Slow cooling of deep crustal granulites and Pb-loss in zircon
Slow cooling of deep crustal granulites and Pb-loss in zircon
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DOI：
10.1016/s0016-7037(99)00166-0
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发表时间：
1999-09
影响因子：
5
通讯作者：
L. Ashwal;R. Tucker;E. Zinner
中科院分区：
文献类型：
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作者：
L. Ashwal;R. Tucker;E. Zinner
我们试图获得马达加斯加西南部Ankafotia体中变质（麻粒岩相，T = 800±90°C; P = 8±1 kbar）块状斜长岩的岩浆结晶年龄。研究样品为粗粒白长石，具有良好的火成岩结构和矿物学保存，但斜长石中含有丰富的金红石和锆石包裹体，并有轻微的再结晶。33份锆石同位素稀释U-Pb分析结果显示，锆石单粒碎片（29份）和多粒组分（4份）的年龄谱在631 ~ 549 Ma之间，时间跨度超过80 myr。背向散射电子和阴极发光图像显示，大多数颗粒要么是均匀的、无结构的碎片（35%），要么是由相对富集U-和/或th的锆石所占据的吻合裂缝（45%）不同程度地渗透；较小比例的颗粒显示残余岩浆分带（20%）。约25%的谷物生长出薄的、U-和/或富th的过度生长。拉曼斑点分析表明，所有的碎片都是高度结晶和非异晶的。锆石粒度与内部特征具有明显的相关性，最古老的锆石粒度较大，具有残余岩浆分带；最年轻的晶粒是含有高u型裂纹网络的小碎片。离子探针斑点分析表明，每个锆石颗粒都保留了独特的微量元素特征；稀土元素模式显示ree富集，Eu负异常，Ce正异常。我们认为，该样品中一致U-Pb年龄的约80 myr分布表明，在一个或多个长时间的麻粒岩相变质作用期间，高温pb丢失，只有少量的幕式或连续变质锆石生长。体积扩散和/或裂缝辅助扩散似乎是铅损失的主要机制。使用最近测量的Pb扩散参数计算的冷却曲线符合年龄-尺寸关系，并且暗示非常缓慢的冷却速率（1-2°C/myr或更低），这可能是对麻粒岩条件维持较长时间的岩层的预期。因此，我们的研究结果建议在解释受长时间高变质作用影响的变质火成岩中的和谐锆石时要谨慎。
We attempted to obtain the magmatic crystallization age of a metamorphosed (granulite facies, T = 800 ± 90°C; P = 8 ± 1 kbar) massif-type anorthosite from the Ankafotia body of southwest Madagascar. The sample studied is a coarse-grained leuconorite with good preservation of igneous texture and mineralogy, although plagioclase, which contains abundant rutile and zircon inclusions, has been slightly recrystallized. Thirty three isotope dilution U-Pb analyses of zircons representing single-grain fragments (29 analyses) and multi-grain fractions (4 analyses) yield a spectrum of concordant ages from 631 to 549 Ma, a time span of more than 80 myr. Back-scattered electron and cathodoluminescence images show that most grains are either homogeneous, structureless fragments (35%), or are permeated to a variable degree by anastamosing cracks occupied by relatively U- and/or Th-enriched zircon (45%); a smaller percentage of grains show relict magmatic zoning (20%). Thin, U- and/or Th-rich overgrowths occur on about 25% of grains. Raman spot analyses demonstrate that all fragments are highly crystalline and non-metamict. There are marked correlations between zircon grain size and internal features, such that the oldest grains are larger, and show relict magmatic zoning; the youngest grains are small fragments containing high-U crack networks. Ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature; rare earth element patterns show heavy REE-enrichment, with negative Eu anomalies and positive Ce anomalies. We suggest that the ca. 80 myr spread in concordant U-Pb ages in this sample is indicative of high-temperature Pb-loss during one or more protracted periods of granulite facies metamorphism, with only minor episodic or continuous metamorphic zircon growth. Volume diffusion and/or fracture-assisted diffusion seems to be the dominant mechanism of Pb-loss. Cooling curves, calculated using recently-measured Pb diffusion parameters, conform to the age-size relationship, and imply very slow cooling rates (1–2°C/myr or less), as might be expected for a terrane in which granulite conditions were maintained for an extended period of time. Our results, therefore, suggest a note of caution for interpretation of concordant zircon ages in meta-igneous rocks affected by high-grade metamorphism of long duration.