Early Mesozoic Time Research Articles

Duraznoscytinum aristovi gen. et sp. nov., a new scytinopterid (Hemiptera: Cicadomorpha) from the Upper Triassic of Argentina

A new scytinopterid (Hemiptera: Cicadomorpha), Duraznoscytinum aristovi gen. et sp. nov., is described. The specimen was recovered from the upper section of the Potrerillos Formation (Upper Triassic) at the Quebrada del Durazno locality, south of the Cerro Cacheuta, Cuyana Basin, in Mendoza, Argentina. This represents the first record of a complete forewing (tegminous + clavus) of a scytinopterid for the Triassic of South America. Additionally, we briefly discuss the Triassic scytinopterid distribution worldwide, and provide preliminary morphological differences observed between the taxa recorded in the Southern Hemisphere. The new fossil described herein contributes to the knowledge on the hemipterofauna diversity present at the south of the Cerro Cacheuta area during Early Mesozoic times.

Paleo-Tethyan Ocean Evolution and Indosinian Orogenesis in the East Kunlun Orogen, Northern Tibetan Plateau

The East Kunlun Orogen on the northern margin of the Tethyan orogenic system records a history of Gondwana dispersal and Laurasian accretion. Uncertainties remain regarding the detailed histories of northern branches of the Paleo-Tethys Ocean in East Kunlun Orogen (Buqingshan Ocean). Based on a synthesis of sedimentary, structural, lithological, geochemical, and geochronological data from the East Kunlun Orogen and adjacent regions, this paper discusses the spreading and northward consumption of the Paleo-Tethys Ocean during Late Paleozoic–Early Mesozoic times. The main evolutionary stages are: (1) during Carboniferous to Middle Permian, the Paleo-Tethys Ocean (Buqingshan Ocean) was in an ocean spreading stage, as suggested by the occurrence of Carboniferous MORB-, and OIB-type oceanic units and Carboniferous to Middle Permian Passive continental margin deposits; (2) the Buqingshan Ocean subducted northward beneath the East Kunlun Terrane, leading to the development of a large continental magmatic arc (Burhan Budai arc) and forearc basin between ~270–240 Ma; (3) during the late Middle Triassic to early Late Triassic (ca. 240–230 Ma), the Qiangtang terrane collided with the East Kunlun–Qaidam terranes, leading to the final closure of the Buqingshan Ocean and occurrences of minor collision-type magmatism and potentially inception of the Bayan Har foreland basin; (4) finally, the East Kunlun Orogen evolved into a post-collisional stage and produced major magmatic flare-ups and polymetallic mineral deposits between Late Triassic to Early Jurassic (ca. 230–200 Ma), which is possibly related to asthenospheric mantle upwelling induced by delamination of thickened continental lithosphere and partial melting of the lower crust. In this paper, we propose that the Wilson cycle-like processes controlled the Late Paleozoic–Early Triassic tectonic evolution of East Kunlun, which provides significant implications for the evolution of the Paleo-Tethys Ocean.

Earliest Tuatara Relative (Lepidosauria: Sphenodontinae) from Southern Continents

The New Zealand tuatara (Sphenodon) is the sole surviving rhynchocephalian of a once thriving group across Pangea during early Mesozoic times. Outside New Zealand, close relatives of the tuatara (sphenodontines) are known from a few Jurassic records in North America and Europe and from end-Cretaceous incomplete remains in Patagonia. Still, the evolutionary relationships of most of them remain elusive. Here we describe a new sphenodontine, Tika giacchinoi gen. et sp. nov., based on well-preserved cranial and postcranial remains from upper levels of the Candeleros Formation (Cenomanian) at the Konservat-Lagerstätte of ‘La Buitrera Paleontological Area’ in northern Patagonia, Argentina. Our phylogenetic analysis recovered Tika as a close relative of the tuatara, together with Laurasian and Patagonian taxa. The new finding represents the oldest certain sphenodontine from the gondwanan continents and reinforces the hypothesis that particular terrestrial ectothermic tetrapods attained a circumantarctic Cretaceous-Tertiary distribution. Tika is inferred to have fed upon a variety of prey items including small vertebrates, similar to the extant tuatara, but ecologically different from the large herbivorous sphenodontians already known from La Buitrera. Therefore the new taxon expands the known diversity of sphenodontians during the Late Cretaceous in Patagonia and indicates that Rhynchocephalia, although declining or extinct in Laurasia, were still taxonomic and ecologically diverse in southwestern Gondwana.

