GEOLOGY AND GEOCHEMISTRY OF THE “SIRUKA ULTRAMAFICS”

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THE ONTONG JAVA PLATEAU (OJP)

Large Igneous Provinces (LIPs)

The Ontong Java Plateau, located in the southwestern Pacific Ocean, is considered to be a giant oceanic plateau, which forms one of three main categories of mafic igneous provinces or LIPs (Coffin and Eldholm, 1991). The other two categories of LIPs include continental flood and volcanic passive margins (Coffin and Eldholm, 1994). The LIPs represent large-scale transient magmatism rooted deep in the Earth’s mantle and therefore not controlled by lithospheric processes predicted by plate tectonic theory (Mann and Taira, 2004). Subcontinental scale-thick crust is produced with huge emplacements of predominantly mafic extrusive and intrusive rocks (Coffin and Eldholm, 1991). An estimation of 25 LIPs have formed since 250 Ma, which are now distributed worldwide (Fig. 1-2A) but most specifically in the Circum-Pacific region (Coffin and Eldholm, 1994) (Fig. 1-2B). Note that no major LIPs are forming today.

Characteristics and structure of OJP

OJP is defined by the 4000 m depth contour, adjacent to the > 4500 km deep Lyra, East Mariana, and Nauru ocean basins and bounded to the southwest by the >3000 m deep North Solomon Trench (Gladczenko et al., 1997). Morphologically, the OJP consists of two parts: the main plateau in the northwest and the eastern lobe or salient (Neal et al., 1997) (Fig. 1-3). The plateau surface rises to depths of 1700m in the central region of the main plateau but lies generally between 2-3km depth. The surface of the plateau appears to be relatively smooth although several seamounts like Ontong Java Atoll (the largest atoll in the world) occur (Berger et al., 1993; Kroenke, 1972).
The Alaska-size OJP is the world’s largest oceanic plateau, covering an area of more than 1.86 * 106 km2 (Coffin and Eldholm, 1994) (Fig. 1-4). Furumoto et al. (1976) has estimated, using seismic and combined seismic-gravity, the crustal thickness of the OJP in the 30 – 43 km range with an estimated average around 36 km, comparable to the continental crusts (Christensen and Mooney, 1995). This translates to a volume of >5 * 107 km3 and makes OJP the most voluminous LIPs (Neal et al., 1997).
The OJP broadly has a similar crustal structure to ‘normal’ Pacific Ocean crust but each layer is abnormally thickened by up to a factor of 5 (Hussong et al., 1979; White et al., 1998). The upper crustal section comprises an upper basaltic lava-still pile with a pelagic sediment cover of > 1 km thick (Mayer et al., 1991). The OJP basement lavas were originally recovered from Ocean Drilling Project sites 209, 803 and 807 (Mahoney et al., 1993) on the high plateau (Fig. 1-3) and also identified outcropping on Malaita (Petterson et al., 1997) and Santa Isabel (Tejada et al., 1996). The origin of the dense lower crustal body with a velocity of 7.0-7.6 km/s remains controversial. Miura et al. (2004) agree that the lower crustal body represents a dense crustal root of garnet granulite or eclogite. However, Neal et al. (1997) suggest that the lower crustal body is wehrilitic to pyroxenite cumulate.

Origin and formation of the OJP

Like the other oceanic plateaus, OJP was formed by thermally induced “active” mantle plume where an inflated plume-head tailored by a narrow feeder conduit (“tail”) develops, rifts the lithosphere (Coffin and Eldholm, 1994) (Fig. 1-5) and causes excessive magmatism (Campbell and Griffiths, 1990) in a geologically short period of less than 3 Ma (Richards et al., 1989).
The 40Ar-39Ar ages of these OJP basalts dated from the OJP drillholes (Mahoney et al., 1993) and outcrops found on northern Santa Isabel and Malaita (Tejada et al., 1996) all suggest a bimodal distribution of ages with the first episode at 1223 Ma and the second at 90 Ma. This means that most of the plateau was formed in these two relatively brief magmatic events related to mantle plume dynamics. Kroenke and Mahoney (1996) and Tejada et al. (1996) suggested that the 122 Ma event was significantly larger than the 90 Ma event. Therefore, the 122 Ma episode was at the origin of the construction of the main high plateau whereas the eastern salient was the result of the volcanic activity at 90 Ma (Neal et al., 1997) (Fig. 1-3).
These massive outpourings of basalt are at the origin of the undeformed Circum-Pacific Ontong Java Plateau, which was formed by mantle processes in the intraplate Pacific Ocean (Mann and Taira, 2004). However the original tectonic setting of the OJP emplacement is far to be resolved. According to the plate reconstruction of the OJP site at 125 Ma (Yan and Kroenke, 1993), the main plateau of OJP was formed at about 42°S, 159°W at the vicinity of a spreading center (Mahoney and Spencer, 1991; Winterer and Nakanishi, 1995) (Fig. 1-6). This location is approximately 1800 km distant from Louisville hotspot, indicating that OJP could be linked to this hotspot (Neal et al., 1997). The Vitiaz Trench has destroyed the portions of the Louisville hotspot older than 70 Ma (Mahoney and Spencer, 1991) and therefore no clear evidence exists to correlate Louisville hotspot to the origin of OJP (Neal et al., 1997). In addition, Mayer and Tarduno, 1993) invoked that true polar wander of ≈10-15° should be executed to accommodate a Louisville hotspot origin for the OJP. Recent studies based on new tectonic reconstructions (Phinney et al., 1999) maintained the origin of the OJP at the Louisville hotspot.

OJP Tectonic evolution (from 125 to ~ 30 Ma)

From when it was formed (125 Ma) until 100 Ma, OJP appears to be very close to the Pacific Plate Euler poles and moved relatively little (Petterson et al., 1993; Yan and Kroenke, 1993). Then from 100 to 85 Ma, the OJP moved steadily northward within the plate after change of plate motion occurring at 100 Ma (Yan and Kroenke, 1993). The 90 Ma event formed the eastern margin of the OJP when OJP passes approximately over the position occupied by the high plateau during the 125 Ma episode (Neal et al., 1997) (Fig.1-6).
From Aptian to Mid-Eocene, deep-water pelagic sedimentation was recorded on Malaita indicating an intra-oceanic environment (Petterson et al., 1997). During this period, OJP was passively carried northwards as part of the Pacific Plate (Fig. 1-7). Throughout the Eocene, the southwards-directed subduction of the Pacific Plate beneath the Australian Plate occurred at the North Solomon Island/Vitiaz Trench, which resulted in Stage 1-arc activity along a northeast-facing Solomon arc (Kroenke, 1984; Petterson et al., 1997).
Between 60 and 30 Ma, OJP passed directly over the Samoan hotspot (Yan and Kroenke, 1993) or possibly the Raratonga hotspot (Tejada et al., 1993) reflecting a subsequent plate motion change (Fig. 1-7). Younger series of Malaita were described by Tejada et al. (1996) and dated by 40Ar-39Ar at 44 Ma. These alkalic lavas are characterised by high vesicularity revealing a shallower –submarine eruptive setting (Petterson et al., 1997) and likely to be erupted from the Samoan hotspot are distinct to the OJP basalts. 44 Ma is also marked by a major change in Pacific plate motion (Fig. 1-7), which initiates a period of extension within the southern OJP. The development of basins allows the egress of magmas relating to both the eruption of volcanic alkaline rocks and intrusion of alnoites, accompanied by consequent intra-basin Eocene-Lower Miocene sedimentation (Neal et al., 1996).

TECTONIC MODELS OF OJP-SOLOMON ARC COLLISION

Timing of the OJP-Arc collision

Previous works (Kroenke and Sager, 1993; Petterson et al., 1997, 1999; Yan and Kroenke, 1993) (Fig. 1-8) suggested that the OJP arrived at the trench at 22 Ma (Early Miocene) in a “soft docking” event with no obvious record of associated deformation of the Solomon arc. The soft docking event of OJP, which was supported by a hiatus in Miocene arc volcanism (∼ 20-15 Ma) and by plate reconstructions (Yan and Kroenke, 1993), was supposed to initiate the polarity reversal in the period ∼12-6 Ma because the OJP was considered too thick to be subducted at the North Solomon trench (Fig. 1-8). According to the authors, this event was followed by a “hard docking” stage of the increased coupling between the OJP and the Solomon arc, which occurred at ∼4-2 Ma (Fig. 1-8). This period coincides with an increasing compressional regime resulting in widespread deformational and uplift events evidenced in the formation of the Malaita anticlinorium (Petterson et al., 1997, 1999). In this scenario of OJP-arc collision, the workers assume that the North Solomon trench became inactive at the time of the first contact between the OJP and Solomon arc and Pacific-Australia motion was accommodated by the subduction of the Australian plate at the San Cristobal trench. This typical arc polarity reversal is well constrained by the dual inwardly dipping Benioff zones of the Pacific and Australian plates (Fig. 1-9) and is the reason that the Solomon Islands arc has been the focus of many seismic, geological and geophysical studies since it was first proposed by Kroenke (1972). However, there is a debate on whether or not the Pacific plate ceases completely to subduct underneath the Solomon arc while the new northeast-directed subduction of the Australian plate beneath the Solomon block along the SSTS is initiated during Late Miocene times (Petterson et al., 1999). Latest studies by Petterson et al. (1997, 1999) suggest the subduction at the Vitiaz trench in the vicinity of Solomon Islands have recommenced during the Mid Miocene (from ~15 Ma) and has continued intermittently and locally until present time based on the formation of Maetambe and Komboro volcanoes (Choiseul) (Ridgway and Coulson, 1987) and recent swath mapping data from Makira (Kroenke, 1995).
There are a lot of controversies about the timing of major events in the Ontong-Java-Solomon island arc convergent (Mann and Taira, 2004). Recent studies (Cowley et al., 2004; Man and Taira, 2004; Phinney et al., 2004), using new marine geophysical and onland geological data, found no evidence for an early Miocene tectonic “soft docking” event and concluded that the OJP converged on the Solomon arc only in the last 5 Ma and collided with the arc causing the accretion of the Malaita anticlinorium (Fig. 1-9). In this single tectonic event of OJP-arc convergence, the authors posit that the subduction occurred along the KKKFS (Kia-Korigole-Kaipito Fault System) (instead of the North Solomon Trench) until the arrival of the OJP at about 5 Ma (Fig. 1-9). The “choking” of this subduction caused the subduction zone to step seaward to form the present-day North Solomon trench.

Accretion models

The exact mechanism of accretion OJP-arc is still a subject of ongoing debate and research (Mann and Taira, 2004; Petterson et al., 1999). Few accretion models have been tested to explain the presence of the OJP-related terranes obducted onto the arc from Malaita and Santa Isabel (Fig. 1-10). All of the models are essentially inferred from the onland geological data from the Malaita anticlinorium. Some authors (Mann and Taira, 2004; Petterson et al., 1999) show the OJP splitting in two with the upper parts “tectonic flakes” obducting and the lower parts subducting southwards on and beneath the arc (Fig. 1-10A). This mechanism is known as tectonic wedging (Oxburgh, 1972; Unruh et al., 1991). However this model is not supported by the geophysical data recently acquired by Mann and Taira (2004), Mann et al. (1998) and Phinney et al. (2004). Another obduction model “Active subduction-accretion of the uppermost crust of the OJP” (Mann and Taira, 2004; Petterson et al., 1999) is proposed where 4-10 km becomes detached from deeper OJP crust and forms a series of NE-directed imbricate structures with accompanying NE-vergent folds and NE-propagating faults (décollements) (Fig. 1-10B). In this model, the lower parts of the OJP are also thought to continue to subduct southwestwards underneath the Solomon arc.

PRESENT-DAY GEOLOGICAL FRAMEWORK OF THE SOLOMON ISLANDS

The islands of New Britain and Bougainville in Papua New Guinea, Solomon Islands and Vanuatu (formerly known as New Hebrides) are the components of the Greater Melanesian Arc System (Kroenke, 1984). The Solomon Islands consist of a linear, NW-SE-trending double chain of islands bounded by two trench systems, the Vitiaz Trench (northeast) and the South Solomon Trench System (SSTS) which marks the collision zone between the Australian and Pacific plates (Fig. 1-11). The complicated geological history of the Solomon Islands is caused in part by the collision between the OJP and the Solomon arc (Petterson et al., 1997). On the basis of different lithological assemblages, the Solomon Islands have been subdivided into several provinces (Coleman, 1960, 1965) or terrains (Petterson et al., 1999).
Coleman (1960) and Coleman and Packlam, 1976) have first described the geological province model (Fig. 1-11), dividing the Solomon Islands into three distinct provinces: 1) The Pacific Province to the east is represented by an uplifted, overthrusted and largely unmetamorphosed portion of OJP and forms the basement of Malaita, Ramos, Ulawa and north of the Kaipito-Korighole Fault System (KKKFS) on Santa Isabel; 2) The Central Province, adjacent to the Pacific Province on the southwest, includes variably metamorphosed Cretaceous and early Tertiary floor seafloor and remnants of the northeast-facing arc sequence that grew during the early to middle Tertiary above the then southwest-plunging Pacific Plate (prior to the arrival of the OJP from the east). The boundary between the Pacific and Central provinces is generally submerged except on Santa Isabel Island where it forms the KKKFS and 3) The Volcanic Province is characterised by an island arc sequence composed by young volcanic and intrusive rocks (<4 Ma), which are exposed along the southwestern flank of the Central Province.
According to Petterson et al. (1999), the Solomon terrains (Fig. 1-12) have a major Cretaceous-aged component and comprise: 1. a plume-related (OJP) terrain including Malaita, Ulawa, and the northeastern part of Santa Isabel; 2. a mid-ocean ridge basalt (MORB) terrain including much of Choiseul and Guadalcanal; 3. a hybrid terrain including both MORB and plume components forming Makira. Two stages of Tertiary arc development are recognized: Stage 1 from the Eocene to Early Miocene, including the Shortlands and southern part of Santa Isabel; Stage 2 from the Late Miocene to the present day, including the active volcanoes of the New Georgia Group and adjacent forearc.

READ  STRATEGY AND STRATEGIC MANAGEMENT

THE GEOLOGY OF CHOISEUL AND SAN JORGE/SANTA ISABEL AND ITS RELEVANCE TO GLOBAL GEOLOGICAL FRAMEWORK OF THE SOLOMON ISLANDS

Geology of Choiseul

Choiseul Island forms part of the NW-SE trending Solomon arc and shows pronounced elongation along this trend. Convergent plate-margin tectonics has dictated the structural development of the island, given rise to two distinct structural units: 1) the Pre-Miocene igneous and metamorphic basement complex and 2) the sedimentary and volcanic cover. The distribution of rock types in the Choiseul Pre-Miocene basement is shown on the geological map of Choiseul (Fig. 1-13). The basement includes the Voza lavas and Choiseuls Schists, the Oaka Metamicrogabbro and Siruka Ultramafites (Ridgway and Coulson, 1987) .The Voza Lavas are the predominant formation of the Choiseul basement together with the Choiseul Schists. The Voza Lavas occur as pillowed, massive and brecciated basalts (Purvis and Kemp, 1975; Ridgway and Coulson, 1987) and can be divided into two groups: Group 1 – unmetamorphosed and low-grade rocks and Group 2 – more highly metamorphosed varieties (up to amphibolite facies). The Choiseul Schists (Fig. 1-13) are considered to be dynamothermally altered Voza Lavas, distinct from them by the presence of a tectonic fabric. Ridgway and Coulson (1987) considered that the Choiseul Schists were formed by deformation and metamorphism of parts of the original Voza lava sequence. The Oaka Metamicrograbbros are only locally exposed at the southeast of Choiseul (Fig. 1-13). Occasionally altered, these microgabbros intruding the Voza Lavas are similar in mineralogy to Group 2 of the Voza lavas and both have comparable metamorphic history. The Siruka Ultramafites (Fig. 1-13) are almost all harzburgites, which have undergone varying degrees of serpentinisation. They form a large sheet lying on Choiseul Schists and Voza lavas with a generally subhorizontal contact (Coleman, 1960; Thompson, 1960) and are believed to be emplaced as a coherent thrust sheet, from the southeast on a subhorizontal plane in Late Miocene to Pleistocene times (Thompson, 1960).
The basement sequence of Choiseul is thought to represent an ophiolite complex with many of the characteristics of MORB, which has developed close to a subduction zone (Ridgway and Coulson, 1987). Unfortunately no radiometric age is available for the Choiseul basement sequence but stratigraphic and structural evidence suggest a probable Cretaceous age (Petterson et al., 1999; Ridgway and Coulson 1987). Petterson et al. (1999) concluded that basement sequence of Choiseul is representative of the SSMT.
The sedimentary and volcanic cover ranges in age from Miocene to Recent and in order of decreasing age (Ridgway and Coulson, 1987), they are Mole Formation (chiefly clastic sediments) (Early to Middle Miocene); Komboro and Maetambe Volcanics (Middle Miocene to Pleistocene); Vaghena Formation (calcareous and tuffaceous sediments) (Early Pliocene); Pemba Formation (calcareous sediments) (Early to Late Pliocene); Nukiki Limestone Formation (backreef and lagoonal deposits) (Pliocene); Holocene Deposits including alluvium, mangrove and freshwater swamp; backreef and lagoonal facies sediments and coralline reef limestones (Holocene). Petterson et al. (1997) inferred that stage 1-arc is represented by crystal- and lithic-rich turbidites from the Mole Formation whereas the Komboro and Maetambe Volcanics constitute the stage 2-arc sequence.

Table of contents :

1. INITIAL PURPOSE OF THIS THESIS xiii
1.1. Problematic
1.2. The choice of the Solomon Islands
1.3. Initial objectives of this study
2. STRUCTURE & SUBJECT OF THIS THESIS
2.1. Structure
2.2. Notes
LIST OF FIGURES AND TABLES
List of Figures
List of Tables
CHAPTER 1: GEOLOGICAL BACKGROUND OF THE SOLOMON ISLANDS
1. REGIONAL SETTING OF THE SOLOMON ISLANDS
2. THE ONTONG JAVA PLATEAU (OJP)
2.1. Large Igneous Provinces (LIPs)
2.2. Characteristics and structure of OJP
2.3. Origin and formation of the OJP
2.4. OJP Tectonic evolution (from 125 to ~ 30 Ma)
3. TECTONIC MODELS OF OJP-SOLOMON ARC COLLISION
3.1. Timing of the OJP-Arc collision
3.2. Accretion models
4. PRESENT-DAY GEOLOGICAL FRAMEWORK OF THE SOLOMON ISLANDS
5. THE GEOLOGY OF CHOISEUL AND SAN JORGE/SANTA ISABEL AND ITS RELEVANCE TO GLOBAL GEOLOGICAL FRAMEWORK OF THE SOLOMON ISLANDS
5.1. Geology of Choiseul
5.2. Geology of Santa Isabel and San Jorge
CHAPTER 2: GEOLOGY AND GEOCHEMISTRY OF THE “SIRUKA ULTRAMAFICS”: EVIDENCE FOR FLUID METASOMATISM IN AN ISLAND ARC SETTING
1. INTRODUCTION
2. GEOLOGICAL BACKGROUND AND SAMPLING LOCATION
3. ANALYTICAL METHODS
4. PETROLOGY
5. MINERAL COMPOSITION
5.1. Olivine
5.2. Spinel
5.3. Orthopyroxene
5.4. Clinopyroxene
5.5. Amphibole
5.6. Chlorite
6. WHOLE ROCK COMPOSITION
6.1. Major elements
6.2. Trace elements
7. CLINOPYROXENE GEOCHEMISTRY
8. GEOTHERMOBAROMETRY CHARACTERISTICS
8.1. Temperature
8.2. Pressure
9. METAMORPHIC HISTORY
10. OXYGEN FUGACITY
11. DISCUSSION
11.1. Choiseul peridotites as residues of melting
11.2. Partial melting characteristics
11.3. Choiseul peridotites: MORB or SSZ or OJP-related?
11.4. Evidence for mantle interaction with a metasomatic fluid
12. TECTONIC IMPLICATIONS
12.1. Formation of the peridotites
12.2. Exhumation and obduction
13. CONCLUSION
CHAPTER 3: SUPRA-SUBDUCTION ZONE PYROXENITES FROM SAN JORGE AND SANTA ISABEL (SOLOMON ISLANDS): A METASOMATIC ORIGIN
1. INTRODUCTION
2. GEOLOGICAL SETTING
3. SAMPLING AND PERIDOTITE-PYROXENITE FIELD RELATIONS
4. ANALYTICAL METHODS
4.1. Whole rock analysis
4.2. Mineral analysis
5. PETROGRAPHY
5.1. Primary assemblages
5.2. Retrograde assemblages
6. BULK ROCK COMPOSITION
6.1. Major Elements
6.2. Trace elements
6.2.1. Rare Earth Elements
6.2.2. Other trace elements
7. MINERAL COMPOSITION
7.1. Orthopyroxene
7.2. Clinopyroxene
7.2.1. Primary clinopyroxene
7.2.2. Secondary clinopyroxene
7.3. Olivine
7.4. Spinel
7.5. Amphibole
8. CONDITIONS OF FORMATION OF THE PYROXENITES
8.1. Temperature
8.2. Pressure
9. DISCUSSION
9.1. Arc- or plume-related pyroxenites?
9.2. Mantle versus crustal arc pyroxenites
9.3. Metasomatic formation of SSZ mantle pyroxenites
10. TECTONIC MODEL OF PYROXENITES FORMATION AND EXHUMATION
10.1. Genesis of the pyroxenites
