Bearing of rare earth patterns of apatites from igneous and metamorphic rocks

Twelve apatite samples from igneous and metamorphic rocks from the Black Forest and igneous rocks from the Kaiserstuhl were analysed for their REE content. The ΣREE range from 0.116 to 1.69 wt.%; the lowest values were found in the metamorphic rocks. All apatites from the various parent rocks show a general enrichment of the lighter rare earths over the heavier and their chondrite-normalized rare earth patterns exhibit a more or less pronounced negative Ce anomaly. This Ce depletion is accompanied by relatively low La and Pr values. In addition, the apatites from igneous rocks from the Black Forest show a marked negative Eu anomaly. An explanation is offered for the simultaneous occurrence of a negative Ce and a negative Eu anomaly in one investigated rock system. The negative Ce anomaly is attributed to the occurrence of phases enriched in Ce (e.g., monazite, allanite) which crystallized prior to or simultaneously with apatite. The negative Eu anomaly in these magmatic apatites most probably is caused by discrimination of Eu²⁺ from the apatite lattice. The result of this discrimination is a selective Eu enrichment in the later crystallizing feldspars, plagioclase and orthoclase. At least in this case, the positive Eu anomaly in feldspars is not a reliable indicator of low oxygen fugacity during their crystallization; the Eu depletion of the earlier crystallized apatites is preferable for this purpose.     

Comparison and interpretation of graphitization in contact and regional metamorphic rocks

Abstract X-ray diffraction (XRD) analyses of carbonaceous materials were carried out in conjunction with petrological studies for selected metamorphic rocks in order to compare the structural state of carbonaceous materials between contact and regional metamorphic rocks. The most extensive study was done for the Daimonji contact aureole in the eastern part of Kyoto city, Japan. The Daimonji contact aureole can be divided into three mineral zones using mineral parageneses of pelitic rocks: chlorite, biotite and cordierite zones. The cordierite zone can be further subdivided into lower- and higher-grade subzones. Petrological considerations allow the two isograd reactions that define the lower- and higher-grade cordierite subzones to be determined and suggest these reactions occurred at 510–560°C and 560–590°C per 2.0-2.3 kbar, respectively. A combination of the petrological studies and the XRD data of carbonaceous materials suggest that fully ordered graphite (FG; defined by d(002) ≤ 3.360 Å following the convention used by many workers), appears around 560–590°C in the Daimonji contact aureole. This data and refinement of geothermometer for published data confirmed that the FG appears at 400–500°C in regional metamorphic rocks, but at higher than 530°C in contact aureoles. One possible explanation for such a temperature difference is the duration of heating. However, the width at half height (WH) of the graphite peak attains a similar value of 0.30° at around 500°C both in contact and regional metamorphic rocks, suggesting that WH value is a more reliable indicator of metamorphic grade than the change of d(002) value. Furthermore, the depressed d(002) data of graphite was observed locally in the higher grade part (≥ 500°C) of the Ryoke regional metamorphic belt, where granitic intrusions exist within a few km distance. These facts indicate that the duration of heating is not an important factor controlling the change of d(002) value. It is possible that interlayered impurities, such as chlorine, which was derived from igneous intrusions, may be an important factor in suppressing the reduction in d(002) at temperatures greater than 500°C.     

Detrital heavy minerals from Lower Jurassic clastic rocks in the Joetsu area,central Japan: Paleo‐Mesozoic tectonics in the East Asian continental margin constrained by limited chloritoid occurrences in Japan

Detrital chloritoids were extracted from the Lower Jurassic sandstones in the Joetsu area of central Japan. The discovery of detrital chloritoids in the Joetsu area, in addition to two previous reports, confirms their limited occurrence in the Jurassic strata of the Japanese islands. This finding emphasizes the importance of the denudation of chloritoid‐yielding metamorphic belts in Jurassic provenance evolution, in addition to a change from an active volcanic arc to a dissected arc that has already been described. Possible sources for the detrital chloritoids from the Jurassic sandstones are the Permo–Triassic chloritoid‐yielding metamorphic rocks distributed in dispersed tectonic zones (Hida, Unazuki, Ryuhozan and Hitachi Metamorphic Rocks), which are in fault contact with Permian to Jurassic accretionary complexes in the Japanese islands. This is because all of these pre‐Jurassic chloritoid‐yielding metamorphic rocks have a Carboniferous–Permian depositional age and a Permo–Triassic metamorphic age, whereas a Permian–Triassic metamorphic age on the Hitachi Metamorphic Rocks remains unreported. In addition, most metamorphic chloritoids imply a former stable land surface that has evolved into an unstable orogenic area. Therefore, the chloritoid‐yielding metamorphic rocks might form a continuous metamorphic belt originating from a passive continental margin in East Asia. Evidence from paleontological and petrological studies indicates that the Permo–Triassic metamorphic belt relates to a collision between the Central Asian Orogenic Belt and the North China Craton. The evolution of the Permian–Jurassic provenance of Japanese detrital rocks indicates that the temporal changes in detritus should result from sequences of collision‐related uplifting processes.     

Microprobe studies of REE-rich accessory minerals: Implications for Skye granite petrogenesis and REE mobility in hydrothermal systems

The REE geochemistry of accessory allanites, sphenes. apatites and zircons from a range of granitic, sedimentary and hydrothermally altered rocks from Skye has been investigated using the electron microprobe. Allanites and sphenes in Skye Tertiary granites are extremely LREE enriched (Ce_N/Y_N= 40–100) and may contain up to 50% of whole rock LREE (La-Nd). These phases are late crystallisation products of redidual magmatic fluids. Earlier-formed apatites (Ce_N/Yb_N = 7.33) and zircons (Ce_N/Yb_N = 0.05) contain insufficient REE to have influenced the REE geochemistry of the Western Red Hills granites by crystal fractionation. However, Y-, Th- and HREE-rich zircons (Ce_N/Yb_N = 0.03–0.12, ΣREE + Y = 16,500–49,500ppm) occur both as detrital grains in Skye Torridonian sediments and in the Coire Uaigneich Granophyre (CUG), suggesting bulk involvement of these sediments in CUG petrogenesis. Hydrothermal allanites in altered Tertiary igneous rocks from Skye are LREE enriched (Ce_N/Y_N = 16–920), whilst allanites formed during alteration of Torridonian arkoses have less fractionated REE patterns (Ce_N/Y_N = 4.4–1.0), as the instability of metamict HREE-rich detrital zircons buffered the hydrothermal fluids in these rocks to more HREE-rich compositions. This buffering indicates that within unveined rocks the scale of REE mobility during hydrothermal alteration was small, even though the occurrence of allanite in hydrothermal veins on Skye suggests that LREE may have been transported for some distance by meteoric-hydrothermal fluids. Zoning of the REE within individual hydrothermal and metamorphic allanites (e.g. coreCe_N/Y_N = 97.56, rim Ce_N/Y_N = 0.22) suggests evolution of their parent geological fluids to more HREE-rich compositions during allanite growth.     

Formation of a high-temperature metamorphic complex due to pervasive melt migration in the hot crust

Abstract   Detailed reviews of metamorphic thermal structures and spatial distributions of granitic rocks in the high- T Higo metamorphic complex (HMC), central Kyushu, Japan, and Ryoke metamorphic complex in the Yanai district (RMC-Y), southwest Japan, reveal that the high-grade metamorphic rocks were associated with migmatites or syn-metamorphic granitoids. The close association of these rocks suggests that transport and solidification of granitic melts are appreciable heat sources. Partial melting of the pelitic host resists heating of the crust due to its endothermic reaction (i.e. absorption of latent heat). However, the effects of slower thermal evolution produced by partial melting are limited by the boundary conditions assumed for the HMC and RMC-Y. New thermal numerical modeling reveals that the volume fraction of the solidified melt products and the duration of melt migration required for crustal heating to a certain temperature decrease with increasing rates of melt migration. The model predicts that the melt migration rate during formation of the HMC is about three to 10 times higher than that of the RMC-Y, and the corresponding duration of melt migration is about one order of magnitude shorter. The model described here also explains the apparently contradictory observations made in different structural units of the RMC-Y: the metamorphic pressure–temperature ( P-T ) ratio of the southern unit is higher than those of the central and northern units, even though the volume fraction of granitic rocks in the southern unit is larger. The model suggests that the melt migration rate in the northern and central units was faster than that in the southern unit. Overall, the model implies that variations of metamorphic field P-T arrays and the spatial distributions of granitic rocks in high- T metamorphic complexes may be explained simply by variations in the rate and duration of melt migration in the hot crust.     

