Development of viscous fingering between mafic and felsic magmas: evidence from the Terra Nova Intrusive Complex (Antarctica)

Summary

A wide range of types of contact morphology among mafic and felsic magmas are observed in outcrops on Vegetation Island (Terra Nova Intrusive Complex, Antarctica). Image analysis and fractal geometry techniques were applied for in-depth study of the mafic/felsic interface, with the aim of studying the origin of the varied morphologies. In particular, the length (IP _N ) and fractal dimension (D_box) of interfaces were measured. Results indicate that there is a close exponential dependence of IP _N on D_box.

The observed morphologies are identical to those observed during viscous fingering processes induced by the displacement of a more viscous fluid by a less viscous one. To test if viscous fingering was responsible in this case too, IP _N and D_box values were measured on viscous fingering structures obtained experimentally using various viscosity ratios (V _R ) from the literature. Results indicate that, as in the natural case, there is an exponential dependence of IP _N on D_box, leading to the conclusion that the varied interface morphologies between mafic and felsic magmas are the result of viscous fingering dynamics. In addition, experimental studies clearly show that there is an exponential relationship between the viscosity ratio of fluids and the interface fractal dimension (D_box), and the ratio between the two types of magma was estimated using this relationship. It is shown that viscosity contrasts between mafic and felsic magmas varied considerably, ratios ranging from ca. 6 to 49. These results, together with outcrop evidence, provide indications regarding the evolution of the magmatic system, which generated the actual mafic/felsic associations on Vegetation Island.