Presentation
Retrograde lawsonite formation in the Franciscan subduction complex
American Geophysical Union Fall Meeting
(2016)
Abstract
Lawsonite [CaAl2Si2O7(OH)2H2O] is an index mineral of low-temperature subduction zones,contains a significant amount of water, and is an important host of rare-earth and trace elements in mafic protoliths.For these reasons, numerous studies have investigated the consequences of lawsonite breakdown during prograde subduction. In the Franciscan subduction complex, however, lawsonite in mafic blueschist largely formed along a retrograde path from pre-existing eclogite.In order to asses the conditions and significance of retrograde lawsonite formation we examined the petrology and geochemistry of lawsonite-bearing assemblages in Franciscan mafic rocks.All of the samples have the common assemblage: lawsonite, glaucophane, and sphene.Quartz is generally absent. Muscovite, chlorite, and relict epidote and rutile are variably present.Different index minerals calcite, aragonite, albite, and jadeitic pyroxene are present within lawsonite assemblages.Garnet occurs in equilibrium with lawsonite, as a relict mineral in lawsonite and the matrix, or is completely absent.Major element compositions vary from typical basalts and are strongly correlated with one another. Chondrite normalized REE compositions are variably LREE depleted or enriched, MREE are flat to enriched, and HREE are generally flat.Trace elements normalized to NMORB show variably enriched and depleted LILE.The petrology suggests lawsonite, glaucophane, and sphene formed from multiple retrograde reactions involving garnet, clinopyroxene, epidote, and rutile, together with significant hydration. Important index minerals imply lawsonite formed over a wide range of pressures within the subduction zone.The major, REE, and trace element compositions suggest lawsonite assemblages were derived from different protoliths or experienced variable amounts of metasomatism and interaction with crustally derived material and serpentinite.
Keywords
- Lawsonite,
- Franciscan subduction complex
Disciplines
Publication Date
Fall 2016
Location
San Francisco, CA
Comments
Section: Tectonophysics
Session: Subduction Top to Bottom (ST2B-2) III Posters
Citation Information
S. Myers, S. R. Mulcahy (2016), Retrograde lawsonite formation in the Franciscan subduction complex, Abstract T31E-2962 presented at 2016 Fall Meeting, AGU, San Francisco, Calif., 11-15 Dec.