high-Al Basalts Research Articles

Island arc volcanic rocks in the north Qaidam UHP belt, northern Tibet plateau: Evidence for ocean–continent subduction preceding continent–continent subduction

A suite of Early Paleozoic island arc volcanic rocks, consisting mainly of basaltic lavas and some intermediate-silicic rocks, crops out in the North Qaidam basin, northeastern Tibet Plateau. These rocks have been extensively metamorphosed under greenschist facies conditions, and hence differ from the older Proterozoic intermediate-mafic volcanic rocks, which have generally experienced amphibolite facies metamorphism. The Early Paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics: (1) VTG-I, consists of island arc tholeiitic basalts, basaltic andesites and andesites that crop out in Jilusu and Shuangkoushan; (2) VTG-II, which are high-Al, alkaline and sub-alkaline basalts in Jilusu; and (3) VTG-III, an assemblage of highly depleted, MORB-type basalts, andesites and dacites that occurs mostly in Shuangkoushan. Groups VTG-I and -II are considered to be the products of mature island arcs produced in two stages. In the first stage, following initiation of ocean–continent subduction, the island arc tholeiites were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc. As subduction progressed and accelerated, calc-alkaline and high-Al basalts were formed and the crust beneath the island arc was thickened. The highly depleted MORB-type lavas (VTG-III) have boninitic affinities and are believed to have been generated from depleted mantle material modified by subduction zone fluids, possibly in the fore-arc region. The island arc tholeiites are dated at 514.2±8.5 Ma whereas UHP eclogite in the North Qaidam Belt has been dated at 494 Ma. These ages suggest that ocean–continent subduction took place in the North Qaidam Basin in the Early Paleozoic and was followed shortly by continent–continent collision and subduction, which produced the UHP rocks.

Petrography and geochemistry of five new Apollo 16 mare basalts and evidence for post‐basin deposition of basaltic material at the site

Abstract— We present the petrography and geochemistry of five 2–4 mm basalt fragments from the Apollo 16 regolith. These fragments are 1) a high‐Ti vitrophyric basalt compositionally similar to Apollo 17 high‐Ti mare basalts, 2) a very high‐Ti vitrophyric basalt compositionally similar to Apollos 12 and 14 red‐black pyroclastic glass, 3) a coarsely crystalline high‐Al basalt compositionally similar to group 5 Apollo 14 high‐Al mare basalts, 4) a very low‐Ti (VLT) crystalline basalt compositionally similar to Luna 24 VLT basalts, and 5) a VLT basaltic glass fragment compositionally similar to Apollo 17 VLT basalts. High‐Ti basalt has been reported previously at the Apollo 16 site; the other basalt types have not been reported previously. As there are no known cryptomaria or pyroclastic deposits in the highlands near the Apollo 16 site (ruling out a local origin), and scant evidence for basaltic material in the Apollo 16 ancient regolith breccias or Apollo 16 soils collected near North Ray Crater (ruling out a basin ejecta origin), we infer that the basaltic material in the Apollo 16 regolith originated in maria near the Apollo 16 site and was transported laterally to the site by small‐ to medium‐sized post‐basin impacts. On the basis of TiO2 concentrations derived from the Clementine UVVIS data, Mare Tranquillitatis (˜300 km north) is the most likely source for the high‐Ti basaltic material at the Apollo 16 site (craters Ross, Arago, Dionysius, Maskelyne, Moltke, Sosigenes, Schmidt), Mare Nectaris/Sinus Asperitatis (˜220 km east) is the most likely source for the low‐Ti and VLT basaltic material (craters Theophilus, Madler, Torricelli), and a large regional pyroclastic deposit near Mare Vaporum (˜600 km northwest) is the most likely source region for pyroclastic material (although no source craters are apparent in the region).

