Calc-alkaline Volcanism Research Articles

Orogenesis without collision: Stabilizing the Terra Australis accretionary orogen, eastern Australia

The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis. At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting. The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis. Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.

Geochemical and Sr-Nd Isotopic Characteristics of Post-Collisional Calc-Alkaline Volcanics in the Eastern Pontides (NE Turkey)

Major, trace element, K-Ar age and Sr-Nd isotopic data are presented for the Eocene Torul volcanics in the eastern Pontide orogenic belt (NE Turkey). The studied rocks are composed of basaltic andesitic, andesitic, trachyandesitic, and minor trachydacitic lavas associated with their pyroclastics. These rocks contain plagioclase (An2-44), hornblende (Mg#= 0.78-0.98), clinopyroxene (Wo43-46 En41-43 Fs10-15), biotite, quartz, and minor sanidine phenocrysts. K-Ar ages on hornblendes ages range from 43.99 (±2.59) to 33.45 (±2.32) Ma, within the Middle to Late Eocene. The volcanic rocks show calc-alkaline affinities and have medium to high K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depletion of high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn= 4.0-9.8) show low to medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values vary between 0.70457 and 0.70511 and initial 143Nd/144Nd values between 0.51264 and 0.51278. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization with minor amounts of crustal contamination ± magma mixing. All evidence supports the conclusion that the parental magma(s) of the rocks probably derived from an enriched upper mantle, previously modified by subduction- induced metasomatism in a post-collisional geodynamic setting.

Paleoproterozoic, turbidite-hosted, gold deposits of the Ashanti gold belt (Ghana, West Africa): Comparative analysis of turbidite-hosted gold deposits and an updated genetic model

The Paleoproterozoic gold deposits of the Ashanti gold belt, Ghana, occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics (alternately block-faulted Tarkwaian sediments). They are suggested in this article to belong to that class of gold deposits recognized as ‘turbidite-hosted’. Hence, evidence from previous investigations of turbidite-hosted and turbidite-associated gold deposits occurring in circumstances roughly similar to the Ashanti deposits are used, where relevant, to produce an updated genetic model for the latter. Given the relatively significant amounts of graphite associated with the major Ashanti deposits, the role of non-carbonate carbon is assessed in the light of conclusions reached by work on other turbidite-hosted and turbidite-associated gold belts. Conclusions as to the origins of mineralizing fluids and temporary repositories of gold (auriferous interflow sediments) reached by work on these other belts are also analysed. A concept of the tectonic development of the Ashanti belt-Kumasi basin area, introduced in this article, is deemed to be critical to the origin of the Ashanti gold deposits. It encompasses the role of non-carbonate carbon, turbidites, and the primary sources and secondary repositories of Au and As in the genesis of the major Ashanti gold deposits. It examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments, the occurrences of felsic metasandstones in transition-zone sediments, and TTG granitic material and fragments of chemical sediments in turbidites: all of which are incompatible with one or the other of existing concepts of tectonic development in the area.

Evolution of Calc-Alkaline Volcanism and Associated Hydrothermal Gold Deposits at Yanacocha, Peru

