Relationships between the Cobo Granite and the Bordeaux Diorite Complex, Guernsey

• Main Author: D'Lemos, R. S.
• Published: Oxford Brookes University 1987
• Subjects: 551; Geology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304090

The main aim of this work was to investigate relationships between granitic and dioritic rocks from northwestern Guernsey, Channel Islands. At the contact between the Cobo Granite and the Bordeaux Diorite Complex there are lithologies within a contact zone which are here collectively termed the Granite/Diorite Marginal Facies. This zone comprises a group of inhomogeneous rock types which are gradational between the two major units, including enclave and microenclave-rich granitic rocks, skialithic granodioritic rocks and members of the Diorite Complex containing proportions of alkali feldspar, quartz and biotite which are atypically high for that Complex. The Cobo Granite itself contains abundant dioritic to granodioritic enclaves believed to have been derived from the Diorite Complex. Detailed field, petrographic and geochemical investigation shows that the contact phenomena have arisen through the interaction between coexisting magmas. Magma-mixing occurred to some degree, but the mixed-magmas were inhomogeneous and modified by infiltration and diffusion of granitic components. The investigation therefore rejects the previously proposed hypothesis that metasomatism (granitisation) was responsible for the contact phenomena. Also investigated in detail were small granitic pipes, and their associated dark margins and felsic aureoles, which occur within the Bordeaux Diorite Complex. These pipes formed through the buoyant intrusion of volatile-rich granite magma into a coexisting, denser, dioritic magma and caused both mineralogical and geochemical modification of the host. Geochemical profiles across felsic aureoles point to both diffusion/infiltration of granitic components from the pipe into the aureole and diffusion/exfiltration of dioritic components outward from the aureole. Rb/Sr isotopic data provide an age (4% ± 13 Ma) for the emplacement of the Cobo Granite and given the contemporaneity of these magmas also provide an age for the Bordeaux Diorite Complex. The magmatism is attributed to the late manifestations of the Cadomian orogenic cycle.