Research Note Additional evidence of pre-Silurian high-pressure metamorphic rocks in Spitsbergen YOSHIHIDE O H T A , YOSHIKUNI HIROl AND T A K A 0 HIRAJIMA Ohta, Y., Hiroi, Y. & Hirajima. T. 1983: Additional evidence of prc-Silurian high-pressure metamorphic rocks in Spitsbcrgcn. Polar Research I n.s., 215-218. It has been previously suggested that the high-pressure metamorphic rock complcx of Motalafjclla. central-western Spitsbergen, is older than Lower Silurian. An unconformity has been discovered at the base of a limestone which contains fossils similar to those reported as Lower Silurian from the same massif. This proves that the high-pressure metamorphics represent an older rock complex. The unconformity surfacc is invcrted in the area, and large recumbent folds involving the Silurian Bulltindcn Formation havc been mapped. These folds are considered to be prc-Carboniferous, k'oshiliide Ohra. Norsk Polarinsriturt. Rolfrraangueieri 12, 1330 Oslo Ltcfthaun. Norway; Yoshikririi Hirrri. Inrtrriire of Eurrh Scierice, Faculty of Education, Kunuzawo Unic~ersify. Kanuzuwa, Y20 J u p u t i , T u k u o Hirulima. Deparrrnent of Geology arid Mineralogy, Fucirlry of Science, Kyoru Uniwrsir)., Kyoro. 606 J a p a n : A u g u r I983 (revised Ocroher 1983). ' d ~ . p ~ L A R l ~ s ~ " unit of mainly grcen and black phyllites with subordinate amounts of mcta-basic rocks (partly showing pillow structure j . A blueschist-eclogite complex has been reported from Mota- quartzite, dolomitic rocks and serpcntinitc. and an uppcr unit lafjclla, central-western Spitsbcrgcn (Horsficld 1972: Ohta of mainly high-pressure metamorphic rocks and schistose lime- 1978: Manby 197s: Ohta 1979). K/Ar ages have been obtained stone with a small amount of chert. The phyllitcs of the lower from the rocks in the range 102-475Ma and at 621Ma (Horsfield unit and the micaccous schists of thc uppcr unit have very 1972). A Flyshoid succcasion, the Bulltinden Formation (Hors- strong diaphtoritic cleavages. field 1972; Hjelle et al. 1979: Harland et al. 1979). is clotely conncctcd with the complex. and fossils from thc formation are Introduction considcrzd to be Lower Silurian in age (Scrutton ct al. 1976) Some pebblec crt altcrcd schists similar to the slaucophane Description of the unconformity schists wcrc found in boulder conglomerates of the Bulltinden Formation. and a pre-Silurian age for the high-pressure meta- morphic rocks has been suggested (Horsficld 1972). During thc 1983 field survcy a n unconformity was discovered at the base of the limestone member of the Bulltinden For- mation. and this protides concrete evidence that the high-pres- sure metamorphism is prc-Silurian. In this paper field obscr- vations of thc unconformity arc described and a preliminary re-interpretation of the geological structure of the area is prexnted. Geological outlirie of Motalafjella (Fig. 1) The Bulltinden Formation make\ up roughly 213 of the north- eastern Motalafjclla. and consists mainly of boulder conglom- crate. black shale. grey \aiid\tone alternations. and limcstonc. Thc remainder of the massif includes phyllitic rocks and the high-prcrsurc metamorphic rocks of the Vestgotabreen For- mation. All planar structures, both bedding and schistosity. show a monoclinical structure with a gentle curve of strihes. The mean dip is moderate to the west and southwest. A 1CL20 m thick limestonc member of the Bulltinden For- mation crosse\ the massif from NW to SE. The boulder con- glomerate changes greatly in thickness, and the shale contains several limcstonc Icnses. most of which are fossilifcrous. The Vestgotahrcen Formation consists of two units: a lower The unconformity has been obscrved clearly at two localitics ( A and B in Fig. 1 ) and supporting evidence has been found at several other localitics (C-G i n Fig. I ) . Locality A. (Fig. 2 ) An about 20 m thick, dark grcy limestone bed of the Bulltinden Formation forms a vertical cliff here and i s overlain by a dolom- itic layer with a characteristic brown, weathered colour. The lower surfacc of the dolomitic layer has a strongly weathered appearance and is brecciatcd into angular-subangular blocks, the interstices of which arc filled with conglomcratic grcy limestone. The limestone is conglomeratic for 2-5 m from its structural upper surface. The clast population includes subangular-rounded hlocks and pebbles. up to 20 ciii. of mninl! dolomitic rocks and subordinate numbers of quartzitc. grcen phyllite, schistose limestone and scrpcntinite. all components derived from the Vestgotabreen Formation. The matrix is dense, grey limestone and contains gastropods similar to thobe reported by Scrutton et al. (1976). Fossil-bcaring limestone pebbles are often found in the boulder conglomerate structurally underlying the limestone. A large recumbent syncline crest can be clearly seen on the eastern cliff in thc conglomerate. Thcsc observations imply that the limestone has an uncomformity at its stratigraphic base, although the whole succession is now overturned. 716 Yoshihide Olira et 01. Locrilir~~ B T h c LLmt of gastropods. cephalopods. and rrilohltea c o n t a c t surface does ncrt show a n y trace of slip. T h e precise in t h c wuthe:ri part. Thin lilnis of grc! l i i n e s t o n e penetrstc a l o n g l o i n r plaiie\ u t the dolamitic tock. Other localities High-pressure metamorphic rocks 217 Fig. 2 . Sketches of the unconformit) at Locality A (after photographs). A : GP-green phyllite. D-massive dolomitic ruck, B - brecciated and brown-weathered dolomitic rock, C - conglomeratic grey limestone, L - grey limestone, W -white lime- stone. Cg- bouldcr conglomerate. B: Contact between the brecciated dolomitic rock and the conglomeratic limcstone. 5 - sandy limestone. C Fossils occurring in the brecciated dolomitic rock. F-gastropods. D -dolomitic rock block. contact is not exposed at Locality D . but many scree blocks of g r q limestone contain subangular blocks and rounded pebbles of quartzite, green phyllite and dolomitic rock On the ridge south of Skipperbreen (Locality E ) . the grey limcstone underlies an epidote-actinolite greenstone of the Ve\tgOtabrcen Formation without any slip plane. Thin layers of green and white limestone, each laver less than lOcm thlck. form a banded structure near the contact for less than 5 m. A stock-like epidote-actinolite greenstone is enclosed by the grey limestonc o n the northern ridge of Skippcrbrsen (Locality F). The limestone overlies the greenstones and high-pressure meta- morphic rocks in the central and western part, showing banded structure as in Locality E , while a fault cuts the eastern part and the limestone is in contact with a dolomitic rock without signs of mobement. Thus, an overturned anticline is exposed here. although being cut by the fault. T h e grey limestone underlies a brown dolomitic layer and greenstones o n the southrrn slopc of Bulltinden (Locality G ) . without slip. The