Small basalt quarry with rhyolitic xenoliths (now closed).

"A quarry on Poihipi Rd, where a section of the northwest dipping flank of the cone is exposed ...... This is a small cone of bedded basalt scoria that is variably welded. It contains fluidal lava bombs and all components contain abundant rhyolite lava lithics." (Leonard, 2003)

Leonard (2003) also reports " The only other exposure is a small cutting (further) along Poihipi Rd." The map reference for the second exposure relates to a point ~500m NNW of the Quarry, well away from the road but this reference coincides with an outcrop of "epiclastic breccia" noted by Stevenson et al (1994).

Note: For some time this locality has been erroneously referred to as the "K" Trig Quarry but that quarry is much further East of here.

The volcanic bombs from this quarry often host cores of rhyolite (and perhaps in some rare cases ignimbrite) which occur as two main varieties:

1. very light alteration - slight colour change at contact and retain many of the original gas phase minerals (tridymite, fayalite, magnetite, phlogopite etc). These bombs are viewed as containing wall rock lithics and many are smooth, rounded "cannon ball" shapes. The lack of melting indicates short residence time in the vent during Strombolian activity.

2. Highly altered - Complete melting and recrystallisation of the rhyolite to a grey, glassy matrix with numerous gas vesicles and with new gas phase minerals deposited (enstatite, magnetite etc). The contact area between the basalt and rhyolite in the bombs is marked by a red oxidation zone where fine grained hematite has been deposited. Complete melting and recrystallisation also occurs to ignimbrite clasts to form an opaque creamy matrix retaining many of the primary ferro-magnesium minerals along with numerous small gas vesicles but there is little if any secondary deposition of minerals. The clasts within these bombs originate in the walls and roof of the magma chamber with sufficient residence time to completely melt. They came to the surface as irregular blocks and the basaltic exterior often shows blisters of rhyolitic glass that have bled through from the molten core.

Other Regions, Features and Areas containing this locality

References