Basic Magma Research Articles

Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet

Postcollisional potassium (K)-rich magmatic rocks have important implications for unveiling the deep geodynamic processes active during the evolution of orogenic belts and the surface uplift of plateaus. However, their petrogenesis remains highly controversial. Here we report on zircon U-Pb and mineral Ar-40-Ar-39 age, zircon Hf-O isotope, mineral composition, whole-rock major and trace element composition, and Nd-Sr isotope data for the postcollisional K-rich lavas in the Gemuchaka area of the central Qiangtang Block (central Tibet). Age dating suggests that these lavas were generated in the late Early Oligocene (ca. 30 Ma). Two series of rocks were recognized: silica-undersaturated and -saturated lavas. Both series are geochemically characterized by continuous variation trends. All rocks are enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs) and depleted in Ta-Nb-Ti, with negligible Sr and Eu and obviously positive Pb anomalies. The rocks have higher Th/ La (0.2-0.8) and Nb/Ta (18.6-19.8) values than asthenospheric mantle-derived rocks but slightly variable values of epsilon(Nd) (t) (-6.0 to -4.6), (Sr-87/Sr-86)(i) (0.7076-0.7083), and zircon epsilon(HF)(t) (-7.2 to +0.8) and delta O-18 (7.1-8.5 parts per thousand). We suggest that the parental magmas of these K-rich lavas were likely derived by variable partial melting of lithospheric mantle metaso-matized by subducting continental sediment-derived melts with the addition of minor asthenospheric components. Element modeling results suggest that the basic end members were likely generated by fractionation processes dominated by olivine and minor clinopyroxene from parental magmas, and some other evolved rocks were generated by crystal (olivine, clinopyroxene, Fe-Ti oxides) fractionation from basic magmas. Combining geological and Cenozoic magmatic rock data in the Qiangtang Block, we propose that the Gemuchaka magmatism was most likely triggered by the delamination of lithospheric mantle thickened by early (Paleocene- Eocene) intracontinental subduction and subsequent asthenospheric upwelling.

Zirconolite, Baddeleyite, Zircon, and Thorite of Island-Arc Quartz Gabbronorite-Dolerites of the Ayu-Dag Intrusive, Crimean Mountains

The Early Bajocian island-arc quartz gabbronorite-dolerites of the Ayu-Dag intrusive contain syngenetic zirconolite and baddeleyite. Zirconolite includes Ca-dominant and atypical Y-dominant (Y, Ca, Th, REE)2FeZr2Ti3O14 varieties. Two genetic types of zircon and thorite occur. The dominant xenogenic zircon-I with thorite inclusions and numerous melting traces (lacunes) is enriched in Hf, Th, Y, and P (up to 6 wt % HfO2, 5 wt % ThO2, 6 wt % Y2O3, 3 wt % P2O5). Thorite contains 7–9 wt % UO2. Zircon-I and thorite-I of similar compositions are typical minerals of highly radioactive granites. The isotope age of xenogenic zircon is more than 2 Ga. Rare syngenetic zircon-II is extremely depleted in Th, U, Y, and Hf. Thus, the Ayu-Dag basic magma was contaminated by Early Precambrian highly radioactive granites, whose bodies probably occur in the basement of Crimean Mountains.

Zirconolite, baddeleite, zircon and thorite of island arc quartz gabbro-norite-dolerites of Ayu-Dag intrusion (Mountain Crimea)

Early Bajocian island arc quartz gabbro-norite-dolerites of Ayu-Dag contain syngenetic zirconolite and baddeleite. Zirconolite presents as Ca-dominant and uncommon Y-dominant (Y, Ca, Th, REE)2FeZr2Ti3O14. Two genetic types of zircon and thorite are developed. Predominant xenogenic zircon-I with thorite inclusions and a mass of lacunes — traces of reflow, is enriched with Hf, Th, Y, P (up to 6 wt.% HfO2, 5% ThO2, 6% Y2O3, 3% Р2О5). Thorite contains 7–9% UO2. Zircon and thorite of such composition are typical minerals of highly radioactive granites. Isotope age of zircon (xenogenic) is >2 billion years. Rare syngenetic zircon-II is sharply poor of Th, U, Y and Hf. So, basic magma of Ayu-Dag was contaminated with substance of Early Precambrian highly radioactive granites, which bodies are probabely located in the Mountain Crimea substructure.

Petrogenesis and Tectonic Implications of Peralkaline A-Type Granites and Syenites from the Suizhou-Zaoyang Region, Central China

ABSTRACT: In this study, we present systematic petrological, geochemical, LA-ICP-MS zircon U-Pb ages and Nd isotopic data for the A-type granites and syenites from Suizhou-Zaoyang region. The results show that the peralkaline A-type granites and syenites were episodically emplaced in Suizhou-Zaoyang region between 450??3 and 441??7 Ma which corresponds to Late Ordovician and Early Silurian periods, respectively. Petrologically, the syenite-peralkaline granite association comprises of nepheline normative-syenite and alkaline granite in Guanzishan and quartz normative syenite and alkaline granite in Huangyangshan. The syenite-granite associations are ferroan to alkali in composition. They depict characteristics of typical OIB (oceanic island basalts) derived A-type granites in multi-elements primitive normalized diagram and Yb/Ta vs. Y/Nb as well as Ce/Nb vs. Y/Nb binary plots. Significant depletion in Ba, Sr, P, Ti and Eu indicates fractionation of feldspars, biotite, amphiboles and Ti-rich augite. The values of ??Nd(t) in Guanzishan nepheline syenite and alkaline granite are +1.81 and +2.26, respectively and the calculated two-stage model age for these rocks are 1 040 and 1 003 Ma, respectively. On the other hand, the Huangyangshan alkaline granite has ??Nd(t) values ranging from +2.61 to +3.46 and a relatively younger two-stage Nd model age values ranging from 906 to 975 Ma, respectively. Based on these data, we inferred that the Guanzishan nepheline syenites and granites were formed from fractional crystallization of OIB-like basic magmas derived from upwelling of metasomatized lithospheric mantle. The Huangyangshan quartz syenite and granite on the other hand, were formed from similar magmas through fractional crystallization with low input from the ancient crustal rocks. Typically, the rocks exhibit A1-type granite affinity and classified as within plate granites associated with the Ordovician crustal extension and the Silurian rifting.

Chapter 2 Scottish mineral Geological Conservation Review Sites – Magmatic Minerals

Following the pioneering work of James Hutton in the 18th century, which was largely carried out on Scottish rocks and minerals, it became generally accepted that igneous rocks formed from liquid magmas. The selected 10 sites cover a wide range of magmatic events, rock types and mineral assemblages. In the case of basic and ultrabasic rocks, prolonged cooling and repeated injections of fresh magma into the magma chamber can lead to settling of early crystals resulting in a rhythmic mineral layering and accumulations of mineral species, such as those bearing chromium and nickel, which have economic importance. The former chromite quarries at Hagdale and Nikka Vord, on Unst ably illustrate the size and podiform nature of the massive chromite bodies and also contain examples of chromium secondaries, such as chromian clinochlore, whereas the water-worn rocks on the shore at Wick of Hagdale provide clean exposures of banded and disseminated chromite. Podiform chromite also features in the Corrycharmaig serpentinite, but the focus of interest there is on the extensive replacement of the original mineralogy by talc and ferroan magnesite. A somewhat different magmatic mineral assemblage is present at Talnotry where contamination of a noritic magma by sedimentary rocks led to the precipitation of Cu-Ni sulphides and platinum group minerals.Prolonged fractionation of mafic minerals in basic magmas can lead to the production of anorthosite. Since such rocks are extremely rare in Scotland the inclusion of the South Harris Anorthosite (which also includes gabbro) in the Scottish Mineral sites is particularly apposite. Mineral chemistries in igneous rocks can provide important indications of cooling histories and formational conditions, particularly with regard to depth of origin. Thus the presence of large zoned clinopyroxene crystals in the volcanic rocks at Bail Hill and sanidine phenocrysts, of appreciable size in a trachybasalt dyke at the Bangley Quarry, both within extremely fine-grained matrices, demonstrate development in a differentiating magma at depth before emplacement. The remaining two sites included in this chapter contain material which provides a window into the upper mantle below Scotland. These comprise pyrope and zircon xenocrysts along with ultramafic xenoliths in the volcanic ventat the Elie Ness site and large corundum (sapphire), sanidine, phlogopite, biotite and magnetite megacrysts in the Cnoc a’ Chapuill-Loch Roag dyke.

The geochemistry, tectonic and palaeogeographic setting of the Karing Volcanic Complex and the Dusunbaru pluton, an early Permian volcanic-plutonic centre in Sumatra, Indonesia

The Karing Volcanic Complex and the Dusunbaru Pluton comprise an Early Permian volcanic-plutonic centre in the Volcanic-Plutonic Arc on the western margin of the Kluet-Kuantan basin of the West Sumatra Block in Sumatra, Indonesia. The Karing Volcanic Complex is mostly composed of ‘Monotonous Intermediate Type’ tuffs which interfinger with the volcaniclastics and coastal and marine sediments of the Mengkarang Formation. Eight lithological Intervals each commencing with tuffs overlain by volcaniclastics and sediments, have been mapped in the Merangin river section. The climax of the volcanicity occurred in Interval IV. In the waning stages of volcanism basalt flows were extruded and the volcanic pile was eroded by lahars and debris flows. Zircon CA-IDTMS dates from tuff beds at the base and top of the section bracket the volcanic activity as having taken place over a period of 630,000 years within the Asselian Stage of the Permian. The Dusunbaru pluton is composed of Gabbroids, Granitoids with subordinate Metabasalts and Metatuffs occurring as xenoliths. These lithologies are present as lithic clasts within the Karing Volcanic Complex tuffs, and it is considered that the Dusunbaru Pluton represents a former magma chamber from which the tuffs were vented. The Mildly-alkaline to Sub-alkaline Metabasalts and Gabbroids, were derived from the N-MORB sector of the lithospheric ‘Mantle Array’ while the compositions of the Granitoids (including gabbroid-granitoid hybrid rocks) were influenced by the reaction of basic magmas with the lower crust. The Sub-alkaline and High–K Late Basalt Lava flows were derived from the melting of basalt below the crust-mantle interface. The rock suites were variably hydrated as a result of alterations. Magmatic alteration has affected the Metabasalts, Gabbroids, Granitoids, their xenoliths and the Late Basalts. Hydrothermal alteration has weakly affected the aubaerial tuffs and strongly changed the mineralogy of some of the Late Basalt flows, in particular the entablatures. The chemistry of the Dusunbaru Pluton is of Cordilleran-type, typical of continent margin volcanic-plutonic arcs. The Dusunbaru Pluton was likely originally emplaced at the interface between the Kuantan Formation (Visean to Permian) and a Proterozoic gneissic basement. A late hydrothermal event/tectonic event during which the Dusunbaru Pluton was faulted against the Karing Volcanic Complex, is attributed to the collision of the Woyla oceanic terrains with Sumatra in the late-mid Cretaceous. The stratigraphy of the West Sumatra Volcanic Arc and the Kluet-Kuantan Basin resembles that of the Sukhothai Volcanic Arc and associated basin sediments in the Indochina Block. A palaeogeographical reconstruction suggests that the West Sumatra Block originally was an appendage to the Indochina Block at a palaeolatitude between 5 and 15° S.

Protracted zircon geochronological record of UHT garnet-free granulites in the Southern Brasília orogen (SE Brazil): Petrochronological constraints on magmatism and metamorphism

The investigation of ultrahigh temperature (UHT) metamorphic rocks, and their corresponding (pressure)-temperature-time (P-T-t) history is critical to distinguish between arc- or collision-related metamorphism. This is a very challenging task if mineral assemblages are highly retrogressed and isotopic systems are disturbed. Garnet-free granulites lacking accessory minerals (chronometers) and metamorphic index minerals (thermobarometers) located in UHT domains are examples of such complex systems. In such cases, zircon may be the main chronometer, although isotopic U-Pb data outline protracted records, making the interpretation of the data complex. This study focuses on the timing of magmatism and metamorphism, as well as on the thermal metamorphic conditions of garnet-free UHT granulites of the Guaxupé nappe, southernmost Brasília orogen, located close to the Paranapanema cratonic block. It presents U-Pb dating, Lu-Hf isotopes and trace element signatures of zircon, and thermometry on metamorphic clinopyroxene and orthopyroxene from granulites. Steady 176Hf/177Hf(t) in zircon cores exhibiting U-Pb dates spreading in the Concordia suggest post-crystallization disturbance. From those disturbed granulitic systems, minimum crystallization ages of ca. 2550 Ma, ca. 790 Ma, ca. 690 Ma and ca. 660 Ma can be retrieved. The juvenile ca. 2.55 Ga granulite is the first evidence of an exposed rock of the Paranapanema cratonic block, previously only inferred from geophysical data. The Guaxupé nappe records arc-related magmatic episodes in the range of 790–640 Ma, partially coeval with a long-lasting (∼80 m.y.) metamorphic event (670–590 Ma) and intrusion of basic magma (ca. 660 Ma). Thermometry on zoned clinopyroxene and orthopyroxene yields UHT conditions around 900–1000 °C. Comparing the distribution patterns of metamorphic zircon rims and newly formed grains, we suggest two distinct metamorphic stages: i) an arc-related metamorphism (670–640 Ma), recorded by domains possibly formed by subsolidus recrystallization; and ii) a continental collision to decompression involving partial melting (630–590 Ma) associated to extensive zircon crystallization. The temporal relationship between magmatic and metamorphic ages suggests an ultrahigh-temperature metamorphic event related to a magmatic arc. This arc was afterwards involved by the Guaxupé nappe stacking during the collisional stage of the southernmost Brasília orogen.

Timescales of pre-eruptive magmatic processes at Vulcano (Aeolian Islands, Italy) during the last 1000 years

An integrated petrological and geochemical study on plagioclase crystals erupted at Vulcano Island (Aeolian Islands, Southern Italy) over the past 1000 years allowed us to draw a detailed model of the internal structure of the volcanic system, in which modes of magma interaction and timescales of storage at crustal depth are shown. The integration of compositional, textural and temporal record preserved in plagioclase crystals provides evidence for an articulated plumbing system constituted by several reservoirs connected at the crust-mantle boundary. Here, a basaltic-shoshonitic magma resides and is thought to feed the shallow magma reservoir of both La Fossa and Vulcanello centers, finally triggering eruptions throughout injections from depth. Textural and micro-compositional data on plagioclase crystals suggest the presence of three main magma levels located between ca. 17 and 2 km of depth beneath La Fossa Cone, which were intermittently reactivated over the whole period of activity considered. Plagioclase textures and compositional zoning indicate that the shallow (<11 km bsl) portions of the La Fossa plumbing system were particularly active over the last 1000 years, as crystals record the ascent and continuous episodes of magma recharge and mixing that affect the shallower reservoirs. The first stage of activity at Vulcanello (i.e. Vulcanello I eruption) was fed by slightly differentiated melts that directly rose from the deep basaltic-shoshonitic reservoir, residing for a short period of time into the crust before the eruption. Indeed, diffusion modeling calculations on Sr zoning in plagioclase indicate shorter timescales of residence (<2 years) for crystals erupted at Vulcanello compared to those of the La Fossa Cone eruptions (ca. 2–10 years). If compared with the time span between eruptions occurred at La Fossa, our time estimations may suggest that magma feeding the activity at La Fossa resides most of the time in reservoirs located below the plagioclase nucleation depth (~11 km of depth), finally rising up only few years before the eruption onset. According to our model, magmatic eruptions at Vulcano Island are related to the ascent of deep basic (basaltic/shoshonitic) magmas that trigger a sort of “reaction chain” through subsequent episodes of recharge and mixing toward the upper magmatic reservoirs.

Petrogenesis and tectonic implications of Late Jurassic – Early Cretaceous granitic magmatism in the Xing’an Block, Northeast China: geochronological, geochemical, and Hf isotopic evidence

This study presents new geochronological, whole-rock geochemical, and zircon Hf isotopic evidence for the age, petrogenesis, and source of Mesozoic granitic rocks of the Xing’an Block, Northeast China. This evidence reveals the Late Mesozoic tectonic evolution of the eastern section of the Central Asian Orogenic Belt. Laser-ablation inductively coupled plasma – mass spectrometryzircon U–Pb age data indicate that the syenogranite, monzogranite, and alkali feldspar granite units, as well as their associated diorite microgranular enclaves, were emplaced between 150–142 Ma, providing evidence of Late Jurassic to Early Cretaceous magmatic events within the Xing’an Block. The granites contain high concentrations of SiO2 (65.24%–75.73 wt.%) and K2O (3.94%–5.30 wt.%), low concentrations of MgO (0.10%–1.30 wt.%), and A/CNK values of 0.92–1.06. In addition, Hf isotopic analysis of zircons from the 150–142 Ma granites yields εHf(t) values of +4.54 to +12.16 and two-stage Hf model aged from 906 to 423 Ma, indicating that they formed from magmas generated by partial melting of a juvenile Neoproterozoic–Phanerozoic accreted crustal source. The basic magma source for the diorite microgranular enclaves most likely formed from partial melting of a depleted mantle that had been metasomatized by subduction-related fluids. Combining these new geochemical data with the geology of this region, Late Jurassic to Early Cretaceous magmatism in the Xing’an Block most likely occurred in an extensional environment associated with closure of the Mongol–Okhotsk Ocean.

Oligocene subduction-related plutonism in the Nodoushan area, Urumieh-Dokhtar magmatic belt: Petrogenetic constraints from U–Pb zircon geochronology and isotope geochemistry

Geochemical data and Sr–Nd isotopes of the host rocks and magmatic microgranular enclaves (MMEs) collected from the Oligocene Nodoushan Plutonic Complex (NPC) in the central part of the Urumieh–Dokhtar Magmatic Belt (UDMB) were studied in order to better understand the magmatic and geodynamic evolution of the UDMB. New U–Pb zircon ages reveal that the NPC was assembled incrementally over ca. 5 m.y., during two main episodes at 30.52 ± 0.11 Ma and 30.06 ± 0.10 Ma in the early Oligocene (middle Rupelian) for dioritic and granite intrusives, and at 24.994 ± 0.037 Ma and 24.13 ± 0.19 Ma in the late Oligocene (latest Chattian) for granodioritic and diorite porphyry units, respectively. The spherical to ellipsoidal enclaves are composed of diorite to monzodiorite and minor gabbroic diorite (SiO2 = 47.73–57.36 wt.%; Mg# = 42.15–53.04); the host intrusions are mainly granite, granodiorite and diorite porphyry (SiO2 = 56.51–72.35 wt.%; Mg# = 26.29–50.86). All the samples used in this study have similar geochemical features, including enrichment in large ion lithophile elements (LILEs, e.g. Rb, Ba, Sr) and light rare earth elements (LREEs) relative to high field strength elements (HFSEs) and heavy rare earth elements (HREEs). These features, combined with a relative depletion in Nb, Ta, Ti and P, are characteristic of subduction-related magmas. Isotopic data for the host rocks display ISr = 0.705045–0.707959, εNd(t) = −3.23 to +3.80, and the Nd model ages (TDM) vary from 0.58 Ga to 1.37 Ga. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with ISr ranging from 0.705513 to 0.707275 and εNd(t) from −1.46 to 4.62. The MMEs have TDM ranging from 0.49 Ga to 1.39 Ga. Geochemical and isotopic similarities between the MMEs and their host rocks demonstrate that the enclaves have mixed origins and were most probably formed by interactions between the lower crust- and mantle-derived magmas. Geochemical data, in combination with geodynamic evidence, suggest that a basic magma was derived from an enriched subcontinental lithospheric mantle (SCLM), presumably triggered by the influx of the hot asthenosphere. This magma then interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on Sr–Nd isotope data indicate that ∼50% to 90% of the lower crust-derived melt and ∼10% to 50% of the mantle-derived mafic magma were involved in the genesis of the early Oligocene magmas. In contrast, ∼45%–65% of the mantle-derived mafic magma were incorporated into the lower crust-derived magma (∼35%–55%) that generated the late Oligocene hybrid granitoid rocks. Early Oligocene granitoid rocks contain a higher proportion of crustal material compared to those that formed in the late Oligocene. It is reasonable to assume that lower crust and mantle interaction processes played a significant role in the genesis of these hybridgranitoid bodies, where melts undergoing fractional crystallization along with minor amounts of crustal assimilation could ascend to shallower crustal levels and generate a variety of rock types ranging from diorite to granite.

Petrogenetic and tectonic implications of major and trace element and radiogenic isotope geochemistry of Pliocene to Holocene rocks from the Tacaná Volcanic Complex and Chiapanecan Volcanic Belt, southern Mexico

New major and trace element, and Sr, Nd and Pb isotope data for samples from the Volcán Tacaná, along with the published data from the Tacaná Volcanic Complex (TVC) and Chiapanecan Volcanic Belt (CVB), were used to better constrain the petrogenesis of the Pliocene to Holocene volcanism in southwestern Mexico. The TVC volcanic rocks sampled in this study were andesite, whereas the literature samples included, besides andesites, one subalkali basalt and a few dacites. The CVB housing the prominent El Chichón volcano, on the other hand, showed more alkalic magmatic products (trachyandesite and basaltic trachyandesite) but also included a few basalts and dacites. The enclaves and lithic fragments sampled from both TVC and CVB showed wide SiO2 compositions similar to the volcanic products. The CVB rocks are more enriched in REE, LILE, and HFSE than the TVC. The combined data of volcanic rocks were evaluated through conventional multi-element diagrams as well as subduction- or crust-sensitive parameters. Although the TVC constitutes the northern end of the Central American Volcanic Arc (CAVA), some chemical differences were documented between them such as the TVC rocks have higher LILE, REE, HFSE, and combined ratio parameters. More importantly, the CVB seems to be significantly different from both TVC and CAVA as well as from other continental arcs such as higher LILE, REE, HFSE, and distinct isotopic signature, which may indicate that the origin of the CVB volcanism represents a direct action of the abundant strike-slip faults and extensional system prevalent in this area. Finally, the relationship of basic and evolved rocks from the CVB, especially on isotope-isotope diagrams, also suggests that the lower crust may have contributed to evolved magmas during the ascent of basic magmas through the crust.

Occurrence of High Level Magma Intrusives and Associated Mineralization near Gani Inlier, Kurnool Sub-Basin, Cuddapah Basin, India

An ensemble of spatially associated high level magma intrusives comprising dykes, laccolith and vent have been identified near Gani inlier in the Kurnool sub-basin. Geological setting, morphology of igneous intrusives and their relationships with the sediment cover have been documented. Diabase in laccolith, dykes and vesicular basalt in vents are prominent rocks and have been subjected to extensive deuteric alterations viz. saussurtisation and uralitisation. It is envisioned that the basic magma was intruded as dykes in strike-slip fault domain, outpoured through the vents as vesicular basalts at the intersection zones of cross faults. The vertical ascending melt was deflected as a sheet due to fault control at a sub-surface level of ∼700m along the lithological discontinuities in country rock sediments. The sheet to laccolith transition has resulted in the uplift of older Tadpatri Formation as an inlier amidst the Neoproterozoic Kurnool sediment cover. The localization of iron ore and copper mineralization adjacent to intrusives is attributed to late stage melt-fluid activity associated with the igneous activity. Based on field relationships it is suggested that the high level magmatic activity had occurred after the Kurnool sedimentation in this part of the Kurnool sub-basin.

Deposits, petrology and mechanism of the 2010–2013 eruption of Kizimen volcano in Kamchatka, Russia

Kizimen volcano in Kamchatka is well known as a source of highly heterogeneous poorly mingled magmas ranging from dacites to basaltic andesites. In 2010–2013, the volcano produced its first historical magmatic eruption with the deposition of 0.27 km3 of block and ash pyroclastic flows accompanied by slow extrusion of a 200-m-thick, highly viscous (1010–1011 Pa s) block lava flow with a volume of 0.3 km3. The total volume of erupted magma comprised approximately 0.4 km3 DRE. We provide description of the eruption chronology, as well as the lithology and petrology of eruptive products. The erupted material is represented by banded dacite and high-silica andesite. The dacitic magma was formed during a long dormancy after the previous magmatic eruption several hundred years ago with mineral compositions indicating average pre-eruptive temperatures of ~ 810 °C, fO2 of 0.9–1.6 log units above the nickel–nickel oxide (NNO) buffer and shallow crustal storage conditions at ~ 123 MPa. The silica-rich andesite represents a hybrid magma, which shows signs of recent thermal and compositional disequilibrium. We suggest that the hybrid magma started to form in 1963 when a swarm of deep earthquakes indicated an input of mafic magma from depth into the 6–11-km-deep silicic magma chamber. It took the following 46 years until the magma filling the chamber reached an eruptible state. Poor mingling of the two melts is attributed to its unusually high viscosity that could be associated with the pre-eruptive long-term leakage of volatiles from the chamber through a regional tectonic fault. Our investigations have shown that shallow magma chambers of dormant volcanoes demonstrating strong persistent fumarolic activity can contain highly viscous, degassed magma of evolved composition. Reactivation of such magma chambers by injection of basic magma takes a long time (several decades). Thus, eruption forecasts at such volcanoes should include a possibility of long time lag between a swarm of deep earthquakes (indicating the recharge of basic magma from depth) and the following swarm of shallow earthquakes (indicating final ascent of the hybrid magma towards the surface). Due to the high viscosity of the magma, the shallow swarm can last for more than a year. The forthcoming eruption can be of moderate to low explosivity and include extrusion of viscous lava flows and domes composed of poorly mingled magmas of contrasting compositions.

Minettes do Stock Monzonítico Glória Norte: evidência de magmatismo ultrapotássico pós-orogênico, com assinatura de subducção, no Sistema Orogênico Sergipano

Lamprófiros ocorrem como enclaves microgranulares no Stock Monzonítico Glória Norte (SGN) (588 ± 5 Ma), localizado na parte central do Domínio Macururé, do Sistema Orogênico Sergipano, no estado de Sergipe. Esses enclaves lamprofíricos foram classificados como minettes e ocorrem concentrados em uma área com aproximadamente 1 km de diâmetro na região sudeste do stock. Os minettes apresentam texturas porfirítica e panidiomórfica, com fenocristais de biotita e matriz com granulação fina. Os minerais máficos presentes, além da biotita, são: diopsídio, augita, hornblenda e titanita. Ortoclásio pertítico, andesina, biotita, carbonato, apatita e zircão também estão presentes na matriz. Os dados dos elementos maiores (K2O/Na2O &gt; 3, 6 &gt; %MgO &gt; 8, K2O &gt; 3%), menores e traços (baixos valores de Ti, Nb, Ta) indicam magmatismo ultrapotássico orogênico. As razões Th/Yb &gt; 9,00 e 0,05 &lt; Ta/Yb &lt; 0,20 apontam para magma formado por fusão de um manto enriquecido. As informações obtidas neste estudo indicam que magmas básicos ultrapotássicos estiveram presentes durante o período pós-tectônico no Sistema Orogênico Sergipano (SOS).

Precambrian protoliths and Phanerozoic overprinting on the Wuyishan terrain (South China): New evidence from a combination of LA-ICPMS zircon and EMP monazite geochronology

Wuyishan is one of the most important terrains exposing the Precambrian basement of the Cathaysia Block, South China. Conventionally, Paleoproterozoic and Neoproterozoic sequences have been proposed in this terrain, however, the thermal histories affecting each unit within these sequences remain unclear. We report here spot analysis of 448 LA-ICPMS U-Pb zircon and >1600 EMP Th-U-Pb monazite ages for twenty samples, including schist, gneiss, migmatite and amphibolite, from different units within the basement. U-Pb zircon dating yield some concordant Neoproterozoic and/or Early Paleozoic age clusters with a wide spread of Neoarchean to Mesoproterozoic inherited ages (except amphibolite), indicating the formation of the Mayuan/Chencai core in the Early Neoproterozoic (887–827 Ma) and peripheral units in two stages of the Late Neoproterozoic (746–731 Ma and 681–660 Ma). The in-situ EMP monazite analyses give consistent Early Paleozoic ages (440–426 Ma) from inclusions in various host minerals for all but two samples from same locality that are Triassic in age (239–237 Ma). A large population of Early Paleozoic interstitial monazites shows breakdown texture and ages of secondary monazites range from 376 Ma to 358 Ma, conforming with the U-Pb zircon age of the amphibolite (370 ± 5 Ma). A few samples exhibit relict ages of ∼540 Ma from monazite cores. We conclude that the Wuyishan terrain is dominated by Early and Late Neoproterozoic basement sequences related to collisional and continental rifting orogenies, respectively, while the Badu Group in Zhejiang as a Paleoproterozoic protolith. The Neoproterozoic basement incorporating some Early Cambrian detritus, has thoroughly metamorphosed during the Silurian and further affected by the regional fluid-aided metasomatism coeval with the basic magma, now represented by amphibolites, around ∼370 Ma. The EMP monazite dating results for the Wuyishan terrain are quite different from the Yunkai massif where Triassic ages predominate, although the Wuyi-Yunkai metamorphic belt is often referred to as the same orogen in the Cathaysia Block.

Space‐Time Evolution of Magma Storage and Transfer at Mt. Etna Volcano (Italy): The 2015–2016 Reawakening of Voragine Crater

AbstractThe eruptions of December 2015 and May 2016 at Voragine crater were among the most explosive recorded during the last two decades at Mt. Etna volcano. Here we present data coming from geophysics (infrasound, LP, VLP, volcanic tremor, VT earthquakes, and ground deformations) and petrology (textural and microanalytical data on plagioclase and olivine crystals) to investigate the preeruptive magma storage and transfer dynamics leading to these exceptional explosive eruptions. Integration of all the available data has led us to constrain chemically, physically, and kinetically the environments where magmas were stored before the eruption, and how they have interacted during the transfer en‐route to the surface. Although the evolution and behavior of volcanic phenomena at the surface was rather similar, some differences in storage and transfer dynamics were observed for 2015 and 2016 eruptions. Specifically, the 2015 eruptions have been fed by magmas stored at shallow levels that were pushed upward as a response of magma injections from deeper environments, whereas evidence of chemical interaction between shallow and deep magmatic environments becomes more prominent during the 2016 eruptions. Main findings evidence the activation of magmatic environments deeper than those generally observed for other recent Etnean eruptions, with involvement of deep basic magmas that were brought to shallow crustal levels in very short time scales (∼1 month). The fast transfer from the deepest levels of the plumbing system of basic, undegassed magmas might be viewed as the crucial triggering factor leading to development of exceptionally violent volcanic phenomena even with only basic magma involved.

Ultrafast syn-eruptive degassing and ascent trigger high-energy basic eruptions

Lithium gradients in plagioclase are capable of recording extremely short-lived processes associated with gas loss from magmas prior to extrusion at the surface. We present SIMS profiles of the 7Li/30Si ion ratio in plagioclase crystals from products of the paroxysmal sequence that occurred in the period 2011–2013 at Mt. Etna (Italy) in an attempt to constrain the final ascent and degassing processes leading to these powerful eruptions involving basic magma. The observed Li concentrations reflect cycles of Li addition to the melt through gas flushing, and a syn-eruptive stage of magma degassing driven by decompression that finally produce significant Li depletion from the melt. Modeling the decreases in Li concentration in plagioclase by diffusion allowed determination of magma ascent timescales that are on the order of minutes or less. Knowledge of the storage depth beneath the volcano has led to the quantification of a mean magma ascent velocity of ~43 m/s for paroxysmal eruptions at Etna. The importance of these results relies on the application of methods, recently used exclusively for closed-system volcanoes producing violent eruptions, to open-conduit systems that have generally quiet eruptive periods of activity sometimes interrupted by sudden re-awakening and the production of anomalously energetic eruptions.

Petrogenesis of composite dikes in granitoids of western Transbaikalia

The performed studies have revealed two varieties of composite dikes differing in morphology and internal structure depending on (1) the proportions of salic and basic components and (2) the rheologic state of the host environment. The latter can be both a solid substrate with open fractures and a melt at different stages of crystallization. The evaluated isotopic age of dikes in the Shaluta massif, 290.8 ± 2.7 Ma and 283.4 ± 3.4 Ma, is correlated with the time of the pluton formation. The age of the composite dike breaking through the metamorphic deposits on the eastern shore of Lake Baikal is 284.10 ± 0.96 Ma. The mass formation of composite dikes in western Transbaikalia is correlated with the Late Paleozoic magmatism, which resulted in one of the Earth’s largest granitoid provinces. The intrusion of dikes was not a single-stage event; it lasted at least 10-12 Myr and was apparently related to the repeated intrusion of large volumes of salic magmas and the formation of granitoid plutons. Early Mesozoic composite dikes associated with alkali-granitoid plutons of the Late Kunalei igneous complex (230-210 Ma) are much scarcer. Basic magmas of the composite dikes were generated at depths greater than 75 km as a result of the melting of a modified (enriched in crustal components) mantle source. Salic components of the dikes, independently of their geologic position, are generally similar in composition to the granitoids of the plutonic facies, and the differences are apparently due to hybridization proceeding at great depths.© 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Spatial and temporal distributions of the late Mesozoic volcanic successions and their controlling effects on the Changling fault depression of the Songliao Basin, northeast China

Volcanic successions, including their spatial, temporal, tectonic, and geochemical attributes, were identified based on well data, 2D and 3D seismic data, U–Pb isotopic ages, and major and trace element data from the Changling fault depression of the Songliao Basin in northeastern China. Three eruption cycles developed in the Changling fault depression: K1h (Huoshiling Formation) (124–118 Ma), K1yc1 (first member of the Yingcheng Formation) (115–106 Ma), and K1yc3 (third member of the Yingcheng Formation) (106–102 Ma). These three eruption cycles comprised seven eruption stages. The volcanic successions in every eruption cycle were bimodal and evolved from basic to acidic. The magma of the basic and intermediate rocks in these cycles was derived from partial melting of the asthenosphere. In K1yc1 and K1yc3, partial melting of the newly formed basic rocks in the lower crust formed the high-SiO2 acidic magma. In addition, fractional crystallization of basic magma formed the small-scale intermediate–acidic magma. In K1h, the most likely origin of the acidic magma was fractional crystallization of basic magma. During the early stage of the fault depression, the volcanic rocks of K1h were controlled primarily by the boundary fault activity. In K1yc1 and K1yc3, basic–intermediate rocks were distributed widely along syngenetic faults, and acidic rocks were concentrated in areas where the syngenetic faults had their largest amplitudes. Multiple cycles and stages of the volcanic successions were controlled by magmatic evolution, and the spatial distribution was controlled by basin tectonics.

Intra-oceanic arc growth driven by magmatic and tectonic processes recorded in the Neoproterozoic Bougmane arc complex (Anti-Atlas, Morocco)

The Bougmane complex forms a Neoproterozoic tectonic unit located south of the Bou Azzer ophiolite in the Moroccan Anti-Atlas orogen. We show here that it corresponds to the lower crustal section of a Neoproterozoic intra-oceanic arc, which recorded 100–120 Ma of arc activity. The Bougmane complex is composed of two units: The 750–730 Ma gneissic unit is made of granodioritic gneiss-amphibolite which recorded a first magmatic pulse in the oceanic arc (IGN1). They are intruded during IGN2 event (710–690 Ma) by undeformed hydrous rocks consisting of hornblende gabbro (dated here at 706 ± 9 Ma, U-Pb protolithic zircons), hornblendite and minor tonalites. The high ƐNd values (+3.6 and +6.7) for all Bougmane rocks and their geochemical similarities ([La/Lu]N > 1.61; high LILE/HFSE ratio) with rocks in modern (Mesozoic to active) oceanic arcs, attest that the parental basic magmas were formed in a mantle wedge in an ocean-ocean subduction zone. The IGN2 event led to partial ‘garnetisation’ of the hornblende-gabbro along contacts with hornblendite bodies. Garnet formed by dehydration (sub-) and dehydration-melting (supra-solidus) of the hornblende-gabbro under HP conditions (>8 kbar). Field and geochemical modelling of Bougmane tonalites, showing low HREE ([La/Yb]N > 11) and high Sr/Y ratio (>1150), suggest that they formed by garnet-present disequilibrium melting of the hornblende-gabbro. U-Pb dating on rutiles in a hornblendite yielded an age of 686 ± 6 Ma, consistent with a post-IGN2 isobaric cooling. Rutiles from a garnet-hornblende gabbro yields a younger age of 658 ± 7 Ma suggesting that a high temperature event locally affected the Bougmane complex, likely related to a third igneous event (IGN3: 660–640 Ma) marked by intrusive diorites in the northern sector of the paleosuture. The building of the Anti-Atlas oceanic arc complexes occurred via three magmatic flare-ups (IGN1-2-3, centred at 750, 700 and 650 Ma respectively) interspersed with an early major tectonic episode (D1: 730–700 Ma). This suggests that the arc thickening in oceanic settings can be driven by magma intrusions at several levels of the crust but it can also be enhanced by intra-oceanic crustal shortening (hence, back-arc on arc stacking) – a still poorly documented geodynamic context in modern oceanic arc settings.