Multistage melt/fluid modification of lithospheric mantle beneath the circum-cratonic orogenic belt: Evidence from the Tuoyun peridotite xenoliths

Abstract

The juvenile and moderate refractory mantle beneath the circum-cratonic orogenic belt is traditionally believed to be hotter and thinner than the ancient refractory cratonic mantle; it is thus more unstable and subject to modification by melts/fluids. Understanding these modification processes would help to elucidate the evolution of Earth’s continents. Peridotite xenoliths carried by the Tuoyun Cenozoic lamprophyre from the southwestern Tianshan belt show evidence of widespread multistage melt/fluid modification of the unstable circum-cratonic orogenic belt mantle. Tuoyun peridotites mainly consist of moderately refractory to fertile lherzolites (Mg# in Ol: 85.5−90.7; Cr# in Sp: 12.7−26.5) and show strong mechanical modification. They can be divided into four groups (A, B, C1, and C2) based on petrography and mineral chemistry. Group A lherzolites show relatively high basaltic components (Al2O3, CaO, TiO2, and FeO) and are enriched in large ion lithophile elements (LILEs) and rare earth elements (REEs), which indicates melt-peridotite reaction processes at high melt/rock ratios. The high modal pyroxene content in Group A suggests that the addition of high-Si melts caused the transition from olivine to pyroxene. Group B lherzolites show high modal pyroxene but relatively depleted incompatible elements, which should be superimposed by later melt extraction. Comparatively, Group C lherzolites exhibit higher modal olivine but lower basaltic components. The clinopyroxene cores of Group C1 are characterized by high (La/Yb)N and low Ti/Eu content, negative high field strength element (HFSE) anomalies, and relatively high 87Sr/86Sr ratios (cores: 0.70331−0.70457), which suggest metasomatism by carbonatite melts originating from recycled sedimentary carbonate. The Group C1 clinopyroxene (spongy rims) and Group C2 clinopyroxene (cores and spongy rims) have positive Sr anomalies, depleted HFSEs, and spoon-shaped REE patterns, which suggest modification by evolved small-volume and volatile-rich silicate melts. In addition, the melt pockets around spinels and the reactive zones of pyroxenes near the lamprophyre reveal the recent incongruent dissolution induced by the host rock. Based on our research and previously reported geological data, we propose that the high-Si melts and carbonatite melts are the products of dehydration and partial melting of the Paleo-Asian oceanic crust, and lithospheric delamination and fracturing (e.g., the Talas-Fergana strike-slip fault) provided the opportunity for small-volume and volatile-rich silicate melts and basaltic melts to modify the peridotites. Multistage melts/fluids and the deformation process are the protagonists in the evolutionary process of the circum-cratonic lithospheric mantle, with important implications for mantle destabilization and multilayered interaction.