Tectonic Juxtaposition Research Articles

Origin and evolution of the granitic intrusions in the Brusque Group of the Dom Feliciano Belt, south Brazil: Petrostructural analysis and whole-rock/isotope geochemistry

In the southern Brazilian state of Santa Catarina the Dom Feliciano Belt, formed by the tectonic juxtaposition of different crustal blocks during the Brasiliano-Pan African Orogenic cycle, can be divided into three domains. In the central domain, three granitic suites intrude the metavolcanosedimentary sequence of the Brusque Group: São João Batista (SJBS), Valsungana (VS) and Nova Trento (NTS), from the oldest to the youngest. This extensive magmatism, here referred to as granitic intrusions in the Brusqe Group (GIBG), is coeval with the thermal peak in the host metamorphic successions, but postdates its main foliation. A progressive deformation starting from the magmatic stage throughout the cooling history points to the influence of the late stages of deformation recorded in the Brusque Group.The SJBS consists of gray to white leucocratic, equigranular granites, with aluminous minerals such as muscovite, garnet and tourmaline. The porphyritic VS is the largest of the suites and is characterized by its cm-sized K-feldspar megacrysts in a coarse-grained biotite-rich matrix. The granites from the NTS are equigranular, light gray to pink in color and have biotite as the main mafic mineral, but magmatic muscovite, tourmaline and hornblende can occur as well.Geochemically, the GIBG are mildly peraluminous and show a calc-alkaline affinity. Most intrusions have a high REE fractionation, but some SJBS granites show a characteristic pattern with no fractionation and strong negative Eu anomalies (“seagull pattern”). Elevated Sr(i) values, between 0.707 and 0.735, and negative εNd values as low as −24 points to the melting of old evolved crust. The Nd (TDM) ages are scattered between 1.54 and 2.76 Ga, with a predominance of values around 2.0 Ga.The GIBG have a strong crustal signature that most closely connects, within the regional units, to that of the metasedimentary rocks of the Brusque Group and its crystalline basement, the Camboriú Complex. All three suites seem to have been produced during a same regional melting event, but at different crustal levels and reflecting heterogeneities within the same source rocks. Most evidences imply that sedimentary source rocks were especially important to the SJBS, which probably originated in a shallower environment, whilst the VS and NTS represent the melting of deeper crystalline crust, probably sharing some magmatic interaction.

Tracing ancient events in the lithospheric mantle: A case study from ophiolitic chromitites of SW Turkey

New major-, minor- and trace-element data on high-Cr chromites from several ophiolitic podiform chromitites from Lycian and Antalya peridotites in southwestern Turkey reveal a polygenetic origin from a range of arc-type melts within forearc and back-arc settings. These forearc and the back-arc related high-Cr chromitites are interpreted to reflect the tectonic juxtaposition of different lithospheric mantle segments during the obduction. The diversity of the γOs(t=0) values (−8.28 to +13.92) in the Antalya and Lycian chromitite PGMs and their good correlations with the sub- to supra-chondritic 187Os/188Os ratios (0.1175–0.1459) suggests a heterogeneous mantle source that incorporated up to 40% recycled crust, probably due to subduction processes of the orogenic events. The few model ages calculated define two significant peaks in TRD model ages at 1.5 and 0.25Ga, suggesting that the chromitites are younger than 0.25Ga and include relics of an at least Mesoproterozoic or older (>1.0Ga) mantle protolith. Eight of the nine zircon grains separated from the chromitites, are interpreted as detrital and/or resorbed xenocrystic relics, whilst a significantly less reworked/resorbed one is considered to be of metasomatic origin. In-situ U–Pb dating of the xenocrystic zircon grains yielded a spread of ages within ca 0.6–2.1Ga, suggesting recycling of crustal rocks younger than 0.6Ga (Late Neoproterozoic). The notable coincidence between the lower age limit of the older zircons (ca 1.6Ga) and the oldest Os model age peak (ca 1.5Ga) from the PGM may suggest a Mesoproterozoic rifting stage. These findings imply a Paleoproterozoic sub-continental lithospheric mantle (SCLM) protolith for the SW Anatolian mantle which was later converted into an oceanic lithospheric mantle domain possibly following a rifting and continental break-up initiated during Mesoproterozoic (ca 1.5–1.0Ga). The single metasomatic zircon of ca 0.09Ga age coinciding with the initiation of the contraction of the Neotethys may be interpreted to date the timing of a metasomatic event beneath the intra-oceanic subduction zone.

