Kinematics of syn-eclogite deformation in the Bergen Arcs, Norway, implications for exhumation mechanisms

The northwestern part of Holsnøy island, in the Bergen Arcs, Norway, consists of a granulite-facies protolith partially transformed at depth in eclogite (700 °C, > 19 kbars) and amphibolite (650 °C, 8-10 kbars) facies during the Caledonian orogenesis. Eclogitized zones are mainly planar objects (fractures with parallel reaction bands and cm-to-100 m-scale shear zones). Eclogitic zones are distributed in two sets of orientations and the associated deformation can be described as 'bookshelf tectonics'. The major shear zones strike around N120 and dip to the North, and show consistent top-to-the-NE shear sense throughout the area. In the large-scale kinematic frame of Caledonian NW-dipping slab, eclogitic shear zones are interpreted as the way to detach crustal units from the subducting slab and to prevent their further sinking. As the retrograde amphibolitic deformation pattern is similar to the eclogitic one, the detached crustal units started their way up along these eclogitic shear zones. Radiometric ages of eclogitic and amphibolitic metamorphism and their comparison with the chronology of Caledonian orogenesis show that the deformation recorded on Holsnøy occurred in a convergent context. The mechanism we propose can thus account for the first steps of exhumation during collision

Origin of Lower Cretaceous ('Nubian') sandstones of North-east Africa and Arabia from detrital zircon U-Pb SHRIMP dating

Lower Cretaceous sandstones of the type exposed in Israel, deposited over much of North Africa and Arabia as widespread sandstone sheets, typically are mineralogically and texturally mature. Previous petrographic examinations suggested that the Lower Cretaceous sandstones are at least partly a product of recycling and the present study supports this notion. The results of U-Pb Sensitive High Resolution Ion Micro-Probe (SHRIMP) dating of detrital zircons from the Lower Cretaceous section exposed in Israel indicate that they are dominated by detrital zircons of Neoproterozoic age, mainly concentrated in the 0·55 to 0·65 Ga interval, with various amounts of older (pre-Neoproterozoic) zircons (of 0·95 to 1·10, 1·7 to 2·0 and 2·6 to 2·65 Ga age groups). The overall age signal is similar to detrital zircon age spectra previously obtained from the Cambrian-Ordovician sections of Israel and Jordan. Remarkably, the detrital zircon spectra remained almost unchanged for nearly 400 Myr. Thus, the most probable provenance of the Lower Cretaceous sandstone is the recycling of relatively proximal Palaeozoic sandstone. Since first unroofed from above pan-African terranes closer to the secession of orogeny, the ensuing siliciclastics were recycled repeatedly throughout the Phanerozoic with little additional basement denudation. The Lower Cretaceous sandstone comprises quartz sand that was first eroded from above pan-African orogens ca 400 Myr prior to deposition

Cadomian basement and Paleozoic to Triassic siliciclastics of the Taurides (Karacahisar dome, south-central Turkey): Paleogeographic constraints from U-Pb-Hf in zircons

The Tauride block in Turkey is a peri-Gondwana, Cadomian-type terrane that rifted from the Afro-Arabian margin of Gondwana in the Permo-Triassic and re-accreted to Arabia in the Neogene. In the Karacahisar dome in the southern-central Taurides, Neoproterozoic basement metasediments and intrusive rocks are overlain by Cambro-Ordovician, Carboniferous and Triassic sediments. We studied U-Pb-Hf in zircons from major rock units exposed in Karacahisar in order to constrain the Cadomian crustal evolution of the Taurides, to evaluate the provenance of the Neoproterozoic and overlying sediments, to constrain the paleogeography of the Taurides, and to assess their linkage to Gondwana

Long-distance transport of North Gondwana Cambro-Ordovician sandstones : evidence from detrital zircon Hf isotopic composition

A voluminous Early Paleozoic sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly during the Neoproterozoic-Cambrian Pan-African Orogeny. Field evidence for the sense of transport indicate that sediments were carried from Gondwana hinterland towards the supercontinent margins in the North (present coordinates). Derivation from Pan-African terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambro-Ordovician sandstone that top the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. Remarkably, the detrital zircon Hf isotopic signal stands in marked contrast with Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambro-Ordovician sandstones in Israel and Jordan yielded negative epsilon (super Hf(t) ) values incompatible with a juvenile source. Therefore, rather than from the adjacent Arabian-Nubian Shield, most of the detrital zircons were derived from distant terrane(s), comprising pre-Neoproterozoic crust reworked during Pan-African orogeny. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Pan-African orogens of NE Africa were completely warned down by the onset of Cambro-Ordovician deposition and that vast areas in the northern part of Gondwana wer e then low-lying such as to allow transfer of sand across the continent.1 page(s

Crustal evolution and recycling in the northern Arabian-Nubian Shield : new perspectives from zircon Lu-Hf and U-Pb systematics

This paper presents laser-ablation (MC)-ICP-MS single-zircon Hf isotopic data coupled with U-Pb geochronology from a unique rock inventory in southern Israel, recording ca. 300m.y. of Neoproterozoic crustal evolution in the northern Arabian-Nubian Shield (ANS). The early, island-arc magmatic cycle in this area is recorded by detrital zircons from arc-derived metasediments, dated between 870 and 760Ma, and by ca. 790 and 740Ma orthogneisses. Zircon e{open}Hf(t) values from these rocks are all positive, supporting previous isotopic evidence for the overall juvenile nature of northern ANS crust. Nonetheless, detrital zircons from the metasedimentary units display a wide range of e{open}Hf(t) values between +1 and +13. The lower values in this range suggest some contribution from an older crustal component. The involvement of pre-Neoproterozoic crustal component is also indicated by the occurrence of rare zircons with pre-Neoproterozoic ages and/or negative e{open}Hf(t) values in younger (post-collisional) magmas that intrude the arc sequence. Arc accretion was followed by a period of ∼60m.y. of relative igneous tranquility interrupted only by the emplacement of local basic dike swarm at ca. 705Ma. Average zircon e{open}Hf (t) value of +10 found in one of these dikes indicates a depleted mantle source possibly implying a mid-Neoproterozoic extensional phase. The later, post-collisional magmatic cycle marks the transition of the accreted arc complexes into a mature continental crust. It consists of voluminous 680-600Ma granitoid plutons of calc-alkaline affinity, and subordinate 610-580Ma intrusions of alkaline affinity. Average zircon e{open}Hf(t) values in these post-collisional rocks are all positive and range between +6 and +9, compatible with both mantle source and juvenile crust reworking.16 page(s

Drainage response to Arabia-Eurasia collision: Insights from provenance examination of the Cyprian Kythrea flysch (Eastern Mediterranean Basin)

The Cenozoic geodynamics of the north-eastern Mediterranean Basin have been dominated by the subduction of the African Plate under Eurasia. A trench-parallel crustal-scale thrust system (Misis-Kyrenia Thrust System) dissects the southern margin of the overriding plate and forms the structural grain and surface expression of northern Cyprus. Late Eocene to Miocene flysch of the Kythrea (Degirmenlik) Group is exposed throughout northern Cyprus, both at the hanging-wall and foot-wall of the thrust system, permitting access to an extensive Cenozoic sedimentary record of the basin. We report the results of a combined examination of detrital zircon and rutile U-Pb geochronology (572 concordant ages), coupled with Th/U ratios, Hf isotopic data and quantitative assessment of grain morphology of detrital zircon from four formations (5 samples) from the Kythrea flysch. These data provide a line of independent evidence for the existence of two different sediment transportation systems that discharged detritus into the basin between the late Eocene and late Miocene. Unique characteristics of each transport system are defined and a sediment unmixing calculation is demonstrated and explained. The first system transported almost exclusively North Gondwana-type, Precambrian-aged detrital zircon sourced from siliciclastic rock units in southern Anatolia. A different drainage system is revealed by the middle to late Miocene flysch sequence that is dominated by Late Cretaceous-Cenozoic-aged detrital zircon, whose age range is consistent with the magmatic episodicity of southeast Anatolia, along the Arabia-Eurasia suture zone. Deposition of these late Miocene strata took place thereupon closure of the Tethyan Seaway and African-Eurasian faunal exchange, and overlap in time with a pronounced uplift of eastern Anatolia. Our analytical data indicate the onset of prominent suture-parallel sediment transport from the collision zone of south-eastern Anatolia into the Kyrenia Range of northern Cyprus, marking the drainage response to the continental collision between Arabia and Eurasia

The tectono-metamorphic evolution of a dismembered ophiolite (Tinos, Cyclades, Greece)

The six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subduction zone (SSZ) affinity for the ophiolitic suite. δ81O values of phyllites, gabbros and serpentinites range from 6 to 15‰. Model calculations indicate that such values are consistent with low temperature (50-200 °C) alteration of parent rocks by sea-water at varying water/rock ratios. This would agree with the early low temperature mineralogy of the serpentinites, but the early high temperature alteration of the gabbros would require the presence of 18O-enriched sea-water. The following overall history is suggested for Tinos ophiolitic slices. (1) Oceanic crust was generated at a supra-subduction zone spreading centre with high temperature alteration of gabbros. (2) Tectonic disturbance (its early hot stages recorded in an amphibolitic shear zone at the base of serpentinites) brought the already cooled ultramafics into direct contact with sea-water and caused low-T serpentinization. (3) Tectonism after cooling involved thrusting which caused repetition and inversion of the original order of the oceanic suite. (4) Regional metamorphism of all the ophiolite components at greenschist-facies conditions (∼450 °C) overprinted the early alteration mineralogy. It was probably induced by continued thrusting and piling up of nappes. The Tinos ophiolite, dated as late Cretaceous and genetically related to other low pressure rock-units of the same age in the Aegean, differs in age and degree of dismemberment and metamorphism from ophiolites in mainland Greece

Cadomian (ca. 550 Ma) magmatic and thermal imprint on the North Arabian-Nubian Shield (south and central Israel): New age and isotopic constraints

The Arabian-Nubian Shield (ANS) is a province of mostly juvenile Neoproterozoic continental crust in NE Africa and Arabia, forming the northern extension of the East-African orogen. Subsequent to widespread, mostly late- to post-orogenic early-Ediacaran calc-alkaline granitoid emplacement, the latest magmatic phase in the ANS saw the intrusion of alkaline shallow plutons alongside volcanic massifs and dikes of variable compositions. While some of these intrusions were dated to 550–530 Ma by Rb-Sr and Ar-Ar, U-Pb zircon data has mostly limited ANS magmatism to no younger than ca. 580 Ma. Here we report new age and isotopic data from latest Neoproterozoic intrusions along a N-S transect within the northern ANS and its periphery, substantiating the record of a ca. 550 Ma igneous-thermal event in the area. In the northern Negev of Israel, mafic igneous rocks intruded into the subsurface late Neoproterozoic clastic wedge of the Zenifim Formation produced apatite U-Pb isochron ages between 557 and 545 Ma. In the Amram alkaline massif, 150 km to the south, an alkali-granite was dated to 605 ± 2.5 Ma by zircon U-Pb, but apatite U-Pb ages from all rocks of the massif concentrate at ca. 550 Ma indicating thermal resetting. Apatites from the zircon dated 611 ± 5 Ma Roded quartz diorite in the Elat area, south of Amram, produced a 600 ± 4 Ma U-Pb age, demonstrating that southern segments were not thermally reset at ca. 550 Ma. Nd isotopes in apatite and whole rock show the ca. 550 Ma igneous rocks to be variable in their isotopic composition, ranging between εNd(t)=(-4) – (+2), far less juvenile than typical ANS magmas. The ca. 550 Ma intrusions resemble in their age and isotopic properties Cadomian granites and volcanic rocks that are known from southern Turkey, Iran and Europe. We interpret the ca. 550 Ma igneous overprint and thermal resetting documented in the northernmost ANS as related to the superposition of Cadomian orogeny on the ANS periphery. This can be related to the late Ediacaran subduction of proto-Tethys accompanying the accretion of the Cadomian basement of the Tauride block to NE Africa and Arabia, before the area was overstepped by Cambrian platform sedimentation

Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian-Ordovician sandstones

A voluminous Cambrian-Ordovician sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly by a series of Neoproterozoic-Cambrian orogenic events. Paleocurrent markers indicate that the sediments were carried from Gondwana hinterland toward the supercontinent margins in the north (present coordinates). Derivation from Neoproterozoic terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambrian-Ordovician sandstone that tops the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. It is remarkable that the detrital zircon Hf isotopic signal is in marked contrast to the Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambrian-Ordovician sandstones yielded negative εHf(t) values incompatible with a juvenile source. Therefore, most of the detrital zircons were derived from distant terranes comprising pre-Neoproterozoic crust reworked during the assembly of Gondwana, rather than from the adjacent Arabian-Nubian Shield. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Neoproterozoic orogens of northeast Africa were completely worn down by the onset of Cambrian deposition and that vast areas in the northern part of Gondwana were low lying at that time.4 page(s