Journal of Petrology

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Journal of Petrology | Volume 44 | Number 10 | Pages 1805-1831 | 2003
© Oxford University Press 2003; all rights reserved

Lawsonite–Omphacite-Bearing Metabasites of the Pam Peninsula, NE New Caledonia: Evidence for Disrupted Blueschist- to Eclogite-Facies Conditions

J. A. FITZHERBERT^1,*, G. L. CLARKE¹ and R. POWELL²

¹ SCHOOL OF GEOSCIENCES, THE UNIVERSITY OF SYDNEY, SYDNEY, NSW 2006, AUSTRALIA
² SCHOOL OF EARTH SCIENCES, THE UNIVERSITY OF MELBOURNE, MELBOURNE, VIC., AUSTRALIA, 3052

^* Corresponding author. E-mail: joel{at}es.usyd.edu.au

RECEIVED NOVEMBER 20, 2001; ACCEPTED MARCH 28, 2003

	ABSTRACT

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
The Diahot terrane of NE New Caledonia contains an interbedded sequence of Cretaceous to Eocene metasediments, felsic and mafic metavolcanics that experienced c. 40 Ma high-P/T metamorphism. Metabasaltic assemblages define two prograde events (M₁ and M₂) and a tectonically disrupted crustal profile that extends from lawsonite–blueschist conditions in the SW to paragonite–eclogite conditions in the NE. Weakly deformed metabasalts from lowest-grade parts of the Diahot terrane contain M₁ omphacite, chlorite, lawsonite and glaucophane-bearing assemblages that partially pseudomorph igneous plagioclase and augite, and reflect P = 0·7–1·0 GPa and T = 350–400°C. M₁ assemblages are enveloped by a steeply SW-dipping S₂ foliation that becomes progressively more intense towards the NE over a distance of c. 15 km. S₂ assemblages are divided into four zones: (1) lawsonite–omphacite; (2) lawsonite–clinozoisite–spessartine; (3) clinozoisite–hornblende–almandine; (4) almandine–omphacite. S₂ assemblages reflect a P–T gradient that spans the exposed 15 km of the Diahot terrane from P = 0·8–1·0 GPa and T = 350–400°C (Zone 1) to P = 1·6–1·7 GPa and T = 550–600°C (Zone 4). The systematic mineralogical changes reflect parts of a P–T array between 1·0 and 1·7 GPa that was extensively disrupted by tectonic thinning during exhumation.

KEY WORDS: blueschist; eclogite; New Caledonia; CNFMASH; pseudosection

	INTRODUCTION

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
The structurally coherent Cretaceous to Eocene high-P metasediments and metavolcanics of the Diahot terrane (Fig. 1) have been the subject of numerous petrological studies (e.g. Brothers & Blake, 1972; Black & Brothers, 1977; Brothers & Yokoyama, 1982; Bell & Brothers, 1985; Brothers, 1985; Yokoyama et al., 1986; Ghent et al., 1987a, 1987b; Black et al., 1988; Maurizot et al., 1989; Clarke et al., 1997). The majority of work concentrated on the high-P regional metamorphic zones, which include strongly deformed metasedimentary lithologies involving lawsonite–albite-bearing schists in the SW and omphacite–garnet-bearing schists in the NE. The Diahot terrane is thought to be tectonically underlain by metabasic eclogites of the Pouébo terrane (Figs 1 and 2a; Clarke et al., 1997; Carson et al., 1999, 2000; Marmo et al., 2002), although relationships between the Diahot terrane blueschists and underlying Pouébo terrane eclogites are controversial. The sequence has been interpreted as representing an intact crustal profile, but the increase in metamorphic grade occurs over such a comparatively short distance that it can only be explained by anomalous pressure and/or temperature gradients (Brothers, 1970, 1985; Yokoyama et al., 1986). On the basis of contrasting P–T estimates obtained from metasedimentary blueschists of the Diahot terrane (P = 1·2 ± 0·1 GPa and T = 570 ± 36°C) and metabasic eclogites of the Pouébo terrane (P = 1·9 GPa and T 600°C), Clarke et al. (1997) inferred that the contact between the Diahot and Pouébo terranes juxtaposed unrelated sections of a subduction zone. Those workers suggested that many of the eclogitic metabasalts within the Diahot terrane represent tectonically derived high-P windows emplaced during late-stage faulting. In the light of new petrological data from the Diahot terrane metabasites, this study re-evaluates the metamorphic zonation and inferred prograde mineralogical changes, and redefines the Eocene P–T profile exposed in NE New Caledonia. A critical aspect of the re-evaluation is the identification of prograde assemblages involving lawsonite and omphacite, which patchily recrystallize coarsely intergrown plagioclase and augite phenocrysts in lawsonite-zone metabasalts. Early omphacite and lawsonite persist in domains of low strain in epidote-zone blueschists, and relics of the igneous texture can be recognized in what have been previously mapped as eclogite-facies metabasite. This study confirms the excellent work done by previous researchers (e.g. Brothers, 1970; Black, 1977; Yokoyama et al., 1986) and the systematic mineralogical changes documented below reflect parts of a blueschist- to eclogite-facies P–T array between 1·0 and 1·7 GPa, which was extensively disrupted by tectonic thinning.

View larger version (45K): [in this window] [in a new window]   Fig. 1. Simplified geological map of New Caledonia, illustrating the major tectonostratigraphic units of the island (after Paris, 1981; Cluzel et al., 1994; Aitchison et al., 1995a; Clarke et al., 1997).

 

View larger version (39K): [in this window] [in a new window]   Fig. 2. (a) Regional map of NE New Caledonia, indicating the distribution of the four regional metamorphic zones and the locations of samples discussed in the text (after Maurizot et al., 1989; Clarke et al., 1997; Rawling & Lister, 2002). (b) Detailed map of the Ouégoa area showing the distribution of metabasic lenses within Zones 1–2b and the locations of samples discussed in the text (after Clark, 1996).

 

	REGIONAL GEOLOGY

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
New Caledonia consists of a mosaic of Cretaceous to Palaeogene terranes that developed along, or were accreted to, the northeastern margin of a rifted fragment of the Australian plate (Aitchison et al., 1995b; Cluzel et al., 1995; Fig. 1). The basement of New Caledonia (Fig. 1) is transgressively overlain by upper Cretaceous to Eocene sedimentary rocks deposited during Gondwana dispersal (Paris, 1981; Aitchison et al., 1995a; Cluzel et al., 1995). Blueschist- to eclogite-facies rocks are exposed in the NE of the island (Fig. 1) and are inferred to have experienced high-P metamorphism in response to the attempted subduction of the crustal fragment that forms the basement terranes of New Caledonia into an Eocene oceanic island-arc subduction system (Brothers & Blake, 1972; Lillie, 1975; Black, 1977; Ghent et al., 1987a, 1987b; Clarke et al., 1997). Two nappes were thrust southwestwards over the basement terranes at this time: (1) the ‘formation de basaltes’ nappe (Paris, 1981), comprising dolerite and pillow basalt overlain by siliceous Late Cretaceous age shales (Aitchison et al., 1995a); (2) an extensive ophiolitic nappe (New Caledonian Ophiolite) that dominates outcrop in the south of the island (Fig. 1). A range of ages, with clusters around 100–77 and 48 Ma, has been documented from the ophiolite (Prinzhofer, 1981) indicating a complex history. The Cretaceous ages are interpreted to represent ophiolite formation whereas the Eocene ages reflect thermal overprinting or degassing associated with obduction (Ghent et al., 1994; Aitchison et al., 1995a).

High-P medium-T rocks of NE New Caledonia may be subdivided into two major terranes (after Cluzel et al., 1994): (1) the Diahot terrane includes Cretaceous to Eocene sediments and volcanics, which experienced peak metamorphic conditions of P 1·2 GPa and T 550°C, that correlate with tectonically disrupted Cretaceous to Eocene sedimentary and volcanic rocks further west that were not significantly affected by the Eocene metamorphism (Maurizot et al., 1989; Figs 1 and 2); (2) the Pouébo terrane, consisting of metabasic eclogite and glaucophanite, which experienced peak metamorphic conditions of P 1·9 GPa and T 590°C. Garnet glaucophanite formed at P = 1·9–1·4 GPa during semi-pervasive fluid influx and recrystallization of the Pouébo terrane (Carson et al., 2000). Later fluid infiltration after further isothermal decompression was focused in shear zones and resulted in chlorite–albite-bearing greenschist-facies assemblages at P 0·9 GPa. ⁴⁰Ar/³⁹Ar studies of white micas from the two terranes yielded consistent cooling ages of 37 ± 1 Ma (Ghent et al., 1994). Previous work documented a progressive northeastward increase in the metamorphic grade of rocks forming the Diahot terrane (Black, 1977; Brothers & Yokoyama, 1982).

The Diahot terrane
The study area (Fig. 2a and b) encompasses the lawsonite to epidote–omphacite zones of Black & Brothers (1977) and Yokoyama et al. (1986) and/or ferro-glaucophane–lawsonite zone, albite–epidote–omphacite zone, and Type II eclogite of Clarke et al. (1997). Petrographic detail of the Pouébo terrane has been covered elsewhere (e.g. Clarke et al., 1997; Carson et al., 1999, 2000).

Textural and mineralogical changes in metabasic lenses distributed throughout most of the Pam–Ouégoa section of the Diahot terrane (Fig. 2a and b), allow the resolution of two discrete prograde metamorphic events (M₁ and M₂). Along the southwestern boundary of the Diahot terrane, M₁ omphacite, chlorite, lawsonite and glaucophane partially to completely pseudomorph igneous plagioclase and augite in weakly deformed metabasalts. M₁ assemblages are progressively enveloped by a SW-dipping S₂ foliation that becomes more intense towards the NE over a distance of c. 15 km. Although S₂ assemblages dominate in most rocks, domains up to 2 cm across that contain relic M₁ mineral assemblages and igneous textures are preserved within most metabasites (see below).

Mineral assemblages defining S₂ vary markedly across the terrane and can be subdivided into four M₂ zones: (1) lawsonite–omphacite–glaucophane; (2) lawsonite–clinozoisite–spessartine; (3) clinozoisite–almandine–hornblende; (4) almandine–omphacite. Metabasalts from Zone 2 are much coarser grained than those from Zone 1, and the boundary between the two zones marks a break between a weak disjunctive S₂ foliation (Zone 1) and a moderate to strong S₂ schistosity (Zone 2). The contact between Zone 1 and Zone 2 (Fig. 2b) is a SW-dipping normal shear zone (Gendamerie Fault, Espirat, 1965; Clarke et al., 1997). Directly north of this shear zone, in the southwestern Pam Peninsula, Zone 1 lithologies are juxtaposed with those from Zone 3 (Fig. 2a). The juxtaposition of Zones 1 and 3 suggests that there was appreciable, post-S₂ tectonic thinning. This resulted in the excision of a substantial portion of the metamorphosed crustal profile (Fig. 2a). A faulted boundary is not obvious between Zones 2 and 3, and it would seem that the zone boundary represents distinctive mineralogical changes that reflect changing P and T conditions across a near-intact portion of the crustal profile. The boundary between Zones 3 and 4 marks a noticeable change in the intensity of the S₂ foliation, from moderately deformed Zone 3 metabasites to intensely deformed mafic gneisses of Zone 4. Clarke et al. (1997) inferred a faulted contact between the mafic gneisses of Zone 4 and the interbedded metasedimentary and metabasic lithologies of Zone 3 (Fig. 2a).

S₂ assemblages in Zones 1–4 may be partially to completely pseudomorphed by combinations of S₃ albite, randomly oriented anhedral phengite, clinozoisite or chlorite. These features reflect the weak effects of a greenschist-facies overprint that has been discussed elsewhere (e.g. Yokoyama et al., 1986; Clarke et al., 1997; Carson et al., 1999, 2000).

	PETROLOGY OF DIAHOT TERRANE METABASITES

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
As M₁ assemblages persist only patchily, references are made below to the degree of their preservation in each zone. Because of their fine grain size and complexity, quantitative element maps provide the best illustration of many mineral textures in the metabasites, as illustrated in grey-scale in Figs 3–6 (full colour versions of these images may be downloaded from the Journal of Petrology website at http://www.petrology.oupjournals.org/). Maps of X-ray intensity were collected using the Cameca SX-50 electron microprobe housed at the Electron Microscope Unit, University of New South Wales. Each wavelength-dispersive spectrometer of the microprobe was set to a wavelength appropriate for recording X-ray intensity as the sample stage was stepped through representative areas of the samples. Maps with 512 x 512 or 256 x 256 analyses were collected at 4 or 6 µm steps for the major oxides: SiO₂, Al₂O₃, FeO, MnO, MgO, CaO, Na₂O and K₂O. The maps were collected with a 15 kV accelerating voltage, a 1–3 µm beam size and count times of 300 ms at each point. Clarke et al. (2001) developed a matrix correction algorithm based on the empirical -factor approach of Bence & Albee (1968), which converts maps of X-ray intensity to maps of oxide weight percent and cation proportion. This method was applied in the maps used below.

View larger version (150K): [in this window] [in a new window]   Fig. 3. (a) Igneous textural characteristics of a Zone 1 metabasalt. The dark mineral is dominantly augite, whereas the interlocking pale rectangular aggregates comprise lawsonite, chlorite and glaucophane. Crossed polars, sample 2008; width of photomicrograph is 4 mm. (b) Close-up photomicrograph of a composite clinopyroxene grain with a relic augite core partially pseudomorphed by omphacite. The augite cores are dark and the omphacite rims are pale to medium grey. Crossed polars, sample 2008; width of photomicrograph is 2 mm. (c) Maps of Na2O and FeO weight percent for part of the Zone 1 metabasalt RS3b. The map shows interfingering clinopyroxene with omphacite rims on augite (unmarked white arrows on the Na2O and FeO maps) and rectangular lawsonite clusters pseudomorphous after plagioclase. Small irregular albite grains can be seen in contact with omphacite and glaucophane (labelled on the Na2O map). Black, unlabelled vertical arrows on the FeO map indicate chlorite partially pseudomorphing augite. (Map is 1·5 mm wide.)

 

View larger version (102K): [in this window] [in a new window]   Fig. 4. (a) Maps of CaO and FeO weight percent covering an area 2 mm across over the S2 assemblage epidote, actinolite, glaucophane, chlorite, garnet and lawsonite in the Zone 2a metabasalt sample 96305b. S2 epidote and garnet envelop fine-grained lawsonite. The white unlabelled arrow in the bottom right indicates a small, relic M1 omphacite grain. Poikiloblastic D3 albite contains inclusions of the S2 minerals in the FeO map. (b) (I) D2 low-strain domain with M1 assemblages in a Zone 2a metabasalt, enveloped by the S2 assemblage epidote, lawsonite, glaucophane, actinolite and chlorite. The dark mineral in the low-strain M1 domain is omphacite, and the interfingering rectangular grains are formed from intergrowths of glaucophane and lawsonite. Crossed polars, sample 96305b; width of photomicrograph is 4 mm. (II) Maps of CaO and FeO weight percent of the Zone 2a metabasalt sample 96305b. The maps cover an area 2 mm across over the M1 low-strain domain represented in (I).

 

View larger version (109K): [in this window] [in a new window]   Fig. 5. (a) Maps of FeO and Na2O weight percent over an area 1·5 mm across covering part of a large S2 epidote grain from the Zone 2b metabasalt sample 96322B2. The FeO map illustrates small, idioblastic lawsonite inclusions in a large optically zoned M2 epidote grain. The Na2O map illustrates M1 omphacite, augite and albite inclusions in the large M1 epidote grain. Dashed white lines on the Na2O map represent the outline of the large epidote grain. (b) Epidote pseudomorphs after lawsonite within the core of a large optically zoned M2 epidote grain. Unlabelled white arrows point to relic lawsonite inclusions. Crossed polars, sample 96322B2; width of photomicrograph is 2 mm. (c) Zoned garnet grain (extinct) illustrating inclusions of lawsonite (arrowed) in the core zone. Coarse epidote envelops the garnet grain and itself contains fine lawsonite inclusions (unlabelled black arrow). Crossed polars, sample 96347; width of photomicrograph is 2 mm. (d) Maps of FeO and CaO weight percent over an area 4 mm across in a D2 low-strain domain in sample 96347, a Zone 2b metabasalt, with interfingering omphacite and rectangular clusters of lawsonite. Omphacite within the low-strain domain is surrounded by a continuous white line. The unlabelled white arrow at the bottom right of the FeO map points to small phengite flakes that, with chlorite, partially replace omphacite. Small, sawtooth-like intergrowths of glaucophane protrude into the lawsonite clusters.

 

View larger version (142K): [in this window] [in a new window]   Fig. 6. (a) Maps of MgO and Na2O weight percent over an area 2 mm across in part of D2 low-strain domain in the Zone 3 metabasalt WC16, with interfingering omphacite and rectangular glaucophane aggregates. A large hornblende grain partially pseudomorphs omphacite. The enveloping S2 foliation is evident at the bottom right-hand corner of the map. The unlabelled white arrows on the Na2O and MgO maps point to phengite pseudomorphing M1 omphacite. (b) Interlocking clinozoisite and omphacite in a D2 low-strain domain from Zone 3. Crossed polars, sample 9925; width of photomicrograph is 2 mm. (c) Large S2 idioblastic omphacite laths and glaucophane from Zone 4. Plane-polarized light, sample 9408c; width of photomicrograph is 4 mm.

 
Zone 1: lawsonite blueschist (lawsonite–omphacite–glaucophane)
Metabasalts of this zone partially retain igneous textures and mineralogy owing to the patchy nature of recrystallization and impersistent effects of deformation (Fig. 3a). Augite occurs in most metabasalts (e.g. samples RS3a, RS3b, 2008, 9911) as euhedral phenocrysts up to 0·5 cm across. It is partially pseudomorphed by clumps of granular titanite and commonly has a pale green rim of intergrown omphacite and chlorite (Fig. 3b and c). These intergrowths of omphacite, chlorite and augite are, in turn, enclosed by a narrow rim of glaucophane and/or actinolite. Igneous plagioclase is less commonly preserved. In most samples, rectangular clusters of fine-grained lawsonite are inferred to pseudomorph plagioclase. Rare samples (e.g. sample RS3a) preserve an igneous plagioclase core to the lawsonite clusters, which also contain quartz, chlorite, minor albite and rare paragonite (e.g. RS3b). The rectangular outlines may interfinger with relict augite (Fig. 3b and c), where lawsonite may be in direct contact with omphacite. However, in many samples the two minerals are separated by sawtooth-like intergrowths of glaucophane and/or actinolite.

Domains of high D₂ strain (e.g. samples RS213, 9911) contain a weakly aligned mosaic comprising combinations of lawsonite, glaucophane, actinolite, chlorite and titanite, with or without omphacite, phengite and relict augite. Augite is always enclosed by pale green actinolite or omphacite, which, in turn, is enclosed by a rim of glaucophane. Where augite–omphacite cores are absent, individual amphibole grains are zoned from a pale green actinolitic core to a pale blue glaucophane rim. Lawsonite in these samples is coarser grained than in the low-strain domains; it occurs as large rectangular grains up to 0·4 cm in length, which may or may not be weakly aligned in S₂. Some, or all of, the omphacite, augite, glaucophane and chlorite may occur as inclusions in S₂ lawsonite.

Zone 2: epidote blueschist (lawsonite–clinozoisite–garnet)
Zone 2 is defined by the first appearance of clinozoisite with lawsonite in S₂, which is accompanied by the appearance of spessartine-rich garnet in some lithologies. Zone 2b is defined by the loss of lawsonite from S₂ assemblages, a change that is accompanied by the appearance of almandine-rich garnet in some lithologies.

Zone 2a (lawsonite–clinozoisite–spessartine)
The fault-bounded southwesterly limit of this zone marks a pronounced grain size coarsening, and the development of a semi-pervasive SW-dipping S₂ foliation in many metabasalts. S₂ assemblages include clinozoisite, glaucophane, chlorite and lawsonite, with or without spessartine-rich garnet, omphacite, actinolite, phengite, quartz and titanite (Fig. 4a and b). Lawsonite occurs in S₂, and as inclusions in S₂ clinozoisite and garnet. Fine (<0·5 mm) spessartine-rich garnet is present only in some lithologies (e.g. 96305b), and always in small proportions. S₂ omphacite is uncommon, and is aligned with chlorite and large lawsonite laths (e.g. sample RS66). The S₂ foliation may envelop centimetre-sized domains of comparatively low D₂ strain, which preserve M₁ mineral assemblages and igneous textural characteristics [e.g. sample 96305b; Fig. 4b(II)]. Although mineral assemblages and textural features within these domains are similar to those described for Zone 1, subtle differences are observed. In most cases, M₁ omphacite completely pseudomorphs igneous augite. Sawtooth-like intergrowths of glaucophane are the dominant constituent of the rectangular lawsonite–glaucophane clusters, and glaucophane always separates M₁ omphacite and lawsonite (Fig. 4d). Coarse-grained clinozoisite has lawsonite inclusions (Fig. 5a).

Zone 2b (clinozoisite–almandine)
All minerals defining S₂ in metabasalts of Zone 2b continue the progressive grain size coarsening. The intensity of S₂ varies in individual metabasalt lenses, with centimetre- to metre-sized domains of high D₂ strain enveloping domains of lower D₂ strain. S₂ in Zone 2b is defined by combinations of clinozoisite, glaucophane and chlorite with or without actinolite, almandine-rich garnet, phengite, titanite, minor rutile and quartz. Importantly, omphacite is never observed defining S₂. Where present, garnet in Zone 2b metabasalts is substantially larger than the spessartine-rich grains in Zone 2a. Garnet cores have inclusions of some, or all of, lawsonite, glaucophane, chlorite, clinozoisite and rare anhedral omphacite and titanite (Fig. 5c). Minerals occurring as inclusions in garnet rims involve combinations of glaucophane, actinolite, clinozoisite and omphacite. Large, euhedral clinozoisite poikiloblasts are abundant in all Zone 2b metabasalts. They may be optically zoned and have inclusions of some, or all, of lawsonite, glaucophane, titanite, augite, albite and omphacite (Fig. 5a). Although lawsonite is generally preserved as inclusions in clinozoisite, an interesting feature of cores in many clinozoisite grains is the partial to complete pseudomorphing of lawsonite inclusions by clinozoisite. The pseudomorphous epidote retains the square form of the lawsonite grain (Fig. 5b). Glaucophane, chlorite and omphacite may occur as inclusions in the comparatively inclusion-poor clinozoisite rims.

In areas of low D₂ strain, S₂ envelops centimetre-sized elliptical domains that preserve M₁ mineral assemblages dominated by intergrown omphacite, glaucophane and clinozoisite (Fig. 5d). Fine trails of M₁ lawsonite may be contained within these domains and are always enveloped by, or included in, glaucophane and clinozoisite. In some samples (e.g. 96417), M₁ omphacite is partly recrystallized to combinations of actinolite, chlorite and phengite.

Zone 3: epidote–hornblende blueschist (clinozoisite–almandine–hornblende)
Zone 3 is defined by the appearance of barroisitic hornblende in all metabasites, although the exact Zone 2–Zone 3 boundary is uncertain, because of limited metabasite outcrop in the appropriate areas. As a result of the intensity of D₂ recrystallization, we are uncertain that mafic rocks in Zone 3 and above were basalts, so they are referred to as metabasite. However, the similarity of textures and semi-continuous nature of mineral assemblages suggest that metabasites in Zones 3 and 4 are equivalent to the metabasalts of Zones 1 and 2. S₂ in Zone 3 is defined by combinations of clinozoisite, hornblende, phengite, glaucophane, almandine-rich garnet, rutile and titanite with or without paragonite. Clinozoisite occurs as abundant, large idioblastic grains in all metabasites, which are comparatively inclusion-poor and contain inclusions of fine-grained glaucophane, hornblende and less commonly omphacite. Quartz and phengite/paragonite are modally more abundant than in previous zones, and rutile is enclosed by titanite as fine lamellae defining S₂. Garnet mode increases markedly in Zone 3 and individual grains may be complexly zoned. The cores of large idioblastic grains >2 cm in diameter are dominated by intergrown garnet and titanite, commonly enclosed by an inclusion-poor zone. Within, or immediately adjacent to the titanite-rich cores, are inclusions of some, or all of, glaucophane, actinolite/winchite, clinozoisite, quartz and lawsonite. Where present, rutile inclusions occur in the outer, relatively inclusion-poor zone of the garnet.

