Rb [sbnd]Sr total rock isotope studies on Precambrian charnockitic gneisses from South Norway: evidence for isochron resetting during a low-grade metamorphic-deformational event

Abstract

The quartzo-feldspathic charnockitic orthogneisses within the Bamble sector of the so-called Sveconorwegian (1.2–0.9 b.y.) zone are highly fractionated in K and Rb such that they comprise two chemically contrasting zones — one highly K, Rb-deficient and the other with values of the same order as upper crustal lithologies. Eight series of samples, each collected from single outcrops, have yielded Rb-Sr total rock apparent ages in two distinct groups, at ∼1540 and ∼1060 m.y. Outcrops in both the K-deficient and normal-K suites have produced examples of each age. The older age relates to the high-grade charnockite event, and the younger to a superimposed low-grade event which occurred at the same time as the intrusion of undeformed granite sheets and pegmatite dikes; one of the granites has yielded an isochron age of 1063 ± 20 m.y. The low-grade event involved only the partial alteration of orthopyroxenes to chlorite and/or serpentine, coupled with some corrosion of biotite; the alterations were initiated along narrow, irregularly spaced, cracks and it was their development which facilitated open system behaviour of the total rock isotopic systems at some localities. The degree of rehomogenisation is a function of the intensity of the secondary alterations. Confirmation of resetting at ∼1060 m.y. is given by four mineral + host rock isochrons all yielding ages within error of the age for the intrusive granite; two of these are from outcrops where the rocks retain the older ∼1540-m.y. age. The secondary total rock isotopic homogenisation cannot be explained adequately by Rb mobility or by simple mixing with a fluid having its own initial 87Sr/ 86Sr composition. The primary mineralogy may have determined whether individual localities and/or samples suffered net increases or net decreases in 87Sr/ 86Sr. An important implication of the results is that in this, or any similar geological situation, there would be a very real possibility of drawing erroneous conclusions from regionally-collected samples, particularly if the full significance of the later, relatively minor P-T event remained undetected and/or the scale of isotopic (re-)homogenisation, were unknown. It is only because of the methods adopted that it can be stated that there is no isotopic evidence for a high grade Sveconorwegian (Grenvillian) event in this part of southern Norway.