Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps, Austria), resolved by automated SEM methods and EMP–Th–U–Pb monazite dating
Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps, Austria), resolved by automated SEM methods and EMP–Th–U–Pb monazite dating
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DOI：
10.1111/jmg.12224
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发表时间：
2017-02
影响因子：
3.4
通讯作者：
B. Schulz
中科院分区：
文献类型：
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作者：
B. Schulz
奥阿尔卑斯山Saualpe Eclopian单元（奥地利Kärnten，东阿尔卑斯山）的多变质石榴石云母片显示出复杂的显微结构和矿物化学关系。自动扫描电子显微镜程序与能量色散X射线（EDX）光谱映射应用于独居石检测和石榴石矿物化学表征。当Fe、Mg、Mn和Ca元素wt%组成用作石榴石EDX光谱的通用标记时，复杂的环带和成斑晶世代可以在完整的薄切片中解析，用于选择性电子探针分析。在低钙和高铝变晶岩中发现了两个石榴石变斑晶世代和不同的独居石年龄种群。石榴石1具有从核到边缘的降低的Mn、恒定的Ca和显著增加的Mg。石榴石1组合的地质温压测量表明，在~650 °C/6-8 kbar下，结晶沿着M1逆冲变质作用。独居石1在c. 320毫安。随后在减压过程中普遍形成的300-250 Ma高Y和高Gd独居石1与二叠纪和早三叠世伟晶岩的侵入相吻合。独居石1沿着石榴石1的边缘结晶。320-250 Ma独居石周围磷灰石和褐帘石的日冕标志着一个退变质阶段。这些显微结构表明，石炭纪至早二叠世的年龄为M1事件与石榴石1。这样一个中等P/T梯度的M1事件尚未在Saualpe中描述，并且在二叠-三叠纪低P阶段之前。含石榴石2的M2事件发生在二叠纪独居石周围的日冕形成之后。石榴石2显示首先增加XCa在减少XMg，然后增加XCa和XMg，并最终减少XCa与增加XMg，总是在高Ca和Mg，和低Mn。这记录了经历榴辉岩相条件的P-T演化，并在减压加热期间达到约750 °C/14 kbar的最高温度。在白垩纪（Eo-Alpine）变质作用M2期间，在高P/T梯度下，Y和Gd含量较低的独居石2种群（94-86 Ma）在降低的压力下结晶。Saualpe Ecological单元在不同的P-T条件下经历了两个不同的顺时针变质旋回，与不同热体制下的大陆碰撞有关。这导致了该单元独居石年龄的特征分布模式，这是不同于其他奥南基底地区。
Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Kärnten, Austria, Eastern Alps) display complex microstructural and mineral–chemical relationships. Automated scanning electron microscopy routines with energy dispersive X‐ray (EDX) spectral mapping were applied for monazite detection and garnet mineral–chemical characterization. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron‐microprobe analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low‐Ca and high‐Al‐metapelites. Garnet 1 has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism at ~650 °C/6–8 kbar. Sporadic monazite 1 crystallization started at c. 320 Ma. Subsequent pervasive 300–250 Ma high‐Y and high‐Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320–250 Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous‐to‐Early‐Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate‐P/T gradient has not yet been described from the Saualpe, and preceded a Permo‐Triassic low‐P stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing XCa at decreasing XMg, then increasing XCa and XMg, and finally decreasing XCa with increasing XMg, always at high Ca and Mg, and low Mn. This records a P–T evolution which passed through eclogite facies conditions and reached maximum temperatures at ~750 °C/14 kbar during decompression‐heating. A monazite 2 population (94–86 Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo‐Alpine) metamorphism M2 at a high‐P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P–T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas.