Takao Hirajima

Occurrence and field relationships of ultrahigh-pressure metagranitoid and coesite eclogite in the Su-Lu terrane, eastern China

Coesite eclogite is associated with metagranitoid in a 50×100 m2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

Decompression P–T path of coesite eclogite to granulite from Weihai, eastern China

Granulitized coesite-bearing eclogite from Weihai, northeastern part of the Shandong peninsula, eastern China was studied in detail to reveal the modification of mineral chemistry during decompression metamorphism. Considerable modification of chemical composition is recorded in clinopyroxene that occurs both as inclusions in garnet and as a matrix mineral. Careful examination of chemical variation with the change in microstructure made it possible to estimate the equilibrium composition of minerals at the coesite eclogite and garnet granulite stages. We were able to define three reference points on the P–T path, namely, coesite eclogite (3 GPa, 660±40°C), granulite (1 GPa, 700±30°C) and amphibolite (0.9 GPa, 600±20°C). The path thus obtained is similar to those obtained by previous workers and supports nearly isothermal decompression of coesite eclogite.

Phase petrology of eclogites and related rocks from the Motalafjella high-pressure metamorphic complex in Spitsbergen (Arctic Ocean) and its significance

Abstract A Caledonian eclogite suite is associated with lower-grade high-pressure rocks in Motalafjella, Oscar II Land, central-western Spitsbergen. A variety of mineral assemblages occur in the suite, depending on the bulk-rock composition; omphacite-paragonite-glaucophane-epidote-garnet in eclogites, jadeite-glaucophane-paragonite-quartz in siliceous schists, and MgFe chloritoid-paragonite-epidote-garnet in pelitic schists. Petrologic and paragenetic studies are presented to characterize this unique set of mineral assemblages in relation to other eclogite suites. Mineralogical data, mainly the distribution of Fe and Mg between coexisting clinopyroxene and garnet, and the existence of impure jadeite+quartz assemblage, give approximately 580–640°C and 18–24 kbar. New criteria for the eclogite classification are derived based on the mineral assemblages and a model petrogenetic grid. The Motalafjella eclogites associated with the glaucophane-epidote assemblage belong to the medium-temperature portion of the eclogite facies.

Superzoned garnets in the coesite-bearing Brossasco-Isasca Unit, Dora-Maira massif, Western Alps, and the origin of the whiteschists

Abstract Rare centimeter-sized superzoned garnets (SZGs) were discovered in two coesite-bearing whiteschists of the Brossasco-Isasca Unit (BIU), southern Dora-Maira massif (DMM), Western Alps. The superzoned garnet consists of a reddish-brown almandine core crowded with inclusions of staurolite, chloritoid, kyanite, chlorite and paragonite, and of a pinkish pyrope rim with sporadic inclusions of kyanite, and magnesian chlorite. The core–rim contact is relatively sharp and marks the termination of the inclusion-rich portion. The core composition of the superzoned garnet is almost identical to, or slightly richer in Mg, than that of the rim of porphyroblastic garnet in metapelites from the same unit. In the rim of the superzoned garnet, Mg–Fe ratio increases abruptly towards the outermost rim, whose composition is identical to that of the common pyrope in the whiteschist. At the core–rim boundary, there is no chemical gap. Chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite. The staurolite composition (Si=2.00 and total R 2+ Grt>Cld>Chl. The resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages, such as St–Cld–Chl vs. Cld–Chl–Ky. New and literature data and results of experiments in the KFASH system suggest that: (1) the superzoned garnet was formed under a single prograde high-pressure/ultra high-pressure (HP/UHP) Alpine metamorphism, (2) the almandine inclusion-rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600°C and less than 16 kbar, probably from a former metapelite xenolith included in a Variscan granitoid, and (3) the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth, i.e., at a stage close to the UHP climax.

Tectonothermal chronology within a blueschist-eclogite complex, west-central Spitsbergen, Svalbard: Evidence from 40Ar39Ar and RbSr mineral ages

Abstract A high-pressure, relatively low-temperature metamorphic complex is exposed at Motalafjella, Spitsbergen. White mica concentrates from the complex record variably discordant 40 Ar 39 Ar age spectra in which apparent ages systematically increase throughout low-temperature portions of the analyses and define intermediate- and high-temperature plateaux. Phengitic concentrates record plateau ages of c. 470 Ma whereas paragonitic concentrates yield c. 460 Ma plateaux. These ages are interpreted to date diachronous cooling through different argon closure temperatures following the high-pressure metamorphism. The slight discordance displayed in low-temperature portions of the experiments is interpreted to reflect a partial rejuvenation of intracrystalline argon systems during an c. 400–425 Ma thermal overprint associated with late Caledonian tectonothermal activity. White mica concentrates and associated whole-rocks yield Rb Sr mineral + whole-rock ages from 457 ± 11 Ma to 474 ± 11 Ma. These are interpreted to date post-metamorphic cooling through Sr blocking temperatures. Because similar ages are recorded by both K Ar and Rb Sr isotopic systems, relatively rapid post-metamorphic cooling is implied. This and relatively rapid depressurization during uplift indicate that the 40 Ar 39 Ar and Rb Sr mineral ages likely closely date the peak metamorphism. The high pressure complex is unconformably overlain by variably cleaved, Upper Ordovician-Lower Silurian flysch which was deformed into regional recumbent folds prior to deposition of unconformably overlying Early Devonian molasse. Two penetratively cleaved slate samples display internally discordant 40 Ar 39 Ar whole-rock age spectra with ages increasing from c. 100 Ma to c. 470 Ma. The spectral discordance is interpreted to reflect the combined effects of: (1) a polymineralic character: (2) a detrital source similar in age to the Motalafjella complex; and (3) a partial, post-Paleozoic thermal rejuvenation of the detrital mica argon systems. No thermal overprint associated with Late Silurian cleavage formation appears to be recorded. This agrees with textural characteristics which suggest that the cleavage largely developed through pressure-solution assisted, grain-boundary sliding.

3 – Diamond and Other Possible Ultradeep Evidence Discovered in the Orogenic Spinel-Garnet Peridotite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Publisher Summary In the Plesovice peridotite, several lines of evidence suggesting an ultradeep condition are found. All of these facts suggest that the Plesovice peridotite once resided in the diamond-stability field, which could not be identified by the previous geothermobarometric studies. This suggests that diamond was stable in the rock before Stage I (high-temperature spinel6garnet peridotite stage), which is the oldest generation of mineral assemblage previously recognized in the Plesovice peridotite. Furthermore, the presence of pyroxene exsolution lamellae in the earlier generation of spinel (Stage I) implies the preexisting high-pressure stage before Stage I. If graphite in MS inclusions were originally crystallized as diamond, parental fluids responsible for MS inclusion was most likely entrapped in the host mineral before Stage I, and they should have crystallized daughter minerals during the cooling/exhumation trajectory from the diamond-stability field, through Stage I, to the chlorite peridotite stage. For MS inclusions within Spinel-I, there is a good possibility original fluid/melt was entrapped in the diamond-stability field, if host Spinel-I was originally its high-pressure polymorph, Ca-ferrite, or Ca-titanite. On the other hand, one could not find the typical pyroxene exsolution texture in garnet. This probably suggests that most garnet was secondary formed at the expense of Spinel-I at relatively low-pressure condition. Therefore, if graphite in garnet was originally diamond, it is most likely that the diamond was graphitized before the entrapment within garnet crystal.

High-pressure marble at Yangguantun, Rongcheng County, Shandong Province, eastern China

SummaryFour generations of minerals have been confirmed in an eclogite-bearing impure marble located at Yangguantun, Rongcheng county, eastern Shandong province, China in the eastern part of the collision zone between the Sino-Korean and Yangtze cratons.(1) Early stage: high-Al-P-F titanite, pure zoisite and jadeite-bearing diopside occur as rounded aggregates surrounded by main stage amphiboles and titanites. (2) The main stage assemblage is characterized by hornblende (I), titanite (II), calcite, dolomite and phlogopite; they are intergrown with each other or occur as corona around the primary diopside, zoisite or titanite. (3) Earlier retrogression stage: hornblende (I) is replaced by hornblende (II) which occurs around hornblende (I). (4) Later retrogression stage: hornblende (II) is replaced by tremolite, chlorite and albite.The early stage is correlated with the eclogitic facies, but the main, earlier retrogression and later retrogression stages reflect retrogressions of eclogitic marble at different depth during decompression. The high pressure evidence and the metamorphic evolution of the marble studied, whose precursor was of crustal sedimentary affinity, indicate that the marble was subducted from the surface to great depth and then uplifted into the country rock gneiss, together with ultra-high-pressure eclogite and ultramafic rocks.ZusammenfassungVier Mineralgenerationen werden in unreinen, Eklogit-führenden Marmoren aus Yangguantun, Rongcheng, in der östlichen Shangdong Provinz Chinas unterschieden. Diese Gesteine sind im östlichen Teil der Kollisionszone zwischen dem Sino- koreanischen und dem Yangtze Kraton aufgeschlossen.(1) Frühes Metamorphosestadium: Hoch-Al-P-F-Titanit, reiner Zoisit und Jadeitführender Diopsid kommen als rundliche Aggregate, die von Amphibolen der Hauptphase und Titanit umwachsen werden, vor. (2) Die Paragenese der metamorphen Hauptphase ist durch Hornblende (I), Titanit (II), Calcit, Dolomit und Phlogopit charakterisiert. Diese Minerale sind eng miteinander verwachsen und bilden Koronartexturen um primären Diopsid, Zoisit und Titanit. (3) Frühes retrogrades Metamorphosestadium: Hornblende (I) wird von Hornblende (II), die sich um Hornblende (I) ausbildet, verdrängt. (4) Spätes retrogrades Metamorphosestadium: Hornblende (II) wird durch Tremolit, Chlorit und Albit verdrängt.Das frühe Stadium wird mit der Eklogitfazies, das Haupt- und die anschließenden retrograden Stadien werden mit der retrograden Metamorphose dieser eklogitischen Marmore in unterschiedlichen Tiefen infolge von Dekompression korreliert. Die hohen Drucke und die Entwicklungsgeschichte der untersuchten Marmore, die sich von krustalem sedimentären Material ableiten, belegen, daß die Subduktion dieses Materials von der Oberfläche in große Tiefen und der anschließende “Uplift” in die umgebenden Gneise, gemeinsam mit der Ultra-Hochdruckmetamorphose der Eklogite und ultramafischer Gesteine erfolgt sein muß.

First in situ X-ray identification of coesite and retrograde quartz on a glass thin section of an ultrahigh-pressure metamorphic rock and their crystal structure details

Abstract To ensure the presence of coesite and its transformed polymorph, quartz, in ultrahigh-pressure (UHP) rocks and to examine the relic of the phase transformation, crystal structures were analyzed by single-crystal X-ray diffraction (XRD) directly using the rock thin section mounted on a slide glass. The rock sample used is a coesite-bearing eclogite from the Sulu UHP terrain, eastern China. The crystal structures were determined successfully by this new method and the presence of coesite and quartz in UHP rocks was identified for the first time by XRD. The R-factor [R(F)] converged to 0.046 for coesite and 0.087 for quartz. The displacement ellipsoids for coesite and quartz are larger than those previously reported for these two phases, which is consistent with expected effects of trapped strain due to the phase transformation from coesite to quartz during exhumation from the Earth.s mantle. This paper is the first report of single-crystal XRD of a rock thin section on a glass slide and establishes the technique, and provides proof-of-concept of the method. Although the mineral species included in a thin section can often be identified by other methods, such as Raman spectroscopy, an advantage of the reported method is that it can be applied to any mineral in a thin section, and not just to the UHP minerals. Moreover, it is applicable to an unknown or new mineral in a thin section, discarding the spots of known minerals and constructing a lattice from the residual spots to find the structure of the unknown phase.

Eclogitic clasts with omphacite and pyrope-rich garnet in the NWA 801 CR2 chondrite

Abstract We report mineral assemblages from three clasts in the Northwest Africa 801 CR chondrite. The clasts, 1-3 mm in size, are ellipsoidal to irregular shaped, and show similar granular texture. The constituent minerals in the clasts are omphacite-rich clinopyroxene and pyrope-rich garnet, in addition to olivine and orthopyroxene. The omphacite contains jadeite (34 mol%) and diopside-hedenbergite (37%), and a significant amount of an enstatite-ferrosilite component (19%), which distinguishes it from terrestrial omphacite. The omphacite has a disordered C2/c structure. Graphite, phlogopite, chlorapatite, Fe-Ni metal, troilite, and pentlandite are present as minor minerals in the clasts. The minerals commonly found in chondrites, such as plagioclase and spinel group minerals, are not found in these clasts. Aluminum and sodium in the clasts are completely partitioned into omphacite and garnet. The mineral assemblages and compositions of the clasts are similar to those in terrestrial eclogite, except for the occurrence of olivine and some mineral chemistry, and this is the first discovery of an extraterrestrial eclogitic mineral assemblage. The clasts formed under high-pressure conditions, 2.8-4.2 GPa and 940-1080 °C, as estimated from a set of conventional geothermobarometers, indicative of formation in a large parent body. Another possibility is impact-induced origin, although the formation conditions would have been different from those for known shock veins. Meteorites usually consist of minerals that formed under low-pressure conditions, except for ultrahigh-pressure minerals found in shock veins. However, this study suggests that the pressure conditions for meteorite formation vary much wider than previously understood.

Geological map of the ultra-high pressure Brossasco-Isasca unit (Western Alps, Italy)

In the southern Dora-Maira Massif, Western Alps, slivers of continental crust with similar lithologies, but recrystallized during the Alpine orogeny at different peak-P conditions, are exposed. They include the Brossasco-Isasca Unit (BIU) where coesite was first discovered in continental crust. A new 1:20,000-scale geologic map and related cross-sections of the whole BIU and adjoining units is presented, in which the most significant features useful to infer the pre-Alpine history and the Alpine tectonic and metamorphic evolution, are summarized. Thanks to detailed petrography and petrology, the geologic map shows the precise location of ultra-high pressure (UHP) minerals (such as coesite), and the locations of the most significant mineral assemblages (such as kyanite + jadeite). This innovative approach is used to distinguish the BIU from the adjacent units. Relict pre-Alpine structures (such as igneous intrusive contacts with basement xenoliths and metagranitoids) are summarized in a sketch illustrating the geologic setting of the UHP metamorphic unit as inferred before the Alpine orogeny.