RbSr whole-rock ages of Precambrian metamorphic rocks in the Kamiaso conglomerate from central Japan

Whole-rock samples of metamorphic and granitic cobbles and boulders from the Kamiaso conglomerate in central Japan yield well-defined RbSr isochron ages of 1985 ± 25 my and 1820 ± 40 my. These ages are the oldest yet obtained for rocks in the Japanese Islands, and provide key evidence for the middle Precambrian metamorphic and igneous events in the provenance of these rocks. The age of 1985 my defined by six samples of quartzo-feldspathic gneiss may be the time of emplacement of the original granitic rocks. The 1820 my age indicates the time of extensive regional metamorphism and igneous activity. Precambrian episodes in the provenance of the Kamiaso conglomerate are summarized as follows: (1) 2000 my — formation of granitic rocks, (2) 1800–1600 my — high grade metamorphism accompanied by igneous activity, (3) 1200–1000 my — some significant thermal event.Judging from rock types and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those of the Matenrei and Nangnim systems in North Korea. The Precambrian complex from which the metamorphic and granitic rocks were derived, was exposed to the north not far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it was probably a part of the large Precambrian continent in East Asia.     

Radiometric ages of Alpine metamorphic rocks in the western and central Alps

Abstract The chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia-Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U-Pb monazite (23-29 Ma), Rb-Sr muscovite (15–40 Ma) and biotite (15–30 Ma), K-Ar biotite (10-30 Ma), muscovite (15–25 Ma) and hornblende (25-35 Ma), and FT zircon (10-20 Ma) and apatite (5-15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U-Pb zircon age for a coesite-bearing whiteschist (metaquartzite) from the Dora-Maira represents the peak of ultra-high pressure metamorphism. Coesite-free eclogites and blueschists related to ultra-high pressure rocks in the Penninic crystalline massifs yield an ⁴⁰Ar-³⁹Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high P/T metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra-high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia-Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium P/T metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid-ocean ridge and Eocene continental collision triggered off the exhumation of the high pressure rocks.     

Triassic warm subduction in northeast Turkey: Evidence from the Ağvanis metamorphic rocks

Within the Tethyan realm, data for the subduction history of the Permo–Triassic Tethys in the form of accretionary complexes are scarce, coming mainly from northwest Turkey and Tibet. Herein we present field geological, petrological and geochronological data on a Triassic accretionary complex, the A?vanis metamorphic rocks, from northeast Turkey. The A?vanis metamorphic rocks form a SSE–NNW trending lozenge‐shaped horst, ～20 km long and ～6 km across, bounded by the strands of the active North Anatolian Fault close to the collision zone between the Eastern Pontides and the Menderes–Taurus Block. The rocks consist mainly of greenschist‐ to epidote‐amphibolite‐facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid‐ocean ridge basaltic and island‐arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes of Early Eocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°C and ～0.60–0.90 GPa. Stepwise ⁴⁰Ar/³⁹Ar dating of phengite–muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ～180 to 209 Ma, suggesting that the metamorphism occurred at ≥ 209 Ma. Thus, the A?vanis metamorphic rocks represent the vestiges of the Late Triassic or slightly older subduction in northeast Turkey. Estimated P–T conditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present‐day Cascadia. Contact metamorphic mineral assemblages around an Early Eocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ～14 km during the Early Eocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐mica Ar–Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision between Eastern Pontides and the Menderes–Taurus Block.     

Orthopyroxene–cordierite mafic gneiss from the Nomamisaki metamorphic rocks,Southern Kyushu,Japan: Implication for western continuation of the Usuki–Yatsushiro Tectonic Line

We describe an orthopyroxene–cordierite mafic gneiss from the Nomamisaki metamorphic rocks in the Noma Peninsula, southern Kyushu, Japan. The mineral assemblage of the gneiss is orthopyroxene, cordierite, biotite, plagioclase, and ilmenite. Thermometry based on the Fe–Mg exchange reaction between orthopyroxene and biotite yields a peak metamorphic temperature of 680°C. The stability of cordierite relative to garnet, quartz, and sillimanite defines the upper limit of the peak metamorphic pressure as 4.4 kbar. These features indicate that the Nomamisaki metamorphic rocks underwent low‐pressure high‐temperature type metamorphism. Although a chronological problem still remains, the Nomamisaki metamorphic rocks can be regarded as a western continuation of the Higo Belt. The Usuki–Yatsushiro Tectonic Line, which delineates the southern border of the Higo Belt, is therefore located on the east of the Nomamisaki metamorphic rocks in southern Kyushu.     

