Structural features and mode of emplacement of part of the blue tier batholith in Northeast Tasmania
Titel:
Structural features and mode of emplacement of part of the blue tier batholith in Northeast Tasmania
Auteur:
Gee, R. D. Groves, D. I.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 18 (1971) nr. 1 pagina's 41-55
Jaar:
1971-05
Inhoud:
The Blue Tier Batholith in northeastern Tasmania is a composite, magmatic intrusion of Upper Devonian age, that intruded a folded, essentially unmeta-morphosed Siluro-Devonian sedimentary sequence at a rather high level in the crust. Aureoles of contact metamorphism in the sedimentary sequence are narrow. The batholith consists of several petrologically and structurally distinct components that intruded in an evolutionary sequence, with the younger units generally being more felsic. The regional stress environment appears to have been one of dilation across pre-existing structures, and regional tension is envisaged. The earliest event was emplacement of a sheet of biotite-hypersthene adamellite porphyry that intruded as a hot, liquid magma. This was followed by emplacement of hornblende and biotite granodiorite plutons, that were apparently structurally controlled by fractures both parallel and perpendicular to the pre-intrusion fold axis. There is local, marginal evidence for “shouldering-aside” of the country rock, but intrusion appears to have occurred mainly by upward displacement or crustal rifting. Large plutons of biotite granite and adamellite discordantly intrude both the country rocks and the earlier granodiorites, and are commonly separated from the granodiorites by thin screens of contact metamorphosed country rock, that are interpreted as remnants of the original walls of the embryonic batholith. The matching of pluton-wall irregularities on a regional scale indicates lateral dilation of at least 4.5 km during intrusion of the biotite granite/adamellite plutons. These plutons appear to have intruded along pre-existing fracture surfaces by a combination of upward displacement of roof rocks and lateral dilation of wall rocks. Finally muscovite-bearing granites and adamellites were intruded as predominantly sheet-like masses that were emplaced by lifting their roofs.
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