Composite Enclaves and Xenoliths in Granite: Kangaroo Island, South Australia

 

Physical disaggregation of preexisting magmatic rocks, with feldspar megacrysts, xenoliths and enclaves into a new magma (remobilization of solidified magmatic rocks).

 

This page describes a number of features found in Six Mile Lagoon in Kangaroo Island, where an older coarse-grained gneiss with large euhedral phenocrysts and xenoliths are transferred into a new unfoliated granite by physical disaggregation of the pre-existing granite.

Figure 1a) Composite enclave formed by a migmatite schist inside a coarse, weakly foliated granite, within an unfoliated and slightly finer granite. This is interpreted as part of a break up process of gneissic granite and transfer of xenolith to a new magma. Notice that there is a ~5cm melanocratic granite rim around the enclave, and that the enclave width is controlled by the size of the internal schist xenolith.	1b) Same but the contact between gneissic granite and new granite is subtle.
Figure 1c) Composite enclave formed by a biotite schist inside a coarse, weakly foliated granite, within an unfoliated and slightly finer granite. The	1d) Same. Notice that in both (c) and (d) the gneissic granite rim of the enclave has been strongly eroded and is controlled by the size of the internal schist block.
Figure 1e) Composite enclave formed by a migmatite schollen inside a coarse, weakly foliated granite, within an unfoliated and slightly finer granite.	1f) Lines drawn on outcrop to mark boundaries.
Figure 1g) Composite enclave formed by a schist inside a coarse, foliated granite, within an unfoliated and slightly finer granite.	1h) Composite enclaves formed by a schist and a migmatite inside a coarse, granite, within a slightly finer and grey granite. Notice that only a very narrow rim of the coarse granite remains on the lower migmatite xenolith
Figure 1i) Gneissic granite with euhedral megacryst of K-feldspar nearly completely transferred to the finer surrounding granite by erosion of the gneiss.	1j) Composite enclaves formed by a schist and a migmatite inside a coarse, granite, within a slightly finer and grey granite. Notice that only a very narrow rim of the coarse granite remains on the lower migmatite xenolith

 

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