Some thoughts on dolerite intrusions with particular reference to marginal features

Some thoughts on dolerite intrusions with particular reference to marginal features

20 Pages · 2000 · 1.82 MB · English

Marginal rocks have been frequently sampled for palaeomagnetic studies because of their even properties, rapid passage through the Curie Point and presumed originality and lack of contamination. Age determinations from such rocks are preferred on the basis that the chilled rock is free of argon los

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u,Q,/97'8_c30 Abstract Some thoughts on dolerite intrusions with particular reference to marginal features DE Leaman The finegrained rocks at the margins of Jurassic dolerite intrusions have been generally regarded as crucial to any understand­ ing of magma composition, temperature conditions and intrusion forms These rocks have also been used for age and magnetic pole determina­ tions While much relevant material can be provided it is often easily misread and must be carefully integrated Several examples of major demonstrably invalid conclusions are included INTPDDUCTION Observations based on contact rocks on both sides of Jurassic dolerite intrusion margins have been used in several fields; these are discussed in order of practical relevance , Understanding of margins is essential to the formulation of structural interpretations in mapping, hydrogeology and engineering projects, where it is necessary to establish the character of the margin contact or fault or spatial orientation of the intrusive body dyke, sill, sheet, wedge or transgression Marginal rocks have been frequently sampled for palaeomagnetic studies because of their even properties, rapid passage through the Curie Point and presumed originality and lack of contamination Age determinations from such rocks are preferred on the basis that the chilled rock is free of argon loss and representative of the original melt­ ing pulse Magma composition may be deduced if it is presumed that the chilled margin represents a leading fluid edge which is singular and continuously typical This implies no contamination Temperature conditions at the tlme of intrusion may also be deduced by consideration of either mineral species and proportions or from the scale and nature of the metamorphic aureole Conclusions of a structural nature are the most important and the most confusing and these are emphasised in the discussion below However, it will be noted that the latter four classes mentioned above depend on the same basic presumption, namely that the samples are derived from the same material, uniformly sampled at both source and in the field, and which were injected in a short time after a rapid unadulterated passage This may be referred to as the uniform leading edge theory LEADING EDGE THEORY Appreciation of the ramifications of the leading edge theory has never be~n discussed in the literature, although there is a general presumption of its truth Early petrologic studies of the Tasmanian dolerites (eg Edwards, 1942) were predicated at a time when a simple differentiation theory was commonly accepted The relationships between analyses of doler­ ite at Ht Nelson and Mt Wellington tended to support this concept Subse­ quent physical and petrographic observations on the same intrusions provided 301 substantial concurrence A measure of agreement was also given by work on the Great Lake intrusions (McDougall, 1964) Given that no other detailed petrologic work had been undertaken, it was reasonable in the circumstances to presume single pulse intrusion followed by simple differentiation 'A corrollary of this presumption was then to examine and compare the marginal materials with a view to the establishment of the properties of the magma The idea of a single intrusion is an old one in Tasmanian geology and may be inferred in the works of Hills et al (1922) and Carey (1958) It was held until Bradley (1965), Leaman (1970, 1975) and Leaman and Naqvi (1968) demonstrated that it could not be so if several sheets occurred in the same section Hydraulically this is no different to multiple intrusion of a 'single' sheet Hale (1953) and McDougall (1959) had suggested sections with two sheets, but Leaman (1970) and Leaman and Naqvi (1968) proved that they were general OVer the years, most field mappers hOive ignored the rich detail often available in areas

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