Paleoproterozoic metamorphism in the northern Fennoscandian Shield: age constraints revealed by monazite

We have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling. U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76‒1.81 Ga with only a few older exceptions in samples showing a spread of ²⁰⁷Pb/²⁰⁶Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum. In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86‒1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of ²⁰⁷Pb/²⁰⁶Pb ages from 1.89‒1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80‒1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of ²⁰⁷Pb/²⁰⁶Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86‒1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes.

我们在芬诺斯坎迪亚地盾（Fennoscandian Shield）北部识别出两种截然不同的古元古代（Paleoproterozoic）变质作用类型。卡累利阿省（Karelia Province）与拉普兰-科拉省（Lapland-Kola Province）出露太古代（Archean）基底及上覆古元古代表壳岩，发育典型的中压巴罗式变质作用（Barrovian-style metamorphism），常见含蓝晶石的矿物组合与等温减压（isothermal decompression，ITD）压力-温度路径。相邻的斯维科芬省（Svecofennia Province）则表现为低压高温的巴肯式变质作用（Buchan style），产出石榴石-堇青石混合岩，以及夹层状红柱石-堇青石片岩、红柱石-十字石片岩与矽线石-白云母片麻岩。其退变质压力-温度路径仅显示冷却过程中存在中等幅度的抬升。研究团队采用激光剥蚀电感耦合等离子体质谱法（LA-ICP-MS）对变沉积岩的80余件样品中的独居石（monazite）开展U-Pb定年，采样覆盖了芬兰境内绝大多数古元古代基岩区域。分析结果显示存在三个年龄峰值：约1.91 Ga、1.86~1.88 Ga以及1.79~1.81 Ga。其中最古老的约1.91 Ga独居石主要产于芬兰最北部的拉普兰-科拉省。卡累利阿省的古元古代地层以太古代基岩为基底，该省产出的独居石年龄集中在1.76~1.81 Ga，仅少数样品存在例外，其²⁰⁷Pb/²⁰⁶Pb年龄分布于约1.92~1.81 Ga之间。卡累利阿省经历了构造加厚作用，约1.80 Ga的独居石年龄大多代表温度峰值附近的地壳折返过程。斯维科芬省的独居石年龄范围为约1.89~1.78 Ga：在芬兰西部次级省，高级变质混合岩中的独居石年龄多为1.86~1.88 Ga，但在较古老的混合岩区域内存在低变质带，其中独居石年龄约为1.80 Ga；部分样品的²⁰⁷Pb/²⁰⁶Pb年龄分布范围为1.89~1.86 Ga至约1.78 Ga。在芬兰南部次级省，多数样品的年龄要么集中在1.80~1.78 Ga（尤其在红柱石级片岩中），要么其²⁰⁷Pb/²⁰⁶Pb年龄分布于约1.88~1.78 Ga之间；仅在芬兰南部次级省的东部区域，岩石仅产出1.86~1.89 Ga的年龄数据。斯维科芬省发育低压高温变质作用且缺乏高或中等P/T比的岩石，其变质作用更倾向于增生型造山作用而非碰撞型造山作用。