Modern metamorphic petrology has founded by Eskora and Goldschmidt in the 1930s, and their basic idea was that the metamorphic facies represent the range of pressure, and the temperature and mineral facies show the compositional space regions. Later, during the 1950s to 1960s the Korzinskii, Thompson and Miyashiro applied solid thermodynamics to metamorphic phase equilibria. Miyashiro presented the new concept of facies - series in a geologic framework. Recent progress in metamorphic petrology has concentrated on methods and applications of exact pressure-temperature paths using differential thermodynamics. This new method of quantitative metamorphic petrology was established by Spear in 1980s. The precise P-T paths of subduction related metamorphic rocks make it possible to discuss the detailed process of subduction. Furthermore, important progress in seismic studies using seismic reflection and refraction methods in the plate boundary regions combines quantitative metamorphic petrology and seismic structure surveys with materials science in the Earth's interior as proposed by Peacock, and Seno and coworkers in the late 1990s. Along the line of the observational but not conceptual metamorphic sciences, the comprehensive field of geosciences focus on the foundations of Observational Metamorphic Sciences including geophysical prospecting, precise metamorphic petrology, and materials sciences of the Earth's interior.
In central Shikoku, SW Japan, eclogitic rock masses expose themselves among the sedimentary schists of the Sambagawa metamorphic belt. When and how they were juxtaposed might reveal the nature of subduction zone tectonics. Garnet is known to have conspicuous zoning in the Sambagawa metamorphic belt. Apart from grains having a normal zoning with a bell-shaped Mn profile, grains with a composite chemical zoning pattern are found exclusively in the schists close to the eastern Iratsu metagabbro mass in the Besshi district. It is reasonable to consider that composite zoning formed as a result of an event related to the juxtaposition. P-T paths of the characteristic zoning in garnet were inversely calculated using differential thermodynamic method (Gibbs' method), to extract the influence of the emplacement of the tectonic block. The derived P-T paths of composite zoning, compared to the prograde P-T path of the Sambagawa metamorphism, showed temperature decreases during their growth interval. It is suggested that the growth interval of garnet corresponds to the juxtaposition of the eclogitic rock masses and the pelitic Sambagawa schists, and that the juxtaposition occurred during subduction.
The differential thermodynamic method (Gibbs method) is applied to amphibole zonings in a metabasaltic system. A common mineral assemblage in metabasites (amphibole+epidote+ plagioclase+chlorite+quartz water) is analyzed in the system of SiO2-Al2O3-Fe2O3-FeO-MgO-CaO-Na2O-H2O. Because the number of end-members of amphibole is larger than the degree of thermodynamic freedom, we can estimate pressure-temperature conditions solely from amphibole chemistries. Analyses for numerous amphiboles from Sanbagawa schists show stable P-T fields of calcic and subcalcic amphiboles, and reveal that typical zoning of the albitebiotite zone (barroisite-hornblende-actinolite) yields the decompression P-T path. The gibbs method analysis minimizes systematic uncertainty when choosing the equilibrium set of mineral compositions, because the compositionset of coexisting minerals are not needed for the analysis. On the other hand, the propagated random uncertainty for calculated dT and dP becomes larger, as the change of monitor parameter becomes larger. To obtain accurate P-T paths, an appropriate way of minimizing total uncertainty is requied.
