Journal of Petrology - Advance Access

Geochemical Evolution of Intraplate Volcanism at Banks Peninsula, New Zealand: Interaction Between Asthenospheric and Lithospheric Melts

Timm, C., Hoernle, K., Van Den Bogaard, P., Bindeman, I., Weaver, S. — 2009-06-24

Intraplate volcanism was widespread and occurred continuously throughout the Cenozoic on the New Zealand micro-continent, Zealandia, forming two volcanic endmembers: (1) monogenetic volcanic fields; (2) composite shield volcanoes. The most prominent volcanic landforms on the South Island of New Zealand are the two composite shield volcanoes (Lyttelton and Akaroa) forming the Banks Peninsula. We present new ⁴⁰Ar/³⁹Ar age and geochemical (major and trace element and Sr–Nd–Pb–Hf–O isotope) data for these Miocene endmembers of intraplate volcanism. Although volcanism persisted for ~7 Myr on Banks Peninsula, both shield volcanoes primarily formed over an ~1 Myr interval with small volumes of late-stage volcanism continuing for ~1·5 Myr after formation of the shields. Compared with normal Pacific mid-ocean ridge basalts (P-MORB), the low-silica (picritic to basanitic to alkali basaltic) Akaroa mafic volcanic rocks (9·4–6·8 Ma) have higher incompatible trace element concentrations and Sr and Pb isotope ratios but lower ¹⁸O (4·6–4·9) and Nd and Hf isotope ratios than ocean island basalts (OIB) or high time-integrated U/Pb HIMU-type signatures, consistent with the presence of a hydrothermally altered recycled oceanic crustal component in their source. Elevated CaO, MnO and Cr contents in the HIMU-type low-silica lavas, however, point to a peridotitic rather than a pyroxenitic or eclogitic source. To explain the decoupling between major elements on the one hand and incompatible elements and isotopic compositions on the other, we propose that the upwelling asthenospheric source consists of carbonated eclogite in a peridotite matrix. Melts from carbonated eclogite generated at the base of the melt column metasomatized the surrounding peridotite before it crossed its solidus. Higher in the melt column the metasomatized peridotite melted to form the Akaroa low-silica melts. The older (12·3–10·4 Ma), high-silica (tholeiitic to alkali basaltic) Lyttelton mafic volcanic rocks have low CaO, MnO and Cr abundances suggesting that they were at least partially derived from a source with residual pyroxenite. They also have lower incompatible element abundances, higher fluid-mobile to fluid-immobile trace element ratios, higher ¹⁸O, and more radiogenic Sr but less radiogenic Pb–Nd–Hf isotopic compositions than the Akaroa volcanic rocks and display enriched (EMII-type) trace element and isotopic compositions. Mixing of asthenospheric (Akaroa-type) melts with lithospheric melts from pyroxenite formed during Mesozoic subduction along the Gondwana margin and crustal melts can explain the composition of the Lyttelton volcano basalts. Two successive lithospheric detachment/delamination events in the form of Rayleigh–Taylor instabilities could have triggered the upwelling and related decompression melting leading to the formation of the Lyttelton (first, smaller detachment event) and Akaroa (second, more extensive detachment event) volcanoes.

Testing the Validity of the Petrological Hypothesis 'No Phenocrysts, No Post-emplacement Differentiation'

Latypov, R. — 2009-06-17

An extravagant hypothesis ‘no phenocrysts, no post-emplacement differentiation’ has been put forward by Marsh, in a series of papers, for the development of mafic–ultramafic intrusions. This hypothesis is based on an assertion that the majority of these intrusions are structureless and undifferentiated because they lack residual granitic rocks. To explain this, the hypothesis postulates that phenocryst-free magmas are not able to differentiate in crustal chambers because all evolved interstitial liquid is locked in solidification fronts, producing compositionally uniform magmatic bodies. Layered, well-differentiated intrusions are attributed to the successive emplacement of magma pulses with phenocrysts of different phase and chemical compositions, rather than to slow magma cooling and fractional crystallization, as conventional models imply. Such phenocryst-laden magma pulses are supposed to be derived from an underlying magmatic mush column. However, structureless, undifferentiated, mafic–ultramafic bodies simply do not exist in nature. All well-studied mafic–ultramafic bodies, with or without residual granitic rocks, that crystallized from parental magmas of non-eutectic composition, tend to reveal clear evidence of internal compositional differentiation in terms of crystallization sequences (e.g. Ol, Opx, Opx + Pl, Opx + Pl + Cpx), mineral compositions (e.g. An in plag, En in cpx) and compatible/incompatible major and trace element geochemistry (e.g. Mg-number, Cr, rare earth elements). This is especially evident in layered intrusions that represent the key evidence against the hypothesis. Essentially, by denying the ability of magma to differentiate in intrusive bodies, the hypothesis forbids magmatic differentiation in any sub-chamber related to the entire magmatic mush column. As a result, magma pulses in which phenocrysts progressively change in composition cannot be derived from the column to form layered intrusions. The hypothesis is thus contradictory, baseless and fundamentally flawed. It should be abandoned in favour of a classical fractional crystallization model based on the pioneering experiments of Bowen and amply confirmed over almost a century by subsequent studies of layered intrusions. The classical model was, is, and will most probably remain, the best explanation for the origin of differentiated magmatic bodies.

Igneous, Alteration and Exhumation Processes Recorded in Abyssal Peridotites and Related Fault Rocks from an Oceanic Core Complex along the Central Indian Ridge

2009-05-13

This paper presents the first detailed studies on the petrology of abyssal peridotites and related fault rocks recovered from an oceanic core complex (OCC) in the southern part of the Central Indian Ridge using the submersible SHINKAI 6500 of the Japan Agency for Marine–Earth Science and Technology. Less deformed, statically serpentinized peridotites were recovered from the ridge-facing slope, whereas highly deformed rocks were recovered from sheet-like structures on the top surface of the OCC. The top surface of the OCC is interpreted to be the main detachment fault. The serpentinized peridotites are consistent with an origin as residues after moderate degrees (13–15%) of partial melting; these were later chemically modified as a result of the infiltration of evolved melts of probable granitic composition resulting in the formation of leucocratic veins. The deformed rocks from the detachment fault are divided into talc-rich and chlorite-rich parts, probably formed as a result of interaction of hydrothermal fluids with peridotite and gabbro precursors along the detachment fault, respectively. Deformation and alteration were locally concentrated along the detachment fault, resulting in mechanical mixing of both altered gabbros and serpentinized peridotites in the deformed rocks during the exhumation of the OCC associated with long-lived fault activity. Our results reveal that gabbros and peridotites are tectonically exposed in oceanic core complexes on the seafloor along the intermediate-spreading CIR, as well as in slow-spreading regions. Fluid-mobile elements such as Li, Rb, Ba, Pb, Sr and U are higher in serpentines than their precursor mantle minerals. The uranium content in serpentine is variable but is abundant in the outermost margin of the precursor minerals. The trace element compositions of serpentine appear to have been continuously changed along with changes in the chemistry of the hydrothermal fluids as temperature decreased. Fluid-mobile elements were thus added and/or leached out during the serpentinization of the peridotite combined with later seafloor weathering.

Continental Flood Basalts and Mantle Plumes: a Case Study of the Northern Ethiopian Plateau

Beccaluva, L., Bianchini, G., Natali, C., Siena, F. — 2009-05-11

New geochemical data integrated in a petrogenetic model indicate that the ~30 Ma Northern Ethiopian continental flood basalts (CFBs) preserve a record of magmas generated from the centre to the flanks of a plume head, currently corresponding to the ‘Afar hotspot’. Basaltic lavas appear zonally arranged with Low-Ti tholeiites (LT) in the west, High-Ti tholeiites (HT1) to the east and very High-Ti transitional basalts and picrites (HT2, TiO₂ 4–6 wt %) closer to the Afar triple junction. Modelling provides estimates of the P–T–X conditions of magma generation showing that the Ethiopian CFBs could be generated in the pressure range 1·3–3·0 GPa at an approximate depth of 40–100 km from mantle sources that were increasingly metasomatized and hotter (1200–1500°C) from west to east; that is, from the outer zones (LT) to the core of the plume head (HT2 ultra-titaniferous basalts and picrites). Metasomatizing agents can be envisaged as alkali–silicate melts that integrate various geochemical components (e.g. Ti and related high field strength elements, low field strength elements, light rare earth elements, H₂O, noble gases, etc.) scavenged and pooled along the plume axis, and derived from heterogeneous mantle materials mixed during the plume rise. This has significant implications for the current debate about mantle plumes, as the modelled compositionally and thermally zoned plume head (T excess ≥ 300°C with respect to ambient mantle) is in accordance with seismic tomography and buoyancy flux, as well as geochemical characteristics, thus supporting a deep provenance of the Afar plume, which possibly originated in the transition zone or lower mantle.

Heterogeneous Lithospheric Mantle beneath Northern Patagonia: Evidence from Prahuaniyeu Garnet- and Spinel-Peridotites

Bjerg, E. A., Ntaflos, T., ThONi, M., Aliani, P., Labudia, C. H. — 2009-05-11

Prahuaniyeu, located on the NW margin of the Somoncura Large Igneous Province in northern Patagonia, is one of two known localities in Argentina where mantle-derived garnet- and spinel-bearing peridotites occur associated with alkali basalts; the other locality is the Pali Aike volcanic field of southern Patagonia. Most of the Prahuaniyeu garnet-bearing peridotites are fertile in terms of their Al₂O₃ and CaO contents, whereas the spinel-bearing peridotites cover a wide range from fertile to depleted compositions. Whole-rock light rare earth element (LREE) enrichment in the garnet-bearing peridotites and partly in the spinel-peridotites is consistent with intergranular percolation of the host basalt melt, as hydrous phases are not present and the clinopyroxenes and garnets are not enriched in LREE. Lack of slab-derived component(s) in the metasomatic products rules out the participation of a subducted slab in the generation of these basalts. In situ clinopyroxene analyses suggest that a group of spinel-peridotites experienced cryptic metasomatism by carbonatitic melts. Non-metasomatized garnet- and spinel-peridotites have experienced fractional melting ranging from 1 to 3% and from 5 to 12%, respectively. The Prahuaniyeu xenoliths lie on an elevated geotherm (high temperatures at low pressure) implying convective heat transport. The two most fertile samples, which indicate apparent internal ‘ages’ between c. 10 and 30 Ma for the sub-Prahuaniyeu lithospheric mantle, suggest resetting of the Sm–Nd isotopic system under a high-temperature regime and most probably reflect closure of the system following this ‘high-T event’, which can be related to extensive magmatic activity within the Somoncura province, starting in the Eocene and finishing in the Miocene.

Transition from Ultrapotassic Kamafugitic to Sub-alkaline Magmas: Sr, Nd, and Pb Isotope, Trace Element and 40Ar-39Ar Age Data from the Middle Latin Valley Volcanic Field, Roman Magmatic Province, Central Italy

Boari, E., Tommasini, S., Laurenzi, M. A., Conticelli, S. — 2009-02-19

The Middle Latin Valley volcanic field forms part of the Roman Magmatic Province and includes Pleistocene monogenetic volcanism characterized by the emplacement of small lava flows and minor pyroclastic ejecta and flows. The absence of a main volcanic edifice and of a large, shallow-level magma reservoir allows the eruption of primitive magmas. Geochemical and petrological data suggest that at least four types of mafic parental magmas are present within the volcanic field: (1) melilite-bearing ultrapotassic (kamafugitic); (2) plagioclase-bearing and -free leucititic (HKS); (3) shoshonitic; (4) sub-alkaline. ⁴⁰Ar–³⁹Ar dating reveals diachronous emplacement of mafic magmas with different levels of K enrichment; the kamafugitic lavas are the oldest and the sub-alkaline lavas the youngest. Incompatible trace element contents strictly follow K₂O, but overall the groups of rocks show similar trace element fractionation, with high field strength elements less enriched than large ion lithophile elements. Despite a restricted range in MgO and SiO₂ contents, the Middle Latin Valley volcanic rocks have highly variable Sr, Nd and Pb isotopic compositions. The sub-alkaline rocks have the lowest ⁸⁷Sr/⁸⁶Sr and the highest ¹⁴³Nd/¹⁴⁴Nd, whereas the kamafugitic rocks have the highest ⁸⁷Sr/⁸⁶Sr and the lowest ¹⁴³Nd/¹⁴⁴Nd. Intermediate isotopic compositions between these two end-members are shown by leucitites–plagio-leucitites and shoshonites. A clear, time-dependent trend of isotopic variation is observed. This also holds true for Pb isotope compositions, with shoshonitic and sub-alkaline rocks showing the most radiogenic signatures and the kamafugitic rocks the least radiogenic signatures. The overall geochemical characteristics of the magmas can be reconciled in terms of a model involving recycling of marly shales within the upper mantle; this overprinted earlier pervasive metasomatism related to melts (supercritical fluids) derived from altered oceanic basalts. The crustal derived (marl) end-member is considered to have been concentrated within a metasomatic vein network within the lithosphere, whereas the supercritical fluid-metasomatized end-member occurs within the surrounding mantle. Early partial melting of veins produced strongly undersaturated melilite-bearing ultrapotassic magma (kamafugitic). The progressive exhaustion of the veined mantle increased the contribution of the surrounding mantle to magma production, explaining the decrease of K₂O with time in the mafic magmas and the geochemical and isotopic transition from leucititic–plagio-leucititic to shoshonitic and sub-alkaline magmas, the latter being the youngest products erupted.

Sr-Nd-Pb-He-O Isotope and Geochemical Constraints on the Genesis of Cenozoic Magmas from the West Antarctic Rift

Nardini, I., Armienti, P., Rocchi, S., Dallai, L., Harrison, D. — 2009-01-22

The West Antarctic Rift System (WARS) represents one of the major active continental extension zones on Earth. The Ross Sea coast in northern Victoria Land (NVL) is littered with alkaline rift-related igneous products (Middle Eocene–Present). This study characterizes the nature of the magma source involved in the rift process through geochemical–isotopic investigation of Cenozoic basalts from NVL, and provides important constraints for the reconstruction of the tectono-magmatic evolution of the Ross Sea region–WARS. The chemical compositions of the basalts (Miocene–Present) display major and trace element characteristics typical of ocean island basalts (OIB), with strong enrichment in the most incompatible elements. Whole-rock isotopic compositions are in the range 0·7028–0·7034 for ⁸⁷Sr/⁸⁶Sr, 0·5129–0·5130 for ¹⁴³Nd/¹⁴⁴Nd (_Nd(t) ~ 4·8–6·7), 19·3–19·7 for ²⁰⁶Pb/²⁰⁴Pb, 15·4–15·6 for ²⁰⁷Pb/²⁰⁴Pb and 38·7–39·3 for ²⁰⁸Pb/²⁰⁴Pb, suggesting a HIMU-like (high U/Pb) signature of the mantle source. Determinations of ³He/⁴He on crushed olivine yielded values between 5·7 and 7·2 times the atmospheric ratio, similar to the lithospheric mantle and in the range of mid-ocean ridge basalts (MORB). The ¹⁸O_ol of olivine separates varies from 4·92 to 5·53 and is positively correlated with Fo content. Integration of our geochemical and isotope data with available geological, geophysical and geochronological data has led to the following reconstruction. The differences in the oxygen isotope values principally reflect the involvement of a heterogeneous mantle source and/or the assimilation of variable amounts of hydrothermally altered crustal rocks from the volcanic edifices. The ³He/⁴He data allow us to exclude a plume-driven model to explain the continuing rifting process. Based on the evidence of metasomatic processes, we propose a model to generate the mantle source(s) of the Cenozoic basaltic melts of the NVL. This is sublithospheric mantle metasomatized during an amagmatic extensional event that affected the WARS in the Late Cretaceous. During Eocene–Oligocene times, mantle flow warmed the mantle at the edge of the thick Antarctic lithosphere, and the reactivation of old translithospheric discontinuities promoted mantle melting and the rise of magmas as plutons and dyke swarms. From the Late Miocene to Present, the continuing craton-directed mantle flow led to normal faulting of the rift shoulder, which favoured the rise of magmas to build up large volcanic edifices.

Formation of Highly Refractory Dunite by Focused Percolation of Pyroxenite-Derived Melt in the Balmuccia Peridotite Massif (Italy)

Mazzucchelli, M., Rivalenti, G., Brunelli, D., Zanetti, A., Boari, E. — 2008-11-14

A 50 m thick and 150 m long dunite body occurs as a subconcordant, tabular structure in the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle. The contacts with the host spinel-facies depleted lherzolite are sharp. The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90·7–93·8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the dunites were generated in a part of the mantle veined by early Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1·5 GPa and focused melt transport occurs at pressures greater than 0·7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P < 1·2 GPa and from pyroxenite at P < 1·1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers. The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites, which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges.

The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications

Griffin, W. L., O'Reilly, S. Y., Afonso, J. C., Begg, G. C. — 2008-07-04

The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM originally consisted of highly depleted dunites/harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes, we suggest a sharp dichotomy between Archean and younger tectonic regimes. The differences in buoyancy and viscosity between these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need to be revised.