Growth of kyanite and Fe-Mg chloritoid in Fe2O3-rich high-pressure–low-temperature metapelites and metapsammites: A case study from the Massa Unit (Alpi Apuane, Italy)

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-05-26 DOI:10.1111/jmg.12736
Samuele Papeschi, Federico Rossetti, Jesse B. Walters
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Abstract

Chloritoid and kyanite coexist in metapelites from the high-pressure/low-temperature Massa Unit in the Alpi Apuane metamorphic complex (Northern Apennines, Italy). The composition of chloritoid is extremely variable throughout the Massa Unit. Fe-chloritoid occurs in association with hematite-free, graphite-bearing schists, whereas strongly zoned Fe-Mg chloritoid is found with hematite and kyanite. We investigated the effect of different bulk Fe2O3 contents in controlling chloritoid composition through phase equilibria modelling of four selected samples, representative of the different chloritoid-bearing parageneses found in the Massa Unit. The ferric iron content, measured through wet chemical titration, ranges from 0 (graphite-chloritoid schist) to 73% of the total iron (hematite-chloritoid schist). We show that Mg-rich chloritoid compositions and stability of kyanite at greenschist to blueschist facies conditions can be reproduced in the MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system only considering the presence of significant amounts of ferric iron as part of the bulk composition. The stabilization of kyanite at lower grade is directly linked to the presence of Fe2O3, which renders the reactive bulk rock composition effectively enriched in Al2O3 with respect to Fe and Mg. We also document that high Fe2O3 contents exacerbate the effect of chloritoid fractionation, producing strongly zoned Fe-Mg-chloritoid grains. Finally, the P–T modelling of the Massa Units performed in this study allows, for the first time, the recognition of a two-stage evolution at peak conditions, with an earlier pressure peak (1.2–1.3 GPa at 350–400°C), and a later thermal peak (0.7–1.1 GPa at 440–480°C), compatible with subduction, underthrusting and exhumation of the Adria continental margin during growth of the Northern Apennine orogenic wedge.

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蓝晶石和Fe - Mg类绿晶石在富Fe2O3的高压-低温变长岩和变长岩中的生长——以意大利Massa单元为例
绿泥石和蓝晶石共存于Alpi-Apuane变质杂岩(意大利亚平宁山脉北部)高压/低温Massa单元的变质精英中。在整个Massa装置中,氯离子的组成变化很大。铁绿泥石与无赤铁矿、含石墨的片岩伴生,而强分区铁镁绿泥石与赤铁矿和蓝晶石伴生。我们通过对四个选定样品的相平衡建模,研究了不同体积Fe2O3含量对控制类氯化合物组成的影响,这四个样品代表了马萨装置中发现的不同类氯化合物。通过湿式化学滴定测量的铁含量范围为0(石墨-氯云母片岩)至总铁(赤铁矿-氯云母云母片岩)的73%。我们表明,在绿片岩至蓝片岩相条件下,只有考虑到大量的铁作为主体成分的一部分,富镁的绿泥石成分和蓝晶石的稳定性才能在MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O(MnNKFMASHTO)化学系统中重现。较低品位蓝晶石的稳定与Fe2O3的存在直接相关,Fe2O3使活性大块岩石成分有效地富含Al2O3(相对于Fe和Mg)。我们还记录到,高Fe2O3含量加剧了氯云母分馏的影响,产生了强烈分区的Fe‐Mg‐氯云母颗粒。最后,本研究中对Massa单元进行的P–T建模首次允许识别峰值条件下的两阶段演变,具有更早的压力峰值(1.2–1.3 350–400°C时的GPa),以及随后的热峰值(0.7–1.1 440–480°C的GPa),与亚平宁造山楔生长期间亚得里亚大陆边缘的俯冲、欠冲和折返相兼容。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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