Primary Basaltic Magmas Research Articles

Why are oceanic arc basalts Ca-rich and Ni-poor? Insights from olivine-hosted melt inclusions from Kibblewhite Volcano in the Kermadec arc

Ankaramites, which are clinopyroxene-rich basalts with primitive whole-rock compositions (Mg# >65), are common in oceanic arcs and are characterized by high whole-rock CaO/Al2O3 (>1.0) ratios and olivine crystals with anomalously low nickel contents (<0.2 wt% NiO). These geochemical characteristics cannot be explained by the melting of ordinary mantle peridotite. However, their origin is critical for understanding the formation of primary magmas in oceanic arcs. Here, we investigated olivine-hosted melt inclusions (MIs) from ankaramites and magnesian andesites of the Kibblewhite Volcano in the Kermadec arc. The MIs from the ankaramites have similar major and trace element characteristics to the host rocks, indicating that the ankaramites did not result from an accumulation of mafic minerals but rather represent the primary magma in the Kibblewhite Volcano. The MIs from the magnesian andesites were hosted in forsteritic olivine xenocrysts with a wide range of NiO contents (Fo90–92; 0.13–0.39 wt% NiO) and have similar major element compositions to the ankaramites but exhibit a wide range of CaO/Al2O3 (0.85–1.54). The trace element characteristics of the MIs from the magnesian andesites do not match those of the host rocks, indicating that they are not primary melts of the magnesian andesites but primitive basaltic melts generated before the magnesian andesites formed.Interestingly, the CaO/Al2O3 ratio of MIs from the magnesian andesites was negatively correlated with the NiO content of their host olivines. This correlation suggests that the composition of the primary basaltic magmas of the Kibblewhite Volcano changed continuously from peridotite-derived to ankaramitic. This correlation could not be explained by grain-scale process, crustal anatexis, or contribution of slab-derived carbonate-rich fluids. Instead, we propose that this correlation can be explained by the interaction of the ascending primary basaltic melts with the lithospheric mantle. During melt-mantle interaction, the assimilation of clinopyroxene and fractionation of olivine and orthopyroxene caused the CaO/Al2O3 ratio to increase in the melt and the Ni content to decrease. Furthermore, because the magnesian andesites have low CaO/Al2O3 ratios and could be derived from a clinopyroxene-poor mantle lithology, the interaction between the melt and mantle may also be closely related to the origin of the magnesian andesites at Kibblewhite Volcano. This interpretation provides a new perspective on the origin of the oceanic arc ankaramites and why primary andesitic and basaltic magmas coexist in the Kibblewhite Volcano.

The Meaning of Pressure for Primary Magmas: New Insights From PRIMELT3‐P

AbstractThis paper reports new software, PRIMELT3‐P, for computing the pressure and temperature range over which a primary basaltic magma was formed by adiabatic decompression and fractional melting of fertile mantle peridotite. The underlying model was developed to explore the meaning of magmatic pressure and its connection to the physics of mantle melting, melt extraction, and melt migration. We present a comparison of the results of the batch melting model FractionatePT3 (Lee et al., 2009, https://doi.org/10.1016/j.epsl.2008.12.020) and the new fractional melting model PRIMELT3‐P, each applied to basalts from various localities, with a particular focus on Hawai'i and Iceland. The unexpected result is that pressures for batch melting and fractional melting are quite similar across a broad range of potential temperatures, agreeing to within ±0.29 GPa (1σ) when applied to magmas that pass tests for derivation from peridotite sources. Although samples from both Hawai'i and Iceland yield a range of final pressures of melting, the minimum final pressure in each location coincides with independent geophysical constraints—the depth to the lithosphere‐asthenosphere boundary beneath Hawai'i and to the Moho below Iceland. These results clarify the meaning of primary magma pressure, but the concept remains complex. We suggest that computed temperature and pressure arrays arising from heterogeneous peridotite‐source primary magmas may reflect melt transport and mixing in a thermally zoned mantle plume with a complex network of branching melt channels that nucleate over a range of depths.

ВСЕЛЕНСКИХ КАТАСТРОФ НА ЗЕМЛЕ НЕ БЫЛО И НЕ БУДЕТ

Based on new ideas on the deep magma formation, according to which t here is only one primary basalt magma, the article interprets the causes of catastrophic phenomena (volcanism, plutonism, earthquakes, tsunamis, glaciation, warming, etc.) exclusively in connection with mantle processes. External man-made causes practically make no matter. And endogenous processes proceed slowly, with long interruptions, locally, discretely. Therefore, there have not been universal catastrophes and there will not be. There is no reason for the climate globe-spanning panic.

Integrating mineralogy, geochemistry and aeromagnetic data for detecting Fe–Ti ore deposits bearing layered mafic intrusion, Akab El-Negum, Eastern Desert, Egypt

This study delineated the Fe–Ti oxide deposit concurrencies on the layered mafic intrusion in Gabal Akab El-Negum (GAN), South Eastern Desert, Egypt, using aeromagnetic mapping and chemical analysis of the hosted mafic rocks and mineralogical studies. Aeromagnetic data was improved using the enhanced horizontal gradient amplitudeto detect the primary structures (edges/contacts/faults) that control the distribution of Fe–Ti ore deposit. GAN layered gabbros are differentiated into troctolite, olivine–, pyroxene–, and hornblende–gabbros. These mafic rocks primarily comprise plagioclase, olivine, pyroxene, and hornblende with Fe–Ti ores (magnetite and ilmenite). The significant variation in Mg# of clinopyroxene between 0.70 and 0.82 indicates the importance of fractional crystallization in developing layered mafic intrusion. Clinopyroxene and plagioclase thermometry yielded low temperatures similar to the fractionated primary basaltic magma. The pairs of magnetite–ilmenite minerals in gabbros provide equilibrium temperatures of 539.44–815.56, and high fO2, reflecting various cooling and subsolidus reequilibration phases of minerals. The enrichment of GAN gabbros in light rare–earth elements relative to heavy rare–earth elements indicates the interaction between the Fe–Ti rich mantle and the fractionated tholeiitic magmas in the back-arc setting, generating Fe–Ti oxide ores.

Na/K Diversity of Primary Basaltic Magmas Induced by the Separation of Slab-derived Supercritical Liquid: Implications from Alkali Basaltic Lavas from Rishiri Volcano, Southern Kuril Arc

Abstract Primary basaltic magmas in subduction zones exhibit chemical diversity even in individual volcanoes. In this study, we aim to elucidate the dominant factors that determine the chemical composition, especially the Na/K, of primary basaltic magmas by conducting a comparative petrological study of the high-Na/K Horyosawa lava and low-Na/K Nozuka lava from Rishiri Volcano in the southern Kuril Arc. Both lava flows are basaltic and are suitable for estimating the magma generation conditions. The whole-rock SiO2 content of the high-Na/K Horyosawa lava, the main target of this study, ranges from 51·9 to 53·4 wt%. The petrological and geochemical features of this lava indicate that the effects of magma mixing and assimilation of crustal materials were limited, and the low-SiO2 magmas were essentially derived via olivine fractionation from the primary magma. The primary magma is estimated to have been generated by ~2·4 % partial melting of the source mantle with ~0·10 wt% water at ~1300 °C and ~2·4 GPa. By comparing the results of the high-Na/K Horyosawa lavas with those published for the low-Na/K basalts (Nozuka lava), we find that (1) the slab-derived fluids involved in their formation were supercritical liquids with similar compositions, including Na/K, when released from the subducting slab, and (2) the Horyosawa primary magma was generated at a shallower level in the mantle than the Nozuka primary magma. From these observations, it is inferred that the primary magma of the Horyosawa lava was formed by the influx melting of aqueous fluid separated from the supercritical liquid at depths shallower than the critical point, whereas the Nozuka lava was generated by supercritical liquid-fluxed melting at depths deeper than the critical point. These findings show that elemental partitioning during the separation of slab-derived supercritical liquid into aqueous fluid and hydrous melt can induce chemical diversity within the primary basaltic magmas of a single volcanic system.

«РУДНАЯ МАГМА». ЧТО ЭТО С ПОЗИЦИИ ГРАНИТНОЙ РУДНО-МАГМАТИЧЕСКОЙ СИСТЕМЫ?

The concept of “ore magma” remains an obscure hypothesis in ore formation. The article considers the process of natural overgrowth of trivial primary basaltic magma into an ore-bearing granite melting and further into the ore-forming “ore magma” as the concentration of volatile and ore components. The dark side of the problem lies in the fact that during the ore formation the “ore magma” liquates into contrasting phases and leaves practically no traces of itself (with rare exceptions). But the concept of the ore magma has received a logical scientific justification from the standpoint of ore-magmatic systems.

