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Olivine-hosted homogenized melt inclusions in a primitive basalt AII32-12-7 from 43oN Mid-Atlantic Ridge have magnesian basalt compositions (10-12 wt% MgO) with high CaO (13.2-15.2 wt%), relatively low Al2O3 (12.8-15.5 wt%), and form a linear array that ranges to extremely high CaO/Al2O3 values (0.8-1.2). These melt compositions are unusual for MORB, as is the observed phenocryst assemblage, which comprises primitive olivine (Fo(87-92) with up to 0.45 wt% CaO), Cr-diopside (Mg# 90-92), and Cr-rich spinel (Cr# 50-70) and directly reflects these melt compositions. The melt compositional array extends from peridotite-saturated compositions formed near 1 GPa to lie well within the clinopyroxene phase volume, or possibly along a clinopyroxene + olivine phase boundary. We interpret the array as either the product of melt-wallrock reaction between a I GPa MORB melt and a clinopyroxene-rich lithology (wehrlite or clinopyroxenite), or of mixing between melt fractions derived separately from these distinct lithologies (i.e. peridotite and clinopyroxenite/wehrlite). Derivation of the melt array from a conventional mantle peridotite source, possibly involving fractional melting near or beyond the point of clinopyroxene exhaustion, is inconsistent with the melt compositions and the trend of the array. Trace element abundance patterns in the melt inclusions range from depleted to highly enriched (e.g. La-n/Yb-n 0.6-7.0), and indicate the generation of compositionally diverse melt fractions via fractional melting processes and/or melting of geochemically distinct source heterogeneities. Most melt inclusions, and the pillow-rim glass, are enriched in the more incompatible trace elements, and have high Nb and Ta contents relative to other highly incompatible elements. These characteristics and the Pb isotopic composition of the pillow-rim glass (Pb-206/Pb-204 = 19.654) indicate the presence of a HIMU mantle source component that can be linked to lateral dispersion of a geochemical s

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclaseþclinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (425) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu– Au mineralised, plagioclaseþhornblendeþquartz-phyric dacites with medium-K calc- alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites ar

The Arthur Lineament is a Cambrian age high-strain metamorphic belt that transects northwestern Tasmania and forms the eastern margin of the Mesoproterozoic and Neoproterozoic Rocky Cape Block. It formed as a result of Middle Cambrian arc–continent collision, and is composed of both allochthonous slices (Bowry Formation and Reece amphibolite), the para-autochthonous ‘eastern’ Ahrberg Group and the autochthonous ‘western’ Ahrberg Group. Amphibolites and mafic schists of the ‘eastern’ Ahrberg Group rocks show a temporal change in composition from transitional alkaline basalt to dominant E-MORB-type rift tholeiites, whereas the ‘western’ Ahrberg Group metabasic rocks are typical E-MORB rift tholeiites. The lithostratigraphy of the Ahrberg Group matches very well that of the Upper Neoproterozoic (650–580 Ma) Togari Group of the Smithton Trough and King Island. In contrast, amphibolites of the allochthonous Bowry Formation, which also show E-MORB-type patterns typical of rift-type tholeiites, are intruded by granitoid sheets dated at 777 ± 7 Ma, similar to the Precambrian granites exposed on King Island (760 ± 12 Ma). The Bowry Formation granitoids have distinctive high Ti, Zr, Nb compositions best matched by granites produced by extended fractionation of rift basalts with significant crustal assimilation. This suggests that the Bowry Formation metabasic rocks may correlate with the Willouran flood basalts in South Australia, and together record the ca 780 Ma breakup event hypothesised to mark the breakup of the Rodinian supercontinent.

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclaseþclinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (425) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu– Au mineralised, plagioclaseþhornblendeþquartz-phyric dacites with medium-K calc- alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites ar

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

Olivine-hosted homogenized melt inclusions in a primitive basalt AII32-12-7 from 43oN Mid-Atlantic Ridge have magnesian basalt compositions (10-12 wt% MgO) with high CaO (13.2-15.2 wt%), relatively low Al2O3 (12.8-15.5 wt%), and form a linear array that ranges to extremely high CaO/Al2O3 values (0.8-1.2). These melt compositions are unusual for MORB, as is the observed phenocryst assemblage, which comprises primitive olivine (Fo(87-92) with up to 0.45 wt% CaO), Cr-diopside (Mg# 90-92), and Cr-rich spinel (Cr# 50-70) and directly reflects these melt compositions. The melt compositional array extends from peridotite-saturated compositions formed near 1 GPa to lie well within the clinopyroxene phase volume, or possibly along a clinopyroxene + olivine phase boundary. We interpret the array as either the product of melt-wallrock reaction between a I GPa MORB melt and a clinopyroxene-rich lithology (wehrlite or clinopyroxenite), or of mixing between melt fractions derived separately from these distinct lithologies (i.e. peridotite and clinopyroxenite/wehrlite). Derivation of the melt array from a conventional mantle peridotite source, possibly involving fractional melting near or beyond the point of clinopyroxene exhaustion, is inconsistent with the melt compositions and the trend of the array. Trace element abundance patterns in the melt inclusions range from depleted to highly enriched (e.g. La-n/Yb-n 0.6-7.0), and indicate the generation of compositionally diverse melt fractions via fractional melting processes and/or melting of geochemically distinct source heterogeneities. Most melt inclusions, and the pillow-rim glass, are enriched in the more incompatible trace elements, and have high Nb and Ta contents relative to other highly incompatible elements. These characteristics and the Pb isotopic composition of the pillow-rim glass (Pb-206/Pb-204 = 19.654) indicate the presence of a HIMU mantle source component that can be linked to lateral dispersion of a geochemical s

The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island are, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain are, and differ significantly from those of MORE-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo(82-92)) Of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na-Ca carbonates, halite and Na-K-Ca alumine-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially from the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO2-H2O fluid, which i