Recent advances in seismic studies have revealed that earthquakes have a close link with chemical processes, i.e. metamorphic dehydration. From this point of view, we provide a new scheme for observation of on-going regional metamorphism in a subduction zone. Combining the phase diagrams of MORB + water and peridotite + water with the thermal structure of the descending Pacific oceanic plate in NE Japan and in the Philippine Sea (PHS) plate in SW Japan, we can draw the distribution of metamorphic facies of regional metamorphism. However, the most uncertain parameter is thermal structure, even though it has been calculated numerically, because of the difficulty of evaluating frictional heating, heat transportation by dehydrated fluids and mantle convection in the hanging wall. To overcome this problem, we have carried out different approach from seismic observations in estimating the thermal structure of a subduction zone, by applying the dehydration-induced earthquake hypothesis. This hypothesis involves the assumptions as follows : 1) any dehydration in the subducted slab induces earthquakes, 2) peridotite of the subducting plate is more or less hydrated, as well as the oceanic crust, and 3) the dehydration reactions proceed in near equilibrium condition. Direct seismic determination of the depths of the blueschist or epidote-amphibolite facies to eclogite transformation, decomposition of serpentine (antigorite), and the stability limit of clinochlore enable us to establish fixed points for the slab temperature. The seismogenic zone (150°C to 350°C), the depth limit of non-volcanic tremor seismicity, and the slab melting in SW Japan (800-900°C) were also used to fix temperatures at given depths. Three profiles in NE-Japan and two profiles in SW-Japan were examined, and their P-T paths along the Wadati-Benioff zone were estimated to be anti-clockwise in all cases. The P-T paths are consistent with those of metamorphic facies series from well-studied on-land regional metamorphic belts. The P-T path of the subducting slab in NE-Japan is colder than that of the eastern-Shikoku section in SW-Japan, except for the Kii-peninsula section which has an almost similar P-T path to that in NE-Japan. Comparison between the on-going metamorphism beneath the Japanese islands and the on-land regional metamorphic belts in the Sanbagawa and Kokchetav shows that the P-T conditions of these two metamorphic belts are located between that of NE-Japan and of the eastern-Shikoku profile. A numerical model for wedge-mantle convection shows that the direction of the small corner flow of the wedge mantle causes a back current along the subducting slab. The area of the corner flow is wider in a shallow subduction zone corresponding to that in eastern Shikoku, and is narrower in a steeper subduction zone represented by NE-Japan. Since the exhumed metamorphic belts have intermediate P-T conditions between those in NE Japan and eastern Shikoku, We suggest that a change in the mode of wedge-corner flow from steep to shallow subduction plays some role in the exhumation of a metamorphic belt. Such a change in Cretaceous time from 120 Ma to 80 Ma may have promoted the exhumation of the Sanbagawa belt.
Garnet-bearing ultramafic rocks including clinopyroxenite, wehrlite, and websterite locally crop out in the Higashi-akaishi peridotite of the Besshi region in the Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins, or layers with a thickness ranging from a few centimeters to 1 meter. The wide and systematic variations of bulk-rock composition and overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite, and magnetite, implying equilibrium at relatively low T and hydrous conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell-shaped Al zoning with a continuous decrease of Al from the core towards the rim, and its rim records peak metamorphic conditions. Estimated P-T conditions imply a high P/T gradient (> 3.1 GPa/100 °C) from 1.5-2.4 GPa/700-800 °C to 2.9-3.8 GPa/700-810°C during prograde metamorphism. Olivines recrystallized at the high P/T prograde metamorphic stage show a B-type lattice preferred orientation (LPO) with a-axis concentrations normal to the stretching lineation. The presence of B-type LPO indicates that deformation of the prograde metamorphic stage possibly progressed under hydrous and high-stress conditions at the wedge mantle adjacent to the subducted slab. The Higashi-akaishi peridotite is a unique example that well records the prograde evolution of subducted ultramafic rocks.
Thermodynamic consideration of equilibria among garnet, biotite, plagioclase, and quartz enables us to estimate pressure-temperature conditions of pelitic and siliceous rocks occurring in a wide range of metamorphic grades, as a difference from the condition of a reference sample. Application of this technique to the Ryoke metamorphic rocks in the Yanai district provides the thermobaric structure of the area in more detail than was available before. The Gibbs method applied to a low-grade siliceous rock of the area yields a decompressional pressure-temperature path. The validity of the result is still ambiguous and should be evaluated petrographically in terms of constancy of mineral assemblage during garnet growth.
One of the important aims of metamorphic petrology is to unveil the physicochemical conditions of the Earth's interior. Decoding the record of metamorphism is a case of the inverse problem based on observations of stable equilibrium mineral assemblages and mineral compositions. Mineralogical forward-modeling is an alternative approach to estimate the metamorphic conditions of a rock. It enables a prediction of equilibrium composition of zoned minerals or mineral inclusions with the matrix phase assemblage. Thus, the forwardapproach tests the assumption of an equilibrium mineral assemblage, which is critical for inversion analysis. In this paper, we introduce thermodynamic forward-modeling in the field of metamorphic petrology. Then, we show an example of its application in estimating a prograde metamorphic P-T path of a whiteschist from the Kokchetav ultrahigh-pressure terrane in Kazakhstan. Garnet in the whiteschist shows prograde compositional zonation and contains mineral inclusions. We carried out geothermobarometric estimates using ilmenite-rutile composite inclusions in garnet, combined with an evaluation of the equilibrium mineral assemblage in a P-T pseudosection of the whiteschist in the K2O-CaO-MgO-FeO-Al2O3-SiO2-H2O system. The result yielded a counter-clockwise prograde P-T path for the rock. The amount and equilibrium composition of garnet were sequentially calculated for model P-T conditions along the P-T path, and the results were compared with a line-profile observation of the garnet. A consistency between the model and the observations was confirmed for XCa in garnet, however XFe and XMg had a large inconsistency. Both uncertainties in the equilibrium model and nonequilibrium effects during the crystal growth are possible reasons for such a discrepancy. Hence, it must be noticed that there are complexities in estimating the equilibrium composition of a zoned garnet and matrix minerals.
In general, derivation of subduction-stage P-T (pressure-temperature) paths for high-P metamorphic rocks is difficult, because in most cases they are affected by subsequent metamorphism, during which the pre-peak P mineral assemblages are significantly modified. However, petrological studies during the last 10 years in the Sambagawa belt have recognized unusually well-constrained sections of the subduction-stage P-T path. In particular, derivation of the subduction-stage P-T path of the Kotsu glaucophane eclogite was backed-up by insights obtained from a combination of petrological and structural analyses. Application of a new thermal model shows that the derived subduction-stage P-T paths are most likely to have been generated in a setting where an active spreading ridge is close to being subducted. This tectonic interpretation suggests that exhumation of high-P/T metamorphic rocks in oceanic subduction zones is triggered by regional heating events such as ridge subduction. A review of recent studies on the Sambagawa belt indicates the significance of promoting a 'comprehensive petrology', which encompasses the full set of constraints provided by disciplines such as petrology, structural geology, geochronology and thermal modeling.
Zircon is an excellent capsule to protect primary evidences of the ultrahigh-pressure (UHP) metamorphism, whereas mineralogic evidences of the UHP conditions were mostly obliterated in the matrix minerals of the rock due to the extensive retrograde overprinting related to exhumation. Zircons from the Kokchetav UHP-HP massif contain numerous inclusions of graphite, quartz, garnet, clinopyroxene, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite and monazite, as well as diagnostic UHP minerals, microdiamond and coesite, which are identified by laser Raman spectroscopy. The diamond inclusions coexist with coesite and garnet, and appear also with low-pressure minerals, especially graphite in single zircon grains with zonal arrangements. Internal structure of zircon displays distinct zonal fabrics, which comprise inherited core, wide mantle and outer rim, each with distinctive inclusion micro-assemblages. These observations indicat-ed that zircon has been grown at several distinct stages including UHP mineral-bearing mantle, low-pressure mineral-bearing rim and inherited core containing low-pressure minerals. Sensitive high resolution ion micro-probe (SHRIMP) analyses of the zoned zircons resulted four discrete ages of the Kokchetav ultrahigh-pressure metamorphic rocks ; (1) Middle Proterozoic protolith age, (2) 537 ± 9 Ma for the UHP metamorphism, (3) 57 ± 8 Ma for the late-stage amphibolite facies overprint, and (4) 456-461 Ma for the post-orogenic thermal events. The zircon inclusion method presented here is a comprehensive technique to demon-strate the history of ultrahigh-pressure metamorphic rocks, and can be applied to understand P-T-time history of other UHP-HP metamorphic terranes.
