Transcrustal magma plumbing process in the Cenozoic Dali potassic lamprophyre dyke formation in the southeastern Tibet Plateau

(Ultra-)potassic lamprophyres are volumetrically rare, but are widely distributed in diverse geological settings and extensively applied to trace the mantle source and melting processes. However, it is still debatable whether the lamprophyre formation involved transcrustal magmatic processes. In this study, we describe the diverse zoning patterns and textures of clinopyroxene (Cpx) from a Cenozoic potassic lamprophyre dyke in southeastern Tibet Plateau. Three types of Cpx macrocrysts with normal- and reverse oscillatory zoning and sieved textures, and Cpx microcrysts from Cpx–Phl nodules are recognized. The Cpx macrocrysts have highly variable Mg# (62–89) and chemical compositions. Their oscillatory-zoned mantle has alternating coarse layers or closely-packed layers, with distinct resorption surface shown in backscattered electron (BSE) images. High-amplitude oscillation of Mg# (±5 to ±12), Al, Cr and Sr at the boundary layers of mantle may reflect multiple magma mixing/recharge events. In contrast, low-amplitude and high-frequency oscillation of Mg# (varying from < ±3) across the closely-packed layers could be attributed to local magma mixing or kinetic effect at the crystal-melt interface. The sieved domains occur in the Cpx macrocrystic core with spongy space and linking the external groundmass by cracks, and have major and minor compositions and 87Sr/86Sr ratios similar to those of the rims. This indicates that the sieved domains are probably originated from dissolution of the Cpx macrocrysts by groundmass melt and later reprecipitation. The estimated pressure and temperature for the Cpx macrocryst high-Mg# cores and low Mg# rims are 580–1160 MPa (avg. 919 MPa) and 30–1270 MPa (avg. 681 MPa), and 1116–1266 °C (avg. 1216 °C) and 955–1215 °C (avg. 1110 °C), respectively, suggesting polybaric differentiation or Cpx-saturated magmas. The estimated pressure and temperature for the reverse-zoned Cpx marcocryst cores are 450–1310 MPa (avg. 953 MPa) and 1097–1209 °C (avg. 1166 °C), respectively, indicating that they are likely formed in equilibrium with a more evolved magma batch, and then were entrapped by more primitive mantle-derived potassic magmas during magma upwelling. The La/Yb and Dy/Yb modelling of equilibrium melts suggest that magmas of the Cpx–Phl nodules and their lamprophyre host were derived from similar mantle sources. Therefore, the complex zoning patterns and compositions of Cpx macrocrysts could indicate transcrustal magmatic processes in the potassic lamprophyre dyke formation.