We present in situ rutile and titanite U–Pb geochronology for three samples from the Ur breccia, which forms the boundary between the Malenco unit and the Margna nappe (Eastern Central Alps) near Pass d’Ur in southeast Switzerland. These sampled both oceanic brecciated material and a blackwall reaction zone in contact with a micaschist and serpentinized peridotite. Peak temperatures during Alpine metamorphism in these units were ~ 460 ± 30 °C. Textural observations combined with new geochronological data indicate that rutile and titanite both grew below their closure temperatures during Alpine metamorphism. We present a technique to calculate the most precise and accurate ages possible using a two-dimensional U–Pb isochron on a Wetherill concordia. Rutile from two samples gave a U–Pb isochron age of 63.0 ± 3.0 Ma. This age conflicts with previous 39Ar–40Ar data on heterogeneous amphiboles from which an age of 90–80 Ma was inferred for the high pressure part of the Alpine evolution, but is consistent with K–Ar ages and Ar–Ar ages on phengitic white mica. Titanite from three samples gave a U–Pb isochron age of 54.7 ± 4.1 Ma. This age is consistent with Rb–Sr isochron ages on mylonites along and in the footwall of the Lunghin–Mortirolo movement zone, a major boundary that separates ductile deformation in the footwall from mostly localized and brittle deformation in the hangingwall. Our ages indicate a Paleocene rather than upper Cretaceous metamorphism of the Pennine–Austroalpine boundary and permit at most ~ 15 Myr, and possibly much less, between the growth of rutile and titanite.

中文翻译：

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沿Penine-Austroalpine缝合线的古新世变质受钛矿和金红石U-Pb年代的约束（Malenco，阿尔卑斯山）

我们介绍了Ur角砾岩的三个样品的原位金红石和钛铁矿U-Pb年代学，这形成了Malenco单位与瑞士东南部Pass d'Ur附近的Margna纳普（东部中部阿尔卑斯山）之间的边界。这些都采样了海洋角砾岩材料和与云母和蛇纹石橄榄岩接触的黑墙反应区。这些单位在高山变质过程中的峰值温度约为460±30°C。纹理观察与新的年代学数据相结合表明，在高山变质过程中，金红石和钛矿都生长在其封闭温度以下。我们提出了一种在Wetherill协奏曲上使用二维U–Pb等时线来计算最精确和最准确年龄的技术。来自两个样品的金红石给出的U–Pb等时年龄为63.0±3.0 Ma。这个年龄与以前关于异质闪石的39Ar-40Ar数据相矛盾，从中推断出高山演化的高压部分的年龄为90-80 Ma，但与斑状白云母的K-Ar年龄和Ar-Ar年龄一致。 。来自三个样品的钛矿的U–Pb等时年龄为54.7±4.1 Ma。该年龄与Lunghin-Mortirolo运动带及其后壁上的镍铁矿的Rb-Sr等时年龄一致，该边界是将后壁的延性变形与上壁的局部变形和脆性变形分开的主要边界。我们的年龄表明，Pennine-Austroalpine边界是古新世而不是上白垩统变质，在金红石和钛铁矿的生长之间最多允许〜15 Myr，甚至可能少得多。