Cadomian Crust Research Articles

Petrogenesis of Miocene igneous rocks in the Tafresh area (central Urumieh‐Dokhtar magmatic arc, Iran): Insights into mantle sources and geodynamic processes

Cenozoic tectono‐magmatism in Iran is widely considered to be related to subduction of the Neo‐Tethys Ocean. We employed whole‐rock and mineral geochemistry and isotopic data of intrusive rocks from Tafresh, central Urumieh‐Dokhtar magmatic arc, to evaluate the role of the mantle in magmatism, to assess the timing of emplacement, and to interpret the tectonic setting. Rock compositions range from gabbro or gabbro‐diorite (plagioclase + pyroxene ± olivine), to diorite (plagioclase + amphibole ± pyroxene), to granodiorite (quartz + plagioclase + K‐feldspar + amphibole + biotite), exhibiting high‐alumina calc‐alkaline affinity. Major oxide and trace element variations vary systematically from less to more evolved rocks suggesting a major role for fractional crystallization processes. Zircon LA‐ICP‐MS U–Pb ages of major rock types are in the range of 24–19 Ma, whereas those of gabbroic dikes are ~17.5 Ma. ԐNd values range between −1.8 and 3.7, and (87Sr/86Sr)i is narrowly restricted to 0.705–0.706, suggesting a common mantle source. The enrichment in light rare earth element (REE) enrichment and flat heavy REE patterns couple depletion of Nb–Ta–Ti indicate that subducting oceanic crust had interacted with the overlying mantle wedge. High‐alumina, mid‐Mg# Tafresh plutonic rocks formed from hydrous melts from which Ca‐pyroxene and magnetite crystallized earlier than plagioclase, whereas late‐crystallizing zircon nucleated while magma traversed through lithospheric mantle and Cadomian crust. Modelling of isotope and incompatible‐element patterns suggests the contribution of no more than ~5% molten sediment or other crustal components in Tafresh magma, at a developmental stage before most plagioclase and amphibole had crystallized. The Miocene Tafresh plutons originated during the final stages of subduction, before the collision between the Arabian and Eurasian plates.

Prolonged magmatism and growth of the Iran-Anatolia Cadomian continental arc segment in Northern Gondwana

Much of the crust of Iran and Anatolia, including their oldest exposed rocks, formed during an episode of intense convergent margin (arc) magmatism as a result of subduction of oceanic lithosphere beneath northern Gondwana from ca 620 Ma to ca 500 Ma, the Cadomian crust-forming event. Most igneous rocks formed between ca 570 and 525 Ma. Cadomian crust is well-known from western and southern Europe and from eastern North America but is much less well-known from Iran and Anatolia. We use published age and compositional data and contribute new data in order to better understand this ancient magmatic system. Cadomian magmatism included calc-alkaline igneous rocks of arc affinity in the main arc and alkalic igneous rocks that formed in a back-arc setting; these igneous rocks are associated with sedimentary rocks. Geochemical and isotopic modelling reveals that basaltic magmas were the main input, that these formed by partial melting in the upper mantle, and that basaltic magmas evolved further in deep crustal hot zones to form granitic magmas through a combination of assimilating older continental crust and fractional crystalization of basaltic magmas.

Constraints from geochemistry, zircon U-Pb geochronology and Hf-Nd isotopic compositions on the origin of Cenozoic volcanic rocks from central Urumieh-Dokhtar magmatic arc, Iran

Considerable debate persists regarding the petrogenesis of high-alumina basalts (HAB) which are purported to occur exclusively in subduction zones. Major and trace element, mineral chemistry, whole-rock Sr-Nd-Hf-isotopes and zircon U-Pb age data are reported for the Cenozoic Eshtehard HABs, in order to constrain the nature of mantle beneath the central Urumieh-Dokhtar magmatic arc, and further investigate the mechanism of HABs generation. Eshtehard HABs, chemically akin to those from continental arcs, include basaltic, basaltic andesite, andesitic and dacitic rocks. U-Pb geochronology of zircon yield ages of 47.2 ± 0.6 Ma, 43.9 ± 0.3, and 40.9 ± 0.5 to 39.4 ± 0.9 for basaltic, andesitic and dacitic samples, respectively. Basaltic andesitic dikes intruded into dacitic hosts yield age of ca. 20–18 Ma. εHf(t) values for Eocene zircons display a range variable from −6.4 to +6.5. Miocene zircons have higher εHf(t), ranging between −1.8 and +10.7. The studied rocks are characterized by enrichment in incompatible trace elements and have relatively homogeneous Sr-Nd isotopes. Integrated studies indicate that Eshtehard HABs were derived from the hydrated, dominantly depleted shallow asthenospheric mantle wedge (and possibly also in the lower lithosphere) overlying subducted oceanic lithosphere. Traversing lithospheric mantle and Cadomian crust, and assimilating crustal material while fractionating plagioclase, high-Ca pyroxene, magnetite, and amphibole, the primary melt formed Eshtehard high-alumina, low-Mg# basalts. We hypothesize that ponding of hydrous magma at the base of the crust allowed for further crustal assimilation and fractionation of Ca-pyroxene and magnetite without plagioclase and zircon nucleation; then, ascending magmas through the crust led to crystallization of plagioclase and eventually zircon. We suggest comparatively high water content (rather than high crystallization pressure), up to 4% sediment melt and less than 10% of continental crust materials were involved, as subordinate components, in the petrogenesis of Eshtehard HABs.

