Flow-banded Rhyolite Research Articles

U–Pb geochronology of Late Silurian (Wenlock to Pridoli) volcanic and sedimentary rocks, central Newfoundland Appalachians: targeting the timing of transient extension as a prelude to Devonian orogenic gold mineralization

Bimodal igneous suites and associated immature clastic sedimentary rocks are characteristic of many orogenic gold-mineralized, crustal-scale fault zones globally. In the central Newfoundland Appalachian orogen, the Rogerson Lake Conglomerate belt and Botwood basin are Late Silurian (Wenlock to Pridoli), fault-controlled sedimentary rock sequences and magmatic suites closely associated with orogenic gold mineralization; however, the spatio-temporal evolution of faulting and associated sedimentation and magmatism are not fully resolved. U–Pb zircon geochronological results were obtained by using an integrated approach employing LA-ICPMS (laser ablation-inductively coupled plasma mass spectrometry) followed by CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) on the same detrital samples. Using this approach, a maximum depositional age for sedimentary rocks of the Rogerson Lake Conglomerate sequence is 421.9 ± 1.0 Ma (Pridoli), which confirms that they are younger than, and stratigraphically overlie, ca. 422–420 Ma igneous rocks exposed along the central Newfoundland gold belt. Towards the stratigraphic middle of the Botwood basin in north-central Newfoundland, a tuffite layer intercalated with graded siltstone produced a maximum depositional age of 427.9 ± 3.1 Ma (Wenlock; Homerian). The age of emplacement of an autobrecciated, flow-banded rhyolite dome of the Charles Lake volcanic belt along the northwestern Botwood basin is 429.3 ± 0.7 Ma (Wenlock; Homerian). The high-precision CA-ID-TIMS zircon data establish a clear link between Wenlock to Pridoli magmatism and sedimentation throughout central Newfoundland. Furthermore, these geochronological results are consistent with a structural model involving the southeastward (present-day coordinates) advancement of a transient extensional fault system across strike of the Exploits Subzone between ca. 429 and 418 Ma, with propagation along strike to the southwest (Rogerson Lake Conglomerate belt) between ca. 422 and 418 Ma. Extensional faulting may have contributed to basin formation, subsidence, and exhumation of pre-Late Silurian rocks of the Exploits Subzone. Time-transgressive, extension-related magmatism and clastic sedimentation appear to mark the transition between the Salinic and Acadian orogenic cycles along the central Newfoundland gold belt. Transient Wenlock to Pridoli lithospheric extension may have been important for increasing heat and fluid flow in the crust as a prelude to Devonian crustal thickening, fluid focussing, and orogenic gold mineralization.

Dike propagation and magma flow in a glassy rhyolite dike: A structural and kinematic analysis

Exhumed magma conduits provide important evidence of the development and evolution of subvolcanic plumbing systems. We use a 5−14-m-thick flow-banded rhyolite dike in Arran (Scotland) to present the first reconstruction of the directions and styles of initial propagation and subsequent magma flow, based on mesoscale kinematic indicators. The dike has concave-inward dike-margin segments with plumose-like structures that record vertical and horizontal propagation of lobes, which inflated and linked to form a through-going sheet. Devitrified rhyolite zones at the dike margins show gentle to open folds. In contrast, glassy central parts of the dike are flow laminated and preserve folded and refolded isoclinal, curvilinear folds and sheath folds that record sustained progressive deformation. The inner interface between the glassy and devitrified facies is abrupt and marked by elongation lineations and mullions. In the dike center, fold axes plunge 27°NE along the dike, and parallel to elongation lineations. Combined with shear sense indicators (σ- and δ-objects, sheared vesicles, and asymmetric folds), these features indicate that magma flow was obliquely upward, to the southwest, and locally ≤60° to the propagation direction of the dike. The distribution of structures within the rhyolite indicates local accretion of the (now) devitrified material to the margins, with localization of flow into the center of the dike. We find that the initial magma flow direction was controlled by fracture propagation and interaction, with the subsequent flow record controlled by accretion and flow localization in the conduit. This study demonstrates that analysis of mesoscopic structural and kinematic features (several of which have not previously been reported from dikes) is a powerful tool that can be used to reconstruct the complex evolution of conduit initiation and magma flow processes.

The Nathrop Domes, Colorado: Geochemistry and petrogenesis of a topaz rhyolite

The four Nathrop Domes (∼30 Ma) are located near Buena Vista, Colorado, within the Arkansas Valley Graben segment of the Rio Grande Rift. The domes are largely composed of sparsely to moderately porphyritic, flow-banded rhyolite, with local vitrophyric margins. Phenocrysts include sanidine, plagioclase, quartz, biotite, and Fe–Ti oxides. Bright red Mn-garnet locally occurs in vapor-phase cavities. Two-feldspar and two-oxide temperatures assuming 1 kb pressure were, respectively, 670°C and 653°C. All four domes erupted rhyolite of essentially identical major-element chemistry, although with substantial trace-element variations, high incompatible trace-element contents (Rb up to 364 ppm; Nb up to 67 ppm), and extreme depletion of Ba, Sr, P, Eu, and Ti. These depletions are consistent with fractionation of observed phenocrysts. A rare-earth element diagram shows parallelism of the rhyolite plots, with sloping light REE values, flat heavy REE values, and a prominent, negative Eu anomaly. The narrow range of major-element compositions within the Nathrop Domes’ rhyolites precludes any fractionation modeling amongst them. Nonetheless, their elevated Rb/Sr ratios (as great as 75–120) strongly suggest that these magmas have undergone extensive fractional crystallization. Neodymium isotope analysis shows ∊ Ndt values of −10.1 for a Bald Mountain sample and −13.9 from a Precambrian granite sample (t = 29 Ma). The similarity of the ∊ Ndt of the Bald Mountain rhyolite and the Precambrian granite suggests that the Nathrop rhyolite magmas were initially formed through partial melting of Precambrian rocks. Neodymium Crustal Index (NCI) calculations were performed using the Precambrian granite to estimate the contribution of crustal sources, assuming lithospheric mantle contributions of basalt with ∊ Nd of 0 to 4, or asthenospheric mantle contributions with ∊ Nd of 5 or greater. The resulting calculations indicate NCI values of 0.727 to 0.789, assuming lithospheric mantle, and 0.799 assuming an asthenospheric mantle contribution with ∊ Nd = 5. Thus, 72.7 to 79.9 percent of the Nd present in the Nathrop rhyolite sample is likely from crustal sources, depending upon what type of mantle contribution was involved. The Nathrop rhyolites may represent the earliest phase of magmatism associated with the northern segments of the Rio Grande Rift.

Microtexture development during rapid cooling in three rhyolitic lava flows

The effects of water concentration and degassing history on the development of spherulites and flow banding were examined in three middle Tertiary rhyolitic lava flows from the Atascosa Mountains of southern Arizona. The Hell’s Gate lava flow and the Atascosa lava flow host spherulites of strongly contrasting texture, and neither are flow banded. The Sycamore Canyon lava flow is a flow-banded rhyolite that hosts two populations of spherulites. Spherulites in the Hell’s Gate lava flow consist of two to four generations of bladed radiating alkali feldspar crystals that increase in water concentration along their length. Needle-like radiating feldspar crystals in spherulites in the Atascosa and Sycamore Canyon lava flows are in some cases punctuated by concentric rinds of glass that are reservoirs for water rejected by the feldspar crystals. The differences in spherulite crystal morphology between the Sycamore Canyon and Atasocosa flows (both needle-like) and the Hell’s Gate flow (bladed) may reflect a more rapid cooling rate of the Sycamore Canyon and Atascosa flows. Thick gray flow bands in the Sycamore Canyon lava flow host higher water concentrations than thin orange flow bands, suggesting that flow bands are zones of greater and lesser volatile concentration, deformed by stretching of the flowing magma. Temperature was uniform across the light and dark flow bands of the Sycamore Canyon flow, indicating that water concentration, one of the variables that controls diffusion coefficients, rather than temperature, controlled spherulite size in this case. Phenocrysts in the Hell’s Gate lava flow are strongly resorbed, probably as a result of magma ascent along a near-adiabatic gradient that resulted in exsolution of water from the melt, and subsequent dissolution of existing quartz phenocrysts by the water-rich melt. Resumption of crystallization of anhydrous phases such as quartz and feldspar would have further enriched the melt in water, facilitating the growth of spherulites. Spherulites in two of the lava flows (Sycamore Canyon and Hell’s Gate) increase in water concentration from core to rim, as would be expected in spherulites growing in melt enriched in water rejected by the growing feldspar crystals. Spherulites in the Atascosa rhyolite flow decrease slightly in water concentration from core to rim, possibly because the magma degassed during spherulite growth. Calculation of water concentration profiles in spherulites from all three rhyolite flows on the basis of Rayleigh fractionation of water between sanidine and rhyolitic melt shows that the very high water concentrations in spherulites (typically >0.6 × water concentration in surrounding glass) cannot be accounted for by Rayleigh fractionation. Instead it is likely that sanidine incorporated water as fluid inclusions and/or as water clusters during rapid crystal growth. Water concentration profiles in the glass surrounding spherulites do not preserve the high concentration zone at the spherulite boundary that has been observed in younger lava flows, so that spherulite growth rates cannot be calculated on the basis of mass-balance calculations of distribution of water during spherulite growth. Rather, the water concentration profile in the surrounding glass is a half plateau, the height of which is approximately equivalent to the far-field water concentration in the surrounding glass, indicating that water that accumulated at the spherulite/magma boundary diffused sufficiently rapidly to equilibrate with the surrounding magma as the lava flow cooled.

A model for submarine rhyolite dome growth: Ponza Island (central Italy)

The Late Pliocene rhyolitic submarine volcanic rocks of Ponza island (Italy) can be interpreted as the subaqueous equivalent of subaerial dome complexes in terms of geometry and structure. Three coalescing domes of about 1km radius and aligned along a NE-trending regional fracture have been identified. The main difference between subaqueous and subaerial lava domes is that in a subaqueous environment, lava is likely to undergo pervasive hyaloclastic brecciation, so that domes are mainly composed of variously brecciated, in situ to clast-rotated hyaloclastite rather than coherent lava. We suggest that the shape and rheologic behaviour through time of submarine domes are closely controlled by the development and thickness of the particulate hyaloclastic carapace, which assumes the role of the solid crust of domes in subaerial environment. The thickness of the hyaloclastic carapace at Ponza is greater than 150m and emplaced during several different pulses (or eruptions). In the earliest pulses, lava is directly extruded on the seafloor and produces hyaloclastite, the degree of brecciation of which decreases inward to the coherent flow-banded rhyolite lava of the feeder dike. Once the hyaloclastic carapace is formed, further pulses of magma, combined with increase in height of the dome result in a local stress pattern characterised by a vertical σ1⪢σ2=σ3, producing concentric and radial fractures and normal faults. The newly rising magma, shielded by the hyaloclastic carapace, can intrude along these fault and fracture systems and invade previously emplaced but still water-saturated hyaloclastite. This produces the characteristic pattern of dikes observed at Ponza as a series of concentric dikes that are progressively less inclined outward with respect to the dome centre. These late stage dikes in turn produce hyaloclastite at their margins, but generally less fragmented than the embedding hyaloclastite, probably because the ascending magma is better shielded from direct contact with sea water. Periodic gravity collapses of the dome maintain the equilibrium between height and radius of the dome, as suggested at Ponza by the presence of mass flow deposits and the development of small topographic basins on top of the domes, probably related to debris removal. Some of these topographic depressions host subaqueous pyroclastic deposits, suggesting that, if due to vertical growth of the dome, water depth above the dome becomes shallower, hydromagmatic explosions may occur.

Precise U‐Pb Zircon Constraints on the Earliest Magmatic History of the Carolina Terrane

The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum-Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U-Pb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Osmond biotite-granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of approximately 25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 Ma and 586+/-10 Ma (reported age of the upper Uwharrie Formation). The U-Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental-type basement. This observation, together with the juvenile Nd isotopic character of the Virgilina volcanic arc sequence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence.

Chert from the Munsungun Lake Formation (Maine) in Palaeoamerican Archaeological Sites in Northeastern North America: Recognition of its Occurrence and Distribution

Numerous fluted point Palaeoamerican sites in the northeast contain significant numbers of chert artefacts whose origin is from the Munsungun Lake Formation in northern Maine. These sites include: Adkins, Bull Brook, Dam, Hedden, Lamareau, Michaud, Morss, Munsungun Lake (three sites), Neponset, Plenge, Point Sebago, Searsmont, Spiller Farm, Vail, Vail Kill (two sites), Wheeler Dam (two sites) and Whipple. Lithic materials in order of decreasing importance at these sites include: (1) chert whose physical characteristics are wholly consistent with chert outcrops and quarries near Munsungun Lake; (2) a variety of cherts whose physical characteristics are inconsistent with outcrops and quarries near Munsungun Lake; (3) a spherulitic, flow-banded rhyolite; (4) miscellaneous fine-grained igneous rocks; (5) clear or milky quartz; (6) miscellaneous very fine-grained or fine-grained metamorphic rocks. Portions of all collections contain artefacts which are completely weathered, and which are not assignable to any specific rock type and cannot be classified as to specific rock type.We developed a hierarchy to classify Munsungun Lake chert, ranging from hand specimen characteristics to polarized light microscopy and X-ray diffraction to chemical analysis. We conclude that hand specimen characteristics combined with polarized light microscopy are viable techniques for assigning Munsungun Lake Formation provenance. Three distinctive hand specimen features are sedimentary lamination, organic burrow structures and colour variation. Using these, chert quarries and outcrops in the geographic vicinity of Munsungun Lake may be assigned to one of the following categories: (A) structureless grayish red, dusky red, blackish red, or dark reddish brown; (B) laminated grayish red, dusky red, blackish red, or dark reddish brown chert with varve-like laminations; (C) mottled or colour laminated red and dark grey, or mottled red and greenish grey chert; (D) colour laminated and burrow mottled chert; (E) greyish to blackish laminated chert; (F) dark grey to black, structureless or weakly and thinly laminated; (G) olive grey to olive black laminated chert which is locally pyrite-rich. The nature of weathered surfaces may be a diagnostic tool for assistance in recognizing Munsungun chert. Chemical weathering tends to produce much lighter colourations and enhances the visibility of sedimentary structures. Munsungun cherts tend to weather to a very light tan, very light grey or greyish white. Radiolarians are uncommon (<10%) in all varieties, and are concentrated in zones parallel to bedding.Chert palaeoquarries are numerous in the Munsungun Lake area. They are of two types. One type occurs on topographically flat areas. Here the quarries are small (1×5m) to large (5×30m) size depressions within and along outcrops. In plan view these range in shape from roughly circular to broad ovals. Along the margins of these depressions are piles of poorly sorted worked and discarded debitage. These “tailing piles” are generally less than 1m in height, but exhibit wide variations in areal extent and shape. Larger tailing piles overlap one another suggesting that they have experienced more than one episode of growth due to repeated quarry activity. The second quarry type occurs on steeper slopes. These are near-vertical quarried faces which range from 1 to nearly 3m in height. Excavation here has occurred both inwards to the slope and downwards. Small cavern or adit-like “mines” have not been recognized.

Geology of Lakeview Uranium District, Oregon: ABSTRACT

About 400,000 lb (181.4 Mg) of U3O8 have been mined from Tertiary volcanic rocks near Lakeview, Oregon, mostly from uraninite-coffinite ore bodies at the White King mine. At this deposit, tuffs and tuffaceous sedimentary rocks are overlain by a basaltic lahar which is capped by basalt flows. The tuffaceous rocks are intruded by flow-banded rhyolite. Most of the ore is in the rholite and adjacent tuffaceous rocks, but uraniferous shear zones cut the lahar, and the basalt is highly altered. Most of the uraniferous rock is silicified and brecciated, and has high As, Hg, Mo, and Pb together with secondary U minerals. Within 10 km of the mine are nine smaller uranium occurrences, four of which are in, or adjacent to, flow-banded rhyolite domes which are considered endogenous. However, five occurrences are distant from surface exposures of rhyolite: three in volcanic sandstone and tuff near contacts with basalt, and two in a thick sequence of rhyolitic to dacitic ash flows. At the White King mine and most of the other occurrences, uranium was probably deposited from epithermal fluids released during rhyolite intrusion. These fluids could have migrated some distance from the intrusions along fractures or through permeable sedimentary rocks. Capping basalts, which are cut by the intrusions, appear to have arrested the movement of uranium at several occurrences. End_of_Article - Last_Page 756------------

Correlation of Pinnacles and Neenach Volcanic Formations and Their Bearing on San Andreas Fault Problem

The Pinnacles Volcanic Formation (San Benito County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the volcanic rocks form a west-dipping homocline which is truncated on the northeast by the Chalone Creek fault. The Chalone Creek fault is interpreted to be an early Miocene trace of the San Andreas fault. The Neenach Volcanic Formation (Los Angeles County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the rocks form a northwest-dipping homocline which is broken by many faults. The formation is truncated on the southwest by the San Andreas fault. The Pinnacles Volcanic Formation and the Neenach Volcanic Formation lie respectively west and east of the San Andreas fault zone and are separated 315 km from each other in a right-lateral sense. Ten rock types with nearly identical field, petrographic, and chemical characteristics are present in each formation in essentially the same stratigraphic order: vitric lapilli-tuff, flow-banded rhyolite, perlite, pumice lapilli-tuff, hypocrystalline hypersthene andesite, augite-olivine andesite, andesite tuff, porphyritic dacite, agglomerate, and porphyritic rhyolite. The Pinnacles Volcanic and Neenach Volcanic Formations were contiguous across the San Andreas fault at the time of their formation and subsequently have been offset 315 km along the San Andreas fault. For several reasons the Pinnacles-Neenach correlation provides some of the most conclusive documentation of large-scale, lateral displacement on the San Andreas fault yet discovered in central California: (1) a precise distance of separation can be determined because both areas are directly adjacent to traces of the fault, thus eliminating the necessity of interpretive projection into the fault; (2) the age of the formations can be determined accurately because the rocks are volcanic and suitable for radiometric dating; (3) the correlation is based on 10 different rock types, each with distinctive petrologic features, thus reducing the likelihood of chance occurrence. Other correlations of pre-Miocene rocks along the San Andreas fault show offsets of the same magnitude as the Pinnacles-Neenach rocks suggesting that 315 km may be the maximum offset along the San Andreas fault proper.

Mono Craters, California: A New Interpretation of the Eruptive Sequence

Recent geologic studies of the Mono Craters in eastern California revealed two types of domal land forms not described in previous investigations: cratered domes and double craters. A pumiceous rhyolite dome in the northern part of the Mono Craters is an example of a cratered dome. This dome has a truncated cone shape and is composed of a funnel-structured rhyolite plug surrounded by a steep talus slope. The summit crater was formed by collapse and explosions. An example of a double crater is West Control Crater (910 m in diameter), which is composed of two funnel-structured pumiceous rhyolite domes, one within the summit crater of the other. Both domes are surrounded by debris and have truncated cone shapes. A four-stage eruptive sequence which includes these two morphologies is: (1) formation of a bowl-shaped rhyolite crater by explosive eruptions, (2) extrusion of a dome of viscous flow-banded rhyolite on the crater floor, (3) formation of a cratered dome by explosion and (or) collapse, and (4) extrusion of a dome in the summit crater of the cratered dome; this dome is cratered, forming a double crater. Flows form during stages 2 and 4 of the sequence. The sequence can be terminated at any stage.

Geology and tectonic setting of the Mule Creek caldera, New Mexico, U.S.A.

The 10-km diameter Mule Creek caldera is the youngest felsic eruptive center in the Mogollon-Datil volcanic field of southwestern New Mexico. The caldera forms a topographic basin surrounded by a raised rim. The caldera wall is well displayed on the south and west sides of the structure where it dips 20–30 degrees toward the center of the basin. Mudflow breccia fills the caldera and is banked up against the caldera wall. Post-caldera porphyritic quartz latite domes and flows crop out along the ring-fracture zone. The caldera is superimposed upon an older volcanic complex of flow-banded rhyolite and porphyritic andesite lava. The Mule Creek caldera probably originated by explosive eruption of about 10 km3 of pumice and ash, in part preserved in the matrix of the mudflow breccia. Periods of explosive volcanism during the deposition of mudflow breccia are documented by tuffaceous beds interbedded with the breccia. A thin rhyolite ash-flow sheet originated in the caldera and overlies the mudflow breccia. The youngest felsic rocks around the caldera are (1) domes and flows of crystal-rich porphyritic quartz latite of variable mineralogy, interpreted as a defluidized magma, and (2) widespread crystal-poor, flow-banded rhyolite, dated at 18.6 m.y., which is not directly related to the caldera sequence. The Mule Creek caldera and other volcanic features farther south represent the only documented overlap of felsic volcanism with early stages of Basin-Range tectonism in the Mogollon-Datil field.