Metamorphic Units Research Articles

Origin of sediments during Cretaceous continent—continent collision in the Romanian Southern Carpathians: preliminary constraints from 40Ar/39Ar single-grain dating of detrital white mica

Abstract Single grains of detrital white mica from the lowermost Upper Cretaceous Sinaia Flysch have been dated using the 40Ar/39Ar technique. The Sinaia Flysch was deposited in a trench between the Danubian and Getic microcontinental pieces after the closure of the Severin oceanic tract. The Danubian basement is largely composed of a Panafrican/Cadomian basement in contrast to the Getic/Supragetic units with a Variscan-aged basement, allowing the distinction between these two blocks. Dating of detrital mica from the Sinaia Flysch resulted in predominantly Variscan ages (329 ± 3 and 288 ± 4 Ma), which prove the Getic/Supragetic source of the infill of the Sinaia Trench. Subordinate Late Permian (263 ± 8 and 255 ±10 Ma), Early Jurassic (185 ± 4 and 183 ± 3 Ma) and Late Jurassic/Early Cretaceous (149 ± 3 and 140 ± 3 Ma) ages as well as a single Cretaceous age (98 ± 4 Ma) are interpreted as representing the exposure of likely retrogressive low-grade metamorphic ductile shear zones of various ages. Ductile shear zones with similar 40Ar/39Ar white mica ages are known in the Getic/Supragetic units. The Cretaceous ages also show that Cretaceous metamorphic units were already subject to erosion during the deposition of the Sinaia Flysch.

First description of a metamorphic sole related to ophiolite obduction in the northern Caribbean: Geochemistry and petrology of the Güira de Jauco Amphibolite complex (eastern Cuba) and tectonic implications

The Güira de Jauco Amphibolite Complex underlies the Mayarí–Baracoa Ophiolitic Belt (eastern Cuba) and is composed of highly foliated amphibolite, locally with blocks of sheared serpentinitie that enclose concordant layers of amphibolite. These amphibolites are interpreted as remnants of a sub-ophiolitic metamorphic sole likely formed during late Cretaceous times before the initiation of the collision between the Caribbean and the North American plates. The complex includes common amphibolites (Hbl+Pl±Ep±Ttn±Rt±Qtz±Ap), garnet amphibolites (Hbl+Grt+Pl+Ep±Ttn±Qtz±Ap) and clinopyroxene amphibolites (Hbl+Pl+Cpx±Ep±Ttn±Qtz±Ap). Geochemical data indicates that the protoliths are igneous mafic rocks of basaltic composition that likely formed part of the upper and lower(?)crust of an oceanic lithosphere. Rare Earth element contents suggest a Mid-Ocean Ridge Basalt (MORB) origin. However, the enrichment in Large-Ion Lithophile elements and the depletion in NbTaTi as compared to Normal-MORB, suggests a suprasubduction scenario, either a back-arc or a fore-arc setting. The pressure–temperature estimates of metamorphism indicate that the entire amphibolite body underwent similar peak metamorphic conditions at 650–665°C and 8.5–8.7kbar (ca. 30km depth), corresponding to a relatively high apparent geothermal gradient of 23°C/km.This data, together with field observations and available age data (KAr ages and the apparent stratigraphy of the metamorphosed (high P) volcanic-arc terrane of eastern Cuba) suggest that the development of the ophiolite and the initial stages of obduction occurred shortly after subduction initiation in a back-arc environment in Late Cretaceous times. Subsequent subduction/collision/accretion of platform- and arc-derived high-pressure metamorphic units took place at the leading edge of the Caribbean plate in response to arc–continent collision (from 70Ma to Eocene).

Major role of shear heating in intracontinental inverted metamorphism: Inference from a thermo-kinematic parametric study

Inverted metamorphism corresponds to the stacking of high-temperature metamorphic units structurally on top of lower-temperature units and is commonly observed along main thrusts in major orogens. Yet, in spite of many existing models, the origin and preservation of the metamorphic inversion in intracontinental collision belts are still debated. In this study, we use a crustal-scale 2D thermo-kinematic model in order to investigate the key parameters controlling the inversion of the geothermal gradient at crustal scale. Our results confirm that the kinematic framework strongly impacts the thermal evolution around the thrust. Erosion velocity and thermal conductivity of rocks are two parameters that control the spatial location of the thermal perturbation and the intensity of inversion, respectively. However, even in extreme kinematic configurations, i.e., convergence velocities > 3 cm ∙ yr− 1 and relatively high thrust dip angles ~ 30°, the thermal inversion is fleeting and thrust temperatures cannot reach the high temperature peak values characteristic of natural occurrences (> 600 °C) if shear heating is not taken into account. Conversion of mechanical energy into heat represents a main contribution to the thermal budget along main crustal shear zones. It leads to high temperature conditions in the thrust zone and our results attest that it is the only process that allows the preservation through time of an intense thermal inversion. Our quantification shows that shear heating is much more efficient than other processes such as accretion and surface denudation and is compatible with the observations of inverted metamorphism in the Himalayan or Variscan belts, for example. This comparison with natural occurrences suggests that the formation and preservation of intracontinental inverted metamorphism require shear zone viscosity values of the order of 1020–1021 Pa ∙ s for convergence velocities between 1 and 3 cm ∙ yr− 1.

Monazite EMP-Th-U-Pb age pattern in Variscan metamorphic units in the Armorican Massif (Brittany, France)

Monazite EMP-Th-U-Pb age pattern in Variscan metamorphic units in the Armorican Massif (Brittany, France)

Meso/Neoarchean crustal domains along the north Konkan coast, western India: The Western Dharwar Craton and the Antongil-Masora Block (NE Madagascar) connection

Synthesis of mesoscopic structures and microscopic analyses of fabric superposition and deformation microstructures are combined with Th–U–Pb (total) ages in monazites to constrain the Meso/Neoarchean crustal domains in the Western Dharwar Craton (WDC) along the western coast of India; the domains are correlated with those in NE Madagascar to configure the assembly of crustal domains in the East Gondwanaland prior to the Mesozoic break up. In the WDC, para-schists and phyllites of the Shimoga schist belt, SSB (unmixed age components, 3067±26 and 3158±110Ma) and the Peninsular gneisses (3138±35Ma) are intruded by granitoid plutons (2924±50Ma) deformed at low-T. Five generations of deformation events are identified in the SSB and Peninsular gneisses. But the two early tectonic fabrics in Peninsular gneisses formed at anatectic amphibolite facies conditions are in sharp contrast to fabrics in SSB formed at greenschist facies. The two lithodemic units are inferred to share a pre-3.2Ga tectonic contact.By contrast, in the Goa schist belt (GSB), phyllites/schists (unmixed ages – 2436±34, 2543±66 and 2625±36Ma) and polymict conglomerates (unmixed ages – 2458±34 and 2566±53Ma) possess two generations of tectonic fabrics and are the youngest dated metamorphic unit in the WDC. The deformed Quepem granitoid pluton located along the axial zone of the regional-scale N-trending gently plunging upright folds in GSB yields comparable chemical age (2565±33Ma). Angular to sub-rounded and boulder to granule sized polymict clasts of Peninsular gneisses (3128±60Ma), foliated granitoids, amphibolites, BIFs and low-grade quartzite/chlorite-muscovite schists in GSB para-conglomerates suggest the sediments deposited in shallow basin were derived from neighboring sources, e.g. the Peninsular gneisses and SSB supracrustals. The Neoarchean deformation-metamorphism marks the closure of the Goa basin synchronous with the 2.4–2.6Ga accretion of the West and the East Dharwar Cratons.Based on the results of this study, the following correlation among Mesoarchean–Neoarchean crystallites in the Western Dharwar Craton and the Antongil-Masora Block, NE Madagascar (in italics below) appears realistic, e.g. 2.4–2.6Ga Goa schist belt≈Mananara Group; 2.5Ga Quepem granitoid≈Masoala Suite; 3.1Ga Shimoga schist belt≈Fenerivo Group; 3.1Ga Peninsular gneiss suite≈Nosy Boraha suite.

Application of ASTER remote sensing data to geological mapping of basement domains in arid regions: a case study from the Central Anti-Atlas, Iguerda inlier, Morocco

