کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
4732908 | 1640487 | 2016 | 16 صفحه PDF | دانلود رایگان |
![عکس صفحه اول مقاله: Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA](/preview/png/4732908.png)
• The age of a regional Acadian (S1) foliation is (411–407 Ma).
• The age of the foliation (S2) in a local shear zone is (377–381 Ma).
• S1 was produced by shortening associated with the arrival of Avalon terrane.
• S2 was produced by dextral transpression caused by the arrival of Meguma terrane.
We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.
Journal: Journal of Structural Geology - Volume 93, December 2016, Pages 51–66