Hornblende Andesite Research Articles

Cerro Quemado, Guatemala: the volcanic history and hazards of an exogenous volcanic dome complex

Cerro Quemado is a Holocene, andesitic to dacitic, exogenous dome complex located in a back-arc volcanic field 5 km west of Quezaltenango, Guatemala. Activity in the back-arc Almolonga volcanic field began prior to 84,000 yr B.P. with growth and collapse of an andesitic stratovolcano and has continued to the present with sporadic eruption of silicic domes and flows. The Cerro Quemado dome complex covers about 12 km 2 , has a volume of nearly 2 km 3 , and comprises at least 8 vents. Historic activity is limited to extrusion of a blocky lava flow during a six-month period beginning in January 1818. Cerro Quemado lavas are biotite-bearing hornblende andesite and dacite. The calc-alkaline lavas have a disequilibrium mineral assemblage that includes: three populations of zoned plagioclase phenocrysts and microphenocrysts, hornblende, biotite, magnetite, two pyroxenes, olivine, and quartz. In addition, spherical inclusions (2 mm to 30 cm) of basaltic andesite (SiO 2 56 wt. %) are ubiquitous in Cerro Quemado lavas. this petrographic evidence points to an origin by mixing of felsic and mafic magma for Cerro Quemado. In the past, Cerro Quemado typically erupted viscous blocky lava flows (up to 120 m thick and 2.5 km long) and plug domes. The presence of tephra and pyroclastic flow deposits, however, indicates that explosive volcanism has also occurred at the dome. About 1150 yr B.P., the west flank of the dome gravitationally collapsed, producing a debris avalanche that underlies 13 km 2 of the Llano del Pinal valley, and avalanche caldera, 1 km × 1.5 km × 90 m deep. The caldera and avalanche deposit have identical volumes of approximately 0.1 km 3 . The collapse apparently precipitated a hot, laterally directed blast that swept about 40 km 2 of rugged terrain that includes Llano del Pinal valley and the east slopes of nearby Siete Orejas to an elevation of 3100 m. A pyroclastic flow overlying the fines-poor lateral blast deposit apparently resulted from remobilization and flow age of hot blast deposits off the steep slopes of Siete Orejas. Subsequently, a small Peleéan dome was emplaced in the upper part of the avalanche caldera. Following the catastrophic events of 1150 yr B.P., Cerro Quemado was dormant until 1818 A.D. Geologic mapping at Cerro Quemado volcano indicates that the principal volcanic hazards includes lava flow and exogenous dome growth, pyroclastic flow, debris avalanche, lateral blast, tephra fall, and mudflow. If Cerro Quemado erupts in the future Quezaltenango and environs (population more then 125,000) will be threatened.

Evolution of calc-alkaline magmas at the early Quaternary Battle Ax volcano, Western Cascade Range, Oregon

Battle Ax volcano is a dissected composite cone of early Quaternary age, located 33 km northwest of Mt. Jefferson in the Western Cascade Range of Oregon. It evolved during three major eruptive episodes, each of which produced lavas with a distinctive set of petrographic, geochemical and paleomagnetic features. The oldest rocks (Series I) consist of olivine-bearing basaltic andesites and two-pyroxene andesites, with the earliest erupted lavas generally being most mafic. The intermediate-age rocks (Series II) are a compositionally uniform suite of two-pyroxene andesites. The youngest rocks (Series III) consist mainly of hornblende andesite, but also include dacitic lavas and rhyolitic tuff. The first eruptions during the Series III episode produced the most silica-rich rocks and succeeding eruptions discharged progressively more mafic lavas. Rocks comprising one eruptive series cannot be related to those of another series by any simple fractionation or mixing process, owing mainly to intra-series differences in the abundances of incompatible trace elements. Compositional variations among the Series I lavas can be attributed primarily to fractional crystallization of plagioclase, olivine (or orthopyroxene), augite, and magnetite (POAM). The most mafic basaltic andesites in this series probably accumulated small amounts of olivine. The Series III magma evolved mainly by fractionation of plagioclase, amphibole, and orthopyroxene, ±magnetite, ±apatite. Compositional differences among the porphyritic andesites of Series III reflect different amounts and proportions of accumulated xenocrysts derived from hornblende-gabbro cumulates. The most silicic rocks in Series III are probably mixtures of fractionated magma and crustal components. Stratigraphic relations among the Series III lavas indicate they represent magmas erupted from progressively deeper levels in a compositionally zoned reservoir.

Double Glacier Volcano, a ?new? Quaternary volcano in the eastern Aleutian volcanic arc

The Double Glacier Volcano (DGV) is a small dome complex of porphyritic hornblende andesite and dacite that is part of the Cook Inlet segment of Quaternary volcanoes of the eastern Aleutian arc. Its discovery reduces the previously described large volcano gap in Cook Inlet segment to a distance similar to that between other volcanoes in the area. DGV lavas are medium-K, calcalkaline andesites and dacites with concentrations of major and minor elements similar to the other Quaternary volcanoes of the Cook Inlet segment. Available K-Ar ages indicate that DGV was active 600–900 ka.

A potassic volcanic front in western Mexico: The lamprophyric and related lavas of San Sebastian

The superposition of two tectonic regimes (rifting and subduction) in western Mexico has led to the development of a lamprophyric volcanic front ∼70 km closer to the Middle America trench than is the main axis of andesitic volcanism. The San Sebastian cindercone field represents the northernmost extent of lamprophyric activity in western Mexico and lies 40 km trenchward of Volcan Ceboruco, a historically active, andesitic stratovolcano. The predominant country rocks are Cretaceous silicic ash flows, although marine sedimentary rocks (emplaced between 45 and 115 Ma) also occur. Minette, absarokite, and kersantite lavas were erupted between 0.48 and 0.26 Ma in the San Sebastian field in close association with flows of basaltic andesite and hornblende andesite. There is a gradual transition observed between the two intermediate lava types, basaltic andesite and kersantite, as plagioclase is suppressed and hornblende and phlogopite are stabilized in melts of intermediate composition. This transition reflects the increasing influence of water during crystallization and documents a wide range of hydrous conditions. Examination of the primary lamprophyric lava types, absarokite and minette, reveals a remarkable range in their oxidation states. If these lavas reflect their mantle source conditions, then the redox state of western Mexico9s sub-arc mantle spans the interval between the Ni-NiO and magnetite-hematite buffers, a variation in fO 2 of five log units. The oxidation and hydrous enrichment of Mexico9s sub-arc mantle probably formed in response to subduction. The eruption of lamprophyric lavas in front of the andesitic suite of the main axis, however, may have depended upon rifting (1) to trigger melting of the modified mantle and (2) to provide an easy passage to the surface through crustal thinning and block faulting.

Tertiary brittle deformation in the central Sierra Nevada, California: Evidence for late Miocene and possibly younger faulting

Tertiary to Holocene(?) strike-slip and oblique-slip faults with minor displacements cut metasedimentary rocks of Snow Lake pendant and surrounding Jurassic to Cretaceous plutonic rocks in the Tower Peak quadrangle. These faults occur in a part of the Sierra Nevada where little Cenozoic deformation has previously been documented. Two sets of faults are present, one set striking northeast and the other striking east-northeast. Pre-Tertiary markers have separations across these faults ranging from 10 to 335 m. Some dextral displacement on both sets of faults postdates an 8.8 Ma hornblende andesite dike. The faults also show evidence of minor Holocene(?) displacements. Displacement histories of these faults are complicated. Geometric relations of markers of different ages indicate that multiple episodes of displacement, including slip reversals, have occurred, and that displacements were not synchronous on all faults. Displacement on the faults was mainly strike slip, although field relations indicate some oblique displacement. The faults may have originated as joint and fracture sets that formed shortly after emplacement of plutonic rocks of the Sierra Nevada batholith (80-90 Ma). Reactivation of the faults then probably occurred during Cenozoic uplift and erosion of the Sierra Nevada, which was probably related to the complex interaction of oblique-convergent to transform margin tectonics and Basin and Range extension.

Emission of charged particles from indentation fracture of rocks

IT has long been realized that earthquakes are often associated with various kinds of anomalous electromagnetic phenomena. These phenomena might be caused by rock failure in the Earth's crust, but the details of the mechanisms are still unknown. Here we describe an attempt to measure the emission of charged particles from rocks undergoing indentation fracture under atmospheric conditions, using a specially designed indentation system. High electron and ion emission intensities could be detected during parts of the loading cycle when cracking occurred around the indent. The emission behaviour of feldspar and quartz in granite was different from that of the hygroscopic matrix of hornblende andesite; moisture in the hornblende andesite enhanced emission appreciably. The different behaviour observed may be attributable to two different mechanisms.

Mineralized and unmineralized calderas in Spain; Part II, evolution of the Rodalquilar caldera complex and associated gold-alunite deposits

The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intru- sions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and pro- vided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sul- fate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, inter- mediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system.

大分県中部，由布・鶴見火山群の地質と岩石

Yufu-Tsurumi volcano group of northeastern Kyushu yields very porphyritic andesite lavas. These lavas show interesting disequilibrium mineral assemblages and contain characteristic dark inclusions. Analyses of phenocryst minerals revealed that basic mineral group have similar compositions of those of dark inclusions. The volcano group consists of three closely associated volcanoes, which we define as Takahira group, Tsurumi group and Yufu group. Volcanic activity of Takahira group was older than those of Tsurumi and Yufu groups. Volcanic activities of both Tsurumi and Yufu groups continued until some time after 6, 000 y. B. P. Pyroclastic flow deposits dominated older volcanic products, whereas lava flows and lava domes dominated younger products. Hornblende andesite to dacite are abundant among the volcanic rocks of Yufu-Tsurumi volcano group. Andesites contain phenocrysts of plagioclase (An>80 and An<50), clinopyroxene (Mg/(Mg+Fe)=86-71), orthopyroxene (Mg/(Mg+Fe)<70), amphiboles (pargasite and magnesio-hornblende) and opaque minerals with rare olivine, biotite and quartz. In contrast, dacites contain phenocrysts of Mg-poor orthopyroxene, magnesio-hornblende and An-poor plagioclase. Dark inclusions are nearly equigranular and consist of Mg-rich clinopyroxene, pargasite and An-rich plagioclase, with ± olivine, ± orthopyroxene and ± quartz. Some inclusions have chilled margin at the circumference, which implies that they were incorporated into the host magma as liquid state. The analyzed samples from Yufu-Tsurumi volcano group have a wide SiO2 range of 56-65 wt. %, but a nearly constant FeO*/MgO ratio. Thus they show a typical calc-alkaline character. The SiO2 content of the dark inclusions ranges from 48 to 55 wt. %. In the variation diagrams, these inclusions and other volcanic rocks form a single trend with the inclusions being the basic end. Both chemistry and mineralogy of these lavas changes systematically with time. Lavas became more basic toward the younger stage. Modal composition of clinopyroxene increases, and the percentage of dark inclusions becomes progressively greater, as the volcanic products became younger.

新潟県，長岡市東方，東山丘陵の地質

Succession of the Middle Miocene to Pliocene series distributed in this area corresponds to the standard succession of the Niigata sedimentary basin, but is complex because these series include larger amounts of volcanic rocks than that of type area and still remain unknown.From detail field and biostratigraphical study, and fission track dating, the succession and age of the Nanatani, Teradomari and Shiiya formations of the Miocene and the Nishiyama Formation of the Pliocene were clarified.Biotite-hornblende dacite and hornblende andesite (9.0Ma) of the Teradomari Formation are interbedded with siltstones deposited in sedimentary environment of upper to middle bathyal zone and pyroxene andesite of the Shiiya Formation are interbedded with siltstones and sandstones deposited in the same environment as the Teradomari Formation. The Teradomari Formation is considered to has deposited on the Nanatani Formation which has occured as topographical high, resulting in clinounconformity between the two formations. The Nokogiriyama fault, which is main geological structure and is related to the Nigoro anticline, may has began to occur in the Shiiya stage as shown in defferences of lithofacies between both flanks of the Nigoro anticline.

Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, southwestern British Columbia

Research Article| April 01, 1988 Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, southwestern British Columbia NATHAN L. GREEN; NATHAN L. GREEN 1Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 Search for other works by this author on: GSW Google Scholar RICHARD L. ARMSTRONG; RICHARD L. ARMSTRONG 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 Search for other works by this author on: GSW Google Scholar J. E. HARAKAL; J. E. HARAKAL 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 Search for other works by this author on: GSW Google Scholar J. G. SOUTHER; J. G. SOUTHER 3Geological Survey of Canada, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 Search for other works by this author on: GSW Google Scholar PETER B. READ PETER B. READ 4Geotex Consultants Limited, Suite 1200, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 Search for other works by this author on: GSW Google Scholar Author and Article Information NATHAN L. GREEN 1Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 RICHARD L. ARMSTRONG 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 J. E. HARAKAL 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 2B4 J. G. SOUTHER 3Geological Survey of Canada, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 PETER B. READ 4Geotex Consultants Limited, Suite 1200, 100 W. Pender, Vancouver, British Columbia, Canada V6B 1R8 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (4): 563–579. https://doi.org/10.1130/0016-7606(1988)100<0563:EHAKAG>2.3.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 NATHAN L. GREEN, RICHARD L. ARMSTRONG, J. E. HARAKAL, J. G. SOUTHER, PETER B. READ; Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, southwestern British Columbia. GSA Bulletin 1988;; 100 (4): 563–579. doi: https://doi.org/10.1130/0016-7606(1988)100<0563:EHAKAG>2.3.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 SocietyGSA Bulletin Search Advanced Search Abstract The Cenozoic Garibaldi belt comprises 6 volcanic fields spaced at irregular intervals along an axis extending 240 km north-northwest from the head of Howe Sound to the Bridge River area; 2 additional fields, the Franklin Glacier and Silvert lie 140 and 190 km west of the north end of the main volcanic belt. The volcanoes erupted lavas ranging in composition from augite-olivine basalt, through hypersthene andesite, hornblende andesite, and hornblende-biotite andesite, to biotite rhyodacite. Many of the volcanic complexes are characterized by geomorphic features which indicate complex interactions between volcanism and the Pleistocene ice sheets, but preglacial and postglacial phases are also present.Whole-rock samples from 18 volcanic complexes have been dated by the K-Ar method. Most of the results are internally consistent with stratigraphic relationships and with limited 14C and paleomaguetic data.They suggest that volcanic activity was episodic; most of the analyzed andesitic and dacitic lavas were erupted in the intervals 2.3 to 1.7 Ma and 1.1 Ma to present in the northern part and in the intervals 1.4 to 1.0 Ma and 0.7 Ma to present in the southern part of the belt. Basaltic volcanism occurred only during the past 0.15 m.y., except in the Salal Glacier area where hawaiite and alkali-olivine basalt, which are perhaps an "edge effect" related to subduction of the Juan de Fuca plate, were erupted as early as 0.97 Ma. The timing of Garibaldi belt volcanism provides information bearing on the distribution of pre-Wisconsin glaciers in southwestern British Columbia and constrains interpretations of late Cenozoic changes in Explorer-Juan de Fuca-North American plate configuration along the continental margin. This content is PDF only. Please click on the PDF icon to access. 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.

Geochemistry and Petrology of Metavolcanic Rocks of the Early Proterozoic Mazaruni Greenstone Belt, Northern Guyana

Summary The dominantly metavolcanic Issineru Formation (IF) forms the lower 5–7 km of the stratotype for the early Proterozoic Mazaruni greenstone belt, N Guyana. Basalts and gabbros comprise the lower part of the section. The upper half includes some basalts, but is predominantly intermediate and felsic rocks. Most of the Issineru Formation volcanics form a tholeiitic series but minor calcalkaline hornblende andesites and two-pyroxene basaltic andesites with alkaline affinities also occur. Issineru Formation tholeiitic basalts have flat, and slightly LREE-depleted patterns with overall abundances of about 10 × chondrites. Some porphyritic basalts have LREE-enriched patterns and may be calcalkaline. The basalts have similar high field strength element (HFS) patterns on multielement MORB-normalized diagrams, with fairly flat patterns and depletion in P and Ti. Their large ion lithophile element (LIL) contents are more variable, due in part to the LIL’s susceptibility to alteration, but range from slightly less than MORB to about 8 × MORB. These patterns are transitional between MORB and island arc tholeiites. Tholeiitic basaltic andesites and andesites have higher REE levels than the basalts and slightly LREE-enriched patterns (Ce/ Yb = 1.3–2). Calcalkaline hornblende basaltic-andesites and andesites have REE patterns with steeper slopes (Ce/Yb=4.2). All of the Issineru felsic rocks have high REE levels (La = 60) and relatively flat patterns with pronounced negative Eu anomalies. Elsewhere in N Guyana there are rhyolites with steeply-sloping REE patterns, which are considered part of the calcalkaline series. The two-pyroxene basaltic andesites with alkaline affinity have very steep REE patterns (Ce/Yb= 10). They may be sills associated with post Trans-Amazonian continental volcanics, rather than coeval members of the greenstone belt volcanism. The IF tholeiitic series can be qualitatively explained by fractional crystallization from a mafic parent with a flat REE pattern.

Ferrierite from Tapu, Coromandel Peninsula, New Zealand, and a crystal chemical study of known occurrences

AbstractFerrierite has been found at Tapu, Coromandel Peninsula, New Zealand, as a mineral vein with calcite in altered hornblende andesite lava of the Miocene Beeson's Island Volcanics. The ferrierite is low in SiO2 (63.67%) and high in Al2O3 (13.75%), MgO (3.48%), and BaO(2.35%). The large a axis and cell volume (a 19.236(4), b 14.162(6), c 7.527(3) Å, V 2050 Å3) are consistent with the low SiO2, high Al2O3, high MgO chemistry. Optical orientation and optical sign (a = Z, b = X, c = Y, 2V(-)55°) of the mineral are different from those of the Lake Kamloops ferrierite reported by Graham (1918). Refractive indices α 1.487, γ 1.489, and density 2.136 were measured. Cleavage observed on (100) is perfect and on (001) is imperfect. Using data from eighteen occurrences so far reported including the Tapu mineral, the crystal chemistry of ferrierite has been studied.

Rare-earth element geochemistry and the origin of andesites and basalts of the Taupo Volcanic Zone, New Zealand

Two types of basalt (a high-Al basalt associated with the rhyolitic centres north of Taupo and a “low-Al” basalt erupted from Red Crater, Tongariro Volcanic Centre) and five types of andesite (labradorite andesite, labradorite-pyroxene andesite, hornblende andesite, pyroxene low-Si andesite and olivine andesite/low-Si andesite) occur in the Taupo Volcanic Zone (TVZ), North Island, New Zealand. Rare-earth abundances for both basalts and andesites are particularly enriched in light rare-earth elements. High-Al basalts are more enriched than the “low-Al” basalt and have values comparable to the andesites. Labradorite and labradorite-pyroxene andesites all have negative Eu anomalies and hornblende andesites all have negative Ce anomalies. The former is probably due to changing plagioclase composition during fractionation and the latter to late-stage hydration of the magma. Least-squares mixing models indicate that neither high-Al nor “low-Al” basalts are likely sources for labradorite/labradorite-pyroxene andesites. High-Al basalts are considered to result from fractionation of olivine and clinopyroxene from a garnet-free peridotite at the top of the mantle wedge. Labradorite/labradorite-pyroxene andesites are mainly associated with an older NW-trending arc. The source is likely to be garnet-free but it is not certain whether the andesites result from partial melting of the top of the subducting plate or a hydrated lower portion of the mantle wedge. Pyroxene low-Si andesites probably result from cumulation of pyroxene and calcic plagioclase within labradorite-pyroxene andesites, and hornblende andesites by late-stage hydration of labradorite-pyroxene andesite magma. Olivine andesites, low-Si andesites and “low-Al” basalts are related to the NNE-trending Taupo-Hikurangi arc structure. Although the initial source material is different for these lavas they have probably undergone a similar history to the labradorite/labradorite-pyroxene andesites. All lavas show evidence of crustal contamination.

47.K-Ar ages of the hornblende andesite lava in Usa Group and of the pyroxene andesite lava in the northern Oita Prefecture. : the Fall Meeting of the Society

47.K-Ar ages of the hornblende andesite lava in Usa Group and of the pyroxene andesite lava in the northern Oita Prefecture. : the Fall Meeting of the Society

Tholeiitic, calc-alkaline and (?)alkaline igneous rocks of the Shortland Islands, Solomon Islands

The Shortland Islands lie in a northeast-southwest line across the western end of Solomon Islands, immediately adjacent to Bougainville. Three major islands dominate the group. Fauro and surrounding islands, in the northeast, have an altered basement suite comprising tholeiite, icelandite and tholeiitic dacite. This is intruded by a high-level calc-alkaline assemblage of microdiorite, hornblende andesite and rhyodacite and overlain by volcanogenic sandstones derived from an andesitic to dacitic volcano. Pyroclastics comprising high-alumina basalt and pyroxene andesite overlie the volcanogenic sandstones. The tholeiitic basement lavas may be of Late Oligocene to Early Miocene age, and the calc-alkaline rocks are probably also pre-Pliocene in age. Alu, in the centre of the group, also has an altered tholeiitic lava basement, which is intruded by a quartz diorite body and overlain by hypersthene-augite basaltic andesite. Pliocene siltstone and Quaternary shallow marine carbonates cover these igneous rocks over much of the island. Mono, in the southwest, has a small basement exposure of altered pillowed hawaiite, overlain by Miocene pelagic limestone, Pliocene siltstone and Quaternary reef limestone. Isolated clasts of pyroxene andesite and ?benmoreite occur in streams and on beaches. The younger, calc-alkaline suites on all islands were formed in an island arc environment, possibly related to subduction from the southwest beneath the New Britain Trench. The basement lavas on Alu are probably early island arc tholeiites, and both these lavas and the calc-alkaline rocks of Alu share a common trend on variation diagrams. The two igneous suites of Fauro, however, have distinctly different trends. The basement lavas have some chemical similarities with oceanic tholeiites, but an early island arc origin for these lavas cannot be ruled out. The altered hawaiite and benmoreite on Mono probably originated in an oceanic island environment.

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).

鹿児島県布計鉱山地域の変質鉱物について

The Fuke mining district is situated in the northern part of Okuchi city, Kagoshima Prefecture. Andesites distributed widely in this district have undergone propylitization, and silicified and argillized zones are formed in them. Distributions of clay minerals and other alteration products were studied with the result that the hornblende andesite intruded in the central part of this area is surrounded by chlorite zone (inner), mixed-layer mineral zone of chlorite-montmorillonite, and the montmorillonite zone (outer). The propylitization is closely related to the intrusion of the hornblende andesite. The propylitization occurred earlier than the silicification and argillization. The values of Ca/Sr and K/Rb in the bulk samples of the altered and unaltered andesites showed that the values of Ca/Sr are almost constant, and the values of K/Rb increase and the K and Rb contents decrease toward the central part from margin of this area.

Petrology of the volcanic rocks of saba, West Indies

Saba is composed of basic and intermediate members of the calc alkaline series with a broad resemblance to volcanic rocks on other islands of the northern Lesser Antilles. The most abundant rock-type is hornblende andesite with relatively high potassium and associated elements. Notable petrographic features are the persistence of magnesian olivine into relatively siliceous whole-rock composition and the prevalence of hornblende even in the more basic rocks. Quartz xenocrysts also occur in the basalts Clinopyroxene phenocrysts are of an gite composition but groundmass grains are sometimes in the range subcalcic augite to magnesian pigeonite. Disequilibrium features in the petrography, together with anomalies in the chemical trends may be due to incorporation of xenocrysts or perhaps in extreme instances to the hybridisation of contrasting magmas.

口ノ津層群における凝灰岩層および竜石層中の安山岩のジルコンのフィッション・トラック年代

Fission track ages of zircon crystals separated from the tuff layers in the Kuchinotsu Group and those from the andesite of the Tatsuishi Formation in the Shimabara Peninsula, northwest Kyushu have been determined in order to bring some light on the history of the Hohi and the Sanin volcanic activities (MATSUMOTO, 1963) in the Early to Middle Pleistocene age in Kyushu and to clarify the absolute ages of the mammalian and floral zones discriminated in the Kuchinotsu Group.As the diameter of the fission track has been considerably shortened by annealing, correction was made on each given age in the present study, based on the results of the annealing examination.The fission track ages of the Kuchinotsu Group and the Tatsuishi Formation are as follows:Fission track ages (m.y.)Tatsuishi Formation 0.24±0.06Kuchinotsu Group {Kitaarima Formation (Saishoji tuff breccia) 1.43±0.27 Oya Formation {Oya tuff (Tuff VIII) 1.76±0.22 Mukaigoya tuff breccia 1.89±0.16The fission track age of the Mukaigoya tuff breccia (1.89±0.16 F.T.m.y) indicates the beginning of the Early Pleistocene Hohi volcanic activities widely recognized as lavas or pyroclastics in the Early Pleistocene formations in north Kyushu. Furthermore, all the fission track ages of the Kuchinotsu Group seem to be consistent with the results of magnetostratigraphy and biostratigraphy based on the mammalian and plant fossils of this group.The Tatsuishi Formation is mainly composed of mud flow of hornblende andesite which belongs to the early volcanic products of the Unzen volcano representing one of the Middle Pleistocene Sanin volcanic activities of west Japan (MATSUMOTO, 1963). The fission track age of the Tatsuishi Formation (0.24±0.06 F.T.m.y) almost corresponds to those of the hornblende andesite of the Tara-dake (MATSUMOTO et al., 1977) of the Nagasaki Prefecture, northwest Kyushu.

