Plutonic Detritus Research Articles

The Late Cretaceous metamorphic rocks of the Akrotiri and Vari subunits on Tinos and Syros, Cyclades, Greece: field observations, geochemistry and geochronology

Abstract The relationship of the Late Cretaceous amphibolite-facies Akrotiri and Vari subunits on the Greek islands of Tinos and Syros to similar occurrences in the Aegean is not fully understood, but a correlation with the Asterousia Crystalline Complex on Crete and corresponding rocks in the Upper Cycladic Unit on other islands of the archipelago is a plausible interpretation. There is currently no clear evidence that the Akrotiri subunit represents a fragment of the metamorphic sole of the nearby Tsiknias ophiolite, as there are differences in field appearance, geochemistry, metamorphic grade and retrograde overprint. Felsic rocks from amphibolite-gneiss sequences of the Akrotiri and Vari subunits are interpreted as reworked igneous rocks with minor admixture of terrigenous material. The Triassic U–Pb zircon age (c. 240 Ma) of such a gneiss from Syros indicates the magmatic crystallization age of the original source rocks. A similar interpretation is suggested for the studied Akrotiri gneiss, but is less clear due to the not fully clarified significance of the presumed Pb loss thought to be responsible for the considerable U–Pb age range of the dated zircons (c. 256–114 Ma). The Vari gneiss sensu strictu is closely associated with metadioritic rocks containing a Triassic zircon population (c. 238 Ma). The protolith is probably plutonic, but a mixture of volcanic and plutonic detritus cannot be excluded. Riebeckitic amphiboles occur in all rock types of the Akrotiri subunit, indicating late overprinting at elevated pressures, which is not known from similar occurrences in the southern Aegean.

Submarine slope–fan sedimentation in an ancient forearc related to contemporaneous magmatism: The Upper Cretaceous Izumi Group, southwestern Japan

AbstractThe Izumi Group in southwestern Japan is considered to represent deposits in a forearc basin along an active volcanic arc during the late Late Cretaceous. The group consists mainly of felsic volcanic and plutonic detritus, and overlies a Lower to Upper Cretaceous plutono‐metamorphic complex (the Ryoke complex). In order to reconstruct the depositional environments and constrain the age of deposition, sedimentary facies and U–Pb dating of zircon grains in tuff were studied for a drilled core obtained from the basal part of the Izumi Group. On the basis of the lithofacies associations, the core was subdivided into six units from base to top, as follows: mudstone‐dominated unit nonconformably deposited on the Ryoke granodiorite; tuffaceous mudstone‐dominated unit; tuff unit; tuffaceous sandstone–mudstone unit; sandstone–mudstone unit; and sandstone‐dominated unit. This succession suggests that the depositional system changed from non‐volcanic muddy slope or basin floor, to volcaniclastic sandy submarine fan. Based on a review of published radiometric age data of the surrounding region of the Ryoke complex and the Sanyo Belt which was an active volcanic front during deposition of the Izumi Group, the U–Pb age (82.7 ±0.5 Ma) of zircon grains in the tuff unit corresponds to those of felsic volcanic and pyroclastic rocks in the Sanyo Belt.

Proto-Pacific-margin source for the Ordovician turbidite submarine fan, Lachlan Orogen, southeast Australia: Geochemical constraints

The Early Palaeozoic proto-Pacific Pacific margin of Gondwana was characterised by a huge turbidite submarine fan with abundant clastic detritus derived from unknown sources within Gondwana. These deposits are widespread in the Lachlan Orogen of southeast Australia and include the Ordovician Adaminaby Group. Here we show that the mudstones and sandstones of the Adaminaby Group have chemical compositions that indicate the detritus in them was derived from a felsic, continental source similar in composition to Post Archean Australian Shales (PAAS). Chondrite normalised REE patterns showing LREE enrichment, flat PAAS normalised patterns and elemental ratios La/Sc, Cr/Th, Cr/V, Th/Sc and Th/U, have been used to support this interpretation. The dominance of quartz, and to a lesser degree plagioclase and biotite in the sandstones, suggests that the source was mainly granodioritic to tonalitic in composition. Th/Yb and Ta/Yb ratios indicate that the source was probably calc-alkaline, continental and shoshonitic. In addition, the presence of detrital muscovite, low-grade metamorphic and felsic volcanic clasts, demonstrates that a low-grade metamorphic terrane and volcanic arc contributed to the detritus observed in the samples. The presence of well-rounded zircons and tourmalines, very high Zr contents, high Zr/Sc and higher Cr/V ratios in some samples particularly in the Shoalhaven River area, indicate that some of the detritus was recycled.SiO2 versus (Al2O3+K2O+Na2O) plots suggest the source areas experienced conditions varying from humid/semi-humid to semi-arid. Textural features and weathering trends of samples from all locations follow a curved pathway on Al2O3 – (CaO*+Na2O) – K2O (ACNK) diagrams, and indicate that the clays formed from weathering had been K-metasomatised prior to penetrative deformation. Chemical indices of alteration (CIA) reveal that even the freshest sandstones are altered and others are moderately to strongly altered.Discrimination diagrams involving major, trace and REE strongly support a collisional/continental volcanic arc setting that was substantially eroded to produce the plutonic detritus observed in the sandstones. The collisional setting accords with that proposed previously by other authors who suggested that it developed during the Delamerian Orogeny, resulting in the uplifted source areas providing detritus that inundated the backarc and forearc sites of the Macquarie Arc. Some of the detritus, however, may have been derived from a continental arc that existed in the late Cambrian along the margin of the Ross Orogen. Based on palaeocurrent analyses in previous studies and shoshonitic signature of the detritus, it is proposed that the Cambrian volcanics along the eastern active margin of Gondwana provided much of the detritus in the Adaminaby Group. Zircons with the Grenvillian signature suggest that some detritus were also derived from the Ross Orogen.

