Basic Volcanic Rocks Research Articles

Geological significance of high-resolution magnetic data in the Quesnel terrane, Central British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.2Geological Survey of Canada Contribution No. 20100225.

The ability of airborne sensors to image the magnetic signatures of prospective Quesnel terrane rocks through ubiquitous Quaternary glacial sedimentary cover in central British Columbia helps target new areas for mineral exploration. Newly acquired high-resolution data provide new perspectives on the nature and probable areal distribution of many geological units, revealing detail and information unattainable by conventional geological mapping. In combination with gravity data, these magnetic data indicate the presence of a granitic intrusion and a development of Nicola Group volcanic rocks, both potential hosts for porphyry- and (or) vein-type mineralization, under younger Tertiary volcanic cover. At a finer scale, magnetic patterns and fabrics permit discrimination between volcanic rocks of the Tertiary Chilcotin and Kamloops groups, and detection of subtle compositional and (or) structural variations within the groups. Contacts between volcanic cover and basement rocks and between basement units are more accurately defined, significantly reducing locally the areal extent of volcanic cover and opening up more ground for exploration. The high resolution of features in images of magnetic vertical derivatives reveals the Naver pluton to be more complex than currently mapped, comprising several integral elements, one of which may be a large roof pendant. Internal subdivisions of the Thuya batholith are defined, and annular marginal phases are proposed within two large granodioritic intrusions. Several new intrusions are proposed within the extensive, mainly sedimentary Devonian–Triassic terrain northeast of Kamloops, internal composition variation is suggested for some larger mapped intrusions, and areas underlain by some intrusions are enlarged.

The Lake El’gygytgyn Scientific Drilling Project – Conquering Arctic Challenges through Continental Drilling

Between October 2008 and May 2009, the International Continental Scientific Drilling Program (ICDP) co-sponsored a campaign at Lake Elgygytgyn, located in a 3.6-Ma-old meteorite impact crater in northeastern Siberia. Drilling targets included three holes in the center of the 170-m-deep lake, utilizing the lake ice cover as a drilling platform, plus one hole close to the shore in the western lake catchment. At the lake's center. the entire 315-m-thick lake sediment succession was penetrated. The sediments lack any hiatuses (i.e., no evidence of basin glaciation or desicca- tion), and their composition reflects the regional climatic and environmental history with great sensitivity. Hence, the record provides the first comprehensive and widely time- continuous insights into the evolution of the terrestrial Arctic since mid-Pliocene times. This is particularly true for the lowermost 40 meters and uppermost 150 meters of the sequence, which were drilled with almost 100% recovery and likely reflect the initial lake stage during the Pliocene and the last ~2.9 Ma, respectively. Nearly 200 meters of under- lying rock were also recovered; these cores consist of an almost complete section of the various types of impact brec- cias including broken and fractured volcanic basement rocks and associated melt clasts. The investigation of this core sequence promises new information concerning the Elgygytgyn impact event, including the composition and nature of the meteorite, the energy released, and the shock behavior of the volcanic basement rocks. Complementary information on the regional environmental history, including the permafrost history and lake-level fluctuations, is being developed from a 142-m-long drill core recovered from the permafrost deposits in the lake catchment. This core con- sists of gravelly and sandy alluvial fan deposits in ice-rich permafrost, presumably comprising a discontinuous record of both Quaternary and Pliocene deposits.

The Lake El'gygytgyn Scientific Drilling Project – Conquering Arctic Challenges through Continental Drilling

Between October 2008 and May 2009, the International Continental Scientific Drilling Program (ICDP) co-sponsored a campaign at Lake El´gygytgyn, located in a 3.6-Ma-old meteorite impact crater in northeastern Siberia. Drilling targets included three holes in the center of the 170-m-deep lake, utilizing the lake ice cover as a drilling platform, plus one hole close to the shore in the western lake catchment. At the lake’s center. the entire 315-m-thick lake sediment succession was penetrated. The sediments lack any hiatuses (i.e., no evidence of basin glaciation or desiccation), and their composition reflects the regional climatic and environmental history with great sensitivity. Hence, the record provides the first comprehensive and widely timecontinuous insights into the evolution of the terrestrial Arctic since mid-Pliocene times. This is particularly true for the lowermost 40 meters and uppermost 150 meters of the sequence, which were drilled with almost 100% recovery and likely reflect the initial lake stage during the Pliocene and the last ~2.9 Ma, respectively. Nearly 200 meters of underlying rock were also recovered; these cores consist of an almost complete section of the various types of impact breccias including broken and fractured volcanic basement rocks and associated melt clasts. The investigation of this core sequence promises new information concerning the El´gygytgyn impact event, including the composition and nature of the meteorite, the energy released, and the shock behavior of the volcanic basement rocks. Complementary information on the regional environmental history, including the permafrost history and lake-level fluctuations, is being developed from a 142-m-long drill core recovered from the permafrost deposits in the lake catchment. This core consists of gravelly and sandy alluvial fan deposits in ice-rich permafrost, presumably comprising a discontinuous record of both Quaternary and Pliocene deposits. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.11.03.2011" target="_blank">10.2204/iodp.sd.11.03.2011</a>