The equisetalean Iberisetum wegeneri gen. nov., sp. nov. from the Upper Pennsylvanian of Portugal

ABSTRACT Equisetales represents a diverse and abundant group of articulate plants with a wide geographical distribution that had their greatest development in late Palaeozoic and early Mesozoic times. The order is represented by 12 families, composed mostly by extinct families of late Palaeozoic age, with only a unique living family, the Equisetaceae. Here we describe the new equisetalean fossil-genus and species Iberisetum wegeneri gen. nov., sp. nov. from the lower Stephanian C (Gzhelian, Upper Pennsylvanian) of intramontane Douro Basin, in northwestern Portugal. This new fossil taxon represents another endemic species for the Douro Basin. Iberisetum wegeneri consists of a leafy articulated stem that shows a unique combination of macro-morphological features that are common in various fossil-genera of different equisetalean families, and that includes exclusive (autapomorphic) characters. The singular morphology of foliar sheaths of Iberisetum wegeneri represents an evolutionary novelty into the Equisetales and suggests a heliotropic organisation (heliotropism) of the plant during its lifetime. Such functional morphology is the result of an evolutionary adaptation to the climatic and ecological conditions restricted to intramontane environments of Douro Basin.

U-Th-Pb Geochronology and Lu-Hf Isotope Geochemistry of Detrital Zircons in Metasedimentary Rocks of the Southern Coast Mountains Batholith

Abstract Most metasedimentary rocks in the southern Coast Mountains batholith are of uncertain tectonic affinity because they occur in discontinuous pendants surrounded by large intrusive bodies, and many protolith features are obscured by regional deformation and metamorphism. This study uses U-Th-Pb ages and Lu-Hf isotope signatures of detrital zircons in metasedimentary rocks in Bute, Loughborough, and Knight Inlets in an effort to test possible correlations with the adjacent Wrangellia, Alexander, Taku, Yukon-Tanana, and Stikine terranes. Detrital zircons from metasedimentary samples yield ages that belong to age groups of 590-528 Ma (peak age of 560 Ma), 485-432 Ma (peak age of 452 Ma), 356-286 Ma (peak age of 307 Ma), and 228-185 Ma (peak ages of 215 and 198 Ma). A small number of ~1.1-1.9 Ga grains are also present. εHft values of the 590-185 Ma grains yield a progression from intermediate (0 to +5) values to more juvenile (mostly +4 to +15) values from Neoproterozoic through early Mesozoic time. The Comparison of these results with similar data sets from adjacent terranes demonstrates that primary connections with the Yukon-Tanana and Taku terranes are unlikely but are consistent with primary connections with the Wrangellia, Stikine, and/or Alexander terranes. Unfortunately, the available constraints are not sufficient to eliminate any of these options or the possibility that the pendants are a unique tectonic fragment. Zircons from the metasedimentary samples also yield U-Th-Pb ages of 165-128 Ma (peak age of 152 Ma) and 114-88 Ma (peak age of 102 Ma). εHft values of these zircon domains are mostly juvenile (+7 to +13). Comparison of U concentrations, U/Th values, and CL textures of zircons from the metasedimentary samples, leucocratic sills that intrude the pendants, and surrounding plutonic bodies suggests that most of the young grains, as well as widespread younger rims on older grains, grew during metamorphism associated with emplacement of the adjacent plutonic bodies. Some young grains were derived from thin felsic sills or veins that were unintentionally included in the sampled material.

Global mantle structure from multifrequency tomography using P, PP and P-diffracted waves

SUMMARYIn global-scale seismic tomography, teleseismic P and PP waves mainly constrain structures in the upper two thirds of the mantle, whereas core-diffracted waves (Pdiff) constrain the lower third. This study is the first to invert a very large data set of Pdiff waves, up to the highest possible frequencies. This results in tomographic resolution matching and exceeding that of global S-wave tomographies, which have long been the models of choice for interpreting lowermost mantle structure.We present three new global tomography models of 3-D isotropic P-wave velocity in the earth’s mantle. Multifrequency cross-correlation traveltimes are measured on all phases in passbands from 30 s dominant period to the highest frequencies that produce satisfactory fits (≈3 s). Model DETOX-P1 fits ≈2.5 M traveltimes from teleseismic P waves. DETOX-P2 fits the same data, plus novel measurements of ≈1.4 M traveltimes of Pdiff waves. DETOX-P3 fits the same data as DETOX-P2, plus ≈ 1.2 M PP traveltimes. Synthetics up to 1 s dominant period are computed by full wave propagation in a spherically symmetric earth using the spectral-element method AxiSEM. Traveltimes are linked to 3-D velocity perturbations (dVP/VP) by finite-frequency Fréchet kernels, parametrized on an adaptive tetrahedral grid of ≈400 000 vertices spaced by ≈80 km in the best-sampled regions. To complete spatial coverage, the waveform cross-correlation measurements are augmented by ≈5.7 million analyst-picked, teleseismic P arrival times. P, Pdiff and PP traveltimes are jointly inverted for 3-D isotropic P-velocity anomalies in the mantle and for events corrections, by least squares solution of an explicit matrix–vector equation.Inclusion of Pdiff traveltimes (in DETOX-P2, -P3) improves the spatial sampling of the lowermost mantle 100- to 1000-fold compared to teleseismic P waves (DETOX-P1). Below ≈2400 km depth, seismically slow anomalies are clustered at southern and equatorial latitudes, in a dozen or more intensely slow patches of 600–1400 km diameter. These features had long been classed into two large low shear velocity provinces (LLVP), which now appears questionable. Instead, patches of intensely slow anomalies in the lowermost mantle seem to form a nearly continuous, globe-spanning chain beneath the southern hemisphere, according to our increased resolution of LLVP-internal subdivisions and newly imaged patches beneath South America. Our tomography also supports the existence of whole-mantle plumes beneath Iceland, Ascension, Afar, Kerguelen, Canary, Azores, Easter, Galapagos, Hawaii, French Polynesia and the Marquesas.Seismically fast structure in the lowermost mantle is imaged as narrowly elongated belts under Eastern Asia and the Americas, presumably reflecting the palaeo-trench geometries of subduction zones and arcs that assembled Eastern Asia and the American Cordilleras in Palaeozoic and early Mesozoic times. Mid-mantle structure is primarily constrained by teleseismic P waves, but Pdiff data have a stabilizing effect, for example, sharpening the geometries of subducted slabs under the Americas, Eurasia and the Northern Pacific in the upper 2000 km. PP traveltimes contribute complementary constraints in the upper and mid mantle, but they also introduce low-velocity artefacts beneath the oceans, through downward smearing of lithospheric structure.Our three new global P-wave models can be accessed and interactively visualized through the SubMachine web portal (http://submachine.earth.ox.ac.uk/).

A major provenance change in sandstones from the Tezoatlán basin, southern Mexico, controlled by Jurassic, sinistral normal motion along the Salado River fault: Implications for the reconstruction of Pangea

During the last decades, authors dealing with the reconstruction of Pangea recognized that restoration of Gondwana to its early Mesozoic position relative to Laurentia produces a continental overlap encompassing most of the Mexican territory. To avoid such a continental misfit, some authors suggested that, during early Mesozoic time, southern and central Mexico were originally located farther to the NW of its present position, and that they were emplaced to its current location by lithospheric-scale, NW-trending, sinistral faults during Jurassic time. However, there are currently few reports of NW-trending sinistral faults that were active in Mexico during Jurassic time.In this work, we present sedimentological and petrological data that document a major provenance change within the Jurassic stratigraphic record exposed in the Tezoatlán basin, southern Mexico. We interpret this provenance change as the result of exhumation of Paleozoic metamorphic rocks of the Acatlán Complex along the Salado River fault, which is a WNW-trending sinistral normal fault that bounds the Tezoatlán basin to the north. Our U-Pb detrital zircon ages integrated with previous biostratigraphic data bracket the age of the Salado River fault between ∼179 and ∼170 Ma. Therefore, we reaffirm that sinistral normal block motion along WNW-trending faults took place in southern Mexico during Jurassic time as predicted by some previous geodynamic reconstructions of Pangea breakup.

Paleomagnetic constraints on the kinematic relationship between the Guerrero terrane (Mexico) and North America since Early Cretaceous time

The North American Cordillera has been shaped by a long history of accretion of arcs and other buoyant crustal fragments to the western margin of the North American plate since early Mesozoic time. The southernmost accreted terrane is the terrane of southwestern Mexico, a latest Jurassic-Cretaceous volcanic arc built on a Triassic accretionary prism. Interpretations of the origin of the terrane vary: Some authors consider it a far-traveled, exotic intra-oceanic island arc, while others view it as the (par)autochthonous, extended North American continental margin. We present new paleomagnetic and U-Pb zircon data from Lower Cretaceous sedimentary rocks of the terrane. These data show that the terrane has a latitudinal plate-motion history equal to that of the North America plate, both before and after accretion. This confirms paleogeographic models in which the arc successions formed on North American crust that rifted away from the Mexican mainland by approximately east-west opening of a back-arc basin above an eastward-dipping subduction zone. Additionally, it renders alternative paleogeographical models in which the terrane is considered to be exotic to the North American continent unlikely. The phase of back-arc spreading resulted in the short-lived existence of an additional Guerrero tectonic plate between the North American and Farallon plates and, upon closure of the back-arc basin, the growth of the North American continent.

NEW RECORD OF HALIMEDACEAN ALGAE FROM THE UPPER TRIASSIC OF THE SOUTHERN ALPS (DOLOMITES, ITALY)

A new record of halimedacean algae is reported from the Upper Triassic (Carnian) of the Dolomites (Southern Alps) near Cortina d’Ampezzo. Based on the gross morphology (non-segmented non-branched thalli) and the internal skeletal arrangement (e.g., medulla with thicker siphons, sometimes bifurcating at high angles; cortex with thinner siphons with multiple Y-like branching), the material can be assigned to the genus Boueina. It is the first record of halimedacean algae from the Upper Triassic of the Southern Alps (Dolomites) and the highly diverse San Cassiano Formation sensu lato (Heiligkreuz Formation). Although the fossilization potential of these algae was low, it is likely that they were much more common than suggested by this sparse fossil record and that they contributed considerably to carbonate production in early Mesozoic times.

Triassic orocline in East Asia: Insights from a transition from passive margin to foreland basin in eastern North China Block

The characteristics and evolution of a continental margin is of key importance in constructing the tectonic framework of a plate boundary. In the eastern North China Block (NCB), there is still controversy regarding the tectonic attribution and types of its continental margin during Paleozoic times, which yielded an understanding of convergence between the NCB and South China Block. In this study, we integrated the previous stratigraphic and sedimentologic results in the NCB to reconstruct the tectono‐paleogeographic evolution of its continental margin. The sedimentary records from Paleozoic to Early Mesozoic times in the eastern and southeastern margin of the NCB can be divided into 3 tectono‐stratigraphic sequences. The Early Paleozoic (Cambrian–Ordovician) sediments that are widespread in the eastern NCB reveal the well‐preserved drifting passive margin successions that consist of enormous thicknesses of limestone and dolomite. During the Late Carboniferous to Early Permian, the eastern NCB was dominated by clastic rocks interbedded with limestones, which implies a convergent passive margin. The Late Permian to Middle Triassic fluvial‐lacustrine units in the eastern NCB have developed in a syn‐collisional foreland basin. We emphasize that the Early Paleozoic passive margin of the eastern NCB directly converted to a convergent platform during the Late Paleozoic, which is consistent with a southeastward subduction rather than a transition to an active continental margin with a northwestward indentation. We suggest that the 3 tectono‐stratigraphic sequences are related to the opening and closure of the Proto‐ and Paleo‐Tethys Ocean in the eastern segment of the northern margin of Gondwana. Moreover, the transition of marine to terrestrial environments is associated with a collisional model about an orocline in East Asia.

Palaeotethys-related sediments of the Karaburun Peninsula, western Turkey: constraints on provenance and stratigraphy from detrital zircon geochronology

Detrital zircon U–Pb geochronology of 15 Late Palaeozoic to Early Mesozoic siliciclastic sandstones from the Karaburun Peninsula in western Turkey determines maximum sedimentation ages, identifies possible source areas, and anchors the study area within the Palaeotethyan realm. Siliciclastic sandstones yielded ages from Triassic to Archean with major input from Palaeozoic to Neoproterozoic sources and very few Mesoproterozoic zircons. The youngest age groups set the new limit of the maximum depositional ages to Late Carboniferous–Early Permian for the Kucukbahce and Dikendagi formations. Detrital zircons from Triassic sandstones are mainly Neoproterozoic and Palaeozoic in age. Zircons from the Scythian–Anisian Gerence Formation are predominantly Devonian and Carboniferous in age, while also Permian and Triassic zircon grains occur in the Carnian–Rhaetian Guvercinlik Formation. According to the zircon age populations and the data available from possible source regions, the Karaburun siliciclastic sediments, with the exception of two samples from the Dikendagi Formation, record sediment supply from units located at the southern margin of Eurasia during Late Palaeozoic and Early Mesozoic times. This interpretation is in agreement with palaeotectonic reconstructions for the closely related Greek islands of Chios and Inousses. The presence of Devonian accompanied by Carboniferous zircons in some of the Karaburun samples reveals similarities with Karakaya Complex sandstones of the Sakarya Zone in NW Turkey.

Provenance analysis of Jurassic sandstones from the Otlaltepec Basin, southern Mexico: Implications for the reconstruction of Pangea breakup

The structural evolution that accompanied the breakup of Pangea during Jurassic time has been constrained in Mexico only at the regional scale on the basis of global plate tectonics and geometric considerations. According to available regional-scale reconstructions, the Jurassic tectonic evolution of Mexico was characterized by: (1) anticlockwise rotation of the Yucatan block along NNW-trending dextral faults and (2) sinistral block motions along W- to WNW-trending faults, which are geometrically needed to restore southern and central Mexico to the northwest of its present position during early Mesozoic time and avoid the overlap between North and South America in the reconstruction of Pangea. Reports of W- to WNW-trending sinistral faults that were active in Mexico during Jurassic time are presently few, and the existence, extension, and age of some of these structures have been questioned by many authors. In this work, we present the provenance analysis from a Jurassic clastic succession deposited within the Otlaltepec Basin in southern Mexico. Whole-rock sandstone petrography integrated with chemical analysis of detrital-garnet and U-Pb detrital-zircon geochronology documents that the analyzed stratigraphic record was deposited during rapid exhumation of the Totoltepec pluton along the Matanza fault, which is a W-trending sinistral normal fault that extends along the southern boundary of the Otlaltepec Basin. U-Pb zircon ages and biostratigraphic data bracket the age of the Matanza fault between 163.5 ± 1 and 167.5 ± 4 Ma. This indicates that the Matanza fault was involved in the crustal attenuation that accompanied the breakup of Pangea and that sinistral motion of continental blocks along W-trending structures was taking place in southern Mexico as predicted by global plate tectonic reconstructions.

Late Paleozoic and Early Mesozoic rare-metal granites in Central Mongolia and Baikal region: review of geochemistry, possible magma sources and related mineralization

The Central Asian Orogenic Belt (CAOB) was a scene of intense granitoid magmatism during the Phanerozoic with formation of vast batholiths: Angara-Vitim and Daurian-Khentei. In the Late Paleozoic and Early Mesozoic times, the peripheral zones of batholiths underwent granitic magmatism associated with rare-metal mineralization. Petrological and geochemical studies show that the rare-metal Li–F granites formed, with a gap about 100 My, large igneous provinces of the Mongol-Okhotsk Belt. Late Paleozoic rare-metal granites build a series of multiphase plutons in the Baikal region (e.g. Kharagul 318 ± 7 Ma, Bitu-Dzhida 311 ± 10 Ma and Urugudei 321 ± 5 Ma). The early medium-grained biotite granites and leucogranites were followed by topaz-bearing microclineand amazonite–albite granites and a series of dikes. The Early Mesozoic epoch was marked by the formation of the Daurian-Khentei Batholith (230–190 Ma) in the center of area and rifting zones with alkaline and rare-metal granite plutons on the peripheries. In contrast to the Late Paleozoic, small Early Mesozoic intrusions (e.g., Avdar Pluton ~10 km2, 212–209 Ma) of rare-metal Li–F granites within the Avdar-Khoshutul series of granitoids coexisted with sizable plutons (e.g., Janchivlan Pluton ~70 km2, 227–195.3 Ma). Rare-metal Li–F granites of the Janchivlan Pluton produced small domal intrusions composed of microcline–albite, amazonite–albite and albite–lepidolite granites. The Sn–Ta–Nb mineralization is associated with albite–lepidolite granites. The rare-metal granites of the Baikal region and Central Mongolia of contrasting ages show an identical geochemical signature of Li–F granites. It is expressed by increase in F, Li, Rb, Cs, Sn, Be, Ta and Pb and decrease in Sr, Ba, Zn, Zr, Th and U contents in course of multiphase intrusions formation. The geochemical data confirm the magmatic model for genesis of the studied rare-metal Li–F granites. However, this process of magma differentiation was terminated with formation of albitites, microclinites and muscovite greisens. The whole-rock geochemistry and isotopic composition of the granites points to the Precambrian crust of the Baikal region (T2DМ = 1.0–1.3 Ga) as the most likely source. We propose the formation of the initial granitic melts due to anatexis of the higher levels of the continental crust, with fluids released during granulite-facies metamorphism in the lower crust. The rare-metal Li–F granites of the studied provinces are intraplate formations geochemically different from the Early Paleozoic collisional granitoids. This could be caused by the influence of deep-seated source on the occurrence of rare-metal magmatism.

Paleomagnetism of Devonian dykes in the northern Kola Peninsula and its bearing on the apparent polar wander path of Baltica in the Precambrian

Mafic dykes and large alkaline and carbonatite intrusions of Middle-Late Devonian age are widespread on the Kola Peninsula in NE Fennoscandia. These magmatic rocks are well characterized with petrographic, geochemical and geochronological data but no paleomagnetic results have been reported yet. We studied dolerite dykes from the northern part of the Peninsula and isolated three paleomagnetic components in these rocks. A low-temperature component is aligned along the present-day field, while a major constituent of natural remanent magnetization is an intermediate-temperature component (Decl.=79.6°, Inc.=78.5°, α95=5,9°, N=17 sites) that is present in most Devonian dykes but is found in some baked metamorphic rocks and Proterozoic dykes too. Finally, a primary Devonian component could be reliably isolated from two dykes only. Rock-magnetic studies point to presumably primary low-Ti titanomagnetite and/or pure magnetite as the main remanence carriers but also reveal alteration of the primary minerals and the formation of new magnetic phases.The directions of a major component differ from the Middle Paleozoic reference data for Baltica but closely match those for the 190±10Ma interval recalculated from the apparent polar wander path of the craton. We assume that this Early Jurassic component is a low-temperature overprint of chemical origin. The main impact of the new results is not to mid-Paleozoic or Early Mesozoic times but to much older epochs. Analysis of paleomagnetic data shows that the directionally similar remanences are present in objects with the ages ranging from 500Ma to ~2Ga over entire Fennoscandia. Hence we argue that an Early Jurassic remagnetization is of regional extent but cannot link it to a certain process and a certain tectonic event. If true, this hypothesis necessitates a major revision of the APWP for Baltica over a wide time interval.

The Mesozoic successions of western Sierra de Zacatecas, Central Mexico: provenance and tectonic implications

AbstractCentral Mexico was subject to active tectonics related to subduction processes while it occupied a position in western equatorial Pangea during early Mesozoic time. The subduction of the palaeo-Pacific plate along the western North American and South American active continental margins produced volcanic arc successions which were subsequently rifted and re-incorporated to the continental margin. In this context, the fringing arcs are important in unravelling the continental accretionary record. Using petrographic analysis, detrital zircon geochronology and structural geology, this paper demonstrates that the Guerrero Arc (Guerrero Terrane) formed on top of a felsic volcaniclastic unit (Middle Jurassic La Pimienta Formation) and siliciclastic strata (Upper Triassic Zacatecas Formation and Arteaga Complex) of continental Mexican provenance, deposited across the continental margin and oceanic substrate. This assemblage was rifted away from continental Mexico to form an intervening oceanic assemblage (Upper Jurassic – Lower Cretaceous Las Pilas Volcanosedimentary Complex of the Arperos Basin), then accreted back more or less at the same place, all above the same east-dipping subduction zone. The accretion of the Guerrero Arc to the Mexican continental mainland (Sierra Madre Terrane) caused the deposition of a siliciclastic unit (La Escondida Phyllite), which recycled detritus from the volcaniclastic and siliciclastic underlying strata.

The closure of Palaeo-Tethys in Eastern Myanmar and Northern Thailand: New insights from zircon U–Pb and Hf isotope data

Two of the major granite belts of Southeast Asia are the Main Range and Eastern Province. Together, these are interpreted to represent the magmatic expression of the closure of Palaeo-Tethys during Late Palaeozoic to Early Mesozoic times. Recent geochronological and geochemical work has better delineated these belts within Peninsular Malaysia, thereby providing important constraints on the timing of Palaeo-Tethys suturing. However, the northern extension of this Palaeo-Tethyan suture is less well understood. Here we present new ion microprobe U–Pb zircon age data from northern Thailand and eastern Myanmar. Measured ages of 219 and 220Ma from the Kyaing Tong granite imply northern extension of the Main Range Province into eastern Myanmar. The Tachileik granite in far eastern Myanmar yields an age of 266Ma, consistent with published Eastern Province ages, and this therefore constrains the northern extension of the Palaeo-Tethys suture in eastern Myanmar. We further discuss how this suture may extend northwards into Yunnan. A Late Cretaceous age (70Ma) measured in Thailand represents later magmatic activity, and is similar to published magmatic ages from central Myanmar. This younger magmatism is interpreted to be related to the subduction of Neo-Tethys prior to India–Asia collision. Further, we present new laser ablation zircon Hf isotope data from eastern Myanmar which suggest that Palaeoproterozoic crust underlies both the Main Range and Eastern Province granites. Our εHf model age of ca. 1750Ma from Sibumasu, the basement underlying eastern Myanmar, lies within the range of other model ages reported thus far for the Baoshan Block north in Yunnan, interpreted by some to be the northern extension of Sibumasu.

Structural characteristics and LA–ICP-MS U–Pb zircon geochronology of the deformed granitic rocks from the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

We describe the recently detected ENE-trending Xingcheng-Taili ductile strike-slip shear zone, which is an important Mesozoic low- to middle-grade tectonic element formed during the reactivation of the North China Craton (NCC) at the eastern termination of the Yanshanian orogenic belt. Within the Xingcheng-Taili ductile shear zone, the degree of deformation weakens gradually from NW to SE. Three main types of banded granitic gneisses represent low- to middle-grade metamorphic tectonites. New U–Pb zircon dating gives a wide range of protolith ages. Neoarchean granitic rocks with an emplacement age of 2498.4±6.3Ma have obvious E–W trending banded structures. Upper Triassic granitic rocks with U–Pb zircon ages of 219.7±1.1Ma, 218.74±0.61Ma, 212.1±1.6Ma and 212.3±1.9Ma are considered to relate to the collision of the NCC and Yangtze craton in early Mesozoic times. The emplacement of biotite adamellite bodies with ages of 159.0±0.8Ma and 152.4±1.9Ma is related to the thinning of the NCC's continental crust. An ENE-trending sinistral ductile shear zone developed within these granites. The strongly deformed biotite adamellite bodies bear a gneissosity with S–C fabrics, ENE-trending sinistral shear zones and show the deformation characteristics of a shallow crustal level, reflecting decratonization of the NCC. Crystal lattice preferred orientations (LPO) of quartz, determined by electron back-scatter diffraction, suggest sinistral strike-slip shear and a deformation temperature at about 400 to 500°C. Quartz mainly shows low-temperature fabrics with a dominant {0001}-slip system similar to the temperature estimated by the micro-structural characteristics of quartz and feldspar. This deformation event might have occurred in Early Cretaceous times and is related to the lithospheric thinning and extension of the North China Craton triggered by an ongoing roll-back of the Pacific Plate beneath the eastern North China Craton.

Wide rift model in Bohai Bay Basin: insight into the destruction of the North China Craton

The Bohai Bay Basin is a Cenozoic extensional basin along the eastern aspect of Asia. Whether the Bohai Bay Basin is a pull-apart or rift basin is controversial. The Bohai Bay Basin exhibits a high density of extensional faults and records destruction of the North China Craton. Many structural analyses have been performed on the Bohai Bay Basin, especially the Tan-L and Taihang Mountain fault systems which control its boundary. The initial deposition of Kongdian Formation was mainly distributed along the boundary of Bohai Bay Basin during the Palaeocene–early Eocene. Subsequently, tectonic activity migrated toward the interior of the basin during deposition of Shahejie Formation in the middle Eocene–early Oligocene. Bohai Bay Basin crust was thickened in early Mesozoic time and has thinned since late Mesozoic time. The crustal strength profile of Bohai Bay Basin is characterized by very weak lower crust, which differs from that of adjacent crust. In regard to the crustal structure, lithospheric thickness, and extensional style, an alternative rift model is proposed. Initial Bohai Bay Basin rifts were characterized by metamorphic core complexes affecting the North China Craton, which reflects collapse of parts of the early Mesozoic intra-plate orogen. Furthermore, westward subduction of the Palaeo-Pacific Plate led to upwelling of asthenosphere mantle. Persistent upwelling of mantle decreased the strength of lower crust and led to the warm heat-flow regime and generation of a lower crustal fluid layer and wide rifting. Outward flow of ductile lower crust following late Cretaceous extension thinned the lower crust and generated the overall sag appearance of the basin in early Cenozoic time. The model supports a model whereby a wide rift narrows with time. For the Bohai Bay Basin, extension and strike-slip faulting were two independent deformation systems superimposed on each other.

Jurassic accretionary complex and ophiolite from northeast Turkey: No evidence for the Cimmerian continental ribbon

Permian-Triassic and Late Cretaceous accretionary complexes, ascribed to the consumption of two distinct oceans, the Paleo- and Neo-Tethys, are exposed over extensive areas in the Eastern Mediterranean region. However, a separating continental ribbon, the so-called Cimmeride continent, between the Paleo- and Neo-Tethys during early Mesozoic time cannot be defined. Here we report a previously unknown Early Jurassic metamorphic oceanic accretionary complex and ophiolite from northeast Turkey, bounded by oceanic accretionary complexes of Permian-Triassic and Late Cretaceous age to the north and the south, respectively, without a continental domain in between. This special tectonic position and widespread coexistence of Permian-Triassic and Late Cretaceous accretionary complexes alongside the Izmir-Ankara-Erzincan suture imply that (1) the southern margin of Laurasia in the eastern Mediterranean region grew by episodic accretionary processes from late Paleozoic to end-Mesozoic time without involvement of a Cimmerian continental ribbon, and (2) the Paleo-Tethys and northern branch of the Neo-Tethys were not distinct oceans in the Eastern Mediterranean region.

Petrogenesis and tectonic implications of early Silurian high-K calc-alkaline granites and their potassic microgranular enclaves, western Kunlun orogen, NW Tibetan Plateau

The western Kunlun orogen occupies a key position along the tectonic junction between the Pan-Asian and Tethyan domains, reflecting Proto- and Palaeo-Tethys subduction and terrane collision during early Palaeozoic to early Mesozoic time. We present the first detailed zircon U–Pb chronology, major and trace element, and Sr–Nd–O–Hf isotope geochemistry of the Qiukesu pluton and its microgranular enclaves from this multiple orogenic belt. SHRIMP zircon U–Pb dating shows that the Qiukesu pluton was emplaced in the early Silurian (ca. 435 Ma). It consists of weakly peraluminous high-K calc-alkaline monzogranite and syenogranite, with initial 87Sr/86Sr ratios of 0.7131–0.7229, ϵNd(T) of –4.1 to –5.7, δ18O of 8.0–10.8‰, and ϵHf(T) (in situ zircon) of –4.9. Elemental and isotopic data suggest that the granites formed by partial melting of lower-crustal granulitized metasedimentary-igneous Precambrian basement triggered by underplating of coeval mantle-derived enclave-forming intermediate magmas. Fractional crystallization of these purely crustal melts may explain the more felsic end-member granitic rocks, whereas such crustal melts plus additional input from coeval enclave-forming intermediate magma could account for the less felsic granites. The enclaves are intermediate (SiO2 57.6–62.2 wt.%) with high K2O (1.8–3.6 wt.%). They have initial 87Sr/86Sr ratios of 0.7132–0.7226, ϵNd(T) of –5.0 to –6.0, δ18O of 6.9–9.9‰, and ϵHf(T) (in situ zircon) of –8.1. We interpret the enclave magmas as having been derived by partial melting of subduction-modified mantle in the P–T transition zone between the spinel and spinel-garnet stability fields. Our new data suggest that subduction of the Proto-Tethyan oceanic crust was continuous to the early Silurian (ca. 435 Ma); the final closure of the Proto-Tethys occurred in the middle Silurian.