnet-granolite zones are among the most chlorine enriched hydroxyfluorapatites known. A later amphibolite facies event has depleted hydrous minerals of fluorine and chlorine, affecting hornblende more strongly than apatite. Based on literature data on F and Cl in coexisting minerals, original hornblende compositions are tentatively restored. It is then possible to derive all the chlorine of the garnet zone apatites from the original rock, with differential loss of H2O and HF over chlorine during the dehydration reaction. Diffusion coefficients would have been larger for H2O and HF than for the large chloride ion, and if the gas phase was in contact with even minor anatectic melts, activity gradients would also have been relatively small for chlorine. Low-Cl scapolite is present in quartz-free pegmatoid veins. Field evidence for a genetic tie between the garnet zones and these veins is inconclusive, but liberation of H2O and HF under granolite facies conditions is likely to have caused limited fusion of the plagioclase. Since many garnet zones do not contain even small pegmatoid veins, melts related to their formation could have collected in veins only after initial diffusion or ‘infiltration’ over a distance, and without leaving segregated mafic residues.