Concentrations Of Large Ion Lithophile Elements Research Articles

Provenance of Early Paleozoic metasediments in the central Chinese Altai: Implications for tectonic affinity of the Altai-Mongolia terrane in the Central Asian Orogenic Belt

The Chinese Altai is one of the most typical segments of the Altai-Mongolia terrane, of which the tectonic evolution and affinity are hotly disputed and still not well constrained. Early Paleozoic metasedimentary rocks are extensive in the Altai-Mongolia terrane and their provenance is a key to unravelling the tectonic history of this terrane. Metasediments from Kulumuti Group were collected from the central Chinese Altai for geochemical study. They have low Chemical Index of Alteration (CIA = 52–54) and high Index of Compositional Variability (ICV = 0.81–1.19) values, different from the mature post-Archean Australian average shale (PAAS), indicating relatively weak chemical weathering and a source compositionally dominated by immature material that lacks alumina-rich minerals. These rocks are moderately enriched in light rare earth element (LREE) and show relatively flat heavy rare earth element (HREE) patterns (LaN/YbN = 2.36–9.80, GdN/YbN = 1.31–2.45). Compared with PAAS, they mostly have lower large ion lithophile element concentrations (e.g. Rb, Sr, Ba, Th, U and Pb) and similar contents of high field strength elements (e.g. Zr, Hf and Y), but with lower Nb and Ta. The metasediments have high Rb concentrations (> 50 ppm), relatively high K2O (> 1.1 wt.%) and low REE contents with negative Eu/Eu* anomalies, indicating that these rocks were derived from an acid-intermediate igneous source. The rocks have slightly enriched Nd isotopic compositions with calculated initial εNd(t) values mostly ranging from − 4.3 to − 0.2 and TDM2 model ages between 1.22 and 1.56 Ga. Detrital zircons from two samples of the Kulumuti metasediments have similar age spectra, dominated by Early Paleozoic to latest Neoproterozoic zircons (465–576 Ma) with a few older grains formed between 766–972 Ma and 1321–2572 Ma, remarkably consistent with those from the low-grade metamorphic Habahe Group in the western Chinese Altai. In combination with previous published data, the detrital zircons from Early Paleozoic metasediments in the Chinese Altai dominantly formed between 430–580 and 750–950 Ma, with only a minor population of Meso- to Paleoproterozoic grains. When compared with adjacent terranes, the age spectrum of the Chinese Altai is most similar to that of the Tarim Craton. This craton provided Precambrian materials for the Early Paleozoic metasedimentary rocks now exposed in the Chinese Altai, and therefore suggests that the whole Altai-Mongolia terrane has a close tectonic affinity to the Tarim Craton.

The Cosmos greenstone succession, Agnew-Wiluna greenstone belt, Yilgarn Craton, Western Australia: Geochemistry of an enriched Neoarchaean volcanic arc succession

Abstract The geodynamic setting of the Neoarchaean Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton is the subject of debate. Some authors propose plume models, while others advocate variants on a subduction accretion model for the origin of mineralised greenstone belt sequences. Felsic volcanism in the Kalgoorlie Terrane, the westernmost terrane of the EGS, is considered to have a tonalite-trondhjemite-granodiorite/dacite (TTG/D) geochemical affinity. The Cosmos greenstone succession, which lies in the Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie Terrane, contains several komatiite-hosted nickel sulphide deposits, the volcanic footwall to which consists of an intercalated succession of fragmental and coherent rocks ranging in composition from basaltic andesite to rhyolite. Light rare earth elements (LREEs) and large ion-lithophile elements (LILEs) are strongly enriched relative to high field strength elements (HFSEs) across all volcanic units, and the rocks display strong positive Pb and negative Nb anomalies. These geochemical characteristics resemble closely those of modern high-K calc-alkaline to shoshonite continental arc successions. Contrasting REE, LILE and HFSE concentrations, coupled with assimilation-fractional crystallisation (AFC) modelling, shows that the intercalated dacitic and andesitic volcanic rocks within the footwall succession are not co-genetic. Xenocrystic zircons within the felsic volcanic lithologies indicate that some assimilation of older continental crust contributed to the generation of the footwall volcanic sequence. The geochemical characteristics of the Cosmos volcanic succession indicate that parental melts were derived via partial melting of enriched peridotite that had been contaminated by subducted crustal material within the mantle wedge of a subduction zone. In contrast, two younger felsic porphyry intrusions, which cross-cut the volcanic succession, have a distinct TTG/D affinity. Therefore, these intrusions are considered to be generated via partial melting of a subducting slab and are related to local high-Ca granitoid intrusions. The Cosmos volcanic succession represents the first extrusive high-K calc-alkaline to shoshonitic volcanic arc sequence described in the Kalgoorlie Terrane and, coupled with age dating of the stratigraphy, is indicative of formation in a long-lived volcanic arc setting active from 2736 Ma to later than 2724 Ma. The composition and geochemical affinity of the volcanic footwall succession to the Cosmos komatiite-hosted nickel-sulphide deposits contrasts with the majority of felsic volcanic rocks within the AWB and also the wider Kalgoorlie Terrane, suggesting that the overall architecture of this region is more complex than is currently thought. Our conclusions not only have consequences for recent models of the tectonic evolution of the EGS but also contribute to the debate on the operation of plate tectonics during the late Archaean in general. The arc affinity of the Cosmos volcanic succession, containing abundant high-K calc-alkaline andesite lavas, provides further support for the operation of plate tectonics in the Neoarchaean.

