高速拡大海嶺のマグマ溜まり構造：オマーンICDP掘削コア解析と野外調査による考察

抄録

The ICDP Oman Drilling Project drilled the lower crustal section of the Oman Ophiolite in Wadi Gideah, north of Ibra and recovered 400 m long cores each from Hole GT1A and GT2A. Field observations confirmed that the lower crust attains 3.5 km in thickness, among which GT1A and GT2A holes range in stratigraphic height above the Moho from 0.4 to 0.8 km and from 2.0 to 1.6 km, respectively. Foliation and hypersolidus deformation structures are ubiquitous. Modal layering is locally present but is much less common. Well developed modal layering is present in the lowermost 125 m thick gabbro above the Moho.

Cored samples are predominantly olivine gabbros with elongated olivine aggregates and clinopyroxene with quarter structures embedded in granoblastic plagioclase and large euhedral plagioclases with resorbed turbid cores. These textures suggest deformation under the presence of melt, which assisted complete recovery of strain through recrystallization of fine-grained plagioclase and overgrowth of euhedral rims around magmatic plagioclase cores. Poikilitic and ophitic clinopyroxene occur sporadically throughout the holes. Hypersolidus deformation structures are present irrespective to depth.

The whole-rock compositions are mainly controlled by accumulation of plagioclase, olivine, clinopyroxene and interstitial melts. The lower GT1A and higher GT2A sections range in whole-rock Mg#s of 72-83 and 68-79, respectively, with the lowest Mg#s overlapping those of the sheeted dikes. The amounts of trapped melt were estimated by assuming La, Pr and Ce abundance in trapped melts to be equivalent to that of the sheeted dikes. Mass balance calculations yield the amount of trapped melt in gabbros to be 5 to 10 mass% in general and 25 mass% at most.

The drilled cores have foliation formed by hypersolidus deformation but only limited modal layering, ubiquitous zoned cumulus minerals and common ophitic clinopyroxenes and trapped melt >5 mass%, which are all characteristics of foliated gabbros. Furthermore, modally layered gabbros occur only in the lowermost 125 m thick gabbro above the Moho.

We concluded that both GT1A and GT2A holes sampled the thick foliated gabbros in Wadi Gideah section, which crystallized on the axial melt lens floor and were transported downward in the lower crust as gabbro glaciers.