Microchemical and Sr isotopic investigation of zoned K-feldspar megacrysts: Insights into the petrogenesis of a granitic system and disequilibrium crystal growth (original) (raw)
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Journal of Volcanology and Geothermal Research, 1994
Rhyolitic lavas and mafic inclusion-bearing dacites (MIBD) form the dominant products of the Monte Arci volcanic complex, one of the most active sites of volcanic activity in Sardinia during the Pliocene. The massif is composed of four distinct eruptive episodes (Phase 1: rhyolites; Phase 2: dacites and andesites; Phase 3: quartznormative trachytes; Phase 4: mafic lavas ranging from subalkaline to mildly alkaline). Monte Arci magmafism has been characterized by open-system behaviour, with both mantle and crustal contributions and magma mixing. Although the mafic products are restricted to the latest stage of activity, mafic inclusions are quite common in many rhyolites and dacites. The mineral assemblages of the inclusions are dominated by plagioclase + orthopyroxene + augite, with minor olivine, Fe-Ti oxides and variable amounts of residual trapped liquid giving rise to a fine-grained groundmass. They represent blobs of magma entrained in a partly molten state and provide evidence of a basaltic contribution to the petrogenesis of their enclosing lavas, both as parental magmas or as a source of heat for partial melting of crustal rocks.
Lithos
The Monte Capanne pluton (Tuscan Magmatic Province, Elba Island, Italy) displays a great variety of magmatic products. Mafic microgranular enclaves (MME) are widespread and display variable size, texture and composition. Petrographic and mineralogical features highlight the importance of magma mixing in their genesis. In particular, patchy-zoned phenocrysts and corroded An-rich plagioclase are common in the MME, which, together with their abundance in the monzogranitic host, suggest that mixing between mantle and crustal-derived magmas occurred relatively early in the crystallisation history of the pluton. A gabbroic enclave is characterised by the abundance of An-rich plagioclase and amphibole, the latter possibly replacing former clinopyroxene. Differences in biotite composition between MME reflect the heterogeneous nature of the mafic magmas prior to the enclave formation. The importance of magma mixing is also evident in K-feldspar megacrysts, which commonly exhibit resorption surfaces followed rimward by a high-Ba zone. Hybrid Orano dykes, which are the latest magmas injected into the Monte Capanne pluton, display specific disequilibrium textures (i.e. sieve-textured plagioclase phenocrysts) and mineral content (i.e. abundance of amphibole) compared to the MME.The genesis of most MME, which display a wide spectrum of major and trace element compositions compared to the host monzogranite, involves a complex interplay between mixing and crystallisation following the input of mantle-derived magma into a silicic magma chamber. Possible loss of the liquid phase from the evolving basic magma may have produced MME distinctively depleted in LREE and enriched in MgO. Fingerprinting the chemical and isotopic signature of the basic magma involved in the genesis of the MME is further complicated by subsequent (but limited) exchange between MME and host. The basic magma is inferred to be similar to coeval high-K calc-alkaline volcanic rocks occurring on Capraia Island.Isotopically, the monzogranite and its MME display distinctive trends pointing towards a low-ɛNd end-member, represented by a garnet mica schist xenolith. We propose that these trends, together with the abundance of variably digested metasedimentary xenoliths dispersed throughout the pluton, reflect assimilation of metapelitic crust early in the evolution of the pluton during storage of the magma within the Tuscan basement, i.e. prior to the final emplacement of the pluton at upper crustal levels.The hybrid Orano dykes, which were injected at the end of the Monte Capanne crystallisation history, display distinctive trace element compositions, similar to REE-Sr-Ba-rich andesites on Capraia. The Orano dykes originated from a deeper level of magma storage and interacted with a crystal-rich enclave-bearing monzogranitic mixture.The proposed scenario involving periodic recharge by mantle-derived magmas and crustal assimilation is also in agreement with isotopic variations observed within a zoned K-feldspar megacryst, which exhibits a core with high 87Sr/86Sr and a rimward decrease of 87Sr/86Sr, arguing for a high 87Sr/86Sr silicic system replenished by lower 87Sr/86Sr mantle-derived magmas.
