Mafic End-members Research Articles

Origin of mafic microgranular enclaves from the Late Triassic Cuojiaoma granitic pluton: Implications for the tectonic evolution of the Yidun arc belt, eastern Tibetan Plateau

In-situ U-Pb ages and Hf-O isotopes of zircon are frequently used to study the petrogenesis of mafic microgranular enclaves (MMEs) in granitoids. However, whether zircons in MMEs have recorded the primary isotopes of their host MMEs is still questionable. In this study, MMEs and host granites from the Cuojiaoma granitic pluton in the Triassic Yidun arc belt, eastern Tibetan Plateau, are studied. LA-ICP-MS zircon U-Pb dating of studied MMEs yields an age of 213 ± 2 Ma. Model calculations indicate that the whole-rock composition of the MMEs was different from the melt composition from which the zircon grains grew. The MMEs crystallized during magma mixing based on analyses of zircon trace element and Zr-Hf isotopes. None of zircons in this study have recorded the primary isotope signature of the mafic endmember of MMEs because they are different from the host granite in terms of zircon morphology, ɛHf values and ΔQFM. The Zr isotopic data demonstrate open-system magmatic crystallization for the host granite and that magma mixing for MMEs play key role in controlling Zr isotopic variations. Initial 87Sr/86Sr ratios of An-rich plagioclase crystals in groundmass from MMEs range from 0.7027 to 0.7058, which are significantly lower than host granites (0.7080 – 0.7089). The lowest initial 87Sr/86Sr ratios (0.7027 – 0.7028) are also lower than arc volcanic rocks and similar to a depleted mantle. Combined with previous studies, we propose that MMEs were generated during post-collision extension by mixing of mafic magma of Xiaxiaoliu basalt and felsic magma from host granite.

Origin of calcite by magma mixing in mingled rocks of the Ghansura Rhyolite Dome, Bathani volcano-sedimentary sequence, eastern India

Abstract The Ghansura Rhyolite Dome (GRD) is an integral part of the Bathani volcano-sedimentary sequence (BVSs), which in turn is a vital component of the Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The rhyolite dome represents a shallow-level felsic magma chamber that underwent intrusion by basaltic magma during its evolution. The rocks present in the felsic dome such as basalts, rhyolites, and intermediate hybrid rocks preserve evidence of magma mixing and mingling. The objective of this contribution is to take into account, for the first time, that magma mixing processes can play an important role in the generation of calcite in magmatic rocks. The present study focuses on the mingled rocks exposed within the rhyolite dome. Previous reports have documented the existence of two distinct zones (mafic and felsic) interfacing each other in the mingled rocks of the rhyolite dome. The mafic zones dominantly consist of mineral phases such as actinolite, hornblende, biotite, plagioclase, ilmenite, calcite, and titanite. The felsic zones comprise quartz, K-feldspar, plagioclase, and biotite. When mafic magma intruded into the felsic magma chamber, interaction between the mafic and felsic magmas caused diffusion of H, Al, and K ions from the felsic to the mafic endmember. Such diffusive activities resulted in the breakdown of augite, already crystallized in the mafic magma, to form newer minerals like actinolite, hornblende, biotite, calcite, and ilmenite. The presence of ilmenite in the mingled rocks indicates the prevalence of reducing conditions during magma interaction and evolution. Breakdown of hornblende to biotite in such a reduced environmental condition led to the formation of calcite in the mingled rocks. From the results presented in this work, we are proposing that magma mixing can be a viable mechanism to form calcite in igneous rocks by magmatic processes.

The process of crustal thickening in the southern Lhasa terrane during India-Eurasia collision: Constraint from Eocene high Sr/Y rocks in the Quxu pluton

The crustal thickening process is the key to understanding global tectonic evolution and climatic patterns. The Sr/Y, (La/Yb)N ratios of granitoids can be used to quantitatively estimate crustal thickness and reconstruct the crustal thickening process. We present the geochemical characteristics, petrogenesis, and geodynamic implications of Eocene dioritic enclaves (monzodiorite), host granitoid rocks (monzonite), and granite dikes of southern Tibet to reveal the crustal thickening process underway during the collision of India and Asia. The Quxu dioritic enclaves were produced by magma mixing and represent mafic end member. The adakitic host rocks were generated by partial melting of lower crust with normal thickness. The high Sr/Y, (La/Yb)N ratios of the Quxu host rocks were inherited from their magma source region. The ∼40% fractional crystallization of amphibole from magma resembling Quxu host rocks formed the Quxu granite dikes. The continuous crustal thickening history determined by the locally weighted regression analysis shows that the crustal thickness of the Lhasa terrane during 50−45 Ma was no more than 36−46 km, and the crust thickened quickly during the syn-collisional period.

Mafic microgranular enclaves (MMEs) trace the origin of post-collisional magmas

Abstract Mafic microgranular enclaves (MMEs) are a ubiquitous feature of post-collisional magmatism, receiving much attention among earth scientists over the last decades. While recent advances point to the large-scale involvement of the lithospheric mantle in granite petrogenesis, MMEs have received less attention in such discussion. Because MMEs are commonly acknowledged to represent the mafic end member with a mantle affinity that is related to early-stage batholith petrogenesis, they constitute a good proxy for the mantle role in the process. Using MME data from Los Pedroches batholith in southwestern Iberia, we conduct a geochemical comparative study between MMEs and the mafic-intermediate (sanukitoid) suite of post-collisional batholiths. An accurate overlap between the two groups is revealed, implying a potential genetic link between MMEs and the sanukitoid suite. Together with evidence from experimental cotectic liquids, the link between the high-Mg signature of postcollisional magmas and the predominance of amphibole in the studied MME samples is used to account for the composition of post-collisional magmatism. Implications for post-collisional batholith petrogenesis is then discussed in a qualitative manner, suggesting a heterogeneous yet common two-stage origin for all post-collisional magmatism in which the relationship between MMEs, sanukitoid, and the host felsic magmas is a differentiation process, thus representing a major input of juvenile magma into the crust.

The ~1.1 Ga St. Ignace Island complex, Northern Ontario, Canada: Evidence for magma mixing and crustal melting in the generation of Midcontinent Rift-related bimodal magmas and implications for regional metallogeny

Abstract The St. Ignace Island complex in Northern Ontario is a package of dominantly felsic rocks emplaced within the upper portions of the Osler Volcanic rocks of the ~1.1 Ga Midcontinent Rift System. The Osler volcanic rocks are predominantly tholeiitic basalts intercalated with rare interflow sediments and rhyolites. The St. Ignace Island complex is a ~26 km2 stock with a felsic core of quartz-feldspar-phyric rhyolites and dacites and an outer ring of anorthosite and gabbro. Textures at a variety of scales within the rocks of the complex show clear evidence of the mingling and mixing of partially crystallized mafic and felsic liquids. Two multigrain (zircon/baddeleyite) fractions from a sample of the gabbro define a Discordia line with an upper intercept date of 1107±8.9 Ma. The core of the complex consists of dacites and rhyolites with similar REE abundances with negative Nb anomalies whereas the surrounding mafic rocks are gabbros to monzogabbros that are less LREE-enriched than the felsic rocks but with similar HREE. Felsic units have a narrow range of 87Sr/86Sri (0.7032-0.7045) and 143Nd/144Ndi (0.51051-0.51057) whereas the mafic end members have similar 87Sr/86Sri (0.7040-0.7061) but more radiogenic 143Nd/144Ndi (0.51067-0.51085). Very well-preserved silicate melt inclusions (MI), many completely glassy, were observed in quartz, clinopyroxene and some plagioclase phenocrysts from the complex. These represent some of the oldest unrecrystallized silicate melt inclusions described to date. Melt inclusions within quartz from the felsic volcanics are broadly rhyolitic in composition whereas MI from plagioclase in the mafic volcanics range from basalt to basaltic andesite; these felsic and mafic melt compositions are interpreted to represent the end-member liquids in the system and bulk rock analyses affirm mixtures of the two. Concentrations of Cu and Ag (in both mafic and felsic MI), and Mo (in felsic MI), are up to an order of magnitude higher in the mafic and felsic MI than in continental crust. Bulk rock metal concentrations are also significantly lower than in the MI, suggesting that the melt inclusions may preserve pre-eruptive metal tenors that were subsequently modified by sulfide saturation, degassing, or post-solidus hydrothermal alteration. The whole rock and MI geochemistry of the St. Ignace complex are broadly similar to the Central Osler Group and, given the broad similar ages, suggests they may have been derived from a similar mantle source, but distinct from the source of rhyolites in the Black Bay Peninsula. The negative Nb anomalies and negative εNd values for the St. Ignace complex are consistent with mixing with older continental crust during ascent and emplacement. The rocks of the St. Ignace Island complex likely formed as the result of emplacement of a large mafic magma chamber at the base of the Osler volcanic pile that triggered partial melting to generate the rhyolite end members. The felsic melts ascended to shallower levels in the crust where they mixed with mafic magmas derived directly from the deeper chamber. Generally, melt inclusions in the complex have very high Cu and Ag contents, similar to those observed in arc-related and extremely oxidized early rift-related rocks and may account for the world-class volcano-sediment-hosted Cu-(Ag) deposits within the rift and the presence of small porphyry-style deposits.

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.

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.

Magmatic drivers of a 200-year-long high-magnitude explosive flare-up from Mt. Tongariro, New Zealand

Subduction zone volcanoes may show irregular bursts of high-frequency or high-magnitude activity. The andesitic Mt. Tongariro (New Zealand) experienced an unusual <200 year-long magmatic flare-up at ~11 ka that produced seven eruption episodes of a higher magnitude (M = 4–5) than seen before or since. This brief sequence produced a total of 4.5 km3 of dominantly tephra fall (Mangamate Formation) sourced by multiple vents aligned along the NNE trending axis of the tectonic Tongariro Graben. The magmatic system responsible for sporadic M = 1–2 eruptions underwent extensive change to feed the flare-up. Petrography and phase equilibria suggest that a coalesced network of magma mush zones formed along the N-S graben axis extending down to ~11 km during the episode. Recharge, mingling and mixing of formerly isolated heterogenous magmas within the plumbing system well before these eruptions is indicated by crystal zonation patterns. Mafic end members are evidenced by Fo86–89 olivine, clinopyroxene with Mg# > 85 and calcic plagioclase (An73–89), while evolved magma end members contained Mg# < 75 clinopyroxene and An56–63 plagioclase. Rim-zoning of these phases reflect timespans for equilibration of evolved and mafic crystals to a hybrid melt. The whole-rock compositions of lapilli reflect the hybrid basaltic andesite to andesite, but show diverse glass compositions (56–72 wt% SiO2) implying that magma homogenisation was incomplete before eruption. Crystal-melt equilibria of olivine and clinopyroxene rims reveal polybaric crystallisation, showing mean depths of ~8.5 km (230 ± 70 MPa) at temperatures between 1000 and 1150 °C. At the northern margins of the system, volatile-rich amphibole-bearing magmas were erupted for the first and last eruption of the series, creating stable Plinian eruption styles. This flare-up was previously interpreted as tectonically controlled, however, there were low tectonic extension rates at that time. Hence, we propose instead that magma pressure build-up and recharge beneath Mt. Tongariro drove the inflation and homogenisation of the magma system, fueling the ~200 year-long flare-up. Subsequently, the magma supply system returned to pre-Mangamate activity levels, so that vigorous recharge would be required for a return to >M 4 eruptions.

Magma hybridization and crystallization in coexisting gabbroic and granitic bodies in the mid-crust, Akechi district, central Japan

Petrological and geochemical features of gabbros and fine-grained mafic rocks (mafic microgranular enclaves; MMEs) in the Inagawa Granite of the Ryoke Plutonic Complex were investigated to assess the interactions between coexisting mafic and silicic magmas, and the petrogenetic relationships between the MMEs and surrounding gabbros. The MMEs exhibit mingling textures that imply the coexistence of mafic and silicic magmas that did not undergo complete mixing, but the geochemical compositions of the MMEs require substantial hybridization and homogenization. The gabbroic rocks exhibit disequilibrium textures and mineral compositions, such as quartz–hornblende ocellar textures and patchy plagioclase crystals with bimodal anorthite contents. These textures and compositions record an abrupt decrease in crystallization temperature and mechanical mixing between crystallizing gabbroic mush and silicic (granitic) melt. Geochemical variations of the gabbroic rocks can be explained by hybridization and fractional crystallization (HFC) processes between crystallizing gabbroic mush and granitic melt. Extrapolation of the mixing trend to a basaltic composition suggests that the primitive mafic end-member was a low-K basaltic magma. Given that HFC yields magnesian andesite by the addition of a small amount of silicic melt to a primitive mafic end-member, the compositional modification of mafic magmas by magma mixing might be an essential process in the formation of andesitic magma in arc crust.

Phenocryst zonation records magma mixing in generation of the Neoproterozoic adakitic dacite porphyries from the Kongling area, Yangtze Craton

Adakites and adakitic rocks are pivotal in understanding the formation and evolution of the early continental crust. However, there remain controversies about the origin of these rocks. This paper reports petrological, mineralogical and geochemical data for the newly identified Neoproterozoic (ca. 819 Ma) adakitic dacite porphyries in the Kongling area, Yangtze Craton, with the aim of constraining their petrogenesis and elucidating the role of magmatic processes in generating the adakitic melts. The dacite porphyries are characterized by high SiO2 (62.2–64.8 wt%), Mg# (52–60), Sr/Y and (La/Yb)N ratios, as well as depleted Nd-isotope compositions (ƐNd (819 Ma) = 0.1–1.6), geochemically comparable to high-Mg# adakitic rocks. Core-rim zoned amphibole and plagioclase phenocrysts are common. Overall, the amphibole cores show lower MgO, FeO, Sr, but higher Al2O3 concentrations than the rims. Similarly, the plagioclase cores exhibit lower An [Ca/(Ca + Na + K), atomic ratios] but more enriched 87Sr/86Sr(i) than the rims. Geochemical modeling demonstrates that the melts in equilibrium with the amphibole cores have high SiO2, low MgO, CaO, Fe2O3T and TiO2 as well as flat REE patterns and low Sr/Y ratios. Such felsic melts were likely generated by low-degree dehydration melting of the Paleo- to Meso-proterozoic amphibolites of the Kongling Complex. On the contrary, the melts in equilibrium with the amphibole rims possess lower SiO2 and higher MgO with elevated (La/Yb)N and Sr/Y ratios, suggesting involvement of more mafic melt components. In combination with regional geology, we propose that this mafic endmember possibly formed through fractional crystallization of original subduction-related basaltic magmas that are geochemically equivalent to the Neoproterozoic Panxi-Hannan arc-type basaltic rocks. Mass-balance calculation further reveals that the Kongling dacite porphyries could form by mixing of the above two geochemically distinct endmembers in ratios of ca.50: 50. Importantly, the adakitic signature of these rocks (i.e., high Sr/Y and (La/Yb)N ratios) was inherited from the fractionated arc-type mafic melts, highlighting the significant role of magma mixing in generating the adakitic melts. We propose that this mechanism may have been much more common than as previously thought and contributes significantly to continental growth and evolution.

Two-stage hybrid origin of Lachlan S-type magmas: A re-appraisal using isotopic microanalysis of lithic inclusion minerals

Microanalysis of accessory minerals was carried out to elucidate the controversial origin of the classical, ~430 Ma (Silurian) S-type granites and related lithic inclusions from the eastern Lachlan Orogen, southeastern Australia. Inclusions were separated into igneous-derived, mafic microgranular enclaves (MME) and metasedimentary inclusions. Apatite ɛNd(t), and monazite ɛNd(t) isotopic values for granite host and inclusions cover a similar range (~ −4 to −13), but their ɛNd(t) peaks are distinct. Whereas similar unimodal peaks exist for both minerals in the granites, suggesting widespread equilibration during hybridisation of two magmatic components in the host S-type magma, skewed or bimodal peaks characterize the MME and metasedimentary inclusions, indicating their different petrogenetic histories. The isotopic data suggests two endmembers: a mature metasedimentary source with ɛNd(t) of −12, represented by ubiquitous Ordovician quartzose turbidites, and a more radiogenic component represented by a − 5 ɛNd(t) peak in the MME. Zircon ɛ(Hf) values from −3 to −10, and δ18O values from 7.5‰ to 10.5‰, reflect variable incorporation (40–80%) of metasedimentary material with a mantle-like endmember. Temperature estimates for magmatic equilibration are 750-800 °C, though evidence exists for transient higher temperatures in some inclusions. Phase equilibria modelling using the average composition of the Ordovician turbidites as a potential source-rock suggests that melt compositions were uniform, silicic (75–76 wt% SiO2) and peraluminous (ASI = 1.13–1.22), for a wide range of temperatures and water contents. However, neither the modelled residual rock (restite) compositions, nor the analysed metasedimentary inclusions, lie along the Lachlan S-type compositional array. Rather, the compositional array projects to a mafic endmember defined by the MME, suggesting widespread, efficient hybridization with weakly peraluminous, andesitic magmas. Source mixing models (without significant mantle-derived magma input) are rejected on petrological and geological criteria. The andesitic MME-type magmas were hybridized in the lower crust, probably during mafic magma underplating, before incorporation into large, cool, midcrustal S-type magma reservoirs, and the observed MME represent the final mingled stage of that interaction during emplacement at shallow crustal levels. Using a modern example from the Andes, a model is presented of 2-stage hybridization, initially near the Moho, then in the midcrust as hot, hybrid andesitic magmas repeatedly infused a resident, giant, low-T, S-type magma body. Late-stage injection of the MME generated localized inclusion swarms, as at Cowra and Deddick, and promoted eruption of vast S-type ignimbrite sheets during a major magmatic flare-up in the Lachlan orogen.

Provenance Characterization and Palaeoenvironmental Analysis of the Meta-Sedimentary Rocks of Sonaghati Formation, Betul District, Madhya Pradesh Using Geochemical Approach

Betul belt, ENE-WSW trending, 135 km long, prominent litho-tectonic unit exposed in the central part of Central Indian Tectonic Zone (CITZ) is composed of meta-sedimentary &amp; meta-volcanic rocks intruded by mafic-ultramafic and granitic suite of rocks, belonging to Palaeoproterozoic to Neoproterozoic age. This belt is traversed by several ENE-WSW trending, sub-vertical ductite shear zones. The meta-sedimentary rocks of Sonaghati Formation were geochemically characterized and their geochemical composition was interpreted for provenance characterization and paleo-environmental assessment. The weathering indices including Chemical index of Alteration, Chemical index of Weathering, Plagioclase Index of Alteration and Weathering Index of Parker indicate that theses meta-sedimentary rocks have witnessed the substantial amount of weathering at the source without any evidence of potash metasomatism. The Bivariate plots using the major and trace element composition show co-linear trends, which reflect that all these samples belong to co-genetic population and the visible compositional variation could be attributed to chemical, mineralogical and textural maturity. The Sonaghati metasedimentary rocks are enriched in REE with negative Eu anomaly. The LREE enrichment varies from 122 to 174 times and that of the HREE enrichment ranges from 12 to 31 times of Chondrite indicating highly varied protoliths. The provenance characterization was attempted using the large ion lithophile elements and high field strength elements. The results show that the precursor for these meta-sedimentary litho-units are mixed source with the major contributor being felsic to intermediate and minor contribution has come from the mafic end members. These meta-sedimentary rocks were deposited in the overall semi arid climate with a sequential transition, suggesting the variable climatic conditions ranging from semi-arid to arid. The Cu/Zn, V/Cr ratios, and presence of pyrites dissemination and stringers eventually indicate the prevalence of reducing environmental conditions during the deposition of these meta-sediments.

Platinum-group elements and gold distribution in ores of the Haftcheshmeh porphyry Cu-Mo-Au deposit, NW Iran

The Haftcheshmeh porphyry Cu-Mo-Au deposit is located in the Arasbaran metallogenic belt, northwest Iran, with mineralization occurring mainly in gabbro-diorite and granodiorite porphyry. Pyrite and chalcopyrite are the most abundant sulfides that occur as disseminated and veinlet type ores in the potassic alteration zone. Platinum-group elements (PGEs), Au, and Cu concentrations of selected pyrite- and chalcopyrite-bearing samples were analyzed using in situ LA-ICP-MS and bulk rock Lead Fire Assay. Relatively high contents of PGEs and other precious elements are present in sulfide minerals, especially in pyrite that is the principal carrier of these elements in the deposit. Palladium is the most abundant of the PGEs in the sulfides. The contents of Pd are systematically higher than Pt in this deposit, consistent with other porphyry deposits, although Os, Rh, and Ir abundances are in general equal to or below the limits of detection in pyrite and chalcopyrite. The most important factors related to potential PGEs and chalcophile elements enrichment in the Haftcheshmeh deposit is the oxidized mafic endmember components of source magmas, i.e., high ƒO2 conditions, possibly with a semi-metal collector melt involved, and the high saline magmatic hydrothermal fractionation process. All these mechanisms seem to have enhanced the concentration of PGEs, Au, and other chalcophile metals. PGEs and other chalcophile elements were transported by exsolved high-T, saline magmatic hydrothermal fluids and then low T reaction and reduction to enhance saturation of sulfide phases during mineralization associated with potassic alteration.

Mush remobilisation and mafic recharge: A study of the crystal cargo of the 2013–17 eruption at Volcán de Colima, Mexico

Volcán de Colima is a highly active stratovolcano at the western end of the Trans-Mexican Volcanic Belt. Present-day activity consists of lava dome growth and destruction cycles, lava flows, small explosions, and larger explosive Vulcanian eruptions. It has been postulated that an increased frequency of more mafic eruptions signals the run-up to the end of c. 100-year eruptive ‘cycles’, terminating with a Plinian eruption such as those in 1818 and 1913. It is therefore important to understand the role played by mafic recharge during interplinian activity. We present new petrological and geochemical data for lava and ash from the 2013–17 phase of eruption. The uniform paragenesis and geochemical homogeneity of bulk rocks indicate efficient long-term homogenisation of magmas within the plumbing system, similar to the previous 1998–2005 eruptive products. Mineral chemistry however preserves complex patterns of magma recharge and mixing. Chemical and textural information support the interpretation of two magmatic end-members – an evolved end-member, saturated with respect to Fe--Ti oxides and apatite and crystallising low-An plagioclase and pyroxenes in the Mg# 69–75 range; and a more primitive, mafic end-member, crystallising high-An plagioclase and pyroxenes in the Mg# 77–88 range. Pyroxene textures and zoning patterns suggest mixing of the mafic melts with the evolved magma and remobilisation of the crystal mush. Two-pyroxene geothermometry constrains magmatic temperatures to c. 980–1000 °C for the evolved end-member, and c. 1020–1080 °C for the mafic end-member. Pressure estimates suggest crystallisation at 4–6 kbar, or c. 12–18 km depth. We interpret this to reflect periodic injections of mafic melts and remobilised crystals into evolved reservoirs in a mushy magma storage system in the mid-crust, in agreement with geophysical data suggesting a semi-molten, partially crystallised body at this depth. An increase in reverse zoned crystals, indicative of mafic injection, from mid-2015 onwards suggests that these melts were injected into the system following the large eruption in July 2015. Our findings suggest that the intense July 2015 eruption may be linked to increased input of mafic magmas into the shallow system, indicating that mafic injections may be a key process governing the timing and style of interplinian eruption at Volcán de Colima.

Early Cretaceous magmatism of the Langxian complex in the eastern Gangdese batholith, southern Tibet: Neo-Tethys ocean subduction re-initiation

新特提斯洋长期俯冲消减作用在早白垩世可能经历二次俯冲启动或板片俯冲几何形态的重大转换。确定西藏南部冈底斯岩基早白垩世岩浆作用的岩石地球化学特征和作用方式是甄别上述过程的关键，对理解新特提斯洋的俯冲演化过程至关重要。本文就冈底斯岩基东段朗县杂岩中保存的各类早白垩世岩浆岩，开展了锆石U-Pb地质年代学和Hf同位素、全岩元素和同位素（Sr-Nd）组成分析。数据结果表明：1）基性岩侵位时代为早白垩世晚期（103.6~100.8Ma），为高钾钙碱性偏铝质岩石，锆石ε_Hf（t）=+0.3~+5.7，全岩ε_Nd（t）=-0.8和-0.3，暗示其岩浆源区具有大量俯冲沉积物或流体的混入，为沉积物熔体和流体交代的地幔楔物质部分熔融的产物，经历了一定程度的角闪石分离结晶作用；2）中性岩形成于99.8~97.6Ma，略晚于基性岩，其主量元素与基性岩具有较好的线性关系，全岩ε_Nd（t）=+1.1，具有较多的地幔物质参与，为基性岩浆进一步演化形成；3）酸性岩（脉体）记录了多阶段岩浆作用（124.1~95.3Ma），根据同位素组成不同进一步划分为两类，第一类具有较低的全岩ε_Nd（t）值（-8.3~-6.0），其岩浆源区显示富集特征，t_DM2=1385~1586Ma，由古老地壳物质的再熔融形成；第二类的锆石ε_Hf（t）值（-2.8~+3.2）变化较大，岩脉的锆石ε_Hf（t）=+0.4~+8.1，t_DM=428~906Ma，全岩ε_Nd（t）=+0.1和+0.8，表明岩浆源区具有不均一性，为古老地壳物质被富流体地幔岩浆改造形成；和4）镁铁质包体的主量元素与寄主花岗岩具有较好的线性关系，锆石的Hf同位素组成变化较大（ε_Hf（t）=-9.3~+4.1），变化范围可达13个ε单位，为岩浆混合成因。寄主花岗岩和角闪辉长岩分别作为酸性和基性端元，是基性岩浆与其诱发古老地壳熔融形成的花岗质岩浆经混合形成。结合冈底斯岩基早白垩世岩浆岩的研究结果，朗县杂岩在早白垩世（124~97Ma）的岩浆作用具有明显的岩浆混合现象，锆石Hf和全岩Sr-Nd同位素组成变化较大，可达13个ε单位，其岩浆源区复杂且富含流体，代表了新特提斯洋在早期（240~144Ma）经历漫长的俯冲之后，在早白垩世时期（~120Ma）俯冲带发生跃迁或俯冲角度达到临界点，导致大量俯冲沉积物和流体沿俯冲带俯冲下去，与发生部分熔融的地幔楔物质混合，底侵导致上覆古老地壳物质的再熔融，形成早白垩世复杂的岩浆岩组合，很可能是新特提斯洋二次俯冲开始的标志。;The subduction of the Neo-Tethyan oceanic lithosphere might experienced a major shift in the subdcution geometry in the Early Cretaceous time during its long-lasting subducion. Knowing the geochemical nature and the mode of the Early Cretaceous magmatism in the Gangdese batholith is critical to deccriminate the subdcution processes in this time. Data from zircon U-Pb geochronology and geochemical (whole-rock element and isotope, zircon Hf) analyses on various rock types preserved in the Langxian complex show that: 1) the mafic rocks formed at 103.6~100.8Ma are of high-K calc-alkaline aluminous in composition. They are characterized by relatively low zircon ε_Hf(t) (+0.3~+5.7) and bulk ε_Nd(t) (-0.8 and -0.3), indicating that they were drived from partial melting of depeled mantle wedge metasomatized by fluids from subducted sediment; 2) the intermediate rocks formed at 99.8~97.6Ma and have slightly higher Nd isotope compositions with ε_Nd(t)=+1.1. They display a good linear relationship in major oxides with those in the mafic ones, suggesting that they are derivative products from the mafic suite; 3) the felsic rock (pluton and dike) crystallized at 124.1~95.3Ma. They could be subdivided into two types based on their isotopic compositions. The first type has a lower ε_Nd(t) (-8.3~6.0) but higher t_DM2 (1385~1586Ma). They formed by remelting of ancient crustal materials. In contrast, ε_Hf(t) and ε_Nd(t) in the second type granitic rocks vary widely with ε_Hf(t) ranging from -2.8 to +3.2 for the plutonic granite and from +0.4 to +8.1 for the dike, respectively. Together with relatively low Nd isotope (ε_Nd(t)=+0.1 and +0.8) and t_DM(428~906Ma), these characteristics suggest a relative young source region due to modification of ancient crustal materials by melts derived from fluid-enriched mantle; 4)enclaves hosted by the granitic rocks show good linear relationship in major oxides with the host rocks. They display a rather large shift of ~13 epsilon units in zircon Hf isotope compositions (ε_Hf(t)=-9.3~+4.1) and might represent the melts formed by various degrees of mixing an acidic end member of the hosted granite wih a mafic end member of hornblende gabbro. Combined with literature data of the Gangdese batholith, magmatic rocks from the Langxian complex preserved a key record of a major magmatic process during the Early Cretaceous (122~97Ma). Melting and mixing of various source components (ancient crustal material, fluid-enriched mantle, and depleted mantle) is one of the important features of the Early Cretaceous magmatism due to subduction of the Neo-Tethyan ocean and resulted in large shifts (up to ~13 epsilon units) in zircon Hf as well as in bulk Sr-Nd isotope compositions. To introduce various components into and achieve a large isotopic heterogeneity in the source region, it requires a major change in the subduction geometry in the long subduction of the Neo-Tethyan subdction system. Since the initation of the subduction at ~240Ma, subdcition of the Neo-Tethyan Ocean might experinecd a critical reorganization enabling the introduction of an increased amount of sediments and fluids into the subduting system and the modification of the mantle wedge in the Early Cretaceous (~120Ma). The mafic magmas caused remelting of the overlying ancient crustal material and finally formed the complex magmatic activity in the Early Cretaceous, which is likely to be a sign of the beginning of the second subduction of the Neo-Tethyan Ocean.

Mineralogical Evidence of Pre-caldera Magma Petrogenesis in the Jemez Mountains Volcanic Field, New Mexico, USA

Abstract The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.

Important role of magma mixing in generating the late Carboniferous Tayuan Complex during subduction of the Paleo-Asian oceanic plate beneath the Xing'an–Erguna Massif, NE China: Evidence from petrology, geochemistry, and zircon U–Pb–Hf isotopes

To constrain the processes of mantle–crust interactions during magma generation and reconstruct the tectonic evolution of the Paleo-Asian Ocean, we present an integrated study involving detailed geological field work, petrological observations, zircon U–Pb–Hf isotope analyses, whole-rock geochemistry, and mineral chemistry for the Tayuan Complex in the northern Great Xing'an Range, NE China. The Tayuan Complex is composed mainly of monzogranites (host granitoids) and gabbros with minor amounts of quartz monzonites, and microgranular enclaves (MMEs) are unevenly distributed within the host granitoids. Our new zircon U-Pb dating results show that the host granitoids were emplaced during the late Carboniferous (318 ± 4 Ma and 319 ± 2 Ma). The MMEs yield similar zircon U-Pb ages of 320 ± 2 Ma and 322 ± 5 Ma, within error of the emplacement ages of the host granitoids. In this study, the petrological and geochemical features of the Tayuan Complex indicate disequilibrium conditions, and we favor a magma mixing model to explain the genesis of various rock types within the complex. Combined with the petrological and geochemical observations, we suggest that the host granitoids can be ascribed to felsic end-member and they originated from the partial melting of the Meso–Neoproterozoic medium- to high-K basaltic lower crust. According to the petrological and geochemical features, the gabbros in Tayuan complex are closely related to the mafic end-member, and we conclude that they were derived from the partial melting of depleted mantle that was metasomatized by subducted slab-derived fluids, and that clinopyroxene, hornblende and Fe-Ti oxides were fractionated during magma generation. The MMEs and coeval quartz monzonites in Tayuan complex represent the mixing of felsic and mafic end-member magmas. Our new data, combined with the spatial distribution and temporal evolution of the Carboniferous magmatism in the Great Xing'an Range, indicate the generation of the late Carboniferous Tayuan complex was related to the northwestward subduction of the Paleo-Asian oceanic plate.

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.

Evidence for a ‘third’ endmember of the Unzen 1991–1995 eruption from amphibole thermometry and crystal clots

Amphibole phenocrysts and crystal clots composed of amphibole, plagioclase, and interstitial glass were observed in dacitic products of the Unzen 1991–1995 eruption and analyzed in order to understand pre-eruptive magmatic processes. Amphibole phenocrysts in the samples erupted at different times are classified by composition as either magnesiohornblendes or tschermakites. Temperatures of 770–830 °C are estimated from magnesiohornblende phenocrysts and amphiboles in the crystal clots, which are consistent with the proposed temperature of the silicic endmember magma. The average approximate composition of the interstitial melt is 67–73 wt% SiO2, ~0.25 wt% TiO2, ~12 wt% Al2O3, ~1.1 wt% FeO, 0.14 wt% MgO, 0.80 wt% CaO, 3.0 wt% Na2O, and 4.0 wt% K2O. The SiO2 plus H2O content (~8.8 wt%) of the melt estimated from these amphiboles (SiO2melt = 71–72 wt%) is consistent with the compositions of interstitial melts observed in the crystal clots. The H2O saturation depth estimated for the interstitial melt and plagioclase compositions is consistent with that of the magma chamber where the silicic endmember magma was likely stored (11–15 km). These results suggest that interstitial melts in crystal clots represent the silicic endmember melt. Temperatures estimated from tschermakite phenocrysts are 870–950 °C, which is lower than the proposed temperature of the mafic endmember magma. In addition, a temperature gap is observed between the tschermakites and magnesiohornblendes. These results suggest the contribution of a previously unrecognized third endmember magma to the 1991–1995 magma, which we term the mid-temperature magma (MT magma).