摘要:

AbstractThe island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite).The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt,F= ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in t

ISSN:0072-1050    

DOI:10.1002/gj.3350270202

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Leannan Fault of north‐west Ireland is a sinistral strike‐slip fault system which juxtaposes Dalradian metasediments of differing structural trends and metamorphic grades. It probably represents a south‐west splay of the Great Glen Fault of Scotland. The recognition and tracing of the Foyle Synform across the fault zone, together with the correlation of regional Dalradian strike swings, lateral sedimentary facies variation and metamorphic grades, suggest a sinistral displacement of 34 km across the fault. Members of the Leannan Fault system displace a Lower Devonian (about 397 Ma) granite, but are overlain by Viséan (about 352 Ma) sandstones, thus constraining major late Caledonian sinistral motions to the Middle to Upper De

ISSN:0072-1050    

DOI:10.1002/gj.3350270203

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

摘要:

Abstract40Ar/39Ar geothermochronology is especially useful within the Scandinavian Caledonides, which experienced deformation and metamorphism as a result of two major tectonothermal pulses during the Early and Middle Palaeozoic. This detailed40Ar/39Ar geochronological study of the Central Norrbotten Caledonides allows examination of the timing of tectonothermal activity within the Lower Köli Nappe, the Seve–Köli shear zone, the Seve Nappe and the shear zone rocks of the Middle Allochthon. The results obtained from a suite of 19 samples from the study area can be incorporated with structural and metamorphic constraints to establish the geological history of the region. Results from this study include: (1) the high grade metamorphism and associated deformation of the Seve units was a Late Cambrian to Early Ordovician event (Finnmarkian) in which the rocks cooled below the respective closure temperatures for hornblende at ≈ 490 Ma and muscovite at ≈ 454 Ma; (2) assuming a simple linear cooling model a cooling rate of 3–6°C/Ma was obtained for the older tectonothermal event; (3) there is evidence for late stage Finnmarkian (450 Ma), relatively high grade shear zones separating different tectonostratigraphic elements within the Seve; (4) the Scandian phase of deformation and metamorphism partially reset some of the Seve hornblendes and most of the muscovites, which indicates that the rocks affected by the Finnmarkian event felt a second tectonothermal pulse of more than 350°C beginning at ≈ 430 Ma; and (5) during the Scandian event all of the far travelled allochthonous tectonic units were juxtaposed and the Middle Allochthon mylonites were formed as these nappes were emplaced above the Baltic Shield. The tectonic units of the Singis‐Nikkaluokta transect were assembled before regional cooling through the closure temperatu

ISSN:0072-1050    

DOI:10.1002/gj.3350270204

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractA number of carbonate buildups in north Co. Dublin, long assigned to the late Viséan (Asbian), are shown on the basis of coral, foraminiferal and algal evidence to be early to mid‐Viséan (late Chadian to Holkerian) in age. They are equivalent in age to beds ranging from the upper part of the Lane Formation to the top of the Holmpatrick Formation.The buildups are poorly exposed and relatively small, probably only a few tens of metres across at most. Buildup sediments are massive to crudely bedded and dominated by peloidal, clotted and dense uniform micrites displaying lime mudstone and bioclastic wackestone textures. Dasycladacean algae are common in the buildups and cryptalgal fabrics are locally important. Cavities in the buildups are generally small (<5 cm) and lined with inclusion‐rich radiaxial calcite cements. Micritization of bioclasts and cements is ubiquitous. Enclosing off‐buildup limestones are skeletal and intraclastic grainstones possessing sedimentary structures indicative of deposition in moderate to high energy environments. Fossil and petrographic evidence from the buildups also indicate a shallow water origin for the north Co. Dublin buildups.Compared with the slightly older Tournaisian (Courceyan to early Chadian) Waulsortian buildups which developed extensively in the Dublin Basin, these younger platform buildups are smaller and more isolated and possess a diverse suite of algal components and cryptalgal fabrics. Nevertheless, components in the north Co. Dublin buildups most closely resemble the shallowest phase D Waulsortian buildups, particularly in the presence of abundant peloids and micritized cements.The north Co. Dublin buildups developed on a carbonate platform (the Milverton Platform), adjacent to the Dublin Basin, whereas the Waulsortian developed in a deeper ramp setting. Following the demise of the Waulsortian in early Chadian time carbonate buildups established themselves on the shallow platforms. It is suggested that the microbial communities responsible for these buildups may have ‘evolved’ from older phase D Waulsortian communities and that he north Co. Dublin platform buildups represent the shallow water end of a spectrum of Vis

ISSN:0072-1050    

DOI:10.1002/gj.3350270205

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350270206

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350270207

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

摘要:

AbstractRecent excavations at Pen‐y‐bryn, Caernarfon have revealed a series of organic deposits below and within a glacial till of Welsh ice provenance. This paper presents the results of14C analyses carried out on tree wood and organic peaty muds, together with a description of the site stratigraphy. Pen‐y‐bryn is the first Welsh site from whichin situbiogenic deposits directly enclosed by glacigenic sediments and yielding finite radiocarbon dates have been recovered. Radiocarbon, plant macrofossil, palynological, fossil coleopteran and lithostratigraphic evidence suggest that this is the first site in Wales at which at least one Early Devensian interstadial episode followed by an Early Devensian stadial ice advance may be represented. Snowdonian glacier ice appears to have buried some of the organic deposits beneath till and incorporated other organic material within the till before a later advance of Irish Sea ice. This evidence is used to construct a Devensian chronostratigraphy which may involve two glacial stadia rather than a single Late Devensian gla

ISSN:0072-1050    

DOI:10.1002/gj.3350270208

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350270209

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350270210

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350270211

出版商:John Wiley&Sons Ltd    

年代:1992

数据来源: WILEY