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

Cook, C.A.; Holdsworth, R.E.; Styles, M.T.

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 degreesC) 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 degreesC), 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 Kermack 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 (< 250degreesC) 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.

Citation

Cook, C., Holdsworth, R., & Styles, M. (2002). The emplacement of peridotites and associated oceanic rocks from the Lizard Complex, southwest England. Geological Magazine, 139(1), 27-45. https://doi.org/10.1017/s0016756801005933

• Journal Article Type: Article
• Publication Date: 2002-01
• Deposit Date: Jul 1, 2008
• Publicly Available Date: Jul 1, 2008
• Journal: Geological Magazine
• Print ISSN: 0016-7568
• Electronic ISSN: 1469-5081
• Publisher: Cambridge University Press
• Peer Reviewed: Peer Reviewed
• Volume: 139
• Issue: 1
• Pages: 27-45
• DOI: https://doi.org/10.1017/s0016756801005933
• Keywords: Geology, Microstructure, Metamorphism.
• Public URL: https://durham-repository.worktribe.com/output/1597002