ACTIVE FAULT SYSTEMS OF THE QUATERNARY VOLCANIC REGION IN THE CENTRAL PART OF OITA PREFECTURE, KYUSHU DISTRICT, SOUTHWEST JAPAN

Recent studies have revealed regional variations in types and intensities of the active faulting in Japan. Normal faults are developed exceptionally well in Central Kyushu, while strike-slip or reverse faults are predominant in the main part of Japan (Matsuda, 1973). The focal mechanisms of several shallow earthquakes in Central . Kyushu showed normal fault type (Yamashina and Murai, 1975), such as the Oita earthquake of April 21, 1975 (M=6.4). The purpose of this paper is to present new data on the active fault system of the Quaternary volcanic region in the central part of Oita prefecture, Central Kyushu, where normal faults are densely developed, and tectonic implications of graben formations are also discussed. The results obtained are summarized as follows: (1) Normal faults with short and irregular traces are densely developed in the central part of Oita prefecture, most of which are integrated into several grabens, such as the Haneyama graben and the Hayami graben (Fig. 3). (2) Judging from the wide-spread distribution of Tsukushi lavas across the present rims of grabens, the Haneyama and Hayami grabens had not yet been formed before the eruptions of Tsukushi lavas. But the faulting had already taken place before the eruptions of Haneyama lavas in the Haneyama graben and before the deposition of Yabakei pyroclastic flow in the Hayami graben. Based on the absolute ages of these volcanic rocks, the faulting of these grabens is estimated to have begun during the period from 800, 000 to 500, 000 years B. P. (3) Haneyama lavas erupted from inside the Haneyama graben and hornblende andesites of San'in volcanic series erupted from inside the Hayami graben. These volcanisms occurred along with faulting. (4) The Hayami graben is developed on the crest of a broad up-warping as indicated by general relief and drainage pattern, and the altitude of its floor is considerably high in spite of local subsidence caused by the normal faulting. Such marginal uplifting is recognized also around the Haneyama graben, though it is less distinct. (5) Among the faults near the northwest rim of the Hayami graben, the faults located on the north (Fault 1-5 in Fig. 6) had finished their activities before the formation of Tateishi-yama volcano. However, the faults located on the south (Fault 8 and 9 in Fig. 6) have been active after the formation of the volcano. This indicates that the location of faulting shifted from north to south, namely inward the graben. The distance of about 2 km between Fault 1 and Fault 8 represents the amount of shifting. (6) The arrangement of volcanic vents inside the grabens suggests the existence of exten. sion fractures underneath the grabens. If we assume the existence of extension fractures, , it becomes considerably easier to understand the phenomena (4) and (5): marginal uplifting of the grabens can be explained as a deformation due to opening of extension fractures, and the opening causes extension of a graben, so that locations of faulting will shift relatively inward the graben (Fig. 9). Faulting itself seems to be a surface effect associated with opening of extension fractures. (7) Surface displacements due to opening of a crack with various depth in an elastic half space are calculated (Fig. 10). By comparing the observed pattern of marginal uplifting with the calculated curves of surface displacements, the depth and the amount of opening of the extension fracture underneath the Hayami graben are estimated to be 8-12 km and 1.7-3.2 km, respectively. The latter value is in well-agreement with the extension of about 2 km inferred from the inward shift of faulting. (8) Based on the arrangement of surface faults and volcanoes, a possible model of subsurface structure of the Hayami graben is presented (Fig. 11).