Ma Time Span Research Articles

The onset of Neo-Tethys subduction in the Early Jurassic: evidence from the eclogites of the North Shahrekord Metamorphic Complex (Sanandaj-Sirjan Zone, W Iran)

AbstractGeodynamic models implying subduction of continental crust either consider this process happening during collision, when the continental margin of the lower plate attempts subduction, or in pre-collisional stages, when tectonic erosion of the upper plate or subduction of continental extensional allochthons drag continental crust in the subduction channel. In the Zagros orogen (W Iran), high-pressure rocks are known only from the Sanandaj-Sirjan Zone, NE of the Main Zagros Thrust. Here, eclogites of the North Shahrekord Metamorphic Complex suggest subduction of continental crust slices derived from the upper plate (Central Iran) during the onset of the Neo-Tethys subduction along the southern margin of Iran. Eclogites record a clockwise pressure-temperature-time path, with pre-eclogitic epidote-amphibolites-facies phase assemblages preserved in garnet cores, a high-pressure stage, and a subsequent retrogression at amphibolite-facies conditions. By means of forward thermodynamic modelling and 40Ar/39Ar geochronology, the peak metamorphism has been constrained at 1.9-2.1 GPa and 550-600 °C, in the 191-194 Ma time span. The following retrogression during exhumation lasted at least until 144 Ma. Our data suggest that the onset of the Neo-Tethys subduction traces back prior to 190 Ma, involving together with the Neo-Tethys oceanic lithosphere also slices of the upper plate continental crust scraped off by means of tectonic erosion processes.

Intrusion‐related Gold Deposits in Egypt

AbstractIntrusion‐related gold deposits (IRGDs) occur in the Eastern Desert (ED) of Egypt within magmatic districts that are exploited for tungsten and tin mineralization. IRGDs and intrusion‐related rare metal deposits (IRRMDs) are almost invariably linked with the late to post collisional Younger Granites (YGs) that have three successive phases (I, II and III). At ∼635–630 Ma, the ED underwent a transition in deformation style from compressional to extensional and a switch from subduction with crustal thickening to delamination with crustal thinning. This transition was concurrent with the emplacement of a short magmatic pulse (∼635–630 Ma) that represents a transition between orogenic gold deposits and IRGDs. K‐rich calc alkaline granites (phase I and II of the YGs) hosting IRGDs like the Hangalia deposit were emplaced during the time span 630–610 Ma. Alkaline magmatism began at 610 Ma, coexisting with the K‐rich calc‐alkaline magmatism over the 610–590 Ma time span, where the Fawakhir (598 ± 3 Ma) and Um Had (596 ± 2 Ma) granites that host the IRGDs were emplaced. In time, the alkaline magmatism became more alkaline giving rise to phase III of the YGs that hosts IRRMDs. A distinct metallogenic epoch comprising both IRGDs and IRRMDs, was undergoing extreme growth at ∼600 Ma.

Paleolatitude estimation and premises for geomagnetic field instability from the Proterozoic drilling core material of the south-western part of the East European Craton

This study addresses new paleomagnetic, geochemical, mineralogical, and rock-magnetic data from four drill cores of the western part of the East European Craton, representing mainly siliciclastics supplemented by volcaniclastics and a few basaltic rock samples. The cores comprise Mesoproterozoic samples of 1400–1030 Ma and Neoproterozoic (Ediacaran) samples of 588–541 Ma. According to the magnetic data, the dominant magnetic carrier in basalts is magnetite, while two genetic types of hematite (detrital and syngenetic/early diagenetic) carry magnetization in sedimentary rocks. The characteristic components of the natural remanent magnetization (NRM), carried by the fine-grained hematite (intermediate temperature component [CM]) and the coarse-grained hematite (high-temperature component [CH]), were identified during thermal cleaning. Mineralogical and geochemical studies provided evidence for detrital and synsedimentary/early post-sedimentary origin of fine-grained hematite and suggested hot and humid environmental conditions on the western part of Baltica during Meso- and Neoproterozoic time. Because of a low-temperature diagenetic history, which affected the Mesoproterozoic and Ediacaran rocks of Pinsk 26, Ochrymy IG-1, and Šaškai 2 boreholes, no post-Ediacaran paleomagnetic overprint is expected there. On the basis of these findings, the CM component in these profiles was classified as nearly primary (synsedimentary/early post-sedimentary) in origin. The CH component in these profiles and the Ediacaran sandstones of the Kaplonosy IG-1 section is also considered primary and acquired as detrital NRM (DRM). Paleomagnetic results yielded a reliable record with the domination of shallow-to-moderate inclinations, showing both polarities. The calculated paleolatitudes for 550–541 Ma time span, regardless of correction for the inclination error, fall within the subequatorial zone. Steeper inclinations, suggesting subpolar paleolatitudes, were observed in the volcaniclastics and basalts interlayering with sedimentary rocks and, sporadically, in the Ediacaran sedimentary successions. The origin of this steep component, however, was attributed to the anomalous behavior of the geomagnetic field, as a consistently warm environmental facies pattern did not support a fast and far-distance, cross-latitudinal, poleward movement of Baltica during the Ediacaran time and then back to the equator. These results complement previous studies that sought to resolve uncertainty of the paleogeographic position of Baltica in the Proterozoic time.

Early stages of the Miocene magmatic arc and related hydrothermal alteration at Valle Hermoso, South Central Andes (35°07′S, 70°17′W)

For the first time, the eastern flank at Cordón Santa Elena, within the Valle Hermoso district, has been mapped and described in detail. Based on key field observations, petrographic, geochemical and geochronological results, a new Miocene volcanic stratigraphy is proposed for this particular area. The Cordón del Burrero Group (Arroyo del Perdido, Puesto Gutiérrez, Bolchiquito and Santa Elena Formations) has been defined for the ~18-13 Ma timespan, whereas the Huincan Formation has been constrained as younger than ~11 Ma. According to La/Yb ratios, Cordón del Burrero volcanics have been emplaced in a relatively thin crust compared to the Huincan dacites, suggesting that the main phase of crust thickening and tectonic deformation took place at ~12-11 Ma. The studies carried out at Cordón Santa Elena revealed a spatial and genetic relationship between the ~13 Ma dioritic subvolcanic pluton and a widespread calcic-sodic hydrothermal alteration. The areal distribution of the hydrothermal alteration shows a core of calcic-sodic alteration accompanied by propylitic alteration, cut by quartz-pyrite veins spatially associated with phyllic alteration and restricted potassic alteration. This hydrothermal system may be considered as the oldest pulse at this latitude.

Evolution of the middle crust of the Pyrenees during the Paleozoic: new data on the plutonic rocks from the North Pyrenean Agly Massif

The Variscan middle crust of the eastern Pyrenees is represented by the Belesta and Caramany gneissic units of the North Pyrenean Agly Massif. In this study, we date and reinterpret the orthogneisses and granites composing these units. We characterize a recurrent plutonic activity spanning all the Paleozoic in this part of the Pyrenean crust. The Belesta unit (amphibolite facies) is mainly composed of sills of (Latest Ediacaran)–Earliest Cambrian (542 ± 4 and 540 ± 4 Ma) orthogneisses (70% of the series), almost unknown in the Pyrenees until now, and Latest Carboniferous–(Earliest Permian) (307 ± 3–298 ± 3 Ma time span) granites (15% of the series). These plutonic rocks were emplaced in probably Ediacaran aluminous pelites later transformed into paragneisses (15% of the series). The Caramany unit (granulite facies) is mainly composed of Precambrian aluminous paragneisses migmatized during the Latest Carboniferous–(Earliest Permian) (299 ± 4 Ma), sills of Earliest Cambrian (529 ± 5 Ma) orthogneisses and sills (about 30% of the series) of Late Carboniferous (308 ± 3–299 ± 3 Ma) opxbearing and opx-free granodiorites, grt-bearing leucogranites and norites. Among these last rocks, the Ansignan charnockite, forming a 800 m-thick laccolith near the base of the Caramany unit, and previously dated at 314 ± 6/7 Ma (Respaut, Lancelot, Neues Jahrbuch Miner Abh 147-1:21–34, 1983), is refined at 307 ± 3 Ma. Our results confirm the importance of the Late Carboniferous plutonism in deep levels of the Pyrenean crust, which largely resulted from recycling of older para- and orthoderived rocks of this crust. The emplacement of hundreds of metres in thickness of granites and other plutonic rocks in the Variscan Pyrenees occurred in a rather short time both as sills in these deep levels and as large plutons in shallower levels, a fact which necessarily had consequences on the HT–LP Carboniferous metamorphism.

Petrogenesis of the Early-Triassic quartz-monzodiorite dykes from Central Jebilet (Moroccan Meseta): Trace element and Nd-Sr isotope constraints on magma sources, and inferences on their geodynamic context

A dyke swarm composed of hypabyssal rocks of intermediate composition cross-cuts Variscan granites and Palaeozoic metasedimentary country-rocks of the Jebilet range. Based on K-Ar isotope dating on kaersutite phenocrysts, these dykes are Early Triassic in age (ca. 240 ± 10 Ma).Due to more or less pervasive alteration, the primary mineralogical assemblages are often difficult to unravel. Plagioclase and amphibole are ubiquitous, but the extent of biotite, for instance, is subject to discussion. Two distinctive types of rocks are clearly present according to their mineralogical composition: most samples are characterized by a light-brown to greenish hornblende, while some samples are characterized by the presence of kaersutite, suggestive of an alkaline affinity.Incompatible trace element data, restricted to those elements which are considered to be relatively immobile during alteration processes, allow two groups of samples to be distinguished. The largest one displays distinctive negative Ti and Nb anomalies similar to those typical for magmas extracted from mantle sources, which interacted with hydrous fluids and silicate melts, and were thus enriched in Th and LREE relative to the HFSE. However, Nd and Sr data preclude late stage crustal contamination as a suitable explanation of these features in all but the most evolved samples. The LREE enrichment combined with relatively radiogenic Nd isotope signatures are interpreted to reflect the characteristics of source materials, which evolved in a rather primitive, ensimatic setting.The second group comprises only three samples but is of major interest as it does not show any clear Nb negative anomaly and its Th/Nb systematics suggests that crustal components were not involved in their genesis. This group is interpreted as having been produced through partial melting of mantle sources which did not suffer hydrous metasomatism nor recycling of old crustal components, but were enriched in a within-plate setting by low-degree mantle melts.Based on these data, the following tentative geological scenario is proposed. Lithospheric stretching and decompression would have triggered partial melting at relatively deep level of the lithosphere, and produced magmas with within-plate, alkaline-affinities. The ascent of these hot mantle melts would have triggered partial melting of fertile, hydrated domains of Late Proterozoic age present in the overlying mantle and/or lower crust, thereby generating the volumetrically predominant group of samples with « calc-alkaline » geochemical features, namely Nb and Ti negative anomalies. The two types of magmas likely interacted to produce hybrid liquids with variably deep Nb anomalies but broadly constant εNd values, as a result of the weak difference of their isotope signatures.These magmas are much younger (ca. 240 ± 10 Ma) than the last Variscan granites dated at ca. 300 Ma. On the other hand, they largely predate the tholeiitic continental flood basalts of the Central Atlantic Magmatic Province (CAMP) emplaced at ca. 200 Ma, and were thus emplaced amidst the 100 Ma time span between the end of the Variscan orogeny and the lithospheric break-up that led to the formation of the Atlantic Ocean. Therefore, the Jebilet dykes neither represent the ultimate manifestation of the Variscan orogeny nor a premise of the Atlantic opening. Instead, they document a particular event within a protracted intra-continental rifting history spanning the whole Permo-Triassic period. In general terms, this distensive event might be viewed as a local consequence of the dextral mega-shear which separated Europe from North Africa in Late Palaeozoic times. In this scope, the studied dykes could be described as a set of tension gashes, resulting from a local change in stress regime. Their formation was accompanied by a small pulse of intraplate magmatism, possibly because favorable fertile materials occurred locally at depth. At a larger scale, the whole dyke swarm cross-cutting the Variscan basement in Morocco is indicative of an aborted rifting event, largely predating the successful break-up of Pangea and the early opening of the Central Atlantic Ocean.

