Renewed Magmatic Activity Research Articles

Off-rift Axis Channelized Melt and Lithospheric Metasomatism along Mid-ocean Ridges—A Case Study from Iceland on the Limits of Melt Channelling

AbstractMagmatism in Iceland is classically explained by the interaction of the mid-Atlantic ridge with the Iceland plume. The growth of Iceland through time is the result of volcanic activity at the rift axis. However, Holocene volcanism (0–11.5 ka) is not restricted to the rift zone (RZ) but also occurs off-axis, specifically in the western Snæfellsnes Volcanic Belt (SVB) and in the Southern Flank Zone (SFZ). The rift and off-axis postglacial volcanic zones are separated by a gap of ~60–80 km. While the volcanic activity of the SFZ seems correlated with the actual location of the Iceland plume, the plume relation to the SVB is uncertain. To address the origin and relationships between rift and off-rift magmas, we present new data from two transects perpendicular to the Reykjanes rift. The lavas in the SVB are characterized by transitional to alkaline compositions, with elevated incompatible trace element content. In contrast, the RZ volcanic rocks have tholeiitic compositions with trace element signatures slightly more enriched than Mid-Ocean Ridge Basalt (MORB). Rift and off-rift Iceland lavas are all characterized by positive Ba and Nb anomalies, particularly in alkaline rocks. Tholeiitic and alkaline lavas show distinct differentiation sequences, with the main difference being the delayed crystallization of plagioclase in the fractionating assemblage of alkaline magmas. We apply these sequences to calculate primary magma compositions, which are then used to constrain melting conditions. Geochemical modelling indicates that Iceland rift and off-rift magmas can be produced from a peridotitic mantle source if lithospheric processes are involved. We demonstrate that recycled crust in the form of pyroxenite is not required to generate Snæfellsnes alkaline lavas. The low solidus temperature and high productivity of pyroxenite favour early and more extensive melting producing primary magmas that are not sufficiently enriched in incompatible trace elements to explain the compositional variation of Snæfellsnes magmas. An alternative mechanism to involve pyroxenite in the source of Snæfellsnes lavas relates to the hybridization of recycled oceanic crust with peridotite, but such reacted pyroxenite requires specific compositions to reproduce the Ba and Nb anomalies. As an alternative, we suggest that Snæfellsnes alkaline lavas are the result of channelized low-degree melts produced on the periphery of the melting column at distances exceeding 65 km from the ridge axis. These melts accumulate and percolate into the lithosphere producing amphibole ± phlogopite-bearing hydrous cumulates. Incongruent melting of these cumulates via renewed magmatic activity and melt-peridotite reaction can reproduce the alkaline compositions observed in the SVB, including the Ba and Nb anomalies. Numerical simulations of melt extraction below mid-ocean ridges suggest that low-degree melts produced as far as ~65 km from the central ridge axis rise vertically to the base of the lithosphere and are then focussed towards the ridge axis in decompaction channels. We propose that these melts interact with hydrous cumulates previously formed during the development of decompaction channels at the lithosphere–asthenosphere boundary and acquire specific Ba and Nb anomalies. The mixing of these distal enriched melts with more depleted melts extracted from the central part of the melting regime explains the composition of RZ lavas. Alkaline lavas observed in the SFZ show numerous analogies with the Snæfellsnes magmas, suggesting that similar lithospheric processes control their chemistry. The high thermal regime in Iceland and thick lithosphere explains the difference between Icelandic tholeiite and typical MORB. Our results highlight the importance of mantle dynamics below mid-ocean ridges and lithospheric interaction to produce off-axis magmatism with enriched alkaline signatures.

Chapter 22 Upper and uppermost thrust sheets in the Caledonide orogen, Sweden: outboard oceanic and exotic continental terranes

Abstract Three separate stacks of thrust sheets (Köli Nappe Complex) constitute the Upper Allochthon in the Caledonide orogen, Sweden. This thrust complex is dominated by late Cambrian–Ordovician successions deposited in subduction-related, marginal oceanic basins. Magmatic activity at c. 488 Ma (Lower Köli) and c. 492–476 Ma (Middle Köli) is linked to rifted volcanic arcs and Zn–Cu–Fe–(Pb–Au–Ag) sulphide mineralization; serpentinite bodies with talc deposits are also conspicuous. Renewed magmatic activity, both plutonic (Upper and Middle Köli) and mafic volcanic (Middle and Lower Köli), occurred at c. 440–434 Ma during crustal extension. Late Ordovician shallow-marine sedimentation, deepening upwards into an early Silurian succession also prevailed (Lower Köli). Silurian ( c. 430 Ma and later) folding, eastwards-vergent thrusting and greenschist or lower amphibolite facies metamorphism preceded upright, orogen-parallel and orogen-transverse open folding. Juxtaposition of an arc-related terrane to an ancient continental margin, comprising slices of gneiss and marble, in the Middle Köli occurred prior to c. 437 Ma and the eastwards-vergent thrusting; remnants of an Ordovician amphibolite facies tectonothermal event are also preserved in the Upper Köli. The tectonic roof to the Köli complex contains amphibolite facies mica schist, gneiss and marble, derived from the Laurentian continental margin, and a major gabbroic pluton (Rödingsfjället Nappe Complex, Uppermost Allochthon).

Decoding sill emplacement and forced fold growth in the Exmouth Sub-basin, offshore northwest Australia: Implications for hydrocarbon exploration

Igneous sills emplaced at shallow levels in sedimentary basins commonly uplift the overburden and free surface. Uplift produces dome-shaped forced folds that may host economic hydrocarbon accumulations. These intrusion-induced forced folds are typically assumed to develop instantaneously, whereby the oldest onlapping strata constrain the age of sill emplacement, and accommodate the entire volume of intruded magma. However, several studies demonstrate that forced folds may grow over geologic timescales, with additional space-making mechanisms (e.g., compaction) partly accommodating the magma volume. It is thus critical to understand when forced fold traps form and how they evolve in relation to the timing of source rock maturation and migration. We analyze two forced folds imaged in 2D seismic reflection data from offshore northwest Australia. Analyzing the seismic stratigraphy of the forced fold overburden allows us to recognize several distinct phases of fold growth. Subhorizontal reflections onlapping onto the lower portion of the forced folds at a high angle indicate that the first phase of sill emplacement and fold development occurred rapidly, facilitated by normal faulting, prior to the deposition of overlying strata during a period of magmatic quiescence and regional hydrocarbon maturation in the Early Cretaceous. Renewed magmatic activity resulted in a final, protracted phase of doming, which is recorded by a package of onlapping growth strata that was incrementally deformed by successive intrusive pulses. We also demonstrate that in addition to folding and faulting, the magma volume was likely accommodated by porosity reduction within the folded strata. Our observations imply that the age of the lowermost onlapping reflections only constrain the onset of sill emplacement and not the duration of magmatic activity. Constraining the dynamic evolution of intrusion-induced forced folds from the structure of onlapping reflections during hydrocarbon exploration can thus provide critical insights into the potential volume and charge history of any hydrocarbon accumulations.

Pyroclastic Flow Deposits on Venus as Indicators of Renewed Magmatic Activity.

Radar-bright deposits on Venus that have diffuse margins suggest eruptions that distribute debris over large areas due to ground-hugging flows from plume collapse. We examine deposits in Eastern Eistla, Western Eistla, Phoebe, and Dione Regiones using Magellan data and Earth-based radar maps. The radar-bright units have no marginal lobes or other features consistent with viscous flow. Their morphology, radar echo strength, polarization properties, and microwave emissivity are consistent with mantling deposits comprised of few-cm or larger clasts. This debris traveled downhill up to ~100 km on modest slopes, and blanketed lava flows and tectonic features to depths of tens of cm to a few meters over areas up to 40×103 km2. There is evidence for ongoing removal and exhumation of previously buried terrain. A newly identified occurrence is associated with a ridge belt south of Ushas Mons. We also note radar-bright streaks of coarse material west of Rona Chasma that reflect the last traces of a deposit mobilized by winds from the formation of Mirabeau crater. If the radar-bright units originate by collapse of eruption columns, with coarse fragmental material entrained and fluidized by hot gases, then their extent suggests large erupted volatile (CO2 or H2O) amounts. We propose that these deposits reflect the early stage of renewed magmatic activity, with volatile-rich, disrupted magma escaping through vents in fractured regions of the upper crust. Rapidly eroding under Venus surface conditions, or buried by subsequent eruptions, these markers of recently renewed activity have disappeared from older regions.

Evidence of recent deep magmatic activity at Cerro Bravo-Cerro Machín volcanic complex, central Colombia. Implications for future volcanic activity at Nevado del Ruiz, Cerro Machín and other volcanoes

In the last nine years (2007–2015), the Cerro Bravo-Cerro Machín volcanic complex (CBCMVC), located in central Colombia, has experienced many changes in volcanic activity. In particular at Nevado del Ruiz volcano (NRV), Cerro Machin volcano (CMV) and Cerro Bravo (CBV) volcano. The recent activity of NRV, as well as increasing seismic activity at other volcanic centers of the CBCMVC, were preceded by notable changes in various geophysical and geochemical parameters, that suggests renewed magmatic activity is occurring at the volcanic complex.The onset of this activity started with seismicity located west of the volcanic complex, followed by seismicity at CBV and CMV. Later in 2010, strong seismicity was observed at NRV, with two small eruptions in 2012. After that, seismicity has been observed intermittently at other volcanic centers such as Santa Isabel, Cerro España, Paramillo de Santa Rosa, Quindío and Tolima volcanoes, which persists until today.Local deformation was observed from 2007 at NRV, followed by possible regional deformation at various volcanic centers between 2011 and 2013. In 2008, an increase in CO2 and Radon in soil was observed at CBV, followed by a change in helium isotopes at CMV between 2009 and 2011. Moreover, SO2 showed an increase from 2010 at NRV, with values remaining high until the present.These observations suggest that renewed magmatic activity is currently occurring at CBCMVC. NRV shows changes in its activity that may be related to this new magmatic activity. NRV is currently exhibiting the most activity of any volcano in the CBCMVC, which may be due to it being the only open volcanic system at this time. This suggests that over the coming years, there is a high probability of new unrest or an increase in volcanic activity of other volcanoes of the CBCMVC.

Magma injection beneath the urban area of Naples: a new mechanism for the 2012-2013 volcanic unrest at Campi Flegrei caldera.

We found the first evidence, in the last 30 years, of a renewed magmatic activity at Campi Flegrei caldera from January 2012 to June 2013. The ground deformation, observed through satellite interferometry and GPS measurements, have been interpreted as the effect of the intrusion at shallow depth (3090 ± 138 m) of 0.0042 ± 0.0002 km3 of magma within a sill. This interrupts about 28 years of dominant hydrothermal activity and occurs in the context of an unrest phase which began in 2005 and within a more general ground uplift that goes on since 1950. This discovery has implications on the evaluation of the volcanic risk and in the volcanic surveillance of this densely populated area.

Young volcanism and related hydrothermal activity at 5°S on the slow‐spreading southern Mid‐Atlantic Ridge

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast‐ and intermediate‐spreading centers but not from slow‐spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow‐spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow‐spreading southern Mid‐Atlantic Ridge near 5°S. Three high‐temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase‐separated, vapor‐type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent‐fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic‐hydrothermal interaction on a slow‐spreading mid‐ocean ridge.

Origin of chemically distinct granites in a composite intrusion in east-central Sweden: geochemical and geothermal constraints

A few tens of millions of years after the intrusion of the Early Svecofennian (∼1.87–1.85 Ma) granitoids in central Sweden, a renewed magmatic activity resulted in the emplacement of the Late Svecofennian granites, the tectonic setting of which remains obscure. S-type granites dominate this group, but both I-type and transitional granites are common. This study deals with one of these intrusions in east-central Sweden; a composite pluton that is insignificantly deformed and hosts both I- and S-type granites. One of the I-type granites shows a compositional trend from granodiorite to granite, which is uncommon among the Late Svecofennian granites. Major element and incompatible trace element compositions and ɛNd data show that two different sources, one igneous and one sedimentary, were involved. An important conclusion is that nearly coeval granites derived from different sources are found in close connection. The granites are suggested to have formed by partial melting in a thickened continental crust that was formed in an early stage of the Svecofennian event. Thermal models suggest that the slightly older, high-temperature I-type granite (granodiorite) was formed deeper in the crust than the S-type granite. The coexistence of essentially pure I- and S-type granites, rather than transitional mixtures, reflects the relative depths of the proposed sources and the varying thermal parameters of the lithologic units in the Svecofennian crust.

New Mapping and Interpretations of the Mount Lyell Mining District, Tasmania: A Large Hybrid Cu-Au System with an Exhalative Pb-Zn Top

New detailed mapping of the Mount Lyell mines area has resulted in a better understanding of the large Cambrian volcanic-hosted alteration-mineralization system. The mapping integrates the exhalative top and silica-rich upper part of the system at Lyell Comstock, the large displaced mass of silica-rich schists with high-grade bornite ores at North Lyell, and the deeper levels of the system exemplified by the low-grade Prince Lyell deposit. The 6-km-long alteration system has a core zone of pyrite-rich sericite-chlorite-silica schists, which contains most of the copper orebodies, flanked by marginal zones of pyrite-poor sericite and chlorite schists. The alteration has been focused along the Great Lyell fault, a major reverse-type growth fault, which forms the contact between the volcanic rocks and the younger (Late Cambrian) siliciclastic conglomerate sequence of the Owen Group. Small lead-zinc-rich massive sulfide lenses, together with lenses of fossil-rich limestone, characterize the exhalative zone, which is hosted within submarine volcaniclastic breccias and sandstones in the basal part of the late Middle Cambrian Tyndall Group at Comstock. The upper part of the alteration zone immediately below this is characterized by numerous cherty silica masses or heads contained within sericite-pyrite-silica schists, culminating in the very large Comstock chert body that appears to have formed a siliceous cap to the system. Ore lenses of disseminated chalcopyrite-pyrite, with some bornite, were located below the silica cap. At North Lyell, a large displaced mass of schists from the upper part of the system has numerous silica heads and a large cap-like silica body, the North Lyell chert, similar to the Comstock chert, which has been generally misinterpreted as silicified Owen conglomerate. Bornite-chalcopyrite ores were located immediately beneath this chert body, and several chalcopyrite-pyrite lenses are present at slightly deeper levels. The schist mass was apparently emplaced into the Owen basin by collapse from the scarp of the Great Lyell fault, after the alteration zone was uplifted and unroofed by erosion. The sulfide- and chert-rich mass was exposed, oxidized, and partially eroded during its slow, nappe-like emplacement, and gossan-like hematite-barite bodies and hematite-chert breccias were developed along the contact in places and interfinger with the contemporaneous Middle and Upper Owen sediments. There appears to be a zonation in the copper ores from disseminated chalcopyrite-pyrite in the deeper levels, as at Prince Lyell, to chalcopyrite with minor bornite in the silica heads zone (exemplified by Comstock and Western Tharsis), to bornite-rich ores with lesser chalcopyrite in the upper levels against the silica cap, as at North Lyell. Pyrophyllite is associated with the bornite mineralization, which appears to represent a high-sulfidation phase, possibly associated with increased or renewed magmatic activity, rather late in the development of this large, hybrid, magmatic-seawater system. Much of the upper part of the system, probably including some rich orebodies, was apparently eroded off during deposition of the Owen Group.

Geochronology of TAG and Snakepit hydrothermal fields, Mid-Atlantic Ridge: Witness to a long and complex hydrothermal history

Geochronological studies of a large number of precipitates from the TAG hydrothermal field and of few samples from Snakepit hydrothermal field of the Mid-Atlantic Ridge show intermittent repeated hydrothermal events at both sites. 210Pb/Pb and 230Th/234U measurements of sulfides, iron and manganese oxides, and 14C measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields. The TAG field shows intermittent activity over the past 120,000 years as evidenced by ages of low-temperature Mn oxides. The presently active black smoker mound first formed about 40,000–50,000 years ago with precipitation of massive sulfides. It has had intermittent, pulsed high-temperature activity every 5000–6000 years over the past 20,000 years which may reflect renewed magmatic activity at the ridge axis. Fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of N-S ridge parallel lystric normal faults and an E-W transform fault. Periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high-temperature event. The Snakepit field had an initial event about 4000 years ago, probably shortly after the eruption of the volcanic ridge on which it sits. A recent renewal, still presently active, was probably initiated by recent fissuring of the volcanic pile.

The Kelian disseminated gold deposit, East Kalimantan, Indonesia

Kelian is one of a number of recently discovered Tertiary volcanic-hosted gold deposits which occur within a 400-km-long, northeast-trending belt in the interior of Kalimantan, the Indonesian part of Borneo. It consists of two main orebodies, namely West and East Prampus, and four smaller mineralized ore zones. These have a combined resource potential of +75 Mt at 1.8 g/t Au, making Kelian the largest known gold deposit in Indonesia. The deposit was discovered in 1976 during follow up work of alluvial gold occurrences within the Kelian River, using traditional exploration techniques of stream sediment, pan concentrate, rock float and outcrop sampling. Subsequent detailed investigations included soil sampling, trenching, deep augering, ground magnetics, induced polarization and 60,000 m of diamond drilling. The geology of the deposit consists of a pile of silicic pyroclastics grading upwards into a series of sediments of Late Eocene age. The sequence was folded and faulted along northerly and northeasterly trends and intruded by a number of subvolcanic andesitic to trachyandesitic bodies in the Early Miocene. Some time after the emplacement of the andesites a hydrothermal system was established. It brought about extensive alteration, mineralization and hydrothermal brecciation. The preserved part of this system is 1 km 2 in area and has a vertical extent of at least 600 m. Limited radiometric age data suggest an age of around 20 Ma for the alteration. Four partly overlapping stages of alteration/mineralization are recognized: 1. Stage I: chlorite+carbonate+sericite+pyrite±epidote alteration in the cores of the larger andesite bodies. 2. Stage IIA: localized fracturing and sericite±quartz deposition. Stage IIB: a major period of polyphasal fracturing and hydrothermal brecciation with associated boiling of hot (290–330°C), dilute (0.5–4.2 eq. wt.% NaCl) and CO 2-rich fluids, and development of replacement/vein alteration of adularia+sericite+quartz+pyrite accompanied by subordinate As-Ag mineralization and possibly some gold mineralization. Progressive loss of CO 2 upon boiling caused a progressive increase in adularia and a corresponding decrease in sericite deposition. Deeper in the system a quartz-sericite-carbonate assemblage was formed without adularia. 3. Stage III: renewed magmatic activity, focused on East Prampus, released volatiles to the hydrothermal system, producing a hot (300–330°C), relatively saline (> 10 wt.% NaCl), CO 2-rich brine, and extensive fracturing and hydrothermal brecciation. This was accompanied by boiling, deposition of various carbonates±quartz±sericite and gold-base metal mineralization; preliminary stable isotope data on the carbonates support the presence of a magmatic component. Mixing of the upwelling hot, saline fluids with descending cooler, relatively low pH, steam-heated fluids has resulted in a complex zonation in carbonate mineralogy and a wide range of fluid salinities and homogenization temperatures. Away from the centre of brecciation and boiling, gold and base metals were deposited in a more passive environment together with carbonate and/or quartz. 4. Stage IV: late-stage widespread influx of relatively cool (<200°C), low pH (3–4), CO 2-rich, steam-heated fluids from the top and margins of the system forming a kaolinite+Fe/Mn carbonate+pyrite ±hematite±cinnabar assemblage. The magmatic-hydrothermal event was followed by uplift and erosion of more than 900 m during the Midddle Late Miocene and basaltic volcanism in the Plio-Pleistocene. A second cycle of erosion, which began in the Pleistocene, has removed most of the young volcanic cover exposing the deposit as known today.