Petrology and Sm–Nd dating of the Genina Gharbia Alaskan-type complex (Egypt): Insights into deep levels of Neoproterozoic island arcs

Abstract The deep levels of Neoproterozoic island arcs are poorly known due to limited accessibility. The Genina Gharbia Alaskan-type complex (south Eastern Desert, Egypt) is the remains of a magma chamber that crystallized at the base (crust–mantle boundary) of a mature Neoproterozoic island arc. The rock assemblage comprises hornblende-bearing harzburgite, lherzolite, pyroxenite, norite and gabbro. All lithologies show cumulus texture with evidence of extensive cumulus mineral–melt interactions. Clinopyroxenes from all lithologies have similar rare earth element (REE) patterns with slight medium-rare earth element (MREE) enrichment. Hornblendes are slightly enriched in MREE and light rare earth elements (LREE). Island arc signatures are indicated by high contents of large ion-lithophile elements and low concentration of high field-strength elements. Positive initial e Nd (+ 5.7 to + 7.0) and Nd model ages (963 ± 81 Ma) are consistent with the Genina Gharbia magma being extracted from a depleted mantle source. Modeling of estimated parental magma indicates 10% partial melting of a 90% depleted mantle source with a 10% (MORB + sediments)-derived fluid, commencing in the spinel stability field (

Conditions of melt generation beneath the Taupo Volcanic Zone: The influence of heterogeneous mantle inputs on large-volume silicic systems

Many arc silicic igneous provinces exhibit compositional variability defined by oscillation between dry and wet rhyolites. The origins of this variability are often uncertain due to the poor constraints on the compositions of the mantle-derived inputs to the lower crustal hybridization zones. The Taupo Volcanic Zone (TVZ) in New Zealand, the most productive of modern silicic igneous provinces, exhibits variability of rhyolite compositions, but small-volume coeval basaltic eruptions also occur, making it an ideal location to study the mantle contributions to these distinctive types of rhyolite. Here we present major and trace element data from 12 magmatic centers that reveal that mafic units from the northern portion of the central TVZ are enriched in large-ion lithophile elements (LILE) in comparison to the southern TVZ. The results of models quantifying slab-derived contributions to the mantle suggest that the observed heterogeneity in LILE concentrations, and volatile fugacities, can be explained by variable amounts of subduction-derived fluid within the melting region of the basalts. These new data and modeling results provide the first direct evidence that the spatial diversity in the flux of mantle-derived basalts and associated volatile elements into the lower crustal differentiation system of the TVZ are coincident with wet to dry rhyolite compositional variability.

Ultramafic xenoliths from Damaping (Hannuoba region, NE-China): Petrogenetic implications from crystal chemistry of pyroxenes, olivine and Cr-spinel and trace element content of clinopyroxene

Olivine, orthopyroxene, clinopyroxene and spinel from 16 different lherzolite mantle xenoliths from Damaping (Hannuoba region, NE-China) have been analyzed by X-ray single crystal diffraction and electron microprobe to define their structure, chemistry and cation distribution. LA-ICP-MS analysis has been carried out on clinopyroxenes to determine their LILE (Large-Ion Lithophile Elements), HFSE (High Field Strength Elements) and REE (Rare Earth Elements) contents. These latter define Depleted, Enriched, U-shaped and Transitional CI-normalized patterns suggesting that the xenolith suite here studied experienced at least two metasomatic events related to the porous flow percolation of melts with tholeiitic and alkaline affinity, respectively. The cation distribution of the mineral phases allowed determination of their intra- and inter-crystalline temperatures of equilibration. The closure temperature of intra-crystalline reactions for the constituent phases is in the range 600–800°C, suggesting a relatively slow cooling rate, while the inter-crystalline temperature interval found out is in the range 866–1048°C. The pressure conditions estimated for Hannuoba xenoliths through structural considerations correspond to a depth of about 50–60km, consistent with the Moho depth beneath the Hannuoba area, which is estimated at about 42km. The pressures of equilibration, as estimated from the cell and T site volumes of the clinopyroxenes, have been compared with the trace element and REE signatures; they demonstrate that Enriched, Depleted, U-shaped and Transitional clinopyroxenes formed at different pressures and are not restricted to a specific region of the mantle, testifying that the metasomatism took place at different depths in the mantle.

Genesis of jadeite–quartz rocks in the Yorii area of the Kanto Mountains, Japan

This paper reports the results of U–Pb dating and REE (rare earth element) analysis of zircons separated from jadeite–quartz rocks within serpentinite mélanges in the Yorii area of the Kanto Mountains, Japan. These rocks contain jadeite, albite, and quartz, with minor aegirine–augite, zircon, monazite, thorite, allanite, and titanite. Mineral textures provide evidence of a jadeite+quartz=albite reaction during formation of these jadeite–quartz rocks. Zircon crystals separated from the jadeite–quartz rocks can be split into two distinct types, here named Types I and II, based on their morphology and REE concentrations. Type I zircons are prismatic and have fluid, jadeite, quartz, and albite inclusions. Those show positive Ce and negative Eu anomalies and HREE (heavy rare earth element) enriched chondrite normalized REE patterns and have higher REE concentrations than those generally found in magmatic zircons. Type I zircons would have precipitated from a fluid. Mineralogical observation provides that Type I zircon crystallized at the same timing of the formation of the jadeite–quartz rocks. Type II zircons are porous and have REE patterns indicative of a hydrothermal zircon. Both types of zircons are fluid-related. Type I zircons yield U–Pb ages of 162.2±0.6Ma, with an MSWD (mean square weighted deviation) of 1.4. At this time, Japan was still a part of the eastern margin of the Asian continent, with the subduction of the oceanic paleo-Pacific Plate leading to the formation of the Jurassic Mino–Tanba–Chichibu accretionary complex in Japan. The age data indicate that the jadeite–quartz rocks formed in a deep subduction zone environment at the same time as the formation of the Jurassic accretionary complex in a shallower near-trench subduction zone environment. The jadeite–quartz rocks contain high concentrations of Zr and Nb, with low LILE (large ion lithophile elements) concentrations, suggesting that the HFSE (high field strength elements) can be concentrated into jadeite–quartz rocks prior to a fluid moving up into the mantle wedge. Typical arc volcanic rocks are depleted in the HFSE, suggesting that the high HFSE concentrations within jadeite–quartz rocks are consistent with fluids being stripped of their HFSE prior to interaction with mantle material during the formation of arc magmas. Although these jadeite-bearing rocks are rare occurrences on the surface exposure, they could be abundant in or above subducted slabs.

The geochemistry of host arc volcanic rocks to the Co–O epithermal gold deposit, Eastern Mindanao, Philippines

Mindanao is the second largest island of the Philippines and is located in the southern part of the archipelago. It comprises the suture zone between the Eurasian and the Philippine plate, which is displayed in the Philippine Mobile Belt. Eastern Mindanao is part of the Philippine Mobile Belt and outcropping rocks are mainly Eocene to Pliocene in age related to episodes of arc volcanism alternating with sedimentation.New high-precision elemental analysis of the Oligocene magma series, hosting the Co-O epithermal Au deposit, which represents an arc segment in the central part of Eastern Mindanao, revealed dominantly calc-alkaline rocks ranging in composition between basalt and dacites. Major element trends (MgO vs. TiO2 and Fe2O3) are comparable to other magmas in Central and Eastern Mindanao as well as other SW Pacific Islands such as Borneo. Rare earth and trace element distribution patterns display typical island arc signatures highlighted by the conjunction of LILE-enrichment with troughs at Nb, Ta, and Ti. Ratios of Zr/Nb in basalts vary between 17 and 39, signifying a depleted subarc mantle wedge comparable to the range of MORB, and other Indonesian island arc basalts. In basalts, Nb/Ta and Zr/Sm ratios are 12–37 and 14–27 respectively indicative of deep melts of rutile-eclogite subducted slab, as well as fluids, infiltrating the mantle wedge source of basalts.Moderate large ion lithophile element contents and low Th/La and Th/Ce ratios suggest no significant slab-derived components such as sediment or crustal fragments. The comparatively low Ce and Yb values in basalts, but also andesites and dacites, are consistent with a thin arc crust related to an intraoceanic convergent margin setting. This is further supported by Nb contents in basalts that range between 1 and 3ppm and are within the range of modern oceanic convergent margin basalts. The range of HREE fractionation signifies that basaltic melts separated at deeper levels of the subarc wedge, possibly between the forearc and arc axis, followed by a calc-alkaline convergent margin magma suite involving shallower crustal AFC near the central arc sector.The analysed Oligocene arc segment is related to a potentially steep to intermediate dipping subduction zone in an extensional to neutral geotectonic regime. The large subduction accretion complex of the Philippine Mobile Belt provides an ideal setting for significant metal deposits during its entire evolution. This is evidenced in the Eastern Mindanao Ridge, which hosts substantial porphyry Cu and epithermal Au deposits.

Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

Barium-rich fluids and melts in a subduction environment (La Corea and Sierra del Convento mélanges, eastern Cuba)

Whole-rock compositions of muscovite-bearing amphibolite, trondhjemite, pegmatite and quartz-muscovite rocks from Sierra del Convento and La Corea melanges (eastern Cuba), as well as mineralogy, record complex circulation of Ba-rich fluids and melts in the subduction environment. Partial melting of fluid-fluxed, MORB-derived amphibolite produced trondhjemite magmas that crystallized at depth, in some cases evolving into pegmatites. Qtz-Ms rocks probably crystallized from primary fluids derived from subducted sediments. All these rocks have elevated concentrations of large-ion lithophile elements, especially Ba (up to 11,810 ppm), presumably released from slab sediments by fluids and/or melts. Fluid–rock interaction produced crystallization of phengite in parental amphibolites. The phengite crystallized in all types of rocks is rich in Ba, with concentric zoning, characterized by Ba-rich cores and Ba-poor rims, indicating a compatible behaviour of Ba in the studied systems. Zoning in phengite is governed primarily by the celadonite (tschermak) exchange vector ((Mg,Fe)Si-(Al)-2), with more moderate contributions of the celsian (BaAl-(KSi)-1) and paragonite (NaK-1) exchange vectors. Late remobilization of Ba at relatively low temperature formed retrograde celsian. The compatible behaviour of Ba in the studied rocks strengthens the importance of the stability of phengite for the transfer of LIL elements from the subduction to the volcanic arc environments.

The geochemistry of mafic gneisses from the Renzy terrane, western Grenville Province, Quebec: Implications for the geodynamic setting of the early Mesoproterozoic Laurentian margin

This study presents new geochemical data on mafic gneisses from the Renzy terrane (RT), located in the Grenvillian Parautochthonous Belt of western Quebec. This region is of particular interest since it contains one of the rare Ni–Cu–Co massive sulfide deposits ever mined in the Grenville Province. RT mafic gneisses have been divided into 3 groups based mainly on major element compositions, trace element profiles normalized to the primitive mantle, and incompatible trace element ratios. The behaviour of high field strength elements (HFSE) (Ta, Zr, Hf, Ti, Th) and large ion lithophile elements (LILE) (Ba, Rb) are particularly useful for discriminating RT mafic gneisses. Major and trace element compositions of group 1 mafic/ultramafic gneisses are very similar to those of the RT ultramafic sheets; the mafic gneisses are therefore interpreted as melagabbroic sills cogenetic with these sheets. The trace element composition of group 1 mafic gneisses is typical of magmas formed from a previously metasomatized depleted mantle source. These rocks have high Th/Ta ratios and show strong to moderate negative anomalies in Ti, Zr, and Hf on multi-element plots normalized to the primitive mantle. Mafic gneisses of group 2 show geochemical features transitional between extensional and compressional settings. These rocks generally have low concentrations of HFSE and high concentrations of LILE, typical features of mafic rocks formed in a compressional environment; however, some of them are Fe–Ti rich, with minor or no negative anomalies in HFSE and low to moderate concentrations of LILE on multi-element diagrams normalized to primitive mantle, features more typical of an extensional environment. Mafic gneisses of group 3 have a geochemical signature typical of extensional environments or of ocean island basalt (OIB). These gneisses have no negative anomalies in HFSE and have low concentrations of LILE. RT mafic gneisses are similar in composition to mafic granulites in the Bondy gneiss complex in the Central Metasedimentary Belt and to Shawanaga Domain amphibolites from the Central Gneiss Belt, two paleoarc/back-arc environments in the Grenville Province. Based on the contrasting geochemical signatures found in the RT mafic gneisses and the similarities with known arc/back-arc settings in the Grenville Province, we propose that the RT was formed by arc/back-arc magmatic activity most likely associated with a period of arc magmatism along the Proterozoic Laurentian margin. OIB-like mafic gneisses found in the RT are similar to those from the Kipawa region, formed by intraplate continental magmatism in the Parautochthonous Belt of the Grenville Province.

