J. geol. Soc. London, Vol. 141, 1984, pp. 37-39, 1 fig. Printed in Northern Ireland.

Hot or cold emplacement of Complex?

R. P. Barnes & J. R. Andrews

SUMMARY: Regional prograde pumpellyite actinolite facies metamorphism (Ml) has affected theGramscatho Group sedimcnts and volcanica structurally underlying thc Lizard Complex. Metamorphism took placesynchronously with the main penetrativedeformation (DI). There is no evidence of any increase in the regional gradient towards the Lizard Complex or of any local thermal overprinting. It is concluded that the Lizard Complex was cmplaced in a cooled thrust sheet during D1 deformation during a regional heat flow which produced very low grade metamorphism. Temperatures of 250-350°C and a pressure of severalkilobars are estimated.

TheLizard Complex isthrust over sediments and obductedhot the ophiolite should have been gener- volcanics of theMeneage Formation (Barnes 1983, atednot long before emplacement, i.e. in Middle 1984)which lies near the top of theGramscatho Devonian or later times. Group (Barnes 1984, fig. 2). The age of the uppermost Meneage Formation is Givetian. possibly extending to Cold emplacement Frasnian(Barnes 1984). The contact is well exposed onlyatPolurrian and inthe coastal Atwo-stage model in which the ophiolite became sections of eitherside of theLizard Peninsula. At coupled with the Old Lizard Head metasediments by Polurrianthis contact is a relativelylate, steeply either hot overthrusting or the Kirby mechanism (op. dipping normal fault. At Porthallow there is a rather cit.)subsequently over-riding the melange in post- gentlydipping fault marked by extensivebrecciation Frasnian times as a cold sheet (Fig. lb). There need be of adjacentlithologies. Most previous workers have no closegeographical relationship with the grade of interpretedthis as a latethrust or reversefault metamorphismin the Meneage Formation and the carrying the Lizard Complex northwards. ophiolite. A MiddleDevonian orolder age of Theestablishment of theLizard Complex as an ophiolitegeneration isindicated. The Sm-Nd and ophiolite (Kirby 1979) and the very low metamorphic Rb-Srisotopic data presented by Davies(1984) and grade of theMeneage Formation leads totwo Styles &L Rundle(1984) point to an approximate alternative views of their relationship. 370 Ma age of ophiolite generation, which would be consistent with either model of final emplacement. Hot emplacement Regional metamorphism in S Themetasediments and volcanics of theMeneage Formation may represent a lower temperature facies Lowgrade metamorphic mineral assemblages de- of the Old Lizard Head metasediments (Flett 1946, p. veloped in a variety of different lithologies occurring 37).brought into juxtaposition by normalfaulting within the Meneage Formation and adjacent rocks are (Fig.la). In this hypothesis the Old Lizard Head described by Barnes(1982) and Barnes & Andrews metasediments and Landewednack hornblende schists (1981). New minerals widely developed during a single areinterpreted as representing a contact aureole to progradeepisode of metamorphism (M1) include hotemplacement of theophiolite during obduction quartz, chlorite, phengitic mica, albite, epidote, pum- ontoa continental margin. This is notfar from the pellyite,actinolite and carbonate. Prehnite has been view of Kirby(1978), who arguedthat the ophiolite isolatedin two widely separate localities. Although mighthave been thrust over Old Lizard Head plagioclase in all lithologies is now albite, where it was metasedimentsand Landewednack schists in a con- originallymore calcic (e.g. in greenstones, amphibo- tinentalmargin environment during regional meta- lite and basalt clasts and scattered grains in greywack- morphismresulting from high heat flow. Kirby es) it contains fine-grained pumpellyite, mica, chlorite rejectedthe primary cause of metamorphism as andepidote. Amounts of thesemetamorphic phases resulting from hot emplacement as he could not find varygreatly. Actinolite commonly occurs in green- anyevidence for inverted metamorphic gradients. A stonesasovergrowths on, or replacing.original hot emplacement model predicts that the metamorphic pyroxene.It also occursin amphibolite rudites as grade of theMeneage Formation should decrease overgrowthson derived amphiboles: there isalso away from the contact as structurally lower levels are evidencethat some of thelatter may have re- reached(Rattey & Sanderson1984). In order to be equilibrated, at least locally. Quartz and chlorite are

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S Porthalloy N S Porthallow N

a Hot Emplacement b Cold Emplacement FIG. 1. Alternative interpretations of field relationships along the E coast section contact of the Lizard Complex. Increasing ornament size represents higher grades of metamorphism. OLH = Old Lizard Head metasediments.

commonlydeveloped interstitially in all lithologies, underlyingthe Lizard Complex. The sediments con- andare widely seen as fibrous D1 pressureshadow tinue 25 km to the E in Roseland (Barnes 1984, fig. 2) fringesin arenites and felsic volcanics. New mica is atthe same metamorphic grade. Similar mineral generallyabundant in sedimentary lithologies and assemblageslacking pumpellyite have been noted in felsites, with a strong preferred orientation parallel to greenstonesin W Cornwall(Floyd & AI-Samman SI. Generallythese new mineral phases are very 1980), and a pumpellyite-bearing assemblage has been fine-grainedandrecrystallization is incomplete. describedfrom the area (Floyd & Row- However, epidote in metabasites and carbonate in all botham 1982). It seems likely that most of S and W lithologiesmayform coarse-grained aggregates Cornwallwas subjected to very low grade regional obscuring alloriginal textures and mineralogies. metamorphismprior to batholith emplacement. As Carbonate, when abundant, is often a later alteration there is noevidence of increasinggrade of meta- feature replacing all other minerals. morphismtowards the Lizard Complex, the hot emplacementmodel forthe Lizard Complex is Pumpellyite-actinolite facies rejected.Rattey & Sanderson (1984) arguethat the regional D1 and D2 structures are consistent with, and The M1 mineralassemblage developed in meta- resultfrom, emplacement of aNNW-directed thrust basites is characterized by pumpellyite and actinolite. complex, of which the Lizard Complex forms a slab. Although difficult to see because of its particularly fine M1 mineral assemblages in D1 extension fractures and grain-size, pumpellyite has been identified at a number pressureshadow regions abutting clasts or pyrite of widespreadlocalities (see Barnes 1984, fig. 2).It crystalsdeveloped synchronously with themain has not been positively identified on the W coast of the penetrativedeformation. Phengitic mica strongly en- Lizard peninsula, probably because of the absence of hances SI by its preferred orientation. Veins of D2 age suitablelithologies orthe presence of abundant also contain identical assemblages suggesting that M1 carbonate which tends to inhibit pumpellyite develop- conditions persisted through D2. Rattey & Sanderson ment (Winkler 1979, p. 200). (1984)attribute D2 deformationgravitationalto Thepumpellyite-bearing assemblages indicate that collapse of D1 nappes.We conclude that cold theMeneage Formation has undergone pumpellyite- emplacement of the Lizard Complex took place under actinolitefacies metamorphism. Microprobe analysis aprevailing heat flow whichproduced the very low of the pumpellyites (Barnes & Andrews 1981) shows grademetamorphism. In theabsence of critical they are of the AI-rich variety, characteristic of higher mineral assemblages we follow Coombs et al. (1976) in grade pumpellyites (Coombs et al. 1976). No systematic suggestingtemperatures of 250-350°C andpressures changes in the mineral assemblages and hence varia- of severalkilobars during metamorphism. If amini- tion in the grade of metamorphism can be identified mum pressure of 3 kbar is assumed during overthrust- within the area studied. ing,then approximately 12 km of overburdenmust subsequently have been removed. Most of this erosion Discussion probablyoccurred synchronously with obduction, producingmarginala overridden ophiolite flysch Pumpellyite-actinolite facies metamorphism character- basin.The Mylor-Gramscatho basin may represent izedthe 3-4 kmwide belt of sedimentsstructurally the remnants of this (but see Barnes 1984).

References

BARNES,R. P. 19x2. The Geology of south Cornish melarzges. associateddeposits in SouthCornwall. England. Pvoc. Thesis. PhD. Univ.Southampton (unpubl.). Geol. Assoc. London, 94, 211-29.

~ 19x3. The stratigraphy of a sedimentary melange and ~ 1984. PossibleLizard-derived material in the underlying

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Meneage Formation. J. geol. Soc. London, 141, 79- secondary mineralsin titaniferous brown amphibole- 85. bearing greenstones from north Cornwall. Proc. Ussher - & ANDREWS,J. R. 1981. Pumpellyite-actinolite grade SOC.5, 296-303. regional metamorphism in south Cornwall. Proc. Ussher KIRBY,G. A. 1978. The petrochemistry of rocks of the Lizard SOC.5, 139-46. Complex, Cornwall. Thesis, PhD, Univ. Southampton COOMBS.D. S.. NAKAMURA.Y. & VUAGNAT,M. 1976. (unpubl.).

Pumpellyite-actinolite faciesschists of the Taveyanne ~ 1979. The Lizard Complex as an opiolite. Nature, Formation near Loeche. Valais, Switzerland. J. Petrol- London, 282, 58-61. ogy, 17, 440-71. RATTEY.R. P. & SANDERSON, D.J. 1984. The structures of DAVIES, G.1984. Isotopic evolution of the Lizard Complex. SW Cornwall and its bearing on the emplacement of the J. geol. Soc. London. 141, 3-14. Lizard Complex. J. geol. Soc. London, 141, 87-95. FLETT,J. S. 1946. The geology of the Lizard and Meneage STYLES,M. T. & RUNDLE, C. C.1984. A whole-rock Rb-Sr (2nd ed.). Mem. geol. Surv. U.K. isochronage of the Kennack gneiss and itsbearing on FLOYD.P. A. & AL-SAMMAN.A. H. 1980. Primary and the age of the Lizard Complex. Cornwall, J. geol. Soc. secondary chemical variation exhibited by some west Lordon, 141, 15-19. Cornish volcanic rocks. Proc. Ussher Soc. 5. 68-75. WTNKLER,H. G. F. 1979. Petrogenesis of Metamorphic Rocks - & ROWBOTHAM.G. 1982. Chemistry of primary and (5th ed.). Springer-Verlag, Berlin, 334 pp.

Revised typescript received 9 September 1983. R. P. BARNES,Institute of GeologicalSciences, Murchison House. WestMains Road. Edinburgh EH9 3LA. J. R. ANDREWS, Departmentof Geology, The University, Southampton SO9 SNH.

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