Felsic End-members Research Articles

Magma mingling in plagiogranites of the Oman ophiolite suggests an origin by fractional crystallisation

Late-stage plagiogranites in the Oman-UAE ophiolite are frequently associated with layered and massive gabbro bodies, intrusive into the mid-crustal section of the ophiolite. This study reports persuasive field evidence from five different localities for magma-mingling between plagiogranites and melts with a mafic and dioritic composition. It is argued from these relationships that mafic and felsic magmas coexisted in the same magma chamber. On this basis it is suggested that the range of melt compositions is related by a process of fractional crystallisation. The plagiogranites are interpreted as the end-product of this fractionation process and the associated gabbros are thought to be related cumulates. The hypothesis is supported with geochemical evidence which shows from the major element chemistry a continuum of compositions between the most mafic and felsic endmembers (47.6–80.6 wt% SiO2). These compositions ‘capture’ the fractionation process in operation. Modelling these variations using the major and trace element chemistry indicates that the fractionating assemblage comprised olivine-clinopyroxene-plagioclase+/−amphibole and that some plagiogranites contained cumulus plagioclase. The mafic rocks associated with late stage plagiogranite formation are derived from a highly depleted mantle source, which implies melting of an already-depleted mantle. This is consistent with previous models for the origin of the Oman ophiolite and implies a supra-subduction setting for this phase of ocean crust evolution in this ophiolite.

Multistage formation of the titanite-centered ocellar texture in magma mixing zones: New insights from field, petrographic and mineralogic evidence in the Paleoproterozoic Mineiro belt, Brazil

The titanite-centered ocellar texture is among the rarest and most intriguing textures associated with magma hybridization zones in plutonic environments. In order to contribute to the discussion regarding its formation process and relation to mixing, we investigated detailed field, petrographic, and mineralogical aspects of two occurrences of ocellar hybrid rocks associated with the Paleoproterozoic Macuco de Minas metagranitoid, Mineiro belt, southern São Francisco Craton. The studied rocks display oriented elliptic leucocratic aggregates composed of quartz, oligoclase (An15-20), microcline, apatite, and zircon, with an up-to centimetric poikilitic or euhedral diamond-shaped titanite crystals in their center (ocelli). The ocelli are often partially connected and immersed in a mesocratic tonalitic matrix composed of quartz, oligoclase (An15-20), Fe-biotite, titanite, allanite, fluorapatite, and zircon. The titanite crystals in the ocelli are enriched in Ti, LREE, Y, Th, and Zr and depleted in Al and F compared to the ones from the matrix, indicating that these compose two distinct groups of titanite. We propose that the development of the titanite-centered ocellar texture is a multi-stage interaction between mafic and felsic end-members, in which a biotite-rich tonalitic hybrid mush is injected by a fractionated leucogranitic melt. The leucogranitic melt migrates through veins that follow the preferred orientation of crystals in the hybrid mush and becomes enriched in Ti by the destabilization of biotite, resulting in the crystallization of the euhedral diamond-shaped titanite crystals. The veins are then disrupted, turning into elliptic ocelli in response to the pressure applied by the denser hybrid mush.

Magma-mixing origin for the Early Jurassic intrusive rocks in the eastern Songnen–Zhangguangcai Range Massif, NE China: Evidence from geochronology, geochemistry and Hf–O–Sr–Nd isotopes

Granites containing abundant dioritic enclaves are widespread in the Songnen–Zhangguangcai Range Massif (SZRM). However, the role of mantle- and crust-derived magmas in the petrogenesis of the dioritic enclaves is debated. We present new zircon U–Pb–Hf–O isotopic compositions, whole-rock geochemistry, mineral geochemistry, and Sr–Nd isotopic compositions of hornblendites and dioritic enclaves and their host granites to elucidate their origins and investigate crust–mantle hybridization. Zircon U–Pb dating suggests that these intrusive rocks were emplaced during the Early Jurassic (197–186 Ma). The geochemical characteristics of hornblendites are similar to those of the Early Jurassic mafic–ultramafic rocks in the Lesser Xing’an–Zhangguangcai Range. They yield low (87Sr/86Sr)i ratios (0.704266–0.704324), high εNd(t) values (3.87–3.91), and are enriched in Ba, Sr, and K and depleted in Nb, Ta, and Ti, suggesting that they were derived from the partial melting of mantle wedge metasomatized by sediment melt. The host granites have high SiO2 (>65.12 wt%) and low MgO (<1.92 wt%) and Al2O3 (<16.23 wt%) contents and are to metaluminous–peraluminous high-K calc-alkaline rocks. In addition, they yield positive zircon εHf(t) values (3.42–7.58), zircon δ18O values (4.87‰–5.85‰) and whole-rock εNd(t) values (0.10–0.44), suggesting that the granitic magma originated from the partial melting of continental crust with the involvement of mantle-derived melts. The field and petrographic observations and geochemical analyses indicate that the enclaves were derived from magma mixing and mingling. The dioritic enclaves contain K-feldspar megacrysts and back-veins, and plagioclase crystals within them are reversely zoned and have ocellar textures. The weighted mean ages of the host granites (186 ± 3 and 188 ± 2.3 Ma) are similar to those of the dioritic enclaves (186 ± 1.8 and 188 ± 1.1 Ma). The major element contents of the intrusive rock are linearly correlated with the SiO2 contents, and the whole-rock geochemical and isotopic characteristics of the hornblendites and host granites show that they are the mafic and felsic end-members, respectively, involved in the magma mixing. Therefore, we propose that underplating by mantle-derived magma likely triggered partial melting of the continental crust to produce felsic magma, and the dioritic enclaves were formed by the mixing and mingling of mantle- and crust-derived magma alongside melting, assimilation, storage, and homogenization. This suggest that the Mudanjiang Oceanic crust was subducted westward beneath the SZRM during the Early Jurassic.

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.

Petrogenesis of low-pressure intra-oceanic arc granitoids: Insights from the late Neoproterozoic Avalonian–Cadomian orogen, Bohemian Massif

The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc – Davle volcanic complex (DVC) situated in the Teplá–Barrandian unit, Bohemian Massif – that formed during the Neoproterozoic Avalonian–Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite–trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (ε Nd +6.0 to +8.7 and ε Hf +10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhjemites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as “low-pressure intra-oceanic arc granitoids” or “LP IOAG”. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite–trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth. • The DVC trondhjemite has flat REE patterns resembling typical LP IOAG. • The LP IOAG play a key role in the post-Archean crustal growth. • The source composition is established by an inverse geochemical modelling. • The trondhjemite formed by fractional crystallization of a tonalitic source.

Petrological and Geochemical Study of Sundoro Volcano, Central Java, Indonesia: Temporal Variations in Differentiation and Source Processes During the Growth of an Individual Volcano

Abstract Volcanic rocks of the Java sector of Sunda arc have a wide range of isotopic compositions that indicate significant addition of subjected sediment. What processes control these geochemical characteristics is a topic of long-standing debate. Here we report Sr–Nd–Pb radiogenic isotope ratios and geochemical data from stratigraphically well-constrained rocks of Sundoro volcano in central Java that represent the volcano’s activity since 34 ka. The rocks range from basalt (51 wt % SiO2) to andesite (63 wt % SiO2) and are dominated by basaltic andesite. We divide them into magma types A, B and C, having low, medium and high 87Sr/86Sr and Pb isotopic ratios, respectively. According to various differentiation indices, the three magma types have separate, parallel 87Sr/86Sr, Ba/Zr and La/Yb trends and disparate Pb isotopic trends. The dominant process of intracrustal differentiation appears to be magma mixing, in which each of the three magma types represents the mixing of a distinct mafic end-member and a distinct felsic end-member. The distinct geochemical profiles of these magma types indicate that the three mafic end-members are genetically unrelated and that their differences may represent characteristics of their magma sources. On the basis of trace element ratios (Ba/Yb and La/Yb) and Sr–Nd–Pb isotopic compositions, we estimate that magma types A, B and C represent mantle wedge materials fluxed by ~1%, ~1.5% and ~2% slab-derived materials containing 50%, 55% and 65% sediment component, respectively, reflecting increasing proportions of sediments and increasing slab flux. Geochemical data from Merapi volcano, interpreted using the same approach, reveal a similar increase in the slab-derived flux to the magma source, raising the possibility that such short-lived variations in magma genesis, perhaps related to the subduction of bathymetric relief features, characterize the unusual magmatism beneath the volcanic front of the central Java sector of the Sunda arc.

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.

Two‐Stage Mafic‐Felsic Magma Interactions and Related Magma Chamber Processes in the Arc Setting: An Example From the Enclave‐Bearing Calc‐Alkaline Plutons, Chinese Altai

AbstractA systematic data set of petrography, mineralogy, geochronology, and geochemistry is reported for typical arc granitoids and associated mafic microgranular enclaves (MME) from the Chinese Altai, Central Asian Orogenic Belt. Zircon U‐Pb dating indicates that they crystallized coevally at ∼395 Ma. Magma hybridization modeling suggests that the granitoid parental magma formed by mixing between a mafic and a felsic endmember. The sharp decrease of An values from cores to rims (e.g., from ca. 80 to 40) of plagioclase crystals points to polybaric crystallization accompanied by degassing. Petrographic evidence and plagioclase in situ Sr isotopic compositions ((87Sr/86Sr)i = 0.7053–0.7071) confirm the interaction of at least two isotopically different magmas during the mineral crystallization. A model for the formation of the enclave‐bearing calc‐alkaline plutons in arc settings is presented: At depth, mantle and crustal melting and efficient magma mixing controlled the principal chemical compositions of the granitoid intrusions. Then in the higher level chamber, decompression‐dominated crystallization and the mingling and limited mixing between blobs of mafic magma and the enclosing granitoid magma ultimately determined the texture, mineral composition, and enclave morphology. The MME have (87Sr/86Sr)i of 0.7048–0.7062, εNd(t) of −0.5 to +2.6, and zircon εHf(t) of +2.3 to +5.4, while the host granitoids have similar Sr isotopic compositions ((87Sr/86Sr)i = 0.7054–0.7064), but generally lower whole‐rock εNd(t) and zircon εHf(t) values (−2.2 to +0.4 and +0.6 to +4.6, respectively). The petrogenetic analysis and isotopic characters suggest that granitoid emplacement was a means of crustal growth in the Chinese Altai orogen.

Geochronology, whole-rock geochemistry, Sr[sbnd]Nd isotopes, and biotite chemistry of the Deh-Bala intrusive rocks, Central Urumieh-Dokhtar Magmatic Arc (Iran): Implications for magmatic processes and copper mineralization

The Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran hosts a variety of volcano-plutonic rocks of different composition and age formed by subduction of Neo-Tethyan Ocean beneath Eurasian supercontinent and the subsequent processes of post-subduction magmatism. This study focuses on mineral chemistry of biotite and amphibole, zircon UPb geochronology, whole-rock geochemistry, and SrNd isotopic geochemistry in the Deh-Bala granitoid, in an attempt to investigate the porphyry-epithermal mineralization potential. The Deh-Bala intrusions in the central part of the UDMA consist mainly of granodiorite and minor tonalite, with microgranular enclaves (MEs) of gabbrodiorite, diorite, and monzodiorite composition. In addition, the main ore minerals associated with more altered and mineralized varieties of these intrusions include chalcopyrite, covellite, malachite, and pyrite. The silicic, argillic, phyllic (sericitic), and propylitic facies of alteration occur in parts of the Deh-Bala area; these types of alteration, which are typically considered the most important predictors of porphyry copper‑gold and (or) epithermal precious metal mineralization.LA-ICP-MS UPb dating of zircons reveals that the pluton was emplaced during Middle-Eocene (ca. 39.0 Ma). The Deh-Bala granitoids (DBG) are calc-alkaline in composition (Na2O + K2O: 4.8–7.3 wt%) and display SiO2 contents from ∼53 to ∼67 wt%. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with marked Eu anomalies. They are also characterized by an enrichment in large-ion lithophile elements (LILEs, such as Rb, K, and U) and a depletion in high-field strength elements (HFSE, such Nb, Y, Ti, and Zr), displaying obviously depletions with Ba, P, and Sc. The DBG shows whole-rock initial 87Sr/86Sr ratios (ISr) of 0.7048–0.7061, εNd(t) values of −1.79 to +1.64, and Nd two-stage depleted mantle model ages (TDM2) from 747 to 1004 Ma. These isotopic data, combined with the geochemical signatures, indicate that the DBG originated through mixing of mafic and felsic end-members, the former was derived from melting of the lithospheric mantle, and the latter derived from partial melting of Neoproterozoic crustal materials in a continental-arc.Biotite compositions analysed by electron microprobe are consistent with the Deh-Bala magmas being oxidized and chlorine-rich, similar to worldwide porphyry copper‑gold systems formed in arc-related settings associated with convergent margin subduction zones. Such a chlorine-rich magmatic-hydrothermal system is known to be effective in scavenging copper and (or) gold from melt and its transfer to shallow levels.

Mesoscopic and Microscopic Magmatic Structures in the Quxu Batholith of the Gangdese Belt, Southern Tibet: Implications for Multiple Hybridization Processes

The Quxu batholith of the Gangdese magmatic belt, southern Tibet, comprises predominantly Early Eocene calc-alkaline granitoids that feature a variety of types of magmatic microgranular enclaves and dikes. Previous studies have demonstrated that magma mixing played a crucial role in the formation of the Quxu batholith. However, the specific processes responsible for this mixing/hybridization have not been identified. The magmatic microgranular enclaves and dikes preserve a record of this magma mixing, and are therefore an excellent source of information about the processes involved. In this study, mesoscopic and microscopic magmatic structures have been investigated, in combination with analyses of mineral textures and chemical compositions. Texturally, most of the enclaves are microporphyritic, with large crystals such as clinopyroxene, hornblende, and plagioclase in a groundmass of hornblende, plagioclase, and biotite. Two types of enclave swarms can be distinguished: polygenic and monogenic swarms. Composite dikes are observed, and represent an intermediate stage between undisturbed mafic dike and dike-like monogenic enclave swarms. Our results reveal three distinct stages of magma mixing in the Quxu batholith, occurring at depth, during ascent and emplacement, and after emplacement, respectively. At depth, thorough and/or partial mixing occurred between mantle-derived mafic and crust-derived felsic magmas to produce hybrid magma. The mafic magma was generated from the primitive mantle, whereas the felsic end-member was produced by partial melting of the preexisting juvenile crust. Many types of enclaves and host granitoids are thus cogenetic, because all are hybrid products produced by the mixing of the two contrasting magmas in different proportions. In the second stage, segregation and differentiation of the hybrid magma led to the formation of the host granitoids as well as various types of magmatic microgranular enclaves. At this stage, mingling and/or local mixing happened during ascent and emplacement. In the final stage, mafic or hybrid magma was injected into early fractures in the crystallizing and cooling pluton to form dikes. Some dikes remained undisturbed, whereas others experienced local mingling and mixing to form composite dikes and eventually disturbed dike-like monogenic enclave swarms. In summary, our study demonstrates the coupling between magmatic texture and composition in an open-system batholith and highlights the potential of magmatic structures for understanding the magma mixing process.

Linking magmatic processes and magma chemistry during the post-glacial to recent explosive eruptions of Ubinas volcano (southern Peru)

Understanding the links between the magma differentiation processes, the magma plumbing system and the magma composition at arc volcanoes is of paramount importance for volcanic hazard assessment. In this work we focus on the post-glacial, Holocene, historical, and recent eruption products of Ubinas volcano (Peru), which display an overall decrease in silica content from the older, plinian (VEI 3–5), rhyolitic eruptions (69–71 wt% SiO2) to the historical and recent (2006–2009, 2013–2017), vulcanian (VEI 1–2) basaltic andesitic eruptions (55–57 wt% SiO2). Based on a comprehensive study of the major and trace elements and the Sr-Nd-Pb isotopes, we conclude that this temporal pattern reflects the evolution of the Ubinas magmas in the middle-to-upper crust by a coupled Assimilation-Fractional Crystallization (AFC) process involving a cumulate composed of plagioclase, amphibole, clinopyroxene, orthopyroxene and Fe–Ti oxides, with minor amounts of olivine and biotite at the mafic and felsic end-members, respectively. Upper crustal assimilation is limited to 5–8 vol%, but the overall radiogenic Sr-Nd-Pb signature of the Ubinas magmas requires a larger crustal component, which must therefore occur at middle to lower crustal depths. The petrology of the Ubinas magmas also points to an overall increase in P-T conditions: the large Holocene dacitic and rhyolitic eruptions record temperatures ranging from 800 to 850 °C and pressures in the range of 200–400 MPa, whereas the historical and recent (2006–2009, 2013–2017) basaltic andesitic eruptions provide higher temperatures and pressures (1000 °C, >300–400 MPa). Overall, the thermo-barometry, phase equilibrium and geochemical constraints allow us to propose the existence of a middle-to-upper crust magma column composed of a highly crystalline magma mush containing batches of liquid magma, which seems to be continually recharged from deeper levels. On the basis of the petrological nature of the historical basaltic andesitic eruptions (1667 CE, 2006–2009, 2013–2017), we postulate that during the last centuries, Ubinas experienced a recharge-dominated process, with no evidence for a rejuvenation of the silica-rich reservoir that fed the large Holocene dacitic to rhyolitic eruptions. This study highlights the importance of detailed petrological studies of Holocene sequences at explosive arc volcanoes in order to constrain the magmatic processes and conditions that control large explosive 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 (&gt; 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 &gt; 71 wt. % SiO2 and &lt; 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.

Provenance and tectonic setting of the Eocene Nanka Sandstone, Anambra Basin, Nigeria

The Anambra Basin is a Cretaceous intracratonic basin, trending NE–SW in the southern part of Nigeria. The basin is believed to have been formed during the Santonian tectonic episode, which affected the entire Benue Trough. Inorganic geochemical data of the Nanka Sandstone was applied in studying its tectonic setting and provenance. The sandstones have fractionated chondrite-normalised rare earth element (REE) patterns, enriched light rare earth elements (LREE) and nearly horizontal REE with negative europium (Eu) anomalies. These patterns as well as a wide range of LREE/HREE (heavy rare earth elements) ratios and negative Eu anomalies suggest a felsic and/or mixed rock type source. A provenance discriminant function diagram utilising major oxides indicates a quartzose recycled sedimentary source. A plot of La/Th vs. Hf suggests a mixed felsic/basic source; a plot of Cr/Th and Th/Sc indicates contributions from felsic and basic end members, suggesting a mixed-source provenance. A Th/Co vs. La/Sc diagram, V-Ni-Th*10 ternary plot and Th/Co vs. La/Co suggest mixed source rock. Provenance investigations allude to sediment derivation from felsic, intermediate and mafic components and probably inputs from recycled older sediments. Tectonic discrimination based on major and trace elements indicates a passive margin setting.

Sources and settings of Ediacaran post-collisional syenite-monzonite-diorite shoshonitic magmatism from southernmost Brazil

The Arroio do Silva Pluton comprises quartz-rich syenite-monzonite-diorite rocks, with subordinate lamprophyre as enclaves. Compositional and textural variations lead to the identification of three main varieties: (i) quartz monzonite to syenite, (ii) quartz monzonite to monzodiorite and (iii) quartz diorite. The transition between all rock types is gradational and marked mostly by the increase of mafic mineral phases, especially biotite, and by the K-feldspar content, which is characteristically euhedral in the quartz monzonite to syenite variety and becomes subhedral to anhedral towards diorite compositions. Geochemical features corroborate field relations and both point to magma mingling as the main differentiation process. Syenite is considered to be the felsic endmember, whilst the mafic one has dioritic composition. Different degrees of interaction between these magmas result in hybrid varieties represented by quartz monzonite to monzodiorite. A third magma type is observed as ultrapotassic lamprophyric enclaves, but its influence in the mixing processes seems to have been minor. Both mafic and felsic magmas have K2O/Na2O ratios between 1 and 2, compatible with their shoshonitic affinity, whilst ratios slightly above 2 characterize lamprophyric enclave magmas as ultrapotassic. The source of shoshonitic and ultrapotassic magmas, according to trace element and isotope evidence, is probably a phlogopite-bearing mantle which was affected by previous subduction of Paleoproterozoic or Neoproterozoic age. The variation of ultrapotassic to shoshonitic magmas is probably controlled by variation of phlogopite contents in the mantle source. UPb zircon crystallization age of 578 Ma (LA-MC-ICP-MS) determined for the ASP quartz monzonite to syenite variety is in agreement with the Neoproterozoic post-collisional period of the Dom Feliciano Belt. This age value is also in agreement with that of other syenitic associations from south Brazil, such as the Piquiri Syenite Massive, that records similar chemical signatures. Therefore, these syenitic associations are taken to represent previously metasomatized mantle sources in late post-collisional setting from the Brasiliano/Pan-African Cycle in southermnmost Brazil.

Mineral and bulk–rock chemistry of Shadli bimodal metavolcanics from Eastern Desert of Egypt: Implication for tectonomagmatic setting and Neoproterozoic continental growth in the Arabian–Nubian Shield

The Neoproterozoic Shadli metavolcanics in the Eastern Desert of Egypt have been conventionally categorized as a bimodal island-arc association. The present study deals with Shadli metavolcanics of three areas: Hamama in the central and Darhib and Atshan in the southern Eastern Desert of Egypt. They contain both mafic and felsic compositions and display a bimodal nature. Geochemical classification indicates that the mafic end-members are tholeiitic basalt and basaltic andesite whereas the felsic ones comprise calc-alkaline dacite and rhyolite.Mafic end-members show flat to slightly LREE-depleted patterns comparable to normal MORB produced from a depleted mantle source. Major element data indicates affinity to arc-related non-cumulative mafic rocks. The obvious negative Nb and positive Pb anomalies, the nearly flat REE patterns together with pyroxene chemistry are characteristic of island-arc tholeiitic suite. Accordingly, we suggest that their primary magma was derived from partial melting of a depleted mantle source metasomatized by subduction-related slab fluids/melts.Felsic end-members exhibit magnesian character consistent with a relatively hydrous, oxidizing magma and subduction-related source. Their weakly peraluminous character and calc-alkalic to calcic compositions are characteristics of arc-related magmas. Moreover, the negative Nb and Ti anomalies suggest an influx of subduction-related slab fluids/melts. Their SiO2 contents are too high to represent magmas directly derived by partial melting of a mantle source. So, the high SiO2 and Al2O3 and the low MgO contents suggest melting crustal source. The high Y/Nb ratios (> 1.2) confirm that they were generated from crustal source. Actually, their Ce/Pb ratios (average of 2.66) are affiliated to those of continental crust and contrast typical mantle values. The weakly peraluminous and sodic nature is a typical feature of I-type magma. Therefore, their magmas were most likely derived by partial melting of thickened low-K mafic lower crust.Several lines of evidence let us speculate a genetic relationship between mafic and felsic end-members and that the felsic rocks were possibly evolved from the mafic melts that derived from metasomatized mantle. These comprise the close proximity and a coeval nature; the subalkaline character with approximately continuous trend; the similarity of trace element patterns; the arc-like signature, and the well fitness of major element data of the felsic rocks with melts derived from low-K mafic lower crust. The general tendency of silica increase with alkalis from mafic to felsic end-members may indicate that the felsic rocks were generated by fractional crystallization processes of the mafic magmas in the area.

Magma mixing origin for the Quxu intrusive complex in southern Tibet: insights into the early Eocene magmatism and geodynamics of the southern Lhasa subterrane

Intrusive complexes with compositional diversity commonly document magma mixing processes and provide insights into regional magmatism and tectonics. Here, we present an integrated study involving detailed field investigations, petrography, mineral chemistry, whole-rock geochemistry, SrNd isotopic compositions, and zircon U–Pb–Hf isotopes for the Quxu intrusive complex in southern Tibet, with the aim of constraining the origin of the complex and gaining insights into the early Eocene magmatism and geodynamics of the southern Lhasa subterrane. The Quxu intrusive complex consists predominantly of tonalites and minor pyroxene–hornblende gabbronorites, with lesser amounts of quartz diorites within the contact zones of these two types of rock. Zircon LA–ICP–MS UPb dating yields crystallization ages of ~51 Ma for all lithologies in this complex. The pyroxene–hornblende gabbronorites show replacement of pyroxene by amphibole and/or biotite and contain plagioclase crystals of variable composition (An38–74) and quartz, characteristics that are consistent with a reactive process via magma mixing. The quartz diorites and tonalites preserve plagioclase crystals with complex oscillatory zoning and repeated resorption surfaces, indicating a magma mixing origin. Geochemically, the pyroxene–hornblende gabbronorites, quartz diorites, and tonalites exhibit continuous variations in most of their major- and trace-element compositions, consistent with published data for mafic microgranular enclaves (MMEs) and host granitoids that resulted from magma mixing. All the lithologies in the Quxu intrusive complex have homogeneous and depleted isotopic compositions, with whole-rock initial 87Sr/86Sr ratios (ISr) of 0.7038–0.7041, εNd(t) values of +5.5 to +6.3, and zircon εHf(t) values of +8.5 to +12.9. These geochemical and isotopic data, combined with the mineralogical features, indicate that the Quxu intrusive complex originated from magma mixing between mafic and felsic end-members in varying proportions. The mafic end-member was derived from the decompression melting of upwelling asthenosphere, and the felsic end-member was derived from the partial melting of juvenile lower crust triggered by underplating of a mafic end-member. Our results also indicate that breakoff of the Neo-Tethyan oceanic slab was responsible for the generation of the Quxu intrusive complex and, more generally, for the early Eocene magmatism in the southern Lhasa subterrane.

Temporal changes in the subduction of the Paleo-Pacific plate beneath Eurasia during the late Mesozoic: Geochronological and geochemical evidence from Cretaceous volcanic rocks in eastern NE China

Subduction of the Paleo-Pacific Plate played an important role in the evolution of the eastern margin of the Eurasian continent. However, the details of the processes of subduction during the late Mesozoic remain poorly constrained. Here we examine the geochronology and geochemistry of Cretaceous volcanic rocks from eastern NE China, with the aim of understanding the variation of subduction process. Cretaceous volcanism in the region mainly occurs in the Raohe–Mishan-Suifenhe–Wangqing and Jiamusi–Jixi areas in eastern Jilin and Heilongjiang provinces, as well as in the Lesser Xing'an Range where volcanism extends from the continental margin farther inland. The Cretaceous volcanic rocks in the Raohe–Mishan–Suifenhe–Wangqing (RMSW) area are composed of basalts, basaltic andesites, andesites, dacites, and rhyolites with late Early Cretaceous (122–106 Ma) ages and dacites that yield early Late Cretaceous (96–93 Ma) ages. A mixed source of compositionally heterogeneous mantle and crustal rocks is assumed based on the compositional range of volcanic rocks. Adakitic andesites (114–110 Ma) were derived from a mantle source modified by slab-derived melt that infiltrated the mantle wedge. Adakitic dacites (122–106 and 96–93 Ma) were produced by partial melting of thickened lower crust that formed in response to earlier subduction of the Paleo-Pacific Plate beneath Eurasia. The rhyolites and dacites that lack adakitic signatures were derived from partial melting of a depleted lower-crustal block that was accreted during the Neoproterozoic. In particular, the late Early Cretaceous (106 Ma) basalt was derived from a depleted lithospheric mantle that was metasomatized by slab-derived fluids and subsequently by asthenospheric melts at a relatively shallow depths, implying rollrback of the subducted Paleo-Pacific Plate. The Cretaceous volcanism in the Jiamusi–Jixi area formed during the late Early Cretaceous (124–110 Ma) and shows a bimodal distribution of compositions with a prominent ‘Daly Gap’. The mafic members were generated by re-melting of depleted lithospheric mantle that was modified by subduction-related fluids and underwent fractional crystallization. The coeval felsic end-members, with or without A-type characteristics, were generated by partial melting of juvenile crustal material, consistent with an extensional environment related to subduction of the Paleo-Pacific Plate. Cretaceous volcanism in the Lesser Xing'an Range shows a bimodal compositional distribution of volcanic rocks with a ‘Daly Gap’ and a period without volcanism at 108–97 Ma. Andesites with and without adakitic signatures were derived from partial melting of mantle that was hybridized by slab-derived melts and metasomatized by a fluid-rich subduction component. In contrast, the coeval A-type rhyolites were generated by partial melting of juvenile crust, implying the migration of the extensional environment. The formation of, and spatio-temporal relationships between, Cretaceous volcanic rocks in NE China, combined with the regional geological data, suggest a transition from low-angle subduction to slab rollback and subsequent changes in the direction of motion of the Paleo-Pacific Plate during the late Early Cretaceous to the Late Cretaceous.

Peralkaline Felsic Magmatism of the Atlantic Islands

The oceanic-island magmatic systems of the Atlantic Ocean exhibit significant diversity in their respective sizes, ages, and the compositional ranges of the magmas they have produced. Nevertheless, almost all of the major Atlantic islands and island groups have produced peralkaline felsic magmas, implying that similar petrogenetic regimes may be operating throughout the Atlantic Ocean, and elsewhere. The origins of peralkaline magmas are frequently linked to low-degree partial melting of enriched mantle, followed by protracted differentiation in the shallow crust. Nevertheless, additional petrogenetic processes such as magma mixing, crustal melting, and contamination have been identified at numerous peralkaline centres. The onset of peralkalinity leads to magma viscosities lower than those typical for metaluminous felsic magmas, which has profound implications for processes such as crystal settling. A literature compilation demonstrates trends which suggest that the peralkaline magmas of the Atlantic Ocean islands are generated primarily via extended (up to ~ 95 %), open system fractional crystallisation of mantle-derived mafic magmas. Crustal assimilation is likely to become more significant as the system matures and low-solidus material accumulates in the crust. Magma mixing occurs between mafic and felsic end-members, and also between variably-evolved felsic magmas, and is often recognised via hybridised intermediate magmas. The peralkaline magmas are hydrous, and are frequently zoned in composition, temperature, and/or water content. They are typically stored in shallow crustal magma reservoirs (~ 2 to 5 km), which are maintained by mafic replenishment. Low melt viscosities (1 × 101.77 to 1 × 104.77 Pa s) facilitate two-phase flow, facilitating the formation of alkali-feldspar crystal mush. This mush may then contribute melt to an overlying melt lens via filter pressing or partial melting. We utilise a three stage model to account for the establishment, development, and termination of peralkaline magmatism in the ocean island magmatic systems of the Atlantic. We suggest that the overall control on peralkaline magmatism in the Atlantic is magma flux rate, which controls the stability of upper crustal magma reservoirs. The abundance of peralkaline magmas in the Atlantic suggests that their development must be a common, but not inevitable, stage in the evolution of ocean islands.

Platinum-group elements, zircon Hf-O isotopes, and mineralogical constraints on magmatic evolution of the Pulang porphyry Cu-Au system, SW China

A new model of porphyry Cu system, which involves pre-enrichment of Cu through sulfide accumulation near the mantle-crust boundary, has been recently proposed to account for variations of Cu endowment in different magmatic arcs. To test this model and explore the possible controlling factors of porphyry Cu metallogeny, we studied the mineralogy and platinum-group elements (PGE) geochemistry of the giant Pulang porphyry Cu-Au system in the Sanjiang Region, SW China. The Late Triassic Pulang intrusive complex comprises five phase of porphyrtic stocks and dikes (ca. 217–212 Ma), which intruded a slightly older (ca. 230–218 Ma) volcanic-sedimentary sequence of the Tumugou Formation. Both the intrusive complex and the related volcanic wall rocks have similar PGE concentrations, Pd/Pt ratios and primitive mantle-normalized PGE patterns, indicating that no discernible sulfide cumulates were lost or gained during magmatic differentiation. The variation in PGE concentrations between different types of rocks at Pulang is probably ascribed to magma mixing between mafic and felsic end-members.Two types of amphibole, i.e., high-Al (Al2O3 = 9.4–11.8 wt%) and low-Al (5.9–7.3 wt%) types, have been identified in the Pulang complex. It is estimated that the magma equilibrated with the high-Al amphibole may have contained up to 6.2 wt% H2O, and was formed under 334–538 MPa (ca. 11–18 km deep eqv.) with oxygen fugacity (fO2) between NNO + 0.2 and NNO + 1.5. In contrast, the magma that equilibrated with the low-Al amphibole was likely formed at a much shallower depth (89–202 MPa, ca. 3–7 km eqv.), and was more oxidized (NNO + 1.0–NNO + 1.7) with a low H2O content (4.2–4.9 wt%). High fO2 would increase sulfur solubility in the magma, thus prohibiting early removal of metals via sulfide precipitation. In addition, the higher H2O content may have enhanced volatile exsolution from the evolving magmas at Pulang. We suggest that oxygen fugacity and H2O content of the magma are two critical factors that controlled the Pulang porphyry Cu-Au metallogeny.