Volcanic Assemblage Research Articles

Predicting Archaean volcanogenic massive sulphide deposit potential from lithogeochemistry: application to the Abitibi Greenstone Belt

Archaean greenstone belts are prime exploration targets for volcanogenic massive sulphide deposits and the use of lithogeochemistry plays an important role in detecting and mapping hydrothermal alteration zones and in recognizing the massive sulphide potential of volcanic assemblages. This study reviews the state of existing lithogeochemical databases and the use of new geochemical analyses for the identification of alteration associated with massive sulphide deposits. This paper also provides a new methodology for the prediction of missing values in geochemical compositions. Through the application of statistical methods, combined sets of lithogeochemical data can be used to create predictive maps of prospective massive sulphide successions. Predictive maps are created through the use of multivariate statistical methods, lithogeochemical alteration indices and normative minerals from which areas of multi-element enrichment and depletion are defined. These indices, when combined in an exploration programme, are potentially useful for massive sulphide discovery. Additionally, the use of rare earth elements can be used to characterize differences in the various supracrustal assemblages and provide a better regional understanding of potential metal endowment. Supplementary material: Provides details on the datasets selected in this study and is available at http://www.geolsoc.org.uk/SUP18712

Permian volcanic rocks from the Apuseni Mountains (Romania): Geochemistry and tectonic constraints

An Early Permian volcanic assemblage is well exposed in the central-western part of the Apuseni Mountains (Romania). The rocks are represented by rhyolites, basalts and subordinate andesites suggesting a bimodal volcanic activity that is intimately associated with a post-orogenic (Variscan) syn-sedimentary intra-basinal continental molasse sequences. The mafic and mafic-intermediate rocks belong to sub-alkaline tholeiitic series were separated in three groups (I–III) showing a high Th and Pb abundances, depletion in Nb, Ta and Sr, and slightly enriched in LREE patterns (LaN/YbN=1.4–4.4). Isotopically, the rocks of Group I have the initial ratios 87Sr/86Sr(i)=0.709351–0.707112, 143Nd/144Nd(i)=0.512490–0.512588 and high positive ɛNd270 values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values 87Sr/86Sr(i)=0.709434–0.710092, 143Nd/144Nd(i)=0.512231–0.512210 and for ɛNd270 the negative values from −1.17 to −1.56; the rocks of Group III display for the initial ratios the values 87Sr/86Sr(i)=0.710751–0.709448, 143Nd/144Nd(i)=0.512347–0.512411 and for ɛNd270 the positive values from 1.64 to 2.35. The rocks resembling continental tholeiites, suggest a mantle origin and were further affected by fractionation and crustal contamination. In addition, the REE geochemistry (1>SmN/YbN<2.5; 0.9>LaN/SmN<2.5) suggests that these rocks were generated by high percentage partial melting of a metasomatized mantle in the garnet peridotite facies. The felsic rocks are enriched in Cs, Rb Th and U and depleted in Nb, Ta, Sr, Eu, and Ti. The REE fractionation patterns show a strong negative Eu anomaly (Eu/Eu*=0.23–0.40). The felsic rocks show the initial ratios the values: 87Sr/86Sr(i)=0.704096–0.707805, 143Nd/144Nd(i)=0.512012–0.512021 and for ɛNd270 the negative values from −5.27 to −5.44. They suggest to be generated within the lower crust during the emplacement of mantle-derived magmas that provided necessary heat to crustal partial melting.The mafic-intermediate and felsic rocks from Apuseni Mts represent a specific example of an association of bimodal volcanic rocks within a post-collisional extensional setting.

Subduction initiation along transform faults: The proto-Franciscan subduction zone

The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones present special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduction initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite melange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that reflect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that reflect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting. The Tehama-Colusa serpentinite melange consists of blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. The melange matrix represents hydrated refractory peridotites with forearc affinities, and blocks within the melange consist largely of upper-plate lithologies (harzburgite, arc volcanics, and arc-derived sediments). Lower-plate blocks within the melange include oceanic basalts and chert with rare blueschist and amphibolite. The abyssal peridotites have low pyroxene equilibration temperatures that are consistent with formation in a fracture-zone setting. However, the current melange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault or fracture zone that evolved into a subduction-zone melange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems and for the Franciscan–Coast Range ophiolite system. Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite melange.

The geochemical characteristics, geochronology and tectonic significance of the Carboniferous volcanic rocks of the Santanghu area in northeastern Xinjiang, China

The Santanghu area is located on the northeastern margin of the Junggar Basin, northern Xinjiang, Northwest China. The Carboniferous volcanic rocks in this area are widely distributed in Kaokesaiergaishan, Santanghu, Daheishan and Naomaohu districts, which are located to the north of the Kalameili Fault. These rocks, sourced from a cognate magma, consist of basic, intermediate, and acidic lavas, and pyroclastic rock. The basic volcanic rocks are enriched with large-ion lithophile elements (LILE), but are relatively depleted in high field strength elements (HFSE), and have an obvious negative Nb-Ta-Ti anomaly. They were most probably derived from a depleted mantle source, and during their ascent, these magmas were not contaminated by the crustal material as they underwent magma crystallization differentiation. Based on the Carboniferous volcanic assemblage and geochemical data, it is apparent that the early Carboniferous volcanism occurred in a subduction-related tectonic setting. New LA-ICP-MS zircon U-Pb analyses constrain the age of the andesite within the volcanic rocks as the early Carboniferous (328.9–331.3 Ma). Combined with the regional geological record, comprehensive analysis of the isotope geochronological data indicates that the subduction of the Junggar Ocean predates the early Carboniferous, and that the Santanghu island arc magmatism was induced by the subduction of the Junggar Ocean in the Carboniferous.

Diamondiferous, Neoarchean fan-delta deposits, western Superior Province, Canada: Sedimentology and provenance

Most fluvial diamond placers occur in non-aggrading braided systems where the diamonds are preferentially trapped by very rough substrates. The recognition that some of the Neoarchean, diamond-bearing breccias near Wawa, Ontario, Canada, are actually fluvial conglomerates, deposited by a mixture of traction current and mass-flow processes, provided the opportunity to add information on aggradational diamond placer systems. The 454m thick Leadbetter Conglomerate was studied to ascertain the depositional processes that led to the development and provenance of the placer. The Conglomerate is composed of a series of weakly sheared and highly viscous debris-flows interbedded with boulder to pebble conglomerates representing longitudinal bars and channels. Pebble conglomerate and sandstone channels become more dominant upwards, with a decrease in mass-flow processes. An abrupt marine transgression flooded this fan-delta assemblage ending deposition of the coarse-grained lithofacies. The lowest 100m of the conglomerate is composed of clasts derived solely from the underlying mafic volcanic assemblage. Above this zone the clast types become more diverse and felsic volcanic clasts become progressively more important upwards. Whole rock geochemistry performed on coarse-grained sandstone samples indicates that major sediment contributions came from mafic rocks and nearby diamondiferous lamprophyres and related breccias, though the possible existence of a localized kimberlitic source cannot be excluded. Geochemistry also indicates that minor enrichments of heavy minerals formed on the alluvial fan-delta surface but the primary cause of the conglomerate's diamondiferous nature was a nearby diamond-bearing source rock.

Petrogenesis and geochronology of Precambrian granitoid gneisses in Western Liaoning Province: Constraints on Neoarchean to early Paleoproterozoic crustal evolution of the North China Craton

The North Chaoyang-Fuxin-Yixian granite-greenstone belt (NCFY-GGB) at the northern margin of the Eastern Continental Block of the North China Craton is composed chiefly of metamorphic volcanic and volcano-sedimentary rocks and granitoid gneisses. The magmatic precursors of these granitoid gneisses are diorites, tonalites, trondhjemites, granodiorites and monzogranites. These granitoid gneisses can be divided into two groups: a high Mg group (HMG) and a low Mg group (LMG) according to their contrasting geochemical characteristics. Petrogenetic studies indicate that the HMG magmas were derived from partial melting of a subducted oceanic slab, which was contaminated by mantle peridotites during their ascent, whereas the LMG magmas were generated from the partial melting of lower continental crust composed chiefly of metamorphosed basaltic and pelitic rocks. LA-ICPMS zircon U–Pb dating reveals that granitoid gneisses in the HMG and low-K granitoid gneisses in the LMG were emplaced at 2511–2521Ma, but that the high-K granitoid gneisses in the LMG formed later at ca. 2495Ma. These Neoarchean to early Paleoproterozoic granitoid gneisses, intrusive into the metamorphic volcanic assemblages of the Yixian-Fuxin greenstone belt, indicate that the NCFY-GGB was developed at a Neoarchean to early Paleoproterozoic Andean-type active continental margin.

Geochemical characteristics of island-arc volcanic rocks in the Nan-Nam Pat-Phetchabun zone, northern Thailand

Late Permian-Early Triassic (P2-T1) volcanic rocks distributed on the eastern side of ocean-ridge and oceanic-island basalts in the Nan-Uttaradit zone were analyzed from aspects of petrographic characteristics, rock assemblage, REE, trace elements, geotectonic setting, etc., indicating that those volcanic rocks possess the characteristic features of island-arc volcanic rocks. The volcanic rock assemblage is basalt-basaltic andesite-andesite. The volcanic rocks are sub-alkaline, dominated by calc-alkaline series, with tholeiite series coming next. The chemical composition of the volcanic rocks is characterized by low TiO2 and K2O and high Al2O3 and Na2O. Their REE patterns are of the flat, weak LREE-enrichment right-inclined type. The trace elements are characterized by the enrichment of large cation elements such as K, Rb and Ba, common enrichment of U and Th, and depletion of Nb, Ta, Zr and Hf. The petrochemical plot falls within the field of volcanic rocks, in consistency with the plot of island-arc volcanic rocks in the Jinsha River zone of China. This island-arc volcanic zone, together with the ocean-ridge/oceanic island type volcanic rocks in the Nan-Uttaradit zone, constitutes the ocean-ridge volcanic rock-island-arc magmatic rock zones which are distributed in pairs, indicating that the oceanic crust of the Nan-Uttaradit zone once was of eastward subduction. This work is of great significance in exploring the evolution of paleo-Tethys in the Nan-Uttaradit zone.

Study on the geochemical characteristics of arc-volcanic rocks in the Chiang Rai-Lampang belt of northern Thailand

On the basis of the petrographic characteristics, rock assemblages, petrochemistry, REEs, trace elements and geotectonic settings, the authors described the characteristics of continental marginal arc-volcanic rocks in the Late Permian-Early Triassic (P2-T1) volcanic rocks distributed on the eastern side of the ocean-ridge/oceanic island basalts in the Chiang Mai belt. The volcanic rock assemblage is basaltic andesite-andesite-rhyolite. The volcanic series is dominated by the calc-alkaline series, with the tholeitic series coming next. The chemical composition of the volcanic rocks is characterized by high Al2O3; the REE distribution patterns are of the LREE-enrichment rightward incline type; the large cation elements are highly enriched, and the volcanic rocks are generally enriched in U and Th and depleted in Ti, Cr and P. The petrochemical plot falls within the field of island-arc volcanic rocks, in consistency with the projected points of continental marginal arc-volcanic rocks in the Lancangjiang belt. These continental marginal arc-volcanic rocks, together with ocean-ridge/oceanic island-type volcanic rocks in the Chiang Mai belt, constitute the ocean-ridge volcanic rock-arc magmatic rock belts distributed in pairs, indicative of eastward subduction of the oceanic crust in the Chiang Mai belt. This result is of great importance in exploring the evolution of the paleo-Tethys in the Chiang Mai belt.

Detrital zircon sedimentary provenance ages for the Eoarchaean Isua supracrustal belt southern West Greenland: Juxtaposition of an imbricated ca. 3700 Ma juvenile arc against an older complex with 3920–3760 Ma components

The amphibolite facies Eoarchaean Isua supracrustal belt (northern part of the Nuuk region, southern West Greenland) is dominated by strongly deformed metabasalts, with chert, banded iron formation, felsic volcanic and volcano-sedimentary rocks and minor gabbro and sedimentary carbonates. It comprises a suture zone between a northern terrane formed at ca. 3700Ma and a southern one formed at ca. 3800Ma. At the junction between these two terranes is a strongly tectonised, thin unit of metachert, BIF and carbonate-bearing rocks with minor detrital components, named the dividing sedimentary unit. Away from the belt, the northern terrane is dominated by ca. 3700Ma tonalites and the southern one by ca. 3800Ma tonalites.The Isua supracrustal belt chemical metasedimentary rocks give low yields of small (commonly ≤50μm long), prismatic, oscillatory-zoned zircons with less common, somewhat rounded zircons of volcanic and/or detrital origin. The most siliceous of the felsic schists give a larger yield of volcanic or detrital zircons. The ages of these zircons are distinct in different parts of the belt. In the northern terrane, most have ages of 3710–3690Ma, with a few slightly older <3750Ma grains. In the southern 3800Ma terrane, a fuchsite-quartzite has 3900–3810Ma zircons. Three samples of the dividing sedimentary unit along the whole length of the belt yielded both 3920–3880Ma (in two samples) and ca. 3750Ma detrital zircons (in all three samples). The part of the ca. 3700Ma northern terrane in the Isua supracrustal belt is interpreted as lithologically diverse and imbricated assemblage of ca. 3700Ma arc-related volcanic rocks (boninitic, island arc tholeiite-picrite and andesite-dacite). In the associated metasedimentary rocks, the lack of >3750Ma detrital zircons suggests that these volcanic assemblages evolved remote from 3800Ma crust. In contrast, the up to 3940Ma detrital zircons from the dividing sedimentary unit and the southern fuchsite quartzite shows that the southern ca. 3800Ma terrane, although dominated by arc-related rocks, might contain older (pre-3800Ma) crust. We propose that the dividing sedimentary unit was deposited on top of already deformed southern terrane rocks, and then acted as a décollement when the northern terrane ca. 3700Ma arc complex collided with the southern terrane by ca. 3660Ma (from previous age constraints). Thus the detrital zircon age data is compatible with proposals from other evidence for juxtaposition of ca. 3700 and 3800Ma terranes in the Isua area.Some thin siliceous metasedimentary rocks along post-assembly mylonite zones in the Isua supracrustal belt yield some <3660Ma zircons. A tentative interpretation of these siliceous rocks (requiring confirmation) is that they could be younger, post-collisional sedimentary rocks, which were incorporated as tectonic lenses along some of the post-terrane-assembly <3660Ma mylonites which partition the collisional assemblage.

A swarm of small shield volcanoes on Syria Planum, Mars

This study focuses on the volcanism in Syria Planum, located at the center of the Tharsis bulge at an altitude of 6 to 8 km above Mars datum. Syria Planum was previously recognized as a center for the tectonic activity of Tharsis, but not as a major locus for volcanic activity, despite its centrality over the bulge. Using high‐resolution images from the high resolution stereo camera on Mars Express combined with Mars Observer Laser Altimeter data, we have characterized a volcanic system that reveals a number of very interesting aspects of Mars volcanism. We identified a swarm of tens of coalesced shallow volcanic edifices, typically 10–30 km diameter, 0.1–0.2 km high, and with slopes around 0.5°. These characteristics are similar to those of small shield volcanoes found in Iceland. In addition, an intermediate‐sized volcano, which is the source of lava flows that extend over &gt;200 km, is observed west of this shield swarm. Our study characterizes a previously unrecognized volcanic assemblage on Mars which appears to be much more developed than was documented before, in terms of morphology, inferred origin, and periodicity of eruption. The estimated lava flux of the Syria Planum volcanoes is of the same order as the lava flux of Tharsis Montes. These characteristics suggest that Syria Planum experienced a very specific style of volcanism, which we dated to the Hesperian period.

Geologic and Structural Setting of Gold Mineralization in the Kirkland Lake-Larder Lake Gold Belt, Ontario

The Kirkland Lake-Larder Lake gold belt includes the giant Kirkland Lake and world-class Kerr-Addison-Chesterville gold deposits, along with several smaller deposits and occurrences. It corresponds to an east-trending band of Timiskaming clastic and volcanic rocks that unconformably overlie older volcanic assemblages in the southern Abitibi greenstone belt of the Archean Superior province. The gold belt is bounded to the south by the Larder Lake-Cadillac deformation zone, which roughly follows the contact between the younger Timiskaming rocks and the older volcanic units. Three generations of fabrics formed during post-Timiskaming regional deformation (D2, D3, and D4) of the belt. North-south shortening during D2 produced a penetrative, generally east-striking, steeply dipping, S2 foliation, and an east-plunging, L2 stretching lineation. S2 is most intense within the syn-D2 Larder Lake-Cadillac deformation zone and its northeast-trending splay, the Upper Canada deformation zone. The D2 structures are overprinted by a north-trending crenulation cleavage S3, which formed during east-west D3 shortening across the belt. A northeast-trending regional S4 foliation associated with Z-shaped F4 folds and overprinting both S2 and S3 formed during northwest-southeast D4 shortening. Gold mineralization is localized along the Larder Lake-Cadillac deformation zone (Anoki and McBean deposits), the Upper Canada deformation zone (Upper Canada deposit), and the brittle Kirkland Lake fault and ‘04 Break (Kirkland Lake deposit). The Upper Canada, McBean, and Anoki deposits formed during D2, and, along with Kerr-Addison-Chesterville, Omega, and Cheminis deposits, are probably related to a regionally extensive hydrothermal system associated with the Larder Lake-Cadillac deformation zone. The sulfide-poor gold- and telluride-bearing quartz veins of the Kirkland Lake deposit are interpreted to have been emplaced during D4, synchronous with reverse-dextral movement along the ore-controlling brittle Kirkland Lake fault. The Kirkland Lake mineralization has a distinct metal signature (Te>Au, Mo, Pb, Ag, high Au/Ag, low As) and probably represents a separate hydrothermal system linked to a deep magmatic (alkalic) fluid source and unrelated to mineralization along the syn-D2 deformation zones.

Linzizong Volcanic Rocks in Linzhou of Tibet: A Volcanic Petrologic Assemblage in Continental Collision Environment

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 128

MAFIC MAGMAS AS SOURCES FOR GOLD IN MIDDLE MIOCENE EPITHERMAL DEPOSITS OF THE NORTHERN GREAT BASIN, UNITED STATES: EVIDENCE FROM Pb ISOTOPE COMPOSITIONS OF NATIVE GOLD

We measured the Pb isotope compositions of traces of lead alloyed by gold, electrum and naumannite, and gangue vein minerals in bonanza epithermal gold deposits of the Northern Great Basin, and compared them to possible sources of lead in the region. Gangue minerals have Pb isotope compositions consistent with hydrothermal leaching of lead from local host rocks. Lead isotope ratios of gold ores, however, have signatures that are distinct from the gangue minerals and common host rocks in the area, such as metasediments and Miocene rhyolites. The gold ores are isotopically similar to local mafic rocks of the middle Miocene bimodal volcanic assemblage, indicating that the latter are the likely source of the precious metals. The local mafic rocks are also similar to the middle Miocene Columbia River Basalts, suggesting that both were derived from a mantle source with similar Pb isotope composition. Our preliminary results indicate that the precious metals were derived from mafic magmas likely related to northern Nevada rifting processes and/or to the arrival of the Columbia River Basalt plume and the emergence of the Yellowstone hot spot. Exsolving volatiles from the mafic magmas most likely transported gold upward to the shallow, meteoric water-dominated epithermal systems, where the precious metals precipitated together with quartz and adularia to form bonanza ores that typify these deposits.

Stratigraphic and U‐Pb SHRIMP Detrital Zircon Evidence for a Neoproterozoic Continental Arc, Central China: Rodinia Implications

The Bikou terrane of central China is located within the southwest Qinling Mountains along the northwest margin of the Yangtze Plate. The terrane consists of a volcanic assemblage, the Bikou Group, and a correlative volcaniclastic turbidite sequence, the Hengdan Group. U‐Pb SHRIMP detrital zircon dating of 181 grains collected from the Hengdan Group indicates ages from ca. 850 to 700 Ma, with a peak near 770 Ma. Given the predominance of volcanic, single‐cycle zircons within the Hengdan Group and the thick succession of coarsening‐upward volcaniclastic marine sediments, the sequence is best interpreted as a fore‐arc basin. This interpretation is supported by previous U‐Pb SHRIMP and conventional U‐Pb zircon dating performed within the Bikou Group that indicates subduction‐related magmatism persisted from at least ca. 850 to as recently as 700 Ma. Together, these data indicate that the Bikou terrane is a fragment of a Neoproterozoic continental arc system and that the Yangtze Plate was bordered by a convergent oceanic margin. The South China Block must therefore occupy an outboard position with respect to Rodinia reconstruction models.

Late Cenozoic sedimentation and volcanism during transtensional deformation in Wingate Wash and the Owlshead Mountains, Death Valley

New 1 : 24,000 scale mapping, geochemical analyses of volcanic rocks, and Ar/Ar and tephrochronology analyses of the Wingate Wash, northern Owlshead Mountain and Southern Panamint Mountain region document a complex structural history constrained by syntectonic volcanism and sedimentation. In this study, the region is divided into five structural domains with distinct, but related, histories: (1) The southern Panamint domain is a structurally intact, gently south-tilted block dominated by a middle Miocene volcanic center recognized as localized hypabyssal intrusives surrounded by proximal facies pyroclastic rocks. This Miocene volcanic sequence is an unusual alkaline volcanic assemblage ranging from trachybasalt to rhyolite, but dominated by trachyandesite. The volcanic rocks are overlain in the southwestern Panamint Mountains by a younger (Late Miocene?) fanglomerate sequence. (2) An upper Wingate Wash domain is characterized by large areas of Quaternary cover and complex overprinting of older structure by Quaternary deformation. Quaternary structures record ∼N–S shortening concurrent with ∼E–W extension accommodated by systems of strike-slip and thrust faults. (3) A central Wingate Wash domain contains a complex structural history that is closely tied to the stratigraphic evolution. In this domain, a middle Miocene volcanic package contains two distinct assemblages; a lower sequence dominated by alkaline pyroclastic rocks similar to the southern Panamint sequence and an upper basaltic sequence of alkaline basalt and basanites. This volcanic sequence is in turn overlain by a coarse clastic sedimentary sequence that records the unroofing of adjacent ranges and development of ∼N–S trending, west-tilted fault blocks. We refer to this sedimentary sequence as the Lost Lake assemblage. (4) The lower Wingate Wash/northern Owlshead domain is characterized by a gently north-dipping stratigraphic sequence with an irregular unconformity at the base developed on granitic basement. The unconformity is locally overlain by channelized deposits of older Tertiary(?) red conglomerate, some of which predate the onset of extensive volcanism, but in most of the area is overlain by a moderately thick package of Middle Miocene trachybasalt, trachyandesitic, ash flows, lithic tuff, basaltic cinder, basanites, and dacitic pyroclastic, debris, and lahar flows with localized exposures of sedimentary rocks. The upper part of the Miocene stratigraphic sequence in this domain is comprised of coarse grained-clastic sediments that are apparently middle Miocene based on Ar/Ar dating of interbedded volcanic rocks. This sedimentary sequence, however, is lithologically indistinguishable from the structurally adjacent Late Miocene Lost Lake assemblage and a stratigraphically overlying Plio-Pleistocene alluvial fan; a relationship that handicaps tracing structures through this domain. This domain is also structurally complex and deformed by a series of northwest–southeast-striking, east-dipping, high-angle oblique, sinistral, normal faults that are cut by left-lateral strike-slip faults. The contact between the southern Panamint domain and the adjacent domains is a complex fault system that we interpret as a zone of Late Miocene distributed sinistral slip that is variably overprinted in different portions of the mapped area. The net sinistral slip across the Wingate Wash fault system is estimated at 7–9 km, based on offset of Proterozoic Crystal Springs Formation beneath the middle Miocene unconformity to as much as 15 km based on offset volcanic facies in Middle Miocene rocks. To the south of Wingate Wash, the northern Owlshead Mountains are also cut by a sinistral, northwest-dipping, oblique normal fault, (referred to as the Filtonny Fault) with significant slip that separates the Lower Wingate Wash and central Owlshead domains. The Filtonny Fault may represent a young conjugate fault to the dextral Southern Death Valley fault system and may be the northwest-dipping fault imaged by COCORP studies. Similarly, younger deformation in upper Wingate Wash is probably broadly related to distributed dextral shear along the Panamint Valley fault system. Earlier deformation (Late Miocene?) is more difficult to constrain because of overprinting but appears to be dominated by an E–W extension recognized by a NNW-striking, northeast-dipping, sinistral-oblique normal faults, ∼N–S striking normal faults that splay in the northern Owlshead Mountains and include the large west-tilted fault blocks of the northern Owlshead Mountains.

Re-Os systematics of komatiites and komatiitic basalts at Dundonald Beach, Ontario, Canada: Evidence for a complex alteration history and implications of a late-Archean chondritic mantle source

Osmium isotopic compositions, and Re and Os concentrations have been examined in one komatiite unit and two komatiitic basalt units at Dundonald Beach, part of the 2.7 Ga Kidd-Munro volcanic assemblage in the Abitibi greenstone belt, Ontario, Canada. The komatiitic rocks in this locality record at least three episodes of alteration of Re-Os elemental and isotope systematics. First, an average of 40% and as much as 75% Re may have been lost due to shallow degassing during eruption and/or hydrothermal leaching during or immediately after emplacement. Second, the Re-Os isotope systematics of whole rock samples with 187Re/ 188Os ratios >1 were reset at ∼2.5 Ga, possibly due to a regional metamorphic event. Third, there is evidence for relatively recent gain and loss of Re in some rocks. Despite the open-system behavior, some aspects of the Re-Os systematics of these rocks can be deciphered. The bulk distribution coefficient for Os (D Os solid/liquid) for the Dundonald rocks is ∼3 ± 1 and is well within the estimated D values obtained for komatiites from the nearby Alexo area and stratigraphically-equivalent komatiites from Munro Township. This suggests that Os was moderately compatible during crystal-liquid fractionation of the magmas parental to the Kidd-Munro komatiitic rocks. Whole-rock samples and chromite separates with low 187Re/ 188Os ratios (<1) yield a precise chondritic average initial 187Os/ 188Os ratio of 0.1083 ± 0.0006 (γ Os = 0.0 ± 0.6) for their well-constrained ∼2715 Ma crystallization age. The chondritic initial Os isotopic composition of the mantle source for the Dundonald rocks is consistent with that determined for komatiites in the Alexo area and in Munro Township, suggesting that the mantle source region for the Kidd-Munro volcanic assemblage had evolved with a long-term chondritic Re/Os before eruption. The chondritic initial Os isotopic composition of the Kidd-Munro komatiites is indistinguishable from that of the projected contemporaneous convective upper mantle. The uniform chondritic Os isotopic composition of the Kidd-Munro komatiites contrasts with the typical large-scale Os isotopic heterogeneity in the mantle sources for ca. 89 Ma komatiites from the Gorgona Island, arc-related rocks and present-day ocean island basalts. This suggests that the Kidd-Munro komatiites sampled a late-Archean mantle source region that was significantly more homogeneous with respect to Re/Os relative to most modern mantle-derived rocks.

Mississippian volcanic assemblage conformably overlying Cordilleran miogeoclinal strata, Turnagain River area, northern British Columbia, is not part of an accreted terrane

A Paleozoic volcanic assemblage exposed in northern British Columbia, near the Turnagain River, previously considered to be part of an accreted terrane, was reported to be in depositional contact with a part of the Cordilleran miogeocline. This paper presents an integrated field, U–Pb geochronology, Sm–Nd isotopic, and geochemical study across the basal contact of the volcanic assemblage. Strongly evolved εNd(T) values, between –13 and –21, from samples of lower Paleozoic sedimentary rocks exposed below the volcanic rocks, and correlated with Atan – Kechika – Road River – Earn strata of the miogeocline farther east, support a North American miogeoclinal affinity, consistent with previously established regional stratigraphic and structural relationships. Nd isotopic data from the volcanic assemblage contrast significantly with data from the sedimentary rocks and record a mantle source (εNd(T) values between +4.0 and +7.0), consistent with a magmatic arc or back arc; negative Nb anomalies are similarly compatible with either arc- or back-arc-related magmatism. A concordant 339.7 ± 0.6 Ma U–Pb zircon date was obtained from the volcanic assemblage. The mixed gradational contact between the miogeoclinal and volcanic rocks is marked by interlayering of finely laminated grey and green phyllites on the scale of centimetres, with no evidence of a tectonic contact. Bedding at the contact is folded into tight outcrop-scale folds that are intruded by an Early Jurassic (187.5 ± 2.9 Ma) granodiorite. On the basis of all available evidence, the contact is interpreted as a facies transition. The Mississippian volcanic assemblage may link the miogeocline with the early development of an Angayucham – Slide Mountain basin.

The Shishkhid ophiolite, northern Mongolia: A key to the reconstruction of a Neoproterozoic island-arc system in central Asia

The Shishkhid ophiolite is a well-preserved 13 km-thick mafic-ultramafic assemblage which comprises (from bottom to top): mantle tectonites (∼6 km), layered and isotropic gabbro (∼4.5 km), sheeted dykes (up to 0.5 km), a bimodal assemblage of basalt and rhyolite (up to 0.7 km), as well as andesitic pyroclastic rocks (∼2 km). The volcanic rocks are overlain by a 3 km-thick sedimentary sequence showing progressive subsidence of the volcanic edifice after cessation of volcanism. The sedimentary unit is unconformably overlain by Ediacaran-Cambrian platform sediments. SHRIMP U-Pb dating of magmatic zircons from a rhyolite of the lower volcanic unit has yielded a concordant 206Pb/ 238U age of 800 ± 2.6 Ma which is interpreted to reflect the time of magma crystallisation. Samples of the gabbro, the bimodal sequence, and andesitic unit show predominantly a calc-alkaline melt fractionation trend. The trace element patterns exhibit subduction-related characteristics such as high abundances of fluid-mobile elements and negative anomalies for Nb relative to La. The initial ɛ Nd values of +6.9 to 0 indicate melting of variably depleted and heterogenous upper mantle sources. These were probably produced by input of a component from subducted sediment. Two samples lacking large negative Nb-anomalies reveal distinct mantle sources not enriched with a slab-derived sedimentary component. We interpret them as melts from an upwelling heterogeneous asthenospheric source (initial ɛ Nd of 3.2 to 6.9) in a rifting arc environment. The Nd-isotopes in a rhyolite sample from the bimodal mafic-felsic volcanic assemblage suggest an origin by melting of gabbroic lower crust, probably due to basaltic injections in the course of arc-rifting. The tectonic setting for the Shishkhid ophiolite is inferred to be similar to that of the Izu-Bonin “back-arc knolls extensional zone”. The Shishkhid arc formed in the mid-Neoproterozoic and evolved through most of the late Neoproterozoic until it collided with a continental block at the end of the Neoproterozoic. This event is termed as the late Baikalian orogenic phase widely recognized in southern Siberia. The Shishkhid ophiolite and related rocks have been traced for about 600 km, and similar units can be found in other Precambrian terrains of central Asia. Thus, the Shishkhid oceanic-arc system was a major tectonic feature of the Neoproterozoic Palaeo-Asian ocean. Its recognition suggests that the central Asian tectonic evolution encompassed the development of three generations of island arc systems replacing each other throughout the Neoproterozoic to early Palaeozoic.