Mechanisms of Crustal Anatexis: a Geochemical Study of Partially Melted Metapelitic Enclaves and Host Dacite, SE Spain

To shed light on the mechanisms of crustal anatexis, a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at El Hoyazo, SE Spain. Anatectic enclaves, composed of plagioclase þ biotite þ sillimanite þ garnet þ glass K-feldspar cordierite þ graphite, formed during the rapid heating and overstepped melting of a greenschist-facies metapelite, and finally equilibrated at 850 508C and 5^7 kbar. Glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves, and has a major element composition similar to that of peraluminous leucogranites. Melt inclusions and matrix glasses have normative quartz^orthoclase^albite compositions that plot in the vicinity of H2O-undersaturated haplogranite eutectics. Melt inclusions show some compositional variability, with high Li, Cs and B, low Y, first row transition elements (FRTE) and rare earth elements (REE), and zircon and monazite saturation temperatures of 665^7508C.They are interpreted as melts produced by muscovitebreakdown melting reactions at the onset of the process of rapid melting and mostly under H2O-undersaturated conditions. Compared with melt inclusions, matrix glasses show less compositional variability, lower large ion lithophile element contents, higher Y, FRTE and REE, and higher zircon and monazite saturation temperatures ( 695^8158C).They are interpreted as former melts recording the onset of biotite dehydration-melting. Matrix glasses in the dacite are compositionally different from glasses in the enclaves, hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest; this opens the possibility for some alternative interpretation. This study shows the following: (1) melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves; (2) melting occurred for the most part under H2O-undersaturated conditions even if, because of the rapid heating, the protolith preserved most of the structurally bound H2O contained at greenschist facies up to the beginning of anatexis, such that the excess H2O maximized the amount of H2O-undersaturated melt generated during anatexis; (3) although a large proportion of accessory minerals are currently shielded within major mineral phases, they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path, as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions; (4) the dacite magma was probably produced by coalescence of melts

Isotopic and geochemical evidence for a recent transition in mantle chemistry beneath the western Canadian Cordillera

New petrologic, geochemical, and isotopic data are reported from a suite of mafic dike and lava flow samples collected from sites within the western Canadian Cordillera. Samples range in age from Eocene to Quaternary and document a significant transition in mantle chemistry that occurred sometime after 10 Ma. Eocene to late Miocene basalts emplaced as dikes within the Coast Mountains Batholith contain abundant hornblende, are enriched in large ion lithophile elements (LILE) (Ba, Rb, K), have negative high field strength element (HFSE) (Nb, Ta) anomalies, and were likely derived from lithospheric mantle (87Sr/86Sr = 0.70353–0.70486; ɛNd = +2.5 to +5.7) that was stabilized after the cessation of arc magmatism in the area. By contrast, Quaternary lava flows have lower LILE concentrations, positive Nb‐Ta anomalies, and were likely generated by upwelling asthenosphere (87Sr/86Sr = 0.70266–0.70386; ɛNd = +7.4 to +8.8) or at least a mantle source with different chemical and isotopic characteristics. A regional comparison of mafic rocks from western Canada that are also Eocene to Quaternary in age indicates that the transition in mantle chemistry after 10 Ma was pervasive and widespread and was not limited to the present study area. This transition occurred ∼40 Ma after the cessation of Cordilleran arc magmatism in central British Columbia, suggesting that large‐scale transitions in mantle chemistry beneath magmatic arcs may occur on the order of tens of millions of years after the final subduction of oceanic lithosphere, in this case as a result of lithospheric thinning by continental extension.

Assimilation of Plutonic Roots, Formation of High-K ‘Exotic’ Melt Inclusions and Genesis of Andesitic Magmas at Volcán De Colima, Mexico

Phenocryst-hosted melt inclusions from the 1998^2005 andesite eruptions of Volcan de Colima (Mexico) show broad ranges of major and trace element contents that do not overlap with the bulk- rock compositions and indicate that melt inclusions can be formed by and record a range of processes involved in the genesis of andesites. The melt inclusions that demonstrably record the evolution of the melt feeding the eruption indicate low-pressure (130^10 MPa) crys- tallization of a dacitic melt despite the monotonous bulk andesitic composition of historical magmas at Volcan de Colima. Mingling of dacite melt with gabbroic fragments in the shallow sub-volcanic system is the process responsible for generating the bulk andesitic composition of the magmas. A significant proportion of the melt inclusions have distinctive high large ion lithophile element (LILE) signatures. These 'exotic' high-K melt inclusions in pyrox- enes are thought to result from incongruent melting of interstitial bio- tite during assimilation of gabbroic fragments in the dacitic melt. A second group of exotic high-K melt inclusions found in plagioclase are likely to result from dissolution of higher-pressure (4200 MPa) amphibole, plagioclase, magnetite and biotite cumulates during assimilation in the ascending dacitic melt. Although they are not volumetrically abundant, high-K melts formed during assimilation of plutonic fragments and crystal cumulates made a significant con- tribution to the LILE contents of the magmas and represent a poten- tial source for this group of elements. The range of melt inclusion compositions in Volcan de Colima magmas emphasizes the impor- tance of mixing between ascending evolved melts and crystal popula- tions formed during previous episodes of magmatism over a range of pressures, temperatures and volatile contents. Cannibalization of plutonic roots appears to be a fundamental process in the genesis of andesite magmas and melt inclusions at continental arc volcanoes.

Post-collisional adakites in south Tibet: Products of partial melting of subduction-modified lower crust

Post-collisional (26.2 to 10.1 Ma) adakites occur within the Lhasa terrane of the southern Tibetan Plateau in an E–W trending, 1500 km long, magmatic belt. Outcrops are small and restricted within N–S-trending rift zones (grabens); they include both extrusive and intrusive facies. The adakites have high SiO 2 (59–70 wt.%), Al 2O 3 (15–18 wt.%) and Sr (317–1133 ppm) contents and Sr/Y ratios (44–162), and low Y (4.2–12.9 ppm) and HREE (e.g. Yb < 0.9 ppm) concentrations. Their MORB-normalised incompatible element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The combined trace element and Sr–Nd–Pb isotope characteristics of the adakites suggest that their source was mafic-intermediate lower crust formed during a preceding stage (153–40 Ma) of active continental margin magmatism. Lower crustal melting was primarily induced by the conduction of heat from contemporaneous potassic–ultrapotassic magmas produced by partial melting in the asthenosphere or lower lithosphere. Trace element modelling calculations suggest that the adakites are the products of 5–10% partial melting of garnet-bearing amphibolite facies meta-igneous rocks. Differences in the LILE contents and Sr–Nd isotope compositions of adakites sampled to the east and west of 86° E can be attributed to variable degrees of partial melting of the lower crust and mixing between potassic–ultrapotassic magmas and lower crustal melts. Extensional collapse of the Tibetan Plateau may have contributed to partial melting of the lower crust and the formation of the potassic–ultrapotassic magmatism by decompression melting of a thin asthenospheric mantle wedge above a subducted slab of Indian continental margin lithosphere. The oldest age of the post-collisional adakites and contemporaneous potassic–ultrapotassic magmatism in the Lhasa terrane may, therefore, provide constraints on the timing of initiation of tectonic collapse in the southern part of the plateau.

Melting and Multi-stage Metasomatism in the Mantle Wedge beneath a Frontal Arc Inferred from Highly Depleted Peridotite Xenoliths from the Avacha Volcano, Southern Kamchatka

Peridotite xenoliths from the Avacha volcano, Kamchatka, derived from the mantle beneath the volcanic front of the Kamchatka arc, are mainly highly depleted clinopyroxene-poor harzburgites with highly forsteritic olivine (Fo 90^92) and high Cr-number spinel (0 5^0 7). The Avacha peridotites have experienced metasomatism to various extents, with the formation of metasomatic orthopyroxene replacing primary olivine, by infiltration of SiO2-rich fluids. The metasomatic orthopyroxenes can be subdivided into two textural types; (1) radially aggregated prismatic grains (opx II-1); (2) stout grains associated with interstitial glass and metasomatic minerals (opx II-2).The Avacha peridotites exhibit light rare earth element (LREE) enrichment relative to heavy REE (HREE), even in primary lithologies with52 vol.% of metasomatic orthopyroxene. However, the metasomatism has not altered the chemical characteristics of the Avacha peridotites significantly, and the Fo contents of olivine do not increase with an increase in the Cr-number of spinel. The clinopyroxene in the most depleted primary peridotite, with the highest spinel Cr-number, has the highest CeN (where subscript N indicates normalized to primitive mantle) and relatively lowYbN contents.We conclude that the Avacha peridotites are residues of high-degree melting induced by infiltration of an aqueous fluid with relatively high Fe/Mg and LREE/HREE ratios. In situ analysis of olivine grain boundaries by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) showed significant concentrations of some trace elements (Ce and Ba), even on glassfree boundaries; such grain boundaries can be a repository for largeion lithophile elements (LILE). Orthopyroxene and Ca-amphibole contribute to the HREE and middle REE (MREE) budget, respectively, of the bulk peridotites.Three types of mantle metasomatism are recorded in the Avacha peridotites. The agent that formed HREE-poor opx II-1 was a slab-derived aqueous fluid rich in SiO2. It was a successor of the fluid influx involved in the initial partial melting event. On the other hand, the metasomatic minerals in highly metasomatized peridotites have high LILE (Th, U and Sr) and LREE contents relative to HREE, indicating involvement of a hydrous melt of adakitic affinity derived from the subducted slab. Glasses associated with opx II-2 are similar in their trace-element characteristics to the host Avacha volcanic rocks, indicating the likelihood that the metasomatic agent was a forerunner of the recent Avacha arc magmatism.

Petrogenesis of a Late Jurassic Peraluminous Volcanic Complex and its High-Mg, Potassic, Quenched Enclaves at Xiangshan, Southeast China

A late Mesozoic belt of volcanic–intrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158–135Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850 C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 65 4% and 76 8% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200 C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial eNd values ( 4 2) and high initial Sr/Sr ratios (0 7081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves.

Eocene shoshonitic mafic dykes intruding the Monashee Complex, British Columbia: a petrogenetic relationship with the Kamloops Group volcanic sequence?

The newly named Three Valley Suite (TVS) kersantite lamprophyre to shoshonitic mafic dykes of the Monashee Complex are inferred to be hypabyssal feeder dykes to an alkaline to calc-alkaline volcanic suite related to the Kamloops Group. These dykes were emplaced in a subvertical north-trending orientation coincident with inferred Eocene crustal extension (~50.0 Ma), based on the flat 40Ar/39Ar step-heating plateau of contact-metamorphic muscovite on the margin of a TVS dyke. These weakly altered mafic dykes are fine grained with phenocrysts (0.5–2.0 mm) of phlogopite, augite, amphibole, and olivine (pseudomorphed by clays), rare labradorite, and both primary and secondary carbonates set in a fine-grained groundmass of similar mineralogy consistent with their classification as plagioclase-bearing potassic diorite to kersantite lamprophyre. The dykes are weakly silica-undersaturated and alkalic (2.8 wt.% K2O, 7.7 wt.% MgO), with high large ion lithophile element contents (~300 times primitive mantle) and elevated high-field-strength element contents, with a prominent negative Nb (Ta) anomaly, and have radiogenic Nd and Sr isotopic signatures; these geochemical attributes are consistent with a calc-alkaline shoshonitic affinity. Therefore, it is inferred that the subducting oceanic plate influenced subcrustal mantle wedge metasomatism in the region. Decompression partial melting of this metasomatised lithospheric mantle was initiated by coupled rapid unroofing, regional trans pression, slab rollback, and slab window development to the south. The TVS is similar to the mafic volcanic rocks within the nearby Eocene volcanic rocks, suggesting that these dykes represent the feeder system to a volcanic field that is now eroded, i.e., a broad-terrane association.

Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian, New England Fold Belt

Geochemical studies of volcanic rocks in the Gamilaroi terrane and Calliope Volcanic Assemblage, New England Fold Belt, eastern Australia, indicate that the setting in which these rocks formed changed in both space and time. The Upper Silurian to Middle Devonian basalts of the Gamilaroi terrane show flat to slightly light rare‐earth element (LREE) depleted chondrite normalised patterns, depletion of high field strength elements (HFSE) relative to N‐MORB, low Ti/V and high Ti/Zr ratios, high Ni, Cr and large‐ion lithophile element (LILE) contents, features characteristic of intra‐oceanic island arc basaltic magmas. They are associated with low‐K, less mafic volcanics, showing moderate LREE enrichment, low Nb and Y contents and Rb/Zr ratios. The depletion of HFSE in the basalts indicates that the magmas were derived from a refractory source in a supra‐subduction zone setting. The presence of such a zone implies that the arc was associated with a backarc basin, the location of which was to the west where a wide backarc region existed from the Middle Silurian. This polarity of arc and backarc basin suggests that the subduction zone dipped to the west. In contrast to their older counterparts, Middle to Upper Devonian basalts of the Gamilaroi terrane have MORB‐like chondrite normalised patterns and higher Ti and lower LILE contents. Moreover, they have low Ti/Zr ratios and MORB‐like Ti/V ratios and HFSE contents, features typical of backarc basins. Dolerites of the Gamilaroi terrane also have predominantly backarc basin signatures. These features suggest that both the basalts and dolerites have been emplaced in an extensional environment produced during the rifting of the intra‐oceanic island arc lithosphere. A progressive increase in Ti/V ratios, and TiO2 and Fe2O3 contents at constant MgO, of stratigraphically equivalent basalts, towards the north‐northwest part of the belt, is consistent with either greater extension to the north or melting of a more fertile magma source. By contrast, basalts in the southeast part of the terrane have moderately high Ti/Zr and low Ti/V ratios and in some samples, exhibit depletion of HFSE, compositional features transitional between island arc and backarc basin basalts. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage show both LREE enriched and depleted chondrite normalised REE patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Early Devonian age and are suggestive of eruption in an arc setting. Thus, the data from this study provide new evidence for the evolution of the New England Fold Belt from the Late Silurian to the Late Devonian and reveal a history more complicated than previously reported.

Structure and performance of PM ultrahard high speed steel: A.L.Wojcieszynski, W.B.Eisen. (Crucible Research, USA.)

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada (∼ 2.75–2.67 Ga).Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at ∼ 2700 Ma at pressures of ∼ 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K2O (< 1.5 wt%), Rb (< 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO2 (0.5 wt%), Fe2O3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment.Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at ∼ 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K2O (1–3 wt%), Rb (10–50 ppm), Ba (< 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5–1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [(LaYb)n = 49-21] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab.Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Watebeag) to stocks (e.g., Garrison), and were emplaced at shallow levels (⩽ 1 kbar) between 2681 to 2676 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO(K2O + Na2O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a “metasomatised” mantle and TGGM-type sources.Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO2 ⩽ 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO2 ⩾ 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab.In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series (∼ 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K2ONa2O ⩾ 1, restricted SiO2 range (69–75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [(LaYb)n = 0.9–16], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone.Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of ∼ 70 Ma.

Adhesive wear of hardmetals and cermets: H.Reshetnyak, J.Kubarsepp. (Tallinn Technical University, Estonia.)

Four distinct granitic magma series developed systematically in space and time during the evolution of the Southern Volcanic Zone of the Archean Abitibi granite-greenstone belt, Canada (∼ 2.75–2.67 Ga).Minor synvolcanic tonalite-trondhjemite-granodiorite (TTG) intrusions, such as the Lac Flavrian and Bourlamaque plutons, were emplaced at ∼ 2700 Ma at pressures of ∼ 1 kbar. Synvolcanic TTG's are chemically similar to Barker's low-Al type Archean TTG, and are characterised by extremely low K2O (< 1.5 wt%), Rb (< 50 ppm) and Sr (< 200 ppm) concentrations, enhanced TiO2 (0.5 wt%), Fe2O3 (4.4 wt%), Nb, Y and Sc, flat REE patterns, and negative Eu anomalies that increase with HREE abundance. The TTG's are interpreted to have originated by fractionation of basaltic magma from partial melting of a depleted mantle source in a supra-subduction zone environment.Voluminous tonalite-granodiorite-granite and quartz monzonite (TGGM series), exemplified by the Round Lake and Lac Abitibi batholiths, developed syntectonically at ∼ 2690 Ma. Compositionally, the TGGM-series granites resemble Archean and Phanerozoic high-Al type TTG; they feature low K2O (1–3 wt%), Rb (10–50 ppm), Ba (< 1000 ppm) and U contents, enhanced Sr, low Ba/Sr (0.5–1.25) ratios, normalised depletion of Ta, Nb and Ti, and strongly fractionated REE patterns [(LaYb)n = 49-21] with minor positive or no Eu anomalies. Syntectonic TGGM's formed by partial melting of an amphibolite and/or quartz eclogite basaltic protolith from a subducting or subcreted slab.Late-tectonic quartz syenite-quartz monzonite-granite (SMG) series range from batholithic dimensions (e.g., Watebeag) to stocks (e.g., Garrison), and were emplaced at shallow levels (⩽ 1 kbar) between 2681 to 2676 Ma. In composition they are similar to the TGGM series, including REE distributions, but feature lower CaO(K2O + Na2O) and greater concentrations of Rb, Ba, Th and U. This granitic series may represent mixtures of magmas derived from a “metasomatised” mantle and TGGM-type sources.Late- to post-tectonic alkali feldspar syenite-alkali feldspar quartz syenite (SS) series (2680-2670 Ma) are one expression of shoshonitic magmatism most prevalent along regional translithospheric strike-slip structures. Comagmatic trachytes unconformably overlie the deformed and metamorphosed greenstone belt. The most primitive magmas (SiO2 ⩽ 65 wt%) are characterised by the conjunction of extreme large-ion lithophile element (LILE) and light REE (LREE) enrichment, strongly fractionated REE patterns, and relative depletions of Ta, Nb and Ti, with enhanced Cr, Co and Ni abundances; evolved phases (SiO2 ⩾ 65 wt%) possess lower contents of compatible and incompatible elements stemming from differentiation. Primitive magmas of the SS series may have been derived from low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab.In the adjacent Pontiac subprovince, two granitic series predominate: a syn- to late-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) series (2690-2670 Ma), and a garnet-muscovite granite (GMG) series (∼ 2645 Ma). The MMGS's have comparable major-element, LILE and REE contents to Abitibi SMG and evolved SS granitic series, but are distinguished by strong correlations of U, Th, Zr and Hf with SiO2, extremely high Ba/Th ratios, and by co-enrichment of Cr, Co and Ni with LREE and Li and Cs in some MMGS phases. GMG series, which are associated with migmatites and Mo,U,Be,Li-pegmatites are characterised by K2ONa2O ⩾ 1, restricted SiO2 range (69–75 wt%), strong enrichments of Rb, Li, Cs, Ta, Nb, Th and U, and moderately fractionated REE's [(LaYb)n = 0.9–16], with pronounced negative Eu anomalies: they are similar to collisional S-type granites and represent infracrustal melting of mature metasediments during a continent-arc collision involving thrusting of the Pontiac subprovince beneath the Abitibi Southern Volcanic Zone.Collectively, the six granitic series reflect a transition from extensional supra-subduction zone magmatism with depleted mantle sources, to volcanic arc magmatism involving subducting slab during early collision, to alkaline granites derived from metasomatised mantle sources beneath a mature arc undergoing extension, and finally to magmatism resulting from terrane-terrane collision over a period of ∼ 70 Ma.