Composite Dykes Research Articles

Germanium and its isotopes as indicators of hydrogeochemical conditions in a terrestrial geothermal system (Karkonosze granitoid, Sudetes, Poland)

The germanium content and its isotope ratio in thermal waters, aquifer rocks and main silicate minerals in a terrestrial groundwater system located in a granitic pluton (Karkonosze granitoid, Sudetes, Poland) have been investigated. This is the first extensive Ge isotope study carried out in both groundwater and aquifer rocks to better understand previously the identified enrichment of groundwater relative to rocks. This research also provides the first data on δ74/70Ge for hybrid acid rocks formed by mixing magmas from two different sources (mantle- and crustal-derived) and main silicate minerals. The Ge isotopic composition in different facies of granite (porphyritic, equigranular) is comparable (δ74/70Ge of 0.43–1.23‰) to its hybrid rocks (quartz diorite–granodiorite, microgranular magmatic enclaves, composite dykes) – 0.79–1.27‰. However, the range of variability of this feature is quite wide. The minerals have δ74/70Ge values of 1.01–1.04‰ in quartz, 0.84–0.90‰ in alkali feldspars, 0.76–0.88‰ in plagioclase, and 0.36–0.39‰ in biotite. The thermal waters are enriched in heavy Ge isotopes (δ74/70Ge of 1.21–2.78‰) relative to the aquifer rocks (δ74/70Ge of 0.43–1.27‰). Currently, the most likely explanation for this is the influence of secondary mineral phases formed in the geothermal granitic system. This comprises the preferential incorporation and adsorption of light Ge isotopes in/onto iron oxide/hydroxides and clay minerals. The other theoretical explanation, i.e., sulfide formation, is unlikely due to the unfavourable hydrogeochemical conditions in the studied system. The δ74/70Ge values of thermal waters are similar to those of previously studied continental geothermal waters (1.65–3.29‰; Siebert et al., 2011) and fresh groundwater (2.24–4.02‰; Baronas et al., 2020), confirming that groundwater enrichment in heavy Ge isotopes relative to aquifer rocks may be widespread in the upper continental crust. Variation in the isotopic composition of the studied thermal waters is a result of the aquifer rock mineralogy and, in some cases, mixing of the old deep-circulating thermal water with the shallow modern cold groundwater. The general similarities in the chemical composition of the studied waters are due to the influence of porphyritic granite, which is the dominant rock type in this aquifer system. Some differences in chemical composition, including differences in the Ge isotopic composition, are related to the local distribution of hybrid rocks and sulfides. The δ74/70Ge and Ge/Si ratios have been shown to be sensitive indicators of local hydrogeochemical conditions in an alimentation zone and a near-borehole zone of particular water intakes.

The Early Neoproterozoic Andean-Type Orogenic and Within-Plate Magmatic Events in the Northern Margin of the Yangtze Craton during the Convergence of the Rodinia Supercontinent

Although considered a crucial component of the Rodinia supercontinent, it remains uncertain how the Yangtze craton relates to the accretion and breakup of Rodinia. Here, the Huanglingmiao granitic complex (HGC), an intermediate-acid rock series that intruded on the southern Kongling terrane of the northern Yangtze craton margin, is investigated to help resolve this conundrum. Our analysis indicates that these rocks consist of tonalite, trondhjemite, granodiorite, oligoporphyritic granodiorite, porphyric biotite granodiorite, and fine- to medium-grained granodiorite dyke compositions. Collectively, this assemblage is further subdivided into two categories by their temporal, spatial, and geochemical features into early TTG-like and later granitic–dioritic units, which are composed of tonalite, trondhjemite, granodiorite, porphyritic granodiorite, and the fine- to medium-grained granodiorite dykes, respectively. Zircon U-Pb dating yields ages of 865~850 Ma for the TTG-like rocks, 844~825 Ma for the porphyritic granodiorites, and ~800 Ma for the granodiorite dykes. Combined with geochemical evidence, the data suggest that the early- and late-series rocks were formed by a partial melting of Mesoproterozoic and Paleoproterozoic crustal materials, respectively, suggesting that the vertical layering of the crust controlled the composition of the independent units. In addition, isotopic evidence points to different sources for the various rocks in the Kongling terrane and that mantle-derived materials influenced the early-series lithologies. Combined with previous studies on the northern margin of the Yangtze craton, it is inferred that the early-series rocks formed in an active continental margin environment, while the late-series rocks display within-plate boundary formation characteristics. The multiple magmatic activities revealed by this study record sequential partial melting with tectonic transition characteristics from an Andean-type to within-plate magmatism in the northern margin of the Yangtze craton. Taken together, these observations point to a strong association between these rocks, convergence, and incorporation of the northern Yangtze craton margin into the Rodinia supercontinent during the Tonian Period.

Cryptic crustal growth identified through Variscan post-collisional lamprophyre-granite composite dykes, French Massif Central

The processes that control crustal growth by addition of new mantle-derived material to the crust and the recycling of crustal components that were introduced into the mantle during subduction remain poorly understood in collisional orogens. This is largely due to the fact that the sub-orogenic mantle has, in many instances, been fertilized by the introduction of crustal components during previous subduction events, and lacks the typical depleted mantle signature. Here, we present a study of lamprophyric–granitic composite dykes from the northern margin of the post-collisional Variscan Aigoual granite, French Massif Central (FMC). At Aigoual, lamprophyres and granites have gradational contacts and similar geochemical characteristics which indicate that the two were co-magmatic. UPb dating of zircon yields crystallization ages between 311 ± 3 Ma and 313 ± 3 Ma for dykes, identical within uncertainty with the ages of 312 ± 3 Ma and 313 ± 3 Ma obtained for the emplacement of the Aigoual granite. The lamprophyres are metaluminous to slightly peraluminous and display enrichment in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K, LILE, LREE) and have crustal isotopic signatures in both radiogenic (Sr, Nd and Hf) and stable (O) isotope systems. This dual crust/mantle signature of the lamprophyres can be interpreted to reflect partial melting, within the spinel stability field, of an enriched mantle that was metasomatised by the addition of subducted sediments. The granites are peraluminous, high-K and have shoshonitic affinities. Their primitive mantle-normalized trace-element patterns are similar to the lamprophyres and the isotopic compositions of the two rock types overlap. The high Cr, Ni, Fe, and Mg contents of the granites are consistent with a mantle component in the magmas. Our results suggest that between 65 and 80 wt% of material in the lamprophyres and granites was derived from the depleted mantle, and the rest from recycled crustal material. Although blurred from an (conventional) isotopic point of view, this magmatism contributed to the crustal growth via the post-collisional magmatism of the FMC. Coupled with their high preservation potential, post-collisional sites might thus represent significant sites of crustal growth.

Tracking the Transition From a Gondwana LIP to the South Atlantic Ocean With Geochronological and Geochemical Indicators

AbstractEvidence of early oceanization during transition from large igneous provinces (LIPs) to new ocean crust is mostly missing in the geological record. Mafic dykes geochemically akin to mid‐ocean ridge basalts (MORBs) occur in late stages of LIPs, but their tectonic role during margin evolution remains poorly constrained. Here we use whole‐rock and high‐resolution clinopyroxene chemistry of two distinct types of mafic dykes from the southeastern Brazilian margin to show that a MORB‐like plumbing system became magmatically active in the central South Atlantic ∼2 Ma after the volcanic peak of the Paraná‐Etendeka Magmatic Province. The LIP basaltic dyke compositions resulted from the mixing between a highly metasomatized lithospheric mantle comprising enriched DMM and Archean contributions. The lack of elemental oscillatory zoning in clinopyroxene crystals suggests that they were produced by a single magma pulse, consistent with the strong magma fluxes in LIPs. In contrast, the MORB‐like dykes, which exhibit strong similarities with South Atlantic MORBs, are interpreted to represent early stages of oceanization deriving from the mixing of enriched asthenospheric melts either with ancient SCLM contributions or small crustal assimilation. The MORB‐like intrusions ascended and cooled rapidly along faults and were fed by numerous magma injections that suggest episodic decompression of the asthenospheric source ∼20 Ma before the birth of central South Atlantic. The combination of subduction‐influenced and MORB‐like dykes is recurrent in ancient and modern rifts globally, suggesting that thinned lithospheric blocks affected by ancient subduction events may become more susceptible to early oceanization in rifted margins.

Geochemistry of Precambrian dyke swarms in the Singhbhum craton, India: Implications for recycled crustal components in the mantle source

The Singhbhum craton, eastern India records multiple stages of emplacement of Precambrian dyke swarms with contrasting petrogenetic models proposed for their formation. In this study, we document elemental and Sr-Nd isotopic data for three major dyke swarms in the southern part of the craton, including the ca. 2.7 Ga Ghatgaon dyke swarm, the Early Proterozoic Keonjhar dyke swarm and the ca. 1.76 Ga Pipilia dyke swarm. Dyke compositions are dominated by basalt and basaltic andesite with minor andesite, showing trace element signatures typical of continental crustal rocks. Age-corrected Nd isotopic data for Ghatgaon (εNdt = −4.8 to + 4.6), Keonjhar (εNdt = −11.9 to + 3.8), and Pipilia (a single sample with εNdt = −8.8) dyke swarms display substantial variations. The lack of magma compositions that could indicate the presence of elevated mantle potential temperature among the rocks suggests melting regime was likely similar to the ambient mantle. The Dy/Yb and Dy/Dy* systematics of the rocks indicates melting occurred between spinel-stable depths and the spinel-garnet transition zone. The dominantly mafic compositions of the rocks and ubiquitous continental crustal trace element signature are best explained by peridotite source with recycled crustal components, probably in the form of pyroxenites. Our new Nd isotopic data, which argue against any simple secular evolution trend invoked in previous studies, indicate that crustal recycling was likely an episodic phenomenon rather than a discrete, single-stage process since the Archean. Geochemical modelling indicates that a sublithospheric mantle source with (10% or less) recycled crustal components satisfactorily explains the trace element variations of the dyke swarms.