Mineralogy, geochemistry and possible provenance of desert sand dunes from western Rub' al Khali area, southeastern Saudi Arabia

The Rub' al Khali desert dune sands are major constitutes of the southern part of the Arabian Peninsula. The investigated dunes in the present work are located in its western extreme where they display three major forms (seif, sigmoidal and barchan) with distinct variations in size and unique homogeneity of mineral composition. The Rub' al Khali sand dunes in its western part are placed in the craton interior and based on the contents of Quartz-FeldsparsLithic fragments, they are originating from a recycled orogen in which intrusive, sedimentary and partly metamorphosed sources have contributed to the composition of the sands. The presence of minerals like clay minerals and amphiboles in the bulk composition of the dune sands supports the influence of granitic, granodioritic and schistose sources. They are all fine sand, moderately well sorted, nearly symmetrical to fine skewed and mostly mesokurtic. The heavy mineral fractions are dominated by a variety of amphiboles (winchite-riebeckite-richterite-arfvedsonite) in addition to cordierite. The geochemistry of the Rub' al Khali dune sands show that the dune sands are associated with acid, felsic plutonic detritus linked to an active continental margin. Correlations between Fe2O3, SiO2, Al2O3, TiO2, MgO, and other oxides in the dune sands indicate the presence of ilmenite, magnetite and zircon in the samples. With respect to the provenance, there are two possibilities which are the Zagros Mountains in Iran and the Sahara Desert in North Africa. Orientation of linear dunes in the Rub' al Khali area is controlled by the dominant shamal (north) wind.

Stratigrafia fisica della successione sedimentaria miocenica del settore Nord-orientale della Stretta di Catanzaro (Calabria centro-orientale)

Physical Stratigraphy of the upper Miocene sedimentary succession in the northeastern Catanzaro Through (Central Calabria, Italy).In the northeastern sector of the Catanzaro Trough, a terrigenous, evaporitic and carbonate mio-pliocenic sedimentary succession overlies the igneous-metamorphic basement of the Sila Unit. These deposits are strongly controlled by NW- strike-slip system, arranged in a right-hand en echelon pattern (Tansi et alii, 2007). Coarse-grained fluvial deposits of the Corace conglomerate Unit represent the onset of the Serravallian – Tortonian sedimentation. Overlying shallow marine arenites and calcarenites, including Porites bodies, pass upward to offshore marly-clays deposits of the Gagliano Unit, evolving upsection to diatomite and organic-rich laminites (Tripoli Fm.) at the Tortonian/Messinian boundary. Evaporitic conditions characterized the sedimentation within the Mediterranean basin during the Messinian as demonstrated by the wide distribution of selenite, micrite and halite deposits.In the northeastern sector of the Catanzaro Trough such conditions are recorded by fi ne grained carbonate deposits, of assumed bacterial origin (Guido et alii, 2007), and can be correlated to CdB type 2 by Manzi et alii (2010), developed during the fi rst MSC stage, from 5,96 Ma to 5,6 Ma. Chaotic deposits of selenitic blocks, gypsrudites and gypsarenites (resedimented evaporites; sub-unit 1 of Riato Conglomerate) and limestone breccias (CdB-2) overlie the Tripoli Fm. along a paraconcordance surface and characterize the depocentral area. The chaotic deposits pass upward to sub-unit 2 of Riato Conglomerate and Arieste Fm characterized by channels and bars facies. They show a fi ning and thinning upward trend represented by a drastic decrease of the intrabasinal evaporitic and carbonate alimentation in favour of metamorphic and plutonic detritus supplied by the Calabrian-Peloritan Arc units.The development of a coarse grained fl uvial-deltaic depositional system characterizes the MSC fi nal stage. Towards the depocentre continental sediments pass to clayey-sandy deposits, containing biofacies with ostracods (Lago Mare) and selenitic bodies with banded and massive facies, referred to Upper Evaporites (Costa, pers. Comm.) The sedimentary succession exposed in the northeastern sector of the Catanzaro Through defi nes a depositional architecture that may be included in the evolution model proposed by Roveri et alii (2008). In this contest the chaotic and terrigenous-evaporitic-carbonate deposits (sub-units 1-2 of the Riato Conglomerate, CdB-2 and Arieste Fm.), originated by the cannibalism of the selenite and carbonates deposits probably between 5.6 and 5.55 Ma, record a gradual return to a extrabasinal alimentation originated by the erosion of the Serravallian and Tortonian sedimentary units and crystalline rocks of the Sila Unit.

Productos de erosión de las rocas intrusivas y plagio- granitos del complejo ofiolítico de Nicoya, Costa Rica

En este estudio se describe por primera vez la existencia de detrito plutónico en areniscas cretácicasdel noroeste de Costa Rica. Los fragmentos de rocas plutónicas son partículas accesorias, comúnmente presentes enlas areniscas piroxénicas que sobreyacen al Complejo Ofiolítico de Nicoya. Con base en el análisis modal de los granos del esqueleto, se analizan tres unidades de areniscas turbidíticas y neríticas de las Formaciones Rivas y El Viejo, respectivamente, cuya edad data del Campaniano Tardío-Maastrichtiano. Considerando los componentes prima-rios, estas areniscas muestran una composición muy similar a aquellas provenientes de arcos magmáticos. Se introducen dos parámetros modales para evaluar las contribuciones plutónicas. Éstos se definen como el índice de granos plutónicos con respecto al total de líticos (Lp+intercrec.QF/Lt) y el índice de piroxeno uralitizado con respectoal piroxeno total (uralPx/Px). Los valores modales del índice Lp+intercrec.QF/Lt indican, que los fragmentos plutó-nicos constituyen hasta un 9% del total de líticos. La relación relativamente proporcional entre ambos parámetrossugiere, que el piroxeno uralitizado también se deriva de la erosión de las rocas plutónicas. Las rocas-fuente de de-tríto plutónico corresponden con rocas intrusivas básicas hasta intermedias: gabros, doleritas y dioritas. Dos compo-nentes en particular - los intercrecimientos micrográficos de cuarzo y albita y el piroxeno uralitizado - son considerados como productos específicos de la erosión cretácica de las rocas intrusivas, principalmente plagiograníticas. Lasunidades de areniscas con detríto plutónico registran una erosión incipiente de los niveles inferiores de la secuenciaofiolítica de Costa Rica durante el Campaniano Tardío y Maastrichtiano. En este sentido, los fragmentos plutónicosy los granos de pedernales radiolaríticos asociados pueden ser usados como indicadores paleotectónicos del levan-tamiento vertical del orógeno de Costa Rica en el Cretácico Tardío. Este evento tectónico se correlaciona con el inicio de la Orogénesis Laramídica. Observaciones de campo evidencian que las rocas intrusivas del Complejo de Nicoya aún hoy día son una fuente miscelánea de sedimento en el noroeste de Costa Rica.

Stratigraphy and Detrital Modes of Upper Messinian Post-evaporitic Sandstones of the Southern Apennines, Italy: Evidence of Foreland-Basin Evolution during the Messinian Mediterranean Salinity Crisis

During the Messinian, the southern Apennines thrust belt experienced a period of strong tectonic rearrangement and accretion, activation of overthrusts, and consequent migration of depocenters. The upper Miocene successions cropping out in the northern segment of the southern Apennine thrust belt have good potential for improving our understanding of the interplay between Messinian salinity-crisis events and foreland-basin evolution. The local Messinian stratigraphy includes: (1) pre-evaporitic thin-bedded euxinic marly clay, interbedded with diatomaceous marls; (2) evaporitic limestone, crystalline gypsum, and reworked gypsum; (3) post-evaporitic deposits subdivided into two main units: the Torrente Fiumarella unit and the Anzano Molasse Formation that grade upward into ostracod-rich deposits (Lago-Mare facies). The evaporitic and post-evaporitic sequences are separated by an angular unconformity. This paper deals with the stratigraphic and petrographic study of the post-evaporitic deposits. The Torrente Fiumarella unit includes lacustrine and alluvial conglomerates, quartzolithic sandstones containing abundant carbonate detritus, shale, and reworked clastic gypsum. The Anzano Molasse Formation includes thick-bedded deltaic to turbiditic conglomerates and sandstones passing upward to thin-bedded turbidite sandstones and marlyclayey siltstones. Sandstones are quartzofeldspathic with variable proportions of sedimentary (both carbonate and siliciclastic) and plutonic detritus. In particular, two populations are present, plutonic-rich and mixed plutonic-sedimentary. Volcaniclastic layers, composed of dominantly vitric particles (shards and pumice), are also interbedded within Anzano Molasse sandstones. The Anzano succession includes rare freshwater ostracods that increase in abundance in the uppermost Lago-Mare facies. The Lago-Mare facies deposits are represented by silty-marly clay with abundant Ostracoda shells (Ilyocypris gibba, Cyprideis torosa and Candona sp.) and intrarenite having abundant intrabasinal carbonate particles (ooids, peloids, and bioclasts) and subordinate extrabasinal noncarbonate and carbonate particles. The post-evaporitic sequences represent an infilled foredeep basin, with a lacustrine environment progressively deepening and experiencing gravity resedimentation. Detrital modes document complex provenance relations from upper Messinian accreted terranes of the southern Apennines thrust belt. Post-evaporitic sandstones in the Irpinia-Daunia sector of the southern Apennines foreland-basin system record both the effects of the foreland tectonic evolution and the Messinian Mediterranean salinity crisis. They may represent alternative models for foreland-basin evolution during a restricted time in late Messinian, which can be applicable also in other portions of the circum-Mediterranean orogen.

