Interpretation of Rb-Sr Dates from the Western Gneiss Region: a Cautionary Note

Interpretation of Rb-Sr dates from the Western Gneiss Region: a cautionary note ALLAN G. KRILL & WILLIAM L. GRIFFIN Krill, A. G. & Griffin, W. L.: Interpretation of Rb-Sr dates from the western Gneiss Region: a cautionary note. Norsk Geologisk Tidsskrift, Vol. 61, pp. 83-86. Oslo 1981. ISSN 0029-196X. Some Precambrian Rb-Sr whole-rock isochrons from the Western Gneiss Region have been interpreted as recording the age of the metamorphism, and some recent geologic models have in turn considered the Caledonian influence in the region to be relativel y minor. However, data from the basement rocks of the Alps clearly show that Rb-Sr whole-rock isochrons survived intense Alpine metamorphism and deformation. Thus, the Precambrian dates from western Norway may record meaningful ages of the Precambrian basement, even though the effects of the Caledonian orogenesis may have been considerable. A. G. Krill & W. L. Griffin, Minearlogisk-geologisk museum, Sars gt. l, Oslo, 5, Norway. The conclusions that recumbently folded and Precambrian dates. Some investigators have in metamorphosed rocks of the Oppdal area repre terpreted such isochrons as recording the sent a Caledonian tectonostratigraphy (Krill magmatic or primary events (Priem et al. 1973, 1980), and that Sm-Nd dates on eclogites indi Brueckner 1972, 1979, Abdei-Monem & Bryhni cate high-grade Caledonian metamorphism fol 1978, Lappin et al. 1979). Others have in lowed by deformation and retrogression (Griffin terpreted similar isochrons as recording & Brueckner 1980), support earlier geologic metamorphic or secondary events (Mysen & models that the Western Gneiss Region is the Heier 1972, Pidgeon & Råheim 1972, Råheim structural and metamorphic core of the 1977, Skjerlie & Pringle 1978, Råheim et al. 1979, Scandinavian Caledonides. These conclusions Solheim 1980). challenge recently suggested revisions that These isochrons generally have low ( <. 706) would question or severely limit the degree of initial 87Sr/ 86Sr rati os. The y can only be in Caledonian influence in the region (Krogh 1977, terpreted as giving metamorphic ages if: (a) the Bryhni & Brastad 1980, Roberts & Stort 1980, metamorphism occurred within a relatively Oftedahll980). Such models, which suggest that short period (ca. 100 Ma?) after separation of the main metamorphism and deformation of the the rock from a 'mantle' source, or (b) selective Western Gneiss Region was Precambrian, were removal of radiogenic 87Sr from all analyzed developed largely from interpretations of Rb-Sr rocks occurred during the later metamorphism. whole-rock dates as representing the age of the The first alternative implies that the isochron last major metamorphic event (eg. Råheim 1977, date records some late stage in a crustal acretion Skjerlie & Pringle 1978). These geochronologic differentiation 'super-event' (Moorbath 1975). interpretations, however, are debatable. The The second requires a process that has been dates may instead record primary igneous/ suggested (Heier & Compston 1969) but never, metamorphic ages related to the original forma to our knowledge, clearly demonstrated. tion of this segment of the continental crust. This Several recent interpretations of the Western interpretation resolves the apparent 'Caledo Gneiss Region suggest that the preservation of nian' vs. 'Precambrian' contradictions in the Precambrian Rb-Sr whole-rock isochrons ex geology of western Norway, and agrees hetter cludes the possibility of major Caledonian with data from gneiss regions of other orogens, metamorphism of the dated rocks (Pidgeon & where geochronologic interpretations are more Råheim 1972, Råheim 1977, Skjerlie & Pringle closely constrained by stratigraphic and fossil 1978,Råheim et al. 1979, Solheim 1980). These evidence. interpretations rest largely on two assumptions: Within the Western Gneiss Region Rb-Sr (a) It is assumed that any strong deformation whole-rock isochrons of gneisses typically give and metamorphism would reset Rb-Sr whole- · 84 Note NORSK GEOLOGISK TIDSSKRIFT l (1981) rock systems, so that the Precambrian isochrons volves homogenization of 87Sr/ 86Sr to the aver could not have survived a strong Caledonian age value for the reset system, and this average recrystallization. (b) Because of the lack of value is necessarily higher than the initial value obvious structural and metamorphic breaks (cf. Faure & Powell 1972). within the Western Gneiss Region, it is assumed In any case, the old isochrons cannot exclude that only one orogeny was significant here. the possibility of a younger metamorphism in the Thus, following the first assumption, the Western Gneiss Region. The resetting of whole orogeny must be dated by the oldest preserved rock isochrons requires the movement and