Ar-40/Ar-39 study of plagioclases from the Rogaland anorthosite complex (SW Norway); an attempt to understand argon ages in plutonic plagioclase

Boven, A.; Pasteels, P.; Kelley, S.P.; Punzalan, L.; Bingen, B. and Damaiffe, D. (2001). Ar-40/Ar-39 study of plagioclases from the Rogaland anorthosite complex (SW Norway); an attempt to understand argon ages in plutonic plagioclase. Chemical Geology, 176(1-4) pp. 105–135.



The combined use of stepwise-heating and UV-laser ablation Ar-40/Ar-39 techniques with microprobe analyses on plutonic plagioclase allows the investigation of complex argon release and retention. The complexity arises from the multiphase nature of the samples comprising plagioclase, K-feldspar (in antiperthite), secondary "sericite" and various inclusions, in which some of the components also exhibit variable ages.

Fragments of plagioclase megacrysts and plagioclase separates from whole rock samples were selected from a well-constrained series of anorthosite-leuconorite intrusions of the Rogaland Igneous Complex (SW Norway) intruded at ca. 930 Ma. All samples exhibit saddle-like staircase-shaped argon-release spectra. Ages rise from around 400-600 to 750-900 Ma, though three samples exhibit excess argon released at high temperatures. The most remarkable aspect of this data is, however, the apparently linear correlation of Ar-40*/Ar-39 With Ca/K. The stepwise-heating data reveal late stage cooling ages, considerably younger than the intrusive age and low temperature reheating of the Rogaland Igneous Complex during Caledonian overthrusting. Arrhenius plots for the stepwise heated samples show a linear trend of diffusion data at low temperatures followed by a systematic break around 900-1060 degreesC.

Single spot ages obtained using a UV-laser show extreme age variations with heterogeneous excess argon contained in inclusions but no correlation of Ar-40*/Ar-39 with Ca/K. This lack of correlation indicates that the two-component mixing model (with two end-members of distinct ages), suggested by the stepwise-heating data, is an artefact resulting from a more complex mechanism of argon release. The most apparent interpretation of this data is that both plagioclase and another potassium-rich minor component, probably K-feldspar antiperthite yield variable but correlated ages. Because the excess argon is found exclusively in samples from marginal leuconorite bodies and never in the core of the megacrysts, it is probably derived from the outgassing of the K-rich country rock.

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