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The ascent history of the Horoman peridotite complex, Hokkaido, northern Japan, is revised on the basis of a detailed study of large ortho- and clinopyroxene grains ∼1 cm in size (megacrysts) in the Upper Zone of the complex. The orthopyroxene megacrysts exhibit distinctive M-shaped Al zoning patterns, which were not observed in porphyroclastic grains less than 5 mm in size described in previous studies. Moreover, the Al and Ca contents of the cores of the orthopyroxene megacrysts are lower than those of the porphyroclasts. The Upper Zone is inferred to have resided not only at a higher temperature than previously suggested but also at a higher pressure (∼1070°C, ∼2·3 GPa) than the Lower Zone (∼950°C, ∼1·9 GPa), in the garnet stability field, before the ascent of the two zones. The Horoman complex probably represents a 12 ± 5 km thick section of lithospheric mantle with an ∼10 ± 8°C/km vertical thermal gradient. The current thickness of the Horoman complex is ∼3 km, which is a result of shortening of the lithospheric mantle by ∼0·25 ± 0·1 during its ascent. The Upper Zone appears to have experienced a heating event during its ascent through the spinel stability field, with a peak temperature as high as 1200°C. The effect of heating decreases continuously towards the base of the complex, and the lowermost part of the Lower Zone underwent very minor heating at a pressure higher than ∼0·5 GPa. The uplift and associated deformation, as well as heating, was probably driven by the ascent of a hot asthenospheric upper-mantle diapir into the Horoman lithosphere.