Schist Clasts Research Articles

УЛАМКИ ЕКЗОТИЧНИХ МЕТАМОРФІЧНИХ СЛАНЦІВ В ОЛІГОЦЕНОВИХ ВІДКЛАДАХ СІЛЕЗЬКОГО ПОКРИВУ (УКРАЇНСЬКІ КАРПАТИ)

The study aims at analyzing the petrographic composition of exotic metamorphic schist clasts in the Oligocene deposits filling the Silesian Nappe in the south-western part of the Ukrainian Carpathians and reconstructing their possible source area. The methodology includes petrographic, sedimentological and comparative geological methods. To achieve this goal, field research was carried out along the first right tributary of the Hysnyi Stream (the village of Uzhok, Velykobereznyanskyi district, Transcarpathian region) and petrographic analysis of the selected samples of the exotic metamorphic schist clasts included in the Oligocene debris-flow deposits was performed. The results of microscopic studies showed that these exotic rocks are represented by mica-quartz schist and garnet-muscovitebiotite-plagioclase-quartz schist with a low content of chlorite, carbonate and epidote. Mineral composition and structure/texture features suggest the rocks were metamorphosed under green schist and epidote-amphibolite facies. Similar exotic rock fragments were identified by Polish scientists in the south-eastern part of the Polish Carpathians in the Silesian Nappe, where possible source area of the exotic rocks is related to the Bukowiec Paleo-Ridge, which had been located between the Dukla and Silesian basins of the Carpathian sedimentary realm. Scientific novelty. For the first time, the petrographic composition and textural and structural features of the exotic schists fragments included in the Oligocene deposits of the Silesian Nappe in the Ukrainian Carpathians (area of the village of Uzhok, Ukrainian Carpathians) are described. These schists are similar to the exotic rocks included in the Oligocene deposits of the Silesian Nappe in the Polish Carpathians, which may indicate the same source area.

An Oligocene olistostrome with exotic clasts in the Silesian Nappe (Outer Ukrainian Carpathians, Uzh River Basin)

In the Ukrainian part of the Silesian Nappe (Outer Carpathians, Uzh River Basin) the exotic clast-bearing Uzhok Olistostrome (up to 60 m thick) occurs within the Oligocene Krosno Formation and underlies the Pikui Sandstone (Otryt Sandstone in Poland). The Uzhok Olistostrome consists of debris/grain/mud flow deposits with clasts of schist and bioclastic limestone. These deposits contain redeposited pelagic sediments with planktonic foraminifers including Parogloborotalia pseudocontinuosa (Jenkins), Ciperoella ciperoensis (Bolli), Globoturborotalita woodi (Jenkins), Chiloguembelina adriatica Premec Fucek, Hernitz Kucenjak and Huber. The age of the Uzhok Olistostrome based on planktonic foraminifers correlates with the middle Oligocene within the middle O2–O5 zones. The source area for the Uzhok Olistostrome and Pikui Sandstone was a mid-Oligocene intrabasinal palaeouplift (the Pikui Ridge) interpreted as the fore-bulge located in the Silesian Sub-basin ahead the emerging Outer Carpathian accretionary prism (including the Dukla Nappe and other West Carpathian inner flysch nappes).

Electrical Conductance Map for the Kachchh Rift Basin: Constraint on Tectonic Evolution and Seismotectonic Implications

Geomagnetic field variations recorded by an array of magnetometers spread across the Kachchh Rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations. A non-uniform thin-sheet electrical conductance model is developed to account for the salient induction patterns. It indicates that the imaged conductivity anomalies can be related to the sediment-filled structural lows in between the fault bounded uplifts. It is suggested that sagging structural lows preserved the marine sediments deposited during the Mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with Late Eocene transgression. Depth integrated electrical conductance helped in mapping two depo-centres: along the ENE-WSW trending Banni half-Graben bounded by the Kachchh Main fault on the south and, second, along the Vinjan depression formed in response to the subsidence between the Vigodi fault and westward extension of the Katrol Hill fault together with the westward bending of the Median High. Presence of metamorphosed graphite schist clasts in shale dominated Mesozoic sequence and/or thin films of carbon resulting from the thermal influence of Deccan activity on Carbonate-rich formations can account for the high electrical conductivity anomalies seen in the depo-centres of thick Mesozoic and Tertiary sediments. Additionally two high conductivity zones are imaged encompassing a block defined by the 2001 Bhuj earthquake and its aftershocks. In agreement with gravity, magnetic and seismic velocity signatures, aqueous fluids released by recrystallizing magmatic bodies intruded in association with Deccan trap activity account for mapped high conductivity zones. High fluid pressure in such a fractured domain, surrounding the intruded magmatic plugs, perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 Bhuj earthquake.

Isotopic ages of the schist clasts from the Kuma Group and Hiwadatoge Formation in Shikoku

Isotopic ages of the schist clasts from the Kuma Group and Hiwadatoge Formation in Shikoku

Lithostratigraphy of gold-bearing Quaternary gravels, middle Manuherikia Valley, Central Otago, New Zealand

The oldest fluvial Quaternary gravels exposed in the middle Manuherikia Valley in the Central Otago goldfield consist predominantly of rounded and angular quartz, with subordinate schist clasts. These sediments occur in a mappable middle Quaternary unit, herein defined as the Tinkers Formation (new name) that is well exposed in historic gold mine excavations. The unit contains placer gold and has been locally mined. The proportion of schist clasts increases up-section in the formation, to make up at least 30% of the gravels. Gravel beds in the formation are interlayered with loess horizons and minor mudstone (1–4 m scale). The Tinkers Formation is unconformably overlain by coarse poorly rounded immature schist gravel that contains dispersed gold but has not been mined. The gravel units have been folded and faulted along the actively deforming Dunstan Range front, from which the gravels have been derived. Ongoing uplift and recycling of sediments and contained gold has yielded younger gold-bearing gravel units. The gravel sequence in the middle Manuherikia Valley is distinctly different from elsewhere in the Manuherikia Valley, where recycled greywacke clasts form a persistent and locally major component of Quaternary sediments. Lithostratigraphic definition and mapping of Quaternary gravels has some advantages over correlation of Quaternary surfaces, which has been the most commonly used approach to Quaternary mapping in the Central Otago goldfield, and the two approaches are complementary.

K-Ar ages of the schist clasts from the Hiwada-toge Formation in the Kumakogen area, Shikoku

K-Ar ages of the schist clasts from the Hiwada-toge Formation in the Kumakogen area, Shikoku

Tectonics of an Early Carboniferous forearc inferred from a high‐P/T schist‐bearing conglomerate in the Nedamo Terrane, Northeast Japan

AbstractThe Nedamo Terrane, an Early Carboniferous accretionary complex, is the oldest biostratigraphically dated accretionary complex in Japan. The purpose of this study is to describe and interpret a conglomerate from the Nedamo Terrane that contains clasts of high‐pressure/low‐temperature (high‐P/T) schist (mainly garnet‐bearing phengite schist) and ultramafic rock, and to infer the tectonics of an Early Carboniferous arc–trench system at the eastern margin of the paleo‐Asian continent. Clasts of high‐P/T schist and ultramafic rock within the conglomerate make up 8.4 and 6.7% of the total clasts, respectively, based on modal counts. These clasts are subangular to subrounded, whereas volcanic clasts are well rounded. The source of the schist clasts, which yield a radiometric age of 347–317 Ma, is considered to be the Renge Metamorphic Rocks of Southwest Japan or equivalent rocks. Based on the chemical composition of chromian spinel, the source of ultramafic clasts is inferred to be the island‐arc‐type Ordovician Miyamori and Hayachine ultramafic complexes in the Kitakami Massif. The conglomerate records multiple provenance regions, including an island arc (South Kitakami Terrane) and a forearc ridge; the high P/T schist and ultramafic rocks were exhumed in the forearc region. The duration of the interval from the early stages of exhumation of the schist to its deposition in the trench as clasts is estimated to have been less than 30 my.

K–Ar and 40Ar/39Ar phengite ages of Sanbagawa schist clasts from the Kuma Group, central Shikoku, southwest Japan

AbstractConglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and 40Ar/39Ar dating of phengite show all the pelitic schist clasts from low‐ to high‐grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. 40A/39Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.

Torlesse greywacke and Haast Schist source for Pliocene conglomerates near Reefton, New Zealand

Late Miocene‐Quaternary conglomerates in the Inangahua valley record provenance changes which can be related to tectonic activity in the northern South Island. Conglomerate clasts within the early Miocene Rotokohu Coal Measures comprise mainly metamorphosed quartzose greywacke with subordinate granitoids, consistent with an essentially local clastic source from west of the Alpine Fault. In contrast, conglomerates of the late Pliocene Old Man Group (Larry Schist and Cronadun Conglomerates) are dominated by Rakaia Terrane (Older Torlesse) greywacke and Haast Schist clasts from east of the Alpine Fault, which is c. 50 km southeast of the study area. A complete range of textural and metamorphic grades is present, confirming that oligoclase zone Alpine Schist was being exhumed during Pliocene time. The 50–70°‐dipping Pliocene beds are unconformably overlain by subhorizontally dipping late Quaternary gravels (Tophouse and Speargrass Formations), which, like the Miocene conglomerates, are composed of quartzose greywacke and granitoid rock types. Significant Quaternary deformation isolated the Inangahua valley area from the Southern Alps and resulted in a return to a local clastic provenance.

Taking the roof off a suture zone: basin setting and provenance of conglomerates in the ORS Dingle Basin of SW Ireland

Abstract The Late Silurian to Mid-Devonian Dingle Basin occupies a central position within the Iapetus Suture Zone of SW Ireland. The basin is believed to have formed from late Silurian times onwards as a product of sinistral transpression along several major faults within this suture zone. Conglomeratic sediments were deposited by moderately large gravelly fans shed into the basin from the NW (Glashabeg Formation) and the SE (Trabeg Conglomerate Formation). The systems fed into a large apparently through-going sandy, axial river (Slea Head Formation) that flowed towards the NE. The lateral, basin-margin systems were sourced from two disparate source terranes. To the NW lay a basic volcanic hinterland with some intermediate volcanic rocks and limestones, mudstones, sandstones and chert. During the Early Devonian time the SE drainage basin was underlain by sandstones, quartzites, phyllites and limestones probably intruded by a granite. Some of these lithologies can be found in outcrop in the pre-Dingle Group of the peninsula. Others need to be correlated with rocks of the oceanic terranes in the northern part of the suture zone. The southerly derived clasts have corollaries in the rocks of the Avalonian Leinster Terrane south of the suture. Following partial inversion of the Dingle Basin, the southerly hinterland was apparently further unroofed during mid-Devonian time, when the Inch Conglomerate Formation was deposited by alluvial fans shed northwards from a source area formed along the Dingle Bay Lineament. The Inch conglomerates are characterized by distinctive clasts of schist, gneiss, mylonite, tourmalinite and granite. The general picture of the Early Devonian deformation, intrusion by granite and unroofing of terranes currently partially exposed in central and southern Ireland within the Iapetus Suture Zone is largely consistent with clast lithotypes. However, some exploratory isotopic data indicate at least two possible vagaries in the interpretation. First, model ND T DM ages of Trabeg sedimentary clasts yield several results older than typical southern Iapetean or Avalonian crustal material. This suggests a complex history of sedimentary mixing of material across the developing Iapetus Ocean. Second, two of three Rb-Sr muscovite-whole-rock dates of Inch metamorphic clasts indicate Silurian ages. These data similar to Rb-Sr dates derived from the Carnsore and Saltees granites in the Rosslare Terrane, perhaps extending the geographical spread of this Silurian deformation.

Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

Abstract Clast compositions of Waiho‐1 conglomerate horizons record dextral movement of the borehole site past different basement terranes on the opposite side of the Alpine Fault. Metavolcanic and volcanogenic metagreywacke clasts in sediments aged c. 4.5 Ma indicate deposition adjacent to exposures of Caples, Dun Mountain—Maitai, or Brook Street basement terranes. Sediments aged c. 4 Ma contain Torlesse Terrane and Haast Schist clasts. Relocation of the Waiho‐1 borehole site using published finite rotations, combined with the observed change in provenance at c. 4.5 Ma, requires that the source rocks had relationships similar to those observed at the present day, that is, that most curvature of basement terranes in southern South Island, New Zealand, was already in place at 4.5 Ma. Major increases in sedimentation rate recorded by Waiho‐1 borehole at c. 12 and 5 Ma are linked to changes in the Australia‐Pacific plate vector. Subsidence before 12 Ma may have been related to thermal subsidence of the Emerald Basin, or/and middle Miocene foreland basin development. Clast compositions of middle—late Miocene (c. 12–5 Ma) sediments record reverse movement on the South Westland Fault Zone and uplift southeast of the Alpine Fault in areas where meagreywacke basement rocks were exposed. Fiordland was low lying and contributing little sediment to the basin in middle—late Miocene time. Large volumes of Pliocene—Quaternary (c. 5–0 Ma) sediment derived from southeast of the Alpine Fault reflect widespread uplift of the Southern Alps. Schist clasts in an early Pliocene sample indicate that amphibolite facies Alpine Schist was exposed by c. 4 Ma.

40Ar/ 39Ar mineral ages within metamorphic clasts from the Kuma Group (Eocene), central Shikoku, Japan: Implications for tectonic development of the Sambagawa accretionary prism

The Sambagawa metamorphic belt exposed in central Shikoku records a high- P/T metamorphism. It is comprised by the Oboke nappe and the structurally overlying, internally imbricated Besshi nappe complex. The Besshi nappe complex locally is unconformably overlain by the Kuma Group (Eocene) which consists of conglomerates and subordinate sandstones and mudstones. Clasts within Kuma conglomerates exposed in the Kamegamori district include low-grade schists (derived from the unconformably underlying Sambagawa complex) and high-grade metamorphic rocks of uncertain provenance (including garnet-amphibolite and oligoclase-bearing pelitic schist). 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope correlation ages recorded by hornblende from two amphibolite clasts are 131.1 ± 4.9 Ma and 156.8 ± 4.3 Ma. These are interpreted to date post-metamorphic cooling through temperatures required for intracrystalline retention of argon, and are older than cooling ages previously reported from any presently exposed segments of the Sambagawa terrane. Plateau ages of 108.8 ± 0.7 Ma and 115.7 ± 0.6 Ma are recorded by muscovite from two clasts of schist. Muscovite within a proximal basement exposure of unconformably underlying Sambagawa pelitic schist records a plateau age of 78.7 ± 0.5 Ma. Combined with isotopic ages previously reported for the Sambagawa terrane, 40Ar/ 39Ar data from clasts in the Kuma Group suggest that metamorphic culmination within a high-grade source terrane occured at c. 185-145 Ma. Sectors of this complex cooled through c. 500°C at c. 150 Ma. Other portions cooled through c. 500°C at c. 130 Ma. Contrasts in the cooling ages likely reflect internal imbrication within the source terrane. The Besshi nappe complex reached peak metamorphic conditions at c. 100-90 Ma and experienced relatively rapid uplift and cooling at c. 85-75 Ma. The Besshi nappe complex was structurally emplaced onto the Oboke nappe which attained peak metamorphic conditions at c. 75 Ma. Subsequently, the entire Sambagawa section was exhumed, and most high-grade metamorphic rocks were removed. The Eocene Kuma Group was unconformably deposited over the Sambagawa terrane at c. 50 Ma.

Horizontal tectonics pre-dating upper gorge creek group sedimentation pilbara block, western australia

Abstract The greenstone belt stratigraphy of the c. 3500 Ma old Pilbara Block has been subdivided into the lower dominantly volcanic Warrawoona Group and the upper mainly-sedimentary Gorge Creek Group. Regional mapping indicated that these units evolved as a near continuous succession with only minor internal unconformities, and that both groups suffered their most intense tectonic imprint together as a result of granite diapirism. However, an upper Gorge Creek conglomerate (a Lalla Rookh correlate), with a basal angular unconformity, contains felsic schist clasts derived from highly strained portions of the Warrawoona Group. The folds in the conglomerate are upright as expected for diapirically generated structures and are as complex as any attributed to this process. The schist clasts were, therefore, probably generated in a pre-doming event. High-strain and high-grade metamorphism are confined to the lower parts of the greenstone stratigraphy where a polyphase deformation history may be clearly defined. The low-strain, low-grade areas are generally regarded as having a simpler tectonic history but some examples contain very tight minor folds that pre-date a widespread subhorizontal cleavage, and both events pre-date the doming. In other low-grade areas thrust structures are truncated by the angular unconformity at the base of the Lalla Rookh correlate and the thrust geometry suggests they formed in already folded rocks.

Depositional Characteristics of Middle to Upper Miocene Point Fermin Submarine Fan, Palos Verdes, California: ABSTRACT

The Point Fermin submarine fan lies within the upper Monterey Formation of the Palos Verdes Peninsula, California. Exposures of sediment gravity flow deposits extend from the upper portion of the tuffaceous lithofacies of the Altamira shale member to the middle of the Valmonte Diatomite. Outstanding sea-cliff exposures display a three-dimensional view of a large (approximately 1000 ft across) backfilled channel, scoured into thin-bedded shale and sandstone deposits. Channel backfill deposits primarily consist of Catalina schist-bearing sandstone and breccia, and lenses of disorganized intraformational breccia and conglomerate. The exposures exhibit an overall thinning and fining-upward sequence. The coarse-grained basal portion of the sequence displays sedimentary structures and bedding characteristics which are indicative of mass deposition by extremely concentrated, viscous dispersions. Channelized breccia beds contain schist clasts up to 9 ft in diameter, and intraformation breccia lenses contain rip-up clasts of Monterey Shale up to 30 ft long. Inferred flow mechanisms are a combination of grain flow, high-density turbulent flow, and sandy debris flow. The coarseness of these deposits is indicative of a proximal source. Higher in the sequence, sedimentary structures in upper portions of individual beds are characteristic of low-density turbulent flows. This upward change in sedimentary structures within individual bedsmore » is indicative of the passage of a high-density gravity flow and the subsequent waning, low-density turbulent flow conditions which follow. Directional features indicate a mean southeasterly flow direction. These results (which contradict data from previous published reports), in conjunction with the overall coarseness of these deposits, suggest the source to be the Palos Verde uplift.« less

Formation of late Caledonian subvolcanic breccia pipes at Cruachan Cruinn, Grampian Highlands, Scotland

ABSTRACTPipe-shaped breccia bodies associated with diorite intrusions are composed mainly of angular clasts of local schists with a few transported clasts of quartzite. Plate shaped fragments are commonly oriented to define planar fabrics in the breccias. These features indicate the operation of gas fluidisation within the pipes and both entrainment and expanded bed conditions are inferred. The fabrics result from the collapse of the fluidised suspensions as the gas flow declined. Dilational fracture patterns in the country rock comparable with the stress release patterns found around mine shafts can be matched with the fractures required to produce the angular schist clasts. It is concluded that fracturing and the introduction of fragments into the fluidised breccia system was a continuous process and that the pipe diameter increased progressively with time. Microdiorite sheets and related stock like bodies of diorite cut and metamorphose the breccias. Compaction, hornfelsing and hydrothermal alteration also contributed to breccia formation.

Late Mesozoic Sedimentation and Orogenesis along Southwestern Oregon Coast: ABSTRACT

Mapping, petrology, and paleocurrent investigations by the speaker and students provide new evidence of sedimentation in a continuously active orogenic belt in southwestern Oregon. Formational units have been traced into California, demonstrating that at least the northern part of the Franciscan terrane is divisible. Intensely sheared, northerly trending zones up to 3 miles wide have been traced from California for 100 miles north along the Oregon coast. They pass offshore beneath the seismically active northeast Pacific Basin. Paleogeology shows that these were superimposed upon arcuate Klamath Province fold trends from late Mesozoic to the present. Extrusion of Jurassic rhyolitic to basaltic marine pillow lava and pyroclastics accompanied deposition of graded and non-graded arkosic wacke, mudstone and bedded chert (Dothan and Rogue? Formations). Associated local conglomerates contain granitic boulders, evidencing hitherto unrecognized pre-Nevadan granites. Laminated quartz arenite and argillite intruded by diorite contain Late Jurassic? Foraminifera (Galice Formation). Over large areas these units all have been metamorphosed to form the Colebrook Schist. Thus Late Jurassic orogeny is recorded by deformation, metamorphism, and ultramafic and dioritic plutons. Latest Jurassic and Early Cretaceous rocks contain excellently graded volcanic wackes, mudstone, and thick chert-volcanic conglomerates (Myrtle Group). Diorite, serpentine and schist clasts together with overlap relationships provide sensitive indicators of diastrophic pulses. Volcanism ceased during Cretaceous. Later orogeny is recorded by severe deformation (but lack of metamorphism) of the Myrtle Group, emplacement of batholiths inland in the eastern Klamath Province, and unconformable overlap by shallow marine Late Cretaceous (Campanian) quartz arenites, quartz wackes, and mudstone. Major shear zones were developed at least by mid-Cretaceous and have been reactivated intermittently since. Post-Cretaceous warping and erosion recurred prior to widespread Eocene transgression, renewed volcanism, and deposition of graded volcanic wackes cross-stratified volcanic arenites, and coal. Regression commenced in Late Eocene. Later Cenozoic movements along the major faults apparently formed the north-trending Coos synclinorium and were related to formation of north-trending Cascade volcanoes and plut ns and to widespread faulting in eastern Oregon and Nevada. End_of_Article - Last_Page 264------------