Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction-exhumation processes (SW Tianshan, China)
Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction-exhumation processes (SW Tianshan, China)
复制标题
俯冲-折返过程中高压变铁岩中鹿岩的稳定性条件和成分变化（中国西南天山）
DOI：
10.1016/j.lithos.2021.106245
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发表时间：
2021
期刊：
影响因子：
3.5
通讯作者：
Gao Jun
中科院分区：
文献类型：
--
作者：
Li Ji-Lei;Klemd Reiner;Hoefer Heidi E.;Gao Jun
变质岩中的指标矿物，例如柯石英、蓝闪石或硬钠石，对于了解高压 (HP) 至超高压 (UHP) 变质作用、热梯度和俯冲带的地球动力学非常重要。鹿岩被认为可以诊断冷俯冲环境中的蓝片岩相变质作用，并且是富铁岩石中氧逸度 (fO2) 的敏感指标。然而，人们对其在俯冲和折返周期中的稳定性和成分变化知之甚少。在这项研究中，我们对西南天山 (U)HP 变质带的榴辉岩相变质铁矿石（鹿辉石-石榴石利贝克铁矿）中的鹿岩进行了首次详细调查。岩石学观察和结构证据表明，变质铁矿石（包围榴辉岩块）构成了最初连贯的火山沉积序列的一部分，该序列经历了共同的构造变质演化，包括高压变质作用和随后的折返作用。两种类型的鹿瑞石，表现出不同的结构（内含物与基体）和成分（TiO2 含量），出现在鹿瑞石 - 石榴石利贝克石中。 Dee_1 和 aegirine-augite 仅以石榴石中的细粒包裹体形式出现，并与石榴石、立贝克石、石英和磁铁矿一起在约 510 °C 和 2.6 GPa 的 P-T 条件下形成峰值变质矿物组合。而基质鹿铁矿（Dee_2）主要以粗粒棒状和针状针状颗粒形式存在于石英中，很少以利贝克石和石榴石中的包裹体形式紧密存在于利贝克石中，代表了变质铁矿石折返过程中蓝片岩相的退变。石榴石-单斜辉石氧压计和热力学计算表明，鹿榴石-石榴石丽贝克石的fO2从榴辉岩相峰值阶段的ΔFMQ-0.64增加到蓝片岩相退变阶段的ΔFMQ+1.5。这种 fO2 变化对应于鹿岭石的化学变化，其中 TiO2 含量从石榴石中 Dee_1 中的 <0.1 wt% 逐渐增加到基质 Dee_2 中的约 5 wt%。我们的研究揭示了俯冲-折返过程中顺行榴辉岩和逆行蓝片岩相含变铁岩的变质铁岩的详细变质 P-T-fO2 演化。这项研究进一步强调了鹿岩揭示俯冲相关高压岩石 P-T-fO2 演化的潜力。
Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that thefO2of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. ThisfO2variation corresponds to the chemical variation of deerite, whereby the TiO2content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2evolution of subduction-related high-pressure rocks.