Calc-alkaline Compositions Research Articles

Petrology, geochemistry, and correlation of tephra deposits from a large early-Holocene eruption of Mentolat volcano, southern Chile

Two correlated tephra deposits, each 13 cm thick in the Aisén region of southern Chile, one in a lacustrine sediment core from the Mallín el Toqui (MET) peat bog and another from a subaerial soil exposure ∼10 km to the west in the Río Maniguales (RM) valley, preserve evidence for a large explosive eruption of Mentolat volcano, one of the five stratovolcanoes of the southernmost portion of the Andean Southern Volcanic Zone (SVZ). This eruption is constrained in age to ≥11,728 cal years BP by radiocarbon dating of organic matter from the MET sediment core and is termed the ∼11.7 ka MEN event. The two tephra deposits are identical and based on their petrology, bulk tephra, glass, and amphibole geochemical characteristics, are attributed to an eruption of Mentolat volcano. Both contain pumice lapilli with glass compositions that ranges from 59 to 76 wt. % SiO2, with medium to low-K2O calc-alkaline composition and trace element abundances similar to both lavas and other tephras derived from Mentolat. They have abundant amphibole, plagioclase, orthopyroxene, and clinopyroxene phenocrysts, with a smaller proportion of olivine and Fe-Ti oxides, and a minor amount of distinctive crustal xenoliths with both unfoliated and foliated textures. Amphiboles have low K2O (0.20–0.37 wt. %) and TiO2 (1.5–3.8 wt. %) and are similar geochemically to amphiboles from other Mentolat-derived tephra (K2O = 0.14–0.43 wt. % and TiO2 = 1.9–2.4 wt. %), but distinct from amphiboles in lavas and tephra derived from other volcanoes in the southernmost SSVZ including Cay (K2O = 0.47–0.55 wt. % and TiO2 = 2.4–3.1 wt. %) and Melimoyu (K2O = 0.39–0.52 wt. % and TiO2 = 2.8–4.5 wt. %). Amphiboles from the ∼11.7 ka MEN tephra formed over a broad range of pressures (154–406 MPa), temperatures (834-969 °C), and magma water contents (4.9–7.0 wt. %), which overlap with the physical-chemical conditions for the formation of amphiboles from other Mentolat-derived tephra. The two correlated tephra deposits, which are located ∼95 km southeast of Mentolat, are correlative with tephra of similar age identified in 12 other lacustrine sediment cores from the region, for which, based on their petrology and the geochemistry of their tephra glass and amphiboles, Mentolat is also the likely source volcano. This eruption produced approximately 1.8 km3 of bulk material with an estimated magnitude of 5.2. Mentolat has produced numerous (>18) explosive eruptions since glacial retreat from the region and future explosive eruptions from this center could potentially impact local population centers and the agricultural industry in southern Chile and Argentina.

Characterization of lacustrine mixed fine-grained sedimentary rocks using coupled chemostratigraphic-petrographic analysis: A case study from a tight oil reservoir in the Jimusar Sag, Junggar Basin

Abstract Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three components, is defined as “mixed sedimentary rocks”. The Permian Lucaogou Formation (P2l) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere.

Петролого-геохимические особенности рудовмещающих эффузивных и интрузивных пород Николаевского месторождения золото-порфирового типа (Южный Урал)

Object. The article considers the results of geochemical studies of ore-bearing effusive and intrusive rocks of the Nikolaevka gold deposit, located in the zone of the Main Uralian fault in the Southern Urals. The deposit belongs to the gold-porphyry type which unconventional for the Urals and poorly studied here. Methods. The rock composition was determined by chemical analysis (IG UFRC RAS), mass-spectrometry with inductively coupled plasma on the ELAH 9000 quadrupole mass-spectrometer (IGG UB RAS) and X-ray fluorescence analysis on the VRA 30 spectrometer (IG UFRC RAS). Results. It is established that gold-porphyry mineralization associated with an island-arc volcanic-intrusive complex that unites plagiophyric and pyroxene-plagiophyric basalts, their tuffs, gabbro intrusions and ore-bearing series of dikes plagiophyric dolerite-porphyrites and gabbro-diorite-porphyrites. The rocks of the complex, including ore-bearing dikes, have normal alkalinity, tholeitic and transition from tholeitic to calc-alkaline composition. Among the volcanogenic formations, faunistically dated in the zone of the Main Ural fault in the Southern Urals, by a number of geochemical parameters (for example, the ratios of Zr/Nb and Nb/Th) the effusive and intrusive rocks of the deposit are the closest to the volcanics of the pyrite-bearing Baymak-Buribay Formation (D 1 e 2 ), lying at the base of the section of the Magnitogorsk island arc, and, in all probability, are their age analog. At the same time, the gold-bearing complex differs from the Baymak-Buribay Formation by a general increased iron and titanium content, with a reduced magnesia of all rock types, as well as the absence of boninites and acid volcanics, the predominance of porphyry rock types, which can be due to the geodynamic conditions of its formation. Conclusion. Geodynamic analysis of geochemical data using diagrams (La/Sm)N-TiO 2 and V-Ti/1000 suggests that the Nikolaevka deposit was formed in the articulation zone of the island arc and the back-arc basin. Gold-porphyry mineralization associated with the Late Emsian volcanic processes was isolated in the Southern Urals for the first time.

Decoding a post-collisional multistage magma system: The Quaternary ignimbrites of Aragats stratovolcano, western Armenia

Quaternary eruptions from an intraplate multistage magma reservoir of the Aragats stratovolcano in western Armenia formed pyroclastic density currents and fallouts deposited in an area around 2,000 km2, comprising six ignimbrite units and three fallout deposits. Published K/Ar and Ar/Ar age data allow for a distinction between older (Calabrian, 1.8–0.90 Ma) and younger (Middle Pleistocene, 0.75–0.65 Ma) ignimbrites. The detailed characterization of the crystal-poor to crystal-rich, trachyandesitic to rhyolitic ignimbrite sequence deciphers the pre-eruptive magmatic conditions. It also contributes to the understanding of Plinian eruptions of the largest volcanic complex in the Lesser Caucasus.The chemical variation of pumice fragments reveals the conditions of both, homogeneous and mingled heterogeneous magma systems. The phenocryst assemblage of the older crystal-poor units (Ab-rich plagioclase + amphibole ± pyroxene) indicates growth in a cooler hydrous magma chamber, whereas the partially resorbed phenocrysts of the younger crystal-rich units (An- to Ab-rich plagioclase + pyroxene) suggest growth in a superheated magma chamber fueled by mafic, mantle-derived melts. In the younger units, external (e.g. sieve fabrics) and internal phenocryst textures have been generated by decompression and convection during ascent. With the older units, the chemical variation in amphibole from core to the narrow break-down rim reveals magma ascent from a deep (13 km depth) to a shallow (up to 8 km) level.Tracking the p-T paths of cumulate fragments (pyroxene, plagioclase) in the younger units, we estimate the depth of the anhydrous crystal-rich magmatic chamber at around 25 km. Based on the composition of clinopyroxene cumulates, we propose a multi-stage evolution of melts from subalkaline and tholeiitic to calc-alkaline composition. The pronounced high Zr content and Zr/Hf ratios of the entire Upper Pliocene to Quaternary post-collisional magmatism could be attributed to partial melting of an enriched mantle source in the Lesser Caucasus.

Age and petrogenesis of the Andagul granodiorite and its implications on gold mineralization of the Kassan region, western Kyrgyzstan Tian Shan

A number of significant gold deposits in the Kassan region in west Kyrgyzstan are associated with the Andagul granitic intrusion. These gold deposits share similarities with the class of “intrusion-related gold deposits”, implying that the gold mineralization may be genetically related to the intrusion. In order to evaluate this potential relationship, it is important to study the petrogenesis of the intrusion. This paper examines the sources and physiochemical conditions of magma generation and crystallization of the Andagul intrusion and their implications for gold mineralization, through an integrated study involving zircon geochronology, mineral chemistry, whole-rock chemistry and isotopic geochemistry. The Andagul intrusion consists mainly of granodiorite, which is characterized by metaluminous and median-K calc-alkaline compositions, high LREE and LILE, low HFSE, right-inclined REE pattern with a flat HREE pattern, and insignificant Eu anomalies ((Eu/Eu∗)N = 0.79–0.85). These geochemical characteristics, together with relatively low positive εNd(t) values (+2.1–+4.3), moderately low (87Sr/86Sr)i ratios (0.7049–0.7059), relatively low initial Pb isotopic ratios ((206Pb/204Pb)t = 17.76–18.51, (207Pb/204Pb)t = 15.56–15.62, and (208Pb/204Pb)t = 37.89–38.33), juvenile εHf(t) (+2.29–+4.80) but relatively old model ages (T2DM(Hf) = 1026–1175 Ma, T2DM(Nd) = 728–907 Ma) indicate that the magma was mainly sourced from partial melting of juvenile basaltic lower crust with the involvement of a small proportion (ca. 5%–10%) of Precambrian basement rocks. The high Zr saturation temperatures (807 °C to 825 °C), relatively high Mg# values (43–52) and presence of mafic enclaves, and development of acicular apatite and oscillatory plagioclase and titanite, suggest that the magmatism was related to injection of hot mantle material. The magma generation and intrusion took place during 299–310 Ma based on LA-ICPMS zircon U-Pb dating, suggesting a post-collisional setting. Remelting of sulfide accumulate residual with high Au/Cu ratios in the juvenile lower crust, formed in the pre-collisional plate subduction-related magmatic arc stage, is proposed to account for the Au mineralization in the Kassan region. The high water contents (4.0–5.2 wt% H2O) and weakly oxidized or slightly reduced redox state (ΔFMQ = −0.33 ∼ +0.49) deduced from mineral composition and zircon Ce anomaly are favorable for the transport and enrichment of gold in the magmatic fluids. These geochemical characteristics suggest that the Andagul granodiorite and related granitic rocks in the Kassan region have the potential to provide ore-forming materials and fluids for gold mineralization.

Sedimentological consequences of arc-arc collision: An example from Mio-Pliocene Miura Group, Japan

Abstract This paper presents brief sedimentological inventory of the Misaki and the overlying Hasse Formation (constituting the Miura Group) based on observations made at Jogashima Island, Miura Peninsula and infers the nature of the stratigraphic transition from tectonic viewpoint. The lower part of the Misaki Formation is made of very coarse to coarse-grained scoriaceous, normally graded sandstones, fine-grained sandstones/siltstones, and mudstones and indicates a deep sea submarine canyon depositional setting. There is no evidence of storm reworking (shallow marine processes) in the lower parts of the Misaki Formation at least in the Jogashima Island as suggested by Stow et al. (1998). However, the upper part of the Misaki succession formed in a shallow marine depositional setting. The overlying Hasse Formation characterized by brownish coloured, very coarse to coarse-grained large-scale trough cross stratified, very poorly sorted sandstones, very poorly sorted polymictic and matrix dominated mass flow conglomerates with siltstone/fine-grained sandstone clasts derived from the underlying Misaki Formation and occasional reverse grading. In significant contrast to the underlying Misaki Formation the Hasse Formation lacks very fine-grained sandstone/siltstone/mudstone facies. Unlike the tholeiitic basalt composition of the lapilli and pebbles of the Misaki sandstone beds, the scoriaceous lapilli and pumiceous beds of the Hasse Formation are of calc-alkaline composition. The Hasse sedimentation probably took place in much shallower paleogeography compared to the underlying Misaki Formation, most probably in an alluvial fan-braided fluvial setting. The Misaki-Hasse stratigraphic contact is very sharp and erosional, and represents a subaerial unconformity (i.e., depositional sequence boundary). Our sedimentological data from the Jogashima Island clearly indicate progressive shallowing of the depositional setting during the terminal phase of the Misaki sedimentation and much abrupt shallowing across the Misaki-Hasse transition possibly as a consequence of ongoing collision between the Izu-Bonin and Honshu arcs.

Holocene eruptions of Mt. Popa, Myanmar: Volcanological evidence of the ongoing subduction of Indian Plate along Arakan Trench

In the western part of Myanmar the geodetic and seismic data indicate the ongoing highly oblique subduction of Indian Plate under Eurasian Plate. Volcanoes of Burma arc, however, did not produce eruptions in the recorded history and are considered extinct. Such perception questions the ongoing subduction, as well as keeping local officials unaware of possibility of future volcanic eruptions in the country. We have investigated the youngest lava flows and pyroclasts of Mt. Popa, which is the best-preserved polygenetic volcanic edifice in Myanmar. The Ar/Ar dating of the youngest products of the volcano provided very young radiometric ages which were unable to be measured accurately. The radiocarbon dating of paleosols intercalated with the most recent ash layers of the volcano has shown that Mt. Popa produced several eruptions in the beginning of Holocene. The youngest eruption occurred ~8000 BP and included the 1.3 km3 gravitational collapse of the volcanic cone with deposition of the 11-km-long debris avalanche, immediately followed by emplacement of 0.1 km3 of pyroclastic flow of calc-alkaline basaltic andesite composition. The collapse direction as well as complex morphology of the resulted crater were prearranged by geometry of the listric fault, which was formed during the pre-collapse asymmetric gravitational spreading of the volcanic cone. The recently erupted products are geochemically similar to products of young monogenetic volcanoes of Monywa area located 150 km north Mt. Popa, and both display patterns consistent with magma generation at an active subduction system. Low magma production rate of Mt. Popa (3 × 10−5 km3/year averaged for the 1 Ma of the volcano history) is in agreement with the highly oblique angle and slow rate of the subduction. The fact of the Holocene eruptions of calc-alkaline composition in Burma arc represents volcanological evidence of the ongoing subduction in this part of the collision zone between India and Asia.

The Belahourou granite pluton (Djibo greenstone belt, Burkina Faso): Emplacement mechanism and implication for gold mineralization along a shear zone

The Belahourou granite pluton (BG pluton) is a magmatic body that belongs to the Djibo greenstone belt, north Burkina Faso. Calc-alkaline in composition and 2132 ± 4 Ma in age (U-Pb zircon), this pluton is bordered by the curved Belahourou-Souma (BS) sinistral shear-zone, NNW-trending in the northern half of the pluton and WNW-trending in the southern part. In order to study the emplacement mechanism relationships with the BS shear-zone and the gold-deposits, the pluton and its host-rocks have been subjected to standard and modern mapping techniques (airborne geophysics, satellite imagery, anisotropy of magnetic susceptibility - AMS - and microstructural observations), in addition to field observations. The pluton, 100 km2 in area, 16 km in length and 7 km in width, is surrounded by metamorphic formations, turbiditic clastic to the west and volcanic material along the southern and eastern sides. To the north the pluton is in contact with granodioritic and gabbroic rocks. Pluton emplacement was likely instigated by opening of a tension gash associated with NW-directed, sinistral shearing along the curved BSS. The overall NW-trending magnetic lineations inside the pluton support this interpretation. The microstructures indicate that shearing continued during pluton cooling, explaining the inside-pluton magmatic to orthogneissic transition, and after cooling, explaining the mylonitic microstructures against the BS shear-zone. A 3D conceptual representation integrates the available data and concludes that the pluton is late-tectonic with respect to the regional D2 deformation event. Once emplaced, the pluton may have provided low pressure zones, at lithological contacts and/or inside brittle fractures in the host formations, concentrating the mineralizing fluids that were pressurized during pluton cooling, and that helped to precipitate the ore.

Tectonic evolution of syn- to late-orogenic sedimentary–volcanic basins in the central Norwegian Caledonides

We present new structural, geochemical and U–Pb zircon data from syn- to late-orogenic sedimentary–volcanic basins in the southwestern part of the Trondheim Nappe Complex, central Norwegian Caledonides. In this area, a succession of enriched mid-ocean ridge basalt type metabasalt, jasper, ribbon chert with associated sandstone and conglomerate, and green siltstone is interpreted to represent volcanism and sedimentation in a hitherto little-known spreading-dominated tectonic environment. This environment is different from the suprasubduction-zone ophiolite setting dominating the Iapetus rock record elsewhere in the Scandinavian Caledonides. This volcanic and sedimentary succession was overturned and isoclinally folded in a pre-427 Ma orogenic phase. Post-427 Ma cross-bedded sandstones were deposited on the eroded surface of the previously deformed rocks, representing a rare example of a late Silurian or younger sedimentary basin within the Scandinavian Caledonides. The cross-bedded sandstones are intercalated with and/or overlain by post-427 Ma intermediate volcanic or subvolcanic rocks of calc-alkaline composition, representing a hitherto unknown volcanic phase within the Trondheim Nappe Complex and elsewhere within the Scandinavian Caledonides. Their particular geochemical signature could be the result of late-stage subduction-zone volcanism just prior to the onset of continent–continent collision between Baltica and Laurentia, or much younger post-collisional extensional melting with inherited subduction signatures. Supplementary material: A description of analytical methods and a table of geochronological data from LA-ICP-MS analysis of zircons are available at https://doi.org/10.6084/m9.figshare.c.3994605 .

Petrogenesis of the late Early Cretaceous granodiorite – Quartz diorite from eastern Guangdong, SE China: Implications for tectono–magmatic evolution and porphyry Cu–Au–Mo mineralization

Comprehensive petrological, zircon U–Pb dating, Hf–O isotopes, whole rock geochemistry and Sr–Nd isotopes data are presented for the Xinwei and Sanrao intrusions in the eastern Guangdong Province, Southeast (SE) China, with an aim to constrain the petrogenesis, tectono-magmatic evolution and evaluate the implication for porphyry Cu–Au–Mo mineralization. The Xinwei intrusion is composed of granodiorite and quartz diorite, whilst the Sanrao intrusion consists of granodiorite. Zircon U–Pb ages show that both intrusions were emplaced at ca. 106–102Ma. All rocks are metaluminous to weakly peraluminous, high-K calc-alkaline in composition, and they are characterized by LREEs enrichment, depletion in Nb, Ta, P, and Ti, and strongly fractionated LREEs to HREEs. The initial 87Sr/86Sr ratios range from 0.7055 to 0.7059, and εNd(t) values range from −3.9 to −3.0. Together with the relatively high εHf(t) values (−3.2 to 3.3) and low δ18O values (4.9‰ to 6.6‰), these data suggest that the Xinwei and Sanrao intrusions were derived from a mixed source: including the mantle-derived mafic magmas and lower continental crustal magmas. Fractional crystallization played an important role in the magmatic evolution of the Xinwei and Sanrao intrusions. The elemental and isotopic compositions of the Xinwei and Sanrao intrusions, as well as the high water content and oxidation state of their parental magmas, are similar to those of the ore-bearing granodiorites of the Luoboling porphyry Cu–Mo deposit in the Fujian Province, neighbouring east to the Guangdong Province, indicating that the late Early Cretaceous granodioritic intrusions in the eastern Guangdong Province may also have Cu–Au–Mo mineralization potential. The late Early Cretaceous magmatic event is firstly reported in eastern Guangdong, and represents a positive response of large-scale lithosphere extension and thinning, triggered by the changing subduction direction of the Paleo-Pacific plate from oblique subduction to parallel to the continental margin during the Early Cretaceous.

Late Cretaceous tectono-magmatic activity in the Nize region, central Tibet: evidence for lithospheric delamination beneath the Qiangtang–Lhasa collision zone

ABSTRACTThe results of zircon U–Pb age dating and whole-rock geochemistry for the Late Cretaceous Nize granodiorite porphyries, combined with analysis of near-coeval structural deformation of the Lower Cretaceous Langshan Formation, provide new data to better understand the tectonic evolution of the northern Lhasa subterrane, central Tibet. Zircon U–Pb ages of 89.2 ± 0.3 Ma to 87.8 ± 0.3 Ma indicate emplacement during the Late Cretaceous. Granodiorite porphyry intrusions were contemporaneous with the development of a regional angular unconformity, overlain by the Upper Cretaceous Jingzhushan (or Abushan) Formation, within the collision zone between the South Qiangtang and Lhasa terranes. Geochemical data for Nize granodiorite porphyries indicate that they have a calc-alkaline composition enriched in large-ion lithophile elements and light rare earth elements and depleted in high-field-strength elements and heavy rare earth elements. High Al2O3 and Sr contents, low Yb and Y contents, and high Sr/Y ratios are similar to adakitic magmas.Structural analysis indicates two stages of deformation (D1 and D2), with D1 forming the focus of the present study. The D1 deformation is represented by large-scale faults and records two periods of faulting. These periods are recognized as early compressional thrust faulting and a dominant late stage characterized by normal faulting and extension, with the latter stages of D1 being near-coeval with the emplacement of the Nize granodiorite porphyries. The combination of zircon ages, geochemical data, and structural analysis indicates that the Nize granodiorite porphyries formed after collision of the South Qiangtang and Lhasa terranes. Adakitic magma derived from partial melting of the thickened lower or middle crust resulted from lithospheric delamination that may have been promoted by the convective removal of deeper lithospheric mantle.

Geochronological and geochemical constraints on the genesis of Cu-Au skarn deposits of the Santa María de la Paz district (Sierra del Fraile, Mexico)

Cu-Au skarn deposits of the mining district of Santa María de La Paz are located in the Sierra del Fraile, in the northern part of San Luis Potosí, Mexico. Samples of four different intrusions and their related skarn deposits were taken underground, in order to obtain whole-rock and mineral geochemical data. U-Pb LA-ICP-MS zircon data indicate crystallization ages of ca. 37.6–35.0 Ma for skarn-related granitic and granodioritic stocks, which show dominant metaluminous to slightly peraluminous, high-K calc-alkaline compositions. These intrusions were emplaced in a post-Laramide continental arc setting under shallow conditions, as suggested by fast cooling inferred from K-Ar biotite ages of ca. 35–33 Ma. Skarn deposits were contemporaneous with emplacement and crystallization of stocks, as revealed by Re-Os molybdenite geochronological data of exoskarn rocks. Skarn rocks show a general depletion of LILE and REE and a slightly enrichment in HFSE in comparison with intrusive rocks. Pyroxene and garnet show dominant hedenbergite and andradite compositions, respectively. The association of hedenbergite, pyrite, chalcopyrite, native bismuth and pyrrhotite during skarn mineralization suggest reducing conditions, typical for Au but not for Cu skarns. Highest metal contents are concentrated in exoskarn rocks, including anomalies of Cu, Fe, Ni, Sn, Sb, Zn, W and Bi. Metal contents, ore minerals and environmental conditions of skarn rocks, together with whole-rock geochemical data of stocks and pyroxene-garnet compositions, reveal some singularities for the Sierra del Fraile deposits, as they exhibit characteristics of both Cu and Au skarn deposits but might be also compatible with Fe, W, and Zn skarns. Hence, skarn and associated vein deposits of the Sierra del Fraile might have a high potential not only for Cu-Au, but also for several base metals, as reported for comparable post-Laramide skarn and hydrothermal deposits of central and northern Mexico.

The onset of the volcanism in the Ciomadul Volcanic Dome Complex (Eastern Carpathians): Eruption chronology and magma type variation

