Sequence Of Schists Research Articles

GEOLOGY OF ALAKNANDA BHGIRATHI AND YAMUNA VALLEY, GARHWAL HIMALAYA, PARTS OF CHAMOLI TEHRI UTTAKASHI & PAURI DISTRICTS, UTTRAKHAND STATE, INDIA

The Alaknanda is the trunk stream of Ganga system it drains the eastern part of the area of study .The rocks of Alaknanda valley and adjoining area consist of three units viz Central Cryastalline , Garhwal Group and Dudatoli Groups which from north to south are separated by thrust or fault. The Central Crystalline Group in this area consist of /comprises of northerly dipping sequence of Kyanite schist,Garnet mica schist quartzites and para amphibolites of Tugnath formation and it is intruded by granite at Ragsi,the main Central Thrust seprates it from Garhwal Group.The Dudatoli Groupis represented Pauri Phyllite and Kirsu Quartzite which forms the northern limb of Dudatoil syncline .The north Almora Thrust makes it boundary with Garhwal group. The later is divisiable in to Rudraprayga ,Lamri , Chamoli and Gawangarh and Patrali Formation which occurs in normal stratigraphic order. It is intruded by biotite granite by biotite graninte at Nainidevi and Mohankal , with tourmaline granite around Chirpatikhal and also by basic intrusive . The Rudraprayg , Lameri and Chamoli formations are equivalent to Uttarkashi Shyalnan and Nagnithhank Formation respectively in Bhagirathi valley. The Garhwal Group of has been subjected to three phases of tectonic deformation. The south east to southerly plunging folds such as Marithanasa and Pingapani synclines .Karanprayg anticline were developed during the second phase of movements. The Alaknada fault which cuts off set of the formation and earlier structures between Sunala is the strike slip fault in western part, appears to be the youngest elements. Geologically , the Bhagirathi valley and adjoining areas comprises of four distinct units namely from north to south the Central Crystalline Group , the Deoban Group the Simla Group,and Krol belt rock separating from one another by thrust or faults. The main Central Thrust passing through Sainj in northern part brings the northerly dipping crystalline rocks in sharp contact with under lying Deoban Group (Garhwal Group) sedimentries which comprises a lower Deoban Formation of Phyllite, slate meta basics, minor quartzite and lime stone, the middle Deoban formation of lime stone and upper Deoban formation of Quartzite and basics. The southern contact of Deoban Group is faulted one with comprising mainly siltstone, greywacks and slates dipping south .This fault called Sringar Nalupani fault of fundamental nature. In the southern and eastern part of the area this fault marks the contact between Deoban and Chandpur formation. In the western part south heading Ton Thrust separates the underlying Chanpur formation from the underlying Simla slates which shows abundant development of slump balls rod etc. indicating syndepositional disturbances in the basin of sedimentation while in Chanpur formation, is manly argillaceous, becoming arenaceous towards the top The Tons thrust passes through Laluli in Nagun Gad and is probably truncated by Tehri Nalupani fault at Chandpur in Bhagirathi valley. The full sequence of Krol rocks is exposed beautifully in Mussoorie syncline. The tectonic succession in Yamuna valley is Naogaon sheet comprising more metamorphosed rocks overly the Deoban group rocks separated from the latter by what is called the Naugaon thrust sheet which is folded in a North to Northwest plunging synform. The western limb of the folded thrust crosses Yamuna near the confluence with Kamola Gad and runs along the Kamola Gad. The rocks of the Naugaon sheet can be seen overlying the Deoban formation limestone along this section. The thrust hade towards east in this section. It follows a southerly trend till Tiyan when it takes an easterly swing, heading northward. It closes around Gair. The eastern limb of the synformal thrust sheet, heading westward crosses the Yamuna River at Kisna, Basic intrusive are seen along the thrust sheet near Kisna and around. The complexity of structural and tectonic set up in the area and lack of fossils make the task of correlation extremely difficult. However, on the basis of lithological similarities as well as structural disposition an attempt has been made to correlate the various litho units in the present area.

Tectono-thermal Evolution of the Proterozoic Mahakoshal Belt along the Northern Margin of Central Indian Tectonic Zone: Constraints from Geochemistry, Phase Equilibria Modelling and in-situ Monazite Dating from a Metapelite Sequence around Jabalpur

ABSTRACT In this contribution, geochemical signatures and petrological evolution of a folded sequence of mica schist from the Jabalpur area of the Mahakoshal Belt in the northern Central Indian Tectonic Zone (CITZ) are described. Multiple tectonic discrimination diagrams incorporating both major and trace element concentrations have constrained back-arc settings for deposition of the sedimentary units. S1 foliation formation marks the beginning of the M1 metamorphic stage. In contrast, the later M2 metamorphic event resulted in garnet and andalusite formation during heating at 2-3 kbar, followed by staurolite-sillimanite, garnet-staurolite assemblages at peak P-T conditions of 5.4 kbar, 550-600°C. Textural evidence suggests that M2 is pre-syn tectonic to D2 deformation, resulting in formation of the NE-SW trending S2 axial planar foliation. From the isopleth thermobarometry, a clockwise P-T path with near-isothermal decompression has been determined for M2. Similar results have been obtained from conventional thermo-barometry performed on multiple samples. Monazite dating (EPMA) of one of the samples suggests 1.9 Ga, age for D1-M1 event. M2 is correlated with the younger age population of monazites yielding 1.5 Ga. A clockwise P-T path constrained for M2 suggests collisional tectonics along the northern margin of the CITZ, as recorded from its southern margin.

Metamorphic origin of stratiform cassiterite mineralization in the Schwarzenberg -Aue district – Clues to the metamorphic history and pre-orogenic Sn enrichment of the Erzgebirge (Germany)

Here we present petrographic and field observations, mineral and whole-rock compositions as well as conventional thermobaromerty and thermodynamic modelling of the Sn-mineralized schists near Bockau, Erzgebirge (Germany). The metamorphic mineral assemblage comprises locally abundant cassiterite (up to 2 wt%), which exhibits metamorphic features indicating pre-orogenic Sn enrichment of the sedimentary protoliths. The results imply that the composite schist sequence of Bockau underwent regional metamorphism at 0.8–0.9 GPa and 550 ± 50 °C and a subsequent relatively hot-exhumation path between 600 and 700 °C. Flat compositional zoning profiles of garnet suggest that these elevated temperature conditions were not short-lived but likely lasted for a time period of 20 to 40 Ma leading to the development of retrograde mineral assemblages randomly overgrowing cassiterite. The textural position of cassiterite indicates that its formation is predominantly related to the prograde/collisional stage of the Variscan orogeny (∼340 Ma) and therefore is linked to metamorphic reactions and hence pre-dates widespread LiSn granites of the area (325–310 Ma). The presented observations of the Bockau schists provide the first direct evidence for pre-orogenic Sn enrichment of their sedimentary protoliths. This has two important implications: first, the meta-sedimentary units of the Bockau series as a part of the Frauenbach super unit may host extensive, yet underexplored Sn resources along its entire strike length (> 40 km). Second, melting and assimilation of cassiterite-bearing Bockau schists may have significantly contributed to the formation of Sn-enriched granites within the Saxothuringian Zone of the European Variscides (and elsewhere). Hence, the presence of Sn-enriched (meta) sedimentary units could be an important prerequisite for Sn fertility of a metallogenic region. The ultimate pre-metamorphic source and the cause of sedimentary Sn-enrichment remain poorly constrained.

Polyakov log coarse garnet occurrence in the South Urals

Ontogenic modeling of the origin of coarse garnet of the Polyakov log occurrence (South Urals) in a fne-grained schist sequence allowed the authors to conclude on its metasomatic growth with the replacement of chlorite aggregates. Muscovite, ilmenite, epidote, quartz and apatite remain preserved in the primary schist orientation of individuals. Accessory andesine, cobaltite, pentlandite, pyrrhotite and zircon crystallized together with garnet. The formation of coarse garnet crystals is probably related to fuctuations of supersaturation relative to garnet in areas of schists with abundant chlorite. This allows us to expect the presence of coarse garnet areas in adjacent parts of the schist sequence. Keywords: South Urals, garnet, ontogeny

A multi-methodological approach for mineral exploration and predictive metallurgy: the case of the Pilar gold deposit at the Quadrilátero Ferrífero, Brazil

