Island Arc Tholeiites Research Articles

A Devonian arc–back-arc basin system in the southern Chinese Altai: Constraints from geochemical and Sr-Nd-Pb isotopic data for meta-basaltic rocks

The early to middle Paleozoic accretionary history of the Chinese Altai orogen remains controversial, especially along its southern margin facing the Paleo-Asian Ocean. This study conducts a systematic investigation on the Devonian meta-volcanic sequences from the Altay Formation in the southern Chinese Altai to constrain their petrogenesis and tectonic implications. LA-ICP-MS zircon UPb dating for rocks from the bottom and top layers of the studied volcanic sequences indicates their eruption age at ca. 380 Ma. The basaltic rocks are low-K tholeiitic with high TiO2 (average = 1.98 wt%), and have rare-earth element (REE) patterns similar to the mid-ocean ridge basalt (MORB), in spite of a large variation in light REE (LREE). They also show enrichment in large ion lithophile elements (LILE; e.g. Ba, U, and Th) and depletion in high field strength elements (HFSE; e.g. Nb and Ta), which are hallmarks of supra-subduction zone basaltic magma. Thus, the elemental data indicate that these rocks are compositionally intermediate between MORB and island-arc tholeiite (IAT).The basaltic samples have positive εNd(t) values (+3.1 to +10.3) and elevated initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotopic ratios. Whole-rock elemental and isotopic data suggest that the rocks were likely derived from a dominant N-MORB type depleted asthenosphere mantle, which was variably metasomatized by or interacted with the relatively enriched sub-arc mantle. In combination with the field relationship and regional geology, our study suggests that the meta-basaltic rocks from the middle Altay Formation formed in a mature back-arc spreading environment. We propose an evolving tectonic scenario from a magmatic arc to arc rifting, and finally back-arc opening in the southern Chinese Altai, which corresponds to an extensional regime and resultant asthenosphere upwelling event in this region during Devonian time, probably in response to a trench retreat.

Neoarchean suprasubduction zone ophiolite discovered from the Miyun Complex: Implications for Archean–Paleoproterozoic Wilson cycle in the North China Craton

The North China Craton (NCC) offers a window to Archean–Paleoproterozoic crustal evolution. Following the amalgamation of microblocks during Neoarchean, the NCC witnessed another protracted Wilson cycle in the Paleoproterozoic with subduction-accretion-collision process. Here we report a newly identified suprasubduction zone ophiolite suite from the Miyun complex in the NCC where a rare and near-complete ophiolite succession is preserved including serpentinite, lherzolite, olivine clinopyroxenite, websterite, gabbro, dolerite, diorite, monzonite, metabasalts and trondhjemite, in association with Banded Iron Formation (BIF). The geochemical features of the suite are consistent with those of suprasubduction ophiolite complexes, with the mafic and intermediate members displaying Island Arc Tholeiite (IAT) and Mid-Ocean Ridge Basalt (MORB) affinity. Zircon grains from lherzolite define three groups of 207Pb/206Pb weighted mean ages at 2500 ± 12 Ma marking the formation age, and 2237 ± 47 Ma, and 1843 ± 30 Ma representing subsequent thermal events. The majority of zircon grains in olivine clinopyroxenite, gabbro, and quartz monzonite yield 207Pb/206Pb weighted mean ages of 2510 ± 19 Ma, 2495 ± 10 Ma, and 2486 ± 21 Ma respectively. Those from the trondhjemite show weighted mean age of 2476 ± 9.4 Ma. The compiled age data from all the rock types in the Miyun suite show two distinct age peaks at ca. 2486 Ma and 1832 Ma, corresponding to the timing of magma differentiation and subsequent metamorphism. The zircon Lu-Hf isotope data display distinct positive ɛHf(t) values in the range 0.35–7.05, suggesting magma derived from Neoarchean to Paleoproterozoic depleted mantle (juvenile) source. The zircon TDM ages show a peak at 2670 Ma, confirming Neoarchean components. The timing of magma formation in the Miyun ophiolite is broadly identical to those of the other rare Neoarchean – early Paleoproterozoic ophiolite suites described from the NCC including those from Yishui and Zunhua. Furthermore, evidence from the recently reported Alaskan-type intrusions and arc root mafic-ultramafic complexes around Miyun complex with ages in the range of 2.4–2.0 Ga suggest continued subduction-related magmatic processes, with final collision in latest Paleoproterozoic at around 1.84 Ga. We propose that following the assembly of ancient microblocks along multiple zones of ocean closure marked by major greenstone belts of 2.6–2.5 Ga, and high-grade metamorphism along the margins of the microblocks during Archean–Paleoproterozoic transition, the NCC went through another major Wilson cycle from early to late Paleoproterozoic involving prolonged subduction, accretion and collision of larger crustal blocks and intervening arc complexes, leading to final cratonization.

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO2 (0.18–0.57 ​wt.%), P2O5 (<0.05 ​wt.%), K2O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu∗ ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)N ​= ​0.10–0.27 and (La/Yb)N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.

Chapter 3 Pre-Late Cretaceous basement terranes of the Gondwana active margin of New Caledonia

Abstract The basement under the Late Cretaceous unconformity in New Caledonia consists of three amalgamated terranes. They are all oceanic, arc-related and developed offshore from the eastern Gondwana active margin during periods of marginal basin development. Téremba Terrane is composed of deep sea Permian to Mesozoic arc-derived volcanic rocks and greywackes. The Koh–Central Terrane includes at its base an ophiolite with island arc tholeiites and boninites (Koh Ophiolite) of Late Carboniferous to Early Permian age overlain by a thick sequence of greywacke (Central Range Volcaniclastic Rocks) of Permian to Late Jurassic age. The Téremba Terrane and the Koh–Central Terrane may be part of the same forearc basin, with the rocks from the Koh–Central Terrane deposited in a deeper environment. The Boghen Terrane is a metamorphic complex composed of schists, broken formations and mafic–ultramafic mélange, derived from mixed terrigenous and volcanic sources. The overall fine grain size and laminar bedding suggest deep sea and more distal deposition than the other terranes. The maximum depositional ages from detrital zircons suggest deposition during the Early Jurassic to Early Cretaceous. The terrane is interpreted as a metamorphosed subduction complex that includes blueschist and greenschist facies metamorphic rocks exhumed through the Koh–Central Terrane. At a regional scale, the nature of these three pre-Late Cretaceous terranes confirms the existing palaeogeographical reconstructions, which locate New Caledonia outboard the ocean–continent subduction that surrounded Gondwana during the Paleozoic and Early Mesozoic. A detailed analysis of these terranes and their relationship with East Australian terranes of the same age shows that a marginal basin system probably existed between mainland Gondwana and proto-New Caledonia and closed before the Late Cretaceous. A tentative detailed reconstruction of this margin during the Carboniferous–Early Cretaceous period is proposed.

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.

The Middle Jurassic and Early Cretaceous basalt-radiolarian chert association from the Tekelidağ Mélange, eastern İzmir-Ankara-Erzincan suture zone (northern Turkey)

Basaltic and deep-sea sedimentary rocks are widely exposed in the Tekelidağ Mélange, eastern part of the İzmir-Ankara-Erzincan (İAE) suture zone. In this study, we present petrogenetic features of the basaltic rocks and new micropalaeontological data from the radiolarian cherts. The basaltic rocks are divided into two different groups based on whole-rock geochemistry data. The first group of basaltic rocks displays MORB-like geochemical characteristics. The second group exhibits island-arc tholeiite (IAT) geochemical characteristics. The cherts, which coexist within the basaltic rocks, yielded Middle Jurassic to Early Cretaceous radiolarian assemblages. Late Bajocian (Middle Jurassic) to Tithonian (Late Jurassic) ages were obtained from the radiolarian cherts observed within the MORB-type basalts belonging to the first group. On the other hand, the supra-subduction zone (SSZ)-type basalts belonging to the second group are associated with upper Aalenian (Middle Jurassic) and lower Aptian (Lower Cretaceous) radiolarian cherts. The new Middle–Late Jurassic and Early Cretaceous radiolarian dates and MORB- and SSZ-type basaltic rocks are consistent with radio-isotopic ages previously obtained from ophiolites and ophiolitic mélanges along the İAE suture zone. The radiolarian ages associated with basaltic rock data indicate that the occurrence of the MORB- and SSZ-type magmatism were simultaneous. The coeval IAT and MORB-like basalt generation inferred to occur in the SSZ environment similar to the South Sandwich back-arc basin system from South Atlantic Ocean, the Mariana Trough (e.g., Fiji and Tonga basin) from the Western Pacific and Coast Range Ophiolite from California.

The Late Jurassic Zedong ophiolite: A remnant of subduction initiation within the Yarlung Zangbo Suture Zone (southern Tibet) and its tectonic implications

The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.

Geochemistry of arc-related mantle peridotites and gabbros from the Chaldoran ophiolite, NW Iran

ABSTRACT The Neo-Tethys-related Chaldoran ophiolite peridotites in NW Iran are remnants of mantle lithosphere, exhumed tectonically during the Late Cretaceous. Harzburgite is the predominant peridotite type, associated with oceanic lower crust cumulate gabbros occasionally. The ophiolite rocks are unconformably overlain by Late Cretaceous-Paleocene sediments. New whole-rock geochemistry of the variably serpentinized harzburgites shows a depleted nature, exemplified by low Al2O3, CaO, TiO2, V and Y and high Ni, Cr and Mg and also low rare earth element (REE) contents. The harzburgites present LREE enrichment. Positive correlations between some LREEs and high field strength elements (HFSE) suggest enrichment of LREEs by melt re-fertilization processes. Cr-spinels have Cr number of [Cr# = Cr/(Cr + Al) = 0.53–0.67], showing medium to high degree of partial melting (F = ~17-20%). Both whole-rock and mineral chemistry data show a supra-subduction zone setting and progressive depletion along with increase in spinel Cr# (MOR to fore arc). The cumulate gabbros have high MgO and SiO2, low TiO2 and Ti/V < 10 and also low chondrite normalized Dy (<8.5). The gabbro samples show enriched LREEs and LILEs and depleted HREEs and HFSEs with respect to MORBs. Subduction initiation (SI) model in a fore-arc/proto-fore-arc environment is suggested for the upper mantle evolution of the Chaldoran ophiolite. The rocks have experienced depletion in a second melting process at the later stages of SI and compositions were probably modified by extraction of island arc tholeiitic (IAT) and possibly boninitic (BON) melts. The chemostratigraphic progression for ‘subduction initiation rule (SIR)’ is likely traceable in Chaldoran mafic-ultramafic sequence, which corresponds to the most Neo-Tethyan ophiolites and is similar to MOR to supra-subduction zone (SSZ) evolution of most Iranian ‘Inner’ and ‘Outer Zagros’ ophiolitic peridotites.

Petrology and Geochemistry of Mafic Intrusive Rocks from the Sapi-Shergol Ophiolitic Mélange, Indus Suture Zone, Western Ladakh: Constraints on Petrogenesis and Tectonic Setting

This article reports the existence of subduction-related tholeiitic, normal mid-ocean ridge basalt (N-MORB)-type mafic intrusives emplaced within the Sapi-Shergol ophiolitic mélange of the Indus Suture Zone, western Ladakh. The Shergol mafic intrusives show Fe enrichment with basalt to basaltic-andesite composition. Based on their mineralogy and textures, these mafic rocks can be identified as fine- to medium-grained gabbros that have undergone greenschist-grade metamorphism. These rocks have N-MORB-type geochemical characteristics, exhibiting nearly flat to depleted light rare earth element patterns ((La/Sm)N=0.66–1.05). Petrogenetic modeling suggests <20% partial melting of a depleted MORB-type mantle source, within the spinel peridotite stability. The presence of slightly negative anomalies of high field strength elements like Nb, Zr, and Ti in multielement patterns reflect the influence of subduction zone magmatism. The presence of low-Ti clinopyroxene (En38–50Fe11–25Wo31–43; enstatite-ferrosilite-wollastonite), Ca-rich plagioclase (An2–36; andesine), and pargasitic amphibole also reflects their subduction-related depleted-mantle origin. The MORB–island arc tholeiite signature displayed by the Shergol gabbros intrusive in Shergol peridotites reflect their generation in a mantle wedge associated with the Early Cretaceous intraoceanic subduction within the Neo-Tethys Ocean. They are similar to Spongtang ophiolite gabbros intrusive in the Spongtang ophiolite mantle peridotites from south Ladakh, ophiolitic mélange gabbros from southern Tibet, and Muslim Bagh ophiolite gabbros from Pakistan. Based on this study, we offer a geodynamic model suggesting that the Sapi-Shergol ophiolitic slice was intruded by mafic intrusive rocks that represent the relict of the intraoceanic substratum of the Cretaceous Dras arc complex.

Neoarchaean subduction tectonics in Western Shandong Province, China: Evidence from geochemistry and zircon U–Pb–Hf isotopes of metabasalts

Neoarchaean supracrustal rocks, which were discovered in the Longhuzhai and Menglianggu areas of the Western Shandong Province (WSP) in the Eastern Block of the North China Craton (NCC), are fine‐ to medium‐grained amphibolites that commonly underwent greenschist‐ to amphibolite‐facies metamorphism. LA‐ICP‐MS zircon U–Pb analyses revealed that these metabasalts erupted at 2,527 ± 5 to 2,514 ± 4 Ma. These supracrustal rocks are tholeiitic basalts in chemical composition and exhibit relatively high SiO2 (49.30–53.34 wt%) and MgO (5.24–11.82 wt%) contents but low TiO2 (0.41–0.75 wt%) contents. They display flat chondrite‐normalized rare earth element (REE) patterns ((La/Yb)N = 1.21–2.60) and negative Nb, Ta, and Ti anomalies ((Nb/La)PM = 0.16–0.54), which are similar to those of typical island arc tholeiites (IATs) in subduction‐related settings. These findings, combined with the positive zircon εHf(t) values (+2.51 to +8.90) and εNd(t) values (+0.19 to +3.65), suggest that these metabasalts were derived from the partial melting of a depleted mantle source that was metasomatized by slab‐derived fluids at shallow levels in the upper mantle. Accordingly, the development of arc‐related tholeiitic volcanic rocks within the interior of the WSP imply that the Neoarchaean (2.6–2.5 Ga) subduction‐related slab–wedge interaction mechanism was widespread in the WSP and played a crucial role in the formation of the continental crust and its growth.

Geochemistry and tectonic setting of the volcanic host rocks of VMS mineralisation in the Qezil Dash area, NW Iran: implications for prospecting of Cyprus-type VMS deposits in the Khoy ophiolite

The uppermost part of the Khoy ophiolite includes submarine volcanic lavas. These rocks are found as two different types including basaltic pillow lavas and andesitic to andesitic basalt massive-sheet lavas known as the Qezil Dash unit. These last rocks host volcanogenic massive sulphide mineralisation (VMS). Concordant small lenticular massive sulphides associated with widespread stockworks, extensive wall rock alteration and simple mineralogical parageneses are characteristic of this mineralisation. The Qezil Dash lavas are characterized by HFSE depletion (such as in Nb, Zr, and Hf) and LILE enrichment, low Yb, Y and Ni contents and low Zr/Hf and Nb/Ta ratios. These rocks have low Ti contents and are of LKT type. These geochemical characteristics can be interpreted as indicating SSZ magmatism and suggesting that the Qezil Dash rocks belong to an early stage of island arc formation. Geochemically, this host rock is comparable with the Lasail unit, part of a volcanic sequence within the Oman ophiolite. In the Troodos and Oman ophiolites VMS mineralisation has occurred at the contacts of MOR-basalt and island arc tholeiite (IAT) lavas, so prospecting for VMS deposits in the Khoy area should be established based on geochemical investigations of the volcanic rocks and recognition of their field boundaries.

Pillow lava basalts with back-arc MORB affinity from the Usagaran Belt, Tanzania: relics of Orosirian ophiolites

The Paleoproterozoic Usagaran Orogenic Belt in East Africa is an exemplary site of preserved relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic crust. This paper describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group). Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist-facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid-immobile elements. The (La/Sm) N ratios (0.62–1.09) and REE patterns point to normal mid-ocean ridge basalt (N-MORB) and transitional MORB (T-MORB) mantle source. The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts are an analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian. The emplacement of the Konse pillow basalts is older than the neighbouring 1920–1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Thus, the Konse pillow basalts might have been displaced from their suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Mid-ocean ridge vs. forearc and subduction settings: Clues from rodingitization of tectonic fragments in the Neoproterozoic ophiolites of the Eastern Desert, Egypt

Tectonic mélange fragments (metabasalt and metadiabase) in the ophiolitic serpentinites of the Egyptian Eastern Desert show peculiar mineral assemblages and they provide useful insights into metamorphism and metasomatism. New field and mineralogical data indicate the formation of rodingite at the expense of mid-ocean ridge basalt (MORB) at three localities, namely Wadi Sikait, Wadi Abu Rusheid and Hafafit (SRH belt). These rocks occupy a lower structural horizon in the tectonic mélange zone(s), whereas ocean-island and arc basalt and diabase (OIB) is relatively younger with no evidence of rodingite formation. Rodingitization starts at slow-ultraslow spreading centers and continued until subduction and exhumation at an accretionary wedge. It is a process that is characterized by Ca-rich fluids and the development of a complicated “blackwall” due to superimposed K+ and Mg2+-metasomatism. Oxygen fugacity is fluctuating where it is high for Hafafit and low for Wadi Abu Rusheid rodingites. The Ta/Yb-Th/Yb ratios suggest that rodingitization is not contemporaneous with serpentinization in an island-arc environment but with serpentinization near seamounts at the spreading centers. Combined field observations, whole-rock geochemistry and mineral chemistry data prove that rodingites are formed at the expense of a MORB (mid-ocean basalt) protolith and occupies a lower structural position than unrodingitized IAT (island-arc tholeiite) metadiabases.

Origin and geodynamic significance of the Siuna Serpentinite Mélange, Northeast Nicaragua: Insights from the large-scale structure, petrology and geochemistry of the ultramafic blocks

The Siuna Serpentinite Mélange (SSM) is located in the boundary zone between continental Chortis block and oceanic Mesquito Composite Terrane in NE Nicaragua. In this paper, we present new large-scale structure, meso-scale kinematic analysis, 40Ar39Ar geochronology, mineral chemistry, in situ trace element composition of clinopyroxene, and bulk rock geochemical data for representative ultramafic rocks preserved as blocks in the SSM. Structurally, the mélange is a > 2.5 km-thick tectonic unit characterized by a disrupted block-in-matrix fabric with occurrence of exotic blocks, which have igneous and sedimentary protoliths and were variably metamorphosed to high-pressure (P) conditions. There are also blocks of ultramafic rocks, which consist mainly of harzburgite and clinopyroxene (Cpx)-rich harzburgite, with minor lherzolite and pyroxenite, and rare dunite and chromitite. In these rocks, relics of bastite and mesh textures of lizardite are replaced by antigorite blades and overprinted by antigorite schistosity.The presented data give rise to argue the following points. (1) The serpentinite matrix of the mélange resulted from hydration, ductile deformation and metamorphism at high-P conditions of abyssal and supra-subduction zone (SSZ) mantle peridotites in a subduction channel. (2) The geochemical characteristics of the ultramafic protoliths can be explained by at least two-stages of melting and refertilization processes: the first-stage occurred in a mid-ocean ridge (MOR) environment and produced Cpx-rich harzburgite and the second-stage took place in an SSZ setting and resulted in harzburgite, dunite and chromitite. (3) The second-stage magmatic processes produced melts that range in composition between island-arc tholeiites (IAT) and boninites, similar to arc-like protoliths of some mafic blocks in the mélange, which establishes a genetic link between blocks and matrix. (4) The presence of ultramafic and mafic blocks in the SSM, derived from both an island-arc upper plate and a MOR-like lower plate sources, suggests its position in an intra-oceanic subduction zone during the late Jurassic-early Cretaceous. (5) By the Hauterivian, the SSM became exhumed and tectonically emplaced onto the Chortis block as a consequence of the collision between the intra-oceanic arc and the southern continental margin of North America.

The origin and emplacement of the Lajishankou ophiolite complex in the South Qilian belt: evidences from geological mapping

AbstractMany ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.

Tipe Magmatik Batuan Beku Formasi Gabon di Tinggian Karangbolong, Kebumen

Karangbolong High located in Java Southern Mountain range is dominated by volcanic rocks of Gabon Formation in the form of volcanic breccia, some intrusion bodies and lavas. Research on the type of magma at Karangbolong high has never been done. This activity is intended to know the distribution, change the type of magma and tectonic position.This research includes field sampling and laboratory work which includes petrographic analysis using polarization microscope and geochemistry analysis 4 litho ICP-WRA for major element and ICP-WRA4B2 for trace elements and REE. Results of geochemistry analysis are.plot in Total Alkali Silica (TAS) diagrams, AFM diagram , SiO2 vs FeO(t) / MgO diagram, SiO2 Vs K2O and the spider diagram.for trace elements and REE.Most of the igneous rocks at Karangbolong Region are andesite pyroxene, basaltic andesite and slightly basalt olivine. To the northward, igneous rocks are tend to more acid as reflected by the increase of SiO2, Na2O and K2O and the decreasing of MgO and Fe2O3 content. The affinity of the magma in the northern part is mostly Calc-Alkaline series (KR-18, KR-28, KR-6, KR-23) and slightly Island Arc Tholeiite as intrusion and lava at southern part (KR-33 and KR- 17). The tectonic position of rock formation resides in Island Arc Plate Margin. The Calk-alkaline magma is enriched with the light rare earth elements (LREE) compared to the Tholeiite magma, while trace element Sr, K, Rb, Ba, and Th strongly enriched in.Keywords : Karangbolong High, igneous rock, magmatic type, tectonic position

Tipe Magmatik Batuan Beku Formasi Gabon di Tinggian Karangbolong, Kebumen

Karangbolong High located in Java Southern Mountain range is dominated by volcanic rocks of Gabon Formation in the form of volcanic breccia, some intrusion bodies and lavas. Research on the type of magma at Karangbolong high has never been done. This activity is intended to know the distribution, change the type of magma and tectonic position.This research includes field sampling and laboratory work which includes petrographic analysis using polarization microscope and geochemistry analysis 4 litho ICP-WRA for major element and ICP-WRA4B2 for trace elements and REE. Results of geochemistry analysis are.plot in Total Alkali Silica (TAS) diagrams, AFM diagram , SiO2 vs FeO(t) / MgO diagram, SiO2 Vs K2O and the spider diagram.for trace elements and REE.Most of the igneous rocks at Karangbolong Region are andesite pyroxene, basaltic andesite and slightly basalt olivine. To the northward, igneous rocks are tend to more acid as reflected by the increase of SiO2, Na2O and K2O and the decreasing of MgO and Fe2O3 content. The affinity of the magma in the northern part is mostly Calc-Alkaline series (KR-18, KR-28, KR-6, KR-23) and slightly Island Arc Tholeiite as intrusion and lava at southern part (KR-33 and KR- 17). The tectonic position of rock formation resides in Island Arc Plate Margin. The Calk-alkaline magma is enriched with the light rare earth elements (LREE) compared to the Tholeiite magma, while trace element Sr, K, Rb, Ba, and Th strongly enriched in.Keywords : Karangbolong High, igneous rock, magmatic type, tectonic position

Tipe Magmatik Batuan Beku Formasi Gabon di Tinggian Karangbolong, Kebumen

Karangbolong High located in Java Southern Mountain range is dominated by volcanic rocks of Gabon Formation in the form of volcanic breccia, some intrusion bodies and lavas. Research on the type of magma at Karangbolong high has never been done. This activity is intended to know the distribution, change the type of magma and tectonic position.This research includes field sampling and laboratory work which includes petrographic analysis using polarization microscope and geochemistry analysis 4 litho ICP-WRA for major element and ICP-WRA4B2 for trace elements and REE. Results of geochemistry analysis are.plot in Total Alkali Silica (TAS) diagrams, AFM diagram , SiO2 vs FeO(t) / MgO diagram, SiO2 Vs K2O and the spider diagram.for trace elements and REE.Most of the igneous rocks at Karangbolong Region are andesite pyroxene, basaltic andesite and slightly basalt olivine. To the northward, igneous rocks are tend to more acid as reflected by the increase of SiO2, Na2O and K2O and the decreasing of MgO and Fe2O3 content. The affinity of the magma in the northern part is mostly Calc-Alkaline series (KR-18, KR-28, KR-6, KR-23) and slightly Island Arc Tholeiite as intrusion and lava at southern part (KR-33 and KR- 17). The tectonic position of rock formation resides in Island Arc Plate Margin. The Calk-alkaline magma is enriched with the light rare earth elements (LREE) compared to the Tholeiite magma, while trace element Sr, K, Rb, Ba, and Th strongly enriched in.Keywords : Karangbolong High, igneous rock, magmatic type, tectonic position

Geochemical constraints on the origin of Neoarchean magmatic rocks in the Lüliang Complex, North China Craton: Tectonic implications

The North China Craton suffered a strong tectonothermal event in the late Neoarchean, with its crystalline basement dominantly composed of ca. 2.6–2.5 Ga granitoid gneisses and metamorphosed volcano-sedimentary sequences. However, controversial issues have long existed on the geodynamic setting of this event. We present new in-situ LA-(MC-) ICP-MS zircon U-Pb and Lu-Hf isotope data, and whole-rock geochemical data for Neoarchean magmatic rocks from the Lüliang Complex. Zircon U-Pb dating reveal that the basalts, andesites and felsic volcanics yield crystallization ages of 2535–2486 Ma, followed by crustal anatexis and metamorphism at ∼2489 Ma. The basalts and andesites yield positive εHf(t) values of 5.1–11.6 and 2.2–6.5 for zircons, corresponding to average Hf model ages of 2547 Ma and 2660 Ma, respectively. The felsic volcanic rock yields positive zircon εHf(t) values of 1.4–6.3, with Hf model ages averaging 2775 Ma. Such ages are consistent with 207Pb/206Pb ages of 2.8–2.7 Ga for relict zircons, indicating reworking of crustal rocks. The basalts and andesites have geochemical compositions of island arc tholeiite (IAT) and calc-alkaline basalt (CAB), suggesting that they were both derived from partial melting of subarc mantle sources that were generated by metasomatic reaction of the mantle wedge peridotite with subducting crust-derived fluids. Compared with the basalts, the andesites were also originated from a subarc mantle source with incorporation of more subducting sediment-derived melts. Thus, both the basalts and andesites would most likely form in a continental arc setting. In this regard, plate subduction and arc magmatism in the North China Craton can be traced back to at least 2.65 Ga in the Lüliang area. The 2.6–2.5 Ga tectonothermal event manifested as widespread accretionary orogeny in the North China Craton, resulting in significant growth of the juvenile mafic crust as well as extensive reworking of the old basement dominated by 2.8–2.7 Ga crustal rocks.

Cretaceous tectonic evolution of the Neo-Tethys in Central Iran: Evidence from petrology and age of the Nain-Ashin ophiolitic basalts

The Nain and Ashin ophiolites consist of Mesozoic mélange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent (CEIM). They largely consist of serpentinized peridotites slices; nonetheless, minor tectonic slices of sheeted dykes and pillow lavas - locally stratigraphically associated with radiolarian cherts - can be found in these ophiolitic mélanges. Based on their whole rock geochemistry and mineral chemistry, these rocks can be divided into two geochemical groups. The sheeted dykes and most of the pillow lavas show island arc tholeiitic (IAT) affinity, whereas a few pillow lavas from the Nain ophiolites show calc-alkaline (CA) affinity. Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting. Petrogenetic modeling shows that these components were represented by pure aqueous fluids, or sediment melts, or a combination of both, suggesting that the studied rocks were formed in an arc-forearc tectonic setting. Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous. Previous tectonic interpretations suggested that the Nain ophiolites formed, in a Late Cretaceous backarc basin located in the south of the CEIM (the so-called Nain-Baft basin). However, recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic, which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location. This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction, below the CEIM, of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia. As a major conclusion of this paper, a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.