Clusters of high-sulfidation epithermal deposits containing more than 50 Moz of gold are hosted by advanced argillic-altered Miocene volcanic rocks in the Yanacocha district, northern Peru (lat. 6°59'30 S, long. 78°30'45 W). We describe the nature of the volcanism and its relation to the gold ores on the basis of new district-scale geologic mapping, 69 40Ar/39Ar ages on igneous rocks and hydrothermal alunite, and petrologic and geochemical investigations. Volcanic rocks of the Calipuy Group are the oldest Cenozoic rocks at Yanacocha, and include the Huambo Cancha andesite, andesitic lahars of Tual and Chaupiloma (19.5–15.9 Ma), and the dacitic Cerro Fraile pyroclastics (15.5–15.1 Ma). The younger Yanacocha Volcanics (14.5–8.4 Ma) form a cogenetic series of lavas and pyroclastic rocks with a cumulative volume of ~88 km3 that represents eruption from a moderate-size volcanic center. Early pyroxene>hornblende-bearing lavas of the Atazaico Andesite (14.5–13.3 Ma) erupted from small stratovolcanoes progressively younger from southwest to northeast. Dacitic dikes followed that are spatially associated with gold deposits at Quilish and Cerro Negro (~7 Moz Au) and stage 1 alunite (13.6–12.6 Ma). The Colorado Pyroclastics erupted in the center of the district and include the hornblende- and biotite-bearing andesitic to dacitic Cori Coshpa (12.6 Ma) and Maqui Maqui (12.5–12.4 Ma) ignimbrites. The Colorado Pyroclastics are overlain by hornblende>pyroxene-bearing andesitic to dacitic lavas, flow-domes, and minor pyroclastic rocks of the Azufre Andesite (12.1–11.6 Ma). Widespread stage 2 alunite (11.5 Ma) and minor gold deposition (~0.5 Moz) closely follow. The San Jose Ignimbrite (11.5–11.2 Ma) overlies the Azufre Andesite and stage 2 alunite and includes three members of hornblende-pyroxene (biotite) dacite and andesite that erupted in the center of the district and flowed southward. Mineralogically similar domes were emplaced into the inferred vents. The Coriwachay Dacite (10.8–8.4 Ma) forms the youngest and most silica rich igneous rocks in the district, and includes intrusions and flow domes of dacite to rhyolite at Corimayo (10.8 Ma), Cerro Yanacocha (9.9 Ma), and Yanacocha Lake (8.4 Ma). Most of the gold (>47 Moz) was deposited at Yanacocha during intrusion of the late Coriwachay Dacite. These late dacites are volumetrically smallest (~2% of the total volume of erupted magma) and are temporally associated with stage 3 to 5 alunites. Stage 3 alunite (11.0–10.7 Ma) developed along a northeast trend for 9 km that includes the gold deposits of Corimayo, San Jose, Carachugo, and Maqui Maqui. The deeper Kupfertal Cu-Au porphyry has an age of 10.7 Ma on hydrothermal biotite and underlies zones of stage 4 and 5 quartz-alunite alternation that are 0.8 and 1.5 m.y. younger, respectively. Stage 4 alunite ranges in age from 10.2 to 10.3 Ma at the Tapado and Chaquicocha Sur gold deposits to Cerro Sugares east of the Maqui Maqui deposit. Stage 4 also includes a younger alunite age of 9.9 Ma from the San Jose gold deposit. Stage 5 alunite ranges from ~9.3 to ~8.2 Ma at Cerro Yanacocha, the largest gold deposit in the district. All these deposits contain massive and vuggy quartz, quartz-alunite, and quartz-pyrophyllite alteration associated with pyrite±enargite-tennantite-covellite. Magmatism in the Yanacocha district lasted for ~11 m.y. The Yanacocha Volcanics spanned the last ~6 m.y. of this period and were associated with long-lived magmatic-hydrothermal activity and episodic gold mineralization. The Yanacocha calc-alkaline suite was oxidized, water and sulfate rich, and evolved from early pyroxene>hornblende andesite to late titanite-bearing dacite and minor rhyolite. Several dacites contain populations of both high- and low-aluminum hornblendes that crystallized in the middle and upper crust, respectively. The variation of Mg, Ti, P, Sr, and Ba contents in these rocks is consistent with a complex magmatic origin via both cooling, fractional crystallization, periodic recharge of deeply derived hydrous basaltic or andesitic melts, and mixing with silicic melts derived by crustal melting. Low eruption rates, high phenocryst contents of the volcanic rocks, and widespread hydrothermal alteration are consistent with the hypothesis that most of the magmas at Yanacocha crystallized in shallow chambers as granitoids that passively degassed ore fluids. The compositional diversity of the volcanic rocks together with an extended magmatic-hydrothermal history reflect complex magmatic processes that were optimum for producing the world-class gold deposits at Yanacocha.

Hercynian post-collisional magmatism in the context of Paleozoic magmatic evolution of the Tien Shan orogenic belt

The Hercynian Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum–Tarim and the Kazakhstan paleo-continents. In order to constrain timing of Hercynian post-collisional magmatism, 27 intrusions were sampled for U–Pb zircon dating along a ca. 2000 km – long profile in Uzbekistan and Kyrgyzstan. The samples were dated utilizing sensitive high resolution ion microprobe (SHRIMP-II). The obtained ages, together with previously published age data, allowed the timing of Hercynian post-collisional magmatism to be constrained and interpreted in the context of the Paleozoic magmatic evolution of the region. Apart from Hercynian post-collisional magmatism, two older magmatic episodes have been recognized, and the following sequence of events has been established: (1) approximately 10 Ma after cessation of continuous Caledonian magmatism a number of Late Silurian–Early Devonian intrusions were emplaced in the Middle and Northern Tien Shan terranes between 420 and 390 Ma. The intrusions probably formed in an extensional back arc setting during coeval subduction under the margins of Caledonian Paleo-Kazakhstan continent; (2) the next relatively short Late Carboniferous episode of subduction under Paleo-Kazakhstan was registered in the Kurama range of the Middle Tien Shan. Calc-alkaline volcanics and granitoids with ages 315–300 Ma have distinct metallogenic affinities typical for subduction-related rocks and are not found anywhere outside the Middle Tien Shan terrane west of the Talas–Farghona fault; (3) the Early Permian Hercynian post-collisional magmatism culminated after the closure of the Paleo-Turkestan ocean and affected the whole region across terrane boundaries. The post-collisional intrusions formed within a relatively short time span between 295 and 280 Ma. The model for Hercynian post-collisional evolution suggests that after collision the Tien Shan was affected by trans-crustal strike-slip motions which provided suitable conduits for ascending asthenospheric material and heat influx in the crust. This produced both granitoid magmas and hydrothermal fluid flow. As a result post-collisional intrusions and orogenic Au deposits, known in the region, formed coevally and were tectonically controlled; (4) between 240 and 220 Ma a Triassic thermal event affected the region resulting in resetting and growth of new zircon grains which is detected on a regional scale. Probably the influx of heat into the crust during the Triassic was tectonically focused and varied significantly in different terranes. In the region under investigation the Triassic thermal event was not accompanied by any significant magmatic activity. Thus, after cessation of Hercynian post-collisional magmatism ca. 280 Ma ago there was a long magmatically quiet period in the Tien Shan.

P.3.c.008 Correlation between psychotic symptoms and sleep disturbances in schizophrenia

The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (∼60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.

Redefining the Waitemata Basin, New Zealand: A new tectonic, magmatic, and basin evolution model at a subduction terminus in the SW Pacific

The early Miocene Waitemata Basin has long been described as an interarc/intra‐arc basin, formed between twin chains of arc volcanoes in Northland, New Zealand. However, deep marine, polymict, volcaniclastic conglomerates within the basin reveal tectonic and magmatic signals that are not evident from neighboring volcanic edifices. The conglomerates were deposited by high‐density turbidity currents and debris flows and include single sediment cycle megaclasts of lava. These basaltic lavas have ocean island basalt (OIB)‐like geochemical affinities and are precisely dated at 20 Ma by 40Ar‐39Ar methods. Their age and the occurrence of subordinate clasts derived from an ophiolitic nappe to the north indicate the basin postdates the initiation of collision in the wider region. Contemporaneous calc‐alkaline volcanism did occur some 250 km NW of the basin. However, the conglomerates lack clasts of calc‐alkaline/arc affinities indicating an absence of arc‐like volcanism in the vicinity of the basin. Recent tectonic models for the SW Pacific region and mantle tomography highlight the importance of wholesale slab detachment in driving early Miocene calc‐alkaline volcanism and basin development. Although such models provide a slab window for the eruption of nonarc OIB‐like magmas, they would not explain their localized occurrence at the proposed leading edge of the tear (Waitemata Basin), rather than progressively along the entire length of the detachment (Northland), as seen in other detachment settings. In addition, OIB‐like volcanism predates the adjacent calc‐alkaline volcanism on the margin of the basin, a transition that is the opposite of that found at other slab detachment settings. The occurrence of OIB‐like volcanism is better explained by a lateral slab termination in the vicinity of the Waitemata Basin that allowed asthenospheric‐derived magmas to erupt. The basin is inferred to have developed in response to asthenospheric upwelling and associated lithospheric extension or deformation associated with a slab termination zone. This study highlights the importance and potential of combined geochemical, geochronological, and sedimentological studies of conglomerates in reconstructing the geodynamic setting of a basin.

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

AbstractNeogene to Quaternary volcanic/magmatic activity in the Carpathian-Pannonian Region (CPR) occurred between 21 and 0.1 Ma with a distinct migration in time from west to east. It shows a diverse compositional variation in response to a complex interplay of subduction with rollback, back-arc extension, collision, slab break-off, delamination, strike-slip tectonics and microplate rotations, as well as in response to further evolution of magmas in the crustal environment by processes of differentiation, crustal contamination, anatexis and magma mixing. Since most of the primary volcanic forms have been affected by erosion, especially in areas of post-volcanic uplift, based on the level of erosion we distinguish: (1) areas eroded to the basement level, where paleovolcanic reconstruction is not possible; (2) deeply eroded volcanic forms with secondary morphology and possible paleovolcanic reconstruction; (3) eroded volcanic forms with remnants of original morphology preserved; and (4) the least eroded volcanic forms with original morphology quite well preserved. The large variety of volcanic forms present in the area can be grouped in a) monogenetic volcanoes and b) polygenetic volcanoes and their subsurface/intrusive counterparts that belong to various rock series found in the CPR such as calc-alkaline magmatic rock-types (felsic, intermediate and mafic varieties) and alkalic types including K-alkalic, shoshonitic, ultrapotassic and Na-alkalic. The following volcanic/subvolcanic forms have been identified: (i) domes, shield volcanoes, effusive cones, pyroclastic cones, stratovolcanoes and calderas with associated intrusive bodies for intermediate and basic calclkaline volcanism; (ii) domes, calderas and ignimbrite/ash-flow fields for felsic calc-alkaline volcanism and (iii) dome flows, shield volcanoes, maars, tuffcone/tuff-rings, scoria-cones with or without related lava flow/field and their erosional or subsurface forms (necks/ plugs, dykes, shallow intrusions, diatreme, lava lake) for various types of K- and Na-alkalic and ultra-potassic magmatism. Finally, we provide a summary of the eruptive history and distribution of volcanic forms in the CPR using several sub-region schemes.

Overview of a Special Issue on “Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part I-III)”

This article provides an overview of papers collected in the special issue titled “Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part I-III)” for international readers. Research on the geologic evolution of the Japanese Islands started from the geotectonic subdivision with definitions of elements and boundaries in the late 19th century. Traditionally, some remarkable faults, such as Median Tectonic Line (MTL), Fossa magna, and Tanakura Tectonic Line (TTL), were regarded as the most important; however, these apparent features were in fact formed during the Miocene opening of the Japan Sea, whereas the major structures of Japan were made by the Pacific-type orogenies throughout the Phanerozoic after the initiation of subduction at 520 Ma (the Cambrian). Marine geophysicists have realized that the formation of accretionary complex (AC) along modern active subduction zones is rather exceptional; instead, tectonic erosion takes place dominantly. The estimated addition rate of juvenile arc crust (composed of tonalite–tronjemite–granodiorite suite; TTG) is negative at present for the entire globe, i.e., the total volume of the continental crust is currently decreasing. This indicates that tectonic erosion must also have occurred effectively in the past, and that its geological remnants need to be investigated carefully. A possible proxy geologic body for ancient tectonic erosion is a serpentinite melange, which often includes calc-alkaline volcanics and coeval high-P/T blueschists within serpentinite matrices as shown in the geology of Japan. The large-scale shortening of a fore-arc crust can occur solely due to tectonic erosion, which may involve more than 150 km-wide areas of missing rocks between volcanic arc and trench. The sporadic occurrence of tectonic blocks of older (520-150 Ma)TTG rocks in serpentinite melange indicates that four arc crusts, out of the five made by Pacific-type orogeny in Japan during the past 500 million years, have already been consumed. The Pacific-type orogeny was revisited after recent critical discoveries of; (1) orogenic core of high-P/T regional metamorphic belt as a thin (< 2 km) solid high-T intrusion, bounded on the top and bottom by a paired fault; (2) rapid increase of 200-300 km-wide TTG belt formed by slab-melting; (3) these culminated with an approaching mid-oceanic ridge, associated with large-scale tectonic erosion; and (4) formation of an AC after ridge subduction and doming up of a sandwiched set of high-P belt and accretionary belt below and above. The Pacific-type orogeny has been ignored since the “terrane plague”; however, Pacific-type orogeny dominated to increase the TTG crust leading to the births of continents and supercontinent by 1.8 Ga. The extensive tectonic erosion and arc subduction through time, suggesting 10 times more TTG materials, enriched with radiogenic elements, floated at the bottom of the mantle transition zone, which generated heat to warm up the mantle up to 200 K within 100 million years in the Archean, and to 100 K in the Phanerozoic. Subducted TTG and presence of second continents in the mid-mantle may explain more general aspects of mantle dynamics, and the concept may become a frontier of solid Earth science in the 21st century. From the viewpoint of the history of science, the previous geological studies in Japan since the Meiji Era are also summarized briefly, with special reference to the orogeny, tectonics, and geotectonic subdivision of the Japanese Islands.

Tectonic significance of changes in post-subduction Pliocene–Quaternary magmatism in the south east part of the Carpathian–Pannonian Region

The south-eastern part of the Carpathian–Pannonian region records the cessation of convergence between the European platform/Moesia and the Tisza–Dacia microplate. Plio-Quaternary magmatic activity in this area, in close proximity to the ‘Vrancea zone’, shows a shift from normal calc-alkaline to much more diverse compositions (adakite-like calc-alkaline, K-alkalic, mafic Na-alkalic and ultrapotassic), suggesting a significant change in geodynamic processes at approximately 3 Ma. We review the tectonic setting, timing, petrology and geochemistry of the post-collisional volcanism to constrain the role of orogenic building processes such as subduction or collision on melt production and migration. The calc-alkaline volcanism (5.3–3.9 Ma) marks the end of normal subduction-related magmatism along the post-collisional Călimani–Gurghiu–Harghita volcanic chain in front of the European convergent plate margin. At ca. 3 Ma in South Harghita magma compositions changed to adakite-like calc-alkaline and continued until recent times (< 0.03 Ma) interrupted at 1.6–1.2 Ma by generation of Na and K-alkalic magmas, signifying changes in the source and melting mechanism. We attribute the changes in magma composition in front of the Moesian platform to two main geodynamic events: (1) slab-pull and steepening with opening of a tear window (adakite-like calc-alkaline magmas) and (2) renewed contraction associated with deep mantle processes such as slab steepening during post-collisional times (Na and K-alkalic magmas). Contemporaneous post-collisional volcanism at the eastern edge of the Pannonian Basin at 2.6–1.3 Ma was dominated by Na-alkalic and ultrapotassic magmas, suggesting a close relationship with thermal asthenospheric doming and strain partitioning related to the Adriatic indentation. Similar timing, magma chamber processes and volume for K-alkalic (shoshonitic) magmas in the South Apuseni Mountains (1.6 Ma) and South Harghita area at a distance of ca. 200 km imply a regional connection with the inversion tectonics.

Plate Tectonics and the Boundary between Alps and Apennines

A new proposal attempting to solve the long-debated issue of the polarity of subduction in the Corsica-Northern Apennine system is presented. Models adopting an original W-dipping subduction and models preferring a flip in the polariy of subduction, from E-dipping to W-dipping, encounter major difficulties at a regional scale. It is considered here that the main inconsistencies faced by both models are due to the two-dimensional approach of reconstructions. The Late Cretaceous to Present-Day kinematics of the Central Mediterranean has been reconstructed using the magnetic anomalies in the Atlantic Ocean and assuming a solid connection between Africa and Adria. Oligocene to Present calcalkaline volcanism and backarc extension in the Balearic and Tyrrhenian basins requires the presence of a wide oceanic embayment to the west of the Adriatic Promontory. It follows that the continental collision that gave rise to the Alps s.s. could not continue SW-ward of Adria. The flip of subduction polarity that can be currently observed, going from the Alps, where Africa is overriding Europe, to the Apennines, where the opposite occurs, was likely on original feature since the beginning of convergence. Kinematic reconstructions allow the point along the plate boundary where the flip of polarity occurs to be tracked back in time. Following the N-ward motion of the colliding Adriatic Promontory, the point of polarity flip moved along the plate boundary from Late Cretaceous to Eocene. As a result, areas that previously experienced continental collision were subsequently affected by oceanic subduction. This sequence of events led to the collapse of the Alpine belt of Corsica and to the opening of the Balearic backarc basin above a retreating oceanic subduction. A similar kinematic evolution is currently ongoing in Taiwan. Finally, the Northern Tyrrhenian basin opened when delamination affected the Adriatic continental margin, following the consumption of oceanic lithosphere at the end of Corsica-Sardinia rotation.

Origin of the deep metamorphic complexes of the Early Proterozoic folded framing, the eastern part of the Aldan shield

It is shown that the base of the geologic section of the deep-seated Early Proterozoic Dzhugdzhur and Sunnagin complexes is made up of granulite-facies crystal schists and plagiogneisses, which are distinguished as the metabasite-enderbite association. As indicated by the major- and trace-element distribution, the predominant protolith of this rock association includes volcanics of the calc-alkaline and komatiite-tholeiitic series. The accepted model of the formation of the primary volcanics of the metabasite-enderbite association includes two stages. The first stage provides formation of melts of komatiite-tholeiitic series via decompressional melting of the material of an ascending mantle plume. The second stage involves the formation of calcalkaline volcanics owing to the partial melting of the initial metabasite crust by the heat of the ascending mantle plume. The igneous protoliths of the metabasite-enderbite association were formed in the Early Proterozoic.

The Choiyoi volcanic province at 34°S–36°S (San Rafael, Mendoza, Argentina): Implications for the Late Palaeozoic evolution of the southwestern margin of Gondwana

The Choiyoi rhyolitic province of Chile and Argentina (23°S–42°S) was emplaced at the SW margin of Gondwana during the Permian. The San Rafael Massif (Mendoza, Argentina, 34°–36°S), is a key area to analyse the relative timing of Choyoi magmatism and related deformation as it bears one of the most complete and well exposed succession. Stratigraphic, structural and magmatic studies indicate that major changes of geodynamic conditions occurred during the Permian since arc-related sequences syntectonic with transpression (lower Choiyoi) were followed by transitional to intraplate, postorogenic suites coeval with transtension (upper Choiyoi). During the Early Permian, a major event of N–NNW dextral transpressional motions deformed the Carboniferous foreland basin in the San Rafael Massif. This event is attributed to the first episode of the San Rafael orogeny and can be related to oblique subduction (Az. 30°) of the Palaeo-Pacific plate. Ca. 280 Ma the inception of voluminous calc-alkaline volcanism (lower Choiyoi) syntectonic with WNW sinistral transpression of the second episode of the San Rafael orogeny, is associated with an eastward migration of the magmatic arc at this latitude. To the southeast of San Rafael, magmatism and transpression continued to migrate inland suggesting that a progressively younger, WNW, sinistral, thick skinned deformation belt broadens into the foreland and can be traced from San Rafael to Sierra de la Ventana, linking the San Rafael orogeny with the Gondwanide orogeny of the Cape Fold Belt in South Africa. This distribution of magmatism and deformation is interpreted as being the consequence of a progressive shallowing of the Palaeo-Pacific plate starting to the north of San Rafael, and culminating with a flat-slab region south of 36°S. Ca. 265 Ma the onset of predominantly felsic volcanism (upper Choiyoi) in San Rafael occurred in a Post-San Rafael extensional setting. Kinematic indicators and strain fabric analyses of San Rafael orogeny transpression and Post-San Rafael extension show a tectonic reversion. The Post-San Rafael event could be the result of the extensional collapse of the San Rafael orogen, triggered by continental-scale clockwise rotations. These rotations would account for subduction ceasing earlier in the north (31°S–36°S) than in the south, thus explaining the coexistence, after ∼ 265 Ma, of extension in San Rafael with compression in the Sierra de la Ventana–Cape Fold Belt area.

A paleointensity study on middle Miocene to Pliocene volcanic rocks from south-eastern Spain

Abstract Thirteen middle-Miocene to Pliocene volcanic sites, belonging to four different types of volcanism (calc-alkaline volcanism, potassic calc-alkaline and shoshonitic volcanism, lamproitic volcanism and basaltic alkaline volcanism), have been sampled in south-eastern Spain for paleointensity determinations. Rock-magnetic studies yield low-Ti titanomagnetite as the main carrier of remanence, showing also the presence of titanohematites in several cases. Analysis of hysteresis parameters indicates a PSD domain structure. Paleointensity determinations were performed with the Coe (1967) method. Of the 90 paleointensity determinations carried out, 29 provide successful determinations which fulfil selection criteria; most of these were in samples of lamproitic volcanism. Only four lamproitic sites out of the 13 studied ones yield reliable results. All have a similar age of approximately 7 M.y. Virtual dipole moments (VDM) of three of these display values between approximately 4·1022 A m2 and 8·1022 A m2, thus providing new paleointensity data for a time interval (between 4 and 8 M.a.), with a lack of Thellier-type paleointensity determinations. The fourth site shows a much lower paleointensity of 1.6·1022 A m2, and may correspond to a polarity transition, in accordance with its paleo-directional results.

Geochemical Characteristics of Mafic and Intermediate Volcanic Rocks from the Hasandağ and Erciyes Volcanoes (Central Anatolia, Turkey)

Hasandağ and Erciyes stratovolcanoes, which are the two important stratovolcanoes in Central Anatolia, erupted volcanic products with both calc-alkaline and alkaline compositions, although the calc-alkaline activity is more widespread. There are three stages of geochemical evolution in the history of the Hasandağ stratovolcanic complex: (1) Keçikalesi tholeiitic volcanism, (2) Hasandağ calc-alkaline volcanism, and (3) Hasandağ alkaline volcanism. The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline volcanism, (2) Koçdağ calc-alkaline volcanism and (3) Erciyes calc-alkaline volcanism. The volcanic rocks from both suites show enrichments in LREE relative to HREE. The rocks as a whole show enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) in N-MORB normalized multi-element diagrams, although the thoeliitic and alkaline rocks have less pronounced effects of HFSE/LILE fractionation comparing to the calc-alkaline rocks. Theoretical fractionation models obtained using the whole-rock trace element data indicate two distinct fractionation trends for the Hasandağ volcanism: amphibole and plagioclase fractionation for the tholeiitic and calc-alkaline rocks and plagioclase, pyroxene and amphibole fractionation for the alkaline rocks. The alkaline and calc-alkaline rocks of Erciyes volcanism, on the other hand, indicate similar fractionation trends that can be explained by plagioclase and amphibole fractionation. AFC modelling indicate that the effect of crustal contamination on parental melt compositions is less pronounced in the Erciyes volcanic rocks compared to the Hasandağ volcanic rocks. Theoretical melting trends obtained using the non-modal batch melting model indicate degree of melting between 8-9% for the Keçikalesi tholeiitic rocks, 4-5% for the Hasandağ alkaline rocks and 3-8% for the Koçdağ alkaline rocks. The modeling also shows that the Central Anatolian volcanic rocks were originated by variable degree melting of a mantle source with garnet+spinel lherzolite composition, although the effect of residual garnet in the source is more pronounced for the Hasandağ alkaline rocks. Geochemical modeling indicates that Central Anatolian volcanic rocks are likely to have originated by partial melting of a metasomatised lithospheric mantle. Delamination of TBL (termal boundary layer) may be considered as a potential mechanism that may cause thermal perturbation melting of continental lithospheric mantle. The distribution of volcanic centres along the region-scale strike-slip fault systems may also indicate additional effects of strike-slip faulting on melt production and eruption.

Geochemistry of the Middle Miocene Collision-Related Yamadaği ( Eastern Anatolia) Calc-Alkaline Volcanics, Turkey

Major, trace element and K-Ar age determinations are reported for a suite from the Yamadağı volcanics in the Eastern Anatolia. The exposed rocks mainly consist of medium-potassium calc-alkaline basaltic andesites, andesites and dacites. Petrographical data exhibit disequilibrium mineral textures, such as resorption of the ferromagnesian phases, clinopyroxene-mantled orthopyroxene, and sieve-textured plagioclases. The Yamadağı volcanics have a calk-alkaline character, and trace element characteristics exhibit that the volcanics resemble subduction zone volcanics and/or volcanics assimilated by continental crust. K/Ar age determinations show that the Yamadağı volcanics were formed during the 12 ± 0.5 - 15 ± 0.5 Ma time interval. Geochemical characteristics of these volcanics can be attributed to complex petrogenetic processes, including magma mixing and crustal assimilation along with fractional crystallization.

Geodynamic evolution of the 2.25-2.0 Ga Palaeoproterozoic magmatic rocks in the Man-Leo Shield of the West African Craton. A model of subsidence of an oceanic plateau

Abstract The West African Craton is defined by the presence of Archaean and Palaeoproterozoic (Birimian) rocks. Since 1990, researchers have published several papers proposing various regional scale accretionary models for the evolution of the Birimian series and the Eburnean orogeny. The Birimian lithostratigraphic succession starts with a major sequence of tholeiitic pillow basalts with intercalations of sediments, overlain by detrital and carbonate sediments, associated with calc-alkaline volcanics and plutons. A global scheme for the geodynamical evolution can be reasonably constrained using the characteristics of early tholeiitic plutono-volcanic and granitoid rocks. Geochemical, geochronological and structural characteristics of Palaeoproterozoic magmatic rocks of the Man-Leo Shield distinguish three tholeiitic (PTH1, PTH2, PTH3) and two granitic (PAG, PBG) series and date three principal events that have characterized the 2250–2000 Ma period: (1) Event I (2250–2200 Ma) characterized by tholeiitic volcanism with a widespread first tholeiite generation (PTH1), interpreted as the eruption of a mantle plume in an oceanic basin floor; (2) Event II (2200–2150 Ma) marked by second (PTH2) and third (PTH3) tholeiite generations and a calc-alkaline magma setting; high granitization and amphibole-bearing (PAG) granitoid emplacement with greenstone belt deformation. This event has been favoured by a crustal subsidence in a broad synclinorium mega-structure followed by vertical tectonics; (3) Event III (2150–2000 Ma) characterized by main biotite±muscovite (without amphibole) bearing (PBG) granitoid emplacement in a context of transcurrent deformation. There are many similarities between West African Palaeoproterozoic rocks and Archaean formations and their crustal evolution also may be similar.

Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO2 ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.

Specifics of magmatism in marginal continental and marginal-sea pull-apart basins: Western periphery of the Pacific Ocean

The paper presents data on pull-apart (synchronous with strike-slip faulting) extensional structures formed in relation to Indo-Eurasian collision and including continental marginal rifts in East Asia and adjacent marginal sea basins. The evolution of Cenozoic pull-apart basins (developing synchronously with strike-slip faulting) in the western surroundings of the Pacific ocean corresponds to a basaltoid sequence in which the onset of rifting and the stage of maximum extension are marked by the first and last members of this sequence that have, respectively, calc-alkaline and tholeiitic depleted composition. The predominance of intermediate members with mixed isotopic-geochemical signatures testifies to the interaction of diverse magmatic melts. The opening of pull-apart basins (including those of marginal sea) was associated with magmatism whose sources were localized, judging from geochemical indicators, in the modified continental lithospheric mantle and depleted asthenosphere. The sources in the lithospheric mantle that was affected by long-lasting metasomatic recycling in the geological past dominated during the initial stages of continental extension and gave way to depleted asthenospheric sources. This model is consistent with the deep structure of the territories: extensional basins correspond to asthenospheric upwelling, with the ascent of asthenospheric diapirs positively correlated with the intensity of extension of the continental lithosphere and the degree of depletion of the accumulated basaltoids. The discovery of widespread calc-alkaline rocks (which are genetically related to the ancient metasomatized lithospheric mantle) in zones of continental rifting and marginal basins of the strike-slip fault nature significantly broadens the compositional range of volcanics typical of extensional geodynamic environments. At the same time, this testifies to the polygeodynamic nature of calc-alkaline volcanics, which can accumulate without any relations with coeval subduction zones.

The stratigraphic response to the Oligo-Miocene extension in the western Mediterranean from observations on the Sardinia graben system (Italy)

Abstract The Sardinian Cainozoic rifted basin is a useful model for studying the stratigraphic response to the Oligo-Miocene structural extension in the western Mediterranean because it allows precise observations on the relationship between sedimentation and normal faulting based on outcrops and seismic reflection data. The purpose of this paper, essentially of stratigraphic nature is to propose a chronology as precise as possible of the tectonic events and of the sedimentary formations. Indeed the tectono-sedimentary framework is complex, characterized by an extreme facies variability, from continental to marginal transitional and to marine environments (shallow-water, hemipelagic). Rifting, active calc-alkaline volcanism and sea-level changes caused rapid physiographical evolution, which controlled progressive marine ingression. New chronobiostratigraphical data presented in this paper allow correlating the sequences, defining their environment and depth of deposition and specifying precisely the timing of pre-, syn-, and post-rift stages in the Oligo-Miocene graben system. In southwestern Sardinia during the middle-late Eocene, after the Pyrenean phase, a continental graben (Cixerri), W-E oriented, preceded the Oligo-Miocene extension, which reactivated inherited Eocene and Palaeozoic faults. The calc-alkaline volcanic activity ranging from 32 to 13 Ma, provides a good estimate for the time span of the west-dipping Apenninic subduction responsible for the continental extension and the oceanic accretion in the western Mediterranean. In Sardinia the Oligo-Miocene extensional tectonics started in a continental environment, preceding the earliest calc-alkaline volcanic products (32 Ma). The marine ingression is dated to the late Chattian-Aquitanian interval and corresponds to a rapid deepening of the Oligo-Miocene graben system of tectonic origin. The end of the rifting i.e. the end of normal faulting activity is pre-middle Burdigalian in age. When Sardinia was in the post-rift stage, extension continued until late Burdigalian – Langhian in the Algero-Provençal basin with oceanic accretion and rotation of the Corsica-Sardinia block (CSB).