Pliocene Volcanism Research Articles

Volcanism in the western part of the rift valley in Southern Kenya

The stratigraphy, tectonic history, petrography and major oxide petrochemistry of the volcanic sequences in the western part of the rift valley in southern Kenya is summarised. Volcanism and rift faulting began 15 and 7 m.y. ago respectively. A recurrent feature of Miocene and Pliocene volcanism was the tendency for salic magmas to be preferentially erupted in the northern part of the area: the accumulation of a northward-thickening Pliocene trachyte pile is particularly notable. Transitional-mildly alkalic basalts and trachytes were erupted only after the onset of rift faulting, from sites within the rift structure, and so can be considered «rift dependent». Nephelinites, melanephelinites, limburgites and phonolites were erupted before and after onset of rift faulting, from sites within and outside the rift structure, and so can be considered «rift independent».

The pliocene volcanism of the voras Mts (Central Macedonia, Greece)

Major element chemistry, K/Ar ages and trace element data are reported for volcanic rocks from the Voras volcanic complex of Central Macedonia (Greece). Petrological data show that the Voras volcanic rocks consist essentially of intermediate members of the high-K calc-alkaline and shoshonitic series, the most abundant rock types being high-K andesites and dacites, latites and trachytes. K/Ar ages determined on selected samples show values ranging between 5.0 and 1.8 m.y., indicating that the Voras volcanism was active from Pliocene up to the Lower Pleistocene. The high Th, Hf, Ba and LREE shown by some representative samples together with their strongly fractionated light and heavy REE patterns and the absence of significant negative Eu anomalies indicate a magma genesis by low degrees of partial melting of a source enriched in large ion lithophile elements, leaving a garnet-bearing and plagioclase-free residue. This source is believed to be represented by a mantle garnet peridotite enriched in incompatible elements.

The Chemistry of Weathering of a Pliocene Volcanic Ash Deposit, National Elk Refuge, Wyoming

Precipitation falls to the earth basically as distilled water, except near population centers where atmospheric pollution contributes solutes and near the ocean which adds sodium and chloride. The waters of springs, however, contain varying amounts of Ca++ , Mg++, Na+, HCO3- and aqueous silica as major constituents, derived from bedrock. How rainwater changes its composition to springwater as it passes through soils and rocks is not well understood. The objectives of our project were to study the initial changes in water chemistry as moisture passed through a "reactive" bedrock and to compare our results with theoretical models (eg. Helgeson 1968, Helgeson et.al. 1969, 1970, Wood and Surdam, 1979).

Paleomagnetism, K-Ar dating and tectonic interpretation of Upper Cretaceous and Cenozoic volcanic rocks of the Chatham Islands, New Zealand

Abstract New K-Ar age determinations on basaltic volcanic sequences in the Chatham Islands, 900 km east of South Island, New Zealand, combined with additional field and paleontologic studies, have led to a stratigraphic revision and the recognition of three distinct volcanic episodes—Late Cretaceous (70–85 m.y.), Eocene-early Oligocene (35–53 m.y.), and late Miocene-Pliocene (2–6m.y.). Paleontology, palynology, and K-Ar dating place the earliest alkali basalt-trachyte volcanism of the Kahuitara Tuff and Southern Volcanics in the early Senonian, at the time of separation of the Chatham Rise-Campbell Plateau block from West Antarctica. The Southern Volcanics were formerly considered to be Eocene and the Kahuitara Tuff was included in the Waihere Bay Group (Albian). The Waihere Bay Group (s.s.) of Albian-Cenomanian age comprises mainly estuarine to deltaic sediments, derived from the Chatham Rise and/or West Antarctica, prior to continental separation. The Eocene and Pliocene volcanics comprise limburgitic p...

Significance of K-Ar Ages of Tertiary Rocks from the Lake Mead Region, Nevada-Arizona

K-Ar ages for 43 igneous and tuffaceous rocks of the Lake Mead region, Nevada-Arizona, are reasonably consistent with mapped stratigraphic and structural relations. They serve to establish age ranges for episodes of Miocene and Pliocene volcanism, plutonism, and tectonism in the region. The late Tertiary geology of the northwestern part of the region contrasts sharply with that of the southeastern part. The northwestern part was characterized by active sedimentation in local basins followed by intense disruption by normal and transcurrent faults. The southeastern part was characterized by brief episodes of overlapping volcanism, plutonism, extreme tensional rifting, followed by severe local uplift. Although diverse in style, the tectonism of the two areas appears to have been largely synchronous. The synchronous character of the brief igneous and tectonic processes suggests strong genetic ties between them. The southeastern part of the region may be a volcanic rift zone that originated as a zone of tension near the end of a major transcurrent fault—the Las Vegas shear zone.

Petrology of Tara-dake volcano, north western Kyushu.

Tara-dake volcano is located in Omura Peninsula, Northwestern Kyushu. This volcano forms the western end of Daisen Volcanic Zone. The basement of this volcano is consisted of early Tertiary sediments and Pliocene volcanics. The volcanic activity of Tara-dake can be devided into four stages. In the first stage, this volcano extruded alkali basalt flows principally with a little of calc-alkali andesite flows. The second stage is represented by numerous andesitic pyroclastic rocks, which directly cover the extrusives of the first stage. Most of the pyroclastic rocks are fit for the pyroclastic flow of the intermediate type (Armakai, 1957). They were formed by several eruptions of many volcanoes, constructing Tara-dake volcano. After this stage, andesitic lava flows were erupted and lava domes were formed finally. In reference to the genesis of calc-alkali rocks, it is the important fact that the alkali basalts and clac-alkali andesites were erupted successively. The successive eruption of calc-alkali rocks and tholeiite (or high-alumina basalt) is known in various volcanoes of Japan. But similar relation between alkali and calc-alkali rocks has not been found out up to this time. In this area, both series of rocks include many xenoliths and xenocrysts, and both were comparatively affected the contaimination effect. So the Tara-dake andesite might be derived from the alkali basalt which assimilated the crustal material.

Une reconnaissance geologique de la peninsule Courbet, archipel de Kerguelen

Abstract Volcanic beds on the Courbet peninsula of the Kerguelen archipelago in the Indian Ocean, are described, placed in stratigraphic order and compared with the beds of the rest of the archipelago. In the western mountainous area between lakes Studer and Margot, there can be seen a massif of modified spilitic lava, 5 km across and bordered on three sides by faults. The greater part of the peninsula is composed of a series of basalt flows of various textures and composition. Detrital features are widespread, especially on the western plains. Moraines, outwash deposits, drumlins, and sand dunes are common. The Kerguelen archipelago was once a large land mass composed of Miocene and lower Pliocene volcanics. The present day islands are the result of severe faulting and subsequent sinking of parts of the area.

Palaeomagnetic Researches for the Pliocene Volcanic Rocks in Central Japan (1)

All the rock specimens collected from middle Pliocene Komoro group and Shigarami group, have shown the marked magnetic polarisations in the direction of highly westward trend ranging roughly from N 60°W to N 110°W. The pole positions indicated by these specimens are calculated to be situated within a range from Lat. 32°N to Lat. 35°S.A series of volcanic rocks including the middle Pliocene rock specimens above-mentioned and ranging from early Pliocene to late Pliocene, seems to indicate that the pole position would have shifted continuously from Lat. 75°N to Lat. 70°S during Pliocene times.As far as the writer's examination goes, it should be stressed that the direction of the earth's magnetic dipole axis in Pliocene age would have shifted from the North polar region, continuously southward to the South polar region.

ORIGIN OF THE CHUSKA SANDSTONE, ARIZONA-NEW MEXICO: A STRUCTURAL AND PETROGRAPHIC STUDY OF A TERTIARY EOLIAN SEDIMENT

The Chuska sandstone as here restricted forms the cap rock of the Chuska Mountains, which extend 60 miles along the Arizona-New Mexico State line north of Gallup, New Mexico. Its maximum preserved thickness is about 1750 feet. It is unfossiliferous, cross-bedded throughout, and contains no shale or conglomerate interbeds; there are several thin ash beds. The cross-bedding dips consistently in the northern quadrants at angles around 25°. Alternating strong to weak chalcedonic and opaline cement in thick layers throughout the formation provides a topography of alternating resistant ledges and gentle slopes. Rosiwal analyses of four samples averaged 74 per cent quartz and 23 per cent feldspar. Sixteen samples have an average median diameter of 0.21 mm and are well sorted and slightly skewed. These values are compared with those of eolian sands described in the literature, and an evaluation is made of the use of particle-size parameters as criteria for postulating depositional environment. The average intercept sphericity for 8 samples is 0.78; the average roundness for 15 samples is 0.51. Nine samples showed that an average of 29 per cent of the grains had frosted or pitted surfaces. Shape, roundness, and frosting are probably inherited from source sediments. The nature of the cross-bedding and the uniformity of texture imply accumulation of dune sand by winds from the south-southwest. Available median diameters, sphericities, roundness, and frosting were plotted against distance in the presumed direction of transport. No firm trends are evident in the 38 miles between outside samples. The restricted Chuska sandstone and the underlying fluvial Deza formation, together forming Gregory9s Chuska sandstone, truncate the Mesaverde group (Upper Cretaceous) and are unconformably overlain by middle or upper Pliocene volcanics. The suggested Miocene (?) age is based on physiographic data.

Epithermal huebnerite from the Monitor District, Alpine County, California

In the Monitor district, are small irregular gold-silver deposits, in which huebnerite is present along with minerals characteristic of the epithermal zone. The gangue is chiefly quartz and rhodochrosite. The paragenesis of the huebnerite indicates that it was deposited quite late. The enclosing Miocene lavas and breccias were extensively altered by hydrothermal solutions and, due to weathering, are vividly colored. The altered rocks are overlain by relatively fresh Pliocene volcanics. The mineralization and other geological features are similar to those accompanying many other bonanza-type gold-silver deposits in the Great Basin region.