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The article presents a reply by authors to a comment on their study related to petrogenesis of rhyolite in Paraná, Brazil. It notes that a fractional crystallization model for a rhyolite can be easily reconciled with highly eruptive temperature and similar incompatible element ratio. It mentions that upper-crustal derived materials can be distinguished by oxygen isotopes.

The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma. [ABSTRACT FROM AUTHOR]

Continental rifting is usually viewed in terms of two contrasting models of active and passive extension. The origin of the Baikal Rift, adjacent to the southern part of the Siberian Craton, has been described by both models in the past. It is expected that basaltic magmatism in an active model scenario should be primarily sourced from a mantle plume or plume-fed asthenosphere, whereas melting of the lithospheric mantle is expected to be a predominant source for magmatism in the passive model. In this paper, we focus on the Miocene volcanic rocks sampled along two 60-km-long profiles that cross the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton. Most of the samples studied are trachybasalts. In terms of trace element concentrations normalised to primitive mantle, the lavas mimic oceanic island basalt-like patterns with troughs at Rb, Th–U, Pb, and Y, and peaks at Ba, Nb, Ta, K, and Sr. Moreover, similar trace element patterns to the studied samples are also observed for Miocene and Quaternary lavas located in the southwestern of the Baikal Rift, and adjacent regions of non-rifted Mongolia. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2, the compositions of the studied lavas coincide with experimental melts derived from mafic lithologies. Trace element data of samples suggest that garnet was a residual phase during partial melting. The Sr-Nd isotopic characteristics of the studied lavas are 87Sr/86Sr 0.70427–0.70469 and 143Nd/144Nd 0.51267–0.51284. They are identical to the coeval Miocene lavas of neighbouring volcanic fields, but they differ from the Quaternary lavas that extend to lower 87Sr/86Sr (0.7038–0.7044) with near identical 143Nd/144Nd. Isotopes of Hf for studied samples show values εHf = 6.0–7.7, except for the two samples taken within the boundary between two lithospheric blocks with εHf 4.6 and 4.8. The δ18O of olivine from lava samples is everywhere higher than that of the asthenospheric mantle and ranges from 5.5 to 6.4‰. Variations of δ18O versus Mg#, 87Sr/86Sr and εHf in the studied samples do not correlate, but do unequivocally rule out crustal assimilation. The isotopic variations are consistent with recycling of mafic crustal lithologies at mantle depths. Lavas from the Tuva-Mongolian massif and the Siberian Craton differ in lead isotopes by lower values of 206Pb/204Pb (< 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples and the reverse relationship for off-cratonic lava (> 17.785 and 55–61), respectively. The equation for Δ8/4Pb = [208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from Hart (Nature , 309 , 753–757, 1984). The correlation of lead isotopes with the mafic recycled component, the sharp change of lead isotopic values at the cratonic boundary and decoupling of lead isotope ratios from other isotopic ratios lead us to suggest that the values of 206Pb/204Pb and Δ8/4Pb are associated with an ancient accessory mineral phase such as sulphide confined within the lithospheric mantle. The predominant role of the lithospheric sources in the formation of the Miocene volcanic rocks indicate that the volcanism of the Baikal Rift was caused by a passive tectonic process, rather than active rifting. [ABSTRACT FROM AUTHOR]

Appinite–granite suites commonly occur in the final stage of collisional orogenic processes, providing a unique opportunity to reveal the properties of continental lithospheric mantle and crust–mantle geodynamics. In this paper, we present a systematic study of the petrology, mineral chemistry, whole-rock geochemistry and geochronology of the Xiong'erling pluton and adjacent appinite dikes in the northern margin of the North Qinling orogenic belt. The pluton is mainly composed of diorites, quartz monzonites and minor granites. The diorites and appinites have LA-ICP-MS zircon U–Pb ages of c. 389 Ma with variable εHf(t) values of −5.58 to +3.36 and TDM1 model ages peaking at c. 1133 Ma. These rocks belong to the shoshonitic series with high Ba–Sr content and were emplaced in an intraplate extensional environment. The quartz monzonites and granites are oxidized A-type granites and have zircon U–Pb ages of c. 396 and c. 378 Ma with negative εHf(t) values of −11.99 to −1.18 and TDM2 model age peaks at 1434, 1720, and 1888 Ma. The in situ 87Sr/86Sr ratios of plagioclase from the mafic and felsic rocks are 0.70565-0.70752 and 0.70695-0.71008, respectively. The Xiong'erling pluton and the coeval appinite dikes represent a Devonian appinite–granite suite in the North Qinling orogenic belt. Asthenospheric upwelling during the intraplate extension triggered partial melting of a phlogopite- and amphibole-bearing garnet lherzolite mantle source that had been previously metasomatized by the subducted oceanic slab, leading to the formation of a primitive hydrous mafic magma. The rapid ascent of the water-rich magma along deep-seated active faults with fast crystallization of amphibole resulted in emplacement of the c. 389 Ma appinite dikes. Replacement of anhydrous minerals by amphibole and biotite via hydrous reaction during magma cooling resulted in the formation of the coeval diorites. The mafic magma underplating triggered episodic remelting of a late Paleoproterozoic to early Mesoproterozoic crustal source, leading to the generation of the c. 396–378 Ma quartz monzonites and granites. Combining our results with existing data, we identify a sequence of (1) northward subduction of the Shangdan oceanic crust beneath the Qinling block at c. 524–438 Ma resulting in island-arc calc-alkaline magmatism, (2) closure of the Shangdan Ocean indicated by collision between the Qinling block and the South Qinling terrane and slab failure magmatism at c. 438–410 Ma, and (3) post-collisional to intraplate extension with alkaline magmatism at c. 410–370 Ma. The Devonian extensive intraplate magmatism marks the end of the Paleozoic orogenesis in the North Qinling belt. [ABSTRACT FROM AUTHOR]

The temporal distribution of enriched source components and magmatism in continental collision zones provides critical information about mantle dynamic processes in collision-related environments. This paper presents petrology, mineralogy, K-Ar ages and whole-rock major and trace elements, as well as Sr-Nd-Pb-Hf isotopic compositions of Melikler volcanism in Kars Plateau (KP) in the East Anatolia Collision Zone, NE Turkey, with the aim to understand the role of the subducting slab, the origin of magmatism and the geodynamic evolution in the collision-related environments. Our K-Ar dating results show the Melikler volcanism erupted between 5.29 and 1.7 Ma and allows us to divide it into an early (5.29–2.53 Ma) and a late (2.24–1.7 Ma) stage. Major-trace element abundances, isotopic compositions, EC(R) AFC (energy-constrained recharge, assimilation, and fractional crystallisation) and MELTS model calculations of both stages indicate that the least evolved samples were not affected by significant crustal contamination and fractional crystallisation. More evolved samples of the late stage underwent AFC processes with up to 6–9% crustal assimilation; however, those of the early stage were differentiated from a parental magma composition via AFC (up to 2–7.5% crustal assimilation) and experienced magma replenishment at pressure of 0.5 kbar; thus, both early and late stages have experienced open system conditions. The least evolved samples of both stages across the KP have arc-enriched geochemical and isotopic signatures, characterised by prevalent negative Nb–Ta anomalies and moderately radiogenic Sr, unradiogenic Nd-Hf and highly radiogenic Pb isotopic compositions. These primary melts could be derived from a depleted MORB mantle source metasomatised by sediment melt from the subducting Neotethys oceanic slab. Combined trace elemental and isotopic modelling results suggest that the least evolved samples of the early stage were formed by 2–4% melting of an amphibole-bearing garnet lherzolitic mantle source, which was metasomatised by 0.3–0.5% contribution of subducted slab component with a ratio of sediment melt/AOC (altered oceanic crust) melt about 90:10. A depleted lherzolitic mantle source containing apatite and garnet through inputs of 0.6–0.8% melts derived from the subducted oceanic slab, with 5–10% partial melting degree, could produce the least evolved samples of the late stage. Thermobarometric calculations reveal that the least evolved samples of the late stage are derived from the lithosphere-asthenosphere boundary at a depth of 77–82 km; in contrast, those of the early stage are produced from the lithosphere at a depth of 66–69 km. Literature data and the findings obtained from this study indicate that the onset of the Arabian-Eurasian collision may have occurred in the Oligocene and lithospheric dripping caused by the hard collision that occurred around the Late Miocene-Early Pliocene may produce the Melikler volcanic rocks. [ABSTRACT FROM AUTHOR]

Thermo-chemical modeling and chemical data suggest that the genesis of arc melts commonly involves re-melting of older intrusions, triggered by the injection of mantle-derived melts, followed by magma mixing. Remelting and mixing may lead to complex zircon populations, which can be used to gain insights into the conditions of mixing. This paper investigates a case where such processes can be studied through the compositional and thermal record provided by zircons preserved in a diorite dyke swarm that remelted host rock tonalites in the Gangdese Batholith in Tibet. Autocrystic zircons from the diorite yield consistent ages of 46 to 47 Ma even though they can be altered, having highly enriched trace elements, reaching ~1 wt % in Fe, Ca, Y, U, Th, and anomalously high values of LREE and Ti. Granitic magmas derived from the tonalite, back-veined the dykes and mixed with the dioritic mush, transferring small quantities of 77–79 Ma zircon xenocrysts. The xenocrysts are euhedral with little evidence for resorption, indicating that they were apparently stable during the process of tonalite anatexis and transfer to the diorite magma. This requires that: (i) tonalite melting occurred at low temperatures with minimal zircon dissolution, and (ii) the diorite either cooled rapidly before significant resorption of the grains or was already saturated in zircon when mixing occurred. Zircon saturation temperatures of the diorite are relatively low, indicating that xenocrystic zircons were transferred to a highly crystalline dioritic mush. This requires either liquefaction by injection of the back-veining felsic magma to allow for mixing, or pervasive throughflow of the diorite mush by the felsic magma leaving behind zircon xenocrysts. The findings suggest that the dykes triggered low-T, water-fluxed melting of the host tonalite and that these anatectic melts invaded the diorite mush initially through the interstices leading to mixing. This may have caused the breakdown of the crystalline framework leading to liquefaction and renewed magma flow. Dyking and assimilation of wall-rock through back-veining as recorded in these rocks could be common in transcrustal arcs. However, this process could be hidden due to a combination of the similarity in the isotopic and chemical nature of arc rocks and resorption of zircon xenocrysts during mixing. This process may explain some complex chemistry of arc magmatic rocks and their minerals that are not easily explained by endmember models, such as pure re-melting of older arc rocks or fractional crystallization of mantle-derived melts. [ABSTRACT FROM AUTHOR]

This paper presents the chemical composition (including H2O and CO2) of matrix glass, melt inclusions in phenocrysts, and their host minerals in eruptive products from Aso caldera. We found a group of melt inclusions with clearly lower potassium (0.6–2 wt % K2O at 50–70 wt % SiO2) than previously reported high-K2O whole-rock compositions (3–5 wt % K2O at 55–70 wt % SiO2). While most of the high-K2O intermediate to felsic melt inclusions are vapor undersaturated and show the features of H2O, CO2, and K2O accumulation, the low-K2O basaltic melt already has higher H2O and CO2. We reconcile this discrepancy with a model in which (1) the volatile-rich basalt magmas degas near the surface, (2) migrate back to depths of |$\ge$| 12 km in the crust, and (3) crystallize feldspar and quartz to produce high-K2O felsic melt, (4) magma mixing among the undegassed, degassed, and evolved magmas. By crystallization, about five times as much low-K2O basaltic magma is required to produce high-K2O felsic magma. The quantity of felsic magma ejected from 270 ka (Aso-1) to 90 ka (Aso-4) requires a basalt magma supply rate of 18 to 31 km3/ka. This magma supply rate is comparable to or less than the present-day production rate of degassed magma (73 km3/ka) at an active center of Aso, Nakadake. These findings suggest volcanic gas flux monitoring has the potential to be a 'basalt usage meter' during the dormant period of caldera volcanoes. [ABSTRACT FROM AUTHOR]

Understanding dendritic crystallization is key to obtaining petrological information about rapid crystallization events. Clinopyroxene dendrites from a basaltic rock fulgurite from Nagpur, India, exhibit curved branches with corresponding lattice rotation that exceeds 180° for some branches. This paper combines crystallographic orientation mapping with microstructural observations and compositional information to determine the dendrites' 3-D morphology and their bending mechanism. Dendrites exhibit a network of branches in the (010) plane, following either {001}* (normal to {001} planes, strong lattice curvature) or < 10–1 > (weak lattice curvature). Three or more orders of branches are observed in the (010) plane, alternating between {001}* and < 10–1>. Side branches with weak lattice curvature extend sub-perpendicular to the (010) plane, following either {021}* (sprouting from {001}* branches) or < 12–1 > (from <10–1 > branches) and defining curved 'ribbons' containing their respective central branch. All branches rotate about [010], with a consistent rotation sense regardless of elongation direction in sample or crystal coordinates. Bending must therefore be caused by local asymmetric thermal and compositional fields in the melt, generated by dendritic growth itself, not by sample-scale compositional, thermal or mechanical gradients. The most likely cause of bending is asymmetric distribution of melt supersaturation around branch tips, related to unequal growth rates perpendicular to different facets. Lattice rotation is inferred to occur via preferential incorporation of high densities of [001] (100) edge dislocations of one sign. High inferred dislocation densities imply that the preservation of bent dendrites requires rapid quenching. Higher inferred degree of undercooling (based on microstructural observations) correlates with greater lattice curvature. Bent dendrites can thus potentially be used to deliver information about spatial variations in degree of undercooling and place limits on the history of a sample after dendritic crystallization. Finally, finding lattice rotation exclusively about [010] is a new criterion to identify cryptic dendritic growth stages in euhedral crystals. [ABSTRACT FROM AUTHOR]

This paper presents and discusses the results of phase diagram (Perple_X) and diffusion modelling (CZGM, or Compositional Zoning and its Modification by diffusion) to constrain the P–T path of metamorphism. The approach is based on the best fits between the zoning profile in measured garnet and that obtained by the intersections of garnet isopleths calculated by phase diagram modelling using whole rock bulk composition. The model was applied to garnets in natural rocks of various metamorphic grades, which were formed within different geotectonic environments. To compare the sequence of compositional change during Barrovian-type metamorphism, well-studied pelitic rocks from garnet–staurolite, kyanite–sillimanite, and sillimanite-K-feldspar metamorphic zones were selected. Garnets with two-stepped core and rim profiles that were formed during two different metamorphic stages or events were used for pressure–temperature (P–T) path constraint of each stage or event. For high-grade rocks, in which the original zoning profile in garnet was severely modified, the diffusion of the initial zoning profile was quantified to estimate the timescale of the metamorphic event. These rocks include high- to ultra-high-pressure rocks, which were subjected to thermal overprinting during collisional orogenesis. The results of the application of this approach allow for deciphering the reason why the calculated profile by phase diagram modelling does not fit with that of the measured garnet from low-grade rocks, in which garnet has preserved the original compositional zoning. This includes garnets whose nucleation was shifted from the garnet-in boundary to higher temperatures and pressures, as well as garnet crystallised during different metamorphic stages or events. Finally, the P–T paths in high-grade rocks were constrained after the multicomponent diffusion in garnet was quantified, and this was used for further P–T-time path constraint of metamorphism in the rocks. [ABSTRACT FROM AUTHOR]

The Qilian orogen marks the northern boundary of the Proto-Tethys Ocean and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The metamorphic P–T–t path of the Central Qilian block and the tectonothermal processes that affected it are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of the metamorphic P–T–t path of amphibolites from the Precambrian Huangyuan Group and Maxianshan Group in the Central Qilian block. An amphibole 40Ar/39Ar age of 456.7 ± 5.2 Ma from a garnet amphibolite and a 427.8 ± 3.8 Ma muscovite 40Ar/39Ar age from an orthogneiss in the Huangyuan Group, and amphibole and biotite 40Ar/39Ar ages of 408.7 ± 2.2 Ma and 349.9 ± 2.0 Ma from an amphibolite and a 361.4 ± 2.0 Ma biotite 40Ar/39Ar age from an orthogneiss in the Maxianshan Group record cooling stages after peak metamorphism. Phase equilibrium modeling of all amphibolite samples indicates clockwise P–T paths. A garnet amphibolite from the sillimanite zone in the Huangyuan Group recorded peak metamorphic conditions of 5.6–6.6 kbar and 730–770°C, reflecting granulite-facies metamorphism. Peak P–T conditions for a garnet amphibolite from the garnet zone are constrained to 580–600°C and 6–7 kbar. The amphibolite sample from the Maxianshan Group indicates peak P–T conditions of ~5.2 kbar and ~630°C. The Central Qilian block was affected by both southward subduction of the North Qilian Ocean and northward subduction of the South Qilian Ocean, which resulted in paired metamorphic belts along the northern margin of the Central Qilian block and accretion of the Hualong block to its southern margin. The subducted North Qilian oceanic slab detached and the remainder was exhumed at 453–445 Ma, accompanied by the extension and retrograde metamorphism of the overlying crust of Huangyuan Group at 457–428 Ma. Subsequent cooling of the Maxianshan Group indicates that the thermal regime relaxed during post-collisional extension at c. 409–350 Ma between the Hualong block and the Central Qilian block. [ABSTRACT FROM AUTHOR]

The Greater Caucasus mountains (Cavcasioni) mark the northern margin of the Arabia–Eurasia collision zone. Magmatism in the central part of the Greater Caucasus began in the Pleistocene, up to ~25 Myr after initial collision. This paper presents bulk-rock and Sr–Nd–Pb isotope geochemistry from 39 Quaternary volcanic rock samples (<450 Ka) recovered from the Mt. Kazbek (Kasbegui) region of the Greater Caucasus, Georgia, to assess the sources and magmatic evolution of these lavas and the possible triggers for melting in the context of their regional tectonics. Compositions are dominantly calc-alkaline basaltic andesite to dacite (57–67 wt % SiO2). Although the lavas were erupted through thick continental crust, there is little evidence for extensive modification by crustal contamination. Trace element and isotopic systematics indicate that the lavas have supra-subduction zone signatures, most likely reflecting derivation from a lithospheric source that had been modified by melts and/or fluids from material subducted before and during the collisional event. Mass-balance modelling of the Sr–Nd isotope data indicates that the lavas require significant input from a subducted slab, with deep-sourced fluids fluxing the slab into the source region. In contrast with published data from Lesser Caucasus magmatism, data from the Mt. Kazbek region suggest that a compositionally distinct sediment source resides beneath the Greater Caucasus, producing characteristic trace element and Pb isotopic signatures. Two distinct compositional groups and therefore primary liquids can be discerned from the various volcanic centres, both derived from light rare-earth element enriched sources, but with distinct differences in Th/Yb and Dy/Yb ratios and Pb isotopes. Rare-earth element modelling of the lava sources is consistent with 3–4% melting starting in the garnet peridotite and continuing into the spinel facies or, potentially, sited in the garnet-spinel transition zone. Small-scale convection related to mantle upwelling provides a plausible mechanism for Greater Caucasus magmatism and explains the random aspect to the distribution of magmatism across the Arabia–Eurasia collision zone. [ABSTRACT FROM AUTHOR]

This paper explores the unusual sulphide–graphite association of a selection of Beni Bousera garnet clinopyroxenites that initially equilibrated within the diamond stability field. Compared with common graphite-free garnet pyroxenites analysed so far, these rocks display tenfold S enrichment with concentrations up to 5550 μg g–1. Fe–Ni–Cu sulphides (up to 1·5 wt%) consist of large (up to 3 mm across), low-Ni pyrrrhotite (<0·1 wt% Ni) of troilite composition, along with volumetrically minor chalcopyrite and pentlandite. Such assemblages are interpreted as low-temperature (<100 °C) subsolidus exsolution products from homogeneous monosulphide solid solution. Troilite compositions of the pyrrhotite indicate strongly reducing conditions that are estimated to be slightly above the iron–wüstite (IW) buffer. Bulk-sulphide compositions are closer to the FeS end-member (i.e. Cu- and Ni-depleted) than other sulphide occurrences in mantle-derived pyroxenites described so far. Moreover, troilite contains trace metal microphases (Pb and Ag tellurides, molybdenite) that have never been reported before from mantle-derived garnet pyroxenites but occur in diamond-hosted eclogitic sulphide inclusions. Beni Bousera sulphides also show strong similarities to diamond-hosted sulphide inclusions of eclogitic affinity for a wide range of chalcophile–siderophile trace element contents. In view of the widespread molybdenite exsolution, coupled with Mo and S/Se/Te systematics of sulphide compositions (7872 < S/Se < 19 776; 15 < Se/Te < 31), black-shale pyrite is a potential sedimentary component to contribute to the petrogenesis of Beni Bousera garnet clinopyroxenites. Black shales would have recycled along with cumulates from the oceanic crust in the mantle source of Beni Bousera pyroxenites. Pyrite underwent desulfidation and replacement by troilite during subduction and prograde metamorphism, releasing its fluid-mobile elements (As, Sb, Pb) while suffering minimum S loss because of the strongly reduced conditions. Taken as a whole, our body of data supports a common origin for carbon (−27 ‰ < δ13C < −17 ‰) and sulphur and concomitant formation of diamond and sulphides. Both elements were delivered by an extraneous sedimentary component mixed with the altered oceanic crust rocks that was involved in the genesis of Beni Bousera garnet pyroxenites, prior to a Proterozoic partial melting event. [ABSTRACT FROM AUTHOR]

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P – T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P – T – t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain. [ABSTRACT FROM AUTHOR]

The sodic amphibole glaucophane is generally considered as indicative of blueschist-facies metamorphism. However, sodic amphiboles display a large range in chemical compositions, owing principally to the Fe2+Mg–1 and Fe3+Al–1 substitutions. Therefore, the whole-rock composition (namely its Na2O and FeO* content, and the Fe2+–Fe3+ ratio), strongly controls the stability field of the sodic amphiboles at the transition from greenschist- to blueschist-facies conditions. Neglecting these variables can lead to erroneous estimates of the metamorphic conditions and consequently the tectonic framework of the rocks. This paper explores the mechanisms that control the development of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as a result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphibole and sodic pyroxene form in different rock types: (1) in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic pargasite; (2) metatonalite displays patches of radiating sodic (magnesioriebeckite) and calcic (actinolite) amphiboles; (3) sodic amphibole (magnesioriebeckite–glaucophane) occurs with high-Si potassic white mica (phengitic muscovite) in fine-grained (blue) schists; (4) in mylonitized granitoids (amphibole-gneiss) metasomatized along the contact with ultramafic cumulates, sodic amphibole (magnesioriebeckite–winchite) mainly forms rosettes or sheaves, generally without a shape-preferred orientation. Only locally are the needles aligned parallel to the stretching lineation. Pale green aegirine–augite is dispersed in an albite–quartz matrix or forms layers of fine-grained fibrous aggregates. The bulk-rock chemical composition of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Na- and Fe-rich systems or in a system with high Fe3+/Fe*. Thermodynamic modelling performed for different rock types (taking into account the measured Fe2O3 contents) reveals that sodic amphibole appears at ∼8 ± 1 kbar and 400–450 °C (i.e. at the transition between the greenschist- and blueschist-facies conditions) about 5 kbar lower than previous estimates. To test the robustness of our conclusion, we performed a review of sodic amphibole compositions from a variety of terranes and P – T conditions. This shows (1) systematic variations of composition with P – T conditions and bulk-rock chemistry, and (2) that the amphibole compositions reported from the studied area are consistent with those reported from other greenschist- to blueschist-facies transitions. [ABSTRACT FROM AUTHOR]

