ALUMINUM ORTHO-PYROXENE IN PYROMETAMORPHOSED GARNET MEGACRYSTS FROM LIAONING AND SHANDONG PROVINCES, NORTHEAST CHINA

• Main Authors: ZANG, QJ, ENAMI, M, SUWA, K
• Other Authors: ZANG, QJ (reprint author), BEIJING UNIV,DEPT GEOL,BEIJING 100871,PEOPLES R CHINA., NAGOYA UNIV,SCH SCI,DEPT EARTH & PLANETARY SCI,NAGOYA 46401,JAPAN., BEIJING UNIV,DEPT GEOL,BEIJING 100871,PEOPLES R CHINA.
• Format: Journal/Newspaper
• Language: English
• Published: european journal of mineralogy 1993
• Subjects: ALUMINUM ORTHO-PYROXENE; GARNET MEGACRYST; PYROMETAMORPHISM; ALKALI BASALT; NORTHEAST CHINA; ENDERBY LAND; ORTHO-PYROXENE; METAMORPHIC EVOLUTION; BEARING GRANULITES; COMPLEX SYSTEMS; ANTARCTICA; GEOTHERMOBAROMETRY; ASSEMBLAGES; OSUMILITE; PRESSURES; Antarc*; Antarctica; Enderby Land
• Online Access: https://hdl.handle.net/20.500.11897/311533

Aluminian subsilicic orthopyroxene (Al2O3 = 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 fine-grained (usually less than 2-3 mum in size) orthopyroxene, spinel and anorthite crystals. The pseudomorphs have many internal cracks, and aggregates of coarser-grained (20-300 mum 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 Al2O3 (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-degrees-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. Mineralogy SCI(E) 0 ARTICLE 1 153-164 5