16:15 〜 16:30
[SCG47-10] THE MAGMATIC CONDITIONS AND DEFORMATION OF LOWER CRUSTAL MAGMA CHAMBER BELOW THE FAST-SPREAD OMAN PALEORIDGE
キーワード：Oman ICDP, GT1A>2A, magma chamber, fast-spreading ridge, foliated gabbro
We present microscope observations, EPMA and La-ICP-MS analyses of the lower crustal section of the Oman Ophiolite in Wadi Gideah to understand the magmatic conditions and deformation operated in the lower crustal magma chamber beneath the Oman paleoridge axis. Samples were obtained by geological mapping along Wadi Gideah from the upper gabbro just below the sheeted dike complex down to the layered gabbro above the Moho transition zone. We also analyzed core samples obtained from ICDP drill holes GT1A and GT2A, each 400 m long, penetrating the middle and lower crust.
Field observation of the gabbros along Wadi Gideah, including the Site GT1A and GT2A, shows that the gabbroic rocks poorly develop modal layering, but foliation defined by shape-preferred orientation of clinopyroxenes in the most stratigraphic levels. Modal layering is locally present, including discontinuous, pinch and swell layer, irregular and tight folding, exhibiting hypersolidus deformation structures. Meanwhile, layered gabbros with well-developed modal layering are only present in the lowermost levels in the crust just above the Moho Transition Zone.
Progressive stratigraphic variations in cumulate gabbro microstructures are observed throughout the lower crust. Plagioclase changes from flat, platy crystals with high aspect ratios to short, stubby crystals with wavy outlines. The proportion of primary cumulus plagioclase decreases from more than 90% to less than 1%, while that of broken plagioclase increases from approximately 7% to 98% throughout the stratigraphy. The amount of recrystallized plagioclase crystals ranges from 0% to 5% at around 500 meter above the Moho upwards, smaller than that in the deeper level ranging from 0% to 10%. Clinopyroxene is deformed to distorted rectangular shapes with a quarter structure, then changes into crystals with sub-grains, which are followed by neoblasts, and eventually forms equilibrium texture shown by polygonal crystals that meet at a triple junction of obtuse angles around 120°. The percentages of primary clinopyroxene decrease from almost 90% at nearly 1500 meter above the Moho to a range of 70%-90% below. In contrast, all the secondary clinopyroxene crystals including subgrains, neoblasts, and recrystallized grains vary around 10% and from 10% to 30%, respectively for the two above stratigraphic intervals. An ophitic texture can be found in all thin sections of GT2A and GT1A drill holes, but not in the layered gabbro of outcrop samples.
Trapped melt fractions were estimated by mass balance calculations on the basis of the modes of olivine, clinopyroxene and plagioclase, and estimated melt compositions on the basis of La, Ce and Pr concentrations in clinopyroxene cores. Most cumulus clinopyroxene shows normal zoning with trapped melt fraction of 3.5 on average.
Clinopyroxenes of the GT1A cores are higher in Mg# and Cr content than those of the GT2A, and plagioclase of GT1A is more calcic than that of GT2A. Generally, GT1A and GT2A cores range in clinopyroxene Mg#s from 80 to 90, and from 75 to 85, respectively. Cr2O3 of clinopyroxene ranges from 0.1 mass% to 0.7 mass% in GT1A, and less than 0.3 mass% in GT2A. Plagioclase of GT1A cores has An80-90, whereas that of GT2A have An70-80.
Discussion
The progressive deformation and recrystallization of cumulus crystals shown above are associated with compaction of cumulus piles that led to entrapment of interstitial melts. Subsequent cooling of cumulus mushes caused in-situ fractionation of the trapped melts to form zoning in the rims. Clinopyroxene of GT2A core sample shows a sector zoning with different REE concentrations, suggesting a cooling rate slower than 250°C/hr (Gary E. Lofgren, 2006).
The presence of ophitic texture and sector zoning of clinopyroxene suggests that cumulus phases crystallized under large degrees of supercooling, which is typically observed in the sheeted dikes and the roof zone upper gabbros. Besides, stratigraphic variations of cumulus plagioclase and clinopyroxene show progressive deformation throughout stratigraphy. Mineral compositions indicate that GT1A cores crystallized from more primitive magma than GT2A. Furthermore, high trapped melt fraction and the common presence of zoned cumulus minerals suggest that both GT2A and GT1A drill holes penetrate the thick foliated gabbros in Wadi Gideah section, which crystallized on the axial melt lens floor and were transported downward in the lower crust.
Field observation of the gabbros along Wadi Gideah, including the Site GT1A and GT2A, shows that the gabbroic rocks poorly develop modal layering, but foliation defined by shape-preferred orientation of clinopyroxenes in the most stratigraphic levels. Modal layering is locally present, including discontinuous, pinch and swell layer, irregular and tight folding, exhibiting hypersolidus deformation structures. Meanwhile, layered gabbros with well-developed modal layering are only present in the lowermost levels in the crust just above the Moho Transition Zone.
Progressive stratigraphic variations in cumulate gabbro microstructures are observed throughout the lower crust. Plagioclase changes from flat, platy crystals with high aspect ratios to short, stubby crystals with wavy outlines. The proportion of primary cumulus plagioclase decreases from more than 90% to less than 1%, while that of broken plagioclase increases from approximately 7% to 98% throughout the stratigraphy. The amount of recrystallized plagioclase crystals ranges from 0% to 5% at around 500 meter above the Moho upwards, smaller than that in the deeper level ranging from 0% to 10%. Clinopyroxene is deformed to distorted rectangular shapes with a quarter structure, then changes into crystals with sub-grains, which are followed by neoblasts, and eventually forms equilibrium texture shown by polygonal crystals that meet at a triple junction of obtuse angles around 120°. The percentages of primary clinopyroxene decrease from almost 90% at nearly 1500 meter above the Moho to a range of 70%-90% below. In contrast, all the secondary clinopyroxene crystals including subgrains, neoblasts, and recrystallized grains vary around 10% and from 10% to 30%, respectively for the two above stratigraphic intervals. An ophitic texture can be found in all thin sections of GT2A and GT1A drill holes, but not in the layered gabbro of outcrop samples.
Trapped melt fractions were estimated by mass balance calculations on the basis of the modes of olivine, clinopyroxene and plagioclase, and estimated melt compositions on the basis of La, Ce and Pr concentrations in clinopyroxene cores. Most cumulus clinopyroxene shows normal zoning with trapped melt fraction of 3.5 on average.
Clinopyroxenes of the GT1A cores are higher in Mg# and Cr content than those of the GT2A, and plagioclase of GT1A is more calcic than that of GT2A. Generally, GT1A and GT2A cores range in clinopyroxene Mg#s from 80 to 90, and from 75 to 85, respectively. Cr2O3 of clinopyroxene ranges from 0.1 mass% to 0.7 mass% in GT1A, and less than 0.3 mass% in GT2A. Plagioclase of GT1A cores has An80-90, whereas that of GT2A have An70-80.
Discussion
The progressive deformation and recrystallization of cumulus crystals shown above are associated with compaction of cumulus piles that led to entrapment of interstitial melts. Subsequent cooling of cumulus mushes caused in-situ fractionation of the trapped melts to form zoning in the rims. Clinopyroxene of GT2A core sample shows a sector zoning with different REE concentrations, suggesting a cooling rate slower than 250°C/hr (Gary E. Lofgren, 2006).
The presence of ophitic texture and sector zoning of clinopyroxene suggests that cumulus phases crystallized under large degrees of supercooling, which is typically observed in the sheeted dikes and the roof zone upper gabbros. Besides, stratigraphic variations of cumulus plagioclase and clinopyroxene show progressive deformation throughout stratigraphy. Mineral compositions indicate that GT1A cores crystallized from more primitive magma than GT2A. Furthermore, high trapped melt fraction and the common presence of zoned cumulus minerals suggest that both GT2A and GT1A drill holes penetrate the thick foliated gabbros in Wadi Gideah section, which crystallized on the axial melt lens floor and were transported downward in the lower crust.