Ophiolites in Pakistan: Key evidence for the geodynamic evolution of the Neo-Tethys Ocean

The Late Triassic-Jurassic magmatic belt and its implications for the double subduction of the Neo-Tethys Ocean in the southern Lhasa subterrane, Tibet

The paper concerns with the problems of tectonic structure and geodynamic evolution of the eastern Paleoasian ocean using a geodynamic map of 1:2,000,000 scale compiled during IGCP Project No. 283 “Geodynamic evolution of the Paleoasian ocean”. Two groups of terranes were distinguished within the territory considered. The first group was referred to composite terranes (or microcontinents) with the participation of Early Precambrian basement blocks: Barguzin, Tova-Mongolian, Argun, Central Mongolian, Khingan-Bureya and Khanka. The second group includes terranes composed of perioceanic complexes (island-arc and accretionary wedge complexes): Dzhida, Khamar-Daban, Eravnin, Kerulen, South Mongolian and Mongolo-Okhotsk. The following regimes were distinguished in the geodynamic evolution of the Paleoasian ocean: formation of an active continental margin of the eastern Pacific type (Riphean) climaxing in amalgamation of microcontinents (Yendian); new oceanic stage and development of intraoceanic island arcs (Yendian-Lower Paleozoic), which is changed by multistage collision of terranes and Siberian craton (Lower and Middle Paleozoic), and active margin of the Andean type (Upper Paleozoic). The structures related to the evolution of the Mongolo-Okhotsk ocean are accretionary wedges with ophiolites and glaucophane schists and Middle-Upper Paleozoic terrigenous series of marginal seas. A prolonged closing of oceanic basin and Late Paleozoic (in the west) to Late Mesozoic (in the east) collision is supposed.

The records of the Jurassic Western Tethys Ocean are the ophiolitic rocks now scattered in the Tertiary orogenic belts of the Alps, Apennines and Betic Cordillera. These ophiolites, involved in a convergent margin environment, are affected I) by HP/LT metamorphism derived from a subduction process or II) by very low grade overprint, corresponding to the tectonic prism at the margin of the overriding plate. On the whole, they share common characteristics: a- The MORB geochemical signature. b- The ophiolitic successions often “reduced” and thin. c- The volcano-sedimentary covers often directly overlying the serpentinised peridotites. d- The widespread occurrence of cherts as Jurassic pelagic sediment. In the thickest “complete” ophiolitic successions, basalt flows, generally thin, are preceded and followed by ophiolitic breccias. Only the basal portion of the breccias on top of the serpentinites (Levanto Breccias) has a tectonic origin, all other levels have a sedimentary origin. These breccia-basalt assemblages are overlain by thick sequences of Mt. Alpe Cherts and Calpionella Limestones, followed by Palombini Shales. In the reduced (or incomplete) successions, thin breccias and cherts were directly deposited, above the Levanto Breccias (ophicalcites pro parte), and followed by Palombini Shales. This stratigraphic pattern seems to be widespread in the whole Western Tethys ocean. In some sequences, transitional mid-ocean ridge (T-MOR) basalts are present and the ophiolitic rocks are associated with Variscan continental slices and debris, as in the Err-Platta succession (Central Alps) and in some exotic blocks in the flysch of the External Ligurides (Northern Apennines). In the Balagne (Corsica) T-MOR basalts are associated with quartzarenites. These occurrences show that an unroofed mantle and sections of oceanic crust evolved very near to a continental margin. The different radiolarian ages of the cherts deposited before, within, or on top of the MOR basalts allow to infer a minimum time interval for the Western Tethys oceanisation. This interval can be considered between 16 and 21 Ma (from Late Bajocian to Kimmeridgian/Tithonian). If we assume 1cm/yr spreading rate during this time, the basin would have reached about 150-200 km width. The same ages suggest that the ocean opening was diachronous along the Western Tethys basin. Mainly on the basis of the Northern Apennines and Corsica data, it is possible to reconstruct the following evolutive geodynamic, paleogeographic and sedimentary evolution of the Western Tethys ocean basin: 1- Bajocian/Bathonian stage: opening of the Ligurian Northern Apennines oceanic segment and, perhaps, also of the Ligurian, Western and Central Alps ones. 2- Bathonian/Callovian stage: opening of all the segments of the Western Tethys ocean basin. The volcano-sedimentary covers formed during these two stages are constituted by breccias, basalts and siliceous pelagites (cherts). 3- Tithonian/Berriasian: end of the ocean spreading (Tithonian) and beginning of the quiescent stage in the whole basin, marked by the lack of any tectonic activity and by the sedimentation of the Calpionella Limestones and, locally, of mixed siliceous-calcareous deposits (Nisportino-Murlo Fm.). 4- Hauterivian/Santhonian: this is the longest quiescent stage of the basin, dominated by the sedimentation of the Palombini Shales and Limestones. Some siliciclastic deposits are shed from both passive continental margin sides. During the Early Cretaceous, there is also evidence of a rare intraplate magmatism in Southern Tuscany. The Western Tethys ophiolitic successions are similar to those of present day, slow spreading oceans, in particular to those of the Atlantic Ocean (Mutter and Karson, 1992; Tucholke and Linn, 1994). The Galician North Atlantic margin provides a model for the process of mantle denudation. For the oceanic evolution, the model of Tucholke and Linn (1994) is particularly taken in consideration. According to this model, tectonic extension was one major process in the Western Tethys oceanic development.

The Meso-Tethys Ocean: The nature, extension and spatial-temporal evolution

Early Jurassic volcanic rocks in the Xiongcun district, southern Lhasa subterrane, Tibet: Implications for the tectono-magmatic events associated with the early evolution of the Neo-Tethys Ocean

The paper reviews and integrates new results on the evolution of the Paleo-Asian Ocean and its related geodynamics and geology of Altai-Sayan Region (ASR) in Central Asia. A revised terrane classification based on Vendian-Cambrian geodynamic units and evolution of terranes is described. Reactivated suture zones along the terrane boundaries are proposed. The obtained data suggest the important role of strike-slip deformations in the formation of mosaic-block structure of Central Asia. Those complicated and multi-stage deformations resulted from the Late Devonian-Early Carboniferous collision of Gondwana-derived terranes. The deformations reached their peak in the Late Carboniferous-Permian due to the collision of the Kazakhstan and Siberian continents. A system of sinistral strike-slip faults formed ASR along the margin of the Siberian continent as a result of the Late Carboniferous-Permian collision. The intrusion of granites occurred in East Kazakhstan and northwestern Gorny Altai in the Late Carboniferous and Permian. This resulted in the formation of the Northern Eurasia continent. Geodynamic evolution of the Paleo-Asian ocean and paleotectonics of ASR allow to recognize in the region the following five geodynamic stages: Vendian-Early Cambrian, Early Ordovician, Early-Middle Devonian, Late-Devonian-Early Carboniferous and Late Carboniferous-Early Permian times.

Problems of geodynamics, tectonics, and metallogeny of orogens

Late Palaeozoic igneous rock associations in response to subduction, accretion, and final closure of the eastern Palaeo-Asian Ocean play a significant role in understanding the geodynamic evolution of the southeastern Central Asian Orogenic Belt. Previous studies have identified a Permian arc magmatic belt associated with the southward-dipping subduction of the eastern Palaeo-Asian Ocean along the Solonker–Changchun suture zone. The genetic mechanism and associated geodynamic settings are of great importance in deciphering the evolution of the eastern Palaeo-Asian Ocean. This paper presents zircon U–Pb–Hf isotope and whole-rock geochemical analyses for a suite of magmatic rocks including the early Permian diorite porphyrites (ca. 281.0 Ma), andesites (ca. 276 Ma) and rhyolites (ca. 275 Ma) in the Kulun region. The diorite porphyrites and andesites have high SiO2and total alkali contents, and low MgO contents and Mg no. values, with enrichments in large ion lithophile elements and depletions in high-field-strength elements. These geochemical characteristics, together with low-Sr and high-Yb contents, a weak concave-upward shape of middle rare earth elements and negative Eu anomalies, suggest that these intermediate igneous rocks were generated by partial melting of amphibolitic lower crust at a crustal depth of 30–40 km. The rhyolites have heterogeneous isotopic compositions, withϵHf(t) values andTDM2ages of –20.8 to +0.5 and 3578∼1494 Ma, implying that they were likely derived from partial melting of a mixed source dominated by recycled ancient crust with minor juvenile crustal materials. The rhyolites show potassic affinity with relatively high K2O and very low Na2O, which was attributed to liquid immiscibility of felsic magma and subsequent limited fractional crystallization of plagioclase. The regional igneous associations, metamorphic events, and coeval sedimentary rocks along the Solonker–Changchun suture zone indicate that the early Permian igneous rocks were formed in an active continental arc environment in response to southward subduction of the eastern Palaeo-Asian Ocean.

Recognition of any intra-oceanic arc-trench system (IOAS) could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean. Four lithotectonic units, including island arc, ophiolite, forearc basin, and accretionary complex, are recognized based on detailed field investigation. They rest on the northern margin of the Qaidam block and occur as allochthons in fault contact with underlying high-grade metamorphic rocks. The ophiolite unit mainly consists of ultramafic rocks, 527–506 Ma gabbro, 515–506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in serpentinite, dolerite with forearc basalt affinity, and boninitic lava collectively indicate a forearc setting. The accretionary complex, exposed to the south of the ophiolite complex and island arc, is highly disrupted and contains repeated slices of basalt, 495–486 Ma tuff, chert, limestone, and mélange. Tuffs with positive zircon εHf(t) values indicate derivation from a nearby juvenile island arc. These lithotectonic units, as well as the back-arc basin, are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean. Combined with regional geology, we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean. The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.

The origin and drift history of the Lhasa block in South Tibet is crucial towards unraveling the evolution of the Neo-Tethys Ocean, which has not yet been well constrained by the paucity of paleomagnetic data, especially for the late Paleozoic. Hence, a systematic paleomagnetic investigation of 50 sandstone samples (6 sites), 166 volcanic samples (21 sites) and 76 limestone samples (9 sites) from the middle Permian (267.8 ± 5.0 Ma) Luobadui Formation was conducted in the Lhunzhub area. The results reveal an Eocene re-magnetization component in the sandstone samples, but stable high temperature (field) components obtained from most volcanic and limestone samples can successfully pass the fold, reversal and paleosecular variation tests, which likely represents primary magnetization. On this basis, the middle Permian paleomagnetic pole position (Plat= 40.9°N, Plong=324.5°E, N=27 sites (dp/dm=3.3/6)) and paleolatitude (~15.9°S) of the Lhasa block are presented. Combined with published paleomagnetic data from other Tethyan continental blocks, this new constraint reveals that the Lhasa block was located in the interior of the Neo-Tethys Ocean at about 268 Ma. In further considering the geological records of the Lhasa block, we propose that the block rifted from the northwestern margin of the Gondwana-Australian plate prior to the middle Permian, and the Neo-Tethys Ocean represented by the Bangong Co-Nujiang and Yarlung-Zangbo Suture Zones coexisted during the middle Permian.

Cretaceous paleomagnetic and detrital zircon U Pb geochronological results from the Tethyan Himalaya: Constraints on the Neo-Tethys evolution

The Bangong-Nujiang Suture Zone, Tibet Plateau: Its role in the tectonic evolution of the eastern Tethys Ocean

An integrated investigation of Lower Jurassic Jialapu Formation (including U–Pb dating of igneous and detrital zircons, geochemical analysis of igneous rocks, and analysis of detrital modes of sandstone interbeds) in the southern margin of the Lhasa terrane, Tibet, provides new constraints on the early evolution of the Neo-Tethyan Ocean. Our new data reveal a stage of silicic magmatism, with an arc affinity along the southernmost Eurasian margin at 193–190 Ma, which is characterized by high SiO2 (76.5–80.0 wt%), Na2O (5.30–7.22 wt%) and low K2O (0.07–0.55 wt%), MgO (0.16–0.54 wt%), Cr (1.44–4.37 ppm), and Ni (0.67–1.62 ppm) contents with low eNd(t) values of + 1.21 to + 2.33. These silicic volcanic rocks are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and can be classified as highly fractionated I-type rhyolites. They show a wide range of zircon eHf(t) values from − 11.7 to + 9.4, suggesting variable contributions from both juvenile and ancient crustal sources. It is concluded that the Jialapu Na-rich rhyolites were derived by the partial melting of a highly differentiated residual melt separated from the K-rich magma, triggered by upwelling asthenospheric mantle. Detrital zircons in the sandstones from the Lower Jurassic Jialapu Formation are dominantly the Mesozoic ones, which are likely derived from Lhasa terrane sources. Considering the associated sedimentary records as well as literature data, it is suggested that an Early Jurassic back-arc basin developed along the southern Lhasa terrane in response to the northward subduction of the Neo-Tethyan Ocean.

Zircon U–Pb geochronology and geochemistry of Early Jurassic granodiorites in Sumdo area, Tibet: Constraints on petrogenesis and the evolution of the Neo-Tethyan Ocean

An evaluation of the potential geodynamic connections between the evolution of Paleozoic oceans in NW Gondwana and NE Gondwana is challenging. Until recently, most syntheses emphasized only two Paleozoic oceans (the Proto-Tethys and the Palaeo-Tethys) in the east Tethys realm. However, the discovery of early Paleozoic ophiolites along Palaeo-Tethys sutures located south of Proto-Tethys sutures challenges these traditional views. After a comprehensive review of relevant early Paleozoic tectonomagmatic events, we herein recognize and propose a model for the tectonic evolution of a hitherto unrecognized early Paleozoic ocean, which we call the Proto-Qiangtang Ocean. This ocean was short lived; it opened in the late Cambrian, began to subduct in the Middle Ordovician, and closed diachronously westwards between the Late Ordovician and the middle Silurian. Its closure by middle Silurian time indicates that was a spatially and temporally distinct ocean from the Palaeo-Tethys Ocean. The early tectonic evolution of the Proto-Qiangtang Ocean shares many characteristics with that of the Rheic Ocean. Both opened in the late Cambrian in the back-arc region of the Iapetus–Proto-Tethys Ocean, and the Proto-Qiangtang Ocean is considered to represent the eastern extension of the Rheic Ocean. This correlation has important implications for the Paleozoic tectonic evolution and palaeogeography of northern Gondwana.