The Identification of a Late Pan-African Granulite Facies Event in Northeastern China: SHRIMP U-Pb Zircon Dating of the Mashan Group at Liu Mao, Heilongjiang Province, China

摘要

The Mashan Group lies within the Jiamusi Massif of northeastern China and is composed of granitic gneisses interdevcloped with khondalitic nietasedimentary rocks, including graphite and sillimanite-bearing schist and diopside marble, that have been metamorphosed to amphibolite/granulite facies. This study was initiated to determine the timing of granulite facies inetamorphism, since previous estimates were inconclusive, with ages ranging from Archaean to Neoproterozoic. Samples were collected from in and around the Liu Mao Graphite Mine for U-Pb single zircon geoclironology using the SHRIMP ion microprobe at Curtin University of Technology. Metamoiphic zircons from a garnet granulite and a metadiorite have weighted M(vPb/2MPbagesof502 ± 8 Ma and 498 ± 11 Ma, respectively. A garnet-bearing granitoid which irregularly cross-cuts the granet granulite, and appears to be of local anatectic origin, contains a population of igneous zircons which define a weighted 20l>Pb/238Pb age of 502 ± 10 Ma, which is identical within error to the granuliles, supporting the field evidence that it formed during the peak of inetamorphism. These results place a tight constraint on the timing of granulite facies metamorphism of the Mashan Group at Liu Mao, with the age of ~ 500 Ma being considerably younger than previously-determined isotopic ages obtained from the area. The metadiorite sample also contains a number of zircon crystals with small, irregular cores which give a spread of ages from 530 to 1274 Ma. These probably represent variously reset zircons from the igneous protolith and are the oldest components recognised. However, they indicate a Mesoproterozoic age and there is no evidence for rocks of Archaean age at the Liu Mao Graphite Mine. Further work is required in order to determine the regional extent of the 500 Ma event, but it is likely to be much more extensive and form part of a major Pan-African terrane that has not previously been recognised. These results have important implications with respect to the reconstruction of the superconlinents of Rodina and Gondwanaland, since they indicate a collisional event between the North China and Siberian Cratons at ~ 500 Ma. A post-tectonic granitoid collected from Sandaogou, 18 km southwest of Liu Mao, has a 20°Pb/23SU weighted age of 264 ± 11 Ma. Such granitoids are widespread in the area and define a major period of Phanerozioic cnistal growth.