Island Arc Magmatism Research Articles

Along-strike island-arc crustal growth rate estimation: case study of the Izu–Bonin–Mariana subduction system

SUMMARYThe island-arc crustal growth rate (IACGR) is the island-arc magma production volume per 1 km width along the arc strike within one million years, and its variations are highly related to slab dehydration and mantle wedge melting. A novel method that includes Earth density modelling, gravity forward and inversion, and arc crustal growth thickness integration is designed to estimate the IACGR. This method can not only estimate the IACGR along the entire arc length but also assess the crustal growth of both remnant and active arcs. Therefore, the estimation result has high coverage and low uncertainty. Here, the Izu–Bonin–Mariana (IBM) subduction zone is taken as a case study region. The estimated time-averaged IACGR along the IBM arc changes between 16 and 59 km3 km−1 Myr−1, with a mean value of 40 km3 km−1 Myr−1, and this result matches the findings of previous studies well. The uncertainties due to crustal thickness inversion are relatively larger than those from flow line reconstruction. The rate results of the Mariana part have lower uncertainties than those of the Izu–Bonin parts since the arc boundaries can be delineated more accurately. The IACGR of the region where a plateau approaches the trench tends to be overestimated because the collision of the plateau with the island arc thickens the island arc crust and bias the uniform pre-existing crustal thickness assumption.

Temporal Evolution of Island Arc Magmatism and Its Influence on Long-Term Climate: Insights From the Izu Intra-Oceanic Arc

Continental arcs have an episodic magmatic activity over long-time periods, which is believed to modulate long-term climate. Island arcs have also the potential to release large amount of CO2 into the atmosphere, but whether they display an episodic magmatic history throughout their lifespan that contributes to the long-term (>10 Ma) climate changes remains an open question. To set additional constraints on the magmatic history of island arcs, here we examine fresh basalts and mineral-hosted melt inclusions from the Izu intra-oceanic arc, shortly after the eruption of boninites (∼45 Ma ago). Using chemical markers, we show that the long-term magmatic activity of the mature Izu arc has been relatively continuous over its lifespan, except during opening of the Shikoku back-arc Basin (∼23–20 Ma). Because slab dehydration and slab melting trigger decarbonation and carbonate dissolution of the subducted plate, we use slab-fluid markers (Ba/Th, Cs/Th, Cs/Ba, Rb/Th, Th/Nb) to examine the variations of slab-derived CO2 captured by the arc magmas. The long-term steadiness in the arc magmatic activity and in the slab-fluid contribution suggests that the CO2 outgassed during mature arc volcanism may have remained relatively homogeneous for the past 40 Ma in Izu. If worldwide mature island arcs also maintain a relatively steady-state magmatic activity over their lifespan, the long-term CO2 outgassed by these arc volcanoes may be rapidly balanced by chemical weathering and tectonic erosion, which rapidly draw down the atmospheric CO2 (within 200–300 kyr). This rapid negative feedback to long-term volcanic degassing permits to sustain a viable atmospheric CO2 for millions of years. The lack of co-variations between the markers of climate changes (δ13O, δ18C) and the long-term averages of the markers of slab fluids further implies that long-term volcanic degassing of CO2 from mature island arcs might play a minor role in the slide into icehouse climatic conditions. This long-term degassing stability may be, instead, a contributor to maintaining a broadly stable climate over long timescales.

Thermal simulation of migration mechanism of the Halmahera volcanic arc, Indonesia

• A thermal evolution model is proposed to explain the activity of the Halmahera volcanic arc. • Our results can explain why there are magmatic gaps and volcanic arc migration during subduction and rollback. • The thermal simulation results show that a smaller subduction rate or a larger convergence rate are favorable to the occurrence of island arc magmatism. The Halmahera volcanic arc has experienced intermittent volcanic activity and location migration since the Pliocene. Two factors affect the volcanic arc magmatism. One is the dehydration depth of hornblende in the oceanic crust and serpentinite in the lithospheric mantle in the subducted slab. The other is the subducted slab and mantle wedge temperature structure, controlled by the subduction rate and plate convergence rate. Thermal simulation results show that conditions are favorable to the occurrence of island arc magmatism at a lower subduction rate with the same convergence rate, or at a higher convergence rate with the same subduction rate. The Miocene–Pliocene favored the occurrence of island arc magmatism in the high temperature region, because there was a lower subduction rate beneath the Halmahera volcanic arc. After the Middle Pliocene, there was cessation of volcanic activity and a volcanic gap, with subsidence of the mantle wedge high temperature region, increase of the partial melting depth, and decrease of the melting amount, because there was an increased subduction rate beneath the Halmahera volcanic arc and a decreased westward convergence rate of the eastern microplate fragments. In the Holocene, the Halmahera arc moved to a new location and resumed volcanic activity. It is because continuous arc–arc collision blocked the westward movement of the Halmahera volcanic arc, the relative convergence rate of the eastern microplate increased, which resulted in the formation of a high temperature region favorable to island arc magmatism.

Geochronology and geochemistry of 2.3 Ga mafic intrusions in the Dengfeng area: Evidence for early Paleoproterozoic subduction in the southern North China Craton

The time of 2.45–2.2 Ga in the early Paleoproterozoic era is called as ‘Quiet Interval’ because of the decreased magmatic activity worldwide. However, tectonic evolution of old continents during this time interval is still in dispute due to the rare geological records, especially for magmatism. In the southern North China Craton (NCC), the Jingwan mafic intrusions intruded into the Songshan Group and its zircon U-Pb dating yielded a weighted mean 207Pb/206Pb age of 2301 ± 16 Ma, and thus they are the only ca. 2.3 Ga mafic magmatic records founded in this region. These diabases belong to low-K tholeiitic series with relatively low K2O (0.1–0.7 wt%) and total alkalis (Na2O + K2O) (2.4–4.7 wt%). They have low SiO2 (44.8–54.1 wt%), slightly variable Fe2O3T (10.1–14.4 wt%), Al2O3 (14.8–17.0 wt%), CaO (5.5–10.6 wt%), and medium to high MgO (6.1–10.8 wt%) and Mg# values (58–65). The rocks display LREE enrichment ((La/Yb)N = 3.02–3.58) without obvious Eu anomaly (Eu/Eu* = 0.87–1.18). They are characterized by obvious depletion in Nb, Zr and Ti, and enrichment of Sr, Ba and Pb, indicative of geochemical features of island arc magmas. The rocks have experienced crystallization from clinopyroxene to orthopyroxene, then plagioclase, suggesting the hydrous fractionation order. They also exhibit variable εNd(t) values ranging from − 0.2 to + 0.8 and zircon εHf(t) values from − 3.0 to + 6.6. These features suggest that the mafic intrusions are formed by partial melting of a garnet-spinel lherzolite and derived from an enriched mantle source metasomatized by slab-derived fluids at a shallow depth. Combined with the previous reported magmatic records, the ca. 2.3 Ga magmatic activity may be a result of the oceanic slab subduction beneath the Eastern Block of the NCC, and the subduction probably continue throughout the early Paleoproterozoic from ca. 2.5 Ga to ca. 2.3 Ga, at least 200 Ma. Therefore, the prevalence of early Paleoproterozoic magmatic activities in the southern NCC indicate that the tectonic activity did not shutdown globally during the interval of the ‘tectono-magmatic lull’.

Subduction initiation of the western Paleo-Asian Ocean linked to global tectonic reorganization: Insights from Cambrian island-arc magmatism within the West Junggar, NW China

Abstract The subduction initiation associated with the beginning of accretionary orogens has been thought to be related to global plate reorganization. To characterize the initial subduction within the western Central Asian Orogenic Belt, this integrated study focuses on Cambrian tholeiitic to calc-alkaline plutons in the Barleik-Mayile-Saleinuohai area of West Junggar, NW China. Zircon U-Pb results of felsic plutons reveal a wide range (511–488 Ma) of ages with older ages up to 514–511 Ma. The felsic rocks exhibit variable SiO2 (53.0–77.4 wt%) and K2O (0.05– 2.24 wt%) contents and can be classified as diorite, granodiorite, trondhjemite, and tonalite. On the basis of their low TiO2 (0.12– 0.71 wt%) contents and characteristic trace element trends as well as high zircon εHf(t) (+10.5 to +14.5) and mantle-like zircon δ18O (5.0 ± 0.48‰ to 5.4 ± 0.43‰, two standard deviations) values, we interpret that the Cambrian felsic rocks have diverse origins, involving differentiation of arc basalts and partial melting of subducted oceanic crust, arc mafic crust, and metasomatized mantle wedge. The Saleinuohai gabbroic pluton shows zircon δ18O ratios from 4.2 to 4.7‰, which are lower than those of igneous zircons in equilibrium with mantle and thus reflect modification of their mantle source by hydrothermal fluids with seawater-like oxygen isotopes at high temperature. Combined with regional data, we propose that the West Junggar arc represents the extending of the Boshchekul-Chingiz arc in the Early Cambrian, defining a long (>1000 km) E-W–trending subduction zone. The earliest island-arc tholeiitic felsic plutons in the West Junggar took place at ca. 514–511 Ma, which, coupled with other early subduction records (e.g., 530 Ma SSZ-type Kopu-relisay ophiolites) in the western Paleo-Asian Ocean, indicates that initial stages of subduction of the western Paleo-Asian Ocean probably occurred in the Early Cambrian. The simultaneity between the initial subduction of the western Paleo-Asian Ocean, Gondwana assembly, and Laurasia breakup suggests a causal link between the three, collectively correlated to a global plate adjustment event.

Geochemical, zircon U–Pb–Hf–O isotopic evidence and molybdenite Re–Os dating from the Cuojiaoma batholith, eastern Tibetan Plateau: Implications for molybdenum potential and tectonic evolution

Subduction of oceanic plates beneath continental lithosphere is critical for understanding tectonic evolution and evaluation of prospecting and exploration. Within the Yidun Terrane (YDT) of southwestern China, a number of Mesozoic to Cenozoic granitoid intrusions are exposed and they are useful for investigating the tectonic evolution of the Paleo-Tethys system. However, Mesozoic magmatism of the northern portion of the YDT, remains ambiguous regarding to their magmatic spatial–temporal evolution and their mineralization potential. As the largest pluton in the northern YDT, the Cuojiaoma batholith mainly consists of monzogranite and granodiorite. In this study, we present new zircon U–Pb and molybdenite Re–Os ages, whole-rock geochemical, and zircon Hf–O isotopic data for the Cuojiaoma batholith. LA-ICP-MS zircon U-Pb dating of granodiorite and monzogranite exhibit ages of 221.8 ± 1.4 Ma (n = 22, MSWD = 2.4) and 216.7 ± 2.2 Ma (n = 14, MSWD = 0.03), respectively. A total of 9 molybdenite samples differing Re-Os model ages of 205.4 ± 3.3 and 220.6 ± 5.5 Ma, yield a robust weighted mean model age of 209.9 ± 1.8 Ma (MSWD = 2.4, n = 9) representing the depositional age of molybdenite. The monzogranite and granodiorite’s mineralogical and geochemical characteristics indicate they are classified as (medium-) high-K calc-alkaline and metaluminous to weakly peraluminous I-type granite. Geochemically, they are enriched in large-ion lithophile elements (LILEs, e.g., Rb, U, K) and light rare earth elements (LREEs), and depleted in high-field-strength elements (HFSEs, e.g., Nb, Ta, Ti and P) and heavy rare earth elements (HREEs), and contain distinctly or slightly negative Eu anomalies and no significant Ce anomalies, indicating an affinity to classical island arc magma. Combined with their negative zircon εHf(t) values (−16.24 to −2.49 and −16.41 to −1.43) and two-stage Hf model ages (2019–1255 Ma and 2027–1200 Ma), plus their zircon δ18O values, which range from 5.96 to 8.01 and from 5.05 to 7.61 for monzogranite and granodiorite, respectively, these geochemical indexes indicate that the Cuojiaoma batholith shares similar petrogenesis to other intrusions within the YDT. The formation of the early granodiorite may be genetically related to the slab subduction and is most likely formed by mixture of lower crustal melts and mafic magma derived from partial melting of mantle wedge induced by the influx of slab derived melt (fluid). Subsequent slab break-off and the upwelling asthenosphere at ∼ 216 Ma to 210 Ma led to high heat flow and extensive melting of the overlying mantle wedge, followed by the highly crystallization differentiation, which finally contributed to the monzogranite and the subsequential disseminated molybdenite in a post-subduction extension setting. The northern YDT possesses high Mo metallogenic prospectivity, especially for the magmatic activity that dominantly by lower crustal melt and genetically related to the slab break-off occurred at ∼ 216 Ma and represented by the highly fractionated granite derived from the large Cuojiaoma granitic batholith.

The initial stage of Early Carboniferous rifting in the Southern Urals: fi rst results of zircon U-Pb dating from granitoids of Neplyuevka complex

Zircons from the Early Carboniferous granitoids of the Neplyuevkа massif of the South Urals have been dated (SHRIMP). Two different age groups are distinguished among the studied zircons: 1 — from 334 to 342 Ma; 2 — from 354 to 356 Ma. The older ages probably correspond to the time of zircon crystallization from the melt, while the younger ages resulted from a partial loss of radiogenic lead by zircon during the cooling of the massif or from thermal impact of Early Carboniferous or even Early Permian intrusions. The formation of the Neplyuevkа complex at the very beginning of the Carboniferous marks the most important stage in the geodynamic evolution of the Southern Urals — a rapid transition from island arc magmatism, which continued during Devonian, to riftogenic magmatism, which ended only in middle Visean.

Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.

Discovery of late Early Cretaceous diorite porphyrite from the Shamuluo Formation in the Gaize area, Tibet: Response to the northward subduction plate rollback event of Bangongco-Nujiang Tethys Ocean

改则地区闪长玢岩位于藏西北班公湖-怒江成矿带西段北缘，呈岩脉状侵位在成岩于浅海环境的沙木罗组地层中，是班公湖-怒江特提斯洋向北俯冲造山消减过程的产物，而其岩石成因、地质意义和深部动力学过程等尚欠缺详细的研究。本文研究了发育于改则地区的闪长玢岩，对其开展了岩相学、锆石LA-ICP-MS U-Pb年代学以及元素和同位素地球化学的研究，对其岩石成因、成岩构造地质背景、深部地球动力学过程等进行了分析和讨论，并对其含矿性进行了评价，为该地区下一步找矿方向提供了初步的设想。锆石U-Pb同位素年代测试显示闪长玢岩具有较多锆石结晶年龄段，相较于研究区出露的沙木罗组碎屑锆石年代学特征，其最新年龄段（117~126Ma）加权平均年龄为120.4Ma，可代表其结晶年龄。元素地球化学结果显示闪长玢岩具有弧岩浆的典型特征，属于高钾钙碱性和准铝质系列岩石，结合锆石微量元素等指标综合判断其属于I型花岗岩类岩石。本文综合全岩Sr-Nd-Pb同位素组成特征等分析认为，闪长玢岩形成于新生玄武质下地壳的部分熔融，岩浆源区残留相以角闪岩相为主，源区物质有少量沉积物组份的参与，侵位过程中分离结晶和地壳混染作用不明显。通过多种方法判别并结合区域构造背景，我们认为闪长玢岩成岩于班公湖-怒江特提斯洋北向俯冲板片折返的地球动力学背景。由于地处俯冲板片后端根部，改则地区闪长玢岩俯冲角度小于成岩于俯冲板片前端同时代的多龙矿集区中酸性岩浆岩，亦小于成岩于相同板片构造位置的103.5Ma的革吉闪长玢岩，暗示120.4~103.5Ma时班公湖-怒江特提斯洋北向俯冲板片仍处于持续折返阶段。由于其俯冲角度较小以及岩浆源区低氧逸度等因素导致其无法成矿。因此建议在多龙矿集区和改则县中间位置寻找可能的与俯冲背景有关的成矿靶区，但是否见矿还应考虑构造、围岩、后期保存等条件的影响。;The diorite porphyrite in Gaize area, located in the northern margin of the western section of the Bangongco-Nujiang metallogenic belt, emplaced into the Shamuluo Formation formed from shallow sea environment. It was the product of the northward subduction of Bangongco-Nujiang Tethys Ocean (BNTO). Taking diorite porphyrite in Gaize area as the research object, this paper studied petrography, zircon U-Pb chronology, elements, and isotope geochemistry, analyzed its petrogenesis and source characteristics, discussed its diagenetic tectonic background, described its deep dynamic process in detail, explained the possible reasons for its non-mineralization, and provided a preliminary idea for the next prospecting direction in this area. Zircon U-Pb isotopic dating shows that diorite porphyrite has many stages of age. Compared with the detrital zircon chronological characteristics of Shamulo Formation in the study area, the weighted average age of the latest age group (117~126Ma) is 120.4Ma, which can represent its crystallizing age. The element geochemical results show that diorite porphyrite is a typical island arc magma with high potassium calc-alkaline characteristics, belonging to quasi-aluminous series. Combined with the characteristics of zircon trace elements, the diorite porphyrite has the characteristics of I-type granitoids. Based on the analysis of the Sr-Nd-Pb isotopic composition of the whole rock, it is considered that the diorite porphyrite was formed in the partial melting of the juvenile basaltic lower crust, the residual facies in the magmatic source are mainly amphibolite facies, with the addition of a small amount of sediment components, and the separation, crystallization and crustal contamination in the emplacement process is not obvious. Through a variety of methods and combined with the regional tectonic background, we present that the diorite porphyrite was formed in the geodynamic background of rollback within the process of northward subduction of the BNTO. Because it is located at the root of the back end of the subducted plate, the subduction angle of the diorite porphyrite in Gaize area is smaller than that of the intermediate acid magmatic rock in the Duolong ore district, which formed from the front of the subducted plate at same time, and is also smaller than that of Geji diorite porphyrite, which is located at the same tectonic location of the subducted plate and formed at 103.5Ma. It is suggested that the northward subduction plate of the BNTO is still in the stage of continuous rollback during 120.4~103.5Ma. It cannot be mineralized because of its small subduction angle and low oxygen fugacity in the magmatic source area. Therefore, it is suggested to look for possible ore-forming targets related to the subduction background between the Duolong ore concentration area and Gaize County, but the influence of structure, surrounding rocks and late preservation should also be considered.

Mobilization and fractionation of Ti-Nb-Ta during exhumation of deeply subducted continental crust

The behavior of Ti-Nb-Ta is crucial to reveal the genesis of island arc magmatism. However, mobilization and fractionation of Ti-Nb-Ta in subduction zone settings remain poorly understood. The discovery of felsic veins rich in coarse-grained rutile within retrograde eclogite of the North Qaidam UHP metamorphic belt provides a unique and novel opportunity to study age variation during rutile formation and alteration, as well as Ti-Nb-Ta mobility and fractionation during fluid/melt-rock interaction. Rutile high-resolution elemental mapping, and U-Pb bulk grain (ID-TIMS), and in-situ U-Pb geochronology have been utilized to focus on the properties of rutile in both, felsic vein and retrograde eclogite host to gain insight into possible similarities and differences. Three groups of rutile were distinguished according to its host rock, trace elements signature, and genetical connection to ilmenite: eclogite-hosted rutile (Rt-1), felsic vein-hosted rutile not associated with ilmenite (Rt-2a), and associated with ilmenite (Rt-2b). Field evidence and rutile trace elements characteristics document the source of vein-hosted rutile to be mainly derived from the eclogite during fluid/melt-rock interaction. Principal Component Analysis reveals that Nb, Ta, Sn, and W are more enriched in Rt-2a compared to Rt-1; Rt-2b has higher Nb, U, and Hf than Rt-2a. High-resolution mapping across large rutile grains shows the enrichment of high field strength elements (HFSEs) in rutile near to ilmenite, which indicates a HFSEs back diffusion from the rutile–ilmenite boundary during the replacement of rutile by ilmenite. The Nb/Ta ratios of Rt-2a are lower than those of Rt-1, which result from different partition coefficients of Nb and Ta during fluid/melt-rock interaction. The diffusion-influenced rutile exhibits suprachondritic Nb/Ta ratios and demonstrates that diffusion of Nb in rutile is higher than that of Ta under identical P-T conditions. Rutiles Rt-1 and Rt-2a yield consistent 206Pb/238U ages of 426–423 Ma, which is similar to the 433 ± 3 Ma determined by ID-TIMS results of bulk rutile grains. This indicates that Ti-Nb-Ta must have been mobilized during the exhumation of deeply subducted continental crust. However, the diffusion-influenced rutile shows a large variation of ages compared to the rutile not associated with ilmenite, demonstrating that the back-diffusion may affect the U-Pb system in rutile. Therefore, when rutile is partially altered into ilmenite or titanite, its dating should be used with caution. Thus, this study demonstrates volume diffusion is a very important geological process to result in extreme HFSEs fractionation and age variation of rutile on the mineral scale. The rutile aggerates that occur in the felsic veins in 3–5 m distance to the adjacent retrograde eclogite suggest that Ti-Nb-Ta-rich melts/fluids were transported over a distance of at least several meters and that rutile does not represent a residual phase of the Na-Si-Al-, F- and CH4-bearing fluid/melt environment that formed during anatexis of the subducted continental crust. The formation of rutile-rich aggregates during the generation, transport, and crystallization of subducted continental crust-derived melts/fluids in the deep roots of orogenic belts may be a critical trigger for the depletion of HFSEs in arc magmatic rocks during the formation of the continental crust.

The role of continental fragments in the formation of intra-oceanic arcs: Constraints from Sr-Nd-Hf-O isotopes of gabbro from the Jiamusi Block, NE China

Intra-oceanic arcs are potential candidates that generate Earth’s continental crust, and serve as active sites for the mixing of juvenile arc and mature continental material. However, whether continental fragments occur beneath intra-oceanic arcs, and how continental materials are involved in island arc magmas remain equivocal. Here we report zircon U-Pb-Hf-O isotopes and whole-rock elemental and Sr-Nd isotopic data on the Silurian island arc gabbro from the southeastern Jiamusi Block in NE China. The gabbro was emplaced at ca. 435–429 Ma associated with the northward intra-oceanic subduction of the Paleo-Asian Ocean, and belongs to the eastern extension of the Bainaimiao arc system. Elevated zircon δ18O (+5.5‰ to +7.9‰) and variable εHf(t) (-2.1 to +4.0) values, and negative whole rock εNd(t) (-1.1 to −1.6) values, suggest an enriched mantle component generated by pelagic and terrigenous sediments input into the mantle wedge. The contribution of terrigenous sediments, together with the recognition of Paleoproterozoic crustal rocks, proves for the first time that continental fragments existed beneath the intra-oceanic arc. In conjunction with published data, we propose that that Tarim Craton-type scattered continental fragments occurred within the Bainaimiao arc system, and that they not only facilitated the initiation of oceanic-oceanic subduction through increasing buoyancy, but also account for the old continental zircon and enriched isotopic variations of the young arc rocks. Our study highlights a fossil example that could help better quantify the scale and role of continental fragments in the origin and evolution of intra-oceanic arcs.

Arc building through bimodal magmatism: The Tsukuba Igneous Complex, Japan, and its correlations and connections

ABSTRACT Subduction zones are the gateways of large-scale material and energy exchange. The SW Japan region is one of the important regions to understand ongoing convergent margin processes with continuous island arc magmatism. Here we investigate the bimodal magmatic suites from the Tsukuba area in Japan based on petrology, geochemistry and zircon U-Pb-Hf isotopes. Magmatic rocks in the Tsukuba area comprise two compositionally different groups: 1) a mafic group comprising Hbl-gabbros of ~66 Ma and 2) a felsic group represented by two pulses of granitoids at ~66 Ma and ~78 Ma, respectively. All these rocks display calc-alkaline affinity and negative Nb, and Ta anomalies, with a bimodal classification represented by a significant silica gap between the mafic and felsic members. The Hbl-gabbro witnessed plagioclase and amphibole fractionation as evidenced by the humped REE pattern and positive Eu and Sr anomalies. Zircon grains from the Hbl-gabbros show ƐHf (t) values in the range of −7.7 to −4.0 with a peak at −6.4, indicating an enriched mantle source modified by slab-derived fluids. The petrological and geochemical characteristics of Hbl-gabbros are similar to those in the magmatic suite of SW Japan, suggesting that Cretaceous arc magmatism in Tsukuba is an extension of the coeval magmatic belt in SW Japan. Two pulses of granitoids identified in this study are similar in composition and are both classified as I-type granite. The relative uniform high SiO2 (70–75%) and large variation of Hf isotope (−21.0 to −4.0) indicates the granitoids formed by lower crustal melting. The Hf model ages (2479–1392 Ma) of granitoids are significantly older than those of coeval granitoids in SW Japan which show Nd model ages at 1435–748 Ma. This indicates that the arc magmatism in the Tsukuba area involved older lower crust linked to a possible micro-continent derived from the Southern Korean Peninsula.

Zircon as a tracer of plumbing processes in an active magmatic system: insights from mingled magmas of the 2010 dome collapse, Montserrat, Lesser Antilles Arc, Caribbean

Soufrière Hills Volcano, Montserrat, erupted from 1995 to 2010, with activity including dome growth, destructive pyroclastic density currents and Vulcanian explosions. Monitoring data, such as gas emissions, show the system is still in a state of unrest. The recent eruptions provide an opportunity to study, in real time, a complex subduction-related subvolcanic transcrustal melt-mush reservoir, its magma fluxes, and the timing of crystal and melt storage prior to eruptive paroxysms. How and when mush destabilisation occurs prior to volcanic eruptions continues to be a question of intense debate. Evidence of mafic magma intrusion, a potential eruptive trigger, is preserved in enclaves with quenched and diffuse margins that are mingled with crystal-rich andesite. Here, in this first study of Soufrière Hills Volcano zircon, we report zircon ages and compositions for mafic-intermediate enclaves and host andesites from the most recent dome collapse in 2010 to place temporal constraints on magma reservoir processes. Zircon 238U-230Th disequilibrium crystallisation ages ranging between c. 2–250 ka constrain the longevity of the magmatic plumbing system. Uniform Hf isotopes, εHf 11.3 ± 1.2 to 14.6 ± 1.5, indicate invariant compositions that are typical for island arc magma sources. Zircon trace element concentrations and Ti-in-zircon crystallisation temperatures indicate crystallisation in isolated, small-volume, lenses with variable fractions of melt of heterogeneous compositions. We suggest amalgamation of assorted crystal cargoes from these lenses occurred prior to eruption during mush destabilisation triggered by mafic magma recharge. Zircon textures, on the other hand, shed light on recent centimetre-scale magma mingling immediately prior to eruption. Euhedral-subhedral zircon is preferentially preserved in or near quenched contacts of the least-evolved enclave and host andesite. By contrast, reheating of the andesite by the mafic magma recharge in the presence of zircon-undersaturated melts promoted zircon resorption. This led to the formation of subhedral-anhedral corroded zircon that is typical in the host andesite mush. Zircon thus reveals processes ranging from 100,000s of years of andesite storage to short-term partial destruction in response to transient heating and magma mixing events.

The first intra-oceanic island-arc porphyry Cu (Au) deposit in the western Bangong-Nujiang suture zone: Evidence from the Saidengnan Cu (Au) deposit

The Saidengnan deposit, located in the Bangong-Nujiang suture zone, is proposed to be the first intra-oceanic island-arc porphyry Cu (Au) deposit in the Bangong-Nujiang metallogenic belt. The Cu (Au) mineralization is hosted in the quartz diorite porphyry as fine grained disseminated form. Zircon U-Pb ages of the ore-forming quartz diorite porphyry and the post-mineralization granodiorite are 118.1–118.8 Ma and 116.2 Ma, respectively, suggesting that mineralization occurred in the Early Cretaceous. The S isotopes (δ34S = −3.8 to −1.5) of ore sulfides and H-O isotopes (δD = −99‰ to −89.9‰, δ18OH2O = 4.9–9.4‰) of ore-bearing quartz veins indicate that the ore-forming fluids was sourced from the ore-forming intrusion. All intrusive rocks are enriched in light REE and LILE (Rb, K) and depleted in HFSE (Ni, Ta, Ti), similar with the island-arc magmas. Whole rock (87Sr/86Sr)i values (0.7040–0.7056), εNd(t) values (−0.35 to 1.94), and significantly positive zircon εHf(t) values (7.37–11.38) and young TDM2 of zircon Hf (455–705 Ma) indicate that the Saidengnan intrusive rocks are dominantly derived from mantle wedge metasomatized by subducted fluids and sediments. This is different from the Duolong continental arc magmas formed by crust-mantle mixing. Combined with stratigraphic data published, we propose that the Saidengnan porphyry Cu (Au) deposit should be formed in the intra-oceanic island-arc setting caused by the oceanic-oceanic subduction of the Bangong-Nujiang Ocean.

Zircon U-Pb and Lu-Hf isotopes and geochemistry of granitoids in central Tibet: Bringing the missing Early Jurassic subduction events to light

Jurassic magmatic and metamorphic rocks in central Tibet are considered to record the subduction history of the Meso-Tethys Ocean (MTO) and have therefore attracted considerable research interest. However, Early–Middle Jurassic island arc magmatism has been reported only from the Amdo and Basu areas in the eastern section of central Tibet. In this paper, we report newly discovered Early–Middle Jurassic granitoids from Lhaguotso in the middle section of central Tibet and from Shiquanhe in the western section of central Tibet. Zircon U–Pb dating and whole-rock geochemical and zircon Lu–Hf isotope analyses were conducted on three groups of samples: plagiogranites from Lhaguotso (LG, 185–181 Ma); granodiorites from Shiquanhe (SGD, 184–183 Ma); and amphibole tonalites from Shiquanhe (SAT, 170 Ma). The three groups of samples are differentially enriched in Mg, Fe, and Ca. In addition, the samples show high SiO2 contents and Mg# values, enrichment in large-ion lithophile elements, and depletion in high-field-strength elements. Combining these characteristics with their differing Th, Ba, and rare earth element contents and Hf isotopic compositions, we suggest that the LG were formed by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids, whereas the SGD and SAT were formed by interaction between subducted sediment melts and the overlying depleted mantle. On the basis of our new data in combination with previous results, we propose that a broad oceanic island arc system existed in the MTO during the Early–Middle Jurassic. Differential influence of ancient microcontinent materials or crust-derived sediment led this intra-oceanic island arc to exhibit clearly different magmatic responses in its eastern and western regions.

Late Cretaceous felsic intrusions in oceanic plateau basalts in SW Ecuador: Markers of subduction initiation?

Felsic magmatic rocks (tonalites and trondhjemites) with calc-alkaline affinity were emplaced in SW Ecuador (Pascuales area; North Guayaquil) at ~87–89 Ma intruding the top of the basalts of the Caribean Large Igneous Province (CLIP), prior to the development of typical well-known island arcs from ~85 to 80 Ma onwards. A clear depletion of REE distinguishes the Pascuales felsic rocks from the more differentiated lavas of the Rio Cala Arc in Ecuador and from other Late Cretaceous arc-like felsic intrusives emplaced within the CLIP in Colombia and the Antilles. In addition, the Pascuales tonalites have very low K2O (<0.4 wt%), TiO2 (<0.4 wt%) and P2O5 (<0.15 wt%) contents compared to dacitic rocks of the Rio Cala Arc. In contrast, their geochemistry and their similar age suggest that the Pascuales tonalites are an equivalent of the Las Orquideas dacitic volcanic rocks. The Pascuales felsic rocks have the less radiogenic Nd compositions of the whole spectrum of Ecuadorian oceanic plateau rocks, nevertheless shearing similarities with some Totoras amphibolites. The geochemical characteristics of the Pascuales intrusives and Las Orquideas dacites are similar to those formed by hydrous partial melting of mafic protoliths. The dacitic magmas must have been in equilibrium with residual mineral assemblages largely dominated by amphibole + Fe–Ti oxides (no garnet). This requires two conditions: 1) low-pressure melting under high water content and 2) a source rock extremely depleted in minor incompatible elements (K, Ti, P, Y) and REE. These conditions are fulfilled by partial melting of mafic-ultramafic cumulates undergoing high temperature and shear deformation at the base of the oceanic plateau triggered by new basaltic intrusions. The Pascuales felsic rocks emplaced at the SW border of the oceanic plateau during its final stage of building; they can thus be related to asthenopheric upwelling during the initial stage of island arc magmatism.

Genesis of Intermediate and Silicic Arc Magmas Constrained by Nb/Ta Fractionation

AbstractResolving the geochemical discrepancies in Nb/Ta between the bulk continental crust and its parental mantle melts can provide insights into the processes by which basaltic magmas differentiated into the felsic crust. Deep‐seated garnet pyroxenite cumulates (i.e., arclogite) have high Nb/Ta values due to the presence of high Nb/Ta‐rutile. For this reason, rutile‐bearing arclogites having Nb‐rich composition were proposed to complement the low Nb/Ta nature of the continental crust and evolved arc magmas, that is to say, continental arc magma evolution dominantly occurred in the stability field of garnet. However, this suggestion is challenged by the incomplete understanding of the role of rutile in the Nb/Ta fractionation during arc magma evolution due to insufficient partitioning data at low temperatures (T ≤ 1,000°C). Here, we determined Nb and Ta partitioning ratios in rutile‐felsic melt systems at 2.0 GPa, 850–1,000°C and ∼4–20 wt.% H2O. Our results confirm that equilibrium crystallization of rutile with respect to Nb and Ta will not impart lower Nb/Ta values to any derivative liquids, whereas diffusive fractionation of Nb/Ta in siliceous melts during the growth of rutile can reduce Nb/Ta ratios in melts. However, a positive correlation between Nb/Ta and Dy/Yb for mature continental arc magmas, incipient continental arc magmas, island arc magmas, and global arc magmas indicates that amphibole rather than rutile‐bearing arclogite was responsible for causing low Nb/Ta ratios of many evolved intermediate rocks. Accordingly, the chemical differentiation from primary basaltic to average andesitic compositions of the continental crust mainly occurred in the stability field of amphibole.

Water transport to the core-mantle boundary.

Water is transported to Earth's interior in lithospheric slabs at subduction zones. Shallow dehydration fuels hydrous island arc magmatism but some water is transported deeper in cool slab mantle. Further dehydration at ∼700 km may limit deeper transport but hydrated phases in slab crust have considerable capacity for transporting water to the core-mantle boundary. Quantifying how much remains the challenge.

Early Cambrian oceanic island-arc magmatism at the paleo-Pacific margin of East Gondwana: Evidence from northern Victoria Land (Antarctica)

We present new geochemical, isotopic and geochronological data for two Cambrian magmatic sequences exposed in northern Victoria Land (Antarctica) at the eastern margin of the Ross-orogenic Wilson Terrane: (i) the metavolcanic and metaplutonic rocks from the Dessent tectono-metamorphic complex, and (ii) the boninite-derived mafic-ultramafic Niagara Icefalls igneous complex.The Dessent metaigneous rocks are subdivided into three main types. Type I metavolcanites show LREE-depleted chondrite-normalized patterns and high initial ɛNd (mostly +6 to +7). Metavolcanic and metaplutonic type II rocks show low MREE-HREE concentrations and initial ɛNd ranging from +2 to +4. Type III metavolcanites mostly differ from type II rocks in the somewhat elevated MREE-HREE abundances. Type I, II and III rocks geochemically resemble forearc basalts, boninites s.l. and firstarc volcanites, respectively, from the Izu-Bonin-Mariana island arc system. Zircon separated from two metaplutonic rocks documents a crystallization age of ~516 Ma.The Niagara Icefalls rocks display a wide range of Nd–Sr isotopic compositions (e.g., initial ɛNd + 5 to −8). Zircons separated from a gabbronorite and from a granitoid vein allowed us to date the building of the Niagara Icefalls igneous complex at ~537 Ma. The melts forming the Niagara Icefalls igneous complex were most likely sourced in a supra-subduction zone mantle wedge, which included both depleted mantle and slab-derived sedimentary components. We propose that the mafic-ultramafic Niagara Icefalls igneous complex and the metaigneous rocks of the Dessent tectono-metamorphic complex are fossil analogues of two distinct oceanic island arcs, which formed in the earliest and the late early Cambrian, respectively.

Porphyry-related high-sulfidation mineralization early in Central American Arc development: Cerro Quema deposit, Azuero Peninsula, Panama

The 70.74 to 70.66 Ma age range for three molybdenite samples accompanying pyrite- and enargite-bearing assemblages effectively constrains an earliest Maastrichtian age for the high-sulfidation Au-Cu mineralization at Cerro Quema, Panama. The epithermal system was contemporaneous with emplacement of a composite dacite dome complex in a geotectonic setting transitional from mafic, primitive intraoceanic (Azuero Protoarc) to more evolved island arc magmatism (Azuero Arc), during initial construction of the Central American land bridge at the trailing edge of the Caribbean Large Igneous Province (CLIP). The molybdenite ages confirm the rapid evolution of the earliest stages of the Central American Arc, from subduction initiation at 75–73 Ma to arc maturation at 71 Ma. A porphyry connection is apparent at Cerro Quema and characterized by highly contorted, banded, and planar quartz-veinlet stockworks and sheeted zones in pyrophyllite- and sericite-bearing patchy-textured rock. These are cut by ledges of quartz, alunite, and dickite, which implies overprinting of the advanced argillic lithocap onto the underlying porphyry environment. Hydrothermal telescoping resulted from synmineralization uplift congruent with an actively emerging volcanic arc, which the Re-Os molybdenite dates accurately constrain at 71 Ma, presumably as a far-field effect of collision between the leading edge of the CLIP with parts of North and South America.