Late Ordovician fore-arc ophiolitic mélange in the southern margin of the Bainaimiao arc: constraints from zircon U–Pb–Hf isotopes and geochemical analyses

The Harihada–Chegendalai ophiolitic mélange, which is located between the Bainaimiao arc and the North China Craton, holds significant clues regarding the tectonic setting of the southern margin of the Central Asian Orogenic Belt. The ophiolitic mélange is mainly composed of gabbroic and serpentinized ultramafic rocks. Here, zircon U–Pb dating, in situ zircon Hf isotopic, whole-rock geochemical and in situ mineral chemical data from the ophiolitic mélange are reported. The zircons in the gabbroic rocks yielded concordia U–Pb ages of 450–448 Ma and exhibited slightly positive ϵHf(t) values (0.87–4.34). The geochemical characteristics of the gabbroic rocks indicate that they were generated from a mantle wedge metasomatized by subduction-derived melts from sediments with continental crust contamination, in a fore-arc tectonic setting. These rocks also experienced the accumulation of plagioclase. The geochemical characteristics of the ultramafic rocks and their Cr-spinels indicate that they may constitute part of residual mantle that has experienced a high degree of partial melting and has interacted with fluids/melts released from the subducted slab in the same fore-arc tectonic setting. The ophiolitic mélange may therefore have formed in this fore-arc tectonic setting, resulting from the northward subduction of the South Bainaimiao Ocean beneath the Bainaimiao arc during Late Ordovician time, prior to the collision between the Bainaimiao arc and the North China Craton during the Silurian to Carboniferous periods.

Potentiality of caro’s acid in leaching of uranium and radioactive measurements from Abu-Rusheid mylonitic gneiss rocks, South Eastern Desert, Egypt

Abu Rusheid area is located at the Southern Eastern Desert of Egypt and composed of Mylonitic gneiss rocks (mineralized rock), Serpentinite rocks, Ophiolitic metagabbro, Ophiolitic mélange, Monzogranites, post- granitic dykes (lamprophyre and dolerite), veins and recent alluvial deposits. This paper is concerned with the study of potentiality of sulphuric and caro’s acid in uranium dissolution from Abu Rusheid mineralized rocks. For this purpose, many batch dissolution experiments were conducted. The obtained results showed that 91.5% and 52% uranium leachability for Caro’s acid and dilute sulfuric acid respectively. The reaction mechanism was described using shrinking core models.

Age and origin of accreted ocean plate stratigraphy in the North Qilian belt, NE Tibet Plateau: Evidences from microfossils and geochemistry of cherts and siltstones

The accretionary complex (AC) in the North Qilian belt comprises coherent and chaotic units consisting of bedded cherts, pelagic mudstone, shale, turbidites, basalt, limestone, blueschist, eclogite lenses, and ophiolitic mélange. Cherts from the Donggoukou and Biandukou outcrops in the north of blueschist belt contain abundant Middle Ordovician radiolarians together with rare conodonts. Well-preserved radiolarians also occur in cherts associated with high-pressure/low-temperature rocks in the Baijingsi AC outcrop. Conodonts of Floian-Dapingian age and Middle Ordovician radiolarians also occur in the Shihuigou AC. Geochemical analysis of 23 cherts reveals variable SiO2 contents (74.56-97.16 wt%) and high mean Al/(Al + Fe + Mn) ratios ranging from 0.35 to 0.85, indicating a non-hydrothermal origin. Ce/Ce* and LaN/YbN ratios of 0.70-1.22 and 0.67-1.59 are high and variable, similar to those of associated muddy siltstone (0.59-0.96 and 1.14-1.55, respectively), suggesting near trench deposition with associated terrigenous input. Together with the metamorphic ages of blueschists and eclogites, the North Qilian belt AC formed by accretion of ocean plate stratigraphic successions in response to subduction of the Proto-Tethyan Ocean prior to 450 Ma.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

The supra-subduction zone ophiolite or ophiolitic mélange formed in the forearc setting is generally considered to be a key geological record for subduction initiation (SI) with petrological characteristics comparable to the SI-related rock sequence from forearc basalt (FAB) to boninite in the Izu-Bonin-Mariana subduction zone. Nevertheless, the standard FAB and boninite are generally difficult to observe in the forearc rocks generated during SI. Yet, a typical rock sequence indicating the SI of the western Proto-Tethys Ocean is reported for the first time in the Qimanyute intra-oceanic forearc system in the western Kunlun Orogen, Northwest Tibetan Plateau. The magmatic compositions, which range from less to more high field strength element (HFSE)-depleted and large ion lithophile element (LILE)-enriched, are changing from oceanic plagiogranites (ca. 494 Ma) to forearc basalt-like gabbros (FAB-Gs, ca. 487 Ma), boninites, and subsequent Nb-enriched gabbros (NEGs, ca. 485 Ma), which are thus consistent with the Izu-Bonin-Mariana forearc rocks as well as the Troodos and Semail supra-subduction zone-type ophiolites. The geochemical data from the chemostratigraphic succession indicate a subduction initiation process from a depleted mid-oceanic-ridge (MORB)-type mantle source with no detectable subduction input to gradual increasing involvement of subduction-derived materials (fluid/melts and sediments). The new petrological, geochemical, and geochronological data, combined with the regional geology, indicate that the well-sustained FAB-like intrusive magmas with associated boninites could provide crucial evidence for SI and further reveal that the SI of the western Proto-Tethys Ocean occurred in the Late Cambrian (494−485 Ma).

Supplemental Material: Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Detailed petrological and geochemical data.

Supplemental Material: Subduction initiation of the western Proto-Tethys Ocean: New evidence from the Cambrian intra-oceanic forearc ophiolitic mélange in the western Kunlun Orogen, NW Tibetan Plateau

Detailed petrological and geochemical data.

Geological Correlation Between Northern Cyprus And Southern Anatolia

The island of Cyprus constitutes a fragment of southern Anatolia separated from the mainland by left-oblique transtension in late Cenozoic time. However, a geological framework of offset features of the south-central Anatolia, for comparison of Cyprus with a source region within and west of the southeastern Anatolian suture zone, has not yet been developed. In this paper, I enumerate, describe, and compare a full suite of potentially correlative spatial and temporal elements exposed in both regions. Northern Cyprus and south-central Anatolia have identical tectonostratigraphic units. At the base of both belts, crop out ophiolitic mélange-accretionary complex generated during the northward subduction of the NeoTethyan Oceanic lithosphere from the Late Cretaceous until the end of middle Eocene. The nappes of the Taurus carbonate platform were thrust above this internally chaotic unit during late Eocene. They began to move as a coherent nappe pile from that time onward. An asymmetrical flysch basin was formed in front of this southward moving nappe pile during the early Miocene. The nappes were then thrust over the flysch basin fill and caused its tight folding. Cyprus separated from Anatolia in the Pleistocene-Holocene when, transtensional oblique faults with dip-slip components caused the development of the Adana and Iskenderun basins and the separation of Cyprus from Anatolia.