SUN Zai-bo, ZENG Wen-tao, HUANG Liang, WANG Xiao-feng, ZHOU Kun

(Yunnan Institute of Geological Survey, Kunming 650216, China)

Abstract: The Tuanliangzi formation complex in southwest Yunnan Province, China is an important tectonic rock unit in the southern Lancangjiang tectonic belt and is closely associated with the evolution of the Lanping-Simao basin. However, due to the lack of marker fossils and difficulties to determine strata sequencing, the age of the Tuanliangzi formation complex lacks effective constraints. The structural differentiation of mafic dykes and quartz dykes is more developed in the Tuanliangzi formation complex. Therefore, based on petrologic observation, zircon samples were selected from greenschist, mafic and quartz dykes and were analysed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Thirty one U-Pb dating points were analysed from two greenschist samples. The results of 206Pb/238U ages can be divided into four groups, which are 1816 to 1542 Ma (weighted average age is 1630 ± 210 Ma, N = 6), 850 to 832 Ma (weighted average age is 844 ± 95 Ma, N = 2), 517 to 422 Ma (weighted average age is 438 ± 26 Ma, N = 4) and 292 to 188 Ma (weighted average age is 219.7 ± 8.5 Ma, N = 19), respectively. The results of 206Pb/238U ages for thirty one U-Pb dating points from two mafic samples can also be divided into four groups, which are 2256 to 1357 Ma (weighted average age is 1646 ± 160 Ma, N = 16), 861 to 750 Ma (weighted average age is 773 ± 92 Ma, N = 2), 484 to 388 Ma (The weighted average age is 433 ± 38 Ma, N = 6) and 320 to 233 Ma (weighted average age is 253 ± 16 Ma, N = 7), respectively. One quartz dyke sample had twenty U-Pb dating points that can be divided into three groups, which are 1244 ± 19 Ma (N = 1), 430 ± 5 Ma (N = 1) and 312 to 212 Ma (weighted average age is 229.6 ± 6.7 Ma, N = 18). The weighted average 206Pb/238U age for the different groups of the three rock types are very similar, suggesting that these ages have different geological implications. The 1646 to 1630 Ma maybe represent the age of formation of the Tuanliangzi formation complex and the 253 to 219 Ma implies that it was impacted by the Indosinian tectonic hydrothermal event, which was associated with the subduction of the ancient Tethys ocean basin from west to east. The zircon U-Pb ages of the three rock types suggest that the Tuanliangzi formation complex might have formed in the Mesoproterozoic and belongs to the Lancang group.

Keywords: Tuanliangzi formation complex; greenschist; mafic dyke; quartz dyke structure differentiation; LA-ICP-MS zircon U-Pb dating; Jiangqiao Puer; Yunnan Province

E-mail: 1553974947@qq.com

ACTA MINERALOGICA SINICA Vol. 35, No. 2, 2015, Page 245-254