Cooperation of upper and lower boundary-layer fractionations in a sheet-like intrusion: Composition and microstructure of the Aosawa dolerite sill in Yamagata Prefecture, northeastern Japan

摘要

Homogeneous fractionation, involving crystal nucleation, growth, and separation from a melt-dominant magma, and boundary-layer fractionation, involving the separation of a fractionated interstitial melt from a crystal-dominant boundary layer and its mixing with the main magma body, have been advocated as two major differentiation mechanisms in a crustal magma chamber. In this study, we focus on one of the dolerite sills in the Aosawa area, Yamagata Prefecture, Japan, to elucidate the roles of the two mechanisms in the differentiation of a sheet-like magma body. The intrusion is concordantly intruded into black mudstone and is ～ 100 m thick and more than 5 km in lateral extension. The chilled margins contain olivine (5.3 vol%) and plagioclase (1.9 vol%) as phenocrysts. There is an absence of clinopyroxene in the chilled margins, and there are systematic sill-scale variations in the whole-rock major- and trace-element contents that require the addition or removal of clinopyroxene. These findings suggest sill-scale differentiation, involving the transportation of crystals that nucleated and grew in the sill, and/or the transfer of the residual melt. The downward increases in mode and size of clinopyroxene and in the size of the most dominant plagioclase, all with maxima near the bottom, and the occurrence of pigeonite rimming augite only near the bottom, suggest a slower cooling rate in the lower boundary layer compared to the upper one. Clinopyroxene crystals with Cr-rich cores that show textures suggesting rapid growth, such as remarkable sector zoning, melt inclusions, and small euhedral plagioclase, are more abundant at 5-15 m above the lower contact. These observations lead us to believe that clinopyroxene nucleated and grew near the upper boundary layer, where large supersaturation occurred, and then settled to concentrate in the lower boundary layer, where slower cooling provided suitable conditions for pigeonite crystallization. The upward increase in the incompatible elements and the decrease in compatible elements from the zone containing a high concentration of Cr-rich clinopyroxene suggest that a fractionated melt, which was formed through enhanced overgrowth on settled clinopyroxene, was transported upwards. The sill-scale magmatic differentiation in the Aosawa dolerite intrusion took place through crystal settling from the roof boundary layer, followed by the upward transportation of the fractionated melt from the bottom boundary layer. The lower-boundary-layer fractionation was probably very effective because the settling of the crystals thickened the bottom boundary layer to facilitate an effective melt-crystal separation.