New data about structure and time of formation of the Khamar-Daban terrane: U-Pb LA-ICP-MS zircon ages

This paper provides new data on the age of detrital zircons from metaterrigenous rocks of the Khamardaban Group (Kornilovskaya and Shubutuiskaya formations) which crown the succession of the Khamar-Daban terrain. It was established that the accumulation of the protoliths of both formations occurred in the interval between the Late Riphean and Early Ordovician. In this case, there is a difference between sequences of Shubutuiskaya and Kornilovskaya formations due to a sharp change in a provenance area and depositional conditions, which is a consequence of the change in the paleogeodynamic environment. In addition, this indicates the tectonic juxtaposition of these sequences and probability of accumulation of deposits of the Shubutuiskaya Formation within Dzhida island arc system.

OIB- and P-type ophiolites along the Yarlung- Zangbo Suture Zone (YZSZ), Southern Tibet: Poly- Phase melt history and mantle sources of the Neotethyan oceanic lithosphere

We present an overview of the internal structure of the ophiolite massifs along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet with a focus on the geochemical character and tectonic evolution of the Ocean Island Basalt (OIB) and mafic alkaline rock assemblages associated with these ophiolites. The Jurassic - early Cretaceous lavas, massive diabase and gabbroic rocks are either tectonically intercalated with the early Cretaceous, subduction-influenced ophiolitic units, or occur as thrust sheets or blocks with an early Cretaceous melange and in a Jurassic-Cretaceous flysch unit structurally beneath these ophiolites. They display uniform chondrite-normalized REE patterns with light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, no obvious Eu anomalies or negative Nb, Ta and Ti anomalies, and primitive mantle normalized trace element patterns with significant large-ion lithophile element (LILE) enrichment, similar to those of modern OIB and the Hawaiian alkaline basalts. These mafic alkaline rock assemblages represent OIB- and Plume-type (P-type) oceanic crustal rocks (with no subduction influence) that formed from magmas produced by partial melting of plume-metasomatized asthenospheric mantle source during the early stages of the opening of a Neotethyan seaway between Proto-India and Eurasia. Subsequent consumption of this OIB-P-type mid-ocean ridge (MOR) oceanic lithosphere at an intraoceanic subduction zone produced the ~130-120 Ma forearc to backarc, SSZ oceanic crust within the same Neotethys. The evolutionary history of the YZSZ ophiolites thus reflects a poly-phase melt history and different mantle melt sources. The final tectonic juxtaposition of the older OIBand P-type oceanic crustal rocks with SSZ-type oceanic lithosphere fragments took place as the northern passive continental margin of Proto-India collided with and underplated the intraoceanic subduction-accretion system in the late Cretaceous. The YZSZ displays a complete Wilson cycle record of the rift-drift, seafloor spreading and subduction zone tectonic evolution of the Mesozoic Neotethys.

Partial melting in amphibolites in a deep section of the Sveconorwegian Orogen, SW Sweden

Garnet amphibolite metataxites at the Steningekusten Nature Reserve in southwestern Sweden contain tonalitic patches and veins. Whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities. Orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others. Orthopyroxene megacrysts were formed by vapor-absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende. The net reaction was Bt+Hbl+Pl+/−Qtz↔Opx+Melt+Cpx+Gt. Melting occurred at pressures of approximately 1GPa and temperatures which probably exceeded 800°C. Pyroxenes are surrounded by hornblende–quartz symplectites, and hornblende in these coronas has distinctly lower concentrations of (Na+K) and Ti than that in adjacent mesosome. The hornblende rims formed upon cooling and reaction with crystallizing melt. This created a barrier to further reaction thus preserving the orthopyroxene megacrysts. Garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor-present melting including lack of peritectic phases predicted by vapor-absent melting reactions, larger amounts of leucosome (14 versus 7%), and less distinct melanosomes. The variation in these migmatites reflects open system behavior, either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites. Zircons from all units have CL-dark core domains that are dated at 1415–1390Ma. The core zones are cut and overgrown by CL-dark and CL-bright rims that are dated at 975–965Ma. The zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization. New growth of zircon associated with migmatization at ca. 970Ma corresponds to the timing of crustal scale partial melting in the deep regions of the Sveconorwegian orogen, synchronous with east–west extension and the intrusion of mafic dykes. If partial melting was driven by an influx of aqueous fluids, they were probably derived from a relatively cool source region, which would indicate tectonic juxtaposition of hotter and cooler terranes.

