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.
The Main Zone of the Hidaka Metamorphic Belt is an uplifted crustal section of island-arc type. The crust was formed during early Tertiary time, as a result of collision between two arc-trench systems of Cretaceous age. The crustal metamorphic sequence is divided into four metamorphic zones (I-IV), in which zone 1V is in the granulite facies.A detailed study of the evolution of the Hidaka Belt, based on a revised P-T-t analysis of the metamorphic rocks, notably a newly found staurolite-bearing granulite, confirms a prograde isobaric heating path, af'ter a supposed event of tectonic thickening of accretionary sedimentary and oceanic crustal rocks. During the peak metamorphic event (c. 53 Ma), the regional geothermal gradient attained 33-40"Ckm-', and the highest P-T condition obtained from the lowest part of the granulite unit is 830" C, 7 kbar. In this part, X,,,, of Gt-Opx-Cd gneiss is about 0.15 and that of Gt-Cd-Bt gneiss is 0.4. The P-T-X,,, condition of the granulite unit is well within a field where fluid-present partial melting of pelitic and greywacke metamorphic rocks takes place. This is in harmony with the restitic nature of the Gt-Opx-Cd gneiss in the lowest part of the granulite unit.The possibility that partial melting took place in the Main Zone is significant for the genesis of the peraluminous (S-type) granitic rocks within it. The S-type granitic rocks in this zone are Opx-Gt-Bt tonalite in the granulite zone, Gt-Cd-Bt tonalite in the amphibolite zone, and Cd-Bt-Mus tonalite in the Bt-Mus gneiss zone. The mineralogical and chemical nature of these strongly peraluminous tonalitic rocks permit them to be regarded as having been derived from S-type granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust. And = andalusite Hbl = hornblende Bt =biotite Hc = hercynite Cd = cordierite Kfs = K-feldspar Chl =chlorite Mus = muscovite Co =corundum Opx = orthopyroxene Cpx = Ca-pyroxene Phe =phengite Cum = cummingtonite PI = plagioclase Ep =epidote Qz =quartz Ged =gedrite Sil = sillimanite Gt = garnet St = staurolite. , , ~ ~~
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