Petrogenesis of the Apollo 14 high-alumina basalts: Implications from ion microprobe analyses

In this study, ion microprobe analyses of individual minerals are used to investigate the petrogenesis of the Apollo 14 high-Al basalts. We use trace element concentrations from individual minerals in the Apollo 14 high-Al basalts to evaluate both endogenic and exogenic models. The data show that if the Apollo 14 high-Al basalts were produced by melting within the lunar mantle, these basalts cannot be related to one another by closed-system fractional crystallization of a single basaltic melt. Rather, the trace element data show that variable amounts of a KREEP component were added to the basalts by either assimilation, mixing into mantle sources, or impact melting. Single-stage assimilation–fractional crystallization models can only explain the data from this study if an excessively large mass of urKREEP is assimilated into the parent magma before olivine crystallization. Alternatively, the trace element data can be explained if the Apollo 14 high-Al basalts were produced by melting multiple Al-rich mantle sources that contain different amounts of urKREEP. Finally, for impact melting to be a relevant process, the data require that multiple large impact melts be formed from mixed KREEP-rich target lithologies. The resulting impact melts must then crystallize to produce basalts with igneous textures, high Al 2O 3 concentrations, uniform major element compositions, and a wide range of incompatible trace element concentrations.

Geochemistry of Late Cretaceous (60–67 Ma) igneous activities in the hebrides terrace seamount (guyot) area, Scotland

Tholeiitic basalts in various stages of alteration were dredged from Late Cretaceous volcanic rocks (60–67 Ma) in the Hebrides Terrace seamount area in the Atlantic Ocean. These rocks are extrusive olivine basalts, including high- and low-Al basalts. High-Al basalts are depleted in MgO, CaO, Cr, Sc, V, Sr, Zr and enriched in TiO2, Na2O, Nb, Rb as compared with low-Al basalts. Petrography and bulk-rock composition (major, trace and rare-earth elements) data defined clear tholeiitic suites displaying possible liquid lines of descent related to different degrees of crystal fractionation and partial melting. Isotopic dating of dredged samples gave the guyot an age of 60–67 Ma, in support of the assumption that it was formed during the Late Cretaceous.

The most reduced rock from the moon, Apollo 14 basalt 14053: Its unique features and their origin

The Apollo 14 high-Al basalt, 14053, is the most reduced lunar rock examined to date. Both fayalite in the mesostasis and spinel minerals have been extensively reduced in the exterior of the rock, whereas the interior contains relatively limited reduction. It is shown that these products are the effects of solar-wind hydrogen that was implanted on the exterior of the “normal” 14053 basalt after it originally crystallized and was weathered to become part of the regolith. Subsequent reheating, probably in an impact-ejecta blanket, caused extreme subsolidus hydrogen reduction, particularly of the weathered exterior of this rock. The limited permeability of the rock prevented the entire rock from being subjected to the same degree of reduction. It is proposed that this extreme reduction, especially of the mesostasis, also affected the phosphate minerals, F-Cl apatite and merrillite, each of which vary greatly in REE contents. This effect on the phosphate minerals could relate to the upset Sm-Nd radiogenetic systematics, whereas the Rb-Sr system may have been largely immune to the subsolidus reduction.

Reply to the comment: Mantle plume-, but not arc-related Neoproterozoic magmatism in South China

The Micangshan terrane is located along the NW margin of the Yangtze block and contains the second oldest Precambrian basement in South China. It comprises the Beiba and the Wangcang-Nanjiang segments, while the oldest suites (∼2.1 Ga Houhe complex) occur in the Beiba. The Hekou and the Wangjiaping metamorphic volcano-sedimentary sequences in the Wangcang are characterized by intensive metamorphism and deformation, long being regarded as the equivalent of the Houhe suite. This contribution presents an integrated study of zircon U-Pb geochronology and Hf isotope, whole rock elemental and Sr-Nd isotope geochemistry on volcanic rocks of the metastrata. It reveals that they formed at ∼ 865–860 Ma and underwent upper amphibolite facies metamorphism at ∼ 815 Ma with lower limits in P-T conditions of ∼ 5.0 kbar and ∼ 650–660 °C. The Hekou metavolcanics comprise dacitic-rhyolitic and alkaline basaltic rocks with minor tholeiitic mafics, whereas those of the Wangjiaping are composed of dacitic and tholeiitic mafic rocks. The Hekou alkaline metamafics show affinity to high-Nb to Nb-enriched basalts (HNB-NEB) with εNd(t) values of + 5.80 to + 6.25, whereas tholeiitic metamafics of the both sequences display characteristics typifying high-Al basalt (HAB) with εNd(t) value ∼+2.66. The Hekou and the Wangjiaping metafelsics have εNd(t) ranges of −2.33 to + 5.60 and −0.82 to + 0.99 (except one + 2.29), respectively, and the dacitic rocks show features akin to the high Mg-number andesites. The ∼ 865–860 Ma magmatism is suggested to have occurred in an extensional basin with a post-subduction affinity. It was induced by upwelling of convective asthenosphere succeed the ceased slab subduction: the Wangjiaping metafelsics were derived from partial melting of juvenile lower crust, the Hekou dacitic rocks were produced by partial melts of the relict of subducted oceanic crust interacted with mantle wedge; the Hekou HNB-BEB were sourced by metasomatized mantle wedge, whereas the HAB lavas were generated by anhydrous partial melts of the upwelling asthenosphere. The ∼ 815 Ma metamorphism was caused by an amalgamation between the South Qinling belt and the proto margin of the northwestern Yangtze block. This event was followed by an expanding rift from the Micangshan successively to the South Qinling.

The nature of melt inclusions inside minerals in an ultramafic cumulate from Adak volcanic center, aleutian arc: implications for the origin of high-Al basalts

In order to characterise the parental melts of crustal ultramafic cumulates from arc environments, we have undertaken a study of melt and fluid inclusions in olivine and clinopyroxene crystals in a typical cumulate xenolith from Adak Island, Aleutian Island Arc. The crystals contain inclusions either of silicate melts plus a H 2O-rich bubble or H 2O-dominated fluids, indicating H 2O saturation of the trapped melt during the entire course of its crystallisation. Homogenisation experiments of the silicate melt inclusions give entrapment temperatures ranging between 940 and 1010 °C. After homogenisation, the melt inclusions range in composition from basalt to dacite ; the Al 2O 3 and SiO 2 contents increase from 18.5 to 26.3 wt.% and 47.1 to 56.4 wt.%, respectively, as MgO and FeO decrease from 6.5 to ∼0.1 wt.% and 6.5 to 0.3 wt.%. The melt inclusions also have high levels of H 2O, ≥6 wt.%. Comparisons of the compositional trends in the melt inclusion suite with those in experimental multiply-saturated liquids of basalts indicate that the compositional variations in the melt inclusions reflect progressive crystallisation of an olivine+clinopyroxene assemblage similar to the host cumulate xenolith, at pressures ≥3.0 kbar and H 2O-saturated conditions. A comparison between lavas from Adak Island and the melt inclusion compositions supports the hypothesis that fractional crystallisation at moderate pressures of hydrous mafic basalts generates high-alumina basalt compositions and leaves large volumes of ultramafic cumulate rocks in the crust beneath arc sections.

Magmatism in the South Sandwich arc

Abstract The South Sandwich Islands are one of the world’s classic examples of an intraoceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas-Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by c. 36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.

The geochemistry of volcanic, plutonic and turbiditic rocks from Sumba, Indonesia

Rocks that reveal the geology of Sumba for times before the Later Miocene (∼16 Ma) are relatively few and are not particularly well exposed. This has led to uncertainty about the nature of the basement rocks of the island and especially about whether Sumba originated as a fragment of Australia, or of that part of southeastern Eurasia which many authors have called Sundaland. A third possibility is that Sumba is underlain by arc material generated on the ocean floor and is not a fragment of either continent. We have studied the geochemistry of volcanic, plutonic and turbiditic rocks collected from Sumba in an attempt to provide additional insight into the island's origin and history between Late Cretaceous (∼86 Ma) and Early Miocene (∼16 Ma) times. Late Cretaceous to Early Oligocene (∼31 Ma) volcanic rocks on Sumba range compositionally from basalts to andesites, and are of typical oceanic island-arc affinity, exhibiting geochemical characteristics similar to those of high-Al basalts and their derivatives. Compositions indicate evolution along both calc-alkaline and tholeiitic trends. Some samples show indications of possible modifications by slab-derived melts and/or related fluids and also of contamination by turbiditic sediments. Gabbros and diorites collected from the Paleocene Tanadaro intrusion are compositionally similar to the associated volcanic rocks and, we consider, represent the plutonic equivalents of high-Al basalt. The geochemistry of Cretaceous turbiditic sedimentary rocks on Sumba indicates close proximity to an intra-oceanic island-arc environment. These results are consistent with the geochemical, sedimentological, stratigraphic, paleontological and paleomagnetic results of other investigators which together indicate that: (1) Late Cretaceous to Early Oligocene volcanic, plutonic and volcaniclastic rocks of Sumba are island-arc- and forearc-related; (2) the arc involved appears to have been what we refer to as the Great Indonesian Volcanic Arc, which had been active in this area from at least ∼86 to approximately ∼31 Ma. The Great Indonesian Volcanic Arc was closely affiliated with, but may have occurred offshore Sundaland. Sumba is therefore a fragment of this oceanic (Aleutian-type) island arc and not a piece of the main Sundaland continent. The geochemistry of the igneous rocks on Sumba is inconsistent with what we would expect from an Andean-type Continental Arc. No continental basement is required to explain the composition and origin of the igneous rocks of Sumba.

Author Index

Anorthite megacrysts from twelve volcanic centers of the Kurile Island Arc were analyzed by the ion microprobe for concentrations of: Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. These 1–2-cm megacrysts are mainly found as early phenocrysts in basalts, but are also present in andesites, rhyolites and hybrid rocks from almost every studied volcanic center, both subaerial and submarine. Electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned, whereas rims often result from overgrowth of sodium-richer plagioclase in equilibrium with its host, silica-richer melt. The cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a `window' into an earlier basaltic stage of magmatic evolution. Petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second (after olivine) liquidus phase in high-Al basalts regardless of varying K2O contents across the arc.Given the narrow compositional range of anorthite megacrysts, the across-arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc. The concentration of K in megacrysts increases linearly by a factor of 11; Ba, 13; Sr, Eu, 4; Be, Ti, P, 3; Li, 2, REE, 5–2; whereas concentrations of Pb, B and halogens and their ratios to HFSE either remain constant or decrease with deepening of the Benioff Zone from 110 to 220 km. We used ion microprobe-derived plagioclase–basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values. The reconstructed concentrations of several key trace elements and their correlation with Benioff Zone depth are consistent with whole-rock basalt data, but all give systematically more primitive compositions. Therefore, anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing. They can be used to assess delicate variations in primary basaltic magma chemistry as a function of Benioff Zone depth. We also report new results for Sr, Nd and Pb isotopes for anorthite-bearing rocks and other rocks from the same volcanoes. Across-arc zoning with respect to trace element concentrations, ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes: trace element transfer by fluids, variation in degree of partial melting and progressive change of the melting source.

The Minoan and post-Minoan eruptions, Santorini (Greece), in the light of melt inclusions: chlorine and sulphur behaviour

Mineralogical and melt inclusion studies were carried out on plinian tephra (Pumice fall unit, Phira) ejected during the first phase of the Minoan eruption, as well as on the most basic pyroclastic ejecta emitted during the recent activity of Nea Kameni. Heating stage experiments were performed on crystallised melt inclusions. The temperature of melting ( T m) varies from 1095±10°C (Fo 80–77, high Al-basaltic melts) to 995±10°C (Fo 76–73, andesitic melts). A subsequent quenching and electron microprobe analysis of glass was performed in order to determine the composition of entrapped magmas. After heating experiments, the melt inclusions are parent liquids of a high Al-basaltic composition; these parent magmas have not been found as whole rocks in post-Minoan deposits. Melt inclusions, whole rocks and matrix glasses define a compositional trend from high Al-basalt to rhyolite, which attests to either mixing/mingling of cogenetic magmas or fractional crystallisation as processes controlling the chemical evolution of these magmas, and corresponds to a fraction of crystallised parent magmas of about 70%. Rhyolitic melts exhibit the highest chlorine contents (3000–3200 μg/g Cl) and the lowest sulphur contents (about 100 μg/g S). Basaltic and andesitic melts show the lowest chlorine contents (900–1000 μg/g Cl) and the highest sulphur contents (1000–600 μg/g S). This shows a difference in behaviour between chlorine and sulphur, which is probably linked to the difference in solubility. This suggests also that the present-day sulphur emission at Santorini does not originate only from differentiated magmas but also from basaltic and andesitic ones. On this assumption, the minimal chlorine and sulphur release to the atmosphere is estimated to be about 2×10 10 and 2.5×10 11 kg for the Minoan eruption. This shows that volcanic hazards related to volatile degassing, especially to SO 2 emissions, could represent a serious risk at Santorini.

The Evolution of Troctolitic and High Al Basaltic Magmas inProterozoic Anorthosite Plutonic Suites and Implications for the Voisey's BayMassive Ni-Cu Sulfide Deposit

The Voisey's Bay massive Ni-Cu sulfide deposit is contained within troctolitic (plagioclase + olivine) intrusions of the 1.34 to 1.29 Ga Nain Plutonic Suite, coastal Labrador, Canada, and represents a new setting for magmatic sulfide deposits. Proterozoic anorthosite plutonic suites consist of vast quantities (as much as 20,000 km 2) of cumulate anorthosite-leucotroctolite-leuconorite, intrusions of troctolite and Fe-enriched dioritic rocks, and large granitic batholiths. Crystallization ages span nearly 1,200 m.y. within the Middle Proterozoic (2.12-0.92 Ga). Many Proterozoic anorthosite plutonic suites were emplaced into relatively young Proterozoic crust and some were intruded along Archean-Proterozoic boundaries (e.g., Nain, Labrador; Laramie, Wyoming; Korosten, Ukraine). Nearly all of the largest plutonic suites contain significant areas (thousands of km 2) of plagioclase-rich cumulate rocks that are olivine bearing (olivine anorthosite-leucotroctolite). Troctolitic rocks (20-30% modal olivine) occur in Proterozoic anorthosite plutonic suites, principally as cumulates in layered or massive intrusions (e.g., Kiglapait, Hettasch, Jonathon-Nain) intruded into anorthositic rocks. With the notable exception of the Voisey's Bay deposit, nearly all troctolitic rocks crystallized late in the evolution of individual plutonic suites. Many of the troctolitic intrusions have fine-grained olivine-rich marginal rocks that closely represent parental magma compositions. Numerous fine-grained olivine-normative dikes with high Al basaltic compositions also occur within many plutonic suites (Nain, Harp Lake, Laramie, Adirondack) and share compositional characteristics with the troctolitic magmas. These olivine-normative high Al basaltic magmas record the least evolved compositions in Proterozoic anorthosite plutonic suites (Mg = 0.43-0.64; An = 45-60), have high Al 2O 3 (15-19 wt %) and FeO(total) (9-15 wt %) contents, and typically show the least contaminated Sr and Nd isotope compositions in a given plutonic suite. Many dikes and marginal rocks have strong positive Eu anomalies, a feature of the melt composition that is not related to plagioclase accumulation. Except at Laramie, the least evolved troctolitic magmas appear to have about 8 wt percent MgO and corresponding Ni and S concentrations between 100 to 150 ppm and 0.02 to 0.05 wt percent, respectively. Based on closed-system fractionation in the Kiglapait intrusion, sulfide saturation occurs late in the crystallization history of troctolitic magmas, after Ni concentrations have been severely depleted by protracted olivine fractionation. The optimal conditions for formation of a Ni-Cu sulfide deposit associated with a Proterozoic anorthosite plutonic suite require a relatively high MgO parental magma that has not crystallized much prior olivine and an external source of S to promote early sulfide saturation, probably through desulfurization reactions as hot troctolitic magmas ascend through the crust. An economically important massive Ni-Cu sulfide deposit on the scale of Voisey's Bay clearly requires large volumes of magma, probably involving continuous replenishment of relatively Ni rich, unfractionated parental magma in staging chambers or mixing between magmas in conduits during ascent. A model for the formation of troctolite-associated massive Ni-Cu sulfide deposits is proposed. Depending on the relative degrees of sulfide saturation and fractionation of the mixed magmas, mixing between troctolitic magmas in a conduit or magma chamber may either poison the system, stopping sulfide segregation, or enhance sulfide segregation.

The Evolution of Troctolitic and High Al Basaltic Magmas in Proterozoic Anorthosite Plutonic Suites and Implications for the Voisey's Bay Massive Ni-Cu Sulfide Deposit

The Evolution of Troctolitic and High Al Basaltic Magmas in Proterozoic Anorthosite Plutonic Suites and Implications for the Voisey's Bay Massive Ni-Cu Sulfide Deposit

Basaltes, andésites, dacites et rhyolites des stratovolcans des Nevados de Chilian et de l'Antuco (Andes méridionales): Ia remarquable illustration d'une différenciation par cristallisation fractionnée

The Nevados de Chilian and Antuco stratovolcanoes (36 °-37 dg S, southern Andes) erupt high-Al basalts, andésites, dacites and rhyolites. The lava series has no indication of contamination by the continental crust. Therefore, its differentiation is very well explained by crystal fractionation of the mineral phases present as phenocrysts (plagioclase, olivine, clinopyroxene, orthopyroxene, magnetite, ilmenite and apatite), taking into account major and trace elements. A fractionation model is presented here.

Trace elements in anorthite megacrysts from the Kurile Island Arc: a window to across-arc geochemical variations in magma compositions

Anorthite megacrysts from twelve volcanic centers of the Kurile Island Arc were analyzed by the ion microprobe for concentrations of: Li, Be, B, F, Mg, P, Cl, K, Ti, Fe, Co, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, and Pb. These 1–2-cm megacrysts are mainly found as early phenocrysts in basalts, but are also present in andesites, rhyolites and hybrid rocks from almost every studied volcanic center, both subaerial and submarine. Electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned, whereas rims often result from overgrowth of sodium-richer plagioclase in equilibrium with its host, silica-richer melt. The cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a `window' into an earlier basaltic stage of magmatic evolution. Petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second (after olivine) liquidus phase in high-Al basalts regardless of varying K 2O contents across the arc. Given the narrow compositional range of anorthite megacrysts, the across-arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc. The concentration of K in megacrysts increases linearly by a factor of 11; Ba, 13; Sr, Eu, 4; Be, Ti, P, 3; Li, 2, REE, 5–2; whereas concentrations of Pb, B and halogens and their ratios to HFSE either remain constant or decrease with deepening of the Benioff Zone from 110 to 220 km. We used ion microprobe-derived plagioclase–basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values. The reconstructed concentrations of several key trace elements and their correlation with Benioff Zone depth are consistent with whole-rock basalt data, but all give systematically more primitive compositions. Therefore, anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing. They can be used to assess delicate variations in primary basaltic magma chemistry as a function of Benioff Zone depth. We also report new results for Sr, Nd and Pb isotopes for anorthite-bearing rocks and other rocks from the same volcanoes. Across-arc zoning with respect to trace element concentrations, ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes: trace element transfer by fluids, variation in degree of partial melting and progressive change of the melting source.

Phase equilibria modeling in igneous petrology: use of COMAGMAT model for simulating fractionation of ferro-basaltic magmas and the genesis of high-alumina basalt

A new version of COMAGMAT-3.5 model designed for computer simulations of equilibrium and fractional crystallization of basaltic magmas at low to high pressures is presented. The most important modifications of COMAGMAT include an ability to calculate more accurately the crystallization of magnetite and ilmenite, allowing the user to study numerically the effect of oxygen fugacity on basalt magma fractionation trends. Methodological principles of the use of COMAGMAT were discussed based on its thermodynamical and empirical basis, including specific details of the model calibration. Using COMAGMAT-3.5 a set of phase equilibria calculations (called Geochemical Thermometry) has been conducted for six cumulative rocks from the Marginal Border Series of the Skaergaard intrusion. As a result, initial magma temperature (1165±10°C) and trapped melt composition proposed to be parental magma to the Skaergaard intrusion were determined. Computer simulations of perfect fractionation of this composition as well as another proposed parent produced petrochemical trends opposite to those followed from natural observations. This is interpreted as evidence for an initial Skaergaard magma containing a large amount of olivine and plagioclase crystals (about 40–45%), so that the proposed and calculated parents are related through the melt trapped in the crystal–liquid mixture. This promotes the conclusion that the Skaergaard magma fractionation process was intermediate between equilibrium and fractional crystallization. In this case the classic Wager's trend should be considered an exception rather than a rule for the differentiation of ferro-basaltic magmas. A polybaric version of COMAGMAT has been applied for the genetic interpretation of a volcanic suite from the Klyuchevskoi volcano, Kamchatka, Russia. To identify petrological processes responsible for the observed suite ranging from high-magnesia to high-alumina basalts, we used the model to simulate the Klyuchevskoi suite assuming isobaric crystallization of a parental HMB magma at a variety of pressures and a separate set of simulations assuming fractionation during continuous magma ascent from a depth of 60 km. These results indicate that the Klyuchevskoi trend can be produced by ∼40% fractionation of Ol–Aug–Sp±Opx assemblages during ascent of the parental HMB magma over the pressure range 19–7 kbar with the rate of decompression being 0.33 kbar/% crystallized (at 1350–1110°C), with ∼2 wt.% of H 2O in the initial melt and ∼3 wt.% of H 2O in the resultant high-Al basalt.

Clinopyroxene geobarometry of magmatic rocks Part 1: An expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasic systems

Crystal-structure modeling of experimental Ca-rich clinopyroxenes [Ca + Na > 0.5 apfu; Mg/(Mg + Fe2+) > 0.7] coexisting with basic and ultrabasic melts was utilized for calibration of geobarometers based on unit-cell volume (Vcell) vs M1-site volume (VM1). The clinopyroxene database includes over one hundred experiments from literature and sixteen previously unpublished experiments on basanite and picrobasalt starting materials. The coexisting melts span a wide range of petrologically relevant anhydrous and hydrous compositions (from quartz-normative basalt to nephelinite, excluding high-Al basalts and melts coexisting with garnet or melilite) at pressure conditions pertinent to the earth's crust and uppermost mantle (P= 0–24 kbar) in a variety of fO 2 conditions (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Lc ± Ne ± Spl ± Amp ± Ilm). As previously found for near-liquidus products of basaltic melts, the experimental clinopyroxenes follow two distinct trends: (i) at a given P, Vcell is linearly and negatively correlated with VM1. This corresponds with the extent of Tschermak-type substitutions, which depends strongly on aSiO2 and a CaO; (ii) for a fixed melt composition, Vcell and VM1 decrease linearly as P increases, due to a combination of M1, M2 and T site exchanges. Despite the chemical complexity of these relationships, P could be modeled as a linear function of Vcell and VM1. A simplified solution for anhydrous magmas reproduced the experimental pressures with an uncertainty of 1.75 kbar (=1; max. dev. = 5.5 kbar; N = 135). An expanded T-dependent solution capable of recovering the measured pressures of both anhydrous and hydrous experiments with an uncertainty of 1.70 kbar (=1; max. dev. = 5.4 kbar; N = 157) was obtained by correcting unit-cell and M1-site volumes for thermal expansivity and compressibility. The corrected formulation is more resistant to the effects of temperature variations and is therefore recommended. Nevertheless, it requires an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase of calculated P. The applicability of the T-dependent formulation was tested on hydrous ultramafic to gabbroic rocks of the southern Adamello batholith for which P-T evolution could independently be constrained by field observation, petrography and experimentally determined phase relations. The pressure estimates obtained by clinopyroxene structural geobarometry closely matched those predicted by phase equilibria of a picrobasaltic melt parental to the investigated magmatic rocks. To facilitate application of the present geobarometers, both anhydrous and corrected solutions were implemented as MS-DOS® and UNIX® software programs (CpxBar) designed to permit retrieval of the pressure of crystallization directly from a chemical analysis or from uncorrected unit-cell and M1-site volume X-ray data.

Petrogenesis of the Paleoproterozoic basalt–andesite–rhyolite dyke association in the Carajás region, Amazonian craton

