摘要:

AbstractThe presence of a dispersed clast fraction in strata near the base of the Cosquer Formation in west Brittany, does not support a glacial origin for this unit. The upper 25 to 30 m of the underlying Kermeur Formation consists of a prograding sequence of very fine to fine sandstones deposited in a mid to distal current swept shelf setting. This sequence shows signs of slope instability, as do the supposed ‘glacial strata’ which overlie it. The upper two thirds of the Cosquer Formation contain spectacular slump‐breccias. Smaller clasts within the laminated mudrocks at the base of the formation are associated with thin graded and non‐graded sandstone laminae. They show no evidence of active penetration into underlying laminae other than can be explained by compaction. Larger clasts are confined to thicker massive beds, or disrupted units with marked internal contorted lamination. This, along with the abundance of slump features within the sequence suggests lateral emplacement by sediment gravity flows in a distal shelf‐slope setting. Surface textures of sand grains within the formation are related to rock disaggregation along fractures developed during post‐depositional deformation and are not related to glacial processes. Distinctive mineralogically immature, poorly‐sorted aggregate sediment pellets, which have been considered as positive proof of glaciation, ar

ISSN:0072-1050    

DOI:10.1002/gj.3350260402

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractExtensive and economically‐important lithium pegmatites have been described from the eastern flank of the large S‐type peraluminous Leinster Granite batholith in southeast Ireland. This flank area is also the locus of a major dip‐slip ductile shear zone which was active during emplacement of the Tullow Lowlands pluton of the batholith. All the known bedrock occurrences of lithium pegmatite are contained within this marginal deformation zone.Most previous studies have suggested derivation of the lithium pegmatites by fractionation from the Leinster Granite itself, though none are conclusive. More recently, an anatectic model has been proposed whereby the pegmatites were derived by melting of Li‐rich sediments adjacent to the batholithic contact. Our investigation has been concerned with a detailed study of the Rb‐Sr isotope systematics and trace element geochemistry of the pegmatites and their host granitoids in an attempt to resolve the problem of pegmatite genesis.The results suggest a strong geochemical and temporal coherence between the Leinster Granite and the lithium pegmatites. A model is proposed in which Li enrichment in residual magmas may be linked to the development of muscovite instead of biotite in the Leinster Granite itself. The Li excluded from entry to biotite then accumulates in highly mobile, low‐viscosity pegmatitic fluids which migrate into the marginal shear zone of the Tullow Lowlands pluton where they crystallize at considerably lower t

ISSN:0072-1050    

DOI:10.1002/gj.3350260403

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe field evidence for the lateral impersistence of the Garnedd‐wen Formation (Hirnantian) and Derwen Formation (Aeronian) immediately northeast of Dinas Mawddwy is reviewed and the evidence for strike faulting as inferred by W. J. Pugh (1928) is reassessed. It is concluded that little if any strata are missing and that surfaces of non‐deposition, rather than fault cut‐out, better explain any strata that are locally abnormally thinned. However strike faulting is newly proven west of Dinas Mawddwy and accounts for much of the rapid apparent thickness variation in the Broad Vein (Rawtheyan) there. The newly constrained thickness estimates strongly imply that sedimentation was influenced by a basement fracture at Dinas Mawddwy. Structural reasoning and palaeocurrent evidence from the Garnedd‐wen Formation suggest that this fracture probably parallels the Corris‐Llangranog fracture to the

ISSN:0072-1050    

DOI:10.1002/gj.3350260404

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractPendent didymograptid graptolites are described from mudstones immediately overlying the Hen‐dy‐capel ironstone (St. Tudwal's Peninsula, Lly̌n, North Wales). The graptolites indicate a Llanvirn age for these beds, implying a Llanvirn or Arenig age for the ironstone, in contrast with the previous interpretation of agracilisBiozone age. The redating of the ironstone horizon suggests that the Pen‐y‐gaer mudstones may be at the base of the local Nant Ffrancon Formation, rather than being equivalent to the lower Caradoc Tyddyn Dicwm beds above the Tremadog ironstone as widely supposed. The ironstones at Trefor, Betws Garmon, and Llandegai, which have been correlated with the Hen‐dy‐capel ironstone on the basis of acritarch assemblages, may also have a similar age. A reinterpretation of the age of these localities as Llanvirn would remove the stratigraphic problems introduced by their previously suggested earlygracilisBiozone age. The Hen‐dy‐capel ironstone forms one of a large number of ironstones recorded from close to the Arenig‐Llanvirn boundary in N. Wales, and elsewhere at high southe

ISSN:0072-1050    

DOI:10.1002/gj.3350260405

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractDioritic plugs (1 km diameter) intrusions show examples of layering and lamination dipping at low to moderate angles. These indicate the beginning of a change from wall‐dominated to floor‐dominated crystallization as intrusion diameter increases. A comparison is made with the nature and origin of layering in other i

ISSN:0072-1050    

DOI:10.1002/gj.3350260406

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260407

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260408

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260410

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260411

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350260401

出版商:John Wiley&Sons Ltd    

年代:1991

数据来源: WILEY