Combined U-Pb geochronology, and trace element analysis (Th, U, Y, Sr, rare earth elements: REE) by LA-Q-ICPMS and halogen chemistry (F, Cl, OH) by SEM-EDX are applied to apatite from a suite of amphibolite- and granulite-facies xenoliths from the Bearpaw Mountains (Montana, USA), to constrain cooling of Paleoproterozoic mid- to lower crust. Xenoliths hosting fluorapatite and hydroxyfluorapatite were emplaced at 54 to 50 Ma within K-rich, host lamprophyres (“minettes”). Granulite xenoliths yield apatite with highly dispersed U-Pb isotopic compositions. But discordia fit through their youngest populations yield Neoproterozoic-Cambrian (c. 650–500 Ma) lower intercepts. A single amphibolite xenolith from a shallower depth contains apatite with older single-grain apatite ages in the range of 1200–890 Ma. These lower intercept ages are much younger than previously reported zircon U--Pb ages from the same samples, which register a thermal maximum in the Palaeoproterozoic (c. 1700–1800 Ma). Trace element analysis of apatite confirms that most grains did not undergo retrogressive recrystallization. Additionally, there is a statistically significant link between apatite size and single grain ages in at least one sample. Our interpretation is that these apatite U-Pb ages represent slow cooling through the apatite Pb partial retention zone (c. 375–450 °C). Whether these data represent a distinct cooling event, or simple protracted cooling from a Palaeoproterozoic thermal peak, cannot be strictly constrained by these data. However, previous evidence from rutile U-Pb in xenoliths from nearby locations implies slow cooling from a Palaeoproterozoic peak. If a slow cooling scenario is assumed, by comparing apatite cooling ages to previous results from Ti-in-zircon thermometry on metamorphic zircon from the same xenoliths, a cooling rate of 0.14--0.33 °C/Myr can be estimated for these granulite xenoliths. Our apatite data thus contribute to a growing consensus on the geodynamic history of the northern Wyoming Craton and Medicine Hat Block, wherein lithospheric stability was maintained on a giga-year timescale from a granulite-facies metamorphic event associated with the assembly of Laurentia up until Laramide orogenesis.

结合U-Pb年代学和LA-Q-ICPMS的痕量元素分析（Th，U，Y，Sr，稀土元素：REE）和SEM-EDX进行卤素化学分析（F，Cl，OH）用于磷灰石中的磷灰石来自Bearpaw山脉（美国蒙大纳州）的一组角闪石-和花岗石相的异岩，以限制古元古代中下地壳的冷却。富含氟磷灰石和羟基氟磷灰石的Xenoliths放置在54至50 Ma的富含钾的宿主鳞茎植物（“小片”）中。粒状异种石产生具有高度分散的U-Pb同位素组成的磷灰石。但是，Discordia在其最年轻的种群中的适合度使新元古代-寒武纪（约650-500 Ma）的截距降低。深度较浅的单个角闪石异岩包含的磷灰石具有较老的单粒磷灰石年龄，范围在1200–890 Ma之间。这些较低的截距年龄比以前报道的相同样品的锆石U-Pb年龄要年轻得多，后者在古元古代（约1700–1800 Ma）记录了最大的热值。磷灰石的痕量元素分析证实，大多数晶粒未经历回退重结晶。另外，在至少一个样品中，磷灰石尺寸与单晶年龄之间存在统计学上的显着联系。我们的解释是，这些磷灰石U-Pb年龄代表通过磷灰石Pb部分保留区（约375-450°C）的缓慢冷却。这些数据是代表明显的冷却事件，还是来自古元古代热峰的简单持续冷却，都不能严格地受这些数据约束。然而，先前来自附近位置的异种岩中金红石型U-Pb的证据表明从古元古代峰开始缓慢冷却。如果假设采用慢速冷却方案，则通过将磷灰石的冷却年龄与以前在相同异种岩中对变质锆石进行锆钛矿测温得到的结果进行比较，可以得出这些粒状异种岩的冷却速率为0.14--0.33°C / Myr 。因此，我们的磷灰石数据有助于人们就怀俄明州克拉通和梅迪辛哈特北部地块的地球动力学历史达成越来越多的共识，其中岩石圈的稳定性从与Laurentia的组装相关的粒岩相变质事件直到拉拉米德（Laramide）一直保持在千兆年的时间尺度上。造山运动。