Geochemical composition of dykes along the Cameroon Line (CL): Petrogenesis and similarities with the Central Atlantic Magmatic Province

We report whole-rock geochemistry and Sr–Nd–Pb isotopic compositions of mafic dykes intruded in the Precambrian granito-gneissic basement complex, exposed at Nyos, Batibo, Dschang and Foumban on the Cameroon Line. The dykes are alkaline (Batibo), transitional (Foumban), and subalkaline (Nyos, Batibo and Dschang) with SiO2 of 45–54 wt% and MgO of 2–9 wt%, similar to dykes reported in other areas of the Cameroon Line (CL) and the Central Atlantic Magmatic Province (CAMP). The abundances of rare earth elements (REE) and the Primitive Mantle normalised patterns for the Nyos, Batibo and Dschang dykes are similar to those of MORB, indicating that the dykes formed at shallower depths by a higher degree of partial melting relative to the Foumban dykes and the alkaline lavas of the CL. The transitional basaltic dykes with steeper REE patterns have their sources at deeper levels in the lithospheric mantle, possibly the garnet-spinel transition zone and were generated by a lower degree partial melting of the lithospheric and plume components. The Nyos and Batibo subalkaline dykes show similar isotopic compositions with a spectrum extending from depleted (DMM-like) to enriched (EM1-like) mantle, indicating the similarity in their source components. The Dschang dykes show distinct isotopic characteristics with relatively unradiogenic Nd-Pb isotope compositions compared to the Batibo and Nyos dykes. The Foumban transitional dykes with characteristic wide ranges in Sr-Nd-Pb isotopic compositions reveal varying contributions from enriched mantle components (EM1 and EM2) in addition to its plume signature similar to those of CL lavas. The Nyos and Batibo dykes alongside other dykes on the CL have low TiO2 abundances (<2 wt%), negative PM-normalised Nb-anomalies, and moderately to strongly enriched REE patterns, and isotopic composition that overlaps with those of CAMP, suggesting a similar lithospheric origin.

Geochemical evolution of a composite pluton: insight from major and trace element chemistry of titanite

Titanite from various rocks of the Karkonosze granitoid pluton (South-Eastern Poland) was studied, in order to evaluate its precision in recording magma evolution processes. The rocks are of lamprophyric, dioritic, granodioritic and granitic composition, including hybrid structures such as microgranular magmatic enclaves and composite dykes. Based on textures, chemistry and Zr-in-titanite geothermometry, titanites can be divided into magmatic and post-magmatic populations. Late- to post-magmatic titanite is present in almost all rock types, especially in the most evolved ones (where magmatic titanite is absent) and can be characterized by low trace element and high Al and F contents. Magmatic titanite crystallized in temperatures between 610 and 870 °C, after apatite and relatively simultaneously with amphibole and zircon. Titanite from lamprophyre exhibits compositional features typical of titanites formed in mafic rocks: low Al and F, high Ti4+/(Al + Fe3+), LREE (light rare earth elemet)-enriched chondrite-normalized REE patterns, low Y/Zr, Nb/Zr, Lu/Hf, high (Ce + Nd)/Y, Th/U and Zr. Titanite from hybrid rocks inherited these characteristics, indicating major contribution of the mantle-derived magma especially during early stages of magmatic evolution. Titanite compositional variations, as well as a wide range of crystallization temperatures in hybrid granodiorites point to prolonged crystallization from distinct magma domains of variable mafic versus felsic melt proportions. The extent of compositional variations decreases through subsequent stages of magmatic evolution, and titanite with the least contribution of the mafic component is characterized by higher total REE, Al and F contents, lower Ti4+/(Al + Fe3+), (Ce + Nd)/Y and Th/U ratios, LREE-depleted chondrite-normalized REE patterns and higher Y/Zr, Nb/Zr and Lu/Hf ratios. Titanite composition from the intermediate and late stage hybrids bears signature of decreasing amount of the mafic melt and higher degree of its evolution, however, the exact distinction between the former and the latter is very limited.

Allanite Geochemical Response to Hydrothermal Alteration by Alkaline, Low-Temperature Fluids

Allanite is one of the main rare earth elements (REE)-rich accessory minerals in composite dykes from the granitoid pluton of Karkonosze. These dykes differ in composition from the bulk of the pluton by elevated rare earth elements (REE), Y, Zr, and alkali contents, suggesting contribution of an additional component. Allanite exhibits complex alteration textures, which can be divided into two stages. The first stage is represented by allanite mantles, formed by fluid infiltration into previously crystallized magmatic allanite. These zones have low totals, are Ca-, Al-, Mg-, and light REE (LREE)-depleted, and Y-, heavy REE (HREE)-, Th-, Ti-, and alkali-enriched. The fractionation between LREE and HREE was caused by different mobility of complexes formed by these elements in aqueous fluids. The second stage includes recrystallized LREE-poor, Y-HREE-rich allanite with variable Ca, Al, Mg, and REE-fluorocarbonates. The alteration products from both stages demonstrate higher Fe3+/(Fe2+ + Fe3+) ratios and a negative Ce anomaly. These features point to the alkaline, low-temperature, and oxidized nature of the fluids. The differences in mobility and solubility of respective ligands show that the fluids from the first stage may have been dominated by Cl, whereas those of the second stage may have been dominated by F and CO2 (and PO4 in case of one sample). The inferred chemistry of the fluids resembles the overall geochemical signature of the composite dykes, indicating a major contribution of the hydrothermal processes to their geochemical evolution.

Evidence of subduction-related components in sapphirine-bearing gabbroic dykes (Finero phlogopite–peridotite): Insights into the source of the Triassic–Jurassic magmatism at the Europe–Africa boundary

A gabbroic dyke swarm containing magmatic sapphirine occurs in the Finero phlogopite–peridotite (FPP), one of the major mantle massifs in the Ivrea–Verbano Zone (IVZ; western Southern Alps). Sapphirine is part of a particular mineral assemblage, including plagioclase, titanian pargasite, titanian phlogopite, and Cl-rich apatite; the latter mineral hosts calcite inclusions. The dykes cut the mantle foliation at a high angle, are bounded by orthopyroxenite layers, and show symmetric internal banding, represented by two outer hornblendite selvages and an inner leucogabbro band. The sapphirine occurs in up to 3 cm-thick irregular patches in both hornblendite salvages, along with Al-rich amphibole and green spinel.We present major and trace elements of minerals and bulk rock, as well as mineral O, Sr, and Nd isotopic compositions of dykes and the host peridotite from two different outcrops in the FPP area.Our data show that early melt migration developed through porous flow within cm-thick channels and was characterised by orthopyroxene dissolution. Following progressive percolation and reaction, the melt became silica saturated with segregation of orthopyroxenite in the centres of the channels. The banded internal structure of the dykes was caused by three different evolutionary stages, involving opening and enlargement of the conduits. The sapphirine and green spinel segregation took place at T > 1000 °C, in the presence of melt with transient composition, which interstitially migrated and reacted with the cumulus minerals to form the hornblendite layers. The mineral chemistry of the newly-formed amphiboles indicates that the sapphirine parental melt was Al-rich, depleted to strongly depleted in Hf, Zr, Nb, Ta, Ti, Sc, V, and middle and heavy rare earth elements, and characterised by a positive Eu anomaly and (Zr/Hf)N < 1. These data suggest a parental melt with a significant amount of normative plagioclase. However, the studied veins do not show evidence of plagioclase assimilation, and we argue that this process could have occurred in magmatic bodies that are not outcropping today to the surface or in the melt source.The δ18O values of vein amphiboles and plagioclases vary from 6.9 to 8.6‰ SMOW, which is well above the mantle range, even when considering fractionation upon cooling. Given that orthopyroxene from the wall has “normal” mantle δ18O values (5.8‰), reaction with the host metasomatised peridotite cannot be responsible for the heavy δ18O signature, and the latter must have been imparted by crustal components deeper in the mantle.Our petrographic and geochemical evidence demonstrates that the northern IVZ records an extremely prolonged release, from the Variscan orogenic cycle to the Mesozoic exhumation, of K-H2O-rich mantle-derived melts, mixed with subduction-related components. This finding provides valuable insights into the Triassic–Jurassic magmatism and the geodynamic environment at the Europe–Africa boundary.

Zircon age of vaugnerite intrusives from the Central and Southern Vosges crystalline massif (E France): contribution to the geodynamics of the European Variscan belt

To provide a better picture of the active geodynamics along the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on vaugnerites, which consist of mafic ultra-potassic magmatic rocks, intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late-collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20–40% vol., 64 &lt; mg# &lt; 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of ultra-potassic mafic to intermediate, metaluminous to slightly peraluminous rocks. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircon grains were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre-Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2σ) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data suggest a relatively early emplacement during the Late Variscan collisional history (i.e. Middle Visean times). These results are in line with previously published ages from the Southern Vosges volcano-sedimentary sequences (Oderen-Markstein) and the nearby ultra-potassic granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a magmatic event of regional significance. Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here that the major ultra-potassic magmatic pulse at 340–335 Ma is a consequence of a significant change into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, magmatism could result from a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian-Moldanubian continental block). This delamination would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.

MECHANISMS OF MAGMATIC MINGLING IN COMPOSITE DYKES: MODELS OF DISPERSION AND SHEAR DILATATION

This article is focused on the intrusion and formation of combined dykes. Two main groups of conventionally magmatic mingling are distinguished: (1) plutonic bodies, and (2) combined dykes. The first group is represented by small basite inclusions that are uniformly scattered in granitoid bodies, and includes elongated swarms and tails of small bodies. The second group includes composite dykes with the indicators of mechanical mingling of basic and acid melts. Despite the similarities in the structural and textural features and the indicators of mechanical mingling of melts, these two groups are characterized by clearly different proportions of the volumes of contrasting melts and differ in the duration of formation, place of melt mingling, and tectonic setting. None of the available models was able to explain the occurrence of magmatic mingling structures in individual dykes. In our study, the mingling mechanisms of contrasting melts are discussed using the data on the geological objects located in West Sangilen, an area of the Central Asian Orogenic Belt (CAOB). The general and specific parameters of combined dikes of the Saizyral and Tavyt‐ Dag sites are considered. The models of shear dilatation and dispersion are proposed for explaining the mechanisms of magmatic mingling in combined dykes.

Age and composition of dykes emplaced before and during the opening of the Tasman Sea—source implications

Dykes are common in the wave-cut platforms along the coast from Newcastle to Sydney. According to some authors, they may be related to the opening of the Tasman Sea that commenced ca 84 Ma ago. However, there are few detailed radiogenic dating and geochemical studies to evaluate this. We attempt to resolve this by K–Ar dating of plagioclase in and geochemical studies of, basaltic dykes intruding Permo-Triassic sequences on the wave-cut platforms and Carboniferous and Permo-Triassic sequences inland. The plagioclase separated from the dykes give K–Ar ages ranging from 266 to 53 Ma with the majority older than 84 Ma indicating that most dykes were emplaced before the Tasman Seafloor formation. The dykes are generally mildly alkaline, high-Ti basalts; fewer are tholeiitic and calc-alkaline, low-Ti basalts. Strongly light rare earth element (LREE)-enriched patterns typify the former and flat, LREE-depleted or slightly to moderately enriched LREE patterns, the latter. High-Ti basalts have ocean-island-basalt-like and low-Ti basalts, calc-alkaline or mid-ocean ridge basalt (MORB)-like patterns. Most high-Ti and some low-Ti basalts show plume-like characteristics, others N-type MORB and arc-like characteristics. Dykes intruding the Carboniferous sequences show a distinct contamination signature that could be crustal or due to subduction-related metasomatism of the subcontinental lithospheric mantle. The sources of the basaltic magmas vary substantially and in places changes with time. All alkali basalts are derived from enriched asthenospheric sources at varying depths (90–147 km) and most tholeiitic, low-Ti basalts have been extracted from asthenospheric and depleted asthenospheric–lithospheric sources indicating substantial compositional heterogeneity of the mantle. Further, Nd model ages varying from Neoproterozoic (940–580 Ma) to Paleozoic (460–370 Ma) suggest variation in the age of mantle sources for the basalts.

Origin of dyke swarms in Wadi El Redi-Wadi Lahami area, southern Eastern Desert of Egypt

Dykes predominate within the Neoproterozoic rocks, especially granites, of Wadi El Redi-Wadi Lahami area in the southern Eastern Desert of Egypt. The dyke swarms form three major suites: from the oldest to the youngest, they are basaltic andesite—Suite 1 (E-W and ENE-WSW), rhyolite—Suite 2 (NE-SW), and andesite—Suite 3 (NNE-SSW, NNW-SSE, and NW-SE). Despite the wide ranges of the dyke compositions, the feldspar and amphibole are usually the essential forming minerals. The plagioclase arrays between Ab0.9An99.10 in the basaltic andesite and Ab98.80An0.70 in the rhyolite, while sanidine ranges from Or44.60Ab49.70 to Or98.40Ab1.60. Amphibole in Suite 1 and 3 (Al2O3, TiO2, Na2O, and K2O are the lowest and those of SiO2 and CaO are the highest) samples are usually magnesio-hornblende, whereas it is edenite and tschermakite in Suite 2 dykes. Despite all parent magmas have calc-alkaline affinity, some elements such as Ni show an erratic behavior against the progressing differentiation from one magma chamber and implying for an assimilation of the country rocks. The high contents of amphibole, the depletion in Ti, and the enrichment in large-ion lithophile elements (such as K, Rb, Ba, Sr, and Ba) compared to the primitive mantle composition are consistent with parent hydrous melts generated due to extension above the subduction zone. The estimated compositions of liquids in equilibrium with amphiboles and the pressures at which they crystallized (4.61–7.8 kbar for the Suite 2 and 1.5–2 kbar for the Suites 1 and 3) are greatly varied. These are indications for a difference in the source regions of the parent magmas of the studied dykes. It is supposed that the Suite 1 and 2 dykes are a conjugate set emplaced due to the NW-SE crustal extension in the Arabian-Nubian shield, whereas the Suite 3 dykes generated due to the rifting along the Red Sea.

Geological, geochemical and isotope diversity of ~ 134 Ma dykes from the Florianópolis Dyke Swarm, Paraná Magmatic Province: Geodynamic controls on petrogenesis

The Florianópolis Dyke Swarm (FDS), one of the major dyke swarms belonging to the Early cretaceous (135–131Ma) Paraná Magmatic Province, is largely dominated by high Sr-Ti-P basalts that are confirmed here as feeders of the unique Urubici (=Khumib) lavas of the Paraná and Edendeka lava piles on the basis of their age and geochemistry. Our study integrates field, petrographic, whole-rock geochemistry, and Sr-Nd-Pb isotope geochemistry of representative samples from three main areas of exposition (Santa Catarina Island, Garopaba and Pinheira beaches), thus encompassing the whole extension of the FDS. Compared to the Urubici lavas, the dykes have usually higher contents of LILE and LREE, more radiogenic Sr and Pb, and more unradiogenic Nd, features attributed to a more pronounced interaction with melts derived from the country rocks registered in the basic magmas that remained in the conduits. Some of these dykes show strongly interactive contacts that must be part of a wider zone of crustal melting, probably more developed at greater depths. Small volumes of intermediate to acidic rocks form the cores of some composite dykes, and correspond to products of fractional crystallization from Urubici basalts contaminated with high Rb/Sr, and U/Th crustal melts (probably derived from Neoproterozoic granites), as indicated by geochemical and Sr-Nd-Pb isotope data. The chemical and isotope signatures of the less contaminated FDS basalts and related Urubici lavas do not show clear evidence of inputs from primitive mantle, and seem heavily influenced by enriched mantle. This suggests that the mantle wedge that was affected by subduction during the Neoproterozoic may have been frozen and coupled to the base of the lithospheric plate where the Early cretaceous magmatism occurred. A control of previous tectonic limits on the sources of the Urubici basalts seems evident, since they seem to be related to the younger lithosphere from the South Domain, related to the Florianópolis Batholith, and no influence from the older “cratonic” lithosphere of the Central Domain can be identified in their feeders.

Tectonic position of mingling dykes in accretion-collision system of early Caledonides of West Sangilen (South-East Tuva, Russia)

Dykes composed of basic rocks and granite are formed due to interactions between melts in a wide range of conditions, from contrasting compositions and fluid saturation rates to various tectonic settings and processes at different depths. Textures and petrochemical characteristics of the dykes are thus widely variable. This paper is focused on composite dykes observed in the West Sangilen region in South-East Tuva, Russia. The Sangilen wedge is a fragment of the Early Caledonian orogenic structure of the Tuva-Mongolia Massif which evolved in a succession of geodynamic settings, from collision (transpression, 570–480 Ma) to transform faulting (transtension, 480–430 Ma). Intensive tectonic deformation facilitated massive basic-rock and granite magmatism at various layers of the crust and associated heating and metamorphism of the rocks (510–460 Ma). Basic-rock–granite composite dykes were formed in the above-mentioned period in various tectonic settings that controlled conditions of dyke intrusions and their compositions. We distinguish two groups of composite dykes observed on two sites, in the area between the Erzin and Naryn rivers and on the right bank of the Erzin river (Strelka and Erzin Sites, respectively) (Fig. 1). The dykes in both groups originated from one and the same basic-rock melt source. However, mingling of the contrasting melts was carried out by different mechanisms as suggested by the proposed intrusion models. In the area between the Erzin and Naryn rivers (Strelka Site), the host rock of the composite dykes is granite of the Nizhneerzin massif. The mingling dykes are composed of amphibole gabbro and monzogabbro, granosyenite and twofeldspar granite. Contacts between basic and felsic rocks vary from smooth contrasting to complex ‘lacerated’ flameshaped, and gradual transition zones are present (Fig. 6). The dykes were formed at mesoabyssal or abyssal depths, and the subliquidus heat regime was thus maintained for a long time, and even the smallest portions of the basic-rock melt were consolidated through quite a long period of time. As a consequence, indicators of deformation are lacking in the composite dykes, while transition zones and hybridization are present. On the right bank of the Erzin river (Ersin Site), the dykes cut through migmatite-granite of the Erzin formation in the same-name tectonic zone. Contacts with host rocks are transverse. Melanocratic rocks are represented by smallgrained diorite and quartz diorite, and the felsic composite dykes are composed of medium- and small-grained twofeldspar granite and leukogranite. Transition zones, hornfelsing and contact alterations are absent at contacts of all the types (Fig. 8). The composite dykes of this type intruded and emplaced when the shear zone was subject to extension and fragmentation, which predetermined active intrusion of basic and, possibly, felsic melts through conjugated faults. Crystallization of the melts was rapid, and their potential heat impact on the adjoining rocks was thus excluded, as evidenced by the presence of oxygonal chips of igneous and host metamorphic rocks, vein pegmatoid intrusions, and composite dykes of the reticulate-cuspate texture with the dominant basic-rock component. The mingling dykes classified in the first group intruded when the Erzin and Kokmolgarga shear zones were formed at the early stage of the tectonic-magmatic evolution of the Sangilen orogen (510–490 Ma). Intrusions of the basic-rock melts were accompanied by the formation of relatively large massifs of the basic composition, i.e. the Erzin and Bayankol gabbro-monzodiorite massifs, as well as by the occurrence of composite dykes that are abundant in the area between the Erzin and Naryn rivers. In the second stage (460–430 Ma), the composite dykes occurred when the orogen was subject to extension along the system of tectonic zones, the Bashkymugur gabbro-monzodiorite massif was emplaced, and fracture-vein structures, including the dykes, were formed.

The Relevance of Crystal Transfer to Magma Mixing: a Case Study in Composite Dykes from the Central Pyrenees

Two composite dykes containing abundant mafic enclaves within a felsic host crop out in the Maladeta Plutonic Complex, Pyrenees, Spain. Field, petrographic and geochemical criteria reveal mixing between gabbroic and aplitic magmas, giving rise to a variety of hybrid compositions. The rocks contain spongy plagioclase, quartz ocelli and amphibole-biotite clots that are interpreted as early crystals destabilized by reaction with the hybrid melts. Spongy plagioclase and quartz ocelli were mechanically transferred from the felsic to the mafic magma, whereas amphibole-biotite clots are former pyroxene crystals from the mafic magma.Multivariate statistics (principal component analysis) have been used to examine variations in the mineral trace element compositions, which are best explained by the crystal transfer process. This study shows that crystal transfer represents a mixing mechanism that can overcome some of the physical limitations of interaction between rheologically contrasting magmas and explain deviations of the hybrid compositions from the theoretical mixed chemical composition. In particular, hybrid wholerock compositions show non-linear correlations in inter-element variation diagrams for elements that are enriched or depleted in preferentially transferred crystals. This effect has been quantified by extending magma mixing modelling to include crystal transfer.The composite dykes studied could be regarded as scale models of the behaviour of larger-scale magmatic systems, so this investigation has important implications for interpreting the petrogenesis of igneous suites by magma mixing.