Intrusion-Related Gold Deposits: New insights from gravity and hydrothermal integrated 3D modeling applied to the Tighza gold mineralization (Central Morocco)

The Tighza (or Jebel Aouam) district is one of the most important polymetallic districts in Morocco. It belongs to the Variscan Belt of Central Meseta, and includes W-Au, Pb-Zn-Ag, and Sb-Ba mineralization types that are spatially related to late-Carboniferous granitic stocks. One of the proposed hypotheses suggests that these granitic stocks are connected to a large intrusive body lying beneath them and that W-Au mineralization is directly related to this magmatism during a 287-285 Ma time span. A more recent model argues for a disconnection between the older barren outcropping magmatic stocks and a younger hidden magmatic complex responsible for the W-Au mineralization. Independently of the magmatic scenario, the W-Au mineralization is consensually recognized as of intrusion-related gold deposit (IRGD) type, W-rich. In addition to discrepancies between magmatic sceneries, the IRGD model does not account for published older age corresponding to a high-temperature hydrothermal event at ca. 291 Ma. Our study is based on gravity data inversion and hydro-thermal modeling, and aims to test this model of IRGD and its related magmatic geometries, with respect to subsurface geometries, favorable physical conditions for deposition and time record of hydrothermal processes. Combined inversion of geology and gravity data suggests that an intrusive body is rooted mainly at the Tighza fault in the north and that it spreads horizontally toward the south during a trans-tensional event (D2). Based on the numerical results, two types of mineralization can be distinguished: 1) the “Pre-Main” type appears during the emplacement of the magmatic body, and 2) the “Main” type appears during magma crystallization and the cooling phase. The time-lag between the two mineralization types depends on the cooling rate of magma. Although our numerical model of thermally-driven fluid flow around the Tighza pluton is simplified, as it does not take into account the chemical and deformation contributions, it provides evidence for abandoning the time-lag as an argument for invalidating the existence of a genetic link between older mineralizations and/or hydrothermal events spatially related to younger parent plutons.

Comments on: Periodicity in the extinction rate and possible astronomical causes – comment on mass extinctions over the last 500 myr: an astronomical cause? (Erlykin et al.)

Comments on: Periodicity in the extinction rate and possible astronomical causes – comment on mass extinctions over the last 500 myr: an astronomical cause? (Erlykin <i>et al</i>.)

The activation and cessation of Late Cenozoic extension in the lithosphere at the margin of the Baikal Rift Zone: Alternating sources of volcanism in the Vitim Upland

A comparative analysis of the concentrations of major oxides, trace elements, and the 143Nd/144Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16–14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14–9 Ma time span was caused by “passive” rifting in the west of the upland and by “active” rifting in the east. The “passive” rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the “active” rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1–0.6 Ma time span caused magma effusion with values of 143Nd/144Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material.

The Paleoproterozoic Fedorov–Pana Layered PGE complex of the northeastern Baltic Shield, Arctic Region: New U–Pb (baddeleyite) and Sm–Nd (sulfide) data

For the first time zircons have been extracted from gabbro–norite of a lower layered horizon of the West Pana Massif in the Pt–Pd Kievei deposit of the Fedorov–Pana Layered Complex. Those zircons have been used for U–Pb dating along with Sm–Nd age determination on sulfide minerals. The obtained new isotopic data are a U–Pb zircon age of 2500 ± 4 Ma, while the Sm–Nd (mineral and whole-rock) isochron yielded 2483 ± 86 Ma. These results correspond to the first phase of the Pt–Pd reef complex formation in the Layered Complex. The Pt–Pd reef formation has been dated by U–Pb baddeleyite and zircon analyses in the East Pana Massif to 2464 ± 12 Ma. The 2485–2464 Ma time span corresponds to the second phase of the Pt–Pd reef formation in the Fedorov–Pana ore cluster.

Large mammal biochronology framework in Europe at Jaramillo: The Epivillafranchian as a formal biochron

European large mammal assemblages in the 1.2–0.9 Ma timespan included Villafranchian taxa together with newcomers, mostly from Asia, persisting in the Middle Pleistocene. A number of biochronological schemes have been suggested to define these “transitional” faunas. The term Epivillafranchian, originally proposed by Bourdier in 1961 and reconsidered as a biochron by Kahlke in the 1990s, is at present widely introduced in the literature. This contribution, after selecting the most representative European large mammal assemblages within this chronological interval, provides a new definition proposal for the Epivillafranchian as a biochron included within the Praemegaceros verticornis FO/Bison menneri FO, and Crocuta crocuta FO.

Geologic History and Timing of Mineralization at the Haile Gold Mine, South Carolina

New constraints on the geologic history and timing of mineralization at the Haile gold deposit in South Carolina are provided by radiometric dating of igneous and metamorphic samples using Re-Os, U-Pb, and 40Ar/39Ar methods. Seven Re-Os measurements were made on impure mineral separates of fine-grained molybdenite containing variable quantities of pyrite, pyrrhotite, muscovite, and illite. Four measurements produced an isochron at 548.7 ± 2.0 Ma (MSWD = 1.04). An additional age of 541 ± 2 Ma suggests that molybdenite may have been produced over a ~8 Ma timespan (541–549 Ma), but the preferred interpretation is that molybdenite formed primarily at ~549 Ma. Broadly correlative Mo and Au abundances in drill samples, and the common presence of molybdenite in Au-bearing zones, indicate that Au mineralization at Haile probably also occurred at ~549 Ma. Contrasting Re-Os ages produced at 564 ± 2 and 529 ± 2 Ma are interpreted to be inaccurate, due to the effects of impure mineral separates, and possibly resulting from loss of radiogenic Os from fine-grained molybdenite to other sulfide minerals. Some mobilization and loss of Re, possibly under oxidizing conditions during late Paleozoic metamorphism, may explain two relatively old Re-Os ages determined here and by Stein et al. (1997). Laser ablation dating of zircons from a metavolcanic sample at Haile yielded a weighted mean age of 560 ± 8 Ma. This age is similar to the published time window of 551 to 555 Ma for magmatism at Haile, based on U-Pb zircon dating by ion probe (Ayuso et al., 2005). Similar timing of magmatism and Au mineralization indicates that the hydrothermal systems at Haile were likely driven by magmatism and were therefore not the product of collision, orogeny, and metamorphism. These results confirm that magmatism and mineralization at Haile were active at a time when the Carolina terrane was still in a peri-Gondwana location, well prior to its accretion to eastern Laurasia. Ages of some detrital zircons imply an anomalously young depositional age for the metasedimentary unit that hosts the Haile orebodies (<540 Ma). These ages conflict with an older age of 560 ± 8 Ma determined for the overlying metavolcanic unit. This conflict is interpreted to indicate that folding has overturned the geologic units at Haile. A possible Devonian-age metamorphic event at ~390 Ma is documented by 40Ar-39Ar dating of porphyroblastic biotite collected from mineralized rock at Haile. Zircon ages from the Dutchman Creek Gabbro (weighted mean of 314 ± 2 Ma) and 40Ar-39Ar ages on biotite separates from dikes that crosscut the Haile property (311 ± 3, 311 ± 2 Ma) document a pulse of mafic and alkaline magmatism in the late Carboniferous. These late-stage metamorphic and magmatic events appear to be unrelated to mineralization at Haile.

Quaternary river incision in NE Ardennes (Belgium)–Insights from 10Be/ 26Al dating of river terraces

Although it constitutes a main tool to unravel the regional recent tectonics, the chronology of the Pleistocene river incision is still poorly constrained within the uplifted Ardennes massif (Western Europe). Here, we use in situ produced cosmogenic 10Be and 26Al concentrations from depth profiles in terrace sediments of several Ardennian rivers (Meuse, Ourthe and Amblève) in order to date the so-called Younger Main Terrace (YMT), a key-level in the drainage network evolution. We present the first absolute dating of the YMT in the lower Meuse valley, where we obtained an age of 725 ± 120 ka for a terrace deposit buried beneath 3 m of Weichselian loess at Romont. This age is consistent with some previously published estimates based on paleomagnetic data and MIS correlations. However, the ages we obtain for the same terrace level in Ardennian tributaries of the Meuse are significantly younger: 390 ± 35 ka in the lower Ourthe, and only 220 ± 31 ka still farther upstream, in the lower Amblève. We thus demonstrate that the post-YMT incision occurred diachronically in NE Ardennes. The ∼0.5 Ma timespan needed by the erosion wave to propagate from the lower Meuse towards the Ardennian headwaters contradicts the long-held statement of a climatically driven incision that would have been synchronous throughout the catchments. Finally, we interpret the strong 10Be enrichment displayed by the lower half of the Belle–Roche (lower Amblève) profile as betraying the long-lasting, slow accumulation of the ∼8 m thick terrace deposit in that place.

Western Palaearctic palaeoenvironmental conditions during the Early and early Middle Pleistocene inferred from large mammal communities, and implications for hominin dispersal in Europe

Large-scale fluctuations in global climate and resulting changes in ecology had a profound effect on human evolution and dispersal. Though hominin remains are scarce, studies focussing on the more abundant records of fossil land mammal communities can contribute greatly to our knowledge of the palaeoenvironmental circumstances that influenced and directed the global spread of hominins. To produce a comprehensive and accurate account of the evolution of western Palaearctic habitat diversity between 2.6 and 0.4 Ma BP, information generated from large mammal communities from 221 key sites has been included in this study. The palaeoecological conditions of the western Palaearctic during the Early and early Middle Pleistocene were principally controlled by the following key factors: (1) a widespread trend of temperature decrease, (2) the periodicity of the global temperature record, (3) the intensity of single climatic stages, (4) the temporal pattern of climatic variation, (5) geographical position, and (6) the distribution of continental water resources. A general picture of the evolution of western Palaearctic habitat diversity saw the replacement of extensive forested terrain by an alternating sequence of varied savannah-like and forested habitats during the 2.6–1.8 Ma span, as well as an alternation between different types of predominantly open habitats between 1.8 and 1.2 Ma. Both of these processes were governed by 41 ka temperature periodicity. During the 1.2–0.9 Ma time span, irregular climatic fluctuations were more common and habitat variability increased. The subsequent 0.9–0.4 Ma interval, a period controlled by 100 ka periodicity, was by comparison more stable, with longer climatic cycles alternating between open and forested landscapes. During the entire Early and early Middle Pleistocene, assemblages of large mammal communities reveal a distinct trend of decreasing continentality between Eastern and South-Eastern Europe on the one hand, and South-Western and North-Western Europe on the other. This trend was due to the effect of the Atlantic Ocean, while in Southern Europe the relatively low continentality was balanced by influences from the Mediterranean Sea. When plotted against evidence of hominin occurrence, the data on western Palaearctic habitat diversity inferred from large mammal communities indicate clear environmental stimuli for the earliest human dispersal in Europe. These are: (1) a wide range of habitats, implying a high diversity of resources; (2) mild climates with low seasonality, implying a lack of strong environmental fluctuations. Around 1.8 Ma at the latest, hominins of African origin entered the western Palaearctic for the first time, taking advantage of the diversity of habitats and resources, particularly along large river systems. Their subsequent westward spread between 1.7 and 1.3 Ma was restricted to Mediterranean-influenced areas, which offered a high variability of habitats and relatively low seasonality. The increase in environmental diversity, which occurred from 1.2 Ma onwards, opened up South-Eastern and Eastern Europe for hominin occupation. According to the available records, North-Western and Central Europe were initially colonized during late Early to early Middle Pleistocene interglacials, when these regions experienced periods of low seasonality and considerable habitat diversity.

