Geochronology of Layered Intrusions
Geochronology of Layered Intrusions
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DOI：
10.1007/978-94-017-9652-1_1
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发表时间：
2015
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影响因子：
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通讯作者：
J. Scoates;C. Wall
中科院分区：
文献类型：
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作者：
J. Scoates;C. Wall
层状侵入体主要由玄武岩浆结晶而成，形成大量的火成岩，这些火成岩具有明显的分层，它们保存了令人惊叹的岩石记录，记录了岩浆在地壳岩浆房中演化的过程。这些侵入体含有世界级的铬、铂族元素（PGE）和钒矿床，这些金属对工业和社会至关重要。尽管它们在科学和实践中具有重要意义，但许多层状侵入体缺乏精确的年龄限制，并且大多数情况下不存在连接结晶和冷却年龄的地质年代学框架。这导致了与其起源和形成有关的严重知识差距。本章概述了测年方法（U-Th-Pb，40 Ar/39 Ar）和矿物计时器（例如，锆石、斜锆石、金红石、磷灰石、钛铁矿），结合现场、结构和岩石学标准进行目标样本选择，以成功实施任何年龄的层状镁铁质-超镁铁质岩石的地质年代学研究。作为一个应用程序，我们演示了如何记录的布什维尔德复杂的热历史的矿物年龄从样品的PGE丰富的Merensky礁。高精度锆石U-Pb年龄，包括化学研磨法对锆石的预处理（退火和浸出）或CA-TIMS技术，因为相隔> 300 km的两个样品彼此无法区分（2056.88 ± 0.41 Ma，东翼2057.04 ± 0.55 Ma，西翼2057.04 ± 0.55 Ma，西翼2057.04 ± 0.55 Ma）。不确定性报告为2s），证实了该层位在整个侵入体的近固相线条件下的同步结晶。同一样品的金红石U-Pb年龄（2052.96 ± 0.61 Ma，2053.0 ± 2.7 Ma）表明该样品经历了快速冷却（~ 125 °C/Ma），温度降至~ 400-450 °C，黑云母40 Ar/39 Ar年龄（1999 ± 10 Ma，2002 ± 10 Ma）表明该样品经历了区域热液事件。层状侵入岩的地质年代学是评估地壳中镁铁质岩浆活动演化的重要工具，其中岩浆分异过程在广泛的岩石结构和结构中被捕获。
Layered intrusions crystallize mainly from basaltic magma to form large bodies of igneous rocks that exhibit prominent layering and they preserve stunning rock records of the processes by which magma evolves in crustal magma chambers. These intrusions contain world-class deposits of chromium, platinum group elements (PGE), and vanadium, metals that are vital to industry and society in general. Despite their scientific and practical importance, precise age constraints are lacking for many layered intrusions, and geochronological frameworks linking crystallization and cooling ages for the most part do not exist. This has resulted in critical knowledge gaps related to their origin and formation. This chapter provides an overview of dating methods (U–Th–Pb,40Ar/39Ar) and mineral chronometers (e.g., zircon, baddeleyite, rutile, apatite, titanite) potentially present in layered intrusions that is coupled with field, textural, and petrographic criteria for targeting sample selection to allow for the successful implementation of geochronologic studies of layered mafic-ultramafic rocks of any age. As an application, we demonstrate how the thermal history of the Bushveld Complex is documented by mineral ages from samples of the PGE-rich Merensky Reef. High-precision U–Pb zircon ages, involving pretreatment of zircon by the chemical abrasion (annealing and leaching) or CA-TIMS technique, for two samples separated by > 300 km are indistinguishable from each other (2056.88 ± 0.41 Ma, Eastern Limb; 2057.04 ± 0.55 Ma, Western Limb; uncertainty reported as 2s) confirming synchronous crystallization of this horizon at near-solidus conditions across the intrusion. Rapid cooling (~ 125 °C/Ma) down to temperatures of ~ 400–450 °C is defined by U–Pb rutile ages from the same samples (2052.96 ± 0.61 Ma, 2053.0 ± 2.7 Ma) and a regional hydrothermal event is signaled in40Ar/39Ar biotite ages (1999 ± 10 Ma, 2002 ± 10 Ma). The geochronology of layered intrusions, where magma differentiation processes are captured in a wide range of rock textures and structures, represents an essential tool for assessing the evolution of mafic magmatism in the Earth’s crust.