PRESSURE–TEMPERATURE EVOLUTION OF GARNET-BEARING BLUESCHIST IN CENTRAL QIANGTANG BLOCK, NORTHERN TIBETAN PLATEAU

A garnet-bearing blueschist from the Lanling in the Qiangtang Block, Tibetan Plateau was studied to elucidate the pressure-temperature (P-T) history of the region. The sample LL-5 contains a mineral assemblage of garnet (Grt), glaucophane, epidote-clinozoisite, rutile, mica, chlorite, apatite, ilmenite, quartz, and pyrite. Garnet porphyroblasts, euhedral to subhedral and up to 1.9 mm in diameter, comprise ~15 vol% of the sample. Inclusions of epidote, clinozoisite and quartz and partial replacement by chlorite are common. Glaucophane is the dominant matrix mineral and anhedral, with inclusions of epidote, rutile, apatite and ilmenite. Epidote-clinozoisite forms euhedral crystals up to 1.3 mm long and typically contain inclusions of rutile, apatite, and ilmenite. Euhedral chlorite occurs locally as pseudomorphs after garnet.

Grt is almandine (Alm) with smaller amounts of grossular (Grs), pyrope (Prp), and spessartine (Sps). The composition of garnet varies from Alm₅₇Prp_6–7Grs₂₁Sps_15–16 at the core to Alm₆₁Prp₁₁Grs_25–26Sps_2–3 at the rim. The glaucophane contains 0.08–0.10 p.f.u. Na in the A-site and up to 0.60 p.f.u. Fe³⁺. Epidote has 0.52–0.76 p.f.u Fe³⁺ and 2.24–2.40 p.f.u Al with an X_Ep [=Fe³⁺/(Fe³⁺+Cr³⁺+Al^-2)] value of 0.61–0.76. Chlorite has a Mg# ranging from 0.56–0.63, with 5.40–6.13 p.f.u. Mg.

Pseudosection of the sample along with garnet chemistry indicates a clockwise P-T path from 2.5–2.6 GPa, 480°C to 2.4–2.5 GPa, 490°C with an assemblage of amphibole, chlorite, omphacite, talc, lawsonite, rutile, and biotite or muscovite. The absence of omphacite, talc and lawsonite is interpreted to be due to retrograde metamorphism. This study indicates that, despite strong retrogression, peak metamorphic conditions can be retained in garnet porphyroblast.