Geochronology and petrogenesis of the Late Neoproterozoic granitic gneisses of Golpayegan metamorphic complex: a new respect for Cadomian crust in the Sanandaj-Sirjan zone, Iran

ABSTRACT The studied granitic gneiss bodies of the Golpayegan metamorphic complex, located in the central part of the Sanandaj Sirjan Zone (SaSZ), in western Iran. Zircon U-Pb dating of two samples shows that the crystallization of the protolith occurred at 557 ± 12 Ma in the Late Neoproterozoic (Ediacaran), broadly coeval to the Neoproterozoic-Early Palaeozoic basement in other parts of Iran. Geochemically, the protolith of the gneisses probably corresponds to differentiated I-type granites with subalkaline affinities in composition. The ratios of Y/Nb >1.2 reveal an affinity to Cordilleran I-type granites for the granitic gneisses. The high Th/U ratios (2.8 to 9), low Eu/Eu* (0.13 to 0.73), with, low contents of FeO (0.55 to 1.72 wt.%), MgO (0.07 to 0.53 wt.%) and MnO (0.01 to 0.04 wt.%) and the high 87Sr/86Sr(i) ratios (0.70693 to 0.73557), negative ƐNd(t) values (−4.4 to −1.7) and Nd model ages (TDM2 = 1.35 to 1.56 Ga) suggest that the protolith may have been derived from partial melting of a pre-existing felsic crustal source (most likely differentiated granitoids). The new results reveal that the granitic source magma has been evolved in an active continental margin tectonic regime during the southward subduction of the Proto-Tethys ocean beneath the northern margin of Gondwana, like other coeval fragments dispersed in the entire Cadomian active continental margins. Also, the Cadomian crust widely extended in western Iran and it confirms these rocks have some clear affinity with Cadomian crust in the world.

Geochronology and geochemistry of exotic blocks of Cadomian crust from the salt diapirs of SE Zagros: the Chah-Banu example

ABSTRACT Cadomian calc-alkaline I-type and within-plate A-type igneous rocks are widespread in the crust of Iran where they are ascribed to a convergent margin associated with the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. These rocks are found as unmetamorphosed magmatic rocks and their metamorphic equivalents (mafic to felsic gneisses) could have been generated in both Cadomian arcs and associated rear-arcs. Nearly all these exposures also contain metamorphosed metasediments, whereas Cadomian igneous rocks of central Iran are associated with thick sequences of unmetamorphosed terrigenous rocks. In the Zagros Fold-Thrust belt of S Iran, salt diapirs contain abundant xenoliths of Cadomian igneous and sedimentary rocks in association with evaporites, dolomites, carbonates, and banded iron-salt deposits. This paper presents new zircon U-Pb and geochemical-isotopic data from igneous clasts in the Chah-Banu salt diapir in SE Zagros. Petrographic and geochemical data indicate two different types of rock clasts; calc-alkaline, I-type dacites-rhyolites, and arc-related to E-MORB-OIB-like gabbros, basalts, and dolerites. New zircon U-Pb ages show that dacites formed at 538.2 ± 2.2 Ma, whereas gabbros show ages of 539.0 ± 1.8 Ma. Zircons from dacites have negative εHf(t) values of – 1.1 to – 8.3. In contrast, zircons from gabbros have higher εHf(t) values of +4.5 to +8.5, indicating crystallization from mantle-derived juvenile magmas. Bulk rock Nd-Sr isotopic data (e.g. ɛNd(t) = +0.3 to +4.0 and 87Sr/86Sr(i) = 0.7059 to 0.70848) for gabbros, dolerites, and basalts confirm that these rocks originated from a mantle source similar to enriched parts of the subcontinental lithospheric mantle, whereas dacites and rhyolites (with εNd(t) = −3.4 to −4.1 and 87Sr/86Sr(i) = 0.70806 to 0.70907) show strong interaction with, and/or re-melting of older continental crust. We suggest that the bimodal calc-alkaline and E-MORB-OIB-like magmatic rocks in salt diapirs as well as associated evaporites and sedimentary rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc.

Cadomian Magmatic Rocks from Zarand (SE Iran) Formed in a Retro-Arc Basin

A major arc-related magmatic episode is recorded in the Ediacaran-Early Cambrian (Cadomian) crust of Iran and Anatolia due to the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. This magmatism was generated at a convergent margin, fragments of which can be traced for thousands of kilometres, from west Avalonia to the Lhasa terrane. Cadomian crustal tracts in Iran and Tauride-Anatolia are represented by abundant arc-like plutonic and volcanic rocks and a series of retro-arc, rifted basins dominated by thick sequences of terrigenous rocks as well as both calc-alkaline and alkaline magmatic rocks. This paper presents new zircon U-Pb as well as geochemical-isotopic data from plutonic (granite to gabbro) and volcanic (basalt to rhyolite) magmatic rocks of a Cadomian retro-arc rifted basin from Zarand in SE Iran. Geochemical data indicate two different geochemical signatures; high K calc-alkaline-shoshonitic, characterized by strong depletions in Nb, Ta, P, Ti, and alkaline rocks (and A1-type intrusions), with strong OIB-like trace element patterns. Cadomian OIB-like rocks- as well as A1-type intrusions- are rare in the basements of Iran and Anatolia and are only documented as minor gabbroic intrusions and/or (meta-) volcanic rocks from Saghand (central Iran), NE Iran and from exotic blocks from Ediacaran salt domes. Moreover, A2-type granites are also present in the Cadomian crust of Iran. Zarand volcanic rocks are interlayered with Rizu-Dezu terrigenous sedimentary rocks, whereas plutonic rocks intruded these metasediments. New zircon U-Pb ages show that high K calc-alkaline-shoshonitic rocks formed ~ 537-536 Ma, whereas A1-type granites crystallized at ~535 Ma. Most zircons from A1-type granites show positive εHf(t) values from +1.1 to +5.1, mostly higher than high K calc-alkaline-shoshonitic rocks with εHf (t) between −6.6 and +8.1. Bulk rock Nd-Sr isotopic data (e.g., ɛNd(t)= +0.3 to +4.0) for OIB-like rocks confirm that these rocks originated from an enriched OIB-like mantle source, whereas high K calc-alkaline-shoshonitic rocks (with εNd(t) = −7.7 to −6.2) show strong interaction with, and/or re-melting of a continental crust. Our results are consistent with an interpretation that the Zarand volcanic-plutonic rocks as well as associated thick sequences of sedimentary strata rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc. This basin seems to have developed during the Ediacaran at ~570-560 Ma with deposition of Ediacaran Morad to Lower Cambrian Rizu-Dezo sedimentary rocks. The retro-arc extensional basin seems to have become magmatically active at 540-535 Ma, as shown by the occurrence of both high K calc-alkaline- shoshonitic and OIB-like magmatic rocks. This basin was flanked by a cratonic hinterland towards Gondwana and a magmatic arc to the N. The cratonic hinterland fed detritus with detrital zircons older than 0.6 Ga into the basin, whereas the magmatic arc shed detritus with 500-600 Ma old zircons. Our compiled and new data from Ediacaran-Early Paleozoic magmatic rocks also show that Cadomian high magmatic fluxes occurred differently in parts of Iran-Anatolia, with overlaps in some areas.

Geochemistry and tectonic significance of the Fannuj-Maskutan SSZ-type ophiolite (Inner Makran, SE Iran)

ABSTRACT The nature and geodynamic setting of Inner Makran ophiolites (SE Iran) are controversial and are the focus of a growing debate. The Fannuj-Maskutan ophiolite complex (central Inner Makran) is made up of lherzolites, harzburgites, layered and isotropic gabbros and diabase dikes and basaltic lavas. Here we present new textural description, mineral and whole-rock chemistry of peridotites together with bulk geochemistry of magmatic mafic rocks and U-Pb geochronology and Lu-Hf isotopes of inherited zircons in gabbros, with the aim to reconstruct the petrogenesis of the Fannuj-Maskutan ophiolite. The results indicate to a subduction-metasomatized depleted mantle source experienced 10–20% of partial melting. A transition from IAT-gabbros to E-MORB-basalts is also outlined. We interpret the obtained results as evidence for the Fannuj-Maskutan genesis in a supra-subduction zone (SSZ) basin developed in the Iranian cadomian crust as indicated by the presence of inherited Cadomian zircons (ca. 515 Ma, εHf(t): −13.6 – +11.2). Comparing the Fannuj-Maskutan ophiolites to well-known SSZ-systems such as the South Sandwich SSZ, and integrating with the existing literature on Inner Makran ophiolite belt we interpret the Fannuj-Maskutan ophiolite as a part of a major SSZ-basin opened in the Iranian cadomian crust and evolved during the Neotethys subduction under Central Iran.

Repeated magmatic buildup and deep “hot zones” in continental evolution: The Cadomian crust of Iran

The generation and differentiation of continental crust by arc magmatism is strongly influenced by episodes of high magmatic flux (“flare-ups”). Magmatic flare-ups encourage the development of deep crustal hot zones where magmatic differentiation and density stratification combine to form the upper felsic and lower mafic continental crust. Such processes, which are responsible for the construction of continental arc crust, are prolonged events, which build a ∼30-40 km arc crust over tens of million years (∼100 Myr). New zircon U-Pb data reveal that the construction of Cadomian crust from NE Iran occurred over ∼15 ± 0.3 Myr. However, compiled zircon U-Pb ages reveal a prolonged magmatic flare-up of ∼45 Myr; ∼570 to 525 Ma. Basement outcrops in NE Iran expose lower- and upper crust that show how magmatic-geochemical differentiation occurred deep beneath a Cadomian continental arc in a crustal hot zone. Isotopic data for igneous rocks produced during this 45 Myr episode reveal interactions between mantle-derived melts and old continental crust. Synthesis of new and published data indicates that this type of interaction is common during periods of high magmatic fluxes. Our results indicate that differentiation of mafic melts in the lower crust during prolonged magmatic flare-ups plays a key role in building a stratified continental crust.

Cadomian crust of Eastern Iran: evidence from the Tapeh Tagh granitic gneisses

ABSTRACT Late Neoproterozoic to Early Cambrian (Cadomian) magmatic rocks are abundant in Iran, but their occurrence in east-central Iran is voluminous. This paper reports, for the first time, on the Cadomian rocks of the eastern Iranian region known as Tapeh Tagh and deals with their geochemistry and uranium-lead (U-Pb) zircon ages. The Tapeh Tagh region contains masses of Cadomian meta-igneous rocks (including various types of granitic gneiss) and overlying meta-sedimentary rocks. The granitic gneisses show tectonic contact with metasedimentary rocks and their connection appears to have been obscured during the younger tectonic movements. The granitic gneisses show mylonitic texture with quartz ribbons and feldspar augen. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U-Pb dating of zircon grains, separated from medium to coarse-grained gneissic rocks, consistently yielded near concordant to discordant pairs of 527 ± 5 and 522 ± 10 Ma. Geochemically, these rocks show a felsic composition with SiO2 content varying from 70.30 to 79.63 wt.%, a high content of FeO(t), alkalis, and incompatible trace elements (Zr, Nb), along with high ratios of Ga/Al and FeO(t)/MgO. They have fractionated rare earth element (REE) patterns with La/Yb = 9.1–24.7. Eu shows depletion with Eu/Eu*<1. The highly fractionated granites of the Tapeh Tagh are geochemically classified into the volcanic arc granites (VAG) and within plate granite.The granitic gneisses are geochemically and geochronologically similar to other Cadomian granites and gneisses in other parts of Iran as well as the Cadomian magmatic rocks from Turkey. These widely disseminated Cadomian basement rocks in Iran and Turkey are considered as fragments of Neoproterozoic to Early Cambrian continental arcs that formed the northern active margin of Gondwanaland.

Jurassic igneous rocks of the central Sanandaj–Sirjan zone (Iran) mark a propagating continental rift, not a magmatic arc

AbstractJurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro‐diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction‐related modifications. The arc‐like features of S‐type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time.

