Kl Ar Ages from the Basal Gneiss Region, Stadlandet Area, Western
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KlAr ages from the Basal Gneiss Region, Stadlandet area, Western Norway DANIEL R. LUX Lux, D. R. KlAr ages from the Basal Gneiss Region, Stadlandet area, Western Norway. Norsk Geologisk Tidsskrift, Vol. 65, pp. 277-286. Oslo 1985. ISSN 0029-196X. KlAr and 40ArP9Ar geochronology of minerals from the Stadlandet region, Norway, is complicated by the presence of excess 40Ar in man y samples. The re is a correlation between excess 40Ar concentration and textural occurrence of samples. Amphiboles and micas closely associated with eclogite pods are likely to contain significant concentrations of excess 40Ar while samples from the country rocks are not. KlAr and 40ArP9 Ar ages for horn blende from the country rocks suggest that post-metamorphic cooling through 500°C occurred about 410 Ma ago, shortly after eclogite formation. Biotites containing no excess 40Ar cooled through 300°C about 370 Ma ago. A post-metamorphic cooling rate of about 5°C/Ma can be inferred. D. R. Lux, Department of Geological Sciences, 110 Boardman Hall, University of Maine- Orono, Orono, ME 04469, USA. Both the petrogenesis and age of the eclogite pods of the Basal Gneiss Region of Western Geologic setting Norway have been the subject of considerable The samples examined during this study were debate. This study was initiated to date the for collected in the Stadlandet peninsula and the mation of the eclogites by the KlAr method. The adjacent Flatraket area of Western Norway (Fig. primary mineralogy of the eclogites is anhydrous 1). Though locally the rocks of the Basal Gneiss and contains no potassium-bearing phases (Lap Region are at granulite grade, they are predomi pin & Smith 1978)- However, in many cases the nately at amphibolite grade in the study area eclogites are retrograded to hydrous, potassium (Lappin 1966). The dominant rock types in the bearing assemblages. Several authors (Lappin & study area are grey gneisses (plagioclase gneiss) Smith 1978, McDougall & Green 1964) have sug and banded and augen gneisses (two feldspar gested that these secondary assemblages formed gneiss). Garnet-mica schist, hornblende schist, at an early stage in the history of the eclogites. If garnet amphibolite, quartzite, pegmatite, and this assumption is correct and if post-metamor eclogite constitute about ten percent of the total phic cooling was rapid, KlAr ages of the secon and are described in detail by Lappin (1966). dary potassium-bearing minerals should closely It is generally believed that the Basal Gneiss approximate the time of formation of eclogites. Complex is the deepest exposed structural leve] Since this study commenced, three point gar of the Norwegian Caledonides (Cuthbert et al. net, pyroxene, and whole-rock Sm/Nd ages have 1983). lts structural and metamorphic history is been determined for three eclogite pods from the complex in that it was affected by the Svecofen immediate study area (Griffin & Bruckner 1980, nian, Sveconorwegian, and Caledonian orogenic Mearns & Lappin 1982). These isochrons firmly events. A peculiar feature of the Basal Gneiss establish the eclogite formation in the Stadlandet Complex is the common occurrence of lithologies area as a Caledonian event (447 ± 20, 408 ± 8, which preserve relict high-pressure mineralogies, 414 ± 31). With this knowledge it is apparent the best known being eclogites. Some eclogites that many of the minerals dated by the KlAr occur in association with other ultramafic rock method have anomalously old apparent ages. bodies, such as dunite, and commonly have the This suggests that 40Ar is present in excess of that form of layers of lenses. Other eclogites occur as produced in situ since eclogite formation. Excess isolated lenses or boudinage layers within the 40Ar is found in both amphiboles and micas. In country rock; this eclogite type is the subject of addition to age information it is the purpose of this paper. this study to explain factors controlling the distri Garnet and clinopyroxene are the dominant bution of excess 40Ar in the eclogites. minerals in the eclogites but orthopyroxene, oli- 278 D. R. Lux NORSK GEOLOGISK TIDSSKRIFT 4 (1985) Stadlandet \ Oslo � 397 � \ Leikanger ("� % (J � Gneiss & Amphibolite D o ... o o "' o 2 3 4 5km 1788 ST/418 68/79A Fig. I. Map of the Stadlandet region showing sample localities and the position of various ultramafic rock localities (after Lappin 1966). NORSK GEOLOGISK TIDSSKRIFf 4 (1985) KlAr ages Basal Gneiss Region, Stadlandet 279 vine, kyanite, quartz, rutile, apatite, and zircon analytical uncertainties. These ages have been may also occur as primary minerals (Lappin & interpreted as the age of formation of igneous Smith 1978). Most of the eclogite pods show precursors of the gneisses (Lappin et al. 1979). some retrograde features. These can be grouped Most previously determined KlAr, 40ArP9 Ar, into three mineral assemblages, each with a dis mod el Rb/Sr, and Rb/Sr mineral isochron ages tinet paragenesis and distinct mineral composi fall within the range 372-495 Ma (McDougall & tions (Lappin & Smith 1978). Early retrograde Green 1964, Bryhni et al. 1971, Lappin et al. minerals are commonly amphiboles, micas, zoi 1979, Bruckner 1972, Pidgeon & Raheim 1972). site, tourmaline, sphene, and carbonates. Am This implies that these minerals formed or their phiboles from this group will be referred to as isotopic systems were largely reset during the early amphiboles. In some instances the early Caledonian Orogeny. Some K-Ar ages for micas, minerals show stable mutual contacts with prima amphiboles, and pyroxene do, however, greatly ry minerals. However, in other cases textural exceed Caledonian ages (McDougall & Green criteria indicate the secondary nature of the early 1964). Ages of 401 and 418 Ma have been deter minerals (Lappin & Smith 1978). Various geo mined for two U-Eoor zircon samples from eclo barometers and geothermometers suggest that gite pods by the 2 8UJ206Pb method (Krogh et al. they formed at lower temperatures and pressures 1974), and were interpreted as crystallization than the primary assemblages (Lappin & Smith ages. More recently, eight Sm/Nd isochrons us 1978). Chemical uniformity of the earl y minerals, ing gamet, clinopyroxene, and whole-rock from regardless of textural differences, suggests that eclogite pods indicate ages of 447 to 408 Ma all of the early minerals represent a retrograde (Griffin & Bruckner 1980, Meams & Lappin adjustment after the initial formation of the eclo 1982), and were also interpreted as crystalliza gite pods (Lappin & Smith 1978). Continued tion ages. Other ages between these two ex retrograde adjustment is characterized by sym tremes (Svecofennian and Caledonian) are ob plectic amphibole and plagioclase replacing both served less frequently for samples from the Basal primary and early minerals. The chemical com Gneiss Complex (Lappin et al. 1979, Bruckner position of these minerals indicates a distinct 1974). mineralogic group (Lappin & Smith 1978). Some Whole-rock dating by the Rb/Sr isochron eclogites are totally recrystallized to form amphi method indicates an age of 1760 ± 70 Ma for bolite. Amphibolite formed from these recrystal gneisses of the Stadlandet Peninsula (Lappin et lized amphiboles usually occurs as rims which al. 1979). Discordant zircons from the same totally enclose the eclogite pods and can easily be gneisses loosely constrain an upper concordia observed in the field (Lappin 1966). The recrys intercept of similar age and this is taken as con tallized amphibolite appears to be in equilibrium firming evidence of a Svecofennian age for for with minerals of the enclosing country rock. Ad mation of the gneisses (Lappin et al. 1979). dition of H20 and K20 is required to complete Three eclogite pods from the immediate study all of the three successive stages of retrograde area have been dated with the Sm/Nd method alteration (Lappin & Smith 1978). and gave similar ages, 447 ± 20, 408 ± 8, and 414 ± 31 Ma (Griffen et al. 1980, Meams & Lappin 1982). KlAr and Rb/Sr apparent ages for miner als from near the study area are largely Caledon Previous isotopic dating ian (Lappin et al. 1979, Bruckner 1972, In recent years geochronology has contributed a McDougall & Green 1964, Bryhni et al. 1971). great deal to the understanding of the timing of Some KlAr age determinations for minerals from events in the Basal Gneiss Region. Different the area are anomalously high, and have been parts of the region apparently record Svecofen attributed to excess 40Ar (McDougall & Green nian (1950-1650 Ma), Sveconorwegian (900- 1964). In most cases the excess 40Ar has been 1200 Ma), or Caledonian (350-650 Ma) events associated with eclogite pods. (Bruckner 1974; Lappin et al. 1979). Whole rock Rb/Sr isochron ages from different parts of the Basal Gneiss Region give Svecofennian ages Results ranging from 1760 to 1682 Ma (Lappin et al. 1979; Pidgeon & Raheim 1972, Mysen & Heier KlAr analytical data for 27 amphiboles and 10 1972) and generally agree within the limits of biotites are presented (Tables l and 2, respec- 19 - Geologisk Tidsskr. /85 4 280 D. R. Lux NORSK GEOLOGISK TIDSSKRIFT 4 (19S5) Tab/e l. Analytical data for KlAr age determinations for am Table 2. Analytical data for KlAr age determinations for bio phiboles. tites. Decay constants are those recommended by Steiger and Jager (1977). Sample • 40 Apparent 40Ar Ar R R Age Textural Sample Apparent Age Textural Number % K % x 10° yrs. Occurr. x yrs. Occurr. Number %K 40A r • % 40Ar Hf R � R 17SB 1.572 2.629 96.2 3S6±11 CR 2.664