Phase Equilibria of Hornblende‐Bearing Eclogite in the Western Dabie Mountain, Central China

Abstract: The high‐pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P‐T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P‐T pseudosection method developed recently showed the following: (1) the growth zonation of garnet records a progressive metamorphic PT path from pre‐peak condition of 1.9–2.1 GPa at 508°C‐514°C to a peak one of 2.3–2.5 GPa at 528°C‐531°C for the HP eclogite; (2) the peak mineral assemblage is garnet+omphacite+glaucophane+quartz±phengite, likely paragenetic with lawsonite; (3) the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post‐peak stage, mostly representing the conditions of about 1.4–1.6 GPa and 580°C‐640°C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.