Geodynamic evolution of the Aegean: constraints from the Plio-Pleistocene volcanism of the Volos–Evia area

Abstract: The Plio-Pleistocene lava flows and domes of the Volos–Evia area were erupted between 3.4 and 0.5 Ma ago on the western continuation of the North Anatolian Fault, in a back-arc position with respect to the active arc. They are mainly high-K calc-alkaline trachyandesites. Based on their Sr–Nd–Pb isotopic compositions, the mantle source of the Volos–Evia area lavas is similar to that of a large volcanic belt that developed north of the Pelagonian–Attic–Cycladic–Menderes massifs, encompassing a 35 Ma timespan and widespread over a large area from NW Greece–Macedonia to the Aegean–western Anatolia. In contrast, southern Aegean arc rocks have a similar subduction fingerprint but distinctly lower Sr and higher Nd isotopic compositions. The geochemical and isotopic differences between southern and northern Aegean rocks may be ascribed to the different nature of the mantle wedge: depleted asthenosphere under the the southern Aegean, and lithosphere northward. The lack of an asthenospheric mantle wedge below the northern Aegean fits with the hypothesis of an almost horizontal subduction of the African slab. In the mantle reference frame the African slab is moving out of the mantle, and a slab-driven suction flow of the underlying mantle may be responsible for the recent development of a thin asthenospheric layer in the southern Aegean mantle wedge.

Thermal metamorphic history of a Ca, Al-rich inclusion constrained by high spatial resolution Mg isotopic measurements with NanoSIMS 50L

Abstract– Mg isotope data were collected by NanoSIMS with high-precision and high-spatial resolution from a coarse-grained type B Ca-, Al-rich inclusion (CAI), EK1-6-3, in the Allende CV3 chondrite to evaluate the time scale of parent body thermal metamorphism. The CAI melilite and fassaite contain excesses of 26Mg (26Mg*) from the in-situ decay of 26Al; the inferred initial ratio, (26Al/27Al)0 = (5.8 ± 2.4) × 10−5, is consistent with many previously reported coarse-grained CAIs from CV chondrites (e.g., MacPherson et al. 1995). However, the anorthite has heterogeneous (26Al/27Al)0, ranging from 1.8 × 10−5 to 3.3 × 10−6. The 26Al-26Mg systematics within the anorthite is consistent with thermal diffusion of Mg isotopes during metamorphism. We also show that the heterogeneous distribution of 26Mg* in anorthite could have resulted from thermal diffusion of 26Mg* over a 0.6–0.8 Ma time span. Mg diffusion thus may be responsible for the (26Al/27Al)0 heterogeneity within anorthite in CAIs.

Magneto-seismic interpretation of subsurface volcanism in the Gaeta Gulf (Italy, Tyrrhenian Sea)

The occurrence of a former subaerial volcanic edifice off the Volturno River (Tyrrhenian Sea, Gulf of Gaeta) in the 41°N parallel is suggested by joint interpretation of multichannel seismic lines and ship-borne magnetic data. In the Campanian region igneous (volcanic) rocks are very close to the carbonate Mesozoic basement and seismics cannot always discriminate between them. A joint seismic-magnetic analysis was very effective in assessing the lithological nature of the bodies evidenced by both geophysical methods. Distortion analysis showed that the main magnetic source in the area is characterised by a not normal-polarity direction of the magnetization, similar to oth- er Pleistocene volcanoes in the Tyrrhenian region. Hence we argued that the overall magmatic emplacement for this source occurred during a reverse-polarity chron, very likely the 0.78-1.78 Ma time span. This magnetically-derived time constraint is in agreement with seismic stratigraphy that shows that the entire volcano is sealed by the Volturno River prograding delta from Middle Pleistocene to Present in age. Our interpreted volcano belongs to a set of in- ferred onshore and offshore volcanic edifices all lying along the 41°N parallel.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

Structural Evolution of the Eastern Qiulitagh Fold and Thrust Belt, Northern Tarim Basin, China

Abstract:The eastern Qiulitagh fold and thrust belt (EQFTB) is part of the active Kuqa fold and thrust belts of the northern Tarim Basin. Seismic reflection profiles have been integrated with surface geologic and drill data to examine the deformation and structure style of the EQFTB, particularly the deformational history of the Dina 2 gas field. Seismic interpretations suggest that Dongqiu 8 is overall a duplex structure developed beneath a passive roof thrust, which generated from a tipline in the Miocene Jidike Formation, and the sole thrust was initiated from the same Jidike Formation evaporite zone that extends westward beneath the Kuqatawu anticline. Dongqiu 5 is a pop‐up structure at the western part of the EQFTB, also developed beneath the Jidike Formation evaporite. Very gentle basement dip and steep dipping topographic slope in the EQFTB suggest that the Jidike Formation salt provides effective decoupling. The strong deformation in the EQFTB appears to have developed further south, in an area where evaporite may be lacking. Since the Pliocene, the EQFTB has moved farther south over the evaporite and reached the Yaken area. Restoring a balanced cross‐section suggests that the minimum shortening across the EQFTB is more than 7800 m. Assuming that this shortening occurred during the 5.3 Ma timespan, the shortening rate is approximately 1.47 mm/year.

Geochemical and K–Ar age constraints on the Late Neoproterozoic (?) gneisses at Um Tenassib area, north Eastern Desert, Egypt

Wadi Um Tenassib metamorphic rocks consist mainly of biotite gneiss and biotite-hornblende gneiss with subordinate intercalations of amphibolite, migmatitic gneiss, and aplitic granite. Biotite-hornblende gneiss, biotite gneiss, and aplitic granite are geochemically characterized and their cooling ages are determined by using the K–Ar method on biotite. The Um Tenassib gneisses (UTG) range in composition from quartz diorite/monzodiorite to granodiorite. They were derived from igneous rocks that pertain to calc alkaline and metaluminous to weakly peraluminous affinities and were generated in continental volcanic arc setting. REE patterns of the UTG are moderately fractionated (La N/Lu N = 5.9–7.5) relative to those of the aplitic granite (La N/Lu N = 33). The similarity in the geochemical characteristics and REE patterns of both gneiss types indicate their magmatic consanguinity. Amphiboles of the UTG biotite-hornblende gneisses are mainly hornblende, together with few paragasitic hornblende and edenite. Plagioclase composition is oligoclase to andesine (An 21–46) in the biotite-hornblende gneiss, and oligoclase (An 11–26) in the biotite gneiss. Mineral chemistry of amphibole and plagioclase indicate that the gneisses were metamorphosed under low- to medium-pressure of 2.6–6.4 kbar and at medium to high temperatures of 660–755 °C. The K–Ar biotite cooling ages (seven samples) range from 585 ± 12 Ma to 598 ± 12 Ma for the UTG, except one biotite-hornblende gneiss sample gives age of 577 ± 11 Ma. These ages suggest a latest metamorphic cooling event at ca. 585–600 Ma time span, which is consistent with the proposed cooling ages of ∼600 Ma for the Elat metamorphic rocks [Cosca, M.A., Shimron, A., Caby, R., 1999. Late Precambrian metamorphism and cooling in the Arabian–Nubian Shield: petrology and 40Ar/ 39Ar geochronology of metamorphic rocks of the Elat area (southern Israel). Precamb. Res. 98, 107–127]. It may indicate that the metamorphism of the UTG might have been contemporaneous with the suggested regional metamorphism at 620 ± 10 Ma for Sinai metamorphic rocks (Cosca et al., 1999) and/or the emplacement age at 614 Ma for the granodiorite in the study area [Stern, R.J., Hedge, C.E., 1985. Geochronological and isotopic constraints on the Late Precambrian crustal evolution in the Eastern Desert of Egypt. Am. J. Sci. 285, 97–127]. These ages also lie within the range of magmatic activity of the Younger Granites in the North Eastern Desert (575–600 Ma).