Supra-subduction Zone Magmatism Research Articles

Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

Abstract Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent SSZ magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~ + 1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of > + 2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution

Fossil seamount in southeast Zagros records intraoceanic arc to back-arc transition: New constraints for the evolution of the Neotethys

The Neotethys ocean is transiently involved in two subduction zones during the Late Cretaceous. While the Northern Neotethys subduction zone (below Eurasia) was active from the early Mesozoic until the Eocene, the intra-oceanic Southern Neotethys subduction zone only developed during the Late Cretaceous. We herein document, through a combination of structural, geochemical and geochronological data, the magmatic evolution of a Late Cretaceous supra-subduction ophiolite fragment of the Neotethys (the Siah Kuh massif, Southern Iran), now sandwiched in the Zagros suture zone. Results show that this ophiolite fragment — a subducted yet exceptionally well-preserved seamount — records an evolution from supra-subduction zone magmatism (including island arc tholeiites, boninites and calc-alkaline transitional magmatism) around 87 Ma, to MORB (from E-MORB to N-MORB) magmatism at 78 Ma, and potentially until 73 Ma. We conclude that this seamount initially formed in an arc context and represents either (i) a non-obducted remnant of the Oman ophiolite that experienced a longer-lived magmatic history (prefered hypothesis) or (ii) a piece from the forearc/frontal arc of the Northern margin of the Neotethys. Regardless of its exact original location, the Siah Kuh seamount was later subducted in the Northern Neotethys subduction zone.

Epithermal systems of the Torud–Chah Shirin district, northern Iran: Ore-fluid evolution and geodynamic setting

The Torud–Chah Shirin (TCS) ore district in northern Iran is defined by an NE alignment of Tertiary Ag-Au and base metal-rich epithermal systems, and it is part of the eastern Alborz orogenic belt of Iran. Intermediate- to high-sulfidation mineralization occurs as veins hosted by the Eocene−Oligocene volcanic, subvolcanic and volcaniclastic rocks. The TCS district is characterized by three fault system populations including ∼70°, ∼270°, and ∼340°−trending faults, and detailed structural mapping show that overall strike of the TCS vein system is 320°–340° but varies from ∼290° to ∼350°. The ∼N70°−trending faults are parallel to the Anjilow and Torud regional faults in the TCS ore district. Green- to grey-schist and metamorphosed dolomite and limestone are the oldest units (Ordovician–Silurian) in the TCS. Sedimentary rocks were initiated by limestone, dolomite and green shale in the Cretaceous and continued with conglomerate into the Paleocene (Fajan Formation). Magmatism mainly occurred sporadically from the Eocene to Oligocene (ca. 55–24 Ma), with two major episodes between early to middle Eocene (ca. 55 and 37 Ma, EME) and early to late Oligocene (ca. 34 and 24 Ma, ELO). Whole rock geochemical data of EME and ELO rocks of the TCS district shows a range from basalts to rhyolites with low-K calc-alkaline and shoshonitic affinity. Their rare earth elements (REEs) and high field strength elements (HFSE) signatures indicate the occurrence of a supra-subduction zone magmatism and all rocks have been sourced from the same parent melt. Samples from ELO display higher alkali contents compare with EME but have a similar trace element characteristics.Hydrothermal alteration is pervasive in volcanic and subvolcanic rocks but is mainly localized near the veins and ore zones. Alteration assemblages include quartz-sericite (±pyrite ± adularia), quartz-illite (kaolinite ± pyrite), carbonate-quartz and chlorite + calcite + epidote (propylitic). Homogenization temperatures of fluid inclusions in the epithermal prospects is in range of 125–375 °C, and ore-forming fluids were mainly of magmatic-hydrothermal origin (e.g., Gandy and Abolhassani prospects), with some contributions from meteoric water. The epithermal prospects have more or less similar salinities, ranging from 2 to 18 wt% NaCl equiv., with a distinct cluster between 6 and 15 wt% NaCl equiv.Our shreds of evidence suggest that the EME and ELO calc-alkaline volcanic activity in TCS occurs occurred in tension fractures related to orthogonal plate convergence that postdates the NE-trending strike-slip regional faults, and plays an important role in the development of the epithermal prospects. Simultaneous with, or soon after crustal heating related to the magmatism, strike-slip faults movement may also have been critical in the ore-forming process, leading to trans-tension of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.

3‐D Structure of the Northern Oman‐UAE Ophiolite: Widespread, Short‐Lived, Suprasubduction Zone Magmatism

AbstractThe Oman‐United Arab Emirates (UAE) ophiolite is the most intensely studied ophiolite on Earth, is commonly used as a template for other poorly exposed and structurally complex ophiolites, and is often used as an analog for fast‐spreading mid‐ocean ridges like the East Pacific Rise. Several recent studies, including the largest ever focused on the UAE, have suggested that the pseudostratigraphy of the ophiolite in the UAE is distinct from Oman. Such interpretations have led researchers to propose that the UAE portion of the ophiolite underwent a separate magmatic and tectonic evolution. We present a profile along a new road section in the UAE that reveals continuous exposure of the entire ophiolite section and places key constraints on the internal architecture. We infer the geometry of the ophiolite thrust sheet and propose a model for the structure, pseudostratigraphy, and evolution of the ophiolite in the UAE that is simplified yet more consistent with geochemical and geochronological data. Our results indicate that (1) the architecture of ophiolite in the UAE is similar to Oman, and there is no reason to infer a different tectonic history, (2) the ophiolite is gently folded but largely intact, (3) extension and magmatism during formation of the ophiolite was broadly distributed, (4) there are no large gabbro bodies in the mantle sequence; they are all above the petrological Moho, and (5) the entire magmatic sequence was generated in a short time (~1 million years), and thus, the ophiolite cannot be compared to the long‐lasting (>100 million years) fast‐spreading East Pacific Rise.

Neoarchean suprasubduction zone magmatism in the Sonakhan greenstone belt, Bastar Craton, India: Implications for subduction initiation and melt extraction

The Neoarchean Sonakhan greenstone belt (SGB) in the north‐eastern Bastar Craton is mainly composed of an association of pillowed and massive tholeiitic basalts emplaced within an intraoceanic environment. Two types of ultrabasic rocks were identified and are designated as TH‐1 and TH‐2 on the basis of geochemical parameters. Both sample suites exhibit depletion of HFSE with reference to LILE and LREE, LREE/HFSE ratios, and negative Nb–Ta–Ti anomalies in the primitive mantle‐normalized multi‐element diagrams. The geochemical characters of TH‐1 and TH‐2 are consistent with Island arc tholeiites (IAT) and boninite‐like rocks of both Archean and Phanerozoic terranes. Mineral chemistry of clinopyroxenes from both volcanic suites indicate an oceanic arc affinity. The chromite chemistry indicate its derivation from a boninite‐like magma in a suprasubduction zone (SSZ) environment. Trace element modelling depicts that source depletion with significant influence of fluids derived from the subducting oceanic slab collectively controlled the trace element inventory of the mantle wedge, which result to the higher abundance of LREE and LILE compared to HREE and HFSE. The occurrence of lower basalt sequence followed by IAT and boninite‐like rocks in the SGB define a magmatic evolution which is comparable to the Phanerozoic ophiolite suites that exhibit subduction initiation prior to forearc rifting and suprasubduction zone magmatism. The subduction initiation in the intraoceanic lithosphere and the initial decompression melting followed by the upwelling of the MORB‐related magmas were attributed to the formation of the lower pillow basalts. Dehydration of the subducting plate and the melt extraction processes in the hydrous lherzolite mantle wedge account for the formation of TH‐1. Subsequent depletion and significant modification of a more refractory harzburgite source by the slab‐derived fluids generated the TH‐2 rocks. The compositionally diverse magmatic imprints with IAT and boninite affinity in the SGB indicate the significance of episodic melt extraction processes, dehydration of the subducting plate, and hydrous fluxing in SSZ tectonic setting.

Vendian Suprasubduction Zone Magmatism in the Southern Urals

Vendian Suprasubduction Zone Magmatism in the Southern Urals

A hafnium isotopic record of magmatic arcs and continental growth in the Iapetus Ocean: The contrasting evolution of Ganderia and the peri-Laurentian margin

We test the sensitivity of combined U-Pb-Hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens, using Ganderia from the Canadian Appalachian orogen as an example. Ganderia hosted a long-lived magmatic arc that began at least by 650Ma and continued until 450Ma, during which time it drifted northward from the Amazonian margin. Its isotopic record suggests arc activity began earlier, possibly at ~750Ma (proto-Ganderia) within an oceanic domain. The U-Pb-Hf zircon isotopic array suggests that Ganderia formed on a sliver of Paleoproterozoic-Mesoproterozoic basement (TDMc=2.2–1.1Ga). A transition towards more evolved Hf isotopic compositions between 650 and 600Ma is interpreted to coincide with its accretion to the Amazonian margin of Gondwana, consistent with other geological evidence. Accretion was followed by increasing amounts of juvenile crustal inputs between ~550 and 500Ma, which is interpreted to reflect subduction roll-back and the onset of drift from the Gondwanan margin. Trench retreat culminated in the separation of the Ganderia arc and formation of the exclusively oceanic ~510–450Ma Penobscot-Popelogan-Victoria arcs on Ganderia. Accretion of Ganderia to the active Notre Dame Subzone along the Laurentian margin occurred in the late Ordovician and is marked by the abrupt termination of Ganderian arc magmatism.The Notre Dame arc evolved along the Laurentian margin between ~515 and 425Ma and was built on Paleoproterozoic-Archean (T(DM)c=2.9–2.5Ga) crust. Supra-subduction zone magmatism occurred at ~490–460Ma during the closure of the Taconic Seaway and was dominated by the recycling of Laurentian crust, with progressive mixing of evolved and juvenile arc magmas over time. The preservation of very evolved Notre Dame arc zircon in Ganderian overstep sequences confirms the arrival of the leading edge of Ganderia to Laurentia by ~450Ma. The hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints, and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems.

Taconic suprasubduction zone magmatism in southern Laurentia: Evidence from the Dadeville Complex

Taconic suprasubduction zone magmatism in southern Laurentia: Evidence from the Dadeville Complex

Hunter–Bowen deformation in South Percy Island, northeastern Australia

ABSTRACTSouth Percy Island is located approximately 50 km off the central Queensland coast and comprises a disrupted ophiolite mass alongside a diverse array of metamorphosed felsic and mafic rocks that record several episodes of magmatism, volcanism and deformation from the Permian to Early Cretaceous. This paper aims to constrain the age, affinity and deformation history of these units, as well as to establish the tectonic significance of the terrane. The trace-element compositions of mafic and felsic meta-igneous rocks record a change from MORB-like prior to ca 277 Ma to subduction-related by ca 258 Ma. Overprinting relationships between intrusive phases and deformation features reveal a relative chronology for the tectonothermal evolution of the area, while U–Pb and 40Ar/39Ar geochronology provides absolute age constraints. Deformation is localised around a NNE-striking tectonic contact that separates serpentinised ultramafic rocks from metamorphosed pillow lavas. Early formed ductile fabrics associated with the main episode of deformation (D1) preserve bulk flattening strains at greenschist-facies conditions. Emplacement and post-kinematic cooling ages of a pre-D1 quartz-monzonite dyke constrain the age of D1/M1 deformation and metamorphism to the period between ca 258 and ca 248 Ma. Minor brittle deformation (D2) occurred at ca 230 Ma, based on U–Pb dating of a syn-D2 diorite dyke (ca 231 ± 10 Ma) and several ca 230 Ma 40Ar/39Ar cooling ages. The deformation, metamorphism, and supra-subduction zone magmatism preserved on South Percy Island is correlated with the nearby Marlborough Terrane and more broadly with the second pulse of the Hunter–Bowen Orogeny, which affected much of the central and northern parts of eastern Australia in the late Permian and Early Triassic. Our results support previous suggestions that the second pulse of the Hunter–Bowen Orogeny involved coeval thrust systems in both the inboard and outboard parts of the orogen.

Supra-subduction zone magmatism of the Koçali ophiolite, SE Turkey

The Late Cretaceous Koçali ophiolite is an important element within the southeastern Anatolian ophiolite belt and contains all components of a complete ophiolite, including mantle peridotites (harzburgites, and subordinate dunite) gabbros, sheeted dyke complexes, lower volcanic unit (pillowed basalts), and upper volcanic unit (andesite and andesitic pyroclastics) plagiogranite and ultramafic rocks intruded into the gabbros. All components of the Koçali ophiolites show strong suprasubduction-zone affinities, from harzburgitic mantle to basaltic lavas. The radiolarian faunas and geochronological data indicate that there are two different volcanic units: Tarasa volcanics and volcanics of the Koçali ophiolite. While the Tarasa volcanics are of Carnian to Rhaetian age, the Koçali ophiolite is of the Late Cretaceous age. The geochemical features indicate an E-MORB source for the Tarasa volcanics, and a forearc source for the Koçali ophiolite.

Tectonomagmatic evolution of the South Dehshir Ophiolite, Central Iran

AbstractThe South Dehshir Ophiolite, exposed along the southwestern margin of the Central Iranian microcontinent, comprises a mantle and a crustal sequence. This massif contains serpentinized peridotites, gabbros and diabase sheeted dykes cutting across wherlite, pegmatitic gabbro and plagiogranite, and is tectonically covered by radiolarites, pelagic limestones with Late Cretaceous microfauna and lavas. Under a microscope, the gabbros exhibit mesocumulate, adcumulate and hetradcumulate textures presumably formed in open magma chambers. Hypabyssal and lava samples show tholeiitic to calc-alkaline affinities, and their compositions cluster in the fields of arc or supra-subduction zone magmatism. Binary diagrams display compositional trends of magmatic differentiation, plotting away from the mid-ocean-ridge basalt (MORB) (tholeiitic) trend and following the arc (calc-alkaline) trend. Rare Earth element (REE) and spider diagrams display various enrichments at different levels. Moderate enrichment of the large-ion lithophile elements (LILE; Rb, Ba, Th, U, La, Sr) relative to the high-field-strength elements (HFSE; Nb, Ta) and slight depletion of Ti and Zr in some patterns may be attributed to variable influences of subduction components over the depleted mantle wedge. Elemental ratios display evidence for the contribution of both fluids and melt released from the subducted slab. Few patterns display ocean-island basalt (OIB) characteristics, possibly attributed to the involvement of local mantle plumes on the partial melting of the mantle wedge. This evidence indicates that the crustal rocks of the South Dehshir Ophiolite formed in a supra-subduction zone.

Supra-subduction zone magmatism of the Neyriz ophiolite, Iran: constraints from geochemistry and Sr-Nd-Pb isotopes

The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al2O3, are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ- peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial 87Sr/86Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207Pb/204Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.

Mineral ages and zircon Hf isotopic composition of the Andong ultramafic complex: implications for the evolution of Mesozoic subduction system and subcontinental lithospheric mantle beneath SE Korea

AbstractThe Phanerozoic subduction system of the Korean peninsula is considered to have been activated by at least Middle Permian time. The geochemically arc-like Andong ultramafic complex (AUC) occurring along the border between the Precambrian Yeongnam massif and the Cretaceous Gyeongsang back-arc basin provides a rare opportunity for direct study of the pre-Cretaceous mantle wedge lying above the subduction zone. The tightly constrained SHRIMP U–Pb age of zircons extracted from orthopyroxenite specimens (222.1±1.0 Ma) is indistinguishable from the Ar/Ar age of coexisting phlogopite (220±6 Ma). These ages represent the timing of suprasubduction zone magmatism likely in response to the sinking of cold and dense oceanic lithosphere and the resultant extensional strain regime in a nascent arc environment. The nearly coeval occurrence of a syenite-gabbro-monzonite suite in the SW Yeongnam massif also suggests an extensional tectonic setting along the continental margin side during Late Triassic time. The relatively enriched ɛHf range of dated zircons (+6.2 to −0.6 at 222 Ma) is in contrast to previously reported primitive Sr–Nd–Hf isotopic features of Cenozoic mantle xenoliths from Korea and eastern China. This enrichment is not ascribed to contamination by the hypothetical Palaeozoic crust beneath SE Korea, but is instead attributable to metasomatism of the lithospheric mantle during the earlier subduction of the palaeo-Pacific plate. Most AUC zircons show a restricted core-to-rim spread of ɛHf values, but some grains testify to the operation of open-system processes during magmatic differentiation.

Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt, Iran): implications for geodynamics of the Bitlis–Zagros suture zone

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.

U–Pb ages, geochemistry, and tectonomagmatic history of the Cambro-Ordovician Annidale Group: a remnant of the Penobscot arc system in southern New Brunswick?1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Penobscot arc system of the northeastern Appalachians is an Early Cambrian to early Tremadocian (ca. 514–485 Ma) ensialic to ensimatic arc–back-arc complex that developed along the margin of the peri-Gondwanan microcontinent Ganderia. Remnants of this Paleozoic arc system are best preserved in the Exploits Subzone of central Newfoundland. Correlative rocks in southern New Brunswick are thought to occur in the ca. 514 Ma Mosquito Lake Road Formation of the Ellsworth Group and ca. 497–493 Ma Annidale Group; however in the past, the work that has been conducted on the latter has been of a preliminary nature. New data bearing on the age and tectonic setting of the Annidale Group provides more conclusive evidence for this correlation. The Annidale Group contains subalkaline, tholeiitic to transitional, basalts to basaltic andesites, picritic tuffs and calc-alkaline to tholeiitic felsic dome complexes that have geochemical signatures consistent with suprasubduction zone magmatism that was likely generated in a back-arc basin. New U–Pb ages establish that the Late Cambrian to Early Tremadocian Annidale Group and adjacent ca. 541 Ma volcanic rocks of the Belleisle Bay Group in the New River belt were affected by a period of younger magmatism ranging in age from ca. 479–467 Ma. This provides important constraints on the timing of tectonism in the area. A ca. 479 Ma age for the Stewarton Gabbro that stitches the faulted contact between the Annidale and Belleisle Bay groups, demonstrates that structural interleaving and juxtaposition occurred during early Tremadocian time, which closely coincides with the timing of obduction of Penobscottian back-arc ophiolites onto the Ganderian margin in Newfoundland.

Architecture of the Oman–UAE ophiolite: evidence for a multi-phase magmatic history

The Oman–United Arab Emirates ophiolite is the world’s largest ophiolite. It is divided into 12 separate fault bounded blocks, of which the northern three lie wholly or partly in the United Arab Emirates. Extensive mapping has shown that the United Arab Emirates blocks contain mantle and crustal sections which correspond to the classic ‘Penrose conference’ ophiolite definition but which are cut by a voluminous later magmatic sequence including ultramafic, mafic and felsic components. Samples from the later magmatic sequence are dated at 96.4 ± 0.3, 95.74 ± 0.3 and 95.2 ± 0.3 Ma; the early crustal section, which has not been dated directly, is thus constrained to be older than c. 96.4 Ma. Petrological evidence shows that the early crustal section formed at a spreading ridge, but the later magmatic sequence was formed from hydrous magmas that produced different mineral crystallisation sequences to normal midocean ridge basalt (MORB). Mineral and whole-rock geochemical analyses show that the early crustal rocks are chemically similar to MORB, but the later magmatic sequence has chemical features typically found in suprasubduction zone (SSZ) settings. The ophiolite in the United Arab Emirates thus preserves clear evidence for two stages of magmatism, an early episode formed at a spreading centre and a later episode associated with the onset of subduction. Similar two-stage magmatism has been recogn sed in the Oman sector, but the United Arab Emirates contains the most voluminous SSZ magmatism yet described from this ophiolite

The mafic rocks of Shao La (Kharta, S. Tibet): Ordovician basaltic magmatism in the greater himalayan crystallines of central-eastern Himalaya

In the Kharta area, east of Mount Everest, the Greater Himalayan Crystallines are significantly richer in mafic rocks than the surrounding areas, Sikkim–West Bhutan and Makalu–Cho Oyu. These rocks are lenses with a complex metamorphic history. The mafic lenses of Shao La, in the Greater Himalayan Sequence south of Kharta, are here considerated as dismembered dykes apparently escaped the Himalayan high-temperature metamorphism and only record a low-grade metamorphic event. They are calc-alkaline medium-K basalts to basaltic andesites, consisting of plagioclase (core 62% An and rim 55% An), augite (Wo 43–47En36 36–37Fs 16–20), hypersthene (Wo 1.6–3.3En 50–52Fs 46–48), and minor brown hornblende, biotite and ilmenite. They show strong enrichment in low ionic potential elements relative to high-field-strength elements, and only minor Ce and P enrichment with respect to MORB. Combined Sr–Nd systematics suggest contamination of a basic magma from a subcontinental mantle source with a small amount of crust (about 4 vol.%). This in turn indicates that the Shao La basalts and basaltic andesites have the geochemical fingerprint of a supra-subduction zone magma. U–Pb dating of zircon from one sample yielded an age of 457 ± 6 Ma for the crystallisation of the Shao La basic rocks, assigning them to the Cambro-Ordovician Bhimphedian orogenic event. The age and geochemical characteristics of the Shao La rocks are similar to those of the basic rocks of the Cambro-Ordovician Mandi pluton further west. This suggests the existence of an extensive supra-subduction zone magmatism along the Indian margin of Gondwana. Like the bimodal granite-gabbro magmatism in the Mandi-Kaplas area, the Shao La basic rocks are contemporaneous with the emplacement of granitic plutons in the Everest-Kharta area. This acid plutonism is interpreted as crustal melt triggered by the upwelling of metasomatised mantle in a back-arc setting. The age of basic and acidic plutonism in the Everest-Kharta area is evidence that the Bhimphedian Orogeny was still active as late as the Late Ordovician

1.3–0.9Ga Oaxaquia (Mexico): Remnant of an arc/backarc on the northern margin of Amazonia

Abstract Rocks with ages of ca. 1 Ga occur in central and southern Mexico as inliers surrounded by ubiquitous Mesozoic and Cenozoic rocks. They appear to share a common history consisting of: (i) ca. 1300–1200 Ma arc magmatism and deposition of sediments including evaporites; (ii) ca.1160–1100 Ma intrusion of syenite, granite and anorthosite, the later part of which is synchronous with migmatization; (iii) intrusion of a ca. 1035–1010 Ma anorthosite–gabbro–charnockite–granite (AMCG) suite; (iv) a 1000–980 Ma granulite facies tectonothermal event with a stretching axis parallel to the long axis of Oaxaquia; (v) gradual exhumation at 750 and/or 545 Ma; and (vi) 517 Ma intrusion of an isolated calcalkaline granitoid pluton. The common Precambrian geological record of these outcrops suggests that they belonged to a single terrane (Oaxaquia) and formed a juvenile arc/backarc bordering a continent that underwent collision with, and overthrusting of, the Avalonian arc at 1000–980 Ma. This buried Oaxaquia to 25–30 km and was followed by further supra-subduction zone magmatism at ca. 917 Ma. These Precambrian rocks are unconformably overlain by uppermost Cambrian and Silurian platform rocks containing Gondwanan fauna and ca. 1 detrital zircons of Oaxacan provenance. The neighbouring Mixteca terrane includes lower Paleozoic, rift-passive margin sedimentary rocks that also contain 900–750 Ma detrital zircons probably derived from the Goias arc in eastern Amazonia. The arc-backarc tectonic setting inferred for the 1300–900 Ma rocks also suggests that Oaxaquia lay on an active periphery of Amazonia until ca. 900 Ma, well after the amalgamation of Rodinia. This precludes a location for Oaxaquia off southern and western Amazonia that are inferred to have been juxtaposed against eastern Laurentia; contiguity with eastern Amazonia is also unlikely given the absence of the 900–750 Ma convergent tectonics in the Goias arc. This leaves northern Amazonia as the most likely position, a location that requires the least relative displacement between Oaxaquia and Amazonia. The inferred 750 and 545 Ma exhumation episodes of Oaxaquia correspond to two proposed breakup stages of Rodinia.

