Subduction-related shoshonitic and ultrapotassic magmatism: a study of Siluro-Ordovician syenites from the Scottish Caledonides   总被引：3，自引：0，他引：3  

R. N. Thompson  M. B. Fowler 《Contributions to Mineralogy and Petrology》1986,94(4):507-522

Syenites are important or predominant components of several plutonic complexes, emplaced between 456 and 415 Ma along the NW margin of the Caledonian orogenic belt, adjacent to the Lewisian foreland, in W and NW Scotland. Although there are, in detail, chemical differences between the syenites from each centre, they form a well-defined compositional group overall. Ratios amongst their trace elements (especially very high values of La/Nb) are quite different from those trachytes and syenites formed by fractional crystallisation of ocean-island basalts and their continental equivalents, emplaced in regions of anorogenic crustal tension. Instead, the Scottish Caledonian syenites closely resemble chemically the fractional-crystallisation residua of potassic subduction-related magmas, such as the shoshonitic series. A comendite minor intrusion from a swarm associated with the Loch Borralan and Loch Ailsh syenitic complexes is remarkably similar in composition to Recent obsidian from the shoshonitic volcano of Lipari, in the Aeolian Arc. Published Sr- and Pb-isotopic ratios preclude a significant component of either upper (Proterozoic Moine schists) or lower crust (granulite-facies Archaean Lewisian or Proterozoic Grenvillian gneisses) in all these syenites, except in local syenitic facies of the Glenelg-Ratagain complex. Fractional crystallisation appears to be the mechanism by which the liquids which formed these syenites evolved from basic parental magmas. The phases involved in this process may have included plagioclase, alkali feldspar, pyroxene, amphibole, biotite, garnet, Fe-Ti oxide, sphene, allanite, apatite, zircon and zirconolite, and therefore all the ratios amongst even the so-called incompatible elements may have changed during the evolution of the leucocratic magmas. Nevertheless, a detailed study of the Glen Dessarry complex shows that the changes are insufficient to disguise the geochemical nature of the parental magmas. These appear to be picritic shoshonite (MgO> 15%, Ni>400ppm, La/Yb20, La/Nb>5) for the Glen Dessarry, Loch Borralan and Loch Ailsh syenites, and picritic ultrapotassic magmas (MgO>15%, Ni>400 ppm, La/ Yb60, La/Nb>7) — such as would crystallise to minettes — for the Loch Loyal and Glenelg-Ratagain syenites. Mafic shoshonites were erupted amongst the 410 Ma Lome lavas of this region and also occur as widespread approximately-contemporaneous volcanic feeder plugs. Minettes of similar age are also common as dykes in NW Scotland. Numerous large tonalite-granite complexes, with minor diorites and gabbros, were emplaced in W and NW Scotland between 435 and 400 Ma. These are generally acknowledged to be mixtures of magmas from crustal and mantle sources. Their more mafic members show compositional features, such as very high Ba and Sr, which group them with the rare syenites and the Lorne lavas of the same region into a distinctive geochemical province, within which shoshonitic and allied magmas were the mantle-derived component. Two models are presented to account for the generation of strongly-potassic, subduction-related magmas several hundred km behind a NW-directed subduction zone, or during the period immediately following continental collision.  相似文献   

Survey of tar,hydrocarbon and metal pollution in the coastal waters of Oman     

K.A. Burns  J.P. Villeneuve  V.C. Anderlin  S.W. Fowler 《Marine pollution bulletin》1982,13(7):240-247

Quantitative beach sampling conducted after the S. W. monsoon season yielded average standing stocks of tar ranging from 5 to 2325 g m^?1 of shoreline with overall average of 224 g m^?1. Values are among the highest reported for any world area and show a trend of increasing levels of oil residues close to the Strait of Hormuz. Data support the premise that tanker deballasting is a major source of oil pollution on the Omani coast. Locally high levels were seen near offshore tanker loading facilities. Petroleum hydrocarbons in rock oysters increased in a south to north gradient consistent with beach tar observations. But relatively low hydrocarbon levels in biota coupled with observations of very little floating tar suggests that oil released in Omani coastal waters aggregates, sinks, and reaches the coast as heavy tar lumps. Except for elevated levels of Cd at two stations, heavy metal and chlorinated hydrocarbon concentrations were low compared to more industrialized world areas. Suggestions are made to improve the effectiveness of future pollution studies in the Gulf Region.  相似文献   

The March 11, 2002 Masafi, United Arab Emirates earthquake: Insights into the seismotectonics of the northern Oman Mountains   总被引：2，自引：1，他引：1  

Arthur Rodgers  Abdel-Rahman Fowler  Abdullah M.S. Al-Amri  Abdullah Al-Enezi 《Tectonophysics》2006,415(1-4):57-64

A moderate (M 5) earthquake struck the northeastern United Arab Emirates (UAE) and northern Oman on March 11, 2002. The event was felt over a wide area of the northern Emirates and was accompanied by smaller (felt) events before and after the March 11 main shock. The event was large enough to be detected and located by global networks at teleseismic distances. We estimated focal mechanism and depth from broadband complete regional waveform modeling. We report a normal mechanism with a slight right-lateral strike-slip component consistent with the large-scale tectonics. The normal component suggests relaxation of obducted crust of the Semail ophiolite (specifically, the Khor Fakkan Block) while the right-lateral strike-slip component of the mechanism is consistent with shear across the Oman Line. Felt earthquakes are rare in the region, however no regional seismic network exists in the UAE to determine local seismicity. This event offers a unique opportunity to study the active tectonics of the region as well as inform future studies of seismic hazard in the UAE and northern Oman.  相似文献   

The effects of burrowing activity on archaeological sites: Ndondondwane,South Africa     

Kent D. Fowler  Haskel J. Greenfield  Leonard O. van Schalkwyk 《Geoarchaeology》2004,19(5):441-470

Burrowing activity is a widely recognized source of site modification. Most taphonomic studies of burrowers emphasize their destructive aspects on the archaeological record. Excavations at Ndondondwane, South Africa, suggest burrowing activity is destructive in some ways, but may also preserve cultural behavior. Drawing on both direct and indirect sources of evidence, we discuss how burrowing activity by rodents, earthworms, and termites can inform about pedogenic and depositional processes at archaeological sites and both preserve and destroy evidence of intra‐settlement patterns and early African cultigens. Specifically, we demonstrate the limited effect of earthworms on site stratigraphy, how the localized activity of termites have preserved casts of early African cultigens, and how the ability of archaeologists to distinguish the devastating effects of rodent burrowing from remains of architectural features have permitted important inferences about social and ritual life in early African farming communities. © 2004 Wiley Periodicals, Inc.  相似文献