Augen Gneiss Research Articles

Cambrian magmatism in the Tethys Himalaya and implications for the evolution of the Proto‐Tethys along the northern Gondwana margin: A case study and overview

The Cuonadong Dome in southern Tibet is a newly discovered gneiss dome in the Tethys Himalaya. Here, we investigate the Late Cambrian augen gneiss (orthogneiss) within the core of this dome to address the controversy surrounding early Palaeozoic tectonic evolution of the northern margin of eastern Gondwana. We report new zircon laser ablation multicollector inductively coupled plasma mass spectrometry (LA‐(MC‐)ICP‐MS) U‐Pb ages, Lu‐Hf isotopes, whole‐rock major and trace element geochemistry, and Sr‐Nd‐Pb data on representative samples from the granitic gneiss. The weighted mean of 116 analyses of zircon grains yields an age of 498.2 ± 1.5 Ma (mean square weighted deviation [MSWD] = 1.2). Forty‐one spots analyses on these grains show consistent εHf (t) values of −2 to +4 (average = +1.1), corresponding to Hf crustal model age (TDM2) of 1.3 to 1.6 Ga (average = 1.39 Ga). The orthogneiss (metagranite) is characterized by high Si and K contents, with low Al, Mg, and Ti, and A/CNK values ranging from 0.87 to 0.98 with an average of 0.92, indicating a metaluminous composition and I‐type granitoid affinity. The granitoid displays significant enrichment in light rare earth elements (LREEs) and relatively flat high rare earth element (HREE) patterns, with strong negative Eu anomalies (Eu/Eu* = 0.26–0.31). The primitive mantle‐normalized trace elements show a relative enrichment in large‐ion lithophile elements, such as Rb, and high‐field strength elements, such as Th, U, Zr, and Hf, with depletion in Ba, Nb, Ta, Sr, P, and Ti. The rocks show high initial 87Sr/86Sr ratios (0.7221–0.7248) and low εNd (t) values (−8.9 to −7.3) with Nd model ages (TDM) of 1.79–1.91 Ga. Their radiogenic Pb isotopic compositions show (206Pb/204Pb)t = 18.804–19.110, (207Pb/204Pb)t = 15.730–15.768, and (208Pb/204Pb)t = 38.409–39.054, indicating an ancient upper middle continental crustal origin. Our study shows that the protolith of the metagranite was most likely derived from the partial melting of upper continental crust with a minor contribution of depleted mantle materials. Through integration of the regional information on early Palaeozoic magmatism and metamorphic events, we contend that the protolith of the Cuonadong granitic gneiss formed in an accretionary setting associated with the early Palaeozoic Proto‐Tethys Oceanic plate subduction beneath the Gondwana continent.

U-Pb zircon and U-Th-total Pb monazite ages from the Phulbani domain of the Eastern Ghats Belt, India: Time constraints on high-grade metamorphism and magmatism in the lower crust

We present zircon U-Pb (SHRIMP) and monazite U-Th-total Pb (EPMA) ages from a suite of high-grade rocks of the Phulbani domain of the Eastern Ghats Belt, India. These ages are integrated with the metamorphic and magmatic histories of the Phulbani domain to put precise time constraints on the tectonothermal evolution of the northern part of the Eastern Ghats Belt. Zircon from the coarse-grained charnockite rock shows crystallization age of ca. 970 Ma. Aluminous granulite contains spinel and quartz-bearing mineral assemblage indicating ultrahigh temperature metamorphism (>900 °C at 8–10 kbar) which possibly occurred during ca. 987 Ma as revealed from monazite included in porphyroblastic garnet. Monazite in the aluminous granulite and the migmatitic felsic gneiss grew dominantly at 966 ± 4 Ma and 968 ± 4 Ma ages respectively, which are interpreted as the cooling ages subsequent to peak metamorphism. Oscillatory-zoned zircon grains of the felsic augen gneiss yield ca. 1173 Ma age which is interpreted as the crystallization age of the granitic protolith. This ca. 1173 Ma age granite possibly composed a part of the Proterozoic basement of the Eastern Ghats Province. A monazite age of 781 ± 15 Ma from the aluminous granulite may indicate the timing of a localized shear-induced thermal process in the Phulbani domain. The presently studied rock suite thus recorded three distinct events (ca. 1173 Ma, ca. 1000–900 Ma and ca. 781 Ma) of magmatism, metamorphism and deformation of Eastern Ghats Belt. Metamorphic and magmatic histories of the Phulbani domain are similar to those of the adjacent Visakhapatnam domain and the Rayner Province of East Antarctica during the time frame ca. 1000–900 Ma.

Dormant landslides distributed in upper course of Sun Kosi Watershed and landslides induced by Nepal Gorkha Earthquake 2015

This study detected 897 dormant landslides of deep-seated type in the upper course of the Sun Kosi Watershed. It also shows geology and slope gradient that are prone to cause deep-seated landslides in the Great Himalaya and Midland zones. The dormant landslides are predominantly distributed in geological areas of the Augen Gneiss, Benighat Slate, Robang Phyllite and the Kuncha Phyllite, considering both landslide area ratio and site number. Landslides of deep-seated type are also found prone to develop on dip-slope. Occurrence of landslides usually increases over 20 degrees of slope angle for each geological type, though the number of landslides in the phyllite area increases below 20 degrees. It is very clear that landslides in phyllite area are prone to occur from relatively lower slope angles compared to those occurring in quartzite, gneiss and the Sermathang Formation. Nepal Gorkha Earthquake 2015 caused 1804 landslides in this study area. Most of the earthquake-induced landslides are of shallow types that occurred just below break of slope, showing a shoulder shaped profile along the deep gorge. They are predominantly distributed on steep slopes in the geological areas of slate, gneiss and dolomite. None of the dormant landslides of large scale was activated by the earthquake.

Anomalous Complex Electrical Conductivity of a Graphitic Black Schist From the Himalayas of Central Nepal

AbstractWe analyzed in the laboratory the frequency‐dependent, complex‐valued, electrical conductivity of a graphitic black schist and an augen gneiss, both collected in the Main Central Thrust shear zone in the Himalayas of central Nepal, which was heavily affected by the deadly Mw7.8 Gorkha earthquake in 2015. We focused on anisotropy and salinity dependence of both cores and crushed material as well as the impact of CO2 on conductivity. This black schist possesses an extraordinarily high polarizability and a highly frequency‐dependent conductivity. Its anisotropy is very pronounced. The investigations can relate the main polarization feature to disseminated, aligned plates of graphite. By contrast, the augen gneiss shows low polarizability and a moderately anisotropic conductivity dominated by the pore‐filling brine. We further demonstrate that neglecting the complex and frequency‐dependent nature of conductivity can lead to serious misinterpretation of magnetotelluric data during inversion if highly polarizable rocks are present.

Regional Middle Paleozoic metamorphism in the southwestern Gyeonggi Massif, South Korea: Its implications for tectonics in Northeast Asia

The Hongseong area in the southwestern Gyeonggi Massif in South Korea is considered to represent the eastward extension of the Qinling–Dabie–Sulu collision belt in China. We have carried out zircon U–Pb SHRIMP dating and P–T estimations of the gneisses and amphibolites in the eastern Wolhyeonri complex within the Hongseong area in order to constrain their metamorphic and tectonic evolutions. The protoliths of the migmatitic biotite gneisses formed during the Neoproterozoic and underwent granulite-facies metamorphism (750–880°C, 12–15kbar) at 442–413Ma. These rocks subsequently experienced amphibolite-facies retrograde metamorphism at 585–660°C and 7.5–10.3kbar. Mylonitic biotite gneiss, hornblende gneiss, and folded amphibolite in the study area yield metamorphic ages that range from 429 to 420Ma. The protoliths of some garnet amphibolites that formed at 470–456Ma are arc magmatic rocks; they experienced metamorphism at the boundary between amphibolite- and eclogite-facies (ca. 625–700°C and 13–15.5kbar) before 418Ma and underwent retrograde amphibolite-facies metamorphism (ca. 625–700°C and 8–9kbar) at 418–405Ma. These data suggest that a regional intermediate-P/T metamorphic event occurred during the Middle Paleozoic. In contrast, Paleoproterozoic augen gneiss blocks enclosed in the Deokjeongri gneiss complex preserve evidence of high-pressure (HP) metamorphism (840–960°C, 17–21.8kbar) at 234–230Ma, which are similar to the previously reported results from eclogite blocks in this area. The occurrence of Middle Paleozoic regional metamorphism before the Permo-Triassic HP metamorphism in the Hongseong area may be correlated with the Middle Paleozoic metamorphism in the Qinling belt in China; such regional metamorphic events were caused by the collision of microcontinents with the North or South China Cratons prior to the collision between the North and South China Cratons in the Permo-Triassic.

Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin

From the Neoproterozoic to the early Paleozoic, the northern Gondwana margin was sequentially shaped by the Cadomian accretionary and the Variscan collisional orogens which offers the opportunity to investigate the relative extent of crust production/reworking in both geodynamic settings. In the eastern part of the Variscan French Massif Central (FMC), the Velay Orthogneiss Formation (VOF) represents a consistent lithological unit of the pre-Variscan basement and comprises augen gneisses and leucogneisses. Such rocks constitute a unique record of the pre-Variscan magmatic history and bear critical information on the crustal evolution of the northern Gondwana margin.Here, we present whole–rock major and trace element compositions indicating that: (i) the VOF shows a remarkable geochemical homogeneity; (ii) the protolith of the augen gneisses corresponds to strongly peraluminous, “S-type” porphyritic granites originating from partial melting of an Ediacaran sedimentary sequence; (iii) the leucogneisses are former leucogranites generated by fractionation of the magma at the origin of the porphyritic granites; and (iv) the whole suite emplaced at shallow crustal levels (<7km). U–Pb LA–(MC–)ICP–MS analyses on zircon yielded similar emplacement ages of c. 542Ma and a narrow range of εHf(t) clustering around 0 for the protoliths of both augen and leucogneisses. This homogeneous Hf isotope signature, notably uncommon for S-type granites, would originate from a sequential process of: (i) inherited zircon dissolution during melting and ascent in the crust due to Zr-undersaturated conditions, (ii) isotopic homogenization of the melt by advection and elemental/isotopic diffusion, followed by (iii) early saturation upon emplacement owing to rapid cooling at shallow crustal levels.We propose that partial melting of Ediacaran sediments occurred during inversion of a Cadomian back-arc basin and was promoted by the high thermal gradient typical of thinned crust domains. Therefore, the VOF and other Cadomian S-type granitoids from the northern Gondwana margin are indicative of substantial crust reworking away from any proper continental collision zone.

From incipient slope instability through slope deformation to catastrophic failure — Different stages of failure development on the Ivasnasen and Vollan rock slopes (western Norway)

The long-term evolution of rock slope failures involves different stages, from incipience of slope instability to catastrophic failure, through a more or less long-lasting slope deformation phase that also involves creeping or sliding. Topography, lithology, and structural inheritance are the main intrinsic factors that influence this evolution. Here, we investigate the role of these intrinsic factors on the rock slope failure development of the Ivasnasen and Vollan rock slopes (Sunndal Valley, western Norway) using a multitechnique approach that includes geomorphologic and structural field mapping, kinematic analysis, terrestrial cosmogenic nuclide exposure dating, topographic reconstruction, and deformation quantification. Ivasnasen is a rock slope failure complex with several past rock slope failures and a present unstable rock slope, located on a cataclinal NW-facing slope and developed in augen gneiss. Vollan on the opposite valley side is a deep-seated gravitational slope deformation (DSGSD) affecting the whole mountainside, developed in quartzite in the upper part and micaschist in the lower part. These different lithologies belong to different nappe complexes that were emplaced and folded into a series of syn- and anticlines during the Caledonian orogeny. These folds lead to different lithologies being exposed in different structural orientations on the opposite valley flanks, which in turn leads to different types and evolution of rock slope failures. At Ivasnasen the 45°–55° NW-dipping ductile foliation allowed for a fairly simple planar sliding mechanism for the 1.2millionm3 post-glacial rock slope failure. Failure occurred ca. 3.3ka ago after a short period of prefailure deformation. For the present 2.2millionm3 unstable rock slope at Ivasnasen, a steepening of the foliation at the toe impedes such a mechanism and up to 10m of displacement has not lead to a catastrophic failure yet. The Vollan DSGSD is characterized by a steep major back scarp with its up to 130-m-wide graben, which opened along the subvertical foliation in the quartzite, and a conspicuous array of counter-scarps in the micaschist. The morphologic features are explained by a flexural toppling mechanism in the micaschist, which likely initiated in the lower slope section and propagated retrogressively until reaching the massive quartzite at the back scarp. Flexural toppling cannot solely explain the total along-slope displacement of up to 200m, i.e., an elongation of 28%. Creep along a basal shear zone, which may have developed in the hinge zone of the flexural toppling, combined with shallow valley-dipping joints, likely accounts for the large elongation. Implications of this study for the hazard assessment of unstable rock slopes in Norway include the relatively rapid development of instabilities in cataclinal slopes and the important insights on long-term displacement rates gained from cosmogenic nuclide dating for a better understanding of the evolution of unstable rock slopes.