Geochemistry of the eclogites of the Maksyutov Complex, South Urals, and genetic nature of their protoliths

Detailed mineralogical-petrographic and petrogeochemical characteristics are reported for eclogites of the Maksyutov Metamorphic Complex (MMC). It was established that protoliths of different varieties of eclogites (high-Ti, moderate- and low-Ti eclogites, graphite eclogites, and eclogites of the layered body) were mafic magmatic rocks of different affinity. The petrogeochemical comparison of MMC eclogites with the Early Paleozoic lithotectonic complexes of the South Urals shows that the eclogites are close to basalts formed in different geodynamic settings that existed in the region in the Cambrian (?)-Ordovician-Silurian(?) time. Subduction of oceanic crust and subsequent rise to the surface resulted in the spatial juxtaposition of bodies, the protoliths of which were formed at the initial and island arc stages of the evolution of the Paleo-Ural Ocean. Thermodynamic calculations of mineral assemblages of eclogites showed that low-Ti eclogites (680–700°C, 24 kbar), graphite eclogites (660–710°C, 17–18.8 kbar), and eclogites of the layered body (610–730°C, 16–18 kbar; 410–430°C, 12.5–13 kbar) were formed at similar temperatures but at a large scatter in pressure. It was concluded that the pressure variations were caused by the tectonic juxtaposition of bodies during exhumation of eclogites formed at different depths of subducted plate.

Chapter 18 Tectonostratigraphic evolution of the Nellore schist belt, southern India, since the Neoarchaean

Abstract Tectonostratigraphic development of the c. 300 km-long Nellore schist belt (NSB) of southern India is described in relation to the post-Neoarchaean growth of the Dharwar cratonic nucleus. Lying along the eastern margin of the Dharwar cratonic nucleus, the NSB comprises several geologically and geochemically distinct tracts of deformed Palaeoproterozoic to Mesoproterozoic volcanosedimentary successions: the Vinjamuru Group, the Kandra ophiolite complex (KOC), the Kanigiri ophiolitic melange (KOM) and the Udaigiri Group, arranged in relative order of younging. The high-grade Eastern Ghats belt occurs further to the east of the NSB with a tectonic contact. Thrust-transported oceanic crust remnants occur in the 1.9 Ga KOC, 1.34 Ga KOM, and the Vinjamuru Group, which show multiple deformation, amphibolite facies metamorphism and granitic intrusions. The available geological, geochemical and geochronological data have been examined to tentatively constrain the relative age of the different tectonostratigraphic units of the NSB, tectonic juxtaposition and implications in relation to global events in the Proterozoic. Subduction-related ocean closures outboard and east of the Dharwar Craton, evidenced by the KOC and KOM, possibly had links with the assembly of Columbia and its final dispersal, respectively.

Exploration of hydrothermal carbonate magnesium isotope signatures as tracers for continental fluid aquifers, Schwarzwald mining district, SW Germany