Mapping radioactivity in groundwater to identify elevated exposure in remote and rural communities

A survey of radioactivity in groundwater (110 sites) was conducted as a precursor to providing a baseline of radiation exposure in rural and remote communities in Queensland, Australia, that may be impacted upon by exposure pathways associated with the supply, treatment, use and wastewater treatment of the resource. Radionuclides in groundwater, including 238U, 226Ra, 222Rn, 228Ra, 224Ra and 40K were measured and found to contain activity concentration levels of up to 0.71 BqL −1, 0.96 BqL −1, 108 BqL −1, 2.8 BqL −1, 0.11 BqL −1 and 0.19 BqL −1 respectively. Activity concentration results were classified by aquifer lithology, showing correlation between increased radium isotope concentration and basic volcanic host rock. The groundwater survey and mapping results were further assessed using an investigation assessment tool to identify seven remote or rural communities that may require additional radiation dose assessment beyond that attributed to ingestion of potable water.

GEOCHEMICAL AND PETROLOGICAL CHARACTERISTICS OF LATE TRIASSIC BASIC VOLCANIC ROCKS FROM THE KOCALI COMPLEX; SE TURKEY: IMPLICATIONS FOR THE TRIASSIC EVOLUTION OF SOUTHERN TETHYS

The Kocali Complex in SE Anatolia is mainly composed of tectonically imbricated slices of pelagic rock suits (pelagic limestones; cherts; etc); platform carbonates; clastics; serpentinites and basic volcanics. It can be subdivided into four main parts as the Tarasa Unit; the Konak Unit; the Cilo Limestones and the Kale Ophiolite. In this paper; geochemical characteristics of basic volcanic rocks associated with pelagic sediments (radiolarian cherts; pelagic limestones; etc) have been studied along four stratigraphical sections (the Tarasa; the Bulam-2; the Korun-1 and the Korun-2) in the Konak and the Tarasa Units. The age of the volcanic sequences have been assigned to Late Triassic (middle Carnian to Rhaetian) based on the radiolarian data from the associated pelagic rock units. Geochemical data indicates the presence of two different types of Late Triassic volcanic rock groups characterized by E-MORB-type (Group-1 volcanites from the Tarasa Unit formed by mixing of OIB and MORB-type melts) and OIB-type (Group-2 volcanites from the Konak Unit) mantle sources. None of the samples from the Kocali Complex has characteristics of N-MORB type mantle source and was affected by crustal contamination. Based on these data; it can be suggested that all these volcanic rocks formed in a marginal oceanic basin away from the ocean ridge. The presence of Radiolaria and Bivalvia-bearing pelagic limestones and cherts associated to the basic volcanic rocks in the Kocali Complex also emphasize that they all formed in deep marine basin. Geochemical characteristics of Late Triassic volcanics in the Kocali Complex have great similarities and well-correlative to the coeval volcanics previously studied in the Southern Tethyan Belt (e.g. the Baer-Bassit region in NW Syria; the Mamonia Complex in SW Cyprus and the Antalya Nappes in SW Turkey). Taking into the consideration of the geochemical data obtained from Late Triassic basic volcanics in the Kocali Complex and geochemical characteristics of the coeval volcanics in the adjacent units in the Southern Tethyan Belt; it can be suggested that rifting age of the Southern Tethyan Oceanic Basin in SE Anatolia is probably earlier than early Late Triassic (middle Carnian time).

Chapter 53 Las Ventanas and San Carlos formations, Maldonado Group, Uruguay

Abstract Together with the Playa Hermosa Formation (Fm.), the Las Ventanas and San Carlos formations constitute the Maldonado Group, which is better developed in the southeastern part of Uruguay and covers an area of c. 200 km 2 . The total thickness of both units (i.e. Las Ventanas and San Carlos formations) reaches c. 1500 m, and comprises mafic and acidic volcanic rocks, pyroclastic rocks, diamictite, sandstone, conglomerate and pelite. Structurally, the Maldonado Group is extensively deformed, although variably, throughout the region. Strike–slip faults, westward-verging detachment faults, and folds with axis sub-parallel to the strike–slip planes are common features. The presence of pumpellyite, prehnite, chlorite and epidote in mafic rocks indicates very low- to low-grade metamorphic conditions. The Las Ventanas Fm. is characterized by basal conglomerate, diamictite, sandstone and siltstone that pass upwards into fine-grained rhythmites (pelite), and is the thickest unit of the Maldonado Group ( c. 1250 m). The San Carlos Formation ( c. 250 m) comprises fine-grained conglomerate, sandstone and mudstone towards the top. Both units lie on an angular unconformity above Palaeo- and Neoproterozoic basement and are overlain unconformably by late Ediacaran–lowermost Cambrian units. Reliable palaeomagnetic data indicate that the Maldonado Group accumulated at high palaeolatitudes; however, the palaeogeographical evolution of the Río de la Plata Craton during the Neoproterozoic remains conjectural. Radiometric data from intrusive bodies and cross-cutting strike–slip faults place the minimum age of the group at c. 565 Ma, whereas basement volcanic rocks dated at 590 ± 2 Ma interbedded with meta-sandstones hosting detrital zircons c. 600 million years old provide the best constraint on the maximum age of deposition. Given the absence of carbonate rocks, no chemostratigraphic studies (e.g. C, O, Sr) are available. The Las Ventanas and San Carlos formations are largely interpreted as units within a thick glacially influenced fan-delta sedimentary system formed during the early Ediacaran in a strike–slip basin. Based on stratigraphic and sedimentological characteristics it has been suggested that this succession, containing glacially influenced diamictite and dropstones, records a glacial period that occurred sometime between c. 570 and 590 million years ago. Ongoing research is focused on establishing the precise age of deposition of the Maldonado Group and on reconstructing the tectonic evolution of the basin. Further palaeomagnetic studies will be especially useful for determining the palaeogeography of the Río de la Plata Craton during the Ediacaran and establishing its relationships with neighbouring strata hosting similar successions.

Palaeoenvironmental controls on the clay mineralogy of Carnian sections from the Transdanubian Range (Hungary)

The Upper Triassic sediments both in NW Europe and in the Mediterranean region imply a generally arid climate regime. In the Carnian, however, a humid phase with a high input of siliciclastics is assumed (‘Carnian Pluvial Event’) but its causes are still highly controversial. The Carnian marine succession of the Transdanubian Range (TR), Hungary also shows a significant change in sedimentation: pelagic carbonate deposition (Füred Limestone Formation) was followed by the formation of marly sediments (Veszprém Marl and Sándorhegy Formations). In order to understand the environmental changes, the clay mineralogy of 240 Carnian samples determined by X-ray diffraction is discussed in the present paper. Additionally, this set of clay mineral data from four boreholes from the Bakony Hills (Bakonyszűcs Bszü–1), Balaton Highland (Balatonfüred Bfü–1, Mencshely Met–1) and Zsámbék Basin (Zsámbék Zs–14) allows us to test the reliability of clay minerals as palaeoclimatic proxies for the Carnian (Late Triassic) by reconstructing spatial and temporal variations. The first increase in clay content of the studied Carnian succession can be detected in the limestone/marl couplets of the‘Cordevolian’ Füred Limestone. Its clay fraction is dominated by illite, mixed-layer illite/smectite (IS) and chlorite without kaolinite, a good indicator of landmasses with a hot climate and constant humidity, thus this compositional change probably related to a sea-level drop. Subsequently, the clay mineral assemblages of the Veszprém Marl sections in the Bakony Hills and the Balaton Highland are composed of illite, poorly ordered (R=0) IS with 60–70% expandable component, chlorite and kaolinite. On the other hand, the clay fraction of the samples from the Zsámbék Basin is composed of illite, IS with an average of 40–50% expandable component and a transitional R=0 to R=1 type of ordering and kaolinite; chlorite does not occur. In the Early Julian, kaolinite enrichments (~10–20%) in the clay fraction can be regarded as a manifestation of the climate shift from the prevailing aridity to the more seasonal climate with enhanced humid season. The overall presence of abundant illite and accompanying chlorite can indicate a high rate of erosion of moderately weathered rocks, while the large amount of smectitic clay (transformed to IS) could be partly derived from transport of weathering products from a distant source area and, subordinately, from diagenetic alteration of intermediate to basic volcanic rocks. Spatial variation in the expandability and ordering of the IS, at least partially, could be caused by different degree of the diagenetic overprint.

The discovery of Palaeoproterozoic volcanic rocks in the Bulunkuoler Group from the Tianshuihai Massif in Xinjiang of Northwest China and its geological significance

In the Dabudaer region of the Tianshuihai Massif (Xinjiang, Northwest China), metavolcanic rocks within the mainly meta-sedimentary Bulunkuoler Group are basalt, basaltic andesite, and rhyolite. In situ zircon U-Pb LA-ICP-MS dating on rhyolite magmatic zircons yielded an age of 2481±14 Ma, interpreted as the eruption age of the Bulunkuoler Group volcanic rocks. The basalt and basaltic andesite are low-Al tholeiite series and the rhyolite is low-Al calc-alkaline series. All of rocks are characterized by enrichment of LREE and LILE (Rb, Th and Ba), depletion of P, Nb, Ta and Ti, and absence of Eu anomalies. The e Nd(t) of basalts ranges from 3.14 to 4.88 indicating a depleted mantle source. The trace element signatures show that these magmas experienced intense crustal contamination during their ascent. Direct evidence for crustal contamination is the xenocrystic zircons in the ryholite, with ages back to ca. 3300 Ma. The primitive-mantle normalized and Zr/Y-Zr diagrams for the basic volcanic rocks show that they formed in an intra-continental tectonic setting. Combined with the previous studies and compared with North China Craton Neoarchean basic volcanic rocks, it is supposed that the Bulunkuoler Group volcanic rocks reflect the Palaeoproterozoic mantle magma underplating and interaction with felsic crust.

Geochemical evidence for the ophiolitic nature of the gabbros, gabbroanorthosites, and ultramafic rocks of the Gerbikan fragment of the Baladek Massif of the southeastern framing of the North Asian craton

The Baladek Massif represents a tectonic block located at the boundary between the SE framing of the North Asian craton and the fold structures of the Galam terrane of the Mongol-Okhotsk fold belt. According to the existing concepts, the rocks of this massif are regarded as Precambrian complexes of the aforementioned craton. This paper reports the results of studying the fragment of the Baladek Massif in the Gerbikan River basin. New geochemical data obtained on the gabbros, gabbroanorthosites, and ultramafic rocks testify to their similarity with typical ophiolite complexes. An underlying complex of metamorphic ultrabasic rocks, a layered complex of pyroxenites and “cumulate” gabbros, gabbroanorthosites, and a complex of isotropic gabbros were distinguished within this fragment. It is highly possible that the basic volcanic rocks and jaspers of the Silurian-Lower Devonian sequences of the Galam segment of the Mongol-Okhotsk fold belt also belong to the inferred ophiolite complex.

Lead Isotopic Composition and Lead Source of the Huogeqi Cu-Pb-Zn Deposit, Inner Mongolia, China

The Huogeqi orefield located on the northern side of Mt. Langshan, Inner Mongolia occurs in the Middle Proterozoic Langshan Group metamorphic rocks, and the orebodies are stratiform. In the past twenty years, many Chinese geologists have conducted researches on the Huogeqi Cu-Pb-Zn deposit, but there has been still a controversy on its origin. Some advocate that the deposit is of sedimentary-metamorphic reworking origin, some hold that it is of sea-floor SEDEX origin, and others have a preference for magmatic superimposition origin. The crux of the controversy is that there is no common understanding about the source of ore-forming materials. In this paper, the Pb isotopic compositions of regional Achaean-Early Proterozoic basement rocks, various types of sedimentary-metamorphic rocks and volcanic rocks in the mining district, Late Proterozoic and Hercynian magmatic rocks are introduced and compared with the ore-lead composition, so as to constrain the source of the ore lead. The result indicates that (1) sulfides in the ores have homogeneous Pb isotopic compositions, showing a narrow variation range. Their 206Pb/204Pb ratios are within a range of 17.027–17.317; 207Pb/204Pb ratios, 15.451–15.786 and 208Pb/204Pb ratios, 36.747–37.669; (2) the Pb isotopic compositions of the regional Achaean-Early Proterozoic basement rocks are characteristic of the old Pb isotopic composition at the early-stage evolution of the Earth, which varies over a wider range, reflecting significant differences in Pb isotopic compositions of the ores. All this indicates that the source of ore lead has no bearing on the basement rocks; (3) the sedimentary-metamorphic rocks in the mining district are characterized by highly variable and more radiogenic Pb isotopic compositions and their Pb isotopic ratios are obviously higher than those of ores, demonstrating that ore lead did not result from metamorphic reworking of these rocks; (4) Pb isotopic compositions of Late Proterozoic diorite-gabbro and Hercynian granite are higher than those of ores. Meanwhile, the Pb isotopic compositions of sulfides in the small-sized strata-penetrating mineralized veinlets formed at later stages are completely consistent with that of sulfides in stratiform-banded ores, suggesting that these veinlets are the product of autochthonous reworking of the stratiform-banded ores during the period of metamorphism and the late magmatic superimposition-mineralization can be excluded; (5) amphibolite, whose protolith is basic volcanic rocks, has the same Pb isotopic compositions as ores, implying that ore lead was derived probably from basic volcanism. So, the source of ore-forming materials for the Huogeqi deposit is like that of the volcanic massive sulfide (VMS) deposits. However, the orebodies do not occur directly within the volcanic rocks, and instead they overlie the volcanic rocks, showing some differences from those typical VMS-type deposits.

Discovery of the Late Triassic (Middle Carnian–Rhaetian) radiolarians in the volcano-sedimentary sequences of the Kocali Complex, SE Turkey: Correlation with the other Tauride units

The Kocali Complex in SE Turkey includes pelagic sediments (pelagic limestones, cherts, etc.), basic volcanic rocks of oceanic crust origin together with platform-derived sediments. Its depositional age was previously assigned as Late Jurassic–Early Cretaceous. In order to study the radiolarian contents of volcano-sedimentary sequences in this complex, six stratigraphic sections have been measured mainly at the NE and NW Adiyaman city. Radiolarian faunas from these stratigraphic sections reveal that the age of these sequences ranges from middle Carnian to Rhaetian. Based on these data, the depositional age of the complex is older than the previously assigned. Lithological characteristics (widespread Triassic basic volcanic rocks associated with pelagic sediments) and radiolarian contents of these sequences have close similarities with the sequences of the Alakircay Nappe of the Antalya Nappes in western and central Taurides. Based on taxonomic studies, 99 taxa have been determined of which one genus ( Adiyamanium) and four species ( Monocapnuchosphaera kocaliensis, Paronaella speciosa, Ferresium okuyucui and Adiyamanium crassum) are described as new.

Plagioclase twin associations from the basic volcanic rocks of the Kamchatka, Russia: growth conditions and formation mechanisms

The plagioclase twin associations found as crystal-lapilli in tephra and megacrysts in high-alumina basalt erupted from volcanoes of the Klyuchevskoy group (Kamchatka, Russia) reveal at least eight twin laws (Albite, Carlsbad, Albite-Carlsbad, Pericline, Baveno- d , - l , Prismatic- d , - l , and Periclinic twins), whose development and hierarchy can be described with the triad theory of L.A. Vardanianz. There are two genetic types of twins: (1) post-growth twins resulted from the thermal stress (Pericline twins and small twins of Albite-Carlsbad triad) and (2) growth twins (large twins of Albite-Carlsbad triad, Baveno and Prismatic twins), whose origin is explained by the action of the autodeformation mechanism. This mechanism implies deformation or stress-induced nucleation of small twin subunits in the subsurface layer of the growing crystal. The high level of stress is provided by the lattice mismatch on the zoning boundaries. Once nucleated, twin subunits grow independently of the original crystal that is favoured by the high growth-rate anisotropy.

In Situ U-Pb and Trace Element Analysis of Accessory Minerals in the Kiruna District, Norrbotten, Sweden: New Constraints on the Timing and Origin of Mineralization

Northern Norrbotten, Sweden is a key part of Baltic Shield and provides a record of magmatic, tectonic and related, superimposed, Fe oxide–apatite and iron oxide–copper–gold (IOCG) mineralization, during the Svecokarelian orogeny. Titanite and allanite from a range of mineral deposits in the area have been analysed for U–Pb isotope systematics and trace element chemistry using laser ablation quadrupole inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses of a single sample from the regional scapolite–albite alteration give an age of 1903 ± 8 Ma (2σ) and may be contemporaneous with the early stages of Fe mineralization (1890–1870 Ma). Analyses of titanite and allanite from undeformed IOCG deposits indicate initial alteration at 1862 ± 16 Ma. In many deposits subsequent metamorphic effects reset titanite isotope systematics from 1790 to 1800 Ma, resulting in a spread of U–Pb isotope analyses along concordia. In some instances core regions may record evidence of early thermal events at around 2050 Ma. Titanite and allanite from deformed IOCG deposits on major shear zones record ages from 1785 ± 21 Ma to 1777 ± 20 Ma, corresponding to deformation, metamorphism and secondary hydrothermal alteration as a result of late orogenic movements. The lack of intracrystalline variations in titanite and allanite trace element chemistry suggests that hydrothermal fluid chemistry and metal source were the main controls on mineral trace element chemistry. Titanite from undeformed Fe oxide–apatite and IOCG deposits is typically light rare earth element (LREE) enriched, and shows low U/Th ratios and low Ni in both intermediate to acid and basic volcanic-hosted deposits. This is consistent with a granitic source for metals. Minor variations in trace element patterns are consistent with the influence of aqueous complex formation on relative REE solubility. Deposits related to the Nautanen Deformation Zone have relatively heavy REE (HREE)-enriched titanite, and LREE-depleted allanite, with high U/Th ratios and elevated Ni contents, consistent with leaching of metals from the local basic volcanic rocks. All hydrothermal titanites are high field strength element enriched (Nb, Ta, Zr) indicating their transport as a result of either high salinities or high F contents, or both. The data overall support models of IOCG-type mineralization as a result of regional circulation of saline hydrothermal fluids in association with major crustal structures, with at least some metallic components derived from the granitoid rocks of the area. All the deposits here show evidence of subsequent metamorphism, although penetrative fabrics are restricted to regional-scale deformation zones.

Trace Elements and Rare Earth Elements Geochemistry and Its Metallogenic Significance for Cu-Zn Ore Deposits in Tongbai Area, Henan Province, China

There are some Cu-Zn polymetallic ore deposits that are of volcanic hosted massive sulfide (VHMS) deposits located in Tongbai region, Henan Province, China. The trace elements and rare earth elements (REEs) geochemistry of ores of Liushanyan Cu-Zn ore deposit, Dalishu–Fanglaozhuang Cu-Zn ore deposit and Yangjuan Cu-mineralizing spot were analyzed by using inductively coupled plasma mass spectrometry (ICP-MS) method to discuss the genesis of these deposits. Four types have been identified on the basis of study on trace elements and REEs geochemistry: the ores of the first type show light rare earth elements (LREE)-enriched REE patterns and have no Eu anomalies and weak negative Ce anomalies, with a weak depleted high field strength elements (HFSE) and smaller anomalies at Zr and Ti relative to normal mid-ocean ridge basalts (N-MORB); the ores of the second type show LREE-enriched REE patterns and have negative Eu anomalies and weak negative Ce anomalies; the ores of the third type show LREE-enriched REE patterns and have positive Eu anomalies and negative Ce anomalies, with a depleted HFSE and small negative anomalies at Zr and Ti relative to N-MORB; and the ores of the fourth type show horizontal distribution REE patterns, have positive Eu anomalies and weak negative Ce anomalies, with a depleted HFSE and no anomalies at Zr and Ti relative to N-MORB. The first type and the second type have similar characteristics to the basic volcanic rocks and acid volcanic rocks, respectively. Characteristics of the third type indicate that the convective mixing of a little amount of sea water with hydrothermal fluids had happened with a lot of minerals which is not dissolved in the solvents in the residual phase. The fourth type has a depleted mantle source. These characteristics indicate that the mineralization material has the same source with wall rocks of the Liushanyan Formation, which is the product of mantle partial melting that experienced early multiple melting events.

Compositional Differences between Felsic Volcanic rocks from the Margin and Center of the Northern Main Ethiopian Rift

Pliocene felsic rift margin and Quaternary rift center volcanic rocks from the northern Main Ethiopian Rift (MER) exhibit contrasts in major and trace element contents and Sr-Nd isotopic ratios. Quaternary rift center felsic volcanic rocks are mainly peralkaline trachytes and rhyolites, whereas Pliocene felsic rift margin volcanic rocks are represented by benmoreites, weakly peralkaline trachytes and rare rhyolites. Most of the felsic rift margin volcanic rocks have greater Al2O3, K2O, Nb, Zr, Rb, and Sr, and lesser CaO, Zr/Nb, and CaO/Al2O3 than rift center volcanic rocks. These contrasts may have been inherited from differences in the compositions of their parental basic magmas, which were produced by variable degrees of partial melting. In both series, the felsic volcanic rocks generally have higher initial Sr- isotopic (0.7038-0.7073) ratios than their basic equivalents (0.7035-0.7046). Nd- isotopic ratios of most felsic rift center samples (0.5129-0.5126) are similar to their associated basic volcanic rocks. In contrast, the Nd-isotopic ratios (0.5128-0.5124) of felsic rift margin volcanic rocks are commonly lower than their companion basic volcanic rocks (0.512806-0.512893), and are relatively lower than rift center equivalents. The elemental and Sr-Nd isotopic compositions of the volcanic rocks suggest that fractional crystallization from differing basic parents accompanied by a limited assimilation (AFC) was the dominant process controlling the genesis of the MER felsic volcanic rocks. Keywords: Ethiopia; Northern Main Ethiopian Rift; Bimodal Volcanism; parental difference; Sr-Nd Isotopes, Fractional Crystallisation

The Late Triassic rift-related volcanic rocks from eastern Qiangtang, northern Tibet (China): Age and tectonic implications

The Late Triassic Nadi Kangri volcanic rocks, with nearly EW trending outcrops within the Qiangtang basin, northern Xizang (Tibet), China, are composed mainly of acid tuff, dacite, rhyolite and minor basic volcanic rocks. There exists a significant depositional hiatus between the Nadi Kangri volcanic rocks and underlying strata. Therefore, the Nadi Kangri volcanic rocks represent a new evolution history of the Mesozoic Qiangtang basin. The magma emplacement age of the Nadi Kangri volcanic rocks in the Geladaindong area is 220.4 ± 2.3 Ma, representing the onset of the Mesozoic Qiangtang basin. The Nadi Kangri basalts have high Nb/Zr (0.049–0.058), Ta/Hf (0.12–0.15) and Zr/Y (4.95–6.01) ratios. In the tectonic discrimination diagrams, such as Zr vs. Zr/Y and Th/Hf vs. Ta/Hf, the Nadi Kangri basaltic rocks mostly plot in the “within-plate” setting field. The geological background and the geochemical characteristics suggest that the Nadi Kangri volcanic rocks were formed in a continental rift setting.

Granitization of the basic volcanic rocks in the contact aureole of the Gabbronorite Intrusion (Ganal Ridge, Kamchatka)

Granitization of the basic volcanic rocks in the contact aureole of the Gabbronorite Intrusion (Ganal Ridge, Kamchatka)

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

AbstractThe Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between −0.19 and +2.22 for the basic volcanic rock and between −2.85 and −4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.

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

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

Puna (Argentina) and northern Chile Ordovician basic magmatism: A contribution to the tectonic setting

Geochemical characteristics of Ordovician basic volcanic rocks help to define the evolving tectonic setting of the Argentine Puna and northern Chile. Four spatially distinct magmatic groups are defined on geological, petrographical, geochemical and isotopic bases, each associated with particular geodynamic environments. The Tremadoc western group of subalkaline low K tholeiites with arc and modified MORB like signatures represent early stages of a back-arc basin, where spreading was incipient. The Arenig western group, medium K calc-alkaline basalts to andesites have volcanic arc in transition to back-arc signatures. The Tremadoc subalkaline basalts of the eastern group have REE patterns similar to E-MORB and at the same time weak subduction characteristics suggesting a rather mature supra-subduction zone (SSZ) basin. In contrast, the Late Tremadocian-Arenig basalts of the same group have intra-plate signatures, interpreted as magmas that ascended along pull apart regions associated with a transtensional regime. The geochemical patterns were applied to correlate basic sequences of doubtful geological setting. So, basalts from Chile were related to the Tremadocian western group, where they represent a slightly more mature stage of spreading of the basin. Basic rocks from Pocitos and part of Calalaste represent pre-Ordovician records of a back-arc system similar to that of the Tremadoc western group. Clearly similar arc patterns to those of the Arenig western group allow extending the arc environment to the southern Puna. The Tremadocian basalts from the eastern group were related to metabasites from the southern Puna, as part of a back-arc environment at that time.