Anthony J. Crawford

Evidence for carbonatite metasomatism in spinel peridotite xenoliths from western Victoria, Australia

The experimental discovery that sodic dolomitic carbonatite melt can be generated in equilibrium with pargasitic lherzolite [1] at pressures corresponding to depths of 90 km or more, has led to the development of models for mantle metasomatism by ephemeral carbonatite melts [2]. A suite of mantle xenoliths (magnesian wehrlites to lherzolites) from Mt Leura and Mt Shadwell (western Victoria, Australia) display the predicted petrographic and geochemical signatures of such metasomatism, namely replacement of primary orthopyroxene by jadeitic clinopyroxene and forsteritic olivine, the presence of significant accessory apatite, unusually high whole-rock CaO/Al2O3 and Na2O/Al2O3 values, and extreme LILE enrichment without concomitant TiO2 enrichment. Isotopic data suggest a petrogenetic association of the carbonatite metasomatic event with the host alkali olivine basalts. We propose a model whereby carbonated, undersaturated silicate melts crystallizing pargasitic amphibole at depths of ⩾ 90 km, produce a mobile carbonatite melt which can segregate and ascend at very low melt fractions, and undergo decarbonation reactions with refractory lithosphere at 55–65 km depth, producing highly magnesian, apatite-bearing wehrlites and lherzolites.

Precious metals in magnesian low-Ti lavas: Implications for metallogenesis and sulfur saturation in primary magmas

Boninites and related magnesian low-Ti magmas are generally regarded as partial melts of a moderately to severely depleted peridotite source. Incompatible lithophile element abundances indicate that this source was variably enriched in LREE, Zr, Sr, Ba and alkalis by some mantle metasomatic process. Low-Ti lavas from the Bonin-Mariana arc system, Cape Vogel, New Caledonia, Cyprus, Newfoundland and SE Australia have been analysed for Pd, Ir, Au, Cu, S and Se. Comparison of fresh glassy material with variably altered samples suggests sporadic loss of Au and Cu and essentially inert behaviour for Pd, Ir and Se during seawater and subsequent alteration. They are uniformly enriched in Pd (mean 15 ppb) and depleted in Cu (mean 20 ppm), S (mean < 54 ppm) and Se (mean 53 ppb) compared to average MORB (<0.8 ppb Pd, 72 ppm Cu, 800 ppm S and 196 ppb Se) and exhibit incompatible-like behaviour for these elements and Au. The data are compatible with fractionation of the chalcophile elements during multi-stage mantle melting. Primary MORB liquids are S-saturated in their mantle source and an immiscible sulfide component is retained in the mantle residue. This results in the preferential removal of metals having low DS/L- values (base metals) and concentration of those metals with high DS/L values (precious metals) in the residual mantle sulfide fraction. Subsequent remelting of this refractory source produces S-deficient precious metal-enriched magmas, as exemplified by boninites. The absence of correlation between incompatible lithophile element enrichment and chalcophile element abundances suggests that the latter were not added to the source during mantle metasomatism. The constraints imposed by the nature of the source region result in two fundamentally contrasting patterns of behaviour for exclusively chalcophile elements. Magmas generated in mildly depleted to undepleted source regions by low to moderate degrees of partial melting (e.g. MORB) are S-saturated and become rapidly impoverished in precious metals during the early stages of silicate fractionation, owing to the co-precipitation of an immiscible sulfide component. Magmas generated from a strongly depleted source are initially S-undersaturated and concentrate chalcophile metals in their liquid residua. The contrasting behaviour of chalcophile metals during the early crystallisation stage of MORB and low-Ti magmas lead to divergent predictions concerning the primary distribution of these metals in oceanic crust generated by these magmas. The similarity in composition of early Bushveld magmas and boninites suggests that these S-deficient, PGE-enriched magmas may be essential to the formation of platiniferous horizons in layered intrusions.

The origin of island arc high-alumina basalts

A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas.Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative.Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass).Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (<17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation.We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.

120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

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 of this subduction zone at 38-34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above which formed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatu and Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first the South Fiji Basin (31-25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong Java Plateau, a large igneous province, with the Solomons section of the Vitiaz arc resulted in a reversal of subduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South Fiji Basin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc and produced the Lau Basin-Havre Trough. This southwest Pacific style of crustal growth above a rolling-back slab is applied to the 600-220 Ma tectonic development of the Tasman Fold Belt System in southeastern Australia, and explains key aspects of the geological evolution of eastern Australia. In particular, collision between a plume-triggered 600 Ma volcanic passive margin and a 510-515 Ma boninitic forearc of an intra-oceanic arc had the same relative orientation and geological effects as that which produced New Caledonia. A new subduction system formed probably at least several hundred kilometres east of the collision zone and produced the Macquarie arc, in which the oldest lavas were erupted ca 480 Ma. Continued slab rollback induced regional extension and the growth of narrow linear troughs in the Macquarie arc, which persisted until terminal deformation of this fold belt in the late-Middle to Late Devonian. A similar pattern of tectonic development generated the New England Fold Belt between the Late Devonian and Late Triassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward to locations in New Zealand, and on the Lord Howe Rise and Norfolk-New Caledonia Rise, during the post- 120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustal accretion like the southwest Pacific since 120 Ma, facing the open Pacific Ocean, we question whether the eastern (Australia-Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined to Laurentia.

Tectonic implications of Late Proterozoic-Early Palaeozoic igneous rock associations in western Tasmania

Abstract In contrast to the mainland Lachlan Fold Belt, Late Proterozoic basement(?) and Cambrian rocks are exposed over large areas of western Tasmania. They offer the possibility of solving the paradox regarding the continental versus oceanic nature of the basement beneath southern Australia, and better understanding the Late Proterozoic and Cambrian tectonic history of this southern part of the Lachlan Fold Belt. Around 600 Ma, attenuation and eventual rifting of Proterozoic continental crust resulted in formation of a thinned passive continental margin transected by small rift basins in which rift tholeiites transitional to MORB accumulated. Eastward-directed intra-oceanic subduction commenced to the east of the passive margin at some time before the Middle Cambrian, forming an oceanic arc with boninites and low-Ti magnesian quartz tholeiites in the forearc region. Continued subduction of intervening oceanic crust between the arc and the passive margin resulted in an arc-continent collision within the Middle Cambrian. One or more extensive sheets of forearc crust, dominated by low-Ti basalts and boninites and their cumulate complements, were overthrust onto thinned continental crust. Continued compression at the suture initiated what was probably a short-lived episode of westward-directed subduction beneath the newly-emplaced allochthon and passive margin basement, and generated medium- to high-K andesites and more felsic rocks of the Central Volcanic Complex of the Mount Read Volcanics. Rebound backthrusting along the eastern side of the Central Volcanic Complex of the Mount Read Volcanics belt exhumed underthrust Proterozoic crystalline crust which now forms the Tyennan region, and a foreland basin half-graben, the Dundas Trough, formed along the western edge of this basement inlier. The upper Mount Read Volcanics are mainly post-collisional high-K to shoshonitic basalts and andesites generated by delayed partial melting of subduction-modified, underthrust passive margin subcontinental mantle. Extension in this post-collision collage of crustal elements led to limited rifting and emplacement of the tholeiitic Henty Dyke Swarm through at least the Central Volcanic Complex. Lower crustal generation of extensive sheet-like intrusive quartz-feldspar porphyrite bodies and associated Tyndall Group felsic explosive volcanism occurred within the Dundas Trough. Uplift and excavation of Tyennan region Proterozoic crystalline crust provided abundant coarse siliciclastic detritus of the Owen conglomerate and correlates which flooded into and gradually filled the Dundas Trough. The major events invoked in the above model to have generated continental crust (allochthon emplacement, reversal of subduction, post-collisional volcanism and graben formation, rebounding and excavation of underthrust crystalline basement, graben filling) all occurred probably within a 20-Ma period. Late Cambrian deformation is well known in the Adelaide Fold Belt (Delamerian Orogeny) and in the New Zealand and Transantarctic Mountains segments of the once-continuous fold belt along the eastern margin of Early Palaeozoic Gondwanaland. It is not recorded from the mainland section of the Lachlan Fold Belt in Victoria, although given the nature of Cambrian outcrops along major Devonian faults in Victoria, this is hardly surprising.

The petrogenesis of high-calcium boninite lavas dredged from the northern Tonga ridge

Abstract In 1984 the research vessel ‘Natsushima’ dredged a fresh suite of MgO- and SiO 2 -rich lavas from the northern termination of the northern Tonga ridge. These lavas are high-Ca boninites and are characterized by the presence of magnesian olivine (up to Fo 94 ), orthopyroxene, clinopyroxene, Cr-rich spinel and calcic plagioclase ( > An 90 ) phenocrysts. Boninite lavas from one dredge site, station 21, range in MgO contents from 3–15 wt% and their major element chemistry appears to be consistent with production of this suite via crystal fractionation. However, large variations in incompatible trace element ratios plus petrographic and mineral chemical evidence demonstrate that magma mixing has been an important process. The isotopic (Sr, Nd) composition of the Tongan high-Ca boninites suggest that their mantle sources are part of a regional OIB mantle domain upwelling beneath the Fiji-Lau-Tonga subduction zone system. The OIB mantle source to the Tongan boninites was of refractory lherzolite composition, depleted in ‘basaltic’ components by prior generation of Lau Basin back arc crust. The mantle sources of the Tongan high-Ca boninites have been enriched in incompatible elements by one or more metasomatic phases, suggested to be a hydrous fluid from the subducting lithospheric slab, a carbonatite melt and a small-degree silicate partial melt both derived from OIB mantle sources. There is no evidence for the involvement of sediment in the source of the Tonga boninites. The presence of the high-Ca boninite lavas at the northern end of the Tonga ridge can be explained by the presence of a northeast-directed spreading ridge in the northern part of the Lau Basin which is propagating into the north Tonga arc. Upwelling asthenospheric mantle beneath the spreading ridge may cause partial melting of refractory peridotite located in the mantle wedge above the subducting Pacific plate at shallow depths (

The Tasman Line: where is it, what is it, and is it Australia's Rodinian breakup boundary?

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.

Field setting, mineralogy, chemistry, and genesis of arc picrites, New Georgia, Solomon Islands

The field setting, petrography, mineralogy, and geochemistry of a suite of picrite basalts and related magnesian olivine tholeiites (New Georgia arc picrites) from the New Georgia Volcanics, Kolo caldera in the active ensimatic Solomon Islands arc are presented. These lavas, with an areal extent in the order of 1002 km and almost 1 km thick in places, are located close to the intersection of the Woodlark spreading zone with the Pacific plate margin. They contain abundant olivine (Fo94-75) and diopside (Cr2O3 1.1-0.4%, Al2O3 1–3%), and spinels characterised by a large range in Cr/(Cr+Al) (0.85–0.46) and Mg/(Mg+ Fe++) (0.65−0.1). The spinels are Fe+++ rich, with Fe+++/ (Fe++++Cr+Al) varying from 0.06 to 1.0. A discrete group of spinels with the highest Cr/(Cr+Al) (0.83–0.86) and lowest Fe+++ contents are included in the most Mg-rich olivine (Fo91–94) and both may be xenocrystal in origin.The lavas, which range between 10–28% MgO, define linear trends on oxide (element) — MgO diagrams and these trends are interpreted as olivine (∼0.9) clinopyroxene (∼0.1) control lines. For the reconstructed parent magma composition of these arc picrites, ratios involving CaO, Al2O3, TiO2, Zr, V and Sc are very close to chondritic. REE patterns are slightly LREE — enriched ((La/Sm)N 1.3–1.43) and HREE are flat. All lavas show marked enrichments in K, Rb, Sr, Ba, and LREE relative to MORB with similar MgO contents, but the TiO2 content of the proposed parent magma is close to those of postulated primary MORB liquids. It is proposed that the arc parent magma was produced by partial melting of sub-oceanic upper mantle induced by the introduction of LILE — enriched hydrous fluids derived by dehydration and/or partial melting of subducted ocean crust and possibly minor sediments.

The H2O content of basalt glasses from Southwest Pacific back-arc basins

The H2O content of 35 glasses from Southwest Pacific back-arc basins (Lau, North Fiji, Woodlark and Manus) have been determined by infrared spectroscopy. On a plot of K2O vs. H2O the glass data define two distinct trends characterized by different slopes. Trend I, with a slope (K2O/H2O) of 0.25, can be explained by addition of a subduction-related component with K2O/H2O = 0.25 to a depleted mid-ocean ridge basalt mantle source (N- or D-MORB-like). Trend II, which coincides with the N- to E-MORB compositional spectrum, can be produced by addition of a non-subduction component, possibly an alkaline magma with K2O/H2O∼ 1.5, to the same depleted mantle source. The K2O/TiO2 and K/Nb values of E-MORB and back-arc basin basalts (BABB) of Trend II suggest that the enriched component involved in their genesis is not derived from a typical ocean island basalt (OIB, e.g. Hawaiian) mantle source. Our data show that the entire spectrum of BABB compositions can be explained by different degrees of mixing of a mantle source of either D-, N- or E-MORB composition with the subduction-related component, characterized by a K2O/H2O value of 0.25. Different BABB types correlate with tectonic setting. Samples from the Trend II are associated with relatively stable spreading ridges, whereas those affected by the subduction-related component are always associated with more complex tectonic settings, or come from young or incipient back-arc basins. Pronounced E-MORB affinities of mantle sources are demonstrated only for samples from the Lau, North Fiji and Scotia Sea basins. The most H2O enriched BABB of Trend I partly overlap in terms of H2O and K2O content and H2O/TiO2 and K2O/TiO2 values with island arc tholeiites. This suggests involvement of similar subduction-related components in the genesis of these two magma types. Because a larger database is now available, the K2O/H2O vs. TiO2 tectonic discriminan diagram of Muenow et al. [2] appears to be less useful than when originally proposed. The very low K2O/H2O value ( O value (0.25) is also of some interest, as the same value occurs in depleted MORB.

Calcic melt inclusions in primitive olivine at 43ºN MAR: evidence for melt–rock reaction=melting involving clinopyroxene-rich lithologies during MORB generation

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 signal commonly attributed to the Azores mantle plume.