Miocene Volcanism Research Articles

Petrogenesis of Miocene volcanism in the Central Anatolia: Geochemical, isotopic and geochronological evidence

This paper presents the first 40Ar/39Ar age data, new bulk-rock major-trace element compositions and NdSr isotope geochemistry, combined with detailed geological mapping of Galatia Volcanic Province (GVP) in the northwest Central Anatolia (Turkey). Here, there are voluminous extrusive rocks in a wide compositional range: a basalt, andesite and trachyte suite, and their pyroclastic equivalents. 40Ar/39Ar dating of two whole-rock samples from the GVP yielded plateau ages of 21.76 ± 0.8 Ma and 20.97 ± 0.5 Ma, constrain the volcanic activity at ca. 21 Ma (Aquitanian). The samples show by sub-parallel light rare earth element (LREE)-enrichment and relatively flat heavy rare earth element (HREE) patterns and moderate fractionation [average (La/Yb)N = 13.5]. Their initial 87/86Sr values vary between 0.704619 and 0.704998, while initial 143/144Nd values lie between 0.512673 and 0.512755. Integration of the geochemical and geochronological with geological data we propose that the volcanic rocks in the northern area of GVP with calc-alkaline affinity were produced by a lithospheric mantle magma in an extensional setting.

Hydrothermal system operating mechanism by 3D gravimetric inversion and Vertical Electrical Soundings: Case of Hammam Righa hot springs, central Algeria

The Hammam Righa thermal site is one of the most important in the central part of the Tellian Atlas in Algeria, where the temperature of the thermal water reaches 68 °C. It is located between the Mitidja Basin to the northeast and the Upper Chelif Basin to the southwest, at about 100 km to the SW of Algiers.In order to characterize the Hammam Righa hydrothermal system and propose a geostructural model that regulates it, a multimethod-multiscale geophysical study was carried out and a three-dimensional gravity data inversion was completed using VOXI Earth Modeling™. 237 gravity data stations covering an area of about 36 km2 and 70 Vertical Electrical Soundings (Schlumberger, AB = 1000 m) covering an area of about 2.5 km2 were collected around the thermal discharge area.The different processing results of the Vertical Electrical Soundings and the gravimetric data allowed us to define the characteristics of the hydrothermal system (geothermal reservoir depth, heat source and faults system). The Hammam Righa thermal springs are structurally controlled by deep NE-SW direction faults that ramify towards the surface. The thermal waters flow into the geothermal reservoir, which is located in the Jurassic formation at a depth of approximately 1300–1500 m, and emerges near the surface from the Upper Cretaceous. The 3D gravity inversion model reveals a dense body with an average density of 2.78 g/cm3 at around 500 m depth, likely to be the source that ensures water heating process and associated with the region's Miocene volcanism.

Born of fire, borne by water – Review of paleo-environmental conditions, floristic assemblages and modes of preservation as evidence of distinct silicification pathways for silcrete floras in Australia

Fossil plants in sub-basaltic andfluvial silcretes differ in their geological setting, silica source, taphonomy, preservation and the plants and communities represented: these factors collectively indicate distinct silicification pathways. This review shows that the floras in sub-basaltic silcretes are mostly autochthonous, being derived from the local vegetation, whereas the floras in inland fluvial silcretes are transported andallochthonous. The floras in both are three-dimensionally preserved. The plantsin sub-basaltic silcretes areusually permineralised, preserving internal anatomy and cellular morphology, whereas the plants in inland fluvial silcretes occur as moulds/impressions that may preserve epidermal andsurface features. Both types of silcrete floras offer insights into past environments and ecological drivers. Terrestrial (ground-dwelling) ferns are frequently preservedin situin sub-basaltic silcretes whilst in the inland fluvial silcretes they are represented only by pinnule fragments or abscised pinnules. The extraordinary anatomical preservation of soft tissues in sub-basaltic silcretes, e.g., exocarps of fruits, bark on wood, actively growing vegetative shoots (fern croziers) and root nodules, is evidence of rapid burial by volcanogenic sediments and subsequent silicification by silica-richgroundwaters. The source of silica was hydrothermal fluids released at the time of eruption and/or the weathering of overlying volcanics, and the acidic (humic) conditions resulting from buried plant material provided ideal geochemical conditions for silica precipitation and plantpermineralisation. The sub-basaltic silcrete floras formed contemporaneously with Eocene–Miocene volcanism in eastern Australia, whereas the formation of inland pedogenic and fluvial silcretes was probably initiated during the Paleocene–Eocene Thermal Maximum (PETM). The fine surface preservation of plants in inland fluvial silcrete floras indicates rapid burial but no organic material is preserved, probably because oxidising groundwaters stripped out the organics before silicification occurred. The comparatively less silica-rich groundwaters associated withfluvial silcretes and the sandy substrates would have further contributed to the loss of plant material.

Petrogenesis of Josoji rhyolite and intrusive rocks of NE Shimane Peninsula, SW Japan: implication on Miocene volcanism related to back-arc rifting in San’in region

Petrogenesis of Josoji rhyolite and intrusive rocks of NE Shimane Peninsula, SW Japan: implication on Miocene volcanism related to back-arc rifting in San’in region

Miocene Volcanism in the Slovenský Raj Mountains: Magmatic, Space, and Time Relationships in the Western Carpathians

The Miocene volcanic-intrusive complex in the Slovenský Raj Mountains, middle Slovakia, comprises a swarm of subalkaline basalts and basaltic andesites with alkaline basalts, trachybasalts and basaltic trachyandesites. Basaltic to doleritic feeder dykes and sporadic hyaloclastite lavas are exposed in contact with the Triassic Bódvaszilas Formation of the Silica Nappe. The primary clinopyroxene, plagioclase, and Fe-Ti oxide assemblage also contains calcite spheroids inferred to represent carbonatitic melt. These spheroids are associated with subsolidus chlorite, actinolite, magnetite, titanite, calcite, and epidote. Micropoikilitic clinopyroxene, albite, and Ti-magnetite formed due to rapid quenching. There was an incorporation of host rock carbonate during the eruption. The erupted products are the result of magmatic differentiation of the parental basaltic tholeiitic magma with a redox of ∆QFM = +1 to +3, affected by varying degrees of 0%–50% fractionation and the assimilation of carbonate material in a shallow magmatic reservoir. REE geochemistry shows N-MORB-like type patterns with both LaN/YbN and LaN/SmN < 1 at near constant Eu/Eu* (~0.9). This is supported by εNd(t=13 Ma) values of +8.0 to +7.4 determined from the basaltic rocks. The REE values can be modeled by 1% fractional melting of garnet peridotite mixed with 7% melting of spinel peridotite of PM composition (1:9 proportions). SIMS and LA-ICP-MS U/Pb analysis of zircons yields a concordant age of 12.69 ± 0.24 Ma and a 13.3 ± 0.16 Ma intercept (Serravallian) age. The Middle Miocene volcanic activity was related to subduction-collision processes along the boundary of the Cenozoic ALCAPA (Alps–Carpathians–Pannonia) microplate and the southern margin of the European plate.

Intra-basin Early Miocene volcanism at the Northern Puna-Eastern Cordillera boundary, province of Jujuy, Argentina

The limit between the Puna and the Eastern Cordillera is an area of geological complexity where Cenozoic tectonic, volcanic and sedimentary processes overlap. In this contribution, we describe the Early Miocene volcanic and subvolcanic manifestations located on the border between Northern Puna and Eastern Cordillera, analyzing their relationship with the foreland basin sedimentary succession. We present a new age of 17.94 ± 0.17 Ma (40Ar/39Ar hornblende) for the volcanic rocks, located in the middle Member of the Moreta Formation, as well as new geochemical analysis, showing andesitic-trachydacitic compositions belonging to the high K-calcalkaline series. In addition, we have studied the subvolcanic bodies intruded in the Río Grande Formation, from we which propose an early post-depositional emplacement. In both cases, we present brief considerations about the chemical composition of both study sites, since in the case of Moreta Formation the rocks are high K-calcalkaline andesites while in the case of Río Grande Formation, they are somewhat more undersaturated in silica and enriched in K (shoshonitic series). The new data we presented evinces an event of intra-foreland basin volcanism, mafic to intermediate in composition, at 18 Ma in the Northern Puna-Eastern Cordillera transition.

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 (< 17.785) and higher values of Δ8/4Pb (61–75) for on-cratonic samples and the reverse relationship for off-cratonic lava (> 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.

The mafic volcanism of Meseta de Coli Toro, Somún Cura massif, Patagonia, Argentina

The Meseta de Coli Toro is a mafic volcanic plateau located in the Río Negro province, north of Patagonia, Argentina. It belongs to the Somún Cura Massif geological province and its origin remains uncertain.The previous information concerning the volcanic rocks of Meseta de Coli Toro is limited to field outcrop and petrographic descriptions and one bulk rock geochemical analysis. Based on that information, the lavas were included in the Basalto Meseta Coli Toro unit. This contribution presents new petrographic and geochemical information from three different areas of the plateau: northeastern, central, and western, contributing to the knowledge of the origin and evolution of this volcanic plateau. Most of the studied lava flows classify as alkali basalts with OIB signature. Regarding their petrological features and geochemical signature the lava flows were linked to the Oligocene-Middle Miocene volcanism of the Meseta de Somuncurá in northern Patagonia, specifically to the Vulcanitas Corona Chico unit. Based on a published isotope analysis (87Sr/86Sr = 0.70439 and 143Nd/144Nd = 0.51275) and the petrological modelling proposed by other authors for the Vulcanitas Corona Chico unit, the lava flows of Basalto Meseta Coli Toro unit derived from 1 to 8% of partial melting of a spinel-bearing pyroxenite source, with an isotopic composition equivalent to 96% of DM and 4% of EM1.

Origin and crustal contamination of magmas of the Chiapanecan Volcanic Arc in Southern Mexico: Constraints from new geochemical, isotopic, and geochronologic data

The processes related to the heterogeneous magma compositions of the Chiapanecan Volcanic Arc (CVA) are still poorly understood. Here we present new field descriptions combined with UPb zircon geochronology and whole-rock major, trace-element, and isotopic (Sr, Nd, Pb) data on poorly studied volcanic structures that surround the active El Chichón volcano in the northwestern part of this arc. These consist of the Santa Fe and La Mina intrusive bodies, volcaniclastic fans (La Catedral, and newly discovered Juárez and El Azufre) as well as Miocene volcanics. The mineralogy of the studied rocks consists of plagioclase + orthopyroxene + clinopyroxene + amphibole + biotite, ± FeTi oxides, ± olivine, ± quartz (this latter one only in the Santa Fe intrusive), and zircon as accessory phase. Young populations of zircons from El Azufre and Juárez lahar deposits were dated by UPb at ~2.1 Ma, being among the oldest yet-found magmatic products of the CVA. Strikingly, the Miocene volcanics have similar chemical characteristics as products of the modern arc. Based on geochemical data, we deduce that magma genesis in the CVA takes place in the mantle wedge, where it is constantly modified by aqueous fluids from the slab, causing relatively high Ba/La and Th/Yb ratios. The 87Sr/86Sr and 143Nd/144Nd ratios, zircon geochronology, and trace elements strongly suggest that crustal contamination was the dominant mechanism of differentiation of the magmas, supported also by Ce/Pb and Nb/U ratios. Furthermore, inherited zircons suggest the presence of Grenvillian and Permian (Chiapas Massif) rocks underneath the CVA. The contamination of the magmas by these basement rocks may be responsible for the K2O-enrichment in all CVA rocks.

New and revised paleomagnetic data from the southern central andes: Testing tectonic rotations

A paleomagnetic study performed in Plio-Pleistocene volcanic rocks of two localities in the Southern Central Andes is presented. The region corresponds to the transition zone between the Central and the Patagonian Andes. In the latter, deformation is almost exclusively controlled by the dextral-strike slip Liquiñe-Ofqui Fault Zone, that extends from ∼47 °S to ∼38 °S and accommodates the deformation generated by the oblique subduction of the Nazca Plate below South America. To the north of ∼36 °S, deformation is mainly contractional. Our study was carried out in the transition zone between both tectonic regimes. Sampling consisted of 31 paleomagnetic sites distributed in two localities: the Loncopué (∼38.0 °S, 70.7 °W) and Andacollo (∼37.2 °S, 70.8 °W) areas. We sampled Pliocene and Pleistocene volcanic rocks with ages younger than 5 Ma. After stepwise demagnetization and principal component analysis, site mean remanence directions were computed. Both localities show non-significant clockwise rotations of 1.7° ± 14.4° (Loncopué) and 8.1° ± 8.4° (Andacollo). Although the presently available database does not support significant tectonic rotation of the Andacollo lavas, preliminary results obtained from upper Miocene volcanics in a nearby area indicating ∼20° of clockwise rotation, encourage further studies to improve the resolution of the paleomagnetic data to determine if crustal block rotations have been, or still are, part of the Andean deformational processes. A revision of previously published paleomagnetic data in the northern Patagonian Andes suggests that, against original claims, no significant tectonic rotations associated to the Liquiñe-Ofqui Fault Zone, between ∼39 and 37.5 °S, can be unambiguously demonstrated.

Origin and geodynamic implications of basaltic rocks intercalated with Miocene turbidites around the İskenderun Basin (Eastern Mediterranean / Turkey)

We report a detailed investigation and the geodynamic implications of a basaltic lava flow that is intercalated with the early–middle Miocene turbiditic sedimentary rocks (Karataş Formation) that are exposed near the İskenderun Basin in southern Turkey. In order to reveal the age and origin of the basaltic rocks, we systematically sampled clastic samples from measured sedimentary sections, which include the basaltic lava flow, from around Ceyhan (east of Adana city). Forty different species belonging to 14 calcareous nannoplankton genera were identified in the samples, representing the Langhian–Tortonian (middle–late Miocene) time interval. Considering the stratigraphic distribution of the calcareous nannoplankton species in the section, the age of the basaltic lava flow is determined to be Serravallian (middle Miocene). The basaltic rocks experienced only minor magmatic evolution and have a restricted compositional range. Analysed in situ major element contents (from electron microprobe analysis, EMPA) for minerals (plagioclase, olivine and clinopyroxene) hosted by the late Miocene basaltic rocks show that they originated in an intraplate setting. Geochemical analysis indicates that olivine have Fo (forsterite) compositions between 57.81% and 83.83%. The EMPA contents (primarily based on pyroxenes) suggests that the late Miocene volcanism was related to the Pleistocene Delihalil–Turunçlu basaltic volcanism. The studied late Miocene volcanism could have originated in two possible tectono-magmatic environments: 1) one involving collisional tectonics (e.g., folding) combined with a change from regional compression to strike-slip; or 2) an intraplate setting. We propose that the tectonic setting of the late Miocene basaltic lava flow was similar to that of the extensive Pleistocene Delihalil volcanism in the same region. This would extend the onset of the regional basaltic volcanism back to the middle–late Miocene, rather than being restricted to the Pleistocene–Quaternary.

Middle-late Miocene rift-related magmatism in Guaymas, Sonora, Mexico at the eastern margin of the Gulf of California: Petrogenetic implications associated to the Pacific-north American plate boundary

Miocene volcanism in the Guaymas region at the eastern margin of the Gulf of California, in coastal Sonora, NW Mexico, was dominated by arc magmatism. However, the presence of rift-related magmatism is also exposed. Rocks associated with subduction of the Farallon plate beneath North America plate are of Oligocene-Miocene ages, while the magmatic rocks with ages between ca. 12.5 to ca. 6 Ma occur in a post-subduction tectonic setting. This period is sometimes referred to as the proto-Gulf phase. The proto-Gulf magmatic rocks are exposed as part of a volcanic igneous plumbing system, herein named Guaymas Group. The Guaymas Group has an age range between ∼12 and 10 Ma and is defined by two magmatic events: an intermediate compositional pulse (Unit 1) and a felsic pulse (Unit 2).The lithological units from the Guaymas Group have similar geochemical features, exhibiting LILE and LREE enrichment and Nb, Ta and Ti depletion. These rocks show a transitional to tholeiitic composition affinity that differs from the arc magmatism from coastal Sonora. The geochemical data suggest that assimilation and fractional crystallization (AFC) processes played an important role in the geochemical variations observed in volcanic and subvolcanic rocks. The geochemical characteristics of the Guaymas Group rocks also could indicate an origin by decompression melting of a mantle source metasomatized by subduction processes.Finally, considering the geological, geochronological and geochemical study, we suggest that the emplacement of the Guaymas Group rocks is contemporaneous with the oblique rifting environment associated with an incipient Pacific-North American plate boundary.

Geochronology and geochemistry of the Miocene volcanics from the Kütahya area: Constraints for post-collisional magmatism in western Anatolia, Turkey

The first geochronological and geochemical data are presented for the Kütahya area volcanics in western Turkey to study mantle sources and magmatic processes. Petrographically, the studied volcanics are composed primarily of olivine augite basalt and augite basalt, with minor basaltic andesites/trachyandesites, and show porphyric, hyalo-microlithic porphyric and rarely glomeroporphyric textures. The rock samples contain plagioclase as phenocrysts and microlites, olivine, augite and biotite as phenocrysts, sanidine, opaques, and volcanic glass. Whole-rock K–Ar dating of the studied volcanics yielded cooling ages between 20.1 ± 0.7 and 17 ± 0.5 Ma (Early Miocene). The volcanics indicate magma evolution from shoshonitic to medium-high-K character. High contents of LILEs and LREEs, and low amounts of Nb, Ta, Zr and Ti indicate that fractional crystallization and assimilation processes had a role in the evolution of these volcanics. The studied volcanics also have moderate to high 87Sr/86Sr (0.70719–0.70971), negative εNd values (−6.27 to −4.63), and high 206Pb/204Pb (18.93–19.05), 207Pb/204Pb (15.69–15.76) and 208Pb/204Pb (38.99–39.32) ratios. Integrated trace element geochemical and Sr-Nd-Pb-O isotopic data suggest that the Kütahya area volcanics evolved via AFC processes from parental magma(s) that originated from a subcontinental veined lithospheric mantle source metasomatized by subduction-related fluids and melts.

Paleomagnetic Study of the Late Miocene Volcanic Sequence in Sonora, Mexico: Clockwise Vertical Axis Rotation Related to the Oblique Rifting in the Pacific‐North American Plate Boundary

AbstractThe evolution of oblique divergence along the Pacific‐North American plate boundary during the late Miocene is well‐preserved along the eastern margin of the Gulf of California. The early phase of this deformation occurred between ∼12.5 and ∼6 Ma in Sonora, NW Mexico, related to oblique rifting of the Gulf of California. The magmatic sequence for this period is exposed in the Cerro El Vigía, Guaymas region. This sequence is characterized by intrusive rocks, lava flows, and pyroclastic flows ranging in age from 12 to 10 Ma. The Cerro El Vigia has a predominant semicircular geomorphology traditionally ascribed to a volcanic caldera structure, such as large ring faults. Rock magnetic analyses of these magmatic products identify the magnetic minerals and their reliability for tectonic interpretations. The mean paleomagnetic direction from 14 sites from the Cerro El Vigia reveals up to 90° clockwise rotation about an inclined axis (65–85° axis plunge) with respect to stable North America. When interpreted in the context of the semicircular structure, these observations suggest that the structure has a tectonic, not volcanic origin. Paleomagnetic results and tectonics during the late Miocene suggest that this crustal block rotation corresponds to the development of large tension gashes deformed within a dextral transtensional system associated with continental deformation of the proto‐Gulf of California rift. Based on the paleomagnetic data, reported ages, and magmatic stratigraphy, we conclude that the magmatism and associated deformation at Cerro El Vigia was synchronous with transtensional tectonics between ∼11 Ma to ∼8 Ma.

Reply to comment on “Preparing the ground for plateau growth: Late Neogene Central Anatolian uplift in the context of orogenic and geodynamic evolution since the Cretaceous”

• Our discussion of plateau margin uplift is unaffected by proposed Paleogene uplift. • Earlier plateau interior uplift supports a conclusion of multiple drivers of uplift. • Earlier lithospheric dripping possible if Miocene volcanism has another cause. • Mechanisms for plateau rise may be evaluated against the orogenic architecture.

Construction and Destruction of Bontău Composite Volcano in the Extensional Setting of Zărand Basin during Miocene (Apuseni Mts., Romania)

The Eastern part of the Miocene Zărand extensional basin witnessed the generation and evolution of the largest composite volcano in Apuseni Mts., named recently Bontău. The volcano is filling the basin at the junction between the South and North Apuseni Mountains. The Bontău Volcano is known to be active roughly between ~14–10. In spite of heavily forested and poorly exposed volcanic deposits, it was possible to identify its complex evolution. The volcano suggests an original oval-shaped edifice base currently showing a north-oriented horseshoe-shaped debris avalanche eroded crater. The early effusive volcanic activity was contemporaneous with the emplacement of individual and/or clustered volcanic lava Domes. Late-stage summit dome generation was followed by several volcanic collapses all around the volcanic edifice producing large volcanic debris avalanche deposits (DADs), accompanied by numerous debris flows all around the volcano periphery. Thick pumice pyroclastic flow deposits found below DADs at the periphery may suggest that the slope failures were proceeded by a Plinian eruption. The debris avalanche crater is the last event in the volcano evolution exposing several intrusive andesitic-dioritic bodies and associated hydrothermal and mineralization processes, most probably including the former central vent area of the volcano. The volcano proximal effusive and explosive deposits display a change in the composition of the erupting magma (increased SiO2 from 53.4% to 60.6%) that resulted in an increase of viscosity and the construction of the summit lava domes. Such domes are however only found as various size blocks in DADs. The volcanism connects with the two steps of geotectonic evolution of the Zărand Basin: The initial construction period during regional extension started ~16 Ma up to 12.3–12.1 when the Bontău volcano and surrounding domes were generated. The second period, younger than 12 Ma, corresponds to NW-SE compressional tectonics developed only in the Bontău volcano with summit dome generation and, finally, assists volcano destruction and DADs generation. Newly performed geochemical and Sr and Nd isotopic data studies attest to a calc-alkaline character and suggest an evolution via assimilation-fractional crystallization processes of a dominant MORB-like mantle source magma. Also, they confirm the amphibole (±pyroxene) andesites to be the most evolved lithology. The stepwise changes in fracture propagation in the Zărand extensional setting along with a change to more hydrated and fractionated magma favored in ~4 Myrs of the evolution of the Bontău volcano lead to multiple pulses of the longest-lived magma chamber in the whole Miocene volcanism of the Apuseni Mts.

