Resistance furnace 40Ar / 39Ar step heating dating of amphibole separates from partially retrogressed High-temperature/Medium-pressure gneisses from a 250 km long transect through the Early Proterozoic Lapland - Kola Orogen yielded both flat and disturbed age spactra. In all cases careful handpicking produced pure amphibole separates for irradiation.
About half of the spectra are flat over at least 90% of 39Ar release with integrated ages between 1.80 and 1.95 Ga and 37ArCa / 39ArK ratios that are essentially constant. These samples are homogeneous (potassic)hastingsitic, pargasitic, edenitic or tschermakitic hornblendes, that are devoid of late-stage retrograde reactions of any importance.
A small subgroup consists of spectra that have apparent ages around 2.6 Ga at intermediate degassing temperature and much lower apparent ages during early and late 39Ar release. The 37ArCa / 39ArK ratio spectra are also dome-shaped. These age spectra are exclusively obtained on samples from Late Archean gneisses that are only slightly influenced by the Early Proterozoic tectono-metamorphic processes, i.e. the Murmansk Domain of the northern Kola Peninsula.
The majority of the disturbed age spectra are characterised by progressively decreasing apparent ages. The main part of these spectra is relatively flat with apparent ages that are typically 5-10% higher than those of the flat age spectra from the same region. However, the apparent ages sharply drop during the final 20-25% of 39Ar release, to apparent ages as low as 1.5 to 1.3 Ga, whereas early degassing increments may yield significantly higher apparent ages. The 37ArCa / 39ArK ratio spectra closely follow the evolution of the apparent ages, although some samples have the lowest 37ArCa / 39ArK ratios during the first 5% of 39Ar release.
All amphiboles with disturbed age spectra are obtained from samples affected by some late-stage retrograde growth of Fe-rich epidote, Na-rich plagioclase, biotite and chlorite. Amphiboles are generally zoned. Atomic number contrast electron back-scatter images reveal that the second type of disturbed amphiboles consist of hornblende with microscopic intergrowths of actinolite as patches or along crack-like zones and (sub)grain boundaries. These intergrowths occur at the scale of several mm to several ten of mm. Electron probe microanalyses show that apart from inverse Si-Al and Mg-Fe variation, Na and K progressively decrease towards actinolite-rich zones, whereas the Ca content remains constant or slightly increases. The K content of
actinolite may be nearly an order of magnitude lower compared to hornblende.
As a result of the chemical changes related to the microscopic actinolite growth a major K contrast occurs over distances of a few mm only. During neutron irradiation recoil is likely to have transferred 39ArK from the K-rich hornblende into the K-poor actinolite. As there is no major Ca-contrast between the two intimately intergrown amphiboles 37ArCa recoil did not produce major disturbances. Degassing of actinolite, that has gained 39ArK by recoil, at higher temperatures than hornblende can explain the abruptly decreasing 37ArCa / 39ArK and 40Ar* / 39ArK ratios during advanced degassing. Accordingly, apparent ages of degassing steps dominated by hornblende are slightly higher, with respect to the ages indicated by the flat age spectra.
The low 37ArCa / 39ArK ratio during early 39Ar release of a number of samples is due to degassing of tiny biotite
inclusions, that are observed along the cleavage and grain boundaries on electron back-scatter images.