Paleoproterozoic basaltic, andesitic and rhyolitic dykes crosscut the Archaean Carajás basement. Basalts are distinguished into a high and a low TiO 2 group (HTi and LTi), each group consisting of geochemically distinct NE- and NW-trending swarms. The HTi dykes are evolved transitional basalts having essentially EMORB-type geochemistry. The LTi basalts are tholeiites (NE-trending swarm) and high-Al basalts (NW-trending swarm) displaying incompatible trace elements patterns with variably negative Nb anomaly, enrichment in Rb, Ba, K (LILE) and La, Ce and Nd (LREE) and positive Sr anomaly. With respect to orogenic analogues, andesites have lower Al 2O 3, CaO and Ni, higher FeO, LILE, LREE, Nb, Zr and Ti and negative Sr anomaly. Rhyolites have geochemical characteristics comparable with those of A-type granites. At 1.8 Ga, 87Sr/ 86Sr ranges from 0.700 to 0.705 in the HTi basalts and from 0.700 to 0.704 in the LTi group. Andesites define an isochron of 1874±110 Ma (Sr o=0.7038±0.0010). Rhyolites from Southern and Northern Carajás define two isochrons of 1802±130 Ma (Sr o=0.7062±0.0046) and 1535±82 Ga (Sr o=0.7625) respectively, the younger date being interpreted as resetting of the Rb–Sr isotopic system. We propose a petrogenetic model relating LTi basalts with melting of lithospheric mantle metasomatized by acid melts derived from incipient melting of eclogites, representing in turn the subsolidus product of basaltic batches trapped in the mantle. The HTi basalts are explained by melting of the lithospheric mantle containing the complementary residual eclogite. Andesite petrogenesis is consistent with crystal fractionation from a high-Mg andesite parent derived from a mantle source more extensively metasomatized by eclogite-derived melts. Rhyolite composition is consistent with low melting degree of the basement rocks. The basalt–andesite–rhyolite dykes may represent the effects of crustal extension and arching in Carajás, which produced the anorogenic acid to intermediate magmatism (Uatumã group) and affecting a large part of the Amazon craton between 1.85 and 1.7 Ga.

Geochemical trends of sequential lava flows from Meseta volcano, Guatemala

The Meseta and Fuego volcanoes closely overlap and collectively are known as the Fuego Volcanic Complex. Historic activity occurs exclusively at Fuego, the southern center, and consists of high-Al basalts. Meseta, the inactive northern center, is predominantly composed of basaltic andesites with minor basalt and andesite. A thick sequence of lava flows and dikes is exposed by a steep collapse escarpment on the east flank of Meseta. The upper 75% of the sequence was sampled from three interfingering stratigraphic sections consisting of 27, 10 and 4 lavas, respectively. Temporal geochemical trends of each section indicates a complex evolutionary history. A major trend toward more evolved compositions upward in the section is consistent with crystal fractionation. This trend is sharply interrupted by the youngest lavas which become distinctly more mafic in composition. Magma mixing is apparently the dominant magmatic evolution process that generated these lavas. The two trends have distinct Sr signatures that suggest a change in parental magma compositions. This abrupt change in composition is interpreted to signal high input rates of mafic magma into the subvolcanic magma chamber. These changes eventually led to sector collapse of Meseta volcano and deposition of the Escuintla debris avalanche. Eruptive activity then migrated to the Fuego volcano where historic activity is similar to that of Meseta immediately prior to its collapse.

Roots of an Archean volcanic arc complex: the Lac des Iles area in Ontario, Canada

The Lac des Iles area is located in the Wabigoon Subprovince, an Archean granite-greenstone terrain in the Superior Province in Ontario, Canada. The area consists of several unmetamorphosed plutons of biotite- and hornblende-tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss. The mafic/ultramafic bodies are concentric in shape and consist of periodotite, pyroxenite, gabbro and hornblende gabbro cumulates and diorite. The largest intrusion, the Lac des Iles complex, consists of several mafic and ultramafic centers. An internal SmNd isochron from the southern ultramafic complex gives an intrusion age of 2736 ± 28 Ma (MSWD = 1.5; ϵ Nd(t) = 1.2). This age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons. The tonalitic gneiss represents older continental crust and had an ϵ Nd value of −1 at this time. The ϵ Nd(t) values of the felsic and mafic/ultramafic suites are positive, ranging from 0 to +2. This range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust, such as the tonalite gneiss, during their ascent. This implies that the tonalite magmas in the Lac des Iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere. The morphology and the rock assemblages of the mafic and ultramafic intrusions in the Lac des Iles area resemble Alaskan-type intrusions which are common components of Phanerozoic magmatic arcs associated with subduction zones. In addition, the crystallization sequence in the mafic/ultramafic intrusions suggests that they are derived from a parental magma with high-Al basalt affinities. The mafic/ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements, and the tonalites belong to calc-alkaline rock series displaying fractionation trends towards trondhjemitic compositions. Thus, the Archean Lac des Iles area and the root zones of Phanerozoic active continental margins share many geological, geochemical and petrological features which would suggest that the Lac des Iles area formed in an Andean-type continental margin.