10.2. Exhumation of the pyroxenites
11. CONCLUSION
CHAPTER 4: MINERAL TRACE ELEMENT COMPOSITIONS, STUDY OF FLUID INCLUSIONS AND WHOLE ROCK RE/OS SYSTEM: INDICATIONS OF A COMPLEX SLAB-DERIVED METASOMATIC ORIGIN FOR THE SAN JORGE/SANTA ISABEL PYROXENITES (SOLOMON ISLANDS).
1. INTRODUCTION
2. PRINCIPLES AND ANALYTICAL METHODS
2.1. Mineral trace element chemistry
2.1.1. Principles
2.1.2. Analytical methods
2.2. Study of Fluid Inclusions
2.2.1. Principles
2.2.2. Analytical methods
2.2.2.1. Raman Spectroscopy
2.2.2.2. LA ICP-MS
2.3. The Re/Os system
2.3.1. Principles
2.3.2. Analytical methods
3. RESULTS
3.1. Mineral trace element chemistry
3.1.1. Clinopyroxene
3.1.1.1. Rare Earth Elements (REE)
3.1.1.2. Other trace elements
3.1.2. Orthopyroxene
3.1.2.1. Rare Earth Elements (REE)
3.1.2.2. Other trace elements
3.1.3. Amphibole
3.1.3.1. Rare Earth Elements (REE)
3.1.3.2. Other trace elements
3.1.4. Other minerals
3.1.4.1. Olivine
3.1.4.2. Spinel
3.1.4.3. Pectolite
3.2. Study of the fluid inclusions
3.2.1. Petrography
3.2.1.1. Criteria
3.2.1.2. Fluid inclusions occurrence
3.2.2. Geochemical study of the fluid inclusions
3.2.2.1. By the Raman spectroscopy
3.2.2.2. By the LA ICP-MS
3.3. The Re/Os geochemistry
4. INTERPRETATIONS
4.1. Mineral Partition Coefficients
4.1.1. Opx/Cpx
4.1.2. Amphi/Cpx
4.2. Whole rock Elemental Budgets
4.2.1. Mass balance: comparison between whole rock and mineral trace element chemistry
4.2.2. Trace element distribution for the pyroxenites
4.3. Fluid inclusions
4.4. The Re-Os System
4.4.1. Evidence for Metasomatism
4.4.2. Re-Os isochron and possible age model
5. DISCUSSION
5.1. Characteristics of the pyroxenites from the Solomon Islands
5.1.1. Similarities between the three groups of pyroxenites: a common SSZ affinity
5.1.2. Differences between the three groups of pyroxenites
5.1.2.1. G1-orthopyroxenites
5.1.2.2. G2-websterites
5.1.2.3. G3-clinopyroxenites (except sample 15.07)
5.1.3. Exceptions within the groups
5.1.3.1. In the G1- orthopyroxenites
5.1.3.2. In the G2-websterites
5.1.3.3. In the G3-clinopyroxenites
5.2. Characteristics of the metasomatism: evidence from the G1-orthopyroxenites and the G2-websterites
5.2.1. Types of metasomatism
5.2.2. Composition of the fluid
5.2.3. Provenance and nature of the fluid
5.3. Sediments: source of the metasomatising agents
6. IMPLICATIONS: GENESIS OF THE SAN JORGE AND SANTA ISABEL PYROXENITES
6.1. Formation of the G1-orthopyroxenites and the G2-websterites
6.2. Formation of the G3-clinopyroxenites
7. CONCLUSION
CHAPTER 5: ORIGIN AND NATURE OF THE MAFIC COMPLEXES OF THE SANTA ISABEL, SAN JORGE AND CHOISEUL ISLANDS: EVIDENCE FROM MINERALOGICAL AND GEOCHEMICAL CHARACTERISTICS
1. INTRODUCTION
2. GEOLOGICAL BACKGROUND
2.1. Choiseul Island
2.2. San Jorge and Santa Isabel
3. SAMPLING AND FIELD RELATION
3.1. Choiseul Island
3.2. San Jorge and Santa Isabel
4. TEXTURAL AND PETROLOGICAL CHARACTERISTICS
4.1. The volcanic rocks and schists
4.2. The gabbros
5. WHOLE CHEMISTRY
5.1. Analytical methods
5.2. Volcanic rocks and schists
5.2.1. Major elements
5.2.2. Trace elements
5.2.2.1. Rare Earth Elements (REE)
5.2.2.2. Other trace elements
5.2.3. Tectonic discrimination Nb-Zr diagram
5.3. Gabbros
5.3.1. Major elements
5.3.2. Trace elements
5.3.2.1. Rare Earth Elements (REE)
5.3.2.2. Other trace elements
6. MINERAL CHEMISTRY
6.1. Volcanic rocks and schists
6.1.1. Pyroxene
6.1.1.1. Clinopyroxene
6.1.1.2. Orthopyroxene
6.1.2. Plagioclase
6.1.3. Amphibole
6.2. Gabbros
6.2.1. Pyroxene
6.2.2. Plagioclase
6.2.3. Amphibole
6.3. Mineral trace element compositions
7. SR AND ND ISOTOPIC COMPOSITIONS
7.1. Analytical procedures
7.2. Volcanic rocks and schists
7.3. Gabbros
8. DISCUSSION
8.1. Chemical mobility
8.2. Volcanic rocks and schists: origin and significance
8.2.1. Voza Lavas and Choiseul Schists: one unit with a NMORB/BABB origin
8.2.2. Extrusive rocks from San Jorge: what origin?
8.2.2.1. Is the 16.13-columnar lava a boninite?
8.2.2.2. The 16.14-dyke: a typical MORB-related Fe-Ti basalt
8.2.2.3. Pillow-basalts from Santa Isabel and schist from San Jorge: a OJP origin
8.3. The gabbros from Santa Isabel/San Jorge and Choiseul: origin and significance
8.3.1. Gabbros from Choiseul
8.3.1.1. The coarse-grained to pegmatitic gabbros: an arc-like cumulate
8.3.1.2. The Oaka Metamicrogabbros: frozen melts from BABB environment
8.3.2. The Gabbros from Santa Isabel and San Jorge
8.3.2.1. A cumulate origin
8.3.2.2. Composition of parental melt: evidence for arc origin
8.3.2.3. San Jorge and Santa Isabel gabbros: evidence for highpressure arc-cumulates
9. CONCLUSION
CHAPTER 6: CONSEQUENCES OF THE OJP-ARC COLLISION
1. IMPLICATIONS FOR THE DEHYDRATION PROCESS
1.1. Before OJP-arc collision
1.2. Chocking of the subduction: OJP arrival at the subduction zone
1.2.1. Melting of the sediment: is it possible?
1.2.2. The transition blueschist-eclogite at high depth?
2. IMPLICATIONS FOR THE COMPOSITION OF THE MANTLE WEDGE
2.1. Before OJP-arc collision: metasomatism and melting of the mantle wedge
2.2. Chocking of the subduction: OJP arrival at the subduction zone
2.2.1. Formation of the G1- and G2-pyroxenites
2.2.2. Genesis of boninite
3. HARD OJP-ARC COLLISION: IMPLICATIONS FOR THE EXHUMATION AND OBDUCTION PROCESSES
3.1. Exhumation of portions of mantle wedge along the slab
3.2. Exhumation of SSZ-related terranes
3.3. Obduction of the OJP-related terranes
CONCLUSION 
References 
Appendices

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