Granite genesis and crustal evolution are closely associated with partial melting in the lower or middle crust and extraction of granite magmas to upper crustal levels. This is generally thought to be the leading mechanism by which the upper continental crust became enriched in incompatible components such as the heat-producing elements U and Th through time. However, field evidence from anatectic terrains, the source rocks of granite magmas, raises doubt about the efficiency of this process. Leucosomes and associated leucogranites, representative of melts in such terrains, are often depleted in U, Th and REE compared to their source and therefore unable to enrich the upper crust in these elements. This paper demonstrates using anatectic turbidites exposed on Kangaroo Island that accessory minerals, the main hosts of U, Th and REE, become preferentially concentrated in the melanosomes, effectively removing these elements from the melt. Whole rock geochemistry and detailed petrography suggests that (1) peraluminous melts dissolve only small fractions of monazite and xenotime, because efficient apatite dissolution saturates melt early in phosphorous; and (2) local melt–host reaction emerging from melt migration may cause melt to crystallize in the magma extraction channelways in or close to the magma source region. Crystallization causes oversaturation of the magma triggering crystallization and capture of accessory minerals in the growing biotite-rich selvedge rather than in the melt channel itself. Crystallization of accessory minerals away from the leucosome explains the apparent under-saturation of elements hosted by these accessory minerals in the leucosome and leucogranites. While intense reworking of thick piles of turbidites, common in accretionary orogens, reflect important processes of crustal formation, the fate of accessory phases and the key elements they control, such as the heat producing elements U and Th, are strongly dependent on the interaction between melt and surrounding solids during segregation and extraction. [ABSTRACT FROM AUTHOR]

The order of crystallization of minerals from melt is of prime importance for an understanding of magma fractionation and chemical differentiation from the magma chamber to the planetary scale. Determination of the crystallization sequence based on petrographic observations, however, is often ambiguous; especially in multiply saturated, nearly eutectic felsic melts. This paper presents a novel approach to estimate the order of crystallization of minerals in igneous systems based on a quantitative study of their spatial distributions. Statistical modelling of crystallization demonstrates that later crystallizing mineral phases are generally more clustered. A simple inversion model is then derived to calculate the crystallization sequence and the volume fraction of older minerals present in the system at the onset of crystallization of a later (younger) phase. Application of the model to a sample of equigranular granodiorite (Fichtelgebirge granite batholith, Germany) indicates that plagioclase was the first liquidus phase. It was followed by biotite, K-feldspar, and quartz at 41, 48, and 63 vol. % crystallized, respectively, which is in qualitative agreement with experimental phase equilibria results for moderately hydrous granitic melts. If phase equilibria for a given composition are known or assumed, the crystallization sequence thus constrains the intensive variables (e.g. water content) and their evolution during magma solidification. The model thus provides a novel and independent approach to reconstruct the magma crystallization path that would be inaccessible by other methods. [ABSTRACT FROM AUTHOR]

The article offers the authors' response to the comment of researcher H. P. Zeck towards their discussion on the structural petrology of the Ronda Peridotite in Southwest Spain. They mention the focus of the remarks from Zeck on the inconsistency of their results with other kinematic data from the peridotite outcrops Sierra Bermeja and Sierra Alpujata as well as their interpretation of garnet-spinel mylonites. They also note the reason of avoiding a priori interpretation of their structures.

Some trace elements have the property that, although they are incompatible with most mineral phases in magmatic systems, they are strongly concentrated in certain minor mineral phases. These minor phases, termed here ‘carrier-phases’, and their associated trace elements include platinum group elements in base metal sulphide and chromite; chromium and vanadium in magnetite; uranium group metals in zircon and monazite; and rare earth elements in monazite and xenotime. Carrier-phases may form only a small fraction of a source rock undergoing partial melting and tend to be eliminated from the residue at an intermediate point in the partial melting history; conversely, those same minor carrier-phases tend to precipitate late during fractional crystallization of a liquid produced in the above manner, but may constitute a high proportion of the cumulate then forming. This paper explores the phase equilibria aspects of such processes in a simple system, outlining a nomenclature which is then used in a mathematical treatment applicable to non-modal melting and crystallization processes involving several crystal species. The treatment at this stage assumes constant individual crystal–liquid distribution coefficients. Equations are developed, which are applied in a companion paper to illustrate the behaviour that can be anticipated when carrier-phases play a significant role in trace element location during melting and crystallization. [ABSTRACT FROM PUBLISHER]

There is much debate about whether carbonatite magmas are derived in ‘secondary’ fashion through the advent of liquid immiscibility operating in the crust on evolved nephelinitic magma, or whether they are derived in the mantle by direct partial melting of a carbonated peridotite. This paper briefly summarizes the εSr–εNd data fo carbonatites in general and evaluates the isotopic relationships between carbonatites and alkaline silicate rocks in several well-studied complexes from Africa. Available data for carbonatites younger than 200 Ma have a range in εSr–εNd that is less than that found in oceanic basalts despite the fact that carbonatites traverse lithospheres that ar much more complex than those in the oceans. By contrast, for the Napak, Kerimasi, Shombole, Dorowa, Shawa and Spitskop complexes the alkaline silicate rocks show greater variability and have more enriched εSr–εNd (higher εSr, lower εNd) values than their associated carbonatites. In general, the carbonatites hav isotopic compositions that are closer to the more primitive silicate rocks, such as melilitites a olivine nephelinites, than to more evolved nephelinites and phonolites. In the case of the Napak Complex the enriched component was introduced from the lower crust whereas for the Dorowa and Shawa complexes of SE Zimbabwe, the component was derived from the sub-continental lithospheric mantle. These relationships indicate that the carbonatites must have existed as discrete magmas i themantle and argue against a derivation by liquid immiscibility in the crust. Although a contrast i isotopic composition does not rule out an immiscibility relationship at mantle depths and early in the evolutionary history of a melilititic or nephelinitic magma, there is little experimental support for it. Existing experimental data indicate that immiscibility between carbonate an silicate liquids is favoured at low, crustal, pressures but that immiscibility is unlikely to occur in realistic mantle melts or their derivatives at mantle pressures. Many experimental data exist to show that magnesian carbonatite liquids form as the near-solidus melts of carbonated mantl peridotite at depths in excess of 75 km. We conclude that the calcitic and dolomitic carbonatite magma discussed in this paper are best considered as being derived from primary carbonatite magmas generated in the mantle by partial melting of carbonated peridotite. [ABSTRACT FROM AUTHOR]

