Fractionation processes in the Boggy Plain zoned pluton

Abstract

The Boggy Plain Granitic Complex is an early Devonian concentrically zoned, level pluton, ranging in composition from a dioritic rim to an aplitic core (Si02 range 50-75%), and located in the southeastern part of the Lachlan Fold Belt. This study of the complex provides fundamental information on the behaviour of trace elements in rocks and minerals fractional crystallisation of granitic magmas. during The intensive parameters, temperature, f(02), and f(H20) have been monitored throughout the crystallisation history by applying thermodynamic treatment of pertinent reactions to compositional changes of the minerals of the complex. The initial magma began crystallisation near ll00°C a f(02) the magnetite wustite buffer and with low f(H20). below As fractionation proceeded, f(02)and f(H 20) increased while at temperatures of about 710°C, The attributed to open system behaviour of increasing f(02) H2 in the magma, with H2 e sea ping to the atmosphere, Such a increasing trend in f(02) is required for fractional calc-alkaline trend - the Bowen Trend. crystallisation along a The Boggy Plain complex is one of the rare examples where such a trend can be verified without resorting to magma mixing or crystal-liquid unmixing. A petrogenetic model has been derived in which the complex represents the cumulate residue in a high level magma chamber beneath a central volcanic complex now removed by erosion. None of the rocks have compositions close to original liquid compositions. Initially the chamber contained a homogeneous magma of andesitic composition with few or no suspended crystals. Crystallisation proceeded from the walls inHards. However only the near-liquidus minerals crystallised at the crystallisation front, before the residual liquid surrounding the crystals became buoyant enough to rise along the chamber walls by boundary layer flow (Turner,1980). Not all the residual liquid escaped, as between 15% and possibly up to 70% in the felsic rocks, of the cumulate residue consists of crystallised trapped interstitial liquid. Nuch of the residual liquid escaped to the top of the chamber where it seems a double diffusive interface formed (Chen & Turner,1980). The liquid above the interface probably escaped as rhyolitic volcanics. Compositional zoning below the interface was produced by mixing of the primary magma with some of the fractionated liquid while the latter was undergoing boundary layer flow. The model can explain compositional zoning in high level magma chambers previously attributed to thermogra vitational diffusion (e.g.the Bishop Tuff); it also provides a mechanism of producing highly fractionated aphy ric rhyolites, as relatively crystal-free lavas can be erupted above a chamber and the lavas can have equilibrated with large quantities of residual solids. The felsic core of the complex has been affected by high temperature hydrothermal alteration as a result of magnetitic fluid pressure reaching confining pressure, which is estimated to have been 1.5±0 .5 Kbars. During alteration, sphene disappears from the rock, and is replaced by ilmenite, scheelite, and a little niobian rutile (containing up to 4% W03). Other effects associated with the hydrothermal alteration are; the crystallisation of the Ti-Nb-U-HREE oxide yttrobetafite, and the rimming of zircon grains by hydrothermal zircon which is richer in Hf, U, Th, and HREE's than the primary igneous cores. Apatites in the hydrothermally altered granites are that the solutions were enriched in u, Th, w, main component of the low fugacities of HF HREE's, Y , Hf, N b ,and T a. Water was the solutions since high fugacities of H20 and and C02 can be demonstrated. The study indicates that many chemical changes take place in granites during high temperature alteration, and it is only with a detailed chemical study of accessory minerals that these effects of alternation can be found.