HR: 1330h
AN: V12A-0558    [PDF]
TI: Sedimentation in Magma Chambers: Evidence From the Geochemistry, Microstructure and Crystallography of Troctolite and Gabbro Cumulates, Rum Layered Intrusion, Scotland.
AU: * Lo R\'{e}, F C
EM: clore@uwyo.edu
AF: University of Wyoming, Dept. of Geology and Geophysics, PO Box 3006, Laramie, WY 82071 United States
AU: Cheadle, M J
AF: University of Wyoming, Dept. of Geology and Geophysics, PO Box 3006, Laramie, WY 82071 United States
AU: Swapp, S M
AF: University of Wyoming, Dept. of Geology and Geophysics, PO Box 3006, Laramie, WY 82071 United States
AU: Coogan, L A
AF: Leicester University, Dept. of Geology, Leicester, LE1 7RH United Kingdom
AB: The formation of igneous cumulates remains poorly understood. In particular, petrologists disagree about the relative importance of crystal accumulation by sedimentation or in-situ growth, partly because post-cumulus processes often overprint evidence for the primary mechanism. We address this problem with a detailed geochemical, microstructural and crystallographic study of gabbros and troctolites from the Eastern Layered Series of the Rum Layered Intrusion, NW Scotland. We collected samples, approximately every 50cm, through the complete 11m sequence of foliated gabbros and troctolites from Unit 9. The samples were quantitatively analyzed for crystal shape and size, shape preferred orientation (SPO), crystallographic preferred orientation (CPO), modal mineralogy, and whole rock and mineral chemistry. We present the following results: Within cumulate layers just cm's apart, olivine crystal shape can vary from sub-equant to tabular, and crystal diameter can vary from 1-8mm. This suggests at least one olivine population was derived elsewhere in the magma chamber. Complex anorthite zoning is also consistent with an extended history of plagioclase crystal transport. A one-crystal thick olivine layer between feldspathic layers of differing grain size is interpreted to be a lag deposit. Two possible examples of cross bedding also exist. Taken together, these observations suggest sedimentation was the primary method of crystal accumulation. The data also provide constraints on post-cumulus processes. Complex plagioclase zoning suggests that processes of viscous compaction and/or recrystallization were not extensive. The preservation of magmatic and deformation twins and absence of a polygonal fabric support this conclusion. In addition, the CPO and SPO are the same. Lacking evidence for compaction and recrystallization lead us to believe similar CPO and SPO are the preserved artifact of a sedimentary foliation. Therefore, we conclude the gabbros and troctolites of Unit 9 formed by crystal sedimentation followed by subsequent overgrowth by porous media convection with limited compaction or recrystallization.
DE: 3640 Igneous petrology
DE: 3694 Instruments and techniques
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting