A study of Devonian syn-extensional silicic volcanism in the Rhenohercynian Zone (RHZ) of Northern Europe was undertaken to determine their evolution and petrogenesis within the changing tectonic and sedimentological environment of a developing intra-plate rift zone. Silicic volcanics in the Lower Devonian of south Devon and Northern Germany are classified as subalkaline rhyolites-rhyodacites with minor peralkaline variants. Middle Devonian silicics from the Lahn-Dill syncline are peralkaline trachytes with minor subalkaline equivalents. All Lower and Middle Devonian Rhenohercynian silicic volcanics are tectonomagmatically classified as within-plate granitoids and are interpreted to represent syn-rift A-type silicic volcanics. However, a distinction is made with respect to age, in that all Lower Devonian silicics are N-type, whereas the Middle Devonian trachytes are AI-type. The McKenzie & Bickle (1988) theoretical evaluation of magmatism associated with crustal extension provides an excellent explanation for the changing geochemical and isotopic signatures of RHZ Lower and Middle Devonian silicic volcanics. The distinct geochemical and isotopic signatures between Lower and Middle Devonian volcanics reflects progressive lithospheric attenuation within the RHZ with the extension factor P<2 for Lower Devonian to P>2 (for Middle Devonian). The Lower Devonian southwest England silicic magmas, generated during the initial stages of extension, show evidence of crustal contamination and were probably derived from subduction-modified lithospheric mantle. Lower Devonian rhyolites from Northern Germany were generated via partial melting of crustal materials, the thermal energy for which was generated by the injection of basaltic melts which ponded at their equilibrated density level. Middle Devonian trachytes display mantle-dominated chemical patterns and represent extreme differentiates of alkali basalts derived from an omlike mantle source. The southwest-northeast trend of the major volcanic masses indicates the direction of faulting which has controlled the timing and extrusion of these syn-extensional melts. The volume of extrusive products appears strongly linked to the rate of extension and sediment accumulation. The greatest period of subsidence (Givetian) and sediment accumulation (200 metres per million years) coincides with the most volumetric period ofRHZ silicic volcanism
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:294194 |
| Date | January 1995 |
| Creators | Jones, Richard Mark |
| Publisher | Keele University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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