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

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

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


Authors

C.A. Cook

M.T. Styles



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 Mar 29, 2024
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.