Rayleigh Fractional Crystallization Research Articles

Geochronological and geochemical constraints on magmatic evolution and mineralization of the northeast Ke’eryin pluton and the newly discovered Jiada pegmatite-type lithium deposit, Western China

The Ke’eryin ore field in the Songpan-Garze Fold Belt (SGFB), western China, is an important area of pegmatite-type lithium accumulation. However, the petrogenesis of the pegmatites and their genetic relationships with the neighboring granitic pluton are still a topic of debate. In this work, LA-MC-ICP-MS zircon and columbite-tantalite U-Pb dating, whole-rock and mica geochemical analysis, and lithium isotopic analysis of the Ke’eryin pluton and pegmatites in the newly discovered Jiada deposit were performed. Zircon U-Pb dating of biotite K-feldspar granite (BG), fine-grained two-mica monzogranite (FTG), and medium-grained two-mica monzogranite (MTG) of the Ke’eryin composite pluton yielded crystallization ages of 219.5 ± 0.7 Ma, 212.8 ± 1.1 Ma, and 209.1 ± 0.7 Ma, respectively. Columbite-tantalite U-Pb dating of albite-spodumene type pegmatite (ASP) from the Jiada deposit yielded crystallization ages of 204.7 ± 1.0 Ma. The results show that the lithium mineralization in the Jiada deposit was temporally closest to the MTG emplacement. Combined with the regional magmatic evolution sequence, it is suggested that the Ke’eryin composite pluton and the Jiada deposit were emplaced during the period of transition from tectonic compression to extension under the effect of lithospheric delamination during post-collision. Geochemically, the compositional variation in the major elements of mica minerals indicate that BG, FTG, and MTG of the Ke’eryin composite pluton and the AP (albite type pegmatite) and ASP in the Jiada deposit were formed via the same continuous magmatic evolution process. The increasing Li, Rb, Cs, Nb, Ta concentrations and decreasing K/Rb, K/Cs, Nb/Ta ratios in mica minerals show that the degree of magmatic differentiation increased in the order BG-FTG-MTG-AP-ASP, indicating that MTG could be the parent rock of the Jiada pegmatites. The increase in the δ7Li values in muscovites from MTG through AP to ASP provides new evidence to support the fact that the fractional crystallization played a significant role in the formation of pegmatites in the Jiada deposit. Alkali metal elements (e.g., K, Rb, Cs) in the muscovites of MTG, AP, and ASP followed the Rayleigh fractionation law, indicating that a Rayleigh-type fractionation process occurred. Rayleigh fractional crystallization modelling of these alkali metal elements showed that the MTG could produce a magma melt of ASP through extreme fractional crystallization (∼98–99 % fractionation). Taken together with associated geological evidence, the aforementioned features indicate that the favorable tectonic setting and the prolonged and extreme fractional crystallization of the granitic melt jointly contributed to the formation of the Jiada pegmatite-type lithium deposit.

The geochemical differentiation of S-type pegmatites: constraints from major–trace element and Li–B isotopic composition of muscovite and tourmaline

Pegmatites often exhibit regional as well as internal zonation. In this study, we use major and trace element and Li–B isotopic composition of muscovite and tourmaline from internally zoned and un-zoned S-type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation. The internally zoned pegmatites comprise three distinct textural zones, namely wall, intermediate and core. Muscovite and tourmaline occur in all the zones of these pegmatites. The trace element concentrations/ratios of muscovite from the three zones form well-defined differentiation trends marked by enrichment of incompatible elements such as Rb, Cs, Sr, B, Zn, Nb, Ta, P and the depletion of Ni, Co, V, Sc, Ti, Ba from the wall zone, through the intermediate zone to the core zone. This is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites. Alkali element ratios such K/Rb and K/Cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of Rayleigh-type fractional crystallization. Fractional crystallization modelling reveals that the formation of the wall zone requires < 69% crystallization, the intermediate zone 85–95% and the core zone ca. 99% crystallization, leading to extreme enrichment of Rb and Cs and other incompatible elements. Muscovites and tourmaline from the un-zoned pegmatites display similar compositional trends as the internally zoned ones, but with a significant compositional gap between the host granite and the pegmatite. Lithium isotopic composition of muscovites and B-isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un-zoned pegmatites. This is suggestive of an important role of vapour exsolution in the formation of the pegmatites. Taken together, the geochemical and isotopic trends in the pegmatites can be explained by Rayleigh fractional crystallization operating in tandem with vapour exsolution.

Géochimie des muscovites comme indicateur du fractionnement des pegmatites de la région de Kabarore-Mparamirundi (nord-ouest du Burundi, Afrique centrale) [Geochemical signature of muscovites as pathfinder for fractionation of pegmatites in the Kabarore-Mparamirundi area (northwestern Burundi, Central Africa)

The Kabarore-Mparamirundi area hosts numerous pegmatites spatiotemporally related to leucogranites dated at 986 ± 10 Ma in Karagwe-Ankole belt. The deposits are intensively exploited for columbite-tantalite and cassiterite. Alkali metals in muscovite (Rb 370–7590 ppm, Cs 8–1470 ppm) are modeled by Rayleigh fractional crystallization from a parental leucogranitic composition (K 4.1 wt%, Rb 321 ppm and Cs 9 ppm). The power law declining behavior of the ratio K/Rb versus Cs indicates the Rayleigh fractional crystallization as the main process of differentiation of the various pegmatite facies. Moreover, the continuous trend from granite to the most evolved, exploited pegmatites demonstrates a co-genetic link among them. The fractionation model shows that unmined and abandoned pegmatites are less fractionated (less than 94% of fractionation) while mined pegmatites are highly fractionated and constitute fractionated products of more than 94% of the initial leucogranite composition. The Rb, Cs, Ta, Sn and Li elements in muscovite can be used as a valuable tool in the exploration of fertile and sterile pegmatites in this area.

Polybaric/polythermal magma transport and trace element partitioning recorded in single crystals: A case study of a zoned clinopyroxene from Mt. Etna

We present elemental maps and intra-crystal compositional profiles conducted on a representative clinopyroxene phenocryst from the 1974 eccentric lava flows at Mt. Etna volcano (Sicily, Italy). The eruption was fed by deep-seated and primitive magmas ascending through pathways bypassing the central volcanic conduits. These magmas show MgO and Cr contents higher (and REE + Y lower) than those characterizing younger and more evolved eruptions, albeit the bulk rock compositions of both primitive and more evolved products are invariably classified as trachybasalts in the TAS (total alkali vs. silica) diagram. Mafic recharge episodes are recorded by the complex textural features of the clinopyroxene, with a subrounded core enclosed within a concentrically zoned mantle. The core is enriched in Mg + Fe2++Na and depleted in Fe3++Ca relative to the mantle. The jadeite (Jd) component decreases from core to mantle and is counterbalanced by higher Ca-Tschermak (CaTs) contents, as the number of TAl cations in tetrahedral coordination increases. The Jd-rich core incorporates high proportions of rare earth elements and Y (REE + Y) and low concentrations of high field strength elements (HFSE) and transition elements (TE, such as Ni, Cr and Sc), whereas the opposite occurs for the CaTs-rich mantle. The decoupling of REE + Y and HFSE argues against simple changes in melt composition and indicates an additional mechanism driving trace element zonations. Thermobarometric calculations indicate that the early-formed Jd-rich core equilibrated with the host magma at mantle depths (750–950 MPa and 1190–1210 °C), whereas the later CaTs-rich mantle formed at shallower crustal levels (400–700 MPa and 1150–1180 °C) after magma recharge. Quantitative modeling of apparent cation partitioning between clinopyroxene and melt (Di) indicates that DHFSE increase from the Jd-rich core to the CaTs-rich mantle. In contrast, DREE+Y increase up to one order of magnitude at the Jd-rich core due to the enhanced stability of an Na0.5REE + Y0.5MgSiO6 end-member. We infer that compositional changes in clinopyroxene due to the different P-T conditions of the plumbing system may control the concentrations of REE + Y in residual melts derived after partial crystallization and differentiation of primitive magmas, such as those feeding the 1974 eccentric eruption. On this basis, we use DREE+Y measured across the core-mantle interface to constrain the geochemical evolution of recent 2000–2013 magmas at Mt. Etna volcano by Rayleigh fractional crystallization. Results indicate that magma dynamics proceed via a stepwise polybaric-polythermal process accounting for 1) crystallization of Jd-rich clinopyroxenes at high-P, high-T conditions, 2) upward migration of crystal-bearing magmas due to replenishment phenomena with input of fresh magmas and 3) crystallization of CaTs-rich clinopyroxene in low-P, low-T reservoirs. The resulting total amount (~40 vol%) of clinopyroxene fractionated agrees with geophysical data suggesting the presence of highly crystalline magmatic bodies at shallow to intermediate crustal levels below Mt. Etna.

Biotite in highly evolved granites from the Shimensi W–Cu–Mo polymetallic ore deposit, China: Insights into magma source and evolution

Highly evolved granites are an important type of granitic rock given their close relationship with W–Sn–Nb–Ta and rare-metal mineralization and unusual geochemistry. Compared with typical granites, highly evolved granites experience a more complex magmatic evolution, which is difficult to investigate with whole-rock analyses. In this paper, we present a comprehensive major and trace element (including halogen) dataset for biotites from highly evolved granites in the Shimensi ore deposit, located in northwestern Jiangxi province, South China, which is one of the largest W deposits in the world, to constrain its magma source and evolution.Biotites in the Shimensi granites have a magmatic origin. They are characterized by low Mg# [ = 100 × MgO/(MgO + FeOT), in mol] (29–43, mean = 34), high A/CNK [ = Al2O3/(CaO + Na2O + K2O), in mol] values (1.60–2.91, mean = 1.78), similar to biotite compositions in S-type granites worldwide, suggesting the parental magmas were of meta-sedimentary origin. Biotite trace element data reveal a differentiation trend from fine-grained granite to granite porphyry and porphyritic granite, as indicated by gradually decreasing K/Rb and Nb/Ta ratios and Ba and Ti contents, and increasing Rb, Cs, W, Nb, and Ta contents. Tungsten contents in biotite increase by two orders of magnitude from ∼0.1 ppm in fine-grained granite to ∼30 ppm in porphyritic granite due to progressive fractional crystallization. The log (fH2O/fHF) and log (fHCl/fHF) values inferred from the biotites increase with progressive fractional crystallization, indicating no obvious fluid exsolution before or during biotite crystallization. High degrees of fractional crystallization and late fluid exsolution in the Shimensi granites were favorable for W mineralization. Nb/Ta ratios of the biotites decreased from 24.3 to 4.30 with progressive fractional crystallization. This result, along with the Rayleigh fractional crystallization modeling results for whole-rock Nb and Ta contents, suggest that fractional crystallization of biotite is the major control on decreasing Nb/Ta ratios in the Shimensi granites.

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

The processes involved in the magmatic–hydrothermal transition in rare-element pegmatite crystallization are obscure, and the role of hydrothermal mechanisms in producing economic concentrations of rare elements such as tantalum remains contentious. To decipher the paragenetic information encoded in zoned minerals crystallized during the magmatic–hydrothermal transition, we applied SEM-EDS and LA-ICP-MS chemical mapping to muscovite- and columbite-group minerals (CGM) from a rare-element pegmatite of the albite-spodumene subtype from Aclare, southeast Ireland.We present a three-stage model for the magmatic–hydrothermal transition based on petrography, imaging and quantification of rare-element (Li, B, Rb, Nb, Sn, Cs, Ba, Ta, W, U) zoning, integrated with geochemical modeling and constraints from published literature. Stage I marks the end of purely magmatic crystallization from a peraluminous granitic melt. In stage II, polymerized silicic melt and depolymerized alkaline aqueous melt coexist as immiscible media, both of which influence muscovite and CGM crystallization. Stage II also marks the onset of phyllic alteration of primary mineral assemblages. Hydrothermal fluid release causes further resorption of primary minerals and eventual precipitation of fine-grained albite in stage III.Muscovite and CGM both exhibit trace-element zoning, while CGM also show major-element zoning. Petrographic relationships and geochemical markers such as Ta# (=Ta/[Ta + Nb]) of individual mineral zones reveal that both mineral species crystallized contemporaneously over all three stages. Furthermore, Rayleigh fractional crystallization of muscovite is efficient in fractionating Ta from Nb. Tantalum and Nb are additionally fractionated by halogen-rich aqueous media, which remobilize both elements, but redeposit preferentially Ta-enriched oxides. Columbite–tantalite mineralization is therefore both magmatic and hydrothermal. Albite associated with stage-III muscovite and CGM was likely precipitated from a hydrothermal fluid when pH changed due to hydrolysis of primary minerals. The complex zoning and resorption of minerals indicates that bulk analyses and conventional LA-ICP-MS spot ablation analyses of muscovite in rare element pegmatites may lead to erroneous modelling of element partitioning and fluid evolution.Combined petrographic and high-resolution geochemical analysis of two mineral species (which co-crystallize and incorporate the same elements of interest) is an effective tool to assess the complex processes of crystal–melt–fluid interaction. Our three-stage model may also be applicable to the still not well understood magmatic–hydrothermal transition from fertile granitic melts to formation of Sn-W veins and greisens or porphyry-type deposits.

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.

Polybaric Fractional Crystallization of High-alumina Basalt Parental Magmas in the Egersund–Ogna Massif-type Anorthosite (Rogaland, SW Norway) Constrained by Plagioclase and High-alumina Orthopyroxene Megacrysts

Bulk analyses of plagioclase and high-alumina orthopyroxene megacrysts (decimetreto metre-sized crystals with plagioclase lamellae) from the late Proterozoic Egersund^Ogna massif-type anorthosite (Rogaland anorthosite province, SWNorway) are used to constrain the parental magma compositions and differentiation processes within this 30 km diameter diapir. Spatial compositional variations show that two types of anorthosite occur: high-Sr (720^1090 ppm) andesine anorthosite (An48 4) in the centre of the intrusion and low-Sr (320^620 ppm) labradorite anorthosite (An59 6) in the margin. Two populations of orthopyroxene megacrysts are clearly discriminated by their Mn and Cr contents, but display similar ranges of Mg-number (55^79). Interpretation of trace element data and comparison with inversion models suggest that the two types of anorthosite result from crystallization of different parental magma compositions with high-alumina basalt affinities and similar Mg-numbers, but different wollastonite, Sr, Mn and Cr contents. A Rayleigh fractional crystallization model of the trace element concentrations vs MgO content in orthopyroxene constrains the cotectic orthopyroxene:plagioclase proportions to 0·33:0·67 in both magma types before the appearance of Fe^Ti oxide minerals on the liquidus at F 0·7 (where F is melt fraction). Polybaric crystallization is recorded by variable alumina contents in the orthopyroxene megacrysts (2·3^8·5 wt %), corresponding to crystallization pressures in the 3^13 kbar range as shown by experimental data. This implies that the high-alumina orthopyroxene megacrysts mainly crystallized en route during diapiric rise of the anorthositic mush. Modelling the plagioclase compositions with experimentally determined partition coefficients for Ca, K, Sr and Ba confirms that pressure variation during polybaric crystallization is the main controlling factor on compositional variations in the anorthosite pluton.

Petrogenesis and geodynamic evolution of the Late Neoproterozoic post-collisional felsic magmatism in NE Afyon area, western central Turkey

Abstract In western Turkey, Late Neoproterozoic basement rocks are represented by variably deformed metasedimentary and meta-igneous rocks within different tectonostratigraphical units that make up the Alpine Tauride–Anatolide Platform. In the Kütahya–Bolkar Dagı unit to the NE of Afyon this basement mainly includes garnet-bearing mica schists intruded by metamorphic granitic rocks with relict porphyritic textures. The youngest zircon ages obtained from the granitic rocks by the single zircon evaporation method are 542±5.0 Ma on average, which correlate with the Late Pan-African–Cadomian granitic magmatism. The granitic rocks are rhyodacitic or dacitic and peraluminous in composition, and display geochemical characteristics of I-type (tonalite–trondhjemite–granodiorite (TTG) source) felsic intrusive rocks. Trace and rare earth element patterns with distinct depletion in Rb, K, Nb, Sr, P and Ti relative to the other trace elements correlate very well with a Proterozoic TTG source. The petrogenetic modelling also implies that they were developed by partial melting of a TTG source by 20% fractional melting plus 20% Rayleigh fractional crystallization. The emplacement temperatures estimated by using zircon (790–820 °C), apatite and monazite saturation thermometry are about 827–1035 °C; these are in accordance with I-type rather than S-type granite melts. A geochemical comparison of the NE Afyon granitic rocks with the coeval quartz-porphyries in the Sandikli area of the Geyik Dag tectonic unit suggests that the latter may represent the more evolved felsic part of the Cadomian magmatism. Hence, both basement complexes are parts of the same Gondwanan terrane and represent the eastern continuation of the North African–Southern European terrane assemblage.

