Phase relations of lawsonite-blueschists and their role as a water-budget monitor: a case study from the Hakoishi sub-unit of the Kurosegawa belt, SW Japan

This study investigated progressive changes in the mineral assemblage of a Palaeozoic lawsonite-blueschist (LBS) unit of the Kurosegawa belt, Kyushu, Japan. The LBS unit is mainly composed of intercalated metachert and metabasaltic rocks surrounded by serpentinite. Based on the spatial distribution of mineral assemblages in the metabasaltic rocks, three mineral zones can be identified in a ∼10 km-long east–west oriented unit: (1) a pumpellyite (Pmp) + Na amphibole (Namp) assemblage dominated area (Zone 1); (2) an intermixed area of lawsonite (Lws) + Na/Ca-Na pyroxenes (Napx) + Pmp and Lws + Namp + Pmp assemblages (transition zone); and (3) an area dominated by Lws + Namp + Napx assemblages (Zone 2); all contain excess chlorite (Chl), albite (Ab), quartz (Qz), titanite (Ttn), and goethite. Napx varies in composition from jadeite(Jd)₂₀-diopside(Di)₂₅ of Zone 1 to Jd₃₅Di₁₅ in Zone 2, with a similar range of aegirine (Aeg) content from 40 to 55 mol%. The Al/(Al+Fe³⁺) ratio of Na amphibole (= Y_Amp) associated with Lws and Pmp increases from Y_Amp = 0.2 in Zone 1 to Y_Amp = 0.8 in Zone 2. The Jd contents and the lack of epidote and zeolite-group minerals suggest that the pressure (P) conditions of the BS increase from Zone 1 (0.50 GPa) to Zone 2 (0.75 GPa) between 200 and 300 °C, suggesting a low geothermal gradient (∼5 to 10 °C/km). Schreinemakers’ analyses in a NaCaMgFe³⁺AlSiH (NCMF3ASH) system, using mean mineral compositions of representative samples and excess Chl, Ab, Qz and H₂O, suggest that, (1) observed variation in mineral assemblages can be explained by the hydration reaction Pmp + Napx + Chl + H₂O = Lws + Namp, which was driven by a P increase from Zone 1 to Zone 2, and represents the transition from the Pmp-BS to the Lws-BS sub-facies; (2) the total H₂O content stored in hydrous minerals increases from 3.6 wt% in the Pmp−Namp assemblage of Zone 1 to 5.0–6.4 wt% in the Lws + Namp + Napx assemblages in Zone 2; and (3) the Napx + Chl assemblage can retain a significant amount of H₂O, along with the Lws−Namp and Pmp−Namp assemblages, and its stability field can expand to higher P conditions by decrease of the Di and increase of the Jd and Aeg contents in Napx.