Injection Of Magma Research Articles

A pluton-dike system as an incomplete assembled pluton: field geology, petrography and geochemistry of the Rodríguez dike swarm and the Kranck pluton, Fuegian Andes

Dikes extending into the host-rock are a common feature at pluton roofs. Typically, the dikes are considered offshoots from differentiated magma reservoirs within the pluton, i.e. late-stage intrusions of the successive discrete magma pulses that built the pluton. We evaluate this hypothesis on a dike swarm and associated pluton, the Kranck-Rodríguez pluton-dike system, by analyzing its field geology, petrography and geochemistry (whole-rock and mineral). This system was emplaced in the shallow crust (∼1.7 kbar) within the fold-and-thrust belt of the Fuegian Andes. The Kranck pluton is a small (∼4.4 km2), composite body composed of a diverse range of lithologies, from mafic to felsic. Internal contacts are mainly sharp and subvertical, suggesting diking as a mode of assembly. The Rodríguez dike swarm extends up to 6 km from the pluton, mainly exposed adjacent to it. This swarm also displays a wide variety of lithotypes, encompassing a set of dikes affected by ductile deformation, and a younger set without it. Geochemical and petrographic analyses, along with thermobarometry, suggest that the Rodríguez dikes and lithotypes within the pluton were fed from different, deeper magma reservoirs, in an inferred trans-crustal plumbing system. We propose that the dikes represent discrete increments of the growing pluton, in a protracted process of successive injections. This ultimately led to the assembly of the pluton in the sector of the system with greater magma flux. The system developed in two major episodes, pre- and post-ductile deformation, each characterized by multiple, discrete magma injections. We consider this system as an embryonic stage of a broader model that would evolve from sheets separated by host-rock, to bodies with sheeted margins containing host-rock rafts, and eventually to relatively inclusion-free plutons, potentially forming larger bodies if provided with a greater magma supply and/or higher accretion rates.

Eruption style and dynamics of the ~ 87 ka Baricha peralkaline rhyolite eruption in Ethiopia

Peralkaline rhyolites are a rare magma type, typically associated with continental rift settings, and characterised by excess alkalis relative to alumina and a moderate-low viscosity compared to calc-alkaline equivalents. Despite their prevalence in extensional rift settings, such as the Main Ethiopian Rift, eruption dynamics of peralkaline magmas are poorly understood and have never been directly observed. To address the knowledge gap, this study investigates the style and dynamics of the ~ 87 ka explosive eruption at Baricha volcano as a case study. This eruption deposited widespread pumice lapilli fall and pyroclastic density currents, which provide valuable information on pre- and syn-eruptive magmatic processes. By examining the physical and textural features of the eruption products at different stratigraphic levels, we reconstruct eruption dynamics over time. Our analysis reveals that the eruption had three distinct phases, each characterised by different types of tephra fall deposits and associated with different plume and vent conditions. Specifically, deposits of phases 1 and 3 were characterised by massive and well-sorted tephra falls indicative of sustained plume behaviour, while phase 2 deposits were bedded, lithic-rich (i.e. non-juvenile fragments) tephra falls, and pyroclastic density current deposit associated with an unsteady plume and vent-widening phase. The pumice (8–16 mm size fraction) from this eruption is microlite-free, with a bulk density of 400–700 kg m−3 and > 60% total vesicularity. The vesicle size distribution is polymodal, with the most frequent size ranging from 0.001 to 2.4 mm and an estimated vesicle number density of 0.07*107 to 1.6*107 mm−3. The textural observations suggest rapid nucleation occurred during the late phases of magma ascent. Calculated decompression rates of the ascending magma were 0.07–5.6 MPa/s and show a variation between the eruption phases. We conclude that the shift in eruption dynamics alternating between steady to unsteady plume behaviour during the eruption was likely driven by changes in conduit geometry, lithic abundance of the eruptive mixture, decompression rate, and fresh magma injection.

Geochemical constraints on reaction temperature and pressure and heat- and mass-transfer efficiency at Rainbow Hydrothermal Field

Seafloor vent fluids hosted by oceanic core complexes (OCCs) are thought to represent circulation of seawater-derived hydrothermal fluid along deeply penetrating, low-angle detachment faults. However, estimation of the source temperatures and pressures of such fluids has been limited because geochemical methods typically require vent fluid silica concentrations to be buffered by quartz, a condition not often met at oceanic core complexes. Here, we extend the calculation of the Si-Cl geothermobarometer to enable predictions of fluid Si concentrations in equilibrium with any Si-buffering mineral assemblage, rather than being restricted to quartz. We apply this method to Rainbow Hydrothermal Field vent fluid compositions and find that they are consistent with buffering by a plagioclase+talc+chlorite+tremolite mineral assemblage at conditions ranging from (430 °C, 359 bar) to (470 °C, 468 bar), which correspond to depths of 1.3–2.4 km below the seafloor. These estimates agree well with the locations of seismically imaged magma chambers within the Rainbow Massif. Additionally, calculations of fluxibility and Fe solubility support this relatively shallow origin for Rainbow vent fluids and imply relatively efficient heat and Fe extraction from the seafloor. We estimate that only 24–30 % of the heat content and almost no Fe is lost during upflow of Rainbow vent fluids.Compared to other OCC-hosted seafloor vents, the source region of Rainbow vent fluids is anomalously shallow, an observation consistent with geological interpretations of the Rainbow Massif. Vent fluids at Rainbow Hydrothermal Field have exhibited apparently stable and greater-than-seawater salinity for over two decades. We interpret these vent fluids as vapors derived from a higher-salinity source fluid that developed over multiple cycles of magma injection and phase-separation inherent to the formation of oceanic crust along slow-spreading, non-volcanic segments of oceanic spreading centers. Alternatively, higher-salinity source fluids could be derived from mineral hydration reactions associated with serpentinization of ultramafic rocks. The occurrence of greater-than-seawater salinity vent fluids is thus predicted to be a common feature of OCC-hosted vent fields, as indicated by several known examples, including the TAG, Kairei, and Edmond vent fields.

The magma evolutional constrains on the genesis of proximal Zn skarn mineralization: A case study from the Yaojialing deposit in Tongling district, eastern China

The major factors especially the roles of the magma evolution controlling the Zn/Cu mineralization in skarn deposits are still controversial. Yaojialing is a large-sized skarn Zn polymetallic deposit (1.74 Mt Zn at 3.6 %, 30.4 t Au at 4.2 g/t and 24.8 t Cu at 0.83 %) located in the Tongling district of the Middle–Lower Yangtze belt (MLYB), both the proximal and distal areas of the deposit exhibit high Zn/Cu mineralization. The intrusions in Yaojialing primarily consist of quartz monzonite porphyry (QMP) and granodiorite porphyry (GDP), and the QMP is related to skarn mineralization. Both the QMP and GDP display relatively high (87Sr/86Sr)i values (0.707907 to 0.709390), low εNd(t) values (−9.05 to −8.17) and negative εHf(t) values (−8.51 to −11.72), suggesting that they originated from a mixed source of enriched mantle and lower crust. Both the QMP and GDP contain type I and type II amphiboles, while type III amphibole exists only in QMP. Type I amphibole is acicular crystals shape, type II amphibole is euhedral in shape and relatively large in size (0.6–2.0 mm), while type III amphibole crystallized around the margin of type II amphibole. The type I amphibole from QMP and GDP show similar calculated temperatures (948–1010 ℃), pressures (2.9–6.5 kbar, corresponding depths at 11.0 to 24.4 km), fO2 (ΔFMQ = 0.2–1.9) and H2O content (4.2–6.8 wt%). Type II amphibole crystallized at 815–941 ℃, 1.2–2.9 kbar (corresponding to depth of 4.6–11.1 km), ΔFMQ = 0.7–2.1, and 4.4–5.7 wt% H2O. Type III amphibole have a lower temperature (679–795 ℃), pressure (<0.5 kbar) and water content (2.5–4.3 wt%) but higher fO2 value (ΔFMQ = 3.1–3.8) compared to type I and type II amphiboles. Reverse zoning of plagioclase and higher Mg# (74 to 89) of type III amphibole in QMP are resulted from injection of mafic magma at shallow depths, which provide sufficient metal and favorable conditions for the formation of QMP parental magma. Modeling of magma H2O solubility indicates that QMP begins to exsolve fluid at depth of 6.2–11.1 km. Initial low oxygen fugacity and ore-forming element differential release during fluid exsolution process resulted in the high Zn/Cu mineralization at Yaojialing.

Tracing Lower Crustal Contamination in Continental Arc Magmas Using Sr–Nd–Hf Isotopes: A Combined In Situ and Bulk Rock Approach Applied to the Adamello Batholith

Abstract The incremental construction of plutons characterises magmatic activity in arc settings, where new continental crust is produced. This polyphasic growth entails interactions with one or more crustal components, which modulate the geochemical and isotopic compositions of the newly formed crust. However, the early stages of magmatism are not always preserved due to obliteration by later magmatic pulses. Spatial migration of magmatism during the construction of the Adamello batholith (Northern Italy) enables the examination of the early pulses of pluton formation, thus allowing a time-integrated study of the relative importance of crystallisation-differentiation and contamination in a continental arc setting. We conducted a detailed textural, major and trace element and Sr isotopic study of plagioclase from the first intrusive pulses of the Adamello batholith, combined with new major, trace element and Sr–Nd isotopic analyses of bulk rock samples across the entire Adamello batholith. We selected well-characterised samples with published CA-ID-TIMS 206Pb-238U ages and Hf isotopic composition for zircons. Strontium isotopes in plagioclase from the same samples were determined by laser ablation multi-collector ICP-MS. The tonalitic samples in the early magmatic stages show elevated but constant Sr isotopic compositions despite large variations in anorthite contents (An90 to An13), indicating that crustal contamination occurred before significant differentiation. Invariant bulk-rock 87Sr/86Sr with variable SiO2 in all superunits of the Adamello batholith further supports contamination preceding significant melt differentiation. Contamination by lower crustal basement lithologies is due to the increasing thermal anomaly triggered by consecutive magmatic injections coupled with the heterogeneous and less restitic nature of the basement in the early stage of the magmatic system (i.e., before consumption of fusible components). In addition, we observe significant variability in crustal contamination proxies (e.g., 87Sr/86Srplag, 87Sr/86Srbulk, εNdbulk, εHfzircon) during the initial phases of magmatism. This variability likely reflects the uneven distribution of positive thermal anomalies in the lower crust during early magmatic stages as well as the diverse lithological and isotopic makeup of the lower crust. The processes identified in our case study are pertinent to continental arc magmatism, particularly where magmas interact with a metapelitic lower crust.

Late Devonian felsic magmatism in southern New Brunswick and its association with a large igneous province that may have contributed to the Frasnian-Famennian extinction

Felsic rocks of the Piskahegan Group and coeval plutons form a major part of a Late Paleozoic epicontinental caldera in southwestern New Brunswick, Canada. The caldera forms an elliptical structure with a length of ∼34 km and a width of ∼13 km. It hosts a significant polymetallic deposit of tin, molybdenum, indium and bismuth associated with mid-sequence granitic intrusions. The caldera formed in the aftermath of the late Early to Middle Devonian Acadian Orogeny during the opening of the late Paleozoic Maritimes Basin in the Northern Appalachian Belt. Its surrounding successions are largely composed of bimodal igneous rocks that were derived from two distinct sources: upper mantle-derived mafic magma and lower crust-derived felsic magma. The rocks that marked the beginning of volcanic activity in the caldera are no younger than 374.2 ± 2 Ma, whereas the uppermost dated volcanic unit in the complex yielded an age of 364.6 ± 0.7 Ma. The latter is conformably overlain by a succession of red beds and undated basalts that also overlie penecontemporaneous felsic volcanic rocks adjacent to the caldera. The post-orogenic felsic rocks are fractionated, peraluminous A-2 type rocks with silica contents ranging from ∼70 to ∼79 wt% along with high K2O contents and ɛNd(t) ranging from −0.10 to +1.05. They were generated by the melting of lower crustal basement rocks linked to a lithospheric plate that was metasomatized during the Neoproterozoic. Based on available geochemical, geophysical and structural data on Late Paleozoic rocks of southeastern Canada, the melting may have been triggered by the injection of profuse mafic magma that eventually formed a thick underplating at the base of the crust in association with heat derived from an underlying mantle plume and conveyed by transtensional structures. Basal rocks of the Piskahegan Group (the Intracaldera Sequence) and penecontemporaneous volcanic rocks in Nova Scotia are interpreted as small erosional remnants of a Large Igneous Province (LIP) based on the extensive exposure of large coeval plutons throughout southeastern Canada. The new geochronological data suggests that profuse late Frasnian magmatism in this LIP could have contributed to the environmental deterioration that led to a significant extinction at the Frasnian-Famennian boundary (the Kellwasser Event).

High-precision U[sbnd]Pb geochronology of an ignimbrite flare-up in a silicic large igneous province (Chon Aike, Patagonia)

Silicic large igneous provinces (SLIPs) are periods of particularly voluminous felsic volcanism in the geologic record. Previous work has suggested an overall long lifespan for SLIPs of 20 to 40 Myr, commonly punctuated by shorter-lived ‘flare-ups’ of higher volcanic productivity (1–5 Myr), but detailed studies of individual flare-up events are lacking. The Jurassic Chon Aike SLIP (CASP) is the product of an exceptional geological event wherein voluminous felsic volcanism (ca. 219,000 km3) was generated predominately via crustal anatexis over 45 Myr. We focus on the Late Jurassic El Quemado Complex (EQC), which marked the final stages of felsic volcanism for the CASP. In-situ U-Pb ages for the EQC previously suggested a duration of ∼5 Myr; however, new high-precision CA-ID-TIMS ages indicate the total durations of ignimbrite successions were shorter than 350 kyr. A compilation of zircon U-Pb ages for the eight CASP formations in Patagonia and the Antarctic Peninsula reveals changes in volcanic duration between formations deposited in the intraplate relative to the continental margin, suggesting a spatial control over the magmatic lifespans of geographically restricted systems. We suggest that the rate of magmatism in the CASP was primarily controlled by heterogeneities in the crust, largely between Proterozoic igneous crust and younger, metasedimentary crust that was recently accreted relative to the timing of Jurassic volcanism. The observed duration of volcanism was modulated by these differences in crustal properties from the injection of mafic magmas into the lower crust during extension and mantle upwelling, following rollback of the subducting oceanic slab towards the Paleo-Pacific margin of Gondwana. The short duration in the EQC is the product of unique overlapping conditions that favored crustal melting and resulted in the voluminous ignimbrite flareup (104 km3) of some of the Earth's highest δ18O magmas measured.

Changes in the magmatic plumbing system associated with the Gotemba sector collapse at Mount Fuji, Japan

It is well known that magmatic plumbing systems change over time, but there is much debate as to why and how. We studied volcanic ejecta continuously deposited in an outcrop at Kagosaka Pass at the eastern base of Mount Fuji to investigate the factors responsible for changes in the magmatic plumbing system. The sample consisted of pyroclastic sediments from explosive eruptions for approximately 3000 y, which sandwiched the time of the Gotemba sector collapse at approximately 2500 BP. Chemical analyses of whole rocks, minerals, and matrix glasses, as well as mode measurements of glass and bubbles, were performed on samples collected from approximately 30 layers; significant changes were observed before and after the collapse. For example, before and after the collapse, matrix glass area increased around 60% to over 80% and anorthite content (Ca / (Ca + Na) * 100) of phenocryst plagioclase decreased from over 80 to below 65. For a period after the collapse, possibly hundreds of years, the plagioclase and olivine phenocrysts exhibited characteristics indicative of crystallization at low temperatures and pressures, and the pyroclast matrix became highly vitreous. Eruptions with ejecta of these characteristics continued more than a dozen times, lasting about 500 years. In addition, the trend in the distribution of the bulk rock chemical composition changed significantly, showing a differentiation trend with only plagioclase and clinopyroxene crystal separation. An investigation using the MELTS software revealed that the phenomenon of direct eruptions from deep magma chambers to the surface, bypassing shallow magma reservoirs, continued for several hundred years after the collapse. This can be interpreted as a decrease in confining pressure associated with the collapse, facilitating the eruption of magma from the depths. Furthermore, based on an examination of the water content in the magma during this period, we posit that the trigger for the rise of magma from the deep magma chamber of Mount Fuji is the acquisition of excess pressure by the injection of magma from a deeper level.

Melt infiltration in a crystal mush and pegmatoid formation in the platiniferous Merensky Reef, Bushveld Complex, South Africa

AbstractGiant mafic-ultramafic layered intrusions of Archaean-Proterozoic age are the fossilised remnants of huge injections of silicate magma in the Earth’s crust and are our most important repositories of platinum-group elements. Magmatic PGE-rich ore deposits, such as the Merensky Reef, are typically hosted in stratiform reefs at the contacts between ultramafic and feldspathic cumulates. The Merensky Reef is commonly characterised by coarse-grained and pegmatoidal textures that may provide important clues to its origin. We present textural and in situ geochemical data for Merensky pegmatoids at Styldrift Mine (Impala Bafokeng) in the Western Bushveld Complex of South Africa. This region is adjacent to an inferred magmatic feeder zone to the Bushveld. The Merensky pegmatoids are characterised by (i) amoeboid olivine inclusions in zoned orthopyroxene megacrysts with increasing molar Mg# of orthopyroxene towards olivine, (ii) fine-grained chains of orthopyroxene in compositional equilibrium with adjacent orthopyroxene megacrysts, (iii) increasing molar Mg# of orthopyroxene megacrysts and increasing molar An with decreasing 87Sr/86Sri (at 2.06 Ga) of plagioclase oikocrysts in pegmatoids laterally across a 10-km section distal to the feeder, and (iv) highly variable molar An and initial 87Sr/86Sri of interstitial plagioclase proximal to the feeder. We interpret the coarse-grained and pegmatoidal textures, their dissolution-reprecipitation features, and lateral chemical variations as the product of lateral melt infiltration and mixing in a crystal mush. We suggest that the platiniferous Merensky Reef was not formed at the base of a large melt-filled magma chamber but was instead the product of non-sequential magma emplacement that rejuvenated the crystal mush.

Petrological and geochemical insights into the magma plumbing system of the Daliuchong dacite eruption, Tengchong Volcanic Field, SW China

The formation of highly evolved, dacitic magmas has been attributed to various processes, including crystal fractionation, partial melting of overlying crust, and/or assimilation of crustal material into an evolving magma chamber. These processes are undoubtedly primary processes involved in the formation of dacites, but they may not be the only mechanism involved in the formation of high-silica dacites. For instance, mafic magma replenishment has been proposed as an additional mechanism but has not been assessed, and thus, its role has not been well-constrained. The Daliuchong volcano is the result of one of the largest eruptive events within the Tengchong Volcanic Field (TVF) in southwest China during the Early-Middle Pleistocene. Here, we conducted detailed mineral textures, mineral chemistry, and geochemical studies on Daliuchong pyroclastic rocks to explore the pre-eruptive storage conditions and evolution processes of the magma. The Daliuchong pyroclastic rocks are dacitic in composition. The samples show porphyritic textures characterized by phenocrysts of plagioclase, amphibole, clinopyroxene, orthopyroxene, and Fe-Ti oxides. Additionally, two distinct types of glomerocryst are identified: a gabbroic glomerocryst containing plagioclase + clinopyroxene + orthopyroxene ± Fe-Ti oxides assemblage and a dioritic glomerocryst containing plagioclase + amphibole ± pyroxene ± Fe-Ti oxides assemblage. Both phenocryst and glomerocryst show rich micro-textures. The Daliuchong dacite exhibits bulk compositional heterogeneity. Analysis of bulk-rock data suggests that this heterogeneity may arise from both the differentiation of the dacite itself and the injection of mafic magma. The compositional similarity between the Daliuchong dacite and experimentally produced partial melts of metamorphic basalt supports that the Daliuchong dacite was predominantly formed through the partial melting of the mafic lower crust. Thermobarometry estimation indicates that clinopyroxenes with high Mg# crystallized at 560–870 MPa, whereas amphibole and clinopyroxenes with low Mg# crystallized at 185–300 MPa. Based on the observed phase relations and the calculated crystallization conditions, we propose that during the differentiation of the Daliuchong dacite, heterogeneous dacitic magma formed by partial melting accumulated in a deep magma reservoir (21–32 km) before subsequently ascending toward shallower depths. Crystallization of plagioclase, amphibole, Fe-Ti oxides, and small amounts of pyroxene and apatite occurred at a shallower depth (7–10 km). The presence of coarse-sieve texture, fine-sieve texture, and oscillatory zoning with high amplitude in plagioclase suggests intermittent injection of mafic magma into the shallow magma reservoir, with the eruption of dacitic magma occurring after the final mafic magma replenishment. The petrological evidence above advocates that primitive magma replenishment could have been involved in the formation and triggered the eruption of dacite in the Daliuchong volcano.

Brittle fragmentation of Fissure 17 enclave magma revealed by fractal analysis

The 2018 LERZ eruption of Kilauea featured a wide range of eruptive styles. In particular, Fissure 17 (F17) displayed activity ranging from Hawaiian fountaining in the eastern part of the fissure to Strombolian explosions in the western part. Lava erupted from F17-West was highly viscous and contained magmatic enclaves. Magmatic enclaves have previously been observed in many other volcanic systems (e.g. Vulcano Island, IT and Sete Cidades Volcano, PT), where they have been attributed to injection of mafic magma into an evolved magma chamber, resulting in viscous fingering, quenching, and break-off into fragments. The F17 enclaves differ from previous studies in that the chemical compositions of the enclave and host magmas are very similar, and that the enclaves have a limited spatial distribution and lack signs of viscous behavior and quenching, pointing to a different formation mechanism than inferred for other volcanic systems.In order to test a different formation hypothesis, we conducted fractal analysis of the size distribution of 84 individual enclaves from F17-West lavas. Our results, including a fractal dimension of fragmentation Df of 2.59, indicate that the F17 enclaves likely formed by brittle fragmentation. Since the enclave and host magmas were at temperatures far above the glass transition during the magma hybridization, high strain rates have to be invoked to explain the brittle fragmentation. This may have caused the enclave magma to transition into solid-state behavior, allowing it to break off into fragments that were subsequently picked up by the host magma and carried to the free surface.The enclaves from F17-West therefore offer a unique insight into the diversity of processes that characterizes the shallow parts of volcanic systems, as well as the importance of strain rates in modulating the rheological behavior of magmas.

Changing magma recharge/discharge dynamics during the 2020–22 lava fountaining activity at Mt. Etna revealed by tilt deformation and volcanic tremor

Mt. Etna exhibited 62 lava fountaining events between December 13, 2020 and February 21, 2022. We analyzed tilt deformations and volcanic tremor amplitude time series, to characterize both eruptions and the preceding preparatory phases in terms of magnitude and speed of development of the volcanic phenomena, as well as to reconstruct the processes that took place inside the plumbing system and drove this intense period of activity. Based on deflation amplitudes associated with lava fountains and according with other retrieved parameters (i.e., magnitude of inflations, inflation and deflation velocities and volcanic tremor amplitudes), three periods have been distinguished. Period I displays higher values of all the aforementioned parameters, interpreted as conspicuous volumes of volatile-rich magma transferred towards the surface. Period II shows lower values evidencing lack of important new injections of magma from depth, whereas period III reveals a general increasing trend possibly related to gas flushing from magma residing in deeper portions of the plumbing system. Detailed elaborations of tilt signals allowed the identification of short-lived inflations accompanying the early stages of lava fountains during period II. Our results reveal significant correlations between amplitude and velocity of tilt and volcanic tremor signals associated with lava fountains and evidence the crucial role of gas in the inflation-deflation cycles.

Magma flux variations triggering shallow-level emplacement of the Takidani pluton (Japan): Insights into the volcanic-plutonic connection

Defining the physico-chemical evolution of felsic magmatic reservoirs in the middle-upper continental crust is crucial for understanding the connection between granitic plutons and silicic volcanic rocks and for research on the trigger mechanisms of volcanic eruptions. In this study, we investigate the formation and evolution of the Hotaka-Takidani volcano-plutonic complex (Japan) using a comprehensive approach that integrates zircon petrochronology with phase-equilibria and thermal modelling. The Takidani pluton is a Pleistocene tilted zoned intrusion of granodiorite-granite composition associated with at least two large caldera-forming eruptions. We have investigated the three uppermost units of the pluton and the oldest and more voluminous Nyukawa eruption, which deposited more than 400 km3 (DRE, dense rock equivalent) of crystal-rich dacitic ignimbrite. Amphibole thermobarometry, U-Pb zircon dates and geochemical data indicate that the Takidani pluton and the Nyukawa dacite were sequentially sourced from a 10–12 km-thick long-lived magmatic reservoir in the middle crust. Thermal evolution of the reservoir led to the extraction of the Nyukawa magma, followed by brief storage in the upper crust and eruption at ca. 1.706 ± 0.091 Ma. For two of the units of the pluton, we combine high-spatial resolution zircon U-Pb geochronology with zircon chemical data. Applying the Ti-in-zircon thermometer, we observe that magma within the reservoir was stored at a relatively constant temperature of ca. 700 °C for 300 kyr and subsequently, starting from 1.4 Ma, the temperature increased progressively reaching 820–850 °C at 1.1 Ma. This temperature shift is inconsistent with a constant magma flux and suggests an increase of the magma injection rate into the reservoir. Using the distribution of zircon ages and thermal simulations, we determined that the reservoir was built by a fairly low average magma flux of ca. 6 × 10−6 km3*km−2*yr−1, shifting to a magma supply rate which is three to four times higher during the last 300 ka of zircon crystallisation. This increase in magma flux caused the final emplacement of the Takidani magma in the upper crust, where it rapidly crystallized. The integration of geochronological data with phase-equilibria and thermo-chemical modelling demonstrates that multiple melt extraction events occurred during the evolution of the Hotaka-Takidani magmatic system. Only the first one triggered a major eruption, while subsequent events resulted in the emplacement of the Takidani pluton in the shallow crust.

Oligocene–Miocene arc magmatic activities associated with the giant Reko Diq porphyry Cu–Au deposit, western Chagai arc, Balochistan, Pakistan

The Chagai belt to the north of the Afghan block and the east of the Iranian block in the western part of Pakistan is known for occurrence of Oligocene to Miocene calc‐alkaline magmatic belt. The present study discusses the characteristics of episodic magmatic activities that contributed to mineralization for the Reko Diq porphyry complex in the western part of the Chagai belt. U–Pb dating of zircons from volcanic rocks of the Reko Diq porphyry complex yielded ages of 25.8 ± 1.8 and 12.29 ± 0.44 Ma for two phases of magmatic crystallization. Geochemical analyses of whole‐rock rhyolite and dacite indicate that the rock units are peraluminous calc‐alkaline, derived from Andean‐type subduction. The magma was formed due to partial melting of the thickened mafic lower crust in a continental arc setting in relation to the subduction of the Neotethys along the Makran subduction zone. The abundance of zircons, Hf, REE, U/Pb and Th indicates high degrees of magmatic evolution. Moreover, the Sr and Nd isotopic data indicate the fractional contribution of depleted N‐MORB mantle to the Reko Diq magmas through bulk mixing with magmas derived from the lower continental crust. Short‐lived magmatic systems repeated magma injection, and various episodes of hydrothermal fluid flow have led to the formation of porphyry mineralization. Emplacement of the Reko Diq porphyry complex and related Cu–Au mineralization is associated with a series of tectonic–magmatic events at different episodes of the Oligocene–Miocene times.

Fracturing and Dome‐Shaped Surface Displacements Above Laccolith Intrusions: Insights From Discrete Element Method Modeling

AbstractInflation of viscous magma intrusions in Earth's shallow crust often induces strain and fracturing within heterogeneous host rocks and dome‐shaped ground deformation. Most geodetic models nevertheless consider homogeneous, isotropic, and linear‐elastic media wherein stress patterns indicate the potential for failure, but without simulating actual fracturing. We present a two‐dimensional Discrete Element Method (DEM) application to simulate magma recharge in a pre‐existing laccolith intrusion. In DEM models, fractures can propagate during simulations. We systematically investigate the effect of the host rock toughness (resistance to fracturing), stiffness (resistance to deformation) and the intrusion depth, on intrusion‐induced stress, strain, displacement and spatial fracture distribution. Our results show that the spatial fracture distribution varies between two end‐members: (a) for high stiffness or low toughness host rock or a shallow intrusion: extensive cracking, multiple vertical surface fractures propagating downward and two inward‐dipping highly cracked shear zones that connect the intrusion tip with the surface; and (b) for low stiffness or high toughness host rock or a deeper intrusion: limited cracking, one central vertical fracture initiated at the surface, and two inward‐dipping fractures at the intrusion tips. Abrupt increases in surface displacement magnitude occur in response to fracturing, even at constant magma injection rates. Our modeling application provides a novel approach to considering host rock mechanical strength and fracturing during viscous magma intrusion and associated dome‐shaped ground deformation, with important implications for interpreting geodetic signals at active volcanoes and the exploitation of geothermal reservoirs and mineral deposits.

Origin of the Permian high silica A-type granites in Halajun, South Tienshan, NW China: Implications for crystal-liquid segregation and rare metal fertilization

High silica A-type granites (HSAGs) are important carriers of rare metal elements. Formation of HSAGs by crystal-liquid segregation would facilitate fertilization of rare metals and rare earth elements (REE). In this work, we present detailed petrology, whole-rock geochemistry, and in-situ zircon UPb ages and HfO isotopic compositions of the Halajun HSAGs in the South Tienshan Orogenic Belt (STOB). In combination with the previous study of the Halajun syenite and monzonite, we unravel the genetic connection of these granitoids and further explore the metallogenetic potentiality of HSAGs along the STOB in China. The Halajun granites have geochemical affinities of HSAGs with high SiO2 contents (> 70 wt%), whole-rock zirconium saturation thermometry (680–860 °C), high Ga/Al ratios and high field strength elements (HFSE) contents. The HSAGs (273–279 Ma), monzonite (280 Ma) and syenite (283 Ma) have similar formation ages, identical mineral assemblages, and consistent variations in major elements, implying a common magma reservoir of their parental magma. The Tamu and Huoshibulake HSAGs have high SiO2 (74.5–78.1 wt%) contents, low Sr (5–28 ppm) and Ba (7–60 ppm) contents, high Rb/Sr ratios (11.6–87.5), and strong negative Eu anomalies (0.01–0.08). Combined with the petrographic features, these geochemical variations suggest that they are the extracted melt. Conversely, the geochemical (high Sr and Ba concentrations, and Eu/Eu* near unity) and petrographic features of monzonite and syenite represent the residual silicic cumulate (crystals ± trapped melt). The Kezi'ertuo and Halajun II HSAGs have SiO2 (71.9–76.5 wt%) contents, Sr (35–101 ppm) and Ba (60–541 ppm) contents, Rb/Sr ratios (1.9–8.9), and negative Eu anomalies (0.04–0.43), indicating the intermediate component (extracted melt + fractional crystals) in the consecutive system of crystal-liquid segregation. In-situ zircon HfO isotopes for the Halajun granitoids indicate a complex trans-crustal magmatic process involving crystal-liquid segregation and wall-rock assimilation. The long-time span of Tarim Large Igneous Province (TLIP) sustained injection of mantle-derived magma and/or advective heat to the magma chamber (> 1 Ma). Owing to the prolonged magma evolution, the highly fractionated silicic magma with high incompatible elements and F contents was extracted from the magma mush, and formed the HSAGs.

A Tall Tale of a Tiny Pluton: The L’Enfer Norite, Grenville Province, Québec

Abstract We present new field observations and mineral and whole-rock geochemical data for the L’Enfer norite, a small pluton in the central Grenville Province of Québec. Although tiny, its plagioclase-rich and noritic character suggested that it might hold clues (possible parental magma?) to the origin of closely associated andesine anorthosites in this part of Québec. The pluton is characterized by spectacular coarse-grained diabasic textures and varies from relatively massive to locally layered varieties. Both coarse- and medium-grained types occur and may be present together in some outcrops. Rare occurrences of snowflake-textured plagioclase are also present. The mafic mineralogy is dominated by orthopyroxene and Fe-Ti oxides (ilmenite and magnetite) with minor clinopyroxene, amphibole (Ti-pargasite), and Ti-rich biotite. Olivine occurs in one of the norites and in a single occurrence of an ultramafic rock that is also enriched in Fe-Ti oxides. Plagioclase compositions range from An32 to An48, but can be divided into two groups on the basis of texture: in one subset of samples, the plagioclase contains exsolution lamellae of orthoclase (i.e., antiperthitic), whereas plagioclase in the remaining samples lacks such lamellae and has not exsolved. Despite these textural contrasts, whole-rock compositions of the two groups are similar. Surprisingly, the compositions of pyroxene, biotite, and ilmenite reveal two non-overlapping compositional groups that correlate with those defined by plagioclase texture. Specifically, mafic minerals in the group with unexsolved plagioclase are consistently enriched in minor elements compared to those in the group with antiperthitic plagioclase. We interpret these contrasts to reflect differences in cooling rate for at least two injections of noritic magma. Moreover, the snowflake plagioclase grains are consistent with some degree of undercooling and supersaturation brought about by magma mixing. Although the norite shares some mineralogical and geochemical features with nearby anorthosites, it does not appear to represent a viable parent magma, primarily due to a pronounced difference in Fe-Ti oxide assemblage (hemo-ilmenite in the anorthosites versus ilmenite + magnetite in L’Enfer).

Mechanism of metamorphic fluid expulsion from ductile contact aureole: Insights from numerical modeling of a growing mid-crustal magma chamber

Metamorphic fluid flow around an igneous intrusion is governed by temperature, rheology, permeability and fluid pressure condition in the contact aureole. In the middle crust, where lithostatic fluid pressure and ductile deformation dominate, the impulse from incremental magma chamber growth facilitates ductile aureole compaction and induces syn-intrusive metamorphic devolatilization. However, the mechanism of metamorphic fluid expulsion from ductile contact aureole is vaguely understood. In this study, we numerically modeled the two-phase solid-fluid flow process in the ductile aureole above the growing chamber to examine the potential for fluid expulsion, the pattern of fluid flow and fluid pressure condition. The numerical model is constructed based on the geological structure of the Papoose Flat pluton in California, applying three different modes of pluton growth: continuous growth, episodic growth with an interval of 100 years and episodic growth with an interval of 1000 years. The results indicate that the metamorphic fluids propagate in the form of porosity wave in the ductile aureole with solid matrix compaction. Continuous pluton growth, with a slow pressurization of aureole, leads to a positive fluid pressure anomaly in most of the aureole, whereas the episodic growth results in a negative fluid pressure anomaly at the base and positive fluid pressure anomaly at the top of the aureole. Hydrofracturing can occur in the inner aureole, and the outer aureole may retain some fluid with low-frequency, high-flux magma injections. The solid-fluid flow models indicate that porosity wave is an important mechanism for metamorphic fluid expulsion from the pluton and wall rock system.

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift

Gedemsa caldera is a peralkaline volcanic depression located in the Central sector of the Main Ethiopian Rift. An integrated volcanological study of stratigraphic sections was carried out in order to constrain the eruptive history of Gedemsa caldera volcano (GCV), Ethiopia. Textural analyses on plagioclase crystals together with field observations shed light on magma chamber processes feeding the volcanic complex. A multi-vent eruption is ascribed to the pre-caldera volcanic products, as can be seen from the varying caldera wall sequences comprising lavas and pyroclastic deposits. At least three major caldera-forming eruptions are identified producing: 1) a lower ignimbrite, 2) extensive pumice fall and pyroclastic density current deposits, and 3) an upper ignimbrite exposed in different areas of the caldera, indicating a sector collapse. We identified 20 individual pyroclastic deposits found outside and within the caldera that erupted after the climactic caldera collapse. The most recent volcanic activity at Gedemsa is characterized by rhyolitic lava domes and pyroclastic deposits from eruptions through numerous vents aligned with WNW-ESE pre-existing cross-cutting structures. Macrocryst disequilibrium textures such as fine-scale oscillatory zoning (FOZ), sieve textures, glomerocryst, and synneusis indicate that magma reservoir is characterized by repeated magma injection, convection flows, and mixing.

Sulfide resorption contributes to porphyry deposit formation in collisional settings

Production of Cu-rich melts via melting of sulfide-bearing lower crustal cumulates is thought to contribute to porphyry Cu formation in the post-collisional Gangdese belt (Tibet). We present new whole rock, including platinum group elements (PGE) data, for ore-causative granites (OCG), ore-related granites (ORG) and barren granites (BG) from the Gangdese belt. The OCG, ORG and BG show indistinguishable Cu, PGE and Cu/Ag behavior during differentiation down to ∼2 wt% MgO. These systematics, together with the occurrence of sulfides, demonstrate that the propensity for some Gangdese magmas to be ore-associated is unrelated to melting of sulfide-bearing cumulates or the relative timing of sulfide fractionation. The behavior of chalcophile and siderophile elements during differentiation of the OCG, ORG and BG diverges when melts evolve past 2–3 wt% MgO and exsolve H2O, S and Cl: the BG show a decrease in Cu, whereas the OCG and ORG show highly variable Cu, Au, and PGE trends. Because S degassing causes melts to become sulfur undersaturated and because Cu is highly soluble in Cl-rich fluids, any sulfides in contact with the melt or magmatic fluids following volatile exsolution would dissolve. Continual injection, cooling and ascent of magmas through the Gangdese crust is expected to have resulted in the development of regions of crystal-rich partially molten crust beneath the intrusive granites. We suggest that prior to porphyry mineralization, extension and erosion caused exhumation of these long-lived sulfide-bearing mush zones. This exhumation may have caused a change in magmatism style from volatile exsolution after the melts transited sulfide-bearing mushes (i.e., the BG), to volatile exsolution during magma transit through sulfide-bearing mushes (i.e., the OCG). We suggest that only the magmas and that exsolved fluids during their ascent through sulfide-bearing mushes were able to assimilate sufficient Cu-rich sulfides to fuel the formation of porphyry Cu deposits in the region.