The first finding of low-temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and ⁄ or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na-poor omphacite and Na-rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peakpressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1-2.2 GPa and 600-620°C, even though thermobarometric results yield higher pressure and temperature conditions (2.6-2.8 GPa and 620-680°C). The eclogites underwent a clockwise P-T trajectory with a post-peak-pressure increase of temperature to a maximum of >750°C at 1.7 GPa and a subsequent cooling during decompression to 650°C and 1.3 GPa, which was followed by additional cooling before close-to-isothermal decompression to 530°C at 0.5 GPa. The surrounding pelitic schist (garnetchloritoid-phengite) records similar metamorphic conditions (580-600°C at 1.9-2.3 GPa) and a monazite chemical age of 243 ± 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet-phengite schist record an older thermal event (424 ± 15 Ma). The present results indicate that the Indochina craton was deeply (>70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non-collisional event in the Indochina craton.

High-pressure mafic granulites containing granoblastic garnet, quartz, and minor hornblende have been found from the Song Ma Suture zone in northern Vietnam, regarded as a microcontinental boundary between the South China and Indochina blocks. Finegrained symplectite formed during the decompression stage is developed in the granulite and is divided into orthopyroxene +plagioclase and orthopyroxene + clinopyroxene + plagioclase AE hornblende. The former replaces garnet and the latter is regarded as a breakdown of sodic clinopyroxene. Detailed observation and careful data selection revealed that the high-Mg and low-Ca garnet should be in equilibrium with the precursor sodic clinopyroxene, and the pair indicates high-temperature and -pressure conditions (910-930 8C at 1.9-2.0 GPa). Although we could not obtain quantitative age data from the high-pressure granulite, the U-Th-Pb age (233 AE 5 Ma) of pelitic gneiss strongly suggests a Middle to Early Triassic metamorphic event. If the age indicates the timing of the high-pressure granulite-facies metamorphism, it might be related to a continental collision setting by following crustal subduction. According to the metamorphic signatures, north to central Vietnam may be regarded as an orogenic belt formed by the micro-continental collision between the South China and Indochina cratons. To cite this article: N.

No abstract

Recent geological and petrological investigations of the metamorphic rocks from Vietnam revealed the following new evidences to understand the tectonic evolution of Southeast Asia; 1) findings of ultrahigh -T (~ 1000 °C) pelitic granulites, high -to ultrahigh -P (~ 40 kbar) mafic metamorphic rocks and high -P/medium -T gneisses from the Kontum Massif, 2) eclogite and high -P granulite from the Song Ma suture zone, and 3) ultrahigh -T aluminous granulite from the Red River zone. These lines of evidence are strongly indicative of the highestmetamorphic conditions in each metamorphic terrane. Estimated P -T conditions and reaction textures from these rocks delineate a characteristic clockwise pressure -temperature -time (P -T -t) path for each other. Based on the combination of P -T paths from these complexes, two -stages of metamorphic field gradient are identified. An earlier M0 -stage of high -P/T gradient is recognized, based on the peak -P conditions from the Kontum Massif and Song Ma suture zone. A later M1 -stage of low -P/T gradient is also identified by linking the peak metamorphic conditions from the Kontum Massif, the Song Ma suture zone and the Red River zone. The former metamorphic field gradient could represent an early continental collision event and the latter would indicate a peak metamorphic stage caused by very high -T magmatic intrusion (asthenosphere upwelling) as a heat source of ultrahigh -T metamorphism. A simultaneous collision metamorphism throughout Vietnam should have taken place during the continental collision between Indochina and South China cratons, which led to the formation of Trans Vietnam Orogenic Belt.

Temperature-programmed desorption of ammonia was applied to gallosilicate with MFI structure using the water vapor treatment method to remove the unnecessary low-temperature peak and the curve-fitting method to calculate the acid strength and its distribution based on the thermodynamics. The determined acid amount approximately agreed with the [Ga]-[Na] content in the low gallium content region, showing the stoichiometric generation of one acid site by the substitution of one gallium atom, and stoichiometric neutralization of one acid site by one sodium atom. The acid strength (adsorption heat of ammonia) due to the framework gallium was ca. 130 kJ mol -1 with the small distribution, close to that on the ZSM-5 type aluminosilicate, and was independent of the composition. This shows that the substitution of Al and Ga into the zeolitic crystal generates the similar acid strength, and the acidic strength is mainly determined by the crystal structure. The extraframework gallium on the ion-exchange site, which was readily formed with the gallium content higher than 0.3-0.4 mol kg -1 , generated the additional type of acid site with the higher adsorption heat of ammonia. IntroductionZeolite, which consists of SiO 2 and Al 2 O 3 , has widely been used as a heterogeneous catalyst. 1-6 In place of the aluminum, various kinds of metals can be incorporated into the framework, and the obtained metallosilicates are being considered as novel functionalized materials. 7 Especially, gallosilicate with MFI structure has a remarkable catalytic activity for the conversion of alkane into aromatics. [8][9][10][11][12][13][14][15][16][17][18] Generation of acidity by isomorphous substitution of silicon by gallium is presumed to be the origin of the catalytic functions. Analysis of the acidic property, namely, the number, strength, and nature (Brønsted or Lewis) of the acid sites, is therefore important to interpret the catalytic functions and, moreover, must be important also from the physicochemical viewpoint.The acidic property of gallosilicate has been investigated by means of the infrared (IR) spectroscopy of surface hydroxide, 19-22 the adsorbed pyridine, 23 carbon monoxide 24,25 and nitrogen, 25 and the nuclear magnetic resonance of proton ( 1 H NMR), 26 supported by quantum chemical calculations. 27-30 These spectroscopic methods mainly deal with the quality of the acid site, but the quantitative measurement is difficult. Traditional indicator 31 and amine titration 32 methods to quantify the acid amount have a disadvantage, because they are confused by the slow diffusion of liquid molecules in the micropore. Test reactions have widely been used, but they cannot be related simply to the acidic property. Measurement of adsorption heat of such a small base as ammonia by calorimetry 33 is one powerful method to evaluate the number and strength of acid sites. By this method, the interaction between the acidic and basic materials is directly evaluated, and this gives an absolute measure of the amount and strength of acid.Temperature...