Abstract One of the first appearances of eclogite-facies mineral assemblages in the geological record occurs in the c. 2000 Ma Palaeoproterozoic Usagaran Belt in central Tanzania, where the extended margin of the Tanzanian Craton is interpreted to have been subducted. Mafic rocks are interpreted to have contained the mineral assemblage garnet + omphacite + rutile + quartz ± amphibole. This high-pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene + plagioclase + hornblende + ilmenite ± orthopyroxene. Mineral equilibria forward modelling indicates that the eclogite-facies assemblages reached minimum peak pressure–temperature (P–T) conditions of ~17 kbar and ~700 °C. Inclusions in garnet document a prograde P–T history consistent with burial through upper amphibolite-facies conditions and possible partial melting. Petrological and compositional evidence from garnet suggests that following peak metamorphism, the eclogite-facies rocks were heated while stalled at approximate peak pressures. Temperature estimates derived from Zr concentrations in interpreted texturally retrograde rutile support a near-isothermal post-peak P–T evolution for the eclogite-facies rocks – an evolution that terminates at retrograde P–T conditions of approximately 7.6–8.2 kbar and 680–790 °C. The relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies (the Isimani Suite). The metapelitic gneisses contain garnet + kyanite + biotite + staurolite + hornblende + plagioclase + muscovite + rutile + quartz and preserve minimal evidence of a high-pressure history, conceivably due to post-peak mineralogical recrystallisation. P–T modelling, inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses — similarly to the relict eclogites — experienced burial to minimum peak pressures of approximately 16.5–17 kbar. Compositional zoning patterns in eclogitic garnet suggest the Isimani system was buried, reached peak metamorphic conditions, and was subsequently exhumed within a timeframe of up to 20 Myr. A tectonic regime involving crustal thickening and subduction, followed by extensional exhumation of the entire Isimani Suite is our preferred model for the development of the c. 2000 Ma Usagaran Belt.

中文翻译：

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20 亿年前榴辉岩及其变泥质寄主埋藏和挖掘的热气压限制

摘要 地质记录中榴辉岩相矿物组合的首次出现是在c. 2000 Ma 坦桑尼亚中部古元古代 Usagaran 带，坦桑尼亚克拉通延伸边缘被解释为已俯冲。镁铁质岩石被解释为含有石榴石 + 绿辉石 + 金红石 + 石英 ± 角闪石的矿物组合。这种高压组合已被包含单斜辉石 + 斜长石 + 角闪石 + 钛铁矿 ± 斜方辉石的次生矿物组合套印。矿物平衡正演模型表明榴辉岩-相组合达到~17 kbar 和~700 °C 的最小峰值压力-温度 (P-T) 条件。石榴石中的内含物记录了与通过上部角闪岩相条件和可能的部分熔融埋藏一致的前级 P-T 历史。石榴石的岩石学和成分证据表明，在峰值变质作用之后，榴辉岩相岩石被加热，同时在近似峰值压力下停滞。从解释的结构逆行金红石中 Zr 浓度得出的温度估计支持榴辉岩相岩石的近等温峰值后 P-T 演化——这种演化终止于大约 7.6-8.2 kbar 和 680-790 的逆行 P-T 条件℃。残存榴辉岩域形成较大组合的一部分，其中包含混合混泥质变泥质岩性（Isimani Suite）。变泥质片麻岩包含石榴石 + 蓝晶石 + 黑云母 + 星形石 + 角闪石 + 斜长石 + 白云母 + 金红石 + 石英，并且保留了极少的高压历史证据，这可能是由于峰值后矿物学再结晶。P-T 模型、包体组合和斑纹石榴石的成分分带模式表明，变泥质片麻岩（类似于残遗榴辉岩）经历了掩埋，最低峰值压力约为 16.5-17 kbar。榴辉岩中的成分分区模式表明 Isimani 系统被掩埋，达到了峰值变质条件，随后在高达 20 Myr 的时间范围内被挖掘出来。涉及地壳增厚和俯冲的构造体制，随后是整个 Isimani Suite 的外延挖掘是我们开发 c. 的首选模型。2000 Ma Usagaran 腰带。