摘要:

AbstractIn the late Precambrian Avalon composite terrane of the Canadian Appalachians, the local juxtaposition of Avalonian successions against gneiss complex–platformal metasedimentary rock associations of uncertain relationship to the Avalonian overstep sequence has raised important questions about the configuration of the composite terrane. Typical of this relationship is the juxtapostion of Avalonian arc‐related packages (Caledonia assemblage) with the migmatitic Brookville Gneiss and metacarbonate–quartzite Green Head Group (Brookville assemblage) along the Caledonia Fault in southern New Brunswick. Polyphase deformation of the predominantly greenschist facies Green Head Group accompanied development of a regional ductile shear zone that separates the group from the amphibolite facies Brookville Gneiss. Heterogeneous ductile flow in carbonate rocks and the development of a regional foliation was followed by NW‐directed shortening and the local development of a penetrative axial planar fabric that intensifies towards the shear zone. Associated structural elements suggest regional dextral transpression, consistent with the metamorphic contrast across the shear zone. Steeply plunging east–west folds may record younger, sinistral movement on associated NE–SW faults. Deformation coincident with metamorphic culmination in the Brookville Gneiss produced a gneissic foliation that was later deformed to produce widespread minor folds of sheath‐like geometry. These folds are best developed proximal to the shear zone where they locally document dextral shear, and probably include several generations that overlap early phases of deformation of the Green Head Group. Kinematic indicators within the gneiss are predominantly dextral.36Ar/40Ar versus39Ar/40Ar isotope‐correlation ages recorded by metamorphic hornblende suggest that regional cooling of the Brookville Gneiss through ca. 500°C occurred at ca. 540 Ma, providing a minimum age for initial deformation and concomitant metamorphic culmination in the gneiss.40Ar/39Ar plateau ages for metamorphic muscovite suggest cooling through ca. 375°C at ca. 500–520 Ma, providing a minimum age for progressive deformation in both lithotectonic sequences. Low temperature age discordance in the muscovite spectra suggest partial rejuvenation in the mid‐ and late Palaeozoic.Protracted Cambrian tectonothermal activity in the Brookville assemblage contrasts with the Avalonian tectonostratigraphic record of the Caledonia assemblage in which late Precambrian arc‐related packages are overstepped by Cambrian–Ordovician shallow marine strata. Significant Cambrian separation between the two assemblages is therefore suggested, despite Precambrian similarities in their tectonothermal evolution. Separation as a consequence of terrane dispersal is suggested, and may imply a significant rearrangement of the Avalon composite terrane at this time. Final juxtaposition of the two assemblages pre‐dates their shared late Palaeozoic rejuvenation, and may correspond to an earlier, mid‐Palaeozoic thermal overprint correlated with tectonothermal activity accompanying accretion of the Avalon and outboard Meguma terranes to more inboard tectonic elements

ISSN:0072-1050    

DOI:10.1002/gj.3350290402

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe polymetaphoric Seve Nappe Complex (SNC) of the Scandinavian Caledonides contains sequences representing the passive margin between continental Baltica and the Iapetus Ocean. The Sarektjåkkå Nappe in the SNC consists of sedimentary rocks intruded by mafic dykes that constitute 70–80 per cent of the nappe and frequently form sheeted dyke complexes. The internal parts of the Sarektjåkkå Nappe escaped penetrative deformation and metamorphism during Caledonian nappe transport and primary igneous relations are preserved. A sequence of nine dyke generations have been identified in a sheeted dyke complex in one well exposed area (Favorithällen). The alteration in this dyke complex was triggered by heat in connection with the dyke emplacement; overprint from Caledonian regional metamorphism cannot be demonstrated. Neither the relative age of a dyke in the dyke complex, nor the contact metamorphism caused by the emplacement of subsequent dykes had any detectable effect on the chemical composition of the dykes. The chemical composition of the diabases thus reflects the composition of the magma. Xenoliths of diabase and various types of gabbro are not uncommon and represent wallrock sampling during intrusion. Irregular patches of static alteration reveal the presence of fluids (brines), probably derived from metaevaporites in the wallrock, during the period of dyke intrusion. Scapolite rich dioritic–quartz dioritic inclusions and veins formed by partial or complete melting of wallrock xenoliths. A Sm–Nd age determination indicates a crystallization age of 573 ± 74 Ma for the Sarek diabase, coinciding with the Vendian–Early Cambrian period of global continental break‐up. Geophysical data from continent–ocean transitions at modern passive margins agree well with the structure of the Sarektjåkkå Nappe. It is suggested that the Sarek dyke complexes were intruded during the initial stages of seafloor spreading in the Iapetus o

ISSN:0072-1050    

DOI:10.1002/gj.3350290403

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractIt is suggested that a decrease in graptoloid diversity in theutilisSubzone (early Telychian) resulted from a eustatic fall in sea level induced by the spread of ice sheets from the highlands of central Gondwanaland. Major extinctions occurred within the retiolites and the generaGlyptograptus, Petalolithus, StimulograptusandRastrites. The selectivity of the extinction may reflect the degree of environmental tolerance of the various graptoloid taxa, with those exhibiting water mass specificity most seriously affected. Lithological changes, here correlated with the glacial interval, are recognized from the Yangtze Platform of China and from Europe.

ISSN:0072-1050    

DOI:10.1002/gj.3350290404

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe disused railway cutting at Ammons Hill, Hereford and Worcester, exposes a sequence of beds belonging to the Devonian St Maughans Formation of Lochkovian (Gedinnian) age. The beds are of Old Red Sandstone facies, but contain brackish water faunas. These faunas occur at a level generally considered to be above the level of marine influence that affected the older Raglan Mudstone Formation of mainly Přídolí Series age. The section, described by King in 1934, is now overgrown, but was excavated in 1986 by the British Geological Survey during its survey of the Worcester 1:50000 sheet. The evidence of the section calls for slight amendment of Allen's (1985) model of an interrupted transition from marine deposition in Ludlow time to freshwater deposition in Gedinnian time that was complete by the time of the formation of a regionally extensive calcrete palaeosol, the Psammosteus Limestone. Subsequent transgressive events took place before the establishment of apparently wholly fluvial and floodplain environmen

ISSN:0072-1050    

DOI:10.1002/gj.3350290405

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290406

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290408

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290409

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290411

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290412

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350290413

出版商:John Wiley&Sons Ltd    

年代:1994

数据来源: WILEY