Protracted fluid-induced melting during Barrovian metamorphism in the Central Alps
Protracted fluid-induced melting during Barrovian metamorphism in the Central Alps
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DOI：
10.1007/s00410-009-0406-5
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发表时间：
2009-05
影响因子：
3.5
通讯作者：
D. Rubatto;J. Hermann;A. Berger;M. Engi
中科院分区：
文献类型：
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作者：
D. Rubatto;J. Hermann;A. Berger;M. Engi
利用锆石年代学和微量元素组成研究了阿尔卑斯山脉中部典型Barrovian层序中流体熔融的时间和动力学。浅色体和围岩中的多个锆石畴产生的U-Pb年龄跨度为~32 ~ 22 Ma。根据年龄、Th/U（<0.1）和微量元素组成，可将阿尔卑斯山熔融形成的锆石与继承性和碎屑岩芯区分开来。锆石中的钛温度测量表明结晶温度约为620-700°C。它们的组成允许区分（1）在早期石榴石存在下形成的锆石，（2）与丰富的L-MREE丰富的副相（褐帘石，钛铁矿和磷灰石）的典型的变质岩平衡的锆石，和（3）在变质沉积物熔融过程中形成的锆石在辉长岩为主的组合。锆石增生和年龄的分布表明，在一个大致恒温的巴罗变质旋回内，曾发生过多次熔融事件，围岩中锆石的形成仅限于熔融的初始阶段，而进一步熔融则集中在分离的浅色体中;由于当地的岩石成分和流体的局部流入，几米之外的样品在不同的时间发生了熔融;当有液体时，白细胞体会反复融化。地质年代学数据迫使在中央阿尔卑斯山的混合岩带的温度-时间路径的修订。在10百万年以上的岩浆熔融之后，同一区域内的阿尔卑斯榴辉岩快速折返，然后快速冷却到地壳上层，冷却速度为100°C/Ma。
The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U–Pb ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U (<0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization temperatures around 620–700°C. Their composition allows discriminating between (1) zircon formation in the presence of early garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites, and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature; that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available. The geochronological data force a revision of the temperature–time path of the migmatite belt in the Central Alps. Protracted melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling in the order of 100°C/Ma to upper crustal levels.