PETROLOGY AND GEOCHEMISTRY OF HIGHPRESSURE METAMORPHOSED PERIDOTITES, HAPPOO’NE COMPLEX, CENTRAL JAPAN

PETROLOGY AND GEOCHEMISTRY OF HIGHPRESSURE METAMORPHOSED PERIDOTITES, HAPPOO’NE COMPLEX, CENTRAL JAPAN

PhD (Doctor of Philosophy) Thesis Submitted to the Graduate School of

Natural Science and Technology, Kanazawa University, Japan

By

MOHAMED ZAKI ABDALLAH MOHAMED KHEDR

Supervised by

PROFESSOR DR. SHOJI ARAI

March 2010

ABSTRACT

The Happo-O’ne peridotite complex is located in the northeastern part of the Hida

marginal tectonic zone, central Japan, characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and high-P metamorphism. Their protoliths, which are mostly lherzolites to harzburgites with subordinate dunites, were formed as refractory residues by near fractional melting, ~ 15-20% melting for lherzolites to harzburgites, and ~ 25% melting for dunites at an earlier stage of the Japanese arc system; then they have been affected by retrograde reactions within the spinel stability field during cooling by slab-derived fluids in the corner of the mantle wedge. Some dunites are intact from hydration; their olivine is Fo92 and their primary chromian spinel shows high Cr#, 0.72 on average, suggesting forearc mantle origin. Tremolite-chlorite peridotites (= lherzolites-harzburgites), which are stable at low-T, from 650 to 750 ºC, and high-P, from 16 to 20 kbar, are restricted only in the tremolite zone. They are composed of olivine + orthopyroxene + tremolite + chlorite + chromian spinel; olivines with Fo88–Fo91 are similar in chemistry to olivines of ordinary Alpine-type peridotites, whereas orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al, Cr, Ca and Ti due to the low equilibration temperature. These orthopyroxenes include euhedral chromian

spinel that has high TiO2 (up to 5.7 wt%) and high Cr# (0.95 on average), but low Fe3+ (<0.3 on average). The high Ti of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-T pyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO2) during cooling. This result is supported by low-TiO2 (£0.02 wt%) in bulk-rock compositions of Ti-rich chromian spinel-bearing peridotites, reflecting no addition of Ti from outside sources. The Happo-O’ne metaperidotites (tremolite-chlorite peridotites) are representative

 of a hydrous low-T, high-P mantle peridotite facies transitional from a higher T anhydrous peridotite facies (spinel peridotites) formed by in-situ retrograde metamorphism influenced by fluids from the subducting slab. The mantle wedge, in which the examined metaperidotites were formed, is proposed as a unique environment for the formation of Tirich chromian spinel. We think that a certain condition (high-P, low-T) in the corner of the mantle wedge, similar to a condition of the Happo-O’ne complex, is proper for the formation of Ti-rich phase, reflecting a specified condition or setting. The Happo-O’ne metaperidotites have suffered from low-T (<600 °C) retrogressive metamorphism to form antigorite and secondary diopside (similar to diopside zone) during their exhumation with the Renge high-P/T metamorphic rocks (e.g., eclogites) in the supra-subduction zone (SSZ). The Low-T (400-600 °C), high-P (<20 kbar) conditions are dominant in the diopside zone of mineral assemblage of olivine + diopside + antigorite. The transition from the tremolite to diopside zone is mainly due to a thermal gradient in the mantle wedge, in which the diopside zone was suffered from high degrees of hydration to form antigorite serpentinites. The bulk-rock compositions of the Happo-O’ne metaperidotites show U-shaped PM (primitive mantle)-normalized REE patterns (0.05–0.5 times PM), and are enriched with Cs, Pb, Sr, Ba and Rb (LILE, 0.2–20 times PM) coupled with depletion of Zr, Hf, Ta and Nb (HFSE, <0.2 times PM), indicating metasomatism by fluids derived from the subducting slab. These results are confirmed by in-situ analysis of tremolites that show Ushaped REE patterns (0.1–3 times PM), and are highly enriched with fluid-mobile elements (B, Li, Cs, Sr and Pb; 1–100 times PM) and Sc relative to HFSE (mainly below the detection limits). Not only tremolites but also all coexisting peridotitic silicates are enriched in fluid-mobile elements (LILE and LREE) relative to HFSE (below the detection limits) because of slab-derived fluids infiltrated into the cooled mantle-wedge peridotites in the Happo-O’ne area.

The Happo-O’ne hydrous peridotites differ in chemistry (e.g., Na, Ca, HFSE, REE

and U) from chlorite harzburgites (prograde peridotites) produced by high-pressure

breakdown of antigorite serpentinite from SE Spain. We think that slab-derived fluids

metasomatized mantle-wedge peridotites in the Happo-O’ne area have different

compositions with those affected on chlorite harzburgites, SE Spain.

In this respect, in-situ analyses of peridotitic silicates from the diopside zone

reaffirmed the previous results; clinopyroxenes as well as other silicates show selective

enrichments of fluid-mobile elements (B, Sr, Pb, Li, Cs, Ba and Rb; 0.1–100 times PM) relative to HFSE (Ta, Hf, Th, Zr, Ti and Nb; <0.7 times PM) + U, indicating that the Happo-O’ne metaperidotites have suffered a slab-fluid metasomatism at high-P, low-T after partial melting stage. We recognized three types of clinopyroxenes that are different in morphology and REE patterns, but show the same major-element chemistry. Nearly euhedral clinopyroxene (Cpx1), mostly with exsolved lamellae, displays spoon-shaped REE pattern (0.2–3 times PM) and reflects the first stage (stage 1) of metasomatism. The apparently primary Cpx1 is considered as recrystallized primary mantle clinopyroxenes (residual Cpx), and may be modified and/or recrystallized at the beginning of metamorphism at slightly high-T/P. Acicular or fibrous clinopyroxene (Cpx2) after tremolite + olivine shows U-shaped REE pattern (0.06–1 times PM) (stage 2), whereas fine-grained clinopyroxene (Cpx3) derived from orthopyroxenes contains very low concentrations of REE (stage 3). Successive three stages of retrogressive clinopyroxene formation possibly denote a retrogressive chemical change of involved fluids during multistage metasomatism. The fluid of stage 1 is highly enriched in LREE (0.2-2 times PM), MREE, Pb, Sr, Li and Rb relative to those of the stage-3 fluid that is free of MREE and Pb, and very low of LREE and Sr, but has high Na, Mn, Ba, B and Cs. The stage-2 fluid,

which is diluted in LREE (0.06-1 times PM), Li and Pb, and free of Cs, Rb and Hf relative to fluid of stage 1, is considered as a transitional fluid between those of stage 1 and stage 3. The retrogressive chemical change of the fluid compositions, which are related to the change of a metamorphic condition, a depth to the subducting slab and an episode of metasomatic stage during the exhumation of the Happo-O’ne peridotites, are possibly equivalent to a transversal change of the subducting slab-derived fluids within the mantle wedge. The Happo-O’ne metaperidotites and their silicates are highly enriched in fluidmobile elements and strongly depleted in HFSE + U. Both textural and chemical evidence suggest the involvement of metasomatic slab-derived fluids rather than melts accompanied by changing in the fluid compositions during the exhumation of the Happo-O’ne peridotites

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