ABSTRACT

At the border between China and Vietnam, a deformed and metamorphosed dome (the Laojunshan-Song Chay Dome) developed at the western prong of the “Caledonian” South China Granitic Fold Belt, north of the Red River fault zone. The main section of the dome, in China, is called the Laojunshan Metamorphic Core Complex. Systematic SHRIMP zircon U-Pb dating has been undertaken on these Caledonian Granites for the first time. The ages of the Nanlao Gneisses (DN4015B), and granites of the Tuantian (LJS-2), Laochengpo (LJS-3) and Nanjiahe (DN4058) Units, are 435.7 ± 6.4, 442.2 ± 8.3, 437.9 ± 9.5 and 418 ± 5.8 Ma respectively. These ages indicate an extended period of igneous activity (ca. 24 million years) that can be divided into two episodes (at ca. 440 and 420 Ma) representing the ages of syn-collisional and post-collisional/late-orogenic granitoid plutons respectively. Inherited zircon with ²⁰⁶Pb/²³⁸U ages of 710 ± 22, 1051 ± 24 and 1657 ± 48 Ma is also present in these samples. The Neoproterozoic zircon might record igneous activity related to the break-up of the Rodinia Supercontinent. Lithogeochemistry results show that most of these rocks are “fractioned” S-type granites related to the Caledonian syn-collisional orogeny. A few of them have affinities to I-type granites produced by partial melting of intermediate-mafic rocks in the upper crust. Based on the geochronology and geochemical data, the inferred provenance and tectonic setting of the granites, and research on geotectonics and regional geology, the granites are interpreted as the products of the Caledonian collision between the Cathaysia and Yangtze blocks, and the fault zones along Shizong-Mile-Luoping-Xingyi-Wangmo-Luodian are proposed to be the southwestern boundary between those two blocks.