Field relations in dissected volcanic terrains and the internal evidence of persistent low-pressure cotectic character in erupted basalts point to the frequent and substantial modification of liquid compositions by some form of partial crystallization within the crust In contrast, the highly compatible trace elements do not display the marked variations and extreme depletions which are predicted to result from perfect fractional crystallization (PFC). Imperfect fractional crystallization, refilling of magma chambers during fractionation and in situ crystallization are important factors which can help to explain this apparent paradox. This paper explores another effect, the integration of residual liquids from differing extents of partial crystallization, which can help to resolve this paradox, even while still permitting perfect fractional crystallization at all points in the magma chamber. Integration of such residual liquids through the thickness of the crystallization zone is explicit, although not implemented, in the model of in situ crystallization proposed by Langmuir ('Nature 340, 199–205, 1989). It may be separated as a process for purposes of mathematical modelling from the basic concept of partial crystallization of small packets of magma with remixing of the residual liquids into the main body of magma. Integration of melts from differing extents of partial crystallization might in principle also be applied to the case of lateral variations in the mass fraction crystallized with position in the magma chamber. Integrated PFC itself can develop residual liquids which differ little from products of equilibrium (batch) crystallization (FTC) at the same average mass fraction of liquid remaining in both incompatible and compatible trace element concentrations. For one specific combination of parameters these integrated liquids are identical in composition at all values of the distribution coefficient to the EPC liquid. At other values of the parameters the integrated liquids may even—a new paradox— have higher relative concentrations of highly compatible elements than the EPC products. Any integration of residual liquids from different mass fractions of PFC rapidly eliminates what have in the past been taken to be the diagnostic differences between PFC and EPC Integration of EPC liquids (towards which the products of imperfect fractional crystallization processes will tend) produces even more pronounced effects, with highly compatible elements less depleted even than in EPC and far less depleted than would be predicted by simple models. When interpreted according to oversimplified models, sequences of residual liquids produced in such processes might appear to be inconsistent with products of a partial crystallization process and to require a process of progressively smaller mass fractions of melting of inhomogeneous and progressively more refractory (higher mg-number) source regions. [ABSTRACT FROM AUTHOR]

The late Miocene and younger mafic back-arc lavas in the southern Puna of the central Andean plateau have been attributed to the aftermath of crustal and mantle lithospheric delamination or foundering. In this paper, we analyze in more detail the nature of the back-arc mafic suite magmas, including the conditions of magma generation in the mantle and of magma evolution during ascent and ponding in the crust, using extensive compositional data for phenocryst minerals and olivine-hosted melt inclusions in combination with published and new whole-rock chemical and isotopic data. We estimate that the primary melts last equilibrated with an enriched mantle source at temperatures near 1375°C and pressures near 2 GPa, which is near the base of the seismically determined ∼60 km thick crust. A mantle source geochemically enriched by continental material introduced through delamination and subducted erosion processes is required to explain the coincidence of the high 87Sr/86Sr ratios (>0·705) and high Sr concentrations (>700 ppm) of the most primitive lavas (e.g. 9–10 wt % MgO, olivine Fo88). The crystallization conditions inferred from mineral–melt equilibria indicate that olivine (T = 1320–1220°C) was followed by clinopyroxene (T = 1230–1140°C). Clinopyroxene–melt equilibration pressures of 0·7 to near 1 GPa in the most mafic samples indicate that the magmas crystallized at mid-crustal depths of 20–35 km, within a region of inferred partial melt accumulation based on the presence of low seismic velocity zones. Olivine-hosted melt inclusions indicate relatively dry melts (maximum 0·5 wt % H2O) with unusual high-Al basaltic compositions, which are attributed to the high-pressure suppression of plagioclase crystallization. A first stage of crustal contamination before mid-crustal accumulation and crystallization of the mafic magmas is suggested by high O-isotope ratios in olivine phenocrysts and negative Eu anomalies in clinopyroxene from the plagioclase-free mafic lavas. Mixing models based on trace elements and radiogenic isotopes suggest assimilation of silicic melt in the lower crust, similar to contemporaneous glassy dacites with steep REE patterns and negative Eu anomalies. A second stage of crustal assimilation at shallower depths is indicated by the mismatch of incompatible elements in clinopyroxene relative to bulk-rock compositions, by strong positive correlations of radiogenic isotopes with wt % SiO2, and by petrographic observation of partly resorbed and reacted quartz xenocrysts. Mixing calculations require the erupted magmas to have assimilated in total some 15–25% crust. [ABSTRACT FROM AUTHOR]

Growth and regrowth textures, trace element and oxygen isotope compositions, and water content or species have been studied in alkali feldspars from the late Archaean Closepet igneous bodies. Feldspar crystals grew from mixed magmas that are characterized by a high degree of homogenization. The 3D depiction of trace element distribution indicates that the crystallization process was followed by interaction with fluids. The magmatic system involved in feldspar formation shows non-linear dynamics. The interaction with fluids is also deterministic, but in contrast to magmatic crystallization, it shows an increasing persistency in element behaviour. The degree of persistency of the element activities in both processes has been calculated using the Hurst exponent. The recrystallization (regrowth) process induced by fluids proceeded along crystal fractures and cleavages, causing selective enrichment in large ion lithophile elements (light rare earth elements), Pb, Y, and in various water species. It did not change the feldspar oxygen isotope signature. In turn, the incorporation of hydrogen species into feldspar domains reduced Al–O−–Al defect densities in the structure, decreasing their luminescence. Water speciation shows persistent behaviour during heating, the process being reversible at least up to 600°C. Carbonate crystals with a mantle isotope signature are associated with the re-equilibrated feldspar domains. The feldspar compositions, the abundance of water species in them and the refractory nature of the residuum after heating, the unchanged oxygen isotopes and the mantle signature of co-precipitated carbonates testify that the observed recrystallization has taken place at temperatures above 600°C with H2O–CO2 fertile, mantle-derived fluids. The paper draws special attention to some methodological aspects of the problem. The multi-method approach used here (major element, trace element and isotope geochemistry, infra-red, cathodoluminescence, 3D depiction of geochemical data and fractal statistics) may help to recognize and separate the various processes throughout the alteration history of the pluton. [ABSTRACT FROM AUTHOR]

Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na2O + K2O) < Al2O3] and peralkaline [(Na2O + K2O) > Al2O3] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa2O, µCaO, µK2O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks. [ABSTRACT FROM AUTHOR]

The mineralogy of the oceanic basalt source region is examined by testing whether a peridotite mineralogy can yield observed whole-rock and olivine compositions from (1) the Hawaiian Islands, our type example of a mantle plume, and (2) the Siqueiros Transform, which provides primitive samples of normal mid-ocean ridge basalt. New olivine compositional data from phase 2 of the Hawaii Scientific Drilling Project (HSDP2) show that higher Ni-in-olivine at the Hawaiian Islands is due to higher temperatures (T) of melt generation and processing (by c. 300°C) related to the Hawaiian mantle plume. DNi is low at high T, so parental Hawaiian basalts are enriched in NiO. When Hawaiian (picritic) parental magmas are transported to shallow depths, olivine precipitation occurs at lower temperatures, where DNi is high, leading to high Ni-in-olivine. Similarly, variations in Mn and Fe/Mn ratios in olivines are explained by contrasts in the temperatures of magma processing. Using the most mafic rocks to delimit Siqueiros and Hawaiian Co and Ni contents in parental magmas and mantle source compositions also shows that both suites can be derived from natural peridotites, but are inconsistent with partial melting of natural pyroxenites. Whole-rock compositions at Hawaii and Siqueiros are also matched by partial melting experiments conducted on peridotite bulk compositions. Hawaiian whole-rocks have elevated FeO contents compared with Siqueiros, which can be explained if Hawaiian parental magmas are generated from peridotite at 4–5 GPa, in contrast to pressures of slightly greater than 1 GPa for melt generation at Siqueiros; these pressures are consistent with olivine thermometry, as described in an earlier paper. SiO2-enriched Koolau compositions are reproduced if high-Fe Hawaiian parental magmas re-equilibrate at 1–1·5 GPa. Peridotite partial melts from experimental studies also reproduce the CaO and Al2O3 contents of Hawaiian (and Siqueiros) whole-rocks. Hawaiian magmas have TiO2 contents, however, that are enriched compared with melts from natural peridotites and magmas derived from the Siqueiros depleted mantle, and consequently may require an enriched source. TiO2 is not the only element that is enriched relative to melts of natural peridotites. Moderately incompatible elements, such as Ti, Zr, Hf, Y, and Eu, and compatible elements, such as Yb and Lu, are all enriched at the Hawaiian Islands. Such enrichments can be explained by adding 5–10% mid-ocean ridge basalt (crust) to depleted mantle; when the major element composition of such a mixture is recast into mineral components, the result is a fertile peridotite mineralogy. [ABSTRACT FROM AUTHOR]

This paper describes in detail an example of a calc-alkaline batholith, located in the Gredos massif (Spanish Central system batholith, Iberian Variscan massif), intruded into pelitic metasediments at shallow crustal levels. The igneous rocks of the study area are divided into three main groups according to their petrographic and geochemical (major, trace and rare earth element) characteristics: calc-alkalic (Qtz diorites and tonalites), transitional (Bt granodiorites), and alkali-calcic series (typically composed of Crd monzogranites). A paradoxical feature of the Crd monzogranites is the presence of euhedral Crd crystals in strong disequilibrium within a non-anatectic monzogranite. The field relations and geochemical data strongly suggest a process of bulk assimilation of the host metasediments by the intruding Bt granodiorites to generate the Crd monzogranites. Geochemical trends defined by the Crd monzogranites clearly depart from cotectic-like evolutions of liquid composition, and point to the composition of the metapelitic host-rocks. Laboratory experiments have been performed to check this hypothesis, and the results show that cordierite forms in a local domain where the reactants of the peritectic reaction Bt + Qtz + Pl + Als → Crd + Kfs + Melt are located. The process includes the isolation of metapelitic xenoliths in the granodiorite magma (a feature observed in the field), partial melting of these xenoliths with generation of cordierite as a peritectic phase in local equilibrium with the melt inside the entrapped xenolith, and the survival of these crystals because the residual melt of the granodiorite magma converges compositionally toward the peraluminous leucogranite generated in the relic xenolith. In turn, the granodiorite gains K released from the xenoliths, inducing the crystallization of K-feldspar and shifting to a Crd monzogranitic composition. Inherited zircons are present in the Crd monzogranites, similar to those found in the host metasediments; these are virtually absent in the intrusive Bt granodiorites. Mass-balance calculations and estimates based on the amount of Crd present have been applied to quantify the extent of assimilation in the petrogenesis of the granitoids; the volume of assimilated material can be high, depending on the proximity to the contacts with the xenoliths and the amount of pelitic material in the metasediments. [ABSTRACT FROM AUTHOR]

This paper gives the results of a set of laboratory experiments designed to analyse the petrological implications of mantle wedge plumes—large buoyant structures predicted by thermomechanical numerical modelling of subduction zones. A particular design of layered capsule was used to simulate the complex multilayer formed by intense flow within the mantle wedge as predicted by numerical models. A basaltic [mid-ocean ridge basalt (MORB)-derived amphibolite] component was sandwiched between two adjacent layers of a sedimentary (Bt-rich metagreywacke) component. Conditions were fixed at temperatures of 1000–1200°C at pressures of 1·5–2·0 GPa. Our results suggest that significant volumes of hybrid, Cordilleran-type granodioritic magmas can be generated by sub-lithospheric partial melting of a mechanically mixed source. Partial melting of the end-members is not buffered, forming granitic (melting of metasediment) and trondhjemitic (melting of MORB) melts in high-variance assemblages Melt + Grt + Pl and Melt + Grt + Cpx, respectively. However, the composition of melts formed from partial melting of metasediment–MORB mélanges is buffered for sediment-to-MORB ratios ranging from 3:1 to 1:3, producing liquids of granodiorite to tonalite composition along a cotectic with the lower-variance phase assemblage Melt + Grt + Cpx + Pl. Our model explains the geochemical and isotopic characteristics of Cordilleran batholiths. In particular, it accounts for the observed decoupling between major element and isotopic compositions. Large variations in isotopic ratios can be inherited from a compositionally heterogeneous source; however, major element compositions are more strongly dependent on the temperature of melting rather than on the composition of the source. [ABSTRACT FROM PUBLISHER]

In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO + MgO) ratio or Fe-index, the modified alkali–lime index, the aluminum-saturation index, the alkalinity index, and the feldspathoid silica-saturation index. The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have not. The transition from tholeiite to ferrobasalt allows us to extend this boundary to silica values as low as 48 wt %. We introduce the feldspathoid silica-saturation index, which, coupled with the alkalinity index, allows us to extend the geochemical classification to alkaline rocks. We show that most alkaline rocks are ferroan and that this probably reflects extensive fractional crystallization of olivine and pyroxene with minimal participation of Fe–Ti oxides. The expanded classification allows us to illustrate the geochemical and petrogenetic relationship of the plutonic rocks from ferroan granites to nepheline syenites that commonly occur in intracratonic environments. It also allows us to distinguish four families of feldspathic rocks: (1) magnesian rocks, which are exemplified by Caledonian and Cordilleran batholiths and are characterized by differentiation under oxidizing and relatively hydrous conditions; (2) ferroan rocks, which include fayalite granites, alkali granites, and nepheline syenites and are characterized by differentiation under reducing and relatively dry conditions; (3) leucogranites, which commonly form by crustal melting; (4) potassic and ultrapotassic rocks, which originate from mantle that has been enriched in K2O. [ABSTRACT FROM PUBLISHER]

This paper presents field, geochemical and isotopic (Sr, Nd, Pb) results on basalts from the Antipodes, Campbell and Chatham Islands, New Zealand. New 40Ar/39Ar age determinations along with previous K–Ar dates reveal three major episodes of volcanic activity on Chatham Island (85–82, 41–35, ∼5 Ma). Chatham and Antipodes samples comprise basanite, alkali and transitional basalts that have HIMU-like isotopic (206Pb/204Pb >20·3–20·8, 87Sr/86Sr <0·7033, 143Nd/144Nd >0·5128) and trace element affinities (Ce/Pb 28–36, Nb/U 34–66, Ba/Nb 4–7). The geochemistry of transitional to Q-normative samples from Campbell Island is explained by interaction with continental crust. The volcanism is part of a long-lived (∼100 Myr), low-volume, diffuse alkaline magmatic province that includes deposits on the North and South Islands of New Zealand as well as portions of West Antarctica and SE Australia. All of these continental areas were juxtaposed on the eastern margin of Gondwanaland at >83 Ma. A ubiquitous feature of mafic alkaline rocks from this region is their depletion in K and Pb relative to other highly incompatible elements when normalized to primitive mantle values. The inversion of trace element data indicates enriched mantle sources that contain variable proportions of hydrous minerals. We propose that the mantle sources represent continental lithosphere that host amphibole/phlogopite-rich veins formed by plume- and/or subduction-related metasomatism between 500 and 100 Ma. The strong HIMU signature (206Pb/204Pb >20·5) is considered to be an in-grown feature generated by partial dehydration and loss of hydrophile elements (Pb, Rb, K) relative to more magmaphile elements (Th, U, Sr) during short-term storage at the base of the lithosphere. [ABSTRACT FROM AUTHOR]

