The Lancang metamorphic terrane consists of an eastern low-P / T belt and a western high-P / T belt divided by a N-S-trending fault. Protoliths of both units are mid-late Proterozoic basement and its cover. The low-PIT belt includes the Permian Lincang batholith, related amphibolite facies rocks of the Damenglong and Chongshan groups, and Permo-Triassic volcanic and volcaniclastic rocks. Most whole-rock Rb-Sr isochron and U-Pb zircon ages of the Lincang batholith are in the range 290-279 and 254-212 Ma, respectively. Metamorphism of the low-P/T belt reaches upper amphibolite with local granulite facies (735" C at 5 kbar), subsequently retrogressed at 450-500" C during post-Triassic time. The high-PIT rocks grade from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies. Estimated P -T conditions follow the high-P intermediate facies series up to about 550-600"C, at which oligoclase is stable. The QAr/'9Ar plateau age of sodic amphibole in blueschist is 279 Ma.The paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of southeast Asia since the latest Precambrian. Tectonic juxtaposition of paired belts with contrasting P-T conditions, perhaps during collision of the Baoshan block with south-east Asia, suggests that an intervening oceanic zone existed that has been removed. The Baoshan block is a microcontinent rifted from the northern periphery of Gondwana. Successive collision and amalgamation of microcontinents from either Gondwana or the Panthalassan Ocean resulted in rapid southward continental growth of c. 500 km during the last 200 Ma. Hence, the Lancang region in southeast Asia represents a suture zone between two contrasting microcontinents.
This paper provides further evidence for the ongoing discussion as to whether the Dabie UHPM belt formed in Triassic or Palaeozoic time, and whether the Sulu UHPM belt formed in Triassic or Neoproterozoic time. Combined use of laser Raman spectrometer (LR), cathodoluminescence imaging (CL), and ion probe U-Pb in-situ dating (SHRIMP) provided accurate ages of UHPM from rocks collected from Weihai, NE Sulu UHPM belt. LR was used to identify coesite and other UHP minerals as inclusions in zircon separates from an amphibolized peridotite and an eclogite. CL was used to examine the zoning structure of these zircon, and SHRIMP dating was performed on specific spots on zircon to obtain ages of different geological events. An age of 221 ± 12 Ma was obtained for coesite-bearing zircon from the amphibolized peridotite; an age of 228 ± 29 Ma for eclogite was obtained from the lower intercept of a concordia plot. These ages are interpreted as the time of UHPM in the Weihai region. Ultramafic rocks to the east of Weihai yield a magmatic age at 581 ± 44 Ma. The zircon in the ultramafic rocks possibly also records a thermal event at c. 400 Ma, but no independent geological evidence for this event has been found. The eclogite protolith formed in the Middle Proterozoic (1821 ± 19 Ma), which is similar to the age of country rock gneisses of 1847-1744 Ma. The new geochronological data confirm that UHPM occurred in the Triassic in the Sulu area when subduction took the ultramafic body and the eclogite protolith, together with the adjacent supracrustal rocks, to mantle depths.
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