Provenance of the Murihiku Terrane, New Zealand: evidence from the Jurassic conglomerates and sandstones in Southland

Sedimentary petrological, geochemical, and geochronological studies were carried out to reveal the provenance of the Murihiku Terrane, Southland, New Zealand. The Murihiku Terrane comprises Late Permian to Early Cretaceous volcaniclastic sediments that were deposited at the fore-arc or back-arc basin of the eastern Gondwana margin. The strata in the Waikawa district, Southland, are Middle or early Late Jurassic and comprise sandstones interbedded with conglomerates and mudstones. The sediments can be petrologically classified into two groups: one dominated by intermediate-mafic volcanic detritus (type-A), and the other more quartz bearing (15–20% in QFL), dominated by felsic volcanic and plutonic detritus (type-B). The chemistry of detrital clinopyroxene in type-A sediments is compatible with their derivation from island arc volcanics. Whole-rock major- and trace-element analyses suggest that granitoid and gabbroid clasts associated with type-B sediments are also of volcanic arc origin. Geochemical characteristics of the plutonic clasts can be compared closely with the Median Batholith (Median Tectonic Zone). Whole-rock K–Ar ages of the clasts (156±8 and 177±9 Ma for gabbroid clasts and 156±8 and 212±11 Ma for alkali feldspar granite clasts) are consistent with the Median Batholith magma cooling. Small quantities of grandite and pyrope-bearing garnet grains in type-B sediments might be derived from metamorphosed intermediate-mafic volcanic rocks to amphibolite facies. Granitic cataclasite and hornfels are also compatible with derivation from the Median Batholith that was accreted to the Brook Street Terrane.

Provenance of plutonic detritus in cover sandstones of Nicoya Complex, Costa Rica: Cretaceous unroofing history of a Mesozoic ophiolite sequence

This study presents new petrologic and sedimentologic data from northwestern Costa Rica concerning the provenance of Cretaceous forearc sandstones that contain plutonic detritus. Plutonic rock fragments are important accessory particles in pyroxene-bearing sandstones overlying the ophiolite named the Nicoya Complex. Through the use of modal analysis of the framework grains, I studied three sandstone suites of the El Viejo and Rivas Formations that include both shallow- and deep-water deposits, ranging from late Campanian to Maastrichtian in age. In terms of primary framework components, the sandstones resemble those derived from magmatic arcs. Two modal parameters are introduced to evaluate detrital plutonic contributions and affinity of source rocks: the ratio of plutonic to total lithic fragments [(Lp + iQF)/Lt], and the ratio of uralitized pyroxene to total pyroxene grains (uralPx/Px). Modal values for (Lp + iQF)/Lt indicate that plutonic fragments comprise up to 9% of total lithic fragments. A strong correlation between these two parameters suggests that uralitized pyroxene grains were also derived from intrusive rocks of probably basic and intermediate compositions. In particular, significant concentrations of lithic fragments exhibiting micrographic textures and uralitized pyroxene grains are interpreted to have been derived predominantly from eroded plagiogranites. Sandstone suites containing plutonic detritus signal an unroofing of deeper levels of the Mesozoic ophiolitic sequence as a consequence of strong uplift of the Costa Rican arc in late Senonian time. This tectonic event began in the Campanian at ca. 75 Ma, ∼9 m.y. after the intrusive magmatic activity on Nicoya Peninsula, and is consistent with the onset of the Laramide orogeny.

Nd-isotope study of provenance patterns across the British sector of the Iapetus Suture

AbstractThe Southern Uplands greywacke succession (Scotland) accumulated at the Laurentian margin of the Iapetus Ocean. It was sequentially incorporated into an imbricate, accretionary thrust complex until closure of the ocean. Thereafter the thrust belt propagated across the suture zone as a foreland thrust belt directed towards the hinterland of Avalonia. A foreland basin migrating ahead of the thrust belt was the depositional site for the southernmost Southern Uplands units and the Windermere Supergroup (English Lake District). A Nd-isotope study has shown that juvenile ophiolitic detritus was introduced into the oldest, mid-Ordovician, Southern Uplands greywackes before two distinct provenance areas evolved: one supplying juvenile andesitic detritus in addition to a quartzo-feldspathic component, the other Proterozoic and exclusively quartzo-feldspathic. Bimodal composition continued into the early Silurian but was overlapped from late in the Ordovician by greywackes with intermediate Nd-isotope composition. This was not a simple mixing effect since the andesitic component is not represented and the necessary juvenile component comes from granodioritic and felsitic lithologies. Intermediate eNd values are then a consistent feature through the Silurian both in the younger strata of the Southern Uplands and in the earliest foreland basin turbidites of the Windermere Supergroup. The transition suggests cessation of volcanicity and erosion of deeper levels of the provenance terrane(s), possibly linked to the evolution of the basin system from active margin, accretion-related, to a foreland setting. To the north of the Southern Uplands terrane, beyond the Southern Upland Fault, a Caradoc to Wenlock turbidite sequence occupies inliers within the Midland Valley. The older greywackes contain abundant juvenile ophiolite and plutonic detritus in addition to a quartzofeldspathic metamorphic component; there are similarities with the most northerly part of the Southern Uplands. From the late Ordovician, εNd values systematically decline so that early Llandovery Midland Valley greywackes are exclusively quartzo-feldspathic, derived from an ancient source indistinguishable in isotopic terms from that periodically supplying the Southern Uplands. In general the Llandovery Midland Valley provenance was significantly more mature than that contemporaneously supplying the Southern Uplands. Thereafter, the Midland Valley latest Llandovery and early Wenlock greywackes contain a higher proportion of a juvenile component, and by the early Wenlock, greywackes from the Midland Valley, Southern Uplands and Lake District terranes are similar in terms of εNd. A common provenance seems likely and suggests that by the mid-Silurian all three terrenes were in close proximity.

Petrofacies and Provenance of the Puente Formation (Middle to Upper Miocene), Los Angeles Basin, Southern California: Implications for Rapid Uplift and Accumulation Rates

ABSTRACT The Puente Formation is a Middle-Upper Miocene clastic unit lying unconformably on the Lower-Middle Miocene El Modeno Volcanics and Topanga Group, in the Los Angeles basin. The Puente Formation, about 3900 m thick, is composed of conglomerate, sandstone, and mudrock deposited as a submarine fan at bathyal depths. Several intrabasinal discordances suggest tectonic activity during deposition. The succession consists of two main upward-thickening and - coarsening megacycles, reflecting submarine-fan progradation. The Puente Formation is characterized up-section by: (1) thin-bedded sandstone and shale (La Vida Member) grading to thick-bedded sandstone and conglomerate (Soquel Member); (2) thin-bedded mudrock and sandstone (Yorba Member) grading to thick- to very thick bedded sandstone and conglomerate (Sycamore Canyon Member). Sandstones of the Puente Formation are quartzofeldspathic (Qm35F54Lt11); their compositions suggest local provenance from the plutonic, volcanic, and metamorphic rocks of the San Gabriel Mountains and surrounding areas. Petrologic parameters, however, suggest variable contribution of these source rocks through time. Four petrofacies, with distinctive parameters, coincide with the lithostratigraphic subdivisions. Coarse-grained plutonic rock fragments are abundant throughout the succession and consist of plagioclase-rich plutonic rocks, probably sourced, in part, from the Lowe Granodiorite. Lathwork, microlitic to felsitic volcanic lithic grains are also present in the lower and middle part. In the La Vida petrofacies, there is also an intrabasinal cont ibution (intraclasts and bioclasts) from shelfal areas. In the Yorba petrofacies there is a local increase of volcanic detritus (Lv/L = 0.81), represented by coarser volcanic lithics and abundant volcaniclastic matrix. Metamorphic detritus is not very abundant. The Sycamore Canyon petrofacies is dominantly plutoniclastic (Rg/R = 0.90; hornblende-bearing plutonic rock fragments), including very abundant hornblende grains. The plutonic detritus is dominantly plagioclase-orthoclase-biotite-bearing in the lower part, and hornblende-bearing in the upper part, suggesting unroofing of the Lowe Granodiorite Complex as a key element of uplift of the San Gabriel Mountains. Other Neogene sandstones deposited in the Los Angeles Basin also consist dominantly of plutoniclastic detritus related to the unroofing of arc-related plutonic rocks (dissected magmatic arc). For example, the up-section increase in plutonic detritus is consistent with the composition of the Upper Miocene-Lower Pliocene Capistrano Formation (Qm50F47Lt3), which has a composition identical to the Sycamore Canyon petrofacies. There is consistent provenance signal in spite of complex transrotational tectonics, responsible for opening of the Los Angeles Basin, and later transpressional processes, which are still active. Detailed provenance study of the Puente Formation and related units provides important constraints on paleogeographic and paleotectonic reco structions of southern California basins and uplifts.

Detrital record of the Gravina arc, southeastern Alaska: Petrology and provenance of Seymour Canal Formation sandstones

Sandstones of the Upper Jurassic to mid-Cretaceous Seymour Canal Formation (northern Gravina belt, southeastern Alaska) are volcaniclastic wackes. They exhibit a variably developed solution cleavage and zeolite- to low-greenschist-grade metamorphic assemblages. Point counts of 37 lithofeldspathic sandstones, spanning the unit9s stratigraphic range, yield framework modes typical of detritus shed from undissected arc sources (Q 4 F 40 L 56 , Qm 3 F 40 Lt 57 , Qm 8 P 91 K 1 , Qp 3 Lvm 91 Lsm 6 ) with only minor detritus characteristic of older terranes or continental-margin sources. Sandstones of the dominant thinly bedded turbidite lithology were derived almost exclusively from the contemporaneous Gravina arc. In contrast, sandstones of channelized conglomeratic facies are richer in quartz, sedimentary, metasedimentary, and plutonic detritus derived from basement terrane(s). The compositional heterogeneity and grain size of conglomeratic facies are consistent with derivation from local source areas, and we interpret them to reflect minor unroofing of the Alexander terrane and the superposed Gravina arc, mostly during Early Cretaceous time. A distinctive, although volumetrically minor, clinopyroxene-rich sandstone petrofacies signals a change in modal expression of source-area volcanism during the Cretaceous, and it probably records initial eruption of pyroxene-rich basalts of the mid-Cretaceous Douglas Island Volcanics. Supracrustal records preserved in the Nutzotin Mountains sequence, Dezadeash flysch, and Gravina sequence are also dominated by shallow-level arc detritus, although coeval supracrustal units on the Insular terrane of British Columbia reflect deeper incision of arc basement, possibly due to along-strike variation in arc geometries or tectonics of the Jura- Cretaceous Insular terrane.

Development of sedimentary basins in the Archean Abitibi belt, Canada: an overview

Sedimentation in the Archean Abitibi greenstone belt occurred during four depositional episodes: (i) sedimentary cycle 1, 2730–2720 Ma; (ii) sedimentary cycle 2, 2715–2705 Ma; (iii) sedimentary cycle 3, 2700–2687 Ma; and (iv) sedimentary cycle 4, 2685–2675 Ma. Records of the first two sedimentary cycles are preserved in basins within the northern volcanic zone, whereas basins formed during the latter two sedimentary cycles are located within the southern volcanic zone of the Abitibi belt. Sedimentary cycles 1 and 3 represent deep-water facies, as indicated by turbidites, resedimented conglomerates, pelagic sediments, and ubiquitous iron-formations; subaerial deposits have not been identified. In contrast, sedimentary cycles 2 and 4 show a prevalence of fluvial to shallow-water marine and (or) lacustrine deposits. Tectono-magmatic influence on sedimentation during cycles 2 and 4 is documented by (i) the presence of numerous unconformities underlain by plutonic and volcanic rocks; (ii) locally voluminous shoshonitic and calc-alkaline volcanic rocks; (iii) abundance of plutonic detritus; (iv) rapid vertical and lateral facies changes; and (v) repetition of successions of large-scale (50–250 m thick) alluvial and shallow-water deposits. Sedimentary cycle 1 represents incipient arc basins dominated by volcaniclastic debris, whereas cycle 2 reflects unroofing of arc volcanoes down to the plutonic roots. The sedimentary basins of cycle 3 have been tentatively interpreted as basins connecting arc terranes, within which small extensional cycle 4 basins of the successor or pull-apart type developed. The sedimentary facies associations, the tectono-magmatic influence on sedimentation, the chronological basin evolution, and overall southward younging of the basins invite comparison with modern island arcs formed by plate-tectonic processes.

Petrology and provenance of the Cretaceous Kodiak Formation, Alaska

Petrographic analysis of 30 sandstone samples from the Cretaceous Kodiak Formation indicates that they consist mainly of intermediate to basic volcanic and plutonic detritus derived from a source area east of Kodiak Island. This source area had extensive volcanic cover with rare exposure and weathering of cogenetic plutons. The petrology of the Kodiak Formation is similar to that of related rocks in the Alaskan Peninsula and is indicative of the longevity of the volcano-plutonic source area.

Detrital record of the early Central American magmatic arc: Petrography of intraoceanic forearc sandstones, Nicoya Peninsula, Costa Rica

The framework compositions of sand-stones from the Costa Rican forearc provide a compositional norm for detrital modes from basins associated with intraoceanic arcs, a poorly documented tectonic setting. Nicoya Peninsula sandstones are made up of mainly volcanic lithic fragments, plagioclase, sedimentary lithic fragments, and mafic accessory minerals. Modal point counts of 70 samples average Q 3 F 32 L 65 , Qm 2 F 32 Lt 66 , Qp 1 Lvm 72 Lsm 27 , and Qm 6 P 94 K 0 , defining these sand-stones as less quartzose than sandstones from most other tectonic settings and less lithic than most quartz-poor sandstones from intraplate oceanic islands. Four newly defined modal parameters quantify temporal changes in composition. These parameters are (1) Vi/V′, the ratio of volcanic fragments with microlitic or felsitic textures to those with lathwork, microlitic, or felsitic textures; (2) Mh/M, the ratio of hydrous to total mafic silicate mineral grains; (3) Po/P, the ratio of oscillatory-zoned to total plagioclase grains; and (4) Sc/S, the ratio of reworked carbonate detritus to total sedimentary lithic fragments. All four parameters increase from negligible values in the basement unit to moderate or high values in cover units. These increases are followed by generally low values in the youngest (Miocene) sandstones. Significant temporal changes are not revealed by previously defined modal parameters that are commonly used to analyze sandstone composition. Petrography of these sandstones provides strong evidence for the initial construction of an andesitic arc edifice by Campanian time (Late Cretaceous). Increases in Vi/V′, Mh/M, and Po/P ratios through most of the section record increased detrital input from evolved volcanic sources. A parallel increase in Sc/S ratios reflects the progressive construction and erosion of a carbonate-rich sedimentary are apron. The Miocene reversal of these trends reflects subaerial erosion of up-lifted basement rocks at the forearc structural high. Miocene sandstones also contain minor plutonic detritus that includes potassium feldspar, signaling incipient unroofing of the arc. Sandstones of the Nicoya Peninsula thus preserve a relatively complete record of the evolution and erosion of the Upper Cretaceous through Miocene intraoceanic arc of southern Central America.

Structure of some Lower Old Red Sandstone conglomerates, Kincardineshire, Scotland: deposition from late-orogenic antecedent streams?

A thick succession of early Lower Old Red Sandstone (ORS) conglomerates and subordinate sandstones are preserved in a small fault-bounded basin in the NE Midland Valley of Scotland. The conglomerates associated with the northern margin of this basin are distinguished by a complex provenance which involved both reworking of existing gravels and addition of contemporary volcanic and plutonic detritus. This may explain their characteristic bimodal texture. Deposition was in large, probably permanent, low sinuosity channels with evidence for both periods of uniform high stage flow and repeated flow stage fluctuations. Variable palaeoflows in the basal part of the succession suggest deposition on wet-type alluvial fans supplied by feeder channels with a high sediment and water discharge. The thickness of the succession, the poorly developed cyclothems (despite active syndepositional faulting), evidence for rapid aggradation, and the inferred scale of channel bars and coarsening-upward units produced by bar migration suggest the basin may have been externally drained. Transverse antecedent streams are invoked to account for the structure of alluvium. During the early Devonian, central Scotland lay close to the boundary between regions suffering Scandian orthogonal shortening and Acadian strike-slip shear. The tectonic controls which governed the preservation of early Lower ORS sediments may have been largely uncoupled from those determining the supply and dispersal of the sediment.

The indus clastics: forearc basin sedimentation in the Ladakh Himalaya (India)

The mid-Cretaceous to Early Tertiary clastic sediments of the Indus Group represent the forearc basin succession of the Transhimalayan arc-trench system. In the late Albian to early Eocene, a shallowing-upward megasequence (Tar turbidites to nummulitic limestones) and alluvial to deltaic sediments (Basgò and Temesgam Formations) were deposited respectively on the southern and northern flanks of the basin. Clastic petrography points to provenance from the largely undissected volcanic cover of the massif, with more important plutonic detritus recorded in the northern alluvial units. After collision with the Indian passive margin in the early Eocene, sedimentation in the whole basin was characterized by huge alluvial fans (Nurla Formation) and then by lacustrine deposits formed in an intermontane depression (Nimu Formation). A rapid change in petrographic and geochemical composition is recorded in post-early Eocene units, with sharp increase in plutonic detritus and trace elements typically associated with ophiolites, testifying to the uplift of both arc massif and subduction complex sources after collision with the Indian passive margin. The petrography of the Indus basin clastics and the evolution of detrital modes compare well with volcano-plutonic suites deposited in modern and ancient forearc basins. The composition of sandstone units contained within the accretionary prism instead suggests provenance from an undissected magmatic arc and deposition in abyssal plain and trench settings (Nindam volcaniclastic turbidites). Reworking of arc-derived sandstones took place in small basins perched on the accretionary trench slope. The Asian continental margin of the Neotethys Ocean was thus an active Pacific-type arc-trench system until the Eocene, when it collided with India, underwent polyphase fold-thrust deformation and was finally uplifted several kilometers to form the Indus suture zone of the Himalayas.

Mesozoic Sedimentary Rocks and Depositional Facies, Vizcaino-Cedros Area, Baja California, Mexico: ABSTRACT

Mesozoic sedimentary rocks in the Vizcaino-Cedros area constitute a 135-m.y. history of arc-related marine sedimentation with a cumulative thickness of 14 km. The Upper Triassic San Hipolito Formation is the oldest sedimentary unit in the region, and is recognized only on the Vizcaino Peninsula. The formation depositionally overlies an ophiolite sequence and consists of 2.4 km of tuffaceous sediment including a limestone megabreccia. The upper two-thirds of the sequence in the type section is now believed to be of Early Jurassic age. On Cedros Island, the basal sedimentary rocks are 1.2 km of the Lower Jurassic Gran Canon Formation. This richly tuffaceous unit depositionally overlies both ophiolite and arc volcanics. Conformably overlying the Gran Canon on Cedros Island is the Coloradito Formation, a spectacular sedimentary olistostrome up to 0.4 km thick, containing Triassic and late Paleozoic metasedimentary blocks. This formation record the first evidence of continental detritus in the region. Conformably overlying the Coloradito and, in part, stratigraphically equivalent to it on Cedros is 0.4 km of volcanogenic-metasedimentary conglomerate designated the Eugenia Formation. The Eugenia is much thicker (to 2.7 km) and widespread on Vizcaino where it unconformably overlies the San Hipolito Formation. On Vizcaino, the lower Eugenia (Upper Jurassic)more » includes spectacular volcanic debris flows interbedded with sandstone, but it is transitional upward into finer facies with increasing plutonic detritus. Time equivalent to the upper Eugenia and locally deposited on tonalite and volcanics is the Lower Cretaceous Asunction Formation (0.8 km thick). This sequence includes calcareous coarse breccias of serpentinized gabbros and tonalite.« less

Provenance of the Plio-Pleistocene sediments in the East Turkana Basin, northern Kenya

The fossiliferous Plio-Pleistocene fluvio-lacustrine sediments in the East Turkana Basin of northern Kenya were derived from two major source areas: (1) Ethiopian plutonic igneous and metamorphic rocks of the Precambrian basement and (2) Kenyan Cenozoic volcanic rocks. Interpretations of provenance are based on heavy mineral analyses of 123 sandstone samples and data collected during stratigraphic studies. Thirty heavy minerals have been identified in the East Turkana Basin and commonly compose 5–20% of the fine sand fraction which is compositionally representative of the entire heavy fraction. Morphological properties of the grains, geometric form, roundness and surface textures indicate that the mineral distribution primarily reflects source rocks lithology. Seven distinct heavy mineral associations have been distinguished which are grouped into two suites: a plutonic suite dominated by blue-green hornblende, apatite, sphene, garnet and clinozoisite-epidote and a volcanic suite dominated by augite and opaque minerals. Both suites form mappable heavy mineral provinces. Plutonic provinces occur throughout the basin interior, and volcanic provinces are distributed along the basin margin in association with alluvial fan deposits. Sedimentation began in the Pliocene following formation of the Turkana half-graben and the Chew Bahir (Stefanie) graben. The sediments are predominantly first cycle with limited reworking in fluvial and lacustrine environments. Two perennial stream systems transported plutonic detritus to the basin interior from basement rocks of high relief exposed to the northeast in the Chew Bahir basin and eastern Omo valley. Short ephemeral streams transported volcanic detritus to alluvial fans along the basin margins from volcanic rocks of low relief which border the basin to the east and south.

Provenance of Franciscan graywackes in coastal California

A systematic comparison of available detrital modes for graywacke sandstones of the Franciscan subduction complex and for coeval sandstones of the Great Valley sequence in the California Coast Ranges indicates that both were apparently derived from the same general sources. The inferred provenance terrane was the ancestral Sierran-Klamath magmatic arc, from which mixed volcanic and plutonic detritus readily entered the adjacent Great Valley forearc basin. At intervals along the trend of the arc-trench system, arc-derived detritus also bypassed the forearc region through submarine canyons that fed the Franciscan trench. Longitudinal flow along both the Great Valley trough and the Franciscan trench achieved wide dispersal of the turbidite sediment. Suites of both Franciscan and Great Valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic. Mean framework modes of 17 Franciscan suites comprising 203 individual samples, and of 23 Great Valley suites comprising 410 individual samples, range from 14% to 44% quartz grains, 15% to 54% feldspar grains, and 7% to 71% total lithic fragments. The ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes. The compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz, and feldspar derived principally from erosion of underlying plutons. Despite major overlap in the compositions of the two sets of samples, some Franciscan sandstones are somewhat more feldspathic and less lithic than any known Great Valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical Great Valley sources. Higher proportions of non-volcanic to volcanic lithic fragments in some Franciscan sandstones probably reflect complex recycling processes on the trench slope. Diagenetic effects in many Franciscan suites include apparent wholesale replacement of K-feldspar by albite. Present age control is inadequate to test fully for time-dependent trends in the compositions of Franciscan sandstones analogous to the known stratigraphic variations in the composition of Great Valley sandstones. This question ought to be investigated in future studies.