Secondary geologically meaningless RbSr isochrons, low 87Sr/ 86Sr initial ratios and crustal residence times of high-grade gneisses

Abstract

Dark green quartzo-feldspathic charnockitic ortho-gneisses from Arendal, southern Norway, have locally been decoloured in the vicinities of intrusive, undeformed granite sheets and pegmatite dikes. The result is a series of pink rocks which are macroscopically indistinguishable from primary augen granite gneisses. The main mineralogical changes associated with the secondary decolcuration occurred under relatively low-grade conditions, and mainly involved chlorite, serpentine ± actinolitic amphibole ± biotite growth. The alterations were initiated along narrow, irregularly spaced cracks. The primary ( ∼ 1540 m.y. ) total rock RbSr isotopic systems which relate to the charnockite mineralogy were disturbed, and sometimes reset, during the secondary event which occurred at ∼ 1060 m.y . Locality by locality, the decoloured samples yield linear arrays corresponding to 1457 ± 200 m.y., 1303 ± 50 m.y. and 1125 ± 185 m.y. The combined data ( n = 19) give an apparent age of 1364 ± 43 m.y. with an 87Sr/ 86Sr initial ratio of 0.7011 ± 0.0018. This apparent age is unrelated to any geological event and there is no overlap at the 2δ level between the 87Sr/ 86Sr initial ratio and that defined by the primary event ( 0.70345 ± 0.00014). Any estimate of crustal residence time based on these decoloured samples would be in error. The weight of evidence is against significant involvement of Rb mobility in the isotopic resetting process which accompanied decolouration. A model involving modification of the 87Sr/ 86Sr ratios is preferred.