Calc-alkaline Suite Research Articles

Generation and evolution of the Choiyoi granitic magmatism based on U-Pb zircon studies, Cordón del Portillo, Frontal Cordillera (Argentina)

Based on a relevant geochronological U-Pb zircon dataset (n = 47) from a sample (PBL-109) of the Cerro Punta Blanca pluton (CPB), which is part of a calc-alkaline suite, we corroborate the development of protracted magmatic activity with three major crystallization events for this Permian magmatism: 278 ± 1, 283 ± 2, and 289 ± 2 Ma, which outcrop in the Cordón del Portillo, Cordillera Frontal (CF) of Argentina. These ages can be assigned to the lower section of the magmatic record of the Choiyoi magmatism (ca., 290-265 Ma), while the age of 289 Ma represents the oldest known age for the Choiyoi, indicating the start of this magmatism during the Artinskiense. Considering these geochronological data, we postulate the presence of a deep mush reservoir where protracted magmatic activity permitted the prolonged crystallization of antecrysts (ca. 283–289 Ma). Migration of the parental magma from the mush reservoir zone occurred near the time of emplacement and culminated in the formation of an ephemeral magma chamber at shallow levels, where zircon autocrysts crystallized (ca. 278 Ma). Age spectra reported within individual samples support the idea of massive magma migration when conditions were favorable (e.g., thermally matured crust). In this view, the studied “older” Choiyoi magmatism represents a continuous magmatic event lasting 11 Ma and corresponds to a single magmatic episode rather than different periods of magmatic activity and subsequent emplacements. A later alkali-calcic magmatic event is recorded at 265 ± 4 Ma from a sample of the Cerro Bayo pluton (MH-0113), which could represent the end of the lower section of the Choiyoi magmatism. Whole-rock Sm-Nd, Rb-Sr, and Lu-Hf data in zircon, along with ages reported for the studied igneous and inherited zircon from the CPB; together with isotopic data and ages from the detrital zircon found in the Carboniferous accretionary complex of Chile, indicate that the source of the Permian parental magma in this region was a heterogeneous continental crust mainly formed by Devonian and Carboniferous rocks, related probably to a magmatic arc. However, some contribution from the Carboniferous accretionary complex of Chile to the parental magma should be consider.

Characteristics of Dioritic Intrusion at Kaligono Area, Kulon Progo Mountains, Indonesia: A Preliminary Study for Petrogenesis Implications

Abstract This study defines the characteristics of dioritic rock intrusion in the Kaligono area, Kulonprogo Mountains, Indonesia. Previous studies suggest the existence of porphyry deposit system in the mentioned area, hosted by the dioritic intrusion. According to the Geologic Map of Yogyakarta Quadrangle, age of the dioritic intrusion is the Miocene. Study of the intrusion characteristics is very important to gain a better understanding of the geological model associated with the deposit system. This preliminary study focused on determining petrogenesis of the dioritic intrusion. Methods carried out in this study were field data collection, petrographic analysis, and geochemical analysis using ICP-AES (Inductive Coupled Plasma-Atomic Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analysis. Petrography analysis showed that igneous rocks in this research area consist of dioritic rock with phorphyritic texture and oscillatory zoning. Based on its geochemical composition, the rock samples can be classified as diorite and granodiorite and calc-alkaline suites. These rocks are enriched in LILEs (Rb, Ba, Sr, and K) and depleted in HFSEs (Nb, Ti, Zr). In addition, REE shows a slight enrichment of light-REE and depletion of HREE. These patterns of the trace elements including REE show a typical pattern of calc alkaline arc. Petrographic observation and geochemical variation indicate that the magma has undergone fractional crystallization.

Partitioning and isotopic fractionation of Li between mineral phases and alkaline to calc-alkaline melts of explosive and effusive eruptions

