Some Thermal Constraints on Crustal Assimilation during Fractionation of Hydrous, Mantle-derived Magmas with Examples from Central Alpine Batholiths

We provide a model for the fractional crystallization of hydrousmantle-derived magma to form calc-alkaline plutons, based uponmass balance for geological examples of fractionation sequencesin the lower continental crust. This is complemented by a thermalmodel for the heat budget obtained from a projected phase diagramand thermodynamic data. Fractional crystallization (FC) andassimilation–fractional crystallization (AFC) paths havebeen calculated with these models and the mass ratio of assimilationto crystallization as a function of parent magma type and temperature,crustal rock fertility and temperature, and mechanism of assimilation,have been determined. When these results are combined with F(melt fraction) and r (ratio of mass assimilated/crystallized)values evaluated from geochemical data then new information,not available with the methods separately, can be deduced. Thisincludes when and at what depth and temperature in the crustthe assimilation took place, as well as the likely parent magmatype and temperature of the assimilant. Our results are presentedin simple graphical fashion to facilitate future studies thatexamine the evolution of individual calc-alkaline plutons andthe mechanisms of crustal contamination, and to improve meltmodels involving hydrous magma in volcanic arcs and in the lowercontinental crust KEY WORDS: assimilation; hydrous mantle magma; thermal models; fractional crystallization; magma mixing; Alpine batholiths; Adamello; Bergell     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

Decompression and H<Subscript>2</Subscript>O exsolution driven crystallization and fractionation: development of a new model for low-pressure fractional crystallization in calc-alkaline magmatic systems

Magma ascent, decompression-induced H₂O exsolution and crystallization is now recognized as an important process in hydrous subduction zone magmas. During the course of such a process calculations suggest that the ascent rate of a degassing and crystallizing mafic magma will be greater than crystal settling velocities. Thus, any crystals formed as a consequence of volatile exsolution will remain suspended in the magma. If the magma erupts before the percentage of suspended crystals reaches the critical crystallinity value for mafic magma (~55 vol.%) it will produce the commonly observed crystal rich island arc basalt lava. If the magma reaches its critical crystallinity before it erupts then it will stall within the crust. Extension of compaction experiments on a 55 vol.% sand-Karo syrup suspension at different temperatures (and liquid viscosities) to the likely viscosities of interstitial andesitic to dacitic liquid within such a stalled magma suggest that small amounts (up to ~10%) can be expelled on a time scale of 1–10 years. The expelled liquid can create a new intermediate to silicic body of magma that is related to the original mafic magma via fractional crystallization. The short time scale for liquid expulsion indicate that decompression-induced H₂O exsolution and crystallization can be an important mechanism for fractional crystallization. Based on this assumption a general model of decompression-induced crystallization and fractionation is proposed that explains many of the compositional, mineralogical and textural features of Aleutian (and other andesites).     

Major,trace element and Sr isotopic composition of lavas from Vico volcano (Central Italy) and their evolution in an open system

Major, trace element and Sr isotopic compositions have been determined on 21 lava samples from Vico volcano, Roman Province, Central Italy. The rocks investigated range from leucite tephritic phonolites to leucite phonolites and trachytes. Trace element compositions are characterized by high enrichments of incompatible elements which display strong variations in rocks with a similar degree of evolution. Well-defined linear trends are observed between pairs of incompatible trace elements such as Th-Ta, Th-La, Th-Hf. A decrease of Large Ion Lithophile (LIL) elements abundance contemporaneously with the formation of a large central caldera is one of the most prominent characteristics of trace element distribution. Sr isotope ratios range from 0.71147 to 0.71037 in the pre-caldera lavas and decreases to values of 0.70974–0.70910 in the lavas erupted after the caldera collapse. Theoretical modelling of geochemical and Sr isotopic variations indicates that, while fractional crystallization was an important evolutionary process, AFC and mixing also played key roles during the evolution of Vico volcano. AFC appears to have dominated during the early stages of the volcanic history when evolved trachytes with the highest Sr isotope ratios were erupted. Mixing processes are particularly evident in volcanites emplaced during the late stages of Vico evolution. According to the model proposed, the evolution of potassic magmas emplaced in a shallow-level reservoir was dominated by crystal fractionation plus wall rock assimilation and mixing with ascending fresh mafic magma. This process generated a range of geochemical and isotopic compositions in the mafic magmas which evolved by both AFC and simple crystal liquid fractionation, producing evolved trachytes and phonolites with variable trace element and Sr isotopic compositions.     

Geochemical effects of decoupled fractional crystallization and crustal assimilation

Most models of crustal assimilation assume that the amount of assimilant added to the magma is proportional to each infinitesimally small amount of solid removed during crystallization (AFC). In some magmatic systems, however, assimilation and crystallization are not strictly related and the mass assimilated is decoupled from, and therefore varies independently of, the mass crystallized (FCA). The geochemical consequences of FCA are examined and compared to those of AFC. The behavior of incompatible elements is identical during AFC and FCA, and ratios of these elements do not allow discrimination between the two processes. Major-oxide least-squares mass-balance models do not discriminate between AFC and FCA at F ≥ 0.7 (F = fraction of melt remaining). However, FCA yields magmas richer in compatible elements and with higher Sr-isotopic ratios than AFC at a given value of F. Repeated cycles of FCA and AFC combined with magma mixing (FAM) may result in unusual geochemical trends, such as the evolution of a calc-alkaline basaltic parent to a tholeiitic daughter magma, or the evolution of low- and medium-K calc-alkaline basalts to high-K andesites, dacites, trachyandesites or trachydacites. Lavas erupted by the volcano Micro Profitis Ilias on Santorini, Hellenic arc, Greece, provide an example of magmas which evolved by combined fractionation, assimilation by FCA and mixing.     

Assimilation and fractionation in adjacent parts of the same magme chamber: Vandfaldsdalen macrodike,East Greenland

The Vandfaldsdalen macrodike, which lies in the Skaergaard region of East Greenland, is a remarkably zoned fossil magma chamber, with a granophyric cap overlying cumulate gabboros. The intrusion is distinctly bimodal, with a large compositional discontinuity at the contact between the gabbro and granophyre. Although the exposed part of the macrodike is in contact with Tertiary basalts and sediments, the granophyre originated by assimilation of xenoliths derived from the underlying Archean basement. Sr and Nd isotopic ratios throughout the cumulate sequence are remarkably similar, indicating insignificant contamination of the gabbro by the granophyre. Modelling of the compositional effects of cooling and crystallization indicate that the cumulate pile resulted from fractional crystallization, with the complicating effects of trapped liquid and post-cumulus fractionation. The uppermost rocks in the mafic part, of the chamber (SiO₂=62%; FeO^*=12.4%) resulted from about 85% fractional crystallization. A transgressive sill of strongly fractionated magma (SiO₂=67%; FeO^*=8.8%) formed from extracted intercumulus liquid that was the result of 90% fractional crystallization of the original magma. Mass-balance indicates that typical granophyre is made up of about 75% dissolved xenoliths, by weight, and 25% mantle-derived basaltic magma. The magmas were not measurably affected by material exchange across the interface between the gabbro and granophyre. This magma chamber evolved by both assimilation and fractional crystallization, but the residual liquids formed by fractional crystallization were unaffected by assimilation. Heat exchange between were unaffected by assimilation. Heat exchange between the two parts of the chamber was obviously important, but there was insignificant material exchange. The inability of fractional crystallization and assimilation to affect the same liquid is related to the dynamic behavior of this particular magma chamber, particularly the buoyancy of granophyre relative to evolving tholeiitic magma.     

Continental back-arc basin origin of some ophiolites from the Eastern Desert of Egypt

Summary Geochemical and petrographical data of three ophiolitic pillow metavolcanic occurrences from the central Eastern Desert of Egypt are presented. The investigated rocks show a subalkaline, tholeiitic affinity. Chemical data indicate that the metavolcanics have transitional within-plate basalt to island-arc basalt features, which are characteristics of basalts formed in ensialic back-arc basins. The association of the investigated ophiolites with volcanoclastic metasedimentary rocks of marine to continental facies is a further confirmation of their ensialic evolution. This suggestion, along with the geochronologic, isotopic and crustal growth rate evidences, revives interest in models that involve contribution from a pre-Pan-African continental crust at least in the southern part of the Egyptian Shield. Mixing between a depleted mantle-derived magma and an enriched crustal melt, a process similar to AFC (assimilation and fractional crystallization), is suggested for the evolution of the investigated rocks. This study provides evidence for formation of some ophiolites in the Eastern Desert of Egypt in continental (ensialic) back arc basins.     

A quantitative approach to trace element and Sr isotope evolution in the Adamello batholith (northern Italy)

A development of De Paolo's mathematical procedure (1981) for magmatic AFC (Assimilation-Fractional Crystallization) processes is discussed with respect to both trace element and Sr isotopic ratio behaviours during the genesis and evolution of Adamello batholith (northern Italy). Resolution of a two equation-system (one relative to ⁸⁷Sr/⁸⁶Sr ratio variation in a magma generated by an AFC process, the other to its trace element content variations) gives the F (mass of magma at time t/mass of initial magma) and D (bulk partition coefficient) values, by which one can deduce the r (rate of assimilation/rate of crystallization) value during each step of magmatic evolution. This quantitative approach suggests that: 1) there was a common precursor magma for all the Adamello granitoids, with a Mg-rich tholeiitic composition; 2) each intrusive unit appears to have been generated by different extents of AFC; 3) the trace element distribution in the magma seems essentially influenced by mineral fractionation, rather than by the composition of the assimilated crustal material.     

Petrogenesis of gabbro and orthopyroxene gabbro from the Phenai Mata Igneous Complex,Deccan volcanic province: Products of concurrent assimilation and fractional crystallization

We report the occurrence of orthopyroxene gabbro from the Phenai Mata Igneous Complex (containing thoeliitic and alkaline rocks) that occur within Deccan Traps. The P-T calculations based on two pyroxene thermometry vary from 8.5±1.0 kbar and 963±39 °C. These gabbroic rocks exhibit high Mg# (0.67 to 0.71). But their primary magma signature can be negated due to their high SiO₂ (> 50 wt %), low Ni (32–35 ppm) and Cr (105–182 ppm) contents. Further, simple fractional crystallization was not responsible for the modification of the magma. Modeling carried out using trace element concentrations revealed that concurrent assimilation and fractional crystallization (AFC) was responsible for the genesis of these rocks. Small pods of magma could have accumulated in the crustal portions and concurrent assimilation and fractional crystallization have taken place in the generation of gabbro and orthopyroxene gabbro in the present study area.     

Composition gaps,critical crystallinity,and fractional crystallization in orogenic (calc-alkaline) magmatic systems

The recognition of a three-way correlation between magmatic SiO₂ content, critical crystallinity, and the size (magnitude) of crystal fractionation-generated composition gaps in calc-alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps. To explain this correlation, it is proposed that fractionation-generated composition gaps are caused by: (1) simultaneous interior (i.e. non-substrate) crystallization and vigorous chamberwide convection which leads to progessive crystal suspension; (2) cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma, and; (3) eventual buoyancy-driven crystal-liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap. This mechanism implies that many, if not most magma bodies are characterized by interior crystallization and vigorous convection, conditions which are not universally agreed upon at present. Given that such conditions characterize natural magma bodies, fractional crystallization through crystal settling in low-velocity boundary layers should be an important mechanism of fractional crystallization. In a crystallizing and convecting body of magma, composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention. As a given volcanic plumbing system matures with time, average settling/retention ratios within individual magma bodies should increase due to higher average wall-rock temperatures. It follows that, within a given volcanic center, early-stage volcanism should be more likely to display fractionation-generated composition gaps than later-stage volcanism. Such a temporal evolution has been documented at at least two Aleutian calc-alkaline volcanic centers.     

甘肃北山黑山岩浆铜镍硫化物矿床橄榄石特征及成因意义

黑山铜镍硫化物矿床是近年在甘肃北山发现的大型岩浆铜镍硫化物矿床,含矿岩体主要由含矿橄榄岩相和南部边缘的角闪辉长岩相构成。研究发现含矿岩体中的橄榄石属贵橄榄石(Fo值为81.54~86.87),其w(Ni)介于(801.53~2 703.19)×10-6;利用橄榄辉长岩中最高Fo值和主量元素反演,表明原始岩浆属高镁玄武质岩浆,w(MgO)=11.65%,w(FeO)=10.12%;橄榄石分离结晶模拟计算结果表明,橄榄石结晶过程中伴随有0.12%~0.17%硫化物熔离,深部岩浆房中橄榄石分离结晶程度小于3%,橄榄石与硫化物最小质量比约14∶1;隙间硅酸盐熔浆和硫化物熔浆作用明显,是造成早期结晶橄榄石成分变化的重要原因。     

A trace-element geochemical model for imperfect fractional crystallization associated with the development of crystal zoning

Although many petrological studies of volcanic rocks have suggested that crystallization proceeds within magma bodies, highly compatible trace elements do not display the marked variations and extreme depletions predicted to result from perfect fractional crystallization. Imperfect crystal-liquid separation is a key process in explaining this paradox. The presence of suspended crystals greatly affects variations in highly compatible elements, and has been quantitatively modeled by assuming perfect equilibrium between the suspended crystals and the liquid (equilibrium crystallization and imperfect separation; ECIS); however, volcanic rocks generally contain zoned phenocrysts that reflect the absence of solid-state equilibration. The present study develops a mass-balance model for zoned crystallization and imperfect separation (ZCIS). The ZCIS process is more efficient than the conventional ECIS process in generating depleted compatible elements. These two end-member models are able to explain the compositional range of igneous rocks that experienced imperfect fractional crystallization under natural conditions. The predicted compositional regions in bivariate trace-element diagrams successfully account for the sizes and shapes of the regions defined by whole-rock and melt-inclusion data from the Bishop Tuff, CA, USA.     

Assimilation of granite by basaltic magma at Burnt Lava flow,Medicine Lake volcano,northern California: Decoupling of heat and mass transfer

At Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material. Burnt Lava contains inclusions of quenched HAB liquid, a potential parent magma of the andesite, highly melted granitic crustal xenoliths, and xenocryst assemblages which provide a record of the fractional crystallization and crustal assimilation process. Samples of granitic crustal material occur as xenoliths in other Holocene and Pleistocene lavas, and these xenoliths are used to constrain geochemical models of the assimilation process.A large amount of assimilation accompanied fractional crystallization to produce the contaminated Burnt lava andesites. Models which assume that assimilation and fractionation occurred simultaneously estimate the ratio of assimilation to fractional crystallization (R) to be >1 and best fits to all geochemical data are at an R value of 1.35 at F=0.68. Petrologic evidence, however, indicates that the assimilation process did not involve continuous addition of granitic crust as fractionation occurred. Instead, heat and mass transfer were separated in space and time. During the assimilation process, HAB magma underwent large amounts of fractional crystallization which was not accompanied by significant amounts of assimilation. This fractionation process supplied heat to melt granitic crust. The models proposed to explain the contamination process involve fractionation, replenishment by parental HAB, and mixing of evolved and parental magmas with melted granitic crust.     

Petro Gram: An excel-based petrology program for modeling of magmatic processes

Petro Gram is an Excel?based magmatic petrology program that generates numerical and graphical models.Petro Gram can model the magmatic processes such as melting,crystallization,assimilation and magma mixing based on the trace element and isotopic data.The program can produce both inverse and forward geochemical models for melting processes(e.g.forward model for batch,fractional and dynamic melting,and inverse model for batch and dynamic melting).However,the program uses a forward modeling approach for magma differentiation processes such as crystallization(EC:Equilibruim Crystallization,FC:Fractional Crystallization,IFC:Imperfect Fractional Crystallization and In-situ Crystallization),assimilation(AFC:Assimilation Fractional Crystallization,Decoupled FC-A:Decoupled Fractional Crystallization and Assimillation,A-IFC:Assimilation and Imperfect Fractional Crystallization)and magma mixing.One of the most important advantages of the program is that the melt composition obtained from any partial melting model can be used as a starting composition of the crystallization,assimilation and magma mixing.In addition,Petro Gram is able to carry out the classification,tectonic setting,multi-element(spider)and isotope correlation diagrams,and basic calculations including Mg^#,Eu/Eu^*,εSrandεNdwidely used in magmatic petrology.     

Quantification of Crustal Contamination in Open Magmatic Systems

The DePaolo (1981a, Earth Planet. Sci. Lett. 53, 189–202)equations for assimilation with fractional crystallization (AFC)are extended and solved to yield directly the fraction of crustassimilated during AFC with or without magma replenishment orrecharge. Errors in the computed crust/magma ratio are alsopresented. Using graphical methods and a sufficient number ofelements no assumptions need be made about the value of theratios r (= rate of assimilation/rate of fractional crystallization)and ß (= rate of recharge/rate of assimilation); infact, the method yields independent estimates of these ratiosin addition to the crust/magma ratio. Thus the average relativesizes of the fluxes of wall-rock melt, replenishing magma, andfractionating phases are known and can be incorporated in abudget for the whole magma-crust system. The method for calculatingthe crust/magma ratio is extended to encompass variable valuesof r, ß, and bulk distribution coefficient (D). Simple(bulk) mixing is effectively a special case of AFC where ßor r and/or D = 0. Only for highly incompatible elements (D<01)do bulk mixing calculations approximate the true crust/magmaratio if AFC processes operated. For more compatible elements(e.g., oxygen) bulk mixing calculations considerably overestimatethe crust/magma ratio if r and ß are constant. Thecomplications of compositional stratification and diffusionin magma chambers are considered qualitatively. Two examples illustrate the application of the methods developedhere. For the upper zone of the Lille Kufjord instrusion innorthern Norway, an example of mid-crustal AFC without magmarecharge, either r = 0.04 and the crust/magma ratio is 0.04,or r = 0.08 and the crust/magma ratio is 0.06. For lavas ofthe Andean Central Volcanic Zone, an example of deep AFC withpossible recharge, r = 0.15, ß = 3.2, and the crust/magmaratio is 0.16.     

Investigation into the petrogenesis of Apollo 14 high-Al basaltic melts through crystal stratigraphy of plagioclase

The petrogenesis of Apollo 14 high-Al basaltic melts was studied using crystal stratigraphy, which involves textural (crystal size distributions — CSDs) and chemical analyses (electron microprobe and laser ablation inductively coupled plasma mass spectrometry). The samples studied here include pristine basalt 14072 and basaltic clasts from breccia 14321, and impact-generated crystalline samples 14073, 14276 and 14310. Plagioclase was the focus of this study because of its relatively high modal abundances and because it was on the liquidus for much of the melt cooling histories. Plagioclase crystals were analyzed (core-to-rim compositions where possible) to test and refine petrogenetic models based upon whole-rock compositions (Groups A, B, and C designations) and to investigate basalt 14072 and impact-melt crystallization. Textural studies have shown that each basalt group has distinctive plagioclase CSDs, which are in turn distinctive from those of the impact melts. Evolution of the individual basaltic melts was studied by comparing the equilibrium-melt compositions (calculated from plagioclase compositions using relevant partition coefficients) to fractional crystallization (FC) and assimilation and fractional crystallization (AFC) models. Petrogenetic modeling of trace elements in Group A basalts revealed that petrogenesis continued beyond 40% total crystallization required to model whole-rock compositions, and that there were open-system processes that affected the magma during plagioclase crystallization. Petrogenetic modeling of pristine high-Al basalts (14072 and Groups A, B and C) using trace elements shows that the equilibrium-melt compositions do not fall on a single AFC or FC trajectory. This is consistent with fluctuating degrees of assimilation (i.e., variable r-values) and/or variable assimilant compositions during petrogenesis. Petrogenetic modeling reveals that the impact melts experienced only closed-system fractional crystallization. This work demonstrates the importance of crystal stratigraphy in revealing the intricacies of lunar basalt petrogenesis.     

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center,eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have _Nd values of -6.2 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the _Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the ⁸⁷Sr/⁸⁶Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO₂ contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.     

Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. Somma–Vesuvius volcano, Italy: Geochemical and Sr isotope evidence

New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma–Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation–Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC–AFC) formulation. J. Petrol. 999–1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation–fractional crystallization (EC–AFC) model to magmatic systems. J. Petrol. 1019–1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9–10 km) is a fundamental process controlling magma compositions at Mt. Somma–Vesuvius in the last 8 ky BP. Contamination in the mid- to upper crust occurred repeatedly, after the magma chamber waxed with influx of new mantle- and crustal-derived magmas and fluids, and waned as a result of magma withdrawal and production of large and energetic plinian and subplinian eruptions.     

Petrochemistry of the south Marmara granitoids, northwest Anatolia, Turkey

Post-collision magmatic rocks are common in the southern portion of the Marmara region (Kap?da?, Karabiga, Gönen, Yenice, Çan areas) and also on the small islands (Marmara, Av?a, Pa?aliman?) in the Sea of Marmara. They are represented mainly by granitic plutons, stocks and sills within Triassic basement rocks. The granitoids have ages between Late Cretaceous and Miocene, but mainly belong to two groups: Eocene in the north and Miocene in the south. The Miocene granitoids have associated volcanic rocks; the Eocene granitoids do not display such associations. They are both granodioritic and granitic in composition, and are metaluminous, calc-alkaline, medium to high-K rocks. Their trace elements patterns are similar to both volcanic-arc and calc-alkaline post-collision intrusions, and the granitoids plot into the volcanic arc granite (VAG) and collision related granite areas (COLG) of discrimination diagrams. The have high ⁸⁷Sr/⁸⁶Sr (0.704–0.707) and low ¹⁴³Nd/¹⁴⁴Nd (0.5124–0.5128). During their evolution, the magma was affected by crustal assimilation and fractional crystallization (AFC). Nd and Sr isotopic compositions support an origin of derivation by combined continental crustal AFC from a basaltic parent magma. A slab breakoff model is consistent with the evolution of South Marmara Sea granitoids.     

Geochemistry of Neogene quartz andesites from the Oaş and Gutâi Mountains,Eastern Carpathians (Romania): a complex magma genesis

The Neogene quartz andesites from the Oa? and Gutâi Mountains (Romania) are mid-K calc-alkaline rocks and contain plagioclase-orthopyroxene-clinopyroxene-amphibole-magnetite phenocrysts as well as quartz crystals. They are associated with a volcanic sequence ranging from basalts and basaltic andesites to dacites and rhyolites, but form a separate magma group, mostly in respect to the trace elements. Based on the geochemical data combined with inferences from complex zoning patterns in plagioclase and pyroxene, the evolution of quartz andesites is interpreted in terms of fractional crystallization, AFC and magma mixing. A parental magma deriving from a MORB- or OIB-type source modified by fluids and melts originating from sediments is envisaged.