Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

Miocene Low-K Dacites and Trondhjemites of Fiji

Abstract: High-Si dacites comprise one mode in each of two bimodal volcanic rock suites in Fiji. Both suites are low-K throughout but differ in level of iron-enrichment and light REE-enrichment. Sparsely-phyric dacite lava and tephra cover 10 2 -10 3 km 2 in each suite, apparently having erupted in shallow marine environments. Tephra predominates. Typical phenocrysts are plagioclase >>quartz >>Fe-Ti oxides and augite; hypersthene and hornblende are rare and biotite is absent. Most samples have >72% SiO 2 , 2 O 3 , and 2 O. Normatively, most have 6% Or and An x 100/(Ab + An) ratios of 20 over a wide range in Ab/Qz ratios; thus they cross the feldspar-quartz cotectic at a high angle. Their compositions are similar to those of 20 to 40% partial melts of basalt at P H 2 O = 5 kb. Tonal ite-trondhjemite and gabbro plutons also form a bimodal suite. Trondhjemite and dacite mineralogy and compositions are analogous although trondhjemites contain primary hornblende. REE patterns vary considerably in the three otherwise similar suites, ranging from light REE-depletion with La e.f. =8, to middle REE-enrichment with Nd e.f. = 360, to middle-REE depletion with Dy e . f = 7 (Fig. 5). The latter characterizes all trondhjemites analyzed and is compatible with fusion of lower Fijian crust.

Magmatism in the eastern Aleutian Arc: temporal characteristic of igneous activity on Akutan Island

Abstract: Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO2). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO2), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO2) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An92−45), olivine (Fo88−51), orthopyroxene (En69−56), and titanomagnetite (15%–21% TiO2) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased87Sr/86Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.

The Campanian Ignimbrite (southern Italy) geochemical zoning: insight on the generation of a super-eruption from catastrophic differentiation and fast withdrawal

Abstract: More than ca 100 km3 of nearly homogeneous crystal-poor phonolite and ca 100 km3 of slightly zoned trachyte were erupted 39 ka during the Campanian Ignimbrite super eruption, the most powerful in the Neapolitan area. Partition coefficient calculations, equilibrium mineral assemblages, glass compositions and texture were used to reconstruct compositional, thermal and pressure gradients in the pre-eruptive reservoir as well as timing and mechanisms of evolution towards magma chamber overpressure and eruption. Our petrologic data indicate that a wide sill-like trachytic magma chamber was active under the Campanian Plain at 2.5 kbar before CI eruption. Thermal exchange between high liquidus (1199°C) trachytic sill and cool country rocks caused intense undercooling, driving a catastrophic and fast (102 years) in situ fractional crystallization and crustal assimilation that produced a water oversaturated phonolitic cap and an overpressure in the chamber that triggered the super eruption. This process culminated in an abrupt reservoir opening and in a fast single-step high decompression. Sanidine phenocrysts crystal size distributions reveal high differentiation rate, thus suggesting that such a sill-like magmatic system is capable of evolving in a very short time and erupting suddenly with only short-term warning.

Multistage magma emplacement and progressive strain accumulation in the shallow‐level Krkonoše‐Jizera plutonic complex, Bohemian Massif

Abstract: [1] Field relationships combined with new U-Pb zircon geochronology suggest that the shallow-level Krkonose-Jizera plutonic complex, northern Bohemian Massif, was assembled successively from bottom to top, starting with emplacement of the separately evolved S-type Tanvald granite (317.3 ± 2.1 Ma), followed by at least two voluminous batches of the I-type porphyritic Liberec (319.5 ± 2.3 Ma) and Jizera (320.1 ± 3.0 Ma and 319.3 ± 3.7 Ma) granites. The intrusive sequence was completed by uppermost, minor intrusions of the equigranular Harrachov (315.0 ± 2.7 Ma) and Krkonose granites. The I-type granites exhibit an unusually complex pattern of superposed feldspar phenocryst and magnetic fabrics as revealed from the anisotropy of magnetic susceptibility (AMS). The outer Liberec granite preserves margin-parallel foliations and lineations, interpreted to record emplacement-related strain captured by cooling from the pluton floor and walls. In contrast, the inner Jizera, Harrachov, and Krkonose granites were overprinted by synmagmatic strain resulting from dextral movements along regional strike-slip faults cutting the opposite ends of the plutonic complex. Late-stage felsic dikes in the Liberec and Jizera granites reorient from horizontal to vertical (lineation-perpendicular) attitude in response to changing the least principal stress direction, whereas mafic schlieren do not do so, representing only randomly oriented small-scale thermal-mechanical instabilities in the phenocryst framework. In general, this case example challenges the common approach of inferring pluton-wide magma flow from interpolated foliation, lineation, and schlieren patterns. More likely, magmatic fabrics in large plutons record complex temporal succession of superposed strains resulting from diverse processes at multiple scales.

Mineralogy and petrology of Grenada, Lesser Antilles island arc

Abstract: The mineralogy and petrology of volcanic and plutonic rocks from the island of Grenada are described The volcanic rocks include basanitoids, alkalic and subalkalic basalts, andesites and dacites Phenocryst phases in the basanitoids and basalts are olivine (Fo90−71), zoned calcic augite, spinel ranging from ferrian pleonaste through chromite to titaniferous magnetite, and plagioclase Some of the basalts contain pargasitic amphibole Andesites and dacites generally contain hypersthene and augite, and one pigeonite-hypersthene-augite-bearing andesite was found Apatite commonly occurs as a phenocryst in the andesites and dacites and quartz is present in some dacites as well as being a possible xenocryst in both alkalic and subalkalic basalts Plutonic cumulates found as ejected fragments in tuffs and ashes are composed of variable proportions of olivine, magnetite, calcic augite, amphibole and plagioclase One peridotitic (ol-cpx-opx) fragment was found but spinel or garnet peridotitis are absent Despite the alkalic nature of the association, calcalkalic characteristics such as calcic plagioclase, restricted Feenrichment in coexisting pyroxenes and generally low TiO2 content relative to oceanic suites are present in Grenada Estimates of conditions of equilibration of the basanitoids with potential upper mantle materials using the results of high-pressure experiments are compared with estimates from thermodynamic data Equating $$a_{{\text{SiO}}_{\text{2}} } $$ and $$a_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} } $$ basanitoid with hypothetical garnet peridotite assemblages gives a pressure and temperature of equilibration in the region of 35–38 kbar and 1550–1625 ° K Experimental results are not supportive of these estimates