Sources of the Nanwenhe - Song Chay granitic complex (SW China - NE Vietnam) and its tectonic significance

The Nanwenhe - Song Chay complex (NSCC) is a large early Paleozoic pluton straddling the border between SW China and NE Vietnam. It is located at the tectonic junction of the Yangtze, Cathaysia and Indochina blocks so that the source of the complex is still enigmatic. The rocks from the complex are high-K calc-alkaline and peraluminous S-type granite. LA-ICP-MS U-Pb dating of zircons from twelve granitic samples in the complex gives crystallization ages in the range of 436-423 Ma. Zircon U-Pb-Hf isotopes and geochemical data show that the Nanwenhe granites in the northern part of the NSCC probably were mainly derived from Neoproterozoic metamorphic pelites analogous to those in the Xiajiang and Danzhou Groups. In contrast, the Song Chay granites in the southern part of the NSCC were probably derived from Neoproterozoic metamorphic psammites analogous to those in the Fanjingshan and Sibao groups in the southwestern Yangtze Block. The granites from the complex contain abundant inherited zircons with a main peak at 836-808 Ma and subordinate peaks at ~ 734 Ma and 528-665 Ma. The inherited zircons have age distribution patterns and Hf-isotopic compositions similar to those for the detrital zircons from Neoproterozoic to Early Paleozoic sedimentary rocks in the southwestern part of the Yangtze Block, but different from those of the Cathaysia Block and Indochina Block. Therefore, the basement of the NSCC is equivalent to the Yangtze Block, and the complex may represent the westernmost tip of the early Paleozoic orogenic belt in the SCB. We thus assume that the boundaries between the Yangtze Block and the Cathaysia Block and between the SCB and ICB should be located to the southeast and southwest of the Nanwenhe - Song Chay area, respectively.