DEFORMATION/FLOW MECHANISMS IN THE 6.8 KM LONG MCGREGOR TRACHYTE FLOW, MAUI, HAWAII, USA

McGregor Point on the SW coast of the island of Maui, Hawaii is composed of massive alkalic trachytic lava belonging to the Honolua volcanic series. This flow has been dated at 1.16 Ma and represents late stage alkalic volcanism associated with the West Maui Volcano. The exposed flow is over 6800 m in length, with an average thickness of 42m and an average width of 160 m. The McGregor Point Trachyte (MPT) travelled from the vent down the southern flank of the volcano over an average slope of 15-18⁰. Compositionally, the MPT has a silica content ranging from 57-64 wt. % SiO₂ yielding temperature dependant viscosities (with 18%H2O) ranging from 10⁶ to 10¹² Pa sec. In hand sample and thin-section, most samples show well developed flow banding defined by elongate (.25-3mm length) plagioclase laths ( average aspect ratio 6.5:1.0) along with rounded (rolled) biotite (with magnetite rims) and magnetite grains. The fine grain matrix lacks a SPO and makes up around 58% of the sample (volume). Textural analyses (intercepts program) of samples through the flow show transition from a brecciated cap at the top to vesiculated trachyte with strong SPO to compact trachyte with well-defined and consistent SPO producing a well-defined foliation in upper and middle portions of the flow, to zones of discrete shear zone development in the lower sections. In these samples, rounded magnetite grains are concentrated in the shear zones and due to their equant shape appear to have facilitated flow causing shear thinning. This allowed the flow to move even at the lower temperatures and hence higher viscosities in the lower portions of the flow near the base. A complex combination of deformation mechanisms evolving through the life of and position in and mineralogy of the flow allowed this highly viscous trachyte lava to flow efficiently.