Evaluation of Pb isotope systematics and metal sources of the Biga Pb–Zn Province (NW Turkey) and comparison with the Pb isotope systematics of the Rhodope Massif

This study presents the Pb isotope values of the galenas and the wall rocks related to the Pb–Zn mineralizations in the Biga Peninsula which include two different continental crusts; Sakarya Zone and Rhodope Massif. Pb–Zn mineralizations in the Sakarya fragment in Biga Peninsula have higher 207Pb/204Pb (15.677–15.710, avg: 15.688) and 208Pb/204Pb (38.895–39.000, avg: 38.935) ratios compared to the Rhodope fragment in Biga Peninsula ratios (15.650–15.709, avg: 15.668 for 207Pb/204Pb, 38.771–38.947, avg: 38.814 for 208Pb/204Pb), though the 206Pb/204Pb ratios of both crusts (Sakarya fragment in Biga Peninsula: 18.749–18.782, avg: 18.763; Rhodope fragment in Biga Peninsula: 18.750–18.839, avg: 18.802) are similar. Also, both crusts have radiogenic 207Pb/204Pb ratios indicating an orogenic source of lead. The Pb isotope ratios of the wall rocks related to the mineralizations in the Sakarya fragment were plotted in close, though different areas on uranogenic and thorogenic diagrams. Only the areas of Oligo-Miocene plutonics and Miocene volcanics, considered to be the main sources of mineralization, coincide with each other. The Pb isotope ratios of galena samples from the Sakarya fragment overlap the Permo-Triassic metamorphic rock (Karakaya Complex) area and seem to be related to the Cenozoic magmatism, both in proximity and trend, in uranogenic-thorogenic diagrams. The intense calc-alkaline magmatism occurrence that followed the closure of the Neo-Tethys Ocean appears to have played an essential role in forming the Pb–Zn mineralizations of the Sakarya fragment by leaching lead from the wall rocks. This study has utilized the Pb isotope ratios of the wall rocks related to mineralizations in the Rhodope fragment from a previous study (one sample for each reservoir). With this data, the magmatic effect on the Pb–Zn mineralizations in the Rhodope fragment is shown to be more dominant compared to those of the Sakarya fragment. In addition, through this study, it was determined that the Pb isotope ratios of two different continental crusts (Sakarya and Rhodope fragments) which came together with the Intra-Pontide suture zone followed two different trends.The Pb isotope compositions of Pb–Zn deposits in Rhodope Massif (both in the fragments of the Balkans and Biga Peninsula) were also compared in this study. According to the Pb isotope ratios, crustal thickness, and amount of the acidic rocks, the crustal materials are dominant in the western part of the massif. These findings also indicate that the western part of the massif is typical for Pb–Zn deposits whereas the eastern part of the massif is typical for the Cu–Au deposits.

Paleomagnetic Results from Western Anatolia: Evidence of Microblock Rotations after Emplacement of the Lower Miocene Yuntdağ Volcanic Rocks

The eastern Aegean region has undergone north dipping subduction in the Oligocene, continental collision and then Miocene-Pliocene extension, which is associated with widespread Miocene volcanism. The aim of this study is to assess the possibility of block rotations due to stress variations in the Dikili (İzmir) province, Western Anatolia, based on paleomagnetic data obtained from 35 independent sites in addition to results from 19 sites in earlier studies. The lower Miocene Yuntdağ volcanic rocks were emplaced in three different structural blocks, the Dikili, Zeytindağ and Bergama blocks. Clockwise rotation is found in the Dikili and Zeytindağ blocks that varies from R (± DR) = 12.5° (± 7.4°) in the west to R (± DR) = 35.6°± (13.2°) in the east, respectively. In contrast, a counterclockwise rotation of R (± DR) =-38.1° (± 6.4°) resulted in the Bergama block, in the north of the Dikili and Zeytindağ blocks. A scissor-like basin evolution is suggested during the opening of the Bakırçay graben which led to counterclockwise rotation of the Bergama block and clockwise rotation of the Dikili and Zeytindağ blocks after lower Miocene to present. The rotation pattern derived from results of this study demonstrates that localized small scale deformation due to basin evolution besides regional affects must be considered as part of the deformation matrix in this area.

Evidence of early Miocene synextensional volcanism and deposition in the northern Calico Mountains, central Mojave metamorphic core complex, southern California, USA

The spatiotemporal link between large-scale continental crustal extension and magmatic activity has been identified by numerous past studies, yet commonly the details of these associations remain unresolved. This study in the central Mojave metamorphic core complex (CMMCC) of southern California (USA) presents new geologic mapping, stratigraphic interpretations, and 40Ar/39Ar geochronology of the Jackhammer and Pickhandle Formations in the northern Calico Mountains to provide additional age constraints on the relative timing of early Miocene volcanism, deposition, and extension. The Jackham­mer Formation, the oldest Tertiary stratigraphic unit, is nonconformable with pre-Cenozoic nonmylonitic metasedimentary and plutonic basement rocks and consists primarily of alluvial deposits and primary to reworked silicic tuffs, interbedded locally with basement-derived avalanche megabreccia, lacustrine limestone, and mafic lava; in addition, the “Mammut ignimbrite”, an ~130-m-thick crystal-rich welded lapilli tuff, is exposed only in the eastern part of the study area and appears to transition laterally into thinner, nonwelded lapilli tuff ~6 km to the west. The Pickhandle Formation conformably overlies the Jackhammer Formation and consists of: (1) a lower assemblage composed of reddish monomictic debris-flow breccias with porphyritic rhyodacitic clasts and silicic block-and-ash-flow deposits of sim­ilar composition; and (2) an upper assemblage of polymictic (metaplutonic basement and rhyodacite) alluvial deposits, primary to reworked lapilli tuff, and local rhyodacitic lava and block-and-ash flows. Rhyodacitic lava domes were emplaced during the final stages of Pickhandle Formation deposition, primarily intruded along preexisting normal fault zones. Sedimentary and volcanic lithofacies suggest that the Jackhammer and Pickhandle Formations were deposited in a volcanic vent–proximal alluvial fan system that formed within a half-graben basin bounded on the east by the southwest-dipping “Amphitheatre fault”. Growth strata within the hanging-wall deposits, primarily southwest-directed paleocurrents, and interbedded alluvial debris-flow, basement-derived megabreccia, and lacustrine deposits adjacent to this fault suggest synextensional deposition in an intra-hanging-wall basin that developed during upper-plate extension in the CMMCC. New 40Ar/39Ar ages for six samples of silicic pyroclastic flows and a lava dome from the synextensional Pickhandle and Jackhammer Formations in the Calico Mountains have a mean age of 20.10 ± 0.06 Ma. This age is 3–4 m.y. younger than the maximum age of initial extension determined by previous studies in other areas of the central Mojave, suggesting that CMMCC extension was not a synchronous large-magnitude regional event. Rather, extension and contemporaneous volcanism was more localized and asynchronous across the region, occurring in many smaller extensional basins that eventually culminated in exposure of the CMMCC mylonitic footwall rocks.

U–Pb geochronology of detrital zircons from San Carlos Basin, Costa Rica: Evidence of Miocene volcanism and implications for the Precambrian and Paleozoic history of the Central American isthmus

U–Pb geochronology on detrital zircons from the Neogene sediments of San Carlos Basin in Costa Rica, yielded a wide range of ages, from Paleoproterozoic to Cenozoic. The most representative is the Miocene population, constrained to a maximum depositional age of 6.9 ± 0.2 Ma. It reflects the local volcanic activity of the evolving island arc and late Miocene to Pliocene basin sedimentation. Cenozoic ages vary from ca. 5.8–56.1 Ma and Mesozoic ages from ca. 68.9–243 Ma. Other zircon grains yielded a variety of older ages: Paleozoic (ca. 264.9–536.5 Ma) and Proterozoic (ca. 552–1910 Ma). Such ages previously undiscovered in Costa Rica bring new insights on provenance and indicate multistage zircon recycling from a variety of sources. The presence of pre-Mesozoic detrital zircons indicates the influx of recycled sediments from older units (continental and sedimentary rocks), possibly from exhumed Precambrian and Paleozoic continental terranes or blocks from the north of Central America (Chortís and Patuca) in Guatemala, Honduras, north of Nicaragua, and probably from southern México.