Zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh-pressure jadeite quartzite in the Dabie orogen
Zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh-pressure jadeite quartzite in the Dabie orogen
复制标题
锆石地球化学记录了大别造山带变质流体对超高压硬玉石英岩形成的作用
DOI：
10.1016/j.chemgeo.2015.10.043
复制
发表时间：
2015-12
期刊：
影响因子：
3.9
通讯作者：
Li, W.C.
中科院分区：
文献类型：
--
作者：
Zheng, Y.F.;Chen, Y.X.;Tang, H.L.;Li, W.C.
对大别造山带含柯石英的硬玉石英岩中的锆石进行了矿物包裹体、U-Pb年龄、微量元素和Hf-O同位素的综合研究。研究结果揭示了超高压（UHP）变质流体在大陆深俯冲至地幔深度过程中的作用，从而对大陆俯冲带硬玉石英岩的成因提出了制约。在阴极发光图像中，锆石多表现为核边结构。增生的边缘含有罕见的矿物包裹体，并表现出一致的U-Pb年龄为225至246 Ma和平坦的重稀土配分模式，可忽略Eu异常。残留岩芯则含有柯石英、硬玉和金红石等超高压变质矿物包裹体，U-Pb年龄为983 ~ 2045 Ma，稀土配分模式陡峭，Eu负异常显著。所有岩芯和边缘的U-Pb同位素数据确定了一条明显的不一致线，其上、下交点年龄分别为2000 ± 43 Ma和234 ± 18 Ma。三叠纪时代的边缘为变质成因的新生代，前寒武纪时代的核部为岩浆成因的原岩遗迹。残余岩浆锆石经历了两种变质重结晶作用，固态转变和交代蚀变。固态重结晶锆石的U-Pb年龄接近原岩年龄，MREE-HREE模式陡峭，Hf同位素比值几乎不变。这些观测结果表明，原岩锆石的地球化学成分的重置程度最低。而交代重结晶锆石在原岩锆石稀土元素组成和U-Pb、Lu-Hf同位素体系中表现出部分重置。所有的锆石域，无论边缘和核心，显示相对一致的δ 18 O值为4.0 ± 0.2‰。这种一致性不仅表明变质流体来自深俯冲的陆壳内部，而且还表明原岩锆石的氧同位素组成与三叠纪超高压变质流体重新平衡。封闭微裂隙中柯石英、金红石、硬玉等超高压包裹体矿物的出现，表明原岩锆石发生了交代重结晶作用。在超高压条件下，流体交代作用首先发生在陆壳俯冲过程中残留锆石的沿着裂隙发生，然后发生变质重结晶作用，导致裂隙封闭。交代重结晶作用使原岩锆石的U-Pb和Lu-Hf同位素体系发生了不均一的重置。残留锆石核内包裹体矿物组成表明，变质流体中富含Si、Ti、Na和Al，这些元素是通过上覆花岗质正片麻岩的变辉岩交代反应获得的。因此，硬玉石英岩可能是在大陆俯冲带变质作用中，由基底正片麻岩脱水生成的超高压变质流体与变质杂岩发生反应而沉淀形成的。
A combined study of mineral inclusions, U–Pb ages, trace elements and Hf-O isotopes was carried out for zircons from a coesite-bearing jadeite quartzite in the Dabie orogen. The results provide insights into the action of ultrahigh-pressure (UHP) metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone. The zircons mostly show core-rim structures in cathodoluminescence images. The overgrown rims contain rare mineral inclusions, and exhibit concordant U–Pb ages of 225 to 246 Ma and flat HREE patterns with negligible Eu anomalies. In contrast, the relict cores contain UHP metamorphic mineral inclusions such as coesite, jadeite and rutile, and show discordant U–Pb ages ranging from 983 to 2045 Ma and steep REE patterns with significant negative Eu anomalies. The U–Pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000 ± 43 Ma and 234 ± 18 Ma, respectively. We interpret the rims with Triassic ages as the new growth of metamorphic origin and the cores with Precambrian ages as the protolith relics of magmatic origin. The relict magmatic zircons underwent two subtypes of metamorphic recrystallization, i.e., solid-state transformation and metasomatic alteration. The solid-state recrystallized zircons exhibit slightly discordant U–Pb ages close to the protolith age, steep MREE–HREE patterns, and almost unchanged Hf isotope ratios. These observations point to the lowest degree of resetting to the geochemical composition of protolith zircons. In contrast, the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon REE composition and U–Pb and Lu–Hf isotopic systems. All of the zircon domains, regardless of the rims and cores, show relatively consistent δ18O values of 4.0 ± 0.2‰. Such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the UHP metamorphic fluids of Triassic age. The metasomatic recrystallization of protolith zircons is indicated by the occurrence of UHP inclusion minerals such as coesite, rutile and jadeite in sealed microcracks. In this regard, the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the UHP conditions. As such, the metasomatic recrystallization has heterogeneously reset the U–Pb and Lu–Hf isotope systems of protolith zircons. The composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in Si, Ti, Na and Al. These elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss. Therefore, the jadeite quartzite would be precipitated from the UHP metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction-zone metamorphism.
登录
查看更多内容
影响因子：
5
作者：
Hoskin, PWO
通讯作者：
Hoskin, PWO
影响因子：
4.5
作者：
Geisler, Thorsten;Schaltegger, Urs;Tomaschek, Frank
通讯作者：
Tomaschek, Frank
DOI：
10.1007/s11430-013-4763-6
发表时间：
2013
期刊：
Science China Earth Sciences
影响因子：
--
作者：
Yanfei Zhang;Yao Wu;Chao Wang;Zhenmin Jin;H. Schertl
通讯作者：
Yanfei Zhang;Yao Wu;Chao Wang;Zhenmin Jin;H. Schertl
影响因子：
4.5
作者：
D. Rubatto;J. Hermann
通讯作者：
D. Rubatto;J. Hermann
影响因子：
2.1
作者：
T. Hirajima;R. Compagnoni
通讯作者：
T. Hirajima;R. Compagnoni