Gold exploration is generally based on methods such as soil, stream and drill core geochemistry and geophysical techniques, but X-ray diffraction (XRD) and the Rietveld method-based mineral quantification are seldomly used. Here, a multi-methodological approach coupling traditional drill core macroscopic description and multi-element analysis, scanning electron microscopy/energy-dispersive X-ray fluorescence spectrometry (SEM/EDS), and electron probe microanalysis (EPMA) with XRD/Rietveld method and statistical techniques is presented as a useful tool for deposit characterization, mineral exploration and predictive metallurgy purposes. The Pilar gold deposit, situated in the Quadrilátero Ferrífero province, Brazil, was used as a case study for the proposed multi-methodological approach. Carbonate-facies banded iron formations (BIFs) and schist sequences showing an intense quartz and sulfide hydrothermal alteration represent the host rocks of Pilar mineralized intervals, which may be identified by three signatures: Au-As-W-S-quartz-chlorite-pyrrhotite-arsenopyrite-calcite for schists, and Au-As-quartz-chlorite-arsenopyrite-pyrrhotite and Au-Ag-S-Te-quartz-chlorite-pyrrhotite for BIFs. Within the mineralized BIFs, the expected average Au grades and gold textures vary according to the signature. In As-poor BIFs, gold is only observed within the structure of sulfide and telluride phases. On the other hand, native gold is present in As-rich BIF samples, which usually contain relatively higher Au grades, compared to the As-poor BIFs. Native gold is also identified in mineralized schists, whose average Au grades are similar to those of As-poor BIFs. Therefore, the proposed multi-methodological approach is useful in providing the following information: geochemical-mineralogical signatures and the expected average Au grades and mode of occurrence of gold for each one of these signatures. At Pilar, the geochemical-mineralogical behavior of mineralized zones is directly linked to Au grades and textures. In terms of a practical workflow, this study illustrates that, after the multi-methodological approach is applied and the representativeness of the ore-petrography is tested, the general described behavior of gold within each group of ores can be used for mineral exploration and metallurgical purposes without the need of repeated SEM/EDS and EPMA analyses. Instead, the average Au grades and the gold textures may be predicted solely by the signature definition based on the drill core geochemical-mineralogical analyses.

Role of fluids on deformation in mid-crustal shear zones, Raft River Mountains, Utah

AbstractFluids are commonly invoked as the primary cause for weakening of detachment shear zones. However, fluid-related mechanisms such as pressure-solution, reaction-enhanced ductility, reaction softening and precipitation of phyllosilicates are not fully understood. Fluid-facilitated reaction and mass transport cause rheological weakening and strain localization, eventually leading to departure from failure laws derived in laboratory experiments. This study focuses on the Miocene Raft River detachment shear zone in northwestern Utah. The shear zone is localized in the Proterozoic Elba Quartzite, which unconformably overlies the Archaean basement, and consists of an alternating sequence of quartzite and muscovite-quartzite schist. In this study, we characterize fluid-related microstructures to constrain conditions that promoted brittle failure in a plastically deforming shear zone. Thin-section analyses reveal the presence of healed microcracks, transgranular fluid inclusion planes and grain boundary fluid inclusion clusters. Healed microcracks occur in three sets, one sub-perpendicular to the mylonitic foliation, and a set of two conjugate microcracks oriented at ∼40–60° to the mylonitic foliation. Healed microfractures are filled with quartz, which has a distinct fabric, suggesting that microcracks healed while the shear zone was still at conditions favourable for quartz crystal plasticity. Transgranular fluid inclusion planes also occur in three sets, similar in orientation to the healed microfractures. Fluid inclusions commonly decorate grain and subgrain boundaries as inter- and intragranular clusters. Our results document ductile overprint of brittle microstructures, suggesting that, during exhumation, the Raft River detachment shear zone crossed the brittle–ductile transition repeatedly, providing pathways for fluids to permeate through this shear zone.

Tectonic transformation from orogen-perpendicular to orogen-parallel extension in the North Himalayan Gneiss Domes: Evidence from a structural, kinematic, and geochronological investigation of the Ramba gneiss dome

The North Himalayan Gneiss Domes consist of a series of isolated domes cored by leucogranite and/or gneiss body. The Ramba dome core comprises gneiss and two-mica granite, mantled mainly by schist sequences, and the degree of deformation decreases outward from pervasive mylonitization to weak mylonitization. Pre-doming deformation is characterized by the formation of regional-scale top-to-the-north shearing (D1) in the Tethyan Himalayan Sequence, locally preserved strong folding (F1), and intrusion of 46.3 Ma-40.6 Ma leucogranite. The D2 deformation consists of map-scale ductile shearing fabrics and top-to-the-north detachment along the South Tibetan Detachment System (STDS) in the middle and lower units of the dome. Intrusion of 26.1–21.0 Ma syn-kinematic leucogranite veins in the schist units occurred during the D2 north-directed extensional episode. The D3 deformation was characterized by top-to-the-east shear during 12.0–10.2 Ma, concurrent with the initiation of activity in the N–S-trending Yadong–Gulu rift. The latest deformation (D4) involved the ascent of two-mica leucogranite through diapirism at about 8.17 Ma. Microstructural analyses, as well as quartz c-axis fabrics measured by EBSD (Electron backscatter diffraction), indicate deformation temperature ranges of D2 extensional shearing from 500 to 600 °C in the middle unit of the Ramba dome to >600 °C in the lower unit. These new data imply that this type of doming in the Ramba region represents a classic type of transformation from orogen-perpendicular to orogen-parallel extensional deformation during Miocene India–Asia collision.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Late Mesoproterozoic–early Neoproterozoic quartzite–schist sequences of the Aktau–Mointy terrane (Central Kazakhstan): Provenance, crustal evolution, and implications for paleotectonic reconstruction

Metasedimentary complexes represented by quartzite–schist Kiik Group quartzite–schists play a significant role in the Aktau–Mointy terrane structure, which is one of the largest Precambrian blocks in the western Central Asian Orogenic Belt (CAOB). U-Pb LA-ICP-MS dating of detrital zircons from quartzites in different Kiik Group areas and related magmatic age data establish that accumulation occurred in the late Mesoproterozoic to early Neoproterozoic (from 1150 to 920 Ma). Concordant age estimates of detrital zircons from Kiik Group quartzites predominately fell within intervals of 1149–2105 and 2272–2850 Ma with the prominent peaks at 1217, 1332, 1466, 1582, 1621, 1748, 1849, 2517 and 2689 Ma, and minor peaks at 1992, 2282, 2791 and 2823 Ma. Hf-in-zircon and Nd whole rock data suggest that the primary stages of continental crustal growth in the Aktau–Mointy terrane were associated with the tectono-magmatic processes of the Neoarchean, Paleoproterozoic, and Mesoproterozoic. Juvenile continental crust production occurred during the Neoarchean and Paleoproterozoic, with older complex tectono-magmatic reworking, and juvenile crust production finally prevailed in the mid-Mesoproterozoic. Based on the age patterns and Hf isotopic compositions of detrital zircons from the Aktau–Mointy terrane and other terranes in the western CAOB, the crustal basement of the Aktau–Mointy terrane exhibits certain similarities with those of the Northern Kazakhstan and Issyk–Kul terranes. They all exhibit a close tectonic affinity with the Chinese Central Tian Shan terranes, Yili Block, North and China Craton, while differing from the Tarim Craton. In the Rodinia supercontinent structure, the Aktau–Mointy terrane was probably located near the Sveconorwegian Orogen in the western Baltica with other Kazakhstan and Northern Tian Shan Precambrian blocks.

Quartzite–Schist Sequences of the Aktau–Mointy Massif (Central Kazakhstan): Structural Position, Provenance, and Formation Stages of the Earth Crust in the Precambrian

Detrital zircons from quartzite–schist sequences of the Aktau–Mointy Massif (Central Kazakhstan), which occur in various structural positions relative to the Early Neoproterozoic felsic volcanics, were studied. The obtained U–Pb zircon dating results suggest similarity of detrital zircon age populations with a lower limiting depositional age of 1220 Ma in all quartzite–schist sequences. The different positions of quartzite–schist sequences relative to the felsic volcanics are due to subsequent deformations. The ages of detrital zircons studied fall mostly within the intervals of 1149–2105 and 2354–3159 Ma with major peaks at 1218, 1338, 1468, 1581, 1628, 1743, 1851, and 2520 Ma and minor peaks at 2016, 2619, 2696, 2791, 2823, and 3114 Ma. The structural features of quartzite–schist sequences and the presence of heavy minerals that are unstable during long-term transport indicate the predominance of local source areas, which either form the buried part of the basement of the Aktau–Mointy Massif or have been eroded completely. Within the Aktau–Mointy Massif, it enables us to use the U–Pb ages and Lu–Hf isotope compositions of detrital zircons from quartzite–schist sequences to reconstruct the history of Precambrian continental crust formation. Results suggest that the production of juvenile continental crust prevailed only during Middle Mesoproterozoic, while formation during the Mesoarchean–Neoproterozoic time accompanied magmatic reworking of the older crustal complexes.