The significance of magnesium isotope (δ26Mg) fractionation in the continental hydrothermal domain is poorly explored. Here, a detailed Mg isotope dataset from various aquifer host rocks and corresponding hydrothermal carbonate precipitates from the Schwarzwald mining area in SW Germany is documented and discussed. This study is motivated by the very considerable existing data set on hydrothermal mineralizations in the study area and the excellent regional control of sampling points. Based on structural, mineralogical and microthermometrical arguments, we here subdivide three clusters of veins: (i) Permian, (ii) Jurassic–Cretaceous and (iii) post-Cretaceous (Cenozoic). The focus is on clusters ii and iii and their corresponding, texturally older hydrothermal fluid inclusion-rich, coarse-grained, low-Mg calcite and dolomite–ankerite solid solutions and younger, oscillatory zoned low-Mg calcite and dolomite–ankerite solid solution crystals precipitated from fluids with temperatures between 50 and 350°C. In terms of their δ26Mg ratios, three characteristic groups of hydrothermal carbonates can be distinguished: (i) Jurassic–Cretaceous veins (δ26Mg=−3.38 to −0.82‰) in the central Schwarzwald precipitated from fluids containing a significant proportion of sedimentary cover-derived waters. They yield 26Mg-depleted Mg isotope signatures typical of sedimentary carbonate lithologies. (ii) Jurassic–Cretaceous vein carbonates in the southern Schwarzwald precipitated from mainly basement-derived fluids with 26Mg-enriched signatures (δ26Mg=−1.22 to +0.05‰). Supporting evidence for the origin of these fluids comes from Sr–C–Pb isotope systematics and trace element compositional variations of fahlore and sphalerite. The Mg isotope variations are controlled by differences in fluid source characteristics dominate over variations exerted by mineralogical differences (where low-Mg calcites are generally, but not in all cases, more depleted in 26Mg compared to dolomite–ankerite solid solutions). (iii) Post-Cretaceous (Cenozoic), Rhinegraben-related veins represent a complex tectonic juxtaposition of different aquifer lithologies. As expected, this phase spans the full range from silicate to carbonate Mg isotope signatures (+0.45 to −3.4‰). Magnesium isotope data are supported by textural analyses (optical and cathodoluminescence microscopy), electron microprobe, microthermometry and published radiogenic 87Sr/86Sr isotope systematics. Our data are not in agreement with a significant temperature-controlled Δ26Mgfluid-calcite and no obvious relation between fluid salinity and hydrothermal carbonate δ26Mg is found. The results of this study suggest that hydrothermal carbonate magnesium isotope ratios have a significant potential as tracer of hydrothermal fluid sources and corresponding aquifer lithologies.

Retrograde metasomatic effects on phase assemblages in an interlayered blueschist–greenschist sequence (Coastal Cordillera, Chile)

Abstract Interlayered blueschists and greenschists of the Coastal Cordillera (Chile) are part of a Late Palaeozoic accretionary complex. They represent metavolcanic rocks with oceanic affinities based on predominantly OIB-type REE patterns and immobile trace element ratios. Both rock types have similar mineralogies, albeit with different mineral modal abundances. Amphibole is the major mafic mineral and varies compositionally from glaucophane to actinolite. The presence of glaucophane relicts as cores in zoned amphiboles in both blueschists and greenschists is evidence for a pervasive high-pressure metamorphic stage, indicating that tectonic juxtaposition is an unlikely explanation for the cm–dm scale interlayering. During exhumation, a retrograde greenschist-facies overprint stabilized chlorite + albite + winchitic/actinolitic amphibole + phengitic white mica ± epidote ± K-feldspar at 0.4 ± 0.1 GPa. Geochemical variability can be partly ascribed to primary magmatic and partly to secondary metasomatic processes that occurred under greenschist-facies conditions. Isocon diagrams of several adjacent blueschist–greenschist pairs with similar protolith geochemistry were used to evaluate metasomatic changes due to retrograde fluid–rock interaction. The most important geochemical changes are depletion of Si and Na and addition of water in the greenschists compared to the blueschists. Transition metals and LILE are mobilized to varying degrees. The unsystematic deviations from magmatic fractionation trends suggest open system conditions and influx of an external fluid. Pseudosection and water isopleth calculations show that the rocks were dehydrating during most of their exhumation history and remained at water-saturated conditions. The mineralogical changes, in particular breakdown of blue amphibole and replacement by chlorite, albite and calcic/sodic–calcic amphibole, are the prime cause for the distinct coloring. Pseudo-binary phase diagrams were used as a means to link bulk rock geochemical variability to modal and chemical changes in the mineralogy. The geochemical changes induced by fluid–rock interaction are important in two ways: First, the bulk rock chemistry is altered, leading to the stabilization of higher modal proportions of chlorite in the greenschists. Second, the retrograde overprint is a selective, layer-parallel fluid infiltration process, causing more intense greenschist-facies recrystallization in greenschist layers and therefore preferential preservation of blue amphibole in blueschist layers. Hence, the distinct colors were acquired by a combination of compositional variability, both primary magmatic and secondary metasomatic, and the different intensity of retrograde fluid infiltration.

P–T–t evolution of eclogite/blueschist facies metamorphism in Alanya Massif: time and space relations with HP event in Bitlis Massif, Turkey

The Alanya Massif, which is located to the south of central Taurides in Turkey, presents a typical nappe pile consisting of thrust sheets with contrasting metamorphic histories. In two thrust sheets, Sugozu and Gundogmus nappes, HP metamorphism under eclogite (550–567 °C/14–18 kbar) and blueschist facies (435–480 °C/11–13 kbar) conditions have been recognized, respectively. Whereas the rest of the Massif underwent MP metamorphism under greenschist to amphibolite facies (525–555 °C/6.5–7.5 kbar) conditions. Eclogite facies metamorphism in Sugozu nappe, which consists of homogeneous garnet–glaucophane–phengite schists with eclogite lenses is dated at 84.8 ± 0.8, 84.7 ± 1.5 and 82 ± 3 Ma (Santonian–Campanian) by 40Ar/39Ar phengite, U/Pb zircon and rutile dating methods, respectively. Similarly, phengites in Gundogmus nappe representing an accretionary complex yield 82–80 Ma (Campanian) ages for blueschist facies metamorphism. During the exhumation, the retrograde overprint of the HP units under greenschist–amphibolite facies conditions and tectonic juxtaposition with the Barrovian units occurred during Campanian (75–78 Ma). Petrological and geochronological data clearly indicate a similar Late Cretaceous tectonometamorphic evolution for both Alanya (84–75 Ma) and Bitlis (84–72 Ma) Massifs. They form part of a single continental sliver (Alanya–Bitlis microcontinent), which was rifted from the southern part of the Anatolide–Tauride platform. The P–T–t coherence between two Massifs suggests that both Massifs have been derived from the closure of the same ocean (Alanya–Bitlis Ocean) located to the south of the Anatolide–Tauride block by a northward subduction. The boundary separating the autochthonous Tauride platform to the north from both the Alanya and Bitlis Massifs to the south represents a suture zone, the Pamphylian–Alanya–Bitlis suture.

Tectonic juxtaposition of crust and continental growth during orogenesis: Example from the Rengali Province, eastern India

The southern boundary of the Singhbhum Craton witnessed multiple orogenies that juxtaposed thin slice of granulite suite of the Rengali Province against the low-grade granite-greenstone belt of the craton along the E–W trending Sukinda Thrust. The strong southerly dipping mylonitic foliation within the granulites along with the prominent down-dip mineral lineation, suggest a northerly-verging thrusting. Mylonitized charnockite at the contact zone contains enclaves of mafic and ultramafic granulite, whereas granitoid gneiss contains enclaves of pelitic granulite. Mafic granulite enclaves preserve an early (S1M) foliation that formed during D1M deformation. This rock, along with the host charnockite, were intensely deformed by the D2M thrusting event and resulting S2M foliation development in both rock suites. Geothermobarometric and pseudosection analyses show that the garnet-clinopyroxene-plagioclase-orthopyroxene-ilmenite-quartz assemblage in mafic granulite was stabilized at high-pressure and temperature conditions (10−12 kbar, 860 °C) and was overprinted by a fine-grained assemblage of clinopyroxene-plagioclase ± hornblende that developed during decompression (down to 5.5–7.5 kbar). Matrix hornblende shows incipient breakdown to garnet-clinopyroxene-quartz intergrowth due to a granulite facies reworking. A contrasting P-T history is preserved in the pelitic granulite. The peak assemblage garnet-orthopyroxene-cordierite-quartz-rutile was stabilized at ∼6.0 kbar, 730 °C which resulted from heating of the mid crust magma during the D2M thrusting. The contrasting P-T histories could result from the tectonic juxtaposition of lower- and mid-crustal section during the D2M event. Evidences of an early orogenic imprint within the mafic granulite imply involvement of deep continental crust during southward growth of the Singhbhum Craton.

The Orekhov–Pavlograd Zone, Ukrainian Shield: Milestones of its evolutionary history and constraints for tectonic models

