A new approach to modeling perpendicular fabrics in porphyritic plutonic rocks using the finite element method
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Phenocryst
Porphyritic
Magma chamber
Matrix (chemical analysis)
The Takidani pluton represents one of a few locations where melt extraction from a crystal mush is preserved in the natural rock record, making it an extremely good case study for investigating the generation of evolved melt reservoirs in the upper crust. Located in the Japan Alps, the Takidani pluton shows a clear vertical zonation consisting of granite and granodiorite in the lower and middle section, a fine-grained porphyritic granitic unit in the upper section and a marginal granodiorite at the roof contact with the host-rock. We present a detailed petrographic and geochemical study using samples collected along a section that traverses the entire vertical section of the pluton. No sharp contacts are found between units. Instead, gradual changes in rock fabric and mineralogy are observed between the lower granodiorite and overlying porphyritic unit. Major and trace element bulk-rock compositions show sigmoidal variations from the bottom to top of the pluton. Incompatible elements and silica contents increase roofwards within the porphyritic unit. Plagioclase chemistry reveals three main crystal populations (P1, P2 and P3) with Fe contents increasing towards the base of the pluton. Comparison with existing crystallization experiments, thermobarometry and hygrometry indicate that the magmas were emplaced at around 200 MPa, 850–900°C and bulk water contents of 3–4 wt %. Whole-rock major and trace element analyses together with mineral chemistry and textural observations suggest that the fine-grained porphyritic unit was extracted from the underlying granodiorite at temperatures between 800 and 740°C and crystallinities of 45–65 wt %. Radiogenic isotopes indicate only minor assimilation (2–6 wt %) and support melt evolution through crystal fractionation. The fine-grained matrix of the porphyritic unit may have been the result of pressure quenching associated with a volcanic eruption.
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Wall rock
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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.
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Pyroxene
Breccia
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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
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Trachyte
Sanidine
Breccia
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Abstract— Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Δ 17 O (=δ 17 O − 0.52 × δ 18 O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, Δ 17 O tends to increase as the FeO contents of the silicates increase. We report a comparison of the O isotopic composition of olivine phenocrysts in low‐FeO (≤Fa 1 ) type I and high‐FeO (≥Fa 15 ) type II porphyritic chondrules in the highly primitive CO3.0 chondrite Yamato‐81020. In agreement with a similar study of chondrules in CO3.0 ALH A77307 by Jones et al. (2000), Δ 17 O tends to increase with increasing FeO. We find that Δ 17 O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high‐FeO chondrules, the difference between Δ 17 O of the late‐formed, high‐FeO phenocryst olivine and those in the low‐FeO cores of relict grains is well‐resolved (although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by Yurimoto and Wasson [2002]). It appears that, during much of the chondrule‐forming period, there was a small upward drift in the Δ 17 O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment.
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Pyroxene
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Phenocryst
Porphyritic
Deccan Traps
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The following remarks are based on five rock-specimens from Clipperton Atoll, referred to by Admiral Wharton in his description of that island. One is a dark brown rock composed of porphyritic crystals of glassy felspar and a compact groundmass; the others are white or cream-coloured, and in some of these the same porphyritic felspars as in the first-mentioned rock may be observed. The examination of a series of microscopic sections shows that the rocks are all more or less altered trachytes. The brown rock is the least altered. It consists of phenocrysts of sanidine (Pl. XXIII, fig. 1) set in a groundmass of microlitic felspars and brown interstitial matter. No ferro-magnesian minerals are recognizable. The conspicuous phenocrysts of sanidine are, as a rule, comparatively free from inclusions; but the rock contains also some patches of felspar, which are so crowded with inclusions of a brown substance that the felspar-material forms only a small portion of the compound mass. An analysis of this rock yielded the following somewhat surprising result:— The phosphoric acid is present in the brown substance, which evidently represents, in a more or less altered form, the interstitial matter of the original trachyte. The microscopic sections of the white or eream-coloured rocks all show the structure of a trachyte. In some specimens the phenocrysts of sanidine have been more or less preserved, while the groundmass has been replaced by isotropic secondary material; in others the phenocrysts have disappeared, and their places have been wholly or partially filled up
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Trachyte
Sanidine
Atoll
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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.
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Chondrule
Melt inclusions
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Watson
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