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[SCG41-01] The structure of magma chamber below a fast-spreading ridge based on field and core analyses of the Oman ICDP drilling
Keywords:Oman ICDP, lower oceanic crust, magma chamber, fast-spreading ridge, upper gabbro, layered gabbro
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. We report field observations of the lower crustal section along Wadi Gideah and reconstructed the vertical stratigraphy in reference to the Moho and describe the microstructures and major and trace element compositions of the whole rocks and constituent minerals. These observations enable us to estimate the amount of trapped melt in the lower crustal gabbro and discuss the conditions of magmatic and hypersolidus deformation that operated in the lower crustal magma chamber beneath the Oman paleoridge axis.
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 deformed 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, which are commonly present in the upper gabbros. Hypersolidus deformation structures are present irrespective to depth.
The lower GT1A and higher GT2A sections range in whole-rock Mg#s of 72-83 and 68-79, respectively. The lowest Mg#s overlap those of the sheeted dikes above. The whole-rock compositions are mainly controlled by accumulation of plagioclase, olivine, clinopyroxene and interstitial melts. The lower whole-rock Mg# and higher REEs and HFSEs of GT2A suggest larger amounts of trapped melt at stratigraphically higher levels. The amounts of trapped melt were estimated by LREE concentrations in the clinopyroxene cores, and by assuming LREE 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 common hypersolidus deformation with foliation but only limited modal layering, ubiquitous zoned cumulus minerals and common ophitic clinopyroxenes as well as the amount of trapped melt >5 mass% are all characteristics of foliated gabbros. In addition to this, field observations of the gabbros in Wadi Gideah show poorly develop modal layering but foliation defined by shape-preferred orientation of clinopyroxenes in the stratigraphic levels including both GT1A and GT2A holes. 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.
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 deformed 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, which are commonly present in the upper gabbros. Hypersolidus deformation structures are present irrespective to depth.
The lower GT1A and higher GT2A sections range in whole-rock Mg#s of 72-83 and 68-79, respectively. The lowest Mg#s overlap those of the sheeted dikes above. The whole-rock compositions are mainly controlled by accumulation of plagioclase, olivine, clinopyroxene and interstitial melts. The lower whole-rock Mg# and higher REEs and HFSEs of GT2A suggest larger amounts of trapped melt at stratigraphically higher levels. The amounts of trapped melt were estimated by LREE concentrations in the clinopyroxene cores, and by assuming LREE 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 common hypersolidus deformation with foliation but only limited modal layering, ubiquitous zoned cumulus minerals and common ophitic clinopyroxenes as well as the amount of trapped melt >5 mass% are all characteristics of foliated gabbros. In addition to this, field observations of the gabbros in Wadi Gideah show poorly develop modal layering but foliation defined by shape-preferred orientation of clinopyroxenes in the stratigraphic levels including both GT1A and GT2A holes. 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.