Dating prograde fluid pulses during subduction by in situ U-Pb and oxygen isotope analysis
Dating prograde fluid pulses during subduction by in situ U-Pb and oxygen isotope analysis
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DOI：
10.1007/s00410-015-1226-4
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发表时间：
2016-02-01
影响因子：
3.5
通讯作者：
Hermann, Joerg
中科院分区：
文献类型：
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作者：
Gauthiez-Putallaz, Laure;Rubatto, Daniela;Hermann, Joerg
多拉-迈拉白片岩来自交代蚀变花岗岩，在阿尔卑斯山造山过程中经历了类似于750℃和40Kbar的超高压变质作用。为了研究白片岩的P-T-时间-流体演化，我们获得了锆石和独居石的U-Pb年龄，并结合副矿物和共生石榴石的微量元素组成和氧同位素。锆石岩芯是花岗岩原岩的唯一残留物，至今仍保存着二叠纪时代、岩浆微量元素组成和约10%的O-18三角洲。富硅和贫硅白片岩组分的热力学模拟表明，在20-40kbar的进行性俯冲过程中，存在两个主要的流体脉冲。在贫硅样品中，绿泥石分解为石榴石+流体的温度约为22kbar。直接覆盖岩心的第一个锆石边缘含有进行性金云母包裹体和富含HREE的图案，表明在石榴石形成之初就有锆石生长。当滑石+蓝晶石与石榴石+柯石英+流体反应时，在富硅白片岩和贫硅白片岩中都记录到了接近峰期变质条件的第二次主要流体脉冲。在富硅白片岩中观察到二次变质过度生长，并亏损HREE，而在富硅白片岩中观察到捕获多硅白云母和滑石包裹体的单一变质过度生长。石榴石、锆石和独居石对氧的化学和同位素平衡，表明它们都形成于35 Ma的峰期变质作用，受独居石(34.7+/-0.4 Ma)和锆石(35.1+/-0.8 Ma)年龄的制约。贫硅白片岩中的前进型锆石边缘的年龄在误差范围内，与峰期变质条件下的年龄难以区分，与Dora-Maira单位的最小俯冲速度2厘米/年一致。锆石、独居石和石榴石的氧同位素值在6.4+/-0.4%误差范围内相等，符合闭合体系平衡分馏。由此产生的均衡。O-18(锆石独居石)在700+/-20摄氏度时为千分之0.1+/-0.7。现场的氧同位素数据与白片岩的外部流体输入相抵触。相反，流体辅助的锆石和独居石重结晶可能与前进俯冲过程中的内部脱水反应有关。我们认为，影响花岗岩原岩的主要交代事件与侏罗纪裂谷作用之后的海底热液改造有关，远在阿尔卑斯俯冲开始之前。
The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahighpressure metamorphism at similar to 750 degrees C and 40 kbar during the Alpine orogeny. In order to investigate the P-T-time-fluid evolution of the whiteschists, we obtained U-Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and delta O-18 of similar to 10 %. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at similar to 22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 +/- 0.4 Ma) and zircon rims (35.1 +/- 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/ year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 +/- 0.4 %, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium. Delta O-18(zircon-monazite) at 700 +/- 20 degrees C is 0.1 +/- 0.7 parts per thousand. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluidassisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction.