Collision‐related granite magma genesis,potential sources and tectono‐magmatic evolution: comparison between central,northwestern and western Anatolia (Turkey)

The central, northwestern and western Anatolian magmatic provinces are defined by a large number of late Mesozoic to late Cenozoic collision‐related granitoids. Calc‐alkaline, subalkaline and alkaline intrusive rocks in central Anatolia are mainly metaluminous, shoshonitic, I‐ to A‐types. They cover a petrological range from monzodiorite through quartz monzonite to granite/syenite, and are all enriched in LILE. Their geochemical characteristics are consistent with formation from a subduction‐modified mantle source. Calc‐alkaline plutonic rocks in northwestern Anatolia are mainly metaluminous, medium‐ to high‐K and I‐types. They are monzonite to granite, and all are enriched in LILE and depleted in HFSE, showing features of arc‐related intrusive rocks. Geochemical data reveal that these plutons were derived from partial melting of mafic lower crustal sources. Calc‐alkaline intrusive rocks in western Anatolia are metaluminous, high‐K and I‐types. They have a compositional range from granodiorite to granite, and are enriched in LILE and depleted in HFSE. Geochemical characteristics of these intrusive rocks indicate that they could have originated by the partial melting of mafic lower crustal source rocks.     

A Review of the Nature of Magmatism in Central Anatolia during the Mesozoic Post-Collisional Period

Magmatism in central Anatolia is characterized by petrographically and chemically distinct granitic and syenitic rocks. The granitic magmatism comprises C-type (crustal-derived) and H-type (hybrid) monzogranites and monzonites. Garnet-bearing C-type leucogranites represent the oldest magmatic phase, but younger hornblende ± biotite ± K-feldspar H-type plutons dominate the geology of the Central Anatolian Crystalline Complex (CACC). These typically include mafic microgranular enclaves. The granitic magmatism predates syenitic intrusions, among which quartz-bearing syenites were emplaced prior to feldspathoid-bearing ones.

The nature of magmatism in central Anatolia varies through time from peraluminous to metaluminous to alkaline. These different magma types reflect distinct stages of postcollisional magmatism, in which interaction between crust and mantle varied considerably. The C-type granites of the early stages of postcollisional magmatism were likely derived by partial melting of the lower continental crust induced by mafic magma underplating as a result of lithospheric delamination. The H-type granites and syenites of the mature and advanced stages of postcollisional magmatism indicate a significant contribution from mande-derived magma within a continuous or episodic extensional tectonic regime.     

新特提斯洋晚白垩世演化特点:来自泽当共国日二长花岗岩年代学、地球化学及Sr-Nd同位素证据

为加深对西藏泽当地区新特提斯洋演化的认识,对西藏泽当蛇绿混杂岩带内的共国日二长花岗岩进行了岩石学、岩石地球化学、同位素及年代学等研究,研究显示:共国日二长花岗岩岩体规模小、岩性稳定,LA-ICP-MS锆石U-Pb年龄为(90.40±0.68)Ma,属晚白垩世;岩石地球化学具高硅、富铝、富钾、低钛和准铝质钙碱性花岗岩特征;轻稀土富集、重稀土亏损,具明显的负Eu异常,微量元素表现为相对富集Rb、K、Ba、Th、U等大离子亲石元素,显著亏损Nb、P、Ti等高场强元素;(⁸⁷Sr/⁸⁶Sr)_i=0.705 708~0.706 284,(¹⁴³Nd/¹⁴⁴Nd)_i=0.512 689~0.512 716,ε_Nd(t)=2.00~2.51。以上特征表明,位于泽当蛇绿混杂岩带内的共国日二长花岗岩属于I型花岗岩,由正常岛弧岩浆形成,应为晚白垩世新特提斯洋向北俯冲形成的岛弧环境下俯冲带上部地壳部分熔融的产物,其不属于泽当蛇绿岩的组成部分,表明在90 Ma前泽当洋内弧已经消失。     

Origin and significance of the Permian high-K calc-alkaline magmatism in the central-eastern Southern Alps, Italy

The Atesina Volcanic District, the Monte Luco volcanics, and the Cima d'Asta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280–260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No.=66 to 70; Ni=25 to 83 ppm; Cr=248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb–Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7057 to 0.7114; ε_Nd values from −2.7 to −7.4; ∂¹⁸O values between 7.6 and 9.5‰) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite–cordierite and orthopyroxene–cordierite bearing peraluminous granites in the Cima d'Asta and Bressanone-Chiusa intrusive complexes can be interpreted as purely crustal melts (initial ⁸⁷Sr/⁸⁶Sr=0.7143–0.7167; initial ε_Nd values between −7.9 and −9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of arc-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts.     

Geochemistry of post-collisional mafic lavas from the North Anatolian Fault zone, Northwestern Turkey

Extensive magmatic activity developed at the northwestern part of the Anatolian block and produced basaltic lavas that are situated along and between the two segments of the North Anatolian Fault zone. This region is a composite tectonic unit formed by collision of continental fragments after consumption of Neotethyan ocean floor during the late Cretaceous. Northwestern Anatolian basalts and evolved lavas exhibit both tholeiitic and calc-alkaline characteristics. Mafic lavas are moderately enriched in LILE (except depleted part of Yuvacık and İznik samples) and depleted in HFSE (but not Zr, Hf) relative to primitive mantle values, suggesting derivation from a MORB-like mantle source that is unexpected in this subduction environment. Sr and Nd isotopes are close to the mantle array and vary beyond analytical error (⁸⁷Sr/⁸⁶Sr 0.70404–0.70546, ¹⁴³Nd/¹⁴⁴Nd 0.51270–0.51289). These geochemical features may result from two possible processes: (1) melting of a MORB-like mantle source that was modified by subduction-released fluids and melts or (2) modification of mafic liquids derived from a dominantly MORB-like source by crustal or lithospheric mantle material. Geochemical characteristics of the lavas (e.g., Ba/Rb, Rb/Sr, Ba/Zr, ⁸⁷Sr/⁸⁶Sr, Sr/P) vary systematically along the fault zone from east to west, consistent with a decrease in the degree of melting from east to west or a change in the nature of the source composition itself. Thus, the difference in incompatible elements and Sr–Nd isotopic ratios seems to result from small-scale mantle heterogeneity in a post-collisional tectonic environment.     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

Modeling of subduction components in the Genesis of the Meso-Cenozoic igneous rocks from the South Shetland Arc, Antarctica

Isotope data and trace elements concentrations are presented for volcanic and plutonic rocks from the Livingston, Greenwich, Robert, King George and Ardley islands (South Shetland arc, Antarctica). These islands were formed during subduction of the Phoenix Plate under the Antarctica Plate from Cretaceous to Tertiary. Isotopically (⁸⁷Sr/⁸⁶Sr)_o ratios vary from 0.7033 to 0.7046 and (¹⁴³Nd/¹⁴⁴Nd)_o ratios from 0.5127 to 0.5129. εNd values vary from +2.71 to +7.30 that indicate asthenospheric mantle source for the analysed samples. ²⁰⁸Pb/²⁰⁴Pb ratios vary from 38.12 to 38.70, ²⁰⁷Pb/²⁰⁴Pb ratios are between 15.49 and 15.68, and ²⁰⁶Pb/²⁰⁴Pb from 18.28 to 18.81. The South Shetland rocks are thought to be derived from a depleted MORB mantle source (DMM) modified by mixtures of two enriched mantle components such as slab-derived melts and/or fluids and small fractions of oceanic sediment (EM I and EM II). The isotopic compositions of the subduction component can be explained by mixing between at least 4 wt.% of sediment and 96 wt.% of melts and/or fluids derived from altered MORB.     

Extrusive Members of Postcollisional A-Type Magmatism in Central Anatolia: Karahidir Volcanics,Idis Dagi-Avanos Area,Turkey

The Idis Dagi Igneous Complex is one of a number of late-stage plutonic bodies within the Central Anatolian Crystalline Complex. It intrudes Paleozoic-Mesozoic metamorphic basement rocks and Late Cretaceous granitoids. The complex comprises mainly quartz syenites and alkali-feldspar quartz syenites known as the Idis Dagi Syenitoids.

Trachydacitic, trachyandesitic, and dacitic rocks (Karahidir Volcanics) have been recently found as dikes cutting the Idis Dagi Syenitoids or as huge blocks within an olistostromal unit of latest Cretaceous(?)-Early Paleocene age that immediately overlies the Idis Dagi Igneous Complex. Petrographic and geochemical data indicate that the Idis Dagi Syenitoids and the Karahidir Volcanics display similar humped patterns on chondrite- and MORB-normalized spider diagrams, with peaks at Rb, Th, and Ce, and also negative Nb anomalies. These features are similar to patterns considered typical of postcollisional, A-type igneous rocks. A postcollisional setting is also suggested by the distribution of data on tectonic discrimination diagrams involving Nb, Y, and Rb. As a whole, the geochemical data suggest that the Idis Dagi Syenitoids and Karahidir Volcanics are cogenetic. Thus, the Karahidir Volcanics represent the shallow intrusive and volcanic equivalents of the deeper-level Idis Dagi Syenitoids.

The syenitoid rocks are considered the final phase of the magmatism in the Central Anatolian Crystalline Complex and are classified as post-orogenic alkaline rocks. Their chemical features suggest a largely mantle-derived magma contribution, together with a noticeable crustal component in their genesis. They were formed during postcollisional uplift that followed crustal thickening related to the southward emplacement of ophiolitic nappes during closure of the Izmir-Ankara-Erzincan Oceanic strand of Neotethys. The Karahidir Volcanics were exposed during regional extension that resulted in the formation of latest Cretaceous(?)-Early Paleocene extensional basins, and were emplaced as huge blocks into the continental clastic rocks by gravity sliding. This extension phase is characteristic of the development of all the other latest Cretaceous-Early Tertiary intracratonic basins in central Anatolia.     

Post-collision neogene volcanism of the Eastern Rif (Morocco): magmatic evolution through time

Neogene volcanism in the Eastern Rif (Morocco) comprises a series of calc-alkaline, potassic calc-alkaline, shoshonitic and alkaline volcanic rocks. According to new stratigraphical, along with new and previous chronological and geochemical data, the orogenic volcanism was successively (1) calc-alkaline (basaltic andesites and andesites: 13.1 to 12.5 Ma, rhyolites: 9.8 Ma), (2) K-calc-alkaline (basaltic andesitic to rhyolitic lavas and granodiorites: 9.0 to 6.6 Ma), and (3) shoshonitic (absarokites, shoshonites, latites, trachytes: 7.0 to 5.4 Ma). The later Pliocene volcanism was basaltic and alkaline (5.6 to 1.5 Ma). The calc-alkaline and K-calc-alkaline series exhibit lower K₂O (0.7–5.3 wt.%), Nb (8–19 ppm) contents and higher ⁸⁷Sr/⁸⁶Sr (0.70773–0.71016) than the shoshonitic series (K₂O: 2.4–7.2 wt.%, Nb: 21–38 ppm, ⁸⁷Sr/⁸⁶Sr: 0.70404–0.70778). Pliocene alkaline basalts have a sodic tendency (Na₂O/K₂O: 1.7–3.5), high Nb content (up to 52 ppm), and low ⁸⁷Sr/⁸⁶Sr ratio (0.70360–0.70413). The variations through time of K₂O, Nb and Sr isotopic ratio reflect different mantle sources: (i) calc-alkaline, potassic calc-alkaline and shoshonitic series are derived from a mantle source modified by older subduction, (ii) alkaline basalts are derived mainly from an enriched mantle source. Through time, incompatible elements such as Nb increased while ⁸⁷Sr/⁸⁶Sr decreased, suggesting a decreasing influence of metasomatized mantle (inherited subduction). Such evolution is related to the post-collision regimes operating in this area, and could be linked to the succession of extensional, compressional and strike-slip fault tectonics.     

Rifting-related volcanism in an oceanic post-collisional setting: the Tabar–Lihir–Tanga–Feni (TLTF) island chain, Papua New Guinea

The Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain occurs in a zone of lithospheric extension superimposed on a post-collisonal tectonic setting along the Pacific and Indo-Australian plates northeast of Papua New Guinea. We present geochemical and Sr, Nd, and Pb isotope data for volcanic rocks from these islands and three recently discovered seamounts located at Lihir island. Major element data document an alkalic affinity of the sample suite and trachybasalts as the predominant rock type. Negative Nb-anomalies in extended trace element patterns, enrichment of the light rare earth elements, and Ce/Pb ratios of about 4 are typical of the values in calc alkaline island arc volcanics and support an origin from subduction-modified mantle. ⁸⁷Sr/⁸⁶Sr ratios of 0.7037 to 0.7044 and _Nd values of +5.6 to +6.8 indicate that the upper mantle evolved with a time-integrated depletion in LREE, however, not as severe as that recorded in basalts from the East Pacific Rise. Variable ⁸⁷Sr/⁸⁶Sr ratios at less variable ¹⁴³Nd/¹⁴⁴Nd ratios suggest that ⁸⁷Sr/⁸⁶Sr ratios of the melts were modified by secondary processes, such as assimilation of seawater Sr from crustal rocks. The Pb isotope ratios are uniform, moderately radiogenic (²⁰⁶Pb/²⁰⁴Pb ca. 18.7 to 18.8), and similar to those reported for the active Mariana arc. Elevated ²⁰⁷Pb/²⁰⁴Pb ratios relative to Pacific MORB suggest melting of small amounts of subducted sediments (ca. 1–2 wt.%). An important control of subducted sediment on the chemistry of the melts can also be inferred from the ratios of highly incompatible trace elements (e.g., Th, U, Pb, La, and Nb). Additional mantle enrichment by subduction derived fluids is reflected in high values of highly incompatible trace element ratios between fluid mobile (e.g., Ba) and fluid immobile elements (e.g., Th, Nb). The results of this study document that the chemical composition of igneous rocks from post-collisional tectonic settings are strongly influenced by previous plate tectonics. This conclusion implies that the information conveyed by tectonic discrimination diagrams for these rocks must be interpreted with care.     

大兴安岭北段新林地区晚石炭世花岗岩的岩石成因及地质意义

大兴安岭北段新林地区晚古生代花岗岩主要出露在大乌苏和富西里附近,岩性主要为二长花岗岩,另有少量花岗闪长岩。对其中二长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,大乌苏和富西里岩体侵位年龄分别为（303.7±2.2）和（300.5±0.5）Ma,均为晚石炭世岩浆活动的产物。花岗岩具有富硅（w（SiO₂）为66.77%~75.85%）、富碱（w（Na₂O+K₂O）为7.41%~8.69%）、高铝（w（Al₂O₃）为12.90%~16.22%）,低MgO、CaO、TiO₂的特点,属于钙碱性系列;铝饱和指数（A/CNK）为1.06~1.44,为过铝质岩石;镜下未见原生白云母、堇青石、石榴石等富铝矿物,不同于富铝的S型花岗岩;而w（P₂O₅）与w（SiO₂）负相关,呈I型花岗岩特征;富集LREE和Ba、Rb、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,与后造山I型花岗岩特征相似,应形成于拉张的构造环境。花岗岩的⁸⁷Sr/⁸⁶Sr为0.712 938、¹⁴³Nd/¹⁴⁴Nd为0.512 386,（⁸⁷Sr/⁸⁶Sr）_i值为0.704 4,ε_Nd（t）值为-1.09,T_DM2=1 172 Ma,源区物质主要为中-新元古代从亏损地幔增生的地壳物质。结合区域研究成果,大兴安岭新林地区晚石炭世岩浆侵位活动与额尔古纳-兴安地块和松嫩地块碰撞拼合后岩石圈伸展环境有关。     

Petrology and Geochemistry of the Volcanic Rocks Dredged from the Geophysicist Seamount in the Kurile Basin: Evidence for the Existence of Thinned Continental Crust

Dredged samples from the Geophysicist seamount volcano in the northeastern part of the Kurile Basin include volcanic and volcanoclastic rocks ranging from basalt to andesite. The rocks have geochemical features typical of high-K island-arc calc-alkaline volcanism. They are enriched in LILE and depleted in Zr, Ti, Nb, Ta and Y. The chondrite-normalized REE patterns are characterized by enrichment of LREE similar to those of island-arc lava from the submarine volcanoes of rear-arc zone of the Kurile Island Arc. The volcanic rocks have a wide range of ⁸⁷Sr/⁸⁶Sr ratios (0.70287-0.70652), varying ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios. Their trace-element compositions and Sr-Nd-Pb isotope signatures may be explained by a small addition of crustal continental component to mantle-derived magmas that suggest the existence of thinned continental basement under the eastern part of the Kurile Basin.     

Magmagenesis in a subduction-related post-collisional volcanic arc segment: the Ukrainian Carpathians

Calc-alkaline magmatism in the south-west Ukraine occurred between 13.8 and 9.1 Ma and formed an integral part of the Neogene subduction-related post-collisional Carpathian volcanic arc. Eruptions occurred contemporaneously in two parallel arcs (here termed Outer Arc and Inner Arc) in the Ukrainian part of the Carpathians. Outer Arc rocks, mainly andesites, are characterized by LILE enrichment (e.g. K and Pb), Nb depletion, low compatible trace element abundances, high ⁸⁷Sr/⁸⁶Sr, high δ¹⁸O and low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios (0.7085–0.7095, 7.01–8.53, 0.51230–0.51245, respectively). Inner Arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas. They also show low compatible element abundances but have lower ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and higher ¹⁴³Nd/¹⁴⁴Nd ratios (0.7060–0.7085, 6.15–6.64, 0.5125–0.5126, respectively) than Outer Arc rocks. Both high-Nb and low-Nb lithologies are present in the Inner Arc. Based on the LILE enrichment (especially Pb), a higher fluid flux is suggested for the Outer Arc magmas compared with those of the Inner Arc.

