Geological Magazine



The emplacement of peridotites and associated oceanic rocks from the Lizard Complex, southwest England


C. A. COOK a1p1, R. E. HOLDSWORTH a1c1 and M. T. STYLES a2
a1 Reactivation Research Group, Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK
a2 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Abstract

Upper mantle peridotites and associated oceanic rocks from the Lizard Complex, southwest England, preserve evidence for a multistage geological history. Steeply dipping pre-emplacement fabrics record high-temperature (900–1100°C) shearing and exhumation of the mantle peridotites apparently formed during localized NE–SW rifting in a pull-apart basin setting (c. 400–390 Ma). Associated oceanic rocks (Landewednack amphibolites) preserve a pre-emplacement prograde brown amphibole-bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during NE–SW rifting. In both the peridotites and Landewednack amphibolites, steep pre-emplacement structures are cross-cut by low-angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra-oceanic setting (c. 390–375 Ma). The fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages (c. 500–800°C), with the amphibolites preserving two superimposed assemblages, green amphibole + titanite and colourless magnesio-hornblende, respectively, that are thought to record progressive down-temperature deformation during thrusting. Emplacement-related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles NW, with down-dip lineations and kinematic indicators showing consistent top-to-the-NW senses of shear. Syn-emplacement magmatism is recorded by intrusions of foliated Kennack Gneiss. Anastomosing serpentine-filled faults mark many existing low-angle contacts between the peridotites and Landewednack amphibolites and appear to represent the final, lowest-temperature (< 250°C) stages of emplacement (c. 370 Ma). This study shows that ‘dynamothermal aureoles’ in ophiolites may preserve evidence for tectonothermal events that pre-date thrust emplacement.

(Received March 5 2001)
(Accepted July 16 2001)


Correspondence:
c1 Author for correspondence: r.e.holdsworth@durham.ac.uk
p1 Present address: Baker Atlas GeoScience, Baker Hughes, Kettock Lodge, Campus 2, Balgownie Drive, Bridge of Don, Aberdeen, AB22 8GU Scotland, UK.


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