Thermal History of the Horoman Peridotite Complex: A Record of Thermal Perturbation in the Lithospheric Mantle

The ascent history of the Horoman peridotite complex, Hokkaido,northern Japan, is revised on the basis of a detailed studyof large ortho- and clinopyroxene grains 1 cm in size (megacrysts)in the Upper Zone of the complex. The orthopyroxene megacrystsexhibit distinctive M-shaped Al zoning patterns, which werenot observed in porphyroclastic grains less than 5 mm in sizedescribed in previous studies. Moreover, the Al and Ca contentsof the cores of the orthopyroxene megacrysts are lower thanthose of the porphyroclasts. The Upper Zone is inferred to haveresided not only at a higher temperature than previously suggestedbut also at a higher pressure (1070°C, 2·3 GPa) thanthe Lower Zone (950°C, 1·9 GPa), in the garnet stabilityfield, before the ascent of the two zones. The Horoman complexprobably represents a 12 ± 5 km thick section of lithosphericmantle with an 10 ± 8°C/km vertical thermal gradient.The current thickness of the Horoman complex is 3 km, whichis a result of shortening of the lithospheric mantle by 0·25± 0·1 during its ascent. The Upper Zone appearsto have experienced a heating event during its ascent throughthe spinel stability field, with a peak temperature as highas 1200°C. The effect of heating decreases continuouslytowards the base of the complex, and the lowermost part of theLower Zone underwent very minor heating at a pressure higherthan 0·5 GPa. The uplift and associated deformation,as well as heating, was probably driven by the ascent of a hotasthenospheric upper-mantle diapir into the Horoman lithosphere. KEY WORDS: Horoman; P–T trajectory; thermal history; Al diffusion in pyroxene; geothermobarometry     

Subhorizontal tectonic framework of the Horoman peridotite complex and enveloping crustal rocks,south‐central Hokkaido,Japan

Geological observations in the Horoman area, south‐central Hokkaido, show that the Horoman peridotite complex of the Hidaka metamorphic belt is a tectonic slice about 1200 m thick. The peridotite slab is intercalated into a gently east‐dipping structure. The underlying unit is a Cretaceous–Paleogene accretionary complex. Riedel shear planes in the sedimentary layers of the accretionary complex near the structural bottom of the peridotite slab indicate top‐to‐the‐west (thrust) displacement. The overlying unit is composed of felsic–pelitic gneisses and mafic–felsic intrusive rocks (the Hidaka metamorphic rocks). The boundary surface between the peridotite complex and metamorphic rocks forms a domal structure. Microstructures of sheared metamorphic rocks near the structural top of the peridotite slab indicate top‐to‐the‐east (normal) displacement. The results combined with previous studies suggest that the Horoman peridotite complex was emplaced onto the Asian margin (Northeast Japan) during the collision between the Asian margin and the Hidaka crustal block.     

Possible Non-melted Remnants of Subducted Lithosphere: Experimental and Geochemical Evidence from Corundum-Bearing Mafic Rocks in the Horoman Peridotite Complex, Japan

We report experimental results and whole-rock trace-elementcharacteristics of a corundum-bearing mafic rock from the Horomanperidotite complex, Japan. Coronitic textures around corundumin the sample suggest that corundum was not stable in maficrock compositions during the late-stage P–T conditionsrecorded in the complex (P < 1 GPa, T < 800°C). Basedon the experimental results, corundum is stable in aluminousmafic compositions at pressures of 2–3 GPa under dry conditions,suggesting that the corundum-bearing mineral assemblages developedunder upper-mantle conditions, probably within the surroundingperidotite. Variations in the trace-element compositions ofthe corundum-bearing mafic rock and related rocks can be controlledby modal variations of plagioclase, clinopyroxene and olivine,suggesting that they formed as gabbroic rocks at low-pressureconditions, and that the corundum-bearing mafic rock was derivedfrom a plagioclase-rich protolith. A complex P–T trajectory,involving metamorphism of the plagioclase-rich protolith ata pressure higher than that at which it was first formed, isneeded to explain the origin of the corundum-bearing mafic rocks.They show no evidence for partial melting after their formationas low-pressure cumulates. The Horoman complex is an exampleof a large peridotite body containing possible remnants of subductedoceanic lithosphere still retaining their original geochemicalsignatures without chemical modification during subduction andexhumation. KEY WORDS: Horoman; mafic rock; corundum; experiment; P–T history; recycling     

40Ar–39Ar analysis of phlogopite in the Horoman Peridotite Complex, Hokkaido, Japan and implications for its origin

Abstract ⁴⁰Ar–³⁹Ar analysis of phlogopite separated from a plagioclase lherzolite of the Horoman Peridotite Complex, Hokkaido, Japan, has yielded a plateau age of 20.6 ± 0.5 Ma in an environment where the metamorphic fluid was characterized by an almost atmospheric Ar isotopic ratio. The age spectrum is slightly saddle-shaped, implying some incorporation of excess ⁴⁰Ar during the formation of the phlogopite at a depth. As the phlogopite has been inferred to have formed in veins and/or interstitials during exhumation of the peridotite body, metasomatic fluids, to which ground- and sea water might have contributed, were probably involved in the formation of phlogopite in the crustal environment. A total ⁴⁰Ar–³⁹Ar age of 129 Ma of a whole rock sample of the plagioclase lherzolite, from which the phlogopite was separated and is representative of the main lithology of the Horoman Peridotite Complex, indicates the occurrence of excess ⁴⁰Ar. Hence, the age has no geological meaning.     

Deformation history and exhumation process of the Horoman Peridotite Complex, Hokkaido, Japan

The Horoman Peridotite Complex is an Alpine-type orogenic peridotite massif in the Hidaka metamorphic belt, Hokkaido, Northern Japan. Because of wide exposure and extremely limited serpentinization, the complex provides important information on uplift and emplacement processes of an Alpine-type peridotite massif into the crust. Based on microstructures, the massif can be divided into five structural units parallel to the lithological layering as follows; (1) Equigranular Zone, (2) Internal Shear Zone (ISZ), (3) Transition Zone, (4) Porphyroclastic Zone and (5) Basal Shear Zone (BSZ). A top-to-the-north sense of shear deformation in the Porphyroclastic Zone and the Basal Shear Zone implies that the Horoman Peridotite Complex had uplifted from the upper mantle to the lower crust along a northward dipping extensional shear-zone systems. After incorporation of the mantle peridotite with lower crustal rocks, the upper part of the massif (i.e. the Equigranular Zone and the Internal Shear Zone) was overprinted by a top-to-the-south sense of shear deformation that was comparable with the sub-horizontal displacement of the crustal granulite sequences in the Hidaka metamorphic belt under transpressive tectonic environment.     

Occurrence and chemical composition of amphiboles and related minerals in corundum-bearing mafic rock from the Horoman Peridotite Complex, Japan

A corundum-bearing mafic rock in the Horoman Peridotite Complex, Japan, was derived from upper mantle conditions to lower crustal conditions with surrounding peridotites. The amphiboles found in the rock are classified into 3 types: (1) as interstitial and/or poikilitic grains (Green amphibole), (2) as a constituent mineral of symplectitic mineral aggregates with aluminous spinel at grain boundary between olivine and plagioclase (Symplectite amphibole) and (3) as film-shaped thin grains, usually less than 10 μm in width, at grain boundary between olivine and clinopyroxene (Film-shaped amphibole). The Film-shaped amphibole is rarely associated with orthopyroxene extremely low in Al₂O₃, Cr₂O₃ and CaO (Low-Al OPX). These minerals were formed by infiltration of SiO₂- and volatile-rich fluids along grain boundaries after the rock was recrystallized at olivine-plagioclase stability conditions, i.e. the late stage of the exhumation of the Horoman Complex.

Chondrite-normalized rare earth element patterns and primitive mantle-normalized trace-element patterns of the Green amphibole and clinopyroxene are characterized by LREE-depleted patterns with Eu positive and negative anomalies of Zr and Hf. These geochemical characteristics of the constituent minerals were inherited from original whole-rock compositions through a reaction involving both pre-existing clinopyroxene and plagioclase. We propose that the fluids were originally rich in a SiO₂ component but depleted in trace-elements. Dehydration of the surrounding metamorphic rocks in the Hidaka metamorphic belt, probably related to intrusion of hot peridotite body into the Hidaka crust, is a plausible origin for the fluids.     

Compositional Continuity and Discontinuity in the Horoman Peridotite, Japan, and its Implication for Melt Extraction Processes in Partially Molten Upper Mantle

A hypothetical model is proposed to explain the origin of compositionaldiscontinuities in the layering observed in orogenic lherzolites.The observed collinearity of the whole-rock peridotite compositionsis best explained in terms of partial melting and melt segregation.The presence of chemical discontinuities implies that melt segregationincludes an abrupt and discontinuous process. A key conceptin the model is the topological transformation of melt geometryin partially molten rocks responding to the equality and inequalityof the fluid pressure and solid pressure, which may be realizedin a gravitational field. It is emphasized that the percolationthreshold is a critical boundary, beyond which a rapid microstructuralchange occurs in response to the change of local fluid pressure,thus causing a rapid increase of permeability. The model impliesthat the mode of melting is closer to batch melting than tofractional melting in the upper mantle. KEY WORDS: critical phenomenon; partial melting; percolation threshold; Horoman peridotite; melt segregation