Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China
Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China
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伟晶岩浆同化分异结晶过程中的锆石微量元素和同位素（Sr、Nd、Hf、Pb）效应——以北秦岭造山带广石沟黑云母伟晶岩为例
DOI：
10.1016/j.lithos.2017.12.022
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发表时间：
2018-03
期刊：
影响因子：
3.5
通讯作者：
Yang Shang-Song
中科院分区：
文献类型：
--
作者：
Feng Yuan;Jia-Jun Liu;Emmanuel John M. Carranza;Shuai Zhang;De-Gao Zhai;Gang Liu;Gong-Wen Wang;Hong-Yuan Zhang;Ya-Zhou Sha;Yang Shang-Song
北秦岭造山带广石沟黑云母伟晶岩中存在与围岩同化伴生分馏结晶（AFC）有关的开放体系岩浆作用的证据。黑云母伟晶岩-片麻岩接触面一般与铀、钍最富集部位重合。采用单键dpb定年法确定了黑云母伟晶岩的结晶年龄（边缘区-415±2.6 Ma，内部区-413.5±2.5 Ma），与黄铁矿pb单键dpb等时年龄（413±22 Ma）一致。锆石变质区微量元素含量普遍升高，放射性铅排出。内带样品为未污染岩浆，锆石(~ 413 Ma) εHf(t)为负-正（- 1.53 ~ + 3.24），εNd(t)值较低（- 2.4），Hf和Nd模式年龄较老（tDM2= 1.5 ~ 1.19 Ga, T2DM= 1.35 Ga），表明源区中元古代下地壳物质以再循环为主，有部分幼体物质参与。岩浆氧逸度（fO2）和结晶温度分别为logfo2的- 24.81 ~ - 13.34℃和570 ~ 793℃。与内部带相比，辉晶岩边缘样品的εNd(t)值（- 3.9 ~ - 2.8）和T2DM值（1.47 ~ 1.37 Ga）变化较小，但Hf同位素组成相似，有利于三组分同位素混合模式（循环中元古代下地壳物质、幼年物质和主片麻质）。锆石中Ti、Y、U、Th、Hf和REE的浓度在单个样品的不同颗粒之间以及在单个颗粒内不同区域之间的显著变化表明，杂化岩浆中存在波动的结晶环境，而边缘锆石正是在这种环境中结晶的。杂化岩浆中黄铁矿形成的可变和高放射性成因铅比值是由基质遗传而来的。两个带的锆石Hf同位素模式相似，表明其在岩浆杂化早期就已结晶。然而，在个别伟晶岩中，不均匀的Nd同位素组成暗示了来自岩壁的熔体与较少放射性成因的Nd的结合。
Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon Usingle bondPb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415 ± 2.6 Ma; internal zone-413.5 ± 2.5 Ma), in line with a pyrite Pbsingle bondPb isochron age (413 ± 22 Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon (~ 413 Ma) εHf(t) (− 1.53 − + 3.24), low εNd(t) values (− 2.4), and old Hf and Nd model ages (tDM2= 1.5–1.19 Ga, T2DM= 1.35 Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from − 24.81 to − 13.34 of logfO2and 570 °C to 793 °C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (− 3.9 to − 2.8) and T2DM(1.47–1.37 Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.
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影响因子：
1.4
作者：
Michelle McKeough;D. Lentz;C. McFarlane;J. Brown
通讯作者：
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影响因子：
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通讯作者：
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影响因子：
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作者：
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通讯作者：
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