Exsolution lamellae in a clinopyroxene megacryst aggregate from Cenozoic basalt, Leizhou Peninsula, South China: petrography and chemical evolution

Petrographic investigations and electron microprobe analyses have been performed on a rare aggregate of clinopyroxene megacrysts collected from Cenozoic basalts in Yinfengling, Leizhou Peninsula of South China. The aggregate, composed of several clinopyroxene megacrysts, shows abundant exsolution lamellae of garnet (Grt) and orthopyroxene (Opx), and granular texture. Cr- and Ti-poor spinels are also present in this sample. They occur predominantly as Sp-Opx-Grt clusters (Cr# = 0.025-0.034) at the interspace between different megacrysts, and subordinately as bleb-shaped (Cr# = 0.025 0.034) or thin-lamella crystals (Cr# = 0.006-0.021) clinopyroxene. Three different assemblages of exsolution are identified, namely (1) Sp (high Cr/Al) and Opx; (2) Grt-Opx; (3) Sp (low Cr/Al) and Opx. In addition, some garnets were likely developed as response to breakdown of the high-Cr/Al Sp. The homogeneous compositions in all constituent minerals and the good agreement between calculated Cpx/Grt partition coefficients (K-d's) for trace elements and reference data strongly suggest a chemical equilibrium among coexisting minerals, probably attained by diffusion after the exsolution. Thermobarometric calculation based on exsolved assemblage yields a temperature of 900 +/- 30 degrees C and a pressure of 12 +/- 2.2 kbar, corresponding to the present-day thermal gradient in the region. Much higher P-T estimates (T = 1,210 +/- 30 degrees C, P = 16.2 +/- 3.5 kbar) are obtained for the reconstructed composition of cpx prior to exsolution. The contrast in thermal state before and after the exsolution might reflect the thermal evolution of the lithosphere beneath South China during the Cenozoic.