Some lavas south of Noranda are composed of relatively mafic matrix and rhyolitic varioles, representing quenched immiscible silicate liquids. Studies of major elements, rare earth elements (REE), Zr, Y, Ni, etc. indicate that the host magmas, derived by varying degrees of partial melting of mantle material at shallow depths, split into two fractions during and/or after fractional crystallization. The two fractions respectively formed the mafic matrix and rhyolitic varioles. Although the varioles are rhyolitic in major element composition, the contribution of this liquid immiscibility process to the origin of the widespread Noranda rhyolitic volcanic rocks is interpreted to be insignificant. This inter-pretation is based on ΣREE-SiO2 correlations which is on the one hand negative between the matrix and varioles but, on the other hand positive between the various volcanic rocks of the area. The structure of the magmatic liquid is sought using available data on the partitioning of elements between immiscible liquid pairs and mineral-liquid pairs. In brief, wavy twoliquid partition coefficient-ionic radius patterns imply the presence of structural sites in the mafic liquid for cations with ionic radius of _??_0.7 Å and _??_0.95 Å The systematic variation, with the varying SiO2 content of liquid phase, of the partition coefficient-ionic radius pattern for apatite-liquid pairs indicates that the above-mentioned 0.95 Å site decreases in amount with increasing SiO2 content of liquid.
Studies on volcanic hornblendes have been carried out with special reference to the relationship in chemistry to their host rocks from Kagawa Prefecture, Japan. The quantity of Si and AlIV in the hornblende seems to depend to a considerable extent on that of free silica in a magma represented by the rock. The groundmass contains about 4 times as much K2O as the hornblende does, and the coefficient of partition of K and Na between the groundmass and the hornblende is 2.2 on an average. It is deduced from considerations of phase relations in the rocks, two-pyroxene geothermometer and the chemistry of hornblendes that the hornblendes ware crystallized at temperatures above 900°C from magmas probably undersaturated with water. A precipitation of some iron-rich phases together with the hornblende must be required to produce a typical cale-alkaline trend. Hornblende-controlled fractionation at a phenocrystic stage of magmatic crystallization must change the residual melt into a peraluminous composition.
New trace-element data on several Ueno basaltic rocks, obtained by instrumental neutron activation and X-ray fluorescence analyses, show significantly low Ta and Nb values, thereby indicating that the rocks are “arc type” basalts. Most of old analyses on the Ueno basaltic rocks, cited by Ujike (1989), seem to have overestimated the Ta content. Therefore, Ujike's (1989) genetic model, in which these rocks could be derived from a “non-arc type” mantle material, a potential source of the Neogene alkalic volcanic rocks in west Japan, should be abandoned.
Research Article| August 01, 1982 Trace-element geochemistry of Archean volcanic rocks and crystal growth in southwestern Abitibi Belt, Canada R. Capdevila; R. Capdevila 1Department of Geology, University of Toronto, Toronto M52 1A1, Canada Search for other works by this author on: GSW Google Scholar A. M. Goodwin; A. M. Goodwin 1Department of Geology, University of Toronto, Toronto M52 1A1, Canada Search for other works by this author on: GSW Google Scholar O. Ujike; O. Ujike 1Department of Geology, University of Toronto, Toronto M52 1A1, Canada Search for other works by this author on: GSW Google Scholar M. P. Gorton M. P. Gorton 1Department of Geology, University of Toronto, Toronto M52 1A1, Canada Search for other works by this author on: GSW Google Scholar Geology (1982) 10 (8): 418–422. https://doi.org/10.1130/0091-7613(1982)10<418:TGOAVR>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. Capdevila, A. M. Goodwin, O. Ujike, M. P. Gorton; Trace-element geochemistry of Archean volcanic rocks and crystal growth in southwestern Abitibi Belt, Canada. Geology 1982;; 10 (8): 418–422. doi: https://doi.org/10.1130/0091-7613(1982)10<418:TGOAVR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Trace-element geochemistry on 176 controlled samples in three adjoining Abitibi (∼2.7 b.y.) volcanic piles confirms the fundamental concept of Archean volcanic cycles composed of lower tholeiitic and upper calc-alkalic parts. The volcanic piles, each as much as 16 km thick, are either unicyclic or multicyclic. In the latter case, the cycles display secular geochemical trends. All the volcanic rocks studied are indicated to have come from mantle sources. A fundamental feature of Abitibi magma genesis is the simultaneous presence of both depleted and undepleted mantle sources. Abitibi tholeiitic basalts closely resemble modern mid-ocean ridge basalts. Abitibi calc-alkalic andesites and alkalic rocks are very similar to modern oceanic island-arc andesites and to some modern volcanic-arc high-K rocks, respectively. The principal constraints to geodynamic processes are interpreted in terms of pulsating migrating mantle diapirism—a type of “hot-spot tectonics” mechanism involving a layered mantle responsible for the early tholeiitic (depleted mantle source) and later calc-alkalic (undepleted mantle source) parts, respectively, of the volcanic cycles. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
One of the so-called ‘lamprophyre dikes’ of Late Cretaceous age on Shodo-shima Island shows as high as 2.7 wt.% K2O content and 2.6 K2O/Na2O ratio. The major-element composition corresponds to absarokite of the shoshonitic association. Discriminating the magma-type using the chemistry of ‘immobile elements’ and the mineralogy of plagioclase and clinopyroxene phenocrysts, however, revealed that the dike was not originally absarokitic (alkalic), but probably tholeiitic like the other ‘lamprophyre dikes’ nearby. The dike seems to have been altered chemically by alteration.
Strontium isotope compositions were determined for 1 dacite, 2 low-silica rhyolites, 1 high-silica rhyolite and 2 phenocrystic hornblendes from Hime-shima volcano, Southwest Japan. The whole rocks increase in Rb and 87Sr/86Sr and decrease in Sr with increasing SiO2. The high-silica rhyolite composition can be closely reproduced in terms of Rb, Sr and 87Sr/86Sr by model calculations of assimilation-fractional crystallization of the dacite magma: the material to be assimilated is granodiorite which is presumably spread beneath the volcano. The low-silica rhyolites have compositions close to a mixing line between the dacite and high-silica rhyolite in a Sr-87Sr/86Sr relation diagram, suggesting that they are mixing products of the dacite and high-silica rhyolite magmas. Hornblende phenocryst and its host dacite have virtually identical 87Sr/86Sr. On the other hand, hornblende in the low-silica rhyolite has 87Sr/86Sr between its host rock and the dacite. It is likely that the hornblende was derived from the end-member dacite magma and on the way to be in re-equilibrium with the melt phase of mixed rhyolitic magma when the magma erupted. Because the hornblende in rhyolite shows no sign of re-equilibration with respect to chemical composition, the tracer diffusivility of 87Sr/86Sr is likely to be higher than the chemical diffusivility.
Compositions of dike rocks of Late Cretaceous age at Shirotori and Hiketa, northeastern Shikoku show a mild relative iron-enrichment, simulating a typical tholeiitic trend. However, the trend is characterized by a stronger silica-enrichment than most tholeiitic series. It is concluded, from petrologic observations and chemical data including Rb, Sr, Y, Zr, Zn, Ni, Co, Cr and V abundances, that the chemical trend displayed by the dikes was due to late precipitation of an Fe-Ti oxide phase in the course of fractional crystallization dominated by amphibole (hornblende). Crystallization of an Fe-Ti oxide phase may have been delayed by a relatively low oxygen fugacity due to external buffering by crustal wall rocks of the magma chamber. Amphibole-dominated fractional crystallization does not always yield a calc-alkalic magma series.
