Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)
Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)
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DOI：
10.1007/s004100050492
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发表时间：
1999-03
影响因子：
3.5
通讯作者：
G. Vavra;R. Schmid;D. Gebauer
中科院分区：
文献类型：
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作者：
G. Vavra;R. Schmid;D. Gebauer
对伊夫雷阿高级变质带中锆石晶体的几种生长形态和蚀变机制进行了区分，并将其归因于岩浆、变质和流体事件。泥质变质沉积物在IZ深熔产生的残留岩的碎屑岩芯上的锆石过度生长和新的结晶岩浆锆石在相关的浅色体。残留岩中锆石增生的原生形态和Th-U化学特征表现出明显的系统性变化，与变质程度相对应：上部角闪岩相为棱柱状（棱柱块状）低Th/U型，过渡相为中Th/U型，麻粒岩相为等轴状（圆带状）高Th/U型。交代交代岩中的辉绿锆石和浅色体中的岩浆锆石的初始结晶年龄不能相互分辨，表明IZ大部分地区的深熔作用是一个单一的、短暂的事件，发生在299 ± 5 Ma（95%c. l.）。来自基性岩组的变辉长岩（293 ± 6 Ma）和变橄榄岩（300 ± 6 Ma）的岩浆锆石U/Pb年龄一致，证实了IZ岩浆底侵作用和麻粒岩相深熔作用的成因背景。变质沉积物中超生锆石U-Pb年龄为299 ± 5 Ma，表明IZ的高级变质（深熔）条件在邻近的Strona-Ceneri带（SCZ）华力西角闪岩相变质作用之后的20 Ma内开始。这清楚地表明，SCZ不能代表中间到上地壳延续的IZ。在麻粒岩相变质作用过程中生长的锆石晶体大部分受到蚀变和Pb亏损的影响。阴极发光成像可以区分两种类型的蚀变和铅损失机制：分带控制蚀变（ZCA）和表面控制蚀变（SCA）。ZCA是二叠纪伸展去顶过程中的热脉冲和/或减压脉冲的结果，时间为249 ± 7 Ma或更早。SCA是210 ± 12 Ma流体侵入的结果，与上三叠世/下侏罗世泛古陆裂解过程中的热液活动有关。
Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic.