The Diancang Shan-Ailao Shan metamorphic complex belt is located on the southeastern margin of Tibetan Plateau, which formed by the Late Permian to Middle-Late Triassic collision of the Yangtze plate and Indochina plate. The complex belt mainly comprises different kind of paragneisses, schists, quartzites, marbles, amphibolites, which have experienced extensive mylonitization and anatectic melting. In this paper a comprehensive study of zircon cathodoluminescence (CL) image, trace element, mineral inclusion, external morphology and LA-ICP-MS U-Pb dating on meta-sedimentary rocks from the Gasa area was accomplished. Two different metamorphic events are revealed by garnet-bearing plagioclase two-mica schist and sillimanite-garnet biotite monzogneiss from the Ailao Shan Complex (ALSC). Thirty metamorphic zircons from the garnet-bearing plagioclase two-mica schist yield consistent ages ranging from 215 +/- 6Ma to 227 +/- 5Ma and give a weighted mean Pb-206/U-238 age of 222.3 +/- 1.2Ma (n=30, MSWD=0.27), In addition, most of these zircons show rounded or ovoid external morphology, flat HREE pattern ((Lu/Gd)N=0.73 similar to 4.08) and weak negative Eu anomaly, similar to metamorphic zircon from typical high grade metamorphic rocks. These zircons gives a large range of Th/U ratios (0.06 similar to 0.84, mean=0.45), which maybe associated with the decompositon of Th-enriched mineral (such as monazite). The nearly identical metamorphic ages with those from the Diancang Shan and Yuanyang areas indicate a metamorphic event associated with the closure of Paleo-Tethys in Middle-Late Triassic and the Diancang Shan-Ailao Shan complex belt is one of essential sutures zones in the Sanjiang area, southeast of Tibetan Plateau. Moreover, twenty-eight metamorphic zircons (rims) in the sillimanite-garnet biotite monzogneiss yield concordant ages ranging from 27.3 +/- 0.5Ma to 31.9 +/- 0.5Ma, and give a weighted mean U-Pb age of 29.4 +/- 0.53Ma (n=28, MSWD=2.0). Petrographic study indicated that the sillimanite-garnet biotite monzogneiss had experienced peritectic reaction and most zircons show rounded or ovoid external morphology. Overgrown rims contain inclusion assemblage of sillimanite+K-feldspar+quartz+biotite+monazite, with low Th/U ratios (0.01 similar to 0.1), flat REE pattern ((Lu/Gd)N=0.45 similar to 7.59) and moderate negative Eu anomalies, all of which demonstrate that the metamorphic zircons were grown in a background coexisting with garnet and feldspar and, the anatexis melting possibly corresponded to the regional ductile strike-slip shear in Oligocene. Besides, six inherited zircons from the two samples have ages from 528Ma to 783Ma, high Th/U ratios (>0.1), steep HREE pattern, exhibit oscillatory zoning in the CL images and prismatic shapes with etching and pitting surface. All these features are corresponding to magmatic zircon underwenting an erosion-transportation-deposition process. As a consequence, the meta-sedimentary rocks in the ALSC should contain the Neoproterozoic and Early Paleozoic source, indicating that the ALSC in studying area do not belong to the basement of the Yangtze plate.
