Mingling Granite-Diatexite, Kangaroo Island, South Australia
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| At Six Mile Lagoon, immediately E of Little Dicky Beach, there is a contact between a diatexite loaded with schollen-schlieren and a cleaner,
coarser, more leucocratic megacrystic granite. The two have complex contacts with the granite intruding and disaggregating the diatexite.
The irregular interleaving of the two, and the transfer
of schollen to the granite suggests the possibility that the two were in the magmatic state. Furthermore, the structures suggest that these rocks were being
sheared at the time on shear planes trending NNE/dipping moderaltely to shallow NW with an oblique dextral-thrust shear sense. The ditatexite granite is fine-grained, Bt-rich with 0.5cm Ms grains, and looks undeformed with foliation related to flow and disaggregation of schlieren. It has phenocrysts rather heterogeneously distributed, locally forming aggregates The megacrystic granite which mixes with diatexite is coarser-grained and more leucocratic with few rounded, metasedimentary xenoliths. |
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| Figure 1a. Irregular narrow sheets of diatexite separated by intrusive megacrystic granite. Notice the thinned out band of the grey diatexite linking small schollen forming the narrow sheet of diatexite. | Figure 1b) Composite xenolith comprising two types of coarse-grained gneissic granite. The one in the core is more mafic and has a K-feldspar megacryst. |
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| Figure 1c. Diatexite disaggregated by megacrystic granite. The curved sub-horizontal surface exaggerates features due to low angle of their mutual contact: here they define a close irregular fold. Note composite gneiss xenolith. | Figure 1d) Megacrystic granite intruding along margin of schollen in diatexite preserving the boudin neck. |
Figure 2a) Disruption of diaxexite by intrusion of megacrystic granite. All that is left of the diatexite granite are narrow fine-grained granitic schlieren linked to pelitic small schollen as well as quartz block and migmatite raft. The original diatexitic magma has been effectively removed by the intrusive magma with a tranfer of schollen from diatexite to granite. Vertical wall, asymmetry of clasts indicate top-to-right translating into a dextral thrust. |
Figure 2b) Detail. The diatexite grey granite matrix is preserved as narrow <1cm darker bands linking schollen blocks. Notice megacryst in granite. |
Figure 2c) Detail. Remnants of grey granite matrix linking schollen and quartz blocks originally present in diatexite. |
Figure 3a) Disruption of diaxexite band (in the middle) by intrusion of megacrystic granite. Towards the right, the diatexite grey matrix is replaced by a lighter, coarser, megacrystic granite matrix. The original diatexitic magma has been effectively removed by the intrusive magma with a tranfer of schollen from diatexite to granite. Photos from a block. |
Figure 3b) Detail (left side of 2a) showing coarser, lighter granite above and below and intruding the grey diatexite band.. |
Figure 3c) Detail (right side of 2a) showing that the grey diatexite matrix has been replaced by the intrusive granite. |
Contemporaneity between mixing with dextral-thrusting on N20E/30-40NW dipping planes
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Figure 3b) Detail (left side of 2a) showing coarser, lighter granite above and below and intruding the grey diatexite band.. |
Figure 3c) Detail (right side of 2a) showing that the grey diatexite matrix has been replaced by the intrusive granite. |