Precambrian rocks in northwestern Chamwood Forest differ markedly
from their lateral equivalents to the east and south. They are subdivided into the
Whitwick Volcanic Complex, of massive to intensely brecciated high-silica andesites
and porphyritic dacites, and the Chamwood Lodge Volcanic Formation, which is a
thickly bedded sequence of mainly andesitic to dacitic volcanic breccias and lapillituffs.
Lithological elements common to both of these units are indicated by field,
petrographical and geochemical evidence, which suggests the existence of two
'genetic associations' of rock-types. These associations, and various other units that
are distinctive to this region, form the basis of a model that views the Whitwick
Complex as an aggregation of magmatic feeder bodies that supplied material, in the
form of blocks and lapilli, to a volcaniclastic apron represented by the Chamwood
Lodge Formation. The analogues for these rocks can be drawn from the axial
magmatic zones of modem or geologically very young volcanic arc systems. The
high-silica (dacitic and rhyolitic) Charnian magmas were intruded into
unconsolidated wet sediments, resulting in physical interactions that generated
peperitic lithologies and related breccias. By contrast, the andesitic magmas may
have extruded subaerially as lava domes that periodically collapsed, giving rise to
block and ash pyroclastic flows and lahars
