Intra-oceanic Island Arc Research Articles

Formation of high-silica adakites and their relationship with slab break-off: Implications for generating fertile Cu-Au-Mo porphyry systems

In recent years, the characteristics and sources of fertile adakites has received considerable attention. As well, most recently the geodynamic environment of convergent margins subducting oceanic crust aiding arc formation, evolving to slab rollback, then slab break-off after collision (i.e. late- to post-collisional slab failure (arc-like magmatism) and transpression) has gained more recognition, although their relationship to each other has yet to be explored. The geochemical characteristics imply that adakites/adakite-like, in particular high-silica adakites (HSA), can form by partial melting of subducting hydrothermally altered oceanic crust in convergent plate boundary settings during the terminal stages of subduction, lithosphere thickening, and then failure (all late to post collisional), while the melting of the mantle wedge during subduction-related dehydration creates more typical calc-alkaline basalt-andesite-dacite-rhyolite series (ADR) to form intraoceanic island arc to intracontinental margin arc systems, before the collisional stage. HSAs are characterized by high-silica (SiO2 > 67 wt.%), Al2O3 > 15 wt.%, Sr > 300 ppm, Y<20 ppm, Yb < 1.8 ppm, and Nb ≤ 10 ppm, and MgO < 3 wt.%, with high Sr/Y (>50), and La/Yb (>10). Some specific geochemical features, such as high Mg# (ave 0.51), Ni (ave 924 ppm), and Cr (ave 36 ppm), in HSAs are typical, in contrast to calc-alkaline arcs, although both groups display similar but less pronounced negative anomalies of Nb, Ta, and Ti in primitive mantle-normalized trace element spider diagram profiles. These unique geochemical features are likely ascribed to the involvement of garnet, hornblende, and titanite either during partial melting of hydrous MORB-like oceanic crust with only minor assimilation and fractional crystallization (AFC) within the mantle and crustal during ascent in a transpressional collisional environment. Hypotheses for origin of HSA derivative from melting in convergent margins from young, hot oceanic plates subducting into the mantle is applicable to only some adakitic systems. The difference in geochemical characteristics of adakites compared to ADR, such as relative higher MgO, Cr, Cu, and Ni, are due to their slab source, as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle; altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements, as well as sulfates and sulfides. HSA magmas related to slab failure have special geochemical properties, such as Sr/Y > 20, Nb/Y > 0.4, Ta/Yb > 0.3, La/Yb > 10, Gd/Yb > 2, and Sm/Yb > 2.5. Slightly higher Nb + Ta is due to high T melting of rutile. Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure (roll back to break-off). High T-P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H2O, SO2, HCl, with chalcophile metals that remain incompatible at higher fO2 (low fH2); these situations happen in late- to post-collisional settings where the subducting oceanic crust experienced slab failure, resulting in advective heat addition to the system from upwelling asthenosphere. In such a slab failure setting, transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust. When oxidized slab melts interact with the subduction-modified lithospheric mantle, the resulting magmas stay oxidized, potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.

Disputed tectonic setting for the late Silurian–early Carboniferous development of the northern New England Orogen: detrital zircon ages suggest a continental margin arc association

Devonian successions in the Calliope and Yarrol provinces of the northern New England Orogen in central to northern Queensland have a lithological assemblage and depositional environments indicative of a close association with a subduction-related volcanic arc. These rocks have been interpreted as related to island arc and backarc settings based on geochemical characteristics of magmatic rocks. We present new detrital zircon ages from the Devonian to lower Carboniferous successions of the Calliope and Yarrol province successions. Most samples have no or relatively few zircon grains extracted except for samples of quartz-rich sandstone in the Middle Devonian Erebus beds of Hunter Island, located 160 km north-northwest of Rockhampton and another sample in the Rockhampton district. The samples with abundant zircon, and some with less common zircon, have prominent peaks of mid Cambrian to Ordovician ages and are consistent with derivation from the northern Thomson Orogen where igneous rocks of this age are documented in the Charters Towers Province. These data indicate a connection between the Devonian arc-flank deposits of the northern New England Orogen and the Thomson Orogen, which had previously been well established for Carboniferous deep-marine sandstones of the subduction complex (e.g. Shoalwater Formation). These results favour an east-facing continental margin arc setting in the Devonian contra to intra-oceanic island arc settings previously proposed. Detrital zircon age spectra from sampled early Carboniferous units of northern New England Orogen association indicate a pulse of felsic magmatism of that age.

