Magma Mingling at Bingie Point, NSW, Australia

 

Coastal outcrops at Bingie-Bingie Point in New South Wales, expose several phases of mingling bewteen mafic and felsic magmas. This site is divided into two parts: a) general mingling features, including the irregular shape of pillows, and the breakup of mafic dykes into angular pieces after it intruded into a granitic magma; b) evolved, leucocratic pegmatitic rock as narrow veins in mafic pillows.

 

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a) General mingling features and the breakup of dykes

 

 

Figure 1a) Net veining of two mafic sills after they intruded into a felsic chamber. Notice how the resident granitic magma becomes increasingly felsic upwards in the lower sill, to leucogranite at its top. There is also, a thin layer of felsic granite below the lower pillows in the upper sill.	1b)
Figure 1c) Net veining of mafic intrusion. Notice how the magma in between the pillows is more felsic than the granitic magma outside the net-veined area. There are also ghosts of partially equilibrated enclaves in the granitic rock in between the two mafic blobs.	1d) Breakup of mafic magma into pillows.
Figure 1e) Complex net-veining.	1f) Pillows impinging into each other.
Figure 2a) The breakup of a late mafic dyke into angular pieces of low aspect ratio. Notice a number of other enclaves in the granitic rock, presumably a result of several phases of earlier mingling events. See also Fig. 3g, h, i for more breakup of dykes.	2b) Same as (a). Notice felsic rock in between the two mafic blocks on the right-hand-side. There are also a number of other enclaves in the granitic rock, presumably a result of several phases of earlier mingling events. See also Fig. 3g, h, i for more breakup of dykes.

 

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b) Evolved, leucocratic pegmatitic rims and narrow veins in mafic pillows. These are interpreted to be evolved interstitial, possibly volatile-rich magma, that is mobilized contraction or flow-related cracks develop in a cooling mafic magma (Figure 3). There is also an asymmetry in the distribution of felsic rims: they are found in the lower-side of flat pillows and indicate way up (Figure 3), possibly having formed as upward migration of fractionated magma from the interstices of the felsic magma.

 

 

 

 

Figure 3a) Flat mafic pillow underlain by a 1cm thick of leucocratic, coarse-grained granite. This rim is absent from the upper surface of the lower pillow, but fills in a tortuous crack.	3b) detail of (a), dykelet developed into a crack in mafic rock is filled with leucocratic, coarse-grained granite.
Figure 3c) Flat mafic pillow: its lower side has smooth contact with granite and cracks indicative of buckling and extension. Cracks are filled with the leucocratic magma. Its upper side by contrast has irregular contact with the granite.	3d) detail of (c).
Figure 3e)	3f)
Figure 3g and h) Dyke frozen in the process of breaking up by influx of surrounding magma and producing angular enclaves. The magma in between the enclaves are dominantly leucocratic. Together with Fig. 3h this is interpreted to indicate that the first magma to be mobilized was felsic, followed by the more melanocratic magma. This process is best seen in Fig. 3i).
Figure 3i) This image indicates the essence of the process: a crack develops, low pressure mobilizes low viscosity, water-rich magma from the interstices of a mush, or even perhaps an exsolved fluid-rich phase. As the crack opens the mush becomes mobilized and intrudes the crack to form a dyke rimmed by leucocratic pegmatitic rock.

 

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