Duration of strike-slip movements in large shear zones: The Red River belt, China

Abstract

Sixty four size fractions of zircon, monazite, xenotime and titanite were dated by the UPb method to temporally and spatially constrain left-lateral movements in the Red River shear zone. Between the depths of 15 and 20 km, the shearing led to the formation of the high-grade metamorphic gneisses that form the bulk of the ∼ 4200 m high Diancang Shan and ∼ 3100 m high Ailao Shan mountain ranges. The omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust, and deformation patterns are very homogeneous over the entire length of the belt. North of the gneisses, subvolcanic bodies intrude unmetamorphosed cover rocks—in particular Eocene red beds—along the projected trend of the Red River shear zone. For a leucogranitic layer in the Diancang Shan gneisses, an age of 24.2 ± 0.2 Ma (2σ) is obtained for monazite and 22.4 ± 0.2 Ma is obtained for xenotime, whereas zircon formed through two growth stages at 24–23 Ma. These ages record solidification of the anatectic melts over a period of at least 1.8 ± 0.4 Ma, during which left-lateral shear was continuously active. In the Ailao Shan gneisses, zircon and titanite ages are 26.3 ± 0.3 and 26.1 ± 0.2 Ma for two monzonitic bodies, and 24.1 ± 0.2 and 22.4 ± 0.2 Ma for two pegmatitic layers that occur within the monzonitic intrusions. These results show that zircon, monazite, titanite and probably also xenotime are closed systems for U and Pb under melting conditions ( > 650°C). Therefore, the ages date mineral formation in the melt, rather than cooling below certain closure temperatures. A significantly older age of 35.0 ± 0.1 Ma is obtained for a monzonitic subvolcanic intrusion north of the Diancang Shan gneisses. Because these intrusions follow the trend of the shear zone, they most likely represent the surface expression of early activity along that zone, indicating that strike-slip movement had already occurred in latest Eocene time. The total set of UPb ages shows that magmatic and metamorphic activity occurred from 35 to 20 Ma, strengthening the idea that the Red River shear zone absorbed most of the intraplate movements required to compensate the opening of the South China Sea between 32 and 16 Ma. The existence of monzonitic-syenitic intrusions along the Red River zone implies that melting affected source rocks other than the Diancang Shan and Ailao Shan gneiss protoliths. Potential sources to produce such rocks lie in the deep crust, possibly including melts from the mantle. Consequently, the Red River shear zone must be a deep-rooted structure, along which melts migrated to the surface. Moreover, the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike-slip activity.