Terrane Assembly Research Articles

First record of the Parabolina Fauna in the Cambrian (Furongian) of Alborz, northern Iran

Abstract A small trilobite assemblage, including Parabolina (Neoparabolina) frequens, assignable to the Parabolina Fauna, has been recovered from the Furongian (Cambrian Stage 10) Sah Member of the Mila Formation in the Tuyeh–Darvar section, the eastern Alborz Mountains, north Iran. The assemblage includes eight genera and species; two of them, Niobella darvarensis n. sp. and Macropyge (Promacropyge) sahensis n. sp., are new to science. The incursion of a Parabolina fauna into Alborz is confined to a significant drowning event with associated dark-gray shale deposition, which most probably occurred in the lower part of the Cordylodus proavus conodont Zone. While the generic composition of the assemblage is mostly cosmopolitan with the exception of the endemic Alborsella, the occurrence of Indiligens, Macropyge (Promacropyge) sahensis n. sp., Agnostotes sp. aff. A. sulcatus, and Leiagnostus bexelli indicates faunal links with South China and Tarim. Parabolina (Neoparabolina) frequens is widespread mainly in offshore deposits from temperate Gondwana (Armorican terrane assemblage, Argentina) and Baltica of about that age. UUID: http://zoobank.org/6b3118d0-5907-41b9-af44-cee838c5e630

A palynological investigation of the Early-Middle Devonian transition and associated Choteč Event in Northern Spain

In Northern Spain there is a near complete sequence of Devonian rocks that accumulated around a relatively isolated land mass, the Armorican Terrane Assemblage, which was separated from Gondwana to the south and Laurussia to the north. The Lower to Middle Devonian transition records a dramatic facies change from carbonate to siliciclastic sedimentation. The Choteč Event, a global anoxic pulse that occurs just above the Lower-Middle Devonian boundary (392.5–393.1 Ma), has been identified within this transition. These strata are included in the Moniello and Naranco fms in Asturias and the equivalent Santa Lucía and Huergas fms in Leon. They are independently age-contrained based on conodont and invertebrate biotas. For the first time we describe palynomorph assemblages recovered from the Lower to Middle Devonian transition in Northern Spain. The assemblages contain terrestrial (spores) and marine (acritarchs, prasinophycean cysts, chitinozoans) elements, thus recording primary productivity on both the land and in the oceans. The older assemblages belong to the annulatus–sextantii Spore Zone and the younger assemblages the douglastownensis–eurypterota Spore Zone, which is concordant with the independent age constraints. Our analysis of the distribution and diversity of marine and terrestrial palynomorphs supports interpretations of the facies change associated with the Choteč Event as being related to a change to increased rainfall and terrestrial runoff. However, this change is associated with only limited taxon turnover in both land plant spores and marine phytoplankton and chitinozoans.

Geochemistry and U-Pb zircon geochronology of the Nkout banded iron formations and associated metasiliciclastic rocks, NW Congo craton, southern Cameroon

ABSTRACT The Nkout banded iron formations (BIFs)-hosted iron deposit is located in the Ntem Complex at the northwestern part of the Congo craton, southern Cameroon. We report geochronological and geochemical data of magnetite BIFs and interlayered mica schists from the Nkout iron ore deposit. The BIFs show typical features of chemical precipitates characterized by seawater-like shale-normalized rare earth elements plus yttrium spectra. Weak positive Eu anomalies indicate the contribution of high-temperature hydrothermal fluids to the BIFs, with low detrital input. The absence of true Ce anomalies indicates that the BIFs were likely to be deposited in anoxic seawater. LA-ICP-MS U-Pb detrital zircon data for the BIFs and associated metasiliciclasltic rocks constrained the maximum depositional age at ca. 2.8 Ga, suggesting that the Nkout BIFs are likely the oldest iron formations in the Ntem Complex. Our zircon U-Pb isotope data indicate that the Nkout succession experienced lower-amphibolite facies metamorphism at ca. 2.0 Ga, which correlates with the regional tectono-thermal event linked to the convergence and assembly of terranes between the Congo and São Francisco cratons during the Eburnean-Transamazonian orogeny.

Correlations on the southern Kaapvaal Craton Margin, 1: Ventersdorp lavas transgressed the Doornberg lineament

Abstract Two small exposures of quartz-porphyritic rocks occur on the farm Zoutpekel 98 in the Marydale Terrane between the Doornberg Fault and Brakbos Shear Zone, apparently overlying Kaapvaal basement granite but lacking clear field relationships due to sand and Dwyka tillite cover. They are lavas and tuffs, metamorphosed in lower amphibolite facies. They contain quartz phenocrysts with a distinctive blue colour, due to metamorphic exsolution of rutile. Microbeam U-Pb zircon dating gives a combined 207Pb/206Pb age of 2 722 ± 3 Ma (seven determinations on four samples), interpreted as the age of extrusion. Three of these samples give the same discordia upper intercept age, but one sample gives discordia intercepts of 2 688 ± 15 and 1 223 ± 120 Ma, thought to reflect metamorphic lead loss related to the ~1 210 Ma Namaqua terrane assembly collisions. The Zoutpekel exposures are coeval with the 2 720 ± 2 Ma Makwassie Formation of the Platberg Group, Ventersdorp Supergroup. They also correspond geochemically to the Makwassie Formation and no other unit of the supergroup. A sample from the T’kuip Formation of the nearest Ventersdorp Supergroup inlier on the Kaapvaal Craton (east of the Doornberg Fault), gives an age of 2 716 ± 8 Ma, also confirming its lithostratigraphic and geochemical correlation with the Makwassie Formation. The Zoutpekel exposures show that not only the Kaapvaal basement granites, but also the supracrustal cover rocks of the Ventersdorp Supergroup, extend southwards across the Doornberg Fault, The Marydale Terrane is thus not an exotic terrane, but probably represents a passive continental margin developed at the beginning of the 1 300 to 1 000 Ma Namaqua-Natal Wilson cycle. The age range of the Ventersdorp Supergroup and the age and stratigraphic correlation of the Marydale Group thrust complex, which straddles the Zoutpekel exposures, will be investigated in two companion papers.

Middle Devonian, late Carboniferous, and Triassic magmatic flare-ups in eastern Armorica (Sakarya Zone, Turkey) as revealed by detrital zircon U-Pb-Hf isotopic data

Abstract The Sakarya Zone of northern Turkey contains a well-preserved Early–Middle Jurassic and Late Cretaceous submarine magmatic arc constructed over pre-Jurassic bedrocks that are considered to be the eastward extension of the Armorican Terrane Assemblage in Europe. In this study, we present U-Pb-Hf isotopic data from the detrital zircons of middle Permian and Lower Jurassic sandstones to reveal episodes of Paleozoic–early Mesozoic magmatic flare-ups. Detrital zircon ages, together with data from the literature, define three major age groups at 400–380 Ma, 326–310 Ma, and 250–230 Ma, which indicates three distinct magmatic flare-ups. In addition, there are minor age clusters at 460–430 Ma and 215–195 Ma. Initial εHf values of the detrital zircons indicate significant juvenile input during the Triassic flare-up, the involvement of significantly reworked crustal material during the late Carboniferous magmatic flare-up, and both juvenile and reworked crustal material during the Middle Devonian magmatic flare-up. Within the pre-Jurassic continental basement rocks of the Sakarya Zone, the late Carboniferous igneous rocks are well documented and most voluminous, and the Middle Devonian rocks are known locally, while the Triassic igneous rocks—apart from those in Triassic accretionary complexes—are hardly known. Because the Sakarya Zone is a Gondwana-derived continental block that was later involved in the Variscan and Alpine orogenies, these magmatic flare-ups cannot be explained by subduction-related processes along a single subduction zone. We propose that the Sakarya Zone rifted from the northern margin of Gondwana during the Late Ordovician–Silurian, the Devonian magmatic flare-up (400–380 Ma) was related to the southward subduction of the Rheic Ocean beneath the Sakarya Zone during its northward drift, the late Carboniferous magmatic flare-up (326–310 Ma) occurred following the collision of the Sakarya Zone with Laurussia, and the Triassic flare-up (250–230 Ma) resulted from northward subduction of the Tethys Ocean beneath the Sakarya Zone. Comparison with data from the literature shows that the Triassic and late Carboniferous magmatic flare-ups are also characteristic features of neighboring Armorican domains, such as the Balkans and the Caucasus; however, the Middle Devonian flare-up appears to be restricted to the Sakarya Zone. Along with the late Carboniferous flare-up, the Late Ordovician–Silurian flare-up, which is locally recorded in the Sakarya Zone, is typical of the Armorican Terrane Assemblage as a whole.

Zircon U-Pb-Hf isotope systematics of southern Black Forest gneiss units (Germany) – Implications for the pre-Variscan evolution of Central Europe

Presently, little is known about depositional ages, geological setting, and provenance of (meta)sedimentary rocks constituting the southern part of the Black Forest Gneiss Complex, preventing detailed reconstructions of the Cadomian to Variscan geodynamic evolution in Europe. Here, we present the first systematic U-Pb-Hf isotope data of detrital zircon grains combined with geochemical data of metasedimentary rocks from the Murgtal, Todtmoos, and Wiese-Wehra gneiss units, preserving different stages of the pre-, syn- and post-Cadomian evolution. Protoliths of Murgtal metagreywackes were deposited during the Ediacaran at <550 Ma and sourced from the Avalonian-Cadomian Belt (550–700 Ma; ∼70 %) and Africa (760–1045 Ma, 1850–2250 Ma, 2720–3230 Ma; ∼30 %). In contrast, metasedimentary rocks of the Wiese-Wehra and Todtmoos gneiss units show late Devonian depositional ages at <375 Ma, but in different geotectonic settings. Data from Wiese-Wehra metagreywackes provide evidence for the existence of a pre-Cadomian oceanic arc formed at 610 Ma (εHft = +5 to +8), accreted to the Avalonian-Cadomian Belt at ca. 540 Ma, and successively reworked until 380 Ma (εHft = +1 to +6). Finally, these rocks became part of an early Variscan arc-back arc system with juvenile input at ca. 375 Ma (εHft = 0 to +10). Todtmoos metaarkoses mainly reflect subduction-related magmatism at 420–490 Ma (∼88 %), and at 380 Ma (∼10 %) in a continental arc setting (εHft = −2 to −8), and rare Cadomian arc input (560–700 Ma; εHft = −10 to +3; ∼2 %). Zircon grains of some samples reveal a Variscan metamorphic overprint at 340–335 Ma. In combination, new and existing data from the Black Forest and other basement units throughout Europe provide evidence for the amalgamation of pre-Cadomian juvenile terranes along the northern margin of Gondwana until 540 Ma, followed by a complex rift history, accompanied by subduction between 500 and 400 Ma. Furthermore, they indicate the existence of an oceanic arc-back arc system, which was located south of the Armorican Terrane Assemblage at 380–350 Ma, immediately prior to Variscan subduction-accretion at <345 Ma.

Origin, nature, and evolution of the northern Pontiac subprovince, Canada: Insights from the intrusive record

The metasedimentary subprovinces of the Superior Province provide critical insights into the tectonic style and thermal state prevailing during its Neoarchean cratonic assembly. The Pontiac subprovince consists of metamorphosed turbiditic sequences with minor interlayered mafic volcanic rocks that are intruded by abundant mafic to felsic plutonic rocks of different geochemical affinities, and for which the source, timing, deformation, and relationship to metamorphism and gold endowment are poorly constrained. Petrological, geochemical, isotopic (Lu-Hf zircon), and geochronological (U-Pb zircon and monazite) data reveal three plutonic events consisting of 1) c. 2764 Ma medium-pressure TTG gneiss, interpreted as the local basement to the Pontiac supracrustal sequences; 2) syn- to late- sedimentary c. 2690–2670 Ma juvenile (mean εiHf = 4.02 ± 1.12) monzonite sanukitoid and highly potassic calc-alkaline sills and plutons; and 3) voluminous c. 2670–2620 Ma S-type granite sourced by protracted partial melting of sedimentary rocks at depth. This succession in the intrusive record combined with syn-depositional volcanism and subsequent Barrovian metamorphism of the metasedimentary host rock is consistent with a transition from a continental rift setting to crustal thickening via terrane assembly and challenges early models of arc accretion.

Divergent double subduction of Bangong-Nujiang Ocean revealed by high-resolution magnetotelluric data at 86°E in the northern Tibetan Plateau

The Tibetan Plateau was mainly produced by the assembly of several terranes resulting from ocean closure and continental collision. The formation and evolution of the Bangong-Nujiang Ocean (BNO) played a prominent role in the Tibetan Plateau accretion before the Cenozoic collision between the Indian and Eurasian continents. The closure mode of the BNO between the Lhasa and Qiangtang terranes has been controversial, involving subduction polarity, i.e., regarding whether there was only northward subduction beneath the Qiangtang terrane or both southward and northward subduction beneath the Lhasa and Qiangtang terranes. To resolve this issue, a dense broadband magnetotelluric (MT) profile transecting the Bangong-Nujiang suture zone (BNS) at 86°E was completed. After standardized data processing and analysis, geological interpretation can be performed on the two-dimensional (2-D) inversion results of the MT data. Through a large number of inversion experiments, we obtained a reliable lithospheric electrical structure beneath the BNS and its adjacent regions. Our results show that the upper crust in our study area is featured by high resistivity overall, but it is cut by several fault zones, indicating that it may be influenced by the late strike-slip fault caused by the India-Asia collision. The middle-lower crust of the northern Lhasa terrane is highly conductive and may represent juvenile crust. The existence of a northward and southward inclined electrical gradient zone in the BNS may indicate that the BNO was closed by north-south bidirectional subduction.

The Paleoproterozoic Zhaigou banded iron formation in the Fuping Complex, North China Craton: Geochemistry, geochronology and implications for genesis and tectonic setting

Precambrian Banded iron formations (BIFs) provide important insights on the chemical composition of ancient seawater, as well as the tectonic setting of their formation. The Precambrian BIFs deposits in the North China Craton are widely distributed in the Archean greenstone belts and are composed predominantly of Neoarchean Algoma-type including those in the Anshan-Benxi and eastern Hebei areas of the Eastern Block, together with minor Paleoproterozoic Superior-type BIFs in the southern segment of the trans-North China Orogen. The Zhaigou BIF deposit is associated with Paleoproterozoic Wanzi supracrustal sequence and amphibolite in the Fuping Complex in central North China Craton. Here, we present petrological, geochronological and geochemical data of the meta-sedimentary rocks, BIF ores and amphibolites from the Zhaigou BIF deposit in the Fuping Complex for the first time to constrain the formation age, source characteristics and depositional setting, and evaluate the redox states of the Paleoproterozoic seawater. Whole-rock geochemical data show that the BIF ores are enriched in HREE with low (La/Yb)PAAS values (0.037–0.073), and exhibit positive Eu (3.34–6.21) and Pr (1.05–1.10) anomalies. The PAAS-normalized REY patterns resemble 1:100 mixture of high-temperature hydrothermal fluid and seawater, indicating the involvement of the two end members. In combination with whole-rock geochemical data of BIF ores, trace elements of magnetite and U-Pb ages of detrital zircons from the meta-sedimentary rocks, we propose that crustal components from the Fuping TTG gneiss also contributed to the BIFs. The geochemical features suggest that the protolith of amphibolite is basalt formed in island arc setting. The negative Ce anomaly (Ce/Ce* = 0.81 ∼ 0.90) indicates the deposition of the Zhaigou BIF ore in an oxidizing environment. Zircon U-Pb data on the BIF ore yield weighted mean 207Pb/206Pb ages of 2028 ± 33 Ma and 1874 ± 52 Ma, representing the formation age of the Zhaigou BIF deposit and the subsequent metamorphic event, respectively. The Zhaigou BIF deposit belongs to the Algoma-type which shows close association with the Paleoproterozoic sedimentary sequence and basaltic magmatism in oceanic island arc setting. The Paleoproterozoic seawater forming the Zhaigou deposit was oxidized after the Great Oxygenation Event (GOE) during the terrane assembly in the trans-North China Orogen.

Dispersed spore assemblages from the Lower Devonian Rañeces - La Vid groups of Northern Spain: Palaeogeographical implications

Dispersed spore assemblages are described from the Early Devonian (late Lochkovian-late Emsian) Rañeces Group (Asturias) and equivalent La Vid Group (Leon) of Northern Spain. The lower part of both groups consists predominantly of limestone and yields only impoverished palynological assemblages. However, the upper parts contain more clastic sediments and yield rich assemblages of well preserved palynomorphs. These are dominated by marine forms but also include abundant spores. Sixty-two spore taxa are reported from the Valporquero Formation (Leon). These all belong to a single assemblage that can be equated with the lindlarensis-sextantii Assemblage Zone (mid-early late Emsian age) erected in northern Gondwana. The sequences are independently age constrained by conodont and invertebrate macrofossil evidence and the ages derived from these studies are concordant with the assigned spore zones. The Spanish spore assemblages are from deposits that accumulated around the Armorican Terrane Assemblage (ATA), a group of islands that lay between the continents of Laurussia and Gondwana. Jacaard Index analysis indicates that the Valporquero Formation spore assemblage is endemic, and not closely related to those previously described from Laurussia or Gondwana, although it has more in common with the latter. This suggests that the Armorican Terrane Assemblage was widely separated from both Laurussia and Gondwana, but was located closer to and had more connectivity with Northern Gondwana.

New constraints on the age of ore at Black Mountain mine, Bushmanland Ore District, South Africa

Abstract The chronostratigraphy of the Bushmanland Ore District and the Namaqua-Natal Province has long been debated, but recent microbeam dating has resolved several issues. An important aspect is the precise age of the sedimentary-exhalative ores and their tectonostratigraphic context. Published constraints on the maximum age of the ores from detrital zircon dating are 1 285 ± 14 Ma (n=4, Gamsberg ore), 1 215 ± 18 Ma (n=6, Wortel Formation) and a tentative 1 118 ± 33 Ma (n=3, Hotson Formation at Black Mountain). The ore is older than the 1 130 ± 35 Ma Koeris Formation metabasalt which unconformably overlies it. Aplite dykes, which intrude the ore of the Black Mountain deposit, provide another potential minimum age constraint on the ore. A sample was dated at 1 175 ± 15 Ma by ion probe U-Pb zircon dating. This shows that the aplite dykes belong to the late-collisional Springputs Suite of granitoids which includes the 1 163 ± 11 Ma Achab and 1 149 ± 15 Ma Hoogoor Gneisses, for which the field relationship with the ores had not been established. The regional M2 metamorphism was recorded in aplite zircon rims at 1 027 ± 9 Ma and at 1 030 ± 6 Ma in monazite and xenotime in the Hotson Formation host rock schists. Detrital zircons, dated by Laser Ablation ICPMS in a host rock schist sample, reflect a dominant Palaeoproterozoic provenance with major age group at 2 003 ± 17 Ma and minor groups at 1 847 and 2 105 Ma. Only 16 analyses were made, which probably accounts for the absence of minor Mesoproterozoic provenance components found in other published datasets. The age of the Black Mountain ore is now constrained between 1 215 ± 18 Ma and 1 175 ± 15 Ma. The tentative 1 118 ± 33 Ma detrital zircon maximum age is shown to be unreliable in view of two younger magmatic rocks with older dates (1 175 and 1 130 Ma). The SEDEX ores thus formed during or just before the ~1 210 Ma assembly of Namaqua terranes and before the ~1 150 Ma syntectonic Springputs Suite granitoid magmatism.

A record of Neoarchaean cratonisation from the Storø Supracrustal Belt, West Greenland

During the late Archaean, exotic juvenile continental (TTG) terranes assembled into stable cratons leading to continental emergence and deposition of shallow-marine sedimentary sequences. This period of cratonisation coincided with crustal reworking and maturation driving the production of granites sensu stricto on most cratons, and may mark a final transition to mobile-lid tectonics. We investigate the relative timing of continental assembly, stabilization, emergence, and maturation, during the formation of the North Atlantic Craton (NAC) in West Greenland from its constituent terranes, using geochemical data from zircon and monazite extracted from its oldest mature metasedimentary unit, the Storø quartzite. Zircons form two U-Pb age groups: (i) an older >2820Ma group with juvenile (elevated) ϵHf(t) and δ18O, derived from weathering surrounding Mesoarchaean terranes; and (ii) a younger <2700Ma group with less radiogenic (lower) ϵHf(t) and elevated δ18O that record post-burial metamorphism peaking ca. 2620 Ma. The quartzite protolith has a maximum depositional age of ca. 2830 Ma, and was deposited after final TTG formation but prior to granite magmatism at ca. 2715 Ma, during which time terranes had sufficiently assembled, stabilized, and emerged to form a common watershed. Cratons form via lateral accretion which requires strong continental lithosphere, for which one agent is crustal reworking and maturation. However, for the NAC, terrane assembly and emergence commenced prior to granite formation, and crustal reworking may be a response to lithospheric thickening. Cratonisation involves a series of complex, intertwined processes operating over 100's of millions of years, which together lead to the development of thick, stable, continental lithosphere. Studies of ancient mature metasediments such as the Storø quartzite can help build timelines for these processes to ultimately better understand their choreography and co-dependencies, that together produced Earth's enduring cratons.

Plateau archives of lithosphere dynamics, cryosphere and paleoclimate: The formation of Cretaceous desert basins in east Asia

