Superficial Geology

Basalt dykes have also been noted intruding the volcanic rocks in the Aberdeen Country Park (3547 1328)
and near the Cape Collinson Training Centre (4419 1259). Basalt dykes cut volcanic rocks, granodiorite and
quartzphyric rhyolite on the D'Aguilar Peninsula (4410 0770, 4450 0804 & 4470 0768). Allen & Stephens
(1971) recognized vogesite lamprophyre at Cape D'Aguilar, but in this survey the separation of lamprophyre
and basalt dykes, both very similar in appearance when weathered, has proved impractical. Where these
dykes are exposed in the shoreline, deep erosion has taken place along the dyke, producing caves and in one
case (4481 0774) a spectacular blowhole feature.
Lamma Island. Basalt dykes have been noted at several localities on Lamma Island, and in many cases the
intrusions appear to be related to and follow faults. Southwest of Mount Stenhouse (3072 0476) a 3.5 m wide
basalt dyke follows a major N-S striking fault, and 1 km to the east (3162 0496) another basalt dyke, 1.5 m
wide, follows an important NNE striking fault. At Sok Kwu Wan Quarry (3180 0832) large, angular and
sub-angular xenoliths of basalt are found in the granite, suggesting that at least some basalt predates the Upper
Jurassic to Lower Cretaceous granites (Plate 25).
Po Toi Island Group. Dykes of basalt extending less than 200 m and less than 2 m wide cut granites on Lo Chau
and Po Toi Island; they generally trend between ENE and NE. At Tai Pai (4605 0358) a number of basalt dykes
cut the coarse-grained granite and fine-grained granite as well as faults and a quartzphyric rhyolite dyke.
/' :%> '
- *3
Plate 37 - Basalt Dykes Cutting Medium-grained Granite at Diamond Hill Quarry (3906 2274)
Pegmatite
Classification, Distribution and Lithology
Pegmatite consists of very coarse-grained granite, with crystals over 20 mm across (Table 3). It
usually occurs as lenticular patches or nests within the granite, but also in thin dykes or veins.
Pegmatite is commonly concentrated at or close to the granite-volcanic contact.
Pegmatites represent the last and most hydrous part of the granite magma to crystallise, and may
contain high concentrations of minerals present only in trace amounts in the surrounding granite.
Alkali feldspars, usually pink in colour, and quartz make up the bulk of the pegmatite, with
occasional biotite and small plagioclase crystals present in some bodies. The alkali feldspar and
quartz crystals often attain 100 mm in length but average 30 to 40 mm.
Pegmatite is least common in the coarse-grained granite, but widespread in the medium-, fine- to
medium-, and fine-grained granites, being most abundant in the latter.