The geology of the northern tip of the Arabian–Nubian Shield

Recently, a detailed (1:50,000) geological map of the Elat area, southern Israel was published. Attached to this map is a stratigraphic table of the Neoproterozoic metamorphic–magmatic complex of the study area. The Neoproterozoic basement in the Elat area encapsulates the Arabian Nubian Shield (ANS) geologic evolution. Uranium–Lead and Lead–Lead zircon ages, included in previous studies and referred to in this paper, reveal that these rocks were formed during more than 300 million years of Neoproterozoic time. The major process controlling the formation of the ANS as part of the East African Orogen is the closure of the Mozambique Ocean. The first orogenic phase in the Elat area, represented by the metamorphic rocks, includes the development of an island arc, erosion of the islands and deposition, and metamorphism. This event took place between ∼950Ma and 780–790Ma. Elat Schist, the oldest metamorphic rock in the area, was deformed and then intruded by quartz dioritic and granitic plutons that were later deformed and metamorphosed. The amphibolite metamorphic rock facies indicate metamorphic conditions of up to 650°C and between 4 and 5kbar. The peak of the metamorphic event was most probably before 750Ma. A gradual change from compressional to extensional stress regime is evidenced by emplacement andesitic magnesium-rich dykes dated to 705Ma that were later metamorphosed to schistose dykes at a greenschist metamorphic facies. The second orogenic phase (terrane amalgamation, main shaping of crust) was associated with the emplacement of large volumes (>50% of area) of calc-alkaline intrusions in a post-collision setting. These very last stages of metamorphism and deformation are characterized by intrusion of ∼630Ma granitoids exhibiting some foliation. Pluton emplacement continued also after the end of deformation. Exhumation and transition to an extensional regime is recorded by the intrusion of shallow alkaline granites in ∼608Ma which were accompanied in ∼609Ma by rhyolite, andesite and composite dykes. The last magmatic event in the Elat area is represented by the volcano-conglomeratic series comprising rhyolites, basalts, andesites, hypabyssal intrusions of monzonite and syenite and conglomerates. The conglomerates, dated to about 590Ma, are the products of a major erosion phase in which about 12,000m of the section were removed. These conglomerates were intruded by 585Ma rhyolite, andesite and composite dykes. The Neoproterozoic basement is truncated by a peneplain whose age, post 532Ma, is constrained by the age of the youngest eroded dolerite dykes. This Early Cambrian peneplain was associated with erosion of 2000m of the section and by chemical weathering. Three major breaks in Neoproterozoic magmatic activity are recognized: the first, occurred in Cryogenian time, lasted ∼60 million years after the amphibolite facies metamorphism and before emplacement of the calc alkaline plutons, separating the first and the second orogenic phases; the second break between the orogenic and the extensional phases occurred in early Ediacaran time, encompassed ∼20 million years between the emplacement of the calc-alkaline and alkaline plutonic rocks and rhyolite, andesite and the composite dykes; and the third, ∼50Ma break, occurred between the emplacement of the last felsic intrusions at ∼585Ma and intrusion of the dolerite dykes in 532Ma, before the Early Cambrian peneplain developed.The great lateral extension of the Cambrian to Eocene sedimentary rocks and their slow facies and thickness changes suggest a stable flat platform area at the northern tip of the ANS. Early Cambrian sedimentation began with fluviatile subarkoses of the Amudei Shlomo Formation. It was overlain by an Early to Middle Cambrian transgressive–regressive lagoonal cycle of dolostones, sandstones, and siltstones of the Timna Formation. Then Middle Cambrian subarkoses and siltstones of the Shehoret Formation and the quartz arenite of the Netafim Formation were deposited in a coastal, intertidal environment representing the southern transgression of a Cambrian ocean.

Monazite to the rescue: U–Th–Pb dating of the intrusive history of the composite Karkonosze pluton, Bohemian Massif

The large (~700km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border between Poland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but contains a range of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism have proved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to 290Ma, with more recent results converging to ~320–300Ma. Dating of zircon and monazite from samples of a variety of major and minor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby and Martin (2008), has narrowed the possible age range further. U–Pb ages measured on eight of ten zircon and monazite samples are in the range ~314–311Ma. Zircon ages measured on the two major types of porphyritic granitoid are 313±3 and 311±4Ma, and monazite ages are 312±2, 313±3 and 311±3Ma. Monazite from one hybrid granitoid has an age of 314±3Ma, and zircon from another an age of 314±4Ma. Zircon from a composite dyke has an age of 311±6Ma. The monazite U–Pb age of an equigranular granite, at 318±6Ma, is consistent with geological evidence that it is older than the porphyritic granitoids but, because of the relatively large uncertainties, is not conclusive. Zircon from one microgranular enclave is anomalously young, 302±4Ma. Evidence is mounting that the main porphyritic granitoids, hybrid granitoids and composite dykes were emplaced within a short time interval between 314±4 and 311±3Ma. Given the uncertainties, emplacement of these units could have been effectively simultaneous. The larger difference between the ages from the equigranular granite and microgranular enclave, however, indicates that the whole Karkonosze thermal episode possibly lasted as long as 15Ma.

The fast evolution of a crustal hot zone at the end of a transpressional regime: The Saint-Tropez peninsula granites and related dykes (Maures Massif, SE France)

Granite and dolerite occurring in the eastern unit of the Maures Massif (southern France Variscan belt) were emplaced in the waning stage of the evolution of a dextral shear zone along the Gondwana border with Laurasia. We present a geochronological and petro-geochemical study (major and trace elements, Sr–Nd isotopes) of (1) the foliated Moulin Blanc cordierite granite and related foliated dykes; (2) the equant Camarat granite and dyke; and (3) the andesite–basalt dolerite dykes (frequently mingled with leucogranite material at Pinet and Capon tips) and cross-cutting leucogranitic dykes. Zircon and monazite dating of the Moulin Blanc granite gives a crystallization age of 301±2Ma. SHRIMP dating on zircons from the leucogranite component of a Pinet composite dyke only reveals zircon core ages of mainly 310±10Ma (age of migmatization of the host gneiss). Undulating contacts of dolerite dykes within the c. 300Ma old Camarat granite suggest penecontemporary relationships between the two intrusions. The age interval between the Moulin Blanc granite and the dolerite dykes would thus be very short. The Moulin Blanc pluton is formed by mingling and mixing of melts of granitic and dioritic composition, the latter occurring mainly as microgranular mafic enclaves. Two other types of diorites are also identified in the pluton. The foliated dykes are similar in composition to the Moulin Blanc granite, which confirms their relationship to the main pluton. The Moulin Blanc granite is a typical example of a pluton formed by repeated recharges coming from a deeper magma chamber. Formation of the Camarat dykes is controlled by hydrothermal fluids that are responsible for the non-CHARAC behavior of trace elements. The Pinet and Capon leucogranite components of the dolerite dykes have distinct geochemical signatures, different from the Camarat granite and dykes. The cross-cutting leucogranitic dykes have much geochemical similarities with the Capon granite. The age of similar dykes in the Tanneron Massif supports the contemporaneity of Camarat granite and dolerites. The dolerite geochemistry was deeply modified by alteration processes, but an andesite–basalt composition is still recognizable by using immobile element ratios. In a εNd vs. Sri diagram, all rocks plot close to a mixing hyperbola between a depleted mantle component and a crustal component, characteristic of the Variscan crust in the French Massif Central. There is a broad correlation between the NdTDM 2-stage model ages and the degree of fractionation of the rocks. Modeling of dehydration melting in a range of P–T conditions reveals that a variety of lithologies were the source of the Saint-Tropez magmas: high-alumina and alkali-basalt amphibolites, metagraywackes, metapelites and meta-arkoses. These sources were melted almost simultaneously at different levels in the crust. The calculated critical dyke width required to transport the granitic melt by buoyancy is far too large compared to the observed granitic dyke thicknesses. Therefore, tectonic processes and/or melt overpressure must have played a role in the emplacement mechanism. Thermo-mechanical constraints on mixing between mafic and felsic melts suggest that, in the Pinet composite dykes, both melts should have mixed. The observed lack of mixing is interpreted as due to quick cooling. The region was affected by a hot zone at the end of a transpressional regime.

Formation of composite dykes by contact remelting and magma mingling: The Shaluta pluton, Transbaikalia (Russia)

A unique opportunity to study the source areas, from which composite dykes were injected, occurs in the Shaluta pluton, Transbaikalia, Russia. The major quartz syenite pluton was intruded by several synplutonic gabbro bodies of various sizes. Investigations of the contact zones between gabbro and host syenite showed that liquid basalt magma intruded the incompletely crystallized coarse-grained quartz syenite with T=700–720°C and caused contact remelting of the silicic rock at about 900–950°C. Mechanical interaction between newly formed silicic melt and partially crystallized mafic magma resulted in extensive magma mingling. Chemical interaction was exhibited by migration of MgO, CaO, FeO∗, Sr, H2O and Cl from the basalt magma, whereas silica, alkalis, Rb and Ba migrated from the silicic refusion zone into the crystallized gabbro. Presence of melt inclusions with homogenization temperature ranging from 640 to 790°C in quartz and attaining 850–900°C in late clinopyroxene indicates that at least part of newly formed minerals crystallized from the hybrid melt. Mingled magmatic material was squeezed out inwards, into the host solid quartz syenite pluton and formed dyke-like apophyses that can be traced for a distance of 60–70m from the contact zone. Apophyses have the same dimensions, structure and composition as typical composite dykes that are common in the roof pendant over the gabbro bodies and nearby the gabbro exposures.