Metamorphic petrology of the Froland corundum­ bearing rocks: the cooling and uplift history of the Bamble Sector, South Norway TIMO G. NIJLAND, FRANK L1 AUW, DIEDERIK VISSER, CORNEllS MAIJ ER & ANTONY SENIOR. Nijland, T. G., Liauw , F., Visser , D., Maijer, C. & Senior, A. 1993: Metamorphic petro logy of the Froland corundum-bearing rocks : the cooling and uplift history of the Bamble Sector , South Nor­ way. Nor. geol. unaers . Bull. 424, 51-63. Corundum-bearing rocks from Kleggasen , Froland, preserve metamorphic textures which reflect the cooling and uplift path of the Bamble Sector. The poT path may be correlated with a T-t path constructed from published ages. The P-T path starts at the thermal climax of prograde metamorphism, M I (c. 750·C, 7 kb), characte rized by Sil, PI, Bt, Rt and Crn. SUbsequent near­ isobaric cooli ng results in M 11 , characte rized by Ky + Ms :t Chi veinlets (600-700·C, 7 kb). This marks the onset of rehydration. M III assemblages of Mrg :t Crn formed around 500-570·C and 3-7 Kb, and are followed by M IV (c. 400·C, 2-4 kb) characterized by Ms, Bt and Ep. The trajectory from M 11 via M III to M IV is believed to reflect the uplift of the area in response to Sveconor­ wegian upthrusting. The uplift was completed before c. 940 Ma, which is the age of post-te cto­ nic granites and K-Ar biotite cooling ages. The latest episode recorded by the Froland rock s, M V (175-280·C, 2-3 kb), is characterized by Prh, Pmp, Scp and Tur , indicating local influx of B­ and Cl-bearing hydrous fluids. MV is interpreted as adapt ion to upper crustal cond itions between c. 920 and 760 Ma. T. G. Nijland, F. Liauw, D. Visser , C. Maijer & A. Senior, Departm ent 01 Geoctiemistry, Utrecht University, P.O. Box 8002 1, 3508 TA Utrecht, The Netherlands. Introduction are composed mainly of plagioclase, corun­ The Bamble Sector (Fig, 1) is part of the South­ dum, biotite and sillimanite, and cut by vein­ west Scandinavian Domain of the Fennoscand ­ lets of green mica and kyanite. They occur in a ian Shield (Gaal & Gorbatschev 1987), It is 2 m-wide , concordant lens, which is separated composed of a Proterozoic supracrustal suite from steeply dipping banded and granitic gneis­ that was metamorphosed and intruded by ses by a 20 cm-thick biotitite. A small, intens­ acidic and basic magmas during the Gothian ively altered part of the outcrop features large (1750-1500 Ma) and Sveconorwegian brown tourma line and a rare chromian rnont­ (1250-950 Ma) oroqen ies. For a detailed des­ morillonite, volkonskoite (Nilssen & Raade cription of the litholog ies and structural geolo­ 1973), The rocks provide information about the gy of the area, the reader ls referred to Star­ successive stages of post-peak metamorphism mer (1985, 1991), and have been studied to unravel the cooling In the Nelaug-Fro land-Arendal area (Fig. 1), and uplift history of the Bamble Secto r. the grade of metamorphism ranges from up­ per-amphibolite facies in the north up to granu­ lite facies in the area around Arendal in the Petrography south (Bugge 1940, Touret 1971, Lamb et al. 1986), poT estimates range from c. 836°C, 7.7 Oldest mineral assemblage (M I) kb in the core of the granulite facies area to The oldest assemblage, M la, cons ists of plagi­ c. 752°, 7.1 kb in the amphibolite facies area oclase , biotite , fibrol itic and euhedral sillimani­ (Nijland & Maijer 1992), te, and rutile, with accessory zircon , allanite Corundum-bearing rocks occur at Klegga­ and apatite (Table 1). Microfolds of fibrolite sen, Froland, 11 km northwest of Arendal, and are enclosed by large plagioclase crysta ls. were first descr ibed by Oftedahl (1963). The Plagioclase is in some cases antiperth itic. rocks have been interpreted as metamorpho­ Large subhedra l brown biotites, which com­ sed kaolinite-bauxite weathering crusts (Serd­ monly contain small sagenite needles , const itu­ yuchenko 1968), They are leucocratic, and te the foliat ion together with fibrolite. Younqer gneissic in their biotite-rich parts, The rocks euhedral sillimanite generally also follows this 52 Timo G. Nijland et al. NGU - BULl. 424. 1993 100 km ~ Caledonian area ITIIIlll Oslo Graben D Precambrian Granites. granitic ~fn f l gneisses Quartzites. cale 1, ::Ili l -silicate rocks. marble Tonalitic to granodioritic gneisses with metape­ A litic intercalations N Amphibolites - Metagabbros 1.5 km II Lakes - "not mapped" I I 64850 Fig. ·1. Geological sketch map 01 the Froland area west of the Nidelva river with relevant localities (modilied after Visser & Senior 1990). Coord inates along the margins of the map are accord ing to the grid of Norge s geogr aliske oppmAling. PKF = Porsgrunn-Kristiansa nd Fault, which is the boundary between the Barnble and Telernark Sectors . Inset: Division 01 the Southwest Scandinavian Domain (modilied after Verschure 1985). Numbers denote : 1 - Western Gneiss Region. 2 - Rogaland I Vest-Agder Sector . 3 - Telemark Sector , 4 - Bamble Sector, 5 - Kongsberg Sector. 6 - 0stfold Sector , 7 . Stora Le­ Marst rand Belt. foliation . but its crysta l faces truncate the fibro ­ prisms at the place where they are overgrown lite (Fig. 2). Large bronze rutile occurs inter­ by corundum. The M Ib corundum occurs in grown with euhedral sillimanite. Rutile shows two differe nt microstructures: either as large lamellar twinning and is common ly conc entra­ subhedral to euhedra l cryst als (up to several ted within the hinges of microfolds. cm) grown in the centre of sigmoidal thick­ Large colourless to pink. pleochroic. corun­ enings of the fibro lite foliation. or as large dum crystals enclose both fibrolite and euhed­ archipelago -like crys tals (Fig. 3). Corundum ral sillimanite. biotite, rutile and plagioclase. shows lamellar twin ning. and displays slight and is therefore denot ed as M lb. Boundaries colour zoning with a bluish tinge. espec ially between corundum and the other minerals are along the margins and accord ing to cry stallog­ usually sharp. and show no signs of reaction; raphic planes; the blue colour become s mor e only sillimanite bottle-neck textures are spora­ intense along altered margins. dically observed. i.e. the thinning of sillimanite NGU · BULL. 424. 1993 Metamorphic pe trology of the Froland 53 M I M 11 M III M IV M V S F P B C R Z A ATK M C M CP B M CC E P P S T i i I t r t r I p u y s h r r I t S h a p r m c u I b n n I r I g n I I h PP r TN75 x x x x x x x x x x x x x TN 76 x x x x x x x x x x x x x TN77 x x x x x x x x x x x x x x x x x x x x x TN 78 x x x x x x x x x x x x x x x x x TN79 x x x x x x x x x TN81 x x x x x x x x x x x x TN82 x x x x x x x x x x x x TN8 3 x x x x x x x x x x TN84 x x x x x x x x x MA993 x x x x x x x x x x x x x FL1 43 x x x x x x x x x x x FL144 x x x x x x x x x x x x x x FL145 x x x x x x x x x FL14 6 x x x x x x x x x x x x x FL1 47 x x x x x x x x x FL14 8 x x x x x x x x x x x x FL 149 x x x x x x x x x x x x x x x FL 150 x x x x x x x x x x x x x x x x FL1 51 x x x x x x x x x x Table 1. Distribution 01 the successive metamorphic assemblages in the different samples. Mineral abbreviations according to Kretz (1983). except Fib - librolite. subhedral grains. Rare relics of biotite and fibrolite occur in the veinlets. Although M 11 III minerals are mainly rest ricted to veinlets, gre­ ;;: .- en muscovite and chlorite also occur disper­ '. sed throughout the rocks. Here too , they for m ' I .". RT relatively large. subhedral grains, in contrast 'i 0 to M IV muscovite and chlorite which usually occur as rims and fine-grained aggregates I S ll around older phases . M 11 Muscovite outside the veinlets has been found to enclose micro­ folds of fibrolite. Pl Margaritization reactions (M Ill) In the host rock (M I assemblage). aggregates of sheaf-like bundles of margarite locally repla­ ced M la plagioclase. These aggregates are usually present in the vicinity of M Ib corun­ Fig . 2. M la fibr olite truncated by M la eu hedral silliman ite . dum crysta ls, but the grain boundaries of cor­ Photomicrogr aph 01 sample TN 84. undum are virtually unaffected. Sporadically. small coru ndum grains occur in the margarite aggregates; these may represent relics. Rutile Kyanite-bearing veinlets (M 11) and biotite occur unaffected in these aggrega­ Veinlets of kyanite and muscovite with minor tes.