As in Zone 2, S₂ envelops elongate domains (up to 2 cm across) of low D₂ strain that preserve M₁ mineral assemblages and igneous textural features. Within these domains, M₁ omphacite is less abundant than in Zone 2; it is partially pseudomorphed by hornblende, phengite and paragonite (Fig. 6a). Omphacite and hornblende within these domains interfinger with rectangular glaucophane clusters (after M₁ lawsonite) and/or complex dendritic to rectangular clinozoisite–paragonite intergrowths (e.g. sample 9925, Fig. 6b). Importantly, lawsonite is observed only as an inclusion phase in coarse garnet from Zone 3 metabasites and is never observed in equilibrium with the matrix, even in domains of low D₂ strain.

Zone 4: hornblende–paragonite eclogite (clinozoisite–almandine–omphacite)
Zone 4 is the highest grade of Diahot metabasites (Fig. 2a), representing lithologies previously termed Type II eclogite by Clarke et al. (1997). The zone boundary is defined by the reappearance of omphacite, now defining S₂. The petrological characteristics of metabasites within this zone do not differ considerably from those of Zone 3, except for the presence of discrete layers (e.g. sample 9408c) that contain abundant omphacite aligned in S₂. S₂ is a well-layered gneissosity, defined by alternating, millimetre- to centimetre-scale garnet–quartz and omphacite–amphibole-rich domains. S₂ assemblages involve combinations of clinozoisite, glaucophane, hornblende, phengite/paragonite (paragonite is modally more significant than phengite), garnet, quartz, rutile, with or without omphacite. Layers rich in S₂ omphacite are deficient in hornblende and clinozoisite compared with surrounding layers, and in these layers M₁ omphacite exists only as inclusions in S₂ garnet. Idioblastic S₂ omphacite laths within these layers are optically and chemically zoned and may contain oriented clinozoisite and glaucophane inclusions (Fig. 6c). Where S₂ omphacite is in low abundance or not present within Zone 4 metabasites (e.g. sample 9409b), centimetre-sized, elongate domains of low D₂ strain are enveloped by S₂. These domains preserve igneous textures, and contain combinations of barroisite, glaucophane and clinozoisite with or without fine-grained relict M₁ omphacite. Lawsonite was not observed in low-strain domains of Zone 4 metabasites and its presence as an inclusion phase in coarse garnet in metabasites from this zone is inferred from ‘box-shaped’ aggregates of clinozoisite and paragonite.

	MINERAL CHEMISTRY OF THE METABASITES

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
Analyses were performed on the Cameca SX-50 Camebax microprobe, at the University of New South Wales, operating with an accelerating voltage of 15 kV, a beam width of 1–5 µm and PAP data reduction techniques supplied by the manufacturer. The major features of mineral compositions in metabasite samples used for thermobarometry are presented below (see also Black, 1973a, 1973b; Clarke et al., 1997) and representative microprobe analyses are presented in Table 1. Many of the minerals are compositionally zoned, and there may be restricted ranges in mineral composition of individual minerals in any one sample. Generalizations are made below for the case of simplicity, but reference is given to type examples on which the generalization is based to allow cross referencing.

View this table: [in this window] [in a new window]   Table 1: Representative microprobe analyses of mineral assemblages in metabasites of Zones 1–4

 
Garnet
Garnet is invariably zoned with cores enriched in spessartine, but the nature of the zoning varies with grain size and metamorphic grade (Fig. 7a; Black, 1973a; Clarke et al., 1997). Garnet in Zone 2a metabasalts is spessartine-rich, pyrope-poor and preserves weak compositional zoning from cores with X_spess = Mn/(Fe + Mn + Mg + Ca) 0·24–0·27 and X_alm = Fe/(Fe + Mn + Mg + Ca) 0·35–0·38 to rims with X_spess 0·20–0·21 and X_alm 0·36–0·41 [Fig. 7a(i) and b(i)]. Pyrope and grossular contents vary only subtly through this profile with X_py = Mg/(Fe + Mn + Mg + Ca) 0·02 and X_gross = Ca/(Fe + Mn + Mg + Ca) 0·30–0·36 (both core to rim). Garnet in Zone 2b has more pronounced, complex zoning profiles [Fig. 7a(ii) and b(i)]. Many grains preserve abrupt steps in the zoning profile, coinciding with changes in the type of mineral inclusions [Fig. 7a(ii) and b(i)]. The cores of large garnet grains have lawsonite and ferro-glaucophane inclusions and compositions in the range of X_spess 0·41–0·25, X_alm 0·28–0·35, X_py 0·02, and X_gross 0·26–0·36. With increasing distance from the core there is a decrease in X_spess 0·12–0·16 and an increase in X_alm 0·49–0·52, X_gross 0·26–0·36 and X_py 0·036–0·12. This trend continues until a decrease in X_gross 0·19–0·29 and X_alm 0·30–0·44 and minor increase in X_py 0·036–0·12. This step in the zoning profile coincides with the appearance of clinozoisite and loss of lawsonite inclusions to the host garnet [Fig. 7a(ii) and b(i)]. The thin clinozoisite-bearing rim has an increased almandine content (X_alm 0·46), whereas X_spess and X_py remain constant up to the grain boundary. Most grains display a further increase in X_gross 0·30–0·36 towards the grain boundary.

View larger version (24K): [in this window] [in a new window]   Fig. 7. (a) Representative zoning profiles of Xalm = Fe/(Fe + Mn + Mg + Ca), Xspess = Mn/(Fe + Mn + Mg + Ca), Xpy = Mg/(Fe + Mn + Mg + Ca) and Xgross = Ca/(Fe + Mn + Mg + Ca) through garnet grains from metabasalts in: (i) Zone 2a, sample 96305b; (ii) Zone 2b, sample 96417; (iii) Zone 3, sample 9950; (iv) Zone 4, sample 9408c. The garnet grains are approximately 0·2 mm (96305b), 1·5 mm (96417) and 2·5 mm (9950, 9408c) in diameter, with analyses spaced evenly through the grains. (b) Ternary plots with apices Xgross, Xspess and (Xalm + Xpy)/2 showing core-to-rim zoning profiles for single garnet grains from metabasalts samples of Zones 2a, 2b, 3 and 4. The profiles are taken from the same grains as in (a) as well as a number of additional grains.

 
The cores of large, idioblastic garnet grains in Zones 3 and 4 are comparatively enriched in almandine content compared with garnet in Zone 2, and display complex zonation profiles [Fig. 7a(iii) and b(ii)]. Subtle changes in the relative proportions of X_alm and X_gross from core to rim coincide with major changes in mineral inclusions. Garnet in Zone 3 and 4 metabasites may preserve inclusion-rich cores of X_alm 0·55, X_spess 0·10, X_py 0·05, X_gross 0·30 intergrown with lawsonite (or clinozoisite–paragonite pseudomorphs after lawsonite in Zone 4), glaucophane, and titanite (e.g. sample WC16 and 9409b). Where present, Zone 3 spessartine-rich cores preserve a subtle step to a thick inner zone of X_alm 0·50, X_spess <0·05, X_py 0·10, X_gross 0·40, that coincides with the disappearance of lawsonite as an inclusion in garnet [Fig. 7a(iii) and b(ii)]. Almandine and grossular increase from the inner zone to inclusion-poor garnet shoulders of X_alm 0·60, X_spess 0·05, X_py 0·10, X_gross 0·25, and then to rims of X_alm 0·58, X_spess 0·02, X_py 0·12, X_gross 0·29 that lack titanite but contain minor rutile [Fig. 7a(iii) and b(ii)].

Clinopyroxene
Clinopyroxene in Diahot metabasites is omphacite or augite. Compositions were recalculated following Morimoto (1988) and plotted on a ternary diagram with apices jadeite, acmite and diopside + hedenbergite (Fig. 8a–c). Clinopyroxene in Zone 1 metabasalt preserves complex textural relationships. The core of large clinopyroxene grains is always augite and may be surrounded by an outer rim of M₁ omphacite (Fig. 8a–c). Thick M₁ omphacite rims (e.g. samples 9911 and RS3b) enclosing augite may be zoned outwards from Jd₁₆ to Jd_32–59 at consistently low acmite content (Fig. 8a). In some samples (e.g. RS3b) an inner omphacite rim involving Acm₃₀(Di + Hed)₃₆ surrounds an augite core and may be zoned to an outer omphacite rim involving Acm₁₈(Di + Hed)₄₁ at slightly increased jadeite content. Minor between-grain and between-sample trends mostly involve decreasing acmite content at constant or increasing jadeite and nearly constant diopside + hedenbergite content [Jd₂₃Acm₃₇(Di + Hed)₄₀ to Jd₆₀Acm₁(Di + Hed)₃₉; Fig. 5a]. Large M₁ omphacite grains in Zone 2a metabasalts display a restricted compositional range [Jd₃₆Acm₁(Di + Hed)₅₃ to Jd₅₆Acm₇(Di + Hed)₃₇; Fig. 8b]. S₂ omphacite in sample RS66 is the exception and some grains may be zoned from cores enriched in diopside + hedenbergite relative to jadeite [Jd₁₉Acm₇(Di + Hed)₇₄], to outer rims enriched in jadeite [Jd₅₀Acm₂(Di + Hed)₄₈; Fig. 8b]. The majority of Zone 2b omphacite grains show no systematic zonation or between-grain variation; they are Jd₅₂Acm₄ (Di + Hed)₄₄ to Jd₄₂Acm₃(Di + Hed)₅₅ (Fig. 8b). M₁ omphacite in Zone 3 has a composition similar to omphacite in Zone 2; individual grains are mostly unzoned [Jd₄₅Acm₁(Di + Hed)₅₄ to Jd₄₀Acm₁(Di + Hed)₅₉; Fig. 8c]. Some grains lie outside this range [e.g. sample 9950, Jd₂₉Acm₆(Di + Hed)₆₅; Fig. 8c]. Zone 4, S₂ omphacite laths (e.g. sample 9408c) have higher jadeite contents than all previous zones, and are strongly reverse zoned from cores of Jd₇₅ to rims of Jd₅₁ (Fig. 8c).

View larger version (28K): [in this window] [in a new window]   Fig. 8. Ternary clinopyroxene plots for jadeite (NaAlSi2O6), acmite (NaFe3+Si2O6) and diopside + hedenbergite (CaMgSi2O6 + CaFeSi2O6) showing omphacites from Zones 1, 2, 3 and 4. Clinopyroxene analyses were recalculated following the method of Morimoto (1988). Primary augite and Ca-rich clinopyroxenes from Zones 1 and 2 are also illustrated.

 
Epidote group minerals
Epidote group minerals show a pronounced decrease in ferric iron content across the Diahot terrane, ranging from Cz = (Al – 2)/(Al – 2 + Fe³⁺) = 0·33 in Zone 2 metabasalt, to almost pure clinozoisite in Zone 4 metabasite. Ferric iron contents in epidote analyses were recalculated assuming single site ordering with a total of six silica, aluminium and ferric iron cations per 12·5 oxygens using the program AX (available at http://www.esc.cam.ac.uk/software.html). Epidote in Zone 2a metabasalt is of restricted composition (Cz = 0·52–0·61). That from Zone 2b metabasalt is commonly zoned and preserves a wider range in Cz content (0·33–0·72). Clinozoisite in Zones 3 and 4 ranges from Cz₈₅ to Cz₁₀₀, but where present as inclusions in Zone 4 garnet it is unzoned and the overall population displays a much wider compositional range (Cz_44–70).

Amphibole
Amphibole in Diahot terrane metabasites is hornblende or magnesioriebeckite/glaucophane (after Leake et al., 1997). Ferric content and site distribution for both sodic and the sodic–calcic amphiboles were calculated using the methods of Robinson et al. (1982). Amphibole in Zone 1 has complex textural relationships. Individual grains have green actinolite cores with blue winchite or glaucophane rims (e.g. samples 2008 and 2044). Alternatively, minor actinolite or winchite is intergrown with glaucophane (Fig. 9a). Sodic amphibole in Zone 1 straddles the divide between magnesioriebeckite and glaucophane with X_Mg ranging from 0·4 to 0·7 (after Leake et al., 1997), whereas Fe²⁺:Fe³⁺ varies substantially (from 5:1 to 2:1; Fig. 9a). Zone 2 amphibole is less complex than that in Zone 1, and mostly involves intergrown glaucophane and actinolite. Texturally complex grains involving actinolitic cores and glaucophane rims may occasionally be present (e.g. sample 96347; Fig. 9b). With the exception of sample 96411, sodic amphibole in Zone 2 is glaucophane with X_Mg = 0·5–0·7 and Fe²⁺:Fe³⁺ ranging from 4:1 to 1:1 (Fig. 9b). Zone 3 and 4 metabasites contain glaucophane with a restricted compositional range (Fig. 9c) and higher X_Mg than that of Zones 1 and 2. Fe²⁺:Fe³⁺ are near 1:1. Ca to Na/Ca amphibole in Zone 3 and 4 metabasites displays a trend of increasing Si (from 7·4 to 7·8 cations) and decreasing Na (X_Na 0·4–0·2) content from the silica-rich end of the barroisite field, through winchite to actinolite (e.g. samples 9950 and WC16; Fig. 9c). This variation in Si content mostly reflects a combination of tschermakitic, edenitic and glaucophane substitutions (Clarke et al., 1997).

View larger version (29K): [in this window] [in a new window]   Fig. 9. Compositions of the amphiboles throughout the Diahot terrane plotted in terms of the number of Si cations per 23 oxygens vs XNa = Na/(Na + Ca) for sodic–calcic amphiboles and XMg = Mg/(Mg + Fe2+) for the sodic amphiboles. Compositional boundaries for Ca amphibole plots follow the nomenclature of Leake (1978) and the compositional boundaries for Na amphibole plots follow the nomenclature of Leake et al. (1997).

 
Phengite and paragonite
The Si and Na contents of phengite define an antithetic trend ranging from 7·0 Si cations and X_Na = Na/(Na + K) 0·05 to 6·6 Si and X_Na = 0·2. Although there are comparatively few analyses, Zone 1 phengite spans the high-Si (6·8–7·8) and low-X_Na (0·05) end of the trend whereas Zone 2 phengite spans the entire Si range. Phengite from Zones 3 and 4 is more restricted in composition, with Si = 6·8–6·5, and has a subtly elevated sodium content (X_Na = 0·08–0·21) compared with phengite in Zone 2. Though less well defined, Si and Na values of paragonite define an antithetic trend ranging from Si 6·2 cations and X_Na 0·85 in Zone 1 to Si 5·9 and X_Na = 1·0 in Zone 4.

	THERMOBAROMERY

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
Table 2 gives the mineral assemblages used for thermobarometry across the Diahot terrane to allow cross-referencing. On the basis of ¹⁶O/¹⁸O ratios, Black (1974) estimated metamorphic temperatures for rocks of the Diahot terrane to range from 250°C in Zone 1 to c. 400°C in Zone 3. The presence of aragonite within lawsonite-bearing metasediments from the Diahot terrane suggests that pressures for Zone 1 assemblages were >0·6 GPa, and most probably about 1·0 GPa (Brothers, 1974; Yokoyama et al., 1986). Clarke et al. (1997) inferred metamorphic conditions of T = 400 ± 58°C and P = 0·6–0·7 ± 0·2 GPa for lawsonite-bearing metasediments from Zone 1, which is very similar to earlier estimates of 0·8 GPa and 400°C (Brothers, 1970). The conditions of formation of M₁ mineral assemblages in Zone 1 metabasalts may be determined using the average pressure–temperature (P–T) method, using the software THERMOCALC (Powell & Holland, 1988), with the internally consistent thermodynamic dataset of Holland & Powell (1990; dated ‘20 April 1996’). End-member activities were calculated using the program AX. Sample RS3b contains the high-P assemblage omphacite–lawsonite–glaucophane–actinolite–albite–chlorite–titanite–quartz and returns average P–T estimates of T = 387 ± 60°C and P = 0·9 ± 0·3 GPa. Pressure estimates can also be made using coexisting albite and omphacite-rim (after Holland, 1988): for T = 350–400°C, the composition of albite and omphacite from sample RS3b returns P = 0·9–1·1 GPa. When all of the above methods are considered, the metamorphic conditions of Zone 1 most probably involved P = 0·7–1·1 GPa and T = 300–400°C, which is remarkably similar to the original estimate of Brothers (1970).

View this table: [in this window] [in a new window]   Table 2: P–T estimates from applying a variety of thermobarometric techniques to the mineral assemblages of metabasalts from Zones 1–4

 
Because of the discontinuous nature of S₂, the interpretation of a ‘stable’ equilibrium mineral assemblage is difficult in many metabasites from Zone 2 and above. Only mineral assemblages that clearly envelop or are external to domains of low D₂ strain were used for estimating P–T conditions that accompanied the development of S₂. Average P–T calculations made using the mineral assemblage garnet–clinozoisite–lawsonite–chlorite–glaucophane–actinolite–quartz in sample 96305b return results of T = 463 ± 44°C and P = 1·5 ± 0·3 GPa. Temperature estimates of 441–464°C were also made using coexisting actinolite and garnet (after Graham & Powell, 1984) in sample 96305b. Average P–T calculations for the S₂ assemblage garnet–actinolite–glaucophane–clinozoisite–phengite–chlorite–rutile–quartz in Zone 2b metabasalts (sample 96322B2) and the assemblage garnet–actinolite–glaucophane–clinozoisite–quartz (sample 96322B1) give results of T = 539 ± 40°C and P = 1·6 ± 0·4 GPa and T = 535 ± 36°C and P = 1·5 ± 0·4 GPa, respectively. Temperature estimates made on the basis of coexisting actinolite and garnet (after Graham & Powell, 1984) for samples 96322B2 and 96321B1 return results ranging from 470 to 500°C. These methods suggest that metamorphic conditions for Zone 2 metabasalts involved T = 450–550°C and P = 1·4–1·6 GPa.

Temperature estimates of T = 500–550°C for S₂ assemblages in Zone 3 metabasites have been made by Clarke et al. (1997) on the basis of garnet–hornblende thermometry (after Graham & Powell, 1984). Temperature estimates made on the basis of coexisting garnet and omphacite have been excluded because of the difficulties of inferring ‘equilibrium’ between M₁ omphacite and S₂ garnet. Temperature estimates of 510–560°C may be made for Zone 3 metabasites (samples 9950 and WC16) on the basis of garnet–hornblende thermometry (after Graham & Powell, 1984). Pressure estimates for conditions that accompanied the development of S₂ in Zone 3 metabasites can be made using the end-member activities for the inferred S₂ assemblages in samples 9950 and WC16 and THERMOCALC (Powell & Holland, 1988). Average P–T estimates made using the mineral assemblages garnet–hornblende–glaucophane–clinozoisite–paragonite–phengite–rutile–quartz (sample 9950) and garnet–hornblende–glaucophane–clinozoisite–chlorite–paragonite–phengite–rutile–quartz (sample WC16) give results of T = 572 ± 76°C and P = 1·6 ± 0·4 GPa and T = 564 ± 28°C and P = 1·7 ± 0·3 GPa, respectively. Yokoyama et al. (1988) and Clarke et al. (1997) estimated P–T conditions for metasedimentary assemblages interbedded with Zone 3 metabasites of P = 1·2–1·5 GPa and T = 550–580°C on the basis of coexisting omphacite–albite, garnet–omphacite and average P–T using THERMOCALC. These P–T estimates are within error of those obtained from the metabasic lithologies. All of the methods suggest that peak metamorphic conditions that accompanied the development of S₂ in Zone 3 metabasites involved P = 1·4–1·7 GPa and T = 550–580°C.

Clarke et al. (1997) estimated temperatures ranging from T = 520–620°C for Zone 4 metabasites on the basis of garnet–amphibole and garnet–clinopyroxene thermometry after Ellis & Green (1979), Krogh (1988) and the average T method using THERMOCALC (Powell & Holland, 1988). Average P–T calculations can also be made using the assemblages of garnet–glaucophane–omphacite–paragonite–rutile–quartz in sample 9408c and garnet–hornblende–glaucophane–clinozoisite–paragonite–phengite–rutile–quartz in sample WC134. These assemblages return estimates of T = 654 ± 72°C and P = 1·7 ± 0·3 GPa and T = 600 ± 66°C and P = 1·5 ± 0·3 GPa, respectively.

	P–T PATH OF THE DIAHOT TERRANE

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
The P–T estimates outlined above imply a regional M₂ P–T array from T = 350°C and P = 0·8–1·0 GPa in Zone 1, to T 620°C and P = 1·7 GPa in Zone 4. Unfortunately, large errors associated with many of these results mean that many P–T estimates for the individual M₂ zones lie within error of each other. P–T pseudosections can be useful to help define the prograde history of Diahot terrane metabasites from Zones 2, 3 and 4, based on M₁ mineral assemblages preserved in domains of low D₂ strain and minerals that occur as inclusions in M₂ garnet. P–T pseudosections display the univariant reactions and multivariant mineral assemblages that are encountered by a particular bulk-rock composition. They are an appropriate way of assessing changing mineral paragenesis in mafic systems, which typically contain high-variance mineral assemblages (Carson et al., 1999). Pseudosections are presented for two high-grade Diahot terrane metabasalts, using the CNFMASH (CaO–Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O) grid of Carson et al. (2000) for H₂O-saturated conditions. For consistency, we used version 2.7 of THERMOCALC and the internally consistent thermodynamic dataset dated ‘20 April 1996’ (Holland & Powell, 1990) that were used for the grid construction by Carson et al. (1999). Activity–composition relationships have been presented by Carson et al. (1999, 2000). The bulk-rock compositions used in the construction of the pseudosections are Al₂O₃:CaO:MgO:FeO:Na₂O = 23·50:20·86:14·72:32·37:8·53 (sample 9408c, after Marmo et al., 2002) and Al₂O₃:CaO:MgO:FeO:Na₂O = 23·00:25·56:26·81:16·82:7·82 (sample 9950; Table 2). Pseudosections were constructed for metamorphic conditions in the range of T = 400–650°C and P = 0·8–2·0 GPa, to adequately model the P–T range indicated by the thermobarometry.

Sample 9408c
The pseudosection presented in Fig. 10a is for a bulk-rock composition appropriate to the Zone 4 metabasite 9408c. The early prograde evolution of sample 9408c is preserved by inclusions of lawsonite, chlorite, glaucophane and actinolite in garnet. This assemblage is best represented by the large trivariant field chlorite–glaucophane–hornblende–lawsonite in the low-T area of Fig. 10a. The appearance of garnet at slightly higher temperatures is controlled by the univariant reaction

(1)

and the divariant and trivariant fields that emanate from it and the low-T side of reaction

(7)

The growth of garnet cores in the presence of M₁ lawsonite is consistent with P–T conditions coincident with the divariant field chlorite–garnet–glaucophane–lawsonite, at P > 1·4–1·5 GPa and temperatures as low as 480°C. Any prograde path at lower pressures than this would pass through the trivariant field chlorite–glaucophane–hornblende–clinozoisite, destabilizing lawsonite in favour of clinozoisite before the growth of garnet. The S₂ assemblage in sample 9408c comprises glaucophane, paragonite and large idioblastic omphacite laths with inclusions of clinozoisite and glaucophane. The observation of clinozoisite inclusions in both garnet and S₂ omphacite suggests that the P–T path passed at pressures below univariant reaction (3) (i.e. P < 1·8 GPa; Fig. 10a) in the divariant field garnet–omphacite–paragonite–glaucophane–clinozoisite. A P–T path that tracks above univariant reaction (3) would result in the exclusion of clinozoisite from all prograde assemblages.

View larger version (45K): [in this window] [in a new window]   Fig. 10. (a) P–T pseudosection in CNFMASH, with H2O in excess, for the bulk-rock composition Al2O3:CaO:MgO:FeO:Na2O = 23·50:20·86:14·72:32·37:8·53, appropriate to the Zone 4 metabasite 9408c (after Marmo et al., 2002). The univariant reactions experienced by this bulk-rock composition, as well as those that bind the higher variance fields that were experienced are shown as bold continuous lines and include: (1) g hb law = chl gl cz; (2) gl law = chl g pa cz; (3) g o law = pa gl cz; (4) gl cz = g o pa hb; (5) g o hb = gl ab pa; (6) gl cz = g pa hb ab; (7) chl gl cz = g pa hb. Divariant fields are unshaded five-phase regions, trivariant fields are shaded four-phase fields and quadrivariant fields are deeply shaded three-phase regions. The dashed black line represents the upper stability of lawsonite, defined by the terminal CASH univariant reaction law = cz + ky + qtz + H2O (Chatterjee et al., 1984). The bold black line represents the P–T trajectory inferred for this rock and the elliptical zone of wavy lines represents the inferred peak mineral assemblage. (b) P–T pseudosection in CNFMASH, with H2O in excess, for a bulk-rock composition Al2O3:CaO:MgO:FeO:Na2O = 23·00:25·56:26·81:16·82:7·82, appropriate to the Zone 3 metabasite 9950. The univariant reactions experienced by this bulk-rock composition, as well as those that bind the higher variance fields that were experienced are shown as bold continuous lines and include: (1) g hb law = chl gl cz; (2) g o hb law = gl cz; (3) gl cz = g o hb ky; (4) g pa hb = o gl ky; (5) g pa hb cz = o ky; (6) gl cz = g o pa hb; (7) chl gl cz = g pa hb; (8) o pa = g hb ab cz; (9) o pa gl = hb ab cz; (10) chl gl ab cz = pa hb. Divariant and trivariant fields as for (a). The dashed black line represents the upper stability of lawsonite, defined by the terminal CASH univariant reaction law = cz + ky + qtz + H2O (Chatterjee et al., 1984). The bold black line represents the P–T trajectory inferred for this rock and the elliptical zone of wavy lines represents the inferred peak mineral assemblage.

 
The inferred peak S₂ assemblage in sample 9408c does not contain clinozoisite and consists predominantly of garnet, glaucophane, paragonite and omphacite. This assemblage is best represented by the large trivariant field garnet–omphacite–paragonite–glaucophane. The striped region within the trivariant field garnet–omphacite–paragonite–glaucophane represents the preferred P–T conditions for sample 9408c (e.g. P = 1·6–1·8 GPa and T = 580–620°C). The near-isothermal decompression path to D₃ greenschist-facies conditions involving chlorite- and albite-bearing assemblages is illustrated for completeness (see also Clarke et al., 1997; Carson et al., 2000; Marmo et al., 2002).

Sample 9950
Figure 10b illustrates a P–T pseudosection appropriate to the Zone 3 metabasalt sample 9950. Early mineral assemblages (low P–T) were controlled by the large trivariant field chlorite–hornblende–glaucophane–lawsonite that emanates from the low-P end of the univariant reaction

(1)

Domains of low D₂ strain in sample 9950 display a mosaic of M₁ omphacite, interlocking with glaucophane, clinozoisite (with rectangular pseudomorphs after M₁ lawsonite) and hornblende. The inferred mineral assemblages are consistent with an early prograde path that tracked through the upper temperature range of the large trivariant field glaucophane–hornblende–chlorite–lawsonite. Petrological evidence in low D₂ strain domains and similar assemblages in lower-grade metabasalts across the terrane are interpreted as representing the recrystallization of M₁ lawsonite to combinations of M₂ clinozoisite, paragonite and glaucophane (e.g. sample 96411), and the replacement of M₁ omphacite by hornblende and chlorite. These progressive mineralogical changes are consistent with P–T conditions having evolved from the trivariant field chlorite–hornblende–glaucophane–lawsonite, through the divariant field chlorite–glaucophane–hornblende–lawsonite–clinozoisite, to the trivariant field chlorite–glaucophane–hornblende–clinozoisite.

The appearance of garnet in sample 9950 was controlled at high P and T by the transition from the trivariant field chlorite–glaucophane–hornblende–clinozoisite and into the divariant field chlorite–garnet–glaucophane–hornblende–clinozoisite which emanates from the low-P termination of the univariant reaction

(2)

and the low-T side of the univariant reaction

(7)

The assemblage glaucophane, clinozoisite, chlorite and hornblende included in garnet cores from sample 9950 is best represented by the trivariant field chlorite–glaucophane–hornblende–clinozoisite. The absence of lawsonite and presence of clinozoisite inclusions in garnet cores refines the position of the high-T portion of the P–T path to below the low-P termination of univariant reactions (1) and (2). The growth of garnet in sample 9950 would have occurred in the divariant field chlorite–garnet–glaucophane–hornblende–clinozoisite. The divariant fields chlorite–garnet–glaucophane–hornblende–clinozoisite and paragonite–garnet–glaucophane–hornblende–clinozoisite represent the most likely conditions for the formation of the inferred peak S₂ mineral assemblage of chlorite, garnet, glaucophane, hornblende, clinozoisite and paragonite in sample 9950 and many of the Zone 3 metabasites (e.g. samples 9925, 9949, WC16, WC82). The P–T region preferred for the development of S₂ in sample 9950 is indicated by the striped region in Fig. 10b. The lack of S₂ omphacite in Zone 3 metabasites constrains the maximum P–T conditions for the development of S₂ to have been on the low-T side of the univariant reaction

(6)

and at pressures less than the divariant field garnet–omphacite–glaucophane–hornblende–clinozoisite.

	DISCUSSION

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
Bulk-rock compositions of the Diahot terrane metabasites vary mostly in the proportions of FeO and MgO, and to a lesser extent Al₂O₃ and CaO content (Table 2). Therefore, at a single P–T, the reactions observed and the mineral assemblages that developed within individual metabasite lenses may differ significantly as illustrated in Fig. 10a and b. The bulk-rock compositions of samples 9408c and 9950 represent two end-member compositions that span the compositional variation: sample 9950 has low FeO and high MgO contents, and sample 9408c has high FeO and low MgO contents. From the textural evolution discussed for Fig. 10a and b, combined with the results of the thermobarometry, we infer that metabasites from Zones 2a, 2b, 3 and 4 represent segments of a single P–T array. Zone 1 metabasalts preserve lawsonite blueschist-facies conditions. Zone 2 metabasalts experienced M₁ lawsonite blueschist-facies conditions before peak conditions that accompanied the development of S₂ in the epidote blueschist facies. Zones 3 and 4 experienced M₁ epidote blueschist-facies conditions, similar to those that accompanied the development of S₂ in Zone 2, before M₂ conditions in the upper blueschist to transitional eclogite facies.

By varying the proportions of the two end-member rock compositions (9950 and 9408c) through steps along the P–T path, a pseudosection can be constructed that represents the regional gradient from Zone 2a to Zone 4, and illustrates the expected reactions and mineral assemblages observed in many of the Diahot metabasalts. Figure 11 is a composite P–X_{bulk rock} and T–X_{bulk rock} pseudosection that reflects a P–T gradient across the Diahot terrane. The horizontal line (X_{bulk rock}) on the diagram represents the relative proportions of two ‘end-member’ bulk-rock compositions. The prograde section of the P–T path defined in Fig. 10a and b is broken up into three isobaric temperature and two isothermal pressure steps that form the vertical axis of the diagram.

View larger version (40K): [in this window] [in a new window]   Fig. 11. Composite P–Xbulk rock and T–Xbulk rock pseudosection in CNFMASH, with H2O in excess, for the spectrum of bulk-rock compositions between those of samples 9950 and 9408c. The pseudosection is drawn for the step-wise P–T trajectory shown in the lower diagrams. At Xbulk rock = 0, the bulk-rock composition is appropriate to that of sample 9950, and at Xbulk rock = 1 the bulk-rock composition is that of sample 9408c. Univariant reactions witnessed by the compositional range are shown as bold continuous lines and include: (1) g hb law = chl gl cz; (2) chl gl cz = g pa hb; (3) gl cz = g o pa hb. Divariant fields are unshaded five-phase regions, trivariant fields are shaded four-phase fields and quadivariant fields are deeply shaded three-phase regions.

 
As discussed above, the bulk rocks used for pseudosection construction were determined by element mapping of selected Zone 3 and Zone 4 metabasites and represent mineral assemblages that are aligned in S₂ and are interpreted to have equilibrated during M₂. M₁ mineral assemblages involving lawsonite and omphacite persist in Zones 2a and 2b as centimetre-sized low-strain domains enveloped by S₂. Omphacite is the only M₁ mineral preserved within low-strain domains in Zone 3 and rarely Zone 4 metabasites. M₁ domains within Zones 2a and 2b have suffered only incipient M₂ recrystallization and as such were preserved as chemically isolated domains during the prograde development of the enveloping S₂ fabric. Through Zones 3 and 4, S₂ becomes increasingly more intense and corresponding M₁ domains display a much greater degree of recrystallization and equilibration with M₂ metamorphic conditions. Therefore, the range of P–T conditions depicted in Fig. 11 does not attempt to model for the development of M₁ chemically isolated mineral assemblages in Zone 2a and 2b metabasites, and only illustrates the M₂ regional mineralogical progression through Zones 2a and 2b, as well as inferred M₂ mineral assemblages of Zone 3 and 4 metabasites.

Between temperatures of 480 and 520°C at P = 1·5 GPa, the mineral assemblages predicted by Fig. 11 range from chlorite–glaucophane–hornblende and lawsonite or clinozoisite, to chlorite–garnet–glaucophane–lawsonite–clinozoisite. These assemblages adequately describe the S₂ mineralogical variation, between the clinozoisite–lawsonite and garnet–lawsonite–clinozoisite-bearing metabasalts in Zone 2a. Evans (1990) inferred that garnet growth in the presence of lawsonite occurred at similar conditions (T = 475°C at P = 1·6 GPa) in the lower epidote blueschist facies. Domains of low D₂ strain in metabasalts of Zone 2a preserve M₁ mineral assemblages dominated by glaucophane, omphacite and lawsonite, the last being partially replaced by S₂ epidote. These assemblages are similar to those described for Zone 1 metabasalts, suggesting that Zone 2a metabasalts passed through the lawsonite blueschist facies (P = 0·8–1·0 GPa and T 350°C) before the development of M₂ epidote blueschist-facies conditions (P = 1·5 GPa and T = 480–520°C).

Between temperatures of 520 and 560°C for P = 1·5–1·6 GPa, garnet is predicted to become increasingly stable across a wide section of the compositional range represented in Fig. 11, and lawsonite is predicted to have been destabilized. Garnet in metabasalt lenses from Zone 2b becomes progressively coarser grained and more abundant with increasing grade, consistent with these predictions. Prograde (M₁) assemblages involving lawsonite and omphacite persist through Zone 2a and 2b metabasalts in domains of low D₂ strain. At higher grade (Zone 2b), sawtooth intergrowths of M₁ omphacite and lawsonite are partially to completely recrystallized in combinations of glaucophane, clinozoisite, chlorite and actinolite. These textures are consistent with M₁ lawsonite persisting in domains of low D₂ strain in Zones 2a and 2b before crossing the degenerative reaction lawsonite clinozoisite + kyanite + quartz + H₂O (Chatterjee et al., 1984). At higher grades (Zones 3 and 4) lawsonite is no longer stable (pseudomorphed by combinations of glaucophane–clinozoisite–paragonite), but early formed (M₁) omphacite may persist in domains of low D₂ strain. Caron & Pequignot (1986) described similar textures in lawsonite-bearing eclogites from Eastern Corsica, involving the growth of garnet and actinolite at the expense of chlorite and lawsonite, and the crystallization of glaucophane at the contact between omphacite and lawsonite through the reactions

The consumption of omphacite during the growth of calcic or sodic–calcic amphiboles, usually in the presence of albite, is generally considered to be a retrograde texture in many eclogite terranes (e.g. Northern Urals, Gomez-Pagnaire et al., 1997; Northern Greece, Liati & Mposkos, 1990; NE Greenland, Thelin et al., 1990; Switzerland, Elvevold & Gilotti, 2000). In contrast, the presence of prograde hornblende inclusions in garnet and clinozoisite in Zone 2b–4 metabasites, the progressive prograde increase in Al and Na contents, and the decrease in Si contents of hornblende from actinolitic (Zones 1 and 2) to barroisitic (Zones 3 and 4) compositions is consistent with prograde (increasing P and T) growth of hornblende through Zones 2–4 at the expense of M₁ omphacite (see also Krogh, 1982; Robinson et al., 1982; Searle et al., 1994; Gomez-Pagnaire et al., 1997; Banno, 2000).

Paragonite is present in all M₂ zones, but does not become abundant until Zone 3. It may be stable in all of the bulk-rock compositions illustrated in Fig. 11 for T = 560–590°C and P = 1·6–1·7 GPa. At T 560°C and P = 1·7 GPa, garnet is stable across the entire bulk-rock compositional range and omphacite becomes stable in compositions close to that of sample 9408c. Mineral assemblages predicted by the divariant fields chlorite–garnet–glaucophane–hornblende–clinozoisite and paragonite–garnet–glaucophane–hornblende–clinozoisite in bulk-rock compositions close to that of sample 9950 are consistent with the assemblages observed in Zone 3 metabasites. However, M₂ omphacite is not observed in Zone 3 metabasites, probably because of the limited range of bulk-rock compositions. Large omphacite, almandine–grossular-rich garnet, paragonite and glaucophane are the dominant S₂ minerals in sample 9408c. Whereas clinozoisite and glaucophane occur as inclusions in large omphacite and clinozoisite grains, clinozoisite is not present in the S₂ matrix. El-Shazly et al. (1990) described a similar textural progression for the development of omphacite–garnet-bearing assemblages in eclogites of NE Oman, and inferred that it was controlled by the reaction gl + cz = grt + cpx + parg + q. This reaction is represented in Diahot metabasites by the transition from the divariant field garnet–omphacite–glaucophane–clinozoisite–paragonite, to the large trivariant field garnet–omphacite–glaucophane–paragonite for a bulk-rock composition close to that of sample 9408c.

Mineral assemblages predicted for temperatures above 610°C at P = 1·7 GPa (Fig. 11) reflect those observed in Zone 4 metabasites. Mineral assemblages in the hornblende-rich, omphacite-poor rock compositions close to that of sample 9950 (e.g. sample 9409b) are represented by the trivariant field garnet–omphacite–paragonite–hornblende–clinozoisite. Mineral assemblage predicted for omphacite-rich, hornblende–clinozoisite-poor bulk-rock compositions close to that of sample 9408c are represented by the large trivariant field garnet–omphacite–paragonite–glaucophane (Fig. 11).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
Metabasite assemblages in the Diahot terrane define four zones: (1) lawsonite blueschists that reflect P = 1·0 GPa and T = 400°C; (2) epidote blueschists that reflect P = 1·4–1·5 GPa and T = 450–500°C; (3) almandine–hornblende blueschists that reflect P = 1·4–1·6 GPa and T = 550–580°C; (4) paragonite–hornblende eclogites that reflect P = 1·7 GPa and T = 600–620°C. On the basis of prograde lawsonite and omphacite inclusions in Zone 2b–4 garnet, and millimetre- to centimetre-sized domains of low D₂ strain that preserve igneous textures and prograde assemblages, we infer that the prograde paths of Zone 2–4 metabasites had a similar P–T trajectory. This P–T path involved early prograde conditions of lawsonite blueschist facies (P = 0·8–1·0 GPa and T = 350–400°C) and passed through epidote blueschist facies before reaching maximum metamorphic conditions in the lower eclogite facies. The rapid change in P–T conditions from lawsonite blueschists of Zone 1 to epidote blueschists of Zone 2 can be explained by the presence of a SW-dipping normal shear zone, and the post-S₂ excision of c. 15 km of the original crustal profile. The section of the terrane between Zones 2 and 4 spans the full range of P–T conditions between T = 470°C and P = 1·4 GPa (Zone 2) to T 600°C and P = 1·7 GPa (Zone 4), and represents a crustal section that has suffered significant tectonic thinning preceding high-P metamorphism.

Peak conditions of T = 600°C and P = 1·7 GPa accompanied the development of S₂ transitional eclogite-facies assemblages in Zone 4 metabasites. The limited contrast in these conditions and those inferred for the Pouébo terrane metabasites suggests that the highest-grade fragments of the Diahot terrane and the metabasic eclogites of the Pouébo terrane developed at similar depths of 50–60 km in the subducted leading edge of NE New Caledonia.

	SUPPLEMENTARY DATA

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
Supplementary data for this paper are available on Journal of Petrology online.

	APPENDIX: MINERAL ABBREVIATIONS

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 

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	ACKNOWLEDGEMENTS

 
J.A.F. acknowledges the support of an Australian Post-Graduate Award from the University of Sydney. Fieldwork and analytical costs were funded through an Australian Research Council Large Grant to G.L.C. and R.P. (A39600827). Many samples used in this work were collected by C. Gerakiteys, H. Clarke, M. Brennan, R. Davies, S. Gotley and W. Reid during the course of fieldwork undertaken for their Honours year. Careful reviews by D. Ellis and R. Arculus greatly improved the text.

	REFERENCES

TOP ABSTRACT INTRODUCTION REGIONAL GEOLOGY PETROLOGY OF DIAHOT TERRANE... MINERAL CHEMISTRY OF THE... THERMOBAROMERY P-T PATH OF THE... DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA APPENDIX: MINERAL ABBREVIATIONS REFERENCES

 
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