HR: 0830h
AN: V11E-0543    [PDF]
TI: Evidence for transition from mid-oceanic ridge setting to supra-subduction zone setting in the lower crustal gabbroic sequence, Haylayn block, Oman ophiolite
AU: * Yamasaki, T
EM: toru@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Hokkaido University, N10W8, Sapporo, Hokkaido, 060-0810 Japan
AU: Maeda, J
EM: jinmaeda@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Hokkaido University, N10W8, Sapporo, Hokkaido, 060-0810 Japan
AB: Origin of ophiolites has been a major focus of research over the past several decades, with most attention focusing on whether they form at mid-ocean ridges or above subduction zones. Oman ophiolite is well known as one of the greatest and the most complete ophiolites in the world, and occurrences of arc tholeiites, calc-alkaline rocks and boninites as well as MORB-type lavas have been reported. In the case of the lower crustal gabbroic sequence, however, clear identification of both arc- and mid-ocean ridge (MOR)- type gabbros has not been documented until now. In the Haylayn block of central part of the Oman ophiolite, we recognized two distinct magmatic suites from the lower gabbroic sequence. The earlier suite (GB1) consists of troctolites and olivine gabbros in the Layered Gabbro Sequence. The GB1 is characterized by MOR gabbro-type trend (Fo87: An93-Fo81: An83) in the plot of olivine (Fo) and plagioclase (An) compositions. On the other hand, the later suite (GB2) consists of olivine gabbros, olivine gabbronorites and gabbronorites from the Layered Gabbro Sequence and also from the upper Laminated Gabbronorite Sequence. The GB2 is characterized by more calcic plagioclase (Fo83: An91-Fo*72: An88, Fo* is calculated from Mg\# of orthopyroxene in olivine-free gabbronorites) and clinopyroxene poorer in Ti and Na compared to those in the GB1. One of the most significant results of our new observations is that the GB2 completely continues in mineral composition to the ultramafic cumulate intrusives (dunites and wehrlites, DW; Fo92-83) within the lower portion of the Layered Gabbro Sequence. Calcic nature of the GB2 plagioclase and the crystallization sequence from the DW and the GB2 suggest that both the DW and the GB2 crystallized from a common hydrous magma. The isotopic compositions are consistent with those observations. The highest value of Fo from the DW is the same as the values from the harzburgites (Fo93-91) in the Mantle Sequence. These lines of evidence imply that the existence of the mantle-derived hydrous primary magma of the DW-GB2 suite rather than the injection of seawater from the roof of the magma chamber by hydrothermal circulation. On the contrary, the GB1 can be explained by crystallization from an essentially anhydrous magma. Since the GB2 constitutes the Layered Gabbro Sequence together with the GB1, very long time interval between the intrusions of the GB1 and the GB2 cannot be suggested.This transition probably resulted from the change from mid-ocean ridge setting to supra-subduction zone setting of the Oman ophiolite.
DE: 1744 Tectonophysics
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting