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Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry

2009-06-08

The Solomon island arc, SW Pacific, is of particular interest for understanding subduction zone volcanism, as magmatism in the active part of the arc is dominated by mafic melts, thus permitting direct insights into mantle processes. Along the Solomon island arc, the Indian–Australian plate is subducting at present beneath the Pacific plate. However, until at least c. 12 Myr ago, the Pacific plate was subducting beneath the Indian–Australian plate until the Cretaceous Ontong Java Plateau collided with the northern Solomon island arc. To evaluate the effects of the changes in tectonic regime on lava compositions, we present a comprehensive Sr–Nd–Hf–Pb isotope, major element and trace element dataset, covering lavas erupted along the entire island arc (c. 1000 km). Basalts and andesites represent the most abundant rock types. Picrites and ankaramites occur in the New Georgia Group of the Solomon Islands, where they erupted above the subducting Woodlark spreading center, and also in the Santa Cruz archipelago, north of Vanuatu, where the Rennell Fracture Zone is subducting. Recent work has also identified the presence of adakites (Sr/Y up to c. 200), and high-Mg andesites (MgO > 5 wt %, Sr/Y c. 11–46). Most of the high-Mg andesites are genetically linked to the adakites, but some of the high-Mg andesites show affinities to boninitic compositions. Large ion lithophile element abundances in most Solomon island arc magmas indicate a strong source overprint by subduction components. ⁸⁷Sr/⁸⁶Sr and Nd values along the arc range from 0·7029 to 0·7052 and from +5·8 to +8·3, respectively. The Sr–Nd values partially overlap the compositions of oceanic basalts from the Indian–Australian plate. Measured Hf values range from +10·5 to +14·6. If corrected for contributions from subduction components, combined Hf–Nd systematics also indicate that most of the studied Solomon arc lavas were generated within the Indian-type mantle domain. However, a few samples display Hf–Nd compositions resembling those of the Pacific-type mantle domain. These samples either originate from older Pacific basement (basalts) or represent melts derived from subducted Pacific crust (adakites). Lead isotope compositions, controlled by subduction components, can be used to identify the presence of two distinct types of subduction components that originate (1) from the Pacific plate including Ontong Java Plateau material (> 6 Myr old) and (2) from the more recently subducted Indian–Australian plate. Combined Hf–Nd–Pb isotope data also reveal that lower parts of the Ontong Java Plateau entered the mantle wedge, as previously postulated by geophysical models.

Differentiation and Compaction in the Skaergaard Intrusion

Tegner, C., Thy, P., Holness, M. B., Jakobsen, J. K., Lesher, C. E. — 2009-06-08

Igneous differentiation processes are constrained from bulk compositions, densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the Layered Series of the Skaergaard intrusion, East Greenland. The stratigraphic distribution of P, U and Rb in cumulates and residual magma, modeled by Rayleigh fractionation, constrains the final porosities or trapped liquid contents to 30–52% in LZa troctolites, decreasing to 4–12% at the top of LZb olivine-gabbros and remaining low (1–13%; 4·6% on average) in the oxide-gabbros of LZc, MZ and UZ. Local variations in trapped liquid content are associated with modal layering: leucocratic, low-density rocks have higher proportions of trapped melt than adjacent melanocratic, high-density rocks. These observations are explained by varying degrees of compaction. Compaction was most important after the onset of Fe–Ti oxide crystallization because of the high densities of the crystal matrix. Here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick. In the basal section (LZa) the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor. In the overlying section (LZb) the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation. The modeling constrains the P₂O₅ content of the residual magma to ~1·7 wt % at the level of apatite-in, suggesting that the magma contained ~49 wt % SiO₂ and followed a trend of iron enrichment. Compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix.

Generation of Tonalitic and Dioritic Magmas by Coupled Partial Melting of Gabbroic and Metasedimentary Rocks within the Deep Crust of the Famatinian Magmatic Arc, Argentina

