An investigation of a polymetamorphic terrain using 40Ar-39Ar geochronology.
This research is an exploration both of the Precambrian metamorphic geology of south-western
Montana and the Wlaser ablation microprobe 4OAt_39At geochronological technique. Using the
high spatial resolution of the UV laser microprobe it has been possible to produce systematic 4OAt_
39Arages within individual mica grains. Further, the UV laser has been used to drill into biotite
inclusions within garnet to look at the shielding effect of the garnet lattice, and to drill depth profiles
into garnet to measure helium diffusion in laboratory experiments.
The south-western Montana region of the Precambrian Wyoming Province was believed to have
undergone at least three separate periods of metamorphism: M1- granulite facies in the Archaean,
M2 - amphibolite facies in the Early Proterozoic, and M3 - greenschist to epidote-amphibolite facies
in the Mid-Proterozoic. Because of the relatively low blocking temperature of the 40Ar- 39Ar
geochronological system in mica (c.300-350 °C), it is readily reset by regional metamorphic events
and previous K-Ar and 4OAr_38
Atgeochronology had found that the Early Proterozoic event
dominated the ages obtained.
UV laser 40Ar-39At dating of the matrix mica constrained the timing of cooling from the Early
Proterozoic metamorphic event to between 1780 to 1740 Ma with a cooling rate between 1 and 8
°C/m.y. 4OAr_39Ar analyses of individual biotite inclusions in garnet also produced similar ages.
However, Pb-Pb step leach dating of a small subset of garnet yielded ages between 1808 and 1765
Ma, demonstrating that the garnet did not grow during an Archaean event but, rather, during the
Early Proterozoic metamorphic event. Thus, the shielding properties of garnet on biotite inclusions
could not be easily assessed in these samples. However, where matrix biotite had been partially
reset by the Cretaceous plutons, there was some evidence to suggest that a minority of the biotite
inclusions in the same sample had been shielded from resetting. The influence of fractures, defects
and other fast diffusion paths is believed to have prevented most of the inclusions from being
shielded. No evidence was found to show that the rocks in south-westem Montana were metamorphosed during the Archaean and it seems likely that the M1 and M2 events were not greatly
separated in time and were both Early ProterozoiC in age.
The high spatial resolution of the UV laser microprobe was used in order to date highly altered
biotite within rocks that had undergone later (M3) greenschist metamorphism. Biotite was
interlayered with chlorite, clinozoisite and prehnite but using the UV laser it was possible to
separately analyse areas of unaltered biotite and areas of alteration within a single mineral and
produce ages from both. It was thus possible to measure two ages from one sample: an unaltered
age consistent with the timing of the Early Proterozoic metamorphism, and a younger age that could
be linked to the c.1S00-1360 Ma timing of Belt basin formation to the north and west of the region.
This technique therefore was able to overcome the difficulties associated with producing meaningful
ages from altered samples whilst constraining the timing of the M3 metamorphic event.