Phenocryst crystallization during ascent of alkali basalt magma at Rishiri Volcano, northern Japan
70
Citation
40
Reference
10
Related Paper
Citation Trend
Keywords:
Phenocryst
Porphyritic
Magma chamber
Igneous differentiation
Liquidus
Caldera
Porphyritic magnesian andesites (PMAs; SiO 2 ∼55 wt. %; MgO ∼7 wt.%) are found in the Miocene Setouchi volcanic belt, SW Japan. The PMAs are characterized by the presence of plagioclase phenocrysts, whereas the rather aphyric, mantle-derived high-Mg sanukitoid andesites (HMAs) found in the region do not contain such phenocrysts. The following petrographic observations suggest a role of mixing of magmas in producing the PMA magma: (1) reversely zoned pyroxene phenocrysts, both clino- and ortho-pyroxenes, are observed in PMAs; (2) normally zoned clinopyroxene may be in equilibrium with olivine but not with normally zoned orthopyroxene in terms of Fe-Mg partitioning; (3) plagioclase displays a wide compositional range (An 80-45 ) with a bimodal distribution; (4) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in HMA sanukitoids and the other identical to those in basalts, are recognized in terms of Ni-Mg and Cr-Al-Fe 3+ relations, respectively. The above petrographic characteristics may be best explained by the presence of three end-member magmas, namely, Mg-rich basalt and HMA magmas, both having olivine and clinopyroxene phenocrysts and a low-Mg, plagioclase-phyric andesite magma. Major, trace, and isotope compositions of these magmas may also support the magma-mixing origin for PMAs.
Phenocryst
Porphyritic
Andesites
Igneous differentiation
Cite
Citations (4)
SUMMARY Some thin basaltic intrusive sheets in south-eastern Iceland consist in cross-section of a porphyritic central zone sharply bounded by non-porphyritic margins. Within the porphyritic zone phenocrysts of plagioclase, augite and olivine are arranged in two main layers, an upper layer containing mostly plagioclase phenocrysts, and a lower layer containing concentrations of augite and olivine phenocrysts. The phenocrysts are considered to have been gravitationally sorted during the passage through the sheets of a highly fluid and strongly flowing porphyritic basalt magma.
Phenocryst
Porphyritic
Pyroxene
Breccia
Cite
Citations (4)
Porphyritic
Magma chamber
Dacite
Igneous differentiation
Fractional crystallization (geology)
Phenocryst
Cite
Citations (155)
Phenocryst
Porphyritic
Magma chamber
Igneous differentiation
Liquidus
Caldera
Cite
Citations (70)
Bangly Quarry is situated about two miles to the north-west of Haddington, and is well known for the fine porphyritic quartz-banakite that occurs there; and also for a so-called very fine porphyritic trachyte carrying large crystals of sanidine up to two inches in length. It is thus described in the Memoir of the Geological Survey for East Lothian, 1910, p. 79: ‶The finest example of a porphyritic trachyte in the Garleton area is met with in the Bangly or Silver Hill Quarry. . . . It belongs to the special group of quartz-banakites, and most of the normal phenocrysts are of plagioclase; but these are associated, in the large quarry, with sanidine crystals which are often two inches long, and in many cases conspicuously twinned according to the Carlsbad law. This rock is probably the finest example of a porphyritic trachyte in the British Isles; yet so local is the development of the late formed giant phenocrysts, that at the west end of the same quarry they have almost entirely disappeared.″ The face of Bangly Quarry is nearly 80 feet high, and looks towards the north. The rock is dark red-brown in colour, and the jointing is in smooth vertical planes, except at one part near the east end. At this place a vertical dyke (Plate XL., Fig. 1), about 12 feet wide, cuts through the whole thickness of the rock, in a nearly north-and-south direction. The jointing of the dyke is conspicuous and horizontal, and there is a chilled
Phenocryst
Porphyritic
Trachyte
Sanidine
Breccia
Cite
Citations (3)
The Ross of Mull Igneous Complex consists of a suite of syn-post tectonic lamprophyric-dioritic-microdioritic-monzogranitic calc-alkaline bodies emplaced hito Moinian metasediments (P=2-3 kbar) throughout the closing stages of the Caledonian orogeny (414±3 Ma; Halliday et al., 1979). The pluton occupies an area approximately 140 km[sup]2, of which only half is exposed on the mainland - the rest is submerged. Microdioritic and dioritic bodies are confined to the core of the pluton and occupy topographic lows, while granites occupy the highs. The distribution of these plutonic rocks is interpreted as a compositionally zoned (reversed) magma chamber. Prior to and throughout the main phase of monzogranite emplacement, a series of basaltic (alkalic) magma pulses intruded the monzogranitic magma chamber, inducing mechanical mixing, homogenisation and the production of a hybrid porphyritic monzogranite. Binary mixing equations allow the proportions of granitic magma involved in the mixing event to be estimated, which vary between 66-80%. Continued injection of basaltic magma into the evolving, crystal-ladden, porphyritic monzogranitic magma chamber resulted in the fragmentation of the basaltic magma and the formation (preservation) of megacrystic, microdioritic enclaves. On the basis of alkali feldspar crystal growth rates in granitic magmas, as well as thermodynamical considerations (e.g. Furman & Spera, 1985), the time elapsed between the formation of the porphyritic monzogranite and the injection of additional basaltic magma pulses was approximately 15000 years. Based on detailed field mapping and petrographic analysis, microdioritic enclaves can be subdivided into four texturally distinct populations, depending on their megacrystic mineralogy. The mineralogy and textures of the enclaves reflect and record the point at which the basaltic magma intruded the crystallising porphyritic monzogranitic magma chamber. Generally, highly megacrystic microdiorites are interpreted as having been intruded relatively early in the crystallisation history of the porphyritic monzogranite. Microdioritic enclaves with fewer megacrysts are likely to have been emplaced late in the crystallisation of the granite, when the rheological differences between the two magmas would have inhibited mingling.
In exceptional circumstances, microdioritic bodies and enclaves become veined by thin (c. 5 mm wide) leucocratic (monzonitic) veins composed of plagioclase + alkali feldspar ± quartz. Typically, these veins occupy 5-30% volume of the microdiorite. Field and mineralogical evidence cannot equivocally explain the formation of the monzonitic veins. Partial melting experiments on megacryst-free microdioritic enclaves at crustal pressures and temperatures (i.e. 750-950 °C, 50 MPa), have therefore been carried out in order to shed light on the origin of the veining phenomena. The composition of the melt generated during these experiments requires high (950 °C) temperatures and is less sodic but richer in quartz than that of the leucocratic veins. Integrated field, mineral chemistry and geochemical data suggests that mechanical mixing of basaltic and porphyritic monzogranite magma (at depth or in a conduit) produced a heterogeneous mixture which was injected into a porphyritic monzogranitic magma chamber. The higher liquidus of the basaltic magma coupled with the input of additional heat from new basaltic magma pulses induced fluid-present partial melting of the more fusible components in the mixture (i.e. the granitic end-member). Where the mixture was almost crystalline prior to incorporation into the porphyritic monzogranite, re-heating of the mixture caused recrystallisation of the microdioritic matrix, partial melting of the granitic material and thermal expansion leading to the formation of a feldspar-rich, pseudo-polygonal monzonitic vein network (e.g. pink veined microdiorites). However, in the case where the mixture was still ,largely molten prior to incorporation into the porphyritic monzogranite, fluid-present partial melting of the granitic material in the mixture caused the formation of feldspar + quartz-rich leucocratic veins without recrystallisation of the microdiorite matrix (e.g. white veined microdiorites). As melting of the granitic magma ensued, monzonitic melt exfiltrated through the partially molten microdiorite matrix via porous flow and deformation enhanced melt segregation mechanisms.
The topology of the vein network will have a fundamental bearing on the efficacy of chemical homogenisation within the microdiorites, as well as controlling the rate of material transport (advection) within the veins. Leucocratic veins are clearly linked in three dimensions and in order to quantify the pore structure of the
veins, a veined microdioritic enclave was collected for serial sectioning. Image analysis software and 3D modelling packages were then used to reconstruct the vein network in 3D. The results show that the vein network posseses a high effective porosity (17%), as well as a complex bifurcating and branching network. Based on the 3D topology, the specific permeability (k) of the vein network has been estimated and ranges from 8x10[sup]-7 to 1x10[sup]-12 m[sup]2. Based on these permeabilities and estimates of granitic melt viscosities (10[sup]4 to 10[sup]8 Pas), Darcian flow velocities range from 10[sup]-6 to 3 m[sup]2 yr[sup]-1. The extensive connectivity of the channel network in the veined microdiorites suggests that element mobility during active flow would have been extensive.
Porphyritic
Magma chamber
Igneous differentiation
Orogeny
Cite
Citations (0)
Phenocryst
Porphyritic
Deccan Traps
Cite
Citations (8)
Phenocryst
Magma chamber
Porphyritic
Leucite
Igneous differentiation
Fractional crystallization (geology)
Incompatible element
Petrogenesis
Cite
Citations (42)
Phenocryst
Porphyritic
Cite
Citations (2)
Abstract Phosphorus X‐ray maps of olivine phenocrysts in many type II (FeO‐rich) porphyritic chondrules in LL3.00 Semarkona and CO3.05 Y 81020 reveal multiple sets of thin dark/bright (P‐poor/P‐rich) layers that resemble oscillatory zoning. Such discrete layers are generally not evident in BSE images or in Fe, Cr, Ca, Al, Mg, or Mn X‐ray maps because rapid diffusion of these cations in olivine at high temperatures smoothed out their initial distributions, thereby mimicking normal igneous zoning. In contrast, the relatively slow diffusion of P in olivine preserves original dendritic or hopper morphologies of olivine crystals; these skeletal structures formed during quenching after initial chondrule melting. The skeletal olivine crystals were filled in with low‐P olivine during cooling after one or more subsequent heating events, mainly involving the melting of mesostasis. Crystallization of mafic silicates depleted the mesostasis in FeO and MgO and enriched it in silico‐feldspathic components. Sectioning of the olivine grains at particular orientations can produce apparent oscillatory zoning in P. Strong evidence of a secondary melting event is evident in Semarkona chondrule H5k. Phenocryst H5k‐2 in this chondrule has a relict core (with rhythmic P zoning layers) that was fractured and severed; it is overlain by a set of differently oriented subparallel P‐poor olivine layers. Chondrule C6f from Y 81020 contains a large multi‐lobed olivine phenocryst that still preserves hopper cavities, partially outlined by P‐poor/P‐rich olivine layers. The thin P‐rich rims surrounding many olivine phenocrysts could reflect a short period of rapid grain growth after a late‐stage chondrule reheating event.
Phenocryst
Porphyritic
Chondrule
Melt inclusions
Cite
Citations (5)