Eclogites and Blueschists of the Pam Peninsula, NE New Caledonia: a Reappraisal

Clarke, G. L.

doi:10.1093/petrology/38.7.843

Cited by 75 publications

(181 citation statements)

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“…Such effects often occur during prograde metamorphism. For example, garnet grains in eclogite and blueschist facies rocks commonly show extensive chemical zoning (Clarke et al ., 1997; Robbo et al ., 1999) as do garnet porphyroblasts in many greenschist to amphibolite facies terranes (Vance & Holland, 1993; Willner et al ., 2000). Factors that may influence this zonation include the comparatively slow nature of intragranular diffusion relative to the growth rate of anhydrous minerals such as garnet, and the transient nature of high‐ P/T metamorphism.…”

Section: Introductionmentioning

confidence: 99%

“…In the New Caledonian eclogites, inclusions of lawsonite, chlorite or glaucophane, which reflect blueschist facies conditions, occur in chemically zoned garnet grains that are texturally equilibrated with matrix eclogite facies assemblages that lack these minerals (e.g. Clarke et al ., 1997). The matrix minerals show restricted compositional ranges that are inferred to reflect extensive recrystallization of these parts of the eclogites either at peak conditions or when the rocks lost free water during decompression (Carson et al ., 2000).…”

Section: Introductionmentioning

confidence: 99%

“…Element mapping undertaken using an electron microprobe provides a powerful tool for examining the nature of chemical zonation in individual crystals and rocks as a whole. Maps of element abundance were prepared for representative samples of the New Caledonian eclogites (Clarke et al ., 1997) following the method of Clarke et al . (2001).…”

Section: Introductionmentioning

confidence: 99%

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Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

Marmo

Clarke

Powell

2002

Journal Metamorphic Geology

235

164

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Chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards. An effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock. To examine this effect on mineral assemblages, the Bence–Albee matrix correction was applied to X‐ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample. The manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix. A series of calculated equilibrium‐volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high‐P conditions. Pressure–temperature pseudosections are calculated for these compositions, in the CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O system. Assemblages, modal proportions and mineral textures observed in the New Caledonian eclogites can be closely modelled by progressively ‘removing’ elements forming garnet cores from the bulk rock composition. The pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage, prograde textures and the development of retrograde assemblages.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

Marmo

Clarke

Powell

2002

Journal Metamorphic Geology

235

164

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“…1994, 2001; Aitchison et al . 1995; Clarke et al . 1997), and a northeast‐dipping subduction system was established beneath the Loyalty Basin.…”

Section: Regional Geologymentioning

confidence: 99%

Very low‐grade metamorphic study in the pre‐Late Cretaceous terranes of New Caledonia (southwest Pacific Ocean)

Potel¹

2007

Island Arc

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Pre-Late Cretaceous terranes from the central part of New Caledonia have been metamorphosed under very low-grade conditions by two high-pressure/low-temperature events. The present study investigates the metamorphic patterns with phyllosilicate crystallinities, electron microprobe analyses and petrography. The first metamorphic event is of Late Jurassic age and is characterized by very low (anchizone) to low-grade (epizone) conditions with a decrease of the illite Kübler Index (KI) and the chlorite Árkai Index (ÁI) values from northeast to southwest. This trend is also confirmed by chlorite thermometry. In the south of the area, un-metamorphosed sediments (diagenetic KI values) are observed in the Senonian 'formation à charbons', post-dating the metamorphism in this region. The second metamorphism is an Eocene high-pressure event, which overprints the Late Jurassic metamorphism in the northern part of the studied area. In this zone, the pattern of KI and ÁI indicates another gradient with increasing metamorphic conditions from southwest to northeast. Temperatures calculated by chlorite thermometry also indicate an evolution from southwest to northeast with slight increase of temperature from 298 Ϯ 8°C to 327 Ϯ 16°C. In both metamorphic zones, the K-white mica b cell dimension calculated on micas analyzed at electron microprobe are in good agreement with high-pressure/lowtemperature metamorphic conditions (b 0 > 9.04 Å). A combination of chlorite thermometry and K-white mica b cell dimension allows estimation of a minimum pressure of 1.3 GPa in the Eocene zone (in excellent agreement with the 1.5 GPa registered in the northern part of New Caledonia) and a minimum of 1.1 GPa in the Late Jurassic metamorphic part.

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“…3a; Appendix 3) because most of the following discussion concerns amphibole composition. The chemistry of sodic‐calcic amphibole can vary from actinolite through winchite to barroisite (Clarke et al ., 1997); for the case of simplicity, they will be referred to as hornblende. To examine hornblende compositional variations in detail, all other minerals may be excluded and a new matrix generated by selecting only data that have between 6.23 and 7.95 Si cations per formula unit (pfu), and more than 2.16 Mg cations pfu.…”

Section: Example: Barroisitic Eclogite From New Caledoniamentioning

confidence: 99%

A method for applying matrix corrections to X‐ray intensity maps using the Bence–Albee algorithm and Matlab

Clarke

Daczko

Nockolds

2001

Journal Metamorphic Geology

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Matlab scripts that apply the Bence & Albee (1968) matrix correction algorithm to X‐ray intensity data collected as element maps on a Cameca SX‐50 microprobe are used to produce two‐dimensional maps of oxide weight percent and cation proportions for SiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O and K2O. Once generated, large data sets of mapped oxide weight percent values or cation numbers that retain spatial information can be used petrologically. The technique is used to evaluate the compositional range of barroisitic amphibole in an eclogite from New Caledonia, to examine aspects of equilibration during the partial hydration of the eclogite facies mineral assemblage.

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Eclogites and Blueschists of the Pam Peninsula, NE New Caledonia: a Reappraisal

Cited by 75 publications

References 0 publications

Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

Very low‐grade metamorphic study in the pre‐Late Cretaceous terranes of New Caledonia (southwest Pacific Ocean)

A method for applying matrix corrections to X‐ray intensity maps using the Bence–Albee algorithm and Matlab

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