The Proterozoic evolution of northern Siberian Craton margin: a comparison of U–Pb–Hf signatures from sedimentary units of the Taimyr orogenic belt and the Siberian platform

Identifying the cratonic affinity of Neoproterozoic crust that surrounds the northern margin of the Siberian Craton (SC) is critical for determining its tectonic evolution and placing the Craton in Neoproterozoic supercontinental reconstructions. Integration of new U–Pb–Hf detrital zircon data with regional geological constraints indicates that distinct Neoproterozoic arc-related magmatic belts can be identified within the Taimyr orogen. Sedimentary rocks derived from 970 to 800 Ma arc-related suites reveal abundant Archean and Paleoproterozoic detritus, characteristic of the SC. The 720–600 Ma arc-related zircon population from the younger Cambrian sedimentary rocks is also complemented by an exotic juvenile Mesoproterozoic zircon population and erosional products of older arc-related suites. Nonetheless, numerous evidences imply that both arcs broadly reworked Siberian basement components. We suggest that the early Neoproterozoic (ca. 970–800 Ma) arc system of the Taimyr orogen evolved on the active margin of the SC and probably extended along the periphery of Rodinia into Valhalla orogen of NE Laurentia. We also suggest the late Neoproterozoic (750–550 Ma) arc system could have been part of the Timanian orogen, which linked Siberia and Baltica at the Precambrian/Phanerozoic transition.     

Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling,China: insights from LA-ICP-MS zircon U–Pb ages,Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

West Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO₂ (63.59–76.22%) and low P₂O₅ (0–0.2%) concentrations, a high K₂O/Na₂O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous ε_Hf(t) (?4.425 to 1.067 for Xujiaba and ?4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.     

Reconstructing the Cryogenian–Ediacaran evolution of the Porongos fold and thrust belt,Southern Brasiliano Orogen,based on Zircon U–Pb–Hf–O isotopes

The Neoproterozoic geotectonic triad of the Brasiliano Orogen is reconstructed in southern Brazil from studies focused on the Porongos fold and thrust belt. We integrate field geology with isotopic studies of zircon U–Pb SHRIMP and Lu–Hf–O laser determinations in seven metasedimentary and three metavolcanic rock samples. The results indicate that the Porongos palaeo-basin was derived from mixed sources (3200–550 Ma), with major contributions from Rhyacian (2170 Ma) and Ediacaran (608 Ma) sources. Minor contributions from Archaean to Tonian sources are also registered. The maximum depositional age of the Porongos palaeo-basin is established by the age range of 650–550 Ma with T_DM model ages between 2.5 and 1.3 Ga. The reworked signature (ε_Hf values = ?34 to ?4) and the characteristic crustal magma reservoirs (δ¹⁸O ≥5.3 ‰) indicate that these sediments are equivalent to Neoproterozoic granites of the Dom Feliciano Belt. The episodic depositional history started in the Cryogenian (650 Ma) and lasted until the Ediacaran (most likely 570 Ma). A magmatic event of Tonian age is recorded in rhyodacite samples interleaved with the metasedimentary rocks and dated at 773, 801, and 809 Ma. The crustal evolution of the Sul-Riograndense Shield included mountain building, folding and thrusting and flexural subsidence in the foreland. An orogenic triad is revealed as the Pelotas Batholith, the Porongos fold and thrust belt and the Camaquã Basin, all part of the Dom Feliciano Belt.     

Constraining the timing of porphyry mineralization in northwest Iran in relation to Lesser Caucasus and Central Iran; Re–Os age data for Sungun porphyry Cu–Mo deposit

The Neo-Tethyan subduction in Iran is characterized by the Urumieh–Dokhtar magmatic arc (UDMA), formed by northeast-ward subduction of the oceanic crust beneath the central Iran. This belt coincides with the porphyry copper metallogenic belt that comprises several metallogenic zones, including Ahar–Jolfa in northwest Iran. The Ahar–Jolfa metallogenic zone encompasses two main batholiths of Qaradagh and Sheyvardagh and numerous intrusive bodies of Cenozoic, which have produced many base and precious metal deposits and prospects. The former is considered as continuation of the Meghri–Ordubad pluton in South Armenian Block (SAB), which also hosts porphyry copper deposits (PCDs). The Sungun PCD is the largest occurrence in northwest Iran. Rhenium-Osmium ages of Sungun molybdenites are early Miocene and range between 22.9 ± 0.2 and 21.7 ± 0.2 Ma. Comparison of the ages obtained here with published ages for mineralization across the region suggests the following sequence. The earliest porphyry Cu–Mo mineralization event in northwest Iran is represented by Saheb Divan PCD of late Eocene age, which is followed by the second epoch of middle Oligocene, including the Cu–Mo–Au mineralization at Qarachilar and the Haftcheshmeh PCD. Mineralization in Sungun, Masjed Daghi, Kighal and Niaz deposits corresponds to the third mineralization event in northwest Iran. The first epoch in northwest Iran postdates all Eocene mineralizations in SAB, while the second epoch is coeval with Paragachay and the first-stage of Kadjaran PCDs. Its third epoch is younger than all mineralizations in SAB, except the second stage in Kadjaran PCD. Finally, the Cu mineralization epochs in northwest Iran are older than nearly all PCDs and prospects in Central Iran (except the Bondar Hanza PCD), altogether revealing an old to young trend along the UDMA and the porphyry Cu belt towards southeast, resulted from diachronous, later closure of the Neo-Tethyan oceanic basin in central and SE Iran.     

Subduction-related 200 Ma Talun metagranite,SE Taiwan: an age constraint for palaeo-Pacific plate subduction beneath South China Block during the Mesozoic

Various tectonic models have been proposed to account for the widely distributed igneous activities in the southeastern part of the South China Block (SCB) during the Triassic–Jurassic period. One of the major contending debates is on the timing of initiation of the palaeo-Pacific plate subduction under the SCB, due to lack of unequivocal evidence for arc magmatism during the period in this region.

The 191 ± 10 Ma (N = 5, MSWD = 12) calc-alkalic high-K I-type Talun metagranite occurs in the southern Tailuko belt of the Tananao metamorphic complex, Taiwan. In terms of age, this metagranite belongs to the Early Yanshanian igneous activity in the southeastern part of the SCB. However, its geographic position does not accord with the well-known general oceanward younging trend of the Yansnanian igneous rocks. In view of the large age uncertainty reported, this metagranite is redated in this study. Some zircons of this metagranite are high in U content and are metamict. Zircons with low U contents are analysed by SHRIMP yielding a more precise age of 200 ± 2 Ma (N = 10, MSWD = 4). In particular, the εHf(t) of these dated zircons ranges from +4.5 to +12.9. The metagranite mainly consists of quartz, K-feldspar, plagioclase, with minor amounts of garnet, biotite, zircon, apatite, and pyrrhotite. Chlorite and calcite are secondary phases overprinted by the later tectonic event(s). Its initial Sr isotope compositional range is 0.70473–0.70588, and εNd(t), +2.4 to +3.6. The results demonstrate that the genesis of this metagranite could be attributed to the assimilation-fractionation of a depleted mantle-derived basaltic magma, which was most likely related to arc magmatism. The present study therefore offers key evidence that during the Mesozoic, the palaeo-Pacific plate subduction underneath the SCB would have taken place no later than the very early Jurassic.     

Geochemistry of late Palaeozoic granitoids of the Balkhash metallogenic belt,Kazakhstan: implications for crustal growth and tectonic evolution of the Central Asian Orogenic Belt

ABSTRACT

The Balkhash metallogenic belt (BMB) in Kazakhstan is a famous porphyry Cu–Mo metallogenic belt in the Central Asian Orogenic Belt (CAOB). The late Palaeozoic granitoids in the BMB are mainly high-K calc-alkaline and I-type granites, with shoshonite that formed during a late stage. Geochemical analyses and tectonic discrimination reveal a change in the tectonic environment from syn-collision and volcanic arcs during the Carboniferous to post-collision during the Permian. The late Palaeozoic granitoids from the Borly porphyry Cu deposit formed in a classical island-arc environment, and those from the Kounrad and Aktogai porphyry Cu deposits and the Sayak skarn Cu deposit are adakitic. The ε_Nd(t) values for the late Palaeozoic granitoids are between ?5.87 and +5.94, and the ε_Sr(t) values range from ?17.16 to +51.10. The continental crustal growth histories are different on either side of the Central Balkhash fault. On the eastern side, the ε_Nd(t) values of the granitoids from the Aktogai and Sayak deposits are very high, which are characteristic of depleted mantle and suggest that crustal growth occurred during the late Palaeozoic. On the western side, the ε_Nd(t) values of the granitoids from the Borly and Kounrad deposits are slightly low, which suggests the presence of a Neoproterozoic basement and the mixing of crust and mantle during magmatism. The granitoids have initial ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of 18.335–20.993, 15.521–15.732, and 38.287–40.021, respectively, which demonstrate an affinity between the late Palaeozoic magmatism in the BMB and that in the Tianshan, Altai, and Junggar orogens.     

The Pelona–Orocopia–Rand and related schists of southern California: a review of the best-known archive of shallow subduction on the planet

ABSTRACT

The Pelona–Orocopia–Rand and related schists of southern California are an archetypal example of an exhumed shallow subduction complex. ‘The schist’ comprises mainly trench materials underthust beneath continental arc rocks during Late Cretaceous–early Cenozoic collision of one or more oceanic plateaux with southern California. The arc-on-trench relationship, without intervening mantle or lowermost crust, implies that significant subduction erosion accompanied shallow subduction. Upsection increases in metamorphic grade (~150 ± 100°C/km) and spatial variations in age and peak temperature provide an ~50 million year long record of tectonic underplating within a cooling system. Evidence for palaeoseismic events in earliest formed and hottest (locally transitional granulite grade) schists provides a possible field-based record of episodic tremor and slow slip events such as detected in several modern shallow subduction zones. Structural ascent of the schist was achieved in distinct Late Cretaceous–early Eocene and late Oligocene–early Miocene extensional pulses, the first during collapse of gravitationally unstable upper plate assemblages and accompanied by trench-directed (top-NE) lower plate extrusion and the second corresponding temporally, spatially, and in character with core complex formation in the SW United States. The line between schist and core complex belts is blurred by the recent discovery of schist within 40 km of the nearest core complex and containing synkinematic Miocene intrusions, a hallmark of SW U.S. core complexes. The history of schist assembly, metamorphism, and exhumation provides the most complete field-based record of thermo-mechanical processes, subduction erosion and tectonic underplating in particular, that operated during a shallow subduction event. Future cross-disciplinary investigations of, and comparisons between, the schist and other possible ancient (e.g. Swakane gneiss, Sanbagawa belt, Qiangtang terrane) and modern (e.g. Cascadia, SW Japan, central Mexico, Chile) shallow subduction zones will yield new insights into the tectonic and petrologic processes that operate within such systems.     

The Cenomanian–Turonian Boundary Event (CTBE) on the southern slope of the Subalpine Basin (SE France) and its bearing on a probable tectonic pulse on a larger scale

The Cenomanian–Turonian Boundary Event (CTBE) event is not associated with a transgression on the southern margin of the Subalpine Basin, but with a steady shallowing-up trend beginning in the lower half of the δ¹³C positive shift. The SW–NE Rouaine Fault had a complex role, first in isolating a black shale basin to the west and a large, deep submarine plateau devoid of black shale to the east, then by a strike-slip movement that induced a forced progradation to the north of the southern platform in the eastern compartment. This compressive tectonic reactivation of the southern margin began around the deposition of the local equivalent of the Plenus bed of boreal basins, as shown by correlation supported by both isotope and palaeontological data. Other local data are pieced together to suggest that the whole of SE France underwent a short-lived transpressive tectonic pulse around the Cenomanian–Turonian boundary, probably connected with the early compressive movement of Africa vs. Europe. On a larger scale, other published data suggest that this pulse could be a global one. It is coeval with renewed thrust loading, volcanism and transgression in the North-American Western Interior, local emergences during the event along the eastern Atlantic margin, suggesting a slight tendency to inversion of the margin, and a tilting to the east of the North-Africa plate that could explain the large transgression recorded from Morocco to Tunisia on the Saharan Craton.New isotope and palaeontological (coiling ratio of Muricohedbergella delrioensis) data from SE France suggest that two coolings of suprabasinal importance occurred just before and during the build-up of the d¹³C shift, including the boreal “Plenus Marls“, especially its middle limestone bed and its SE France equivalent.Regarding the extinction of the genus Thalmaninella and Rotalipora and during the event, neither anoxia nor climate changes can fully explain the palaeontological crisis, given that Rotalipora cushmani crosses the first phase of anoxia without harm, as well as the two coolings, not only in SE France but on a large scale, as shown by the correlation of the published data. This extinction needs alternative explanations as we challenge both anoxia and climate as major causes.     