Partitioning of trace elements among coexisting crystals, melt, and supercritical fluid during isobaric crystallization and melting

The distribution of trace elements among coexisting crystals, melt, and supercritical fluid during melting and crystallization is a critical constraint for understanding the evolution of magmatic systems, including the origin and development of continental and oceanic crust. Although trace-element partitioning between crystals and melt during Rayleigh fractional crystallization or melting is well-known, partitioning among co-existing melt, crystals, and supercritical fluid during anatexis or crystallization is less explored despite the ubiquity of magmatic fluids. Here we develop the trace-element differential equations governing solid-melt-fluid equilibria for melting and crystallization under fluid-present conditions and provide analytical solutions for fractional and equilibrium crystallization and melting. A compilation of solid-fluid and melt-fluid distribution coefficients for about 30 trace elements in olivine, clinopyroxene, garnet, plagioclase, alkali feldspar, biotite, amphibole, apatite, and silicic melts is provided. Forward modeling demonstrates the conditions under which fluid-melt-solid partitioning will impact trace-element signatures in magmatic systems. We show that for trace elements soluble in aqueous fluids, the composition of a melt derived by fluid-present fractional crystallization or by fluid-present fractional melting will be significantly different than in otherwise comparable fluid-absent systems. Ignoring the partitioning of soluble elements into the fluid phase leads to large errors in concentrations (over 100%) and ratios and consequent misinterpretation of the trace-element character of source material and/or the processes of fractional crystallization and melting. Although significant in any setting involving fluid-present equilibria, this analysis may have a most profound influence on fluid-present subduction zone magma generation and the evolution of shallow level fluid-saturated silicic magmatic systems.

Trace element zoning in pyroxenes from ODP Hole 735B gabbros: diffusive exchange or synkinematic crystal fractionation?

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.

Petrogenesis and tectonic setting of Cadomian felsic igneous rocks, Sandıklı area of the western Taurides, Turkey

In the Sandikli (Afyon) region, western Taurides, the Late Proterozoic rocks of the Sandikli basement complex are composed of low-grade meta-sedimentary rocks (Guvercinoluk Formation) intruded by felsic rocks (Kestel Cayi Porphyroid Suite, KCPS). The KCPS is a deformed and highly sheared, dome-shaped rhyolitic body with a granitic core. Quartz porphyry dikes intrude both the slightly metamorphic igneous and the sedimentary rocks of the basement complex. Both the quartz porphyries and rhyolites were converted into mylonites with relict igneous textures. Geochemical data show that these felsic igneous rocks are subalkaline and mainly granitic in composition with SiO2 >72 wt% and Al2O3 >11.5 wt%. The chondrite-normalized incompatible trace element patterns are characterized by distinct negative Rb, Nb, Sr, P, Ti, and Eu with enrichment in Th, U, La, Ce, Nd, Sm, and Zr. The REE patterns of the felsic rocks indicate a strong enrichment in LREE but display slightly flat HREE patterns. According to geochemical characteristics and petrogenetic modeling, extrusive and intrusive rocks of the KCPS were probably derived from an upper continental crustal source (partial melting of granites/felsic rocks) by 18–20% fractional melting plus 18–20% Rayleigh fractional crystallization, which seems to be the most effective igneous process during the crystallization of the KCPS. Single zircon age data from the granitoids and fossils from the disconformably overlying sedimentary successions indicate that the metamorphism and the igneous event in the Taurides are related to the Cadomian orogeny. Based on the geological, geochemical and petrogenetic correlation of the post-collisional granitoids it is further suggested that the Tauride belt in western central Turkey was in a similar tectonic setting to the Gondwanan terranes in North Africa (Younger Granitoids) and southern Europe (Spain, France, Bohemia, Brno Massifs) during the Late Cadomian period.

Processes and timescales in the evolution of a chemically zoned trachyte: Fogo A, Sao Miguel, Azores

U-series disequilibria analyses have been combined with chemical and petrographic analyses in order to assess both the timescales and processes involved in the formation of the chemically zoned Fogo A trachytes. Least squares major element modelling demonstrates that the mafic trachytes could have evolved from a parental alkali basalt via trachybasalt with ∼70% fractionation of augite (35–36%), plagioclase (23%), magnetite (16%), kaersutite (15%), olivine (8%) and apatite (2–3%). Derivation of the mafic trachytes from a basanite parent is inconsistent with calculated fractionation paths. Major and trace element variations in 25 pumice samples collected from throughout the stratigraphic extent of the Fogo A deposit show that the trachytes represent the inverted, extrusive equivalent of a strongly chemically zoned magma chamber. The zonation is attributed to 70–75% Rayleigh fractional crystallization of the observed phenocryst phases. Wallrock assimilation and magma mixing did not contribute significantly to the observed chemical trends. The maximum age of the Fogo A trachytic magma based on radioactive disequilibria between 230Th and 238U is 300000 years. However, a calculated model age suggests that the time of evolution of the Fogo A trachytes from a parent alkali basalt is only 90000 years. Constant element variations and Th-isotopic ratios in Fogo C, Fogo A and 1563 A.D. trachytes suggest that a single long-lived trachytic magma chamber has been the source of at least the past 15.2 Ka of trachytic volcanism from Agua de Pao. After each eruption an evolved cupola reformed and became zoned prior to the next eruption. The maximum time necessary to form the zonation is 4600 years, the time between the Fogo A and 1563 A.D. eruptions. Low (226Ra)/(230Th)i ratios in the Fogo A and 1563 A.D. trachytes suggest that alkali feldspar fractionation continued up to the time of the respective eruptions.

Modelling the petrogenesis of high Rb/Sr silicic magmas

Rhyolites can be highly evolved with Sr contents as low as 0.1 ppm and Rb Sr > 2,000 . In contrast, granite batholiths are commonly comprised of rocks with Rb Sr < 10 and only rarely > 100. Mass-balance modelling of source compositions, differentiation and contamination using the trace-element geochemistry of granites are therefore commonly in error because of the failure to account for evolved differentiates that may have been erupted from the system. Rhyolitic magmas with very low Sr concentrations (⩽1 ppm) cannot be explained by any partial melting models involving typical crustal source compositions. The only plausible mechanism for the production of such rhyolites is Rayleigh fractional crystallization involving substantial volumes of cumulates. A variety of methods for modelling the differentiation of magmas with extremely high Rb/Sr is discussed. In each case it is concluded that the bulk partition coefficients for Sr have to be large. In the simplest models, the bulk D Sr of the most evolved types is modelled as > 50. Evidence from phenocryst/glass/whole-rock concentrations supports high Sr partition coefficients in feldspars from high silica rhyolites. However, the low modal abundance of plagioclase commonly observed in such rocks is difficult to reconcile with such simple fractionation models of the observed trace-element trends. In certain cases, this may be because the apparent trace-element trend defined by the suite of cognetic rhyolites is the product of different batches of magma with separate differentiation histories accumulating in the magma chamber roof zone.

Structural setting and magmatic evolution of Montefiascone volcanic complex, vulsini district, central Italy

Abstract A comparative study concerning volcanological, petrographic and geochemical aspects of Montefiascone Volcanic Complex has been carried out. This complex, together with those of Paleo-Bolsena, Bolsena and Latera, forms the Vulsini Volcanic District, located in the northernmost part of Roman Comagmatic Province. Four groups of lavas, belonging to Montefiascone Volcanic Complex, have been recognized on petrographic and geochemical grounds: Tephritic Leucitites (TEL), Leucite-Bearing Basanites (LBSN), Evolved Tephritic Leucitites (ETEL) and Leucititic Tephrites (LT). Lavas clearly show Ti and Nb anomalies, further supporting the genesis of these magmas from a subduction-related metasomatized mantle source. The majority of the lava erupted, comparable in volume with the pyroclastic products, appears to be of leucititic affinity, having suffered only a slight amount of differentiation. Major- and trace-element compositions very similar to those of ETEL and LT have been obtained from TEL and LBSN by means of simple mass balance and Rayleigh fractional crystallization model. According to petrographic observations, main liquidus phases are clinopyroxene, leucite, olivine and magnetite, and magmatic differentiation acts essentially by fractional crystallization. Eruptive activity is spread over a large area and the volcanic centres are aligned following NNW-SSE, WNW-ESE and E-W directions. Explosive activity, on the other hand, was mainly located at Montefiascone Caldera. The great abundance of nearly primary magmas in the area leads us to think that the structural setting was not suitable for developing a great, unique magma reservoir, but rather a series of small, elongated bodies.

