Replenishment, Crystal Accumulation and Floor Aggradation in the Megacrystic Kameruka Suite, Australia

Detailed field evidence indicates that the Kameruka Suite plutonsof the Bega Batholith, eastern Australia, grew by crystal accumulationon the floor of a magma chamber. Depositional features in theplutons, including mafic enclave channels, asymmetric enclavepillows and exotic rafts, load casts and flame structures, andgraded and trough cross-beds, indicate that the pluton builtprogressively upward. The general eastward dip of depositionalfeatures in the main pluton implies a lower western and uppereastern contact, consistent with a basal granite–migmatitecontact in the west and a sharp hornfelsic sidewall contactin the east. Mafic, felsic and composite dykes, most commonnear and below the basal western contact, are interpreted asconduits for magma chamber replenishment and imply open-systembehaviour during pluton construction. Textural relations arealso consistent with an open-system, cumulate origin. Typically,centimetre-scale grains of quartz, plagioclase and megacrysticalkali feldspar form a touching framework with interstices filledwith smaller biotite flakes and smaller intercumulus quartzand feldspar crystals. Alkali feldspar megacrysts vary fromeuhedral and unzoned, to mantled and partially replaced by plagioclase,to ovoid and completely pseudomorphed by quartz–albiteaggregates. The common occurrence of mantled and pseudomorphedalkali feldspar in mafic enclaves, and in hybrid tonalitic rocksforming the matrix to enclave swarms, suggests that replacementor resorption of granitic primocrysts was associated with maficreplenishments. The occurrence of all megacryst types at outcropscale implies extended alkali feldspar crystallization in differentparts of the chamber, thorough stirring during, or after, periodicreplenishment, and final settling in a cumulate mush. The bulkcomposition of the cumulate mush, represented by granodiorite,cannot represent the emplaced magma. Compositional variationcan be modelled by variable degrees of crystal accumulationfrom a parental, silica-rich melt represented by the silicicdykes. As dykes periodically fed the magma chamber, crystalsaccumulated on the floor, and more evolved melts probably eruptedfrom its roof. Thus, the average composition of the magma, andthe cumulus minerals, may have remained relatively constant,and the sublinear chemical trends that typify the Kameruka Suitesimply reflect differing proportions of melt and cumulate material.Sublinear chemical trends can also be explained by a restitemodel; however, the distinctive Ba, light rare earth elementand Zr spikes at high silica can be explained only by a cumulatemodel, which also explains why the low-silica granites of thesuite share the same chemical characteristics as the high-silicagranites. KEY WORDS: crystal accumulation; magma chamber; open system; granitoids; Kameruka; Australia     

Mechanism of Crystal Redistribution in a Sheet-like Magma Body: Constraints from the Nosappumisaki and Other Shoshonite Intrusions in the Nemuro Peninsula, Northern Japan

Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (‘phenocrysts’)and those that crystallized in situ after intrusion. From thevertical change in mode and size of ‘phenocrysts’,it is shown that (1) augite ‘phenocrysts’ were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase ‘phenocrysts’, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite ‘phenocrysts’.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystal–crystaland crystal–melt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki     

用地震走时反演计算长白山天池火山区二维地壳泊松比

尝试应用一种同时获得地壳二维速度结构和界面形态的地震走时反演算法， 通过分别反演P波和S波速度结构， 计算深地震测深剖面的泊松比分布的方法， 并对长白山天池火山区测深剖面（L1）进行处理解释. 计算表明， 该方法克服了传统基于正演计算方法的诸多缺陷， 是计算测深剖面泊松比分布的行之有效的方法， 特别是在对火山岩浆系统及地壳断裂带研究中具有重要意义. 研究结果显示， 天池火山口下的地壳低速体内有一宽约30 km、 高约12 km的高泊松比异常区， 其泊松比值高于周围介质8％左右， 应为高温熔融岩石介质， 即岩浆囊. 在其上方直至上地壳顶部有一高泊松比条带， 代表了热物质上移的通道. 在其下部高泊松比区域， 局部深达莫霍面形成了地幔热物质底侵的可能通道. 三维台阵观测的层析成像和大地电磁测深研究结果均与上述结论相印证.      

The Negra Muerta Volcanic Complex, southern Central Andes: geochemical characteristics and magmatic evolution of an episodically active volcanic centre

This petrologic analysis of the Negra Muerta Volcanic Complex (NMVC) contributes to understanding the magmatic evolution of eruptive centres associated with prominent NW-striking fault zones in the southern Central Andes. Specifically, the geochemical characteristics and magmatic evolution of the two eruptive episodes of this Complex are analysed. The first one occurred as an explosive eruption at 9 Ma and is represented by a strongly welded, fiamme-rich, andesitic to dacitic ignimbrite deposit. The second commenced with an eruption of a rhyolitic ignimbrite at 7.6 Ma followed by effusive discharge of hybrid lavas at 7.3 Ma and by emplacement of andesitic to rhyodacitic dykes and domes. Both explosive and effusive eruptions of the second episode occurred within a short time span, but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber. Our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber, pressurising the magmatic system and triggering explosive eruption of rhyolitic magma. Chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition, which could have been strong enough to inhibit the interaction between the two magmas. After the initial explosive stages of the eruption at 7.6 Ma, the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation. Restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber. At 7.3 Ma, increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera. Assimilation during turbulent ascent (ATA) is invoked to explain a conspicuous reversed isotopic signature (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) in the entire volcanic series. Therefore, the 7.6 to 7.3 Ma volcanic rocks of the NMVC resulted from synchronous and mutually interacting petrological processes such as recharge, fractional crystallization, hybridisation, and Assimilation during Turbulent Ascent (ATA).Geochemical characteristics of both volcanic episodes show diverse type and/or depth in the sources and variable influence of upper crustal processes, and indicate a recurrence in the magma-forming conditions. Similarly, other minor volcanic centres in the transversal volcanic belts of the Central Andes repeated their geochemical signatures throughout the Miocene.     

Magma mixing recorded in intermediate rocks associated with high-Mg andesites from the Setouchi volcanic belt, Japan: implications for Archean TTG formation

Calc-alkaline intermediate rocks are spatially and temporally associated with high-Mg andesites (HMAs, Mg#>60) in Middle Miocene Setouchi volcanic belt. The calc-alkaline rocks are characterized by higher Mg# (strongly calc-alkaline trend) than ordinary calc-alkaline rocks at equivalent silica contents. Phenocrysts in the intermediate rocks have petrographical features such as: (1) coexisting reversely and normally zoned orthopyroxene phenocrysts in single rock; (2) sieve type plagioclase in which cores are mantled by higher An%, melt inclusion-rich zone; and (3) reversely zoned amphibole phenocrysts with opacite cores. In addition, mingling textures and magmatic inclusions were observed in some rocks. These petrographic features and the mineral chemistry indicate that magma mixing was the most important process in producing the strongly calc-alkaline rocks. The core composition of normally zoned orthopyroxene phenocrysts and the mantle composition of reversely zoned orthopyroxene phenocrysts have relatively high Mg# (85–90) in maximum. Although basaltic and high-Mg andesitic magmas are candidate as possible mafic end-member magmas, basaltic magma is excluded in terms of phenocryst assemblage and bulk composition. HMA magmas are suitable mafic end-member magmas that precipitated high Mg# (90) orthopyroxene, whereas andesitic to dacitic magma are suitable felsic end-members. In contrast, it is difficult to produce the strongly calc-alkaline trend through fractional crystallization from a HMA magma, because it would require removal of plagioclase together with mafic minerals from the early stage of crystallization, whereas the precipitation of plagiolase is suppressed due to the high water content of HMA magmas. These results imply that Archean Mg#-rich TTGs (>45–55), which are an analog of the strongly calc-alkaline rocks in terms of chemistry and magma genesis, can be derived from magma mixing in which a HMA magma is the mafic end-member magma, rather than by fractional crystallization from a HMA magma.     

Origin of micro-layering in a deep magma chamber: Evidence from two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central)

The origin of magmatic layering is still hotly debated. To try to shed some light on this problem, two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central) were investigated in detail. The nodules belong to a stratiform intrusion emplaced in the deep crust during the Permian (257 ± 6 Ma; Féménias, O., Coussaert, N., Bingen, B., Whitehouse, M., Mercier, J.-C., Demaiffe, D., 2003. A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central). Chem. Geol. 199 293–315.). The 3 to 5 cm thick nodules have, in common, a central orthopyroxenite layer; the succession of layers is, respectively, norite–orthopyroxenite–norite (PBN 00-01) and norite–orthopyroxenite–harzburgite (PBN 00-03). The variations of both major (by electron microprobe) and trace, essentially the RE, elements (by LA-ICP-MS) were measured in major mineral phases (orthopyroxene, clinopyroxene, plagioclase, spinel) along cross-section perpendicular to the layering. Strong grain size, chemical and textural variations occur along these sections: they can be continuous or discontinuous, symmetrical or asymmetrical. Such complex variations cannot be solely related to a single magmatic history (fractional crystallisation, mineral sorting). Other processes such as element enrichment by residual liquid channelling along layer boundaries and/or sub-solidus recrystallisation and element redistribution must be invoked. It appears, in particular, that element distribution in the central orthopyroxenite layer could result from the injection of micro-sills of orthopyroxene-rich liquid between previously consolidated layers.     

Cambro-Ordovician Magmatism in SE Brazil: U-Pb and Rb-Sr Ages, Combined with Sr and Nd Isotopic Data of Charnockitic Rocks from the Varzea Alegre Complex

In the central-southern region of Espírito Santo State, southeastern Brazil, several granitoids with a variable composition intruded high-grade metamorphic rocks, in the northern segment of the Ribeira fold belt. A close relationship between hydrous and anhydrous facies is present in some of the plutons, including the Varzea Alegre Igneous Complex, which has an inner domain formed by gabbro, diorite and granite, and an irregular outer ring of charnockitic rocks. These green megaporphyritic charnockites have primary anhydrous mineral assemblage, I-type and metaluminous character, and high-K calc-alkaline signature. U-Pb zircon single crystal ages obtained by TIMS indicate crystallization at about 500 Ma, similar to other late tectonic plutons of this part of the Ribeira belt. Sr and Nd isotopic ratios ranging from 0.7078 to 0.7096 and 0.5114 to 0.5116 respectively, are interpreted to be indicative of a hybrid origin from crustal and mantle-derived magmas. A binary diagram using Sr isotope ratios also demonstrates that the genesis of the charnockites probably included both magma mixing and fractional crystallization processes.     

Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part I: Phase Relations, Phase Composition and Pre-eruptive Conditions

Crystallization experiments were conducted on dry glasses fromthe Unzen 1992 dacite at 100–300 MPa, 775–875°C,various water activities, and fO₂ buffered by the Ni–NiObuffer. The compositions of the experimental products and naturalphases are used to constrain the temperature and water contentsof the low-temperature and high-temperature magmas prior tothe magma mixing event leading to the 1991–1995 eruption.A temperature of 1050 ± 75°C is determined for thehigh-temperature magma based on two-pyroxene thermometry. Theinvestigation of glass inclusions suggests that the water contentof the rhyolitic low-temperature magma could be as high as 8wt % H₂O. The phase relations at 300 MPa and in the temperaturerange 870–900°C, which are conditions assumed to berepresentative of the main magma chamber after mixing, showthat the main phenocrysts (orthopyroxene, plagioclase, hornblende)coexist only at reduced water activity; the water content ofthe post-mixing dacitic melt is estimated to be 6 ± 1wt % H₂O. Quartz and biotite, also present as phenocrysts inthe dacite, are observed only at low temperature (below 800–775°C).It is concluded that the erupted dacitic magma resulted fromthe mixing of c. 35 wt % of an almost aphyric pyroxene-bearingandesitic magma (1050 ± 75°C; 4 ± 1 wt % H₂Oin the melt) with 65 wt % of a phenocryst-rich low-temperaturemagma (760–780°C) in which the melt phase was rhyolitic,containing up to 8 ± 1 wt % H₂O. The proportions of rhyoliticmelt and phenocrysts in the low-temperature magma are estimatedto be 65% and 35%, respectively. It is emphasized that the strongvariations of phenocryst compositions, especially plagioclase,can be explained only if there were variations of temperatureand/or water activity (in time and/or space) in the low-temperaturemagma. KEY WORDS: Unzen volcano; magma mixing; experimental study     

Microchemical and Sr Isotopic Investigation of Zoned K-feldspar Megacrysts: Insights into the Petrogenesis of a Granitic System and Disequilibrium Crystal Growth

K-feldspar megacrysts (Kfm) are used to investigate the magmaticevolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba,Italy). Dissolution and regrowth of Kfm during magma mixingor mingling events produce indented resorption surfaces associatedwith high Ba contents. Diffusion calculations demonstrate thatKfm chemical zoning is primary. Core-to-rim variations in Ba,Rb, Sr, Li and P support magma mixing (i.e. high Ba and P andlow Rb/Sr at rims), but more complex variations require othermechanisms. In particular, we show that disequilibrium growth(related to variations in diffusion rates in the melt) may haveoccurred as a result of thermal disturbance following influxof mafic magma in the magma chamber. Initial ⁸⁷Sr/⁸⁶Sr ratios(I_Sr) (obtained by microdrilling) decrease from core to rim.Inner core analyses define a mixing trend extending towardsa high I_Sr–Rb/Sr melt component, whereas the outer coresand rims display a more restricted range of I_Sr, but a largerrange of Rb/Sr. Lower I_Sr at the rim of one megacryst suggestsmixing with high-K calc-alkaline mantle-derived volcanics ofsimilar age on Capraia. Trace element and isotopic profilessuggest (1) early megacryst growth in magmas contaminated bycrust and refreshed by high I_Sr silicic melts (as seen in theinner cores) and (2) later recharge with mafic magmas (as seenin the outer cores) followed by (3) crystal fractionation, withpossible interaction with hydrothermal fluids (as seen in therim). The model is compatible with the field occurrence of maficenclaves and xenoliths. KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis