ABSTRACT

Lithological, petrological and structural observations and measurements of graphite thermometry, Si-in-phengite barometry, and ⁴⁰Ar/³⁹Ar phengite thermochronology were performed along two transects across the south-central Brooks Range of northern Alaska. The integrated data set reveals a history of accretion, protracted thermal overprinting of high-pressure/low-temperature (HP/LT) metamorphic assemblages, and subsequent exhumation. Tectonic domains characterized by upper greenschist facies metamorphic rocks display a dominant E-W axial planar cleavage/foliation fabric that overprints bedding and earlier metamorphic foliations. Raman maturation thermometry and Si-in-phengite barometry document a southward increase from greenschist facies conditions (ca. 360–400°C; 3–7 kbar) in the Doonerak window and Endicott Mountains allochthon to epidote-blueschist facies conditions (~515°C and 10–13 kbar) within the Schist Belt. Within the Central and Schist belts albite porphyroblasts overgrow the earlier deformation fabric and quartz microstructures exhibit evidence for diffusive recovery. This indicates thermal overprinting of deeper portions of the accretionary complex caused protracted (ca. 25 m.y.) greenschist facies overprinting of HP/LT assemblages within the Ar partial retention zone. The Schist Belt – Central Belt boundary is defined by a singular, penetrative foliation that displays a weak shear-band fabric. ⁴⁰Ar/³⁹Ar results along this boundary indicate that the fabric records deformation along the lower boundary of a nappe that began to move up the subduction channel as subduction ceased by ~135-130 Ma. Subsequent deformation along the Schist Belt – Phyllite Belt boundary reflects the exhumation of HP/LT rocks along a south-dipping normal-sense shear zone. Phengite ⁴⁰Ar/³⁹Ar age gradients coupled with the first arrival of coarse-grained Brooks Ranges detritus within the adjacent Yukon Koyukuk Basin indicate that tectonic denudation along this shear zone partly exhumed the Schist and Phyllite belts to mid-crustal depths by 110–105 Ma. Final exhumation of the HP/LT rocks was delayed until the Cenozoic which resulted in < 15 km of additional denudation.

摘要

岩性、岩石学和结构观察和石墨温度测定法、硅铁含量测定法和⁴⁰ Ar/ ³⁹沿着横跨阿拉斯加北部布鲁克斯山脉中南部的两条横断面进行了 Ar phengite 热年代学。综合数据集揭示了高压/低温 (HP/LT) 变质组合的吸积、长期热叠印和随后的挖掘历史。以上绿片岩相变质岩为特征的构造域显示出占主导​​地位的 EW 轴向平面解理/叶理结构，覆盖层理和早期变质叶理。拉曼成熟温度测定法和硅酸镁气压测定法记录了从 Doonerak 窗口和恩迪科特山脉异质带中的绿片岩相条件（约 360–400°C；3–7 kbar）向南增加到绿帘石-蓝片岩相条件（~515° C 和 10-13 kbar）在片岩带内。在中央和片岩带内，钠长石斑状细胞过度生长，早期变形结构和石英微结构显示出扩散恢复的证据。这表明增生复合体较深部分的热套印导致了在 Ar 部分保留区内 HP/LT 组合的长期（约 25my）绿片岩相套印。片岩带 - 中央带边界由显示弱剪切带结构的单一穿透叶理定义。沿该边界的⁴⁰ Ar/ ³⁹ Ar 结果表明，当俯冲停止约 135-130 Ma 时，织物记录沿推覆下边界的变形，推覆开始向上移动到俯冲通道。随后沿片岩带-千枚岩带边界的变形反映了 HP/LT 岩石沿南倾法向剪切带的折返。Phengite ⁴⁰ Ar/ ³⁹Ar 年龄梯度加上粗粒布鲁克斯山脉碎屑在邻近育空 Koyukuk 盆地内的首次到达表明，沿该剪切带的构造剥蚀使片岩和千枚岩带在 110-105 Ma 时部分地挖掘到地壳中部深度。HP/LT 岩石的最终挖掘被推迟到新生代，这导致了 <15 公里的额外剥蚀。