Back-arc Basin Setting Research Articles

The effect of redox on the relative solubilities of copper and iron in Cl-bearing aqueous fluids at elevated temperatures and pressures: An experimental study with application to subseafloor hydrothermal systems

A series of hydrothermal experiments were performed to evaluate the role of redox, temperature, and fluid chemistry on the solubility of Cu- and Fe-bearing sulfide minerals in Na-K-Cl aqueous fluids. The experiments made use of potassic aluminosilicate minerals + quartz to maintain fluid pH at values generally appropriate for subseafloor hydrothermal systems. To avoid uncertainties in pH resulting from non-ideal activity-composition relations involving potassium aluminosilicate phases in aqueous fluids dominated by NaCl, however, fluid pH values were explicitly determined from magnetite solubility constraints. Total dissolved Cl concentrations of 0.3 to 1.94 molal were utilized, while redox constraints were established by different assemblages of oxide and sulfide phases. Relatively oxidizing and low dissolved S concentrations were maintained by the assemblage hematite-magnetite-pyrite (HMP), while the assemblage pyrite-pyrrhotite-magnetite (PPM) was used to maintain relatively reducing conditions and high dissolved S concentrations. Chalcopyrite was utilized in all experiments as a source of dissolved Cu. The use of flexible cell hydrothermal apparatus permitted sampling of fluids at experimental conditions. Phase equilibria constraints at 400°C, 500 bars indicate pH values between 4.8 and 5.5 for all dissolved Cl concentrations investigated. At a pH of 4.8, and dissolved NaCl equivalent to that of seawater (0.55 m), dissolved Cu decreased, and Fe increased with decreasing f o 2 . For f o 2 values fixed by Hm-Mgt-Py and Py-Po-Mgt, dissolved Cu and Fe concentrations ranged from approximately 0.12 to 0.032 and 5.01 to 9.52 mmolal, respectively. As a consequence of this, Fe Cu ratio increased from approximately 50 to 250. Increasing pH from 4.8 to 5.0 affected significantly absolute metal concentrations, but this had only a slight affect on Fe Cu ratio. Dissolved concentrations of Cu and Fe were also affected by changes in total dissolved Cl and temperature, but again, Fe Cu ratio changed only slightly. The dominating effect of redox on Fe Cu ratio suggests that this parameter may be usefully applied to constrain redox in subseafloor hydrothermal systems. Fe Cu ratios of hot spring fluids at mid-ocean ridges and in back-arc basin settings reveal a relatively narrow range of values which are consistent with hydrothermal alteration under moderately oxidizing conditions. Fe Cu ratios measured for EPR, 21°N vent fluids approach values of 50, while Lau Basin vent fluids reveal values as high as 77. It is suggested here that anhydrite formation in subseafloor reaction zones helps to maintain relatively oxidizing conditions for these hot spring systems. In contrast, hot spring fluids from axial locations on the Mid-Atlantic Ridge (MARK) reveal Fe Cu ratios which indicate redox reactions buffered by the assemblage Py-Po-Mgt. Alternatively, hot spring fluids at MARK may have experienced extreme heat-loss effects with attendant loss of Cu very near the seafloor. Data from the present investigation and results of earlier basalt-fluid alteration experiments have shown that dissolved Cu is particularly sensitive to temperature at temperatures lower than approximately 350°C, and is lost from the fluid relative to Fe.

Accretionary lapilli associated with Archaean banded iron formations of the Kraaipan Group, Amalia greenstone belt, South Africa

Abstract The Kraaipan Group in the Amalia greenstone belt, South Africa, consists of an Archaean volcano-sedimentary assemblage of mafic metavolcanic rocks and banded iron formations, together with subordinate greywacke, slate and carbonate rocks. The sequence is deformed and intruded by a variety of granites and gneisses. Interlayered with the volcano-sedimentary rocks, in close proximity to a small gold prospect, are pyroclastic rocks in which are described well-developed “coated” or “rim-type” accretionary lapilli of a variety hitherto rarely, if ever, recorded in association with oxide-facies banded iron formations. The accretionary lapilli are discussed in terms of their significance and as palaeoenvironmental indicators. It is concluded that the accretionary lapilli probably developed in a proximal setting with respect to an ancient eruptive centre and that the lapilli themselves do not constitute exceptional indicators of sedimentary environments but are useful mainly as time or stratigraphic markers. Their association with banded iron formations suggests the accretionary lapilli here described were deposited from volcanic centres close to some shallow-marine or barred lagoonal basin, the latter possibly analogous to a playa-lake complex developed in a primitive inter-arc or back-arc basin setting in existence during the evolution of the Kraaipan volcano-sedimentary assemblage found in the Amalia greenstone belt.

A general review of volcanogenic massive sulphide deposits in China

Volcanogenic, massive sulphide deposits in China have, for the past forty years, been an important source of Cu, Zn, Pb and sulphur, and of a number of associated elements such as Au, Ag and Co. The areal distribution of the deposits is determined by the different geological settings in which they occur and which range in time from Archean to Triassic. Host-rock lithologies comprise felsic, mafic and ultramafic metavolcanic rocks, with or without subsidiary, intercalated metasedimentary rocks; four groups of ore associations, based on chemical composition, have been distinguished: Cu, CuZn, CuFe and PbZnCu. Only one deposit of economic significance (Hongtoushan) has been recognised so far in an Archean greenstone belt. This ore is hosted by metafelsic volcanic rocks of a calc-alkaline suite. The Proterozoic was an important period for the formation of two groups of volcanogenic massive sulphide deposits in China. Early Proterozoic CuFe deposits, such as Dahongshan and Lalachang, are predominantly distributed in graben-like depressions within cratons. Late Proterozoic CuZn deposits, e.g., Xiqiu and Liujiaping, occur mainly along the edges of the craton. The ores of all Proterozoic deposits are hosted by quartz keratophyre. The most important metallogenic epoch for this type of ores in China was the Lower Palaeozoic and is represented by a number of deposits situated in the northern Qilian Caledonian orogenic belt. Within this belt, the various groups of deposits can be related to specific plate-tectonic settings. CuZn or PbZnCu ores occur in quartz-keratophyric tuffite, with or without intercalated sediments, of island-arc environments (Houyanshan, Zheyaoshan, Xiaotieshan); Cu ores are hosted by spilite, generated probably in back-arc basin settings (Honggou); while CuZn ores presumably occur in metabasalts of oceanic ridge or off axis settings (Yendonggou). Volcanogenic, massive sulphides are relatively unimportant in the Late Palaeozoic, both with regard to size and numbers, but a rather unusual and important CuZn deposit, hosted by ultramafic rocks (Deerni) is ascribed to this epoch. An important PbZnCu deposit (Gacun) that is comparable with the Kuroko deposits was formed in the Early Mesozoic Yidun orogenic belt, which is considered to represent a mature arc environment. The ores are hosted by rhyolites within calc-alkaline volcanites of dominantly andesitic and rhyolitic composition. The very important sedex-type stratabound and stratiform base-metal deposits in China seem to have occurred mainly at times when the volcanogenic massive sulphide deposits became relatively unimportant. For instance, the Early Proterozoic, Early Palaeozoic and Early Mesozoic, are the main periods for the formation of volcex-type massive sulphides, while the Middle Proterozoic and Middle-Late Palaeozoic are most important for sedex-type, stratiform base-metal sulphides.

Bimodal magmatism in northeast Palmer Land, Antarctic Peninsula: Geochemical evidence for a Jurassic ensialic back-arc basin

Major- and trace-element (XRF and INAA) and Nd-isotope analyses are presented on a Jurassic bimodal association of basic greenstones and silicic metavolcanic rocks from the Black Coast, northeast Palmer Land, Antarctic Peninsula. The greenstones are divided into three sub-groups, indistinct in the field, but which have geochemical characteristics of island arc tholeiites (group I), E-type MORB (group II), and continental arc basalts (group III). The tholeiites of group I and II have a similar range of ϵ Nd values (+3.7 to −1.2) and were produced from a heterogeneous, large ion lithophile element (LILE)-enriched, mantle source. The rocks of group III show a much stronger enrichment in LILE than groups I and II, and were derived from a mantle source with slightly lower ϵ Nd values (−2.3 to −5.0). The silicic volcanic rocks have low ϵ Nd values (−7.1 to −8.7) indicating a dominant crustal source, although trace element concentrations show a within-plate affinity. The origin and tectonic setting of this bimodal suite is discussed in relation to Mesozoic subduction along the proto-Pacific margin of the Antarctic Peninsula and southern South America and intra-continental extension associated with the break-up of Gondwana. It is concluded that the mafic greenstones and silicic metavolcanic rocks formed in an ensialic back-arc basin setting where, during a period of continental lithospheric attenuation, the rise of a mantle diapir may have caused widespread bimodal magmatism. The geographical extent of the Palmer Land basin is unknown at present, but it may be part of a much larger Weddell Sea or proto-Weddell Sea back-arc basin system.

Abundances of volatiles and genetic relationships among submarine basalts from the Woodlark Basin, southwest Pacific

Glasses from submarine lavas of the Woodlark Basin (Southwest Pacific) have been analyzed for volatiles using dynamic high-temperature mass spectrometry. Abundances for H 2O, Cl, F, S, and CO 2 in basaltic glasses with typical mid-ocean ridge basalt (MORB) compositions and rare high Na-high Ti(NaTi) basalts are reported. Compared to similar incompatible element-depleted MORB, the Woodlark Basin basalts have Cl contents (average 0.046 ± 0.005 wt%) which are enriched and, at similar Mg#, are even higher than those from most back-arc basin (BAB) settings. Glasses from near the Woodlark Spreading Center (WSC) have H 2O abundances (average 0.210 ± 0.020 wt%) typical of most MORB, but those for the “NaTi” basalts recovered from a transform that intersects the Solomon Island arc are significantly higher (average 0.605 ± 0.020 wt%). The volatile data in the MORB are consistent with previous results from trace element and isotope studies which suggest that fluids introduced by subduction are not actively involved in the genesis of magmas along the present spreading center. Elevated Cl abundances may reflect some crustal assimilation during magmatic evolution, but the high H 2O contents in NaTi basalts appear to be a primary feature associated with the enrichment of moderately incompatible elements in mantle-derived liquids. Least-squares mixing and Rayleigh fractionation calculations were used to evaluate the roles of fractional crystallization and partial melting. It appears that the Woodlark Basin basalts have at least three different sources. The petrogenesis of “NaTi” basalts remains enigmatic as no simple model for their origin can explain their chemical characteristics.

Amphibolites of the Ashe and Alligator Back Formations, North Carolina: Samples of Late Proterozoic-early Paleozoic oceanic crust

The amphibolites of the Late Proterozoic-early Paleozoic Ashe and Alligator Back Formations in western North Carolina represent metamorphosed tholeiitic basalts, unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the Grenville-age basement rocks. On the basis of their geochemistry, the amphibolites are divisible into three compositional groups (I, II, and III), the protoliths of which appear to have evolved from different parental magmas at a spreading center. Group II (intermediate-Ti) amphibolite, the predominant variety in the study area, constitutes a highly fractionated suite (Zr ≃ 40-200 ppm, TiO2 ≃ 0.7-2.3 wt.%) with geochemical characteristics similar to those of N-type MORB formed from a heterogeneous mantle source. Group III (high-Ti) amphibolite is enriched in high-field-strength incompatible elements (Zr, Nb, Ti, Y, REE's), especially Ti (TiO2 = 2.8-3.4 wt.%), and is interpreted as T-type MORB on the basis of its Zr/Nb and Y/Nb ratios. Group I (low-Ti) amphibolite, restricted to the Ashe Formation, is characterized by extremely low abundances of incompatible elements, including Ti (TiO2 < 0.45 wt.%), and U-shaped REE profiles, requiring the mixing of a depleted MORB-type mantle source with a fluid phase enriched in LREE's. The juxtaposition of depleted, low-Ti basalt (group I amphibolite) and MORB-like basalts (group II and group III amphibolites) is suggestive of a back-arc basin setting, as has been postulated for many ophiolites, but it can also be accounted for by multi-stage melting of an upper-mantle source adjacent to a mantle plume in a mid-oceanic-ridge environment. In either case, the Ashe and Alligator Back amphibolites represent metamorphosed oceanic crust material generated at a spreading center.

Geochemistry and tectonic environment of basaltic rocks from the Misis ophiolitic mélange, south Turkey

The Miocene Misis tectonosedimentary mélange was formed during the closing stages of the Neotethys in what is now the Adana basin of southern Turkey. A major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas, associated hyaloclastites, serpentinites and various acidic tuffs. All the basaltic rocks are mildly metamorphosed (zeolite facies), variably vesicular and differentially altered to smectite, chlorite, Fe-oxides, zeolites and carbonate. Petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine-plagioclase-clinopyroxene-phyric tholeiites and minor amphibole-phyric basaltic andesites. The basaltic clasts in the mélange matrix are genetically related to the pillow lava sequences, with the observed variation throughout the suite being governed by low-pressure fractional crystallization. As a group, the basalts are characterized by low Zr/Y (2–4), relatively high Th/Yb (0.5–0.7) and La/Nb (1.5–2.5) ratios, and moderate, chondrite-normalized, light REE enrichment [ ( La Yb ) N = 3 ] patterns. The basalts have a mildly enriched, subduction-related chemistry and were possibly formed initially in a back-arc basin setting. The supra-subduction zone characteristics of the Misis basaltic units are similar to other Tethyan basalts in eastern Mediterranean ophiolites.

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with K Ar ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin. These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures. Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H 2O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al 2O 3 depletion and delays the initiation of Fe 2O 3 (tot) and TiO 2 enrichment. However, unlike arc basalts,Fe 3+/ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe 2O 3 (tot) and TiO 2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H 2O, CO 2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H 2O-rich mixed gas phase. HighH 2O/S, highδD, and low (MORB-like)δ 34S ratios are considered primary and distinctive of the back-arc basin setting.

The petrology of the Harkerbreen Group, Ny Friesland, Svalbard: protoliths and tectonic significance

AbstractThe late Precambrian–early Palaeozoic rocks of Ny Friesland, which have been subjected to Caledonian deformation and metamorphism, constitute part of the Eastern Province or Terrane of Svalbard. The Harkerbreen group and other divisions of the Stubendorffbreen supergroup form a high-grade and intensely deformed core complex to this terrane which is bounded to the west by the Billefjorden Fault Zone and to the east by a major north–south shear zone. The Stubendorffbreen rocks exhibit two gneissic foliations, one axial planar to a large scale, F1fold nappe closing to the east and the other axial planar to kilometre-scale upright F2folds subsidiary to the Atomfjella Arch. Metamorphism in the mid-amphibolite facies range coincided with generation of these folds, and F3.crenulation folding was accompanied by waningP–Tconditions. A significant proportion of the gneisses within the Harkerbreen group display silica–major element covariation patterns consistent with their position in the granodiorite field on the AFM plot. Incompatible, immobile element ratios Zr/Ti v. Nb/Y indicate affinities with rhyolites to rhyodacites which is also suggested by their REE profiles. Normalized multi-element plots of the gneisses are similar to those of granites from attenuated within-plate settings such as Mull and Skaergaard. The amphibolites which were intruded in the D1–D2interval appear to be derivatives of fractionated basalts. They plot across the calk-alkaline tholeiite boundary on the AFM diagram, and the calc-alkaline character of some of the amphibolites is further suggested by their Yb-normalized Ce-Ta abundances. Zr-Ti-Y and REE abundances would support their derivation from a related suite of fractionated basalt liquids. On the Zr/Y v. Zr discrimination diagram the amphibolites appear to have compositions transitional between Mid Ocean Ridge and Within-Plate basalts whilst the Zr-Nb-Ta plot indicates Volcanic Arc Basalt affinities. Th-Hf-Ta and multi-element plots, however, indicate a marginal to back-arc basin setting possibly above a mature subduction zone. The late Caledonian Chydenius granite is an adamellite with mixed within-plate and syn-orogenic characteristics typical of post-collisional granites.

Geochemistry of mafic rocks from the Coto Block, Zambales ophiolite, Philippines: trace element evidence for two stages of crustal growth

Slightly to pervasively metamorphosed basaltic, diabasic and gabbroic rocks from the central eastern (Coto Block) portion of the Zambales ophiolite and fresh basalts from the Mariana Trough provide new evidence than the Coto Block did not form in a simple, evolved back-arc basin setting. Although the majority of the Zambales basalts and diabases have slight LREE depletions [ ( La Sm ) ch < 1 ], REE abundances 4–15 × chondrites, and magmatic concentrations of Sr Y. Hf, Th and TiO 2 similar to back-arc spreading center basalts (N-MORB), other data, including bimodal Cr and Ni abundances and wide ranges in some incompatible element ratios (e.g., ( La Ta ) n = 0.2–21.0 ) are not typical geochemical characteristics of basalts dredged from modern back-arc basins. Furthermore, comparison of the Zambales trace element data to fresh basalts from the Mariana Trough and other ocean basins shows that such geochemical differences are not the result of low grade, sub-seafloor metamorphism. Synthesis of these geochemical findings with the currently available geological data indicates that the Coto Block of the Zambales ophiolite has experienced a two-stage crustal history. During the initial stage, dominantly N-MORB type ocean crust was formed at a large or back-arc basin spreading center but soon thereafter Coto Block crust was chemically modified by incipient island arc magmatism, most probably in a proto-forearc setting.

Geochemistry of volcanic rocks from the Naga Hills Ophiolites, northeast India and their inferred tectonic setting

Highly disrupted and deformed slices of ophiolitic rocks occurring along a linear belt in Nagaland and Manipur states, NE India, are known as the Naga Hills Ophiolites (NHO). The principal rock types include dunite, harzburgite, lherzolite, wehrlite, pyroxenite and mafic volcanics. The volcanic rocks of NHO have been classified into low-Ti and high-Ti groups with 2 wt.% TiO 2 as the boundary, which is corroborated by the differences in their immobile trace element and Rare Earth element characteristics. The two groups are probably not cogenetic; their chemical differences reflect variable source composition. Within group variation of incompatible elements such as La/Ta in the high-Ti group may be related to varying degrees of partial melting. The Ti/V ratios and REE distribution pattern of low-Ti group show overlapping Mid-Ocean Ridge Basalt and island arc-like characteristics, and possibly suggest back-arc basin setting. The high-Ti group, on the other hand, has similarities with within-plate basalts erupted at off-axis sea-mounts and ocean islands. The sea-mounts and ocean islands brought to the subduction zone acted as barriers. Within the imbricate fault slices are components from the oceanic crust which were accreted to the continental margin and which represent clipped off portions from these barriers.

The Sulu Sea, a back-arc basin setting within a Neogene collision zone

The Southeast Sulu Sea is a small oceanic basin that opened in Middle Miocene time in a back-arc setting along the Sundaland margin during subduction of the Celebes Sea plate beneath the Sulu arc. This back-arc extension post-dates compressive tectonism present in this back arc area in early Middle Miocene time. Progressive and still incomplete closing of this basin, initiated in Late Miocene time, is the result of collision of exotic terranes with the Sundaland margin. This collision also induced flipping of the Sulu subduction zone. The Neogene geodynamic evolution of the Sulu Sea provides a modern example of how young oceanic crust can be incorporated within an orogenic belt.

Dredged igneous rocks from the northern termination of the Tofua magmatic arc, Tonga and adjacent Lau Basin

Fresh igneous rocks were dredged from six stations during the 1984 cruise of the Natsushima to the North Tonga Ridge and Lau Basin. Samples were recovered from inner trench wall, forearc, arc ridge and backarc basin settings in water depths >2000 m. The volcanics are unlike those of the subaerial volcanoes of the Tofua magmatic arc, and provide a glimpse of the petrological diversity present in the submarine levels of this intra‐oceanic arc‐backarc system. Amongst the dredged lavas are unusual olivine‐ and pyroxene‐bearing vitrophyres with high‐MgO (13–20 wt%) and SiO2 (52–56 wt%) contents. These primitive lavas are distinct petrographically and geochemically from boninites in their high CaO/Al2O3ratios (0.7–0.9), high‐CaO (>7 wt%) contents, and the presence of abundant olivine phenocrysts, but are similar to low‐Ti ophiolitic basalts such as those of the Upper Pillow Lavas of the Troodos ophiolite, Cyprus. The dredged Tongan high‐Mg lavas are therefore an in situ occurrence of a low‐Ti ophiolitic lava su...

Sedimentation, Metallogenesis, and Tectonic Origin of the Basal Galice Formation Overlying the Josephine Ophiolite, Northwestern California

Basal sediments overlying the Late Jurassic Josephine ophiolite consist of interbedded siliceous argillite, radiolarian chert, and rare graywacke overlain by a thick flysch sequence; together the pelagic rocks and flysch comprise the Galice Formation in northwestern California. The transition between pelagic rocks and overlying flysch is comprised of radiolarian argillites containing abundant silt-size detritus. Deposition of many of the argillites and cherts by turbidity currents is indicated by sedimentary structures such as graded bedding. Some of the cherts are tuffaceous. Metalliferous rocks (up to 22% and 6% MnO) occur in some sections and have geochemical characteristics indicative of a substantial hydrothermal component formed by precipitation from heated seawater circulated through the underlying ophiolite. These rocks are found approximately 8-21 m above the base of the sequence, suggesting an origin from off-axis hydrothermal vents. A massive sequence of metalliferous sediments in one of the sections studied is probably a fossil mound. The presence of graywackes within the pelagic sequence, the local occurrence of an olistostrome rather than pelagic rocks directly overlying the ophiolite, and the common high detrital contents indicate deposition of the basal Galice on a topographic high (spreading ridge) within a sediment-choked basin. Previous work strongly suggests a back-arc basin setting.

Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada

Research Article| October 01, 1981 Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada ELIZABETH L. MILLER; ELIZABETH L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar JAMES BATESON; JAMES BATESON 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar DAVID DINTER; DAVID DINTER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar JAMES R. DYER; JAMES R. DYER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar DWIGHT HARBAUGH; DWIGHT HARBAUGH 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar D. L. JONES D. L. JONES 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94325 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1981) 92 (10): 730–737. https://doi.org/10.1130/0016-7606(1981)92<730:TEOTSS>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 ELIZABETH L. MILLER, JAMES BATESON, DAVID DINTER, JAMES R. DYER, DWIGHT HARBAUGH, D. L. JONES; Thrust emplacement of the Schoonover sequence, northern Independence Mountains, Nevada. GSA Bulletin 1981;; 92 (10): 730–737. doi: https://doi.org/10.1130/0016-7606(1981)92<730:TEOTSS>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 Preliminary paleontologic data from the northern Independence Mountains, Nevada, suggest that at least part of the Schoonover sequence is time equivalent to the lithologically and structurally similar upper Paleozoic Havallah sequence. The base of the Schoonover is a major thrust fault that juxtaposes paleogeographically distant but, in part, time-equivalent rocks. Along the west side of the range, the Schoonover sequence structurally overlies Late Mississippian nonmarine to shallow-marine rocks that unconformably overlie deformed Ordovician strata of the Roberts Mountains allochthon. This nonmarine to shallow-marine sequence is correlative to overlap assemblage sequences elsewhere in Nevada. Along the east side of the range, the Schoonover is thrust over shallow-marine to slope-facies Permian rocks that unconformably overlie deformed lower Paleozoic miogeoclinal rocks as well as the Roberts Mountains allochthon.The allochthonous Schoonover sequence is a heterogeneous assemblage of deformed deep-water sedimentary rocks and greenstone units. Isoclinally folded chert in the structurally lower half of the Schoonover yields ages based on radiolaria that range from earliest Mississippian to possibly latest Devonian at the very base of the sequence to as young as Early Pennsylvanian in structurally higher parts of the sequence. Clastic rocks within the lower part of the sequence contain debris derived from erosion of Roberts Mountains allochthon-type rocks. Rare volcaniclastic and volcanogenic units in the Schoonover indicate an andesitic-dacitic source. The structurally higher part of the allochthon contains a thick succession of silty-limestone turbidite deposits that represent yet a third, carbonate source terrane for the Schoonover sequence.Structural data indicate southeast-directed thrusting of the Schoonover sequence. Thrusting postdates deposition of Late Permian rocks and is inferred to predate intrusion of Late Jurassic plutons present to the north of the map area. Similarity of structural style and constraints on the timing of thrusting in the Independence Mountains strongly suggest that the Schoonover sequence may represent the northeasternmost exposures of the Golconda allochthon.The age of at least some of the pelagic sediments in the Schoonover sequence may overlap the age of the youngest rocks known in the Roberts Mountains allochthon as well as the age of the Antler Orogeny. This suggests that the Schoonover sequence was deposited, at least in part, in a location that was paleogeographically distant from the edge of the Nevadan portion of the North American shelf during the Antler Orogeny. No volcanism occurred on the continental shelf during the emplacement of the Roberts Mountains allochthon; therefore, the presence of tuffaceous turbidites within the Schoonover also suggests deposition far away from the continental margin. On the other hand, the composition of coarse clastic rocks within the Schoonover is compatible with their derivation from the eroding Roberts Mountains allochthon, suggesting proximity to the North American shelf. If sedimentary rocks from dual sources (volcanic arc and Roberts Mountains allochthon) are indeed interbedded in the Schoonover, rather than tectonically interleaved, then this might argue in favor of deposition of the Schoonover-Havallah sequences in a back-arc basin setting. This content is PDF only. 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