Volcanogenic Sedimentary Rocks Research Articles

Sr-isotope chemostratigraphy of carbonate deposits from the Yenisei Group, Azyrtal ridge, eastern slope of Kuznetsk Alatau

This study uses Sr isotope chemostratigraphy to place constraints on the depositional age of carbonate rocks from the Yenisei Group, which are considered to be the Precambrian reference section in the western Altai-Sayan Folded Area. The least altered samples of carbonate rocks were selected on the basis of strict geochemical criteria, Mn/Sr 0.2 , Fe/Sr 5.0 for limestone and Mn/Sr 1.0 , Fe/Sr 4.0 for dolomite. Preliminary leaching of samples in 1N solution of ammonium acetate was used to enrich these samples in primary carbonate matter. Sr chemostratigraphic signatures of carbonate deposits from the Yenisei Group revealed that deposition took place during the Late Vendian and Early Cambrian, in the time span of 580‐530 Ma. The southwestern fold rim of the Siberian platform was formed as a result of the Salair and Caledonian tectogenesis. The accretion-collision series of the AltaiSayan Folded Area contain thick blocks of carbonate rocks indicative of the marine and ocean basins that existed in the Cambrian at the periphery of the Siberian craton. Isotopic age dating of carbonates is an important task in the reconstruction of the Paleoasian Ocean. The thickest carbonate complexes situated in the western Altai-Sayan Folded Area within Kuznetsk Alatau are the subject of this investigation (Fig. 1). The southeastern boundary of Kuznetsk Alatau runs along the Azyrtal ridge, which, from the viewpoint of tectonics, is a structurally complicated anticlinorium, covering Devonian rocks of the Minusa intermountain trough [1, 2]. A thick (5.5‐6.0 km) section of carbonate deposits exposed within the spur of the Azyrtal ridge forms the Yenisei Group and is considered to be the reference for the western part of the Altai-Sayan Folded Area [1‐3]. With its base not exposed in the present-day section, this group is conformably overlapped by Cambrian volcanogenic sedimentary rocks of the KutenBuluk Formation. This group is subdivided into four formations: mostly dolomitic Charyshtag (~2900 m), bituminous mostly calcareous Bidga (1800‐2000 m),

Volcanic Framework of the Pliocene El Dorado Low-Sulfidation Epithermal Gold District, El Salvador

The Pliocene El Dorado low-sulfidation epithermal Au-Ag vein system, located within an 84-km2 district in north-central El Salvador, is hosted by volcanic rocks formed along the Caribbean plate margin in response to the subduction of the Cocos plate during the Tertiary. The principal volcanic host rocks to epithermal veins, and effective basement in the district, consist of a >400-m-thick sequence of basaltic to andesitic lava flows and volcanogenic sedimentary rocks. Locally, porphyritic basaltic to andesitic domes and dikes intruded this sequence during the late Miocene (10.7 ± 1.9 Ma). The mineralized volcanic basement rocks, outcropping mainly in north and central El Dorado, represent different parts of a complex volcanic sequence composed of multiple superimposed volcanic centers. Vein formation in the district occurred in the Pliocene between 4.7 ± 0.2 Ma and 4.06 ± 0.29 Ma, based on K-Ar and 40Ar-39Ar plateau ages on vein adularia. Vein formation overlapped with Pliocene felsic volcanism (3.94 ± 0.29 Ma to 3.36 ± 0.49 Ma), based on field relationships and 40Ar-39Ar geochronology. Pliocene felsic volcanic rock sequences crop out widely in southern El Dorado at lower elevations relative to central and northern El Dorado. The felsic rocks have sinters intercalated with pyroclastic and sedimentary rocks at the base of the sequence that define the paleosurface of the epithermal vein system, at least in this part of the district. The depth of erosion in central and northern El Dorado, the primary Au-Ag resource, is below the paleosurface, as only the low sulfidation–style epithermal veins are present. Field relationships and geochronology suggest that the onset of extensive felsic volcanism marks the waning stage of hydrothermal activity associated with vein formation and mineralization. From north to south, the El Dorado district represents an oblique cross section through the volcanic and epithermal vein system formed during east-west extension. Northern and central El Dorado, dominated by mineralized mafic to intermediate volcanic basement rocks, represent the deeper parts of the volcanic and hydrothermal system. Southern El Dorado is the shallow level of the volcanic field and epithermal vein system, preserving the surface and shallow subsurface environments, including the sinters and contemporaneous to postmineral felsic volcanic rocks. Although we cannot preclude that the distribution of volcanic rocks represents lateral facies changes in a Pliocene felsic dome field or that the variations in depth within the low-sulfidation epithermal system represent simply lateral flow driven by topography in the geothermal system, the preponderance of evidence suggests that the oblique section is principally due to postmineral deformation associated with formation of large Pliocene and Pleistocene Rio Titihuappa basin lying to the immediate south.

Pedrolampi: A precambrian gold sulfide deposit, Karelia

The Precambrian gold deposit Pedrolampi belongs to the structure controlled type and is located within the intersection between the submeridianal and northwestern schistosity and rock metasomatical alteration zones of the Elmus area in Central Karelia. Structural and mineralogical investigations performed allowed us to confirm that gold-sulfide ore formation occurred in the Late Archean within the submeridianal zone, and more late silver and copper mineralization overlapped along the zones of NW schistosity in the post-Yatulian period. Submeridianally striking quartz veins of the Pedrolampi deposit are boudinaged by NW deformations, and along these zones chlorite‐tourmaline-bearing metasomatites with Ag‐Au‐Cu mineralization developed. The formation temperature of metasomatites, veins, and ores descended from 400 up to 110 ° C and lower in the zone of oxidation. The ore-formation temperature is close for both types of ores and accounts for 270‐ 290 ° C (ore type-1) and 240 ° C (ore type-2). The simple mineral composition (gold‐pyrite), the presence of platinoids and Hg, and the low content of Ag in gold in ore type-1 prove their more high-temperature character and additional source of metals at the early ore-formation stage. Gold‐chalcopyrite mineralization in the ores of ore type-2 formed under lower temperatures and was accompanied by Cu and Ag afflux. Pedrolampi is a Precambrian mesothermal genetically polystage small gold deposit with resources accounting for about 10 t. The Precambrian gold deposit Pedrolampi is located in the intersection between submeridianal and northeasternly directed zones of schistosity and metasomatical alteration of Central Karelia rocks. New data on the geology and magmatism of the Elmus area belonging to the Segozersk-Vedlozersky greenstone belt, ore mineralogy, and ore-boundary alterations of the Pedrolampi area make it possible to consider it as a polygenetic mesothermal gold-sulfide deposit (with resources accounting for about 10 t) formed in the late AR and altered in the early PR. The geological structure of the Elmus area is composed of Neoarchean and Paleoproterozoic rocks (Fig. 1). Upper Archean (Lopian) strata strike in the submeridianal direction. Metabasalts and tuffs in the lower part of the section are overlapped by volcanogenic sedimentary rocks (banded and aglomerated tuffs and volcanites represented by quartz‐carbonate‐mica‐chlorite, chlorite‐cericite shales, sometimes with veinletimpregnated pyrite ores). In the upper part of the section occur metasandstones and polymictic conglomerates. These conglomerates belong to the molasse formations (accumulated in the basin of a strike-slip fault nature) similar to the strata occurring in the Koykarsky structure located to the south. Weakly rounded pebbles of 1‐ 10 cm in size within these conglomerates are presented by apo-tuff schists, porphires, and quartz. Archean strata are broken by bodies and dikes of pyroxenites, gabbro, rhyodazites, granite-porphyres, diorites, and

Geochemical systematics of komatiite–tholeiite and adakitic-arc basalt associations: The role of a mantle plume and convergent margin in formation of the Sandur Superterrane, Dharwar craton, India

The ∼ 2.7 Ga Sandur Superterrane is located within the central belt of the ∼ 2.6 Ga Closepet granite that divides the Dharwar craton into eastern and western sectors. The composite SST includes multiple terranes defined by distinct lithological associations, and metamorphic-deformational histories, demarked by accretionary structures. The Sultanpura volcanic terrane includes well preserved spinifex textured komatiites and komatiitic-basalts, with pillowed tholeiitic basalts. Komatiites and komatiitic-basalts have Mg# of 0.82–0.84 and 0.55–0.64 respectively, and plot near the olivine control line, whereas basalts have Mg# 0.53–0.69. All three volcanic types can be divided into two populations based on Nb/Th ratios: for rocks with Nb/Th < 8, there is covariation with Th, and (La/Sm) N interpreted to be the result of crustal assimilation fractional crystallization (AFC), whereas those rocks with Nb/Th > 8 plot along the Mid Oceanic Ridge Basalt–Oceanic Island Basalt array in Th/Yb vs. Nb/Yb coordinates. Collectively, the data are interpreted as signatures of a zoned mantle plume, having multiple sources that erupted through, or at the margin of, continental lithosphere. Felsic flows associated with arc basalts of the eastern felsic volcanic terrane, tectonically juxtaposed to the Sultanpura volcanic terrane, have adakitic compositional characteristics: elevated Al 2O 3 but low Yb (0.30–0.50 ppm) contents, coupled with high (La/Yb) N (43–71) and Zr/Sm (37–41) ratios, but low Nb/Ta (5–12). These features, in conjunction with mostly positive Eu anomalies, rule out detectable crustal contamination, such that adakitic flows and associated basalts and volcanogenic sedimentary rocks having normalized anomalies at Nb–Ta–P–Ti, represent an arc association. Consequently, the distinctive magmatic associations of the Sultanpura and eastern felsic volcanic terranes are consistent with the Sandur Superterrane being tectonic fragments of distinct continental and oceanic provenance tectonically juxtaposed in a Cordilleran type, accretionary orogen at ∼ 2.7 Ga.

Geological structure of the submarine Vityaz Ridge within the Seismic Gap area (Pacific slope of the Kurile island arc)

Results of geological research conducted by the Pacific Oceanological Institute of the Far East Division of the Russian Academy of Sciences and the Institute of Oceanology of the Russian Academy of Sciences on the submarine Vityaz Ridge during Cruise 37 of R/V Akademik M.A. Lavrentyev in 2005 are discussed. Various rocks constituting the basement and sedimentary cover of the ridge were dredged in three areas of the ridge. Based on isotope geochronology, petrogeochemical, petrographic, and paleontological data and comparison with similar rocks available from the adjacent land and Sea of Okhotsk, they are subdivided into several age complexes. Late Cretaceous, Eocene, Late Oligocene, Miocene, and Pliocene-Pleistocene complexes are defined among the igneous rocks, while volcanogenic-sedimentary rocks are united into Late Cretaceous-Early Paleocene (late Campanian-Danian), undivided Paleogene (Paleocene-Eocene?), Oligocene-early Miocene, and Pliocene-Pleistocene complexes. The obtained data on the age and formation settings of the defined complexes made it possible to reconstruct the geological evolution of the central Pacific slope of the Kurile island arc.

Geologic implications of new zircon U‐Pb ages from the White Mountain Peak Metavolcanic Complex, eastern California

The NNW‐trending White‐Inyo Range includes intrusive and volcanic rocks on the eastern flank of the Sierran volcano‐plutonic arc. The NE‐striking, steeply SE‐dipping Barcroft reverse fault separates folded, metamorphosed Mesozoic White Mountain Peak mafic and felsic volcanic flows, volcanogenic sedimentary rocks, and minor hypabyssal plugs on the north from folded, well‐bedded Neoproterozoic‐Cambrian marble and siliciclastic strata on the south. The 163 ± 2 Ma Barcroft Granodiorite rose along this fault, and thermally recrystallized its wall rocks. However, new SHRIMP‐RG ages of magmatic zircons from three White Mountain Peak volcanogenic metasedimentary rocks and a metafelsite document stages of effusion at ∼115–120 Ma as well as at ∼155–170 Ma. The U‐Pb data confirm the interpretation by Hanson et al. (1987) that part of the metasedimentary‐metavolcanic pile was laid down after Late Jurassic intrusion of the Barcroft pluton. The Lower Cretaceous, largely volcanogenic metasedimentary section lies beneath a low‐angle thrust fault, the upper plate of which includes interlayered Late Jurassic mafic and felsic metavolcanic rocks and the roughly coeval Barcroft pluton. Late Jurassic and Early Cretaceous volcanism in this sector of the Californian continental margin, combined with earlier petrologic, structural, and geochronologic studies, indicates that there was no gap in igneous activity at this latitude of the North American continental margin.

Geochemistry of black shales from the Neoarchaean Sandur Superterrane, India: First cycle volcanogenic sedimentary rocks in an intraoceanic arc–trench complex

The ∼2.7 Ga Sandur Superterrane (SST), of the western Dharwar craton, is a collage of greenstone terranes having distinct lithotectonic associations; volcanic associations are prevalent. Fine-grained metasedimentary rocks, which are optimal for provenance studies, are sparse in greenstone terranes of this craton. However, extensive shale sequences are present in the eastern volcanic terrane (EVT) and the eastern felsic volcanic terrane (EFVT) of the SST. Within the EVT, the black shales are stratigraphically associated with black cherts, metabasalt and banded iron formation (BIF), and underlain by greywackes. Shales have compositions of tholeiitic basalt in terms of TiO 2, Cr, Co, Ni, V, and Sc contents, and plot near the arc basalt endmember on the Th/Sc versus Sc mixing hyperbola. In contrast, Archean average upper continental crust of Taylor and McLennan [Taylor, S.R., McLennan, S.M., 1985. The Continental crust: Its Composition and Evolution. Blackwell, Oxford, 307p.; Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of the continental crust. Rev. Geophys. 33, 241–265], plots mid-hyperbola indicative of bimodal arc magma provenance. Accordingly, the Sandur shales likely had a catchment in an oceanic arc or back-arc dominated by tholeiitic basalts. Specifically, Nb/Th ratios 1.5–2.5 in shales are close to those of Archean arc basalts (1–4), so a plateau or ocean island basalt source, where Nb/Th >8, can be ruled out. Compositionally, cherts are shale highly diluted by silica, with positive Eu anomalies, and are interpreted to be hydrothermal sediments precipitated from reduced fluids during periods of limited siliciclastic input. In the shales, variable SiO 2 and Fe 2O 3 contents, depletions of MnO, MgO, and Na 2O, and positive to negative Eu anomalies, but gains of K relative to arc basalt compositions, are interpreted as due to hydrothermal alteration. Greywackes underlying the shales have two compositions. Type I is similar to the shales, whereas Type II has fractionated REE with negative Eu anomalies consistent with a cratonic granitoid catchment [Manikyamba, C., Naqvi, S.M., Moeen, S., Gnaneswar Rao, T., Balaram, V., Ramesh, S.V., Reddy, G.L.N., 1997a. Geochemical heterogeneities of metagreywackes from the Sandur schist belt: implications for active plate margin processes. Precambrian Res. 84, 117–138]. Collectively, the results are in keeping with an intraoceanic arc outboard of a continental margin. During transgression the trench has a low energy shale facies with dominant arc contribution, but for regression high energy greywackes are deposited from a cratonic provenance.

Evidence for two episodes of volcanism in the Bigadiç borate basin and tectonic implications for western Turkey

Western Turkey has been dominated by N–S extension since the Early Miocene. The timing and cause of this N–S extension and related basin formation have been the subject of much debate, but new data from the Bigadiç borate basin provide insights that may solve this controversy. The basin is located in the Bornova Flysch Zone, which is thought to have formed as a major NE-trending transform zone during Late Cretaceous-Palaeocene collisional Tethyan orogenesis and later reactivated as a transfer zone of weakness, and which separates two orogenic domains having different structural evolutions. Volcanism in the Bigadiç area is characterized by two rock units that are separated by an angular unconformity. These are: (1) the Kocaiskan volcanites that gives K/Ar ages of 23 Ma, and (2) the Bigadiç volcano-sedimentary succession that yields ages of 20.6 to 17.8 Ma. Both units are unconformably overlain by Upper Miocene-Pliocene continental deposits. The Kocaiskan volcanites are related to the first episode of volcanic activity and comprise thick volcanogenic sedimentary rocks derived from subaerial andesitic intrusions, domes, lava flows and pyroclastic rocks. The second episode of volcanic activity, represented by basaltic to rhyolitic lavas and pyroclastic rocks, accompanied lacustrine–evaporitic sedimentation. Dacitic to rhyolitic volcanic rocks, called the Sındırgı volcanites, comprise NE-trending intrusions producing lava flows, ignimbrites, ash-fall deposits and associated volcanogenic sedimentary rocks. Other NE-trending olivine basaltic (Gölcük basalt) and trachyandesitic (Kayırlar volcanites) intrusions and lava flows were synchronously emplaced into the lacustrine sediments. The intrusions typically display peperitic rocks along their contacts with the sedimentary rocks. It is important to note that the Gölcük basalt described here is the first recorded Early Miocene alkali basalt in western Turkey. The oldest volcanic episode occurred in the NE-trending zone when the region was still experiencing N–S compression. The angular unconformity between the two volcanic episodes marks an abrupt transition from N–S collision-related convergence to N–S extension related to retreat of the Aegean subduction zone to the south along an extensional detachment. Thrust faults with top-to-the-north sense of shear and a series of anticlines and synclines with subvertical NE-striking axial planes observed in the Bigadiç volcano-sedimentary succession suggest that NW–SE compression was reactivated following sedimentation. Geochemical data from the Bigadiç area also support the validity of the extensional regime, which was characterized by a bimodal volcanism related to extrusion of coeval alkaline and calc-alkaline volcanic rocks during the second volcanic episode. The formation of alkaline volcanic rocks dated as 19.7 ± 0.4 Ma can be related directly to the onset of the N–S extensional regime in western Turkey. Copyright © 2005 John Wiley & Sons, Ltd.

Thrusting and Extension in the Scandian Hinterland, Norway: New U-Pb Ages and Tectonostratigraphic Evidence

A new regional compilation map and U-Pb ages on a suite of variably deformed, Ordovician, calc-alkaline intrusive igneous rocks requires a reinterpretation of the nature of continental collision and extensional exhumation of deep-seated rocks of the Western Gneiss Region west and northwest of Trondheim. A suite of calc-alkaline plutonic rocks, in the age range 482 to 438 Ma, previously known from the region of Smola-Hitra-Orlandet-Froan above the Hoybakken extensional detachment fault associated with the Devonian ‘Old Red Sandstone’ basins, is shown to extend over wide areas below the fault, commonly as strongly foliated and lineated gneisses that had been previously mistaken for parts of the Proterozoic Western Gneiss Region. At Follafoss, a member of this intrusive suite is unconformably overlain by weakly metamorphosed conglomerate and volcanogenic sedimentary rocks of probable Late Ordovician age, suggesting that both the sedimentary rocks and the underlying intrusions correlate with the Storen Nappe in the upper part of the local sequence of Caledonide nappes. U-Pb evidence of metamorphic ages from deep-seated rocks of the Western Gneiss Region include: 1) zircon reaction rims on Proterozoic igneous baddeleyite in the Selnes Gabbro at 401 ± 2 Ma; 2) widespread development of zircon overgrowth and metamorphic zircon with omphacite and coesite inclusions from the Hareidlandet eclogite at 402 ± 2 Ma; and 3) extreme Devonian thermal resetting and neocrystallization of titanite over a wide area of the Proterozoic basement gneisses ending at 395 ± 3 Ma, here fully documented for the first time. At Kjorsvika, west of Trondheimsfjord, a ductilely deformed gneiss of the Ordovician intrusive suite contains igneous titanite dated at 455 Ma that shows little evidence for Devonian thermal resetting. This gneiss lies only 1 to 2 km northwest from a large area of Proterozoic gneisses with 100 percent Devonian reset titanite across a ductilely deformed contact that must represent a phase of extensional detachment (Agdenes detachment) much older than the more brittle detachments (compare Hoybakken detachment) associated with some of the present outcrops of the Devonian clastic basins. These and other relationships suggest the following broad sequence of Siluro-Devonian events in the region: A) Early Scandian (430 - 410 Ma) thrusting with emplacement of the composite Storen Nappe onto the relatively cool Baltoscandian margin of Baltica during contemporaneous subduction of its distal part, locally producing eclogites, possibly representing the earliest initiation of high-level ‘post-orogenic’ clastic basins. B) Early mid-Scandian (410 - 406 Ma) continued subduction and imbrication of a more proximal part of the Baltoscandian margin, with its heating and high-pressure metamorphism. Continued deep-level thickening by imbrication providing the gravitational potential for foreland- and hinterland-directed extension at high levels, while deposition in Devonian clastic basins probably continued. C) Late mid-Scandian (406 - 396 Ma) continued subduction of more proximal Baltican continental basement with common production of HP and UHP eclogites. Imbricate thrusting of a subducted and heated proximal continental slab provided the gravitational potential to initate hinterland backsliding and extensional emplacement of the cool high-level Storen Nappe against the cooling basement. Deposition in high-level Devonian clastic basins was active. D) Early late-Scandian (396 - 390 Ma). The extension and cooling initiated in C) brought an enormous volume of Proterozoic basement from high amphibolite facies to low amphibolite facies and through about 600°C, terminating Pb loss and neocrystallization of titanite at 395 ± 3 Ma. During this phase, ductile extensional deformation progressed into a sinistral shear field, producing folds and subhorizontal stretching lineations within the previously thrust and extensionally juxtaposed, deep and high-level packages. E) Late Scandian (390 - 375 Ma) extension in a sinistral shear field bringing the ductilely deformed package into contact with brittle, highest-level rocks along detachments associated with outcrop areas of the Devonian clastic basins. In this sequence, the brittle detachments (E) played a relatively minor role, whereas the mid-Scandian deep-crustal imbrication and related high-level collapse (B, C and early D) were the main events that brought deep metamorphic rocks relatively close to the surface.

Conglomerates of Stopnik

In the area between Stopnik, Šebrelje and Pick of Šebrelje the Ladinian clastic, carbonate and volcanogenic rocks, in the thickness of more then 600 m, are very diversely developed. They are situated on diferent structural blocks bounded by the middle Triassic faults and overlay partly eroded Anisian dolomite in the footwall. Variegated volcanogenic sedimentary rocks with conglomerate lenses comprising the middle part of the Ladinian succession in the broader sorounding of Stopnik are very important for paleogeographic interpretation of the area. Ten conglomerate lenses were separated in the middle of volcanoclastic and volcanogenic rocks on the Šebrelje structural block. Cross sections of the conglomerate lenses are some 10 m to 550 m long and 15 to 60 m thick. Their axes are arranged in the direction NW-SE. In the lower and upper part of the lithological succession in the lenses prevail conglomerates with more ordered internal structure, denser packing of pebbles and more distinct bedding but in the middle part of the succession m lenses prevail sandy conglomerates and pebbly sandstones with less ordered internal structure, less denser packing of pebbles and less distinct bedding. The pebbles consist of diferent types of limestone, dolomite, volcanic rocks, tuffs, tuffaceous sandstone and conglomerate. They represent prevailing resedimented lower Ladinian rocks. The described sedimentary rocks are interpreted as a product of alluvial fan and fan delta sedimentary complexes, which were transgressively covered by tuffaceous muddy marine sediments passing into the Cordevolian dolomite.

Silurian deformation and metamorphism of Ordovician arc rocks of the Casco Bay Group, south-central Maine

The Casco Bay Group in south-central Maine consists of a sequence of Late Cambrian to Early Ordovician interlayered quartzofeldspathic granofels and pelite (Cape Elizabeth Formation) overlain by Early to Late Ordovician back-arc volcanic (Spring Point Formation) and volcanogenic sedimentary rocks (Diamond Island and Scarboro formations). These rocks were tightly folded and subjected to low-pressure amphibolite-facies metamorphism in the Late Silurian. This phase of deformation and metamorphism was followed by the development of a variety of structures consistent with a period of dextral transpression in Middle Devonian – Early Carboniferous time. Previously dated plutons within the sequence range in age from 422–389 Ma and record a period of prolonged intrusive activity in the region. Similarities in age, volcanic rock geochemistry, and lithologic characteristics argue strongly for a correlation between rocks of the Casco Bay Group and those in the Miramichi belt of eastern Maine and northern New Brunswick. The Cape Elizabeth Formation correlates with Late Cambrian to Early Ordovician sediments of the Miramichi Group (Gander Zone) and the Spring Point through Scarboro formations correlate with Early to Late Ordovician back-arc basin volcanics and volcanogenic sediments of the Bathurst Supergroup. The folding and low-pressure metamorphism of the Casco Bay Group is attributed to Late Silurian to Early Devonian terrane convergence and possible lithospheric delamination that would have resulted in a prolonged period of intrusive activity and elevated temperatures at low pressures. Continued convergence and likely plate reconfigurations in the Middle Devonian to Carboniferous led to widespread dextral transpression in the region.

The geotectonic evolution of the Western Ethiopian shield

The western Ethiopian Shield comprises three lithotectonic units. The Birbir domain, an assemblage of mafic to felsic intrusive and extrusive rocks and mainly volcanogenic sedimentary rocks, is enclosed between the dominantly orthogneissic Baro and Geba domains. The earliest recorded deformation event (D1) resulted in the formation of a subhorizontal gneissosity within the gneissic terranes which was synchronous with an early upper amphibolite-facies metamorphic peak (M1) at 800–770 Ma which locally caused partial melting. All terrains were subsequently deformed in the D2 event which was the result of severe E-W crustal shortening. An anticlockwise P-T-t path is implied. Subsequent D3 deformation was concentrated within mylonitised domain boundaries which record major transcurrent movement. These structures were reactivated and suffered fluid incursion resulting in isotopic reequilibration at 635–580 Ma. A second metamorphic event, M2, related to crustal thickening and consequent granite genesis, occurred after the cessation of D3 shearing. Bulk chemical analyses show that the metamorphosed plutonic and volcanic rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern magmatic arcs. They belong in part to the low-K series, suggesting an oceanic environment. The evolution of the region can be explained in terms of the melting of a subducting slab, intrusion, metamorphism and the formation of an oceanic island arc complex. Continued plate convergence caused severe east-west shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx, a second thermal event and accretion of microcontinents. Key words/phrases: Accretion, cratonisation, isotope systems, subduction, transcurrent movement SINET: Ethiopian Journal of Science Vol.25(2) 2002: 227-252

Ammonite-radiolarian assemblage, Tobago Volcanic Group, Tobago, West Indies—Implications for the evolution of the Great Arc of the Caribbean

An unusually fossiliferous sedimentary sequence is exposed near Scarborough, Tobago, West Indies. It occurs within the Tobago Volcanic Group above an undifferentiated sequence of volcaniclastic breccia and lava and includes volcanogenic sedimentary rocks at the base of the Bacolet Formation. An ammonite-radiolarian assemblage has been recovered from this sedimentary interval in the Tobago Volcanic Group. Ammonites from the Spring Gardens quarry are moderately evolute and have unbranched ribs; they are probably juveniles of Mojsisoviczia , of middle Albian age. Another ammonite from a stratigraphically higher locality (i.e., volcanogenic sedimentary rocks at the base of the Bacolet Formation) is identified as Manuaniceras decsernae Young, of early late Albian age. Radiolarian assemblages from intercalations of black siliceous argillite of the Tobago Volcanic Group are characterized by abundant A rchaeospongoprunum spp. and the common occurrence of Pseudodictyomitra pentacolaensis , Dictyomitra montisserei , Thanarla brouweri , and Pantanellium ventosum . Also present in lesser abundances are other members of the family Archaeodictyomitridae, Xitus , Napora durhami , and Sciadiocapsa speciosa . This assemblage correlates with the Romanus subzone of [O'Dogherty (1994)][1] and Kozurium zingulai zone of [Pessagno (1977)][2], both of which are dated as early to middle Albian. These newly reported fossils indicate an Albian age for deposition of the Tobago Volcanic Group. Earlier published 40Ar/39Ar radiometric ages from the group are consistent with the stratigraphic age assignment of the fossils and thereby suggest that volcanism associated with the Tobago Volcanic Group began ca. 105 Ma. Possible correlative, accreted fragments of the Mesozoic oceanic-arc of the southern Caribbean include the Curacao Lava Formation, the lower part of the Washikemba Formation (Bonaire), the Tiara Formation of north-central Venezuela, and possibly the subsurface Mejillones complex of the Carupano basin. The Mesozoic rocks of Tobago consist of two distinct, oceanic-arc rock sequences. The older North Coast Schist (≥120 Ma) exhibits polyphase deformation and lower greenschist facies metamorphism. The structurally overlying Tobago Volcanic Group is not penetratively deformed or folded; it has undergone prehnite-pumpellyite facies metamorphic conditions and has been tilted to moderate dips by various brittle faults. We interpret the hiatus that separates these oceanic-arc rock sequences as representing a significant event in the evolution of the Great Arc of the Caribbean. Consequently, we propose that the commonly postulated reversal in subduction-zone polarity of the Great Arc occurred prior to deposition of the Tobago Volcanic Group (ca. 105 Ma), and that older, more highly deformed and metamorphosed oceanic-arc sequences (e.g., North Coast Schist) served as a basement for younger sequences or as wall rock for intrusive elements of younger magmatic suites of the Great Arc. [1]: #ref-23 [2]: #ref-26

K-Ar dating of Mesta volcanics (SW Bulgaria)

The Mesta volcanic massif (SW Bulgaria) belongs to the Macedonian-Rhodope-North Aegean Magmatic Zone. It crops out along the middle course of Mesta River and covers about 200 km2 of the homonymous graben. The bаsеmеnt of Mesta graben and the neighbouring elevated blocks of Pirin and Western Rhodope mountains are composed of high-grade metamorphics, hosting granitoidic plutons of diverse ages, including the Paleogene Central Pirin and Teshevo plutons. Apart from the volcanic bodies, the graben-fill consists of complexly interfingered Paleogene stratified continental terrigenous sedimentary rocks, volcanogenic-sedimentary rocks and volcaniclastics (both руrо-, and epiclastics), overlain unconformably bу Neogene and Quaternary deposits. The Mesta volcanics are represented by two main types: high-K calc-alkaline to shoshonitic dacites (in some places with а transition to latites) and high-K calc-alkaline rhyodacites. The magmatic bodies of subvolcanic and extrusive facies are represented by а number of morphologic varieties, which belongs to two polygonal caldera structures (Кrеmеn and Banichan) and two linear magmato-tectonic zones (Gostoun and Dobrinishte). This paper presents 12 К-Аг ages obtained for 11 lауа bodies of different geological position and for one re-deposited volcanic clast from the middle part of the stratified Paleogene section. The ages are in support of the suggestion that the Mesta volcanics originated from а zoned magma chamber. More silicic (rhyodacitic) eruptions and subvolcanic intrusions are related to earlier volcanic events in the northern part of the massif - оn the territories of the Кrеmеn caldera, Gostoun and Dobrinishte zones. These events (approximately 33-31 Ма) are рге-, and sincollapse ones in respect to the Кrеmеn caldera formation. The later (sin- and post-collapse) rhyodacitic eruptions were accompanied and followed by eruptions and subvolcanic intrusions of less silicic, dacitic melts (31-28 Ма). The main portion of the dacites is related to the centres in the Banichan caldera (29-28 Ма). The time-span obtained covers the events of the paroxysmal volcanic activity in the Mesta graben only. Thus it is shorter than the real time-interval of the volcanism. This time-span is comparable with that of the voluminous Раlеоgene volcanism in the Smolyan region (Central Rhodopes). The Rb-Sr ages of neighbouring Teshevo and Central Pirin plutons plot in its boundaries as well. Some recommendations for more detailed and precise future radiometric studies are proposed to define the age of the initial and the final volcanic events in the Paleogene Mesta graben.

Upper Triassic Takla Group volcanic rocks, Stikine Terrane, north-central British Columbia: geochemistry, petrogenesis, and tectonic implications

The Upper Triassic Takla Group volcano-sedimentary assemblage is part of the Stikine Terrane of the Intermontane Belt in the Canadian Cordillera and covers an area of more than 30 000 km2 in a belt up to 50 km wide and more than 800 km long. In the McConnell Creek area of north-central British Columbia, the assemblage consists of plagioclase-clinopyroxene-phyric, dominantly basaltic to andesitic flows and pyroclastic rocks, interlayered with volcanogenic sedimentary rocks. Compositionally, the volcanic rocks are intermediate between tholeiitic and calc-alkaline. Their mantle-normalized trace element patterns are characterized by a moderate large-ion lithophile element enrichment and Nb and Ti depletion, suggesting that magmatism occurred in a volcanic-arc setting. Flat, heavy rare earth element chondrite-normalized patterns with (La/Yb)n ratios from 2 to 4.5 suggest that the parent magma was produced by mantle melting in the spinel stability field. The low Sr isotopic ratios (87Sr/86Sri approximately equal to 0.7033-0.7043) and positive εNd values (~ +7) indicate that an older sialic crust was not involved in their genesis. A coeval and compositionally similar volcano-sedimentary assemblage, also of the Takla Group, occurs in the adjacent Quesnel Terrane, in fault contact with the Stikinian Takla Group. Chemical resemblances between the Takla Groups of the Stikine and Quesnel terranes suggest that the volcanic assemblages may have had similar source compositions and melt histories. These results emphasize larger scale similarities between the Stikine and Quesnel terranes and suggest the Upper Triassic volcanic suites represent different fragments of the same early Mesozoic arc system.

EVOLUÇÃO DO VULCANISMO ALCALINO DA PORÇÃO SUL DO PLATÔ DO TAQUAREMBÓ, DOM PEDRITO-RS

The alkaline acid volcanism of the Southern part of the Taquarembo Plateau, Dom Pedritro, RS, is located at the Western portipn of the Sul-rio-grandense Shield. This Neo-Proterozoic unit, named Acampamento Velho Formation, is represented by effusive and pyroclastic rock succession, referred as Acid Volcanic Sequence (SVA). The volcanic activity was characterized by pulse successions, effusive and explosive, as registered by the presence of lava flows, flow and surge pyroclastic deposits. Re-worked volcanoclastic deposits, with abundant juvenile clasts and volcanogenic sedimentary rocks are also observed. The SVA stratigraphy shows an early trachytic followed by rhyolitic volcanism. Themain geochemieal characteristics of the SVA are the high values of SiO 2 , FeO t , Na 2 O, K 2 O, Rb, Zr, Nb, Y, ETR, high FeO t /FeO t +MgO ratio and low values of Al 2 O 3 , MgO, CaO, Sr and Ba, which characterize this magmatism as of alkaline affinity and peralkaline character (comendiitic). The magmatic evolution was dominated by mineral fractionation, involving mainly plagioclase, alkali-feldspar, pyroxene and magnetite. The REE pattern and the tectonic environmental discriminants diagrams (e.g. Nb x Zr) classify this volcanism as typical post-orogenic magmatism, associated with the later stages of the Brasiliano Cycle at the Sul-rio-grandense Shield.

La roca magica: uses of natural zeolites in agriculture and industry.

For nearly 200 years since their discovery in 1756, geologists considered the zeolite minerals to occur as fairly large crystals in the vugs and cavities of basalts and other traprock formations. Here, they were prized by mineral collectors, but their small abundance and polymineralic nature defied commercial exploitation. As the synthetic zeolite (molecular sieve) business began to take hold in the late 1950s, huge beds of zeolite-rich sediments, formed by the alteration of volcanic ash (glass) in lake and marine waters, were discovered in the western United States and elsewhere in the world. These beds were found to contain as much as 95% of a single zeolite; they were generally flat-lying and easily mined by surface methods. The properties of these low-cost natural materials mimicked those of many of their synthetic counterparts, and considerable effort has made since that time to develop applications for them based on their unique adsorption, cation-exchange, dehydration-rehydration, and catalytic properties. Natural zeolites (i.e., those found in volcanogenic sedimentary rocks) have been and are being used as building stone, as lightweight aggregate and pozzolans in cements and concretes, as filler in paper, in the take-up of Cs and Sr from nuclear waste and fallout, as soil amendments in agronomy and horticulture, in the removal of ammonia from municipal, industrial, and agricultural waste and drinking waters, as energy exchangers in solar refrigerators, as dietary supplements in animal diets, as consumer deodorizers, in pet litters, in taking up ammonia from animal manures, and as ammonia filters in kidney-dialysis units. From their use in construction during Roman times, to their role as hydroponic (zeoponic) substrate for growing plants on space missions, to their recent success in the healing of cuts and wounds, natural zeolites are now considered to be full-fledged mineral commodities, the use of which promise to expand even more in the future.

Cretaceous volcanogenic sedimentary basins and folding in the Komsomolsky tin-ore region, Khabarovsk Territory, Russia

At the western margin of the northern Sikhote-Alin, the Cretaceous (Aptian—Campanian) basins of the Komsomolsky tin-ore region occur within large linear gentle NE-trending depressions of the Late Triassic—Valanginian consolidated terrigenous basement. The depressions are conjugate with the basement-arched uplifts, forming a single system. The Cretaceous basins are represented by an uninterrupted stratigraphic succession of the Aptian—Campanian volcanogenic sedimentary rocks, whose composition regularly and gradually changes upwards from purely clastics (mainly the continental molasse) to pyroclastics and lava of acidic to intermediate volcanics.

Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins

Sedimentary rocks contained in basins adjacent to the Tian Shan provide a long and complex record of the late Paleozoic continental amalgamation of northwestern China, complementing that provided by rocks preserved within the range. This record, which comprises dramatic changes in sedimentary facies, sediment dispersal patterns, sandstone provenance, and basin subsidence rates, broadly supports previous interpretations of a two-part evolution of the Tian Shan: Late Devonian to Early Carboniferous collision of the Tarim continental block with the Central Tian Shan, followed by collision of this combined block with island arcs in the north Tian Shan and Bogda Shan in Late Carboniferous–Early Permian times. The first collision resulted in widespread angular unconformities within the Tarim basin. Continued convergence following the collision created a long-lived flexural foredeep along the northern margin of the Tarim block, which received at least 2000 m of Lower Carboniferous through Lower Permian fluvial and marine sediment derived from the interior of Tarim. Subsequent Early Permian continental extension of the northern Tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks. Sandstone within this interval was derived from the paleo–Tian Shan, and is composed predominantly of lithic volcanic grains similar to the rhyolite. In contrast to the Tarim basin, calc-alkaline volcanic rocks and volcanogenic sedimentary rocks dominated Carboniferous and Permian sedimentation in the northern Turpan and northwestern Junggar basins. Volcanic arcs remained active in the North Tian Shan and Bogda Shan through the early Late Carboniferous, depositing a kilometers-thick interval of deep marine sediment-gravity flows in the northwestern Junggar basin. Major arc magmatism ceased in the Late Carboniferous in response to closure of the oceanic basin between the combined Tarim/Central Tian Shan block and the North Tian Shan/Bogda Shan arcs. Upper Carboniferous through Lower Permian rocks in the northwestern Junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth (on the northern side of the volcanic arcs), culminating in a 1000-m-thick marine regressive sequence. Middle to Upper Permian sandstones were derived from the uplifted paleo–Tian Shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range. Late Permian subsidence of the Junggar basin accommodated >5 km of nonmarine sediments; however, the cause of this subsidence and its relationship to regional tectonic events remain controversial.

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.