Miocene Volcanic Rocks Research Articles

Mineralogy and Geochemistry of Upper Miocene Igneous Rocks, Kos Island, Greece: Extension during Strike-Slip Faulting and Subduction Rollback

Upper Miocene volcanic and plutonic rocks on Kos island preserve a record of magmatic and tectonic events in the transition zone between the Aegean and Anatolian microplates. Their field setting, syn-intrusion deformation, mineralogy, and geochemistry were investigated. Volcanic rocks, including trachyandesite flows and trachyandesite to rhyolite domes, were extruded on a central E–W horst and directly overlie Alpine basement. Thick successions of trachytic flow tuffs are interbedded with fluvial and lacustrine basinal sediments to the south of this horst. Volcanism was synchronous with the emplacement of the Dikeos monzonite pluton, which is geochemically similar to some lithic clasts in the thick flow tuffs and is cut by mafic dykes including lamprophyres. Two main types of mafic magma were present: a K-rich lamprophyric magma that evolved to trachyandesite and more calc-alkaline magma similar to mafic enclaves in the monzonite. Syn-intrusion structures in the monzonite indicate emplacement during E–W sinistral strike-slip faulting that created local transtensional deformation, providing accommodation for a Dikeos magma reservoir. A change in the style of deformation in the Late Miocene led to NW-striking extension in the footwall, occupied by mafic dykes and mineralized veins, and extensional detachment of the hanging wall, resulting in unroofing of the monzonite.

Post-collisional calc alkaline-adakites from a metasomatized subcontinental lithospheric mantle: Sr-Nd isotopes and bulk-rock geochemistry of Miocene volcanic rocks from the Varzaghan area, NW Iran

ABSTRACT The NW Iran is located at the western termination of the Alborz Mountain range and continuation of the Sanandaj-Sirjan magmatic-metamorphic zone at the NW. The volcanic rocks in the Varzaghan area are distributed over different compositional ranges from high-K basalt, andesite, dacite, and trachyandesite, dacite, to rhyolite with adakitic signatures. Chondrite-normalized REE diagrams show flat MREE and HREE patterns for calc-alkaline volcanic rocks and enrichment in LREE, indicating a garnet-free source. Adakitic rock sample chondrite-normalized REE patterns are relatively steep with significant enrichment in LREE and depletion in HREE, indicating a garnet-bearing source or garnet fractionation process. The arc trace element signatures, narrow range of initial87Sr/86Sr(i) = 0.70425–0.70487, different initial143Nd/144Nd(i) = 0.51264–0.51286 and the presence of adakitic rocks reveal that the volcanic rock sources were not the same and can be related to the partial melting of metasomatized mantle and lower continental crust sources. Slab break-off and upwelling of hot asthenosphere in a post-collisional tectonic regime resulted in the melting of the subcontinental lithosphere and lower continental crust which produced calc-alkaline-adakitic volcanic rocks in the Varzaghan area. The main petrological processes involved in the volcanic rocks petrogenesis were fractional crystallization and crustal assimilation.

Magmatic Evolution of the Fossil Melanesian Island Arc: Evidence From Lower Miocene Lavas of Malekula Island (Vanuatu)

AbstractThe subduction zones in the SW Pacific Ocean are some of the most dynamic plate boundaries on Earth with changes in subduction polarity and subduction initiation processes. The New Hebrides island arc formed some 10 million years ago after the Melanesian island arc was abandoned due to the cessation of subduction of the Pacific Plate after collision of oceanic plateaus with the island arc. Parts of the Melanesian island arc occur within the New Hebrides island arc and we show that Miocene volcanic rocks exposed on Malekula island in the New Hebrides island arc consist of island arc tholeiites to alkaline basalts with variable slab contribution. The lava erupted between 22.5 and 16.3 Ma, similar to the Wainimala Group lavas on Fiji representing the eastern portion of the Melanesian island arc. Partial melts from subducted sediments affected a Pacific MORB‐type mantle, but we do not find evidence for a slab component derived from the Ontong Java Plateau or for assimilation of continental crust. Thus, no Miocene subduction of the Ontong Java Plateau occurred beneath Malekula and the basement of the island probably does not consist of continental crust rifted from Australia. The Malekula lava succession resembles that of other portions of the Miocene Melanesian island arc between New Britain and Fiji, indicating continuous subduction of the Pacific plate along this arc.

Mineral chemistry and pre-eruptive crystallization conditions of Miocene volcanic rocks in the Kütahya (West Anatolia) and surrounding area

Mineral chemistry and pre-eruptive crystallization conditions of Miocene volcanic rocks in the Kütahya (West Anatolia) and surrounding area

Type of Rockslide along the Ponorogo – Pacitan Roads, East Java, Indonesia

The Pacitan Regency, located in the southwestern part of East Java Province, is one of the areas with low economic growth. Difficult accessibility is one of the reasons for the slow development of this region. To reaching the Pacitan Regency it can take through the Ponorogo – Slahung – Arjosari - Pacitan roads. Based on the geological condition most of roads are built on geologic formations composed of Oligocene - Miocene volcanic and intrusive rocks. Mass movement often occurs along roads and disturbs the transportation of goods and people. This paper discusses the types and mechanisms of slope failure along roads. Recognizing these two aspects of slope failure plays important role to treat them. The field survey identified 13 landslide zones with a 50–200 m bright. Based on rock type, weathering and alteration levels, and geological structure, slope failure can be classified into three types. First, the wedge rockslide occurred on the low-altered dacitic lapilli tuff. The planar – toppling type of rockslide occurred in the residual soil of the andesitic basalt lava with columnar and sheeting joint. Third, complex circular debris slides occur in highly jointed and altered dacitic intrusive rocks, which contain a large amount of clayey material. The third slide type that occurred at km 226 requires further research with an interdisciplinary approach because of moving the 200-meter-long road mass.

Along- and across-strike variation of damage zone parameters in the Kornos-Aghios Ioannis normal fault, Lemnos Island, Greece

The Kornos-Aghios Ioannis Fault (KAIF) is an extensional fault system that deformed Lower Miocene volcanic rocks and Middle Eocene to Lower Miocene turbidite sandstones at shallow depth (<1 km). The fault system is exposed for about 10 km and consists of two major NW-SE-striking segments connected by an E-W-striking one. This structural architecture provides the opportunity to study the variation of damage zone parameters (i.e., damage zone width, fracture density, attitude of deformation structures, and fracture connectivity) in wall, tip and intersecting damage zones. The >122 m wide tip damage zone has a width greater than the wall and intersecting damage zones (72.6 and 50.9m, respectively) and exhibits the highest average fracture density recognized along the entire KAIF. In tip and intersecting damage zones, damage structures exhibit a larger azimuthal variability and hence a greater fracture connectivity (average number of connections per branch (CB) 1.60 and 1.53, respectively) compared to wall damage zones (average CB value 0.88). Accordingly, significant along-strike variations of damage zone parameters occur in the absence of a clear dependence on fault displacement. Particularly, fault intersections and tip regions represent areas of enhanced secondary permeability potentially controlling fluid circulation in the subsurface.

Geochemical Characteristics of the Volcanic Rocks Associated with Boron-Rich Deposits from the Xiongba Basin, Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) hosts significant lacustrine sedimentary boron-rich deposits, with the Xiongba Basin being a prominent region housing two large sedimentary boron-rich deposits. These deposits are closely associated with extensive Neogene volcanic rocks. This study investigates the origin and boron sources of Miocene volcanic rocks in the Xiangqu River area, located within the Xiongba Basin. The volcanic rocks in the basin comprise ultrapotassic andesites, ultrapotassic trachyte, potassic trachyte, and potassic trachyandeiste. The trace element content and the active/inert elements ratios of the studied volcanic rocks have indicated that they were generated in a subduction environment and were influenced by enrichment fluids derived from deep-sea sediments or altered oceanic crust during their formation. Accordingly, the studied volcanic rocks exhibit significant boron enrichment. The eruption of magma and subsequent hydrothermal activity released boron, which became the primary source for the lacustrine sedimentary boron-rich deposits within the basin. The arc-like trace element features (e.g., Nb-Ta depletion relative to La and K) and high B concentrations in these rocks were inherited from the mantle source, which had been enriched by melt/fluid of the subducted sediments. A two-stage evolutionary model is proposed to explain the enrichment of B in subduction environments, as well as the subsequent melting of the B-enriched source during a post-collisional setting. These findings highlight the potential for boron and lithium mineralization in similar volcanic rock-bearing regions across the QTP. Future exploration efforts in such areas could provide valuable insights into the formation processes of lacustrine sedimentary boron-rich deposits and contribute to the understanding of boron and lithium resource potential.

Miocene Volcanism of the Baikal Rift Across the Boundary of the Siberian Craton: Evidence for Lithospheric Mantle Melting

Abstract Continental rifting is usually viewed in terms of two contrasting models of active and passive extension. The origin of the Baikal Rift, adjacent to the southern part of the Siberian Craton, has been described by both models in the past. It is expected that basaltic magmatism in an active model scenario should be primarily sourced from a mantle plume or plume-fed asthenosphere, whereas melting of the lithospheric mantle is expected to be a predominant source for magmatism in the passive model. In this paper, we focus on the Miocene volcanic rocks sampled along two 60-km-long profiles that cross the boundary between the Neoproterozoic Tuva-Mongolian massif and the Archean-Paleoproterozoic Siberian Craton. Most of the samples studied are trachybasalts. In terms of trace element concentrations normalised to primitive mantle, the lavas mimic oceanic island basalt-like patterns with troughs at Rb, Th–U, Pb, and Y, and peaks at Ba, Nb, Ta, K, and Sr. Moreover, similar trace element patterns to the studied samples are also observed for Miocene and Quaternary lavas located in the southwestern of the Baikal Rift, and adjacent regions of non-rifted Mongolia. According to the ratio of CaO to MgO, and TiO2/Al2O3 to SiO2, the compositions of the studied lavas coincide with experimental melts derived from mafic lithologies. Trace element data of samples suggest that garnet was a residual phase during partial melting. The Sr-Nd isotopic characteristics of the studied lavas are 87Sr/86Sr 0.70427–0.70469 and 143Nd/144Nd 0.51267–0.51284. They are identical to the coeval Miocene lavas of neighbouring volcanic fields, but they differ from the Quaternary lavas that extend to lower 87Sr/86Sr (0.7038–0.7044) with near identical 143Nd/144Nd. Isotopes of Hf for studied samples show values εHf = 6.0–7.7, except for the two samples taken within the boundary between two lithospheric blocks with εHf 4.6 and 4.8. The δ18O of olivine from lava samples is everywhere higher than that of the asthenospheric mantle and ranges from 5.5 to 6.4‰. Variations of δ18O versus Mg#, 87Sr/86Sr and εHf in the studied samples do not correlate, but do unequivocally rule out crustal assimilation. The isotopic variations are consistent with recycling of mafic crustal lithologies at mantle depths. Lavas from the Tuva-Mongolian massif and the Siberian Craton differ in lead isotopes by lower values of 206Pb/204Pb (&amp;lt; 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples and the reverse relationship for off-cratonic lava (&amp;gt; 17.785 and 55–61), respectively. The equation for Δ8/4Pb = [208Pb/204Pb-(1.209*(206Pb/204Pb) +15.627)] *100 is from Hart (Nature, 309, 753–757, 1984). The correlation of lead isotopes with the mafic recycled component, the sharp change of lead isotopic values at the cratonic boundary and decoupling of lead isotope ratios from other isotopic ratios lead us to suggest that the values of 206Pb/204Pb and Δ8/4Pb are associated with an ancient accessory mineral phase such as sulphide confined within the lithospheric mantle. The predominant role of the lithospheric sources in the formation of the Miocene volcanic rocks indicate that the volcanism of the Baikal Rift was caused by a passive tectonic process, rather than active rifting.

Vanished evaporites, halokinetic structure, and Zn-Pb mineralization in the world-class Angouran deposit, northwestern Iran

The Angouran deposit (19.3 Mt at 23.4% Zn and 4% Pb) is the second-largest Zn-Pb deposit in Iran. The deposit is hosted in a Neoproterozoic−Cambrian marble-schist sequence within a breccia pipe in a domal structure, with sulfide mineralization under low-temperature hydrothermal conditions (&amp;lt;200 ºC). The features of the ore-hosting breccias are similar to known halokinetic diapir breccias in the world but evaporite minerals are subtle. The common types of breccia clasts in the Angouran breccia pipe include a matrix-supported angular clast (float breccia) with highly variable sizes and orientations and exotic volcanic clasts. The volcanic clasts were derived from the underlying Miocene volcanic rocks, evidenced by the consistent petrography and zircon U-Pb ages dated at 20−19 Ma. Abundant smithsonite pseudomorphs after anhydrite and anhydrite inclusions within sphalerite and pre-ore marcasite in the breccia matrix indicate that the breccia pipe contains abundant anhydrite prior to the Zn-Pb mineralization. The enrichment of evaporite is also supported by the occurrence of considerable double-terminated quartz crystals that contain spherical and tabular carbonate inclusions and anomalously high Li, Na, and K concentrations, relatively high B concentration, and high δ18O values (up to 28.3‰). These observations suggest the Angouran deposit formed in a former halokinetic diapir breccia pipe. The halokinetic diapirism was possibly triggered by thrust loading of the marble-schist sequence over the Miocene evaporite beds during the Arabia-Eurasia continental collision. Halokinetic structures elsewhere in the Angouran region warrant this consideration. Most of the evaporite minerals in the breccia pipe were dissolved and replaced before and/or during subsequent Zn-Pb sulfide and smithsonite mineralization events. This study provides a good example for the identification of vanished evaporites, halokinetic structure, and associated Mississippi Valley-type mineralization.

New paleomagnetic results from Neogene to Quaternary volcanic rocks of north of the Lake Van, Eastern Turkey

The Eastern Anatolia is an active tectonic region where the collision between the Arabian and Eurasian plates take place. Due to the subduction of Arabian plate’s oceanic lithosphere under Eurasian plate, widespread volcanism observed in large areas began in Serravallian. There is no consensus in the literature for the tectonic evolution of the region. Therefore, there are many geological and geophysical studies conducted with the intention of explaining the tectonic evolution of Eastern Anatolia by geodynamic models. Our paleomagnetism study aims to reveal the tectonic rotations in order to better understand the development of the prevailing tectonism in the region from the volcanic rocks. Paleomagnetic samples were collected from 86 sites of the Late Miocene–Pleistocene volcanic rocks located at the north of Lake Van. Isothermal remanent magnetization studies show that magnetite is the mineral responsible for magnetization in most rocks, while both magnetite and hematite are responsible for the rest of the rocks. Curie temperatures and alteration degrees of rock samples were also determined by high-temperature susceptibility (HTS) studies. In some samples, titanomagnetite component was observed in the heating phase of the HTS measurements. The absence of this component in the cooling step indicates that Ti-magnetite is transformed into magnetite by alteration. The Pleistocene volcanics show counterclockwise rotation of R ± ΔR = 13.4° ± 3.8°. The Pliocene volcanic rocks were defined in four different groups: south of Erciş Fault, north of Erciş Fault, around Muradiye and north of Van. Also, the remarkable clockwise rotation is observed in the north of Van and near Muradiye R ± ΔR = 24.4° ± 17.0° and R ± ΔR = 6.9° ± 9.4°, respectively. In addition, counterclockwise rotation (R ± ΔR = 14.5° ± 6.1°) is obtained in the southern part of the Erciş Fault, while there is no significant rotation (R ± ΔR = 0.6° ± 7.4°) on the northern side. Late Miocene volcanic rocks show no significant rotation either (R ± ΔR = 1.8° ± 13.7°). Our new paleomagnetic results indicate that the left-lateral strike-slip Çakırbey Fault, located to the east of the Erciş fault and extending roughly in the northeast–southwest direction, may be active.

Mapping hydrothermal alterations and lineaments associated with epithermal and massive sulphides deposits of Tifraouine (northwest Algerian coast): Use of Landsat 8 OLI data and remote sensing

Within the context of studying mineralizations related to the Miocene volcanic rocks from the northern part of Algeria, Landsat 8 OLI data and remote sensing techniques are carried out to characterize and map hydrothermal alteration outcrops in the Miocene volcanic rocks of Tifraouine on the northwestern Algerian coast, where several indications of mineralization are known. Short-wave infrared (SWIR) bands allowed for the identification of clay alterations, while the visible and near-infrared bands help in the detection of iron alterations. Red-green-blue color combination images of 6/7, 4/2 and 5/6 ratios were produced. Principal component analysis (PCA) was computed to characterise altered zones and extract lineaments. Matched filtering technique (MF) allows direct detection after applying minimum noise fraction (MNF) transformation and pixel purity index to the spectra in order to obtain a distribution map of the target mineral species in studied area. The remote sensing results combined with the structural geological data determine a relationship between mineralization and structural features and show that hydrothermal alterations are clearly controlled by tectonic. They are highly concentrated in the calderas zone and extend towards the Bouzedjar area which allowed checking the map performed according to the field studies. This will contribute to preliminary studies related to mineral exploration and enhance the mining potential of Tifraouine. The acquisition of supplementary data would allow us to extend this study in coming works.

Was cratonic Asia deeply subducted beneath the Pamir? Evidence from P–T conditions and tectonic affinities of Cenozoic Pamir crustal xenoliths

AbstractOne of the most striking geological features of the Pamir is the south‐dipping lithospheric slab beneath the orogen characterized by an intracontinental Wadati‐Benioff zone. A widely accepted hypothesis over the past 40 years interprets the slab to represent southward subducted cratonic Asian continental lithosphere, which predicts significant cratonic Asia‐sourced crustal materials (e.g., Tarim Basin) beneath the Pamir. Alternatively, recent studies have interpreted the slab to be lithosphere delaminated from the base of the Pamir. To test these hypotheses, depth–tectonic affinity relations of crustal xenoliths carried by Miocene volcanic rocks in the eastern Pamir, interpreted to be sourced from the Pamir deep lithosphere, are used to determine whether they represented Asian affinity cratonic crust. Thermodynamic calculations, zircon U–Pb geochronology combined with rare earth element analysis, and whole‐rock major‐trace element and Sr–Nd isotopic analyses document that (1) eclogite and pyroxenite xenoliths (~31–43 kbar/~960–1170°C) are the deepest sourced portions of the lithosphere from ~100 to 140 km depth, the protoliths of which represent the mid‐lower crustal rocks of the Cretaceous Pamir magmatic arc, rather than material from cratonic Asia, and (2) granulite xenoliths (~20 kbar/~900°C) represent the Cenozoic lower crustal rocks of Pamir terranes from ~70 km depth. These results indicate the south‐dipping slab represents delaminated Pamir lower crust and mantle lithosphere, rather than intracontinental subduction of Asian lithosphere, and further support the hypothesis of minimal Cenozoic northward translation of the Pamir.

Detection of hydrothermal alteration and structural characteristics of Miocene volcanic rocks using remote sensing in the M’sirda region (northwestern Algeria)

Detection of hydrothermal alteration and structural characteristics of Miocene volcanic rocks using remote sensing in the M’sirda region (northwestern Algeria)

Geology and Structural Evolution of the La Huifa Ore Deposit, Central Chile: A Newly Discovered Porphyry Cu-Mo System in the El Teniente District

Abstract La Huifa is a breccia-related Cu-Mo porphyry deposit located in the Andes of central Chile, 3 km northeast of the giant El Teniente porphyry Cu-Mo orebody. It was discovered as part of CODELCO’s (Corporación Nacional del Cobre de Chile) brownfield exploration activities in the El Teniente district. It is the first major discovery in the area, and this work presents its first detailed geologic description. The La Huifa orebody is related to a complex of late Miocene diorite and granodiorite intrusions and hydrothermal breccias, the latter characterized by abundant tourmaline-anhydrite cement, emplaced in middle to late Miocene volcanic and subvolcanic rocks. The structural architecture of La Huifa is dominated by WNW-and NE-striking high angle faults. Usually, slip on the WNW-striking faults involves a sinistral component, whereas slip on the NE-striking faults is predominantly dextral. The main hydrothermal breccia body at La Huifa is located at the intersection of WNW- and NE-striking faults. Four stages are proposed for the tectono-magmatic-hydrothermal evolution of La Huifa:Premineralization, involving tourmaline-albite veins and sodic calcic alteration emplaced under a transpressive tectonic regime with a subhorizontal, ~E-W–trending σ1 (~N-S–trending σ3);Early mineralization, involving a polydirectional vein system associated with poorly developed potassic and transitional alteration zones with a scarce presence of Cu and Mo sulfides;Main mineralization, involving Cu- and Mo-rich anhydrite-tourmaline hydrothermal breccias and polydirectional veins associated with strong chlorite-sericite alteration with a 270° to 290°C emplacement temperature, obtained from chlorite geothermometry. The regional stress field during this period was exceeded by the local fluid pressure. These breccias and veins are followed by the emplacement of NE-striking C and D type veins associated with gray and white sericite alteration, respectively, which show a similar temperature range.Late mineralization, involving a NE-striking, carbonate-rich vein system containing polymetallic sulfides. It is associated with argillic alteration, whose temperature was 200° to 260°C. This stage occurred under a compressive tectonic regime with NE-trending σ1. The relatively low temperature at which mineralization was introduced in this deposit (~270°–290°C) and the predominantly vertical fluid flow pattern typical of hydrothermal breccia systems allow us to infer the presence of more mineralized rock at depth, below the recognized portion of the system. The association of Cu and Mo enrichment with chlorite-sericite alteration, instead of higher-temperature potassic or gray sericite alteration events, makes La Huifa a unique case among the porphyry deposits of central Chile and broadens the scope of mineralization styles that could be targeted in this belt. Finally, the intersection of arc-oblique, high-angle faults and, in particular, the presence of fault systems orthogonal to the main compression direction, favorable for magma storage and differentiation and metal concentration, constitute a valuable tool for exploring porphyry Cu deposits in similar geologic contexts.

Genesis of alunite-bearing kaolin deposit in Mudamköy member of the Miocene Göbel Formation, Mustafakemalpaşa (Bursa), Turkey

Tectonic-controlled hydrothermal alteration of Lower Miocene volcanic rocks in the Mustafakemalpaşa (Bursa) region resulted in the development of kaolin deposit, which have an economical potential. This study will be important for the mineralogical, geochemical, and genetic contributions to the ongoing mining operation of this kaolin deposit and its possible application fields. The upward mineralogical and geochemical zonation of the kaolin deposit is as follows: kaolin, alunite-bearing kaolin, silicified kaolin, and silcrete; associated with Fe-oxide bearing veins and silicification. The characteristics of the kaolinized and alunitized units and their parent rocks were examined using polarized-light microscopy, X-ray powder diffractometry (XRD), scanning electron microscopy (SEM–EDX), chemical (ICP–AES and –MS), and isotopic (H–O, S–O) methods. Kaolinite and alunite are abundant and generally associated with quartz, feldspar, volcanic glass, and mica, locally smectite, accessory halloysite, and goethite. The kaolinite content increases up to 75% in the kaolin zone. Kaolinite shows both ideal hexagonal book-like and non-idiomorphic (initial stage) crystal forms associated with rhombohedral blocky alunite are formed authigenically on altered volcanic materials and locally covered by accessory star-like goethite and tube-like halloysite crystals. Under the open hydrological system and acidic conditions, the Al ± Fe/Si ratio and Al + K + S values increased compared to the parent rocks, favoring the formation of kaolinite and alunite, respectively. The enrichment of Sr, Ba, and depletion of Fe, Rb, Ce, Y, and La, and the negative Eu anomaly shows that the fractionation of feldspar and mica originated from Miocene volcano-sedimentary components during the magmatic and hydrothermal alteration process(es) were the main sources for the formation of kaolinite and alunite. The δD and δ18O composition and formation temperatures of kaolinite are 106–167 °C, suggesting an epithermal alteration process(es). The negative δ34S and positive δ18O values for alunite exhibit local contribution of meteoric water mixed with magmatic fluid during hydrothermal activity.

An unconformity in the Pole Creek area (Sevier Plateau) west of Antimony, western Garfield County, Utah, and its bearing on the Sevier gravity slide

Pole Canyon is cut into the western backslope of the Sevier Plateau of southwestern Utah, a gently-tilted, block-faulted range that extends north from Bryce Canyon National Park through the eastern part of the Marysvale volcanic field. The canyon exposes a spectacular angular unconformity that separates brecciated, intensely deformed, and steeply dipping Eocene to Oligocene sedimentary and volcanic rocks below, from gently east-dipping Miocene volcanic rocks above. Although identified in 1968 by the senior author, it took renewed geologic mapping in 2015 by all three authors to discover that the rocks below the unconformity were deformed by gravity sliding. We named it the Sevier gravity slide, one of the largest terrestrial landslides on Earth. The ages of the volcanic rocks above and below the unconformity constrain the age of sliding at between 25.8 and 23.1 Ma; later dating elsewhere put the slide movement at between 25.2 and 25.1 Ma.

Highly siderophile element and Os isotope systematics of the Cenozoic volcanic rocks from the Eastern Pontides, NE Turkey: Constraints on the origin and evolution of subcontinental mantle-derived magmas

The Cenozoic subalkaline to alkaline volcanic rocks from the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey exhibit wide range of highly siderophile element (HSE) contents and Os isotope compositions that can be related to their mantle sources and parental melt evolution. Although some Eocene rocks show signs of late-stage modification, most of the studied volcanic rocks possess primary HSE and Os systematics. The Eocene volcanic rocks have largely variable PdN/IrN (~0.25–751), PdN/PtN (~0.25–87) and (187Os/188Os)i ratios (~0.113 to 0.51). The Miocene volcanic rocks have high PdN/IrN (~2.6–441), low PdN/PtN (~0.11–1.7) and radiogenic (187Os/188Os)i ratios ranging from ~0.166 to 0.52 while the Mio-Pliocene volcanic rocks show high PdN/IrN of ~7.2–29, low PdN/PtN (~1.2–2.7), and only mildly radiogenic (187Os/188Os)i values between ~0.132 and 0.177. Finally, the Quaternary volcanic rocks have low PdN/IrN (~2.3–3.6), PdN/PtN (~0.9–1.6) and radiogenic (187Os/188Os)i values ranging from ~0.133 to 0.194. Overall, the primitive mantle-normalized HSE patterns of the studied volcanic rocks are characterized by PPGE (Pt, Pd) and Re enrichment with respect to IPGE (Os, Ir, Ru). The variations in HSE and radiogenic Os isotope compositions of the relatively differentiated volcanic rocks indicate that crustal assimilation played active role in their parental melt evolution, especially in the Eocene and Miocene period. Besides, the obtained HSE data also suggest that the parental magmas of all the studied volcanic rocks were generated mainly in the S-saturated conditions. The temporal and spatial HSE and Os isotope variations observed in the studied Cenozoic volcanic rocks reflect the heterogeneity of the mantle source beneath the EPOB, which could be attributed to variations in response of different fluid/melt metasomatism of the underlying SCLM.

Petrological characteristics and K-Ar age of Sakiura welded tuff from the Komegawaki Formation, Fukui prefecture, central Japan

Major and trace element compositions and a K-Ar age are reported for the Sakiura welded tuff of the Miocene Komegawaki Formation in the Mikuni coastal area, Fukui prefecture, central Japan. The rock samples are densely welded, generally show eutaxitic texture, and contain 9-17 vol.% plagioclase, minor (<1 vol.%) hornblende, and 1-14 vol.% lithic fragments. Plagioclase in the Sakiura welded tuff yields a K-Ar age of 13 Ma, which is within the 12-14 Ma age range of Komegawaki volcanic rocks. Sakiura welded tuff samples are rhyolitic (SiO2 = 73.3-74.5 wt.%) and are near the SiO2-rich end of the compositional trend of Komegawaki volcanic rocks on major element versus SiO2 cross-plots. The MORB-normalized trace element patterns of Sakiura welded tuff samples are similar to those of the Komegawaki volcanic rocks. These observations indicate that the origin of the rhyolite magma that produced the Sakiura welded tuff is associated with the Komegawaki volcanic rocks. Evidently, multiple rhyolitic eruptions occurred concurrently with the formation of the Miocene volcanic rocks after the opening of the Japan Sea.

Zircon U–Pb ages and petrogenesis of late Miocene adakitic rocks from the Sari Gunay gold deposit, NW Iran

AbstractLate Miocene volcanic rocks host the Sari Gunay epithermal gold deposit in NW Iran. These rocks are located within the Hamedan–Tabriz volcanic belt and occupy the northwestern part of the Sanandaj–Sirjan zone (SaSZ). The volcanic rocks span in composition from latite to dacite and rhyolite. Plagioclase, hornblende, biotite and quartz are the main phenocrysts in a fine-grained and glassy matrix. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating yielded crystallization ages of 10.10 ± 0.01 Ma and 11.18 ± 0.14 Ma for rhyolite and dacite, respectively. High ratios of Sr/Y (&gt; 20) and La/Yb (&gt; 20), high contents of Sr (≥ 400 ppm), low contents of MgO (≤ 6 wt%), Y ≤ 18 ppm (c. 16.5 ppm), Yb ≤ 1.9 ppm (c. 1.53 ppm) and weak negative Eu anomalies (Eu*/Eu c. 0.81) are compatible with a high-silica adakitic signature of the rocks. Regarding the location of the study area nearly 100 km from the Zagros suture zone, we argue that delamination of lithospheric mantle beneath the SaSZ has played a key role in the development of the adakitic rocks in a post-collision tectonic regime. The adakitic melts are suggested to have formed by partial melting of delaminated continental lithosphere and/or lower crustal amphibolite following the collision of the Arabian and Iranian plates.

Geochronology, geochemistry and petrogenesis of Miocene adakitic rocks in Milashan, Southern Tibet

中新世是青藏高原隆升、增厚的重要时期，并且在这一时期内拉萨地块广泛发育碰撞后岩浆岩。本文对南拉萨地块米拉山地区的钙碱性钾质火山岩进行了锆石U-Pb年代学、Lu-Hf同位素和全岩主量、微量元素的测定与系统研究。米拉山中新世火山岩为粗面英安岩、英安岩和流纹岩（SiO₂=59.89%~71.78%）。锆石U-Pb定年结果为16.1±0.2Ma~20.4±0.3Ma，表明其喷发时代为中新世。岩石具有较高的Al₂O₃含量（13.54%~16.31%），低MgO（0.46%~1.95%）、高Sr（388×10^-6~804×10^-6）、低Y（6.55×10^-6~11.20×10^-6）和Yb（0.70×10^-6~1.07×10^-6）的特征，具有较高的Sr/Y值（51~80）、低相容元素（Cr=4.26×10^-6~32.53×10^-6，Ni=4.16×10^-6~25.75×10^-6）和弱Eu负异常。岩石具有轻稀土元素和Rb、Th、U、K等元素富集、重稀土元素和高场强元素Nb、Ta、Ti亏损的特征。米拉山中新世火山岩显示出埃达克质岩石的地球化学特征，可能来自于的镁铁质加厚下地壳的部分熔融，推测下地壳源区是石榴石角闪岩。锆石ε_Hf（t）值为+2.2~+7.8，表明源区为新生地壳物质，有俯冲板片熔体加入。米拉山中新世火山岩的喷发时代与米拉山断裂活动时间一致，二者可能同为拉萨地块岩石圈拆沉的结果。;Miocene is an important period for the uplifting and thickening of the Tibetan Plateau, and meanwhile, post-collisional igneous rocks are widely developed. The zircon U-Pb dating and Lu-Hf isotopes, and whole rock major and trace element compositions were analyzed on the calc-alkaline potassic volcanic rocks in the Milashan area, Southern Lhasa terrane. Milashan Miocene volcanic rocks are trachydacite, dacite and rhyolite, with intermediate to acid composition and SiO₂ ranging from 59.89% to 71.87%. The zircon U-Pb dating yielded ages from 16.1±0.2Ma to 20.4±0.3Ma. All samples exhibit high Al₂O₃ (13.54%~16.31%), low MgO (0.46%~1.95%), high Sr(388×10^-6~804×10^-6), low Y (6.55×10^-6~11.20×10^-6) and Yb (0.70×10^-6~1.07×10^-6), high Sr/Y ratios, low compatible element concentration (Cr=4.26×10^-6~32.53×10^-6, Ni=4.16×10^-6~25.75×10^-6) and slight negative Eu anomaly. The samples are characterized by enrichment of light rare earth elements and large ion lithophile element (LILEs, Rb, Th, U, K), and depletion of heavy rare earth elements and high field strength element (HFSEs, Nb, Ta, Ti). These Miocene volcanic rock samples show adakitic affinity, and may have been formed by partial melting of mafic thickened lower crust. The source region is probably garnet amphibolite. Lu-Hf isotopic determination yielded positive zircon ε_Hf(t) values ranging from +2.2 to +7.8, indicating that the source was juvenile crust with the addition of subducted slab melt. Milashan Miocene volcanic rocks were formed at the same time as the action of Milashan fault, which probably suggests that both Milashan Miocene volcanic rocks and Milashan fault are caused by the delamination of Lhasa terrane lithosphere.