Shigenori Maruyama

Paleogeographic reconstruction and origin of the Philippine Sea

Abstract Our reconstruction of the Philippine Sea suggests that it formed by two distinct episodes of back-arc spreading, each of which resulted from seaward retreat of the trench. In the first episode, the protoIzu-Bonin Trench retreated northward and the West Philippine Basin formed behind the northern half of the Palau-Kyushu Ridge. In the second episode, the Izu-Mariana Trench retreated eastward and the Shikoku and Parece Vela Basins formed behind it. During the last 17 Ma, the Philippine Sea basin has been moving northwestward with respect to Eurasia shifting the TTT triple junction off central Japan westward by about 50 km. The motion of the Philippine Sea with respect to Eurasia at the triple junction changed from north-northwestward to west-northwestward 10-5 Ma ago. For the period before 17 Ma ago, we construct two models, retreating trench model and anchored slab model. The Izu-Bonin Trench migrated from south to northeast rotating in a clock-wise sense since 48 Ma ago in the retreating trench model. In the anchored slab model, the trench has been fixed with respect to Eurasian margin since 43 Ma ago. We prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of Eurasia during 30-17 Ma ago is much simpler for this model than for the anchored slab model. Furthermore rotations of the Bonin-Mariana islands are consistent with those predicted from the retreating trench model. The 48 Ma ages of the northern part of the Palau-Kyushu Ridge and of Chichi-Jima of the Bonin Islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 Ma ago. From this and the subparallelism in trend between the northern part of the Palau-Kyushu Ridge and the Central Basin Ridge, we propose that the major part of the West Philippine Basin formed by back-arc spreading in a N-S direction behind the northern part of the Palau-Kyushu Ridge. The Pacific plate was moving northward with respect to hot-spots from 48 to 43 Ma ago, which implies that the Pacific plate is not likely to have been subducting beneath the West Philippine Basin during this time. We speculate that another plate existed south of the Pacific plate and thai it was subducting beneath both tile West Philippine Basin and Australia. The annihilation of this plate might he a cause for the sudden change of the Pacific plate motion at 43 Ma ago.

Orogeny and relative plate motions: Example of the Japanese Islands

Abstract The paleogeography of the eastern margin of Asia, near Japan, for the past 250 Ma has been reconstructed on the basis of the relative motions between the oceanic plates and the Eurasian plate (Engebretson et al., 1982) and on-land geology. The relationships between the plate kinematics and orogeny are as follows: Calc-alkaline magmatism is related to the presence of subduction. When a plate boundary is a transform fault, there is no volcanism. Very slow convergence rates do not give rise to magmatism. Migration of RFT-type triple junctions or ridge subduction produces forearc volcanism near the trench, and also a wider volcanic zone. Uplift of low-P/T-type regional metamorphic belts is related to collision episodes. The uplift of the high-P/T blueschist belts, on the contrary, is not contemporaneous with them. They appear to be related to the start of the subduction of a young plate after the migration of an RFT-type triple junction. Because the formation of a voluminous accretionary complex is contemporaneous with this, and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed, the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high P/T-type metamorphic belt. The subduction of a young plate would thus be an important factor causing these orogenic events. Highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists, presumably along a mechanical plate boundary in the forearc.

The peristerite gap in low-grade metamorphic rocks

Coexisting Na-plagioclases from greenschists both in the thermal aureole of the Kasugamura Granite, Japan, and in the low-P metamorphic zone of Yap Island, western Pacific were analyzed in great detail; the peristerite solvus was determined for each suite. The asymmetric solvus has steep albite-rich and gentle oligoclase-rich limbs that are similar to those for higher pressure series. The present results together with those from Vermont, New Zealand, and the Sanbagawa belt indicate that the peristerite solvus shifts toward the albite component and higher temperature with increasing pressure. With increasing pressure, albite co-existing with oligoclase (An=100 Ca/Ca+ Na=20) varies in composition from An 8–9 (in Kasugamura), through An 3 (in Yap Island and Vermont), to An 1 (in New Zealand) and An less than 0.5 (in the Sanbagawa belt). The consolute temperatures for the peristerite solvus estimated from available geothermometry are 420° C in Kasugamura, 450–550° C in Vermont and 550°–600° C in the Sanbagawa belt. The variation of plagioclase composition in progressive metamorphic zones is explained by intersection of a plagioclase-forming reaction and the peristerite immiscibility gap in an isobaric T-XAn diagram. The greenschist zone is characterized by albite, the transition zone by occurrence of peristerite pairs and the amphibolite zone by plagioclase of An 20–50.

Kurosegawa zone and its bearing on the development of the Japanese Islands

Abstract The Kurosegawa zone in southwest Japan is a 600 km long serpentinite melange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone: 1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma, 2. (2) Siluro-Devonian formations, 3. (3) Upper Carboniferous to Jurassic formations, 4. (4) Upper Jurassic to Lower Cretaceous formations, 5. (5) serpentinite and 6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar). The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units. Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.

An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar Pfluid

AbstractThe univariant reaction governing the upper stability of heulandite (CaAl2Si7O18·6H2O), heulandite=laumontite+3 quartz+2H2O (1), has been bracketed through reversal experiments at: 155±6° C, 1000 bar; 175±6° C, 1500 bar; and 180±8° C, 2000 bar. Reversals were established by determining the growth of one assemblage at the expense of the other, using both XRD and SEM studies. The standard molal entropy of heulandite is estimated to be 783.7±16 J mol−1 K−1 from the experimental brackets. Predicted standard molal Gibbs free energy and enthalpy of formation of heulandite are −9722.3±6.3 kJ mol−1 and −10524.3±9.6 kJ mol−1, respectively. The reaction (1), together with the reaction, stilbite=laumontite+3 quartz+3 H2O, defines an invariant point at which a third reaction, stilbite=heulandite+ H2O, meets. By combining the present experimental data with past work, this invariant point is located at approximately 600 bar and 140° C. Heulandite, which is stable between the stability fields of stilbite and laumontite, can occur only at pressures higher than that of the invariant point, for

Phase equilibria and mineral parageneses of metabasites in low-grade metamorphism

P_{H_2 O}

Greenschist–Amphibolite Transition Equilibria at Low Pressures

= Ptotal.These results are consistent with natural parageneses in low-grade metamorphic rocks recrystallized in equilibrium with an aqueous phase in which