Late Silurian Middle Devonian Research Articles

Proto-Tethys magmatic evolution along northern Gondwana: Insights from Late Silurian–Middle Devonian A-type magmatism, East Kunlun Orogen, Northern Tibetan Plateau, China

The East Kunlun Orogen records the geological evolutions of the Neoproterozoic – Early Paleozoic Proto-Tethyan Ocean and Late Paleozoic–Mesozoic Paleo-Tethys Ocean along northern Gondwana. However, the late-stage evolution of the Proto-Tethyan Ocean and the configuration of peri-Gondwana microcontinents during the Silurian – Devonian is under debate. Here we report new geochronological and geochemical data of A-type granites from the western Wulonggou and the eastern Gouli areas in the East Kunlun Orogen to deepen our understanding of these problems. Zircon LA-ICP-MS UPb data reveal that the Danshuigou monzogranite and Shenshuitan syenogranite from the western Wulonggou area were emplaced simultaneously at 418 ± 3 Ma, while the Niantang syenogranite from the eastern Gouli area was emplaced at 403 ± 2 Ma. All these rocks display high-K calcic-alkalic to shoshonitic and metaluminous to slight peraluminous signatures, with relatively low CaO, Al2O3, MgO and Sr, and high FeOt/MgO, Ga/Al, Zr, and Nb, indicating their A-type affinity. Their moderate whole-rock εNd(t) (−5.3 to −0.6) and zircon εHf(t) (−6.3–6.4) are different from those of depleted mantle and old basement rocks, but similar to those of the Ordovician–Silurian granitoids in the East Kunlun Orogen. These chemical signatures, together with the anhydrous, low-pressure and high-temperature characteristics of the magmas, indicate that partial melting of the Ordovician–Silurian granitoids generated these A-type granites. Regionally, these A-type granites and previously reported A-type granites in the East Kunlun Orogen compose a Late Silurian – Middle Devonian A-type granite belt. This belt, together with the regionally coeval molasse formation and mafic-ultramafic rocks, indicate a post-collisional extensional regime for the East Kunlun Orogen during the Late Silurian – Middle Devonian. Given that extensive contemporaneous post-collision-related magmatic rocks have also been revealed in the neighboring West Kunlun, Altyn, Qilian and Qinling blocks/terranes, we contend that the Neoproterozoic – Early Paleozoic Proto-Tethyan Ocean that separated these blocks/terranes from Gondwana had closed by the Late Silurian – Middle Devonian, which]resulted in the re-welding of the above blocks/terranes to northern Gondwana or Gondwana-derived microcontinents.

Early–Middle Paleozoic subduction–collision history of the south-eastern Central Asian Orogenic Belt: Evidence from igneous and metasedimentary rocks of central Jilin Province, NE China

To constrain the Early–Middle Paleozoic tectonic evolution of the south-eastern segment of the Central Asian Orogenic Belt (CAOB), we undertook zircon U–Pb dating and analyzed major and trace elements and zircon Hf isotope compositions of Late Cambrian to Middle Devonian igneous and metasedimentary rocks in central Jilin Province, NE China. LA–ICP–MS zircon U–Pb dating indicates that the Early–Middle Paleozoic magmatism in central Jilin Province can be divided into four episodes: Late Cambrian (ca. 493Ma), Middle Ordovician (ca. 467Ma), Late Ordovician–Early Silurian (ca. 443Ma), and Late Silurian–Middle Devonian (425–396Ma). The progression from subduction initiation to maturity is recorded by Late Cambrian low-K tholeiitic meta-diabase, Middle Ordovician medium-K calc-alkaline pyroxene andesite, and Late Ordovician to Early Silurian low-K tonalite, which all have subduction-related characteristics and formed in an evolving supra-subduction zone setting. Late Silurian to Middle Devonian calc-alkaline igneous rocks, with the lithological association of granodiorite, monzogranite, rhyolite, dacite, and trachydacite, show progressively increasing K2O contents from medium K to shoshonite series. Furthermore, the Early–Middle Devonian monzogranites are characterized by high K2O, Sr/Y, and [La/Yb]N values, indicating they were generated by the melting of thickened lower crust. These results suggest a transition from subduction to post-orogenic setting during the Late Silurian–Middle Devonian. Our interpretation is supported by the maximum age of molasse deposition in the Zhangjiatun member of the Xibiehe Formation. Overall, we suggest that Late Cambrian tholeiitic meta-diabase, Middle Ordovician pyroxene andesite, and Late Ordovician–Early Silurian tonalite formed above the northward-subducting and simultaneously seaward-retreating of Paleo-Asian Ocean plate. Subsequently, the northern arc collided with the North China Craton and post-orogenic extension occurred during the Late Silurian–Middle Devonian.

Middle Paleozoic initial amalgamation and crustal growth in the West Junggar (NW China): Constraints from geochronology, geochemistry and Sr–Nd–Hf–Os isotopes of calc-alkaline and alkaline intrusions in the Xiemisitai-Saier Mountains

As one of the important parts of the Central Asian Orogenic Belt, the Early–Middle Paleozoic subduction–accretion history of the West Junggar is still needed to be further constrained although major progress has been made in the past years. In particular, the geodynamic setting for Late Silurian–Early Devonian A-type granites and associated mafic to felsic magmatism in the northern West Junggar is still controversial. In this study, we report new geochronological and geochemical results for Middle Paleozoic plutons in the Xiemisitai-Saier Mountains, northern West Junggar, including undeformed peralkaline granite, alkali feldspar granite, monzogranite, quartz syenite, syenite, monzodiorite, diorite, and hornblende gabbro. These plutons crop out in the eastern segment of the Early–Middle Paleozoic Boshchekul–Chingiz arc and invade into Early–Middle Paleozoic marine volcanic and pyroclastic sequences. New and published zircon U–Pb ages indicate that these intrusions were emplaced during the Late Silurian–Middle Devonian (424–392Ma). Late Silurian hornblende gabbro, diorite, and monzodiorite commonly contain abundant hornblendes and exhibit mantle-like Nb/Ta ratios (15.9–18.7), negligible Eu anomalies, with primitive Sr–Nd–Hf [I(Sr)=0.70370–0.70375, εNd(t)=+5.1 to +6.0 and εHf(t)=+14.3 to +16.2] and radiogenetic Os (I(Os)=0.2099–0.8795) isotopic signatures. They may be originated from a fluid metasomatized, H2O-rich and isotopically depleted mantle source and suffered some crustal contamination. Early–Middle Devonian calc-alkaline monzogranite and alkaline–peralkaline granitoids may have distinct origins. The monzogranite is I-type granite with high La/Yb ratios, depleted εNd(t) values (+5.1), and crust-like Nb/Ta ratios (8.5–8.9), which is suggested to be partial melt of juvenile lower crust containing massive hornblende±garnet. The peralkaline–alkaline granitoids include A-type and highly fractionated I-type granites, which contain mafic–intermediate enclaves and chemically exhibit variable element ratios, analogical εNd(t) values (+5.1 to +6.2), and significantly high I(Os) value (2.1247). They are interpreted to be produced by fractional crystallization of the hybrid magma contributed from mantle and crustal sources.Based on our new data, along with other geological evidence, it is speculated that these A-type granites and associated mafic to felsic intrusions may occur in a post-collisional extension setting since the end-Silurian, after the initial amalgamation of the West Junggar terrane during the Middle–Late Silurian. On-going lithosphere extension during the Early–Middle Devonian would result in the rifting of newly amalgamated West Junggar and thus induce asthenosphere upwelling, which gave rise to the mantle-derived magmatism in the northern and southern West Junggar and juvenile oceanic crust in central West Junggar. It is suggested that the Middle Paleozoic is an important episode of crustal growth in the West Junggar.

The late Silurian–Middle Devonian long-term eustatic cycle as a possible control on the global generic diversity dynamics of bivalves and gastropods

Abstract A long-term eustatic cycle (fall and subsequent rise of the global sea level) embraced the late Silurian-Middle Devonian time interval. Potentially, these sea-level changes could drive global biodiversity. The stratigraphic ranges of 204 bivalve genera and 279 gastropod genera included into the famous Sepkoski database allow reconstructing changes in the total diversity and the number of originations and extinctions of these important groups of marine benthic macro- -invertebrates during this interval. None of the recorded parameters coincided with the long-term global sea-level cycle. It cannot be not excluded, however, that the global sea-level changes did not affect the regions favourable for bivalve and gastropod radiation because of regional tectonic mechanisms; neither can it be excluded that the eustatic control persisted together with many other extrinsic and intrinsic controls. Interestingly, the generic diversity of gastropods increased together with a cooling trend, and vice versa. Additionally, the Ludlow, Eifelian, and Givetian biotic crises affected, probably, both fossil groups under study. There was also a coincidence of the relatively high bivalve generic diversity, initial radiation of gastropods and the entire biota, and the diversification of brachiopods with the Early Devonian global sea-level lowstand, and this may be interpreted as evidence of a certain eustatic control on the marine biodiversity.

Late Silurian–Middle Devonian long-term shoreline shifts on the northern Gondwanan margin: eustatic versus tectonic controls

Abstract Long-term shoreline shifts reflect eustatic changes, tectonic activity, and sediment supply. Available lithostratigraphical data from northern Africa, Arabia, and the Tethys Hymalaya, coupled with facies interpretations, permit us to trace late Silurian–Middle Devonian long-term shoreline shifts across the northern Gondwanan margin and to compare them with constraints on global sea-level changes. Our analysis establishes a regression–transgression cycle. Its coincident global sea-level changes reveal the dominance of the eustatic control. A transgression–regression cycle observed in Arabia is best explained by regional subsidence. Our study highlights the importance of constraining the role of regional tectonics when interpreting shoreline shifts.

The Baikal-Vitim Fold System: Structure and geodynamic evolution

New data on the stratigraphy, structure, isotopic age, geochemistry, and geodynamic characteristics of the lithotectonic complexes of the Baikal-Vitim Fold System are reported. In particular, it is shown that Middle and Upper Paleozoic rocks are widespread along with Precambrian and Lower Paleozoic sequences. The Baikal-Vitim Fold System is characterized by cyclic evolution and comprises four structural stages: Baikalian (Riphean-Vendian), Caledonian (Cambrian-Early Silurian), Variscan (Late Silurian-Early Carboniferous), and Hercynian (Middle Carboniferous-Permian). A specific set of lithotectonic complexes formed in certain geodynamic settings corresponds to each stage. According to the proposed model, the Variscan and Hercynian complexes developed under conditions of progressively changing geodynamic settings of passive (Late Silurian-Middle Devonian), Andean-type active (Middle Devonian-Early Carboniferous), and Californian-type (Middle Carboniferous-Permian) continental margins. The Middle and Late Paleozoic evolution of the Baikal-Vitim Fold System is correlated with that of the Mongolia-Okhotsk Belt (Aga paleooceanic basin).

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.

Synthesis of Palaeozoic deformational events and terrane accretion in the Canadian Appalachians

Plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion, with some deformation associated with transform/transcurrent movements. Deformation associated with subduction varies between two end members: (1) where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes, a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over-riding plate; and (2) where continent-continent collision is occurring, a wide surface zone of accretionary deformation starts synchronously or with limited diachronism. Palaeozoic deformational events in the Canadian Appalachians correspond to narrow diachronous events in the Ordovician and Silurian, whereas Devonian, Carboniferous and Permian deformational events are widespread and broadly synchronous. Along the western side of the Canadian Appalachians, the Taconian deformational event starts diachronously throughout the Ordovician and corresponds to the north-north-west accretion of the Notre Dame, Ascot-Weedon, St Victor and various ophiolitic massifs (volcanic arc and peri-arc terranes) over cratonic North America. Within the eastern half of the Central Mobile Belt, the Late Cambrian-Early Ordovician Penobscotian deformational event corresponds to the ?south-easterly accretion of the Exploits subzone (various volcanic are and peri-arc terranes) over the Gander Zone (?continental rise). In the centre of the orogen, the Late Ordovician-Silurian Beothukan deformational event corresponds to the south-easterly accretion of the Notre Dame over the Exploits-Gander subzones. Along the south-eastern side of the Central Mobile Belt, the Silurian Ganderian deformational event corresponds to the north-north-east, sinistral transcurrent accretion of the Avalon Composite Terrane (microcontinent) over the Gander-Exploits zones. Along the south-eastern half of the orogen, the Late Silurian-Middle Devonian Acadian deformation event corresponds to the westerly accretion of the Meguma terrane (intradeep or continental rise) over the Avalon Composite Terrane. Affecting the entire orogen, the Late Devonian, Carboniferous and Permian, Acadian-Alleghanian deformational events correspond to the east-west convergence between Laurentia and Gondwana (continent-continent collision).

40Ar/39Ar Mineral Ages from the Southern Cape Breton Highlands and Creignish Hills, Cape Breton Island, Canada: Evidence for a Polyphase Tectonothermal Evolution

$^{40}Ar/^{39}Ar$ ages for hornblende and muscovite from plutons and metamorphic country rocks in the southern Cape Breton Highlands and Creignish Hills, together with previously published U-Pb, Rb-Sr and $^{40}Ar/^{39}Ar$ ages, document a complex tectonothermal history that included (1) episodes of plutonic activity at ca. 600, 565-560, 555, 500-490-(?470), and 410-395 Ma (all of which cooled through 500-400°C within ca. 20 m.y.); and (2) deformation in the late Precambrian (>550 Ma) and the Late Silurian-Middle Devonian (415-380 Ma). The latter was associated with evelopment of a positive flower-like structure during maintenance of medium-pressure, greenschist-amphibolite facies, regional metamorphic conditions that may be traced northward along the length of the Cape Breton Highlands. It may be correlated with the Kingston Complex and associated mylonite zone exposed in the Avalon Composite Terrane of southern New Brunswick. Northward it merges with a major high-grade, poly deformed belt within the Gan...

Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: Comparisons of paleomagnetism with paleoclimatology and biogeographical patterns

New paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the Silurian and Devonian. In this paper, the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data. The data from the three fields of paleomagnetism, paleoclimatology, and biogeography are generally in excellent agreement, and an internally consistent paleogeographic evolutionary picture of the interactions between North America, Gondwana, and intervening displaced terranes is emerging.During the interval of the Ordovician, Silurian, and Devonian, North America stayed in equatorial paleoposition, while rotating counter-clockwise. The northwest African part of Gondwana was in high southerly latitudes during the Late Ordovician and was fringed by peri-Gondwanide terranes, such as southern Europe (Armorica) and Avalonian basement blocks now found in eastern Newfoundland, Nova Scotia, the Boston Basin, the Appalachian Piedmont, and northern Florida. Subsequently, Gondwana and the peri-Gondwanide terranes displayed rapid drift with respect to the pole. This drift translates into the following pattern of movement for northwest Africa. During the latest Ordovician-Early Silurian, this area moved rapidly northward from polar to subtropical latitudes, followed by equally rapid southward motion from subtropical to intermediate (about 50°S) paleolatitudes during the Late Silurian-Middle Devonian. It is likely that significant east-to-west motion accompanied the latter shift in paleolatitudes, with the Caledonian-Acadian orogeny the result of Silurian to Early Devonian convergence and collision between Gondwana and North America. This collision sandwiched several of the intervening displaced terranes between Gondwana and North America. Subsequent to this collision, Gondwana was separated in the Late Devonian by a medium-width ocean from North America and the Avalonian and southern European blocks which were left behind adjacent to North America. This new ocean closed during the Carboniferous, and the resulting convergence and collision were the cause of the Hercynian-Alleghanian orogenic belt. Problems remaining for future research, besides the further gathering of reliable paleopoles, involve the uncertain pre-Devonian position of the southern British Isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid Late Ordovician-Silurian apparent polar wander for Gondwana.