Island arc tholeiite to boninitic melt evolution of the Cretaceous Kizildag (Turkey) ophiolite: Model for multi-stage early arc–forearc magmatism in Tethyan subduction factories

The Kizildag ophiolite in Turkey is a remnant of the late Cretaceous suprasubduction zone (SSZ) oceanic lithosphere that was developed in Southern Tethys, and is part of a peri-Arabian ophiolite belt including the Troodos (Cyprus), Baër-Bassit (Syria) and Semail (Oman) ophiolites in the eastern Mediterranean region. The internal structure, petrology and geochemistry of the Kizildag intrusive and extrusive rocks show a magmatic progression from island arc tholeiite (IAT) to boninitic compositions. Similar structural architecture and geochemical trends in the coeval Troodos and Baër-Bassit ophiolites suggest that this multi-stage SSZ magmatism was common in most Tethyan ophiolites. The sheeted dikes and the majority of extrusive rocks in Kizildag are composed of basalt, basaltic andesite and andesite with relatively low La/Sm N (0.5–0.7), low MgO (7–9 wt.%) and high TiO 2 (0.7–1.0 wt.%) and exhibit high Dy/Yb N ratios. These IAT suites also include rare, younger dacitic dikes showing strong enrichment in the LREE. The late-stage basaltic dikes and lavas (sakalavites) have high La/Sm N (0.8–1.0), high MgO (9–11 wt.%) and low-TiO 2 (0.3–0.4 wt.%) contents and display strong depletion in both LREE and HREE. This suite of rocks represents the boninitic component of the Kizildag ophiolite that developed from shallow and greater degrees of melting of hydrous and highly refractory peridotites beneath the extended forearc environment. The progressive evolution of Kizildag magmas from IAT to boninitic compositions in time and space was a result of variable mixing, aggregation, and differentiation of geochemically different melt batches, which formed in different levels (polybaric) within the melting column. Rapid slab rollback-induced corner flow, mantle return flow, and asthenospheric diapirism (providing more fertile melt) directly affected the evolution of this melting column and may have triggered advanced degrees of shallow melting of the hydrous and highly depleted forearc mantle, producing refractory liquids of the boninitic stage. These geochemical patterns of the Kizildag ophiolite reflect a multi-stage evolution of SSZ magmatism in Tethyan subduction factories that is analogous to the earlier arc volcanism in the Eocene–Oligocene Izu–Bonin–Mariana system.

Isotopic evidence for rapid continental growth in an extensional accretionary orogen: The Tasmanides, eastern Australia

The trace element signature of Earth's continental crust resembles that of arc lavas, but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra-subduction zone magmatism. We explore the role of coupled arc–back-arc accretionary processes in crust generation by considering the tectonic context of whole rock Nd isotope and zircon Hf–O isotope data from igneous rocks of the Australian Tasmanides (515–230 Ma), which is thought to have evolved by the repeated opening and closure of sediment-filled back-arc basins behind a long-lived subduction zone. The significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective. Granitic rocks in this area define striking secular ε Nd– ε Hf–δ 18O trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity. These trends reveal that juvenile magmatic input was enhanced during extensional, back-arc rifting episodes that immediately followed crustal thickening, suggesting a relationship between slab rollback and continental growth. Interaction between juvenile magma and sedimentary units deposited during a preceding back-arc rifting cycle was integral to the formation of stable continental material. This highlights the importance of back-arc environments for both the generation and differentiation of continental crust. The juvenile component within the Tasmanide igneous rocks increased from the Cambrian to the Triassic, consistent with a diminished input from craton-derived metasedimentary material as the subduction zone migrated outboard of the Gondwana margin. Subduction zone retreat formed large tracts of new crust in eastern Australia at comparable rates to crust generation at modern island arcs, providing a mechanism for rapid continental growth at convergent margins. Using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation.