摘要:

AbstractThe Droimchogaidh sill, south‐central Mayo, is sheeted at high‐level in the Ordovician sedimentary pile of the South Mayo Trough. This association now lies raised on the eastern fringe of the Connemara‐Murrisk Tertiary uplift. The sill is basin‐shaped with a maximum thickness of about 100 m. It is composed of isotropic or weakly layered olivine gabbro, and subordinate, locally‐developed picritic cumulates and syenitic differentiates. The chemistry of the gabbro reveals the effects of post‐emplacement hydrothermal circulation, but the magma that supplied the sill was probably similar in composition to that of the analcime‐olivine dolerite dikes further west. The40Ar/30Ar dating gives an age of 55 ± 1 Ma for the sill, consistent with a younger set of ages from the Irish/British Ter

ISSN:0072-1050    

DOI:10.1002/gj.3350190102

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractNeedle‐fibre calcite occurs in association with root structures in a Lower Carboniferous calcrete crust. The origin of this form of calcite is discussed and the scattered literature from soil science, microbiology and geology is reviewed. It is concluded that while needle‐fibre calcite is not diagnostic of any environment its origin is related to the activities of various micro‐organisms, especially fungi. The use of needle‐fibre calcite as a climatic indicator is reviewed and it is used, with other criteria, as evidence of fluctuating climates in the Lower Carbon

ISSN:0072-1050    

DOI:10.1002/gj.3350190103

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractThe Lleiniog Sand and Gravel Formation, of late Devensian age, has been divided into four Members. The sediments are interpreted as the sequential products of the wasting of a single ice lobe. Member I formed where subglacial streams debouched into a pro‐glacial lake and is noteworthy for the occurrence of a bedform, hitherto undescribed but which may be common in many outwash sequences; Member 2 consists of channel sands and gravels deposited by pro‐glacial streams; Member 3 has the characters of a melt‐out till deposited after a readvance; and Member 4 accumulated in pools of meltwater on the irregular surface of Member 3.Clasts above granule grade are dominantly of locally‐derived Carboniferous rocks although a far travelled component from northern England and southern Scotland can be distinguished as well as a small but constant input of Lower Palaeozoic rock types from the Conwy area. The sand grade is dominated by quartz grains released from Permo‐Triassic sandstones. Clay grade material has been derived from Carbonifero

ISSN:0072-1050    

DOI:10.1002/gj.3350190104

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

摘要:

AbstractThe Elchuru alkaline igneous intrusion is an arcuate‐shaped ring complex, approximately 16 km2in area, cropping out in the Eastern Ghats high grade metamorphic series. It is part of an alkaline province composed of a number of intrusions which range from ijolite‐melteigite to alkali gabbro (viz. malignite, melalusitanite, shonkinite) and then to hypersolvus nepheline syenite followed by subsolvus nepheline syenite. The complex is cut by late lamprophyric dykes. A nephelinized alkaline gneiss, within the investigated complex, is the only deformed rock type and is regarded as an older unit not related to the comagmatic series. The remainder of the complex was emplaced post‐tectonically. Sovitic carbonatite is a conspicuous Iithologic unit associated with the complex.Chemical analyses of 19 selected samples for 13 major oxides and 5 trace elements (Rb, Ba, Sr, Zr and Nb) are presented to establish a geochemical model for the investigated complex.The mineralogy, petrography and geochemistry of the rocks of the Elchuru Alkaline Complex suggest that it was formed by differentiation of an initially alkali‐rich ijolitic magma as reflected in the crystallization of nepheline, kaersutitic amphibole and alkali feldspar. With progressive increase in alkali feldspar content (volume percentage) the ijolite passes to malignite and then nepheline syenites. Amphibole shows sodic enrichment from a dominant calcic variety. Pyroxene, likewise, shows sodic enrichment following the scheme salite‐ferrosalite to aegirine‐augite. The igneous cycle closes with the intrusion of biotite lamprophyre.There is a systematic increase in total alkalies (Na2O+K2O) and decrease in CaO from the early mafic rocks to the syenitic rocks. The alkali‐lime index of the complex is 48 indicating its strongly alkaline nature (Peacock 1931), and they are miaskitic in character (agpaicity index<1, Currie 1976). Such miaskitic complexes are associated with carbonatites (H

ISSN:0072-1050    

DOI:10.1002/gj.3350190105

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350190107

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350190109

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350190110

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350190112

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350190114

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350190101

出版商:John Wiley&Sons Ltd    

年代:1984

数据来源: WILEY