Calc-alkaline Differentiation Trend Research Articles

СЕРЕБРЯНАЯ МИНЕРАЛИЗАЦИЯ В ГЛУБИННЫХ МАГМАТОГЕННЫХ СИСТЕМАХ ДРЕВНИХ ОСТРОВНЫХ ДУГ: ИЛЬДЕУССКИЙ УЛЬТРАОСНОВНОЙ МАССИВ, СТАНОВОЙ МОБИЛЬНЫЙ ПОЯС (ДАЛЬНИЙ ВОСТОК РОССИИ)

The Triassic (232–233 Ma) Ildeus mafic-ultrabasic massif is a fragment of a large mineralized magmatogenic system formed within the Stanovoy mobile belt at the Mesozoic stage of its evolution. Ultrabasic rocks are represented by cumulative plagioclase- and amphibole-bearing dunites, peridotites, websterites, and pyroxenites; basic rocks are gabbro-anorthosites, norites, and two-pyroxene gabbro. Most intrusive rocks of the Ildeus magmatogenic system experienced ultramafic (serpentine, chlorite, talc, carbonates) and acidic (quartz, biotite, potassium feldspar, secondary micas) metasomatism. Geochemical characteristics (calc-alkaline differentiation trend, negative anomalies of highly charged incoherent elements) indicate the subduction nature of the primary magma in the Ildeus system. The ultrabasic rocks are intruded by intermediate-acid intrusions with geochemical characteristics of adakite. Magmatic-stage silver mineralization is represented by microinclusions of cuprous silver, alloys of silver, copper, gold and zinc, acanthite and silver halides in association with pentlandite, chalcopyrite, bornite, and pyrrhotite. These rocks are characterized by microinclusions of native platinum, gold, tungsten, bismuth, lead, PGE alloys, W-Co-Ti, Sb-Pb, Cu-Zn-Sn, minerals of Ag-Cu-Sb-Se-S and Zn-Ni systems -Co-Fe-S, barite, chlorapatite and Bi-Sn-Pb chlorides. At the metasomatic stage, native silver, Au-Ag alloys, silver and copper chlorides, acanthite, silver and lead tellurides coexist with digenite, heathlewoodite, pyrite, sphalerite, galena, native nickel and gold, Cu-Zn alloys, sulfides of the Pb-As- Cu-Fe-S and bismuth chloride. At the magmatic stage, silver minerals crystallized under reducing conditions in the presence of high-temperature aqueous fluids enriched in sulfide sulfur, halogens, and volatile siderophile (W, Pt) and chalcophile (Ag, Bi, Sn, Sb, Pb) metals that entered the primary melt from serpentinites, metamorphosed gabbroids, and metal-bearing pelagic sediments of the subducting oceanic plate. Metasomatic parageneses of silver minerals were formed under oxidized near-surface conditions with the participation of low-temperature water-salt solutions with elevated concentrations of sulfate sulfur, arsenic, and tellurium. A two-stage model is proposed for the formation of silver mineralization in island arc magmatic systems, in which the high-temperature magmatic stage of formation in deep crustal parts is replaced by low-temperature hydrothermal ore formation processes in near-surface conditions.

ПРОСТРАНСТВЕННО-ВРЕМЕННАЯ СМЕНА ГЛУБИННЫХ ИСТОЧНИКОВ КАЙНОЗОЙСКИХ ВУЛКАНИЧЕСКИХ ПОРОД ВОСТОКА КОРЯКСКОГО НАГОРЬЯ

A spatiotemporal analysis of the trace element ratios and the isotopic composition of Sr, Nd and Pb in Cenozoic post-accretionary volcanic rocks in the east of the Koryak Highlands (Northeastern Russia) has been carried out. The Early Paleogene volcanic complex is shown to be represented by moderately Ti-rich aluminous tholeiites with elevated contents of high field strength elements (HFSE) (except for Ta and Nb), which makes them similar to E-MORBs. Low (La/Yb)n and high Zr/Nb (25–35) ratios characterize one of the source components as depleted and similar to MORBs. At the same time, low ratios of Nb/Ta, Ce/Pb, Nb/La with a high ratio of K/Nb were determined in basalts, which indicates the presence of a subduction component in the source. Low Zr/Hf and (Dy/Yb)n ratios indicate melting of the garnet-free substratum. The second (Miocene) stage is represented by the subalkaline basalt flows and dacite extrusions and dikes forming a bimodal series. Basalts are characterized by low concentrations of large ion lithophile elements (LILE), enrichment in HFSE, and fractionated distribution of REE, with (La/Yb)n ratios varying from the values characteristic of E-MORBs to the values corresponding to intraplate tholeiites. The Nb/Ta, Ce/Pb, La/Ta, and Hf/Th ratios tend to exhibit compositions of intraplate tholeiites. The (Dy/Yb)n and (La/Yb)pm - Ybpm ratios indicate the formation of Miocene basalts during the selective melting of garnet peridotite with varying garnet contents in the source. Dikes of dacites, in comparison with effusive facies, differ in calc-alkaline differentiation trends and other absolute concentrations of a number of elements. Early Quaternary alkaline olivine basalts and basanites of the Navarin area are high in HFSE, LILE, and REE and are compositionally similar to intraplate volcanics of oceanic islands and continental rifts. Data points of these rocks plotted on discriminant diagrams are confined to the field of the garnet-rich intraplate source with low degrees of partial melting. The initial isotopic ratios of Sr, Nd, and Pb in the Cenozoic basaltoids characterize the deep sources of Cenozoic rocks in the eastern part of the Koryak Highlands as depleted. The similarity of isotope ratios in the basalts of the Miocene and Early Quaternary stages of volcanism suggests that the mantle region was isotopically homogenized as a result of local convection.

Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems

Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO2) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO2 corresponding to the Re-ReO2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.

Mantle source heterogeneity in a Neoproterozoic back-arc basin: Geochemical and thermodynamic modeling of the volcanic section of Wadi Ghadir ophiolite, Egypt

Wadi Ghadir ophiolite in the Egyptian Eastern Desert, which forms the northern part of the Nubian Shield, is considered as one of the best-preserved segments of the Neoproterozoic oceanic lithosphere on Earth. Primary melt calculations and thermodynamic modeling of new geochemical data for the Wadi Ghadir pillow lavas and dike complexes, integrated with comprehensive petrographic investigations, are here employed to unravel the geodynamic evolution of this ophiolitic section.Whole-rock geochemical and clinopyroxene data indicate that the pillow lavas and sheeted dike complexes preserve geochemical signatures of N-MORB, E-MORB and OIB-like basalts. Less abundant, discrete dikes have a distinctly LILE-enriched island arc tholeiite (IAT) composition. The pillow lavas and sheeted dike complexes replicate mixed liquid lines of descent, whereas the discrete dikes exhibit a calc-alkaline differentiation trend. The pillow lavas were produced by limited (5–7%) mantle partial melting as a result of isothermal decompression over a wide pressure range (5 – 19 kbar) and restricted mantle potential temperatures (Tp ≈ 1260–1300 °C), suggesting ponding of rising melts from depths of ∼60 to 15 km in dry mantle. Primary melts of the sheeted dike complexes were produced by 9–11% partial melting at P ≈ 9.7 ± 2.5 kbar, Tp ≈ 1290 °C, and P ≈ 14.7 ± 1.4 kbar, Tp ≈ 1325 °C, suggesting a role of varying hydrostatic pressure. The magma source for the discrete dikes occurred at 10.7 ± 0.6 kbar and Tp of 1230–1300 °C by 11–13% partial melting of a metasomatized mantle region.The calculated Tp ranges for the different melts are consistently lower than temperatures of sub-ridge ambient mantle. Together with the variable concentrations of K2O and other incompatible elements, the estimated low temperatures of melt generation emphasize mixed sources and wet peridotite melting. The modeled primary melts designate crustal growth during back‐arc basin opening and closure. During the basin opening, decompression melting of mildly enriched mantle produced high-Ti tholeiitic (pillow lavas) and transitional (sheeted dike complexes) melts, whereas subduction initiation during basin closure promoted melt-peridotite interaction. Lowering of the solidus by subduction-related components triggered melting of a spinel lherzolitic mantle and produced low-Ti calc-alkaline melts that sourced the discrete dikes. The mixed MORB and SSZ geochemical characteristics of the studied volcanic rocks, coupled with the various modeled melts and lack of significant thermal anomalies in the mantle conditions, are here interpreted as manifestations of mantle source heterogeneity in a marginal oceanic basin during the accretionary stages of the Arabian-Nubian Shield.

Crystallisation and zircon saturation of calc-alkaline tonalite from the Adamello Batholith at upper crustal conditions: an experimental study

The understanding of the geochemical and petrophysical evolution of magmas forming intermediate calc-alkaline batholiths at shallow crustal levels critically depends on knowledge of the phase equilibria relations along the liquid line of descent. Here, we present experimental results for a tonalitic system at a pressure of 200 MPa and under water-saturated conditions. Melting experiments were performed at temperatures between 700 and 1000 °C in externally heated HCM pressure vessels, with oxygen fugacity controlled close to the Ni–NiO buffer equilibrium (NNO) employing an argon–methane mixture as pressure medium and Co–Pd redox sensors to verify fO2 conditions. Natural rock powder of a medium-K tonalite from the Adamello Batholith in Northern Italy served as experimental starting material. Based on compositional data of stable phases in the run products and images of entire run charges, mass balance calculations as well as image processing were performed to investigate the evolution of the crystal/melt ratio with respect to temperature. Furthermore, compositional trends of minerals as well as the liquid line of descent of residual melts were obtained. Orthopyroxene, clinopyroxene and plagioclase were identified as near-liquidus phases (below 990 °C). At 900 °C, amphibole joins the solid-phase assemblage at the expense of clinopyroxene, indicating the existence of a peritectic relationship. After an initial near-linear decrease with temperature, residual melt fractions exhibit a plateau of 45–55 vol. % between 750 and 850 °C, followed by a rapid decrease coinciding with quartz saturation at 725 °C. Compositions of residual liquids evolve along a typical calc-alkaline differentiation trend with decreasing temperature (increasing SiO2 and decreasing TiO2, Al2O3, CaO, MgO and FeO contents) and become peraluminous below 900 °C. Intermediate to acidic rocks from the Adamello follow the experimental liquid line of descent indicating that the observed compositional spread of the natural intermediate composition rock record can be explained by low-pressure magma differentiation and liquid extraction. Experimentally determined zircon saturation levels are at low temperatures distinctly lower compared to existing and often used Zr-saturation models, but fully consistent with observed and modelled Zr-evolution trends from the natural rock record of the Southern Adamello Batholith inferring that zircon saturation in these intermediate to felsic plutonic rocks occurred at 800–830 °C corresponding to a melt fraction of about 50 vol. %.

The Triassic magmatism and its relation with the Pre-Andean tectonic evolution: Geochemical and petrographic constrains from the High Andes of north central Chile (29°30′ – 30°S)

The western border of South America has been an intermittently active convergent margin since the late Paleozoic, with the recognition of two major orogenic stages: Gondwanian (Early Carboniferous –Early Permian) and Andean (Early Jurassic–Recent), which differ in dominant tectonic regime, magmatism affinities and composition, and inferred crustal thickness. The time span between these stages, known as the Pre-Andean cycle, coincides with the period when the continental drift of Gondwana declined. Arrested subduction and the development of a continental rift along the southwestern margin of Gondwana has been suggested for this stage. However, new petrographic and geochemical data from the Upper Triassic (Carnian – Norian) magmatism exposed between 29°30′ – 30°S in the Chilean High Andes do not support this geotectonic scenario. The Upper Triassic rocks show features that are typical of a magmatism generated by the melting of an asthenospheric mantle triggered by fluids derived from dehydration of the subducted slab, including a broad and continuous compositional range, calc-alkaline differentiation trends, enrichment of LILE in relation to HFSE, and negative anomalies of Nb-Ta, and Ti. The less evolved isotopic composition of the Upper Triassic magmatism compared to the Paleozoic crust, suggests that the studied magmatism could not have been generated exclusively by crustal anatexis, as has been interpreted previously, and that requires a mixture with a more depleted source, hydrated and with a high LILE content. Subduction-related magmatism with similar characteristics is described for the Late Permian to Middle Triassic period in the studied area, which argues against the hypothesis of arrested subduction during the Pre-Andean cycle. Our data indicate that subduction was an active process at least in this segment of the southwestern margin of Gondwana during the Pre-Andean cycle.

Generation Conditions of Dacite and Rhyodacite via the Crystallization of an Andesitic Magma. Implications for the Plumbing System at Santorini (Greece) and the Origin of Tholeiitic or Calc-alkaline Differentiation Trends in Arc Magmas

Andesitic arc volcanism is the most common type of subduction-related magmatism on Earth. How these melts are generated and under which conditions they evolve towards silica-rich liquids is still a matter of discussion. We have performed crystallization experiments on a representative andesite sample from the Upper Scoriae 1 (USC-1) eruption of Santorini (Greece) with the aim of understanding such processes. Experiments were performed between 1000 and 900 °C, in the pressure range 100–400 MPa, at fO2 from QFM (quartz–fayalite–magnetite) to NNO (nickel–nickel oxide) + 1·5, with H2Omelt contents varying from saturation to nominally dry conditions. The results show that the USC-1 andesitic magma was generated at 1000 °C and 12–15 km depth (400 MPa), migrated to shallower levels (8 km; 200 MPa) and intruded into a partially crystallized dacitic magma body. The magma cooled to 975 °C and generated the phenocryst assemblage and compositional zonations that characterize the products of this eruption. An injection of basaltic magma problably subsequently triggered the eruption. In addition to providing the pre-eruptive conditions of the USC-1 magma, our experiments also shed light on the generation conditions of silica-rich magmas at Santorini. Experimental of runs performed at fO2 ∼ NNO + 1 (± 0·5) closely mimic the compositional evolution of magmas at Santorini whereas those at reduced conditions (QFM) do not. Glasses from runs at 1000–975 °C encompass the magma compositions of intermediate-dominated eruptions, whereas those at 950–900 °C reproduce the silicic-dominated eruptions. Altogether, the comparison between our experimental results and natural data for major recent eruptions from Santorini shows that different magma reservoirs, located at different levels, were involved during highly energetic events. Our results suggest that fractionation in deep reservoirs may give rise to magma series with a tholeiitic signature whereas at shallow levels calc-alkaline trends are produced.

Tschermak fractionation in calc-alkaline magmas: the Eocene Sabzevar volcanism (NE Iran)

Calc-alkaline arc magmatism at convergent plate margins is volumetrically dominated by metaluminous andesites. Many studies highlighted the importance of differentiation via fractionation processes of arc magmas, but only in the last decades, it has been demonstrated that not all rock-forming minerals may affect the evolution of calc-alkaline suites. In particular, a major role exerted by Al-rich hornblende amphibole as fractionating mineral phase has been documented in many volcanic arc settings. The aim of this work is to understand the role of the Tschermak molecule (CaAlAlSiO6) hosted in the hornblende and plagioclase fractionation assemblage in driving magma differentiation in calc-alkaline magmatic suites. We explore this issue by applying replenishment–fractional crystallization (RFC) and rare earth element–Rayleigh fractional crystallization (REE-FC) modeling to the Sabzevar Eocene (ca. 45–47 Ma) calc-alkaline volcanism of NE Central Iran, where hornblende-controlled fractionation has been demonstrated. Major element mass balance modeling indicates RFC dominated by a fractionating assemblage made of Hbl52.0–52.5 + Pl44.1–44.2 + Ttn3.3–3.9 (phases are expressed on total crystallized assemblage). REE-FC modeling shows, instead, a lower degree of fractionation with respect to RFC models that is interpreted as due to hornblende and plagioclase resorption by the residual melt. Calculations demonstrate that fractionation of the Tschermak molecule can readily produce dacite and rhyolite magmas starting from a calc-alkaline andesite source (FC = ca. 30 %). In particular, the Tschermak molecule controls both the heavy rare earth elements (HREE) and light rare earth element (LREE) budgets in calc-alkaline differentiation trends.

Generation of continental crust in the northern part of the Borborema Province, northeastern Brazil, from Archaean to Neoproterozoic

This work deals with the origin and evolution of the magmatic rocks in the area north of the Patos Lineament in the Borborema Province (BP). This northeastern segment of NE Brazil is composed of at least six different tectonic blocks with ages varying from late-Archaean to late-Palaeoproterozoic. Archaean rocks cover ca. 5% of the region. They were emplaced over a period of 700 Ma, with at least seven events of magma generation, at 3.41, 3.36, 3.25, 3.18, 3.12, 3.03, and 2.69 Ga. The rocks are subalkaline to slightly alkaline, with affinity to I- and M-type magmas; they follow trondhjemitic or potassium calc-alkaline differentiation trends. They have epsilon Nd(t) of +1.4 to −4.2 and negative anomalies for Ta–Nb, P and Ti, consistent with a convergent tectonic setting. Both subducted oceanic crust and upper mantle (depleted or metasomatised) served as sources of the magmas. After a time lapse of about 350 m y., large-scale emplacement of Paleoproterozoic units took place. These rocks cover about 50% of the region. Their geochemistry indicates juvenile magmatism with a minor contribution from crustal sources. These rocks also exhibit potassic calc-alkaline differentiation trends, again akin to I- and M-type magmas, and show negative anomalies for Ta–Nb, Ti and P. Depleted and metasomatised mantle, resulting from interaction with adakitic or trondhjemitic melts in a subduction zone setting, is interpreted to be the main source of the magmas, predominanting over crustal recycling. U–Pb ages indicate generation of plutonic rocks at 2.24–2.22 Ga (in some places at about 2.4–2.3 Ga) and 2.13–2.11 Ga, and andesitic volcanism at 2.15 Ga. Isotopic evidence indicates juvenile magmatism (epsilon Nd(t) of +2.9 to −2.9). After a time lapse of about 200 m y. a period of within-plate magmatic activity followed, with acidic volcanism (1.79 Ga) in Orós, granitic plutonism (1.74 Ga) in the Seridó region, anorthosites (1.70 Ga) and A-type granites (1.6 Ga) in the Transverse Zone (TZ). Early Neoproterozoic volcanism at 1091 Ma, and A-type plutonism, from 920 to 775 Ma, mark the intracontinental magmatism in the TZ. In the Seridó Domain, the Late Neoproterozoic registers several events of plutonism, at 600–593, 575–560, 548–533, 528–510, 495–450 Ma. These rocks cover ca. 15% of the area, while Neoproterozoic supracrustal rocks cover ca. 30%. The most important magmatic event is that at 575 Ma, consistent with the peak of widespread transpression and synchronous high temperature metamorphism. The Neoproterozoic rocks are mostly K-enriched alkaline or transitional to calc-alkaline. Inherited Archaean and Palaeoproterozoic zircons and Nd model ages, as well as moderate to strongly negative (−21 to −9) epsilon Nd, and persistent negative anomalies for Ta–Nb, Ti and P indicate significant crustal contributions in their genesis. While a convergent setting (subduction zone) could explain the Archaean and Palaeoproterozoic units, this is not so for the Neoproterozoic rocks which mimic the geochemical and isotopic features of the older sources. In the study area, the peak of juvenile accretion (mantle derived magmas) took place in the Archaean (3.4–2.7 Ga) and Palaeproterozoic (2.4–2.11 Ga), whereas crustal recycling predominated in the Neoproterozoic.

The Late Cretaceous igneous rocks of Romania (Apuseni Mountains and Banat): the possible role of amphibole versus plagioclase deep fractionation in two different crustal terranes

We provide new whole-rock major and trace elements as well as 87Sr/86Sr and 143Nd/144Nd isotopic data of a suite of samples collected in the Late Cretaceous volcanic and plutonic bodies of the Apuseni Mts. (Romania) that belong to the Banatitic Magmatic and Metallogenic Belt, also called the Apuseni–Banat–Timok–Srednogorie belt. The samples define a medium- to high-K calc-alkaline differentiation trend that can be predicted by a three-step fractional crystallization process which probably took place in upper crustal magma chambers. Published experimental data indicate that the parent magma (Mg# = 0.47) of the Apuseni Mts. trend could have been produced by the lower crustal differentiation of a primary (in equilibrium with a mantle source) magma. The Late Cretaceous magmatic rocks of the Apuseni Mts. and Banat display overlapping major and trace element trends except that Sr is slightly lower and Ga is higher in the Apuseni Mts. parent magma. This difference can be accounted for by fractionating plagioclase-bearing (Apuseni Mts.) or amphibole-bearing (Banat) cumulates during the lower crustal differentiation of the primary magma to the composition of the parent magma of both trends. This, together with results obtained on the Late Cretaceous igneous rocks from the Timok area in Eastern Serbia, further suggests variation of the water content of the primary magma along and across the belt. The Apuseni Mts. versus the Banat samples display different isotopic compositions that likely resulted from the assimilation of two distinct crustal contaminants, in agreement with their emplacement in two separate mega-units of Alpine Europe.

Shallow submarine volcano group in the early stage of island arc development: Geology and petrology of small islands south off Hahajima main island, the Ogasawara Islands

Small Islands south off Hahajima, the southernmost of the Ogasawara Archipelago, consist of primitive basalts (<12wt.% MgO) to dacite erupted during the transitional stage immediately following boninite volcanism on the incipient arc to sustained typical oceanic arc. Strombolian to Hawaiian fissure eruptions occurring on independent volcanic centers for the individual islands under a shallow sea produced magnesian basalt to dacite fall-out tephras, hyaloclastite and a small volume of pillow lava, which were intruded by NE-trending dikes. These volcanic strata are correlated to the upper part (<40Ma) of the Hahajima main island. Volcanic rock samples have slightly lower FeO*/MgO ratios than the present volcanic front lavas, and are divided into three types with high, medium and low La/Yb ratios. Basalt to dacite of high- and medium-La/Yb types show both tholeiitic (TH) and calc-alkaline (CA) differentiation trends. Low-La/Yb type belongs only to TH basalt. The multiple magma types are coexistence on the each island. TH basalts have phenocrysts of olivine, clinopyroxene and plagioclase, while CA basalts are free from plagioclase phenocrysts.

Internal architecture and Fe–Ti–V oxide ore genesis in a Variscan synorogenic layered mafic intrusion, the Beja Layered Gabbroic Sequence (Portugal)

The Beja Layered Gabbroic Sequence (LGS) intruded the SW border of the Ossa Morena Zone at ca. 350Ma during the early stages of the Variscan oblique continental collision in Iberia. It represents an exceptional case-study for the petrogenesis and metallogenesis of the rare class of synorogenic layered intrusions.LGS oxidised parental magmas (+1.7±0.5 ΔFMQ) evolved to relatively reduced ferro-basaltic magmas (±0.5 ΔFMQ) after extensive differentiation (SB I–SB II–ODV I Series: Fo88-54) and gave rise to ODV I Series oxide-rich ferro-gabbros and massive oxide ores, that represent a potential economic resource for Fe, Ti and V. Vanadium enrichment in ODV I melt peaked at the formation of basal ores and was followed by heterogeneous depletion at prevailing high Fe–Ti contents within other oxide-rich domains of the Series. Geological, geophysical and geochemical evidence suggest that density driven processes were fundamental to oxide accumulation and segregation within ODV I basal domains.The SB I–SB II–ODV I suite was followed by monotonous gabbroic Series (ODV II, III, BRG I, II) developed from repeated magma replenishments with similar initial composition (Fo78). The prevailing oxidising conditions (Fo76-66 ΔFMQ+.5 to+1.0) and the buffering effect of recurrent replenishments constrained the Fe–Ti enrichment within a typical calc-alkaline differentiation trend; decreasing recharge/crystallisation rates locally produced oxide accumulations at ≈+1Δ FMQ. The coupled ƒO2 decreasing with Fe-enrichment throughout LGS is consistent with crystallisation under conditions closed to oxygen exchange. It is inferred that the large amount of oxides that precipitated throughout ODV I Series required transient, episodic openings of the system favoured by the tectonically active character of the magma chamber.Modelling of melt compositions supports that LGS parental magmas were high-alumina, primitive basalts (#Mg0.63), which after some differentiation gave rise to SB II Series (#Mg0.59) preserved as a chilled margin. The SB I parental magmas may account for the monotonous Series compositional spectrum, whereas ODV I Series model melts may be derived as residual magmas from SB II Series. ODV I melts show strong Fe–Ti enrichment and Si depletion, consistent with a Fenner trend, wherein the formation of extreme differentiation products was precluded by the arrival of new magma batches related with the monotonous Series.

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K 2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization. The low abundances of trace elements, together with low K 2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO 2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.

Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California

Research Article| September 01, 2010 Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California Brandon Browne; Brandon Browne † 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA †E-mail: bbrowne@fullerton.edu Search for other works by this author on: GSW Google Scholar Marcus Bursik; Marcus Bursik 2Department of Geology, State University of New York, Buffalo, New York 14260, USA Search for other works by this author on: GSW Google Scholar Justin Deming; Justin Deming 2Department of Geology, State University of New York, Buffalo, New York 14260, USA Search for other works by this author on: GSW Google Scholar Michael Louros; Michael Louros 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA Search for other works by this author on: GSW Google Scholar Antonio Martos; Antonio Martos 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA Search for other works by this author on: GSW Google Scholar Scott Stine Scott Stine 3Department of Geography and Environmental Studies, California State University, Hayward, California 94542, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Brandon Browne † 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA Marcus Bursik 2Department of Geology, State University of New York, Buffalo, New York 14260, USA Justin Deming 2Department of Geology, State University of New York, Buffalo, New York 14260, USA Michael Louros 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA Antonio Martos 1Department of Geological Sciences, California State University, Fullerton, California 92834, USA Scott Stine 3Department of Geography and Environmental Studies, California State University, Hayward, California 94542, USA †E-mail: bbrowne@fullerton.edu Publisher: Geological Society of America Received: 06 Apr 2009 Revision Received: 30 Oct 2009 Accepted: 01 Nov 2009 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2010 Geological Society of America GSA Bulletin (2010) 122 (9-10): 1401–1422. https://doi.org/10.1130/B30070.1 Article history Received: 06 Apr 2009 Revision Received: 30 Oct 2009 Accepted: 01 Nov 2009 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brandon Browne, Marcus Bursik, Justin Deming, Michael Louros, Antonio Martos, Scott Stine; Eruption chronology and petrologic reconstruction of the ca. 8500 yr B.P. eruption of Red Cones, southern Inyo chain, California. GSA Bulletin 2010;; 122 (9-10): 1401–1422. doi: https://doi.org/10.1130/B30070.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Red Cones are a pair of basaltic cinder cones located 5 km SSW of Mammoth Mountain at the southern end of the Mono-Inyo volcanic chain, in eastern California. Charcoal recovered at two separate locations beneath the Red Cones scoria-fall deposits indicates that the eruption most likely occurred shortly after 8490 ± 90 14C yr B.P. and no later than 9325 ± 83 14C yr B.P., which implicates Red Cones as the most recent eruption of basalt in the Mono-Inyo volcanic chain. Results from geologic field mapping combined with geochemical and petrologic analysis suggest that the ca. 8500 yr B.P. eruption produced 10.1 × 106 m3 of magma, possibly beginning from south Red Cone and later from north Red Cone via Hawaiian, Strombolian, and violent Strombolian eruptions over a minimum of 28 days. All deposits contain plagioclase, olivine, clinopyroxene, chrome-spinel, and titanomagnetite. Material erupted from each cone can be classified as high-aluminum basalts that exhibit calc-alkaline differentiation trends and belong to the medium-K series. Red Cone basalt samples are generally similar in terms of many major and trace element concentrations, but south Red Cone samples typically contain more SiO2, Sr, Zr, Rb, and Ba, and less MgO, FeO, CaO, Ni, and Cr than north Red Cone samples. Clinopyroxene-liquid thermobarometry calculations indicate that the majority of Red Cones clinopyroxene crystal cores crystallized at temperatures of 1160–1210 °C and pressures equivalent to 10–25 km depth, which supports the possibility of a basaltic dike and sill plexus located 10–25 km beneath the west and southwest flanks of Mammoth Mountain. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harşit Pluton, Eastern Turkey

We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb) n = 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd isotopic data for all of the samples display I Sr = 0.70676–0.70708, ε Nd(79 Ma) = −4.4 to −3.3, with T DM = 1.09–1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79–18.87, (207Pb/204Pb) = 15.59–15.61 and (208Pb/204Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.

Geology and Lithostratigraphy of the Ultramafic-Mafic Rocks and Associated Mineralizations, Wadi Khamal Area, West-Central Arabian Shield

Field, petrography and major elements geochemistry of the ultramafic -mafic rocks of Wadi Khamal area led to the recognition of the following rock groups, namely: 1) ophiolitic suite, melagabbro- norite and cumulate gabbronorite to the south of Wadi Mahalah (southern ultramafic-mafic rocks) and 2) Wadi Khamal anorthosite- gabbro complex to the north. The ophiolitic rocks in the southern area include serpentinites, peridotites and pyroxenites. The serpentinites contain large blocks of amphibolites, schists, gneisses, and metagabbros. These rocks were in- truded by melagabbronorite and cumulate gabbronorite. On the AFM diagram, these rocks plot along the MgO-FeO* side in the tholeiitic field except those of the melagabbronorite which plot within the calc- alkaline field. The Wadi Khamal complex consists of four major units; mar- ginal gabbro unit, anorthosite unit, central gabbroic unit and northern gabbronorite unit. The marginal gabbro unit and anorthosite unit have a calc alkaline differentiation trend while those of the central gabbro unit (olivine gabbro, melagabbronorite and Fe-Ti oxides-apatite gab- bronorite) and the northern gabbronorite show a distinct different trend parallel to the FeO*-MgO side line and belong to the tholeiitic differentiation trend. Based on major element geochemistry, the southern ultramafic - mafic rocks were derived from a more mafic liquid source rather different from magma source(s) which formed the Khamal anorthosite-gabbro complex.

Geochemical and Sr–Nd–Pb isotopic compositions of the Eocene Dölek and Sariçiçek Plutons, Eastern Turkey: Implications for magma interaction in the genesis of high-K calc-alkaline granitoids in a post-collision extensional setting

The major and trace elements and Sr–Nd–Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at ∼ 43 Ma at shallow depths (∼ 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO 2 = 56.98–72.67 wt.%; Mg# = 36.8–50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO 2 = 53.21–60.94 wt.%; Mg# = 44.4–53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb) N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu ⁎ = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd–Sr isotopic compositions for the host rocks are ε Nd(43 Ma) = − 0.6 to 0.8 and mostly I Sr = 0.70482–0.70548. The Nd model ages ( T DM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are ( 206Pb/ 204Pb) = 18.60–18.65, ( 207Pb/ 204Pb) = 15.61–15.66 and ( 208Pb/ 204Pb) = 38.69–38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with I Sr ranging from 0.70485 to 0.70517, ε Nd(43 Ma) − 0.1 to 0.8 and with Pb isotopic ratios of ( 206Pb/ 204Pb) = 18.58–18.64, ( 207Pb/ 204Pb) = 15.60–15.66 and ( 208Pb/ 204Pb) = 38.64–38.77. The MMEs have T DM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr–Nd isotope data indicates that ∼ 77–83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime.

An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200 MPa in the temperature range 1150–950°C at oxygen fugacities corresponding to the quartz–fayalite–magnetite (QFM) and MnO–Mn3O4 buffers to assess the role of water and fO2 on phase relations and differentiation trends in mid-ocean ridge basalt (MORB) systems. Starting from a primitive (MgO 9·8 wt %) and an evolved MORB (MgO 6·49 wt %), crystallization paths with four different water contents (0·35–4·7 wt % H2O) have been investigated. In primitive MORB, olivine is the liquidus phase followed by plagioclase + clinopyroxene. Amphibole is present only at water-saturated conditions below 1000°C, but not all fluid-saturated runs contain amphibole. Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer). Residual liquids obtained at low fO2 show a tholeiitic differentiation trend. The crystallization of magnetite at high fO2 (MnO–Mn3O4 buffer) results in a decrease of melt FeO*/MgO ratio, causing a calc-alkaline differentiation trend. Because the magnetite crystallization temperature is nearly independent of the H2O content, in contrast to silicate minerals, the calc-alkaline differentiation trend is more pronounced at high water contents. Residual melts at 950°C in a primitive MORB system have compositions approaching those of oceanic plagiogranites in terms of SiO2 and K2O, but have Ca/Na ratios and FeO* contents that are too high compared with the natural rocks, implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites.

Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H2O contents. The experimental results are used to explore the influence of H2O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H2O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO2. We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H2O-rich (>4.5--6 wt% H2O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H2O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustal-level differentiation occurs under near-H2O saturated conditions producing the distinctive high SiO2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H2O contents (>10 wt% H2O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite--tonalite--granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.

Geochemistry, mineralogy and petrogenesis of the northeast Niğde volcanics, central Anatolia, Turkey

AbstractThe volcanics exposed in the northeast Niğde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end‐member compositions and a continuity of trace element behaviour exists through the basaltic andesite–andesite–dacite–rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high‐K calc‐alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Niğde magmas.The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Niğde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Niğde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Niğde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley &amp; Sons, Ltd.