Zircon U-Pb Dating and Its Geological Implications of the Metamorphic Rock Series in Ailao Shan Ranges, Western Yunnan
详细信息
- 责任者：Wang Dongbing (Chengdu Institute of Geology and Mineral Resources ; Chengdu 610081 ; China) ; Tang Yuan
- 刊名：Acta Petrologica Sinica
- 出版年：2013
- 卷期：29(4)
- 关键词：变质岩 ; metamorphic rocks ; 铀—铅计时 ; U-Pb dating ; 哀牢山 ; Ailao Mountains
- 页码：p.1261-1278
- 图表：illus., 1 table, 57 refs.
- 分类号：0802
- issnNo：ISSN1000-0569
- isbnNo：CN11-1922/P
摘要
The Ailao Shan-Red River metamorphic belt consists largely of high-grade metamorphic rocks of the Ailao Group, which were previously thought to be the Paleoproterozoic basement rocks of the Yangtze Craton. We present new results of zircon U-Pb dating on four samples from the representative lithologies of the Ailao Shan metamorphic belt to explore their possible regional implications. Two groups of ages are identified in granitic gneiss sample 11AL17-1 of the Ailao Shan high metamorphic rock series. The older magmatic group yields an age of 700±6Ma (MSWD=1.4, n=14) and the clearly metamorphic younger group yields an age of 27.4±1.2Ma (MSWD=1.9, n=3). A similar situation exists in granitic gneiss sample 11AL09-1, with magmatic zircons clustered around concordia at 220±3Ma (MSWD=3.1,n=14) and metamorphic zircons at 31.2±2.3Ma (MSWD=6,n=5). The quartzite sample11AL08-1 is also collected from the Ailao Shan high metamorphic rock series. All zircons extracted from quartzite sample 11AL08-1 have distinct core-rim structure. A total of 92 spots analyses on cores define six age populations at 493~528Ma (n=42), 635~640Ma (n=2), 701~784Ma (n=44), 976~980Ma (n=2), 1839Ma (n=1) and 2487Ma (n=1). All the 92 cores have high Th/U ratios (>0.23) and were probably derived from igneous rocks. Forty-two of the youngest spots have a weighted mean age of 509Ma, representing the maximum depositional age of the protolith of the quartzite 11AL08-1. Seven spots on metamorphic rims yield much younger ages ranging from 26Ma to 75Ma, which is interpreted to reflect subsequent metamorphic event. A weighted mean 206Pb/238U age of 750±4Ma (MSWD=0.6,n=22) was obtained from a granite sample 11AL12-1 adjacent to the Ailao Shan high metamorphic belt and is considered to represent the intrusion age. The data described above demonstrate that the Ailao Group is not Paleoproterozoic basement but younger metamorphic complex, the protolith of which consists of at least three units including the ~700Ma and 220~240Ma igneous rocks, as well as ~509Ma sedimentary rocks. They have undergone metamorphism as a result of the Indo-Asian collision at 26~31Ma, and acquired the current morphological features that is hard to be distinguished from the basement rocks. The zircon age spectra of the Ailao Shan-Red River metamorphic belt are roughly consistent with those of the western Yangtze Craton, suggesting a Yangtze Craton affinity of the Ailao Shan-Red River metamorphic belt.