Stability of Al- and F-rich titanite in metacarbonate: petrologic and isotopic constraints from a polymetamorphic eclogitic marble of the internal Sesia Zone (Western Alps)
Stability of Al- and F-rich titanite in metacarbonate: petrologic and isotopic constraints from a polymetamorphic eclogitic marble of the internal Sesia Zone (Western Alps)
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DOI：
10.1007/s00410-001-0317-6
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发表时间：
2002-03-01
影响因子：
3.5
通讯作者：
Rubatto, D
中科院分区：
文献类型：
--
作者：
Castelli, D;Rubatto, D
为了确定高压变质过程中钛矿的行为，研究了西阿尔卑斯山塞西亚带多变质大理岩中富铝和富F钛矿（3.86 < Al2O3 < 9.33 wt%, 0.93 < F < 2.53 wt%）。细观构造-微观构造关系、矿物组合和岩石学资料显示了前阿尔卑斯（低压、高温）到早阿尔卑斯（高压、中温）的压力-温度-时间演化。背散射电子图像、x射线定性元素图和电子探针分析表明，大多数富Al和富f钛矿以分离晶体的形式存在，其组成相当均匀。相比之下，与典型的早阿尔卑斯高压矿物伴生或边缘的钛矿的特征是单晶内可变的Al-1(F， OH)(1) ti - 10 -1取代，特别是在与辉长石和/或辉长石的晶界处。原位离子探针U-Pb分析发现，不同Al和F含量的钛矿结构域的Pb-206/U-238年龄分布在283 ~ 153 Ma之间。Pb-206/U-238比值与Al含量具有良好的相关性，表明富Al和富f钛矿形成于前高山变质期（大于或等于281 +/- 11 Ma）。在Al和F含量下降的区域中，年龄逐渐变小，表明部分化学再平衡是阿尔卑斯变质过程中不完全同位素重置的原因。岩石学和U-Pb数据表明，在推断多变质碳酸盐岩体系高压条件时，应谨慎使用富Al和富f的钛矿。
Al- and F-rich titanite (3.86 < Al2O3 < 9.33 wt%,, 0.93 < F < 2.53 wt%) from a polymetamorphic marble of the Sesia Zone (Western Alps) has been investigated in order to determine the behaviour of titanite during high-pressure metamorphism. Mesostructural to micro-structural relationships, mineral assemblages and petrological data indicate a pre-Alpine (low-pressure, high-temperature) to early-Alpine (high-pressure, medium-temperature) pressure-temperature-time evolution. Backscattered electron images, X-ray qualitative elemental maps and electron microprobe analysis show that most Al- and F-rich titanite that occurs as isolated crystals is rather homogeneous in composition. In contrast, titanite associated with, or rimmed by, typical early-Alpine, high-pressure minerals is characterized by variable Al-1(F, OH)(1)Ti-1O-1 substitution within single crystals, particularly at grain boundaries with omphacite and/or phengite. In-situ ion microprobe U-Pb analysis of titanite domains that have various Al and F contents yielded apparent Pb-206/U-238 ages scattering between 283 and 153 Ma. Chemical and petrological data are indispensable to interpret this complex age distribution, and the good correlation between Pb-206/U-238 ratios and Al content indicates that the Al- and F-rich titanite was formed during pre-Alpine metamorphism (greater than or equal to281 +/- 11 Ma). Progressively younger ages are obtained in domains with decreasing Al and F content, suggesting that partial chemical re-equilibration was responsible for the incomplete isotopic resetting during Alpine metamorphism. Petrological and U-Pb data show that Al- and F-rich titanite should be used with caution to infer high-pressure conditions in polymetamorphic carbonate systems.