Melt inclusions are small portions of liquid trapped by growing crystals during magma evolution. Recent studies of melt inclusions have revealed a large range of unusual major and trace element compositions in phenocrysts from primitive mantle-derived magmatic rocks [e.g. in high-Fo olivine (Fo > ∼85 mol %), spinel, high-An plagioclase]. Inclusions in phenocrysts crystallized from more evolved magmas (e.g. olivine Fo < ∼85 mol %), are usually compositionally similar to the host lavas. This paper reviews the chemistry of melt inclusions in high-Fo olivine phenocrysts focusing on those with anomalous major and trace element contents from mid-ocean ridge and subduction-related basalts. We suggest that a significant portion of the anomalous inclusion compositions reflects localized, grain-scale dissolution–reaction–mixing (DRM) processes within the magmatic plumbing system. The DRM processes occur at the margins of primitive magma bodies, where magma is in contact with cooler wall rocks and/or pre-existing semi-solidified crystal mush zones (depending on the specific environment). Injection of hotter, more primitive magma causes partial dissolution (incongruent melting) of the mush-zone phases, which are not in equilibrium with the primitive melt, and mixing of the reaction products with the primitive magma. Localized rapid crystallization of high-Fo olivines from the primitive magma may lead to entrapment of numerous large melt inclusions, which record the DRM processes in progress. In some magmatic suites melt inclusions in primitive phenocrysts may be naturally biased towards the anomalous compositions. The occurrence of melt inclusions with unusual compositions does not necessarily imply the existence of new geologically significant magma types and/or melt-generation processes, and caution should be exercised in their interpretation. [ABSTRACT FROM AUTHOR]

This paper presents the first comprehensive major and trace element data for ∼130 abyssal peridotite samples from the Pacific and Indian ocean ridge–transform systems. The data reveal important features about the petrogenesis of these rocks, mantle melting and melt extraction processes beneath ocean ridges, and elemental behaviours. Although abyssal peridotites are serpentinized, and have also experienced seafloor weathering, magmatic signatures remain well preserved in the bulk-rock compositions. The better inverse correlation of MgO with progressively heavier rare earth elements (REE) reflects varying amounts of melt depletion. This melt depletion may result from recent sub-ridge mantle melting, but could also be inherited from previous melt extraction events from the fertile mantle source. Light REE (LREE) in bulk-rock samples are more enriched, not more depleted, than in the constituent clinopyroxenes (cpx) of the same sample suites. If the cpx LREE record sub-ridge mantle melting processes, then the bulk-rock LREE must reflect post-melting refertilization. The significant correlations of LREE (e.g. La, Ce, Pr, Nd) with immobile high field strength elements (HFSE, e.g. Nb and Zr) suggest that enrichments of both LREE and HFSE resulted from a common magmatic process. The refertilization takes place in the ‘cold’ thermal boundary layer (TBL) beneath ridges through which the ascending melts migrate and interact with the advanced residues. The refertilization apparently did not affect the cpx relics analyzed for trace elements. This observation suggests grain-boundary porous melt migration in the TBL. The ascending melts may not be thermally ‘reactive’, and thus may have affected only cpx rims, which, together with precipitated olivine, entrapped melt, and the rest of the rock, were subsequently serpentinized. Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios are observed, which are unexpected. The correlation between the two ratios is consistent with observations on basalts that DZr/DHf < 1 and DNb/DTa < 1. Given the identical charges (5+ for Nb and Ta; 4+ for Zr and Hf) and essentially the same ionic radii (RNb/RTa = 1·000 and RZr/RHf = 1·006–1·026), yet a factor of ∼2 mass differences (MZr/MHf = 0·511 and MNb/MTa = 0·513), it is hypothesized that mass-dependent D values, or diffusion or mass-transfer rates may be important in causing elemental fractionations during porous melt migration in the TBL. It is also possible that some ‘exotic’ phases with highly fractionated Zr/Hf and Nb/Ta ratios may exist in these rocks, thus having ‘nugget’ effects on the bulk-rock analyses. All these hypotheses need testing by constraining the storage and distribution of all the incompatible trace elements in mantle peridotite. As serpentine contains up to 13 wt % H2O, and is stable up to 7 GPa before it is transformed to dense hydrous magnesium silicate phases that are stable at pressures of ∼5–50 GPa, it is possible that the serpentinized peridotites may survive, at least partly, subduction-zone dehydration, and transport large amounts of H2O (also Ba, Rb, Cs, K, U, Sr, Pb, etc. with elevated U/Pb ratios) into the deep mantle. The latter may contribute to the HIMU component in the source regions of some oceanic basalts. [ABSTRACT FROM AUTHOR]

The largest accumulations of rhyolitic melt in the upper crust occur in voluminous silicic crystal mushes, which sometimes erupt as unzoned, crystal-rich ignimbrites, but are most frequently preserved as granodioritic batholiths. After approximately 40–50% crystallization, magmas of intermediate composition (andesite–dacite) typically contain high-SiO2 interstitial melt, similar to crystal-poor rhyolites commonly erupted in mature arc and continental settings. This paper analyzes the feasibility of system-wide extraction of this melt from the mush, a mechanism that can rationalize a number of observations in both the plutonic and volcanic record, such as: (1) abrupt compositional gaps in ignimbrites; (2) the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths; (3) the observed range of ages (up to 200–300 ka) recorded by zircons in silicic magmas; (4) extensive zones of low P-wave velocity in the shallow crust under active silicic calderas. We argue that crystal–melt segregation occurs by a combination of several processes (hindered settling, micro-settling, compaction) once convection is hampered as the rheological locking point of the crystal–melt mixture (≥50 vol. % crystals) is attained. We constrain segregation rates by using hindered settling velocities and compaction rates as end-members. Time scales estimated for the formation of >500 km3 of crystal-poor rhyolite range from 104 to 105 years, within the estimated residence times of mushes in the upper crust (>105 years, largely based on U/Th and U/Pb dating). This model provides an integrated picture of silicic magmatism, linking the evolution of plutonic and volcanic systems until storage in the upper crust, where granitoids become the leftovers from rhyolitic eruptions. [ABSTRACT FROM AUTHOR]

Following previous publication of majorminor element data, this paper presents rare earth element (REE) data for heterogeneous (chemically zoned) garnets belonging to the peridotite suite of mantle xenoliths from the Jagersfontein kimberlite pipe, South Africa. The rim compositions of the garnets in the highest temperaturepressure (deepest) deformed peridotites show a typical megacryst-like pattern, of very low light REE (LREE) increasing through the middle REE (MREE) to a plateau of heavy REE (HREE) at c. 20 times chondrite; these compositions would be in equilibrium with small-volume melts of the mid-ocean ridge basalt (MORB) source (asthenosphere). With decreasing depth the garnet rims show increasing LREE and decreasing HREE, eventually resulting in humped relative abundance patterns. A set of compositions is calculated for melts that would be in equilibrium with the garnet rims at different depths. These show decreasing relative abundance of each REE from La to Lu, and the La/Lu ratio of the melts increases with decreasing depth of formation. Modelling of the effects of crystal fractionation shows that this process could largely generate the sequence of garnet rim and melt compositions found with decreasing depth, including the humped REE patterns in high-level garnets. Considering the behaviour of majorminor elements as well as REE, a process of percolative fractional crystallization is advocated in which megacryst source melts percolate upwards through peridotites and undergo fractionation in conjunction with exchange with the peridotite minerals. The initial megacryst melt probably includes melt of lithospheric origin as well as melt from the MORB source, and it is suggested that the process of percolative fractional crystallization may form a variety of metasomatic and kimberlitic melts from initial megacryst melts. Repeated metasomatism of the lower lithosphere by such differentiating melts is suggested by consideration of garnet core compositions. Such metasomatism would progressively convert harzburgites to lherzolites by increasing their CaO content, and this may account for the fact that the Cr-rich diamondgarnet harzburgite paragenesis is commonly preserved only where it has been encapsulated in diamonds. [ABSTRACT FROM AUTHOR]

Two hundred years of magmatic history are documented by the lavas and tephra sampled from Kīlauea's historical summit eruptions. This paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near Kīlauea's summit caldera since the 17th century. Our results elucidate the range of magmatic processes that operate within the volcano's summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano. Prior to the 1924 summit crater collapse, a trend of increasing incompatible element and CaO and decreasing SiO2 abundances (at a constant MgO) prevailed. Thereafter, the trend reversed direction and has persisted for the rest of the 20th century, including during the current Pu`u `Ō`ō eruption. The rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single, relatively small body rather than a plexus of dikes and sills. Olivine fractionation is the dominant petrologic process within this reservoir. Petrographic observations and olivine and whole-rock geochemical data suggest that the summit reservoir has a crown of aphyric, more evolved, low-density magma. Differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in Kīlauea summit lavas. The effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new, mantle-derived magmas with more fractionated reservoir magma. Frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition. These compositional variations correlate with magma supply rate; both are thought to be influenced by the degree of melting of small-scale source heterogeneities within the Hawaiian plume. However, Kīlauea's source compositions and partial-melting processes have varied only within a narrow range over the past 350 kyr. [ABSTRACT FROM AUTHOR]

The lower-crustal section exposed in the Serre, southern Italy, consists mainly of Al-rich metasediments, which underwent granulite-facies metamorphism, partial melting and melt extraction. The paper considers the formation of melts in metapelites and metagreywackes. Leucosomes and host rocks have been studied to investigate the melting process. Biotite-rich and biotite-free melanosomes with scarce felsic components are present; the biotite-rich types are widespread in the upper part of the section and the two types may occur side by side in the lower part. Na-rich and K-rich leucosomes including residual phases are interspersed within the metasediments; on the whole they do not show geochemical signatures suggestive of magmatic fractionation. Leucotonalitic types prevail among the sampled leucosomes, which generally are rare earth element (REE) depleted with positive Eu anomalies whereas the host rocks are REE enriched with overall negative Eu anomalies. Melanosomes and migmatites show restitic chemistries. The precursor metagreywackes underwent depletion in Na2O and enrichment in K2O. The precursor metapelites document general depletion in Na2O and they may be enriched or depleted in K2O. All the characteristics of the migmatites and of their components reflect a two-stage melting: (1) H2O-present melting, involving mainly plagioclase, and (2) dehydration melting of micas. All the metasediments underwent H2O-present melting, forming mostly sodic melts which, owing to their removal from the source as fast as they formed, did not accumulate in such proportions as to allow migration and mostly remained within the lower-crustal metasediments; metapelites also underwent variable dehydration melting, depending on chemical features and physical conditions, forming larger volumes of mobile granitic melts, most of which migrated far from the source. Extractions of 57–66 vol. % of total melts (sodic + potassic) from the most residual metapelitic melanosomes and of about 27–44 vol. % of potassic melts from metapelitic migmatites have been calculated. Higher volumes of the extracted melts have been calculated for the metapelites of the lower part of the section; the most depleted metagreywackes underwent melt extraction of about 9–13 vol. %. The two-stage melting occurred during the prograde metamorphism and continued during the isothermal decompression. [ABSTRACT FROM PUBLISHER]

A key element in achieving Ocean Drilling Program Leg 183 science objectives is determining the age of volcanism at different locations across the Kerguelen Plateau and Broken Ridge. This paper reports crystallization ages derived from 40Ar–39Ar incremental heating experiments, for whole rocks and feldspars separated from basement units recovered at Sites 1136, 1137, 1138, 1139, 1140, 1141 and 1142. The subaerial environment of eruption at most sites and the generally evolved, high K content of these lava flows contributed to precise and reproducible age determinations. Volcanic activity at southern Kerguelen Plateau Site 1136 occurred at 118–119 Ma; at Elan Bank Site 1137, 107–108 Ma; at central Kerguelen Plateau Site 1138, 100–101 Ma; at Skiff Bank Site 1139, 68–69 Ma; at northern Kerguelen Plateau Site 1140, 34–35 Ma; and at Broken Ridge Sites 1141 and 1142, 94–95 Ma. The new ages allow calculation of melt production rates through the ∼120 Myr history of the Kerguelen plume, adjustments to plate reconstructions for the eastern Indian Ocean region in the hotspot reference frame, and assessment of proposed links between large igneous province (LIP) magmatism and environmental crises. [ABSTRACT FROM PUBLISHER]

The article discusses various papers published within the issue including one about the ages of Ocean Drilling Program (ODP) Leg 183 igneous basement drill sites, and another about the eruption of the Rajmahal flood basalt.

This paper presents new geochemical data from Kistufell (64°48′N, 17°13′W), a monogenetic table mountain situated directly above the inferred locus of the Iceland mantle plume. Kistufell is composed of the most primitive olivine tholeiitic glasses found in central Iceland (MgO 10·56 wt %, olivine Fo89·7). The glasses are interpreted as near-primary, high-degree plume melts derived from a heterogeneous mantle source. Mineral, glass and bulk-rock (glass + minerals) chemistry indicates a low average melting pressure (15 kbar), high initial crystallization pressures and temperatures (10–15 kbar and 1270°C), and eruption temperatures (1240°C) that are among the highest observed in Iceland. The glasses have trace element signatures (Lan/Ybn <1, Ban/Zrn 0·55–0·58) indicative of a trace element depleted source, and the Sr–Nd–Pb isotopic ratios (87Sr/86Sr 0·70304–0·70308, 143Nd/144Nd 0·513058–0·513099, 206Pb/204Pb 18·343–18·361) further suggest a long-term trace element depletion relative to primordial mantle. High He isotopic ratios (15·3–16·8 R/Ra) combined with low 207Pb/204Pb (15·42–15·43) suggest that the mantle source of the magma is different from that of North Atlantic mid-ocean ridge basalt. Negative Pb anomalies, and positive Nb and Ta anomalies indicate that the source includes a recycled, subducted oceanic crustal or mantle component. Positive Sr anomalies (Srn/Ndn = 1·39–1·50) further suggest that this recycled source component involves lower oceanic crustal gabbros. The δ18O values (4·2–4·7‰), which are lower than those observed in mantle peridotites but similar to those observed in ophiolites and in situ oceanic gabbros, are consistent with this interpretation. The elevated 3He/4He ratios are primarily attributed to a primitive, relatively undegassed component in the Iceland mantle plume, which dominates the He isotope signature as a result of long-term depletion of U, Th and He in the recycled gabbroic component. [ABSTRACT FROM PUBLISHER]

The granulites of the Saxon Granulite Massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near-isothermal decompression. This raises the question of whether P–T–t data on the peak metamorphism may still be retrieved with confidence. Felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate P–T estimates with isotopic ages presented in a companion paper. The assemblage garnet + clinopyroxene in mafic granulite records peak temperatures of 1010–1060°C, consistent with minimum estimates of around 967°C and 22·3 kbar obtained from the assemblage garnet + kyanite + ternary feldspar + quartz in felsic granulite. Multiple partial overprint of these assemblages reflects a clockwise P–T evolution. Garnet and kyanite in the felsic granulite were successively overgrown by plagioclase, spinel + plagioclase, sapphirine + plagioclase, and biotite + plagioclase. Most of this overprinting occurred within the stability field of sillimanite. Garnet + clinopyroxene in the mafic granulite were replaced by clinopyroxene + amphibole + plagioclase + magnetite. The high P–T conditions and the absence of thermal relaxation features in these granulites require a short-lived metamorphism with rapid exhumation. The ages of peak metamorphism (342 Ma) and shallow-level granitoid intrusions (333 Ma) constrain the time span for the exhumation of the Saxon granulites to ∼9 my. [ABSTRACT FROM PUBLISHER]