The Shalap Mélange of the Salairian Alambay Ophiolite Zone (Northwestern Central Asian Orogenic Belt), Geological Structure and Compositional Features of Amphibolites and Greenstone Basalts

The Alambay ophiolite zone (AOZ) is located in the axial part of the Early Paleozoic Salair orogen and includes the northern extension of the Alambay-Kaim zone, Salair and Altai Mountains. The Shalap area of the AOZ is predominantly composed of clastic mélange with occasional serpentine mélange. The geological and geochemical studies showed that in the Shalap mélange there are basalt blocks of the Alambay formation whose petrogeochemical features are similar to those of the oceanic island basalts (OIB). Metamorphic rocks of the Angurep complex, represented by garnet and non-garnet amphibolites, form a tectonic slab which is a part of the accretionary complex east of the Shalap mélange area. Metamorphic rocks also form blocks in the Shalap mélange. The amphibolites of the Angurep complex are similar in their petrogeochemical features to the basalts of intraoceanic island arcs. The Shalap mélange is a fragment of the Salairian Cambrian paleosubduction zone. The subduction and exhumation processes in this paleosubduction zone terminated by the 500 Ma time stage.

Geochemical fingerprinting of continental crust trapped in Cadomian volcanic arcs along northern Gondwana

During the Ediacaran to Cambrian periods, the extensive Avalonian–Cadomian accretionary orogen formed along the northern margin of the Gondwana (Pannotia) supercontinent. One of the best-preserved magmatic arcs within this orogenic system is the Davle volcanic complex (DVC) in the Teplá–Barrandian unit of the Bohemian Massif. Using detailed field observations, petrography, major/trace element concentrations, Nd–Hf isotopic compositions as well as U–Pb zircon dating, we interpret a complex evolutionary history of the DVC as comprising three main stages. The first stage (∼610–570 Ma) is represented by a volcano-plutonic association of andesite, dacite, rhyolite and tonalite–trondhjemite–granodiorite (metamorphosed to orthogneiss). A wide range of major/trace element contents together with highly variable εNd values (+3.6 to –5.1) suggest derivation from a juvenile (mantle-derived) source with subsequent assimilation of cratonic crust during magma emplacement, also supported by the modelling of the assimilation–fractional crystallization (AFC) process. The second stage (∼580–560 Ma) is characterized by a gradual termination of the volcanic arc activity, followed by a deposition of marine siliciclastic succession comprising, from the bottom to the top, laminated tuffs, silicified black shales (Lečice Member), and a thick turbidite greywacke–shale–conglomerate sequence (Štěchovice Group). The third stage of poorly constrained age (∼570–500 Ma) is represented by an intra-arc extension and intrusion of juvenile trondhjemite and related rhyolite dykes (εNd +4.8 to +8.4). Finally, we compare these findings with zircon U–Pb ages and whole-rock Nd isotopic data from other Cadomian terranes and propose that the cratonic crust played an important role in recycling along the whole orogenic belt. Thus, we conclude that the Cadomian Orogen formed as a collage of fringing (such as modern Japan, Taiwan, or Philippines) and continental (as modern Andes) arcs of variable evolutionary stages rather than as a belt dominated by intra-oceanic island arcs.

A remnant root of a Neoproterozoic island arc in the Northern Eastern Desert of Egypt: Evidence from the whole-rock and amphibole chemistry of the Gattar gabbro

The tectonic settings and petrogenesis of Neoproterozoic gabbroic rocks are critical to understand the formation and evolution of the Arabian-Nubian accretionary orogen. We present the whole-rock chemistry and amphibole chemistry of the Gattar gabbro in the northern Eastern Desert of Egypt to better understand the formation of this part of the orogen. The Gattar gabbro is composed of variable proportions of amphibole and plagioclase, which imply the hydrous nature of its parent magma. Trace element patterns of the gabbro and the calculated liquids in equilibrium with its amphibole show enrichment in large ion lithophile elements relative to high field strength elements, which indicate that the Gattar magma formed above a subduction zone. The low abundances of high field strength element in the gabbro are consistent with a depleted mantle source similar to intra-oceanic island arc rocks. However, the high Nb/La ratio of the Gattar gabbro (0.46–10.71) is reminiscent to Nb-enriched mafic rocks from subduction settings. The amphibole chemistry suggests that the hydrous magma of the gabbro crystallized at temperature and pressure estimates of 930 °C and 8 kbar and under oxidizing conditions. The Gattar gabbro is affiliated with the Tonian-Cryogenian arc-related mafic gabbro, which is rarely recorded in this northernmost segment of the Arabian-Nubian orogen. In terms of comparison with the Ediacaran post-orogenic gabbros from the northern Eastern Desert, the Gattar gabbro shows lower concentrations of incompatible elements and Ti/V ratio.

Tungsten stable isotope composition of the upper continental crust

Tungsten (W) stable isotope data for twenty-four composites made from the fine-grained matrix of glacial diamictites, deposited between the Mesoarchean and the Paleozoic, as well as two Archean tonalite-trondhjemite-granodiorites (TTG) are used to refine the W isotope compositions (expressed as δ186W deviations from the NIST 3163 W standard) of the upper continental crust (UCC). To do this we evaluate W isotope fractionation in the diamictites as the result of both magmatic and low-temperature processes. The δ186W values of the diamictites are heterogeneous (0.017 ± 0.011 ‰, 2SE to 0.182 ± 0.007 ‰, 2SE), encompassing the range of previously published values for igneous rocks. We find that δ186W correlates positively with the diamictite’s chemical index of alteration (CIA), a measure of the degree of chemical weathering in the provenance, suggesting that the continental regolith (i.e., surface material composed of weathered residues) is isotopically heavy for W. Since W in rivers and ocean water is also isotopically heavy, this suggests that equilibration with Fe-Mn-oxides controls the W isotope composition of surface waters and that such oxides are not important constituents of the regolith, whose W is likely accommodated in clays. Using diamictites with low CIA (<60, n = 9), we calculate an average δ186W of 0.046 ± 0.036 ‰ (2SD) for the UCC, from 2.3 to 0.3 Ga. This average is combined with the TTG data of this study, as well as data from the literature for upper crustal rocks to obtain a representative δ186W value of the UCC of 0.046 ± 0.046 ‰ (2SD). The W stable isotope composition of the UCC is lighter than that of mantle-derived melts (MORB and OIB, with an average δ186W = 0.082 ± 0.026 ‰, 2SD), and much lighter than the weighted average δ186W of intra-oceanic arcs (δ186W = 0.104 ± 0.052 ‰, 2SD), though the spread in the arc rocks is large. The significant difference in δ186W between average intra-oceanic island arc lavas and UCC is intriguing given that continental crust is thought to form primarily via arc magmatism. This difference suggests that not all modern intra-oceanic arcs are representative of those that formed the continental crust. Rather, only arcs that are enriched in incompatible trace elements, which also tend to have lighter W isotope compositions, may have been involved in making new continental crust. Alternatively, there may have been a secular change in the W isotope composition of arc magmas.

The Yubileinoe Porphyry Gold–Copper Deposit (Western Kazakhstan): Geological Position and Conditions of Formation

In the Uralian Fold Belt, there are quite numerous and well-studied porphyry copper (±Mo) deposits corresponding to the traditional “diorite” (most) or “monzonite” (Talitsa, Verkhneuralskoe) models. Along with them, there are relatively small but gold-rich massifs of porphyric granitoids, including the large Yubileinoe porphyry Au–Cu deposit, which is located at the southernmost extremity of the Urals. In this study, two main types of regional hydrothermal–metasomatic alteration were distinguished based on applying quantitative petrography and areal multielement geochemical studies in the scale of the ore district: (1) an earlier synvolcanic secondary alteration of volcanics, similar to those observed in volcanic massive sulfide-bearing fields (albitization, propylitic, and listvenitic alteration) and (2) a later plutonogenic alteration of the porphyry type. The plutonogenic hydrothermal–metasomatic (HM) complex is represented by K-feldspathization, hornfels and skarn alteration at the progressive phase, and propylitization, sericitization, and beresitization at the regressive phase. They are caused by hydrothermal alteration in the apical part of the stock, composed of the mineralizing Frasnian granite porphyry complex that hosts the Yubileinoe gold deposit. A lateral series of geochemical zonality (from the periphery of volcanotectonic structures to their center) has been established for the volcanogenic stage of hydrothermal activity: CrNiCo → PbZnCuCrNi → AuAg (CrNi) → BaAuAg. A large positive anomaly of the lithochalcophilic type was found for the HM plutonogenic complex in the ore field of the Yubileinoe deposit. The concentric zonality of this anomaly is characterized by the development of Ag, W, Sn, Pb, As, and Sb halos at its periphery, and Au, Cu, Bi and Mo at its focus (“core”). The stable and radiogenic isotope geochemical data for most of the porphyry copper deposits of the Urals indicate the predominant mantle source of their rocks and ore matter. Their paleotectonic position is reconstructed as a mature stage of intraoceanic island arcs. Unlike many other porphyry objects in the Urals, the totality of geochemical, isotope–geochemical and geological features of the Yubileinoe deposit indicate the predominantly crustal magma source. According to these features, this deposit is closer to Andean-type porphyry deposits, and its position can be reconstructed as an active margin of the Mugodzhar microcontinent, i.e., a suprasubduction regime, transitional from a mature island arc to the marginal continental one. According to the complex of features, this deposit in the Urals is a close analogue of the porphyry gold deposits of the Maricunga Belt in Chile. The magmatic complexes from the Silurian (Wenlock) to the Devonian (Frasnian) age, which are parental to the porphyry gold–copper systems of the Urals, correspond to the early phase of the Wilson cycle. This phase is the most ore-productive with the formation of giant Cr and Fe–Ti–V deposits associated with ultramafic–mafic complexes. It is likely that the differentiation of mafic magmas in the large-volume chambers occurring in the lower part of the lithosphere causes the appearance (as an extreme member) of diorite melts with a noticeable enrichment in gold and copper.

The Geodynamic Evolution of Intraoceanic Island‒Arc Systems: Expansive (Izu–Bonin‒Mariana), Accretionary (Nemur‒Olutor) and Stationary (Aleutian) Types

The Geodynamic Evolution of Intraoceanic Island‒Arc Systems: Expansive (Izu–Bonin‒Mariana), Accretionary (Nemur‒Olutor) and Stationary (Aleutian) Types

Geodynamic Evolution of Intra-Oceanic Island‒Arc Systems: Expansive (Izu-Bonin‒Marian), Accretionary (Nemuro‒Olutorsky) and Stationary (Aleutian) Types

The authors propose a typification of intra-oceanic island‒arc systems according to the geodynamics of their development in the oceanic space. The currently existing and reconstructed (represented by terranes on the margins of the continents) intraoceanic island-arc systems of the late Mesozoic-Cenozoic are subdivided into expansive, accretionary, and stationary types. Systems of the expansive type (Izu-Bonin–Marian and Lesser Antilles) grow both towards the subducting oceanic plate and towards the free oceanic space – their geodynamics is determined by processes in the oceanic plates. The mantle currents under the overhanging lithospheric plate are directed towards the subducting plate. Accretionary systems such as the Olyutor–East Kamchatka, Nemuro–Lesser Kuril, and Talkitna systems have completed their development as part of active continental margins. The paleotectonic reconstruction of such systems shows that these systems in the course of their development were reduced to relict terranes, tectonically aligned with continental margins. The geodynamics of intra-ocean systems of the accretion type also depends on processes in oceanic plates, but leads to the opposite result compared to expansive systems. This is due to the direction of mantle flows under the overhanging plate, which is opposite to the expansion type, i.e. coinciding in direction with the mantle flow under the absorbed plate. The stationary Aleutian island-arc system is intercontinental and its development in space, as well as the formation of internal structures (the Paleogene island arc of the Bowers Ridge), depended on the difference in the relative movement of the Eurasian and North American lithospheric plates. The most specific feature of this system is the absence of signs of back-arc basin opening, which invariably characterizes expansive and accretionary island-arc systems. It is assumed that this specific feature of the system may be related to the mantle flow under the overhanging slab, which has a transverse direction with respect to the direction of the subducting slab. The Aleutian system, from the moment of its formation, was and remained autochthonous in relation to the North American and Eurasian continents.

Tonian-Cryogenian bimodal volcanism and related sulfide ores in the Arabian-Nubian Shield: An example from the Dendekan prospect in the Eastern Desert of Egypt

The Dendekan prospect is located in the southern segment of the Eastern Desert of Egypt, which constitutes the northwestern part of the Arabian-Nubian Shield. The volcanic rocks of the Dendekan prospect are considered to be the eastern part of the Shadli-Ranga bimodal volcanic belt. These volcanic rocks include massive basaltic flows that are intruded by porphyritic rhyolite. The porphyritic rhyolite yielded a SIMS U-Pb zircon age of 750 ± 10 Ma. Both basalt and rhyolite have a low-K tholeiitic affinity and their mantle-normalized trace element patterns show enrichment of large ion lithophile elements (LILE), U and Th over high field strength elements (HFSE) and pronounced negative Nb anomaly. These geochemical characteristics along with the bimodal nature of the volcanic rocks are consistent with incipient rifting in an intra-oceanic island arc tectonic setting. The onset of rifting promoted hydrous melting of depleted mantle as indicated by the low Nb/Yb ratio of the Dendekan basalt. Geochemistry of the Dendekan rhyolite and its high zircon εHf (T) values (9.5 average) suggest melting of mainly juvenile mafic lower crust. The Hydrothermal alteration in the prospect is represented by patchy and vein-like epidotization as well as silicification of basalt, while the porphyritic rhyolite exhibits extensive sericitization along the wall of old exploited trenches extending close to the contact between the rhyolite and the basalt. Pyrite occurs as disseminated crystals and aggregates in association with minor pyrrhotite and chalcopyrite. Pyrite is commonly altered to ferric oxyhydroxide, locally with native gold inclusions. Sulfides in the Dendekan prospect may represent a pyrite-rich deep part of a volcanic-associated massive sulfide system that was affected by weathering. Compared with the Tonian bimodal volcanic rocks of the Dendekan prospect, the volcanic assemblage from the western part of the Shadli-Ranga bimodal belt exhibits a younger Cryogenian age and a less pronounced subduction geochemical signature suggesting the evolution form Tonian incipient arc rifting to Cryogenian rifting in a back-arc setting.

Geochemistry and petrogenesis of ophiolitic rocks from the Indus Suture Zone (ISZ), Ladakh Himalaya: insights for depleted mantle beneath an intra-oceanic island arc complex

ABSTRACT The Indus Suture Zone preserves the ophiolitic remnants of the eastern part of the Neo-Tethys Ocean. Parts of these ophiolitic remnants are exposed along the Khangral-Chiktan and Dras-Kargil road sections of western Ladakh. The observed rock types include ultramafic-mafic cumulates, gabbros, and volcanics emplaced as faulted blocks over the Mesozoic Dras arc complex. Geochemically, these rocks show sub-alkaline tholeiitic characteristics with basalt to basaltic-andesite compositional variation. Based on modal mineralogy, the ultramafic cumulates classify as olivine-websterite, the mafic cumulates as olivine-norite, and the gabbros as norite to hornblende-gabbro. The ultramafic cumulates depict depleted chondrite normalized REE patterns [(La/Yb)N = 0.6–1.1] while the mafic cumulates display depleted to enriched REE patterns [(La/Yb)N = 0.6–3.2]. Fractionated patterns are observed in gabbros [(La/Yb)N = 1.6–4.1], while flat and enriched chondrite normalized REE-patterns in volcanics [(La/Yb)N = 1.0–12.3] similar to NMORB, and EMORB. The multi-element patterns depict subduction-related geochemical characteristics such as enriched LILE and depleted HFSE compared to the primitive mantle. Presence of Mg-rich olivines, orthopyroxenes, and clinopyroxenes while Ti-poor clinopyroxenes in ultramafic-mafic cumulates reflect their derivation from previously depleted mantle sources at high-pressure and temperature comparable to the base of modern intra-oceanic island arc tholeiitic sequences. The studied rocks exhibit close similarity to an intra-oceanic subduction system contemporaneous to the Dras-Shergol-Suru-Thasgam ophiolitic slices of western Ladakh.

Final Amalgamation Processes of the Southern Altaids: Insights from the Triassic Houhongquan Ophiolitic Mélange in the Beishan Orogen (NW China)

AbstractThe Permian–Triassic tectonic setting is still controversial in the southern Altaids. The Beishan orogen is an ideal region to address the final tectonic of the Altaids. These systematic mapping, geochemistry, and geochronology studies on the Houhongquan ophiolitic mélange in the south Beishan are conducted to address this issue. New mapping reveals that the Houhongquan ophiolitic mélange consists of blocks of gabbro, basalt, chert, granite, and strongly deformed and cleaved sandstone in the southern Beishan. The studies reveal that the mafic fragments are relics of normal-mid-ocean ridge (N-MOR) and suprasubduction zone (SSZ) types of oceanic lithosphere. The four sandstone matrix samples yield the maximum depositional ages of 222±5 Ma, 233.8±2.3 Ma, 263.4±2.5 Ma, and 263.5±2.8 Ma, respectively, indicating that the youngest sandstones were tectonic emplaced in the Houhongquan ophiolitic mélange after ca. 222 Ma. The sandstone matrices display two types of age spectra. Early Permian sandstones have a single Devonian to Early Permian peak age patterns, indicating the existence of an independent Permian intraoceanic arc. In contrast, Late Triassic sandstones have multiple peaks with some Precambrian zircons, suggesting that they were sourced from a continental arc. Accordingly, we consider that the Houhongquan ophiolitic mélange tectonic was emplaced in the intraoceanic island arc during the Middle Permian and docked to a continental margin arc during the Late Triassic. Thus, we argue that the terminal amalgamation timing of the southern Altaids was probably during ca. 222-217 Ma.

Two different types of provenances and the amalgamation of subduction complexes in the Eastern Tianshan of the Southern Altaids

The nature and final closure of the northern Tianshan Ocean have been debated in regard to the eastern Tianshan orogen, southern Altaids. The Kanguer subduction complex of the eastern Tianshan is the key to addressing these issues. In this study, we report new mapping, geochemical and geochronological results on the Kanguer subduction complex in the Haluo area. Our new results show that upper Permian (257 Ma) basaltic blocks emplaced in a sandstone matrix in the northern HL area are fragments of normal-mid-ocean-ridge-basalt (N-MORB)-type oceanic crust. The geochronological results indicate that the sandstone matrices display two different types of provenances. The first type in the northern part of the cross-section (till Sample YY12) has maximum depositional ages ranging from 316 Ma to 238 Ma. Their depositional settings varied from intraoceanic island arcs to continental Andean arcs after ca. 244 Ma, their detrital zircon age patterns vary from a single peak to multiple peaks, and their zircon εHf(t) values vary from uniquely high positive to some negative values. The second type is in the southern part of the cross-section with Samples YY12 to 21Kg05, which have the geochemical signatures of continental island arc sandstone, multiple-peak detrital zircon-age patterns, and positive to negative zircon εHf(t) values. The youngest sandstone sample has a maximum depositional age of 241 Ma. The above provenance results indicate that all mélanges and coherent units north of Sample YY12 belong to an accretionary complex of the Dananhu intraoceanic arc, and those south of Sample YY12 belong to an accretionary complex of the Yamansu-central Tianshan arc. According to the youngest components in both accretionary complexes, which suggest the latest subduction events, we conclude that the final amalgamation timing was after 238 Ma and that the Paleo-Asian Ocean closed during the Middle to Late Triassic.

Geologic setting of the Pueblo Viejo Au–Ag–Cu-(Zn) mining district, Dominican Republic - Links to volcanic domes and volcanogenic massive sulfide mineralization

Geologic mapping at the Pueblo Viejo Au–Ag–Cu-(Zn) mine, Dominican Republic, and across the surrounding Pueblo Viejo district, reveals the geologic setting at the time of mineralization. Ore deposits consisting of disseminated sulfide, bedded massive sulfide, and high-grade massive sulfide veins formed (∼112 Ma) in a volcanic dome field during a period of extension across an emergent, intraoceanic island arc. Geologic maps reveal the links between volcanic domes and structure, hydrothermal alteration and mineralization, and ore deposits and gossans. Whole-rock geochemical data establish a tholeiitic signature for Early Cretaceous magmatism including the volcanic dome field. Northerly-striking, high-angle faults controlled the emplacement of volcanic domes and served as feeders for hydrothermal solutions. The largest ore deposits formed in a subsiding, marginal marine, volcano-sedimentary basin. Stacked intervals of bedded massive sulfide mineralization indicate that the basin was subsiding while mineralization was underway. Calc-alkaline dikes and plutons in the Pueblo Viejo district intruded at ∼88 Ma, well after mineralization had ended. Low-angle reverse faulting, greenschist facies metamorphism, and milky quartz veining and replacement were also Late Cretaceous (post-mineral) events. Ore deposits in the Pueblo Viejo district are best described as hybrid epithermal – VMS deposits, formed during a period of Early Cretaceous extension, tholeiitic volcanism, and volcanic dome emplacement in a shallow, marginal marine environment. Exploration for Au–Ag–Cu-(Zn) deposits like Pueblo Viejo should focus on intraoceanic island arcs, tholeiitic volcanic dome fields, extensional volcano-sedimentary basins, and VMS mineral occurrences.

Petrogenesis of the Córrego das Campinas Gabbro-Anorthosite Suite: Characterization of a Neoproterozoic massif-type anorthosite in the Goiás Magmatic Arc and its significance in the evolution of the Brasília belt, Brazil

Córrego das Campinas Anorthosite is a rare example of a massif-type anorthosite developed in the late Proterozoic (∼650 Ma) during the amalgamation of West Gondwana. This undisturbed, oval, and homogeneous massif body primarily consists of labradorite (An49-63) with a porphyritic and cumulatic inequigranular texture. Other rocks are associated with anorthosite, such as a gabbroic body, consisting of olivine gabbro, leucotroctolite and gabbro with granular, ophitic and cumulatic textures. The olivine gabbro presents a more primitive character, showing minerals enriched in Mg and Ca, such as olivine Fo76-78 intercumulates and anorthitic plagioclase (An90-97), albeit with an Al-rich diopside as well. The anorthosite body is also bordered by tonalite and quartz monzodiorite. These leucocratic and granular rocks contain An26-46 plagioclase in tonalite and An12-18 plagioclase in quartz monzodiorite. Given the signature of these rocks, they can be considered residual liquids from anorthosite crystallization, more enriched in Na and Fe. Metamorphosed ferrodiorites, amphibolites and ilmenite-rich levels occur in the form of dikes and veinlets associated with anorthosite and also seem to represent differentiated liquids from anorthosite crystallization. A quartz syenite body also occurs, consisting of perthitic orthoclase cumulates with an albite mantle, whose genesis is also associated with a process of accumulation in a magma chamber. A albite granite plug occurs intrusively. This albite granite presents enriched in enriched in Na, Si, Rb, Th, Ta, Nb, Ce, Zr, Hf, Y and Yb showing typical signature of an anorogenic granite. Zircon U–Pb dating of olivine gabbro, tonalite, quartz monzodiorite, ferrodiorite and albite granite indicate that these rocks are contemporaneous, with crystallization ages ranging from 648 to 661 Ma, showing older zircon grains indicating Neoproterozoic crustal material contamination (∼870–900 Ma) and even older in the genesis of these rocks. Sm–Nd model ages around 0.75 Ga and positive ϵNd(t) suggest that these rocks are juvenile and derive from depleted mantle, whereas another set of results around 1.0 Ga and less positive ϵNd(t) values indicate that these rocks are juvenile but show crustal contamination. The high percentages of plagioclase found in the rocks, combined with high Al2O3, CaO and Na2O concentrations in total rock, indicate that the rocks of this suite derive from calc-alkaline or Al-rich basaltic magmas. The formation of the suite is in accordance with the classical petrological model of plagioclase accumulation in magma chambers and its subsequent diapiric rise. The age of the suite and its positioning indicate a correlation with an important mafic magmatic event that occurred in the Brasília Belt between 670 and 600 Ma. The interpretation of geological data on the Brasília Belt, associated with the present study, indicates that the rocks of this suite initially developed in a back-arc setting during the pre-tectonic stage of an active continental margin and that the bodies ascended to shallower regions of the crust during the post-collisional stage, through pressure relief, at approximately 650 Ma. The evolution of the Brasília Belt was marked by a constantly elevated geothermal gradient and the Córrego das Campinas massif-type anorthosite supports this inference. Therefore, we suggest that the constantly elevated geothermal gradients in the Brasília Belt were caused by multiple, concurrent geodynamic processes during its evolution, such as i) blocked mantle convection currents triggered by oceanic crust subduction, which formed an intraoceanic island arc (Goiás Magmatic Arc) between 900 and 800 Ma; this physical barrier limited the heat release from these currents, generating a “heat trap” and thus accounting for the elevated geothermal gradients; ii) thickened crust caused by collisional and accretionary events; iii) increased heat of Rodinia break-up plume; and iv) elongated microplate of the Goiás Massif.

The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Growth of the upper crust in intra-oceanic island arcs by intrusion of basaltic magmas: the case of the Koloula Igneous Complex, Guadalcanal, Solomon Islands (SW Pacific)

The Pleistocene (2.2–1.5 Ma) Koloula Igneous Complex (KIC) on Guadalcanal in the Solomon island arc consists of a low-K calc-alkaline sequence of ultramafic to felsic plutonic rocks. We present whole-rock major and trace element and Sr–Nd-Pb isotope data, as well as mineral compositions that record the magmatic evolution of the complex. The intrusive sequence is grouped into two cycles, Cycle 1 and 2, comprising gabbroic or dioritic to granodioritic rocks. The major and trace element data of each cycle forms a single calc-alkaline fractional crystallisation trend. The distinct radiogenic isotope and incompatible element compositions of the Cycle 1 and 2 intrusions imply slightly different mantle sources. The KIC formed by shallow (0.1 GPa) fractional crystallisation of mantle-derived Al-rich basaltic parental magmas (6–8 wt.% MgO) that were formed by deeper-level (0.7 GPa) fractionation of olivine and pyroxene from Mg-rich (~ 11 wt.% MgO) primary magmas in the Solomon intra-oceanic island arc. Olivine, clinopyroxene, plagioclase, amphibole, biotite, apatite, and Fe–Ti oxides fractionated from the KIC’s high-Al basaltic parental magmas to form calc-alkaline magmas. Liquid line of descent trends calculated using mass balance calculations closely match major element trends observed in the KIC data. The KIC crystallised at shallow, upper crustal depths of ~ 2.0–3.0 km in ~ 20 km-thick island arc crust. This complex is typical of other Cenozoic calc-alkaline ultramafic to felsic plutons in Pacific intra-oceanic island arcs in terms of field relationships, petrology, mineral chemistry and whole-rock geochemistry. Hornblende fractionation played a significant role in the formation of the calc-alkaline felsic plutonic rocks in these Cenozoic arc plutons, causing an enrichment of SiO2 and light rare earth elements. These plutons represent the fossil magma systems of arc volcanoes; thus, the upper arc crust is probably generated by migration of magmatic centres.

Petrogenesis of low-pressure intra-oceanic arc granitoids: Insights from the late Neoproterozoic Avalonian–Cadomian orogen, Bohemian Massif

The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc – Davle volcanic complex (DVC) situated in the Teplá–Barrandian unit, Bohemian Massif – that formed during the Neoproterozoic Avalonian–Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite–trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (ε Nd +6.0 to +8.7 and ε Hf +10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhjemites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as “low-pressure intra-oceanic arc granitoids” or “LP IOAG”. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite–trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth. • The DVC trondhjemite has flat REE patterns resembling typical LP IOAG. • The LP IOAG play a key role in the post-Archean crustal growth. • The source composition is established by an inverse geochemical modelling. • The trondhjemite formed by fractional crystallization of a tonalitic source.

Hydrothermal activity and associated subsurface processes at Niuatahi rear-arc volcano, North East Lau Basin, SW Pacific: Implications from trace elements and stable isotope systematics in vent fluids

Hydrothermal activity is abundant in the area between the North Eastern Lau Spreading Center and the Tofua intra-oceanic island arc with multiple active sites in the rear-arc at the Mata and Niuatahi volcanoes. We report geochemical data for high-temperature vent fluids sampled from within the caldera of Niuatahi volcano. Hydrothermal fluids were sampled from three vent sites: South Central, Southwestern Cone and Northern Cone located in water depths between 1607 and 1699 m. Maximum temperatures of 334 °C were measured and pH values were as low as 2.8. The vent fluids were characterized by depletions in Mg, SO4 and U as well as an enrichment of (trace) metals (e.g., Fe, Mn, K, Li) and dissolved gases (e.g., H2S, CO2, H2) relative to seawater. Water-rock ratios calculated based on concentrations (K, Li, Rb, Cs, REE) and isotope ratios (δ7Li, δ11B, 87Sr/86Sr) suggest fluid-rock interactions under rock-dominated conditions at all three vent sites.The South Central vents lie closest to the site of most recent volcanic activity in the Niuatahi caldera. Vent fluids are characterized by relatively low Cl concentrations (as low as 292 mmol/kg) that are indicative of sub-critical phase separation. These fluids also had the lowest pH values (2.8–3.1), highest H2S and lowest H2, CH4 and CO2 concentrations of the three sites. The δD and δ18O values suggest that H2O and CO2 were added in small amounts by subduction-related volcanic vapors. However, there was no evidence for magmatic SO2 input in the vent fluids at the time of sampling in 2018. Vent fluids from the Northern and Southwestern Cone sites on the caldera ring fault had a similar chemical composition, despite being situated at opposite sides of the caldera. Fluids from these sites had lower Fe/Mn ratios (<1) and H2S concentrations than those from South Central suggesting that they were affected by subsurface cooling and sulfide precipitation. This study indicates variations of the Niuatahi hydrothermal vent sites depending on the location within the caldera with variable effects of fluid-rock interaction and magmatic input on fluid compositions in agreement with previous work on fluid S isotopes and sulfides.

Mineralogy, Geochemistry, and Geochronology of the Yehe-Shigna Ophiolitic Massif, Tuva-Mongolian Microcontinent, Southern Siberia: Evidence for a Back-Arc Origin and Geodynamic Implications

The new results have been represented of mineralogical–geochemical and geochronological studies of rocks of the Yehe-Shigna ophiolite massif located in the Tuva-Mongolian microcontinent in the northern part of the Central Asian orogenic belt (Eastern Sayan, Southern Siberia). The Yehe-Shigna ophiolite massif is part of the Belsk-Dugda ophiolite belt. The structural position, age, and geochemical characteristics of the belt indicate its formation in the setting of the back-arc basin of the Shishkhid intraoceanic island arc, developing in the period of 810–750 million years. It is assumed that together with the same-age formations of the Oka accretion wedge and the Sarkhoi active margin, it formed on the convergent margin of the Gondwana supercontinent. Its basement is represented by the Archean-Early Precambrian crystalline rocks and carbonate cover (“Gargan Glyba”). The gold-bearing Neoproterozoic deposits with dominant gold-telluride assemblages are localization in large ophiolites thrust zones along with the frame of the “Gargan Glyba”. They are allochthonous with respect to the Late Neoproterozoic-Cambrian Tuva-Mongolian island arc of the Siberian continent. A similar type of gold deposit is probably worth looking for ophiolites thrust zones in other Precambrian Gondwana-derived microcontinents.