Biostratigraphy and facies analysis of the Upper Cretaceous–Danian? platform carbonate succession in the Kuyucak area,western Central Taurides,S Turkey

The Upper Cretaceous succession outcropping in the Anamas–Akseki Autochton, consists of approximately 500 m thick purely platform carbonate sediments. It begins with Cenomanian limestones intercalated with limestone breccias (Unit-1) containing mainly Pseudorhapydionina dubia, Pseudonummoloculina heimi, Spiroloculina cretacea (Assemblage I) and unconformably overlies the Lower Cretaceous (Barremian–Aptian) limestones with Vercorsella laurentii, Praechrysalidina infracretacea and Salpingoporella hasi. The Cenomanian limestones include foraminiferal packstone–wackestone, peloidal packstone–wackestone and mudstone microfacies deposited in restricted platform conditions. The Cenomanian succession is truncated by an unconformity characterised by locale bauxite deposits. Immediately above the unconformable surface, dolomitic limestones and rudistid limestones (Unit-2) are assigned to the upper Campanian based on the benthic foraminiferal assemblage (Assemblage II) comprising mainly Murciella gr. cuvillieri, Pseudocyclammina sphaeroidea, Accordiella conica, Scandonea samnitica and Fleuryana adriatica (smaller-sized populations). The upper Campanian limestones composed of dominantly foraminiferal-microbial packstone–wackestone microfacies deposited in shallow water environment with low energy, restricted circulation. The following limestones of the Unit-2 is characterised by sporadic intercalation of “open shelf” Orbitoides, Omphalocyclus, Siderolites assemblage (Assemblage III), assigned to the Maastrichtian, in addition to pre-existing “restricted platform” species. In the upper part of this biozone, the Rhapydionina liburnica/Fleuryana adriatica concurrent range subzone (Assemblage IIIb) is distinguished by the presence of Valvulina aff. triangularis, Loftusia minor as well as the nominal species. The Maastrichtian limestones with sporadically open marine influence consist of bioclastic (rudist-bearing) packstone–floatstone, foraminiferal packstone–wackestone with rudist fragments and peloidal/intraclastic packstone–wackestone microfacies deposited in shallow subtidal–subtidal (lagoonal) environments. The Upper Cretaceous succession passes upwardly into 70 m thick limestones and clayey limestones (Unit-3) which do not contain rudists and pre-existing foraminiferal assemblage with one exception Valvulina aff. triangularis. Variable amounts of ostracoda, discorbids, miliolids, dasycladacean algae and Stomatorbina sp. (Assemblage IV) occur into mud-rich microfacies suggesting restricted conditions with low water energy. A probable Danian age is proposed for the Unit-3 based on the occurrence of Valvulina aff. triangularis and Stomatorbina sp. which were previously recorded from Danian of peri-Tethyan platforms.     

A 2.5-D coupled FE–BE model for the dynamic interaction between saturated soil and longitudinally invariant structures

A two-and-a-half-dimensional (2.5-D) coupled finite element–boundary element (FE–BE) model is presented to simulate the three-dimensional dynamic interaction between saturated soils and structures with longitudinally invariant geometries. A regularized 2.5-D boundary integral equation for saturated porous media is derived that avoids the evaluation of singular traction integrals. The 2.5-D coupled FE–BE model is established by using the continuity conditions on the soil–structure interface. The developed model is verified through comparison with an existing semi-analytical method. Two case studies of a tunnel embedded in a poroelastic half-space and the efficiency of a vibration isolating screen are presented.