Combined trace element and Sr–Nd–O isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex. Compositional differences between the Inner and Outer Arcs were produced by introduction of variable proportions of slab-derived sediments and fluids into a heterogeneous mantle wedge, and by different extents of upper crustal contamination. Degrees of magmatic fractionation also differed between the two arcs. The most primitive magmas belong to the Inner Arc. Isotopic modelling shows that they can be produced by adding 3–8% subducted terrigenous flysch sediments to the local mantle wedge source. Up to 5% upper crustal contamination has been modelled for fractionated products of the Inner Arc. The geochemical features of Outer Arc rocks suggest that they were generated from mantle wedge melts similar to the Inner Arc primitive magmas, but were strongly affected by both source enrichment and upper crustal contamination. Assimilation of 10–20% bulk upper crust is required in the AFC modelling, assuming an Inner Arc parental magma. We suggest that magmagenesis is closely related to the complex geotectonic evolution of the Carpathian area. Several tectonic and kinematic factors are significant: (1) hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes; (2) thermal disturbance caused by ascent of hot asthenospheric mantle during the back-arc opening of the Pannonian Basin; (3) clockwise translational movements of the Intracarpathian terranes, which facilitated eruption of the magmas.     

Origin and geodynamic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (⁸⁷Sr/⁸⁶Sr_i=0.7047–0.7064, ¹⁴³Nd/¹⁴⁴Nd_i=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La_N up to 400). The isotopic ratios approach crustal values (⁸⁷Sr/⁸⁶Sr_i=0.7059–0.7115 and ¹⁴³Nd/¹⁴⁴Nd_i=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).     

Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes

Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (0.5×N-MORB), especially Nb (0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. ¹⁴³Nd/¹⁴⁴Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas ⁸⁷Sr/⁸⁶Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (⁸⁷Sr/⁸⁶Sr)₀ up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies.

The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.     

Neoproterozoic alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton: petrography,geochemistry, and geochronology

The formation and evolution conditions for alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton are shown by the example of alkaline and subalkaline intrusive bodies of the Yenisei Ridge. Here we present petrographic, mineralogical, geochemical, and geochronological data for the rocks of the Srednetatarka and Yagodka plutons located within the Tatarka–Ishimba suture zone. Ferroan and metaluminous varieties enriched with rare elements (Nb, Ta, Zr, Hf, and REE) are making up most of the studied rocks. They formed at the stages of fractional crystallization of alkaline magma in a setting of active continental margin in the west of the Siberian craton in the Late Neoproterozoic (710–690 Ma). As differentiates of mantle magmas, these rocks associate with Nb-enriched rocks—A-type leucogranites and carbonatites. Sm/Nd and Rb/Sr isotopic data imply a predominance of the mantle component in the magmatic sources of the mafic and intermediate rocks as well as contamination processes of various volumes of continental crustal material by this magma.     

The Dodecanese Province, SE Aegean: A model for tectonic control on potassic magmatism

In the south-eastern Aegean several composite Upper Miocene volcanoes have erupted a variety of extrusive and intrusive rocks of mainly intermediate composition with potassic affinities. This study discusses the tectonic setting of this distinct igneous province (Dodecanese Province, DP) and presents mineralogical, geochemical and isotopic (Sr, Nd) characteristics of mafic rocks from two of its centers (Bodrum, Turkey and Samos, Greece). The mafics fall in two groups: ultrapotassics in Bodrum and shoshonitic rocks in Bodrum and Samos, with their geochemical signature varying from typical arc-like (Bodrum) to weakly orogenic (Bodrum, Samos).

The Bodrum ultrapotassic rocks are unusual and important in that while they display the petrological and geochemical characteristics of primary mantle-derived magmas they are also extraordinary LIL element-enriched. Their initial Sr and Nd isotopic compositions (⁸⁷Sr⁸⁶Sr =0. 7071; ¹⁴³Nd/¹⁴⁴Nd = 0.512465) lie at one extreme of the Bodrum-Samos range (⁸⁷Sr⁸⁶Sr = 0.7052−0.7071; ¹⁴³Nd/¹⁴⁴Nd = 0/51246−0.51264) and are evidence for the existence of an “enriched mantle” component.