The 40Ar–39Ar dating and geochemistry of the Carpathian C1 obsidians (Zemplín, Slovakia)

In situ 40Ar–39Ar UV laser ablation dating of the Carpathian C1 obsidians from the Slovakian part of the Zemplin –Tokaj area yielded new 40Ar–39Ar obsidian glass ages that fall in a narrow time interval of 12.07 ± 0.37 to 11.44 ± 0.39 Ma. This indicates that most of the Zemplin obsidian findings come from one short-time monogenic volcano, forming part of a long-lasting volcanism over the 15–10 Ma period. Chemical compositions of the Carpathian C1 obsidians clearly indicate common similarities between all examined localities (Brehov, Cejkov, Hraň, and Vinicky). Geochemically, these obsidians belong to the silica-rich, peraluminous, high-potassium, calc-alkaline rhyolite series volcanic rocks of ferroan character. They were derived by multi-stage magmatic processes, from mixed mantle and crustal sources, and generated during subduction in a volcanic arc tectonic setting. The primary basaltic magma formed from the melting of the lower crustal source at the mantle/crust boundary. Subsequent formation of melt reservoirs in the middle and upper crust, accompanied by secondary melting of the surrounding rocks with continual addition of ascending melt, and repeated processes of assimilation and fractionation produced rhyolitic rocks with obsidians in the Zemplinske vrchy Mts.

Extremely low-Ni olivine as a tracer of carbonate-rich sediment recycling beneath the southwestern Okinawa Trough

ABSTRACT Carbonate recycling in subduction zones has a strong influence on arc magmatism and carbon reservoirs in the mantle. However, geochemical evidence of carbonate recycling during subduction, especially at sub-arc depths, is rare and the related process is poorly understood. In this study, we document high-Mg olivines (81–91 mol.% forsterite) with extremely low Ni concentrations (mostly <600 ppm), which occur in host dacites recovered from the southwestern Okinawa Trough (SWOT). The temperature–pressure of crystallization had a negligible effect on the Ni contents of the olivines, and reactions between primary basaltic magmas and the lithospheric mantle were also insignificant. Small sulphide droplets are present in the olivines, which suggest that sulphide–silicate melt immiscibility occurred during the early stages of magmatic evolution. Although early segregation of sulphide melts could reduce the Ni contents of the primitive magmas, our forward modelling suggests that sulphide segregation cannot solely explain the low Ni contents. Therefore, the most likely explanation for low Ni concentrations is the interaction between Ca-rich carbonate melts and the mantle wedge, forming an olivine-rich mantle source by consumption of orthopyroxene, as indicated by the extremely low Ni/Mg and high Ca/Fe ratios of magmatic olivines. Given that the SWOT is located just ~100 km above the Wadati–Benioff zone, our results provide rare evidence for carbonate recycling at sub-arc depths in an active subduction zone.

Late Eocene post-collisional magmatic rocks from the southern Qiangtang terrane record the melting of pre-collisional enriched lithospheric mantle

Abstract Diverse rock types and contrasting geochemical compositions of post-collisional mafic rocks across the Tibetan Plateau indicate that the underlying enriched lithospheric mantle is heterogeneous; however, how these enriched mantle sources were formed is still debated. The accreted terranes within the Tibetan Plateau experienced multiple stages of evolution. To track the geochemical characteristics of their associated lithospheric mantle through time, we can use mantle-derived magmas to constrain the mechanism of mantle enrichment. We report zircon U-Pb ages, major and trace element contents, and Sr-Nd isotopic compositions for Early Cretaceous and late Eocene mafic rocks in the southern Qiangtang terrane. The Early Cretaceous Baishagang basalts (107.3 Ma) are characterized by low K2O/Na2O (&amp;lt;1.0) ratios, arc-like trace element patterns, and uniform Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7067–0.7073, εNd(t) = −0.4 to −0.2]. We suggest that the Baishagang basalts were derived from partial melting of enriched lithospheric mantle that was metasomatized by subducted Bangong–Nujiang oceanic material. We establish the geochemistry of the pre-collisional enriched lithospheric mantle under the southern Qiangtang terrane by combining our data with those from other Early Cretaceous mafic rocks in the region. The late Eocene (ca. 35 Ma) post-collisional rocks in the southern Qiangtang terrane have low K2O/Na2O (&amp;lt;1.0) ratios, and their major element, trace element, and Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7042–0.7072, εNd(t) = −4.5 to +1.5] are similar to those of the Early Cretaceous mafic rocks. Based on the distribution, melting depths, and whole-rock geochemical compositions of the Early Cretaceous and late Eocene mafic rocks, we argue that the primitive late Eocene post-collisional rocks were derived from pre-collisional enriched lithospheric mantle, and the evolved samples were produced by assimilation and fractional crystallization of primary basaltic magma. Asthenosphere upwelling in response to the removal of lithospheric mantle induced the partial melting of enriched lithospheric mantle at ca. 35 Ma.

A tale of five enclaves: Mineral perspectives on origins of mafic enclaves in the Tuolumne Intrusive Complex

AbstractThe widespread occurrence of mafic magmatic enclaves (mme) in arc volcanic rocks attests to hybridization of mafic-intermediate magmas with felsic ones. Typically, mme and their hosts differ in mineral assemblage and the compositions of phenocrysts and matrix glass. In contrast, in many arc plutons, the mineral assemblages in mme are the same as in their host granitic rocks, and major-element mineral compositions are similar or identical. These similarities lead to difficulties in identifying mixing end members except through the use of bulk-rock compositions, which themselves may reflect various degrees of hybridization and potentially melt loss. This work describes the variety of enclave types and occurrences in the equigranular Half Dome unit (eHD) of the Tuolumne Intrusive Complex and then focuses on textural and mineral composition data on five porphyritic mme from the eHD. Specifically, major- and trace-element compositions and zoning patterns of plagioclase and hornblende were measured in the mme and their adjacent host granitic rocks. In each case, the majority of plagioclase phenocrysts in the mme (i.e., large crystals) were derived from a rhyolitic end member. The trace-element compositions and zoning patterns in these plagioclase phenocrysts indicate that each mme formed by hybridization with a distinct rhyolitic magma. In some cases, hybridization involved a single mixing event, whereas in others, evidence for at least two mixing events is preserved. In contrast, some hornblende phenocrysts grew from the enclave magma, and others were derived from the rhyolitic end member. Moreover, the composition of hornblende in the immediately adjacent host rock is distinct from hornblende typically observed in the eHD. Although primary basaltic magmas are thought to be parental to the mme, little or no evidence of such parents is preserved in the enclaves. Instead, the data indicate that hybridization of already hybrid andesitic enclave magmas with rhyolitic magmas in the eHD involved multiple andesitic and rhyolitic end members, which in turn is consistent with the eHD representing an amalgamation of numerous, compositionally distinct magma reservoirs. This conclusion applies to enclaves sampled &amp;lt;30 m from one another. Moreover, during amalgamation of various rhyolitic reservoirs, some mme were evidently disrupted from a surrounding mush and thus carried remnants of that mush as their immediately adjacent host. We suggest that detailed study of mineral compositions and zoning in plutonic mme provides a means to identify magmatic processes that cannot be deciphered from bulk-rock analysis.

Geochemistry and Sr–Nd–Pb isotope characteristics of Miocene basalt–trachyte rock association in transitional zone between the Outer Western Carpathians and Bohemian Massif

Representative samples covering all petrographic types of the Miocene subvolcanic (dykes and sills) basalt-trachyte rock association from the Uherský Brod area (UB, Moravia) in the Czech Republic were studied. The UB area is associated with the Klippen Zone in the neighbourhood of the contact of the Carpathian-Pannonian Block with the Bohemian Massif. The petrographic and geochemical features of the subvolcanic rocks from the UB area only partly resemble those of the calc-alkaline rocks in a similar structural position in the coeval Pieniny area in Poland as well as of the transitional calc-alkaline rocks from the Central Slovakian volcanic field. The Miocene subvolcanic rocks were generated by melting of the variably metasomatised sub-lithospheric source and subsequent fractional crystallisation of the primary basaltic magma. The compositional characteristics of this rock series straddle the alkaline and subalkaline volcanic rock fields, while the rocks from the Pieniny area and from the Central Slovakian volcanic field are predominantly calc-alkaline andesites. The distribution of incompatible elements such as U, Th, REE, Nb, Ta, Sr, Ba as well as LaN/YbN ratios and Sr-Nd-Pb isotope composition of the rocks from the UB and Pieniny areas differ substantially, whereas the geochemical patterns of the UB rocks are slightly similar to those of the transitional rocks from the Central Slovakian volcanic field. The modest contents of incompatible trace elements in the andesitic rocks from the Pieniny area share several characteristics of the prevalent calc-alkaline andesitic rocks from the Carpathian-Pannonian region. Basaltic-trachytic series from the UB area are characterised by higher epsilon Nd (similar to 2.5) and lower Sr-87/Sr-86 (similar to 0.704) and Pb-207/Pb-204 (similar to 15.65) initial values in comparison with the calc-alkaline rocks from the Carpathian-Pannonian region including the Pieniny area. Nevertheless, the basaltic-trachytic rocks from the UB, richer in some of the incompatible trace elements (e.g., U, Th, REE, Sr and Ba) partly resemble (including Sr-Nd-Pb isotopes) the Na-alkaline Miocene volcanic rock series of the NE shoulder of the Cheb-Domažlice Graben in the Bohemian Massif.

A new concept for the genesis of felsic magma: the separation of slab-derived supercritical liquid

Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas, partial melting of crustal materials, partial melting of subducting slabs, and partial melting of pyroxenitic mantle wedge components have been proposed. Recent experimental studies have predicted that felsic melt can also be produced in the mantle wedge by the separation of slab-derived supercritical liquid beyond depths corresponding to the critical point. To date, however, the presence of felsic magma of this origin has not yet been reported. In this study, we investigated dacitic lavas and preceding calc-alkaline andesite lavas from the Rishiri Volcano, located at the rear of the Kuril arc. We show that hydrous felsic melt and aqueous fluid were separated from slab-derived supercritical liquid in the mantle wedge. The former erupted as dacitic magma whilst the aqueous fluid induced the generation of primary basaltic magma involved in creating calc-alkaline andesite magma. We infer that slab-derived supercritical liquid is an efficient transport medium for moving silicate-rich components from subducting slabs to the Earth’s surface, and that this process may have contributed to the growth of the continental crust.

Generation of calc-alkaline andesite magma through crustal melting induced by emplacement of mantle-derived water-rich primary magma: Evidence from Rishiri Volcano, southern Kuril Arc

Calc-alkaline andesite is a typical rock series in many island and continental arcs, and it commonly coexists with tholeiitic series rocks in individual volcanoes. In this study, the main controlling factors on the occurrence of calc-alkaline magmatism are examined for andesitic lavas from Rishiri Volcano, northern Japan. The whole-rock SiO2 contents of lavas range from 57.9 wt% to 61.9 wt%, and they are classified as calc-alkaline series. The lavas have petrographical and mineralogical features that show evidence of magma mixing. The whole-rock compositional variations of the lavas indicate that three end-member magmas were involved in their generation, a primitive mafic end-member magma, an evolved mafic end-member magma, and a felsic end-member magma. We suggest that the primitive mafic end-member magma represents a near primary magma generated in the upper mantle, the evolved mafic end-member magma was derived from the primitive magma by olivine fractionation, and the felsic end-member magma was produced primarily by partial melting of the crust. The primitive end-member magma is estimated to contain ~5 wt% water, which is much higher than those of the primary basaltic magmas of other volcanic stages of Rishiri that do not include calc-alkaline magmas. Based on this observation, we suggest that the water content of the primary magma was the main factor controlling the genesis of calc-alkaline magmatism at Rishiri. The more hydrous the primary magma and its derivative magma, the more extensive the melting of the crust due to the higher water flux from the solidifying magma chamber.

Geochemical constraints on the petrogenesis of Triassic alkaline basalts of Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina: implications for their origin, evolution and tectonic setting

The first geochemical and Sr-Nd isotope data of a series of Triassic basaltic rocks from Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina, are herein presented. The mafic rocks are the result of continental intraplate magmatism related to an extensional tectonic setting after the climax of the Gondwanide orogeny (Upper Carboniferous-Permian). Major minerals phases of these rocks are olivine, clinopyroxene, plagioclase and Fe-Ti oxides. Olivine and clinopyroxene occur both as phenocrysts and as groundmass, while feldspar is mostly restricted to the groundmass. The rocks belong to a typical alkaline series displaying SiO2 contents of 43.9–51.6 wt%, Na2O between 3.1 and 5.4 wt%, K2O between 0.9 and 2.9 wt% and high TiO2 2.2–3.6 wt% and P2O5 0.5–1.2 wt% contents. Nepheline on its CIPW normative composition indicates that it crystallized from silica-undersaturated magmas. Samples have moderate to low Mg# (58.1–15.9) values and Cr and Co contents that indicate they are not primary magmas but have undergone fractional crystallization with earlier fractionation of olivine and clinopyroxene. Geochemical patterns are similar to those found in the ocean island basalts (OIBs), with enrichment of alkalis and incompatible elements: LILE, HFSE and LREE, strong fractionation of HREE relative to LREE [(La/Yb)N = 9.6–22.4], no depletion of Nb and Ta and absence of negative Eu anomaly. Observed LILE-enrichment is indicated by the alkaline nature of the basaltic rocks and suggests melting of an enriched mantle. The Eu and Sr positive anomalies, not related to plagioclase accumulation, argue against significant plagioclase fractionation and are original geochemical characteristics of the primary magma. The chemical features, together with high incompatible-element ratios, are consistent with low degrees of partial melting of a dominantly garnet-pyroxenite mantle source. The trace-element patterns and isotopic data with 143Nd/144Nd(i) (0.51257–0.51267), positive εNd(i) and low 87Sr/86Sr(i) (0.70292–0.70339) values, suggest a mantle source influenced by an enriched-component. Active subduction-related mechanisms promoted the conditions for decompression melting in the asthenospheric mantle. Primary basaltic magma experienced subsequent compositional modifications by fractional crystallization in shallow-level low-pressure magma storage reservoirs. There is no evidence of crustal contamination during magma ascent.

Early Cretaceous Post-Collisional Collapse of the Yidun Terrane: Geochronological and Geochemical Constraints from Calc-alkaline to Alkaline Basalts in Xiqiu Area, Southwest China

Several Cretaceous Carlin-like or hydrothermal gold deposits along the Garze-Litang suture zone and Early Cretaceous hydrothermal copper mineralization along the southeastern margin of the Songpan-Garze fold belt were presumed to have a magmatic heat source. However, no actual coeval magmatic events nearby were discovered. Here, we report zircon SIMS U-Pb age, whole-rock geochemical and Sr-Nd isotopic data of the Xiqiu basalts in the southern end of the Yidun terrane, eastern Tibetan Plateau. New zircon U-Pb ages yield weighted mean 206Pb/238U age of 117.7±1.6 Ma. The basalts are classified as calc-alkaline to alkaline and have relatively high MgO (4.77 wt.%–10.84 wt.%) and Mg number values (Mg#=(100×Mg/(Mg+Fe2+)); 45.35–67.28) and positive eNd(t) (t=118 Ma) values (+1.86 to +3.2), suggesting a OIB-like mantle source that is consistent with the normalized patterns of trace elements and rare earth elements (REEs). Geochemical data suggest that the primary basaltic magma was generated by low degree partial melting of a peridotite-dominated mantle source with a minor component of garnet-eclogite or pyroxenite and experienced olivine+clinopyroxene dominated fractional crystallization. The primary melt compositions calculated from the high MgO samples, in turn, suggest that the Xiqiu basalts were generated at 1.6–2.9 GPa with abnormally hot mantle potential temperatures from 1 465 to 1 540 oC. The melting temperatures are similar to the abnormally hot mantle underneath the Colorado Plateau and hotter than the mid-ocean range basalt (MORB) mantle and normal intra-continental mantle. Combined with previous studies, the Cretaceous Xiqiu basalts allow us to reconstruct a tectonic and geodynamic evolutionary model responsible for the Late Jurassic to Late Cretaceous geological records (magmatism, ore deposits and enhanced exhumation) in the Yidun terrane and southern Songpan-Garze fold belt.

Arc crust formation and differentiation constrained by experimental petrology

We review igneous experimental data designed to understand fractionation processes in the roots of magmatic arcs, focusing on fractional crystallization experiments in dry and hydrous basalts to andesites to simulate the liquid and cumulate lines of descent of primary mantle derived magmas. An evaluation of the major element composition indicates that the cumulate line of descent (CLD) of hydrous systems is fundamentally different from dry systems. Cumulates derived from hydrous experiments display elevated Al2O3 and CaO contents at low SiO2, producing voluminous andesitic to rhyolitic liquids that closely overlap with compositions of natural systems. The mineralogical and chemical composition of cumulates converges for very different hydrous primary mantle magmas, indicating that fundamental phase equilibria under the conditions prevailing in the roots of magmatic arcs exert a strong control on the compositions of derivative andesitic to rhyolitic liquids. Melting experiments on amphibolite or basaltic to andesitic compositions equally produce granitic – rhyolitic liquids of similar composition yet their restites do not present the same variability of the CLD of hydrous fractional crystallization experiments. The comparison of our experimentally produced cumulates with natural rock suites from the Jurassic Talkeetna arc (South Central Alaska) and the Cretaceous Kohistan arc (Pakistan) show that fractionation experiments closely reproduce the natural rock compositions and in particular the cumulate sequences exposed in the roots of island arcs. Both dry and hydrous fractionation series are preserved yet only the hydrous fractionation series produces voluminous ultramafic cumulates (50–70%) that are negatively buoyant with respect to mantle peridotite. Delamination of dense ultramafic roots can explain the andesitic composition of continental crust. Our fractionation experiments show that generation of granites is a natural consequence of fractional crystallization of primary basaltic magmas. Discrepancies of liquid compositions produced in the lower crust and the arc volcanic record can readily be explained by further differentiation, partial assimilation and mingling and mixing processes during ascent and decompression in transcrustal magmatic systems. We therefore favor the interpretation that Earth-like continental crust is primarily produced by fractional crystallization in the lower crust and upper mantle, and that magma mixing, assimilation, metamorphism and crustal melting are important secondary processes that refine and amplify the difference and stratification between mafic lower crust and a granitic upper crust.

Geochemistry, origin, and nature of metamorphic rocks of the Batomga granite-greenstone terrane (Aldan Shield)

Two series of volcanic rocks with different petrochemical affinities—calcalkaline and komatiite- tholeiitic series—were identified as protoliths for the Early Proterozoic metamorphic rocks of the Batomga granite-greenstone terrane. The metavolcanic rocks of the calcalkaline series comprise metabasalts, meta� andesites, metadacites, and metarhyolites. The distribution of the trace element abundances in the felsic metavolcanic rocks is similar to that of the Archean grey gneisses from the platform basements, thus suggest� ing a similar petrological mechanism for the formation of their protoliths. The protoliths for the komatiite-tholeiitic metavolcanic rocks include komatiite and tholeiite basalts. The chemical behavior of the tholeiites tends to support the fractionation of primary highMg basaltic magmas in a transient magma chamber at low pressures. The variations in the Nb, Y, and Zr contents of the metatoleiites indicate the derivation of their parental magmas from a plumerelated source.

Mg-(Fe + Ti)-Al petrochemical diagram for the melting of mantle pyrolite: Implications for the derivation conditions of the parental magmas of major volcanic series

The Mg-(Fe + Ti)-Al melting diagram for pyrolite based on experimental data from literature shows the composition of the liquid as a function of pressure and the degree of pyrolite melting. Three mechanisms of liquid separation from a mantle source material are discussed: (i) gravitational mechanism, which works at a degree of source material melting of 25%, (ii) filter pressing mechanism, which is efficient at degrees of melting lower than 2%, and (iii) nearly complete local melting of mantle material. Garnet in the solid residue is thought to play an important role by affecting the chemistries of mantle magmas. The comparison of petrochemical and experimental data in a Mg-(Fe + Ti)-Al ternary plot shows that picrite and ferropicrite alcaline primary magmas are segregated at depths of 120 and 210 km, respectively, in the garnet stability zone, at degrees of melting lower than 2%; and tholeiite basalt magmas are segregated above this zone. At degrees of melting of 25%, picrobasalt, komatiite-basalt, picrite, and ferropicrite primary magmas of the tholeiite series are derived at depths of 80, 130, 240, and 300 km, respectively. Ultrabasic komatiite magma is generated at high degrees of mantle source melting, with the solid residues devoid of garnet. The tholeiite basalt series can be produced by two parental melts: aluminous and magnesian basaltic, both separated from the mantle sources via the filter pressing mechanism: the former at depths shallower than 30 km in ocean spreading zones (TOR-2), and the latter at depths of 50–60 km in oceanic spreading zones (TOR-1) and in the subcontinental lithosphere. Primary magnesian basalt magmas of the calc-alkaline and tholeiite series are derived in the lithospheric mantle at the same depths and low degrees of melting. Different evolutionary trajectories of compositionally similar primary magmas are controlled by the conditions of their further fractional crystallization: in compressional environments and with fluids saturating the melts in subduction zones for the former and in extensional environments and free magma ascent to the surface for the latter. Ultrapotassic rock series, such as lamprophyres, leucitites, kamafugites, lamproites, and kimberlites, are most probably generated via the melting of the metasomatized subcratonic mantle.

