Gramscatho-Mylor facies relationships; Hayle, south Cornwall

The Gramscatho Group and the Mylor Slate Formation are the two main lithostratigraphic groups that outcrop in south Cornwall. Their mutual relationship has been disputed for some time because of poor palaeontological control and the effects of polyphase deformation. A study of the lithofacies exposed around Hayle suggests a breakdown of the generally valid lithostratigraphy. A transitional zone occurs in which Gramscatho and Mylor lithofacies are seen to be contemporaneous and are interpreted as a rise-slope association. On the scale of south Cornwall, the Gramscatho Group comprises basinal and rise deposits whilst the Mylor Slate Formation comprises rise, slope and possibly outer shelf deposits

An evaluation of flysch provenance - example from the Gramscatho Group of south Cornwall

Recent work has suggested that plate tectonic environments can be reconstructed utilizing clast mode petrography and bulk geochemistry of flysch sandstones. An assessment of the relative effects of processes that may modify sedimentary composition is made by examining the clast modes, bulk geochemistry and general petrography within a graded turbidite unit of the Gramscatho Group. Within the medium-fine sand lower half of the turbidite unit, clast mode variation is limited. The geochemistry of the sample suite is dominated by the non-framework matrix mineralogy with progressive element enhancement in the more muddy fractions of the unit. Although variations in element absolute abundances occur as a function of grainsize-related matrix content, the ratios of many element pairs (e.g. Ti-K, Cr-Y) remain constant, implying no major change in matrix composition through the unit. Diagenetic, metamorphic and microstructural effects are the dominant controls on the modification of original sediment composition. These processes are qualitatively evaluated in terms of the relative effects they will have on the endmember compositions of the various provenance diagrams

Late- to post-Variscan structures on the coast between Penzance and Pentewan, south Cornwall

The occurrence of two generations (D1/D2) of folds and cleavage, compatible with a top to the north-north-west sense of shear during Variscan convergence is confirmed. A subsequent change in the stress regime (sigma(1) approximate to vertical, sigma(3) approximate to north-north-west-south-south-east) brought about the extensional reactivation of convergence-related features. The resultant D3 structures are diverse and include zones of distributed shear within the footwall of the Carrick Thrust, together with detachments and high angle brittle extensional faults within the hangingwall. D3 deformation probably initiated during the Stephanian, prior to lamprophyre intrusion, but persisted into the early Permian and was partially synchronous with granite emplacement and high temperature mineralization. Changes in the stress regime during the Permian (sigma(1) approximate to east-north-east-west-south-west, sigma(3) approximate to north-north-west-south-south-east to sigma(1) approximate to north-north-west-south-south-east, sigma(3) approximate to east-north-east-west-south-west) resulted in strike-slip faulting and the formation of steeply dipping cleavages. Triassic rift-related extension is also recognised (sigma(1) approximate to vertical, sigma(3) approximate to east-north-east-west-south-west). Low temperature base metal mineralization was in part synchronous with Permian and Triassic faulting. This study demonstrates that the Variscan basement in south Cornwall preserves a valuable record of the late Palaeozoic to Mesozoic tectonic evolution of the region

Late- to post-Variscan extensional tectonics in south Cornwall

Faults which post-date Variscan contractional deformation are ubiquitous in south Cornwall. Three broad geometric/kinematic types can be defined. The first type comprises low angle (dip <45°) extensional faults which exhibit listric or ramp and flat geometries and may display complex arrangements of secondary faults and folds in their hangingwalls. In some instances they host elvans and "mainstage" magmatic-hydrothermal lodes (more rarely lamprophyres). The second type are moderate to high angle (dip >45°) extensional faults which generally offset earlier low-angle faults; they frequently host elvans and "mainstage" magmatichydrothermal lodes. The third type are high-angle faults which usually exhibit dominant strike-slip displacement, offset all previous faults and are often associated with base metal mineralization. Collectively these structures exerted a strong control on granite emplacement, mineralization and the development of offshore sedimentary basins. The approximately coeval association of potassic volcanicity, granite magmatism, extensional faulting and sedimentary basin development was probably a consequence of late Carboniferous to early Permian collapse of previously thickened lithosphere

Devonian rift-related sedimentation and Variscan tectonics – new data on the Looe and Gramscatho basins from the resurvey of the Newquay District

The geological resurvey of the Newquay District (Geological Survey Sheet 346) has resulted in stratigraphical and structural revision. The Devonian successions form part of the Looe and Gramscatho basins and broadly young to the south throughout the area. Deposition of the green to purple mudstones and sandstones of the Whitsand Bay Formation (Dartmouth Group) had initiated by the latest Lochkovian and was conformably succeeded by the sandstones, mudstones and bioclastic limestones of the Bovisand Fomation (Meadfoot Group). The newly defined Trendrean Mudstone Formation (Meadfoot Group) is dated as mid-Emsian or younger on the basis of palynological studies. These three formations respectively record the transition from lacustrine/fluvial through shallow marine to outer shelf/slope depositional environments during Lower Devonian rifting and the development of the Looe Basin. The lowermost part of the Gramscatho Basin succession is represented by the undated Grampound Formation (Gramscatho Group) that has a faulted contact with the underlying Looe Basin succession. It predominantly comprises mudstone but includes sandstone-dominated ‘packets’ (Treworgans Sandstone Member) consistent with an outer shelf and/or slope depositional environment along the northern margin of the Gramscatho Basin. The conformably overlying Porthtowan Formation (Gramscatho Group) comprises mudstones and sandstone-mudstone couplets and is entirely deep marine. Variscan primary deformation (D1) resulted in isoclinal folding and an associated axial planar cleavage throughout both successions. The ‘Watergate Bay Antiform’ of earlier workers is discounted; the associated outcrop geometry of the Dartmouth and Meadfoot groups is thrust-controlled. D2 deformation is developed around Porth Joke (Looe Basin succession) and intensifies southwards towards the Gramscatho Basin, probably in response to the NNW thrusting of the northern ‘parautochthonous’ margin of the Gramscatho Basin over the southern margin of the Looe Basin. An anomalous 900 m wide zone of steeply dipping S2 cleavage around Penhale Point is interpreted as primarily reflecting reorientation by a large-scale southwards-verging monoformal F3 fold. The structural complexity within the boundary zone possibly reflects a pre-Devonian basement fault influence upon: (i) the transition from shelf to deep marine depositional environments during the Lower-Middle Devonian, (ii) Variscan thrust juxataposition (D1 and D2) of the Looe and Gramscatho basin successions, and (iii) D3 post-Variscan extensional reactivation and reorientation of earlier fabrics

Monazite in polymetallic chlorite-(tourmaline)-quartz-(fluorite)-cassiterite-sulphide lodes and its potential for constraining the chronology of magmatic hydrothermal mineralisation in Cornwall.

Monazite [(Ce, La, Th, Nd, Y) PO4] occurs far more widely within the magmatic hydrothermal lode systems of Cornwall than has been previously recognised. In situ and mine dump specimens analysed by binocular and transmitted light microscopy, SEM, XRD and whole rock XRF reveal a near ubiquitous association of hydrothermal monazite with polymetallic chlorite-(tourmaline)-quartz- (fluorite)-cassiterite-sulphide mineralisation. The assemblage is interpreted to represent an early lode paragenesis formed by mixing of magmatic hydrothermal fluids with meteoric and/or metamorphic fluids derived from the country rocks. Such a model may explain why these assemblages are not observed in mines that have worked at greater depths within the granite. Major advances in understanding the chronology of granite magmatism across SW England using U-Pb monazite and xenotime methods have not, to date, been matched by the same level of success with respect to the development of the productive major lode systems. This discovery of widespread hydrothermal monazite raises the potential for high precision U-Pb dating of such mineralisation across the Cornubian Orefield. Where paragenetic relationships can be established it may also be possible to construct a fine-scale chronology for individual lode systems