Felsic End-member Magmas Research Articles

Genesis of Andesitic Magma Erupted at Yufu Volcano, Kyushu Island, Southwest Japan Arc: Evidence from the Chemical Compositions of Amphibole Phenocrysts

Abstract The major- and trace-element compositions of amphiboles in andesite from Quaternary Yufu Volcano, northeastern Kyushu, Japan were analysed to investigate the generation processes of andesitic magma from Yufu Volcano. The amphiboles in andesite from Yufu volcano can be divided into two groups based on major-element composition: pargasite and magnesio-hornblende. To estimate temperature, pressure, and major- and trace-element compositions of melts in equilibrium with amphiboles, we used the recently proposed methods that can calculate temperature, pressure, major element compositions, and partition coefficients of trace-element between amphibole and melt using only the major-element compositions of amphibole. The estimated temperature, pressure, and major-element composition of melt in equilibrium with the amphibole phenocrysts indicate that each group crystallised under different conditions. These differences suggest that two magma chambers at different depths existed beneath Yufu Volcano and that the andesitic magma of Yufu Volcano was formed by mixing of the two magmas. The trace-element compositions of melts in equilibrium with the pargasite and magnesio-hornblende, estimated by applying the partition coefficients calculated from major-element compositions of amphibole to trace-element compositions of amphiboles, indicate magma derived from slab melt and the partial melting of crustal material, respectively. Because magma is a mixture of minerals and melt, we estimate the chemical compositional ranges of the two end-member magmas on the Y versus SiO2 diagram from the mixing relationship between amphibole and estimated melt, as well as phenocrysts of plagioclase, clinopyroxene, and orthopyroxene. The overlap of the estimated compositional range with the trend of whole-rock composition represents the chemical compositions of the end-members of magma mixing, yielding estimates of the mafic (SiO2 ≈ 45 wt %) and felsic (SiO2 ≈ 68 wt %) end-member magmas. Furthermore, we estimate the concentrations of other elements in the end-member magmas by substituting the estimated SiO2 concentrations of the magmas into linear regression equations between the whole-rock contents of other elements and SiO2. The trace-element compositions of the mafic and felsic end-member magmas, as estimated in this study, have similar features to those of gabbroids and Cretaceous granitic rocks, respectively, that are presumed to lie beneath Yufu Volcano. These similarities could be explained by the possibility that the compositions of the end-member magmas were influenced by basement rocks.

Geochemical and thermodynamic constraints on Archean comagmatic volcanic and cumulate rocks from southern West Greenland

The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid- to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.

Late Carboniferous bimodal volcanic rocks of the West Junggar Terrane, NW China: Implications for the postcollisional tectonic setting of the southwestern CAOB

ABSTRACT The West Junggar terrain (WJT), as a crucial part of the Central Asian Orogenic Belt (CAOB), is distributed with numerous igneous rocks, which provide critical information for crustal growth. However, the closure of the Junggar Ocean (JO) and the beginning of the postcollisional tectonic stage of the WJT have been controversial. This study delimited the regional lithologic units based on remote sensing geological mapping and recognized a series of bimodal volcanic rocks (BVR) in Hala’alate Mountain of the WJT. LA-ICP‒MS zircon U‒Pb geochronology and geochemistry were used to discuss the petrogenesis of the BVR and determine the stage of regional tectonic evolution. Geochronological results yield crystallization ages of 302 ± 4 Ma, 298 ± 2 Ma, 304 ± 1 Ma, and 303 Ma± 2 Ma for the basaltic andesite, basalt, and two rhyolitic samples, respectively. Basalts and basaltic andesites are calc-alkaline, and display enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Notably, basaltic andesites in this area were once misjudged as sanukitoids owing to their low contents of Mg#, Ni, Cr, and other characteristics that are inconsistent with the typical definition of sanukitoids. The rhyolites are A2-type granitoids with high SiO2 contents and are depleted in Nb, Ta, P, Ti, and Sr, showing enriched LREE patterns with negative Eu anomalies. These features indicate that the magma of the mafic end-member of the BVR is derived from the partial melting of the depleted lithospheric mantle, whereas the felsic end-member magma can be associated with the remelting of the lower crust due to the upwelling and underplating of mafic magma. In combination with previous studies of simultaneous basic dikes and felsic rocks, a postcollisional tectonic stage was proposed for the WTJ during the late Carboniferous, suggesting that the JO was closed.

Temporal Changes of Magmas That Caused Lava-Dome Eruptions of Haruna Volcano in the Past 45,000 Years

We have studied four lava dome eruptions which occurred in 45–10 ka in Haruna volcano. Enclave parts (SiO2 50.9–55.1 wt%) and host parts (SiO2 59.5–64.5 wt%) in lava samples are all products of magma mixing. Characteristics of felsic endmember magmas are the same among four eruptions, while those of mafic endmember magmas vary slightly in terms of bulk composition. The felsic magma had SiO2 ≥ 63 wt% and a temperature of 760°C–860°C, and contained ≥60 vol% of orthopyroxene, amphibole, plagioclase, quartz, and Fe-Ti oxides. The mafic magma had SiO2 48–51 wt% and contained 0–10 vol% of olivine. The enclave magmas resulted from higher contribution of mafic magma and thus had higher temperature than the host magmas, which led to formation of enclave upon their interaction. Similarities of endmember magmas between the four eruptions and the Futatsudake-Ikaho eruption (late 6th–beginning of 7th century) suggest structure of magma plumbing system and eruption triggering process have been basically unchanged in past 45,000 years. The felsic magmas were commonly mush-like and had high viscosity. Therefore, generation of low-viscosity magma through magma mixing, and vent-opening by the low-viscosity magma are mandatory for eruption to initiate. Unlike the Futatsudake-Ikaho eruption, the older four eruptions did not proceed to eruptive phase where felsic magma erupts without mixing and explosively. The absence of quartz only in felsic magma of Futatsudake-Ikaho eruption is consistent with its less-evolved bulk composition and slightly higher temperature than those of older four eruptions.

Temporal Variations of Petrological Characteristics of Tangkil and Rajabasa Volcanic Rocks, Indonesia

DOI:10.17014/ijog.7.2.135-159Tangkil and Rajabasa Volcanoes are neighbouring subduction-zone volcanoes located on the southeast tip of Sumatra Island (Sunda Strait, Indonesia). Stratigraphic correlation of lavas in Tangkil-Rajabasa volcanic area was established from field observations, morphological analysis, and K-Ar dating analysis. Detailed petrography and geochemical data of two and eleven lava units from Tangkil and Rajabasa, respectively, were then integrated with the stratigraphy to show the temporal variations. Early stage (> 4.3 Ma) effusives of Tangkil Volcano are dacitic to rhyolitic (67-71 wt. % SiO2; Tklf), whereas the later (c. 4.3 Ma) rocks are basalt to basaltic andesite (c. 52 wt. % SiO2; Tklm). Tangkil shows bimodal magmatism, of which the felsic endmember is > 71 wt. % SiO2 and < 0.1 wt. % MgO. Lavas of Rajabasa Volcano are comparatively younger (c. 0.3 to 0.1 Ma) with compositions ranging from basalt to andesite (51-62 wt. % SiO2; Rbs). Chemical variations of Rajabasa accounts for the interactions of at least three endmembers: Mg-rich medium-K basalt magma, low-Mg medium-K basalt magma, and high-K andesitic magma. During the long evolution of Rbs magma system, the temporal chemistry shows rising-falling variation in SiO2 and MgO indicating the three magmas were active. The felsic endmember magma of Rajabasa is fixed in composition (at ~62 wt. % SiO2; ~2.2 wt. % MgO). The rocks from the last Tklf and Rbs indicate open system processes by containing plagioclase and pyroxene phenocrysts that show resorption of evolved core and overgrowth of less evolved mantle. The multiple zones of dissolution-overgrowth in plagioclase crystals and the fluctuating trend in temporal whole-rock variation suggest that the changes of magmatic condition in temperature, H2O, or chemical composition were repetitive.

Origin of the Heping granodiorite pluton: Implications for syn-convergent extension and asthenosphere upwelling accompanying the early Paleozoic orogeny in South China

We present geochemical and geochronological data of host granodiorites and mafic microgranular enclaves (MMEs) from the Heping pluton, which is situated in the central part of the Wuyi-Yunkai Orogen (WYO) in the South China Block (SCB), and reveal syn-convergent extension and asthenosphere upwelling during the early Paleozoic intracontinental orogeny. Two host and two MME samples from the Heping pluton yield LA-ICP-MS zircon UPb ages of ca. 445 Ma, coincident with the peak magmatism-metamorphism of the WYO. The host granodioritic samples are metaluminous (A/CNK = 0.68–0.91) and have abundant amphibole and low-moderate SiO2 (59.1 to 67.3 wt%), belonging to high-K calc-alkaline I-type granite, whereas the MMEs are more basic with SiO2 of 56.3 to 59.6 wt%. Both the host and MME samples display weakly negative Eu anomalies and distinctly relative depletions of Ta, Nb, and Ti, and most of them share similar initial 87Sr/86Sr ratios of 0.7089–0.7112 and εNd(t) values of −6.52 to −7.12, except one MME sample with a lower initial 87Sr/86Sr of 0.70758 and a higher εNd(t) of −4.33. The zircon εHf(t) values exhibit a wide range from −13.68 to −0.87. Petrological, geochemical, and chronological data suggest that the Heping pluton were generated by mingling of mafic and felsic magmas. The mafic endmember magma was originated mainly from the enriched subcontinental lithosphere mantle at the spinel–garnet transitional zone (~70 km), subsequently underplated/intraplated into the lower-middle crust resulting in the melting of the intermediate rock at pressure < 8 kbar to produce the felsic endmember magma. We proposed a new tectonic model “syn-convergent extension and asthenosphere upwelling during the intracontinental orogeny”. The syn-convergent extension zones, which include a series of NW-trending transverse faults and the NE-trending reactivated pre-exsiting suture and rift zone (i.e. Shaoxing-Jiangshan -Pingxiang- Chenzhou fault) within the WYO, are in favor of asthenosphere upwelling leading to intensive crust-mantle interaction.

Multiple processes in the genesis of the Pohorje igneous complex: Evidence from petrology and geochemistry

The present contribution reports new whole-rock major and trace element data, REE, and Sr–Nd isotopic ratios for the early Miocene Pohorje igneous complex (PIC) in Slovenia (Eastern Alps), which, along with published data, are used to investigate its genesis and evolution. The complex comprises three main rock-groups. The largest one is made of granodiorite and minor tonalite (GTD) enclosing mafic microgranular enclaves (MME), and a small Q-dioritic body. The second group prevailingly consists of dacite stocks and dykes, and porphyritic microgranodiorite (DAMG). Andesitic dykes (AD), intruding metamorphic rocks and less often the GDT, constitute the third group. Aplites and pegmatites intrude both the Pohorje igneous complex and the country rocks.Evidence of interaction between magmas has been observed only in the GDT rocks. In the southeastern part of the complex the GDT rocks are less evolved and emplaced at greater depth (ca. 16.5–17.6 km) than the rocks cropping out in the northwestern part (ca. 12.4–13.8 km). DAMG rocks can be divided geochemically into two groups, high-Ga DAMG and low-Ga DAMG, having similar chemical composition with the most evolved GDT, and the most evolved andesitic dykes, respectively. The andesitic dykes can be divided into three groups that differ in LILE contents from each other up to three orders of magnitude.A Mixing plus Fractional Crystallization (MFC) process between the least evolved MME and the most evolved DAMG rocks is considered responsible for the formation of the two diversely evolved GDT. Two of the three AD groups (AD1, AD3) originated in the mantle, whereas the third one (AD2) is considered as the result of an AFC process between the less enriched andesite (AD1) and continental crust (gneiss). The same process is considered responsible for the low-Ga DAMG genesis.The most mafic magma is presumably the result of melting of mantle wedge differently metasomatized and enriched in LILE due to fluids, sediment melts, and bulk sediments. The felsic end-member magma of the MFC process originated from partial melting of intermediate-lower crust having intermediate to basaltic composition. Geodynamic sequence for the formation of the PIC comprehend mantle metasomatism during Alpine subduction, thickening of crust during the collision that followed subduction, and production of mantle melts in response to delamination caused by opening of the Pannonian basin and asthenospheric upwelling. Crustal derived melts resulted by underplating of mantle-derived magmas.

Petrogenesis of early Cretaceous granitoids in the southern Great Xing’an Range, NE China: Constraints from the Haliheiba pluton

Early Cretaceous granitoids are widespread in the southern Great Xing’an Range. However, the petrogenesis of these rocks is poorly understood. We therefore present field investigations, petrological observations, zircon U–Pb ages, whole-rock geochemistry and Hf isotope compositions for the Haliheiba pluton. The pluton comprises fine-grained biotite granite, medium- to coarse-grained biotite granite and porphyritic biotite granite, and mafic microgranular enclaves (MMEs) are unevenly distributed within the host granites. Zircon LA-ICP-MS U–Pb dating yields emplacement ages of ∼139 Ma for the three units. The host granites are characterized by high SiO2 contents (73.16–76.55 wt%) and alkali concentrations (7.92–8.62 wt%), enrichment in light rare earth elements (LREEs), Rb, U, Th, K, and Pb, depletion in heavy rare earth elements (HREEs), Sr, Ba, Ti, and P, and intensely negative Eu anomalies with δEu values from 0.14 to 0.47. The zircons in the host granites yield positive εHf(t) values of +5.0 to +11.2 and fairly young Hf model ages of 408 Ma to 731 Ma. These results indicate that the host granites have high-K calc-alkaline, metaluminous to weakly peraluminous A-type affinity and were mainly derived from partial melting of juvenile crustal materials originated from the depleted mantle, and fractional crystallization was involved in magma evolution. The MMEs have SiO2 contents of 63.45–64.14 wt% and have higher MgO, CaO, tFe2O3 and TiO2 contents than the host granites. The MMEs are enriched in LREEs relative to HREEs with slightly negative Eu anomalies (δEu values of 0.77–0.79) and are slightly enriched in Rb, U, Th, K, Sr, Pb and strongly to slightly depleted in Nb, Ta, Ti and P. These results, together with field investigations and petrological observations, indicate that crust-mantle magma mixing played an important role during the petrogenesis. Furthermore, the MMEs were intermediate members after magma mixing between mafic end-member magmas derived from lithospheric mantle and felsic end-member magmas derived from juvenile crustal materials. Combined with regional geology, our results suggest that a substantial crustal accretion in the southern Great Xing’an Range that occurred during the Neoproterozoic–Carboniferous, and the formation of the Early Cretaceous granitoids in this region was closely related to the lithospheric delamination triggered by the subduction of the Paleo-Pacific Ocean beneath Eurasia.

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.

New evidence for a continental rift tectonic setting of the Neoproterozoic Imorona-Itsindro Suite (central Madagascar)

The Neoproterozoic Imorona-Itsindro Suite of central Madagascar represents one of the crucial petrological records of the supercontinent transformation from Rodinia into Gondwana, whereas its tectonic setting remains open to debate. Here newly reported and existing zircon U/Pb-Hf-O isotope data are used to better understand the tempo-spatial features and petrogenetic mechanisms of this enigmatic suite. The following results are at least more compatible with, if not suggestive of, a continental rift setting than the traditionally accepted continental arc setting. (1) This suite is present throughout central Madagascar, including the Manampotsy-Anaboriana Domain and probably also the Antongil Sub-domain. Particularly, the high-flux magmatic pulse at ∼790 Ma has a width of more than 300 km. (2) This suite is in essence an igneous expression of lithospheric reworking with no asthenosphere contribution being clearly detected. (3) The felsic and mafic end-members of this suite have independent sources and were derived from the crustal basement and lithospheric mantle, respectively. (4) Input of mantle materials (via magma mixing process) into the crust-derived felsic end-member increased at the ∼790 Ma high-flux period, as indicated by a discernible positive zircon εHf(t) excursion. (5) Felsic end-member magma systems may have interacted with meteoric water at a high temperature, as implied by a previously reported zircon low-δ18O signature. Thus, there is perhaps no Neoproterozoic Mozambique Ocean suture across the Archean basement of central Madagascar. The most important implication of this study is that an elongate Andean-type orogenic system assumed in many Neoproterozoic paleogeography maps may not have been developed along the western margin of the supercontinent Rodinia.

Evaluation of magma mixing in the subvolcanic rocks of Ghansura Felsic Dome of Chotanagpur Granite Gneiss Complex, eastern India

The subvolcanic rocks exposed in the Ghansura Felsic Dome (GFD) of the Bathani volcano-sedimentary sequence at the northern fringe of the Rajgir fold belt in the Proterozoic Chotanagpur Granite Gneiss Complex preserves evidence of magma mixing and mingling in mafic (dolerite), felsic (microgranite) and intermediate (hybrid) rocks. Structures like crenulated margins of mafic enclaves, felsic microgranular enclaves and ocelli with reaction surfaces in mafic rocks, hybrid zones at mafic-felsic contacts, back-veining and mafic flows in the granitic host imply magma mingling phenomena. Textural features like quartz and titanite ocelli, acicular apatite, rapakivi and anti-rapakivi feldspar intergrowths, oscillatory zoned plagioclase, plagioclase with resorbed core and intact rim, resorbed crystals, mafic clots and mineral transporting veins are interpreted as evidence of magma mixing. Three distinct hybridized rocks have formed due to varied interactions of the intruding mafic magma with the felsic host, which include porphyritic diorite, mingled rocks and intermediate rocks containing felsic ocelli. Geochemical signatures confirm that the hybrid rocks present in the study area are mixing products formed due to the interaction of mafic and felsic magmas. Physical parameters like temperature, viscosity, glass transition temperature and fragility calculated for different rock types have been used to model the relative contributions of mafic and felsic end-member magmas in forming the porphyritic diorite. From textural and geochemical investigations it appears that the GFD was a partly solidified magma chamber when mafic magma intruded it leading to the formation of a variety of hybrid rock types.

Textures, trace element compositions, and U–Pb ages of titanite from the Mangling granitoid pluton, East Qinling Orogen: Implications for magma mixing and destruction of the North China Craton

The Mangling granitoid pluton, located along the southern margin of the North China Craton, consists mainly of monzogranite with minor amounts of diorite. The monzogranite contains abundant mafic microgranular enclaves (MMEs) and is intruded by numerous mafic dikes, providing an opportunity to study magma mixing and its role in the formation of the granitioid pluton. In this paper, we present in situ analysis of U–Pb isotopes and trace element compositions of titanite from the MMEs and the host monzogranite using Laser Ablation–Inductively Coupled Plasma-Mass Spectrometry to document the role of magma mixing in the formation of the Mangling granitoid pluton. Titanite grains from the MMEs (type 1) are euhedral with patchy zoning, whereas the varieties from the closely associated host monzogranite are euhedral and consist of two types (type 2 and type 3). Type 2 titanite is more abundant and has oscillatory zoning without Fe–Ti oxide inclusions, whereas type 3 grains commonly have Fe–Ti oxide inclusions in the core that is rimmed by inclusion-free overgrowths with weak oscillatory zoning. Titanite from monzogranite without MMEs (type 4) is euhedal and has weak oscillatory zoning, with rare ilmenite inclusions in the core. Titanite from a mafic dike intruding the monzogranite (type 5) is anhedral and has sector zoning. Titanite grains from MMEs and the monzogranite (type 1, 2 and 3) have U–Pb ages that are indistinguishable (149±1Ma, 148±1Ma and 148±2Ma, respectively). These ages agree well with zircon U–Pb ages of 150±1Ma, 150±1Ma and 149±1Ma for the MMEs, host monzogranite, and mafic dike, respectively. The age consistency thus confirms coeval formation of the MMEs, the host monzogranite, and the mafic dikes, demonstrating a mafic magmatic, rather than extraneous or restitic origin for the MMEs.Titanite grains from the MMEs (type 1) and mafic dike (type 5) have much lower Al2O3, REE, Nb/Zr, Y/Zr, and Lu/Hf, but higher (Ce+Nd)/Y and La/Ce ratios compared to those from the monzogranite (type 2 and type 4). These compositional differences suggest that titanite grains from the MMEs and the host monzogranite crystallized from different magmas. Type 3 titanite contains abundant Fe–Ti oxide inclusions and is characterized by dissolution and reprecipitation textures; the composition of its core and rims are comparable to those of type 1 and type 4 titanites, respectively. The compositional variation between the core and rims of the type 3 titanite grains implies that they were re-equilibrated with high temperature mafic magma and then continued to crystallize within the granitic magmas with low fO2. The textural, geochronological, and geochemical data of titanite grains can be best explained in term of mixing of mafic and felsic magmas. We conclude that both the fine-grained MMEs and the host monzogranite are hybrid products of mafic and felsic end-member magmas. When combined with other geological and geochronological data in the North China Craton, our results suggest that the widespread late Jurassic to early Cretaceous granitoid intrusions were generated in an extensional setting related to craton destruction.

Archaean andesite petrogenesis: Insights from the Grædefjord Supracrustal Belt, southern West Greenland

We present new whole-rock major, trace and platinum-group element data, as well as Sm–Nd and Lu–Hf isotope data for meta-volcanic rocks from the Mesoarchaean Grædefjord Supracrustal Belt (GSB), located within the Tasiusarsuaq terrane, southern West Greenland. We also present new in situ zircon U–Pb isotope data (by LA-ICP-MS) for associated felsic rocks. This region has experienced amphibolite to lower granulite facies metamorphism, causing re-equilibration of most mineral phases (including zircon).An intrusive tonalite sheet with a zircon U–Pb age of 2888 ±6.8Ma, yields a minimum age for the GSB. The Sm–Nd and Lu–Hf isotope data do not provide meaningful isochron ages, but the isotope compositions of the mafic rocks are consistent with the ca. 2970Ma regional volcanic event, which is documented in previous studies of the Tasiusarsuaq terrane. The major and trace element data suggest a significant crustal contribution in the petrogenesis of andesitic volcanic rocks in the GSB. The trace element variation of these andesitic leucoamphibolites cannot be explained by bulk assimilation–fractional-crystallisation (AFC) processes involving local basement. Rather, the observed patterns require binary mixing between basaltic and felsic end-member magmas with between 50% and 80% contributions from the latter (depending on the assumed felsic composition). Hf-isotope constraints point to contamination with pre-existing continental crust with an age of ca. 3250Ma. Basement gneisses of this age were previously described at two localities in the Tasiusarsuaq terrane, which supports the mixing hypothesis. Thus the felsic end-member likely represents melts derived from the local basement.Ultramafic rocks (18.35–22.80wt.% MgO) in GSB have platinum-group element (PGE) patterns that are similar to magmas derived from high-degree melting of mantle, but they have relatively enriched trace element patterns. We propose that the ultramafic rocks represent arc-related picrites or alternatively were derived by melting of metasomatised sub-continental lithospheric mantle.Overall these new geochemical data from the Mesoarchaean Grædefjord Supracrustal Belt and the petrogenetic mixing model in particular, are similar to observations from modern continental subduction zone environments, which also require large degrees of mixing with felsic basement melts. Therefore, we propose that the metavolcanic rocks formed in a modern-style subduction zone geodynamic setting, which due to the hotter Archaean mantle conditions allowed for substantial amounts of partial melting and magma mixing, rather than assimilating pre-existing continental crust.

Formation and evolution of silicic magma plumbing system: Petrology of the volcanic rocks of Usu volcano, Hokkaido, Japan

The whole-rock composition of the historical rocks of Usu volcano, which has erupted nine times since AD 1663, has changed from rhyolite (SiO 2 = 75.8 in AD 1663) to dacite (SiO 2 = 69.1 in AD 2000). Temporal changes in the composition of the eruptive magma have been explained as due to repeated withdrawals from a single, zoned magma chamber. Nevertheless, new petrographical and geochemical data suggest that the juvenile materials are composed of three groups: Group-1 is nearly aphyric hornblende–augite-bearing hypersthene rhyolite (SiO 2 = 74.3–75.8) from the 1663 eruption; Group-2 is porphyritic augite–hornblende–quartz-bearing hypersthene rhyodacite (SiO 2 = 70.5–72.7) from four eruptions during the 18–19th centuries; and Group-3 is slightly porphyritic, augite-bearing hypersthene dacite (SiO 2 = 69.1–70.5) from three eruptions in the 20th century. All the rocks show wide compositional distributions of phenocrystic minerals and coexistence of normally and reversely zoned phenocrysts, which suggest that the rocks originated from magma mixing. Many variation diagrams of the whole-rock chemistry show distinctive mixing trends with possible compositional gaps. The groups differ—especially Group-3 from Group-1 and Group-2—not merely in terms of several oxide–oxide diagrams of matrix glass chemistry, but also in terms of SiO 2 vs. Sr isotopic ratios of the whole-rock chemistry. These geochemical features suggest that the end-member magmas of each group are different from those of the other groups. Felsic end-member magmas, which have been voluminous, could not have been formed from a single magma by fractional crystallization or assimilation and fractional crystallization (AFC) processes: the felsic end-member magma in the magma plumbing system must have been replaced twice, and the three resulting magmas could have been active ever since. Compositional variations of these felsic magmas could have been caused by partial melting of the same source by different degrees. During the 350 years of volcanic activity, the temperature and degree of melting of the magma progressively increased from Group-1 to Group-3, as reflected by successive melt segregations from partially melted crustal materials by a single heat source.

Origin of felsic magmas in a large-caldera-related stratovolcano in the central part of NE Japan — Petrogenesis of the Takamatsu volcano

Quaternary stratovolcanoes in NE Japan are sometimes underlain by large-caldera volcanoes. Rocks of the stratovolcanoes range from basalt (< 10%), andesite (> 80%) to high-silica dacite (< 10%), in contrast to those of the large-caldera volcanoes which are dominated by high-silica dacite. Thus the petrologic study of post-caldera volcanoes provides useful information on the genesis of felsic magmas in large-caldera volcanoes. The Takamatsu volcano, which is composed of calc-alkaline andesite to dacite, is one of the post large-caldera volcanoes ca. 40 km westward from the volcanic front in the central part of NE Japan. The Takamatsudake lava, composed of olv(±)–qtz(±)–opx–cpx–andesite to dacite (61.6–66.5 wt.% SiO 2), is covered by the Yamabushidake dacite composed of hbl(±)–qtz–opx–cpx–dacite (68.4–70.2%). Mafic inclusions (50.9–55.6%), which have a quench texture, having olv, opx(±), cpx(±) and plg phenocrysts, are found in these lavas. The intermediate rocks (Takamatsudake lavas and silica-rich mafic inclusions) possess features suggesting magma mixing, while such features are generally lacking in silica-poor mafic inclusions and the Yamabushidake dacite. All rocks plot on the same linear trends in co-variant diagrams, thus the intermediate rocks are formed by mixing between mafic and felsic end-member magmas. These are similar to silica-poor mafic inclusions (olv basalt, 1070–1080 °C) and the Yamabushidake dacite (hbl dacite, 820–850 °C). In the Aoso and Gassan volcanoes, examples of post large-caldera volcanoes from the front and rear arc of the central part of NE Japan, most of the rocks are also formed by mixing between mafic and felsic end-member magmas, although, in the case of the Aoso volcano, composition of the felsic end-member changed temporally. The following data on these post-large-caldera volcanoes suggest the mafic and felsic end-member magmas are cogenetic. (1) Increasing potassium content in mafic end-members of the three volcanoes from front to rear arc is a remarkable feature, but this feature is observed also in felsic end-members. (2) Sr isotopic data for the Takamatsu and Aoso volcanoes are rather constant in basalt to dacite. Fractional crystallization of phenocrystic minerals from the mafic end-member cannot explain the whole rock compositions of the felsic end-member. An alternative process to produce the felsic end-members is partial melting of solidified mafic end-member magmas and trace element compositions are consistent with this if hornblende gabbroic or amphibolitic residues are produced. This result, coupled with the compiled Sr isotopic data, show that the same process would take place in the generation of felsic magmas in the large-caldera volcanoes.

Volcanic–plutonic links, plutons as magma chambers and crust–mantle interaction: a lithospheric scale view of magma systems

ABSTRACTThe northern Colorado River extensional corridor (NCREC, USA) provides an excellent record of coeval volcanic and mid- to upper-crustal (&lt;13 km) plutonic suites. The NCREC is a 50–100-km-wide zone that records late Tertiary lithospheric extension, volcanism, continental sedimentation and plutonism. Compilation of published studies of NCREC magmatic rocks permits an assessment of volcanic–plutonic links, magma sources and magmatic processes. The volcanic sections provide an excellent record of magma compositions (basalt, trachyandesite, trachyte and rhyolite) which span a 9-million-year period in the Miocene age (20–11 Ma).Contemporaneous Miocene plutons span a similar compositional range (gabbro, diorite, quartz monzonite and granite) and were emplaced during a 4·5-million-year interval from 17 to 12·5 Ma. Geochemical and isotopic compositions and compositional trends allow direct correlation between plutonic and volcanic suites across the entire compositional range. Petrogenetic models demonstrate that intermediate magmas formed by a combination of magma mixing and fractional crystallisation involving mantle-derived mafic with crustal-derived felsic end-member magmas. Plutons exhibit a variety of features which suggest magma chamber processes, including (1) mafic cumulate sequences, (2) felsic cumulate sequences, and (3) magma mingling and advanced stages of magma mixing. Thus, the NCREC plutonic-volcanic record provides a link between magmatic processes recorded in pluton magma chambers and magmatic products in the form of extrusive igneous rocks. The NCREC plutons represent upper crustal magma chambers which connected volcanic eruptive centres to deeper-level magma chambers, and ultimately, to zones of mantle and crustal mel

Petrological study of Nasu-Chausudake Volcano (ca. 16 ka to Present), northeastern Japan

Chausudake Volcano is representative of the active volcanoes in northeastern Japan, and has a record of many historical eruptions. Because its 16-ky eruptive history is well documented, Chausudake is well-suited for examining the temporal change of magma chamber processes and for assessing potential hazards. The activity of the Chausudake Volcano can be divided into six magmatic units (CH1–CH6). Most of its products have similar characteristics, but those from unit CH1 show wider variation. Most rocks are andesite and have plagioclase, clinopyroxene, orthopyroxene, and Fe–Ti oxides as phenocrysts, with or without olivine or quartz. Mafic inclusions, which are observed in most products, are basaltic andesites that have various combinations of the same phenocryst species. Petrographic features observed in host rocks and mafic inclusions, such as disequilibrium phenocrysts and resorbed textures, suggest magma mixing/co-mingling. Whole rock compositions of both host rocks and mafic inclusions show linear trends in variation diagrams, which suggest that the rocks are derived from the mixing/co-mingling between mafic and felsic end members. Bulk silica content of the mafic end-member magma is estimated to be ca. 52%, and contains Mg-rich olivine and An-rich plagioclase. The temperature of this end member is estimated to have been higher than 1,100 °C. Bulk silica content of the felsic end-member magma is estimated to be ~66%, and contains Mg-poor pyroxenes, An-poor plagioclase, and quartz phenocrysts, with a temperature of between 800 and 900 °C. Trace element compositions show that the end members have different origins, but have changed little over the entire 16-ky of activity. The mafic end-member magmas might come from a lower-crustal homogeneous, large magma chamber, whereas the felsic end-member magmas may be partial melts of crustal materials produced by the heat of the mafic end member. Felsic end-member magma may have accumulated in the middle crust before CH1 activity. The mixing ratio of the felsic to mafic end members was 0.5:0.5 to 0.4:0.6 for the CH1 unit, and ca. 0.4:0.6 for the other units. Considering that ca. 75% of the total volume of the eruptive products form the first unit, its wider compositional variation is attributed to more heterogeneous mixing ratios.

Origin of some icelandic pitchstones

The whole-rock composition of the historical rocks of Usu volcano, which has erupted nine times since AD 1663, has changed from rhyolite (SiO2 = 75.8 in AD 1663) to dacite (SiO2 = 69.1 in AD 2000). Temporal changes in the composition of the eruptive magma have been explained as due to repeated withdrawals from a single, zoned magma chamber. Nevertheless, new petrographical and geochemical data suggest that the juvenile materials are composed of three groups: Group-1 is nearly aphyric hornblende–augite-bearing hypersthene rhyolite (SiO2 = 74.3–75.8) from the 1663 eruption; Group-2 is porphyritic augite–hornblende–quartz-bearing hypersthene rhyodacite (SiO2 = 70.5–72.7) from four eruptions during the 18–19th centuries; and Group-3 is slightly porphyritic, augite-bearing hypersthene dacite (SiO2 = 69.1–70.5) from three eruptions in the 20th century. All the rocks show wide compositional distributions of phenocrystic minerals and coexistence of normally and reversely zoned phenocrysts, which suggest that the rocks originated from magma mixing. Many variation diagrams of the whole-rock chemistry show distinctive mixing trends with possible compositional gaps. The groups differ—especially Group-3 from Group-1 and Group-2—not merely in terms of several oxide–oxide diagrams of matrix glass chemistry, but also in terms of SiO2 vs. Sr isotopic ratios of the whole-rock chemistry. These geochemical features suggest that the end-member magmas of each group are different from those of the other groups. Felsic end-member magmas, which have been voluminous, could not have been formed from a single magma by fractional crystallization or assimilation and fractional crystallization (AFC) processes: the felsic end-member magma in the magma plumbing system must have been replaced twice, and the three resulting magmas could have been active ever since. Compositional variations of these felsic magmas could have been caused by partial melting of the same source by different degrees. During the 350 years of volcanic activity, the temperature and degree of melting of the magma progressively increased from Group-1 to Group-3, as reflected by successive melt segregations from partially melted crustal materials by a single heat source.