Two Primary Basalt Magma Types from Northwest Rota-1 Volcano, Mariana Arc and its Mantle Diapir or Mantle Wedge Plume

Primitive basalts are rarely found in arcs.The active NWRota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51^52 wt % SiO2) with 7·5^9·5 wt % MgO and Mg-number of 61^67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo93 and contain 0·4 wt % NiO; the Cr/(CrþAl) values of spinels are mostly 0·5^0·8, indicating equilibrium with depleted mantle.There are three petrographic groups, based on phenocryst populations: (1) cpx^olivine basalt (COB); (2) plagioclase^olivine basalt (POB); (3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3·1^3·2 and 1·2^1·5, respectively) than in COB (Zr/Y1⁄4 2·8^3·0 and Nb/ Yb1⁄4 0·7^0·9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COB have Ba/Nb (70^80) and Th/Nb (0·4^ 0·5) that are higher than for POB (Ba/Nb1⁄4 30^35 and Th/ Nb1⁄4 0·1^0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched Sr/Sr and Nd/Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. Hf/Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr,Yand HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB.The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2^1·5 GPa (equivalent to 65^50 km depth).These magmas formed by 24^18% melting of mantle peridotite having Mg-number 89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at Sumisu andTorishima volcanoes in the Izu^Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents.

Extension in the Baikal rift and the depth of basalt magma generation

It is considered that the opening of the Baikal riftsystem occurred as a result of counterclockwise rotation of the Amur microplate relative to stable SiberianCraton [1]. The pole of rotation is located on thenortheastern termination of the rift system in the areaof the Chara depression [1, 2] (Fig. 1). According togravimetrical data, it was established that the value ofthe extension increases linearly from the pole of rotation towards the South Baikal depression [2]. Therecent data of the seismic profiling across the SouthBaikal depression show that under this depression therise of the Moho boundary is absent. However, in thebottom parts of the Earth’s crust, a highvelocityanomaly, interpreted as a series of deep mafic intrusions, is established [3]. The Crustal extension is compensated by magma intrusion, and not its thinning [3].At the same time, within the largest Baikal depressions, volcanism manifestations [4] do not occur (Fig. 1).Thus, the problem of interrelation of volcanism andrifting still remains open. In this work, the depth offormation of primary basalt magmas under differentvolcanic fields of the Baikal Rift System is determinedon the basis of the chemical composition of volcanicproducts. It is shown that between the depth of magmageneration and the crustal extension value a reversecorrelation exists; i.e., the stronger the extension, thelower the depth that testifies that rifting processes control volcanism. The role of the inflow of deep mantlematter in generation of basalt magmas is also considered.In the northeastern Baikal rift system, there are tworelatively small Late Cenozoic volcanic fields (~100 kmin diameter), the Udokan and the Vitim (Fig. 1). Theformer is situated on the southern slope of the Charadepression; the latter one, in the area of small depressions outside the axial zone of the rift system. In thesouthwestern rift system, the Late Cenozoic volcanismwas manifested on the vast territory (more 450 km indiameter) within the depressions (Tunka and Khubsugul), on their slopes (as an example, on the KhamarDaban Ridge), as well as in areas without visible extension structures (the East Sayan Ridge).The calculation of the pressure of generation of primary magma was carried out using the proceduredescribed in [5] with insufficient modifications, whichinclude the following: (1) the assumption was usedthat the magma source is similar in chemical composition to enriched KLB–1 lherzolite, but not to the intermediate composition between KLB–1 and HK–66lherzolites; (2) the correction for olivine and clinopyroxene fractionation was not made. The first calculation modification was made on account of the comparison of calculated pressure values obtained formantle xenoliths and the host basalts of the Oka highland [6]. The second modification is connected withnonessential change in the calculated values for compositions of real basalts and compositions, corrected forolivine and clinopyroxene fractionation [5]; i.e., it simplifies the calculation procedure. Finally, the equationwas used based on experimental data for melting ofKLB–1 lherzolite [7]: