Geochemical and Geochonological Constraints on the Tectonic Outline of the Dabie Mountains, Central China: A Continent MicrocontinentContinent Collision Model

Abstract

The Qinling Dabie orogenic belt is considered to be a collision zone between two continental blocks, i.e. North China Block (NCB) and Yangtze Block (YB). However, this model is difficult to reconcile many apparent contradictional observations in this orogenic belt, such as termination of island arc magmatic activity in the middle Devonian and absence of deep water deposits and fragments of oceanic crust after Devonian on the south margin of NCB versus the Triassic ultrahigh pressure metamorphism and molasse formation appeared only after the upper Triassic in the north margin of YB. This paper argues that there is also a microcontinent in the Dabie belt between the NCB and YB. The Wuhe Shuihou ductile shear zone is a boundary between the North and South Dabie terranes. The Triassic high and ultrahigh pressure metamorphic rocks were only developed in the South Dabie terrane, but no such rock has been observed in the North Dabie terrane. On the other hand, the Jurassic syncollisional mafic ultramafic intrusions (～182 Ma) were only developed in the North Dabie terrane. Their very low e Nd values ( 15～ 18) and negative Nb anomaly suggest that they were derived from an enriched mantle which may be metasomatized by fluid released from subducted continental crust. The above observations suggest that the South Dabie terrane is subducted continental crust, whereas the North Dabie terrane is overthrust crust wedge, and the Wuhe Shuihou shear zone is a Triassic suture line between them. However, the later Paleozoic cold eclogite with U Pb zircon ages of 400 ± 2 to 377 ± 2 Ma in the Sujiahe tectonic melange suggest that there should be another Paleozoic suture line between the Beihuaiyang and North Dabie terranes. The Ordovician to Silurian magmatic arc and Devonian flysch formation with relative lower Nd isotope model ages (T DM = 1 6 1 9 Ga) suggests that the Beihuaiyang should be an active continental margin of the NCB. Its Paleozoic magmatic arc and metamorphism can be compared with that of the North Qinling which is considered to be a Paleozoic island near the NCB. In contrast, no Paleozoic magmatism and metamorphism has been observed in the North Dabie terrane. Very old Nd model ages (T DM = 2 0 2 6 Ga) of orthogneiss in the North Dabie terrane and late Proterozoic U Pb zircon ages (679 756 Ma) of TTG (tonalitic trondhjemitic granodioritic gneisses) rocks suggest that the North Dabie terrane is not like to be a Paleozoic island arc near the NCB, but more likely to be a microcontinent with an ancient basement. Based on the above observations, a continent (NCB) microcontinent (North Dabie terrane) continent (YB) collision model is proposed in this paper. This model can reconcile the above contradictory.