Mantle peridotites exposed on the Earth's surface are “fossilized” mantle materials. Records of pressure-temperature-deformation histories (P-T-d-t paths) preserved in mantle peridotites reveal ...dynamic motion within the mantle with association of deformation, changes in pressure and temperature, and magma-related processes. Reconstruction of such P-T-d-t paths for mantle peridotites backward from the moment of “fossilization” through exhumation as far back in time as possible, will expand the current understanding of the long-term dynamic behavior of the mantle. The Horoman peridotite complex in the southern Hidaka metamorphic belt of Hokkaido, Japan, has long been examined using various approaches to decode past P-T-d-t paths recorded in the peridotite and mafic rocks. Here we critically review previous work related to reconstructions of P-T-d-t paths for the Horoman peridotite complex, and use the data to outline the dynamic history of the complex from the oldest events until their final ascent leading to emplacement within the crust.
We examined vertical variations of the petrological characteristics of a 33-m-thick peridotite section under the layered gabbro section along Wadi Fizh of the northern Oman ophiolite to understand ...the formation mechanism of the Mohorovicic discontinuity (Moho) beneath a spreading center. Here, we refer to the base of the layered gabbro section as "L-Moho" for the sake of simplicity. Network-like gabbro sills in peridotites increase in frequency upward to the L-Moho. The L-Moho is underlain by a 1-m-thick wehrlite layer, under which exists a 10-m-thick dunite layer, overlying a harzburgite layer where total pyroxenes slightly increase downward. Wehrlite is also found as screens between gabbro layers above the L-Moho. The mineral chemistry indicates systematic variations toward the L-Moho within the peridotite section; the Fo content (91 to 85) and NiO (0.4 to 0.2 wt%) of olivine decrease; the TiO2 content of clinopyroxene (0.1 to 0.6 wt%) and spinel (nil to 1.4 wt%) and atomic ratios of Cr/(Cr + Al) (0.5 to 0.6) and Fe3+/(Cr + Al + Fe3+) (0.05 to >0.1) in spinel increase upsection from the base (harzburgite) to the around L-Moho wehrlite via dunite. These variations are essentially similar to those observed in harzburgite/MORB reaction products from Hess Deep, East Pacific Rise, and possibly indicate that the lithological and mineral chemical variations within the examined peridotite layer resulted from the reaction between a harzburgite and a melt that produced the layered gabbros.
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