摘要:

AbstractThe Pb + Zn + Ba veins of West Shropshire, England, occupy fractures in Ordovician and Precambrian rocks of the Shelve Inlier. Precipitation of sphaleritic ores was succeeded by galena + baryte mineralization, with chalcopyrite also occurring late in the mineralizing episode. Three generations of sphalerite are recognized, and second being chemically zoned with distinct Fe + Cd‐rich growth zones. Associated with these iron‐rich bands are smaller (∼ 10 μm) zones rich in indium (>1.0 wt%) and copper, electron electron probe microanalyses suggesting coupled substitution of Cu++ In+3in the ZnS. Based on the sequence of growth zones and their chemistry a sphalerite stratigraphy can be recognized within the orefield.Fluid inclusions studies reveal the mineralizing fluids to be highly saline (18–30 wt% CaCl2equivalent) Na + Ca‐richbrines, with mineralizing temperatures in the range 200–120°C. A trend from higher temperature‐low salinity to low temperature‐high salinity fluids with time is recognized. Sulphide sulphur isotopic ratios are consistent and suggest a ΔS34SH2Sof the mineralizing fluid of 10%, while34Sbarytevalues are in the range + 14 to + 19%, indicating separate sulphate and sulphide sulphur sources. The mineralogical, fluid inclusion, and isotopic data suggest the saline fluids rose into an open plumbing system where mineral precipitation was mainly controlled by fluid cooling. The baryte however, formed due to mixing with oan overlying sulphate‐bearing reservioir.Theree possible fluid sources are considered, namely: Lower Carboniferous seawater, basinal brines, and metamorphic fluids. However, the information available does not allow the source to be p

ISSN:0072-1050    

DOI:10.1002/gj.3350260202

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe concept of suites within granitoid batholiths must be treated with extreme caution, especially when petrogenic interpretation of identified suites is involved (e.g. infrastructural control of granitoid suite composition). Without stated, necessary, and sufficient operational definitions of the parameters (and their weighting) used in suite definition, suite/s should be avoided.

ISSN:0072-1050    

DOI:10.1002/gj.3350260203

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe fluviatile Pleistocene of the lower Wye valley, below the Devensian glacial limit near Hereford, consists almost entirely of coarse gravels, situated both above and below the level of the modern flooplain. Those above the floodplain are largely composed of pebbles from the Ordovician and Silurian of east‐central Wales. Around Hereford they are associated with well‐preserved terraces; further downstream they are fragmentary and have also undergone some displacement. It is suggested that the high‐level gravels comprise remants of four distinct aggradational units. Correlations are proposed with the Severn valley Pleistocene sequence, from which it is concluded that the lower Wye gravels represent a series of cold, perhaps glacial, periods ranging in age from pre‐Hoxnian to Devension. This confirms the pre‐Hoxnian age of fragmentary glacial deposits in northeast Herefordshire. Relations between the gravels and some geomorphological features are discussed, with special attention to abandoned incised

ISSN:0072-1050    

DOI:10.1002/gj.3350260204

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA ‘Gwynedd readvance’ of the last Irish Sea glacier, terminating on the north coast of the Lleyn peninsula in northwest Wales, has been proposed on the basis of differences in ice flow direction and in drift landforms on either side of the proposed limit. However, detailed drift mapping of western Lleyn reveals erosional and depositional evidence of ice flow direction which are consistent with a single advance and retreat. Differences in drift landforms are explained by the influence of topography on glacier dynamics and sedimentation. There is, therefore, no reason to invoke a ‘Gwynedd readvance’ of the last Irish Sea glacier and the marked climatic deterioration which a substantial readvance of such a large glacier woul

ISSN:0072-1050    

DOI:10.1002/gj.3350260205

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractVolcanic rocks from three Palaeozoic low‐grade metamorphic sequences of different age from the Peloritan Mountains, Sicily, have been analysed for major and trace elements. On the basis of the relative abundances of certain comparatively immobile elements (especially Zr, Ti, Nb, and Y) the petrogenetic affinities of the volcanic rocks in each area have been established. The Fiumara Fitalia sequence contains Lower Devonian within‐plate alkaline basalts, the Ogliastrello sequence contains Upper Devonian continental within‐plate calc‐alkaline basalts and the Randazzo‐Floresta road sequence contains Lower Carboniferous calc‐alkaline basalts probably of island arc affinity.When account is taken of their stratigraphic positions, geochemical characteristics of the volcanic rocks indicate Devonian extensional conditions and Lower Carboniferous compressional conditions. Evidence for Lower Devonian extension is present in several circum‐Mediterranean basins (i.e. Calabria, Sardinia, Carnic Alps). In the Peloritan Mountains, as well as in Calabria and Sardinia, extensional conditions were already present in the Cambro‐Ordovician. The compression in the Calabrian‐Peloritan Arc was probably related to a period of subduction during the Hercynian orogeny and resulted in the closure of the basins in

ISSN:0072-1050    

DOI:10.1002/gj.3350260206

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe overall lithological succession in the Millstone Grit of the southern Pennines was most probably controlled by glacial‐eustatic sea‐level oscillations and may be interpreted in terms of the systems tracts of sequence stratigraphy. Bands containing thick‐shelled goniatites represent ‘condensed sections’ formed at times of maximum flooding. Turbidite‐fronted deltas, marking major basin‐filling episodes, resemble ‘lowstand systems tracts’. However all sand influxes did not necessarily occur during lowstands. Millstone Grit sheet deltas and elongate deltas resemble lobate and birdfoot deltas in Recent ‘highstand systems tracts’. The prograding fluviodeltaic complexes at the tops of some major turbidite‐fronted deltas were braid deltas. So was the sheet‐like channel complex of the Rough Rock at the top of the Millstone Grit, but this was probably part of a

ISSN:0072-1050    

DOI:10.1002/gj.3350260207

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260208

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractMajor revisions are proposed to the stratigraphy of the late Precambrian Longmyndian Supergroup of the Welsh Borderland. The stratigraphic relationship between it and the adjacent, late Precambrian Uriconian volcanic complex is reviewed and reinterpreted. Contrary to previous belief it is proposed that the Wentnor Group of the Longmyndian Supergroup does not overlie the Stretton Group unconformably. An apparent unconformity is interpreted as the result of post‐Longmyndian tectonism. The Ragleth Tuff Formation, previously included with the Uriconian volcanic complex, is argued to be part of the Longmyndian Supergroup. The Helmeth Grit Member, previously thought to be the base of the Longmyndian Supergroup, is incorporated within the Ragleth Tuff Formation. These modifications necessitate a revision of the previously held belief that the Helmeth Grit Member is the base of the Longmyndian Supergroup which overlies the ‘Ragleth Tuffs’ of the Uriconian volcanic complex unconformably. The Ragleth Tuff Formation and the Stretton Shale Formation are thought to be faulted against the Eastern Uriconian, and the Wentnor Group is thought to be faulted against the Western Uriconian. Juxtaposition of the Longmyndian Supergroup with the Uriconian volcanic complex across faults is the result of strike‐slip movements along the Church Stretton and Pontesford‐Linley fault systems. Lithological and petrographical evidence suggests that the Longmyndian Supergroup is partly coeval with, and partly younger than, the Uriconian volcanic complex, which acted as a source. The Willstone Hill conglomerate, previously thought to be interbedded with the Eastern Uriconian, may be a representative of the Wentnor Group which overlies the Eastern Uriconian unconformably. As such, it may represent a late onlap of Longmyndian sediments onto the Eastern Uriconian at the margins of the Longmyndian basin. A new formation, the Linley Formation, is recognized. This occurs as fault‐bounded slivers and lenses within the Pontesford‐Linley fault system, and correlation with the rest of the Longmyndian

ISSN:0072-1050    

DOI:10.1002/gj.3350260209

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260210

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260211

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY