Rhyolite Sequence Research Articles

Geochemistry of West-central Chihuahua Obsidian Nodules and Implications for the Derivation of Obsidian Artefacts

Obsidian nodules (marekanites or “Apache tears”) weathered out of volcanic units west of Chihuahua were utilized by the precontact native population for the manufacture of small stone implements. We subjected 77 specimens, including both unworked nodules and bipolar percussion debitage, collected in a 16,000sqkm area, to standard ICP–AES, XRF glass bead and C-H-N combustion analyses. The chemical compositions of the samples form four groups related to differing fractional crystallization histories which developed in four discrete magma chambers. K2O and Nb/Y versus SiO2diagrams define the volcanics as high-potassium, calc-alkalic rhyolites, similar to a regionally extensive sequence of rhyolites related to Farallon Plate subduction, and erupted between 36 and 27 Ma. Harker and ratio diagrams indicate that amphibole crystallization was a major control on the chemistries of the northern, southern and western volcanic sequences. The geochemical signature of the central area volcanic rocks was mainly the result of plagioclase and K-feldspar crystallization in the magma chamber. Attributing variance in the data set to differing fractional crystallization histories allows elements and element ratios to be selected for grouping techniques which maximize the effectiveness of these techniques. In this study, Sr/Y versus Ba/Zr is the best choice, reflecting the crystallization histories of plagioclase, amphibole, K-feldspar and possibly sphene, apatite and zircon.Some 90% of samples were collected in the area which matches their geochemistry. In the southern area the ratio plot is capable of distinguishing between the geochemical signatures of individual sites, which indicates that most procurement was within approximately 1km of where the nodule was worked. Of 10 anomalous, apparently exotic, samples, six were probably transported a minimum of 25km by humans, and the means of transport of four is unclear. The northern area had no exotic fragments, though a small sample size (N=11) makes this finding only suggestive.

Proximal stratigraphy and syn-eruptive faulting in rhyolitic Grants Ridge Tuff, New Mexico, USA

Proximal deposits of the 3.3 Ma Grants Ridge Tuff, part of a 5-km 3 topaz rhyolite sequence, are composed of basal pyroclastic flow, surge, and fallout deposits, a thick central ignimbrite, and upper surge and fallout deposits. Large lithic blocks (≤2 m) of underlying sedimentary and granitic bedrock that are present in lower pyroclastic flow and fallout deposits indicate that the eruptive sequence began with explosive, conduit-excavating eruptions. The massive, nonwelded central ignimbrite displays evidence for postemplacement deformation. The upper pyroclastic surge deposits are dominated by fine ash, some beds containing accretionary lapilli, soft-sediment deformation features, and mud-coated lithic lapilli, indicating an explosive, hydromagmatic component to these later eruptions. The upper fall and surge deposits are overlain by fluvially reworked volcaniclastic deposits that truncate the primary section with a relatively planar surface. The proximal, upper pyroclastic surge and Plinian fall deposits are preserved only in small grabens (5–8 m deep and wide), where they subsided into the ignimbrite and were protected from reworking. The pyroclastic surge and fall deposits within the grabens are offset by numerous small normal faults. The offset on some faults decreases upward through the section, indicating that the faulting process may have been syn-eruptive. Several graben-bounding faults extend downward into the ignimbrite, but the uppermost, fluvially reworked tephra layers are not cut by these faults. The faulting mechanism may have been related to settling and compaction of the 60 m thick, valley-filling ignimbrite along the axis of the paleovalley. Draping surge contacts against the graben faults and brittle and soft-style disruption of the upper pyroclastic surge beds indicate that subsidence was ongoing during the emplacement of the upper eruptive sequence. Seismicity accompanying the late-stage hydromagmatic explosions may have contributed to the abrupt settling and compaction of the ignimbrite.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite [formula omitted] age constraints on tectonic evolution of Southwestern Gondwanaland

Zircon U-Pb and muscovite 40Ar 39Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic “back stop” of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the 40Ar 39Ar age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Allochthonous 2.78 Ga oceanic plateau slivers in a 2.72 Ga continental arc sequence: Vizien greenstone belt, northeastern Superior Province, Canada

Embedded within the vast granitoid terrane of the Minto block of northeastern Superior Province are Late Archean greenstone belts of the Goudalie domain that preserve a long-lived record of continent-ocean interaction. The Vizien greenstone belt is one such belt and it contains four fault-bounded structural panels. The 2786 Ma mafic-ultramafic sequence is an allochthonous package of pillowed basaltic andesite, komatiite and volcaniclastic rocks cut by peridotite and gabbro sills. The mafic rocks are LREE-depleted tholeiites which have primitive mantle (PRIM)-normalized abundances of Th < Nb < La, and ϵ Nd values of +1.5 to + 3.2 reflecting extraction from a depleted mantle source. The 2724 Ma lac Lintelle continental calc-alkaline volcanic sequence consists of massive basalt, plagioclase-porphyritic andesite, dacite, rhyolite, capped by quartz-rich sandstones/conglomerates with 2.97 Ga Nd model ages. Lac Lintelle volcanic rocks are LREE enriched, with low TiO 2 (< 1%) and Zr (< 200 ppm), PRIM-normalized enrichment in Th > La > Nb, and a range of ϵ Nd values from −0.1 to +1.7. The ~ 2722 Ma lac Serindac bimodal, subaerial tholeiitic volcanic sequence contains andesite (locally with tonalite xenoliths), basalt, gabbro sills, lenses of quartz-rich sedimentary rocks and a thick, upper rhyolite sequence. The lac Serindac tholeiites are LREE-enriched, have PRIM-normalized Th > La > Nb, high Zr (to 300 ppm) and Ti contents, and low ϵ Nd values from +0.8 in basalt to −1.4 in rhyolite. The < 2718 Ma basement-cover sequence comprises 2.94 Ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 Ma gabbro, siliceous high-Mg basalt (SHMB) and andesite. The SHMB are characterised by LREE depletion and ϵ Nd values of +2.6, whereas the andesite is LREE-enriched and has ϵ Nd values of −0.3. The 2786 Ma mafic-ultramafic sequence is interpreted as a sliver of plume-related oceanic plateau crust. The 2724 lac Lintelle sequence represents a continental arc formed on the eastern protocraton. The ~ 2722 Ma lac Serindac volcanic sequence represents late continental rift deposits. The various 2.8-2.7 Ga supracrustal sequences were accreted, deformed and metamorphosed to mid-amphibolite facies during late-stage assembly of the Minto block between 2.718 and 2.693 Ga.