MANGALA VALLES, MARS: ASSESSMENT OF EARLY STAGES OF FLOODING AND DOWNSTREAM FLOOD EVOLUTION

The Mangala Valles system is an ∼ ∼900 km fluvially carved channel system located southwest of the Tharsis rise and is unique among the martian outflow channels in that it heads at a linear fracture within the crust as opposed to a collapsed region of chaos as is the case with the circum-Chryse channels. Mangala Valles is confined within a broad, north–south trending depression, and begins as a single valley measuring up to 350 km wide that extends northward from a Memnonia Fossae graben, across the southern highlands toward the northern lowlands. Approximately 600 km downstream, this single valley branches into multiple channels, which ultimately lose their expression at the dichotomy boundary. Previous investigations of Mangala Vallis suggested that many of the units mapped interior to the valley were depositional, related to flooding, and that a minimum of two distinct periods of flooding separated by tens to hundreds of millions of years were required to explain the observed geology. We use infrared and visible images from the THermal EMission Imaging System (THEMIS), and topographic data from the Mars Orbiting Laser Altimeter (MOLA), to investigate the nature of the units mapped within Mangala Vallis. We find that the geomorphology of the units, as well as their topographic and geographic distribution, are consistent with most of them originating from a single assemblage of volcanic flow deposits, once continuous with volcanic flows to the south of the Memnonia Fossae source graben. These flows resurfaced the broad, north–south trending depression into which Mangala Vallis formed prior to any fluvial activity. Later flooding scoured and eroded this volcanic assemblage north of the Mangala source graben, resulting in the present distribution of the units within Mangala Vallis. Additionally, our observations suggest that a single period of catastrophic flooding, rather than multiple periods separated by tens to hundreds of millions of years, is consistent with and can plausibly explain the interior geology of Mangala Vallis. Further, we present a new scenario for the source and delivery of water to the Mangala source graben that models flow of groundwater through a sub-cryosphere aquifer and up a fracture that cracks the cryosphere and taps this aquifer. The results of our model indicate that the source graben, locally enlarged to a trough near the head region of Mangala, would have required less than several days to fill up prior to any spill-over of water to the north. Through estimates of the volume of material missing from Mangala (13,000–20,000 km3), and calculation of mean discharge rates through the channel system (∼ ∼5 × 106 m3 s−1), we estimate that the total duration of fluvial activity through the Mangala Valles was 1–3 months.

Neoarchaean volcanic rocks, Central Hearne supracrustal belt, Western Churchill Province, Canada: geochemical and isotopic evidence supporting intra-oceanic, supra-subduction zone extension

The Kaminak segment of the Central Hearne supracrustal belt (CHSB) Western Churchill Province, Canada, comprises a diverse sequence of Neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695–2711 Ma and 2681–2686 Ma, respectively. These are intruded by uncommon pre-tectonic diorites and tonalites (ca. 2691 Ma), voluminous syn-tectonic tonalites and granodiorites (ca. 2679–2686 Ma) and, rare post–tectonic potassic monzogranite and syenite (ca. 2659–2666 Ma). Metasedimentary rocks include turbidites, epiclastic tuffs, minor iron formation (oxide) and rare volcanic conglomerates. Volcanic rocks of assemblage I include abundant pillowed and massive basalts to andesites with less common silicic lavas, tuffs and volcaniclastic debris flow deposits. Assemblage II contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows. The critical diagnostic feature of the CHSB is the stratigraphic intercalation of compositionally diverse basaltic, andesitic and felsic volcanic rocks throughout both assemblages. Mapping, U–Pb geochronology and lithogeochemistry suggest that an initial MORB-like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca. 2688 Ma by a relatively short-lived, dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686–2679 Ma tonalitic to granodioritic plutons. Basaltic to andesitic rocks are dominated by iron-rich tholeiites, although the proportion of calc-alkaline rocks increases with silica content. Felsic volcanic rocks all exhibit calc-alkaline affinities. The wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous MORB-, with less common ARC-, NEB(OIB)- and rare BABB-like rocks. Similarly, the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers. Two geochemically distinct, arc-like mafic suites were generated through contamination of primary mantle-derived magmas by juvenile, ca. 2700 Ma silicic crust either in their mantle source or through assimilation upon ascent. ϵNd t isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older, ⋙2700 Ma crust. The CHSB may have formed via tectono-magmatic processes comparable to those of the Eocene, infant-arcs of the SW Pacific, whereby the formation of a thick sequence of coeval, intercalated, compositionally diverse mantle- and crustal-derived rocks, are generated in an extensional supra-subduction setting. The cessation of supra-subduction zone extension at ca. 2688 Ma, was followed by the short-lived development of felsic volcanic edifices (incipient arc), the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn-kinematic tonalitic plutons. This accompanied a major change in the tectono-magmatic setting accompanying and presumably following the termination of extensional, supra-subduction zone processes.