Showing papers on "Basalt published in 2012"

Copper systematics in arc magmas and implications for crust-mantle differentiation.

Abstract: Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

Early differentiation and volatile accretion recorded in deep-mantle neon and xenon

Abstract: Noble gas contents of the Iceland mantle plume show that neither the Moon-forming impact nor billions of years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation. This analysis of noble gas content in Icelandic basaltic rocks — material produced by the melting of a deep upwelling of hot rock in Earth's mantle — indicates that their source is less degassed than that of mid-ocean-ridge basalts and that Earth's mantle has accreted volatiles from at least two separate sources. The author concludes that neither the Moon-forming impact nor billions of years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation. The isotopes 129Xe, produced from the radioactive decay of extinct 129I, and 136Xe, produced from extinct 244Pu and extant 238U, have provided important constraints on early mantle outgassing and volatile loss from Earth1,2. The low ratios of radiogenic to non-radiogenic xenon (129Xe/130Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir1. However, the low 129Xe/130Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir3,4. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and 20Ne/22Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower 129Xe/130Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because 129I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth’s mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation.

Statistical geochemistry reveals disruption in secular lithospheric evolution about 2.5 Gyr ago

Abstract: Statistical sampling of a large geochemical database reveals a pervasive discontinuity about 25 billion years ago, indicating marked changes in mantle and deep-crustal melting, and providing a link between deep Earth processes and the rise of atmospheric oxygen on the Earth Brenhin Keller and Blaire Schoene apply statistical sampling techniques to a geochemical database of about 70,000 samples from continental igneous rocks to produce a record of secular geochemical evolution throughout Earth's history They find, superimposed on the expected gradual geochemical evolution attributable to secular cooling of Earth, a pervasive geochemical discontinuity approximately 25 billion years ago This discontinuity is indicative of dramatic decreases in mantle melt fraction in basalts and in deep crustal melting/fractionation indicators The Archaean/Proterozoic geochemical transition revealed by this analysis coincides with sudden atmospheric oxygenation at the end of the Archaean aeon, providing a temporal link between deep Earth geochemistry and the rise of atmospheric oxygen The Earth has cooled over the past 45 billion years (Gyr) as a result of surface heat loss and declining radiogenic heat production Igneous geochemistry has been used to understand how changing heat flux influenced Archaean geodynamics1,2, but records of systematic geochemical evolution are complicated by heterogeneity of the rock record and uncertainties regarding selection and preservation bias3,4,5 Here we apply statistical sampling techniques to a geochemical database of about 70,000 samples from the continental igneous rock record to produce a comprehensive record of secular geochemical evolution throughout Earth history Consistent with secular mantle cooling, compatible and incompatible elements in basalts record gradually decreasing mantle melt fraction through time Superimposed on this gradual evolution is a pervasive geochemical discontinuity occurring about 25 Gyr ago, involving substantial decreases in mantle melt fraction in basalts, and in indicators of deep crustal melting and fractionation, such as Na/K, Eu/Eu* (europium anomaly4) and La/Yb ratios in felsic rocks Along with an increase in preserved crustal thickness across the Archaean/Proterozoic boundary6,7, these data are consistent with a model in which high-degree Archaean mantle melting produced a thick, mafic lower crust and consequent deep crustal delamination and melting—leading to abundant tonalite–trondhjemite–granodiorite magmatism and a thin preserved Archaean crust The coincidence of the observed changes in geochemistry and crustal thickness with stepwise atmospheric oxidation8 at the end of the Archaean eon provides a significant temporal link between deep Earth geochemical processes and the rise of atmospheric oxygen on the Earth

Zinc isotopic evidence for the origin of the Moon

Abstract: Lunar magmatic rocks are shown to be enriched in the heavy isotopes of zinc and to have lower zinc concentrations than terrestrial or Martian igneous rocks; these variations represent the large-scale evaporation of zinc, most probably in the aftermath of the Moon-forming giant impact event. The heavily favoured theory for the origin of the Earth–Moon system is a giant impact between the proto-Earth and a Mars-sized body. Such a cataclysmic event would have left its mark on the isotopic composition of the Moon, because light isotopes evaporate more readily than heavier ones. Zinc in particular is a powerful indicator of the volatile histories of planets — it undergoes strong isotopic fractionation in planetary rocks, but is hardly fractionated following volcanic activity on Earth. This study compares high-precision zinc isotopic data for lunar basalts, Martian meteorites and terrestrial igneous rocks, and finds that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or Martian samples. The authors conclude that these variations are the result of large-scale evaporation of zinc in the aftermath of the Moon-forming giant-impact event. Volatile elements have a fundamental role in the evolution of planets. But how budgets of volatiles were set in planets, and the nature and extent of volatile-depletion of planetary bodies during the earliest stages of Solar System formation remain poorly understood1,2. The Moon is considered to be volatile-depleted and so it has been predicted that volatile loss should have fractionated stable isotopes of moderately volatile elements3. One such element, zinc, exhibits strong isotopic fractionation during volatilization in planetary rocks4,5, but is hardly fractionated during terrestrial igneous processes6, making it a powerful tracer of the volatile histories of planets. Here we present high-precision zinc isotopic and abundance data which show that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or Martian igneous rocks. Conversely, Earth and Mars have broadly chondritic zinc isotopic compositions. We show that these variations represent large-scale evaporation of zinc, most probably in the aftermath of the Moon-forming event, rather than small-scale evaporation processes during volcanism. Our results therefore represent evidence for volatile depletion of the Moon through evaporation, and are consistent with a giant impact origin for the Earth and Moon.

Temperature, Pressure, and Composition of the Mantle Source Region of Late Cenozoic Basalts in Hainan Island, SE Asia: a Consequence of a Young Thermal Mantle Plume close to Subduction Zones?

Abstract: Basaltic lavas from Hainan Island near the northern edge of the South China Sea have an age range of between late Miocene (about 13 Ma) and Holocene, with a peak age of late Pliocene to middle Pleistocene. The basaltic province is dominated by tholeiites with subordinate alkali basalts. Most analysed samples display light rare earth element (LREE) enriched REE patterns and ocean island basalt (OIB)-like incompatible element distributions. The basalts contain abundant undeformed high-Mg olivine phenocrysts (up to Fo90•7) that are high in CaO and MnO, indicating high-magnesian parental magmas. Independent barometers indicate that clinopyroxenes in the basalts crystallized over a wide range of pressures of 2–25 kbar (dominantly at 10–15 kbar) and that the melt cooled from about 1350°C to 1100°C during their crystallization. The compositional characteristics of the basalts indicate that their generation most probably involved both low-silica and high-silica melts, as represented by the alkali basalts and tholeiites, respectively. Our results show that the source region for the Hainan basalts is highly heterogeneous. The source for the tholeiites is mainly composed of peridotite and recycled oceanic crust, whereas the source for the bulk of the low-Th alkali basalts consists predominantly of peridotite and low-silica eclogite (garnet pyroxenite). Some high-Th (≥ 4 ppm) alkali basalts may have been produced by partial melting of low-silica garnet pyroxenite (eclogite). We estimated the primary melt compositions for the Hainan basalts using the most forsteritic olivine (Fo90•7) composition and the most primitive bulk-rock samples (MgO > 9•0 wt % and CaO >8•0 wt %), assuming a constant Fe–Mg exchange partition coefficient of KD = 0•31 and Fe3+/FeT = 0•1.

Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust

Abstract: The earliest compounds forming Earth9s first continental crust were magmatic rocks with tonalitic-trondhjemitic-granodioritic composition (TTGs). TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust. To simulate formation of Eoarchean TTGs in different tectonic regimes, we combine for the first time the thermodynamic calculation of residual assemblages with subsequent modeling of trace element contents in TTGs. We compare water-absent partial melting of two hydrated starting compositions, a modern mid-oceanic-ridge basalt (MORB) and a typical Eoarchean arc tholeiite from the Isua Supracrustal Belt that represents the country rock of Earth9s oldest TTGs in southern West Greenland. At 10 kbar, partial melting of MORB-like residues results in modeled TTG compositions that are very different from natural ones. Melting at higher pressures (14 and 18 kbar) leads to a better match, but several key trace element parameters in TTGs are still amiss. A perfect fit for trace element compositions is achieved by melting of Eoarchean arc tholeiites at 10 and 14 kbar. These protoliths contain less Al and Na and more Fe and Mg as compared to present-day MORB and form amphibole-rich and plagioclase-free residues even at low pressures. Formation of Earth9s oldest continental crust is therefore best explained by melting within tectonically thickened mafic island-arc crust.

A Short Review on Basalt Fiber

Abstract: A hard, dense, inert rock found world wide, basalt is an igneous rock, wh ich is solid ified volcanic lava. Cast basalt liners for steel tubing shows very high abrasion resistance in various industrial applications. In recent years, continuous basalt fibers ext ruded fro m naturally fire-resistant basalt are attracted attention as a rep lacement fo r asbestos fibers. In the last decade, basalt has emerged as a contender in the fiber reinforcement of co mposites. Some manufacturer of basalts claims it offers performance similar to S-2 glass fibers at a price point between S-2 glass and E-g lass, and may offer manufacturers a less -expensive alternative to carbon fiber. Basalt fibre (BF) is capable to withstand very high temperature and can act as fire blocking element.

Eastern Dharwar Craton, India: Continental lithosphere growth by accretion of diverse plume and arc terranes

Abstract: Greenstone belts of the eastern Dharwar Craton, India are reinterpreted as composite tectonostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data. The former are dominated by a komatiite plus Mg-tholeiitic basalt volcanic association, with deep water siliciclastic and banded iron formation (BIF) sedimentary rocks. Plumes melted at Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts compositionally similar to recent intraoceanic arcs. As well, boninitic flows sourced in extremely depleted mantle are present, and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young ( Archean lithospheric mantle, distinctive in being thick, refractory, and buoyant, formed complementary to the accreted plume and convergent margin terranes, as migrating arcs captured thick plume-plateaus, and the refractory, low density, residue of plume melting coupled with accreted imbricated plume-arc crust.

Melt Segregation in Deep Crustal Hot Zones: a Mechanism for Chemical Differentiation, Crustal Assimilation and the Formation of Evolved Magmas

Abstract: Mantle-derived basaltic sills emplaced in the lower crust provide a mechanism for the generation of evolved magmas in deep crustal hot zones (DCHZ).This study uses numerical modelling to characterize the time required for evolved magma formation, the depth and temperature at which magma formation occurs, and the composition of the magma.The lower crust is assumed to comprise amphibolite. In an extension of previous DCHZ models, the new model couples heat transfer during the repetitive emplacement of sills with mass transfer via buoyancy-driven melt segregation along grain boundaries.The results shed light on the dynamics of DCHZ development and evolution.The DCHZ comprises a mush of crystals plus interstitial melt, except when a new influx of basaltic magma yields a short-lived (20^200 years) reservoir of melt plus suspended crystals (magma). Melt segregation and accumulation within the mush yields two contrasting modes of evolved magma formation, which operate over timescales of c. 10 kyr^1 Myr, depending upon emplacement rate and style. In one, favoured by emplacement via over-accretion, or emplacement at high rates, evolved magma forms in the crust overlying the intruded basalt sills, and is composed of crustal partial melt, and residual melt that has migrated upwards out of the crystallizing basalt. In the other, favoured by emplacement via underor intra-accretion, or by emplacement at lower rates, evolved magma forms in the intruded basalt, and the resulting magma is composed primarily of residual melt. In all cases, the upward migration of buoyant melt yields cooler and more evolved magmas, which are broadly granitic in composition. Chemical differentiation is therefore driven by melt migration, because the melt migrates through, and chemically equilibrates with, partially molten rock at progressively lower temperatures. Crustal assimilation occurs during partial melting, and mixing of crustal and residual melt occurs when residual melt migrates into the partially molten crust, yielding chemical signatures indicative of a mixed crustal and mantle origin. However, residual melt is volumetrically more significant than crustal melt, except at the highest emplacement rates. Contamination of crustal melt by residual melt from basalt crystallization appears to be an inevitable consequence of melt segregation in DCHZ, and can explain the mixed crust^mantle origin of many granites.

Hydrovolcanic processes forming basaltic tuff rings and cones on

Abstract: Tuff rings and tuff cones are small volcanoes produced by explosive magma-water interactions and have been regarded as resulting from relatively dry and wet eruptions, respectively, which are related to low and high mixing ratios of water to magma. However, comparative work on four Pleistocene basaltic tuff rings and cones on Cheju Island, Korea, shows that there are dry and wet types in both tuff rings and tuff cones, and their variations are not satisfactorily explained by the prevailing model. Instead, it is inferred that the morphological variations are directly caused by depositional processes (pyroclastic surge‐dominated in tuff rings and fallout-dominated in tuff cones), irrespective of watermagma mixing ratios. The depositional processes are interpreted to be in turn controlled by a number of fundamental controls, which include depositional settings, type, level, and lithology of aquifers, strength of country rocks, ground-water behavior, and properties and behavior of magma. These controls determine the explosion depth, conduit geometry, mode of magma-water interaction, magnitude of explosion, eruption-column behavior, and subsequent depositional processes. The Suwolbong and Songaksan tuff rings, which formed almost entirely on land above fragile and permeable sediments and granites with some aquiclude beds, were produced by contact-surface steam explosivity at depth because of the fragility of country rocks, insufficient and inhibited supply of shallow-level external water into the vents, and interaction of nonvesiculated magma with interstitial water. These conditions led to generation of buoyancy-dominated eruption columns and pyroclastic surges, resulting in tuff rings. On the other hand, the Ilchulbong and Udo tuff cones formed in shallow seas above extremely permeable but rigid basalt lavas. The explosions occurred at shallow depths mainly by bulk-interaction steam explosivity because of the rigidity of country rocks, sustained supply of shallow-level external water into the vents, and interaction of vesiculated magma with free water. This process resulted in the generation of dense, inertia-dominated jets and the formation of tuff cones mainly by fallout processes. It is thought that the morphological and sedimentological variations of these volcanoes are more successfully explained by the fundamental controls rather than solely by the water-magma ratio. It is suggested that the water-magma ratio can explain the evolution of a single volcano or a group of volcanoes under otherwise identical conditions, but cannot explain the variability of tuff rings and cones in different hydrogeologic settings because the nature of hydroeruptions is governed by a number of fundamental controls.

The global pattern of trace-element distributions in ocean floor basalts

Abstract: The magmatic layers of the oceanic crust are created at constructive plate margins by partial melting of the mantle as it wells up. The chemistry of ocean floor basalts, the most accessible product of this magmatism, is studied for the insights it yields into the compositional heterogeneity of the mantle and its thermal structure. However, before eruption, parental magma compositions are modified at crustal pressures by a process that has usually been assumed to be fractional crystallization. Here we show that the global distributions of trace elements in ocean floor basalts describe a systematic pattern that cannot be explained by simple fractional crystallization alone, but is due to cycling of magma through the global ensemble of magma chambers. Variability in both major and incompatible trace-element contents about the average global pattern is due to fluctuations in the magma fluxes into and out of the chambers, and their depth, as well as to differences in the composition of the parental magmas.

Origin of Columbia River flood basalt controlled by propagating rupture of the Farallon slab

Abstract: A model of subduction that reveals a long tear under Oregon and Nevada provides a new mechanism for the origin of Columbia River flood basalt, resolving previous hypotheses. A flood basalt is the product of a volcanic eruption that covers large areas with basalt lava. The origin of the Steens–Columbia River flood basalts, thought to be associated with the onset of volcanism within the Yellowstone hotspot, has remained controversial. Using geodynamic modelling, Lijun Liu and Dave Stegman show that an episode of slab tearing about 17 million years ago under eastern Oregon quickly ruptured north and south, covering a distance of about 900 kilometres along eastern Oregon and northern Nevada. This is consistent both in space and time with the Steens–Columbia River–Northern Nevada Rift flood basalt event. The model also predicts the sequence of flood basalt composition, which should help to reconcile controversies regarding the origin of these flood basalts and involvement of the putative Yellowstone plume. The origin of the Steens–Columbia River (SCR) flood basalts, which is presumed to be the onset of Yellowstone volcanism, has remained controversial, with the proposed conceptual models involving either a mantle plume1,2,3,4,5 or back-arc processes6,7,8. Recent tomographic inversions based on the USArray data reveal unprecedented detail of upper-mantle structures of the western USA9 and tightly constrain geodynamic models simulating Farallon subduction, which has been proposed to influence the Yellowstone volcanism5,6. Here we show that the best-fitting geodynamic model10 depicts an episode of slab tearing about 17 million years ago under eastern Oregon, where an associated sub-slab asthenospheric upwelling thermally erodes the Farallon slab, leading to formation of a slab gap at shallow depth. Driven by a gradient of dynamic pressure, the tear ruptured quickly north and south and within about two million years covering a distance of around 900 kilometres along all of eastern Oregon and northern Nevada. This tear would be consistent with the occurrence of major volcanic dikes during the SCR–Northern Nevada Rift flood basalt event both in space and time. The model predicts a petrogenetic sequence for the flood basalt with sources of melt starting from the base of the slab, at first remelting oceanic lithosphere and then evolving upwards, ending with remelting of oceanic crust. Such a progression helps to reconcile the existing controversies on the interpretation of SCR geochemistry and the involvement of the putative Yellowstone plume. Our study suggests a new mechanism for the formation of large igneous provinces.

Oxidation state of subarc mantle

Abstract: The subarc mantle is a primary control on the composition of arc magmas and the formation of arc-related ore deposits. Elevated ferric iron contents in arc lavas have been interpreted as a record of subarc mantle that is oxidized relative to mid-oceanic ridge basalt (MORB), but this conclusion is controversial. Measurements of spinel compositions in primitive arc lavas imply an arc magma source 1-4 log units more oxidized than MORB and ocean island basalts analyzed using the same technique. Samples from seven arcs show a significant correlation (P < 0.0005) between redox budget, subduction zone convergence rate, and subduction zone age. These results support the notion of oxidized arc lavas in the mantle source zone, but resolution of contradictory evidence regarding subarc mantle redox state requires further work. © 2012 Geological Society of America.

Picrites from the Emeishan Large Igneous Province, SW China: a Compositional Continuum in Primitive Magmas and their Respective Mantle Sources

Abstract: Flood basalts are one of the remaining enigmas in global mantle petrology. They come in enormous quantities of up to 10. 6 km. 3 of mantle-derived melt, and they erupt in rather short time intervals of only a few million years. Throughout geological history, all continents have been periodically flooded by dominantly basaltic and rare picritic magmas that can differ widely within the same province in terms of their major element (e.g. high- and low-Ti series), trace element, and radiogenic isotope compositions, suggesting significant compositional heterogeneity within the mantle source regions tapped. In this study of the Late Permian Emeishan large igneous province (ELIP) in SW China picrite lavas from thick stratigraphic successions at Binchuan and Yongsheng represent the low-Ti and high-Ti 'classic' end-members of continental flood basalt magmatism, respectively. This study focuses on the petrochemical variability of the Emeishan magmas, and the genetic links between the suites and their respective mantle sources, based upon estimates of the chemical compositions of the primary melts represented by homogenized melt inclusions hosted by exceptionally primitive olivine (up to 92 mol % Fo in both suites) and Cr-spinel (Cr# 64-72 mol % in Binchuan and 65-80 mol % in Yongsheng) phenocrysts. The average compositions of the melt inclusions and their overall chemical variability, together with the presence of picrites in the province (e.g. Lijiang and Dali localities) with compositions intermediate between the low- and high-Ti end-members, suggest that numerous parental magma batches contributed to a diverse spectrum of more differentiated basaltic magmas within the ELIP. The end-member and intermediate magma compositions are confirmed by the compositions of phenocrysts (Ni and Mn abundances in olivine, Ti abundances in Cr-spinel and clinopyroxene, and trace element abundances in clinopyroxene). The end-member melt and phenocryst compositions (e.g. Gd/Yb in bulk-rocks, melt inclusions and clinopyroxene and Ni-Mn systematics in olivine) suggest a peridotite and garnet pyroxenite mantle source for the low- and high-Ti end-members, respectively. The Sr and Nd isotopic compositions of the two end-member magmas are similar [. 87Sr/. 86Sr. i ∼0·7045; e{open}Nd(t) ∼ +1·7] and are considered to reflect a source in the subcontinental lithospheric mantle rather than the convective asthenosphere or a deep mantle 'plume'. © The Author 2012. Published by Oxford University Press. All rights reserved.

The density and porosity of lunar rocks

Abstract: [1] Accurate lunar rock densities are necessary for constructing gravity models of the Moon's crust and lithosphere. Most Apollo-era density measurements have errors of 2–5% or more and few include porosity measurements. We report new density and porosity measurements using the bead method and helium pycnometry for 6 Apollo samples and 7 lunar meteorites, with typical grain density uncertainties of 10–30 kg m−3 (0.3–0.9%) and porosity uncertainties of 1–3%. Comparison between igneous grain densities and normative mineral densities show that these uncertainties are realistic and that the helium fully penetrates the pore space. Basalt grain densities are a strong function of composition, varying over at least 3270 kg m−3 (high aluminum basalt) to 3460 kg m−3 (high titanium basalt). Feldspathic highland crust has a bulk density of 2200–2600 kg m−3 and porosity of 10–20%. Impact basin ejecta has a bulk density of 2350–2600 kg m−3 and porosity of ∼20%.

A case for hornblende dominated fractionation of arc magmas: the Chelan Complex (Washington Cascades)

Abstract: Amphibole fractionation in the deep roots of subduction-related magmatic arcs is a fundamental process for the generation of the continental crust. Field relations and geochemical data of exposed lower crustal igneous rocks can be used to better constrain these processes. The Chelan Complex in the western U.S. forms the lowest level of a 40-km thick exposed crustal section of the North Cascades and is composed of olivine websterite, pyroxenite, hornblendite, and dominantly by hornblende gabbro and tonalite. Magmatic breccias, comb layers and intrusive contacts suggest that the Chelan Complex was build by igneous processes. Phase equilibria, textural observations and mineral chemistry yield emplacement pressures of ∼1.0 GPa followed by isobaric cooling to 700°C. The widespread occurrence of idiomorphic hornblende and interstitial plagioclase together with the lack of Eu anomalies in bulk rock compositions indicate that the differentiation is largely dominated by amphibole. Major and trace element modeling constrained by field observations and bulk chemistry demonstrate that peraluminous tonalite could be derived by removing successively 3% of olivine websterite, 12% of pyroxene hornblendite, 33% of pyroxene hornblendite, 19% of gabbros, 15% of diorite and 2% tonalite. Peraluminous tonalite with high Sr/Y that are worldwide associated with active margin settings can be derived from a parental basaltic melt by crystal fractionation at high pressure provided that amphibole dominates the fractionation process. Crustal assimilation during fractionation is thus not required to generate peraluminous tonalite.