2009-06-08

The source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). The source zones of intermediate igneous rocks are located at the transition between a gabbro-dominated mafic unit and a tonalite-dominated intermediate unit. In the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall-rocks, crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments. In turn, the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks. The gabbroic rocks became mafic migmatites comprising amphibole-rich pyroxene-bearing mesosomes and leucotonalitic veins. Significantly, most of the mafic migmatites have isotopic compositions [⁸⁷Sr/⁸⁶Sr(T) < 0·7063 and _Nd(T) = –0·94 to +2·24] similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins [⁸⁷Sr/⁸⁶Sr(T) = 0·7075–0·7126 and _Nd(T) < –2·65], providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts [⁸⁷Sr/⁸⁶Sr(T) = 0·715 and _Nd(T) = –6·21]. Mass-balance calculations show that the model reaction plagioclase + amphibole + leucogranitic melt -> leucotonalitic melt + clinopyroxene ± orthopyroxene can better explain the partial melting of the gabbroic rocks. Based on field observations, we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt-dominated leucotonalitic pools. However, the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically (elemental and isotopic) more evolved than the dioritic and tonalitic rocks. We envisage that once detached from their source region the leucotonalitic magmas were able to react, commingle and mix with entrained fragments of both mafic and metasedimentary rocks. This process gave rise to melts that became tonalitic and dioritic magmas. This study concludes that the generation of intermediate magmas is a multistage process with three critical steps: (1) influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks; (2) physically and chemically coupled melting of mafic and metasedimentary rocks, leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies; (3) retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors. The dimensions of the source zone seem to be insufficient to generate crustal-scale volumes of intermediate igneous rocks. However, the Famatinian paleo-arc crust would expose only those magma source zones that were still active during the tectonic closure of the arc. Ultimately, a time-integrated perspective indicates that early active source zones were cannibalized during the downward expansion of the plutonic bodies already dominated by intermediate plutonic rocks.

Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U-Th Model-age Spectra in Zircons

Wilson, C. J. N., Charlier, B. L. A. — 2009-06-08

New and/or enlarged datasets of U–Th disequilibrium model ages from secondary ionization mass spectrometry (SIMS) analyses of zircons in eight eruptive units from the area of Taupo volcano, New Zealand, highlight the behavioural contrasts of two closely adjacent, contemporaneous but independent magma chambers. One yielded closely similar crystal-poor (‘Oruanui-type’) rhyolites, sampled in three small precursor eruptions (Tihoi, ‘New plinian’, Okaia) from ~45 to 30 ka, then the major 27 ka Oruanui eruption. Three of the four eruptions had vents within the modern Lake Taupo, whereas the fourth (‘New plinian’) was sourced ~20 km NNE of the other vents, fed by lateral magma migration. Samples from all four eruptions share a common model-age peak at ~95 ka of antecrystic zircons. However, three of the four differ in younger pre-eruptive model-age peaks that require their parental melt-dominant bodies to have been physically extracted independently from a common mush zone represented by the ~95 ka peak. A sample from a fifth eruption (‘New phreatoplinian’, also at ~45 ka) shares an older 80–100 ka peak but has numerous older grains and distinctly contrasting Sr-isotopic characteristics to the ‘Oruanui-type’ magmas. The 530 km³ Oruanui melt-dominant body was produced in at most ~3000 years as shown by differences in zircon model-age spectra and average ages between it and the 30 ka Okaia eruption, despite their coincidence in vent locations. The second suite of eruptions at ~47, 28 and 16 ka ejected moderately crystal-rich biotite rhyolites from a second source chamber, which vented over a 15 km wide area NE of Taupo (overlapping with Maroa volcano). This second chamber is inferred to have comparable horizontal dimensions to the vent spacing. The three biotite rhyolites show unimodal model-age spectra that peak at 30, 15–25 and 6 kyr prior to each eruption, respectively, and underwent single cycles of melt generation and eruption with no recycling of significantly older antecrysts or xenocrysts (< 1% equiline grains). Crystallization peaks defined by probability density function curves are not in phase between the two magma chambers and they had wholly independent thermal and chemical histories, despite their close geographical proximity. Post-Oruanui activity involved recycling of Oruanui-age zircons, but these crystals are xenocrystic, as the host melts show no lineage towards or mixing with the Oruanui compositions. Magma chambers at Taupo accumulated melt-dominant bodies as rapidly as > 5 m³/s (Oruanui) and effectively drained the mush of melt in doing so (Oruanui vs post-Oruanui activity), probably mediated by active rifting processes and tectonic disruption of the mush pile. Comparisons of ‘magma residence times’ and discussion of the growth histories of large silicic chambers represented by volcanic or plutonic rocks are self-limited by the uncertainties in the respective SIMS analyses. Growth times of Miocene and older plutons dated by SIMS U–Pb techniques are comparable with the 2 Myr lifetime of the whole Taupo Volcanic Zone, and the associated 1 SIMS analytical uncertainties exceed the lifetime of a volcano such as Taupo. Subtle details that indicate the rapidity of magma accumulation and recycling of crystals in the young Taupo system cannot be discerned in most pre-300 ka silicic systems. Averaging of SIMS model-age data further obscures subtle details that would allow discrimination of newly crystallized versus recycled zircons. Discussions of volcano–plutonic relationships and accumulation rates for large silicic melt-dominant bodies cannot rely on age data in isolation, but require knowledge of the stratigraphic and compositional settings.

An Experimental Determination of the Effect of Bulk Composition on Phase Relationships in Metasediments at Near-solidus Conditions

Ferri, F., Poli, S., Vielzeuf, D. — 2009-06-08

Phase relationships in metapelites were experimentally investigated in the model system CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O on four synthetic compositions differing in their K₂O content and K₂O/Al₂O₃ ratio. Experiments were carried out at pressures ranging from 0·8 to 1·4 GPa and temperatures from 620 to 740°C under different fluid-buffered conditions. The stability of the assemblage garnet–biotite–staurolite/Al₂SiO₅ at the investigated conditions largely controls the absence of muscovite in K-poor compositions. At 1·2 GPa and 700°C staurolite is present with garnet + biotite + muscovite, replaced by Al₂SiO₅ at lower pressures through the reaction staurolite + muscovite + quartz = garnet + biotite + Al₂SiO₅ + H₂O. In K-poor bulk compositions, garnet + biotite + staurolite + gedrite coexist up to 730°C. A compositional reversal of the Mg–Fe partitioning between garnet and staurolite is observed with decreasing pressure and is responsible for the singular equilibrium staurolite + quartz = Al₂SiO₅ + garnet + H₂O that governs the high-temperature breakdown of staurolite. Melting is found to depend both on bulk composition and on fluid speciation. The wet solidus is represented by the reaction muscovite + anorthite + garnet + quartz + H₂O = melt + biotite + Al₂SiO₅. Even though staurolite is never recovered at supersolidus conditions, the large pressure–temperature stability field of staurolite + muscovite and of staurolite + gedrite suggests that staurolite can be directly involved in melt production through fluid-present or fluid-absent reactions.

Archean Accretion and Crustal Evolution of the Kalahari Craton--the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton/Swaziland to the Francistown Arc

Zeh, A., Gerdes, A., Barton, J. M. — 2009-06-08

U–Pb and Lu–Hf isotope analyses, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry on zircon grains from 37 granitoid samples indicate that the Kalahari Craton consists of at least five distinct terranes—Barberton South (BS), Barberton North (BN), Murchison–Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistown—which underwent different crustal evolutions, and were successively accreted at c. 3·23 Ga, 2·9 Ga and 2·65–2·7 Ga. The investigated granitoids were emplaced over a period of c. 1.5 billion years, and are exposed along a c. 1000 km long traverse from the Barberton Mountain Land/Swaziland to the Francistown arc complex, Botswana. The presented datasets reveal that most granitoids of the BS (3·45–3·10 Ga), MNK (2·93–2·67 Ga), Francistown (2·70–2·65 Ga) and LCZ terranes (3·2–2·03 Ga) show near-chondritic to subchondritic Hf_t (BS = –1·7 to + 0·5; MNK = –3·4 to + 0·7; Francistown = –0·5 to + 1·1; LCZ = –12·4 to –1·8), indicating that crustal recycling—perhaps by mixing of an older crust with a depleted mantle reservoir—played an important role during their formation and growth. Higher, superchondritic Hf_t values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (Hf_3·23Ga = +2·5 ± 0·8), the Gaborone Granite Suite (Hf_2·80Ga = +2·0 ± 1·6), and from a few detrital zircons from the Mahalapye complex of the Limpopo Belt. In addition, the datasets show that the internal Hf isotope variation of magmatic zircon domains from most granitoids is commonly less than ±1·5 -units, and only in rare cases up to ±3·1 -units. The rare significant Hf_t variations may be ascribed to incomplete mixing of different sources during magma crystallization. It is also shown that the combined approach of cathodoluminescence imaging with U–Pb and Lu–Hf isotope analysis provides a powerful tool to distinguish zircon domains formed and/or altered at different times.

The Complex Hydrothermal History of Granitic Rocks: Multiple Feldspar Replacement Reactions under Subsolidus Conditions

Plumper, O., Putnis, A. — 2009-06-08

Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid–rock interactions that affect large volumes of the Earth's continental crust. This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replacement reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts. This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (~An₉) along polysynthetic twinning and intragranular fractures. The features of this replacement are characteristic of a dissolution–reprecipitation mechanism. Fine-grained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid–feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered.