Intrusive Phases Research Articles

U–Pb and 40Ar/ 39Ar geochronology of the Saint-Urbain and Lac Allard (Havre-Saint-Pierre) anorthosites and their associated Fe–Ti oxide ores, Québec: Evidence for emplacement and slow cooling during the collisional Ottawan orogeny in the Grenville Province

Intrusive suites containing anorthosite–mangerite–charnockite–granite (AMCG) are a characteristic feature of Mesoproterozoic magmatic activity in the Grenville Province of eastern North America. In this study, the U–Pb and 40Ar/ 39Ar geochronology of two AMCG suites (Saint-Urbain and Havre-Saint-Pierre), and associated rutile-bearing hemo-ilmenite deposits, within the allochthonous polycyclic belt of the Grenville Province in Québec are used to constrain their crystallization ages and cooling histories, and to assess their tectonic setting. Both AMCG suites, separated by 700 km along the St. Lawrence River, have similar crystallization ages (ca. 1050–1060 Ma) and durations of magmatism (10–15 million years). The 450 km 2 Saint-Urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite (1046.2 ± 3.1 Ma) compared with anorthosite to the north and south (1055.0 ± 2.4 Ma and 1053.6 ± 2.6 Ma, respectively). The age of a deformed oxide–apatite gabbronorite (1057.4 ± 1.5 Ma) along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites, whereas the oldest intrusion in the Saint-Urbain area is the regionally extensive Saint-Anne du Nord orthopyroxene granodiorite (1060.8 ± 2.8 Ma). The large 11,000 km 2 Havre-Saint-Pierre anorthosite suite comprises a number of intrusive phases, including the 2700 km 2 Lac Allard anorthosite, which contains the giant Lac Tio ilmenite deposit and Big Island ilmenitite dyke. U–Pb zircon ages of three anorthosites from the Lac Allard intrusion, including the host to the Lac Tio deposit, overlap within error (1061.6 ± 3.0 Ma, 1060.5 ± 1.9 Ma, 1057.4 ± 8.4 Ma) and the crosscutting Big Island dyke yields an age of 1052.9 ± 6.5 Ma. These results indicate that the Havre-Saint-Pierre anorthosite should be divided into an older geon 11 part (ca. 1129 Ma) and a younger geon 10 part (ca. 1060 Ma). Average cooling rates of 3–4 °C/million years were estimated for the two AMCG suites by combining crystallization ages (U–Pb zircon) and cooling ages (U–Pb rutile, 40Ar/ 39Ar biotite/plagioclase) and reflect emplacement of the intrusions into a hot, long-duration orogen. Emplacement of these two AMCG suites at ca. 1050–1060 Ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional Ottawan orogenic phase of the Grenville orogeny. The results of this study provide strong support for proposals that link AMCG magmatism associated with the Grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle.

Linking changes in tectonic style with magmatism in northern Europe during the late Carboniferous to latest Permian

Early Carboniferous to Permian magmatism associated with rifting within the northern foreland of the Variscan Orogen was widespread across Europe. During the long period of magmatic activity the regional tectonic setting changed across the region from early Carboniferous extension and basin formation to a rifting–wrenching style of deformation in the late Carboniferous (Stephanian) to early Permian. Wrenching and faulting were accompanied by widespread, voluminous and episodic magmatic extrusion, intrusion and underplating. This was followed by thermal relaxation and the development of the Northern and Southern Permian Basins in later Permian times. Thermal relaxation was punctuated by a Permo-Triassic phase of extension and graben formation. 40Ar/ 39Ar Ar step-heating dating for mineral separates and whole-rock samples of magmatic rocks from southern Scandinavia (Oslo Graben and south Sweden) and Rügen (north Germany) provides further radiometric evidence for three of the proposed periods of magmatic activity in the region. Latest Carboniferous to earliest Permian ages (c. 300–310 Ma) were obtained for volcanic rocks in the Oslo Graben and dolerite sills and dykes in south Sweden and north Germany. This phase can be time-correlated with magmatic activity that occurred throughout Europe during large-scale dextral wrenching that followed the Variscan Orogeny. A second phase of alkaline intrusions is confined to the Oslo Graben and related to caldera collapse around c. 275 Ma. The third, Permo-Triassic phase (c. 250 Ma) is considered to be related to a new tectonic cycle involving extension that triggered minor melting of enriched, fertile mantle.

Trace element ratios as indicators of source mixing and magma differentiation of alkali granitoids and basites of the Haldzan-Buregtey massif and the Haldzan-Buregtey rare-metal deposit, western Mongolia

The Haldzan-Buregtey group of alkali granitoid massifs with an age of 391–395 Ma is located among the Early Caledonides of the Ozernaya zone of western Mongolia and consists of seven intrusive phases, including two rare-metal phases with Zr, Mn, Y, and REE mineralization. In order to identify the magma sources of the massifs, the abundances and canonical ratios of incompatible trace elements in the rocks of various intrusive phases are analyzed and compared with those in the volcanic rocks of Pantelleria island. The latter rocks were taken as the reference association of rocks linked through crystallization differentiation. The rocks of the Haldzan-Buregtey Complex were formed by mixing an OIB source (with participation of MORB) and host ophiolites, while alkali granitoids of phase 2 originated via mixting these sources with the host non-alkaline granitoids. Practically all rocks have mixed sources, with all transitional varieties from OIB, MORB to ophiolites. OIB was the main source for the rocks, while the host ophiolites could serve as sources for anatectic magmas or contaminants of the magmas of other considered rocks. The rare-metal granitoids were produced from the same sources as the barren magmatic rocks of the Haldzan-Buregtey Complex. The rocks of the Haldzan-Buregtey Complex show a bimodal distribution, with the practically complete absence of intermediate varieties between basite dikes and syenite-granite rocks. This seems to be related to the formation of the least differentiated sialic rocks (nordmarkites, pantellerites, some alkali granites) by anatexis of their own parental basite rocks (dolerites and basites), their cumulates, or ophiolites. Most of the phase-2 alkali granites likely resulted from the differentiation of the phase-1 nordmarkites coupled with assimilation of the host ophiolites. Ekerites are geochemically similar to the nordmarkites and can be interpreted as their residual in situ melts or their anatectic melts.

New massif of Archean alkaline syenites in the Murmansk domain of the Kola peninsula

Alkaline magmatism typical of a mature continental crust is among the most important geodynamic markers in the Early Precambrian. The Late Archean alkaline rocks represented by alkali granites, alkali gabbros, syenites, nepheline syenites, and carbonatites are known at the ancient shields [1]. In the Kola Peninsula, such magmatism (with an age of 2.75‐2.61 Ga) in the form of alkali granite and alkali gabbro‐nepheline syenite complexes occurs in the Archean Keivy and Ingozero structures [2, 3]. No Precambrian alkaline complexes have been found yet in the Archean Murmansk domain of the Kola peninsula. The Late Archean Iokan’gskii intrusive complex of porphyritic lepidomelane‐ferrohstingsite granites was previously ascribed by Batieva [2] to the alkali granite complex. However, later studies [4] showed that it belongs to the complex of ancient rapakivilike granites of normal alkalinity. During recent studies of the granitoids from the Barents Sea shore of the Kola Peninsula in the area of Gremikha settlement and Mt. Ostrovnaya, the small Late Archean alkaline massif called Panejavr Massif after an nearby lake was discovered and studied. The Panejavr Massif is located within the Iokan’gskii block of the Murmansk domain, and its position is controlled by the junction of two faults (Fig. 1). The massif is contained among Late Archean migmatite‐granites and metabasite dikes that fill the NE-trending faults. The Panejavr Massif is a complex stock having an autonomous internal structure discordant to the host rocks (Fig. 1) and consisting of two chemically and mineralogically contrasting intrusive phases.

The Zr/Hf ratio as a fractionation indicator of rare-metal granites

The Zr-Hf geochemical indicator, i.e., the Zr/Hf ratio (in wt %) in granitic rocks is proposed to be used as the most reliable indicator of the fractionation and ore potential of rare-metal granites. It was empirically determined that the fractional crystallization of granitic magma according to the scheme granodiorite → biotite granite → leucogranite → Li-F granite is associated with a decrease in the Zr/Hf ratio of the granites. The reason for this is the stronger affinity of Hf than Zr to granitic melt. This was confirmed by experiments on Zr and Hf distribution between granitic melt and crystals of Hf-bearing zircon (T = 800°C, P= 1 kbar). The application of the Zr/Hf indicator was tested at three classic territories of rare-metal granites: eastern Transbaikalia, central Kazakhstan, and the Erzgebirge in the Czech Republic and Germany. The reference Kukul’bei complex of rare-metal granites in eastern Transbaikalia (J3) is characterized by a uniquely high degree of fractionation of the parental granitic melt, with the granites and their vein derivatives forming three intrusive phases. The biotite granites of phase 1 are barren, the leucogranites of phase 2 are accompanied by greisen Sn-W mineral deposits (Spokoininskoe and others), and the final dome-shaped stocks of amazonite Li-F granites of phase 3 host (in their upper parts) Ta deposits of the “apogranite” type: Orlovka, Etyka, and Achikan. The Kukul’bei Complex includes also dikes of ongonites, elvanes, amazonite granites, and miarolitic pegmatites. All granitic rocks of the complex are roughly coeval and have an age of 142±0.6 Ma. The Zr/Hf ratio of the rocks systematically decreases from intrusive phase 1 (40–25) to phases 2 (20–30) and 3 (10–2). Compared to other granite series, the granites of the Kukul’bei Complex are enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but are depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From earlier to later intrusive phases, the rocks become progressively more strongly enriched or depleted in these elements, and their Zr/Hf ratio systematically decreases from 40 to 2. This ratio serves as a reliable indicator of genetic links, degree of fractionation, and rare-metal potential of granites. Greisen Sn, W, Mo, and Be deposits are expected to accompany granites with Zr/Hf < 25, whereas granites related to Ta deposits should have Zr/Hf < 5.

Timing and duration of Palaeoproterozoic events producing ore-bearing layered intrusions of the Baltic Shield: metallogenic, petrological and geodynamic implications

Abstract There are two 300–500 km long belts of Palaeoproterozoic layered intrusions in the Baltic (Fennoscandian) Shield; the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt. New U–Pb (TIMS) ages and radiogenic isotopic (Nd–Sr–He) data have been determined for mafic-ultramafic Cu–Ni–Ti–Cr and PGE-bearing layered intrusions of the Kola Belt. U–Pb zircon and baddeleyite data from gabbronorite and anorthosite bodies of the Fedorovo-Pansky, Monchepluton, Main Ridge (Monchetundra and Chunatundra) and Mt Generalskaya intrusions, and from gabbronorite intrusions and dykes associated with the Imandra lopolith, yield ages from c . 2.52–2.39 Ga. The age range of 130 Ma recorded in the Kola Belt samples, associated with at least four intrusive phases (three PGE-bearing and one barren), is significantly greater than that for intrusions of the Southern (Fenno-Karelian) Belt which clusters at 2.44 Ga. Nd isotopic values for the Kola Belt range from −1.1 to −2.4 and indicate an enriched mantle ‘EM-1 type’ reservoir for these layered intrusions. Initial Sr isotopic data for the Kola intrusions are radiogenic relative to bulk mantle, with I Sr values from 0.703 to 0.704, but geochemical data and 4 He/ 3 He isotopic ratios of various minerals record a significant contribution from a mantle source rather than simply crustal melting. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic‐ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.

Rare-earth elements as indicators of ore sources and the degree of differentiation and ore potential of rare-metal granite intrusions ( eastern Transbaikalia)

The meteoritic-material-normalized REE patterns of rare-metal granite intrusions of the ore-bearing Kukul'bei complex (J2–J3), eastern Transbaikalia, were studied. It is shown that the intrusions were initially enriched in granitophile volatiles and trace elements (rare metals), i.e., this phenomenon is not related to the differentiation of their parental magma chambers. On the differentiation of the Kukul'bei rare-metal intrusions, the REE contents decrease in passing from granites of the main intrusive phase (MP) to late leucocratic differentiates (muscovite and amazonite granites), the differentiates become more enriched in granitophile elements, and their rare-metal contents drastically increase as compared with the MP granites. The ore-bearing bodies of muscovite and amazonite granites have extremely low REE contents and the highest contents of granitophile (including ore-forming) elements. The REE patterns of the Kukul'bei intrusive differentiates are not universal among rare-metal intrusions. By the example of highly ore-bearing rare-metal granite intrusions of the Erzgebirge, Central Europe, it has been established that their late deep-seated differentiates (ultrarare-metal lithionite-zinnwaldite Li-F-granites) accompanied by highly productive Sn-W mineralization concentrate both granitophile elements and REE (particularly HREE). Among the studied Transbaikalian rare-metal intrusions of the Kukul'bei complex, only the differentiates of the most ore-bearing Sherlovaya Gora intrusive system belong to the above type. The analysis of the REE patterns of the Kukul'bei granites confirmed the earlier conclusions on the low ore potential of the rare-metal mineralization of the studied intrusive complex.

Thermodynamic model of REE leaching from monazite by hydrothermal fluids

Monazite is the most abundant REE mineral that is formed in a wide range of geochemical conditions. This mineral is the most widespread in the felsic magmatic rocks, and it occurs in alkaline rocks, their pegmatites, and definite metamorphic complexes. Since, besides REEs, it contains significant amounts of Th and U cations, an increasing number of works in recent decades have focused on the use of this mineral for geochronological dating. Moreover, monazite and, more precisely, its compositional specifics in different phases of granitoid intrusions are employed not only for dating, but also for understanding the physicochemical evolution of magmatic complexes. In particular, significant differences were found between the compositions of predominant primary monazite in the two-mica Ireteba granites, southern Nevada (with an age of ~64 Ma), and late (secondary) monazite, which replaces it and is restricted to the influence zones of Late Miocene granite plutons (~14‐18 Ma). The replacement occurred at temperatures below 400 ° C and was expressed in a significant decrease in the content of the huttonite (ThSiO 4 ) end member with a simultaneous increase in the La/Sm and Sm/Y ratios. There is an article [2] dedicated to a detailed SHRIMP study of monazite from the Middle Archean greenschist metamorphic rocks of the Pilbara craton, Western Australia. It was shown that metamorphic monazite is represented by three generations corresponding to different “growth stages.” The La, Sm, Y, and Th distribution mapping combined with U‐Pb dating revealed the presence of distinct cores and rims in some crystals and provided insight into a hidden spatiotemporal pattern of distribution of fluid flows related to tectonic processes. Additionally, the authors of [3] demonstrated that a similar technique can be applied to determine the main trends in monazite evolution during formation of highgrade granulite-facies rocks in gneisses contained in the Challenger Au deposit (South Australia). The determination of Ce, Nd, Sm, Eu, and Gd contents in this mineral made it possible to trace the formation of its compositional domains towards the core: growth during partial melting in the closed system.

Garnetization as a ground preparation process for copper mineralization: evidence from the Mazraeh skarn deposit, Iran

The Mazraeh Cu–Fe skarn deposit, NW Iran is the result of the intrusion of an Oligocene–Miocene granitic pluton into Cretaceous calcareous rocks. The pluton ranges in composition from monzonite to quartz monzonite, monzogranite, tonalite and granodiorite with I-type, calc-alkaline, and weakly peraluminous characteristics. The Mazraeh pluton was emplaced in a volcanic arc setting in an active continental margin at a depth of ~8 km. Pyroxene skarn, garnet skarn, and epidote skarn zones were formed during the intrusive phase. The garnet skarn developed as exoskarn and endoskarn from the calcareous wall rocks and the pluton, respectively, prior to mineralization. Garnet skarn from the exoskarn zone is identified by relict layering inherited from the precursor calcareous lithologies. Mass balance calculation of garnet skarn in the endoskarn zone indicates that hydrothermal fluids originating from the cooling magma introduced Si, Fe, Mn, Ca, Mg, P, Ag, Cu, Zn, La, Pb, Cd, Mo, and Y. The main mass loss in the garnet skarn was due to destruction of feldspars in the Mazraeh plutonic rocks and leaching of K2O and Na2O. Released Ca has been fixed in the andraditic garnet. Garnetization of the Mazraeh pluton was accompanied by mass and volume increase. The magnitude of these changes depends mainly on the degree of alteration and composition of the precursor. The brittle behavior of the endoskarn zone was increased due to formation of massive garnet which subsequently fractured. These fractures not only facilitated movement of hydrothermal fluids but also provided new locations for Cu mineralization. Therefore locating strongly garnetized zones may be a vector to ore in skarn deposits.

Zr/Hf ratio as an indicator of fractionation of rare-metal granites by the example of the Kukulbei complex, eastern Transbaikalia

The concept of granitic melt fractionation as the main process in the concentration of rare elements in granites calls for the development of a reliable method to determine the evolutionary sequences of granite series. We propose to use for this purpose a zirconium-hafnium indicator, the Zr/Hf weight ratio in granitic rocks (Zaraisky et al., 1999, 2000). By the example of three classic regions of rare-metal deposits, eastern Transbaikalia, central Kazakhstan, and Erzgebirge (Czech Republic and Germany), it was empirically shown that the Zr/Hf ratio of granites decreases during the fractional crystallization of granite magmas in the sequence granodiorite → biotite granite → leucogranite → lithium-fluorine granite. The reason is the higher affinity of Hf compared with Zr to a granite melt. This implies that the crystallization and settling of accessory zircon will cause the progressive enrichment of Hf relative to Zr in the residual melt. As a result, the Zr/Hf ratio decreases regularly in the series of sequential phases of granite intrusion related to a single magma chamber from granodiorite to biotite granite, leucogranite, and Li-F granite (from 45-30 to 10-2). Our experimental investigations supported the preferential enrichment of haplogranite melt in Hf and zircon crystals in equilibrium with melt in Zr (T= 800°C and P = 1 kbar). The Zr/Hf indicator was tested by the example of the wellknown Kukulbei rare-metal granite complex of eastern Transbaikalia (J3), which is unique in the degree of fractionation of initial granite melt with the formation of three phases of granite emplacement and vein derivatives. An important feature of the complex is its “short” differentiation trend. It was supposed that the granite magma of the first phase is parental, and the later phases forming small intrusive bodies in large massifs of biotite granites of the first phase are sequential products of its crystallization differentiation in a magma chamber. The biotite granites of the first phase are barren. The leucocratic granites of the second phase are accompanied by tin-tungsten greisen deposits (e.g., Spokoininskoe), and the upper part of cupola-like stocks of Li-F amazonite granites of the third phase host apogranite-type tantalum deposits (Orlovka, Etyka, and Achikan). In addition to three granite phases, the Kukulbei complex includes dikes of ongonites, elvans, amazonite granites, and chamber miarolitic pegmatites. All of the granitic rocks of the complex have similar isotopic ages of 142± 0.6 Ma. The Zr/Hf ratio decreases systematically from phase 1 (40–25), to phase 2 (20–10), and phase 3 (10–2). The ongonites, elvans, and pegmatites have similar Zr/Hf ratios (15-5), falling between the ranges of leucocratic muscovite granites and Li-F granites. Compared with other granite series, the granitic rocks of the Kukulbei complex show specific petrographic and geochemical features: they are strongly enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From the early to late intrusion phases, the degree of enrichment and depletion in these element groups increases regularly. This is accompanied by a significant decrease (from 40 to 2) in Zr/Hf, which can be used as a reliable indicator of genetic relations, degree of fractionation, and rare-metal potential of granites. Granites with Zr/Hf values lower than 25 are promising for prospecting for Sn, W, Mo, and Be greisen deposits, whereas the formation of Ta deposits requires Zr/Hf values lower than 10.

Intrusion of eccentric dikes: The case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano

The 2001 eruption represents one of the most studied events both from volcanological and geophysical point of view on Mt. Etna. This eruption was a crucial event in the recent dynamics of the volcano, marking the passage from a period (March 1993–June 2001) of moderate stability with slow, continuous flank sliding and contemporaneous summit eruptions, to a period (July 2001 to present) of dramatically increased flank deformations and flank eruptions. We show new GPS data and high precision relocation of seismicity in order to demonstrate the role of the 2001 intrusive phase in this change of the dynamic regime of the volcano. GPS data consist of two kinematic surveys carried out on 12 July, a few hours before the beginning of the seismic swarm, and on 17 July, just after the onset of eruptive activity. A picture of the spatial distribution of the sin-eruptive seismicity has been obtained using the HypoDD relocation algorithm based on the double-difference (DD) technique. Modeling of GPS measurements reveals a southward motion of the upper southern part of the volcano, driven by a NNW–SSE structure showing mainly left-lateral kinematics. Precise hypocenter location evidences an aseismic zone at about sea level, where the magma upraise was characterized by a much higher velocity and an abrupt westward shift, revealing the existence of a weakened or ductile zone. These results reveal how an intrusion of a dike can severely modify the shallow stress field, triggering significant flank failure. In 2001, the intrusion was driven by a weakened surface, which might correspond to a decollement plane of the portion of the volcano affected by flank instability, inducing an additional stress testified by GPS measurements and seismic data, which led to an acceleration of the sliding flanks.

Paleomagnetism and geochronology of sills of the Doherty Mountain area, southwestern Montana: Implications for the timing of fold-and-thrust belt deformation and vertical-axis rotations along the southern margin of the Helena salient

Paleomagnetic and 40Ar/39Ar data from intermediate composition sills exposed along the southern margin of the Helena salient in southwestern Montana help refine the age of Late Cretaceous contraction in this part of the North American Cordillera. The structures in the Doherty Mountain fold complex deform Cambrian through Mississippian strata in the hanging wall of the Jefferson Canyon–Lombard fault system. Sills exposed within these strata, which are complexly deformed into northeast- to northwest-plunging anticlines and a northwest-plunging syncline give well-defined remanent magnetizations and, after correction for fold plunge followed by bedding tilt, paleomagnetic data from 20 of 21 sites from four folds yield positive fold tests at 100 percent unfolding. The magnetizations from these sites are interpreted to be primary remanent magnetizations acquired during sill emplacement prior to fold-and-thrust deformation, although pre-folding remagnetization due to widespread latest Cretaceous magmatism in the area cannot be ruled out. One site yields a magnetization that appears to postdate folding; this site has either been remagnetized or is in a distinct intrusive phase emplaced subsequent to folding. Biotite mineral separates from sills from two folds yield identical 40Ar/39Ar plateau dates of 77 Ma. Although the group mean directions from sills in each of the folds may represent incomplete samplings of the geomagnetic field and the number of independent readings is low, their grand-mean direction (declination [Dec.] = 347°, inclination [Inc.] = 66°, k = 71.9, α95 = 9°, N = 5 independent observations based on results from 21 sites) is similar to the expected Late Cretaceous reference direction for the study area, based on results from the essentially coeval Adel Mountain Volcanics. Therefore, these data suggest that deformation began after 77 Ma (Campanian-Maestrichtian) and that upper plate strata in the Lombard thrust sheet of the Helena salient in this area have not experienced significant local vertical-axis rotation due to thrust movement and impingement with foreland structures along the Southwest Montana Transverse Zone. The absence of appreciable vertical-axis rotations is consistent with data obtained from other paleomagnetic studies in the region.

Shallow magma transport for the 2002–3 Mt. Etna eruption inferred from thermal infrared surveys

The 26 October 2002–28 January 2003 eruption of Mt. Etna volcano was characterised by lava effusion and by an uncommon explosivity along a 1 km-long-eruptive fissure on the southern, upper flank of the volcano. The intense activity promoted rapid growth of cinder cones and several effusive vents. Analysis of thermal images, recorded throughout the eruption, allowed investigation of the distribution of vents along the eruptive fissure, and of the nature of explosive activity. The spatial and temporal distribution of active vents revealed phases of dike intrusion, expansion, geometric stabilization and drainage. These phases were characterised by different styles of explosive activity, with a gradual transition from fire fountaining through transitional phases to mild strombolian activity, and ending with non-explosive lava effusion. Here we interpret the mechanisms of the dike emplacement and the eruptive dynamics, according to changes in the eruptive style, vent morphology and apparent temperature variations at vents, detected through thermal imaging. This is the first time that dike emplacement and eruptive activity have been tracked using a handheld thermal camera and we believe that its use was crucial to gain a detailed understanding of the eruptive event.

Geology and Re‐Os Geochronology of Mineralization of the Miduk Porphyry Copper Deposit, Iran

AbstractThe Miduk porphyry copper deposit is located in Kerman province, 85 km northwest of the Sar Cheshmeh porphyry copper deposit, Iran. The deposit is hosted by Eocene volcanic rocks of andesitic–basaltic composition. The porphyry‐type mineralization is associated with two Miocene calc‐alkaline intrusive phases (P1 and P2, respectively). Five hypogene alteration zones are distinguished at the Miduk deposit, including magnetite‐rich potassic, potassic, potassic–phyllic, phyllic and propylitic. Mineralization occurs as stockwork, dissemination and nine generations (magnetite, quartz–magnetite, barren quartz, quartz‐magnetite‐chalcopyrite‐anhydrite, chalcopyrite–anhydrite, quartz‐chalcopyrite‐anhydrite‐pyrite, quartz‐molybdenite‐anhydrite ± chalcopyrite ± magnetite, pyrite, and quartz‐pyrite‐anhydrite ± sericite) of veinlets and veins. Early stages of mineralization consist of magnetite rich veins in the deepest part of the deposit and the main stage of mineralization contains chalcopyrite, magnetite and anhydrite in the potassic zone. The high intensity of mineralization is associated with P2 porphyry (Miduk porphyry). Based on petrography, mineralogy, alteration halos and geochemistry, the Miduk porphyry copper deposit is similar to those of continental arc setting porphyry copper deposits. The Re‐Os molybdenite dates provide the timing of sulfide mineralization at 12.23 ± 0.07 Ma, coincident with U/Pb zircon ages of the P2 porphyry. This evidence indicates a direct genetic relationship between the Miduk porphyry stock and molybdenite mineralization. The Re‐Os age of the Miduk deposit marks the main stage of magmatism and porphyry copper formation in the Central Iranian volcano‐plutonic belt.

Long-term geochemical variability of the Late Cretaceous Tuolumne Intrusive Suite, central Sierra Nevada, California

Abstract This study investigates the internal anatomy and petrogenesis of the Tuolumne Intrusive Suite (TIS), which comprises metaluminous, high-potassium, calc-alkaline granitoids typical of the Sierra Nevada batholith. Although the TIS has often been cited as an example of a large magma chamber that cooled and fractionated from the margins inward, its geochemistry is inconsistent with closed-system fractionation. Most major elements are highly correlated with SiO 2 , but the scattered nature of trace elements and variations of initial Sr and Nd isotopic ratios indicate that fractional crystallization is not the predominant process responsible for its chemical evolution. Isotopic data suggest mixing between melts of mantle-like rocks and a granitic melt similar in composition to the highest-silica TIS unit. Monte Carlo models of magma mixing confirm that such processes can reproduce the observed variations in major elements, trace elements and isotopic ratios. Thermobarometry suggests emplacement at depths near 6 km and crystallization temperatures ranging from 660 to 750 °C. Feldspars, hornblende, biotite and magnetite exhibit evidence of extensive low-temperature subsolidus exsolution. The TIS as a whole trends toward more evolved isotopic compositions and younger U–Pb zircon ages passing inward. This pattern indicates a general increase in the proportion of felsic, crustally derived melt in the mixing process, which may have resulted from net accumulation of heat added to the lower crust by intrusion of mantle-derived mafic magma. However, the bulk geochemical and isotopic compositions of the equigranular Half Dome Granodiorite, the porphyritic Half Dome Granodiorite and the Cathedral Peak Granodiorite overlap one another and the contacts between them are commonly gradational. We interpret these map units to represent a single petrological continuum rather than distinct intrusive phases. The textural differences that define the units probably reflect thermal evolution of the system rather than distinct intrusive events.

The Rhyolite-Hosted Volcanogenic Massive Sulfide District of Cuale, Guerrero Terrane, West-Central Mexico: Silver-Rich, Base Metal Mineralization Emplaced in a Shallow Marine Continental Margin Setting

The Cuale mining district, situated in the Cordillera Madre del Sur, ~30 km southeast of Puerto Vallarta, Mexico, comprises a number of small, polymetallic Pb, Zn, Cu, volcanogenic massive sulfide deposits (VMS), and vein and stockwork-hosted deposits with low-sulfidation epithermal characteristics. Massive sulfide mineralization contains high Ag ± Au grades when compared to similar deposits in western Mexico. The massive sulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession, herein termed the Cuale volcanic sequence. Regionally, the Cuale volcanic sequence is part of the Zihuatanejo subterrane belonging to the Guerrero terrane, which is interpreted to be a complex Mesozoic accreted arc terrane hosting the majority of VMS deposits in Mexico. The rocks of the Cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibit intense chlorite, sericite, and quartz hydrothermal alteration. Three stratigraphic units are identified within the Cuale volcanic sequence: (1) a more than 400-m-thick footwall unit of quartz and plagioclase phyric rhyolite flows, related hyaloclastites, and subordinate heterolithic volcaniclastic breccias; (2) an ore horizon characterized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales, as well as aphyric to quartz and plagioclase phyric rhyolite; and (3) late intrusions composed of quartz and plagioclase phyric rhyolite and subordinate andesitic dikes intruding all units. The sedimentary rocks of the ore horizon are intercalated with rhyolite tuff horizons, some of which exhibit welding or accretionary lapilli that are interpreted as evidence for shallow submarine or subaerial deposition. The rhyolites of the Cuale volcanic sequence are calc-alkaline with only limited compositional variations. However, the trace element signatures reveal a weak trend from the footwall rhyolites, which are characterized by low Zr/Y (1‐1.9) and low chondrite normalized La/Yb ratios (9 of 11 samples between 1.24 and 3.3), to slightly higher values in the ore horizon and late intrusive phase (Zr/Y = 1.6‐7.6 and chondrite normalized La/Yb ratios between 2.25 and 15). This compositional evolution can be explained by slightly decreasing degrees of partial melting related to a transition from amphibole-free assemblages for the footwall to amphibole-bearing assemblages in the residuum for the ore horizon and intrusive rhyolites. This interpretation is supported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation as the volcanic edifice was built. Zircons from five samples of the Cuale volcanic sequence have been dated by the U-Pb method, using isotope dilution thermal ionization mass spectrometry (ID-TIMS). Although some samples show evidence of a minor inherited Pb component, an age range of 157.2 ± 0.5 to 154.0 ± 0.9 Ma has been established. Cuale represents the oldest dated VMS camp in western Mexico as other VMS districts of western Mexico are between 151.3 and 138 Ma. Two styles of mineralization are recognized: volcanogenic massive sulfide mineralization (VMS) and late lowsulfidation epithermal vein and stockwork mineralization characterized by high Au (1.89 ppm) and Zn (2.35%) but low Cu (0.2%) contents. The VMS mineralization is contained in more than 15 small orebodies, which exhibit a range of mineralization styles. Orebodies interpreted as proximal are characterized by stockwork vein networks with high Au and Cu contents, underlying massive pyrite bodies, which grade laterally and vertically into sphalerite- and galena-rich massive sulfide ore. Stockwork mineralization is locally hosted by aphyric rhyolite. More distal orebodies, rich in Pb, Zn, and Ag, are hosted by black shale and locally exhibit laminated or brecciated sedimentary textures in the galena-sphalerite-pyrite ore. The sedimentary textures observed in some massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basins adjacent to the rhyolite domes. Some of the more significant orebodies show evidence of both distal sedimentary ore textures and proximal features indicated by intense quartz-sericite-pyrite alteration in the immediate footwall.

Petrology of the Early Paleoproterozoic Burakovsky complex, southern Karelia

The Early Paleoproterozoic Burakovsky complex contains Europe’s largest layered mafic-ultramafic pluton and giant Avdeevo gabbronorite dike. The pluton consists of two independent bodies of different age (the Aganozero and Shalozero-Burakovsky bodies), each having its own internal structure and contacting each other in their apical parts. Both bodies have a similar rock sequence including five differentiated zones (from bottom upward, based on the predominant cumulus assemblages): ultrabasic, pyroxenite, gabbronorite, pigeonite gabbronorite, and magnetite gabbronorite-diorite. Being generally similar to each other owing to a common differentiation trend of similar melts, these bodies differ notably in the character of their cumulus stratigraphy and, to a lesser extent, in mineral and geochemical composition. The pluton is distinguished by the presence of marker horizons—singular interlayers of high-temperature mafic cumulates emplaced in the sequence of lower-temperature gabbroid. Their origin is believed to have been associated with the influxes of fresh magma portions into the crystallizing magma chambers. Based on petrological, geochemical, and mineralogical data, the Aganozero and Shalozero-Burakovsky bodies, as well as additional intrusive phases, were derived from the compositionally similar but not identical melts of siliceous high-Mg (boninite like) series. The Avdeevo dike (2436 ± 46 Ma at ɛNd(T) = −1.5) extends along the southeastern contact of the Shalozero-Burakovsky body and was formed almost simultaneously with it. The dike is made up of pigeonite gabbronorites, which are almost identical to the rocks of the Pigeonite gabbronorite zone of the Burakovsky pluton in geochemistry and mineral composition. It was concluded that the Burakovsky Complex was a long-lived igneous center, whose origin was related to the activity of an Early Paleoproterozoic mantle superplume.

An explosive–intrusive subglacial rhyolite eruption at Dalakvísl, Torfajökull, Iceland

This paper describes unusual rhyolitic deposits at Dalakvisl, Torfajokull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km3), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.

Advanced post-processing and correlation analyses in high-velocity air–water flows

The interest in air-water flows has not diminished in recent years, but it is accompanied by frequent citations of early, sometimes outdated articles. A basic issue is the inadequate, incomplete interpretation of air-water flow instrumentation by hydraulic engineers and researchers. This article comments on high-velocity air-water flow measurements by means of intrusive phase detection probes. This article focus on the bubbly flow structure of high-velocity air-water flow based upon measurements by means of intrusive phase detection probes. It is shown that some advanced post-processing techniques may yields expanded information on the air-water turbulent flow properties and bubbly flow structures. The outcomes demonstrate simple techniques in high-velocity air-water flow analysis.

Trace elements and evolution of granite melt as exemplified by the Raumid Pluton, Southern Pamirs

New trace element data were obtained by ICP-MS for 58 samples representing eight intrusive phases of the Raumid granite Pluton. All of the rocks, except for one sample that was deliberately taken from a greisenized zone, were not affected by postmagmatic fluid alteration. The sequential accumulation of incompatible trace elements (Rb, Ta, Nb, Pb, U, and others) in the Raumid Pluton from the early to late phases coupled with a decrease in incompatible element contents (Sr, Eu, Ba, and others) indicates a genetic link between the granites of all phases via fractional crystallization of a granite melt. The REE distribution patterns of final granite phases are typical of rare-metal granites. The Ta content in the granites of phase 8 is only slightly lower than that of typical rare-metal granites. Greisenization disturbed the systematic variations in trace element distribution formed during the magmatic stage. The ranges of trace element contents (Rb, Sr, Ta, Nb, and others) and ratios (Rb/Sr, La/Lu, Eu/Eu*, and others) in the Raumid granite overlap almost entirely the ranges of granitic rocks of various compositions, from the least differentiated with ordinary trace element contents to rare-metal granites. This indicates that the geochemical signature of rare-metal granites can develop at the magmatic stage owing to fractional crystallization of melts, which is the case for the melt of the Raumid granite.