Trace element geochemistry of Hannuoba ultramafic inclusion-bearing alkali basalts

Presented in this paper are the trace element abundances of 16 samples of Hannuoba ultramafic inclusion-bearing alkali basalts, which were determined by instrumental neutron activation analysis and X-ray fluorescence spectrometry. The petrogenesis of the alkali basalt suite has been modeled by batch partial melting and Rayleigh fractional crystallization processes. The geochemical characteristics of the mantle source from where alkali basalts were derived are described in terms of variations in trace element abundances of the alkali basalt suite.

Rectilinear partition coefficient function of REE pertaining to the formation of basaltic rocks from the New Georgia Group, the Solomon Islands.

Nine basaltic rocks from the New Georgia Group, the Solomon Islands, have been analyzed for REE, Ba and Sr abundances, Sr and Nd isotopic ratios, and major element compositions. Nd and Sr isotopic ratios of four olivine basalts and two picrite basalts fall in restricted ranges, suggesting that these rocks have a common source. REE patterns of six of these basaltic rocks have the following features: (1) The patterns have no intersection with each other, (2) All of the patterns possess a common fine structure of convex curve around Nd-Gd-Dy, and (3) Inclination of light REE span increases with increase of REE concentration. REE pattern analysis of the basalts leads to inferences that (A) a linear REE partition coefficient function with a sharp break at Ho has operated in the generation of the olivine basalts by the Rayleigh fractional crystallization of a picrite basalt magma, and (B) the absolute magnitudes of the partition coefficients did not change substantially during the magmatic differentiation.

Geochemistry and origin of Archean felsic metavolcanic rocks, central Noranda area, Quebec, Canada

Felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled Archean andesite-to-rhyolite lava flows of both tholeiitic and calc-alkalic affinity from the upper Noranda Subgroup, Quebec, using instrumental neutron activation and X-ray fluorescence techniques. The lavas have moderate values of [La/Yb]N (0.9–3.8) and low values of 100 × Th/Zr (~1). According to calculations following batch partial melting and Rayleigh fractional crystallization models, both the calc-alkalic and tholeiitic felsic volcanic rocks are probably products of shallow-level fractional crystallization of mafic parental magmas formed respectively by lower (~7 % for calc-alkalic) and higher (~14% for tholeiitic) degrees of partial melting of a primitive mantle source.Contribution to the magma genesis from plausible crustal materials was negligible. A back-arc-type diapirism is geochemically suggested for the tectonic model of origin of Noranda felsic magmas, in conformity with geological observations. Felsic volcanic rocks with compositions analogous to the studied samples exist in several other Archean terrains of the Canadian Shield, suggesting thereby that the late Archean sialic crust was at least in part produced by volcanic rocks ultimately derived from the primitive mantle.

Evolution of a high-level, high-silica magma chamber: the Pattison pluton, Nisling Range alaskites, Yukon

The Pattison pluton is a high-level, high-silica Tertiary alaskite intruding rocks of the Yukon Crystalline Terrane. The pluton is roughly circular in form with a diameter of about 18 km. The intrusion is cut by a series of shallowly dipping aplite dikes. Mineralogically, the alaskite body and the aplite dikes are very similar, but the pluton is divided into a series of phases based on texture and "stratigraphic" position: a fine-grained upper border phase, a medium-grained, graphic, strongly miarolitic, porphyritic phase, and a lower, coarser grained phase, contacts being gradational.No strong major element trends are found within the pluton (including the aplite dikes) because of eutectic crystallization. The trace elements, particularly Ba, Sr, Rb, Zr, and the REE, however, do show a liquid line of descent, with the upper border phase as the least differentiated and the aplite dikes as the most differentiated phases. Rayleigh fractional crystallization of quartz, K-feldspar, and plagioclase (and accessory allanite) in eutectic proportions readily accounts for the observed trends.The parental magma was water undersaturated (&lt; 1.2% H2O). Crystallization of anhydrous phases led to water-saturated conditions late in the pluton's history. Volatiles migrated to and concentrated in the upper parts of the chamber where the melting temperature was depressed, causing the pluton to crystallize from the bottom up. The volatile pressure eventually exceeded the confining pressure, resulting in failure of the surrounding rocks, retrograde boiling, vertical extension, and lateral injection of residual magma caused by filter pressing to form the aplite dikes. The escaping volatile phase resulted in quartz veining and Mo mineralization.