The Tongkuangyu Cu Deposit, Trans-North China Orogen: A Metamorphosed Paleoproterozoic Porphyry Cu Deposit

AbstractThe Tongkuangyu copper deposit in the Zhongtiaoshan region, southern Trans-North China orogen, is hosted by a poorly constrained sequence of Paleoproterozoic volcano-sedimentary (quartz-sericite schist and biotite schist) and granitic rocks that have been metamorphosed to lower greenschist facies and variably deformed. The deposit has previously been proposed to be either a porphyry-type or a sediment-hosted stratiform Cu deposit, and its age of formation has been debated.The quartz-sericite schist is interpreted to be a felsic crystal tuff and consists of angular quartz crystals in a fine-grained sericite-altered matrix. Two quartz-sericite schist samples yielded zircon U-Pb upper concordia intercept ages of 2512 ± 12 (2σ, mean square of weighted deviates [MSWD] = 0.19) and 2335 ± 16 Ma (2σ, MSWD = 0.80). Biotite schist, which is interleaved locally with the quartz-sericite schist and is interpreted to be a basaltic-andesitic sill, yielded a younger zircon U-Pb upper concordia intercept age of 2191 ± 10 Ma (2σ, MSWD = 1.7). Five samples of granodiorite and granodiorite porphyry that intruded the schist sequence yielded similar zircon U-Pb ages, with a weighted mean upper concordia intercept age of 2182 ± 7 Ma (2σ, MSWD = 1.3). These results suggest that the volcanic sequence was deposited between ∼2.5 and 2.3 Ga and was intruded by basaltic-andesitic sills and a suite of granodiorite and granodiorite porphyry intrusions at ∼2.19 to 2.18 Ga.Two stages of copper mineralization are interpreted to have formed after pervasive sericite alteration of the felsic volcanic rocks. Stage 1 mineralization includes disseminated and deformed quartz veinlets containing chalcopyrite ± pyrite ± magnetite ± molybdenite associated with biotite ± K-feldspar alteration in granodiorite porphyry and schist. Stage 2 comprises undeformed quartz-chlorite-carbonate veins with bornite ± chalcopyrite ± magnetite associated with local chlorite and silicic alteration. Allanite crystals intergrown with chalcopyrite in the granodiorite porphyry yielded an approximate concordia U-Pb age of 2115 ± 31 Ma (2σ, MSWD = 2.3). Two molybdenite samples in a deformed quartz-chalcopyrite-molybdenite vein yielded Re-Os model ages of 2106 ± 9 and 2089 ± 9 Ma (2σ), consistent with previously published results.Hydrothermal monazite grains with Cu-Fe sulfide inclusions in the granodiorite porphyry, quartz-sericite schist, and undeformed chlorite-bornite-quartz veins yielded much younger U-Pb upper concordia intercept ages of 1832 ± 16 (2σ, MSWD = 0.48), 1810 ± 14 (2σ, MSWD = 0.92), and 1809 ± 12 Ma (2σ, MSWD = 0.38), respectively. The results are in agreement with four Re-Os model ages for pyrite mineral separates from undeformed quartz-sulfide veins, which yielded a weighted mean age of 1807 ± 4 Ma (2σ, n = 4, MSWD = 0.42). In contrast, hydrothermal rutile crystals in the quartz-sericite schist and biotite schist yielded a range of roughly concordant ages between 2.1 and 1.8 Ga, reflecting isotopic disturbance.We interpret these results to indicate original copper mineralization at ∼2.1 Ga that is significantly later than the granodiorite (∼2.18 Ga) and schists (∼2.5–2.2 Ga), followed by hydrothermal remobilization and metamorphism at ∼1.8 Ga. The metavolcanic and granodiorite porphyry host rocks, alteration styles, and disseminated and veinlet form of the earlier mineralization are strongly reminiscent of porphyry Cu deposits, and ages of ∼2.1 Ga have been reported for one intrusion and three volcanic rock samples from the district. The Tongkuangyu, therefore, represents one of the oldest known porphyry copper deposits. Remobilization of copper occurred at ∼1.8 Ga during the Zhongtiao orogeny.

First Results of Dating Detrital Zircons from the Late Precambrian Quartzite–Schist Sequences of the Chu Block (Southern Kazakhstan)

U-Pb geochronological studies of detrital zircons from quartzite-schist sequences of the Akbastau Formations of the Chu block (northwest of the Chu-Kendyktas terrane, Southern Kazakhstan) have been provided. The concordant ages of detrital zircons are predominantly within the intervals of 1672-2115 Ma with peaks at 1697, 1780, 1857 and 2066 Ma. Individual zircon grains display ages of 2291-2332 Ma with peaks at 2303 and 2322 Ma. Neoarchean ages 2608-2747 with peak at 2681 Ma characterize another significant zircon population. The lower limit of deposition for the Akbastau Formations of the Chu block, corresponding to the youngest statistically significant zircon population, is estimated at 1,7 billion years.

Cretaceous tectonic evolution of the Sava-Klepa Massif, Republic of North Macedonia – Results from calcite twin based automated paleostress analysis

The Sava-Klepa Massif represents an approximately 5 × 2 km sized fault-bounded block of dominantly basaltic rocks located within the Sava-Zone, an important suture zone between the Eurasian (Europe) and Gondwana (Adria) continental plates in the Balkans. Its nature and tectonic evolution is controversial: It is either interpreted as a remnant of the youngest Tethyan oceanic realm left behind after the main closure in the Late Jurassic or as the delimiter of a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic and was dominantly controlled by transtensional tectonics during Cretaceous times.In order to strengthen one or the other model, we concentrate on the Sava-Klepa Massif enclosing Paleozoic basement rocks, which mainly consist of a layered sequence of metamorphic marbles and mica schists. The metamorphic basement shows deformation structures clearly related to the Klepa event. Our results are based on paleostress and paleostrain analysis of twinned calcite from deformed marbles, and field observations, microstructural analysis, and 40Ar/39Ar dating of white mica. Paleostress and paleostrain analysis was enhanced by using an automated fabric analyser microscope, and developing the herein presented freeware standalone Windows® executable PACT software (Paleostress Analysis with Calcite Twins).Our results support the hypothesis that the Sava-Klepa Massif was formed during transtension as a pull-apart basin in the Late Cretaceous. This could have happened either after the collision of the European and Adriatic plates, or along a transtentional zone in the fore-arc region during subduction of the last Tethyan oceanic lithosphere.

Ore mineralization and genesis of the Yuzhka gold–limonite occurrence, Elmus area, Karelia

Yuzhka gold-limonite occurrence is the first one to have been found in the linear-type brecciation and oxidation zone in the Archean greenstone rock oxidation zone, Elmus area, Central Karelia. The oxidation zone formed along the broken quartz-sericite-chlorite schist, which contains a pyrite ore horizon and broken quartz veins. The mineral limonite mixture was studied using Raman, thermal, X-ray, chemical, ICP-MS and microprobe analyses. The upper portion of the oxidation zone consists primarily of unconsolidated yellow-brown ochre, sintered packages and gossan (consolidated rocks). Iron oxides and hydroxides in the upper unconsolidated portion of the limonite zone were found to consist of goethite (hydrogoethite) and hematite. Pyrite ores of chalcopyrite-pyrite composition occur in lower horizons; they are broken and replaced by either hematite or both hematite and goethite. Broken quartz veins in the oxidized and ferruginized schist sequence form breccia with angular fragments of varied size, which contain scarce sulphide (pyrite, chalcopyrite, galena), sulphosalt and sulphoarsenide inclusions and are cemented by goethite and hematite. Persisting non-oxidized sulphosalts and sulphoarsenides in the ores and broken veins are composed of semseyite, tetrahedrite, jamsonite, plagionite, cobaltite and gersdorffite; melonite is occasionally encountered. The gold is fine-textured and high-grade (Ag 1–10 %). It is intergrown with goethite near the oxidized pyrite ore horizon; its concentration is 1–2.6 g/t. The geochemistry of the limonite zone (As, Sb, Pb, Ni, Cu, Co) reflects the composition of primary pyrite ores and quartz vein associations in the Elmus area, suggesting that these oxidized rocks were the source of the gold.

Structure, Age, and Settings of Formation of Ordovician Complexes of the Northwestern Frame of the Kokchetav Massif, Northern Kazakhstan

This work presents the data on the structure, geochronology, and formation settings of the Ordovician sedimentary and volcanogenic-sedimentary complexes of the Sterlitamak, Mariev, and Imanburluk structural and formational zones located in the western and northwestern frames of the Kokchetav massif (Northern Kazakhstan). In addition, the results of detailed stratigraphic, geochemical, and geochronological studies of the reference section of the Ordovician deposits of the Mariev Zone are given. The studied section is composed of carbonate, terrigenous, and less commonly volcanogenic-sedimentary deposits, confined to a wide stratigraphic interval from Tremadocian Stage of the Lower Ordovician to the lower Sandbian Stage of the Upper Ordovician. For the first time, the study of conodont assemblages made it possible to establish the Early to Middle Ordovician age of the most ancient limestone–dolomite sequence, which was previously conventionally attributed to the Cambrian. The above-lying tuffaceous–terrigenous Kupriyanovka Formation is now attributed to the Middle Ordovician. On the basis of compositional features of the lithoclastic tuffs composing the middle part of the formation, we assume that it was formed within the island arc zone. Limestones from the base of the youngest terrigenous–carbonate Kreshchenovka Formation are attributed to the lower part of the Sandbian Stage of the Upper Ordovician. The study of the geochronology of detrital zircons from terrigenous rocks of the limestone–dolomite sequence has shown that the Early Neoproterozoic quartzite–schist sequences of the Kokchetav massif were the most probable provenance area during its deposition. It was established that there was the change of sedimentation environments from closed lagoons to a relatively deep sea basin with normal salinity and intense circulation of water masses in the northwestern frame of the Kokchetav massif during the Ordovician. During this period of time, there was a sufficiently high level of erosion of provenance areas that resulted in the deposition of thick strata of terrigenous material. A general tendency of the deepening of sedimentation environments from the Early to Late Ordovician was interrupted by sea level rises in the Dapingian and early Darriwilian ages.

First Results of Study of Detrital Zircons from Late Precambrian Quartzite–Schist Sequences of the Aktau–Mointy Block, Central Kazakhstan

For the first time, the U–Pb age is determined for detrital zircons of quartzite–schist sequences, which are part of the Precambrian basement of the Aktau–Mointy Block (Central Kazakhstan) along with Neoproterozoic felsic volcanic (925–920 Ma) and granitic (945–917 Ma) rocks [6]. We analyzed 219 zircon grains from small-grained quartzites of the northern part of the block (Mt. Bol’shoi Alabas) including 206 grains with concordant age (1149–1273, 1276–1975, 2354–2592 Ma). These ages indicate the Mesoproterozoic, Paleoproterozoic, and Neoarchean rocks as provenances. The youngest statistically significant age peak of 1209 Ma indicates that the quartzite–schist sequences accumulated 1200–900 Ma ago (at the end of the Mesoproterozoic and beginning of the Neoproterozoic) prior to the formation of the Early Neoproterozoic felsic rocks and granites.

The Ticunzal Formation in central Brazil: Record of Rhyacian sedimentation and metamorphism in the western border of the São Francisco Craton

The Ticunzal Formation is exposed in the northern portion basement of the external zone of the Brasília Fold Belt and corresponds to a sequence of biotite-garnet paragneisses and mica-graphite schists with mineral assemblages indicating retrograde metamorphism under greenschist facies conditions. The formation displays local mylonitic structures and is intruded syn-to post-tectonically by 2.11–2.16 Ga peraluminous plutons of the Aurumina Suite. Detrital zircon U-Pb ages, Nd model ages between 2.23 and 2.88 Ga and geochemical data reflect a provenance probably represented by the 2.20 and 2.46 Ga tonalitic/granodioritic rocks of the Almas-Dianópolis terrane, exposed northwards of the study area. Zircon U-Pb geochronology and field observations suggest an age interval for the deposition of the precursor sediments of the Ticunzal Formation between 2.16 and 2.19 Ga. Raman spectroscopy analyses on graphite indicate high-crystallinity internal structures consistent with peak metamorphic temperatures between 620 and 630 °C. Trace element geochemical data of Ticunzal Formation samples suggest that precursor sediments were deposited in a continental arc-related basin, defining a setting consistent with an active mobile belt at the western margin of the São Francisco Craton during Rhyacian times, as inferred also from the characteristics of rocks from the Almas-Dianópolis terrane and the Aurumina Suite.

Intra-continental transpression and gneiss doming in an obliquely convergent regime in SE Asia

The Tengchong terrane comprises a sequence of linear dome-like zones, cored by granite and migmatitic layers. These cores are mantled by predominantly gneiss and subordinate schist sequences, decreasing in deformation intensity outward from extensive mylonitization to weak mylonitization. The pre-doming deformation was characterized by the formation of large-scale top-to-the-east shearing (D1) in the gneiss terrane, locally preserved flat-lying foliation (S1), weak folding (F1) and emplacement of the Mangbang granite during the Cretaceous (114-104 Ma). The second stage of deformation (D2) consisted of map-scale east-verging folds (F2, dome amplification) and minor lateral strike-slip shear zones between the anticlines in the gneiss and migmatitic sequences. Extensive partial melting and emplacement of 67-30 Ma synkinematic granitoid bodies/veins occurred, leading to the emplacement of wedges of granite into the easterly directed F2 fold cores. These wedges formed kilometer-scale granitoid domes. The post-doming D3 deformation with transpression recorded strain partitioning with simple shear-dominated high-strain zones along the Gaoligong and Nabang dextral lateral strike-slip shear zones (active during 30-11 Ma). Late transtensional deformation (D4) during cooling of the entire terrane involved the localized low-temperature Gaoligong west and east detachment faults that controlled the late exhumation of the Gaoligong metamorphic zone (since 10 Ma). Our structural observations, combined with previous studies, suggest that this style of doming is a representative type of intra-continental deformation in the Cenozoic during the oblique India-Asia collision. The actual dome shapes reflect formation of antiforms during compression-dominated transpression, prior to localized strike-slip shearing, in the accommodation belt around the Eastern Himalayan Syntaxis. Vertical exhumation of crustal material by contractional doming played an important role in absorbing the vast majority of the internal deformation of crustal fragments during oblique collision.

Age and sources of Precambrian zircon–rutile deposits in the Kokchetav sialic massif (northern Kazakhstan)

The U–Pb geochronological data on detrital zircons from placers confined to Neoproterozoic quartzite–schist sequences, which are widespread in the Kokchetav massif of northern Kazakhstan, are discussed. Detrital zircons (332 grains in total) originate from the ore occurrences in the central, northern, and western parts of the massif. The concordant ages of detrital zircons from all the examined occurrences largely correspond to intervals of 1017–1528, 1628–1946, and 2653–2739 Ma. The obtained data imply that material of quartzite–schist sequences of the Kokchetav massif was provided by Mesoproterozoic, Paleoproterozoic, and Neoarchean rock complexes. The lower age limit determined for these sequences is approximately 1.06 Ga. The dates obtained for detrital zircons are most consistent with events that took place in Laurentia. They correspond to the formation and breakup of the Columbia/Nuna supercontinent (approximately 1650–1580 and 1450–1380 Ma ago, respectively) and formation of the Rodinia supercontinent in the period of 1300–900 Ma ago.

Provenance of the Passo Feio complex, Dom Feliciano Belt: Implications for the age of supracrustal rocks of the São Gabriel Arc, southern Brazil

The Passo Feio complex (PFC) is a sequence of metapelite, amphibolite, metavolcanic/metavolcanoclastic rocks, marble, calc-silicate rocks, quartzite and magnesium schist. It is part of the São Gabriel Terrane, a Neoproterozoic juvenile arc formed during the early stages of evolution of the Neoproterozoic Dom Feliciano Belt (DFB), in southern Brazil. This belt corresponds to the southernmost portion of the Mantiqueira Province, an important Neoproterozoic orogenic system exposed in the NNE direction along the southeastern coast of Brazil. The geotectonic significance of the original Passo Feio basin in the tectonic evolution of São Gabriel Terrene is not well understood. It has been considered as part of a passive margin sequence or as a back-arc sequence. Geochronological and isotopic data are very scarse for the Passo Feio rocks and this has hampered the better understanding of its significance in the Neoproterozoic tectonic evolution of southern Brazil. In the present study the age and significance of metasedimentary rocks of the PFC were investigated. The provenance study was carried out in four metapelite samples from the southern and northern Passo Feio complex, using U-Pb dating of detrital zircon. The results showed varied provenance patterns and zircon ages range from 3637 to 803 Ma. Paleo- and Mesoproterozoic detrital zircon grains are present in all samples, but an important Neoproterozoic population has been identified in one of them. 3.5 Ga old zircon grains form the oldest population of detrital zircon ever reported in southern Brazil. Paleoproterozoic/Archean terranes within the Rio de la Plata Craton may represent the main source of detrital sediments and this suggests that part of the Passo Feio complex might have been a passive margin sequence, developed along the northeastern margin of that paleocontinent. However, the presence of a Neoproterozoic zircon population is not consistent with derivation solely from the craton and indicates contribution from younger sources, such as the Neoproterozoic São Gabriel Arc itself. This Neoproterozoic zircon population with ages ranging from 948 to 803 Ma, suggests that the onset of the magmatic/tectonic evolution of the São Gabriel Arc might have started as early ca. 0.95 Ma.