GRANITES AT WILSONS PROMONTORY
Copyright 1999 by Patrice Rey. All right reserved.
Unlimited permission to copy or use is hereby granted for non-profit driven enterprise subject to inclusion of this copyright notice and my World Wide Web URL: www.geosci.usyd.edu.au/users/prey/. An email stating where this material is going to be used would be very appreciated. Thanks PR
Wilsons Promontory is one of the most famous Australian National Parks. It is located on the coast of Victoria, 180 km southeast of Melbourne. Although this Park is reputed for its sandy beaches and spectacular sceneries, it is also a very interesting place for geologist interested in granites. The Wilsons Promontory Batholith is one the largest and best exposed Lower Devonian batholith in Victoria. This region is part of the Lachlan Orogen developed along the Pacific margin of Gondwana during the Palaeozoic. This Batholith is formed by a number of sheet like granitic bodies which dip shallowly eastward. This virtual field-trip will bring you there...
a1/ Yes, it is that nice... a2/ and a3/ are two pictures showing the same outcrop in two perpendicular planes. a2/ we are looking here at a foliation surface (XY plane). The large yellowish minerals are potash felspars. Because of their tabular shape K-feldpars tend to develop a preferred orientation during the flow of the magma. a3/ shows a section perpendicular to the foliation plane. K-felspars are now seen on their sides, they 'float' in a matrix of quartz, plagioclase and biotite. Note the preferred orientation of the K-feldspars, the long axis of the tablets are statistically oriented parallel to the long side of the picture. The dark blob in the center is a mafic enclave, which long axis is also parallel to the foliation.
Pictures b1 to b3 are perpendicular to the foliation plane. They show what happens when numerous solid particles, such as K-felspars, interact during the flow of the magma. Interactions between particles impede their rotation. b1/ Two K-felspars are imbricated like tiles on a roof. This "tilling" impedes further rotation therefore stabilizing the magmatic foliation that develops during the flow of the magma. b2/ A spectacular example of tilling, the direction of the flow of the magma can be deduced from this structure. Here, the flow is top to the right. b3/ This picture shows again tilling between K-felspars, but if you look closely you will see that one of the felspar (in the center of the picture) impinges on the one underneath. This suggests that the local stress related to the tilling was strong enough to trigger deformation via a pressure-solution process.
c1 and c2 show more pressure-solution. c3 shows a coronitic reaction where a thin biotite-rich layer (dark mineral) wraps around a K-felspar. Biotite is a potasic mineral that incorporates iron and magnesium as well. Here the biotites "pump" potasium from the K-felspar and iron and magnesium from nearby garnets (the red spots on the left side). This reaction suggests that the felspar was not in chemical equilibrium with the surrounding magma.
Granites may contain many compositional heterogeneities in the form of enclaves, xenoliths, schlierens and dikes. Pictures form d1 to d3 show a pipe (tube-shaped conduit) cutting through the granite. Pictures d1 and d2 show a section of the pipe parallel to its long axis. The third picture shows a section perpendicular to the pipe. This pipe is filled of mafic enclaves.
The majority of mafic enclaves does not appear in pipes, but are distributed within the felsic magma. In addition to the enclaves (genetically related to the granite), there are also some xenoliths (rocks coming from the surrounding host rock in which the granite intruded). e1/ shows both enclaves (top two blobs) and a xenolith (the one at the bottom is a metasedimentary xenolith). One very common feature is the occurence of large K-felspars from the felsic magma in inclusion within mafic enclaves (e2 and e3). This suggests that both mafic and felsic materials were still partially melted allowing for K-felspar to move from the felsic magma into mafic enclaves, and that there was little viscosity contrast between them at the time when K-felspars (that crystallised at an early stage within the felsic magma) were included in the mafic enclaves.
f1/ shows a mafic enclave full of K-felspars. This enclave was isoclinally folded during the flow of the magma. Note that other mafic enclaves have little or no K-felspar inclusions and that they are not strongly deformed. One can argue that the isoclinally folded enclave was incorporated in the magma at a very early stage when both mafic and felsic material had a similar rheological behaviour and K-felspars could easily move inside the mafic enclaves. As the magma cooled, the mafic enclaves crystallised and deformed plastically (isoclinal fold). At that stage, new mafic enclaves were incorporated in the now cooler felsic magma. As they crystallised more rapidly K-felspar could no longer find their way into the mafic enclaves. f2 and f3 show felsic dikes. In the picture f2, the dike has a different composition from that of the host rock, however K-felspars from the host granite were able to move into the felsic dike. This suggests that the intrusion of the felsic dike occurred at a stage when the host granite was still a melt-rich magma. In f3, the dike is the result of melt segregation process that developed during the crystallisation of the felsic magma. Its composition is similar to that of the coarse grained host rock, and its margins are rather transitional.
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