Massive sulfide deposits of the Noranda area, Quebec. IV. The Mobrun mine

The Mobrun polymetallic deposit near Rouyn–Noranda comprises two complexes of massive sulfide lenses within mainly felsic volcanic rocks of the Archean Blake River Group. The Main lens contained 3.37 Mt of massive sulfides, with 1989 reserves of 0.95 Mt at 0.81% Cu, 2.44% Zn, 30.3 g/t Ag, and 2.2 g/t Au. The 1100 complex, located ~250 m to the southeast of the Main complex, contains estimated 1989 reserves of 10.4 Mt at 0.76% Cu, 5.43% Zn, 37.4 g/t Ag, and 1.35 g/t Au.Host volcanic rocks of the Main complex are mostly massive, brecciated, and tuffaceous rhyolites. The rhyolites are commonly strongly sheared parallel to lithological contacts, which are locally displaced by high-angle faults. Immobile-element plots such as Y–Zr and Nb–Zr show a separation of rhyolite data into two distinct alteration trends that generally correspond to massive and in situ brecciated rhyolite of the footwall, and tuffaceous rhyolite of the hanging wall. The hanging wall has tholeiitic Zr/Y ratios (3–5), whereas the footwall has mildly calc-alkaline Zr/Y ratios (7–9). Several immobile-element trends indicate that there was a subtle but clear change in rhyolite composition near the time of ore deposition. Identification of chemically distinct footwall and hanging wall rhyolites allows these units to be recognized and traced along strike, even where alteration is strong. Sericitization and silicification extend at least 100 m from the orebody, with local chloritic zones in the upper footwall. Calculated mass changes indicate that the footwall generally has lost silica mass relative to the hanging wall. Alteration zones associated with mineralization have mass gains in FeO + MgO and K2O gains, but mass loss in silica.The 1100 complex, located stratigraphically below the Main complex, is hosted by rhyolite, with one main andesite interval in the footwall. The footwall contains three chemically distinct rhyolite types, all tholeiitic. Hanging-wall rhyolites are, however, mildly calc-alkaline, and thus are chemically comparable to, and correlated with, the footwall of the Main complex. Rhyolites within ~100 m stratigraphically of the Main and 1100 complexes commonly have positively shifted δ18O whole-rock values of 11–13‰. These high values are interpreted as the result of an initial, widespread phase of low-temperature hydrothermal alteration that increased δ18O values by 3–5‰ relative to unaltered rhyolites. Some footwall rhyolites, however, are relatively depleted in 18O, strongly depleted in Ca–Na and depleted in Eu2+. Rhyolites with these chemical features have been overprinted by higher temperature alteration, presumably in localized feeder zones. All four rhyolite types near the 1100 complex are chemically recognizable despite contrasting alteration.The orebodies are interpreted as synvolcanic, based on their occurrence along distinctive volcanic contacts, and the presence of primary sulfide textures where deformation is minor. The chemostratigraphic framework defined for the host rhyolite sequence can be used to trace critical volcanic contacts through lithologically monotonous, strongly altered, and faulted stratigraphy.

Precious-Metal Deposits in Relation to Volcanic and Sedimentary Stratigraphy: Examples from the Oregon Plateau, U.S.A.: ABSTRACT

Miocene to Recent volcanic and sedimentary rocks of the Oregon Plateau host precious-metal deposits. The deposits, formed within an extensional tectonic setting, illustrate the relation among volcanic and sedimentary stratigraphy, regional structural development, and hydrothermal systems. Gold deposits hosted in felsic volcaniclastic sediments in the Owyhee region formed during or shortly after sedimentation. Uniformly silicified tuffaceous mudstone and siltstone interbedded with siliceous and sulfide-rich sediments and bedded hydrathermal breccia, hydrothermal explosion craters infilled by breccia fragments, and stacked buried sinter deposits attest to mineralization near the time of sedimentation. The regional structures that controlled the distribution of volcanic activity and location of sedimentary basins changed through time. In the middle Miocene, basin-and range faults controlled the location of basalt and rhyolitic volcanism and fluvial and lacustrine basins. In the late Miocene and Pliocene, faults parallel to those of the Western Snake River Plain are more prominent and interfingering volcanic and volcaniclastic sedimentary deposits characterize the stratigraphic section. Fault-controlled mercury mineralization in quartz within opalized high-silica rhyolite flow-domes produces an obscure precious-metal prospect at Glass Buttes in the High Lava Plains. Glassy high-silica rhyolite flow-domes were erupted between 5.8 and 5.0 ma but alteration and mineralization occur along faults of themore » Brothers fault zone that cut the high-silica rhyolite sequence. Younger porphyritic rhyolite to dacite domes, spines, and intrusions controlled by faults of the Brothers fault zone are believed to be the heat source for the hydrothermal system. Precious-metal mineralization may occur in the subsurface where hydrothermal solutions boiled beneath the paleowater table.« less

The Zadinian Group (late Proterozoic, Zaire) and its bearing on the origin of the west-congo orogenic belt

The West-Congo orogenic belt is a Late Proterozoic chain that extends parallel to the western coast of central Africa from Gabon to Angola for a length of more than 1000 km. Its origin is still debated and two contrasting viewpoints have been proposed: plate collision (Vellutini et al.) versus ensialic thermal orogeny (Schermerhorn). The question is reinvestigated in the light of new field work and structural, petrological and geochemical studies carried out in Zaïre. We focus on the Zadinian Group that outcrops between Boma and Matadi. We compare it with the Mayumbian Group occurring to the east of Matadi, and we have come to the conclusion that they are stratigraphically equivalent. The metavolcanic rocks ranging in composition from basalt to rhyolite have trace element signatures comparable with those of Mesozoic and Cenozoic continental tholeiites that erupted in rift zones. The most primitive metabasalts have a Nb anomaly relative to Th-La, which suggests a ‘supra subduction’ setting for the postulated spreading centre. The rift system developed inside the 2100 Ma old Eburnean basement, which is recognized here and there at the base of the Zadinian Group. The presence of this continental crust is also deduced from the thick sequence of Mayumbian rhyolites, which could not have been produced without crustal melting due to uprise of mantle-derived basaltic magmas. Structural investigations reveal two compressive structural types F1 and F2, different in trend but similar in style. The F1 type, trending E-W, is restricted to the Boma area, while the F2 type affected the whole West-Congo belt. The structural and metamorphic asymmetry of this orogenic belt results from: (1) the strong eastward vergence of F2 folds and N-S tectonic thrust faults; (2) the eastward decreasing metamorphic grade (lower part of the amphibolite facies to greenschist facies). The P-T paths of regional metamorphism have been estimated using the parageneses of metapelites, calc-silicate rocks and metabasalts. They characterize a low- P/high- T facies series with a probable average thermal gradient of 60° C km −1. The syntectonic single-phase metamorphism took place before the amplacement of the 733 Ma old peralkaline Noqui granite. The various geological constraints are integrated in a geotectonic model: the West-Congo orogenic belt is likely to have evolved from a wide intracratonic marginal basin that has been subsequently deformed in response to the collision between the Brazilian and the Congo plates. The suture zone is located farther west than the amphibolitic migmatites of Boma, probably near the present continental margin of central Africa or its Brazilian counterpart.

Petrology of Upper Ordovician – Lower Silurian rocks of the Antigonish Highlands, Nova Scotia

Upper Ordovician to Lower Silurian rocks in the Antigonish Highlands consist of interlayered basalts, rhyodacites, arkoses, and conglomerates overlain by a thick sequence of marine clastic rocks and minor rhyolites. The stratigraphy documents a marine transgression. The volcanic rocks were deposited in a within-plate, continental, extensional environment. The basalts display alkalic and tholeiitic affinities, and the rhyodacites were formed by anatexis of the crust. The origin of the younger rhyolites is not clear: they are compositionally distinct from the rhyodacites but may be related to them as late-stage differentiates. At present, it is not possible to evaluate whether the tectonic setting and magmatic affinities are regionally or locally controlled.The geological history is very similar to that of Lower Silurian rocks immediately north of the Antigonish Highlands at Arisaig. In the simplest sense, this indicates these areas may have been juxtaposed prior to the Late Ordovician and limits cumulative post-Silurian movement on the boundary (Hollow) fault to about 40 km.

Explosive eruptions of the Ayarza calderas, southeastern Guatemala

The Ayarza calderas, southeastern Guatemala, form a figure “8”-shaped depression that is now occupied by Laguna de Ayarza. The smaller eastern caldera is also the older. It formed upon eruption of a “mixed” plinian air fall that is locally overlain by a comagmatic ash-flow deposit. Called the “Mixta” unit, the air-fall portion contains rhyolitic (75.2 wt.% SiO2) pumice, basaltic (52 wt.% SiO2) blobs and compositional hybrids that are either spectacularly banded or homogeneous in color. Although of limited aerial extent and volume (∼ 0.1 km3, dense rock equivalent), the unit is of considerable petrologic interest because the basalt contains pristine phenocrysts and quench-crystallites of calcic hornblende. Virtually identical 87Sr/86Sr ratios (0.70373) of both rhyolite and basalt end members suggest a common magmatic source. A log from the Mixta ash-flow deposit has been C14-dated at 27,000 ± 1,600 y.B.P.Eruption of the younger pyroclastic sequence at Ayarza culminated in formation of the western caldera. This rhyolitic sequence also consists of a plinian air fall overlain by a comagmatic ash-flow deposit. The ash-flow unit is quite fine-grained and probably phreatomagmatic in origin. A thin horizon of air-fall material is locally interbedded within the ash unit. Volume (DRE) of the younger sequence has been estimated at a minimum 2 km3. A log from the younger ash-flow unit has been C14-dated at 23,100 ± 500 y.B.P., the age of formation of the western caldera and the last recognized activity at Ayarza.Although major-element geochemistry of the Ayarzan rhyolites is very similar to that of other silicic units of northern Central America, minor- and trace-element contents permit all the rhyolites to be distinguished from each other. Sr-isotopic ratios of the Mixta rhyolite reaffirm that older radiogenic crustal rocks are not involved in the petrogenesis of rhyolites in northern Central America.

Aborted Proterozoic rifting in eastern Newfoundland

Recent geological mapping and geochemical studies of the late Proterozoic rocks of the Burin Peninsula in the southwestern part of the Avalon Zone of Newfoundland show the following sequence: (1) The basal Rock Harbour Group, a sedimentary assemblage ranging from conglomerates to turbidites, was derived from a subaerial volcanic–plutonic source region. (2) The overlying Burin Group, a belt of mafic volcanic and intrusive rocks that have alkalic affinities at the base but are dominated by oceanic tholeiite. (3) The Marystown Group, a bimodal sequence of alkalic basalts and rhyolites deposited in a subaerial environment unconformably above the Burin Group, and apparently conformably overlain by Infracambrian to Cambrian sedimentary rocks. Stromatolites occur within resedimented limestone breccias and conglomerates at the top of the Rock Harbour Group and throughout the Burin Group.These assemblages are interpreted to indicate that after a period of continental type bimodal volcanism, the Avalon terrain was rifted in Burin Group times to form some kind of small ocean basin, which was abruptly closed with a reversion to continental type bimodal volcanism of the Marystown Group.

Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahia Kino

Research Article| February 01, 1977 Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahia Kino R. GORDON GASTIL; R. GORDON GASTIL 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar DANIEL KRUMMENACHER DANIEL KRUMMENACHER 1Department of Geological Sciences, San Diego State University, San Diego, California 92182 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1977) 88 (2): 189–198. https://doi.org/10.1130/0016-7606(1977)88<189:RGOCSB>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. GORDON GASTIL, DANIEL KRUMMENACHER; Reconnaissance geology of coastal Sonora between Puerto Lobos and Bahia Kino. GSA Bulletin 1977;; 88 (2): 189–198. doi: https://doi.org/10.1130/0016-7606(1977)88<189:RGOCSB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Coastal Sonora between Puerto Lobos and Bahia Kino can be subdivided into four structural-petrographic subprovinces: an inland subprovince in which unmetamorphosed upper Precambrian and Cambrian strata rest on older Precambrian gneiss and three other subprovinces in which Cenozoic volcanic strata rest on metamorphosed strata of post-Precambrian age intruded by granitic rocks of Mesozoic age. One of these latter subprovinces displays basin-and-range fault blocks; a second has northwest-trending strike-slip(?) faults; and the third, Isla Tiburón, shows structure related to the Neogene dilation of the Gulf of California depression.The pregranitic rocks include upper Precambrian and Cambrian carbonate rocks, a chert-graywacke-volcaniclastic sequence of probable Carboniferous age, and volcanic-volcaniclastic rocks of Jurassic age. The granitic rocks range from gabbro to granite and have K-Ar cooling ages of from 91 to 30 m.y. Dikes of basaltic to dacitic composition and quartz porphyritic bodies of late Mesozoic or early Cenozoic age cut the granitic rocks.The lowermost Cenozoic volcanic strata (pre–22 m.y. B.P.) are predominantly composed of rhyolite and basalt. These are followed by a sequence of predominant andesite (∼20 to 18 m.y. old), a sequence of partially marine conglomerate and pyroclastic deposits, and a widespread, predominantly rhyolitic sequence (∼14 to 10 m.y.). All strata 10 m.y. old or older are involved in basin-and-range–type tilting. Volcanic strata less than 8 m.y. old are nearly flat-lying. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isotopic and Paleontologic Evidence for Correlating Three Volcanic Sequences in the Maine Coastal Volcanic Belt

The correlation of the Castine Volcanics, the Thorofare Andesite–Vinalhaven Rhyolite sequence, and the Cranberry Island Series of Shaler (1889), suggested by their similar lithologies, appearance, and structural histories, is supported by the results of Rb-Sr whole-rock isotopic analyses and by the faunal assemblages from old and new fossil localities in the Castine Volcanics, Ames Knob Formation, and Thorofare Andesite. The volcanic rocks are partly Late Silurian, but mostly Early Devonian in age and yield an average radiogenic age of 390 ± 5 m.y. The use of calcite-bearing volcanic samples for whole-rock Rb-Sr dating degrades the method by greatly increasing the uncertainty of the isochron and initial Sr 87 /Sr 86 . Lower to Middle Devonian granitic plutons have initial ratios of Sr 87 and Sr 86 similar to those in the volcanic formations. The Castine Volcanics and the Lower Devonian granite of Sedgwick may be comagmatic, but the time interval between the extrusion of the Vinalhaven Rhyolite and its intrusion by the Middle Devonian granite of Vinalhaven Island is too long to support the comagmatic hypothesis.

The eruptive history and depositional environment of the Devonian extrusive rocks, Eden, New South Wales

Abstract The Devonian, terrestrial Eden Rhyolite attains a thickness of 156 m and in stratigraphic order consists of the following units: Flow Two followed Flow One rapidly enough to form one cooling unit and these two flows represent the bulk of the Eden Rhyolite sequence. The sources of these acidic eruptions were probably north-south trending fissures formed in an elongated, terrestrial foredeep in front of an orogen toward the west. Post-Eden Rhyolite deformation is thought to have been restricted to faulting resulting in the development of an asymmetrical graben which cuts across the north-south orogenic trend of the underlying (?) Ordovician Mallacoota Beds and the depositional trend of the Eden Rhyolite. The southern, east-west to west-northwest trending boundary fault of this graben (the Twofold Bay Fault) is delineated by the contact between the Mallacoota Beds and the Devonian rocks near Eden. The Twofold Bay Fault may be the Devonian termination of the Berridale Wrench Fault. The development of...

Tertiary Volcanic Center, West-Central Nevada

The Clan Alpine Mountains, Churchill County, Nevada, contain two sequences of Tertiary rhyolite tuffs and flows which are contemporaneous but markedly different in structure and thickness. The sequence in the northern part of the range consists of about 500 m of well-stratified ash-flow sheets of age range 22 to 30 m.y. The southern part of the range contains an assemblage of lava flows, domes, and ash-flow and epiclastic tuff beds which is here subdivided into five mappable units. Radiometric ages are 22 to 30 m.y. Based on gravimetric analysis, the thickness of the southern sequence is at least 3,000 m and perhaps 5,000 m. The contact of the two rhyolite sequences, which is now largely eroded, was probably an abrupt gradation close to the present northern margin of the southern sequence. Structural evidence indicates that the voluminous eruptions of the southern sequence were probably from local sources and were concurrent with faulting such that the rocks were deposited in one or more volcano-tectonic depressions. Recognition of the north wall of at least one caldera within the southern sequence seems clear. Age relations indicate that the vents for the southern sequence were probably the source of at least some of the ash flows of the northern sequence. Similar age and structural relations between different sequences of rhyolite tuffs and flows in the next range east, the Desatoya Mountains, suggest that the volcano-tectonic feature of which the southern sequence is a part extended east of the Clan Alpine Mountains.

Karroo lavas and associated igneous rocks of Southern Africa

This review deals mainly with the petrography, geochemistry and stratigraphic development of the Karroo lavas of southern Africa and, more briefly, with the post-Karroo lavas of the area. The great majority of the Karroo lavas are tholeiitic basalts either poor in olivine or olivine-free, though great thicknesses of glassy, olivine-rich basalts, also of tholeiitic affinities, form the lower part of the sequence in certain restricted areas. Small amounts of alkalic types (nephelinite, shoshonite) are associated with the tholeiites. The basalts are overlain by a thick sequence of rhyolites with interbedded basalts in the eastern (monoclinal) part of the outcrop area. Overlying the rhyolites a younger basaltic sequence, including both tholeiitic and alkali basaltic types is locally exposed in southern Mozambique, while the Lupata Series of phonolites, trachytes and rhyolites of Cretaceous age is found further north in the Zambezi valley section of Mozambique. Certain tholeiites, formerly regarded as normal Karroo basalts, occuring in South West Africa, have recently been shown also to be of Cretaceous age. A brief description of the intrusive rocks associated with the Jurassic and Cretaceous vulcanism is also given. The rocks include widespread sills and dykes of tholeiitic composition (the Karroo Dolerites) and numerous intrusive complexes covering a broad compositional range and including tholeiitic gabbros, granites and granophyres, quartz-syenites, nepheline syenites, carbonatites, ijolites, and other undersaturated types.

Jebel Khariz: an upper miocene strato-volcano of comenditic affinity on the South Arabian coast

Jebel Khariz, the largest central vent volcano on the south Arabian coast, lies 60 miles (95 km) to the west of Aden and was probably active during the Upper Miocene. The volcanic edifice originally covered some 350 sq. miles (900 km2) and consists of an older, radially dipping main cone sequence of rhyolites, trachytes, basalts and olivine basalts and a younger, horizontal caldera sequence mainly of intermediate lavas, that infilled the caldera subsequent to its formation at a late stage in the history of the volcano. The Khariz volcanic suite, ranging in composition from olivine basalts to per-alkali rhyolites of comenditic affinity, was probably produced by fractionation, in a low pressure regime, of a mildly alkali olivine basalt magma. Noting the abundance of peralkali volcanics associated with the African rift system in Kenya, Ethiopia and on the margins of the Red Sea and the Gulf of Aden, it is tentatively suggested that, at times, the sub-crustal mechanism, responsible for the rift development, might also produce an environment where fusion of the earth’s mantle gave rise to a relatively rare, mildly alkali, ‘parental’ basaltic magma.

Geochemical features of a strongly fractionated alkali igneous suite

Geochemical data for 37 igneous rocks from the Paresis volcanic complex, South West Africa, are presented and discussed. Unusual abundances of certain minor constituents and variation trends of element-ratios support the view that the central rhyolites and syenitic dykes are products of a strongly differentiated magma—probably originating by palingenesis of the basement granite. Granitophile elements are strongly enriched in the rhyolite sequence but their variation in the holocrystalline dykes is more subdued. Differences in trace element distribution between rhyolites and syenitic dykes are attributed largely to the relative prominence of the forces governing diadochy. The abundances and distribution of V, Cr, Ni, Co, Sr, Ba, Ga, Zr, Pb, La, Li, Rb. and Cs are discussed in terms of their relationships to the major rock-forming constituents.