Cooling and contamination of mafic and ultramafic magmas during ascent through continental crust
Earth and Planetary Science Letters, 1985
When magma ascends turbulently through continental crust, heat transfer can be rapid and the wall rocks of the conduit can melt and be assimilated into the magma. Calculations are presented for cooling, crystallization and contamination during the turbulent ascent of a komatiite, a picritic basalt and a tholeiitic basalt. Primitive magmas, like komatiite and pieritic basalt, are predicted to erupt with moderate to large amounts of olivine phenocrysts except at very high flow rates. Little crystallization takes place in a basalt on ascent, and any phenocrysts are likely to be inherited from the magma chamber. The erosion rate of the conduit walls and amount of contamination are greatest in primitive magmas and least for cool, fractionated magmas. Contamination is also affected by flow rate. For low flow rates, where the Reynolds number is significantly less than 2000, movement is laminar and the magma is likely to solidify against the dyke walls and so the amount of contamination is negligible. Maximum contamination will occur for flow rates at Reynolds number around 2000 and the total amount of contamination will decrease as the Reynolds number increases above this value. This kind of contamination can produce trends on geochemical diagrams which are opposite to those produced by assimilation and fractional crystallization processes in magma chambers. Indices of crustal contamination such as STSr, REE, K20 and other incompatible elements can be greatest in magmas with high values of Mg/(Mg + Fe) and low SiO 2. Both highly incompatible and highly compatible trace elements can show positive correlation with one another (for example, Ni and K20). These features are shown by the Plateau lavas of Skye, Scotland, and some lava groups of the Deccan Traps. Curved trends produced in this way on many types of geochemical diagrams are not mixing hyperbolas and do not necessarily point towards contaminant compositions. Thermal erosion rates are proportional to the difference between the magma temperature and the fusion temperature of the wall rocks. Contamination will thus tend to be selective towards rocks of low fusion temperature. Because of their high temperatures, komatiites are relatively indiscriminate in what they assimilate, while basalts are highly selective. The calculations show that komatiites are highly susceptible to contamination by both continental and oceanic crustal components. Under suitable'flow conditions they can be contaminated with up to 3070 of crustal material. Contamination could result in spurious conclusions about their age and mantle-source characteristics. The geochemical differences between komatiites and closely associated basaltic komatiites can often not be attributed to fractionation of olivine. These basaltic komatiites may represent highly contaminated komatiite rather than an unrelated magma type derived from a different mantle source.
Journal of Volcanology and Geothermal Research, 1978
. Magma mixing in mafic alkaline volcanic rocks: the evidence from relict phenocryst phases and other inclusions. J. Volcanol. Geotherm. Res., 4: 315--331. Green clinopyroxenes, commonly rounded and anhedral and richer in Fe, Na and Mn than the pyroxenes of the surrounding groundmass are a common feature of mafic alkaline volcanic rocks (e.g. basanites, monchiquites, leucitites). Some are accompanied by one or more of the following phases: Fe-rich kaersutite and biotite, anorthoclase, sodic plagioclase, apatite, magnetite, sphene, which are believed to be cognate with the green pyroxenes. We review evidence that these minerals have crystallized from mugearite, trachyte or phonolite magmas, and their presence in mafic alkaline rocks is due to magma mixing. The intermediate and salic magmas may sometimes be generated at mantle depths, possibly by melting of mantle material enriched in Fe, Na and volatiles. *By evolved phases we mean those phases which have a Mg/(Mg+Fe) ratio less than 65 and are unlikely to have been in equilibrium with normal mantle material Walton and Arnold (1970) K. Hansen (personal communication) J. Gutzon Larsen , and personal communication) Brooks and Rucklidge (1973 C.IL Brooks, A.K. Pedersen and D.C. Rex (in preparation) Carmichael (1969), Smith and and found reverse-zoned clinopyroxenes in calc-alkaline volcanic rocks and Anderson and Wright (1971) in tholeiitic lavas from Hawaii, but these are not considered here as we restrict our discussion to alkaline rocks. However, these cases probably also reveal magma mixing.
Contributions to Mineralogy and Petrology, 2011
The Sesia magmatic system of northwest Italy allows direct study of the links between silicic plutonism and volcanism in the upper crust and the coeval interaction of mafic intrusions with the deep crust. In this paper, we focus on the chemical stratigraphy of the pre-intrusion crust, which can be inferred from the compositions of crustal-contaminated mafic plutonic rocks, restitic crustal material incorporated by the complex, and granitic rocks crystallized from anatectic melts. These data sources independently indicate that the crust was compositionally stratified prior to the intrusion of an 8-km-thick gabbroic to dioritic body known as the Mafic Complex, with mica and K-feldspar abundance decreasing with depth and increasing metamorphic grade. Reconsideration of published zircon age data suggest that the igneous evolution initiated with sporadic pulses at around 295 Ma, when mafic sills intruded deep granulites which provided a minor amount of depleted crustal contaminant, very poor in LIL elements. With accelerated rates of the intrusion, between 292 and 286 m.y, mafic magmas invaded significantly more fertile, amphibolite-facies paragneisses, resulting in increased contamination and generating hybrid rocks with distinct chemistry. At this point, increased anatexis produced a large amount of silicic hybrid melts that fed the incremental growth of upper-crustal plutons and volcanic activity, while the disaggregated restite was largely assimilated once ingested by the growing Mafic Complex. This ''igneous climax'' was coincident with an increasing rate of intrusion, when the upper Mafic Complex began growing according to the ''gabbro glacier'' model and, at about the same time, volcanic activity initiated. Cooling lasted millions of years. In the coupled magmatic evolution of the deep and upper crust, the Mafic Complex should be considered more as a large reservoir of heat rather than a source of upper-crustal magma, while the fertility of ''under/intra-plated'' crust plays a crucial role in governing the generation of large volumes of continental silicic magmas. Keywords Ivrea zone Á Sesia magmatic system Á SLIP Á Igneous evolution Á Sesia caldera Communicated by J. Blundy.
Geochimica et Cosmochimica Acta, 1993
and Sr-Nd-0 isotopic data presented for basaltic andesitic to rhyolitic and for quartz noritic to monzogranitic rock suites from the Late Hercynian talc-alkaline Atesina volcanic complex (AVC) and the Cima d'Asta pluton (CAP), Southern Alps (northern Italy), provide information on both the primary magmatic processes and the effects of (mainly Triassic) hydrothermal overprint. Fluid infiltration led to mobilization of major and trace elements ( KzO, NazO, CaO, Rb, Sr, and Ba), opensystem behavior in total-rock Rb-Sr, and shift in d "0 to elevated values (total rock up to 16.6%0 and volcanic matrix up to 17.8460). Oxygen isotopic disequilibrium between quartz-feldspar pairs suggests water-rock interaction at medium/ low temperatures. The 6 "0 values of quartz, the REE characterized by regular LREE enrichment/HREE depletion, and the Sm-Nd isotopic signatures, however, remained virtually unaffected by secondary processes. The initial +d values (at 270 Ma) of the AVC and CAP magma&es are restricted to overlapping ranges of -3.6 to -6.5 and of -2.7 to -6.5, respectively, indicating significant crustal contribution; these values and associated T nM model ages of 1 . l-1.6 Ga agree well with those of typical South Alpine lower crustal magmatites. The AVC and CAP rocks do not follow the "normal" trend of increasingly crustal Nd isotopic signatures with progressive degree of magma evolution expected for a single-stage AFC-type process, but instead display an inversion of this relationship. Geochemical and isotopic constraints favor a model of a large-scale MASH-type melting and mixing zone at or near the base of the continental crust. Distinct elemental emichment/depletion and REE crossover patterns displayed by high-silica as compared to less silicic AVC rhyolites suggest subsequent magma evolution within a shallow-level compositionally zoned chamber.
The International Workshop on Petrological Analysis of Pre-eruptive Magma (PAPEMP) abstracts
BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
近年急速に進みつつある．こうして得られた最新の知見の共有と，関連研究者間の相互交流等を目的とし，国際 ワークショップ "International Workshop on Petrological Analysis of Pre-eruptive Magma Processes (PAPEMP)" ［噴火準 備過程の岩石学的解析に関する国際ワークショップ］ を，活断層・火山研究部門主催で行った．ワークショップ は，所内 34 名・所外 42 名・合計 76 名を集め，活発な議論が行われた．以下では，国内外から招聘した 5 名の 招待基調講演 (30 分) ，および総合討論の要旨を報告する．それ以外のショートトーク (5 分) ・ポスター発表に ついては，タイトルと著者名のみを挙げる．なお，発表は全て英語で行われたが，利便性を考え，各発表の英文 タイトルに和訳を付けるとともに，国内招待講演者の要旨の一部は日本語で掲載する． Keywords: magma process, magma system, time scale, pre-eruptive magma condition, magma decompression 講演要旨 -Abstract ＊ 平成 28 年 11 月 9 日 産業技術総合研究所つくばセンター中央 第一事業所 ネットワーク会議室において開催
Journal of Asian Earth Sciences, 2015
The Salmas plutonic complex, in the northernmost part of Sanandaj-Sirjan Zone of Iran, provides evidence for magma interaction processes. The complex contains maficintermediate, hybrid and felsic rocks which intruded into the Paleozoic metamorphic complex. They show typical relationships described in many mafic-felsic mingling and mixing zones worldwide, such as mafic microgranular enclaves (in felsic and hybrid rocks), mafic sheets, and hybrid rocks. The mafic microgranular enclaves (MMEs) are characterized by fine-grained, equigranular and hypidiomorphic texture and some special types of microscopic textures, e.g., quartz xenocrysts, oscillatory-zoned plagioclase, small lath-shaped plagioclase in large plagioclase, spike zones in plagioclase and spongy-cellular plagioclase textures, rounded plagioclase megacrysts blade-shaped biotite, acicular apatite. The mafic sheets and MMEs in granites (MME-Gr), which indicated magma mingling structures, show I Sr values and εNd(i) similar to diorites. The hybrid rocks and their mafic enclaves (MME-H) show isotope signatures similar to each other. Granites have isotope signatures [higher 87 Sr/ 86 Sr (i) (0.70788 to 0.71075) and lower εNd(i) (-2.4 to-4.2)] distinct to those of the all rock types and MMEs. Major, trace and REE modeling show that hybrid rocks are generated via 40-60% mixing of mafic (dioritic) and felsic (granitic) end-members. All the geochemical data suggest that underplating of dioritic magma, which has been produced by fractional crystallization of gabbros, under the lower crust caused its melting to make felsic (granitic) magma. Injection of dioritic magma into the base of the felsic magma chamber and a limited mixing of two end-members, the lower crustderived magma and mantle-derived melts, formed hybrid magma and their enclaves. Injections of new mafic magma pulses into hybrid magma generated mafic enclaves into them. The injections of denser dioritic magma pulses into a felsic magma chamber and spreading of it at the level of rheological contrast have been formed mafic sheets and MMEs in granites.