Ophiolitic Melange Research Articles

Geochronology, geochemistry and tectonic significance of the Dashizhai ophiolitic mélange belt, southeastern Xing’an–Mongolia orogenic belt

The Xing’an–Mongolia orogenic belt places at the southeastern margin of the Central Asian orogenic belt, is important for unraveling the tectonic evolution of the accretionary orogens. The validity of existing models that account for the development and closure of the Paleo-Asian Ocean critically depends on the assumed initial configuration and evolution process of the various microcontinents that were separated by the ophiolitic belts or suture zones, and deep faults. In this work, we present a newly discovered Dashizhai ophiolitic melange belt in the Ulanhot region, and systematically investigate the occurrence through petrology and geochronology of this ophiolitic melange. The Dashizhai ophiolitic melange belt is a typical SSZ-type ophiolitic melange, has a block-in-matrix structure, the blocks include ophiolitic ultramafic rock, volcanic rock, siliceous rock and gabbro in a matrix of deformed Early Permian slate. Our new geochronology data suggest that the Dashizhai ophiolitic melange belt should form in the early Permian or younger and emplaced before the late Permian deposition. By focusing on the regional geologic history of the southeastern Xing’an–Mongolia orogenic belt, the Dashizhai ophiolitic melange belt together with northeast-migrating Hehei fault zone were suggested as the tectonic boundary between the Xing’an and Songliao–Xilinhot blocks.

Geochemical and Sr-Nd-Pb isotopic characteristics of extrusive rocks within the Çetmi Ophiolitic Mélange, Biga Peninsula

The different geochemical characteristics of the extrusive rocks of the Cetmi Ophiolitic Melange (COM) a remnant of the Intra-Pontide Suture (IPS) in the Biga Peninsula, NW Turkey, evidence the episodic formation of oceanic crust. Geochemically, volcanic rocks are divided into three groups: ocean island basalts (OIB), enriched mid-ocean ridge basalts (E-MORB) and basalts with supra-subduction zone (SSZ) affinity. While the OIB type samples were produced by small degrees partial melting from an enriched source, the E-MORB type samples point to mixing of enriched and depleted sources. In the OIB-type samples, the depletion of Zr and Hf compared to Nd as well as enrichment of Nb-Ta are attributed to presence of the amphibole in the mantle source. Radiogenic Pb isotope compositions towards EMII support metasomatic processes related to fractionation in the mantle reservoir. The SSZ samples are characterized by enriched light rare earth elements (LREE) and depleted high field strength elements (HFSE). The trace element characteristics of the SSZ samples indicate that a depleted MORB mantle source interacted with subduction-derived fluids due to the slab dehydration processes. Besides, the Sr-Nd-Pb isotope characteristics suggest that these samples originated from a MORB-type mantle source metasomatized by fluids or melts from subducted sediments. The major, trace element and isotopic data of the volcanic rocks in the COM indicate that the oceanic lithosphere is generated along a SSZ-type spreading centre at an arc-basin system within the Intra-Pontide Ocean.

Structure and tectonics of a Mesoproterozoic ophiolite – Insight from Kanigiri Ophiolite with a mélange zone, southern India

The ~1334 Ma Kanigiri Ophiolite (KO) occurring within the Nellore Schist Belt (NSB) at the Eastern Dharwar Craton (EDC) margin, India provides clues to tectonics of the paleo-ocean outboard of Mesoproterozoic Indian craton. The KO shows a diverse assemblage of fragmentary remains of the then oceanic crust including massive gabbro-anorthosite from the lower part, through a tectonic mélange, to 2–3 km thick slivers of upper oceanic crust with vesicular to pillowed basalt, mafic agglomerate, and mafic to felsic tuff with thin sedimentary intercalation. The mélange consists of up to meter size blocks of mafic igneous rocks and larger rafts of pelagic chert set in a fine grained, weakly foliated mafic matrix showing low grade metamorphism and hydrothermal alteration. The volcanic components of the KO are ultrapotassic to potassic, while the gabbroic rocks show high Ti/V, high Th/Nb ratios, high Y, and enrichment in Th compared to Hf and Ta, suggesting suprasubduction zone oceanic arc setting. Restricted to N-S elongated fault bounded block, the KO shows internal deformation with two sets of folds and tectonic foliation in the upper part and common block-in-matrix structure with gabbroic blocks in the middle part, possibly representing an accretionary wedge. The KO is tectonically juxtaposed against multiply deformed shallow marine succession of the Udaigiri Group (NSB), where second generation folds and crenulation cleavage compare with those in the KO metavolcanics. Post- tectonic granitic stocks of ~1284 Ma age intrude the ophiolitic melange and adjoining rocks of the NSB, suggesting ocean closure and accretionary growth of the EDC margin during the Mesoproterozoic. Components of the EDC margin were fused together during the 1100–900 Ma Rodinia amalgamation with collisional suturing between India and Antarctica. The earlier supercontinent build-up namely Columbia (or Nuna), probably left traces of an archipelago like paleogeographic scenario outboard of the EDC.

New radiolarian data from the Jurassic ophiolitic mélange of Avala Mountain (Serbia, Belgrade Region)

An ophiolitic melange of Avala Mountain located south of Belgrade was studied to obtain age data from blocks of radiolarian cherts and to enable better understanding of age and nature of this melange. The studied area is located south of Avala Mt. and its geotectonic position is rather obscure. The obtained data allow us to determine radiolarians from five samples; one is of Late Anisian to Early Ladinian, and four of them are of Middle Jurassic (Late Bathonian–Early Callovian) age. The finding of Triassic as well as Jurassic age radiolarians in one and the same melange suggests that this melange formed during obduction of the Western Vardar ophiolites.

Geochemistry and geochronology of the Shihuiyao ophiolite from the Kelameili suture in East Junggar: Constraint on the tectonic evolution of the Kelameili Ocean

The Kelameili suture located between the eastern Tianshan Junggar Plate and the Harlik island arc in east Tianshan. This suture is well known as identifying the Kelameili ophiolite and the Tucker Takezale ophiolitic melange. This paper first reports a new discovery of the Shihuiyao ophiolite, which is 70 km away from the Takezale ophiolitic mélange to the east. It is mainly composed of the variable serpentinized peridotite, metagabbro, and minor siliceous rocks. The serpentinite and gabbros are characterized by high content of Al2O3 and normal mid‐ocean ridge basalt (N‐MORB)‐type rare earth element compositions. These rocks also show a typical characteristic of origin of island arc setting with depletion of high‐field strength element (HFSE) and enrichment of large‐ion lithophile element (LILE). The LA‐ICP‐MS dating of zircon grains from gabbros gave a weighted mean 206Pb/238U age of 352 Ma, which was interpreted to represent the age of the Kelameili Ocean. This paper provided an important constraint on the tectonic evolution of the Kelameili Ocean.

Transformation from Neoproterozoic Sinistral to Early Paleozoic Dextral Shearing for the Jingdezhen Ductile Shear Zone in the Jiangnan Orogen, South China

The Jingdezhen ductile shear zone is evolved from the Neoproterozoic Zhangyuan ophiolite melange belt in the eastern Jiangnan Orogen, South China. Comprehensive study of geometry, kinematics, quartz c-axis fabric, temperature-pressure conditions and geochronology were conducted in this study. The Jingdezhen shear zone extends ∼180 km along the NE orientation with two groups of subvertical foliation and subhorizontal lineation. One group of foliation strikes NEN orientation whereas another one NEE orientation. Field investigation, microscopic observation and quartz c-axis fabric show that sinistral shearing along NEN-striking foliation occurred earlier than dextral shearing along NEE-striking foliation. Syn-tectonic staurolite porphyroblasts and deformation manner of feldspar imply that sinistral shearing occurred at 530–420 oC and 6–2 kbar. Deformation manner and c-axis fabric of quartz and pre-tectonic staurolite porphyroblasts indicate that dextral shearing took place at 420–300 oC. LA-ICP-MS zircon U-Pb and mica 40Ar/39Ar dating indicate that the sinistral shearing occurred during Neoproterozoic orogeny (830–800 Ma) whereas the dextral shearing at 447±12 Ma. The sinistral shearing resulted from the Neoproterozoic final assembly between the Yangtze and Cathaysia blocks. The dextral shearing was caused by Early Paleozoic orogen parallel extension and clockwise rotation.

Detrital Zircon U-Pb Geochronology from Greywackes in the Niujuanzi Ophiolitic Mélange, Beishan Area, NW China: Provenance and Tectonic Implications

The Niujuanzi ophiolitic melange represents the remnant of oceanic crust between the Dunhuang massif and Mingshui-Hanshan massif. Greywacke from different tectonic slices in the Niujuanzi ophiolitic melange were analyzed for detrital zircon U-Pb geochronology, trace elements and whole-rock trace elements to infer their provenance and the evolution of the Niujuanzi Ocean. Sample N-76s containing Carboniferous spores has the youngest zircon age of 323 Ma, while sample N-85s without fossils has the youngest zircon age of 449 Ma. The two samples were deposited no earlier than 323 and 449 Ma, respectively. The greywackes are depleted in large ion lithophile elements, and are relatively enriched in high field strength elements. The age spectra and trace element concentrations indicate that the sediments may have been deposited near the trench. The Hf, U, and Yb contents of zircons from sample N-76s vary widely, whereas those from sample N-85s have a narrow range. Sample N-76s contains both continental and oceanic zircons, while sample N-85s contains only continental zircons. The sediments were derived from the continental arc and accretionary wedge. The Paleozoic oceanic crust zircons have ages of 430–500 Ma, indicating the timing of the expansion of the Hongliuhe-Niujuanzi-Xichangjing Ocean expansion. The oldest Paleozoic continental zircon has an age of 470 Ma, suggesting that the northward subduction of the oceanic crust may have started at that time.

Emendation of the Grivska formation in its type area (Dinaridic Ophiolite Belt, SW Serbia)

Age, microfacies and depositional realm of the Grivska Formation is controversially discussed due to the fact that detailed investigations are missing. Based on reinvestigations of the type locality of the Grivska Formation and in adjacent areas, the following results can be drawn: 1) The Grivska Formation is of Late Triassic (Early Carnian to Rhaetian) age according to conodont dating. 2) Sedimentological and microfacies studies evidenced that the Grivska Formation was deposited on the continental slope and transitional to the Neo-Tethys Ocean. Based on the results of these investigations in the type area and several reference sections in the Dinaridic Ophiolite Belt, the Grivska Formation is emended and clearly defined. In the Dinaridic Ophiolite Belt, the Grivska Formation occurs only as clasts and blocks in the ophiolitic melange.

FORMATION MECHANISMS OF FE-MN CONCRETIONS IN THE VIJENAC QUARRY, DINARIC OPHIOLITE ZONE

The Vijenac limestone quarry, near Tuzla in Bosnia and Herzegovina, is composed of carbonate rocks locally embedding tectonically disturbed siltite and sandstone with Fe-Mn concretions. The quarry itself represents a part of Dinaric overstep sequences (the Pogari Formation) unconformably overlying ophiolite melange and ophiolite trust-scheets. Petrographic, chemical and mineralogical analyses had shown that the concretions may be divided into two types: (i) Mn-rich concretions with ≈ 17 wt.% of Mn and compact texture and (ii) Mn–poor with ≈ 8 wt.% of Mn and porous texture. The amount of Ca, Fe and Mg in both concretion types are similar. Nickel and Cr positively correlate with Fe and Mn, respectively. According to petrographic and mineralogical analyses, concretions are composed of calcite, dolomite, hematite, todorokite and takanelite. Petrographic study confi rmed the development of concretions within three stages including two generations of calcite. Studied concretions are formed within consolidated sandstones inheriting their sedimentary textures – therefore a late diagenetic process is assumed.

Late Paleozoic and Mesozoic evolution of the Lhasa Terrane in the Xainza area of southern Tibet

Models for the Mesozoic growth of the Tibetan plateau describe closure of the Bangong Ocean resulting in accretion of the Lhasa terrane to the Qiangtang terrane along the Bangong-Nuijiang suture zone (BNSZ). However, a more complex history is suggested by studies of ophiolitic melanges south of the BNSZ “within” the Lhasa terrane. One such mélange belt is the Shiquanhe-Namu Co mélange zone (SNMZ) that is coincident with the Geren Co-Namu Co thrust (GNT). To better understand the structure, age, and provenance of rocks exposed along the SNMZ we conducted geologic mapping, sandstone petrography, and U-Pb zircon geochronology of rocks straddling the SNMZ. The GNT is north-directed and places Paleozoic strata against the Yongzhu ophiolite and Cretaceous strata along strike. A gabbro in the Yongzhu ophiolite yielded a U-Pb zircon age of 153Ma. Detrital zircon age data from Permian rocks in the hanging wall suggests that the Lhasa terrane has affinity with the Himalaya and Qiangtang, rather than northwest Australia. The Late Jurassic- Early Cretaceous Rila Formation overlies the ophiolite and shows signatures of syntectonic deposition. Detrital zircon ages from the Cretaceous Duoni Formation in the footwall of the GNT suggest that the provenance is mainly Early Cretaceous igneous rocks in the northern Lhasa terrane and Paleozoic strata in the hanging wall of the GNT. Our results support the interpretation that the SNMZ is a Late Jurassic-Early Cretaceous suture zone that resulted from closure of a small Jurassic ocean basin, thereby suggesting more Jurassic shortening than conventional models. The GNT developed along the SNMZ in the Early Cretaceous and facilitated exhumation of Paleozoic strata, which became a source for Cretaceous basins along the GNT. Early Cretaceous igneous rocks in northern Lhasa are interpreted to be a consequence of subduction of ocean lithosphere along the SNMZ and its later delamination.

In situ gamma ray measurements for deciphering of radioactivity level in Sarıhan pluton area of northeastern Turkey

The present work utilizes in situ gamma ray spectrometric measurement data to map the surface geology of Sarihan Granitoid and its surrounding area. The study area comprises three different lithological units, namely, Hozbirikyayla Formation (limestone and sandy limestones), Sarihan Granitoid (consist of quartz monzodiorite, granodiorite and quartz diorite) and the Ophiolitic olistostromal melange (andesite, basalt, sandstone, gravelly sandstone). When lithological units are assessed according to the radioactivity characteristics, natural radionuclide contents (40K and radionuclides from 238U and 232Th series) of Hozbirikyayla limestones and ophiolitic melange rocks are lower than the Sarihan pluton. The U, Th and K radionuclide contents were found to be 0.8–5.4 ppm, 10.1–33.6 ppm and 1.29–4.41% in the Sarihan plutonic area and 0.9–5.3 ppm, 1.1–20.3 ppm and 0.04–2.71% in Horzbirikyayla formation, respectively. The element concentrations of 238U, 232Th and 40K of the Ophiolitic melange are 1.1–4.5 ppm, 1.6–25.3 ppm and 0.09–3.63%, respectively. Radioelement ratio maps are created for the studied area, because the parameters of radioelement ratios, eU/eTh, eU/K and eTh/K, reflect the radioactive characters of the rock and soil. The Hozbirikyayla Formation is characterized by the highest value of eU/eTh and lowest value of eTh/K. While the lowest eU/eTh and eU/K ratios were observed in the Sarihan Granodiorite, the highest value of eU/K and eTh/K were obtained in the Ophiolitic olistostromal melange. By comparing these maps with the geology, it was found that the radioelement concentrations are in good agreement with the geological properties of the region. In addition to this, the radiation hazard parameters were evaluated to assess the radiation hazard for people living in this area. It has also been found that there is no significant radiologic hazard for humans and the environment in and around studied area.

Open-marine Hallstatt Limestones reworked in the Jurassic Zlatar Mélange (SW Serbia): a contribution to understanding the orogenic evolution of the Inner Dinarides

Varied west-transported and far-traveled Jurassic melanges in southwestern Serbia represent a key to understand the geodynamic history and to solve paleogeographic questions and reconstructions in the Triassic–Jurassic passive and active margin arrangement of the Inner Dinarides. Of special interest are the carbonate-clastic radiolaritic melange areas in the Zlatar Mountain below the Dinaridic Ophiolite nappe. The present study reports from a Middle Jurassic sedimentary melange in the area of Vodena Poljana. Carbonate components and blocks of the mass-flow deposits consist exclusively of a reworked Middle/Late Anisian to Early Jurassic distal shelf succession. Ophiolite components from the Dinaridic Ophiolite nappe stack are missing in the spectrum. The underlying series of the Zlatar Melange belong to Early/Middle Anisian shallow-water carbonates and to Late Anisian to Middle Jurassic deep-water sedimentary rocks of the Hallstatt facies zone. South of Vodena Poljana in the overlying ophiolitic melange occur Late Triassic radiolaritic components from the sedimentary cover of the Late Triassic ocean floor, beside ophiolite clasts and limestone components from the continental slope. A comparison with preserved Hallstatt Limestone successions and Jurassic melange complexes from the Eastern Alps, Western Carpathians, and Albanides strengthen the interpretation of a provenance of the Zlatar melange from the distal passive margin facing the Neotethys Ocean to the east. An autochthonous Dinaridic Ocean west of the Drina-Ivanjica Unit cannot be confirmed.

Petrography, mineral chemistry and genesis of peridotite in the Ratouk ophiolitic melange (East of Iran)

Petrography, mineral chemistry and genesis of peridotite in the Ratouk ophiolitic melange (East of Iran)

THE GEOLOGICAL EVOLUTION OF SORGUN (YOZGAT)-YILDIZELİ (SİVAS) FORELAND BASIN, PETROGRAPHIC, GEOCHEMICAL ASPECTS AND GEOCHRONOLOGY OF VOLCANISM AFFECTING THE BASIN

Sorgun-Yıldızeli basin is an east-west trending asymmetric marginal foreland (peripheral foreland basin) formed as a result of the consumption of oceanic crust of the northern branch of Neotethys due to the collision of Sakarya continent in the north and Kırşehir Block in the south. It provides much information about the geodynamic evolution of the region. The basement of the study area consists of Late Palaeozoic-Mesozoic Akdağmadeni Massif. Akdağmadeni Massif was intruded by CenomanianMaastrichtian granitoids and is overlain tectonically by Late Cretaceous Artova ophiolitic melange within the İzmir-Ankara-Erzincan suture zone and by Cenomanian-Maastrichtian Darmik Formation. The volcano-sedimentary sequence deposited in Sorgun-Yıldızeli foreland basin developed on the relicts of the İzmir-Ankara-Erzincan Suture Zone along the northern margin of Kırşehir Block and was named the Boğazköy Formation. The pelitic, clastic and coarse clastic levels of Upper Palaeocene-Middle Eocene Boğazköy Formation including İzmir-Ankara-Erzincan Suture Zone slices and olistostromes are differentiated as the Dolak member. Toward the inner part of the basin, a turbiditic sequence of conglomerate, sandstone, claystone including rare limestone beds was deposited, consisting of conglomerate, sandstone, claystone, rarely limestone and alternations of three different volcanic rocks (acidic, basic and intermediate) related to subduction and/or collision. Calc-alkaline lava and pyroclastics of basaltic and basalt-andesite composition were differentiated as the Pazarcık volcanic member, with lava and pyroclastics of dacitic and rhyolitic composition called the Sarayözü volcanic member and calcalkaline lava and pyroclastics of andesitic, trachyandesitic and dacitic composition called Kiremitlik volcanic member in Sorgun-Yıldızeli basin. According to the 40Ar/39Ar geochronologic method, the ages of 57.2±2.0 Ma and 56.7±1.8 Ma were obtained for Pazarcık volcanics, 48.8±1.5 Ma for Sarayözü volcanics and 45.1±1.3 Ma and 47.3±0.6 Ma for Kiremitlik volcanics. The reefal limestones which are not thick and occur in higher parts of the Boğazköy Formation were called the Limestone member. The uppermost section of the sequence which consists of generally clastic and coarse clastics is named the Konacı member. When the sedimentation and volcanism continued in the basin, gabbroic intrusions occurred as sills and laccoliths cutting the Boğazköy Formation. These gabbroic rocks were named the Yaycılar Gabbro. The age of 51.0 ± 0.7 Ma was found for the Yaycılar Gabbro with the 40Ar/39Ar geochronologic method. As a result of slab breakoff, the relicts of the consumed oceanic crust and the released massif were uplifted rapidly and the units of Middle-Late Eocene Tokuş Formation were deposited transgressively on the Boğazköy Formation and on outcropping basement rocks in the basin which was controlled by an extensional tectonic regime. Upper Miocene-Pliocene (İncesu Formation), Pliocene and Quaternary terrestrial sediments were deposited unconformably on all older units.