The Orekhov–Pavlograd Zone (OPZ), which is located in the Ukrainian Shield, represents the reworked western margin of the Archaean Azov Province and separates it from the Archaean Middle Dnieper Province. In the Vasilkovka area, which is located in the northern OPZ, Archaean tonalites and amphibolites and Palaeoproterozoic quartzites, high-Al paragneisses, and mafic granulites were studied. Structural observations allow the OPZ to be interpreted as a steep transpressional ductile shear zone. The tonalites and amphibolites were dated at 3.50 and 2.88Ga, respectively (SIMS zircon ages hereafter). The metasedimentary rocks originated from mature sediments from Archaean continental sources and were deposited in a shallow intracratonic basin. The basaltic protoliths of the mafic granulites were produced by a mantle-plume-related source in a quiescent intraplate setting. Several metamorphic events were recognised (3.5, 3.4, 3.3, 2.85 and 2.1–2.0Ga). The main metamorphic reworking occurred in the Palaeoproterozoic; its onset was dated at 2.08–2.07Ga, and it ended at 2.00–2.01Ga. The earliest Palaeoproterozoic metamorphism occurred at P=ca. 8.5kbar and T=ca. 600°C, then at P=5–7kbar and the peak metamorphic temperature T=830°C, and the final stage was at T=ca. 600°C and P=3.3–4kbar (based on the TWEEQU and THERIAK-DOMINO methods), which suggests a clockwise P–T–t loop. In small shear zones, the Archaean tonalites were converted to garnet–biotite gneisses that inherited the tonalitic geochemistry and Palaeoarchaean magmatic zircons. Subduction-related igneous rocks and accretionary prism sediments are absent in both the Vasilkovka area and the entire AP, which makes subduction-related models questionable. An alternative tectonic model for the OPZ could be an intracratonic (intraplate) ductile shear zone that developed due to far-field stresses due to the tectonic juxtaposition of the Sarmatia and Volgo-Uralia continental blocks, which were parts of the East European Craton. Regional metamorphism and the coeval emplacement of mantle-derived igneous rocks and crust-derived granites are suggested to have been produced by magmatic underplating at ca. 2.02–2.10Ga. This model needs to be developed further.

Introduction to this special section: Gravity and potential fields

Tien Grauch and her coauthors describe an integrated study of high-resolution aeromagnetics, ground gravity, airborne gravity gradiometry, TEM, and 2D seismic data for imaging not only basement faulting and relief, but also shallow faulting within the sedimentary section. Rather than invoking magnetic mineralization along fault zones (or other geochemical explanations) as an explanation for the observed sedimentary magnetic anomalies, they show that the anomalies can be completely explained by tectonic juxtaposition of strata of differing magnetic properties across the fault zones. They document, through 2D forward magnetic modeling, a faulted geologic column of juxtaposed magnetite-enriched clastics with magnetite-depleted clastics. These laterally varying magnetite-bearing horizons stack to produce super-positioned magnetic anomalies of relatively low amplitude and short wavelength which are superimposed on the longer wavelength, higher-amplitude magnetic anom-alies sourced by basement composition heterogeneities and structural relief.

Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectonics in the Chinese Tianshan

We dated and geochemically characterized peridotite-bearing mélanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan mélange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10km long; protolith age≥ca. 445–466Ma). Metamorphic zircons of a mafic granulite (εNd(t)=5.0) yielded a weighted mean 206Pb/238U age of 341±8Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (εNd(t)=−4.3) have a mean age of 332±13Ma that records a retrogressive metamorphic event. These ages determine the timing (ca. 341–332Ma) of mantle diapirism through continental crust. A dolerite dike (εNd(t)=2.3) emplaced into metaperidote has a crystallization age of 335±5Ma, that, on the basis of geochemistry, we interpret as representing E-MORB-OIB magmatism that accompanied mantle diapirism. An undeformed pink granite (εNd(t)=−3.6) intrudes the mélange matrix and has an emplacement age of 324±5Ma, thus providing an upper time limit for a tectonic movement that led to mélange formation. The older time limit of deformation (ca. 362–352Ma) is constrained by the youngest ages of thermo-tectonically modified zircons in a mylonitized metagabbro and a foliated meta-andesite. Magmatic zircons in the meta-andesite (εNd(t)=−3.3) and a dacite (εNd(t)=−5.6) have eruption ages of 433±4Ma and 435±3Ma that date formation of the mélange matrix. A microgabbro dike (εNd(t)=10.0) cutting metasediments (i.e. mélange matrix) has an emplacement age of 279±3Ma and contains abundant zircon xenocrysts ranging in age from Archean to late Paleozoic. Similarly, a dolerite dike, a meta-andesite, an intermediate granulite, and a metagabbro all contain inherited Precambrian to Paleozoic zircons. We accordingly conclude that the mélange was formed by mantle diapirism in the early Carboniferous and was subsequently affected by an early Permian volumetrically minor mafic episode. Two foliated garnet granites (εNd(t)=−7.0 and −6.9), in direct contact with metaperidotite within the Main Tianshan Shear Zone have emplacement ages of 341±2Ma and 329±4Ma that reflect an early Carboniferous episode of crustal anatexis. They also contain Neoproterozoic zircon xenocrysts. Also within the Main Tianshan Shear Zone, a gabbro (εNd(t)=8.7) has an emplacement age of 297±3Ma and contains abundant Neoproterozoic to late Paleozoic zircon xenocrysts. Based on the regional geological relationships, we associate early Carboniferous mantle diapirism and crustal anatexis with lower crustal delamination-induced asthenospheric upwelling. The numerous Precambrian zircon xenocrysts support a Chinese Tianshan–Tarim connection. The early Permian mafic episode coincided in time with the development of an early Permian unconformity in the Chinese North Tianshan.

Zircon Nano-SIMS U-Pb dating from the country gneiss surrounding Horoman peridotite –age constraint for tectonic juxtaposition of peridotite body into granulite in Arc-Arc collision zone-

Zircon Nano-SIMS U-Pb dating from the country gneiss surrounding Horoman peridotite –age constraint for tectonic juxtaposition of peridotite body into granulite in Arc-Arc collision zone-

A deep, stratigraphically and structurally controlled landslide: the case of Mount La Civita (Molise, Italy)

The present paper illustrates the results of an integrated study of a large landslide located on the southern slope of Mount la Civita (Molise, Southern Apennine), an E–W elongated, SSE dipping and 890-m-high monocline carbonate ridge. The upper part of the slope affected by the landslide is largely controlled by strata attitude while its basal part is marked by a strike–slip fault causing the tectonic juxtaposition of the carbonate successions against predominantly clayey flysch units. An integrated study, including geological, geomorphological and geotechnical investigations, was carried out to determine the features of the landslide and to plan further investigation and monitoring. In particular, from 2002 to 2004, Differential Global Positioning System monitoring and core drillings, coupled with inclinometer measurements, were carried out to determine the landslide’s kinematics, extent, depth to the surface of rupture and rates of movement. Inclinometer data revealed the presence of the rupture surface at a depth of about 20 m. DGPS monitoring allowed rates of movement up to several tens of centimetres per year to be recorded. The nearby village of Civitanova del Sannio can still be considered at risk due to the landslide, as recent remedial works, consisting mainly of very shallow re-shaping of the slope by blasting and partial filling of trenches, did not succeed in stopping its movement.

The Murchison Greenstone Belt, South Africa: Accreted slivers with contrasting metamorphic conditions

This paper presents new petrological and geochronological data for the ∼3.09–2.92Ga Murchison Greenstone Belt (MGB), located in South Africa's Kaapvaal Craton, and discusses their geotectonic implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure–temperature–deformation (P–T–D) histories, and are separated from each other by relatively narrow, high-strain shear zones, with a sinistral, transpressive top-to-the-south movement, consistent with the deformation patterns observed throughout the belt. These patterns are explained by a N–S compressional stress field, affecting the Murchison Belt between 2.97 and 2.92Ga. Results of new petrological investigations indicate that ultramafic to felsic volcano-sedimentary rocks of the Murchison Unit underwent a greenschist- to lower-amphibolite-facies metamorphism at maximum P–T conditions of 5.6±0.6kbar at 570°C, along a relatively hot, minimum apparent geotherm of ∼30°C/km. In contrast, the Silwana Amphibolites and the La France Formation were metamorphosed at much higher peak metamorphic conditions of 8.7–10kbar, 630–670°C, and 8–9kbar, 600–650°C, respectively, and require a colder apparent geotherm of ∼20°C/km. A retrograde, nearly isothermal–decompression P–T path followed by isobaric cooling is also inferred for the La France Formation. The timing of the structural–metamorphic overprint is bracketed between 2.97 and 2.90Ga, which is constrained by U–Pb zircon ages of a syn-deformation granite within the Murchison Unit and the post-deformation Maranda granite, respectively. Monazite and xenotime from La France metapelites yield much younger ages of ca. 2.75Ga, with few inherited components at 2.92 Ga. They point to a later activation of the MGB, perhaps related with tectono-thermal events in the Rooiwater Complex and the Pietersburg Greenstone Belt. The relatively cold apparent geotherms recorded in the Silwana and La France rocks, the contrasted peak P–T conditions between the different units, and the near isothermal decompression of the La France Formation indicate that the Kaapvaal craton crust must have been cold enough to enable significant crustal thickening and strain localisation along narrow shear zones and, as a consequence, fast tectonic juxtaposition of rocks metamorphosed at different crustal depths. These features are similar to those observed along Palaeozoic or modern day, oblique subduction–collision zones, but different to those of hot Archaean provinces. We therefore interpret the MGB as representing part of an oblique collision-zone between two terrains of the Kaapvaal craton: the Witwatersrand and Pietersburg terrains.

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt, Southwestern Transbaikalia: Age and conditions of the formation

This paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.

Fault-related rocks: deciphering the structural–metamorphic evolution of an accretionary wedge in a collisional belt, NE Sicily

The Alpine chain exposed in the Western Mediterranean area represents a front several kilometres in width, dismembered by more recent tectonics and by opening of the Tyrrhenian Basin. In most exposures of this mountain belt, relics of older metamorphic rocks occur. The deformational sequence of events may be revealed by the recognition of metamorphic records associated with different structures. Within a tract of the Alpine front cropping out in the Peloritani Mountains (NE Sicily), we distinguished two metamorphic complexes characterized by different tectonometamorphic histories. Their present tectonic juxtaposition is a cataclastic thrust linked to the recent Africa-verging Sicilian–Maghrebian fold-and-thrust belt. The Lower Complex is characterized by Hercynian metamorphism (P > 0.2 GPa and T ≈ 350°C) exclusively. It essentially consists of very low-grade metapelites and metavolcanic rocks overlain by an unmetamorphosed sedimentary cover. The Upper Complex, comprising different tectonic slices, consists of medium- to high-grade Hercynian metamorphic rocks (P = 0.3–0.8 GPa and T up to 650°C) with Alpine metamorphic overprint (T > 250°C) affecting also the Mesozoic–Cenozoic cover. Lithotypes, structures, and inferred P–T conditions of investigated rocks suggest the existence of an Alpine accretionary wedge during the Cretaceous deformational collision. Within the Upper Complex, a polyphase Palaeogene mylonitic horizon involving rocks belonging to different tectonic slices fully preserves the tectonometamorphic evolution. For this reason, we focused our attention on these sheared rocks in order to reconstruct the entire tectonic history of this geologically complex area. Our new basic model allows the complex structure of the nappe-pile edifice of the Peloritani Mountains to be simplified, casting new light on the tectonic evolution of this key sector of the southern Calabrian-Peloritani Orogen.

Tectonic evolution of Early Paleozoic island-arc systems and continental crust formation in the Caledonides of Kazakhstan and the North Tien Shan

The extended Saryarka and Shyngyz-North Tien Shan volcanic belts that underwent secondary deformation are traced in the Caledonides of Kazakhstan and the North Tien Shan. These belts are composed of igneous rocks pertaining to Early Paleozoic island-arc systems of various types and the conjugated basins with oceanic crust. The Saryarka volcanic belt has a complex fold-nappe structure formed in the middle Arenigian-middle Llanvirnian as a result of the tectonic juxtaposition of Early-Middle Cambrian and Late Cambrian-Early Ordovician complexes of ensimatic island arcs and basins with oceanic crust. The Shyngyz-North Tien Shan volcanic belt is characterized by a rather simple fold structure and consists of Middle-Late Ordovician volcanic and plutonic associations of ensialic island arcs developing on heterogeneous basement, which is composed of complexes belonging to the Saryarka belt and Precambrian sialic massifs. The structure and isotopic composition of the Paleozoic igneous complexes provide evidence for the heterogeneous structure of the continental crust in various segments of the Kazakh Caledonides. The upper crust of the Shyngyz segment consists of Early Paleozoic island-arc complexes and basins with oceanic crust related to the Saryarka and Shyngyz-North Tien Shan volcanic belts in combination with Middle and Late Paleozoic continental igneous rocks. The deep crustal units of this segment are dominated by mafic rocks of Early Paleozoic suprasubduction complexes. The upper continental crust of the Stepnyak segment is composed of Middle-Late Ordovician island-arc complexes of the Shyngyz-North Tien Shan volcanic belt and Early Ordovician rift-related volcanics. The middle crustal units are composed of Riphean, Paleoproterozoic, and probably Archean sialic rocks, whereas the lower crustal units are composed of Neoproterozoic mafic rocks.