Prograde P-T paths were obtained by Gibbs method using zonings of garnet in the Annapurna area of central Nepal Himalaya. The results show different P-T paths in the uppermost and lower Lesser Himalayan Sequence (LHS), at the footwall of the Main Central Thrust (MCT). The uppermost LHS garnet shows paths with adiabatic compression (ΔP=90 MPa, ΔT=7°C) followed by heating (ΔT=20e), whereas the lower LHS garnet simply shows a sub-adiabatic compression (ΔP=350 MPa, ΔT=70°C) path. Tectonically, adiabatic compre-ssion is supposed to correspond with subduction, and subsequent heating with thrusting along the MCT. Heating by the MCT faulting seems to have occurred in a narrow zone of approx. 100 m in thickness.
The Cretaceous Abukuma metamorphic terrane consists of the oceanic Gosaisyo Series overthrust onto the terrigenous Takanuki Series. Although the dominant mineralogy defines one of the classic andalusite-sillimanite-type progressive metamorphism, there are several lines of evidence suggesting an earlier higher-pressure history of the Takanuki Series. Garnet in the Takanuki pelitic rocks commonly shows textural sector zoning and preserves growth compositional zoning despite the high metamorphic grade, suggesting rapid and continuous changes in P-T conditions and a relatively short duration of high-temperature conditions. The common high grossular content of garnet interiors (up to ca. 30 mol%) overgrown by Ca-poor rims (less than 3 mol% grossular) in the pelitic rocks containing Al2SiO5 minerals, plagioclase, and quartz indicates high-pressure (> 12 kbar) and subsequent low-pressure (ca. 6 kbar) conditions. In addition, the occasional presence of low Ca cores with sillimanite-bearing plagioclase inclusions suggests earlier high-temperature and low-pressure conditions. Thus, the Abukuma (Takanuki) metamorphic rocks are inferred to have experienced rapid hightemperature loading and subsequent unloading, which may be in common with some Cretaceous metamorphic rocks in the circum-Pacific region.
Boundary metamorphic rocks as exemplified by Sanbagawa metamorphic rocks have chemical records accompanied by physical processes in garnet and amphiboles. Garnet showing chemical zoning from core to rim grew during prograde metamorphism, which is the process of boundary rock subduction, and the pressure-temperature-water production rate path can be precisely determined by the modified differential thermodynamic method with the volume fraction of garnet. The results of the paths can be discussed in a dynamic system involving mechanical coupling between slab and arc crust, thermal interaction, and dehydration reaction, suggesting that mechanical coupling is weakend by a dehydration reaction.
The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolu-tionized the classic interpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regional metamorphic belts, (2) geochronology of that metamorphism, (3) origin of metamorphic textures, (4) P-T-t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids during regional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, and the Franciscan and Sanbagawa belts, we point out or predict the following seven revolutionary paradigm shifts. So-called mineral isograds defined on the maps of regional metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction, because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic-psammitic, and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should be carefully re-examined. The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP-UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23-40 m.y. from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP-HP unit occurred due to fluid infiltration underneath, when the UHP-HP unit intruded a low-grade to un-metamorphosed unit at a mid-crustal level. Most deformation textures such as mineral lineations, porphyroblasts, pull-apart or boudinaged amphiboles, formed during extensive hydration at the late-stage, hence do not indicate a progressive stress regime. The P-T-t path determined by thermobarometry using mineral inclusions in garnet and forward-modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise path in the P-T space, and it follows an identical P-T change in the metamorphic facies series. This is consistent with the numerically calculated geotherm along the WadatiBenioff plane. Collision-type orogenic belts have long been regarded as being characterized by the intermediate-type metamorphic facies series. The kyanite-sillimanite-type is an apparent type facies series formed by late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type facies series with an anticlockwise kink-point at around 10 kb is a progressive type. A collision-type regional metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P or weakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1 : 100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending a depth of from 100-200 km into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite-sillimanite type facies series. The major reason for the misunderstanding of the progressive metamorphism in collisiontype orogenic belts is the underestimation of the role of fluids derived from the underlying the low-grade metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids at a plate boundary is more important than a P-T change.