Relationships between the Sakarya Zone and the Ankara–Erzincan suture (central-northern Turkey): geological and petrographic data from the Ankara–Çankiri, Çorum and Amasya areas

AbstractThe study was performed in central-northern Anatolia (from Ankara to Amasya) to investigate the relationships of the Sakarya Zone units and the Izmir–Ankara–Erzincan suture (IAES) melange. It reveals that all the Sakarya Zone units are metamorphic and three main tectonostratigraphic units have been distinguished for the first time: the BAA (metasiliciclastic rocks capped by metacarbonates and varicoloured phyllite), the BKC (poly-metamorphic garnet-bearing micaschist and metabasite with a well-preserved relict HP–LT amphibole in a low-amphibolitic to greenschist-facies framework) and the AMC (meta-arkose passing vertically to carbonate–phyllitic alternations and, then, to a thick succession of prevailing acidic to intermediate–basic metavolcanites and volcanic-rich metasediments). The BAA and AMC, whose metamorphic frameworks are of Cimmerian age, underlie the Mesozoic carbonate cover sequences (e.g. t2-3, j3–k1) that often show tectonic detachments and slicing. The piling up of the BAA above the HP–LT BKC can be correlated to the tectonic superposition of two similar units (i.e. the Cimmerian Çangaldağ Complex and the Alpine Middle–Upper Cretaceous Domuzdağ Complex, respectively) defined by previous authors in other sectors of the Central Pontides front. The ophiolitic melange generally underlies the Sakarya Zone, but locally (e.g. SE of Amasya) tectonically rests above the latter, probably owing to back-thrusting that occurred during the Tertiary syn-collisional shortenings and the later strike-slip tectonics. We hypothesize that, also in these areas, the Sakarya Zone–IAES consists of a complex tectonic stack of different units, belonging to different palaeogeographic domains and orogenic events (Cimmerian versus Alpine orogenies), but originated within a single long-lived (since Late Triassic to Paleocene/Eocene times), prograding subduction–accretion system in front of the Laurasian continent.

Geochronology and geochemistry of the Niujuanzi ophiolitic mélange, Gansu Province, NW China: implications for tectonic evolution of the Beishan Orogenic Collage

The Niujuanzi ophiolitic melange (NOM), located in the Beishan Orogenic Collage, marks the termination between the Huaniushan arc and Mingshui-Hanshan Massifs. The NOM is mainly composed of gabbros, diabases, plagiogranites, basalts, and greywacke. Two gabbros have ages of 433.8 ± 3.1 and 354.0 ± 3.3 Ma, two plagiogranites have ages of 429.8 ± 2 and 448.7 ± 2.0 Ma, and a diabase has an age of 433.4 ± 3.2 Ma. The gabbros and diabases are calc-alkaline and tholeiitic, with high Al2O3, CaO, and TiO2 contents and low FeOT contents. The gabbros have high Mg# values (49–82), while the diabases have relatively low Mg# values (46–61). The plagiogranites are calc-alkaline and metaluminous, with high SiO2 and Na2O contents and low Al2O3 and K2O contents. The gabbros and diabases are enriched in large iron lithophile elements and slightly depleted in high field strength elements relative to N-MORB and their trace element characteristics are similar to E-MORB. With respect to rare earth element (REE), they have slightly enriched LREEs relative to HREEs. The majority of the plagiogranite trace elements approximate those of the volcanic arc granite. The plagiogranites have obviously enriched LREEs relative to HREEs, with a slightly to strongly negative Eu anomaly, which is similar to ORG but distinct from volcanic arc and within plate granite. The NOM was formed from the Ordovician to the Carboniferous, representing the expansion period of the Niujuanzi Ocean. The gabbros, diabases, and plagiogranites were formed in a mid-ocean ridge environment. The gabbros and diabases were generated by different degrees of partial melting of the mantle, and the plagiogranites derived from both the crystallization differentiation of basaltic magma and the partial melting of amphibolites in the crust.

The Petrology and Geochemistry of Listwaenite in the Sartohay Ophiolitic Melange of West Junggar, Xinjiang, China

The west Junggar, located in the eastern part of Balkash‐Junggar tectonic province, is a major component of the core of the Central Asian metallogenic region. This area is characterized by occurrences of ophiolitic mélanges, such as the Sartohay ophiolitic mélange in the NE and the Tangbale ophiolitic mélange in the west. As a hydrothermal alteration product of serpentinite in the Sartohay ophiolitic mélange, listwaenite lenses are gold‐mineralized and crop out on surface in the ophiolitic mélange via weathering of exhumated hanging wall of fault zone. Listwaenite is mainly composed of magnesite, quartz, dolomite, and trace amounts of mariposite, chromian spinel, talc and sulfide. A vertical thermal gradient model for the hydrothermal alteration shows that serpentinite would first be transformed to talc schist, then into listwaenite as the ophiolite slices continued to rise along shear zone, with XCO2, oxygen and sulfur fugacity increase and temperature decrease. Both serpentine and magnetite were progressively destroyed during the transformation from serpentinite to talc schist, and completely vanished in listwaenite, while mariposite generated in weakly deformed to mylonitized listwaenite. Concentrations of most trace elements including high field strength elements and metallogenic elements, increasing from undeformed, through weakly deformed, to mylonitized listwaenite, show a positive correlation with deformation degree and content of apatite, rutile, monazite, zircon and sulfide in listwaenite. The shear zone served as pathways for percolation and accumulation of fluid and trace elements during the metasomatism from serpentinite to listwaenite. Compared to undeformed listwaenite, mylonitized listwaenite will be more favorable to be fractured and brecciated due to more intense shearing, which caused strong metasomatic reaction and then induced trace element‐bearing mylonitized listwaenite.

Paleo‐Tethyan Oceanic Crust Subduction in the Eastern Section of the East Kunlun Orogenic Belt: Geochronology and Petrogenesis of the Qushi'ang Granodiorite

AbstractThe Qushi'ang granodiorite (QSG) is located at the central east of the ophiolitic melange belt in the East Kunlun Orogenic Belt (EKOB) in the northern margin of the Qinghai‐Tibetan Plateau. LA‐MC‐ICP‐MS zircon U–Pb dating suggests that the granodiorite and mafic microgranular enclaves (MMEs) crystallized 246.61±0.62 and 245.45±0.9 Ma ago, respectively. Granodiorite, porphyritic diorite, and MMEs are metaluminous and medium‐K calk ‐ alkaline series, with island‐arc magma features, such as LILE enrichment and HFSE depletion. The porphyritic diorite has high Cr (13.50 ppm to 59.01 ppm), Ni (228.53 ppm to 261.29 ppm), and Mg# (46–54). Granodiorite and porphyritic diorite have similar mineral compositions and evolved major and trace elements contents, particularly Cr and Ni, both of which are significantly higher than that in granites of the same period. The crystallization age of MMEs is close to that of granodiorite, and their major and trace elements contents are in‐between porphyritic diorite and granodiorite. The results suggest that the original mafic magma, which was the product of mantle melting by subduction process, intruded into the lower crust (Kuhai Rock Group), resulting in the formation of granodiorite. Countinous intrusion of mafic magma into the unconsolidated granodiorite formed MMEs and porphyritic diorite. The granodiorite reformed by late‐stage strike‐slip faulting tectonic event indicates that the strike‐slip fault of Middle Kunlun and the collision of the Bayanhar block with East Kunlun were later than 246 Ma. Therefore, the formation of the QSG not only indicates the critical period of evolution of East Kunlun but also represents the tectonic transition from oceanic crust subduction to slab breaking.

Mineralogy and radioactivity of anomalous sites of Wadi Murrah post-collision granites, south Eastern Desert, Egypt

Wadi Murrah area is located in the extreme south Eastern Desert of Egypt. The area comprises four distinct tectono-stratigraphic units beginning with the oldest as follows: ophiolitic assemblage (dismembered ophiolitic rocks and ophiolitic melange), arc assemblage (metavolcanics, metagabbros, and older granites), post-collision granitoids (monzogranite, syenogranite, and alkali feldspar granite), and associated with dykes and quartz veins. The identified minerals in the samples taken from the anomalous sites of syenogranite and alkali feldspar granite in the northern part of the study area are represented by uranophane, autonite, meta-autonite, uranothorit, columbite, zircon, fluorite, monazite, xenotime, sphene, allanite, and apatite. The anomalous samples have U and Th average contents for syenogranites and alkali feldspar granites 82 and 104.4 ppm and 209 and 115.2 ppm, respectively. The high radioactivity in the anomalous sites is controlled lithologically and structurally. The secondary processes played a main role in uranium enrichment (uranium added to these granitic sites post-magmatically).

Early Jurassic tectonism occurred within the Basu metamorphic complex, eastern central Tibet: Implications for an archipelago-accretion orogenic model

The Basu metamorphic complex, surrounded by ophiolitic melanges and intruded by a large volume of undeformed granitoid rocks along the eastern segment of the Bangong-Nujiang suture, holds one of the keys to understanding the pre-Cenozoic tectonic evolution of central Tibet. Zircon U-Pb dating of rocks from the Basu metamorphic complex reveals that meta-igneous rocks yield Early Paleozoic crystallization ages of 500–492Ma and an Early Jurassic metamorphic age of ~173Ma, and that undeformed granitoid rocks yield crystallization ages of approximately 186–174Ma. Whole rock geochemical and zircon Lu-Hf isotopic data indicate that the undeformed granitoid rocks originated mainly from partial melting of ancient crustal sources, which may reflect a collisional orogenic setting. 40Ar/39Ar dating of biotite from a sillimanite-garnet-biotite paragneiss shows cooling to 300±50°C at ~165Ma. These data indicate significant Early Jurassic tectonism, during which most of the Basu metamorphic complex was formed. Furthermore, the age data resemble those of the Amdo metamorphic complex located approximately 500km to the west along the Bangong-Nujiang suture. Together, these complexes may represent a “destroyed or unrecognized” block, i.e., the Amdo-Tongka block, which may be the eastern extension of the South Qiangtang terrane. Based on the tectonic outlines of the multiple ophiolitic zones and magmatic belts, we suggest a new archipelago-accretion model that attributes the Early Jurassic tectonism to an arc-continent/micro-continent collision. This model further enables the reconstruction of the eastern Tethyan Ocean and the orogenic processes of central Tibet during the Mesozoic.