Zircon growth in UHT leucosome: constraints from zircon-garnet rare earth elements (REE) relations in Napier Complex, East Antarctica
Zircon growth in UHT leucosome: constraints from zircon-garnet rare earth elements (REE) relations in Napier Complex, East Antarctica
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DOI：
10.2465/jmps.99.180
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发表时间：
2004-08
影响因子：
0.7
通讯作者：
T. Hokada;S. Harley
中科院分区：
文献类型：
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作者：
T. Hokada;S. Harley
东南极纳皮耶杂岩中的长石质浅色体产于超高温变质的含石榴子石的副片麻岩/石英岩中。浅色体中的锆石粗粒（200-400 μm）显示出3种结构域：（I）暗阴极发光结构的内核，（II）亮阴极发光结构的外核，（III）暗阴极发光无结构的边缘。化学成分，特别是稀土元素的二次离子质谱（西姆斯）标准化模式与这三个区域相关：内核（I区）表现为高稀土元素富集，Yb（n）/Gd（n）= 3.3，而外核（II区）和边缘（III区）表现为平坦或相对亏损的高稀土元素模式，Yb（n）/Gd（n）= 0.7-0.8。Th/U比值从锆石内核（I）的3.2下降到外核（II）的1.2和边缘（III）的0.3。在这些锆石颗粒附近的石榴石显示出两种微量元素组成特征。首先，高核心Zr含量（300 ppm）在50-100 μm的晶粒边缘内降低到100 ppm。锆石内核（I）与石榴石核之间的重稀土配分在Gd处为2（GdDZrn/Grt = 2），在Lu处上升到8（LuDZrn/Grt = 8），而锆石外核（II）或边缘（III）与石榴石核或边缘之间的重稀土配分则低得多，Gd至Lu的重稀土配分一般小于1（GdDZrn/Grt = 0.8-1.2; LuDZrn/Grt = 0.6-0.7）。锆石和石榴石之间重稀土元素分布的这种显著变化反映了锆石生长或被改性的矿物的变化，锆石生长的物理和化学条件的变化，或两者的组合。基于最近的平衡锆石/石榴石重稀土元素分配系数的估计值的比较，我们推断，内核（I）没有增长的石榴石发生在副片麻岩，但增长的石榴石不存在的熔体，然后注入到片麻岩。所产生的无色体，然后进行围岩反应与封闭的石榴子石轴承片麻岩，导致减少石榴石锆含量的值接近平衡与熔体，并沉淀锆石外核（II）。最后，锆石边（III）和后来的独居石形成于HREE亏损环境。熔融注入、反应和结晶作用发生在纳皮耶杂岩超高温历史末期的2496-2471 Ma之间。
Feldspathic leucosomes occur in an ultrahigh-temperature (UHT) metamorphosed garnet-bearing paragneiss/quartzite in the Napier Complex, East Antarctica. Coarse (200-400 μm) zircon grains occurring in the leucosomes display 3 textural domains: (I) dark-CL (cathodoluminescence) structured inner-core, (II) bright-CL structured outer-core, and (III) dark-CL structureless rim. Chemical compositions, especially chondrite-normalized REE patterns obtained by SIMS analysis, correlate with these three domains: the inner-core (domain-I) shows HREE-enrichment with Yb(n)/Gd(n) = 3.3, whereas the outer-core (II) and rim (III) have flat to relatively depleted HREE patterns with Yb(n)/Gd(n) = 0.7-0.8. Th/U ratios decrease from 3.2 in the zircon inner-core (I) to 1.2 in outer-core (II) and to 0.3 in the rim (III). Garnets near such zircon grains display two trace element compositional features. Firstly, high core Zr contents (300 ppm) decrease to 100 ppm within 50-100 μm of grain rims. Secondly, the HREE distribution between zircon inner-core (I) and garnet core is 2 at Gd (GdDZrn/Grt = 2), rising to 8 at Lu (LuDZrn/Grt = 8), whereas those defined from zircon outer-core (II) or rim (III) and garnet core or rim are much lower and generally below 1 for Gd thruogh to Lu (GdDZrn/Grt = 0.8-1.2; LuDZrn/Grt = 0.6-0.7). This marked change in the HREE distribution between zircon and garnet must reflect either a change in the minerals with which the zircon was growing or being modified, a change in the physical and chemical conditions of zircon growth, or a combination of the two. Based on comparisons with recent estimates of equilibrium zircon/garnet HREE distribution coefficients we infer that the inner-core (I) did not grow with the garnet that occurs in the paragneiss but grew within a garnet-absent melt that was then injected into the gneiss. The resulting leucosome then underwent wall-rock reaction with the enclosing garnet-bearing gneiss, causing a decrease in garnet to Zr contents to values approaching equilibrium with melt, and precipitating the zircon outer-core (II). Finally, the zircon rim (III) and later monazite formed in a HREE depleted environment. Melt injection, reaction and crystallization of the leucosomes took place within the time interval 2496-2471 Ma at the end of the UHT history of the Napier Complex.