Melting Of Mantle Wedge Research Articles

Quaternary calc-alkaline and alkaline volcanism in an hyper-oblique convergence setting, central Myanmar and western Yunnan

Abstract Quaternary to recent volcanic centres located in Central Myanmar basin and in western Yunnan (Tengchong rift) display contrasted geochemical signatures. Mt. Popa lavas range in composition from calc-alkaline to potassic calc-alkaline and derive from partial melting of a subduction-modified mantle. The Monywa Holocene maars are made up of absarokites thought to result from low degrees of melting of a similar source. The Tengchong rift potassic calc-alkaline andesites and dacites also display typical subduction-related imprints. On the contrary, the geochemical signature of the Singu alkaline mafic basaltic trachyandesites which are offset by the Sagaing Fault suggests that they derive from the melting of deep enriched intraplate-type mantle, similar to the source of common Plio-Quaternary alkali basalts and related rocks from the Sundaland. The origin of this uncommon spatial and temporal magmatic association is connected to an unusual tectonic setting of hyper-oblique convergence. The collision between India and the Sundaland is marked by shear partitioning within Myanmar, a significant part of the right lateral motion being accommodated by the Sagaing Fault. Rapid uprise of high temperature alkaline magmas derived from the melting of enriched Sundaland-type mantle and channelled by Neogene fault planes parallel to the Sagaing Fault led to the emplacement of the Singu lavas. Active subduction of the Gulf of Bengal oceanic lithosphere resulted in the development of a Wadati-Benioff plane located 100 to 140 km beneath Mt. Popa and Monywa. The dehydration of this slab led to metasomatism and subsequent melting of the Central Myanmar mantle wedge, and finally to the emplacement of calc-alkaline and shoshonitic magmas in those volcanoes. The origin of the subduction-related signature of the Tengchong lavas has to be related to an older subduction event.

PETROGENESIS AND ORIGIN OF THE CHENAR GRANITOID STOCK, NW OF KERMAN, IRAN: EVIDENCE OF NEOTETHYS SUBDUCTION RELATED ARC MAGMATISM

The lower to middle Miocene Chenar granitoid stock, a part of Central Iranian volcanic belt, is intruded into Eocene volcano-sedimentary complex in northwestern Kerman in south Central Iran. The Chenar granitoid stock consists mainly of coarse to medium grain granodiorite and monzogranite with subordinate tonalite and syenogranite. Alkali granites as aplitic veins and quartz dioritic dykes frequently cut through the granitoid stock. Enclaves of various types and sizes (andesitic and microgranular mafic, few meters to several centimeters wide) also occur near the margin. The granitoid rocks represent convergent margin magmas enriched in large ionic lithophile elements (LILE) such as Rb, Ba, K, Ce and depleted in high field strength elements (HFSE) such as Y, Nb and Zr. The REE patterns have relatively smooth LREE enriched chondrite normalized REE profiles with (La/Yb)n between 4.28 to 14.66. A lack of Eu anomalies along with a continuous increase in the slope of LREE profile from tonalite to syenogranite exhibit a relatively low fractionated chondrite normalized pattern [(La/Yb)n] and also involvement of plagioclase in the melting processes. Small negative Eu anomaly in alkali granite is an indication of more differentiated rock type [(La/Yb)n=42.78] in Chenar granitoid stock. Geochemical data, various trace element discriminant diagrams, common igneous microgranitoid enclaves, and ocean ridge granite normalized multi-element diagrams indicate that the Chenar granitoid stock has characteristics of metaluminous to slightly peraluminous, calc-alkaline, I-type granite of a volcanic arc settings and is formed in an active continental margin environment. In this model, a NE-dipping subduction zone of Neotethys could have accounted for the arc volcanism of the Central Iran where felsic, Andean-type magmatic arc volcanics and plutonics were formed. As a result of subduction mafic arc magmas with significant fluids were produced following the dehydration of oceanic crust and partial melting of mantle wedge which in turn provoke partial melting of considerably metasomatised and enriched subcontinental lithosphere, leading to generation of siliceous magma. Its low-pressure crystal fractionation gave rise to generation of hydrous calc-alkaline magmas, represented in part by Chenar granitoid stock.

Temporal control of subduction magmatism in the eastern Trans‐Mexican Volcanic Belt: Mantle sources, slab contributions, and crustal contamination

The magmatic record of the easternmost part of the Trans‐Mexican Volcanic Belt elucidates how temporal changes in subduction parameters influence convergent margin volcanism. In the Palma Sola massif, three phases of magmatic rocks with distinct chemical characteristics were emplaced in a relatively short time span (∼17 Ma): Miocene calc‐alkaline plutons, latest Miocene‐Pleistocene alkaline plateau basalts, and Quaternary calc‐alkaline cinder cones. Plutons have arc‐like trace element patterns (Ba/Nb = 16–101), and their Sr, Nd, and Pb isotopic compositions become more “depleted” with increasing SiO2 contents. Their Pb isotopes are bracketed by the subducted sediments and Pacific mid‐ocean ridge basalts (MORB), requiring the participation of an unradiogenic component that mixes with a sediment contribution. High Sr/Y and Gd/Yb ratios in the least radiogenic pluton might indicate a melt coming from the subducted MORB. Trace element patterns of the plateau basalts show moderate or negligible subduction contributions (Ba/Nb = 6–31). Rocks without subduction signatures are similar to ocean island basalts, indicating melting of an enriched mantle wedge. The plateau basalts form an array in 206Pb/204Pb‐207Pb/204Pb space that trends toward the composition of the subducted sediment. The sediment component is also indicated by the inverse correlations between Pb isotopes and subduction signals. This component has high Th/Nd coupled with low 143Nd/144Nd, but lower Pb/Nd and Sr/Nd ratios than the bulk sediment. These suggest melting of a sediment that has lost fluid mobile elements prior to melting. The Quaternary cinder cones have moderate subduction signals (Ba/Nb = 16–41), and their isotopic compositions correlate with differentiation indices. Contamination with the local Paleozoic basement can explain the petrogenesis of the youngest rock suite. The geochemical differences among the suites indicate temporal modifications in the chemical characteristics of the slab input. These variations can be associated with modifications in the Pacific subduction regime. We suggest the Miocene magmatic phase was formed by an essentially flat subduction angle that favored melting of the subducted oceanic crust. Slab rollback in the Pliocene allowed melting of deeper portions of the wedge by the injection of dehydrated sediment melts. In the Quaternary, an even steeper subduction angle provided negligible slab contributions to the Palma Sola region, and upper crustal contamination largely controls the petrogenesis.

Paleoproterozoic crustal genesis: calc-alkaline magmatism of the Torngat Orogen, Voisey’s Bay area, Labrador

Abstract A suite of metaplutonic rocks representing a mafic to intermediate calc-alkaline batholith has been identified in the Voisey’s Bay area of northern Labrador. The suite represents the southern extension of similar metaplutonic rocks occurring along some 500 km of the Paleoproterozoic Torngat Orogen. The rocks have been metamorphosed at upper amphibolite to granulite facies conditions that only mildly disturbed their igneous characteristics. Two magma series are present, one comprising gabbros, diorites and quartz diorites, and another composed of tonalites. The gabbro to quartz diorite series evolved by fractional crystallization of a mantle-derived mafic magma. Two samples of the metaplutonic suite yield U–Pb zircon thermal ionization mass spectrometry (TIMS) ages of 1893±1 and 1890±2 Ma. Based on U–Pb zircon laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) geochronology, one of these samples contains inherited cores, some 50–100 Ma older than the igneous population, but no Archean cores were found. Metatonalite rocks formed from a separate tonalitic magma during a slightly younger magmatic event dated at 1883±5 Ma. Both the gabbro to quartz diorite and tonalite series rocks are enriched in Rb, Ba, Sr, Zr and light rare earth elements relative to primitive mantle; and have negative Nb–Ta anomalies, which are thought to reflect subduction processes. They have fractionated REE patterns, with the extent of fractionation greatest for the tonalites (chondrite normalized La/Yb=30–50), suggesting that their parent magmas left more residual garnet in their source regions than did the gabbro to quartz diorite magmas. Sr–Nd–Pb isotope compositions of the metaplutonic suite indicate significant involvement of a lower crustal component in their genesis. When combined with previous data for the suite, and excluding data thought to be disturbed by metamorphism, 87 Sr / 86 Sr (1890 Ma ) =0.7036 –0.7048, e Nd (1890 Ma ) =+4.7 to −6.9, and μ∗ (time-integrated 238 U / 204 Pb of the mantle and crustal sources) = 7.66–7.98. A model for the metaplutonic suite is presented in which the gabbro to quartz diorite series formed by partial melting of the mantle wedge in a subduction zone and interacted with a lower crustal contaminant, possibly in the source region. The tonalites were derived by partial melting of a mixture of subducting oceanic crust and lower crustal detritus. An important role for lower crustal recycling is thus indicated. Crustal genesis by both mantle wedge and slab melting in the Paleoproterozoic may mark the transition from Archean to post-Archean subduction processes.

Origin of Geochemical Variability by Arc-Continent Collision in the Biru Area, Southern Sulawesi (Indonesia)

Analyses of igneous rocks from the Eocene calc-alkaline and Miocene geochemistry can be modelled by varying contributions potassic volcanic arc in southwest Sulawesi indicate that magmas from the mantle wedge, fluid from the subducted slab, became more heterogeneous in their trace element and Pb–Sr–Nd and the presence or absence of a subducted sedimentary isotopic signature following the collision of the Buton microcontinent component (White & Patchett, 1984; Gill & Williams, with the arc at >15 Ma. Isotopic ratios become more ‘continental’ 1990; Woodhead & Johnson, 1993; Kepezhinskas et al., 4 my after the collisional event ( Sr/Sr Ζ0·7085, Nd/ 1997; Peate et al., 1997; Alves et al., 1999; Hoogewerff, Nd [0·5125, Pb/Pb Ζ 19·2, Pb/Pb Ζ 15·73, 1999; Pearce et al., 1999; Eiler et al., 2000). Despite Pb/Pb Ζ39·4). As the overriding plate consists of young agreement on the importance of these components, there Sundaland crust, whereas the subducted sediment is likely to is still some dispute about the importance of an ocean have been shed from a compositionally distinct microcontinent of island basalt (OIB) component in the wedge (Stolz et al., Australian derivation, we can be certain that the continental isotopic 1990; Edwards et al., 1991) and the implied role of signature reflects subduction of continental material rather than residual rutile (Ryerson & Watson, 1987; Foley et al., crustal contamination. The isotopic compositions of the magmas 2000). If an arc is built upon continental crust, or can be explained by the melting of a mixed mantle wedge, consisting overlain by a thick layer of sediment, there is a potential of fluid-fluxed and sediment-modified MORB mantle. In this model, contribution from the upper plate via assimilation– the maximum amount of sediment added to the mantle source is fractional crystallization (AFC) processes, which can 10%. sometimes be difficult to distinguish from the signature of subducted sediment (Davidson, 1987; Thirlwall et al., 1996; Gasparon & Varne, 1998). The only unambiguous way to distinguish between subducted sediment and

Petrogenesis of the tholeiitic basalt, calc-alkaline basaltic andesite and high magnesian andesite lava succession of the Oligo-Miocene Anamizu Formation in northeastern Noto Peninsula, central Japan.

The Oligo-Miocene Anamizu Formation in the Ushitsu-Matsunami area consists of a lower volcano-sedimentary member and an upper volcanic member. The upper member is essentially composed of three lava series; tholeiite (basalt and basaltic andesite), calc-alkaline (basaltic andesite) and high-magnesian andesite (bronzite andesite), in ascending order. The tholeiitic basalt is further divided into K-poor and K-rich (K-feldespar bearing) types. The tholeiitic basaltic andesite has higher FeO*/MgO (∼3) than the others (1-1.5). The calc-alkaline basaltic andesite has higher Cr and Ni than tholeiitic basaltic andesite. Bronzite andesite contains higher MgO, Cr and Ni than common calc-alkaline andesite of the same SiO2 content (∼60 wt%), but lower than those of boninite and sanukite. All the rock series are depleted in HFSE (Nb and Ti) in comparison with N-MORB and OIB, suggesting typical subduction-related arc magmas. High Zr/Y ratios of the tholeiitic basalt resemble those of active continental margin magmas rather than island-arc magmas. The HFSE, Ni and Cr compositions indicate a progressive depletion or increasing degree of partial melting of the mantle wedge source in the order tholeiite basalt (basaltic andesite)→calc-alkaline basaltic andesite→bronzite andesite. Spinel in the three series shows different trends: Cr-poor (Cr# 0.49 to 0.54) in the tholeiitic basalt, fairly Cr-rich (Cr# 0.61) in the calc-alkaline basaltic andesite, and Cr-rich (Cr# 0.73) in the high-magnesian andesite. These trends indicate different mantle restites; lherzolite for the tholeiitic basalt and harzburgite for the calc-alkaline basaltic andesite and high-magnesian andesite, and increasing degree of depletion in the order as above. On the contrary, LILE and LREE exhibit a gradual enrichment of the source in the same order. Corresponding decrease of TiO2/K2O suggests that the enrichment has been due to addition of fluids derived from the descending slab. These data indicate that the K-poor tholeiitic basalt magma has been formed by partial melting of the lherzolitic upper mantle wedge under almost anhydrous conditions; whereas the K-rich tholeiitic basalt magma may have been produced by partial melting of the metasomatized lherzolitic mantle source under slightly hydrous conditions. The calc-alkaline and bronzite andesite magmas have been produced by partial melting of the hydrous, metasomatized, harzburgitic mantle wedge. The stratigraphy of the lava succession in the studied area (tholeiite basalt-basaltic andesite→calc-alkaline basaltic andesite→bronzite andesite) indicates that depletion and hydration (metasomatism) of the mantle source have progressed simultaneously.

Geochemical evidence for arc-type volcanism in the Aegean Sea: the blueschist unit of Siphnos, Cyclades (Greece)

Blueschists, eclogites, chlorite–actinolite rocks and jadeite-gneisses of the blueschist unit of Siphnos have been investigated for their geochemical composition. Their protolith nature is characterised and a geodynamic model for the pre-metamorphic evolution of these metavolcanic rocks is proposed on the basis of immobile elements, especially trace elements and rare earth elements (REE). The protoliths of the eclogites are characterised as calc-alkaline basalts, andesites and Fe-rich tholeiites evolving in an island-arc setting. Trace element data indicate that subducted marine sediments were assimilated in the magma chamber, enriching the protoliths in LILE and Pb. Produced in the early stage of back-arc basin opening, a protolith with affinities to both island-arc and MORB formed the precursor of the chlorite–actinolite rocks. They were created by low degrees of partial melting of very primitive magmas, akin to spinel-peridotites and have affinities to boninites, probably through melting of the peridotitic mantle wedge. Tholeiitic basalts and andesites with N-MORB affinity, especially in their REE-patterns, were then produced by partial melting, possibly in an embryonic back-arc basin. These rocks were the protoliths of the blueschists of Siphnos. Their enrichment in some LILE and Pb indicates a N-MORB source contaminated by marine sediments, probably shales or other Pb-rich sediments. Because the jadeite-gneisses show affinities to MOR-granites and volcanic arc granites, intrusion of their protoliths in a back-arc environment is likely. The protoliths of the quartz-jadeite gneisses are rhyodacites/dacites and rhyolites, those of the glaucophane-jadeite gneisses were andesites. The proposed geodynamic model, solely based on geochemical data, is consistent with geochemical data from neighbouring islands, though those rock units show much higher chemical variability. Consistent with geotectonic models, which are based on structural and geophysical data, the volcanic protoliths of the Siphnos blueschist unit reflect the transition from subduction to spreading environment and record in detail: subduction, formation of an island-arc, and the evolution of a back-arc basin.