Satellite remote sensing is shown to provide critical support for geological and structural mapping in semiarid and arid areas. In this work, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to clarify the geological framework of the Precambrian basement of the Iguerda Proterozoic inlier in the Moroccan Central Anti-Atlas. In this study, the interpretation of the processed digital data has been ground truthed with geological field data collected during a reconnaissance-mapping program in the Central Anti-Atlas. The Iguerda inlier offers a deeply eroded Precambrian massif dominated by a Paleoproterozoic basement composed of supracrustal metasedimentary units intruded by various Eburnian granitoids. Impressive mafic dyke swarms mainly of Proterozoic age crosscut this basement. Eburnian basement rocks are unconformably overlain by Lower Ediacaran volcanosedimentary rocks of the Ouarzazate Group and Upper Ediacaran–Lower Cambrian carbonates. The applied ASTER analyses are particularly effective in the lithological differentiation and discrimination of geological units of the Iguerda inlier. The spectral information divergence (SID) classification algorithm coupled with spectral angle mapper and maximum likelihood classification effectively discriminates between metamorphic rocks, granitoid bodies, and carbonate cover. SID classification improves geologic map accuracy with respect to the spatial distribution of plutonic bodies and metamorphic units. In addition, Paleoproterozoic granitoids have been well discriminated into separate distinct suites of porphyritic granites, granodiorites, and peraluminous leucogranite suites. This discrimination was initially identified via remote sensing analysis and later ground truthed in the field. This methodology enhances geological mapping and illustrates the potential of ASTER data to serve as a vital tool in detailed geologic mapping and exploration of well-exposed basement of arid regions, such as the Proterozoic of the Anti-Atlas Mountains of Morocco.

Late Neoproterozoic extensional detachment in eastern Sør Rondane Mountains, East Antarctica: Implications for the collapse of the East African Antarctic Orogen

A geological study of the Balchenfjella area of the eastern Sør Rondane Mountains forms the basis of a revised tectonic history for eastern Dronning Maud Land in Antarctica. The Balchenfjella area can be structurally divided into two metamorphic units that are separated by a SE-dipping ductile extensional shear zone (the Balchen detachment fault). The mafic- and ultramafic-dominant part of the footwall side is exposed as a tectonic window adjacent to the detachment. Metamorphic and structural data indicate that the eastern Sør Rondane Mountains were affected by four phases of deformation (D1–D4). Phases D1 and D2 are developed in the footwall side of the shear zone, whereas D3 and D4 are recorded in both the footwall and hangingwall sides. Mineral assemblages in mafic lithologies are characteristic of granulite facies (D1 and D2) to amphibolite facies (D2 and D3) metamorphism. D1 is characterized by recumbent folds and this phase is interpreted as a compressional tectonic setting coincident with granulite facies metamorphism. D2 represents a transpressional deformation phase characterized by upright folds and a large-scale dextral shear zone. The Balchen detachment fault was formed during the extensional tectonic setting of D3. The timing of D3 extensional deformation is bracketed by the age of D2-related metamorphism (at c. 600Ma) and the timing of post-kinematic intrusion of granites (at c. 550Ma). The present results demonstrate that the eastern Sør Rondane Mountains underwent Late Neoproterozoic extensional collapse similar to that observed in other parts of the East African–Antarctic Orogen including the Arabian–Nubian Shield and Madagascar.

Neotethys closure history of Anatolia: insights from40Ar–39Ar geochronology andP–Testimation in high‐pressure metasedimentary rocks

AbstractThe multiple high‐pressure (HP), low‐temperature (LT) metamorphic units of Western and Central Anatolia offer a great opportunity to investigate the subduction‐ and continental accretion‐related evolution of the eastern limb of the long‐lived Aegean subduction system. Recent reports of theHP–LTindex mineral Fe‐Mg‐carpholite in three metasedimentary units of the Gondwana‐derived Anatolide–Tauride continental block (namely the Afyon Zone, the Ören Unit and the southern Menderes Massif) suggest a more complicated scenario than the single‐continental accretion model generally put forward in previous studies. This study presents the first isotopic dates (white mica40Ar–39Ar geochronology), and where possible are combined withP–Testimates (chlorite thermometry, phengite barometry, multi‐equilibrium thermobarometry), on carpholite‐bearing rocks from these threeHP–LTmetasedimentary units. It is shown that, in the Afyon Zone, carpholite‐bearing assemblages were retrogressed through greenschist‐facies conditions atc. 67–62 Ma. Early retrograde stages in the Ören Unit are dated to 63–59 Ma. In the Kurudere–Nebiler Unit (HPMesozoic cover of the southern Menderes Massif),HPretrograde stages are dated toc. 45 Ma, and post‐collisional cooling toc. 26 Ma. These new results support that the Ören Unit represents the westernmost continuation of the Afyon Zone, whereas the Kurudere–Nebiler Unit correlates with the Cycladic Blueschist Unit of the Aegean Domain. In Western Anatolia, three successiveHP–LTmetamorphic belts thus formed: the northernmost Tavşanlı Zone (c. 88–82 Ma), the Ören–Afyon Zone (between 70 and 65 Ma), and the Kurudere–Nebiler Unit (c. 52–45 Ma). The southward younging trend of theHP–LTmetamorphism from the upper and internal to the deeper and more external structural units, as in the Aegean Domain, points to the persistence of subduction in Western Anatolia between 93–90 andc. 35 Ma. After the accretion of the Menderes–Tauride terrane, in Eocene times, subduction stopped, leading to continental collision and associated Barrovian‐type metamorphism. Because, by contrast, the Aegean subduction did remain active due to slab roll‐back and trench migration, the eastern limb (below Southwestern Anatolia) of the Hellenic slab was dramatically curved and consequently teared. It therefore is suggested that the possibility for subduction to continue after the accretion of buoyant (e.g. continental) terranes probably depends much on palaeogeography.

Timing of metamorphism of the Lansang gneiss and implications for left-lateral motion along the Mae Ping (Wang Chao) strike-slip fault, Thailand

The Mae Ping fault (MPF), western Thailand, exhibits dominantly left-lateral strike-slip motion and stretches for >600km, reportedly branching off the right-lateral Sagaing fault in Myanmar and extending southeast towards Cambodia. Previous studies have suggested that the fault assisted the large-scale extrusion of Sundaland that occurred during the Late Eocene–Early Oligocene, with a geological offset of ∼120–150km estimated from displaced high-grade gneisses and granites of the Chiang Mai–Lincang belt. Exposures of high-grade orthogneiss in the Lansang National Park, part of this belt, locally contain strong mylonitic textures and are bounded by strike-slip ductile shear zones and brittle faults. Geochronological analysis of monazite from a sample of sheared biotite-K-feldspar orthogneiss suggests two episodes of crystallization, with core regions documenting Th–Pb ages between c. 123 and c. 114Ma and rim regions documenting a significantly younger age range between c. 45–37Ma. These data are interpreted to represent possible magmatic protolith emplacement for the Lansang orthogneiss during the Early Cretaceous, with a later episode of metamorphism occurring during the Eocene. Textural relationships provided by in situ analysis suggest that ductile shearing along the MPF occurred during the latter stages of, or after, this metamorphic event. In addition, monazite analyzed from an undeformed garnet-two-mica granite dyke intruding metamorphic units at Bhumipol Lake outside of the Mae Ping shear zone produced a Th–Pb age of 66.2±1.6Ma. This age is interpreted to date the timing of dyke emplacement, implying that the MPF cuts through earlier formed magmatic and high-grade metamorphic rocks. These new data, when combined with regional mapping and earlier geochronological work, show that neither metamorphism, nor regional cooling, was directly related to strike-slip motion.

Evidence of Eocene high-temperature/high-pressure metamorphism of ophiolitic rocks and granitoid intrusion related to Neotethyan subduction processes (Doğanşehir area, SE Anatolia)

Abstract New data for regionally important granulite facies metaophiolitic rocks and cross-cutting granitoids rocks are presented and discussed. The high-temperature/high-pressure Berit metaophiolite is cut by unmetamorphosed Eocene (51–45 Ma) granitoid rocks. The highest metamorphic grade occurs in blocks of mafic granulites. Enveloping amphibolite facies rocks reflect retrograde metamorphism related to exhumation. Sm–Nd (pyroxene–garnet–amphibole–whole rock) isochron ages of 52–50 Ma for the granulite facies rocks are interpreted to represent the time of cooling of the granulite facies rocks. The over-riding Malatya metamorphic unit to the north is also intruded by Eocene granitoid rocks. The granulite facies metamorphism of the meta-ophiolitic rocks is inferred to have formed in the roots of an Eocene magmatic arc, with accentuated heat flow being provided by subduction of a spreading ridge, or rupture of the subducting slab. The high-temperature/high-pressure metamorphism was followed by exhumation, as indicated by field structural relations and the evidence of retrograde metamorphism. The Eocene arc magmatism can best be explained by northward subduction of the Southern Neotethys, which persisted after the time of latest Cretaceous regional ophiolite emplacement until the collision of the Eurasian (Anatolian) and Arabian continents during the Early–Mid Miocene. Subsequent Plio-Quaternary left-lateral strike-slip strongly affected the area. Supplementary-material: Four supplementary tables giving the whole rock geochemistry of the granitoids, mineral geochemistry of the granulite facies rocks, LA-MC-ICP-MS zircon U–Pb data belonging to granitoids and Sm–Nd data belonging to granulite facies rocks and two documents giving the detailed analytical procedures and detailed petrography of the granitoids are available at www.geolsoc.org.uk/SUP18588

Cimmerian evolution of the Central Iranian basement: Evidence from metamorphic units of the Kashmar–Kerman Tectonic Zone

The Kashmar–Kerman Tectonic Zone (KKTZ) is an arcuate zone located between the Tabas and Yazd blocks in Central Iran. Detailed structural analysis and mapping of the metamorphic basement rocks of the KKTZ on the Boneh-Shurow, Tashk, Saghand and Sarkuh Metamorphic Complexes, as well as 40Ar/39Ar cooling ages on the Boneh-Shurow Metamorphic Complex indicate three deformation stages in the tectonometamorphic evolution of the KKTZ during the Cimmerian orogeny. The D1–1 event corresponds to continental accretion through the progressive formation of dextral shear zones, which is equivalent to an Early Cimmerian event during Late Triassic–Early Jurassic times. The D1–2 event is characterized by top-to-NE normal shear zones due to syn-collisional exhumation within the KKTZ and is considered to be a Mid-Cimmerian Middle Jurassic event at ca. 168Ma. The progressive regional compressional deformation is continued by the D2–1 and D2–2 events through formation of reverse shear zones and faults, with their SW movement. The progressive D2 event is considered to be Late Cimmerian and has taken place during Early Cretaceous. It is proposed that the mapped long-lived dextral shear zones within the KKTZ acted as block bounding faults between the Tabas and Yazd blocks in Central Iran.

Zircon U–Pb age and Lu–Hf isotope constraints on Precambrian evolution of continental crust in the Songshan area, the south-central North China Craton

A combined whole-rock geochemical and zircon U–Pb and Lu–Hf isotope study on two Precambrian lithotectonic units in the Songshan area, south-central North China Craton (NCC), was performed to constrain the tectonic evolution of the Trans-North China Orogen (TNCO) during collision between the Eastern and Western Blocks of the NCC. The metamorphic unit at Shipaihe consists of dioritic gneiss with minor amphibolitic enclave, whereas the magmatic unit at Shicheng is composed of granite. Zircon U–Pb dating demonstrate that the dioritic gneiss and enclave amphibolite were originally emplaced contemporaneously at ca. 2.5Ga. The magmatic zircons exhibit ɛHf(t) values of 2.2–7.8 and Hf model ages of 2.52–2.73Ga, suggesting that the dioritic gneiss was produced by reworking of the juvenile crust. The magmatic zircons of amphibolite have ɛHf(t) values of 0.7–8.1 and Hf model ages of 2.51–2.79Ga. The granite primarily gave zircon U–Pb ages of ca. 1.78Ga, with inherited zircons in ca. 1.87Ga. Zircon Lu–Hf isotopic analyses yield negative ɛHf(t) values of −16.7 to −1.8 and Hf model ages of 2.55–3.47Ga, indicating that the granite was mainly derived from reworking of ancient Archean crust. The Neoarchean dioritic gneiss and amphibolite show enrichment of LREE and LILE but depletion of HREE and HFSE, suggesting their derivation from anatexis of juvenile arc-type crust and enriched lithospheric mantle, respectively. The dioritic gneiss also has highly fractionated REE patterns and negligible negative Eu anomalies, implying anatexis at high pressures, where garnet and possibly amphibole as residual phases. The Paleoproterozoic granite has high K2O+Na2O and Zr, high ratios of total FeO/MgO and Ga/Al, and low contents of CaO, Al2O3, Ba, Sr and Eu/Eu* values, consistent with the characteristics of A-type granites. The negative Eu, Ba and Sr anomalies suggest that plagioclase acted as a residual phase during partial melting at relatively low pressures. Taken together, the Songshan metamorphic complex would originally derive from the reworking of juvenile arc-type crust and coeval partial melting of juvenile, enriched lithospheric mantle. Thus, an active continental margin was present in the late Neoarchean in south-central NCC, generating arc-type magmatism between the Eastern and Western Blocks. During the collisional orogeny, the arc-type crust in southern NCC underwent intensive metamorphism that followed by post-collisional extension at ca. 1.78Ga, and the reworking of the old continent in the southern TNCO yield Shicheng A-type granite.

U-Pb zircon geochronology of northern metamorphic massifs in the Biga Peninsula (NW Anatolia-Turkey): new data and a new approach to understand the tectonostratigraphy of the region

Metamorphic massifs of the Biga Peninsula can be divided into two approximately ENE–WSW-trending belts. It is believed that these two belts represent two different tectonic zones separated by a NE-trending Alpine ophiolitic suture. The Sakarya Zone lies to the S–SE of this Alpine suture, and consists of the Kazdağ metamorphic complex which is tectonically overlain by the Permo-Triassic Karakaya Complex. The metamorphic rocks, as an Alpine edifice located N–NW of the suture, have been assigned to the Rhodope and Serbo-Macedonian massifs of Bulgaria and Greece. The northern metamorphic belt is represented by the Karadağ Massif in the west, which has been evaluated as a different unit from the others, the Karabiga Massif in the east and the Çamlıca Massif between them. All three massifs are mapped in detail in light of previous studies and LA–ICP–MS U-Pb zircon dating was applied to stratigraphically compare them. Contrary to previous studies, our data indicate that the basement metamorphic associations in these three areas show similar characteristics concerning their stratigraphical and lithological aspects and also spatial distribution of their outcrops. U-Pb LA–ICP–MS dating of zircons from the three individual metamorphic massifs yielded the following data: maximum sedimentation ages of the protolith of mica schists in the range of 559 ± 17 to 582 ± 30 Ma; crystallisation age of the protolith of metavolcanic rocks of 577 ± 20 Ma; and crystallisation age of the protolith of eclogites at 565 ± 9 Ma. These ages clearly show that the metamorphic units of the northern massifs are comparable to each other. Also, the U-Pb zircon concordia diagrams from these three metamorphic massifs show remarkably similar patterns. In addition to the similar maximum sedimentation ages for all mica schists with crystallisation ages of the protoliths of the metabasic rocks, there are two major complex tectono-thermal overprints (episodic lead loss events), at c. 330–300 Ma (Variscan?) and c. 100–10 Ma (Alpine and late Alpine?), respectively. Field mapping and analytical data indicate that the basement rocks of the northern massifs in the Biga Peninsula have a correlative Late Ediacaran to Early Cambrian stratigraphic range. In part, Permian strata unconformably overlie basement rocks in the Karadağ Massif. Both tectono-thermal events are demonstrated by coeval episodic lead loss of many zircons in all samples from all areas. Finally, the results of this research do not support an Alpine suture between the two metamorphic belts of the Biga Peninsula. Therefore, the geological evolution of the region might require reevaluation.

Geological map of the ultra-high pressure Brossasco-Isasca unit (Western Alps, Italy)

In the southern Dora-Maira Massif, Western Alps, slivers of continental crust with similar lithologies, but recrystallized during the Alpine orogeny at different peak-P conditions, are exposed. They include the Brossasco-Isasca Unit (BIU) where coesite was first discovered in continental crust. A new 1:20,000-scale geologic map and related cross-sections of the whole BIU and adjoining units is presented, in which the most significant features useful to infer the pre-Alpine history and the Alpine tectonic and metamorphic evolution, are summarized. Thanks to detailed petrography and petrology, the geologic map shows the precise location of ultra-high pressure (UHP) minerals (such as coesite), and the locations of the most significant mineral assemblages (such as kyanite + jadeite). This innovative approach is used to distinguish the BIU from the adjacent units. Relict pre-Alpine structures (such as igneous intrusive contacts with basement xenoliths and metagranitoids) are summarized in a sketch illustrating the geologic setting of the UHP metamorphic unit as inferred before the Alpine orogeny.

Petrology of the İspendere (Malatya) ophiolite from the Southeast Anatolia: implications for the Late Mesozoic evolution of the southern Neotethyan Ocean

Abstract The İspendere ophiolite forms part of the Tauride active continental margin assemblage in SE Anatolia. The ophiolite exhibits an intact oceanic lithosphere section and is intruded by Late Cretaceous calc-alkaline granites. The ophiolite comprises mantle tectonites, ultramafic to mafic cumulates, isotropic gabbros, isolated diabase dykes, a sheeted dyke complex, plagiogranite and volcanic rocks. The volcanics and the sheeted dyke complex exhibit (1) similar rare earth element patterns, with flat to light rare earth element depletion (La–Yb) N =0.71–1.14 and 0.65–1.22, (2) negative Nb anomalies and (3) flat-lying high field strength element trends. These features differ from a typical Normal--Mid Ocean Ridge Basalt fractionation trend and could have resulted from c. 15% partial melting of a previously depleted mantle source. The whole-rock chemistry and the mineral chemistry of the ultramafic to mafic cumulates [high Ca plagioclases (An 89–81 ), magnesian olivines (Fo 88–81 ) and clinopyroxenes (Mg# 90–83 )] show that the primary magma of the plutonic suite is compositionally similar to modern island arc tholeiites. The available evidence suggests that the İspendere ophiolite formed at a northerly supra-subduction zone spreading centre of the Southern Neotethys, between the Taurides and the Bitlis–Pütürge metamorphic units, during the Late Cretaceous. Comparison with the adjacent Göksun, Kömürhan and Guleman ophiolites suggests that the İspendere ophiolite represents part of a single regional-scale sheet of oceanic lithosphere that was accreted to the base of Tauride active continental margin where it was cut by arc-type magmatic rocks.

Geochemical and structural constraints on the magmatic history of the Chandman Massif of the eastern Mongolian Altay Range, SW Mongolia

2� CzechGeologicalSurvey,�Leitnerova�22,�658�69�Brno,�CzechRepublic� 3� SchoolofGeologyandPetroleumEngineering,�MongolianUniversityofScienceandTechnology,�Box�520,�P.O.�46,�Ulaanbaatar,�� � Mongolia�210646 4� InstitutfurGeowissenschaften,�GoetheUniversitat,�Altenhoferallee�1;�60438�FrankfurtamMain;�Germany * �Correspondingauthor In the Mongolian Altay range, immediately south of the Bogd fault zone, the Chandman Massif intruded the Chandman Khayrkhan Crystalline Complex to the NE. This complex consists of migmatized biotite gneisses, orthogneisses and amphibolites. To the south, the Massif cuts the chlorite schists and quartzites of the Tugrug Formation. The Massif mainly consists of diorites to granites with rare gabbro bodies interspersed with metamorphic host rock screens, generally of amphibolite with calc-silicate lenses. Granitoids are further categorized based on petrography and whole-rock geochemical data into peraluminous and metaluminous groups. Granites comprise the peraluminous group, and possess high (71-77 wt. %) silica contents with abundant modal K-feldspar. Metaluminous granitoids span a wide range in silica contents, from diorite to granite. Also their feldspars are compositionally variable. Both these granitoid units are of volcanic-arc character and display a calc-alkaline geochemical trend. Granodiorites of the metaluminous group contain widespread planar rhythmic schlieren layering. Metaluminous units slightly predate the intrusion of peraluminous granites, but the two suites are occasionally co-magmatic. The Al-in-hornblende barometric analysis combined with plagioclase thermo- metry reveals a depth of emplacement of 11.5-13.7 km and magma temperatures of 725-775 oC. Chandman Khayrkhan metamorphic foliations strike NNW. Magmatic fabrics in the Chandman Massif roughly E-W and subparallel E-W solid-state foliations overprint magmatic foliations. All foliations are moderately to steeply dipping. Fabric analysis shows a discordance of structures to the north and south of a fault that cross-cuts the field area. This fault is interpreted to be related to motion on the Bogd Fault. Thus, this area experienced 1) amphibolite-facies metamorphism of the Chandman Khayrkhan Crystalline Complex, 2) exhumation to higher crustal levels, 3) juxtaposition against the greenschist-facies Tugrug Formation, 4) intrusion of the Chandman Massif under tectonic strains that continued through the solidification of plutons, and 5) late block-style rotation related to motion on recent faults. These age and geological constraints iden- tify the Chandman Massif as an intrusion of substantially younger age than the Caledonian association into which it was previously placed. It is thus far the only arc-type intrusion in the earliest Hercynian age range identified in the Gobi-Altay Terrane. Its metamorphic and magmatic history of migmatization followed by intrusion of metaluminous and peraluminous plutons are similar to those of rocks to the west, in the Tseel Terrane, and may represent its easternmost counterpart. The exact juxtaposition mechanism for metamorphic units of different grade and the formation of schlieren layering in the Chandman Massif granodiorites remain enigmatic.

Early Cretaceous closure of the Intra-Pontide Ocean in western Pontides (northwestern Turkey)

The Intra-Pontide suture is the boundary between the İstanbul and Sakarya terranes in northwest Turkey. We provide new isotopic and stratigraphical data from the Intra-Pontide suture zone, which indicate Early Cretaceous collision between the Sakarya and İstanbul terranes. These two terranes along with the Strandja Massif make up the Pontides. Metamorphic units of the Intra-Pontide suture zone are best exposed in the Armutlu Peninsula. In the eastern part of the Armutlu Peninsula, three metamorphic units crop out forming an eastward dipping thrust stack. At the base of the thrust stack there is a metaclastic-marble sequence, which is tectonically overlain by a Cretaceous subduction-accretion complex, farther up in the thrust stack there is a high-grade metamorphic unit, which represents the Proterozoic basement of the İstanbul Zone. New clastic zircon ages from the metaclastic-marble sequence indicate that deposition of the sandstones must be later than Permian (∼264Ma) possibly during Triassic. Similar Triassic metasediments are also reported in Strandja Massif. We interpret that these metasediments were deposited during Triassic along the rift flanks leading to the opening of the Intra-Pontide Ocean, which suggest a possible Early Triassic opening for the Intra-Pontide Ocean. Our new Rb–Sr mica and Sm–Nd garnet ages dates the regional metamorphism between Late Jurassic–Early Cretaceous (158–111Ma) along the Intra-Pontide suture zone, similar ages are also reported in the Strandja Massif. The metamorphic rocks of the Intra-Pontide suture are unconformably overlain by Campanian–Ypresian clastics. The collision between the İstanbul and Sakarya–Strandja terranes has occurred during Early Cretaceous and the Proterozoic basement of İstanbul terrane was reheated during this collision.