Crustal Evolution of NW Iran: Cadomian Arcs, Archean Fragments and the Cenozoic Magmatic Flare-Up

The Cadomian orogen of NW Iran includes a series of metamorphic rocks with zircon U-Pb ages between ca 562 and 505 Ma (Ediacaran to middle Cambrian). The Ediacaran-Cambrian basement is intruded by a series of Late Eocene-Late Oligocene I-type granitic rocks. U-Pb geochronology, integrated with geochemical and isotopic data for the basement rocks in NW Iran, provides further evidence of a Cadomian (562-505 Ma) arc-related magmatic event lasting ∼ 60 Myr. Cadomian magmatism in Iran was a part of a ∼ 100 Myr long episode of subduction-related arc magmatism at the northern margin of Gondwana. Zircon Hf-isotope compositions show that during Cadomian magmatic arc activity, juvenile arc magmas interacted with reworked Archean crust to generate the Ediacaran-Cambrian igneous rocks. Our results document both inheritance of old zircons and the presence of zircons with juvenile signatures in NW Iran, suggesting that the geotectonic setting for the Cadomian rocks was an Ediacaran continental magmatic arc and probably a neighboring back-arc basin. The occurrence of Ediacaran ophiolitic slices in NW Iran may provide evidence of back-arc basin opening at that time. Cenozoic plutonism in NW Iran is part of an Eocene-Oligocene magmatic ‘flare-up’ along the Urumieh-Dokhtar Magmatic Belt in central Iran, which lasted for ca 30 Myr. The melts responsible for the formation of these rocks had an essentially juvenile signature with minor contamination by Archean to Cadomian middle-lower continental crust. Continuous convergence between Arabia and Iran was accompanied by the transition of SW Eurasia from a compressional to an extensional convergent plate margin in Eocene-Oligocene times, leading to orogenic collapse, core-complex formation, exhumation of Cadomian crust and a major increase in arc magmatism.

Failed Silurian continental rifting at the NW margin of Gondwana: evidence from basaltic volcanism of the Prague Basin (Teplá–Barrandian Unit, Bohemian Massif)

The Silurian volcanic rocks of the Prague Basin represent within-plate, transitional alkali to tholeiitic basalts, which erupted in a continental rift setting through the thick Cadomian crust of the Tepla–Barrandian Unit (Bohemian Massif). Despite the variable, often intense alteration resulting in post-magmatic replacement of the basalt mass due to carbonatization, the geochemical signatures of Silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting. The studied interval of Silurian volcanic activity ranges from Wenlock (Homerian, ~431 Ma) to late Ludlow (Gorstian, ~425 Ma) with a distinct peak at the Wenlock/Ludlow boundary (~428 Ma). Trace-element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source. Wenlock basalts are similar to alkaline OIB with depleted radiogenic Nd signature compared to Ludlow basalts, which are rather tholeiitic, EMORB-like with enriched radiogenic Nd signature. The correlation of petrogenetically significant trace-element ratios with Nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous, possibly two-component mantle source during the Wenlock–Ludlow melting. Lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills. The latter represent olivine-rich cumulates of basaltic magmas of probably predominantly Ludlow age. Meimechites with dolerites and, to a lesser extent, some lavas were subject to alteration due to wall-rock–fluid interaction. The trigger for the Wenlock-to-Ludlow (431–425 Ma) extension and related volcanism in the Prague Basin is related to far-field forces, namely slab-pull regime due to progressive closure of the Iapetus Ocean. The main stage of the Baltica–Laurentia collision then caused the Prague Basin rift failure at ca. 425 Ma that has never reached an oceanic stage.

Neoproterozoic magmatic flare-up along the N. margin of Gondwana: The Taknar complex, NE Iran

Magmatic “flare-ups” are common in continental arcs. The best-studied examples of such flare-ups are from Cretaceous and younger continental arcs, but a more ancient example is preserved in Late Ediacaran–Cambrian or Cadomian arcs that formed along the northern margin of Gondwana. In this paper, we report new trace-element, isotopic and geochronological data on ∼550 Ma magmatic rocks from the Taknar complex, NE Iran, and use this information to better understand episodes of flare-up, crustal thickening and magmatic periodicity in the Cadomian arcs of Iran and Anatolia. Igneous rocks in the Taknar complex include gabbros, diorites, and granitoids, which grade upward into a sequence of metamorphosed volcano-sedimentary rocks with interlayered rhyolites. Granodioritic dikes crosscut the Taknar gabbros and diorites. Gabbros are the oldest units and have zircon U–Pb ages of ca 556 Ma. Granites are younger and have U–Pb zircon ages of ca 552–547 Ma. Rhyolites are coeval with the granites, with U–Pb zircon ages of ∼551 Ma. Granodioritic dikes show two U–Pb zircon ages; ca 531 and 548 Ma. Geochemically, the Taknar igneous rocks have calc-alkaline signatures typical of continental arcs. Whole-rock Nd and zircon O–Hf isotopic data show that from Taknar igneous rocks were generated via mixing of juvenile magmas with older continental crust components at an active continental margin. Compiled geochronological and geochemical data from Iran and Anatolia allow identification of a Cadomian flare-up along northern Gondwana. The compiled U–Pb results from both magmatic and detrital zircons indicate the flare-up started ∼572 Ma and ended ∼528 Ma. The Cadomian flare-up was linked to strong crustal extension above a S-dipping subduction zone beneath northern Gondwana. The Iran–Anatolian Cadomian arc represents a site of crustal differentiation and stratification and involved older (Archean?) continental lower–middle crust, which has yet to be identified in situ, to form the continental nuclei of Anatolia and Iran. The Cadomian crust of Anatolia and Iran formed a single block “Cimmeria” that rifted away from northern Gondwana and was accreted to southern Eurasia in late Paleozoic time.

Late Ediacaran crustal thickening in Iran: Geochemical and isotopic constraints from the ~550 Ma Mishu granitoids (northwest Iran)

ABSTRACTThe aim of this article is to examine the geochemistry and geochronology of the Cadomian Mishu granites from northwest Iran, in order to elucidate petrogenesis and their role in the evolution of the Cadomian crust of Iran. The Mishu granites mainly consist of two-mica granites associated with scarce outcrops of tonalite, amphibole granodiorite, and diorite. Leucogranitic dikes locally crosscut the Mishu granites. Two-mica granites show S-type characteristics whereas amphibole granodiorite, tonalities, and diorites have I-type signatures. The I-type granites show enrichment in large-ion lithophile elements (e.g. Rb, Ba and K) and depletion in high field strength elements (e.g. Nb, Ti and Ta). These characteristics show that these granites have been formed along an ancient, fossilized subduction zone. The S-type granites have high K, Rb, Cs (and other large ion lithophile elements) contents, resembling collision-related granites. U–Pb zircon dating of the Mishu rocks yielded 238U/206Pb crystallization ages of ca. 550 Ma. Moreover, Rb–Sr errorchron shows an early Ediacaran age (547 ± 84 Ma) for the Mishu igneous rocks. The two-mica granites (S-type granites) show high 87Sr/86Sr(i) ratios, ranging from 0.7068 to 0.7095. Their ɛNd values change between −4.2 and −4.6. Amphibole granitoids and diorites (I-type granites) are characterized by relatively low 87Sr/86Sr(i) ratios (0.7048–0.7079) and higher values of ɛNd (−0.8 to −4.2). Leucogranitic dikes have quite juvenile signature, with ɛNd values ranging from +1.1 to +1.4 and Nd model ages (TDM) from 1.1 to 1.2 Ga. The isotopic data suggests interaction of juvenile, mantle-derived melts with old continental crust to be the main factor for the generation of the Mishu granites. Interaction with older continental crust is also confirmed by the presence of abundant inherited zircon cores. The liquid-line of descend in the Harker diagrams suggests fractional crystallization was also a predominant mechanism during evolution of the Mishu I-type granites. The zircon U–Pb ages, whole rock trace elements, and Sr–Nd isotope data strongly indicate the similarities between the Mishu Cadomian granites with other late Neoproterozoic–early Cambrian (600–520 Ma) granites across Iran and the surrounding areas such as Turkey and Iberia. The generation of the Mishu I-type granites could be related to the subduction of the Proto-Tethyan Ocean during Cadomian orogeny, through interaction between juvenile melts and old (Mesoproterozoic or Archaean) continental crust. The S-type granites are related to the pooling of the basaltic melts within the middle–upper parts of the thick continental crust and then partial melting of that crust.

Origin of the Eastern Mediterranean: Neotethys rifting along a cryptic Cadomian suture with Afro-Arabia

The Eastern Mediterranean is a land-locked basin, a remnant of the Neotethys Ocean. It was formed in the Permian−Triassic as a result of the drift of the Tauride block from the Afro-Arabian margin of Gondwana. Herein, we show that rather than being a genuine Afro-Arabia crustal fragment, the Tauride block is underlain by late Neoproterozoic Cadomian basement, which differs significantly from the Neoproterozoic “Pan-African” basement of NE Africa from which it was detached. Resembling other Cadomian terranes of western Europe, the Tauride basement is chiefly a graywacke succession deposited in a mid- to late Ediacaran back-arc basin formed on the periphery of Afro-Arabia, above the southward-subducting proto-Tethys Ocean. The back-arc region was deformed and metamorphosed to various degrees and intruded by latest Ediacaran−Cambrian granites and volcanics during the Cadomian orogeny. Unlike the protracted (∼300 m.y.) Neoproterozoic crustal evolution recorded in Afro-Arabia, the Cadomian basement of the Taurides evolved briefly, over ∼50 m.y. We show that the entire cycle of sedimentation, metamorphism, and magmatism in the Tauride basement took place in the late Ediacaran−Cambrian and lagged after Neoproterozoic Pan-African orogeny and igneous activity in Afro-Arabia. The Cadomian orogeny accreted the Taurides, and adjoining peri-Gondwana Cadomian terranes, with an already-consolidated Afro-Arabian continent. Permian−Triassic rifting of the Eastern Mediterranean occurred close to the transition between these two domains. Rifting was thus inherited from, and superimposed on, late Ediacaran structures formed in front of the current Afro-Arabia margin of Gondwana during Cadomian orogeny. The boundary between the Cadomian edifice and the Pan-African crust of Afro-Arabia appears to lie presently on the southern margin of the Mediterranean, extending from Morocco in the west to Arabia in the east. Hence, the continental margin of the Eastern Mediterranean, including in the Levant Basin, is probably underlain by a thinned Cadomian crust.

Cadomian (∼560 Ma) crust buried beneath the northern Arabian Peninsula: Mineral, chemical, geochronological, and isotopic constraints from NE Jordan xenoliths

In order to better understand the nature and formation of the lower continental crust beneath northern Arabia, we studied lower crustal xenoliths brought up by Neogene basalts in NE Jordan. Most of these xenoliths are comprised of primary phases plagioclase + two-pyroxenes with magnetite and ilmenite. Most clinopyroxene are augite whereas orthopyroxene mostly are hypersthene (Mg# = 50–80). Plagioclase feldspar is dominantly andesine–labradorite; pyrope-rich garnet and Fe-rich olivine (Fo75 to Fo62) are rare. These xenoliths represent cumulates formed from intermediate magmas that pooled in the lower crust. Many xenoliths also contain small, fine-grained K-rich zones interpreted as melt pockets reflecting late magmatic infiltration of the lower crust. The xenoliths display a wide range in major element compositions (37–51 wt.% SiO2, 4–15 wt.% MgO and 0.1–6.3 wt.% TiO2), enrichment in Ba, K, Sr, Pb and Eu, and some trace element ratios atypical of bulk continental crust (e.g., K/Rb=1265±565, K/U=63000±60080 and Th/U=0.96±0.56); these extreme ratios reflect widespread K-metasomatism associated with melt pockets. The magmas from which these cumulates formed may have been generated at a reararc convergent margin setting. Four U–Pb zircon populations yield indistinguishable ages of 554±4 Ma; 559±5 Ma; 559±6 Ma, and 563±5 Ma. Initial 87Sr/86Sr values (0.70260–0.70352) and positive εNd(560) (with the exception of a single, more radiogenic sample (+9.6), range=+1.3 to +4.8) indicate that the lower crust sampled by the xenoliths originated in the asthenospheric mantle, with little or no interaction with older crust, although Pb isotopic compositions allow for some interaction with older or subducted crustal materials. We interpret the geochemistry and mineralogy of these xenoliths to indicate that the lower crust beneath NE Jordan is mafic and comprised of plagioclase-rich 2-pyroxene igneous rocks. The lower crust of this area formed by magmatic underplating over less than 18 Ma. The crust of NE Jordan is significantly younger than the crust of the northernmost Arabian–Nubian Shield and represents a fragment of Cadomian (600–520 Ma) crust that may make up the NE margin of the Arabian Plate.

Topography of the Variscan orogen in Europe: failed–not collapsed

The Variscan orogenic collage consists of three subduction-collision systems (Rheno-Hercynian, Saxo-Thuringian and Massif Central-Moldanubian). Devonian to early Carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts, but also in post-tectonic basins within these foreland belts and on the Cadomian crust of peri-Gondwanan microcontinental fragments, which represent the upper plates of the subduction/collision zones. These marine basins preclude high elevations in the respective areas and also in their neighbourhood. Widespread late Carboniferous intra-montane basins with their coal-bearing sequences are likewise incompatible with high and dry plateaus. While narrow belts with high elevations remain possible along active margins within the orogen, comparison of the Variscides with the Himalaya/Tibetan plateau is unfounded. Plausible reasons for the scarcity of high Variscan relief include subduction of oceanic and even continental crust, subduction erosion, orogen-parallel extension and—most important—lithospheric thinning accompanied by high heat flow and magmatism. In many areas, timing and areal array of magmatism and HT metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse. It is suggested, instead, that lithospheric thinning and heating are due to mantle activities caused by the Tethys rift. The lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift: in terms of topography, the Variscides represent a failed orogen. The HT regime also explains the abundance of granitoids and HT/LP metamorphic rocks typical of the Variscides. Melting in the HT regime extracted mafic components from Variscan and Cadomian crust as well as from Cadomian metasomatized lithospheric mantle, thus mimicking subduction-related magmatism. The onset of the HT regime at c. 340 Ma may also have triggered the final ascent of HP/UHP felsic metamorphic rocks.