Nonchondritic Pt/Pd ratios in arc mantle xenoliths: Evidence for platinum enrichment in depleted island-arc mantle sources

Mantle-derived spinel harzburgites from the Kamchatka arc have fractionated Pd- group element patterns, and Pt is clearly enriched relative to Pd and Rh. This fractionation is also consistent with chondritic Ir-group–platinum-group element distribution and super- chondritic Pt/Pd, chondrite-normalized (Pt/Os)N and (Pt/Ir)N found in harzburgite xenoliths from the Tubaf seamount in the Lihir island group of the Tabar-Lihir-Tanga-Feni island arc in Papua New Guinea. The nonchondritic Pt/Pd ratios and Pt enrichment in the island-arc mantle are best explained by extraction of melt in a back-arc volcanic-arc setting, which left refractory Pt-Fe alloys in the residual mantle. Pt-Fe alloys selectively concentrate Pt relative to Pd and, overall, increase the Pt/Pd in a subarc mantle wedge. Partial melting of this Pt-enriched mantle wedge above the subduction zone, due to hydrous flux from the subducting lithosphere, is potentially capable of producing Pt-rich primitive melts parental to the Alaskan-type ultramafic-mafic complexes.

Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas

On the basis of geological, geochemical and geophysical data of Late Mesozoic igneous rocks in SE China, we suggest that during the period from 180 to 80 Ma, the slab dip angle of Paleo-Pacific plate subduction underneath SE China increased from a very low angle to a median angle. Consequently, magmatic activity of the SE China continental margin migrated oceanward to the southeast. Initially, magmatism was concentrated in the region as far as 800∼1000 km northwest of the ocean–continent boundary zone, which is located in the eastern flank of the Central Ranges, Taiwan. As the slab dip angle increased, this magmatic belt migrated rapidly to the region only 100–200 km away from this boundary zone. During subduction process, various degrees of mantle wedge melting and basaltic underplating provided the necessary heat to cause partial melting of lower- and middle- crust, and generation of voluminous felsic magmas. A combination of these processes is responsible for the formation of the famous Yanshanian granitoids, volcanic rocks and related ore deposits in SE China. Mantle input may have played an important role in sustaining magma fractionation and eventually leads to release of magmatic fluids and formation of world class W, Sn and other types of ore deposits in SE China.

Pan-African, post-collisional, ferro-potassic granite and quartz–monzonite plutons of Eastern Nigeria

Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. 40 Ar/ 39 Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa.

Mineral-aqueous fluid partitioning of trace elements at 900–1200°C and 3.0–5.7 GPa: new experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism

In order to constrain the role of fluid phases during metasomatic processes in the upper mantle, trace element partition coefficients for Ba, Sr, Pb, Nb, Ta, Zr, Hf, Ti, La, Ce, Sm, Tb, and Yb between aqueous fluids and eclogite assemblage minerals (garnet, clinopyroxene, and rutile) have been determined experimentally at 900–1200°C and 3.0–5.7 GPa. Using a new experimental technique in which diamond aggregates are added to the experimental capsule set-up, the fluid was separated from the solid residue so that both quenched solute and residual minerals could be analysed directly. Trace element concentrations were determined in situ by laser ablation microprobe (LAM). The partitioning behaviour is controlled by temperature, pressure, and crystal chemistry; whereas fluid composition is not as crucial. Neither addition of hydrochloric acid nor high silica concentrations in the fluid have strong effects on trace element partitioning. Results indicate that in the presence of garnet or clinopyroxene, Nb and Ta are highly soluble in aqueous fluids, whereas Zr and Hf show variable solubilities. Low field strength elements (LFSE) and light rare earth elements (LREE) are always enriched in the fluid (D (fluid/Min) > 1). Generally, D (fluid/Cpx) is positively correlated with temperature only for high field strength elements (HFSE), but positively correlated with pressure for all other elements. Therefore, the lowest Nb/La is achieved at high pressures and low temperatures. However, even the highest pressures and lowest temperatures examined did not exhibit strong negative HFSE anomalies in the fluid. Garnet retains compatible trace elements at 3 GPa and 1000°C much more effectively (D (fluid/gt)Yb= 0.002) than at 5.7 GPa at the same temperature (D (fluid/gt) Yb = 0.04). Decreasing temperature results in a lowered D (fluid/gt) , particularly for Zr, Hf, and heavy rare earth elements (HREE). At 5 GPa and 900°C a strong intra-REE fractionation is observed (D (fluid/gt)Sm/Yb around 100) and significantly negative anomalies for Hf and Zr, but not for Nb and Ta, are developed. Only residual rutile fractionates all HFSE from all other trace elements. Tantalum and niobium are retained most effectively by rutile, as is the case for rutile/melt partitioning. Fluid/mineral trace element partitioning has important implications for mantle metasomatism in subarc regions. A model is proposed in which HFSE depletions, as observed in island arc volcanic rocks, could originate from a selective enrichment of the mantle wedge in LFSE and LREE by aqueous fluids derived from a rutile-bearing subducted slab. It is shown that melting of the enriched mantle wedge, which had previously been depleted by melt extraction (depleted MORB mantle) can produce magmas with trace element patterns similar to those of subduction-related volcanic rocks.

Phenocryst and melt inclusion chemistry of near-axis seamounts, Valu Fa Ridge, Lau Basin: insight into mantle wedge melting and the addition of subduction components

Phenocryst assemblages, and mineral and melt inclusion compositions of magmas erupted at near-axis seamounts on either side of Valu Fa Ridge provide a hitherto unprecedented insight into the complexity of magma generation in this back-arc basin tectonic setting. Two fundamentally different primitive primary melt compositions are identified based on melt inclusion compositions, olivine phenocryst chemistry, and the early co-crystallisation of either magnesian clinopyroxene (Mg# to 93) or magnesian orthopyroxene (Mg# to 93.5) with magnesian olivine (to Fo 94) and Cr-rich spinel (Cr# = 0.78–0.87). One magma type is a H 2O-rich (∼ 2.5 wt%), high-CaO (∼ 14 wt%), low-Al 2O 3 (∼ 8 wt%) magnesian basalt, variants of which occur in both the eastern and western seamounts. The other is a low-Ca boninite-like magma that only occurs as a component of the western seamount magmas. Large and systematic variations in incompatible trace-element compositions of melt inclusions trapped in primitive olivine phenocrysts, reflect an integration of diverse but geochemically related melt fractions to produce the magmas at each seamount. Trace-element systematics require the variable addition of a LILE-, Pb-, and Cl-rich component to the mantle wedge source with increased influence toward the Tofua arc. This component, as invoked in most models of arc magma genesis, is likely to be a supercritical aqueous fluid released by dehydrating subducting ocean crust beneath the volcanic arc front. We propose that southward propagation of the back-arc basin spreading center mantle provided heat necessary to generate both magmatic suites by decompression melting of refractory hydrated sub-arc lithosphere, probably veined by clinopyroxene-rich dykes in the case of the high-CaO magma series. These near-ridge seamount lavas are very similar to those drilled at ODP Site 839 in the Lau Basin, and we suggest that the Site 839 basalts, as well as other Lau Basin seamount arc-like magmas, were produced from sub-arc lithosphere during southward propagation of the Eastern Lau Spreading Center ∼ 2–3 Ma.

Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg ∼80 to 93.4) and clinopyroxene (mg ∼80 to 92), and include accessory Cr-rich spinel (cr ∼50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to ∼20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with ∼10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2 ∼FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has ∼15 wt% MgO and an mg Fe2 value of ∼81. It requires deep (∼3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T p ∼1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.

The Atnarpa Igneous Complex, southeast Arunta Inlier, central Australia: implications for subduction at an Early-Mid Proterozoic continental margin

Three distinctive igneous suites have been identified from the Atnarpa Igneous Complex, southeast margin of the Arunta Inlier, central Australia. The 1879 +11, −10 Ma gabbro-diorite-tonalite-trondhjemite suite (SiO 2=49–78%) shows both calc-alkaline and trondhjemitic affinities. The overall depletion of Nb, Ti, Y, K and Rb in this suite indicates that its parental magma must have been depleted in these elements. It is the oldest zircon age so far obtained in the Arunta Inlier. The 1873 +11 −10 Ma low-Al tonalite-trondhjemite-granodiorite suite is also depleted in K and Rb. However, unlike either the 1879 Ma or the 1751 Ma suites, it has lower Al 2O 3 and Sr at comparable SiO 2 levels and display flat HREEs and negative Eu anomalies. The 1751±12 Ma younger tonalite suite is high in Sr, Sr/Y and Al 2O 3, and low in Zr, Ti, Nb, Y, Th and U, and displays a strongly fractionated REE pattern with HREE depletion, typical of Archaean high-Al tonalite-trondhjemite-granodiorite suites. Such geochemical features contrast with the 1850–1880 Ma old K, Rb, REE, Th-enriched, Na, Sr-depleted Barramundi Igneous Association, widespread in Proterozoic terrains of northern Australia. The parental magma of the 1879±11 Ma suite is considered to be derived by subduction-related partial melting of a metasomatised-hybridised mantle wedge with phlogopite retained in the residue, the parental magma of the 1873±11 Ma low-Al suite, by relatively low-pressure partial melting of a mafic source with residual plagioclase left in the residue, and the 1751±12 Ma younger tonalite suite, by partial melting of a subducted oceanic crust at mantle depth with residual garnet±amphibole. These processes contrast with models of restite unmixing of a geochemically uniform, but fractionated, preexisting mantle-derived underplate previously proposed for the K-rich Barramundi Igneous Association. Combining geochemical features of the Atnarpa Igneous Complex with reported data from the Harts Range and the Alice Springs areas, we propose that the southeast margin of the Arunta Inlier represents a convergent continental margin during the Early to Middle Proterozoic which has undergone at least two episodes of subduction-related magmatism (1860–1880 Ma and 1730–1770 Ma). Such continental margin subduction could provide additional driving force for crust-mantle delamination and ensialic A-subduction within the preexisting continental crust further north. Both within-plate vertical underplating and marginal lateral accretion could be important processes for Proterozoic crustal evolution.

Eocene potassic magmatism in the Highwood Mountains, Montana: Petrology, geochemistry, and tectonic implications

Potassic volcanics and intrusives of Eocene age (52±1 Ma) in the Highwood Mountains of north‐central Montana provide evidence for interaction of asthenospheric magmas with Archean mantle lithosphere of the Wyoming Craton. Diverse rock types were produced by shallow level degassing, fractional crystallization and magma mixing within two separate magma series whose parental liquids were latite and olivine minette. Halogen systematics of apatite microphenocrysts and other evidence have established that shallow degassing of the phlogopite‐diopside‐phyric minettes to yield heteromorphic leucite‐salite‐phyric mafic phonolites and shonkinites was an important process. Geochemical and mineralogical data suggest that fractional crystallization of olivine, clinopyroxene, mica and leucite, accompanied by widespread magma mixing, produced a spectrum of more evolved magmas such as leucite phonolites, malignites, alkali syenites and trachytes along with mica clinopyroxenite cumulates. Sr, Nd and Pb isotopic compositions and trace element data for the primitive olivine minette magmas are explicable by a multistage model involving three source components. One component is ancient Ba‐LREE‐enriched, U‐Th‐HFSE‐depleted subcontinental lithospheric mantle, which has been recognized in other alkalic rocks of the region, including those from the Crazy Mountains and Smoky Butte. Glimmerite‐veined harzburgite and phlogopite dunite xenoliths (one with ‐εNd of −33 at 52 Ma) found in the most primitive Highwood olivine minette are probably samples of this material. The other two components are asthenospheric mantle with isotopic composition near that of bulk earth (and dominant in Montana alnöitic diatremes) and a young subduction‐related component (probably Eocene, but possibly as old as late Cretaceous), which is required to explain the Rb/Sr‐87Sr/86Sr systematics of the Highwood rocks. A consistent model for the petrogenesis of the Highwood parental mafic magmas involves partial melting of asthenospheric mantle wedge triggered by infiltration of melts released from the metasomatized carapace above a low‐angle subducted slab of Farallon Plate lithosphere, followed by assimilative interaction of these melts with ancient, phlogopite‐rich, metasome veins upon ascent through the Wyoming Craton mantle keel.

Partial melting of mantle wedge and evolution of island arc crust

Evolution of the crust in the northeast Honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data. The chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure‐temperature conditions for shallow parts of the mantle wedge (1.2–2.0 GPa) have been experimentally determined with the “sandwich experiment” technique. The melts range from olivine tholeiitic to magnesian andesitic compositions (SiO2 48–56 wt %; MgO 9–14 wt %) beneath the volcanic front in the northeast Honshu arc and are slightly less silicic beneath the coast of the Sea of Japan. The crustal compositions of the volcanic zone in the northeast Honshu arc estimated from deep‐seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high‐alumina basaltic near the coast of the Sea of Japan. The compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions. Consequently, if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge, thick (up to 30 km) ultramafic cumulates including olivine pyroxenite must have been formed beneath Moho during fractional crystallization and would constitute the upper part of the mantle wedge. The volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast Honshu arc. A static mantle wedge model is, therefore, not satisfactory for the growth of the crust in northeast Honshu arc. A dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow.

Some Geochemical Applications of a Semi-Empirical Approach for Estimating Distribution Coefficients of a Series of Cations between Liquid Aluminosilicates and their Crystallization Products

The distribution coefficients of alkali, alkaline earth, rare earth and a series of other elements between solid and liquid natural aluminosilicate systems can be estimated by a semi-empirical equation which can be derived by extension of Schuetze's incremental method for calculating oxygen isotope effects in silicate systems. This equation is used for investigating the partial melting of mantle wedge and subducted oceanic crust as potential sources of magmas giving rise to continental growth and for evaluating the history of igneous rocks. A series of geochemical criteria are derived for attributing igneous rocks to common parent magmas and for contributing to the estimation of the length of intrusive pathways. Particular attention is devoted to the evaluation of the geochemical behaviour of the rare earth elements.

Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.

Bull. Geol. Soc. Amer. : Shipek C.G., 1962. Photographic survey of sea floor on southwest slope of Eniwetok Atoll. 73 (7): 805–812

Basalts and basaltic rocks are the most abundant igneous rocks on the earth and their petrologic and geochemical studies have formed our knowledge base on the thermal structure and composition of the mantle with which we have developed workable models on the chemical differentiation of the earth. All this would not have been possible without innovative and painstaking experimental petrology on mantle peridotite melting, basaltic magma generation and evolution largely done in the period of 1960s -1980s. However, the ~30 year lively debate on the nature of “primary magma” among experimental petrologists and the petrology community during this time had inadvertently shelved the development of consensus models on mantle melting in the context of plate tectonics. Continued experimental petrology in parallel with worldwide sampling and study of mid-ocean ridge basalts (MORB) brought about new insights, culminating with a model in 1980s that mantle potential temperature (TMP) variation controls the extent and pressure of mantle melting and basalt compositions. The tenet of this model is that hotter rising mantle begins to melt deeper and thus has greater decompression depth interval to melt more with the melt having the petrological signature of higher extent and pressure of melting than cooler mantle. This model has gained wide acceptance in MORB studies and has also been invoked in the study of intra-plate basalts in ocean basins and in continental settings. Basalt generation above subduction zones, on the other hand, has been generally accepted as resulting from slab-dehydration induced mantle wedge melting since early 1980s, but recent studies also advocate mantle temperature variation as the primary control on the extent of mantle wedge melting. All these views with laudable merits have formed a paradigm on mantle melting and basaltic magmatism. In this paper, I review the historical developments towards this paradigm and demonstrate in simple clarity that it is the lithosphere thickness, not TMP, that controls the extent of mantle melting, depth of melt extraction and basalt compositions, i.e., the lid effect. The lithospheric lid caps the rising melting mantle, thus limiting the extent of decompression melting and equilibrium pressure/depth of melt extraction, which is well registered in the compositions of MORB, intra-plate ocean island basalts (OIB), volcanic arc basalts above subduction zones (VAB) and basalts in continental interiors (CIB). Hence, lithosphere thickness is the governing variable that controls mantle melt compositions in all tectonic settings on earth. Major element compositions (e.g., Si-Mg-Fe) of erupted basalts have no memory of initial depth of melting because of effective and efficient melt-solid (e.g., olivine [Mg,Fe]2SiO4) equilibration in the rising melting mantle. Therefore, basalt-olivine based thermobarometry, albeit useful, supplies no information on TMP. It is also the lithosphere thickness that controls whether “mantle plumes” can surface or not and the large igneous provinces (LIPs) serve as effective manifestations for thin or thinned lithosphere at the time of emplacement. This new understanding based on global observations, well-understood experimental petrology and rigorous analysis is fundamental and requires a major change to the current paradigm.