Zircon U-Pb dating for metavolcanites in the Laochanghe Formation of the Dahongshan Group in southwestern Yangtze Block, and its geological significance
扬子地块西南缘大红山群老厂河组变质火山岩的锆石U-Pb定年及其地质意义

Dahongshan Group is one of the oldest stratigraphic units in western Yangtze Block, which experienced metamorphism of greenschist-lower amphibolite facies. Depositional ages (~1.68Ga, the Late Paleoproterozoic) of the Manganghe and Hongshan formations in the middle of this group have already been achieved, while that of the Laochanghe Formation at the bottom of the group has not been reported until now. Compared to its depositional ages, no accurate statistics have been reached for the metamorphic ages. This study works on the metavolcanites as thin layers in the thick metasedimentary rocks of the Laochanghe Formation. Petrography, in situ LA-ICP-MS zircon U-Pb dating and the related trace element and REE analyses provided insights into the protolith and metamorphic ages of the rocks. Results are as follows. (1) Magmatic zircon domains of the meta-intermediate-acid and metabasic rocks in the Laochanghe Formation have the weighted average 207Pb/206Pb ages of 1711±4Ma and 1686±4Ma respectively, and zircon ages constrain the timing of the Laochanghe to the range from 1711Ma to 1686Ma; (2) Metamorphic zircon domains in a metabasic rock (garnet amphibolite) has 206Pb/238U zircon age of 849±12Ma. The results show that the starting age of the Dahongshan Group is advanced to 1711±4Ma which confirms again the existence of the old crystalline basement in western Yangtze Block, and that there was a metamorphic event around~850Ma in the Neoproterozoic which must be a regional metamorphism related to the Neoproterozoic magmatism.