Magmatic history of granite-derived mylonites from the southern Desná Unit (Silesicum, Czech Republic)

In the southern part of the Hrubý Jeseník Mts. (Silesicum, Bohemian Massif) granitic orthogneisses and quartz-feldspars mylonites occur that were variously deformed, metamorphosed and imbricated with the overlaying Devonian volcano-sedimentary complex during the Variscan orogeny. Based on combined mapping, petrologic and geochemical studies, three main rock groups are distinguished. The fairly primitive nature of the tonalite suite is shown by low 87Sr/86Sr550 ratios (0.7034–0.7038) and positive ε Nd 550 values (+3.8 and +3.1). Isotopic composition of the metagranite suite is similar (ε Nd 550 = +1.9 to +2.9). Both suites probably belong to a single Cadomian calc-alkaline tonalite–granite association, which can be correlated with acid-intermediate plutonic rocks of the eastern Brunovistulicum (Slavkov Terrane). All these units are interpreted as representing dismembered fragments of the same continental margin magmatic arc system. The leucogranite suite is most likely early Variscan in age. The initial Nd isotopic composition (ε Nd 330 = +0.8 and +2.3) is consistent with its formation by a Variscan remelting of the metaigneous Cadomian crust represented by the rocks of the tonalite–granite association. The within-plate granite affinity of the leucogranite suite is most likely related to the break-up of the Brunovistulicum during the Variscan orogeny.

Gondwana-derived microcontinents — the constituents of the Variscan and Alpine collisional orogens

The European Variscan and Alpine mountain chains are collisional orogens, and are built up of pre-Variscan “building blocks” which, in most cases, originated at the Gondwana margin. Such pre-Variscan elements were part of a pre-Ordovician archipelago-like continental ribbon in the former eastern prolongation of Avalonia, and their present-day distribution resulted from juxtaposition through Variscan and/or Alpine tectonic evolution. The well-known nomenclatures applied to these mountain chains are the mirror of Variscan resp. Alpine organization. It is the aim of this paper to present a terminology taking into account their pre-Variscan evolution at the Gondwana margin. They may contain relics of volcanic islands with pieces of Cadomian crust, relics of volcanic arc settings, and accretionary wedges, which were separated from Gondwana by initial stages of Rheic ocean opening. After a short-lived Ordovician orogenic event and amalgamation of these elements at the Gondwanan margin, the still continuing Gondwana-directed subduction triggered the formation of Ordovician Al-rich granitoids and the latest Ordovician opening of Palaeo-Tethys. An example from the Alps (External Massifs) illustrates the gradual reworking of Gondwana-derived, pre-Variscan elements during the Variscan and Alpine/Tertiary orogenic cycles.

Refraction/wide-angle reflection investigation of the Cadomian crust between northern Brittany and the Channel Islands

A seismic refraction experiment has been carried out off the coast of northern Brittany (NW France) to investigate the deep structure of the North Armorican block. The seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations. We present 2D velocity models computed from two crossing seismic refraction profiles (labelled P1 and P2 and each about 135km long) oriented NE–SW and NNW–SSE, i.e. parallel and perpendicular, respectively, to the regional basement structures. Wide-angle reflection profiles were also performed to extend the 2D structures to a 3D crustal interpretation. Observed and calculated P-wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models. The model which best fit the data consists of: (1) high basement velocities at the top of the crust, except in an area with shallow seismic velocity which can be as low as 3.0km/s at the surface and possibly related to the existence of shallow Paleozoic sedimentary basins; (2) the mean crustal velocity ranges from 5.5 to 6.8km/s at a depth of 15km depth; (3) lower crustal velocities range from 7.0 to 7.5km/s at a depth of 35–38km and then from 7.5 to 8.1km/s at the crust–mantle boundary located between 35 and 38km depth. Comparisons with the crustal section imaged by the coincident seismic vertical reflection profile SWAT 10 show a good correlation between velocity gradients and lower crustal reflectivity. When compared to surrounding basement areas, the North Armorican Cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this Upper Proterozoic block from later Paleozoic and Mesozoic tectonic episodes. The crustal structure of the North Armorican block thus appears to be an intercalated feature between the surrounding Paleozoic areas and older shield or platform domains.