isochrons in the region (Svecofennian) and homogenization of Sr isotopes on a scale equi younger isochrons must record isotopic dis valent to the maximum distance between sam turbance and resetting due to secondary events ples. Evidence from other polymetamorphic of relative! y minor geologic significance. orogens demonstrates that this requirement is A close look at geochronologic and geologic seldom roet, especially in orthogneisses. The studies of other orogens discredits these as consensus among geochronologists working in sumptions. The Alps are especially suited for such terrains is that extensive movement of a such a comparison and for understanding the fluidphase is necessary for the resetting ofRb-Sr meaning of geochronologic data (cf. Jager 1977). systems. Dehydration during the original igne Many Rb-Sr studies have been made in the Alps ous/metamorphic event would therefore greatly and, because of the young age of the Alpine reduce the likelihood of isotopic resetting of orogeny, absolute errors of isochrons are rela whole-rock systems during subsequent events tively small. Furthermore, abundant fossil evi (Jager 1979). dence provides independent control on the age It is important to note that all known Rb-Sr interpretations. The degree of Alpine deforma mineral dates from the Western Gneiss Region tion and metamorphism of the basement and (north of Sognefjorden) are 'Caledonian' (35� cover is well documented, and the influence of 500 Ma). These data have been widely disre the Alpine orogenesis on the Rb-Sr mineral and garded in the interpretation of whole-rock data, whole-rock systems of basement rocks has been on the assumption that mineral dates record only studied in detail. The results from many studies weak reheating, rather than metamorphic re of basement rocks show that Rb-Sr whole-rock crystallization. This asumption is based largely systems remained closed despite deformation on the relatively low 'blocking temperatures' and recrystallization, even in the staurolite, recorded in some mineral Rb-Sr systems (and kyanite, and sillimanite zones of regional K-Ar systems) in studies of contact metamorph metamorphism (Jager 1970, 1977). ism. However, other arguments may be In the Pennine zone of the Alps, basement advanced. (a) Muscovite 'blocking temperatures' rocks were strongly foliated, recumbently folded are as high as 500°C (Purdy & Jager 1976, Jager with cover rocks, and strongly metamorphosed, 1979), suggesting minimum temperatures on locally forming Alpine eclogites, yet Rb-Sr this order throughout the Western Gneiss Region whole-rock isochrons commonly yield meaning in Caledonian time. (b) Regional metamorphism ful pre-Alpine ages (e.g. Hunziker 1979, Jager to temperatures up to l 00°C above the assumed 1970, Gulson 1973, Satir 1974). It is reasonable 'blocking temperature' rnay not reset Rb-Sr and to compare the Western Gneiss Region to the K-Ar systems in biotite and muscovite (cf. Pennine Zone (Holtedahl 1938, Muret 1960), and Verschure et al. 1980, Chopin & Maluski 1980). to interpret the Precambrian dates as meaningful Recrystallization, rather than simply heating, ages for the pre-Caledonian basement. appears to be the critical factor. (c) Small, late Isotopic disturbance of whole-rock specimens to post-tectonic granite or granite-pegmatite is common in the Alps, and isolated cases of dikes are common in the western parts of the completely reset or secondary whole-rock iso Gneiss Region, and give Caledonian (<400 Ma) chrons are known (Hanson et al. 1969, Hunziker Rb-Sr whole-rock dates with high inital 87Srf!6Sr 1970). As with secondary mineral isochrons, the (i.e.;;;:. .712) (Pidgeon & Råheim 1972, S. Hebekk, well documented secondary whole-rock iso pers. co mm. 1981). The existence of the se dikes chrons are characterized by higher initial ratios. implies local anatexis of older rocks not far from. Common models of isotopic behavior require the site of emplacement. These data thus suggest that secondary isochrons should indeed have that the Caledonian Rb-Sr mineral dates from high initial ratios, since isotopic resetting in- the Western Gneiss Region reflect high tempera- NORSK GEOLOGISK TIDSSKRIFT l (1981) Note 8S tures (> 600°C) and probably extensive re Tafjord-Grotli area of the Basal Gneiss Region, west Nor crystallization during the Caledonian event. way. Nor. Geo/. Tidsskr. 59, 141-153. The absence of obvious structural and meta Bryhni, I. & Brastad, K. 1980: Caledonian regional metamorphism in Norway. J. Geo/. Soc. London 137, morphic breaks within the Western Gneiss Re 251-259. gion does not eliminate the possibility of both Chadwick, B. 1968: Deformation and metamorphism in the Precambrian and Caledonian orogenic events. Lukmanier region, central Switzerland. Bull. Geo/.

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