Physical volcanology and geochemistry of a monogenetic lava shield

Below is a physical excerpt from my honours thesis submitted on the 25/10/13. It is such a tedious chore completing such a project, and requires a vast amount of time and energy - trust me, I almost went crazy haha

Hope you enjoy the read!

Physical Volcanology and Geochemistry of a monogenetic lava shield, Mount Napier Volcanic Complex, Newer Volcanics Province

S. D. DYER (B. A. Sc.)
SUPERVISOR: R. A. F. CAS & C . FOLKES
School of Geosciences, Monash University, Vic. 3800, Australia.

Despite their name, monogenetic volcanoes display great complexities in their variety of magma composition, and pre-eruptive magmatic evolution. The Mt.Napier Volcanic Complex (MNVC) is a monogenetic volcanic centre in S.E. Australia’s Newer Volcanics Province (NVP) that displays a transition from largely effusive lava flows to a scoriaceous spatter rampart related to a Hawaiian fire-fountaining event. The Harman’s Valley flow flowed down the Harman’s Valley in a west south-west direction after being erupted from Mt. Napier. Within the Harman’s Valley, several important lava flow structures have been formed and preserved including the nationally and internationally renowned Wallacedale tumuli, and Byaduk Caves. Minute changes in the crystal content of the various lava flows have affected the varying viscosities of the different flow types – a’a and pahoehoe and consequently led to the variance in eruptive style of the complex. Major and trace element analysis of selected samples from the volcanic stratigraphy/succession identified a single co-magmatic magma batch, related by processes of fractional crystallisation, ~20% olivine, ~10% clinopyroxene and ~2% magnetite. Trace element modelling and Rare Earth element (REE) analysis suggest melting originated in the lower lithospheric mantle at depths of 73km, by ~8% partial melting of an enriched metasomatized garnet-bearing lherzolite source.

Key words –Mount Napier Volcanic Complex (MNVC), Newer Volcanics Province (NVP), monogenetic volcanism, geochemistry

1. INTRODUCTION

Most monogenetic basaltic volcanoes are traditionally thought to experience one short-lived eruptive event, sourced from a single magma batch. However, recent research demonstrates that monogenetic volcanoes may have complexities in their evolution, behaviour and origin. This includes the depositional facies present, their related eruption styles (Van Otterloo et al. 2012), and variations in composition (Brenna et al. 2010; Needham et al. 2011).

The Mount Napier Volcanic Complex (MNVC) is the largest eruption centre in the late Cenozoic Newer Volcanic Province (NVP) of south-eastern Australia, and shows a transition from shield volcano with an extensive lava flow field to a summit scoria cone complex. The Newer Volcanics Province (NVP) is an intra-plate basaltic, monogenetic province and represents the youngest volcanic activity on the Australian landmass (Johnson 1989).

Recent work on the MNVC includes physical volcanology studies by Gill and Elmore (1974), King (1978) and Whitehead (1991). Whitehead (1991) conducted a preliminary study of the volcanic facies associated with both the initial shield building stage and then later the scoria cone building eruptive sequence. Whitehead (1991) identified three separate lava flow facies, representing eruption styles transitional between effusive magmatism and Hawaiian-style fire fountaining.Although previous physical volcanology has been undertaken on the MNVC, it has been at a fairly general level, and therefore there is scope for more detailed description and geochemical characterization of the complex volcanic deposits, and understanding the factors influencing eruption styles and changes therein.

Therefore the aims of this study are to determine:

• The causes of the interchangeable eruption styles and the variety of the volcanic deposits preserved
• If the MNVC is a product of a single or multiple eruption phases, and
• Whether single or multiple magma batches were involved