Combined zircon U-Th-Pb and (U-Th)/He dating was applied to refine the eruption chronology of the last 2Myr for the andesitic and dacitic Pilişca volcano and Ciomadul Volcanic Dome Complex (CVDC), the youngest volcanic area of the Carpathian-Pannonian region, located in the southernmost Harghita, eastern-central Europe. The proposed eruption ages, which are supported also by the youngest zircon crystallization ages, are much younger than the previously determined K/Ar ages. By dating every known eruption center in the CVDC, repose times between eruptive events were also accurately determined. Eruption of the andesite at Murgul Mare (1865±87ka) and dacite of the Pilişca volcanic complex (1640±37ka) terminated an earlier pulse of volcanic activity within the southernmost Harghita region, west of the Olt valley. This was followed by the onset of the volcanism in the CVDC, which occurred after several 100skyr of eruptive quiescence. At ca. 1Ma a significant change in the composition of erupted magma occurred from medium-K calc-alkaline compositions to high-K dacitic (Baba-Laposa dome at 942±65ka) and shoshonitic magmas (Malnaş and Bixad domes; 964±46ka and 907±66ka, respectively). Noteworthy, eruptions of magmas with distinct chemical compositions occurred within a restricted area, a few km from one another. These oldest lava domes of the CVDC form a NNE-SSW striking tectonic lineament along the Olt valley. Following a brief (ca. 100kyr) hiatus, extrusion of high-K andesitic magma continued at Dealul Mare (842±53ka). After another ca. 200kyr period of quiescence two high-K dacitic lava domes extruded (Puturosul: 642±44ka and Balvanyos: 583±30ka). The Turnul Apor lava extrusion occurred after a ca. 200kyr repose time (at 344±33ka), whereas formation of the Haramul Mic lava dome (154±16ka) represents the onset of the development of the prominent Ciomadul volcano. The accurate determination of eruption dates shows that the volcanic eruptions were often separated by prolonged (ca. 100 to 200kyr) quiescence periods. Demonstration of recurrence of volcanism even after such long dormancy has to be considered in assessing volcanic hazards, particularly in seemingly inactive volcanic areas, where no Holocene eruptions occurred. The term of ‘volcanoes with Potentially Active Magma Storage’ illustrates the potential of volcanic rejuvenation for such long-dormant volcanoes with the existence of melt-bearing crustal magma body.

Petrogenesis and Geochemical Properties of Dome-shaped Subvolcanic Complexes in Southwest of Shahrab (Northeast of Isfahan)

<p>The studied area is located in southwest of Shahrab village near Ardestan city. This zone is part of Uremia- Dokhtar magmatic belt. Outcrops composed of rhyolite and rhyodacite dome-shaped volcanic complexes are scattered in the studied area; some of which are exploited as ornamental stone. The main rhyolite minerals include quartz, plagioclase and alkali feldspar. Minor minerals include Apatite, Sphene and opaque minerals and of the secondary minerals in these rocks Christie, Chlorite, Epidote and calcite could be mentioned. Calcite exists in rocks in form of filler of micro-fissures. The ignimbrite presence in this group of rocks in form of xenolith is one of the features of this rock group. The main primary texture in rhyolite and rhyodacite is porphyritic and the secondary texture includes pull-apart, snow flake and spherulitic textures. Geochemical evidences indicate that these rocks are sub-alkaline, Calc-alkaline compositions with high potassium and meta-alumina. These rocks have negative EU anomaly that is the feature of acidic igneous rocks. The studied rocks show high enrichment of LREE and LILE elements. The primary magmas constituting these rocks have mantel origin raised under extreme compressional conditions on continental crust in a tectonic environment of volcanic arc. It seems that these rocks are formed in connection with continuance of volcanic activities associated with subduction of Neolithic oceanic plate beneath continental plate of Iran. </p>

Mid–Late Cretaceous igneous activity in South China: the Qianjia example, Hainan Island

ABSTRACTBoth Pacific and Neo-Tethys plates had major influences on the Cretaceous magmatisms in southeastern China. The subduction of the Neo-Tethys plate is, however, not well studied. This paper reports zircon U–Pb ages, Lu–Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes for the Qianjia intrusive rocks in Hainan Island, southeast China. LA-inductively coupled plasma mass spectrometry zircon U–Pb dating of granites and dark enclave monzonite in the area yield magmatic crystallization ages of ca. 100 Ma, which are consistent with other Late Cretaceous granites, e.g. Baocheng, Tunchang, and Yaliang. Both rocks show high-K calc-alkaline compositions and metaluminous to weakly peraluminous signatures belonging to I-type rocks. They are enriched in the alkalis, Rb, Th, U, K, and light rare earth elements, depleted in Nb, Ta, Ti, and P, and characterized by high Al2O3 contents (14–15 wt%) and high Mg# values (50–53). Among them, some of granodiorites have geochemical affinities of adakitic rocks. Zircon εHf(t) values range from −5.97 to −1.18, with fairly constant whole-rock Sr–Nd isotopes (ISr = 0.7084–0.7086; εNd(t) = −4.97 to −4.29) similar with those of the Cretaceous mafic dikes (136–81 Ma) in Hainan Island, which are the result of partial melting of subduction-related sub-continental lithospheric mantle. Combined with Sr–Nd isotopes and negative Hf isotope, Qianjia intrusive rocks were likely derived from hybrid melts of underplated continental crust-derived with mantle-derived, then experienced varied degrees of fractional crystallization. According to the latest geophysical, sedimentological, and geochemical data, previous authors identified a Cretaceous E–W-trend subduction zone in the northern margin of the South China Sea. Combined with the southern margin magmatisms (110–80 Ma) and magmatisms of ~120 Ma distributed east–west ward from the Philippines to the Vietnam, We preferred that the subduction of the E–W-trend Neo-Tethys plate was the main geodynamic mechanism which induced the Cretaceous large-scale magmatisms in the southern margin of South China Block.

Platinum-group element geochemistry of the Forest Reef Volcanics, southeastern Australia: Implications for porphyry Au-Cu mineralisation

Platinum-group element concentrations in felsic to intermediate rocks from the Forest Reef Volcanics, Cadia-Neville region, southeastern Australia have been analysed by the Ni-S fire assay-isotope dilution method. The Forest Reef Volcanics are shoshonitic to calc-alkaline in composition and fractionated to produce a wide range of compositions, with MgO varying between 9.7 and 1.8 wt.%. The interest in this suite is that it is coeval with Au-Cu porphyry-style mineralisation in the Cadia mineral district. This study uses PGE geochemistry to determine the timing of sulfide saturation, relative to volatile (ore-fluid) saturation, in the magma that gave rise to the Forest Reef Volcanics and, in turn, to assess how this timing affected the mineralisation potential of the evolving magmatic system.The Forest Reef Volcanics can be subdivided, on the basis of their contrasting PGE geochemistry, into high-Mg (>6.8 wt.% MgO) and low-Mg suites (≤6.8 wt.% MgO). Platinum, Pd and Re concentrations increase in the high-Mg samples, whereas Ir and Ru decrease and Rh concentrations remain steady, with decreasing MgO. The coupled Ir, Ru and Rh depletion is attributed to the partitioning of these elements into magnetite. The rate of Pt and Pd enrichment is not possible by closed-system fractional crystallisation alone, which suggests that the parent magma was replenished by a Pt-Pd-rich melt. In contrast, the PGE concentrations in the low-Mg samples decrease with decreasing MgO indicating the onset of sulfide saturation at 6.8 wt.% MgO, which is confirmed by the presence of spheroidal sulfide inclusions in liquidus crystals (i.e. clinopyroxene, plagioclase, magnetite). The rate of Pd depletion is appreciably less than for any other sulfide saturated felsic system for which data are available. This requires either that the amount of sulfide melt to have precipitated was unusually low, or that the rate of Pd depletion was limited by the mass of silicate melt the sulfide melt reached equilibrium with, or both. In any event, the fraction of sulfide melt that precipitated was too small to have had a significant effect on the Cu and Au content of the magma so that both Cu and Au were available to enter the ore-forming fluid when the magma became volatile saturated at, or shortly after, it reached ca. 2.9 wt.% MgO.

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt, NW China: Constraints on the initiation of a magmatic arc in the southern Central Asian Orogenic Belt

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt (ETOB) have been studied in order to constraint the initiation of a magmatic arc formed in this region. Zircon U–Pb dating indicates that two dioritic plutons in the northern ETOB were generated in the Late Ordovician (452±4Ma) and the Early Silurian (442±3Ma), respectively. Diorites from the two plutons are characterized by enrichments in large ion lithophile elements (LILE) and highly incompatible elements, with depletions in high field strength elements (HSFE) displaying typical geochemical features of a subduction-related origin. They have positive εNd(t) values (+5.08–+6.58), relatively young Nd model ages (TDM=0.71–1.08Ga), with Ta/Yb (0.05–0.09) and Nb/Ta ratios (12.06–15.19) similar to those of depleted mantle, suggesting a juvenile mantle origin. Their high Ba/La (13.3–35.9), low Th/Yb (0.72–2.02), and relatively low Ce/Th (4.57–14.7) and Ba/Th (47.8–235) ratios indicate that these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids. Zircon U–Pb dating of a granitic pluton in the northern ETOB yielded a Late Ordovician intrusion age of 447±5Ma. Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites. They also show positive εNd(t) values (+6.49–+6.95) and young Nd model ages (TDM=0.69–0.87Ga), indicating that the granites were most likely derived from juvenile lower crust. Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern ETOB began prior to or at the Late Ordovician (452–442Ma). In addition, north-dipping subduction of the Kangguertage oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the ETOB in the Early Paleozoic.

Paleoproterozoic andesitic volcanism in the southern Amazonian craton, the Sobreiro Formation: New insights from lithofacies analysis of the volcaniclastic sequences

The Sobreiro Formation (SF) records one excellent and well-preserved example of subaerial Precambrian (ca. 1.88Ga) volcanism on earth. It is located in the São Felix do Xingu region (SFX), in the eastern part of Pará State, southern Amazonian craton, northern of Brazil. The high-K calc-alkaline composition of the Sobreiro rocks indicates that this formation likely generated in an ocean-continent convergent margin. This paper documents the architecture of a series of basaltic-andesite to andesite and minor dacite lava flows and associated volcaniclastic rocks. These rocks are divided into primary and secondary lithotypes, depending if they resulted from a direct volcanic activity (pyroclastic) or reworked processes and massive to stratified, depending on the different transport and emplacement mechanisms. Primary lithofacies is formed of pyroclastic flow and surge deposits, volcanic breccias and welded ignimbrites; secondary volcaniclastic lithofacies consist of reworked debris. Mass-flows, hyperconcentrated flows and stream floods are interpreted as fluvial/alluvial deposits representing periods of stream and river reworking and re-establishment after an eruptive phase or an edifice failure event. These different lithofacies record the volcanic history of the Sobreiro Formation. A complex volcanic environment is thought to have existed with emission of large lava flows and explosive eruptions. The modern volcanological approach used here can serve as a model for the evolution of Precambrian volcano-sedimentary basins. Our approach sheds new light on the different processes operating on volcanic edifices and to constrain the depositional environment and thus geodynamic setting of Precambrian continental volcanic belts.

Petrogenesis and mineralization of REE-rich granites in Qingxi and Guanxi, Nanling region, South China

Mesozoic granites in the Nanling region, South China, are important primary rocks for ion-adsorption rare earth element (REE) deposits. Moreover, metallic deposits of W, Sn, Nb and Ta also develop in these granites, most of which formed in the Late Jurassic. The Qingxi and Guanxi granites in the Nanling region contain abundant REE minerals and have zircon U-Pb ages of 223.0±0.7Ma and 198.8±1.4Ma, respectively. They are of metaluminous to slightly peraluminous in character having a high K calc-alkaline composition, and have high K2O+Na2O and FeOT+MgO contents and a Ga/Al ratio similar to A-type granites. These granites plot into the post-collision field, similar to the A2-type granites. The Qingxi and Guanxi granites indicate a crust-dominant origin by having high levels of Rb and Th relative to Nb and Ta normalized to ocean-ridge granites. They are characterized by LREE enrichment and the majority of REE accumulating in the primary REE minerals include REE-phosphates and fluorocarbonates. The Guanxi rocks have a stronger negative Eu anomaly, but lower REE composition than the Qingxi granites, implying that the basement may have played an important role in the REE mineralization rather than fractional crystallization. This further implies that the basement in South China may have been subjected to different evolutionary geodynamics, resulting in different multi-metallic mineralization of W-Sn-Nb-Ta-REE hosted by large scale Mesozoic granites.

Mineralogy, geochemistry, and petrology of Neoarchean ferroan to magnesian granites of Carajás Province, Amazonian Craton: The origin of hydrated granites associated with charnockites

2.75 to 2.73Ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the Carajás Province. The Vila Jussara Suite was recognized recently and its geologic, mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the Estrela Complex, Serra do Rabo and Igarapé Gelado granites and Planalto Suite. Monzogranites and syenogranites are dominant in most units but in the Igarapé Gelado Granite and Vila Jussara Suite granodiorites and tonalites are also relatively abundant. The main mafic phases in all these granitoids are amphibole [potassian hastingsite with subordinate magnesiohastingsite; 0.95≥Fe/(Fe+Mg)≥0.47] and biotite [0.88≥Fe/(Fe+Mg)≥0.52] with rare occurrence of clinopyroxene relict crystals. Magmatic epidote (% of ‘pistacite’ component from 26 to 31) occurs only in the Vila Jussara Suite. The rocks of Estrela, Serra do Rabo, and Planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced-ferroan granites [whole rock FeOt/(FeOt+MgO)≥0.89] that evolved at low fO2 (FMQ±0.5). The Igarapé Gelado Granite needs additional studies but it is formed apparently by reduced-ferroan and oxidized-ferroan granites. The Vila Jussara Suite is also composed of reduced-ferroan granites that are similar to those of the other granite units. However, it has additionally oxidized-ferroan and magnesian granitoids which contain magnetite (±ilmenite) as the main Fe-Ti oxide phase and evolved, respectively, at moderate fO2 (NNO±0.5) and at comparatively higher fO2 (NNO to NNO+1). The magmas of these granites were formed at temperatures≥900°C and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc-alkaline in composition. Plagioclase, orthopyroxene, clinopyroxene and Fe-Ti oxides were the main phases in the melt residues and the liquids were enriched in Ba, Zr, Y, and REE (except for Eu), but not in Sr compared to their sources. The magmas should have relatively high water content (>4wt.%). The magmatic evolution was strongly influenced by partial melting processes. The different varieties of the Vila Jussara Suite are not linked by fractional crystallization and derived from distinct magmas. The Neoarchean granites of the Carajás Province were emplaced in a collisional tectonic setting. They are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling. The magmas were generated in the crust at pressures of ~800 to 1000MPa and emplaced at pressures of 200 to 500MPa. The studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites. The magnesian variety of the Vila Jussara Suite is similar to the pyroxene-free granitoids of the Neoarchean Matok Pluton of the collisional Limpopo Belt. The Neoarchean granites of Carajás are hydrated granites emplaced at comparatively higher crustal levels when compared with the Matok pluton.