Paleozoic adakitic rocks in the northern Altyn Tagh, northwest China: Evidence for progressive crustal thickening beneath the Dunhuang Block

To constrain the Phanerozoic crustal evolution of the Dunhuang Block, new whole-rock geochemistry, zircon U–Pb ages and Hf isotopic data were determined for three Paleozoic granitic plutons in the northern Altyn Tagh, northwest China. Zircon U–Pb dating of these plutons, including gneissic granite, mylonitic granite and augen gneiss, yielded Middle Devonian to Early Carboniferous crystallization ages of 358±4Ma, 382±4Ma and 332±2Ma, respectively. The gneissic granites are characterized by a small SiO2 variation (69.11–70.33wt.%) with moderate K2O (3.07–3.55wt.%), Na2O (3.61–4.08wt.%), low Fe2O3T (2.25–2.84wt.%) and MgO (1.04–1.25wt.%). The granites are weakly peraluminous with moderate aluminium saturation indexes (ASI=1.02–1.08) and exhibit the geochemical features of high-K calc-alkaline igneous rocks. Although the major element contents of the mylonitic granites and the augen gneisses are different from those of the gneissic granites, all three plutons show similar REE patterns and trace element variations. Rocks from these plutons are all LREE-enriched and show relatively flat HREE patterns with pronounced depletions in Ba, Nb, Ta, Ti and negative Eu anomalies. Moreover, the rocks are characterized by high Sr (330.7–656ppm), low Yb (0.513–1.521ppm) and Y (5.199–13.73ppm), and thus have high Sr/Y ratios (32–122), showing geochemical affinities of adakitic rocks. Their negative εHf(t) values, low MgO, Cr, Co and Ni contents indicate a continental crust origin without significant involvement of mantle-derived materials. The negative Eu anomalies, low Yb and Y contents of these granitic rocks reveal that plagioclase and garnet are major residue minerals in the magma source. All the geochemical characteristics demonstrate that these adakitic plutons were produced by partial melting of thickened lower crust at relatively shallow depths under high amphibolite or granulite facies and did not reach the eclogite facies. Combining this with the formation ages, we suggest that the southern margin of the Dunhuang Block was thickened during the late Paleozoic. The gradual variations in the (La/Yb)N ratios of these rocks indicate that the continental crust of the southern Dunhuang Block was progressively thickened from the Late Devonian to Early Carboniferous. This thickening process was most likely triggered by continental collision between the Dunhuang Block and the Central Altyn massif.

Tin-tantalum-niobium mineralization in the Penouta deposit (NW Spain): Textural features and mineral chemistry to unravel the genesis and evolution of cassiterite and columbite group minerals in a peraluminous system

The Sn-Ta-Nb Penouta granite is a highly evolved rare-metal post-kinematic leucogranite located in north-western Spain, which was intermittently mined from Roman times until 1985, when falling metal prices led to the closure of most mines in the country.Mineralization consists mainly of cassiterite (cassiterite I) and columbite group minerals (CGM) disseminated in the leucogranite, that increase in abundance and crystal size towards the upper part of the granitic cupola, especially in banded pegmatite-like dikes. Upwards quartz veins and greisen zones associated with the wall rock (mainly augen gneisses) were developed, containing coarse-grained cassiterite (cassiterite II). In contrast to nearly homogenous cassiterite chemistry, CGM may show complex compositional zonation patterns in relation with compositional variations of the leucogranite. Taking into account the mineral chemistry and textural features of CGM, two main groups were distinguished: (i) the first one (Group I) follows a general main trend from columbite-Fe to columbite-Mn, and finally to tantalite-Mn, which is in accordance with the evolution of the magma observed from the less to the more evolved zones of the leucogranite and from bottom to top of the body, and responds to common fractionation patterns of decreasing Nb/Ta ratios in the melt; and (ii) a second group (Group II), with intermediate values of Mn/(Mn+Fe) and Ta/(Ta+Nb) due to local supersaturation effects of the melt during crystallization, resulting in complex oscillatory zoned CGM crystals.Textural and chemical features of cassiterite and CGM support the magmatic origin for both minerals, although cassiterite crystallized later in the sequence in both generations. Mineral chemistry evolution of CGM is in good agreement with a greater solubility of tantalite in the melt at the upper levels, where, besides, the fluorine contents of apatite and muscovite were increased and the onset of fluorite crystallization occurred. So, the fluorine enrichment in the upper levels seems likely to be an important component to explain the mineral chemistry of the Penouta granite, although the same mineral chemistry scheme could also be explained in terms of white mica fractionation upwards. Furthermore, the absence of typical textures originated by fluids in the CGM as well as the lack of these minerals in the quartz veins point to a low influence of the hydrothermal fluids in dissolution and recrystallization processes of the primary oxides.

