The upper mantle beneath the continental rift of Tanlu, Eastern China: evidence for the intra-lithospheric shear zones

Abstract

Spinel-lherzolite xenoliths from four Neogene basanite volcanic cones of Yitong within the Tanlu Rift of eastern China can be divided into two groups according to their grain sizes: coarse-grained peridotites (> 1.5 mm) and mylonite peridotites (<0.1 mm). Coarse-grained samples display various texture types ranging from protogranular to tabular granoblastic, implying a virtually steady-state stress in the mantle. Some recovery of olivines (up to 4 mm) in tabular granoblastic matrix (1 mm) reflect a stress relaxation, followed by another stress increase inferred from some rare kink bands in these crystals. Such cyclic stress variation is probably related to the multistage of formation of the Yitong graben which has been subject to both tensile and compressive stress. Two subtypes are found for mylonite peridotites: porphyroclastic and mosaic. The extremely fine grain size ( < 0.01 mm) yield the highest stress estimates ( > 400 Mpa) among those reported in literature so far. One spinel-mylonite peridotite contains minor plagioclase, indicating their shallow depth of origin. Electron microprobe analysis reveals distinct chemical features for these two different textural groups. Coarse-grained peridotites are compositionally homogeneous. Sample to sample variations are ascribed to different degrees of extraction of basalt during ancient partial melting events related to the regional upwelling of the upper mantle during subduction of the Pacific plate beneath the eastern continent of China. Geothermometric calculations show that all coarse-grained samples as well as the intruded pyroxenite are equilibrated between 900–1050°C. Conversely, in the mylonite samples, compositional zoning is found at the rims (200 μm) of the pyroxenes with decreasing Ca, AI and Cr from cores to rims, whereas the composition profile of the porphyroclast cores indicate that these fine-grained samples were in equilibrium some time prior to mylonitization. Compositional disequilibrium at porphyroclast rims reflects the kinetics of diffusion-controlled reactions due to a cooling event associated with later intense shearing deformation some time before eruption. This cooling history (from 980 to 740°C) is further revealed by the geothermometric study of the core/rim and porphyroclast/neoblast compositions in these mylonite peridotites. These highly deformed peridotites indicate the presence of an intra-lithospheric shear zone(s) near the crust/mantle boundary, which formed synchronously with the fullest development of Yuan-Yitong rift during Oligocene and Eocene times. We infer, then, that these Yitong Iherzolite xenoliths record at least two superimposed episodes of deformation and chemical evolution which were closely related to the regional geodynamic evolution.