Overthrust shear folding and contrasting deformation styles in a multiple decollement setting, Gibraltar Arc external wedge

The deformation style distribution and related finite strain exhibited by the tectonic units of the Western Gibraltar Arc depict a multiple decollement setting, which was developed during Miocene westward Arc migration. In the external wedge, alternating ductile and brittle–ductile deformation bands occur, which are mainly controlled by differences in primary rheological properties and structural position. In the northern branch of the Gibraltar Arc, the metamorphic units of the hinterland (Alboran Domain) fully overthrust their peripheral non-metamorphic Dorsal units, producing NW-vergent folds. Similar structures (foliated NW-vergent chevron folds) developed on the Paleomargin units as a result of the overriding Flysch Trough unit emplacement. This scenario could be explained by overthrust shear produced by an overriding mass, which generated a progressively higher shear strain of the Paleomargin units towards the hinterland. This model is supported by (a) changes in axial surface attitude, which becomes steeper towards the foreland; (b) tighter folds towards the hinterland, and (c) stronger evidence of fold flattening towards more internal positions. In contrast, coeval thrust systems shortened and displaced the overriding Flysch Trough units under quite brittle conditions. This indicates that they were not sheared below the Alboran Domain, but essentially deformed by a push-from-behind mechanism, where the Alboran Domain acted as a relatively rigid backstop.

Neogene deformation and granite emplacement in the metamorphic units of northern Apennines (Italy): Insights from mylonitic marbles in the Porto Azzurro pluton contact aureole (Elba Island)

In the northern Tyrrhenian Sea, late Miocene intrusions (Monte Capanne pluton and Porto Azzurro pluton) were emplaced at upper crustal levels (

The Chortis Block--southwestern Mexico connections: U-Pb zircon geochronology constraints

The proposed connections between basement terranes of southwestern México and the Chortís Block of Central America are tested in this paper, which focuses on metamorphic units in northern Chortís that outcrop in southeastern Guatemala. Two chief units are recognized: the mid- to high-metamorphic grade Las Ovejas Complex made up of ortho-, para-gneisses, schists, amphibolites and marbles; and the tectonically juxtaposed, low-metamorphic grade San Diego Phyllite. U-Pb geochronology carried out by LA-ICPMS on zircons separated from both units reveal major differences between them. The high-grade metamorphism and deformation of Las Ovejas Complex is bracketed between ∼27 and ∼37 Ma, which correspond to the U-Pb zircon ages of the oldest cross-cutting dikes and youngest zircons in metasedimentary samples. Las Ovejas samples share xenocrystic and detrital zircon populations of Middle-Late Jurassic, Middle-Late Triassic and Permo-Carboniferous age. In contrast, the San Diego Phyllite contains zircons that overlap at ∼830 to 870 and ∼1150 to 1220 Ma, with the youngest concordant zircons yielding Cambrian age. The contrast in population ages supports the idea that Las Ovejas Complex and San Diego Phyllite are different lithotectonic units tectonically juxtaposed during or after their exhumation. When compared with available U-Pb zircon data for southern México, Las Ovejas Complex show similarities to samples of the Guerrero terrane, mainly the Arteaga Complex and Zihuatanejo subterrane, as well as to the southern and northern portions of the Cuicateco terrane. The San Diego Phyllite shows instead similarities with the Teloloapan subterrane of the Guerrero terrane, the Mexcala Formation, the low-grade units of the Cuicateco terrane and the Grenvillian ages of the Oaxacan Complex metaigneous samples, as well as some of the samples belonging to the Maya Block. We propose a tectonic model in which the Las Ovejas Complex was a portion of a fringing arc located in front of the Cuicateco terrane of southwestern México, removed during the Early Tertiary by docking of the Chortís Block represented by the San Diego Phyllite, and tentatively by similar units recognized elsewhere in the Chortís Block in Honduras and Nicaragua. Eocene-Oligocene metamorphism and deformation of the Las Ovejas Complex would be a result of its displacement off southern México, mainly characterized by sinistral stretching and tectonic Cenozoic transport.