Dichotomy of The Messada Pluton, Serbo-Macedonian Massif, Greece: From Rifting to Subduction

The Messada pluton is a mafic intrusion that is located about 12 km SW of Serres town, (Macedonia Greece) that intrudes the two mica, biotite and the augen gneisses of the Vertiskos formation (Serbo-Macedonian massif). The aim of this study is to investigate, define and evaluate the geochemical characteristics of the pluton in order to determine the geotectonic environment in which the parental magma has been formed. The Mesada pluton is a mid to coarse grained intrusion presenting petrographic variety from diorite and quartz diorite to tonalite and granodiorite. The variety in petrography reflects its chemical inhomogeneity in major and trace elements. It is suggested that parts of pluton have been formed by distinctly different types of magmas originated in diverse geotectonic settings. Those parts of quartz diorite and tonalite composition, present similar geochemical characteristics, LILE/HFSE ratios and negative Nb, but no Ti anomalies in their primitive mantle normalized trace elements spider grams. They exhibit higher HFS values than those of granodioritic composition. Moreover, their ORG normalized spider grams not only suggest that they have been evolved by a common parental magma, but also present the typical characteristics of a “crust dominated” within plate pluton that may have been formed in an early stage during rifting, prior to a subsequent subduction episode. This interpretation may be in accordance with the suggestion for the Gondwanian origin of the more silicic Triassic rift related meta-granites (e.g. Arnea plutonic complex) of the Serbo-Macedonian massif. In contrary; the parts of Mesada pluton of granodioritic composition, exhibit a calc-alkaline to high K calc-alkaline magmatic suite and present higher LILE/HFSE and LREE/HREE ratios, related to a higher crustal component contribution for the magma genesis. Furthermore, their primitive mantle normalized spider grams’ present negative anomalies at Nb and Ti. These characteristics indicate that those granodioritic parts have been formed by the crystallization of a calk-alkaline magma, produced by the partial melting of lower crust, lithospheric mantle and asthenospheric mantle components, in a volcanic arc geo-tectonic setting. Their geochemical characteristics have close similarities to those of the collision related granitoids that have intruded the Serbo- Macedonian during Tertiary.

Tectono-sedimentary evolution of the Miocene Oued Dayr Formation (Ghomaride Complex, Internal Domain of the Maghrebian Rif Belt, Morocco)

This work deals with sedimentological, petrographic, and structural analyses of a middle Miocene late-orogenic sedimentary cycle, denoted Oued Dayr Formation, recognized in the Rifian sector of the Maghrebian Chain (Morocco). The analyzed Formation (75 m thick) starts with 15–20 m of light colored polymict conglomerates, with minor sandstone beds, lying on the Paleozoic basement and Mesozoic cover of the Ghomaride Nappe. Facies analysis indicates a fining-upward deposition in a marine environment characterized by increasing deepening, reflecting a subsidence rate that exceeds sedimentary supply. Petrographic analysis points out that sandstones are represented by litharenites originated by erosion of recycled orogen. The conglomerates pebbles and cobbles consist of Alpine low- to high-grade metamorphic rocks as metarenites, phyllites, mylonitic quartzites, micaschists, augen gneisses deriving from the exhumed deep metamorphic basement, the overlying metasedimentary of the Sebtide Nappes and of sedimentary rocks as sandstones, jaspes, limestones, and shales deriving from the Ghomaride Nappes and their sedimentary cover. Data reveal mixed provenance indicating that the Oued Dayr Formation was fed by the Internal Nappes stack of the Maghrebian Chain. Structural analysis shows that the Oued Dayr Formation accumulated in a Thrust-Top basin, during an early extension (D0 phase), recorded by synsedimentary normal faults within middle Langhian deposits on the rear of the Internal Nappes stack. Subsequent ductile and brittle compressional (D1, D2, D3) and extensional (D4) deformation phases occurred during and/or after the stacking, exhumation, and early unroofing of Sebtide Complex coeval with the opening of the western Mediterranean back-arc basins since middle Miocene time.

Zircon U-Pb age and geochemical constraints on the origin and tectonic implication of Cadomian (Ediacaran-Early Cambrian) magmatism in SE Turkey

The Bitlis-Pütürge Massifs and Derik volcanics that crop out in the Southeast Anatolian Belt are parts of the Cadomian domain in Anatolia where relicts of the oldest continental crust of Turkey are exposed. The Bitlis-Pütürge Massifs contain a Neoproterozoic basement, with overlying Phanerozoic rocks that were imbricated, metamorphosed and thrust over the edge of Arabia during the Alpine orogeny. The basement consists mainly of granitic to tonalitic augen gneisses and metagranites, associated with schists, amphibolites and paragneisses. Based on whole-rock geochemical data, the augen gneisses are interpreted to have protoliths crystallized from subduction zone magmas. This study conducted the first zircon dating on two augen gneisses that gave 206Pb/238U dates of 551±6 and 544±4Ma, interpreted as the formation ages of the Pütürge Massif, broadly coeval to those of the Bitlis metagranites and the Derik volcanics that occurred from ca. 581 to 529Ma (the Ediacaran-early Cambrian). The ɛHf(t) values (+1.2 to −5.3) of the dated zircons, with crustal model ages (TDMC) from 1.4 to 1.8Ga, indicate that formation of the Pütürge Massif involves an older, most likely the Mesoproterozoic, continental crust component. Similar to the Bitlis-Pütürge gneisses, coeval basement rocks are widespread in the Tauride-Anatolide platform (e.g., the Menderes Massif). All these dispersed Cadomian basement rocks are interpreted as fragments of the Ediacaran-Early Cambrian continental arcs bordering the active margin of northern Gondwana.

Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth

The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model.Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite–trondhjemite–granodiorite (TTG) suites, with slightly magnesian character (average Mg#=33), moderate LREE (average LaN=154), low HREE (average YbN=6) and Y (1–53ppm; average 11ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu*=0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg#=22), low to moderate degrees of REE fractionation (average [La/Yb]N=34.84), high contents of Y (5–128ppm; average 68), and low Sr/Y (1–98) ratios. Significant negative Eu anomalies (Eu/Eu*=0.18–0.91; average 0.50) and low Sr (65–690ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg#=21), low to moderate degrees of REE fractionation (average [La/Yb]N=26), high contents of Y (71–99ppm; average 87), and low Sr/Y (average 2) ratios with significant negative Eu anomalies (Eu/Eu*=0.03–0.31; average 0.23) and low Sr (average 160ppm) contents. The protoliths of the TC are interpreted as being derived from partial melting of thickened oceanic-arc crust composed of Archaean mafic composite source rocks (i.e., eclogite and/or garnet amphibolite) with a garnet amphibolite residue. Geochemical features of the GC, such as high Rb/Sr (average 1.80) and Ba/Sr ratios (average values >6), are considered as evidence for crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields.The geochemical features of the felsic ortho-granulite suite, substantiated with published geochronological data on members of the TC, GC, and AC suites, suggest a four-stage crustal evolution of the KKB. The first stage is marked by the formation of an over-thickened oceanic-arc. Zircon Hfc model ages of the TC and GC suites constrain the time of this juvenile magmatic crust-forming event as Meso- to Neoarchaean (2.8 to 2.6Ga). The second stage corresponds to the production of TTG magmas by melting of the over-thickened oceanic-arc crust, subsequent to basaltic underplating during Palaeoproterozoic (ca. 2.1Ga). The third stage was initiated by a transition in subduction style from shallow to steep due to continent-arc accretion. This stage is marked by the formation of granitic magmas through partial melting of the TTG crust and their differentiation into GC and TC. The zircon crystallization ages (1.89 and 1.85Ga) of the GC indicate arc accretion occurred during the Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic (~1.5Ga) emplacement of megacrystic K-feldspar granites (protoliths to the AC and augen gneisses). The distinct petrography, geochemistry and crystallization ages of the AC suggests recurrence of megacrystic, high-K calc-alkaline granitoids as the product of final phases of crustal-remelting marking subduction cessation. All these magmatic events are fairly well correlated with the major episodes of crustal growth observed in the once contiguous continental fragments (Sri Lanka and Madagascar) and worldwide events (2.7, 1.9, and 1.2Ga) implying similar episodic nature in the lower crustal evolution of the KKB.

Surface exposure dating of Holocene basalt flows and cinder cones in the Kula volcanic field (Western Turkey) using cosmogenic 3He and 10Be

Abstract The Kula volcanic field in Western Turkey comprises about 80 cinder cones and associated basaltic lava flows of Quaternary age. Based on geomorphological criteria and K-Ar dating, three eruption phases, β2–β4, were distinguished in previous studies. Human footprints in ash deposits document that the early inhabitants of Anatolia were affected by the volcanic eruptions, but the age of the footprints has been poorly constrained. Here we use 3 He and 10 Be exposure dating of olivine phenocrysts and quartz-bearing xenoliths to determine the age of the youngest lava flows and cinder cones. In the western part of the volcanic field, two basalt samples from a 15-km-long block lava flow yielded 3 He ages of 1.5 ± 0.3 ka and 2.5 ± 0.4 ka, respectively, with the latter being in good agreement with a 10 Be age of 2.4 ± 0.3 ka for an augen gneiss xenolith from the same flow. A few kilometers farther north, a metasedimentary xenolith from the top of the cinder cone Cakallar Tepe gave a 10 Be age of 11.2 ± 1.1 ka, which dates the last eruption of this cone and also the human footprints in the related ash deposits. In the center of the volcanic field, a basalt sample and a metasedimentary xenolith from another cinder cone gave consistent 3 He and 10 Be ages of 2.6 ± 0.4 ka and 2.6 ± 0.3 ka, respectively. Two β4 lava flows in the central and eastern part of the volcanic province yielded 3 He ages of 3.3 ± 0.4 ka and 0.9 ± 0.2 ka, respectively. Finally, a relatively well-preserved β3 flow gave a 3 He age of ∼13 ka. Taken together, our results demonstrate that the penultimate eruption phase β3 in the Kula volcanic field continued until ∼11 ka, whereas the youngest phase β4 started less than four thousand years ago and may continue in the future.

Early Paleozoic Magmatism in Himalayan Orogen: The Geochronological Study on Augen Gneisses from Gyirongand Nyalam Areas, Southern Tibet

Early Paleozoic Magmatism in Himalayan Orogen: The Geochronological Study on Augen Gneisses from Gyirongand Nyalam Areas, Southern Tibet

Late-Proterozoic to Paleozoic history of the peri-Gondwana Calabria-Peloritani Terrane inferred from a review of zircon chronology.

U–Pb analyses of zircon from ten samples of augen gneisses, eight mafic and intermediate metaigneous rocks and six metasediments from some tectonic domains along the Calabria–Peloritani Terrane (Southern Italy) contribute to knowledge of peri-Gondwanan evolution from Late-Proterozoic to Paleozoic times. All samples were equilibrated under amphibolite to granulite facies metamorphism during the Variscan orogeny. The zircon grains of all considered samples preserve a Proterozoic memory suggestive of detrital, metamorphic and igneous origin. The available data fit a frame involving: (1) Neoproterozoic detrital input from cratonic areas of Gondwana; (2) Pan-African/Cadomian assemblage of blocks derived from East and West African Craton; (3) metamorphism and bimodal magmatism between 535 and 579 Ma, within an active margin setting; (4) rifting and opening of Ordovician basins fed by detrital input from the assembled Cadomian blocks. The Paleozoic basins evolved through sedimentation, metamorphism and magmatism during the Variscan orogeny involving Palaeozoic and pre-Paleozoic blocks. The Proterozoic zircon records decidedly decrease in the high grade metamorphic rocks affected by Variscan pervasive partial melting. Electronic supplementary materialThe online version of this article (doi:10.1186/s40064-016-1839-8) contains supplementary material, which is available to authorized users.

A new petrogenetic model for meta-granitic rocks in the central and southern Menderes Massif – W Turkey: Implications for Cadomian crustal evolution within the Pan-African mega-cycle

A new petrogenetic model is presented for genesis of the Late Neoproterozoic meta-granitic rocks in the central and southern Menderes Massif (MM). This study evaluates the Sr and Nd isotope geochemistry of 12 meta-granite rock samples collected from the central and southern MM to constrain the late-stage evolution of the Cadomian Orogeny in western Turkey. LA-ICP-MS analyses in the literature yield an age interval of 544.9–554.4Ma for crystallization of the meta-granitic rocks in the MM, western Turkey. The meta-granitic rocks are classified as augen gneisses, bi-rich granitic mylonites that are cross-cut by trm-rich granitic mylonites. Geochemically, the biotite-rich granitic mylonites and augen gneisses have high HFSE and HREE-enriched multi-elemental patterns, whereas tourmaline-rich granitic mylonites display less enriched HFSE and LREE, with negative anomalies in Nb, Ti and Eu. The studied rocks are not juvenile and have a crustal component and residence history based on the initial 143Nd/144Nd(t) (0.51169–0.51190) ratios and negative ɛNd(t) (−4.63 to −0.54) values. The geochemical trends of the four biotite-rich granitic mylonite samples and three augen gneiss samples indicate that they may have been derived from mafic sources followed by mixing of a continental crust source. In contrast, the five trm-rich granitic mylonites display no mixing effect of upper continental crust in their genesis. The secondary, intense alteration/fluid-ingress process strongly changed the primary geochemical features of these rocks during or after mylonitization. The 147Sm/144Nd ratios of the four bi-rich granitic mylonite samples and three augen gneiss samples have typical crustal values (0.12–0.13). In contrast, four of the five trm-rich granitic mylonite samples yield 147Sm/144Nd ratio ranges of 0.20–0.28 (the fifth trm-rich granitic mylonite sample yielded a value of 0.12). The 0.20–0.28 147Sm/144Nd ratios are higher than crustal values. All three rock suites may have been emplaced during late- to post-collisional back-arc extension at 545–560Ma behind the Cadomian arc concomitant with southward subduction of the Proto-Tethys Ocean along the northern margin of Gondwana. This has also been observed in the peri-Gondwanan terranes in W and E Europe, Iran, Tibet and SE Asia.

Proterozoic magmatism in the southern Scandinavian Caledonides, with special reference to the occurrences in the Eikefjord Nappe

Caledonian nappes of southern Norway comprise basement slices and sedimentary cover, and generally have close affinity with autochthonous terranes in the Fennoscandian foreland. The nappes can be distinguished in those having Gothian (1700–1600 Ma) character and those of Telemarkian (1550–1480 Ma) type. There were also several periods of variously intense magmatism between these early accretionary events and the end of the Sveconorwegian Orogeny. Most prominent features are the anorthositic-noritic massifs, which formed mainly during the Telemarkian period and in the late stages of the Sveconorwegian Orogeny between 980 and 920 Ma. In this paper, we review the main events affecting the crystalline parts of the nappes and report new results from the Eikefjord Nappe. An augen gneiss yields a crystallization age of 1191 ± 2 Ma, which is an event rarely seen in nappes or the autochthon. An amphibolitic gneiss formed at about 1500 Ma and underwent high-grade metamorphism at 986 ± 3 Ma, followed by lower grade overprints as reflected by titanite dates ranging to 915 Ma. The Sveconorwegian metamorphism was ubiquitous in the nappes, although its intensity was locally highly variable. The association of metamorphism with the second main anorthosite generation event points to intense subcrustal magmatism, heating and deformation of the lithosphere. The various crustal forming events have different geneses, some are clearly related to subduction processes, others more consistent with localized mantle melting. Those closer in character to classical Large Igneous Provinces are the events at 1470–1420, 1280–1240 and 1220–1180 Ma.

Two garnet growth events in polymetamorphic rocks in southwest Spitsbergen, Norway: insight in the history of Neoproterozoic and early Paleozoic metamorphism in the High Arctic

Geochronological studies in northern Wedel Jarlsberg Land, southwestern Svalbard (Norway), showed that the Tonian (c. 950 Ma) igneous rocks were subjected to metamorphism during the Torellian (c. 640 Ma) and early Caledonian (470–460 Ma) events. Predominant augen gneisses, derived from a Tonian protolith, are intercalated in that area, with schists comprising two distinct metamorphic mineral assemblages. The M1 (Torellian) assemblage containing garnet-I + quartz + plagioclase-I + biotite-I + muscovite-I was formed under amphibolite-facies conditions at c. 550–600 °C and 5–8 kbar (1 kbar = 100 MPa). The M2 (Caledonian) assemblage comprising garnet-II + quartz + plagioclase-II + biotite-II + muscovite-II + zoisite + chlorite crystallized at c. 500–550 °C and 9–12 kbar, corresponding to epidote–amphibolite facies conditions. The M2 mineral assemblage constitutes the pervasive Caledonian fabric of the schists that was subsequently reactivated in a left-lateral strike-slip shear regime. The subsequent c. 70° clockwise rotation of the original structure to its present position was caused by a large-scale passive rotation during the Paleogene Eurekan orogeny. The new pressure–temperature estimates suggest that metamorphic basement in the study area was consolidated during the Torellian middle-grade event and then overprinted by Caledonian moderate- to high-pressure subduction-related metamorphism. A following sinistral shear zone assembled the present structure of basement units. Our results pose a question about the possible extent of Torellian precursor to the Caledonian basement across the High Arctic and the scale of its subsequent involvement in early Caledonian subduction. In conjunction with previous studies, the results suggest a possible correlation between southwestern Spitsbergen and the Pearya Terrane in Ellesmere Island.

Strain and flow in the metamorphic core complex of Ios Island (Cyclades, Greece)

We have analysed strain and flow kinematics in the footwall of the South Cyclades Shear Zone (SCSZ), an important tectonic boundary within the Attic-Cycladic Crystalline Complex exposed on Ios Island, Cyclades, Aegean Sea. Coarse-grained augen gneisses in the basement unit flooring the SCSZ and forming a metamorphic core complex are excellently suited to measure finite strain using the Fry method and estimate the vorticity number (Wk) of flow with the “blocked-object” method. The results show that Oligo-Miocene exhumation of the basement unit during extension brought approximately 70 % N–S crustal stretching and up to 40 % subvertical shortening in a plane strain environment (k = 0.99). Linear down-section strain decrease constrains a zone of contact deformation of the SCSZ of about 1.5 km thick. Kinematic vorticity number estimates suggest little deviation from pure shear (Wk = 0.26). Finite strain and Wk are not correlated, indicating that the Ios basement and the overlying cover units were stretched compatibly. While the SCSZ is a localized zone of high strain, net displacement, however, may be restricted to about ten kilometres. This has important repercussions on large-scale tectonic models for extension in the Aegean.