Evaluating the lithium (Li) budget of alkaline to calc-alkaline melts and their co-existing mineral phases is crucial to understanding which magmatic or post-eruptive processes can influence the Li contents and isotopic compositions of volcanic deposits. This is also important to be able to better constrain how economically relevant Li deposits form. The lithium contents of many different eruptions from different tectonic settings (intraplate, subduction zone, extensional environment) and cooling environments (quenched, quickly cooled and slowly cooled) were studied by combining new and literature data that were obtained using LA-ICP-MS. We found that the Li concentrations in melt inclusions and re-entrants are independent of tectonic setting (hotspot, subduction zone and rifting) and rarely exceed 100 ppm. When melt inclusions are re-homogenised the Li concentration increases which can lead to the overestimation of the Li budget of a magma.The highest Li concentrations in the alkaline suite of samples were found in groundmass glass (reaching up to 318 ppm). The Li contents in the co-existing mineral phases decrease in the following order: biotite (0.8–99.3 ppm), plagioclase (0.9–25.1 ppm), clinopyroxene (0.6–31.0 ppm) and sanidine (0.5–38.7 ppm). The highest Li concentrations in the calc-alkaline suite of samples were found in the biotite crystals (10.2–2314 ppm) followed by the groundmass glass (2.1–176 ppm). The Li contents are lower in the other mineral phases, from plagioclase (1.4–35.0 ppm), quartz (3.2–21.1 ppm), olivine (1.0–16.3 ppm), sanidine (0.3–8.1 ppm) to clinopyroxene (0.7–21.8 ppm). The Li concentrations in groundmass glass and co-existing crystal phases are independent of the tectonic setting but appear to be more closely related to their cooling environment. To evaluate the influence of the different cooling environments the Li isotopic compositions (per mil deviation of 7Li/6Li ratio relative to L-SVEC reference material) of different eruptions were also determined. The Li isotopic compositions of groundmass glass and mineral phases reveal a range in δ7Li composition spanning up to 6 ‰ for alkaline samples and >10‰ between bulk sample and mineral phases for calc-alkaline samples. The cooling environment of the samples has a large impact on the Li isotopic compositions of the mineral phases with only quartz seemingly unaffected. Rapidly quenched samples may retain their magmatic δ7Li compositions or will only experience small amounts of diffusional overprinting (e.g., by degassing) whereas slowly cooled samples will experience a prolonged diffusional exchange between minerals and matrix overprinting their magmatic δ7Li compositions.The Li inventory in quartz crystals may be influenced by the tectonic setting and water concentration in the crystals. To further assess the influence of tectonic settings on Li behaviour we used a large data set which is based on the 13 alkaline to calc-alkaline eruptions from different tectonic settings and calculated the apparent partition coefficients (Kds) between groundmass glass and co-existing mineral phases. Biotite crystals (alkaline systems) return the highest Kds (subduction-extensional setting: 0.01–0.57, oceanic hotspot: 0.17–2.65) whereas sanidine crystals yield the lowest Kds. Sanidines from the subduction-extensional setting (alkaline trend) have the lowest Kds (0.01–0.07; one outlier ranging from 0.02 to 0.24), followed by the oceanic hotspot setting (alkaline trend, 0.02–0.09) and the extensional setting (calc-alkaline trend; 0.01–0.15). Plagioclase crystals from the subduction-extensional setting (alkaline trend) yield Kds between 0.01 and 0.29, crystals from subduction zone settings (0.06–0.81) and extensional settings (0.09–0.50) have larger ranges in their Kds. The clinopyroxene crystals from the subduction zone settings (calc-alkaline trend) have the largest range in Kds (0.03–0.99), followed by the oceanic hotspot setting (alkaline trend, 0.06–0.48) and subduction-extensional setting (alkaline trend, 0.01–0.22). Quartz crystals (only from the calc-alkaline trend) from subduction zone settings have lower Kds (0.09–0.13) compared to the extensional settings (Bishop Tuff: 0.14–0.28; Caetano Tuff: 0.73–0.94). The wide variety of processes that can affect final Li abundances in melt and crystals (degassing, scavenging into fluids, intra-crystal diffusion, hydration of glasses) makes estimating the partition coefficients particularly difficult and results in the wide ranges observed here. Targeted experimental work would thus help to better constrain the behaviour of Li in alkaline compositions.

Arc-like magmatism in syn- to post-collisional setting: The Ediacaran Angra Fria Magmatic Complex (NW Namibia) and its cross-Atlantic correlatives in the south Brazilian Florianópolis Batholith

Ediacaran syn-tectonic plutonic rocks (amphibole gabbros, quartz diorites/tonalites to biotite- and muscovite-bearing granites) of the Angra Fria Magmatic Complex (Kaoko Belt, north-western Namibia) belong to two compositionally similar, magnesian, transitional tholeiitic–calc-alkaline suites, the Older (∼625–620 Ma) and the Younger (∼585–575 Ma). Both have counterparts in the broadly contemporaneous Florianópolis Batholith (southern Brazil), from which they were separated during the Cretaceous opening of the southern Atlantic. In the Angra Fria Magmatic Complex, the only unequivocal mantle contributions are identified in mingling zones of the Younger Suite and hybrid mafic–intermediate dykes of uncertain age. Previously published Hf-in-zircon isotopic data, together with new whole-rock geochemical and Sr–Nd isotopic signatures, underline an important role of crustal anatexis of a material with late Palaeoproterozoic to early Mesoproterozoic mean crustal residence (1.9–1.5 Ga). This interval resembles some of the published Nd model ages for Tonian ‘Adamastor Rift’-related felsic magmatic rocks in the Namibian Coastal and Uruguayan Punta del Este terranes. In detail, the Older Suite probably originated mainly by fluid-present melting of metabasalts and metatonalites, followed by (near) closed-system fractional crystallization (with or without accumulation) of amphibole ± plagioclase. For the Younger Suite, the principal process was the dehydration melting of relatively felsic lower crustal protoliths (metagreywackes or intermediate–acid orthogneisses >> metapelites), leaving garnet in the residue. Based on the geological context, the conspicuous enrichment of hydrous-fluid-mobile large ion lithophile over the conservative high field strength elements is not interpreted through a classic model of oceanic plate subduction, devolatilization, and fluxed-melting of the overriding mantle wedge. Instead, it is thought to reflect high-grade metamorphism of deeply buried continental crust and attendant water-fluxed melting of the overlying crustal lithologies, connected with inversion of the Tonian ‘Adamastor Rift’.

Interplay between oceanic subduction and continental collision in building continental crust

Generation of continental crust in collision zones reflect the interplay between oceanic subduction and continental collision. The Gangdese continental crust in southern Tibet developed during subduction of the Neo-Tethyan oceanic slab in the Mesozoic prior to reworking during the India-Asia collision in the Cenozoic. Here we show that continental arc magmatism started with fractional crystallization to form cumulates and associated medium-K calc-alkaline suites. This was followed by a period commencing at ~70 Ma dominated by remelting of pre-existing lower crust, producing more potassic compositions. The increased importance of remelting coincides with an acceleration in the convergence rate between India and Asia leading to higher basaltic flow into the Asian lithosphere, followed by convergence deceleration due to slab breakoff, enabling high heat flow and melting of the base of the arc. This two-stage process of accumulation and remelting leads to the chemical maturation of juvenile continental crust in collision zones, strengthening crustal stratification.

Alkalic pyroclast morphology informs on fragmentation mechanisms, Trindade Island, Brazil

Quantitative morphological analysis of juvenile pyroclasts is commonly used to understand fragmentation mechanisms and eruptive styles. Pyroclast morphology has been chiefly studied on conventional tholeiitic and calc-alkaline suites, while the relationship is more poorly constrained for alkaline melt compositions. Here, we analyze the shape of high-Na alkalic juvenile pyroclasts from the Trindade Complex, Desejado, Morro Vermelho, and Paredão Formations on Trindade Island, Brazil, and classify particle populations using multivariate cluster analysis. We expand the field of particle morphology to low-viscosity alkaline suite and interpret fragmentation mechanisms and eruptive styles on products of dry magmatic and phreatomagmatic explosive eruptions. In contrast to tholeiitic and calc-alkaline series magmas, low-viscosity alkaline olivine nephelinitic compositions generate a wide variety of fragments. It includes achneliths, Pele's tears, spheres, sideromelane shards, and tachylyte, formed either by lava fountaining, Strombolian eruptions, or ash explosions, through ductile or ruptile magmatic and secondary fragmentation mechanisms. Phonolitic coarse ash and lapilli fall deposits are dominated by angular juvenile glassy pyroclasts with irregular contours and slightly rounded shapes resulting from dry magmatic fragmentation. The fine ash fall and surge phreatomagmatic deposits chiefly contain lithic clasts, crystal fragments, and blocky and angular juvenile glassy fragments.

Basaltic volcanism of Medvezhia caldera on the Iturup Island of Kurile Isles: impact of regional tectonics on subduction magmatismMartynov Yu.А., Rybin А.V., Chibisova М.V., Ostapenko D.S.,Davydova M.Yu

ABSTRACT We present newly acquired data on the trace element composition and Sr–Nd–Pb-O isotopes of Pliocene – Holocene basalts of the Medvezhiy caldera located on the Iturup Island, Kuril arc. With its unique rhenium mineralization and high-Mg basalt volcanism, it is the largest caldera on the volcanic front of the Kurile Island arc. The calc-alkaline suite of rocks spanning the range from high-Mg basalts to high-Al basalts, has typical arc rock features, such as Nb-Ta depletion and LILE enrichment. The whole set of the caldera geochemical data is divided into two age units, pre- and postcaldera basalts. The precaldera basalts typify low-K, high-Al, basaltic suite of the frontal zones of «cold» subduction. PRIMACALC2 code and Arc Basalt Simulator version 3.1 forward model points to ~21.7% of partial melting of moderately depleted mantle peridotite with relatively high water content (~0.34 wt%) at the pressure of ~1.37 GPa. With similar isotope and trace element signatures, Late Pleistocene–Holocene basalts are marked by the high abundances of MgO (up to 11 wt %), but low Al2O3. The primary magmas were generated from depleted mantle with lower H2O content (~0.13%) and degree of partial melting (~9.5%), but at higher pressure (~1.77 GPa). The decompression scheme reproduces the fractionation trends of postcaldera magnesian basalts for most major elements but fails to explain the composition variations of the precaldera lavas. Their origin has been the matter of some complex processes in the shallow magma chamber. The abrupt change in the type of eruptions and magma composition of the caldera in Late Pleistocene–Holocene were related to a transformation of both the magma-feeding and melt generation systems as the result of geodynamic reorganization of the vast territory adjacent to the Sea of Japan and Kamchatka.

Integrated petrological and Fe-Zn isotopic modelling of plutonic differentiation

The upper continental crust is formed from chemically diverse granitic plutons. Active debate surrounds the range of physical conditions (P-T-X-fO2) and differentiation processes which occur in mush bodies that solidify to form plutons. Transition metal stable isotopes are increasingly employed to trace magmatic processes in both extrusive lavas and intrusive plutonic suites, with a focus on analysis of whole rock powders. However, studies of plutonic suites often overlook the complex textures represented within coarse grained samples, and how these will influence whole rock isotopic compositions.Here we examine the calc-alkaline Boggy Plain Zoned Pluton, SE Australia, which closely approximates closed system behaviour during magmatic differentiation. We combine petrological examination with Fe and Zn isotopic analysis of biotite, hornblende and magnetite mineral separates and whole rock powders. Whole rock Fe isotopic composition (as δ56Fe) increases from 0.038‰ to 0.171‰ with decreasing MgO content, while mineral separates display heavy Fe isotope enrichment in the order magnetite > biotite = hornblende > pyroxene. A lack of correlation between whole rock Fe and Zn isotopic compositions suggests that the Fe isotopic variation is predominantly driven by closed system fractional crystallisation: specifically by the balance between crystallisation of isotopically heavy magnetite, and isotopically light silicates. To demonstrate this quantitatively, temperature dependent mineral-melt fractionation factors were derived from the mineral separate data (Δ56Femag-melt = 0.17 × 106/T2 and Δ56Febt/hbd-melt = −0.12 × 106/T2) and used to construct models that successfully reproduce the observed Fe isotopic variation during fractional crystallisation. These fractionation factors are compared to theoretical and empirical estimates from previous studies. We highlight that accurate determinations of temperature and modal mineralogy are critical when modelling Fe isotopic variations in plutonic suites. Successful interpretation of equilibrium Fe isotopic fractionation in a relatively simple calc-alkaline suite like the Boggy Plain Zoned Pluton paves the way for Fe isotopes to be used to investigate more complex mush bodies.

Age, chemistry, and tectonic setting of Miramichi terrane (Early Paleozoic) volcanic rocks, eastern and east-central Maine, USA

Volcanic rocks in the Miramichi inlier in Maine occur in two areas separated by the Bottle Lake plutonic complex: the Danforth segment (Stetson Mountain Formation) north of the complex and Greenfield segment to the south (Olamon Stream Formation). Both suites are dominantly pyroclastic, with abundant andesite, dacite, and rhyolite tuffs and subordinate lavas, breccias, and agglomerates. Rare basaltic tuffs and a small area of basaltic tuffs, agglomerates, and lavas are restricted to the Greenfield segment. U–Pb zircon geochronology dates Greenfield segment volcanism at ca. 469 Ma, the Floian–Dapingian boundary between the Lower and Middle Ordovician. Chemical analyses reveal a calc-alkaline suite erupted in a continental volcanic arc, either the Meductic or earliest Balmoral phase of Popelogan arc activity. The Maine Miramichi volcanic rocks are most likely correlative with the Meductic Group volcanic suite in west-central New Brunswick. Orogen-parallel lithologic and chemical variations from New Brunswick to east-central Maine may result from eruptions at different volcanic centers. The bimodal Poplar Mountain volcanic suite at the Maine–New Brunswick border is 10–20 myr younger than the Miramichi volcanic rocks and more likely an early phase of back-arc basin rifting than a late-stage Meductic phase event. Coeval calc-alkaline arc volcanism in the Miramichi, Weeksboro–Lunksoos Lake, and Munsungun Cambrian–Ordovician inliers in Maine is not consistent with tectonic models involving northwestward migration of arc volcanism. This >150 km span cannot be explained by a single east-facing subduction zone, suggesting more than one subduction zone/arc complex in the region.

Petrogenesis of Botorubuh igneous rocks at Gunungkidul: A preliminary study for paleomagmatism phenomenon in the southern beach of Yogyakarta - Indonesia

Botorubuh Beach is located at Southern Beach of Gunungkidul, in the Southern Mountain of Yogyakarta - Indonesia and is a promontory composed of Middle - Late Miocene igneous rock with a columnar jointing structure. This isolated igneous rock area is surrounded by limestone. Therefore, the regional geological map of Surakarta-Giritontro classifies this area as the Punung-Wonosari Formation which is dominated by limestone. Because of the geological phenomenon of the isolated igneous rock area, it is necessary to study the petrogenesis of the igneous rocks at the area. The petrogenesis research is based on a preliminary study of the petrographical and geochemical characteristics of this igneous rock samples. The petrographic identification of andesite samples shows porphyritic, trachytic, oscillatory zoning, and sieve textures. The results of geochemical analysis (major and trace elements) show that the rock samples are are classified to andesite rocks and calc-alkaline suites. These rocks are enriched in LILEs (Rb, Ba, K) and depleted in HFSEs (Nb, Ti, Ce). Additionally, REE shows a slight enrichment of light-REE and a slight negative anomaly of Eu. The patterns of the trace elements including REE show a typical pattern of calc alkaline arc. Petrographical and geochemical characteristics suggest evidence of magma differentiation process, that is by a mechanism of crystallization fractionation. The andesite was formed in relation to a Middle – Late Miocene paleomagmatism and the Late Miocene-Pliocene subduction zone.

Petrogenesis of Mediterranean lamproites and associated rocks: The role of overprinted metasomatic events in the post-collisional lithospheric upper mantle

Abstract High-MgO lamproite and lamproite-like (i.e. lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine–Himalayan collisional margin, which followed the closure of the Tethys Ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterranean region in the form of shallow level intrusions (e.g. plugs, dykes and laccoliths) and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (northern Italy), through the Late Miocene in Corsica (southern France) and in Murcia-Almeria (southeastern Spain), to the Plio-Pleistocene in Southern Tuscany and Northern Latium (central Italy), in the Balkan peninsula (Serbia and Macedonia) and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al 2 O 3 , CaO and Na 2 O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent, and it is rarely found only in the groundmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotope compositions, high 207 Pb over 206 Pb and high time-integrated 232 Th/ 238 U. Their composition requires an originally depleted lithospheric mantle source metasomatized by at least two different agents: (1) a high Th/La and Sm/La (i.e. SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; (2) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterized by network of felsic and phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melting of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterize areas that were affected by multiple Wilson cycles, as observed in the Alpine–Himalayan Realm.

