Multiple exsolutions in a rare clinopyroxene megacryst from the Hannuoba basalt, North China: Implications for subducted slab-related crustal thickening and recycling

Abstract

A rare large clinopyroxene megacryst (referred to as type 1) collected from the Hannuoba basalt, North China was studied. It is distinguished from the prevalent clinopyroxene megacrysts (type 2) by garnet and orthopyroxene exsolutions and by elemental and Sr–Nd isotopic compositions. The type 1 clinopyroxene megacryst has higher Cr and Nb contents and Mg# (82.9) than the type 2 clinopyroxene megacrysts as well as more evolved Sr and Nd isotopic compositions (87Sr/86Sr=0.704520, 143Nd/144Nd=0.512350) than clinopyroxene megacrysts elsewhere. These characteristics suggest that the type 1 clinopyroxene megacryst could have been formed by a recycled crust-related melt–peridotite reaction and that the melt formed in the rutile unstable field. The type 2 clinopyroxene megacrysts exhibit good correlations between Mg# and major and trace element compositions. Type 2 Sr–Nd isotopic compositions cluster at the least evolved end of the Hannuoba basalt composition. These observations imply that the type 2 clinopyroxene megacrysts were crystallized from the host lava at high pressure.The type 1 clinopyroxene megacryst contains abundant coherent cryptocrystalline lamellae and orthopyroxene exsolutions. The bulk composition of the cryptocrystalline lamellae, composed of fine plagioclase and olivine, shows typical chemical features of garnet with a Sr isotopic composition similar to the clinopyroxene host. These observations indicate that the cryptocrystalline lamellae are decomposition products of garnet exsolutions in the clinopyroxene megacryst. P–T estimates suggest that garnet exsolution in the clinopyroxene megacryst could have occurred at 2.75GPa and 1290°C. This garnet exsolution could be caused by increasing pressure or decreasing temperature, as indicated by experimental results. Although the temperature decreases during basalt eruption, the much quicker decrease in pressure will suppress the garnet exsolution in clinopyroxene. Therefore, we suggest that the type 1 clinopyroxene megacryst could have experienced pre-Mesozoic crustal uplifting and thickening at the north margin of the North China Craton. Garnet decomposition could have taken place prior to orthopyroxene exsolution during the eruption of the host lava.