Gabbroic lithologies of the dike-gabbro transition, Hole GT3A, Oman Drilling Project
Abstract
Hole GT3A intersects 400 m of oceanic crust providing unique insight into the dike-gabbro transition and the variability of the high level gabbros in the Samail ophiolite. Olivine gabbro and olivine bearing gabbro occur exclusively within the Upper Gabbro Sequence (16 % thickness; 111.02 m - 127.89 m) whereas oxide gabbro and disseminated oxide gabbro represent ca 5 % of the Lower Gabbro Sequence (233.84 m - 398.21 m). Gabbro with less than 1 vol. % olivine and oxide is the most common lithology in both Gabbro Sequences (10-13 %). Most gabbroic rocks were classified as "varitextured" due to textural and grain size macroscopic variations forming irregular domains/patches. Varitextured gabbros are medium-grained (1-5 mm), with seriate grain size distribution and subophitic/poikilitic to granular textural domains. Poikilitic domains comprise clinopyroxene with plagioclase chadacrysts, whereas in granular domains plagioclase interstices are filled by green-brown magmatic hornblende; plagioclase is zoned in both domains. Olivine (bearing) gabbros have 4-8 mm skeletal olivine pseudomorphs with roundish inclusions of chromite and plagioclase. Oxide (disseminated) gabbros comprise variable amounts of plagioclase, clinopyroxene, <5 % oxides and up to 2 % apatite. Oxides form skeletal grains included in silicates and comprise magnetite with sets of ilmenite lamellae and modally subordinated, optically homogenous ilmenite. Ilmenite is the dominant oxide phase in olivine gabbros and is usually also included in silicates. Oxides become gradually interstitial with depth until magnetite appears as the dominant oxide above the first oxide gabbro occurrence. Many gabbros show evidences of rapid crystal growth in plagioclase and oxides (rarely in clinopyroxene) such as needle to skeletal habit, branching tips, embayments and curved shapes. All studied gabbros show evidence for low-temperature hydrothermal alteration (20 to 90 % altered) and Fe-Ti oxides to hematite (along exsolution planes or as patches), ultimately to titanite. Granoblastic hornfels patches (plagioclase + two pyroxenes) showing an intersertal/intergranular texture occur as relics in oxide gabbro. These are interpreted as former basalts metamorphosed to granulite facies during overhead stoping of the axial magma chamber at the Oman paleo ridge.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V43G2953J
- Keywords:
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- 3036 Ocean drilling;
- MARINE GEOLOGY AND GEOPHYSICS;
- 4863 Sedimentation;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL;
- 8115 Core processes;
- TECTONOPHYSICS;
- 8140 Ophiolites;
- TECTONOPHYSICS