The Mozambique Ocean closed as Gondwana formed. Its suture has been identified in Madagascar (Betsimisaraka suture), but its continuation, into India, is controversial. The Palghat-Cauvery shear system appears an ideal candidate as it: (i) lies along strike of the Betsimisaraka suture in Gondwana; (ii) forms a high-pressure granulite belt; and (iii) separates crustal domains with different geological histories. However, existing age constraints have been used to suggest that the structure is Archaean/Palaeoproterozoic. Here we date metamorphic zircons using secondary ion mass spectrometry (535.0 ± 4.9 Ma) and monazites using electron probe micro-analysis (537 ± 9, 532 ± 8, 525 ± 10 Ma). No evidence for an earlier metamorphic event was found. The identification of Palghat-Cauvery highpressure metamorphism as Cambrian, and recognition that it bounds crustal domains of contrasting origin, points to it being the southern continuation of the Betsimisaraka suture and southern margin of Neoproterozoic India.

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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.

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