Aluminian orthopyroxene in pyrometamorphosed garnet megacrysts from Liaoning and Shandong provinces, northeast China

Abstract Aluminian subsilicic orthopyroxene (AI2O3 =16.8 wt% maximum) was found in pseudomorphs after garnet megacrysts in alkali basaltic rocks occurring at the Mt. Huangyishan, Liaoning province and at Mt. Chishan, Shandong province of northeast China. The orthopyroxene-bearing pseudomorphs are mainly composed of finegrained (usually less than 2-3 µm in size) orthopyroxene, spinel and anorthite crystals. The pseudomorphs have many internal cracks, and aggregates of coarser-grained (20-300 µm in size) orthopyroxene, spinel and anorthite occur sporadically along the cracks. Olivine or clinopyroxene occurs rarely in the coarser-grained aggregates of a Huangyishan megacryst. In this sample, orthopyroxenes in the olivine- or clinopyroxene-bearing coarser-grained aggregates have fairly restricted Al2O3 contents of 10.5 ±1.2 and 10.7 ± 1.2 wt%, respectively, and that in the olivine- and clinopyroxene-free aggregates is richer in AI2O3 (12.3 ± 1.2 wt%). The Al enrichment of orthopyroxene is strongly controlled by the Tschermaks substitution [4]Al[6]Al(Mg, Fe)-1Si-1. The equilibrium conditions of the orthopyroxene-bearing assemblages are estimated at T = 1180-1270°C and P = 9-15 kbar. The orthopyroxene-bearing pseudomorphs have been formed by solid-state and isochemical breakdown of original garnet megacrysts during the transportation by the alkali basaltic magma. The transportation speed was high and is estimated at 3.5-7.0 km/h from a calculation of the velocities of the host magma ascent and gravitational sedimentation of the garnet megacryst. This suggests that the transportation time of the garnet megacryst from the garnet-stable depth to the surface was about 6-14 hours, which is probably sufficient for its partial decomposition.