Appinitic and high Ba[sbnd]Sr magmatism in central Brazil: Insights into the late accretion stage of West Gondwana

Large amounts of high BaSr magmatism can be produced as the result of late accretion or decompression in the final stages of continental arc evolution in orogenic systems and leads to continental crustal growth. This type of magmatism often occurs associated with mega-shear zones and displays geochemical signatures that indicate magmas deriving from the melting of enriched lithospheric mantle. Such enrichment may be an indication of slab-derived fluid and/or melt interaction, released from subducted material. The high BaSr granitoids of the Brasiliano Paraguay Belt, Tocantins Province, central Brazil, comprise two suites that crystallized approximately 40 My apart from each other. Both suites are composed of metaluminous to slightly peraluminous rocks (ASI = 0.64–1.09) with magnesian character, and present high Ba (~1430 ppm), Sr (~800 ppm), K2O (~4 wt%), LREE and LILE contents. The negative εNd(T) values (−6.32 and − 6.65) associated with high contents of incompatible elements indicate an evolution from partial melting of mantle-derived magma previously metasomatized by subducted slab fluids. Crystallization ages of 557 ± 5.7 Ma and 554 ± 2.9 Ma have been obtained for the older, high MgK calc-alkaline suite. It evolved from high Cr and Ni mantle-derived appinitic magma to coeval granodiorites by crystal fractionation accompanied by minor crustal contamination. The younger, 515 ± 3.6 Ma-old suite displays an elevated concentration in some HFSEs, show alkali-calcic affinity and is represented by high-K monzogranites. A model proposed for the two hybrid suites is based on a slab break-off and a subsequent lithospheric delamination after 40 My, where the heat flux generated by mantle upwelling led to a partial melting of the enriched subcontinental lithospheric mantle beneath the continental crust. The older calc-alkaline suite represents the end process of magmatic arc accretion in the central Tocantins Province. The younger one is compatible with a transitional shoshonitic to A-type affinity, associated to post-collisional collapse stage of the orogen at Cambrian times. The petrogenesis of the investigated rocks requires an ocean closure phase at ca. 590 Ma, which could be linked to the consumption of the Clymene Ocean toward Amazonian Craton, recording a new collisional event in the central West Gondwana.

Compositional Evolution of the Variscan Intra-Orogenic Extensional Magmatism in the Valencia del Ventoso Plutonic Complex, Ossa-Morena Zone (SW Iberia): A View from Amphibole Compositional Relationships

The Ossa-Morena Zone (OMZ), SW Iberia, has numerous Lower Carboniferous compositionally zoned plutons that formed in a Variscan intra-orogenic extensional setting. This magmatism shows a wide compositional variation comprising alkaline, transitional, and calc-alkaline suites. The calc-alkaline suite was produced by hybridization of alkaline magmas with felsic melts generated by crustal anatexis related to the intrusion of mafic magmas in the middle crust. In this work, we present a textural and mineralogical study of the Variscan Valencia del Ventoso main pluton from the OMZ to track the compositional evolution of magmas during hybridization using constraints from amphibole compositions and to determine the P-T conditions of emplacement using amphibole-based thermobarometry. This pluton exhibits reverse zoning with an inner facies containing alkaline dolerites, gabbros, and quartz diorites, an intermediate facies with transitional diorites, and an outer facies with calc-alkaline quartz diorites to monzogranites. Magmas from the intermediate and border facies crystallized under oxidizing conditions at relatively low temperatures (range: 640–760 °C) and ca. 280–300 MPa, implying near H2O-saturated conditions. These rock facies show mineralogical evidence of hybridization between alkaline to mildly alkalic and calc-alkaline magmas. The former is inferred from the occurrence of antecrysts of labradorite-andesine, high-Ti pargasite-hastingsite, and biotite with deficiency in tetrahedral-site occupancy, a distinctive feature of biotite from the inner facies alkaline dolerites. This contrasts with later crystallization from the calc-alkaline magma of andesine-oligoclase, low-Ti magnesiohornblende-edenite, and biotite with full tetrahedral-site occupancy. Constraints from amphibole-melt compositional relationships in antecrystic high-Ti amphibole suggest that the alkaline magmatic component could have a high- to ultra-K affinity.

A New Magma Type in the Continental Collision Zone. The Case of Capraia Island (Tuscany, Italy)

The Tuscany Magmatic Province consists of a Miocene to Pleistocene association of a wide variety of rock types, including peraluminous crustal anatectic granites and rhyolites, calcalkaline and shoshonitic suites and ultrapotassic lamproites. In addition to the magma types already recognised, the occurrence of a new, distinct magma type at Capraia and Elba islands and in mafic enclaves in the San Vincenzo rhyolites has been suggested by recent studies. This particular type of magma, represented by intermediate to acidic calcalkaline rocks showing high Sr, Ba, and LREE, is restricted to the northwestern sector of the province and to a time interval of about 8 to 4.5 Ma. New data obtained on rocks from Capraia Island have allowed for the verification of the occurrence of this new magma type, the exploration of its origin and a discussion of its possible geodynamic significance. The high-Sr-Ba andesite-dacite rocks occurring in the Laghetto area at Capraia display a composition that is intermediate between adakitic and calcalkaline rocks. It is suggested that they represent a distinct type of magma that originated at mantle pressure by melting of the lower continental crust, followed by mixing with other Capraia magmas. The geodynamic model that best explains the composition of the studied rocks is the thickening of the continental crust during continental collision, followed by extension that favoured melting of the lower crust.

Isotopic and geochemical record of the active to passive margin transition in NW Iberia during the Cambrian-Ordovician: vestiges of a waning continental arc

Time-resolved geochemical data from the orthogneisses of the basal units of the Allochthonous complexes of NW Iberian Massif allow us to track the changes in composition of the magmas generated in a subduction-related setting that affects a continental margin, which evolves from the building of a volcanic arc to its dismantlement during a rifting episode. The basal units are interpreted as a peripheral section of the North African margin of Gondwana and record Neoproterozoic to Cambrian arc-related magmatism, followed by an episode of Cambro-Ordovician rifting. The intrusive rocks of the basal units, preserved as orthogneisses, can be grouped into two magmatic suites: calc-alkaline (c. 493–475 Ma) and alkaline to per-alkaline (c. 475–470 Ma). The calc-alkaline association has a chemical composition ranging from magnesian calcic metaluminous types to ferroan calc-alkalic and alkali-calcic peraluminous types. The moderately alkaline association and per-alkaline metagranites have ferroan alkalic and per-alkaline to metaluminous composition. The chemical characteristics of the metaigneous rocks from the basal units indicate that the older members of the calc-alkaline suite (tonalites and granodiorites) were generated in a continental arc environment, while the alkaline and per-alkaline rocks were formed in an intraplate extensional setting. The negative values in eNd (− 7.8 to − 1.5) of the calc-alkaline suite suggest a crustal influence in its generation. The eNd values in the alkaline orthogneisses range between − 2.0 and 2.2, and are positive in the per-alkaline group, suggesting a greater participation of a mantle component. The transition from the calc-alkaline suite to the alkaline suite is represented by alkali-granites, which are coeval with the intrusion of basic magmas, and likely derived from a mantle magma underplating that triggered the rifting episode after the cessation of continental arc magmatism during the Ordovician.

Melt chemistry and redox conditions control titanium isotope fractionation during magmatic differentiation

Titanium offers a burgeoning isotope system that has shown significant promise as a tracer of magmatic processes. Recent studies have shown that Ti isotopes display significant mass-dependent variations linked to the crystallisation of Fe-Ti oxides during magma differentiation. We present a comprehensive set of Ti isotope data for a range of differentiation suites from alkaline (Ascension Island, Afar and Heard Island), calc-alkaline (Santorini) and tholeiitic (Monowai seamount and Alarcon Rise) magma series to further explore the mechanics of Ti isotope fractionation in magmas. Whilst all suites display an increase in δ49/47Ti (deviation in 49Ti/47Ti of a sample relative to the OL-Ti reference material) during magma differentiation relative to indices such as increasing SiO2 and decreasing Mg#, our data reveal that each of the three magma series have contrasting δ49/47Ti fractionation patterns over comparable ranges of SiO2 and Mg#. Alkaline differentiation suites from intraplate settings display the most substantial range of variation (δ49/47Ti = +0.01 to +2.32‰), followed by tholeiites (−0.01 to +1.06‰) and calc-alkaline magmas (+0.06 to +0.64‰). Alkaline magmas possess high initial melt TiO2 contents which enables early saturation of ilmenite + titanomagnetite and a substantial degree of oxide crystallisation, whereas tholeiitic and calc-alkaline suites crystallise fewer oxides and have titanomagnetite as the dominant oxide phase. Positive slopes of FeO*/TiO2 vs. SiO2 during magma differentiation are related to high degrees of crystallisation of Ti-rich oxides (i.e. ilmenite). Bulk solid-melt Ti isotope fractionation factors co-vary with the magnitude of the slope of FeO*/TiO2 vs. SiO2 during magma differentiation.This indicates that the modal abundance and composition of the Fe-Ti oxide phase assemblage, itself is controlled by melt composition, governs Ti isotope fractionation during magma differentiation. In addition to this overall control, hydrous, oxidised calc-alkaline suites display a resolvable increase in δ49/47Ti at higher Mg# relative to drier and more reduced tholeiitic arc suites. These subparallel Ti isotope fractionation patterns are best explained by the earlier onset of oxide segregation in arc magmas with a higher oxidation state and H2O content. This indicates the potential of Ti isotopes to be utilised as proxies for geodynamic settings of magma generation.

Neoproterozoic magmatic evolution of the southern Ouaddaï Massif (Chad)

This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the so-called Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre- to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage.

Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a suitable area for considering the effects of tectonic structures on development of the volcanic facies in the magmatic provinces. The main structure in the area is a dextral fault (so called Rahjerd Fault) that has produced the local extensional basins and also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline suite that had been changed and evolved in the magma chambers of local extensional basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite and magnesiohornblende) reveal that the AFC process is a dominant process in the magma chamber. Also, according to geothermobarometric calculations, the investigated volcanic rocks of the area could be grouped into two types: one type with higher P-T (>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600- 800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA) and the dioritic stock.

Exploration and Sources of Bauxite Deposit in the Boloven Plateau, Southern Laos

The Boloven bauxite deposit occurs either in the weathered basalt (alkali basalt and tholeiite), or in the Cretaceous sandstone. It is generally agreed that the bauxite deposits/laterites overlying the alkali basalt and tholeiite were derived from weathering of underlying basalt, however, the origin of bauxite deposit overlying the sandstone remains controversial. Chondrite-normalized REE patterns show that the bauxite ores/laterites overlying the sandstone exhibit quite similar chondrite-normalized REE patterns to those overlying the alkali basalt. Diagram of Al-Ti-Zr shows that the bauxite ores/laterites overlying the sandstone, tholeiite and alkali basalt are close to each other and to the calc-alkaline suite, however, significantly different from the sandstone and shales. Binary diagram of log Nb/Y vs log Zr/Ti further indicates that the parent rocks of bauxite ores/laterites overlying the sandstone belong to the suit of ultra-alkali to alkali basalt. Multivariate statistical analysis of geochemical data exhibit that the geochemical characteristics of HREE, Y, LREE and Al2O3 for bauxite ores/laterites overlying the sandstone are similar to those overlying the alkali basalt (15.7 Ma), obviously different from those overlying the tholeiite (1.2 to 0.5 Ma). Consequently, it can be inferred that the bauxite deposits/laterites overlying the sandstone were derived from the alkali basalt.

Cargo Porphyry Cu-Au Deposit – Where is the High Grade Core?

The Cargo Copper-Gold Porphyry deposit lies within the world class porphyry belt of the Ordovician Macquarie volcanic arc. It is one of several porphyry complexes in the Lachlan Orogen including Cadia Valley (42.8 MOz Au), Northparkes, Cowal and Copper Hill.The Cargo Porphyry Intrusive Complex is a calc-alkaline suite of late Ordovician age (467 Ma) intrusives comprising quartz monzodiorite and diorite intruded by coeval andesitic and trachy-andesitic volcanics. The most prominent NW trending structural zone is characterised by areas of strong silicification, pyritisation and tectonic brecciation together with stockwork and sheeted quartz veining. It is up to 300 m wide bounded on the southern side by a major 60o SW dipping normal fault and on the Northern side by 75o SW dipping shear zones.Mineralization and alteration is zoned from a western core of fracture controlled, potassic altered porphyry (Cu-Mo-Au) to a peripheral zone of phyllic altered gold rich quartz-sulphide veining up to 200 m wide, surrounded by an outer propylitic zinc rich halo.The peripheral zone of Cargo contains gold rich sheeted quartz veins which hosted 14 small gold workings in the late 1800’s. Two lode systems, Dalcoath and Spur, have JORC inferred resource of 4 Mt @ 1.19g/t gold for 154,000 Oz Au.A classically zoned porphyry Cargo’s geochemical footprint is comparable in size to the famous Bingham Canyon and the Bougainville Panguna deposits despite the fact that Cargo’s porphyry deposits’ western half has been faulted away.