Geochemical characteristics, including Nd- and Sr-isotope data, are used to discuss source component mixing arrays defined by a wide range of circum-Mediterranean igneous provinces including the DP suites. At least three endmembers are required: (1) enriched mantle, (2) depleted mantle and (3) continental crust. The enriched mantle is most probably part of the sub-continental lithosphere which may be regionally distributed throughout the Mediterranean. Enrichment by emplacement of small fractions of melts of the depleted mantle can yield such a source if the enrichment is ancient (≈1.25 Ga). Crustal involvement may be the product of the extensive role of AFC processes operating both close to the Moho and in higher level magma chambers.

The location of the DP in the transitional margin of the Aegean zone of extension may partly explain the survival to upper crustal levels of emplacement, of unmixed, ultrapotassic melts of the enriched heterogeneities in the lithospheer. Changes in Ti/Zr ratio implicate the buffering role of a titanate in the lithosphere. Loss of orogenic geochemical signature and depletion in potassium content in recent volcanics in Western Anatolia imply an increased role of depleted mantle.     

Spatial, temporal and geochemical characteristics of Silurian collision-zone magmatism, Newfoundland Appalachians: An example of a rapidly evolving magmatic system related to slab break-off

Silurian plutonic suites in the Newfoundland Appalachians include abundant gabbro, monzogabbro and granite to granodiorite and lesser quartz diorite and tonalite. Most are medium- to high-K, but included are some low-K and shoshonitic mafic compositions. Felsic rocks are of both alkaline (A-type or within-plate granite (WPG)) and calc-alkaline volcanic arc granite (VAG) affinity. Mafic rocks include both arc-like (Nb/Th < 3) calc-alkaline and non-arc-like (Nb/Th > 3) transitional calc-alkaline basalt to continental tholeiitic affinity compositions. ε_Nd(T) values range from − 9.6 to + 5.4 and δ¹⁸O (VSMOW) values range from + 3.1 to + 13.2‰.

A rapid progression from exclusively arc-type to non-arc-like mafic and then contemporaneous WPG plus VAG magmatism has been documented using precise U–Pb zircon dating. Earlier arc-like plutonism indicates subduction, while asthenosphere-derived mafic magmas support slab break-off, due to subduction of a young, warm back-arc basin. Contemporaneous mafic magmas with arc and non-arc geochemical signatures may reflect tapping of asthenospheric and subcontinental lithospheric mantle (SCLM) sources and/or contamination of asthenosphere-derived magmas by SCLM or crust.

The brevity (< 5 Ma) of the mafic magmatic pulse agrees with the transient nature of magmatism associated with slab break-off. The subsequent ca. 1 to 2 m.y. period of voluminous WPG and VAG plutonism likely reflects mafic magma-driven partial melting of both SCLM and crustal sources, respectively. Continuation of VAG-like magmatism for an additional 2 to 5 m.y. may reflect lower solidus temperatures of crustal materials, enabling anatexis to continue after mantle melting ceased. East to west spatial variation of ε_Nd and (La/Yb)_CN in Silurian plutons suggests a transition from shallow melting of juvenile sources proximal to the collision zone to deeper melting of old source materials in the garnet-stability field further inboard.

Previous work has demonstrated that geochemical discriminaton of post-collisional granitoid magmatism (PCGM) is difficult in the absence of other constraints. Our example should contribute to the understanding and identification of PCGM if it can be employed as a ‘fingerprint’ for slab break-off-related PCGM within the Paleozoic geological record.     

Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S)

Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S–71°30′W), and occur together with scarce trachytic lava flows. Whole-rock K–Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic–felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A₁-type granitoid affinities and are characterized by high silica and alkali contents (60–68 wt.% SiO₂; 8.7–10.8 wt.% Na₂O + K₂O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts ((⁸⁷Sr/⁸⁶Sr)_o = 0.70488–0.70571; (¹⁴³Nd/¹⁴⁴Nd)_o = 0.512603–0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province ((⁸⁷Sr/⁸⁶Sr)_o = 0.70556–0.70571; (¹⁴³Nd/¹⁴⁴Nd)_o = 0.512603–0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A₁-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings.

In Central Patagonia, these  3–4 Ma old alkaline intrusions occur aligned along a  N160–170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench.     

Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin

The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have ⁸⁷Sr/⁸⁶Sr (0.7040–0.7042) and ¹⁴³Nd/¹⁴⁴Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher ⁸⁷Sr/⁸⁶Sr (0.7044–0.7047) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization (M_a/M_c) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current.