Variscan high-pressure metamorphism of kyanite-bearing paragneisses hosting eclogites in the Veporic unit, Western Carpathians: Evidence from Th–U–Pb dating of monazite
Variscan high-pressure metamorphism of kyanite-bearing paragneisses hosting eclogites in the Veporic unit, Western Carpathians: Evidence from Th–U–Pb dating of monazite
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西喀尔巴阡山脉 Veporic 单元中含有榴辉岩的含蓝晶石的副片麻岩的瓦里斯坎高压变质作用：来自独居石 Th-U-Pb 测年的证据
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Š. Méres
中科院分区：
文献类型：
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作者：
I. Petrik;M. Janák;T. Vaculovič;P. Konečný;Š. Méres
考虑到西喀尔巴阡山脉北部Veporic前阿尔卑斯基底的多相发育，从该地区榴辉岩中确定高压变质作用的年龄具有重要意义。为此，对几种含单氮石的副长岩进行了岩石学研究，并用化学（U-Th-Pb）法测定了单氮石的年代。鉴定出高压阶段的残余，即白云石（Si apfu = 3.2 ~ 3.3）、蓝晶石、金红石和富钙石榴石镶边。目前的矿物组合中，部分可能是前进阶段保存下来的（斜长石），部分反映了退退过程中的再平衡（黑云母，石榴石岩心中的主要元素）。然而，石榴石岩心中的Y和HREE并没有均匀化，它们的剖面表明它们是在升温过程中形成的。利用金红石、石榴石和独居石中的锆（zr -in-金红石）、钇（y -in-石榴石）和独居石的测温、辉云石组成和伪剖面模拟，计算了变质峰条件（P = 21 kbar, T = 650℃）和退变阶段（P = 9 kbar, T = 520℃）。独居石以早石炭世（354 Ma）为主，奥陶系（485 Ma）和寒武系（505 Ma）次之，无高山记录。石榴石岩心中独居石的位置早于Ca边缘的生长，表明石炭纪354 Ma的年龄可能早于峰值条件，是Variscan变质演化的进阶阶段。
In view of a polyphase development of the northern Veporic pre-Alpine basement of the Western Carpathians it is important to determine the age of high pressure (HP) metamorphism known from eclogites occurring in this area. To do this, several monazite-bearing paragneisses were studied petrologically and monazite was dated by chemical (U–Th–Pb) method. Identified were remnants from high-pressure stage, i.e. phengite (Si apfu = 3.2–3.3), kyanite, rutile and Ca-rich garnet rims. Part of the present mineral assemblage may have been preserved from prograde stage (plagioclase) and part reflects re-equilibration during retrogression (biotite, major elements in garnet cores). However, Y and HREE in garnet cores were not homogenised and their profiles indicate origin during temperature rise. Peak metamorphic conditions (P = 21 kbar; T = 650 °C), and retrogression stage (P = 9 kbar; T = 520 °C) were calculated using Zr-in-rutile, and Y-in-garnet and monazite thermometry, phengite composition and pseudosection modelling. Monazite yielded dominant Early Carboniferous age (354 Ma) with subordinate amounts of Ordovician (485 Ma) and Cambrian (505 Ma) ones, and no Alpine record. The monazite position in garnet cores predating the growth of Ca rims suggest that the Carboniferous age of 354 Ma probably predates the peak conditions and refers to the prograde stage of the Variscan metamorphic evolution.