Orogenic eclogites record relative magnitude of deep crustal flow and extent of migmatite-eclogite interaction
Orogenic eclogites record relative magnitude of deep crustal flow and extent of migmatite-eclogite interaction
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DOI：
10.1016/j.lithos.2022.106917
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发表时间：
2022-10
期刊：
影响因子：
3.5
通讯作者：
Hamelin Clémentine;L. Whitney Donna;Roger Françoise;Teyssier Christian
中科院分区：
文献类型：
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作者：
Hamelin Clémentine;L. Whitney Donna;Roger Françoise;Teyssier Christian
在已发掘的造山带中，难熔基性岩石有可能保存岩石成因和高压变质作用的记录，而高压变质作用通常在石英岩中由于高温、低压（lb - ht）条件下的再平衡而被湮没。在法国蒙塔涅黑岩穹窿中，榴辉岩暴露于穹窿的核心和边缘，位于瓦立斯坎造山带的前陆。本研究结合原位U-Pb岩石年代学和关键榴辉岩相氧同位素分析，表明两个不同域的榴辉岩具有不同的原岩和源区，在地壳深处移动的距离相对不同，在挖掘前与周围混辉岩的相互作用也不同。圆顶边缘榴长岩锆石体积小（~ 40 μm），继承核保存完好，边缘薄（<15 μm），而体积大（40 ~ 120 μm）的新和再结晶的圆顶核锆石，遗留核小，边缘宽（15 ~ 30 μm）。采用原位锆石和金红石岩石年代学（LASS-ICP-MS）测定了原岩和hp变质作用年龄。两种榴辉岩均形成于大陆环境；丘顶边缘原岩锆石岩心形成于442.5±3.4 Ma （ree陡坡，无eu异常），而丘顶岩心榴辉岩的锆石岩心产生于~ 500-400 Ma （ree陡坡，eu异常明显）。两种榴辉岩均经历了高温变质作用。320-310 Ma石榴石稳定，斜长石无条件。大多数分析金红石的日期为307-304 Ma，与冷却有关。通过石榴石和锆石的o同位素（SIMS）分析确定了高压流体条件的记录。圆顶边缘锆石岩心和边缘的δ18O值为~ 8.2-8.5‰，在不确定度范围内难以区分，与同位素无带的石榴石（δ18O ~ 8.0-8.2‰）处于同位素平衡状态。相比之下，圆顶核榴辉岩中锆石的δ18O值相对于其边缘和新晶颗粒有系统的降低，锆石核在氧方面与主阳离子带石榴石处于平衡状态。两个圆顶核榴辉岩样品的锆石和石榴石δ18O值分别为~ 8.6 ~ 9.5‰和~ 9.7 ~ 10.5‰。基于以上研究结果和已有的辉长岩HPfabric资料，我们认为：(1)两种辉长岩原岩位于寒武-奥陶系大陆地壳包体的不同深度；(2)在埋藏和前陆辐散地壳流动过程中，穹隆-岩心榴辉岩与周围片麻岩有广泛的相互作用，而穹隆-边缘榴辉岩则来源于穹隆-侵位附近，与周围片麻岩的化学相互作用最小。黑山混辉岩穹隆至少有部分来源较深，但岩心处的岩石比边缘处的岩石具有更广泛、更持久的深地壳流动历史。
In exhumed orogens, refractory mafic rocks have the potential to preserve a record of petrogenesis and high-pressure (HP) metamorphism that is commonly obliterated in quartzofeldspathic rocks owing to re-equilibration at high-temperature, low-pressure (LP–HT) conditions. In the Montagne Noire (France) migmatite dome, located in the foreland of the Variscan orogen, eclogite is exposed in both the core and margin of the dome. In this study, we combine in situ U–Pb petrochronology and oxygen-isotope analyses of key eclogite phases to demonstrate that eclogites from the two distinct domains had different protoliths and source regions, traveled relatively variable distances in the deep crust, and differentially interacted with surrounding migmatite prior to exhumation. Dome-margin eclogite zircons are small (∼40 μm) with well-preserved inherited cores and thin (<15 μm) rims, compared to larger (40–120 μm) neo- and recrystallized dome-core zircons with small relict cores and wide (15–30 μm) recrystallized rims. Protolith and HPmetamorphism ages were determined using in situ zircon and rutile petrochronology (LASS-ICP-MS). Both eclogites formed in a continental setting; dome margin protolith zircon cores formed at 442.5 ± 3.4 Ma (steep HREE slope, no Eu-anomaly) whereas zircon cores of the dome-core eclogites yielded scattered dates suggesting protolith crystallization between ∼500–400 Ma (steep HREE slope, pronounced Eu-anomaly). Both eclogites experienced HPmetamorphism atc.320–310 Ma in garnet-stable, plagioclase-absent conditions. Most analyzed rutile yielded dates of 307–304 Ma associated with cooling. The record of HPfluid conditions was determined by O-isotope (SIMS) analyses of garnet and zircon. Dome-margin zircon cores and rims have δ18O of ∼8.2–8.5 ‰, indistinguishable within uncertainty, in isotopic equilibrium with isotopically unzoned garnet (δ18O ∼ 8.0–8.2 ‰). In contrast, zircons in dome-core eclogites have systematically lower zircon-core δ18O values compared to their rims and neocrystallized grains, and zircon cores were in equilibrium with major-cation zoned garnet with respect to oxygen. The two dome-core eclogite samples yielded zircon and garnet δ18O values of ∼8.6–9.5 ‰ and ∼ 9.7–10.5 ‰. Based on these results and existing HPfabric data for these eclogites, we propose that (1) gabbro protoliths for the two eclogites were emplaced at different depths in a Cambro-Ordovician continental crustal package; and (2) dome-core eclogites interacted extensively with surrounding gneiss during burial and foreland-vergent crustal flow, whereas the dome-margin eclogite was sourced proximally to the dome-emplacement location and had minimal chemical interaction with surrounding gneiss. At least parts of the Montagne Noire migmatite dome were deeply sourced, but rocks exhumed in the core had a more extensive and protracted history of deep-crustal flow than deep-crustal rocks exhumed at the margin.