Phenocryst assemblages, and mineral and melt inclusion compositions of magmas erupted at near-axis seamounts on either side of Valu Fa Ridge provide a hitherto unprecedented insight into the complexity of magma generation in this back-are basin tectonic setting, Two fundamentally different primitive primary melt compositions are identified based on melt inclusion compositions, olivine phenocryst chemistry, and the early co-crystallisation of either magnesian clinopyroxene (Mg# to 93) or magnesian orthopyroxene (Mg# to 93.5) with magnesian olivine (to Fo(94)) and Cr-rich spinel (Cr# = 0.78-0.87). One magma type is a H2O-rich (similar to 2.5 wt%), high-CaO (similar to 14 wt%), low-Al2O3 (similar to 8 wt%) magnesian basalt, variants of which occur in both the eastern and western seamounts, The other is a low-Ca boninite-like magma that only occurs as a component of the western seamount magmas.Large and systematic variations in incompatible trace-element compositions of melt inclusions trapped in primitive olivine phenocrysts, reflect an integration of diverse but geochemically related melt fractions to produce the magmas at each seamount. Trace-element systematics require the variable addition of a LILE-, Pb-, and Cl-rich component to the mantle wedge source with increased influence toward the Tofua are. This component, as invoked in most models of are magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting ocean crust beneath the volcanic are front.We propose that southward propagation of the back-are basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting of refractory hydrated sub-are lithosphere, probably veined by clinopyroxene-rich dykes in the case of the high-CaO magma series, These near-ridge seamount lavas are very similar to those drilled at ODP Site 839 in the Lau Basin, and we suggest that the Site 839 basalts, as well as other Lau Basin seamount are-like magmas

Compositions of spinel and grassy melt inclusions in primitive olivine (FO89.3-91) from basalt AII32 D11-177 at 43°N, Mid-Atlantic Ridge fall into two principal groups. The dominant (similar to 90%) Group-I spinel and melt inclusions have typical MORB compositions. In contrast, Group-II. Cr-spinels are strongly enriched in TiO2 (2.6-4.1 wt%), and Group-II melt inclusions show significant enrichment in SiO2 (54.6-58.4 wt%), TiO2, Na2O and K2O, whereas their CaO contents (9.3-11.1 wt%) are unusually low. Group-II melts are also remarkable in crystallizing high-Mg orthopyroxene (Mg# 91). These mineral associations and melt compositions are unusual for MORB, and are interpreted to result from interaction between MORB-like melts and harzburgitic peridotite at low pressure.

At its southernmost end, the main spreading centre of the North Fiji Basin is propagating into arc crust of the poorly-known Hunter Ridge. We define nine magmatic groups from major element glass chemistry and olivine and spinel compositions in samples dredged from twenty six sites in this area by the ''R/V Academician A. Nesmeyanov'' in 1990. These include groups of boninites, island arc tholeiites (IAT), mid-ocean ridge basalts (MORB), enriched mid-ocean ridge basalts (E-MORB), olivine porphyritic andesites and basaltic andesite and Na-rhyolites. Primitive lavas containing highly forsteritic olivine phenocrysts are common in all the groups, except for the rhyolites.We report over 100 glass analyses for dredged rocks from this region, and about 300 olivine-spinel pairs for representatives of all the magmatic groups identified, except the Na-rhyolites.The MORB in this region are probably produced at the propagating spreading centre in the southern part of the North Fiji Basin. Juxtaposition of shallow, hot MORB-source diapirs supplying the MORB in this area, and the sub-arc damp, refractory upper mantle beneath the Hunter Ridge, provides suitable petrogenetic conditions to produce a range of magma types, from island arc tholeiites through to high-Ca boninites. The latter were recovered in eleven dredges.The E-MORB lavas recovered from the extreme southern margin of the North Fiji Basin are shown to be essentially identical to those dredged from adjacent older South Fiji Basin crust. It is hypothesized that the former were either scraped off the South Fiji Basin crust during an episode of oblique subduction that may have generated the Hunter Ridge during the last 5 Myr, or alternatively, that slices of the South Fiji Basin crust were trapped and incorporated into the North Fiji Basin as the subduction zones fronting the Vanuatu arc stepped or propagated southward.

TheOrdovician volcanic and intrusive rocks of the Cadia –Neville region, in the southern Molong Volcanic Belt section of the Macquarie Arc in central-western New South Wales, display a temporal progression from shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornian (ca 460 – 453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-type dacite) in the late Eastonian to early Bolindian (ca 450 – 445 Ma) to large-volume, high-K calc-alkaline to shoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia Intrusive Complex) at about 445 – 440 Ma. The two episodes of shoshonitic magmatism are separated by at least 15 (but up to *20) million years, during which time limestones, associated with regional uplift, were deposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions. Despite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism have strikingly similar geochemistry, indicating that a similar mantle source and partial melting processes were involved over this time interval. The arc-type geochemistry of the volcanic and intrusive rocks in the Cadia–Neville region is very similar to the lavas and intrusions forming the Pliocene Tavua Caldera in Viti Levu, Fiji. The Pliocene shoshonitic rocks at Tavua Caldera were emplaced following cessation of arc magmatism when the arc moved into an extensional phase. By analogy with the Pliocene tectonic setting in Fiji, the arc-like signature of the Ordovician magmas of the Macquarie Arc may be due to preprocessing of the mantle above a subduction zone prior to extension and fragmentation of arc lithosphere that led to generation of the subduction-modified shoshonite-dominated magmas.

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclaseþclinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (425) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu– Au mineralised, plagioclaseþhornblendeþquartz-phyric dacites with medium-K calc- alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites ar

The eastern coast of King Island in southeastern Australia exposes a thick, well-preserved sequence of latest Neoproterozoic volcanic, and related shallow intrusive rocks. These rocks are associated with shallow marine carbonates and siltstones and pass up into massive conglomerates representing a marine flooding event and unconformity, during continental break-up and subsequent volcanic passive margin formation. Unusual differentiated sills (Grimes Intrusive suite) with extreme internal variation (wehrlite to andesite compositions) intrude deformed Proterozoic metasediments of the Rodinian basement. A thin, basal tholeiitic basaltic volcanic unit (City of Melbourne Volcanics) is less contaminated than the underlying sills, and preceded eruption of a thick sequence of highly depleted picritic pillows, sub-aerial flows and hyaloclastites (Shower Droplet Volcanics). The picrite sequence is overlain by thick tholeiitic basalts and reworked volcanogenic conglomerates (Bold Head Formation) that show a strong compositional similarity to enriched mid ocean ridge basalts. Both the picrites and the upper tholeiitic basalts are not crustally contaminated and have an Nd-Sm isochron age of 579 +/- 16 Ma with initial epsilonNd of +4.2. The lithostratigraphy and range of compositions represented are analogous to early magmatism associated with continental break-up and volcanic passive margin formation, including voluminous Seaward Dipping Reflector Sequences, in the Mesozoic North Atlantic volcanic margins.

Al-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island are volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Ball in the Sunda are, and Epi, Merelava, and Ambrym islands in the Vanuatu are. More commonly, relatively Cr-rich spinels also occur as inclusions in the same olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localised Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) incongruent breakdown of amphibole in amphibole-rich cumulates in sub-are, or sub-GIB volcano magma chambers. The latter reaction produces a melt with similar to 20-22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

Basalts of the Efate Island Group ( less than 0.7 Ma) occur in the central part of the Vanuatu island arc between the normal southern section of the arc and the central part of the arc affected by collision with the Eocene DEntrecasteaux Ridge. These basalts form small cones constructed upon a volcaniclastic apron principally composed of trachydacitic breccias. Temporal and compositional features allow identification of three basalt groups. Subtle compositional differences exist between two of these suites, whereas the third shows pronounced enrichment in K and Rb, with few differences for other elements. These variations are taken to record slight compositional variations in the ambient mantle wedge composition, with the high-K, high-Rb suite deriving from localised zones relatively enriched in phlogopite. Compared with other Vanuatu arc basalts, the Efate basalts show appreciable enrichment in Sr and P2O5. The Pb isotopic characteristics of the Efate basalts are commensurate with the transitional position of this island within the Vanuatu arc, between the southern arc lavas and those erupted in the region affected by DEntrecasteaux Ridge collision. However, 87Sr/86Sr values (0.70387-0.70424) are slightly higher and 143Nd/144Nd values slightly lower than predicted by the spatial change of these values along the arc. These features are taken to indicate derivation of the Efate basalts from a local, unusual mantle source. Two contrasting models are considered to account for the unusual features of the source mantle of these basalts. Model 1 suggests that the high Sr and P2O5 characteristics of the peridotite source of the Efate basalts were produced by subduction of an intraplate volcano similar to ORSTOM seamount, currently subducting immediately northwest of Efate. Recent studies have demonstrated massive P2O5 and Sr enrichments in highly altered formerly glassy seamount lavas and hyaloclastites. Model 2 proposes that the ambient strongly depleted mantle wedge has

Uncertainty about the timing and location of the initiation of convergence in the western and south-western Pacific greatly hinders accurate plate tectonic reconstructions of subduction systems in that area. The chemistry and age of dikes intruding mantle peridotite in the ophiolite of New Caledonia infer that subduction-related magmatism began before 53 Ma. These new results infer that obduction in the south-west Pacific is unrelated to the reorientation of the Pacific plate motion that occurred c. 43 Ma and confirm new interpretations showing that changes in mantle flow, hotspot and plate motion may have occurred as soon as late Paleocene or early Eocene.

The Tasman Line, a much-discussed concept in the geology and tectonics of eastern Australia, has a long and chequered history of interpretation. This extends to current debates regarding the age and position of the Tasman Line in Gondwana-Rodinia reconstructions. We present constraints, from mapping, geochemistry and geophysics, on the interpretation of gravity and magnetic lineaments attributed to the Tasman Line in New South Wales, South Australia, Victoria and Tasmania. These pieces of evidence suggest a protracted and complex latest Neoproterozoic to Carboniferous geological history that produces a variety of geophysical responses, rather than a simple 'Line'. We also find no evidence of Rodinian breakup age activity responsible for any of the anomalies. In light of these findings, our preference is that the Tasman Line concept be abandoned as misleading, especially with regard to models of Rodinia-Gondwana breakup, which must have occurred elsewhere, possibly well to the east. Instead, the rocks preserved in the westernmost part of the Tasmanides are consistent with previously proposed 'Southwest Pacific'-style models for Neoproterozoic continental breakup, margin formation and reaccretion of continental fragments in the Early Palaeozoic.

Taiwan is an active mountain belt formed by oblique collision between the Luzon are and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio-Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at similar to0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO approximate to 15 wt.%) and potassic (K2O approximate to 5 wt.%, K2O/Na2O approximate to 1.6-3.0). Whereas these basic rocks (SiO2 approximate to 48 wt.%) have relatively low Al2O3 approximate to 12 wt.%, total Fe2O3 approximate to 7.5 wt.% and CaO approximate to 7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, > 1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high Field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu * approximate to 0.90 - 0.84), and Sr and Nd isotope ratios are uniform (Sr-87/Sr-86 approximate to 0.70540-0.70551; Nd-143/(144) Nd approximate to 0.51268-0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4 +/- 1.8; 1 sigma deviation), which are in agreement with the whole rock Mg-values (83 - 80). Spinel inclusions in olivine are characterized by high Cr/Cr+Al ratios (0.94-0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet, We therefore suggest that the T

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of~9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive eNd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee-Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee-Narromine Volcanic Belt have ages that cluster in the 470-460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calcalkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley-Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee-Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley-Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian-Devonian opening of the Hill End Trough. P

Much of the northern and southern sections of the Junee-Narromine Volcanic Belt of the Ordovician Macquarie Arc in central-western New South Wales are buried beneath the sediment cover of the Great Artesian Basin. Exploration drilling of these aeromagnetically defined blocks has provided important new material to assess the temporal and magmatic affinities of the rocks in the Narromine Igneous Complex (northern end) and the Cowal and Fairholme Igneous Complexes (southern end). Basement rocks are representative of Phase 1 magmatism in the Macquarie Arc, and consist of Lower Ordovician basalts and andesites and common volcaniclastic rocks, all with high-K calc-alkaline affinities. These are very similar compositionally to the Lower Ordovician Nelungaloo Volcanics that outcrop in the central part of the Junee-Narromine Volcanic Belt west of Parkes. Intruding the basement volcanic-volcaniclastic package are three distinct igneous suites. The oldest of these, dated at ca 466-460 Ma (Middle Ordovician) and representative of Phase 2 magmatism in the Macquarie Arc, consists of stocks of monzogabbro, monzodiorite and monzonite with high-K calc-alkaline affinities, and is well represented in the Narromine and Cowal Igneous Complexes. Phase 2 suite rocks were, in turn, intruded in the Bolindian (445 plus or minus 5 Ma) by kilometre-size stocks, narrow sheets and dykes made up of hornblende gabbro, diorite and granodiorite, and including quartz+plagioclase+hornblende-phyric dacitic porphyries in the Narromine Igneous Complex. These medium-K calc-alkaline rocks, assigned to the Phase 3 Copper Hill Suite that also occurs in the Molong and Rockley-Gulgong Volcanic Belts further east, crystallised from andesitic or more evolved magmas, probably derived via partial melting of low-K rocks in the arc basement, or amphibolites in the subducting slab. The youngest rocks in these igneous complexes are believed to be the high-level shoshonitic intrusives in the Cowal and Fairholme Igneous

Detrital spinel is a widespread heavy mineral in sandstones from the Maastrichtian-Middle Eocene sedimentary basins in the SE Alps. Chemistry of detrital spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slovenia) is used to constrain petrological and geochemical affinities and tectonic provenance of the source rocks. In addition, we have analysed melt inclusion compositions in the detrital volcanic spinels to better constrain the nature of their parental magmas. This is the first study of melt inclusions in detrital spinels. Two principal compositional groups of detrital spinels are recognised based on their TiO2 and Fe2+/Fe3+; one derived from peridotites, the other from basaltic volcanics. Peridotitic spinels are more abundant and have TiO2 < 0.2 wt% and high Cr/Cr + Al(40-90), characteristic of suprasubduction zone harzburgites. Significant chemical variations among volcanic spinels (TiO2 up to 3 wt%, Al2O3 12-44 wt%) suggest multiple sources, with geochemically distinct characteristics, including MORE-type and backarc basin basalts, subduction-related magmas and tholeiites produced during early continental rifting. Compositions of homogenised melt inclusions in spinels with TiO2 > 0.2 better distinguish the differences between the compositions of their host spinels and help to further clarify the geodynamic provenance of extrusive source rocks. Several compositional groups of melt inclusions have been recognised and represent diverse magmatism of marginal basins, including MORB- and subduction-related geochemical types, as well as magmas characteristic of early continental rifting. These results, combined with the data on regional ophiolitic complexes and tectonic reconstructions favour the Internal Dinarides of Yugoslavia as a possible source area for the SE Alps sediments.

Melt inclusions are small portions of liquid trapped by growing crystals during magma evolution. Recent studies of melt inclusions have revealed a large range of unusual major and trace element compositions in phenocrysts from primitive mantle-derived magmatic rocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-An plagioclase]. Inclusions in phenocrysts crystallized from more evolved magmas (e.g. olivine Fo < 85 mol %), are usually compositionally similar to the host lavas. This paper reviews the chemistry of melt inclusions in high-Fo olivine phenocrysts focusing on those with anomalous major and trace element contents from mid-ocean ridge and subduction-related basalts.We suggest that a significant portion of the anomalous inclusion compositions reflects localized, grainscale dissolution-reaction-mixing (DRM) processes within the magmatic plumbing system. The DRM processes occur at the margins of primitive magma bodies, where magma is in contact with cooler wall rocks and/or pre-existing semi-solidified crystal mush zones (depending on the specific environment). Injection of hotter, more primitive magma causes partial dissolution (incongruent melting) of the mush-zone phases, which are not in equilibrium with the primitive melt, and mixing of the reaction products with the primitive magma. Localized rapid crystallization of high-Fo olivines from the primitive magma may lead to entrapment of numerous large melt inclusions, which record the DRM processes in progress. In some magmatic suites melt inclusions in primitive phenocrysts may be naturally biased towards the anomalous compositions. The occurrence of melt inclusions with unusual compositions does not necessarily imply the existence of new geologically significant magma types and/or melt-generation processes, and caution should be exercised in their interpretation.

The composition of primary magmas and their mantle sources can be successfully inferred from the study of melt inclusions trapped in spinel phenocrysts. This is particularly true in the case of severely altered rocks, in which spinel and spinel-hosted melt inclusions usually retain primary magmatic information. We report the results of the study of melt inclusions in high-Cr (Cr# 90-95), primitive (Mg# 65-78) spinel in the Palaeocene low-Ca boninites from Cape Vogel, Papua New Guinea. Melt inclusions are represented by the aggregate of skeletal olivine crystals in the residual glass. Raster beam electron Microprobe analyses of melt inclusions demonstrated that they are broadly similar in composition to primitive orthopyroxene and to the most primitive boninites in this area, having (in wt.%): very high MgO (18-30), SiO2 (53-61) and very low TiO2 (0.04-0.19), Al2O(3) (3-9), CaO (2-4), Na2O (<0.9), K2O (0.05-0.15) and CaO/Al2O3 (0.4-0.6). H2O abundances in melt inclusions, analysed by an ion probe, are very high (1-2 wt.%), and they could have been even higher (similar to3.5 wt.%) if the melts lost H2O before crystallisation. Trace elements in melt inclusions, analysed by laser ablation ICPMS, have exceptional depletion in HREE (<1 PM) and significant enrichment in LREE over HREE (La/Yb 5-12), and Ph (Ce/Pb 2-12) and Zr (Zr-N/Sm-N 2-3.4) over REE. The compositions of melt inclusions correlate well with the compositions of host spinel showing the fractionation path of initial ultramafic melt. Cape Vogel primary melts could have originated from melting of extremely refractory hot (> 1500 degreesC) harzburgitic mantle fluxed by subduction-related, H2O-bearing enriched components. Trace element composition of these enriched components is estimated from melt inclusion compositions by mass balance calculations.

Twenty-one alkalic porphyry deposits of Late Ordovician to Early Silurian age occur in two mineral districts in New South Wales. The Cadia and Northparkes districts formed in shoshonitic volcanic centres where a major basement structure (the Lachlan Transverse Zone) cut the Ordovician Macquarie Arc obliquely. Processes of mineralisation in both districts were centred in and around quartz monzonite porphyry complexes that intruded the volcanic centres. These composite intrusive complexes comprise pipes, dykes and stocks. Hydrothermal alteration in and around the intrusions produced a complex sequence of potassic, calc-potassic, sodic, propylitic and late stage, typically fault- and fracturecontrolled phyllic assemblages. Hematite dusting was a common alteration product giving the intrusions and the altered volcano-sedimentary host sequences a distinctive pink-orange coloration. Several of the deposits have bornite-rich cores, chalcopyrite-dominant annuli and pyritic outer haloes. Gold is well correlated with bornite in most of the deposits, and with chalcopyrite at Cadia Hill. The mineralising intrusions have Sr and Nd isotopic compositions consistent with derivation from a depleted mantle source regime, although the Nd data are permissive of limited crustal contamination. Epidote peripheral to the porphyry deposits has Sr isotopic compositions indistinguishable from the host intrusions, precluding the involvement of external seawater in the mineralising processes. In contrast, slightly elevated initial Sr values have been detected in epidote from nearby skarn deposits, indicative of incorporation of a minor component of Sr from limestone dissolution or seawater mixing. The alkalic porphyry deposits are difficult exploration targets because intensely developed hydrothermal alteration zones are restricted to within a few hundred metres of the monzonite complexes.

Major advances have been made over the last decade in our understanding of the distribution, compositions, age and significance of Late Neoproterozoic and Cambrian rocks in Victoria.These advances have been driven mainly by the new generation of geological mapping carried out by the Geological Survey of Victoria, using detailed aeromagnetic, gravity and radiometric datasets covering much of the state. In addition, detailed work by universities over certain areas, particularly the Glenelg region in the far west and locally around Stawell, have made significant contributions to the knowledge of the Cambrian geology. In western Victoria, Cambrian rocks comprise all the known sedimentary and volcanic bedrock as well as numerous granites. Although there is some possibility of Proterozoic rocks occurring here, no dated rocks have returned ages older than Cambrian. The rock units and geological histories of the Glenelg, Grampians, Stavely and Stawell zones, which were poorly known until recently, are now much better understood. In central and eastern Victoria, the Cambrian rocks generally lie at deeper structural levels and are only exposed in the hanging walls of major faults. Important new information is available for the Glenelg River Complex in westernmost Victoria, the exposed and drilled volcanic belts in western Victoria, and the Cambrian greenstones around Pitfield and on Phillip Island.

Hypotheses for the formation of many types of hydrothermal ore deposits often involve the direct contribution of magma-related fluids (e.g., Cu-Mo-Au porphyries) or their superimposition on barren hydrothermal cells (e.g., volcanic-hosted massive sulfide deposits). However, the chemical and phase compositions of such fluids remain largely unknown. We report preliminary results of a comprehensive study of fluid bubbles trapped inside glassy melt inclusions in primitive olivine phenocrysts and pillow-rim glasses from basaltic magmas from different tectonic environments, including mid-ocean ridges (Macquarie Island, SW Pacific and Mid-Atlantic Ridge 43°N Fracture Zone), ocean islands (Hawaii) and a variety of modern and ancient backarc-island arc settings (eastern Manus Basin, Okinawa and Vanuatu Troughs, Troodos, New Caledonia and Hunter Ridge-Hunter Fracture Zone). Fluid bubbles from all localities, studied using electron microscopy with EDS and laser Raman spectroscopy, are composed of CO2-(± H2O ± sulfur)-bearing vapor and contain significant amounts of amorphous (Na-K-Ca-Fe alumino-silicates and dissorded carbon) and crystalline phases. The crystals are represented mainly by carbonates (magnesite, calcite, ankerite, dolomite, siderite, nahcolite and rhodochrosite), sulfates (anhydrite, gypsum, barite and anglesite), and sulfides (pyrite, arsenopyrite, chalcopyrite and marcasite), though other minerals (brukite, apatite, halite, clinoenstatite, kalsilite, nepheline, amphibole and mica) may occur as well. We argue that chemical components (e.g., C, H, S, Cl, Si, Al, Na, K, Fe, Mn, Cr, Ca, Mg, Ba, Pb and Cu) that later formed precipitates in fluid bubbles were originally dissolved in the magmatic fluid, and were not supplied by host glasses or phenocrysts after entrapment. Magma-related fluid rich in dissolved metals and other non-volatile elements may be a potential precursor to ore-forming solutions.

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental ribbons, including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These microcontinental ribbons were isolated from Australia and from each other during a protracted extension episode from ca 120 to 52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major change in Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spreading centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty-Entrecasteaux arc. Continued subduction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 70-60 million years old western volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorly exposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty-Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist and eclogite facies metamorphic assemblages. Following locking o

TheOrdovician volcanic and intrusive rocks of the Cadia –Neville region, in the southern Molong Volcanic Belt section of the Macquarie Arc in central-western New South Wales, display a temporal progression from shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornian (ca 460 – 453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-type dacite) in the late Eastonian to early Bolindian (ca 450 – 445 Ma) to large-volume, high-K calc-alkaline to shoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia Intrusive Complex) at about 445 – 440 Ma. The two episodes of shoshonitic magmatism are separated by at least 15 (but up to *20) million years, during which time limestones, associated with regional uplift, were deposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions. Despite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism have strikingly similar geochemistry, indicating that a similar mantle source and partial melting processes were involved over this time interval. The arc-type geochemistry of the volcanic and intrusive rocks in the Cadia–Neville region is very similar to the lavas and intrusions forming the Pliocene Tavua Caldera in Viti Levu, Fiji. The Pliocene shoshonitic rocks at Tavua Caldera were emplaced following cessation of arc magmatism when the arc moved into an extensional phase. By analogy with the Pliocene tectonic setting in Fiji, the arc-like signature of the Ordovician magmas of the Macquarie Arc may be due to preprocessing of the mantle above a subduction zone prior to extension and fragmentation of arc lithosphere that led to generation of the subduction-modified shoshonite-dominated magmas.

The Tasman Line, a much-discussed concept in the geology and tectonics of eastern Australia, has a long and chequered history of interpretation. This extends to current debates regarding the age and position of the Tasman Line in Gondwana-Rodinia reconstructions. We present constraints, from mapping, geochemistry and geophysics, on the interpretation of gravity and magnetic lineaments attributed to the Tasman Line in New South Wales, South Australia, Victoria and Tasmania. These pieces of evidence suggest a protracted and complex latest Neoproterozoic to Carboniferous geological history that produces a variety of geophysical responses, rather than a simple 'Line'. We also find no evidence of Rodinian breakup age activity responsible for any of the anomalies. In light of these findings, our preference is that the Tasman Line concept be abandoned as misleading, especially with regard to models of Rodinia-Gondwana breakup, which must have occurred elsewhere, possibly well to the east. Instead, the rocks preserved in the westernmost part of the Tasmanides are consistent with previously proposed 'Southwest Pacific'-style models for Neoproterozoic continental breakup, margin formation and reaccretion of continental fragments in the Early Palaeozoic.

Major advances have been made over the last decade in our understanding of the distribution, compositions, age and significance of Late Neoproterozoic and Cambrian rocks in Victoria.These advances have been driven mainly by the new generation of geological mapping carried out by the Geological Survey of Victoria, using detailed aeromagnetic, gravity and radiometric datasets covering much of the state. In addition, detailed work by universities over certain areas, particularly the Glenelg region in the far west and locally around Stawell, have made significant contributions to the knowledge of the Cambrian geology. In western Victoria, Cambrian rocks comprise all the known sedimentary and volcanic bedrock as well as numerous granites. Although there is some possibility of Proterozoic rocks occurring here, no dated rocks have returned ages older than Cambrian. The rock units and geological histories of the Glenelg, Grampians, Stavely and Stawell zones, which were poorly known until recently, are now much better understood. In central and eastern Victoria, the Cambrian rocks generally lie at deeper structural levels and are only exposed in the hanging walls of major faults. Important new information is available for the Glenelg River Complex in westernmost Victoria, the exposed and drilled volcanic belts in western Victoria, and the Cambrian greenstones around Pitfield and on Phillip Island.

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of~9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive eNd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee-Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee-Narromine Volcanic Belt have ages that cluster in the 470-460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calcalkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley-Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee-Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley-Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian-Devonian opening of the Hill End Trough. P

Much of the northern and southern sections of the Junee-Narromine Volcanic Belt of the Ordovician Macquarie Arc in central-western New South Wales are buried beneath the sediment cover of the Great Artesian Basin. Exploration drilling of these aeromagnetically defined blocks has provided important new material to assess the temporal and magmatic affinities of the rocks in the Narromine Igneous Complex (northern end) and the Cowal and Fairholme Igneous Complexes (southern end). Basement rocks are representative of Phase 1 magmatism in the Macquarie Arc, and consist of Lower Ordovician basalts and andesites and common volcaniclastic rocks, all with high-K calc-alkaline affinities. These are very similar compositionally to the Lower Ordovician Nelungaloo Volcanics that outcrop in the central part of the Junee-Narromine Volcanic Belt west of Parkes. Intruding the basement volcanic-volcaniclastic package are three distinct igneous suites. The oldest of these, dated at ca 466-460 Ma (Middle Ordovician) and representative of Phase 2 magmatism in the Macquarie Arc, consists of stocks of monzogabbro, monzodiorite and monzonite with high-K calc-alkaline affinities, and is well represented in the Narromine and Cowal Igneous Complexes. Phase 2 suite rocks were, in turn, intruded in the Bolindian (445 plus or minus 5 Ma) by kilometre-size stocks, narrow sheets and dykes made up of hornblende gabbro, diorite and granodiorite, and including quartz+plagioclase+hornblende-phyric dacitic porphyries in the Narromine Igneous Complex. These medium-K calc-alkaline rocks, assigned to the Phase 3 Copper Hill Suite that also occurs in the Molong and Rockley-Gulgong Volcanic Belts further east, crystallised from andesitic or more evolved magmas, probably derived via partial melting of low-K rocks in the arc basement, or amphibolites in the subducting slab. The youngest rocks in these igneous complexes are believed to be the high-level shoshonitic intrusives in the Cowal and Fairholme Igneous

Twenty-one alkalic porphyry deposits of Late Ordovician to Early Silurian age occur in two mineral districts in New South Wales. The Cadia and Northparkes districts formed in shoshonitic volcanic centres where a major basement structure (the Lachlan Transverse Zone) cut the Ordovician Macquarie Arc obliquely. Processes of mineralisation in both districts were centred in and around quartz monzonite porphyry complexes that intruded the volcanic centres. These composite intrusive complexes comprise pipes, dykes and stocks. Hydrothermal alteration in and around the intrusions produced a complex sequence of potassic, calc-potassic, sodic, propylitic and late stage, typically fault- and fracturecontrolled phyllic assemblages. Hematite dusting was a common alteration product giving the intrusions and the altered volcano-sedimentary host sequences a distinctive pink-orange coloration. Several of the deposits have bornite-rich cores, chalcopyrite-dominant annuli and pyritic outer haloes. Gold is well correlated with bornite in most of the deposits, and with chalcopyrite at Cadia Hill. The mineralising intrusions have Sr and Nd isotopic compositions consistent with derivation from a depleted mantle source regime, although the Nd data are permissive of limited crustal contamination. Epidote peripheral to the porphyry deposits has Sr isotopic compositions indistinguishable from the host intrusions, precluding the involvement of external seawater in the mineralising processes. In contrast, slightly elevated initial Sr values have been detected in epidote from nearby skarn deposits, indicative of incorporation of a minor component of Sr from limestone dissolution or seawater mixing. The alkalic porphyry deposits are difficult exploration targets because intensely developed hydrothermal alteration zones are restricted to within a few hundred metres of the monzonite complexes.

Basalts of the Efate Island Group ( less than 0.7 Ma) occur in the central part of the Vanuatu island arc between the normal southern section of the arc and the central part of the arc affected by collision with the Eocene DEntrecasteaux Ridge. These basalts form small cones constructed upon a volcaniclastic apron principally composed of trachydacitic breccias. Temporal and compositional features allow identification of three basalt groups. Subtle compositional differences exist between two of these suites, whereas the third shows pronounced enrichment in K and Rb, with few differences for other elements. These variations are taken to record slight compositional variations in the ambient mantle wedge composition, with the high-K, high-Rb suite deriving from localised zones relatively enriched in phlogopite. Compared with other Vanuatu arc basalts, the Efate basalts show appreciable enrichment in Sr and P2O5. The Pb isotopic characteristics of the Efate basalts are commensurate with the transitional position of this island within the Vanuatu arc, between the southern arc lavas and those erupted in the region affected by DEntrecasteaux Ridge collision. However, 87Sr/86Sr values (0.70387-0.70424) are slightly higher and 143Nd/144Nd values slightly lower than predicted by the spatial change of these values along the arc. These features are taken to indicate derivation of the Efate basalts from a local, unusual mantle source. Two contrasting models are considered to account for the unusual features of the source mantle of these basalts. Model 1 suggests that the high Sr and P2O5 characteristics of the peridotite source of the Efate basalts were produced by subduction of an intraplate volcano similar to ORSTOM seamount, currently subducting immediately northwest of Efate. Recent studies have demonstrated massive P2O5 and Sr enrichments in highly altered formerly glassy seamount lavas and hyaloclastites. Model 2 proposes that the ambient strongly depleted mantle wedge has

Melt inclusions are small portions of liquid trapped by growing crystals during magma evolution. Recent studies of melt inclusions have revealed a large range of unusual major and trace element compositions in phenocrysts from primitive mantle-derived magmatic rocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-An plagioclase]. Inclusions in phenocrysts crystallized from more evolved magmas (e.g. olivine Fo < 85 mol %), are usually compositionally similar to the host lavas. This paper reviews the chemistry of melt inclusions in high-Fo olivine phenocrysts focusing on those with anomalous major and trace element contents from mid-ocean ridge and subduction-related basalts.We suggest that a significant portion of the anomalous inclusion compositions reflects localized, grainscale dissolution-reaction-mixing (DRM) processes within the magmatic plumbing system. The DRM processes occur at the margins of primitive magma bodies, where magma is in contact with cooler wall rocks and/or pre-existing semi-solidified crystal mush zones (depending on the specific environment). Injection of hotter, more primitive magma causes partial dissolution (incongruent melting) of the mush-zone phases, which are not in equilibrium with the primitive melt, and mixing of the reaction products with the primitive magma. Localized rapid crystallization of high-Fo olivines from the primitive magma may lead to entrapment of numerous large melt inclusions, which record the DRM processes in progress. In some magmatic suites melt inclusions in primitive phenocrysts may be naturally biased towards the anomalous compositions. The occurrence of melt inclusions with unusual compositions does not necessarily imply the existence of new geologically significant magma types and/or melt-generation processes, and caution should be exercised in their interpretation.

Uncertainty about the timing and location of the initiation of convergence in the western and south-western Pacific greatly hinders accurate plate tectonic reconstructions of subduction systems in that area. The chemistry and age of dikes intruding mantle peridotite in the ophiolite of New Caledonia infer that subduction-related magmatism began before 53 Ma. These new results infer that obduction in the south-west Pacific is unrelated to the reorientation of the Pacific plate motion that occurred c. 43 Ma and confirm new interpretations showing that changes in mantle flow, hotspot and plate motion may have occurred as soon as late Paleocene or early Eocene.

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental ribbons, including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These microcontinental ribbons were isolated from Australia and from each other during a protracted extension episode from ca 120 to 52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major change in Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spreading centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty-Entrecasteaux arc. Continued subduction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 70-60 million years old western volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorly exposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty-Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist and eclogite facies metamorphic assemblages. Following locking o

Al-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island are volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Ball in the Sunda are, and Epi, Merelava, and Ambrym islands in the Vanuatu are. More commonly, relatively Cr-rich spinels also occur as inclusions in the same olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localised Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) incongruent breakdown of amphibole in amphibole-rich cumulates in sub-are, or sub-GIB volcano magma chambers. The latter reaction produces a melt with similar to 20-22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from

Compositions of spinel and grassy melt inclusions in primitive olivine (FO89.3-91) from basalt AII32 D11-177 at 43°N, Mid-Atlantic Ridge fall into two principal groups. The dominant (similar to 90%) Group-I spinel and melt inclusions have typical MORB compositions. In contrast, Group-II. Cr-spinels are strongly enriched in TiO2 (2.6-4.1 wt%), and Group-II melt inclusions show significant enrichment in SiO2 (54.6-58.4 wt%), TiO2, Na2O and K2O, whereas their CaO contents (9.3-11.1 wt%) are unusually low. Group-II melts are also remarkable in crystallizing high-Mg orthopyroxene (Mg# 91). These mineral associations and melt compositions are unusual for MORB, and are interpreted to result from interaction between MORB-like melts and harzburgitic peridotite at low pressure.

Hypotheses for the formation of many types of hydrothermal ore deposits often involve the direct contribution of magma-related fluids (e.g., Cu-Mo-Au porphyries) or their superimposition on barren hydrothermal cells (e.g., volcanic-hosted massive sulfide deposits). However, the chemical and phase compositions of such fluids remain largely unknown. We report preliminary results of a comprehensive study of fluid bubbles trapped inside glassy melt inclusions in primitive olivine phenocrysts and pillow-rim glasses from basaltic magmas from different tectonic environments, including mid-ocean ridges (Macquarie Island, SW Pacific and Mid-Atlantic Ridge 43°N Fracture Zone), ocean islands (Hawaii) and a variety of modern and ancient backarc-island arc settings (eastern Manus Basin, Okinawa and Vanuatu Troughs, Troodos, New Caledonia and Hunter Ridge-Hunter Fracture Zone). Fluid bubbles from all localities, studied using electron microscopy with EDS and laser Raman spectroscopy, are composed of CO2-(± H2O ± sulfur)-bearing vapor and contain significant amounts of amorphous (Na-K-Ca-Fe alumino-silicates and dissorded carbon) and crystalline phases. The crystals are represented mainly by carbonates (magnesite, calcite, ankerite, dolomite, siderite, nahcolite and rhodochrosite), sulfates (anhydrite, gypsum, barite and anglesite), and sulfides (pyrite, arsenopyrite, chalcopyrite and marcasite), though other minerals (brukite, apatite, halite, clinoenstatite, kalsilite, nepheline, amphibole and mica) may occur as well. We argue that chemical components (e.g., C, H, S, Cl, Si, Al, Na, K, Fe, Mn, Cr, Ca, Mg, Ba, Pb and Cu) that later formed precipitates in fluid bubbles were originally dissolved in the magmatic fluid, and were not supplied by host glasses or phenocrysts after entrapment. Magma-related fluid rich in dissolved metals and other non-volatile elements may be a potential precursor to ore-forming solutions.

Detrital spinel is a widespread heavy mineral in sandstones from the Maastrichtian-Middle Eocene sedimentary basins in the SE Alps. Chemistry of detrital spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slovenia) is used to constrain petrological and geochemical affinities and tectonic provenance of the source rocks. In addition, we have analysed melt inclusion compositions in the detrital volcanic spinels to better constrain the nature of their parental magmas. This is the first study of melt inclusions in detrital spinels. Two principal compositional groups of detrital spinels are recognised based on their TiO2 and Fe2+/Fe3+; one derived from peridotites, the other from basaltic volcanics. Peridotitic spinels are more abundant and have TiO2 < 0.2 wt% and high Cr/Cr + Al(40-90), characteristic of suprasubduction zone harzburgites. Significant chemical variations among volcanic spinels (TiO2 up to 3 wt%, Al2O3 12-44 wt%) suggest multiple sources, with geochemically distinct characteristics, including MORE-type and backarc basin basalts, subduction-related magmas and tholeiites produced during early continental rifting. Compositions of homogenised melt inclusions in spinels with TiO2 > 0.2 better distinguish the differences between the compositions of their host spinels and help to further clarify the geodynamic provenance of extrusive source rocks. Several compositional groups of melt inclusions have been recognised and represent diverse magmatism of marginal basins, including MORB- and subduction-related geochemical types, as well as magmas characteristic of early continental rifting. These results, combined with the data on regional ophiolitic complexes and tectonic reconstructions favour the Internal Dinarides of Yugoslavia as a possible source area for the SE Alps sediments.

The composition of primary magmas and their mantle sources can be successfully inferred from the study of melt inclusions trapped in spinel phenocrysts. This is particularly true in the case of severely altered rocks, in which spinel and spinel-hosted melt inclusions usually retain primary magmatic information. We report the results of the study of melt inclusions in high-Cr (Cr# 90-95), primitive (Mg# 65-78) spinel in the Palaeocene low-Ca boninites from Cape Vogel, Papua New Guinea. Melt inclusions are represented by the aggregate of skeletal olivine crystals in the residual glass. Raster beam electron Microprobe analyses of melt inclusions demonstrated that they are broadly similar in composition to primitive orthopyroxene and to the most primitive boninites in this area, having (in wt.%): very high MgO (18-30), SiO2 (53-61) and very low TiO2 (0.04-0.19), Al2O(3) (3-9), CaO (2-4), Na2O (<0.9), K2O (0.05-0.15) and CaO/Al2O3 (0.4-0.6). H2O abundances in melt inclusions, analysed by an ion probe, are very high (1-2 wt.%), and they could have been even higher (similar to3.5 wt.%) if the melts lost H2O before crystallisation. Trace elements in melt inclusions, analysed by laser ablation ICPMS, have exceptional depletion in HREE (<1 PM) and significant enrichment in LREE over HREE (La/Yb 5-12), and Ph (Ce/Pb 2-12) and Zr (Zr-N/Sm-N 2-3.4) over REE. The compositions of melt inclusions correlate well with the compositions of host spinel showing the fractionation path of initial ultramafic melt. Cape Vogel primary melts could have originated from melting of extremely refractory hot (> 1500 degreesC) harzburgitic mantle fluxed by subduction-related, H2O-bearing enriched components. Trace element composition of these enriched components is estimated from melt inclusion compositions by mass balance calculations.

The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island are, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain are, and differ significantly from those of MORE-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo(82-92)) Of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na-Ca carbonates, halite and Na-K-Ca alumine-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially from the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO2-H2O fluid, which i

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

At its southernmost end, the main spreading centre of the North Fiji Basin is propagating into arc crust of the poorly-known Hunter Ridge. We define nine magmatic groups from major element glass chemistry and olivine and spinel compositions in samples dredged from twenty six sites in this area by the ''R/V Academician A. Nesmeyanov'' in 1990. These include groups of boninites, island arc tholeiites (IAT), mid-ocean ridge basalts (MORB), enriched mid-ocean ridge basalts (E-MORB), olivine porphyritic andesites and basaltic andesite and Na-rhyolites. Primitive lavas containing highly forsteritic olivine phenocrysts are common in all the groups, except for the rhyolites.We report over 100 glass analyses for dredged rocks from this region, and about 300 olivine-spinel pairs for representatives of all the magmatic groups identified, except the Na-rhyolites.The MORB in this region are probably produced at the propagating spreading centre in the southern part of the North Fiji Basin. Juxtaposition of shallow, hot MORB-source diapirs supplying the MORB in this area, and the sub-arc damp, refractory upper mantle beneath the Hunter Ridge, provides suitable petrogenetic conditions to produce a range of magma types, from island arc tholeiites through to high-Ca boninites. The latter were recovered in eleven dredges.The E-MORB lavas recovered from the extreme southern margin of the North Fiji Basin are shown to be essentially identical to those dredged from adjacent older South Fiji Basin crust. It is hypothesized that the former were either scraped off the South Fiji Basin crust during an episode of oblique subduction that may have generated the Hunter Ridge during the last 5 Myr, or alternatively, that slices of the South Fiji Basin crust were trapped and incorporated into the North Fiji Basin as the subduction zones fronting the Vanuatu arc stepped or propagated southward.

Taiwan is an active mountain belt formed by oblique collision between the Luzon are and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio-Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at similar to0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO approximate to 15 wt.%) and potassic (K2O approximate to 5 wt.%, K2O/Na2O approximate to 1.6-3.0). Whereas these basic rocks (SiO2 approximate to 48 wt.%) have relatively low Al2O3 approximate to 12 wt.%, total Fe2O3 approximate to 7.5 wt.% and CaO approximate to 7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, > 1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high Field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu * approximate to 0.90 - 0.84), and Sr and Nd isotope ratios are uniform (Sr-87/Sr-86 approximate to 0.70540-0.70551; Nd-143/(144) Nd approximate to 0.51268-0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4 +/- 1.8; 1 sigma deviation), which are in agreement with the whole rock Mg-values (83 - 80). Spinel inclusions in olivine are characterized by high Cr/Cr+Al ratios (0.94-0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet, We therefore suggest that the T

The eastern coast of King Island in southeastern Australia exposes a thick, well-preserved sequence of latest Neoproterozoic volcanic, and related shallow intrusive rocks. These rocks are associated with shallow marine carbonates and siltstones and pass up into massive conglomerates representing a marine flooding event and unconformity, during continental break-up and subsequent volcanic passive margin formation. Unusual differentiated sills (Grimes Intrusive suite) with extreme internal variation (wehrlite to andesite compositions) intrude deformed Proterozoic metasediments of the Rodinian basement. A thin, basal tholeiitic basaltic volcanic unit (City of Melbourne Volcanics) is less contaminated than the underlying sills, and preceded eruption of a thick sequence of highly depleted picritic pillows, sub-aerial flows and hyaloclastites (Shower Droplet Volcanics). The picrite sequence is overlain by thick tholeiitic basalts and reworked volcanogenic conglomerates (Bold Head Formation) that show a strong compositional similarity to enriched mid ocean ridge basalts. Both the picrites and the upper tholeiitic basalts are not crustally contaminated and have an Nd-Sm isochron age of 579 +/- 16 Ma with initial epsilonNd of +4.2. The lithostratigraphy and range of compositions represented are analogous to early magmatism associated with continental break-up and volcanic passive margin formation, including voluminous Seaward Dipping Reflector Sequences, in the Mesozoic North Atlantic volcanic margins.