Abstract:
The stratigraphy and structure of the Cambrian and Ordovician rocks of
the Blaenau Ffestiniog area are described briefly.
This forms a background to a more intensive study of the four types of
intrusive igneous rocks which are emplaced among the sedimentary and igneous
rocks of the region.
The intrusive rocks are: The Tan-y-Grisiau, Microgranite. Microgranodiorite and Quartz Latite. Dolerite. Multiple Intrusions. (Dolerite-quartz latite). The Tan-y-Grisiau microgranite is intruded into Cambrian rocks and has an extensive metamorphic aureole. Structural evidence indicates that it was
emplaced prior to the imposition of the regional cleavage (Caledonian) and
faunal evidence from outside the area under consideration shows that its emplacement occurred during post-Harnagian, pre-Longvillian times. The mineral assemblage in the main part of the intrusion (hornblende
- biotite
- plagioclase - orthoclase - quartz) can be equated, in terms of metamorphic
facies, with a relict assemblage discovered in the hornfelses (andalusite-
- Ooridrite - biotite - plagioclase - quartz). The north-western part
of the microgranite has suffered extensive post-magmatic modification
as a result of the operation of volatiles. Its mineral assemblage (chlorite
- sericite - calcite - albite - quartz) can be equated with the ubiquitous
retrograde assemblage in the hornfelses (chlorite
- sericite - epidote - albite
quartz). This assemblage is attributed to the presence of the slowly
cooling microgranite and to the operation of volatiles derived from it
rather than to retrogression brought about later by the imposition of cleavage
or folding. The microgranodiorite and the quartz latites are considered to be
closely related to the pyroclastic rocks of the Moelwyn Volcanic Series.
They were intruded at very shallow depth and in some cases intrusion was
continued after the rocks had consolidated. This gave rise to the development of
autobreccias which have previously been interpretted as agglomerates. The amount of metamorphism associated with these intrusions is very slight.
The dolerites occur both as thin dykes and sills of varying dimensions.
They are petrographically identical to the dolerites which are ubiquitous
throughout the Cambrian and Ordovician successions in the rest of North
Wales.
The multiple intrusions are thought to have been formed by rising
dolerite magma remelting quartz latite through which it passed. This in turn was intruded into the centre part of certain dolerite sills. Petrographic evidence suggests that acid and basic magmas were held in juxtaposition at high temperatures without any significant mixing taking place.
