Volcanic Arc System Research Articles

Arc adakitic-like signatures caused by amphibole fractionation: An example from 20 Ma granitoids, Charco Rico district, Panama

The Charco Rico prospective comprises a segment of the Sierra de Majé massif in Central Panama, an intrusive complex constructed during a lull in volcanic activity between 40 and 16 Ma at the 75–0 Ma Central American Volcanic arc system (CAVAS). Charco Rico represents an intermediate suite (55–65 wt% SiO2) but with inclusion of related Majé intrusives, range from gabbro to granodiorite. Zircon UPb dating of Charco Rico granitoids yield LA-ICP-MS ages of 20.2 ± 0.9 to 19.1 ± 1.5 Ma. Their formation is consistent with maximum partial melting of ∼10% of a Sr-enriched (i.e., amphibolite) N-MORB like source followed by fractional crystallization of hornblende at about 0.8 GPa (∼29 km depth). Zirconium saturation thermometry shows a temperature of magma crystallization of 700–800 °C which essentially is a record of amphibole crystallization temperature. Whole rock chemistry of the Charco Rico and Majé intrusives and other 32–20 Ma arc-related suites in Panama record chemical parameters similar to adakite, e.g., Sr/Y that ranges to 80, high LREE/HREE ratios, and depletion in HREE (Yb, Lu). A fundamental chemical attribute of these suites, however, is depletion of MREE, and particularly Ho, relative to LREE and HREE, and the absence of Eu-anomalies. These features, together with positive and negative correlations of La/Yb and Dy/Yb with SiO2, respectively, infer a dominant role of amphibole fractionation from basaltic magma and/or residue during partial melting of older calc-alkaline bodies in the lower crust. 176Hf/177Hf ratios in zircon grains range from 0.282925 to 0.283192 and record εHft between +5.8 and + 15.3, which are indicative of a depleted mantle (juvenile) source and/or recycling (partial melting) of older juvenile crust. TDM incubation ages range to higher than 300 Ma, which may reflect old basement recycling. Partial melting of the depleted mantle ± lower crust may have been facilitated by a combination of hot asthenospheric upwelling during slab rollback, slab tear or lithospheric delamination, and simultaneous thinning of the continental crust under extension. Whole rock chemistry and reported cumulate hornblende in host granites with adakitic-like signatures, infer that hornblende differentiation controlled the magma chemistry and migration to high-SiO2 rocks with ‘adakite’ signatures.

A new species of Arenallianassa (Decapoda: Axiidea: Callianassidae) from hydrothermal vents with notes on its ecology and a redescription of Arenallianassa arenosa (Poore, 1975)

The hitherto monotypic genus Arenallianassa Poore, Dworschak, Robles, Mantelatto and Felder, 2019 is revised following the discovery of a new species, Arenallianassa katrinae sp. nov. from New Zealand and Tonga. The original description of the type species Arenallianassa arenosa (Poore, 1975) is augmented. The new species has been found in localised high abundance in the vicinity of shallow (111–181 m depth) hydrothermal vents at each end of the Kermadec–Tonga Volcanic Arc system. This brings the number of callianassoids in New Zealand to six.

Trace Metal Dynamics in Shallow Hydrothermal Plumes at the Kermadec Arc

Hydrothermal vents are a source of many trace metals to the oceans. Compared to mid-ocean ridges, hydrothermal vent systems at arcs occur in shallower water depth and are much more diverse in fluid composition, resulting in highly variable water column trace metal concentrations. However, only few studies have focused on trace metal dynamics in hydrothermal plumes at volcanic arcs. During R/V Sonne cruise SO253 in 2016/2017, hydrothermal plumes from two hydrothermally active submarine volcanoes along the Kermadec arc in the Southwest Pacific Ocean were sampled: (1) Macauley, a magmatic dominated vent site located in water depths between 300 and 680 m, and (2) Brothers, located between 1,200 and 1,600 m water depth, where hydrothermalism influenced by water rock interactions and magmatically influenced vent sites occur near each other. Surface currents estimated from satellite-altimeter derived currents and direct measurements at the sites using lowered acoustic Doppler current profilers indicate the oceanic regime is dominated by mesoscale eddies. At both volcanoes, results indicated strong plumes of dissolved trace metals, notably Mn, Fe, Co, Ni, Cu, Zn, Cd, La, and Pb, some of which are essential micronutrients. Dissolved metal concentrations commonly decreased with distance from the vents, as to be expected, however, certain element/Fe ratios increased, suggesting a higher solubility of these elements and/or their stronger stabilization (e.g., for Zn compared to Fe). Our data indicate that at the magmatically influenced Macauley and Brothers cone sites, the transport of trace metals is strongly controlled by sulfide nanoparticles, while at the Brothers NW caldera wall site iron oxyhydroxides seem to dominate the trace metal transport over sulfides. Solution stabilization of trace metals by organic complexation appears to compete with particle adsorption processes. As well as extending the generally sparse data set for hydrothermal plumes at volcanic arc systems, our study presents the first data on several dissolved trace metals in the Macauley system, and extends the existing plume dataset of Brothers volcano. Our data further indicate that chemical signatures and processes at arc volcanoes are highly diverse, even on small scales.

