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Megacrysts and Megeacryst Transfer Across Magmas, Kangaroo Island, South Australia

 

megacryst

Roberto Weinberg, Monash University, Australia

 

 

 

Along the south coast of Kangaroo Island, we found three distinct coarse-grained Bt-Ms granites with large megacrysts of K-feldspar. All three granites are essentially very similar and so are the megacrysts. This page describes their diffences and similarities. These granites are found in two separate areas and are found intruding broadly contemporaneous migmatites. One area is west of Vivonne Bay (Seal Rock) and the other is close to Little Dicky Beach in Six Mile Lagoon. In the latter, an older coarser-granite with a gneissic foliation is partly digested by a younger finer-grained and more felsic granite which seems to inherit some megacrysts through assimilation of the gneissic granite (megacryst transfer).

 

Megacrysts in all granites are large (upto 10 cm), and tending to equidimensional (aspect ratios close to unity). They are typically euhedral, some times rounded. They have inclusions of Bt, generally randomly distributed, and less commonly in zonal arrangements. The megacrysts in both the older and younger granites at Little Dicky Beach differ in that they also have inclusions of Ms. Bt inclusions here tend to be single grains, whereas at Seal Rock there are also fine-grained 0.5cm clots of Bt+Qtz similar to clots found in the surrounding granite.

 

 

 

Megacrystic granite
Figure 1a) K-feldspar megacrysts in granite in Seal Rock, close to Vivonne Bay. Notice presence of round xenoliths or schollen of metasedimentary rocks including migmatite blocks. Lens cap is 5.5cm in diameter. 		Megacrystic granite
Figure 1b) K-feldspar megacrysts in Dicky Beach Granite (to the right) and older gneissic granite (to the left), close to Six Mile Lagoon.
Megacrysts in Seal Rock granite west of Vivonne Bay
Megacryst
Figure 2a) Example of round megacryst with concentric Bt inclusions. 		Megacryst
Figure 2b) Example of round megacryst with concentric Bt inclusions (scale: ~ 4cm in diameter).
Megacryst
Figure 2c) Megacryst with fine-grained Qz-Bt clot comparable to those in the surrouding granite. 		Megacryst
Figure 2d) Fine-grained Qz-Bt clot comparable to those inside the megacrysts.
Megacryst
Figure 2e) Megacryst with a rim of fine quartz-biotite grains. 		Megacryst
Figure 2f) Megacrysts within a xenolith rich in biotite (we are not sure of its origin).
Megacrysts in Dicky Beach Granite and older gneissic granite, Six Mile Lagoon
Quartz Porphyroclast Mylonite megacryst transfer to granite
Figure 3a) Contact between intrusive dyke of megacrystic Dicky Beach granite into coarse-grained, clot, megacrystic gneissic granite. Figure 3b) Composite xenolith comprising two types of coarse-grained gneissic granite. The one in the core is more mafic and has a K-feldspar megacryst. Click on photo to see higher resolution image with no lines.
megacryst transfer to granite megacryst transfer to granite
Figure 3c) Euhedral megacryst with Bt inclusion bordered by a thin rim of coarse gneissic granite forming a xenolith in the Dicky Beach megacrystic granite: evidence for Kfs transfer one to the other granite. Click on photo to see higher resolution image with no lines. Figure 3d) Euhedral megacryst with Bt inclusion bordered by a thin rim of coarse gneissic granite forming a xenolith in the Dicky Beach megacrystic granite: evidence for Kfs transfer one to the other granite. Click on photo to see higher resolution image with no lines.
megacryst transfer to granite megacryst transfer to granite
Figure 3e) Euhedral megacryst with Bt inclusion within a dark granitic xenolith inside a the Dicky Beach granite. Figure 3f) Megacryst in Dicky Beach granite with a thin rim rich in biotite that could be interpreted as a thin slice of the gneissic granite after assimilation.