Ultra-high pressure metamorphic rocks in the Dabie-Su-Lu region, China: Their formation and exhumation

Abstract Based on petrological, structural, geological and geochronological research, the authors summarize the progress of ultra-high pressure (UHP) metamorphic rock study since 1989 by Chinese geoscientists and foreign geoscientists in the Dabie-Su-Lu region. The authors introduce and discuss a two-stage exhumation process for the UHP metamorphic rocks that have various lithologies; eclogite, ultramafics, jadeitic quartzite, gneiss, schist and marble. The metamorphic history of UHP metamorphic rocks is divided into three stages, that is, the pre-eclogite stage, coesite eclogite stage, and retrograde stage. Prior to UHP metamorphism, the ultramafics had a high temperature environment assemblage of mantle and others had blueschist facies assemblages. The granulite facies assemblages, which have recorded a temperature increase event with decompression, have developed locally in the Weihai basaltic rocks. Isotopic ages show a long range from > 700 Ma to 200 Ma. The diversity in protoliths of UHP metamorphic rocks may be related to the variation of isotopic ages older than 400 Ma. The Sm-Nd dating of ~ 220 Ma could reflect the initial exhumation stage after the peak UHP metamorphism in relation to the collision between the Sino-Korean and Yangtze blocks and subsequent events. Petrological and structural evidence imply a two-stage exhumation process. During the initial exhumation, the UHP metamorphic rocks were sheared and squeezed up in a high P/T regime. In the second exhumation stage the UHP metamorphic rocks were uplifted and eventually exposed with middle crustal rocks.     

Subhorizontal boundary between ultrahigh-pressure and low-pressure metamorphic units in the Sulu–Tjube area of the Kokchetav Massif, Kazakhstan

Abstract The Kokchetav Massif of Kazakhstan includes high to ultrahigh-pressure (HP–UHP) metamorphic rocks (some of which were recrystallized at depths in excess of 150 km), juxtaposed against much lower pressure metamorphic components. We investigated the relationship between the HP–UHP metamorphic unit and the low pressure (LP) unit (Daulet Suite) in the Sulu–Tjube area, where the metamorphic rocks have previously been interpreted as constituting a megamelange with subvertical structural attitudes. Analyses of fold structures suggest that the HP–UHP metamorphic unit overlies the LP unit across a west-dipping subhorizontal boundary. In addition, kinematic indicators display top-to-the-north senses of shear along the tectonic contact between the two units, indicating that the HP–UHP unit has been extruded northward onto the LP unit. Following the juxtaposition of the two units, upright folds developed in both units, and these are associated with the previously reported steeply dipping metamorphic foliations. These data have important implications for the mode of exhumation of the UHP rocks from upper mantle to shallow crustal depths.     

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.     

Extensive normal faulting during exhumation revealed by the spatial variation of phengite K–Ar ages in the Sambagawa metamorphic rocks,central Shikoku,SW Japan

Metamorphic rocks experience change in the mode of deformation from ductile flow to brittle failure during their exhumation. We investigated the spatial variation of phengite K–Ar ages of pelitic schist of the Sambagawa metamorphic rocks (sensu lato) from the Saruta River area, central Shikoku, to evaluate if those ages are disturbed by faults or not. As a result, we found that these ages change by ca 5 my across the two boundaries between the lower‐garnet and albite–biotite, and the albite–biotite and upper‐garnet zones. These spatial changes in phengite K–Ar ages were perhaps caused by truncation of the metamorphic layers by large‐scale normal faulting at D₂ phase under the brittle‐ductile transition conditions (ca 300°C) during exhumation, because an actinolite rock was formed along a fault near the former boundary. Assuming that the horizontal metamorphic layers and a previously estimated exhumation rate of 1 km/my before the D₂ phase, the change of 5 my in phengite K–Ar ages is converted to a displacement of about 10 km along the north‐dipping, low‐angle normal fault documented in the previous study. Phengite ⁴⁰Ar–³⁹Ar ages (ca 85 to 78 Ma) in the actinolite rock could be reasonably comparable to the phengite K–Ar ages of the surrounding non‐faulted pelitic schist, because the K–Ar ages of pelitic schist could have been also reset at temperatures close to the brittle–ductile transition conditions far below the closure temperature for thermal retention of argon in phengite (about 500–600°C).     

Thermal regimes and regional metamorphism in the vicinity of overthrust faults: an example of shear heating and inverted metamorphic zonation from southern California

Thermal models are developed and temperature-depth profiles computed to account for the development of sharply inverted metamorphic zonations and estimatedP-T conditions beneath deep-seated thrust faults, with particular reference to the metamorphism of the Pelona Schist-Vincent Thrust system of southern California.Models dependent only upon conductive redistribution of heat from an initially hot upper plate are inadequate to account for the observed temperature distributions and theP-T time constraints can only be satisfied if a total shear heating of 3 hfu or more occurs in the region of the thrusts. These heating rates are consistent with known rates of plate movement, shear stresses along major fault zones, and shear strengths of rocks.Petrological constraints indicate the likelihood of buffering of the maximum temperature attained by shear heating due to marked reduction of shear strengths of rocks upon increase of temperature and release of water during metamorphic dehydration reactions.These models may be of general applicability to inverted metamorphic zonations, notably those beneath obducted Newfoundland ophiolites, and are consistent with our present understanding of the thermal properties of the upper parts of subduction zones.     

A high‐T metamorphic complex derived from the high‐P Suo metamorphic complex in the Omuta district,northern Kyushu,southwest Japan

New U–Pb ages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐P Suo metamorphic complex to a high‐T metamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca 1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐T metamorphism. The high‐P to high‐T transformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐T metamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐T metamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐P to high‐T transformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐T metamorphic field.     

SHRIMP U–Pb zircon ages of the Hida metamorphic and plutonic rocks,Japan: Implications for late Paleozoic to Mesozoic tectonics around the Korean Peninsula

A new U–Pb zircon geochronological study for the Hida metamorphic and plutonic rocks from the Tateyama area in the Hida Mountains of north central Japan is presented. The U–Pb ages of metamorphic zircon grains with inherited/detrital cores in paragneisses suggest that a metamorphic event took place at around 235–250 Ma; the cores yield ages around 275 Ma, 300 Ma, 330 Ma, 1 850 Ma, and 2 650 Ma. New age data, together with geochronological and geological context of the Hida Belt, indicate that a sedimentary protolith of the paragneisses is younger than 275 Ma and was crystallized at around 235–250 Ma. Detrital ages support a model that the Hida Belt was located in the eastern margin of the North China Craton, which provided zircon grains from Paleoproterozoic to Paleozoic rocks and also from Archean and rare Neoproterozoic rocks. Triassic regional metamorphism possibly reflects collision between the North and South China Cratons.     

Parentage of low‐grade metasediments in the Sanbagawa belt,eastern Shikoku,Southwest Japan,and its geotectonic implications

This study examines the geology of low‐grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS‐II (Coniacian–Campanian) and KS‐I (late Albian–early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low‐grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north‐younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low‐grade metamorphic rocks.     

Evolution processes of Ordovician–Devonian arc system in the South‐Kitakami Massif and its relevance to the Ordovician ophiolite pulse

The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.     

Properties of some rocks from the section of the Kola ultradeep borehole as a function of the P-T parameters

Using core samples of the Kola ultradeep borehole (SG-3) and their surface analogues, variations in the density and elastic properties of some crystalline rocks of the Earth’s crust are estimated by modeling of in situ conditions. It is shown that the bulk density and the elastic wave velocities in the rocks have a weak depth gradient. In the SG-3 section under consideration, this gradient is negative. The resulting dependences for estimating the variations in the properties of the crystalline rocks are suitable for the depth range from the surface to 20–30 km. The initial data for the linear approximation of the characteristics can be obtained from the results of tests of surface analogue samples. It is shown that the velocity anisotropy of the metamorphic rocks can vary within wide limits.