Multi-episodic formation of baddeleyite and zircon in polymetamorphic anorthosite and rutile-bearing ilmenitite from the Chiapas Massif Complex, Mexico

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2022-06-28 DOI:10.1111/jmg.12683
Alejandro Cisneros de León, Axel K. Schmitt, Bodo Weber
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引用次数: 1

Abstract

Massif-type anorthosite and comagmatic associations of rutile-bearing ilmenitite (RBI) and oxide-apatite-rich amphibolite (OARA) from the Chiapas Massif Complex (CMC) in southeastern Mexico display a protracted billion-year accessory mineral record encompassing magmatic crystallization at c. 1.0 Ga to recent ductile shear deformation at c. 3.0 Ma. Multiple discrete zircon populations between these age end-members resulted from neoformation/recrystallization during local to regional metamorphism that affected the southeastern portion of the CMC. The ubiquitous presence of relict baddeleyite (ZrO2), along with various zircon generations spatially associated with pristine to partly retrogressed Zr-bearing igneous and metamorphic minerals (e.g., ilmenite, rutile, högbomite and garnet), suggests significant Zr diffusive re-equilibration (exsolution) during slow cooling and mineral breakdown followed by crystallization of baddeleyite. The subsequent transformation of baddeleyite into zircon was likely driven by reaction with Si-bearing fluids in several geochronologically identified metamorphic stages. Strikingly contrasting compositional signatures in coeval zircon from anorthosite (silicate-dominated) and comagmatic RBI (Ti-Fe-oxide-dominated) indicate a major role of fluids locally equilibrating with the rock matrix, as indicated by distinct zircon trace element and oxygen isotopic compositions. A high-grade metamorphic event at c. 950 Ma is likely responsible for the formation of coarse-grained rutile (~0.1–10 mm in diameter), srilankite, zircon and garnet with rutile inclusions as well as metamorphic högbomite surrounding Fe-Mg spinel. Zr-in-rutile minimum temperatures suggest >730°C for this event, which may correlate to rutile-forming granulite facies metamorphism in other Grenvillian-aged basement rocks in Mexico and northern South America. A younger generation of baddeleyite exsolution occurred during post-peak cooling of coarse-grained rutile, reflected in rimward Zr depletion and formation of discontinuous baddeleyite coronas. Baddeleyite around rutile was then transformed into zircon possibly during subsequent metamorphism at c. 920 or 620 Ma, resulting from syn-kinematic and contact metamorphism, respectively. Regional metamorphism at c. 450 and 250 Ma extensively overprinted the existing zircon population, especially during the Triassic event, as suggested by a significant presence of zircon with this age. Nearly pristine baddeleyite occurring interstitial to ilmenite yielded an isochron age of c. 232 Ma according to in situ U–Pb secondary ion mass spectrometry (SIMS), suggesting either formation during metamorphic peak conditions or post-peak cooling. Zircon with ages of c. 80–100 Ma in anorthosite is identified for the first time within the CMC and coincides with cooling ages of c. 100 Ma for coarse-grained rutile. This age is similar to those of rocks occurring ~200 km further to the east in Guatemala, which are also bounded to the Polochic fault system but overprinted by eclogite facies metamorphism. A high-pressure event in the southern CMC after 200 Ma, however, is presently unsupported. Although the abundance of rutile and ilmenite is unusually high in the CMC anorthosite assemblage compared with common igneous rocks, the reactions documented here nonetheless stress the importance of these phases for generating Zr-bearing accessory minerals over a wide range of metamorphic conditions.

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墨西哥恰帕斯地块杂岩多变质斜长岩和含金红石钛铁矿中坏辉石和锆石的多期形成
墨西哥东南部恰帕斯地块杂岩(CMC)中含金红石的钛铁矿(RBI)和富含氧化磷灰石的角闪岩(OARA)的地块型斜长岩和岩浆组合显示了一段长达10亿年的辅助矿物记录,包括1.0 Ga的岩浆结晶和3.0 Ma的近期韧性剪切变形。在这些年龄端元之间的多个离散锆石种群是由局部到区域变质作用期间的新形成/再结晶造成的,这些变质作用影响了CMC的东南部。残余坏辉石(ZrO2)的普遍存在,以及与原始到部分退变含锆火成岩和变质矿物(如钛铁矿、金红石、högbomite和石榴石)相关的各种锆石代,表明在缓慢冷却和矿物分解过程中,锆的扩散再平衡(析出)显著,然后是坏辉石的结晶。在地质年代学上确定的几个变质期中,变质岩与含硅流体的反应可能导致了变质岩向锆石的转化。从锆石微量元素和氧同位素组成可以看出,同时期的斜长岩(硅酸盐为主)和岩浆型RBI(钛铁氧化物为主)锆石的组成特征明显不同,表明流体在与岩石基质的局部平衡中起主要作用。c. 950 Ma的高变质事件可能形成了粗粒金红石(直径~0.1 ~ 10mm)、含金红石包裹体的硅蓝石、锆石和石榴石,以及围绕铁镁尖晶石的变质岩högbomite。金红石中锆的最低温度为>730°C,这可能与墨西哥和南美洲北部其他格伦维里亚时代基底岩石中的金红石形成麻粒岩相变质有关。粗粒金红石峰后冷却过程中出现了较年轻一代的坏辉石析出,表现为Zr向内耗竭和不连续坏辉石日冕的形成。金红石周围的坏辉石可能在随后的约920或620 Ma变质作用中转化为锆石,分别是同步变质作用和接触变质作用的结果。c. 450和250 Ma的区域变质作用广泛地覆盖了现有的锆石种群,特别是在三叠纪事件期间,这表明这个时代的锆石大量存在。根据原位U-Pb二次离子质谱分析(SIMS),在钛铁矿与钛铁矿之间出现的近乎原始的坏辉石的等时线年龄为c. 232 Ma,表明它可能是在变质峰条件下形成的,也可能是峰后冷却形成的。斜长岩的锆石年龄在80 ~ 100 Ma之间,与粗晶金红石的冷却年龄在100 Ma之间一致。这一年龄与危地马拉东部约200公里的岩石年龄相似,这些岩石也被波洛奇断裂系统所束缚,但被榴辉岩相变质作用所覆盖。然而,在200ma之后在中压中心南部发生的高压事件目前还没有得到支持。尽管与普通火成岩相比,CMC斜长岩组合中金红石和钛铁矿的丰度异常高,但这里记录的反应仍然强调了这些相在广泛的变质条件下产生含锌副矿物的重要性。
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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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