U-Pb zircon geochronology of late Archaean metamorphic rocks in the Taihangshan—Wutaishan area, North China

Abstract

A geochronological study has been made in the Taihangshan—Wutaishan area, Shanxi Province, on a sequence of Precambrian supracrustal rocks and stratigraphically related intrusives. These constitute part of the North China basement block.The oldest rocks of the Fuping Group consist predominantly of mafic granulites, paragneisses, quartzites and amphibolites. This Group was always considered Archaean, and early K-Ar measurements yielded some minimum ages as old as 2300 Ma and others as young as 1500–1900 Ma. The unconformably overlying Wutai Group is considered as the base of the Proterozoic succession and consists of amphibolite-grade paragneisses, carbonates and amphibolites in its lower part, and greenschist-grade clastic sediments and volcanics in its upper part. It has given apparent K-Ar mineral ages between 1500 and 2000 Ma.Present investigations have employed the U-Pb zircon approach on selected parts of the sequences to provide direct depositional ages or age constraints for the Fuping and Wutai Groups and to find the age(s) of subsequent metamorphic processes. A minimum age for the Fuping Group and maximum age for the Wutai Group has been derived from zircon age measurements on the Lanzhishan Granite (2560 ± 9 Ma) which intrudes the Fuping Group and is unconformably overlain by the Wutai Group. This late Archaean result can be interpreted as the age of an igneous crystallization event and therefore a maximum for the depositional age of the Wutai Group.The above result is well substantiated by zircon dating of the Wutai Group itself. Zircons from a low-grade metamorphosed keratophyric lava in the upper part of the Group give a volcanic crystallization age of 2520 ± 17 Ma. This provides close constraints for the age of deposition of the Wutai Group at between 2560 and 2520 Ma ago. If the Wutai Group is indeed basal Proterozoic, the Archaean—Proterozoic boundary in this part of China is also dated at between 2560 and 2520 Ma. Further evidence as to the antiquity of the Wutai Group comes from a U-Pb zircon age of 2520 ± 30 Ma for the Ekou Granite which intrudes the Wutai Group sequence.The age of the underlying late Archaean rocks in the Tainhangshan—Wutaishan area, the Fuping Group, could not be directly determined because of the paucity of volcanic markers and the higher grade of metamorphism. Its minimum age is 2560 Ma (see above), but only an approximate maximum age can be gleaned from the zircon systematics in granulite and amphibolite-grade paragneiss samples. These data indicate a degree of Pb-inheritance which, combined with metamorphic disturbance and more recent Pb loss, inhibit any further interpretation other than that they indicate an approximate maximum age of around 2800 Ma.Several of the paragneisses and one amphibolitic schist contain zircon systems which are interpreted as having completely responded to a well-defined amphibolite facies metamorphic event at ∼2500 Ma ago. A younger but less well-defined metamorphism caused complete Pb loss from zircon and recrystallization in the amphibolite and granulite facies terrains at between 2000 and 1800 Ma ago. Rocks which show evidence of this younger metamorphic event were not recognized from the field or structural data base, but from the distribution of the few such samples analyzed it seems possible that this younger terrain may now be interpretable as a cross-cutting Proterozoic mobile belt. This Proterozoic metamorphic province is also reflected by K-Ar mica ages between 1600 and 2000 Ma and some preliminary Rb-Sr total-rock ages at ∼1700 Ma on nearby paragneisses whose U-Pb zircon age is 2500 Ma.