Southeastern Eurasia is a global window to the Cretaceous paleoclimate and lithosphere coupling. China contains one of the most complete and complex sedimentary records of Mesozoic desert basins on planet Earth. In this study, we perform the spatio-temporal tracking of 96 Cretaceous palaeoclimate indicators during 79 Myr which reveal that the plateau paleoclimate archives from East Asia resulted from an Early to Mid-Cretaceous ocean–atmosphere coupling and a shift to a preponderant role of Late Cretaceous lithosphere dynamics and tectonic forcing on high-altitude depositional systems linked to the subduction margins of the Tethys and Paleo-Pacific realms beneath the Eurasian plate. The crustal response to tectonic processes linked with the spatio-temporal evolution of the Tethyan and Paleo-Pacific margins defined the configuration of major sedimentary basins on this region. The significant increase and decrease in the number of active sedimentary basins that occur during the Cretaceous, from 16 in the Early Cretaceous, to 28 in the Mid-Cretaceous, and a decreasing to 20 sedimentary basins in the Late Cretaceous, is a direct response of lithospheric dynamics associated with the two main subduction zones (Tethys and Pacific domains). A shift in subduction style from an Early Cretaceous Paleo-Pacific Plate slab roll back to a Late Cretaceous flat-slab mode might have triggered regional plateau uplift, blocked intraplate volcanism, thus enhancing the denudation and sediment availability, and created wind corridors that led to the construction and accumulation of extensive Late Cretaceous aeolian sandy deserts (ergs) that covered Mid-Cretaceous plateau salars. At the same time, plateau uplift associated with crustal thickening following terrane assembly in the Tethyan margin triggered altitudinal cryospheric processes in sandy desert systems. Evidence of an active Cretaceous cryosphere in China include Valanginian-Hauterivian glacial debris flows, Early Aptian geochemical signature of melt waters from extensive ice sheets, and Cenomanian–Turonian ice-rafted debris (IRD). These cryospheric indicators suggest an already uplifted plateau in southeastern Eurasia during the Cretaceous, and the marked correlation between cold plateau paleoclimate archives and marine records suggests a strong ocean-atmosphere coupling during Early and Mid-Cretaceous cold snaps. We thus conclude that lithospheric tectonics during Cretaceous played a fundamental role in triggering high-altitude basin desertification and spatio-temporal plateau paleohydrology variability in the Cretaceous of south-eastern Eurasia.

Mercury evidence of intense submarine volcanism and hydrothermal activity during a mid-Tournaisian anoxic event in the Carnic Alps

The mid-Tournaisian Event, or Lower Alum Shale Event (LASE), is linked to severe facies changes including pelagic carbonates and widespread black siliceous shales and radiolarites in many parts of the world. For the first time, high-resolution inorganic geochemistry and framboidal pyrite analyses were applied to decipher depositional condition changes during the mid-Tournaisian event in the Carnic Alps. The predominance of tiny pyrite framboids, high Corg/P, U/Th, and V/Cr ratios, generally higher enrichments of redox-sensitive metals such as Mo, U, V, and depletion in Mn suggest anoxic/euxinic conditions at the sea bottom. Moreover, we found very large anomalous Hg spikes (with maximum values reaching 3650 ppb) for the first time in the mid-Tournaisian deep-water marine succession of the Carnic Alps. Our results suggest intense volcanism during the LASE, which triggered the mid-Tournaisian anoxic event. The most likely candidate for the observed Hg anomalous contents is submarine arc volcanism and hydrothermal activity connected with the collision of the peri-Gondwanan terranes Paleo-Adria and probably Armorican Terrane Assemblage (ATA) and/or Noric terrane during the first phase of the Variscan orogeny.

Provenance of lower Palaeozoic metasediments of the East Odenwald (Mid-German-Crystalline Zone, Variscides)\u2014a correlation with the East European Platform (Poland)

U–Pb age spectra of detrital zircons related to the East European Platform could be traced in paragneiss through the whole Mid-German-Crystalline Zone (Variscides, Central Europe) from the Odenwald via the Spessart to the Ruhla crystalline forming an exotic unit between Armorica and Laurussia. The depositional ages of the paragneiss are defined by the youngest age of the detrital zircons and the oldest intrusion ages as Ordovician to Silurian. The Ediacaran dominated age spectrum of detrital zircons from the paragneiss of the East Odenwald suggests the latter to be derived from the shelf of the East European Platform (Baltica), which was influenced by the 1.5 Ga old detritus delivered from a giant intrusion (Mazury granitoid, Poland). The detrital zircon age spectrum of the lower Palaeozoic paragneiss of the East Odenwald and sandstone of the northern Holy Cross Mountains are identical. The pure Sveconorwegian spectrum of the lower Palaeozoic quartzite from the Spessart, (Kirchner and Albert Int J Earth Sci 2020) and the Ruhla (Zeh and Gerdes Gondwana Res 17:254–263, 2010) could be sourced from Bornholm and southern Sweden. A U–Pb age spectrum with 88% Palaeozoic detrital zircons from a volcano-sedimentary rock of the East Odenwald is interpreted to be derived from a Silurian magmatic arc (46%), which was probably generated during the drift of the Mid-German-Crystalline Zone micro-continent to the south. A tentative plate tectonic model of Mid-German-Crystalline Zone is presented taking into account (a) the East European Platform related age spectra of the detrital zircons (b) the Ordovician to Silurian depositional age of the metasediments (c) the Silurian and Early Devonian intrusion age of the plutonic and volcanic rocks and (d) the U–Pb ages of the Middle Devonian high-grade metamorphism. The East European Platform-related part of the Mid-German-Crystalline Zone is interpreted as a micro-continent, which drifted through the Rheic Ocean to the south and collided with the Saxothuringian (Armorican Terrane Assemblage) during the Early Devonian. Such large-scale tectonic transport from the northern continent to the southern continent is also known from the SW Iberia, where Laurussia-related metasediments of the Rheic suture zone are explained by a large scale tectonic escape (Braid et al. J Geol Soc Lond 168:383–392, 2011).

Triassic–Jurassic Accretionary History and Tectonic Origin of Stikinia From U‐Pb Geochronology and Lu‐Hf Isotope Analysis, British Columbia

AbstractThe timing of assembly and tectonic origins of terranes in the northern Cordillera of Alaska, British Columbia, and the Pacific Northwest are debated. Stikinia, a long‐lived arc terrane, has an enigmatic regional Mesozoic accretionary history and its tectonic origins remain unconstrained. Zircon U‐Pb geochronology and Lu‐Hf isotopic data on Triassic–Jurassic sedimentary and igneous rocks from central Stikinia shed light on the terrane‐scale effects of a latest Triassic–Early Jurassic collision between Stikinia and pericratonic Yukon‐Tanana terrane. Main age peaks from central Stikinia are 250–160 Ma, reflecting ongoing Mesozoic arc‐related igneous activity within Stikinia. Comparison of isotopic evolution and unconformity development between central Stikinia and northern Stikinia (Whitehorse trough) provide new constraints on regional latest Triassic–earliest Jurassic deformation. We attribute the shortening‐related deformation to variable along‐strike interactions during end‐on collision with the Yukon‐Tanana terrane, with significant crustal thickening at the northern apex of Stikinia that did not persist farther south. A small pre‐Devonian zircon population is significant, as the oldest exposed rocks in Stikinia are Early Devonian. Pre‐Devonian age peaks differ from those of the northern Yukon‐Tanana terrane, but resemble zircons from southern Wrangellia. These zircons are likely multi‐cyclic, derived from crust that originated in the Arctic region near the northern end of the Caledonide orogeny. We suggest that Stikinia was an independent crustal block prior to latest Triassic onset of collision with Yukon‐Tanana terrane. The ongoing, end‐on collision in turn promoted oroclinal assembly of the peri‐Laurentian terranes.

Timing of formation and tectonic setting of Paleozoic granitoids in the eastern Mongol–Okhotsk Belt: Constraints from geochemical, U[sbnd]Pb, and Hf isotope data

The Mongol–Okhotsk Belt is one of the major structural elements in East Asia. However, many aspects of the evolution of this structure from the oceanic domain to the suture zone remain unresolved. The most controversial of these aspects are the timing and polarity of subduction processes as well as the timing of the final closure of the Mongol–Okhotsk Ocean. Here, we present whole-rock geochemical and UPb geochronological data for granitoids in the eastern Mongol–Okhotsk Belt to constrain their timing of formation and tectonic setting. Our new data suggest three stages of granitic magmatism in the eastern part of the Mongol–Okhotsk Belt: Cambrian (ca. 511 Ma), Middle Devonian (ca. 393 Ma), and Late Permian–Early Triassic (257–250 Ma). Cambrian (511 ± 4 Ma) plagiogranites in the Galam Terrane have adakitic geochemical signatures, and εHf(t) values of zircons from these granites vary mostly from +8.8 to +14.8 with Hf model ages (tHf(DM)) ranging from 0.77 to 0.53 Ga. These plagiogranites most likely represent the roots of an island arc that formed above a NW-dipping (present-day coordinates) subduction zone beneath the edge of the Siberian Craton. Middle Devonian (393 ± 7 Ma) granites from the Selemdzha Terrane, as well as Late Permian–Early Triassic (257 ± 4, 253 ± 3, 253 ± 2, 252 ± 3, and 250 ± 4 Ma) granodiorites and granites from the Selemdzha and Tokur terranes, have supra-subduction whole-rock geochemical signatures. Slight negative and positive zircon εHf(t) values of −6.1 to +8.4, tHf(DM) ages of 1.1–0.6 Ga, and tHf(C) ages of 1.4–0.6 Ga suggest that the studied rocks contain a continental crustal component. The Middle Devonian and late Permian–Early Triassic granodiorites and granites formed either in a continental island arc or on an active continental margin. During the Middle Devonian and at the late Permian–Early Triassic boundary, the Mongol–Okhotsk oceanic crust was subducting beneath the edge of the Siberian Craton. Thus, middle Cambrian, Middle Devonian, and Permian–Triassic granitoids in the eastern Mongol–Okhotsk Belt reflect three stages of subduction, meaning that the accretionary wedge terrane assemblages started to form during the Cambrian then continued during the Devonian and at the Permian–Triassic boundary, but was not started at Jurassic as previously proposed.

Paleotectonic and paleogeographic history of the Arctic region

Paleogeographic maps represent the ultimate synthesis of complex and extensive geologic data and express pictorially the hypothetical landscape of some region during a given time-slice of deep geologic time. Such maps, presented as paired paleogeographic and paleotectonic reconstructions, have been developed to portray the geologic history of the greater Arctic region over the past 400 million years. Collectively they depict four major episodes in the development of the Arctic region. The first episode witnessed early and middle Paleozoic terrane assembly and accretion during the Caledonian and Ellesmerian orogenies, which brought together many pieces of the Arctic collage along the northern margin of Laurussia. During the second phase, the assembly of Pangea in the late Paleozoic joined Siberia to Laurussia, an entity that became Laurasia during the subsequent break-up of Pangea. Then, Mesozoic subduction and terrane accretion constructed the Cordilleran margin and opened the Canada Basin. Finally, Cenozoic North Atlantic sea-floor spreading fully opened the Arctic Ocean.

The Balkan terranes: a missing link between the eastern and western segments of the Avalonian–Cadomian orogenic belt?

ABSTRACT The Alpine–Himalayan collision zone involves a number of crustal fragments that originated in the Neoproterozoic to Cambrian Avalonian–Cadomian belt of northern Gondwana. We use the detrital zircon U–Pb geochronology to examine four of these lithotectonic units, now exposed in the Balkans in Bulgaria and Serbia. The obtained age spectra suggest that the Diabase–Phyllitoid Complex (the maximum depositional age, MDA, estimated at 540 + 5/-9 Ma) was presumably an accretionary wedge or a forearc basin sourced from a nearby volcanic arc, however, its palaeo-position remains uncertain. The Vlasina Complex (MDA of 577 + 5/-6 Ma) was the most ʽwesterly’ terrane, adjacent to the Trans-Saharan belt, whereas the Sredna Gora and Stara Planina complexes (MDAs of 546 ± 7 Ma and 579 + 4/-5, respectively) were positioned next to the Saharan Metacraton and Arabian–Nubian shield. To put the Balkan terranes into a broad context, we statistically compare the detrital zircon ages in other terranes from the Eastern Alps to Iran with igneous and metamorphic U–Pb zircon ages from North African source areas. The statistical comparison is done through multi-dimensional scaling (MDS), a more rigorous method than a visual comparison of age spectra, to examine the degree of inter-sample similarity. This information is then transferred to a tentative palaeogeographic map showing position of each terrane with respect to its most likely source region. As a result, we define a ʽwesterly’ terrane assemblage, characterized by Mesoproterozoic ages and sourced from the West African craton and the Trans-Saharan belt and an ʽeasterly’ assemblage formed next to the Saharan Metacraton and the Arabian–Nubian shield. The present-day position of some of these terranes implies significant dextral strike-slip displacement, perhaps due to movement on the Pangea megashear during the Carboniferous and Permian.

Mesozoic intraplate tectonism of East Asia due to flat subduction of a composite terrane slab

The widespread Middle Jurassic to Early Cretaceous (J2-K1) intraplate tectonism of East Asia has recently been proposed to reflect the flat subduction of the Izanagi plate, whose mechanism, however, remains poorly constrained. Here, we compare these geological records with those within the western United States (U.S.) during the Laramide flat subduction. Two prominent differences are identified during slab advance: 1) western U.S. witnessed continental-scale subsidence and marine inundation, while East Asia experienced mostly basin inversion implying uplift; 2) the landward migrating magmatism in western U.S. quickly evolved into a regional magmatic lull that lasted for >40 Myr, but the East Asian magmatism was largely continuous in space and time. Assisted with numerical models, we show that a flat slab could indeed reproduce the observed East Asian crustal deformation and topographic history during the advance and retreat of the slab. We further find that 1) this slab should be more buoyant than the Laramide slab so that the former oceanic slab could break off which led to surface uplift instead of subsidence, and 2) the crust of the flat slab melted more easily than an oceanic one. The observation that the J2-K1 tectonism was largely contemporary with the assembly (ca. 180–137 Ma) of exotic terranes along the east coast implies that the flat sab underplating North China and Northeast China likely represented subduction of the buoyant continental lithosphere associated with these terranes. The terrane slab could have carried and flattened portions of the nearby oceanic slab with it, forming a giant composite flat slab that defined the J2-K1 intraplate tectonism within East Asia.