Comparison Study of Crustal Structure in Aceh Region based on Volcanic Arc System using Receiver Function Method

Aceh region has a very complex crustal structure from the forearc ridge to the backarc basin. This study aims to determine the velocity model of P and S waves and the depth of Moho discontinuity. This research was conducted using teleseismic earthquake data (30°-90° from the station) with M>6 from four seismic stations belonging to the BMKG in Aceh region. The stations are qualified based on the volcanic arc system zone. Furthermore, the velocity model determined by result of forward modelling, while the depth of the Moho layer estimated by migrated receiver function from time domain to the depth domain. At station SNSI that represented the forearc ridge zone, the depth of Moho is ±28 km, at station TPTI represent the forearc basin is ±16 km, while at zone with higher topography, namely volcanic arc zone represented by station KCSI, the Moho depth was identified at ±38 km, and the backarc basin represented by station LASI with ±40 km depth of Moho. This variation occurs because the composition of the earth’s layers below the station is diverse also different topography for each station.

Geochemical and mineralogical composition of ferromanganese precipitates from the southern Mariana arc: Evaluation, formation, and implications

The Mariana intraoceanic volcanic arc system in the western Pacific Ocean hosts abundant ferromanganese (Fe-Mn) precipitates. A suite (n = 22) of Fe-Mn precipitates were collected from the southern portion of the arc and their mineralogies and chemical compositions were determined. These results were used to decipher their genetic assemblage, assess their potential as a source of trace metals, and place them into context with respect to Fe-Mn precipitates sampled at higher latitudes in the Mariana arc and from other locations, globally. Minerals identified include vernadite, birnessite, 10 Å manganate, manganite, hematite, goethite, maghemite, calcite, rhodochrosite, quartz, phillipsite, various feldspar, pyroxene, and clay minerals. Element discrimination diagrams indicate that the samples are predominantly of hydrothermal and hydrogenetic-hydrothermal (i.e., mixed) origin, with most reflecting some influence of both. Rare earth element and Y (REY) profiles are distinguished by negative Ce and Y anomalies and positive Eu anomalies. Together, the samples form a continuum from the hydrogenetic to hydrothermal endmembers. Samples with the largest hydrogenetic component are friable with branching oxide/oxyhydroxide growth structures, contain mostly vernadite, and have the greatest concentrations of most metals (including the REY elements). Hydrothermal input produces denser, cemented deposits that contain more 10 Å and 7 Å manganate minerals and lower minor-metal contents. Calculated growth rates range from 6 mm to >190 m Ma−1. Average metal contents of Fe-Mn precipitates from the southern Mariana arc are low relative to hydrogenetic Fe-Mn crusts and hydrothermal Fe-Mn deposits from the northern Mariana arc and elsewhere, globally, and are therefore unlikely to be viable exploration targets.

Zircon geochronology and geochemistry of the Ward Hunt pluton, Pearya terrane, Canadian High Arctic: Insights into its age, origin, and circum-Arctic Timanide connections

The northern margin of the Neoproterozoic Timanide Orogen is truncated by Paleozoic deformation of the Caledonian Orogen. Evidence for dispersion of terranes affected by the Timanide Orogen is documented through contemporaneous tectonothermal activity, and by detrital zircon in sedimentary rock from across the Arctic Ocean margins. However, directly tying these terranes to the Caledonide realm is hindered by the paucity of appropriate events in proximal terranes. The Ward Hunt Pluton, a previously undated syenite–monzodiorite intrusion located on Ward Hunt Island, northern Pearya terrane, yields a crystallization age of 542 ± 2 Ma. Trace-element data from the igneous zircon suggest that the pluton intruded older metasedimentary rocks of the terrane as part of a volcanic arc system, indicated by juvenile Hf isotopic signatures and trace-element data. The data support links between the Pearya terrane and other Neoproterozoic–Cambrian arc systems, such as those proposed in Arctic Alaska-Chukota and the Alexander terrane.

Paleobiogeographical inferences of Indian Late Cretaceous vertebrates with special reference to dinosaurs

ABSTRACT The Infra- and Intertrappean deposits have yielded diverse vertebrates, especially dinosaurs, mammals, snakes, turtles, crocodiles, and invertebrates. The biotic assemblages demonstrate a remarkable degree of resemblance between these deposits. The palaeobiogeographical affinities of the paleobiota are more intricate, yielding remains of Gondwanan and Laurasian affinities, and some endemic forms. In order to explain the presence of such a complex biota during the northward drift of the Indian plate (Late Cretaceous period), different palaeobiogeographical models have been proposed. Special emphasis has been given in this paper to the palaeobiogeographical implications of Indian Late Cretaceous dinosaurs. The size of the animal played an essential role in determining the nature of the biotic interchange between India and its nearby landmasses. The faunal exchange between India and Asia through the Kohistan Dras volcanic arc system has been considered as the superlative migratory route, which favoured the small fauna during trans-maritime dispersal. Conversely, it was difficult for small animals to cross huge marine boundaries, other than for the very large vertebrates (especially dinosaurs). Consequently, a straight terrestrial course, especially in the northern India, is a lesser probability, and the dispersal of these huge vertebrates ought to be seen as part of a ‘Pan Gondwanan’ model.

Early central American forearc follows the subduction initiation rule

The “subduction initiation rule” (SIR) (Whattam and Stern, 2011) advocates that proto-arc and forearc complexes preserved in ophiolites and forearcs follow a predictable chemotemporal and/or chemostratigraphic vertical progression. This chemotemporal evolution is defined by a progression from bottom to top, from less to more depleted and slab-metasomatized sources. This progression has been recently documented for other igneous suites associated with subduction initiation. The Sona-Azuero forearc complex of southern Panama represents the earliest magmatic arc activity at the Central American Volcanic Arc system. Comparison of new and existing geochemical data for the circa 82-40 Ma Sona-Azuero Proto-Arc/Arc, its underlying 89-85 Ma “oceanic plateau” of SW Panama and the 72-69 Ma Golfito Proto-Arc of southern Costa Rica with the 70-39 Ma Chagres-Bayano Arc of eastern Panama exhibits a chemotemporal progression as described above and which follows the SIR. Sona-Azuero lavas are predominantly MORB-like, whereas those of the younger Chagres-Bayano complex are mostly VAB-like; lavas of the Golfito Proto-Arc typically show characteristics intermediate to that of the Sona-Azuero and Chagres-Bayano proto-arc/arc complexes. On the basis of isotope evidence as shown in other studies, lava types of all three complexes are clearly derived from a source contaminated by the Caribbean Large Igneous Province plume; we term these “plume-contaminated” forearc basalts and volcanic arc basalts, respectively. Apart from a plume-induced subduction initiation origin for the Panamanian forearc, these insights suggest otherwise similar petrogenetic origins and tectonic setting to lavas comprising earliest-formed forearc crust, and most ophiolites, which follow the SIR.

Evaluating tectonic models for the formation of the North American Cordillera using multivariate statistical analysis of Late Triassic conodont faunas

The tectonic processes that led to the current configuration of the Quesnel and Stikine terranes within the North American Cordillera are contentious, with competing models favouring either northward, strike-slip movement or southward, rotational movement of the Stikine terrane. Multivariate statistical analysis of Late Triassic conodonts faunas from Canada and the USA has been used to evaluate these competing models and to constrain the movement of terranes relative to the North American continent. Late Carnian to early Norian conodonts are present in both the Stikine and Quesnel terranes, as well as at high and low latitudes on the North American continental margin, in northeastern British Columbia and Nevada respectively. The conodont faunas from each of these four geographic areas have been compared using the Dice, Jaccard, Ochiai and Simpson similarity indices. The highest values of similarity were found to be between the faunas of the Stikine and Quesnel terranes, supporting previous conclusions that these belonged to a single volcanic arc system. The conodont faunas of both of these terranes were more similar to that of northeastern British Columbia than either were to that of Nevada, suggesting that during the Late Triassic the terranes were more likely located at higher latitudes than lower latitudes. Finally, the conodont fauna of the Stikine terrane was significantly more similar to that of Nevada than the fauna of the Quesnel terrane was, which indicates that of the two terranes, the Stikine terrane lay farther south than the Quesnel terrane during the Late Triassic. This interpretation of the orientation of these terranes supports tectonic models that favour northward strike-slip translation of the Stikine terrane, rather than those models that require southward rotation of the terrane.

Faunal composition of deep-sea hydrothermal vent fields on the Izu–Bonin–Mariana Arc, northwestern Pacific

Hydrothermal vent fields in the deep-sea are highly productive and host highly specific animals adapted to chemosynthesis-based ecosystems. Knowledge on the distribution range of dominant fauna in these ecosystems help elucidate their biogeography, as well as contribute key baseline data for managing conservation strategies. Here, we report faunal compositions for two hydrothermal vent fields, Myojin-sho Caldera and the Bayonnaise Knoll, on the Izu–Bonin–Mariana (IBM) Arc, a submarine volcanic arc system in the western Pacific, and examined biogeographical subdivisions within. Similarity indices and subsequent analyses based on species presence/absence data revealed three groups, namely those in the Izu area, shallower vents in the Bonin–Mariana area, and deeper vents in the Bonin–Mariana area. This pattern of subdivision indicates that water depth and tectonic discontinuity, in this case the Sofugan Tectonic Line, contribute to the distribution ranges of vent specific animals on the IBM Arc, but the conjunction of Mariana and West Mariana ridges have little contribution. This is the first data for faunal subdivision of vents on the 9000 km-long submarine volcanic chain of the western Pacific extending further south of the IBM Arc, and further analyses on faunal distribution of other vents in this area and larval ecology of vent animals are required to understand subdivisions across the entire chain.

Recognition of Pan-African-aged metamorphism in the Fisher Terrane, central Prince Charles Mountains, East Antarctica

The Precambrian tectonic architecture of the East Antarctic Shield is subject to considerable uncertainty because of limited outcrop. The Fisher Terrane, located in the central region of the Prince Charles Mountains, evolved during the Mesoproterozoic as a volcanic arc system and provides key data about the evolution of this region. Our study provides evidence that the Fisher Terrane formed in proximity to a Proterozoic plate boundary and was subsequently metamorphosed during the late Neoproterozoic to early Cambrian during the final stages of Gondwana amalgamation. U–Pb detrital zircon geochronology reveals that metasedimentary rocks from within a metasedimentary–metavolcanic package cropping out at Fisher Massif were deposited after c. 1300 Ma, and contain detritus derived from the Rayner–Eastern Ghats Terrane. This suggests that the Fisher Terrane was not an isolated oceanic arc but rather formed on the same tectonic plate as the Rayner Complex. Metapelitic schists from within the same metasedimentary package yield metamorphic U–Pb monazite ages of c. 512–509 Ma, corresponding to a regionally recognized Pan-African-aged event. This event has not been previously identified in the Fisher Terrane, and demonstrates that Pan-African-aged metamorphism affected all parts of the Prince Charles Mountains. Calculated phase equilibria modelling constrains the metamorphic conditions during this event to 2.5–4.0 kbar and 550–615°C, corresponding to apparent thermal gradients of 146–220°C kbar −1 . Such conditions plausibly relate to metamorphism taking place in an extensional setting. Supplementary material: Methods and summarized mineral chemistry data, sample locations, U–Pb zircon and monazite data, and representative electron microprobe mineral analyses are available at https://doi.org/10.6084/m9.figshare.c.4392710

Character and tectonic setting of plutonic rocks in the Gällivare area, northern Norrbotten, Sweden

ABSTRACTPetrographical and lithogeochemical investigations in combination with mapping in the Gällivare area, northern Norrbotten, Sweden, have led to the identification of several igneous intrusive rock types. These include: (1) ultramafic-mafic complexes, (2) mafic-intermediate rocks, (3) dolerites and (4) felsic plutons. The ultramafic-mafic rocks include the ca. 1.88 Ga Dundret complex and ca. 1.80 Ga Vassaravaara complex. The Dundret complex has tholeiitic to calc-alkaline affinity, shows a primitive mineral content and was formed in an extensional tectonic setting. The Vassaravaara complex has a similar chemical signature as the Dundret complex. The mafic-intermediate plutons vary in composition from gabbro to diorite. The chemical signature of the dioritic rocks indicate formation in a volcanic arc setting. Dolerites occur as solitary dikes and have calc-alkaline affinity. The felsic plutons include granite and syenite of ca. 1.88, 1.80 and 1.78 Ga age. The felsic plutons have calc-alkaline to shoshonitic affinity and mostly show a metaluminous I-type character. Results indicate subduction at 1.90 Ga resulting in a volcanic arc system, and including extensional events generating back-arc environments leading to mafic, intermediate and felsic magmatism in the Gällivare area. Subduction at 1.80 Ga is suggested to have caused a similar process generating mafic and felsic magmatic rocks in the same area. A subsequent collision event finally generated 1.78 Ga granitic rocks.

Primitive Magmas in the Early Central American Volcanic Arc System Generated by Plume-Induced Subduction Initiation

Primitive lavas with Mg# ≥60 have been identified in the literature from the Sona-Azuero, Golfito and Chagres-Bayano forearc segments of southern Costa Rica and Panama of the early (75-39 Ma) Central American Volcanic Arc system (CAVAS). Primitive CAVAS basalts are remarkably similar in terms of major and many trace elements to primitive MORB and primitive basalts of the Mariana Arc. Significant differences however, are apparent between primitive lavas of Chagres-Bayano relative to those of Sona-Azuero and Golfito. Primitive Chagres-Bayano lavas record much higher degrees of partial melting and higher amounts of shallow and deep subduction additions (e.g., higher Ba/Th, Th/Nb) than those of the older Sona-Azuero and Golfito arc segments which reflects the early petrologic and tectonic evolution of the CAVAS which is similar to other forearc systems. As shown in previous studies, early CAVAS lavas are exceptionally similar in radiogenic isotopic composition to those of the Caribbean Large Igneous Province (CLIP) oceanic plateau at 90 Ma, the approximate timing of onset of the volumetrically dominant phase of CLIP magmatism. This recognition is consistent with subduction initiation models which require plume-subduction interaction. Such models also dictate that an elevated sub-arc thermal anomaly was present at inception of the CAVAS. It is postulated that in addition to providing an anomalously high temperature regime in the sub-arc mantle at inception of the CAVAS, that extrusion of the CLIP also resulted in lithospheric weakening and extension which allowed for production and rapid ascent of a high percentage of primitive lavas.

A compilation of metamorphic pressure–temperature estimates from the Svecofennian province of eastern and central Sweden

Here, we present a compilation of 44 metamorphic pressure–temperature (P–T) estimates from 31 localities in the Svecofennian province of eastern and central Sweden. Based on these P–T estimates, which were obtained using the average P–T method of the computer programme THERMOCALC, we calculated an apparent metamorphic field gradient of 54 ± 4 °C/km for the Svecofennian province. This is typical for low-medium P/T (Buchan) metamorphism and supports tectonic models that imply Svecofennian crustal growth by accretion of volcanic arc systems. In general, estimated P and T conditions range from 0.2 to 0.6 GPa and from 400 to 800 °C, respectively; i.e., from greenschist to granulite facies conditions. Metamorphic grade is generally higher, reaching upper amphibolite or granulite facies in northern and southwestern parts of the Svecofennian province, whereas metamorphism in Bergslagen was at greenschist to lower amphibolite facies conditions. The higher metamorphic temperatures recorded by rocks in the southwestern part of the province might relate to magmatic activity associated with the Transscandinavian Igneous Belt (TIB). Higher pressure, epidote amphibolite facies metamorphic conditions in the western part of the province probably reflect Sveconorwegian overprinting. Finally, local upper amphibolite and granulite facies conditions probably reflect contact metamorphism.

Early 2017 Activity of the Barren Island Volcano:Facts Versus Hype

Barren Island is the northernmost active stratovolcano of the Sunda Volcanic Arc system in the northeastern Indian Ocean. It is the only active volcano in the Indian territory. The arc is a result of subduction of the Indian plate beneath the Burma plate, a sliver of the Eurasian plate. This subduction zone is a southward extension of the Himalayan convergent boundary that takes a turn at the Eastern Syntaxial Bend. Located ~135 km northeast of Port Blair (12°70'N/93°51'E), the Barren Island volcano covers a subaerial area of ~8 km.

Late Cretaceous volcanic arc system in southwest Korea: Distribution, lithology, age, and tectonic implications

In the southwestern part of the Korean Peninsula, Cretaceous volcanic rocks occur along a NE–SW-trending, sinistral strike-slip fault (Hamyeol Fault). Based on their spatial distribution, the volcanic rocks are classified into three lithostratigraphic units: the Buan, the Seonunsan, and the Beopseongpo Volcanics. These volcanics are composed of various types of pyroclastic, sedimentary, and lavas/intrusive rocks. SHRIMP UPb ages of zircon crystals were isolated from each unit. For the Buan Volcanics, Cheonmasan Tuff = 87.23 ± 0.92 Ma; Udongje Tuff = 86.79 ± 0.71 Ma; Seokpo Tuff = 87.30 ± 0.99 Ma; and Yujeongje Tuff = 86.66 ± 0.93 Ma. For the Seonunsan Volcanics, Gyeongsusan Tuff = 84.9 ± 1.1 Ma and Yeongije Tuff = 86.61 ± 0.67 Ma. For the Beopseongpo Volcanics, Seongsan Tuff = 87.73 ± 0.89 Ma and Gyema Rhyolite = 86.2 ± 1.7 Ma. The ages correspond well to recent mapping results of volcanic rocks in the southwest and south parts of the Korean Peninsula. In addition, the age data are comparable to those of the Aioi and Arima volcanic rock groups in southwestern Japan. This suggests that regional volcanic activity in the Japanese Islands and the southwestern part of the Korean Peninsula is related, and results from magmatism together with crustal deformation due to slab rollback of the Izanagi Oceanic Plate underneath the Eurasian Plate.

Links between sedimentary basin development and Pacific Basin plate kinematics recorded in Jurassic to Miocene strata on the western Alaska Peninsula

Late Jurassic to Miocene strata exposed on the western Alaska Peninsula record major changes in plate kinematics and sedimentary basin development along the northern Pacific region. These changes include Late Jurassic to Late Cretaceous accretion of the oceanic Wrangellia composite terrane, subsequent establishment of a Late Cretaceous continental volcanic arc, and middle Eocene Pacific plate reorganization and subduction initiation marked by the Alaska Peninsula–Aleutian volcanic arc. Stratigraphic descriptions and detrital zircon geochronology allow reconstruction of sediment dispersal and basinal response to these plate-scale events. Upper Jurassic strata, for example, contain a dominant population of Late Triassic;Late Jurassic detrital zircons that reflect sediment input from the adjacent oceanic Talkeetna arc located to the north into a marine forearc basin situated on the outboard side of the Wrangellia composite terrane. The dominant population of 180–140 Ma detrital zircons in the Jurassic strata is related to a large-magmatic-flux event that extended throughout the northwestern Cordillera. By Late Cretaceous time, after final suturing of the Wrangellia composite terrane, detrital zircon ages indicate the presence of a new coeval continental volcanic arc system to the north with minor sediment input from older inboard terranes. With final subduction of the Resurrection plate during early Eocene time, the southern margin of the northwestern Cordillera was again reconfigured. Along the western Alaska Peninsula, the Aleutian and Meshik volcanic arcs initiated in response to a shift toward more orthogonal subduction. These arcs are part of a mainly oceanic arc system that initiated in middle-late Eocene time and extend over 3000 km on the northern rim of the Pacific Basin. The basinal response to this event was a shift from nonmarine to marine depositional systems and to a southerly provenance. Middle Eocene through Miocene strata have detrital zircon ages that indicate recycling of the older Mesozoic forearc strata into a developing backarc basin. Dynamic subsidence associated with the change in subduction parameters and flexural subsidence associated with loading from the volcanic arc and backarc basin strata produced the southward-thickening asymmetric sedimentary package observed today. Results from this study provide insight into both the development and the reconfiguration of sedimentary basins associated with changing plate parameters along a convergent margin.

Arc magmatic evolution and the construction of continental crust at the Central American Volcanic Arc system

ABSTRACTWhether or not magmatic arcs evolve compositionally with time and the processes responsible remain controversial. Resolution of this question requires the reconstruction of arc geochemical evolution at the level of a discrete arc system. Here, we address this problem using the well-studied Central American Volcanic Arc System (CAVAS) as an example. Geochemical and isotopic data were compiled for 1031 samples of lavas and intrusive rocks from the ~1100 km-long segment of oceanic CAVAS (Panama, Costa Rica, Nicaragua) built on thickened oceanic crust over its 75 million year lifespan. We used available age constraints to subdivide this data set into six magmatic phases: 75–39 Ma (Phase I or PI); 35–16 Ma (PII); 16–6 Ma (PIII); 6–3 Ma (PIV); 5.9–0.01 Ma (PVa arc alkaline and PVb adakitic); and 2.6–0 Ma (PVI, Quaternary to modern magmatism, predominantly ≪ 1 Ma). To correct for magmatic fractionation, selected major and trace element abundances were linearly regressed to 55 wt.% SiO2. The most striking observation is the overall evolution of the CAVAS to more incompatible element enriched and ultimately continental-like compositions with time, although magmatic evolution took on a more regional character in the youngest rocks, with magmatic rocks of Nicaragua becoming increasingly distinguishable from those of Costa Rica and Panama with time. Models entailing progressive arc magmatic enrichment are generally supported by the CAVAS record. Progressive enrichment of the oceanic CAVAS with time reflects changes in mantle wedge composition and decreased melting due to arc crust thickening, which was kick-started by the involvement of enriched plume mantle in the formation of the CAVAS. Progressive crustal thickening and associated changes in the sub-arc thermal regime resulted in decreasing degrees of partial melting over time, which allowed for progressive enrichment of the CAVAS and ultimately the production of continental-like crust in Panama and Costa Rica by ~16–10 Ma.

Rifting, subduction and collisional records from pluton petrogenesis and geochronology in the Hindu Kush, NW Pakistan

New U-(Th)/Pb geochronology and geochemical analyses of plutonic bodies in the Hindu Kush range, NW Pakistan, provide insight on the crustal growth and tectonic evolution of the southern Eurasian margin. These new data outline a protracted magmatic history that spans the Cambrian to the Neogene (ca. 538 to 23Ma) and record a variety of petrogenetic associations variably influenced by within plate, volcanic arc, and collision tectonic environments. The Kafiristan pluton (538±4 to 487±3Ma) yields geochemical signatures consistent with extensional plutonism and rifting of the Hindu Kush terrane from Gondwana. The Tirich Mir (127±1 to 123±1Ma) and Buni-Zom (110±1 to 104±1Ma) plutons have geochemical signatures that can be attributed to a subduction related continental volcanic arc system that developed along the southern margin of Eurasia in the Mesozoic. The Garam Chasma pluton, the youngest body in the study area (27.3±0.5 to 22.8±0.4Ma), yields a geochemical signature consistent with widespread anatexis during crustal thickening related to the development of the Himalaya. The present geochemical and geochronological analysis from the Hindu Kush have produced important new constraints on the timing of tectonic events and variable tectonic settings along the south Eurasian margin before and after the continued India–Asia collision.

40Ar/39Ar geochronology of subaerial lava flows of Barren Island volcano and the deep crust beneath the Andaman Island Arc, Burma Microplate

Little was known about the nature and origin of the deep crust beneath the Andaman Island Arc in spite of the fact that it formed part of the highly active Indonesian volcanic arc system, one of the important continental crust forming regions in Southeast Asia. This arc, formed as a result of subduction of the Indian Plate beneath the Burma Microplate (a sliver of the Eurasian Plate), contains only one active subaerial magmatic center, Barren Island volcano, whose evolutional timeline had remained uncertain. In this work, we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island, by incremental heating 40Ar/39Ar method, in an attempt to understand the evolutionary histories of the volcano and its basement. Based on concordant plateau and isochron ages, we establish that the oldest subaerial lava flows of the volcano are 1.58 ± 0.04 (2σ) Ma, and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 ± 3 (2σ) Ma. Mineralogy (anorthite + Cr-rich diopside + minor olivine) and isotopic compositions (87Sr/86Sr 7.0) of xenoliths not only indicate their derivation from a lower (oceanic) crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the Andaman accretionary prism. We speculate that the basements of the forearc and volcanic arc of the Andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the Eurasian Plate.