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High-Grade Contact Metamorphism of Calcareous Rocks from the Oslo Rift, Southern Norway

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High-Grade Contact Metamorphism of Calcareous Rocks from the Oslo Rift, Southern Norway American Mineralogist, Volume 82, pages 1241±1254, 1997 High-grade contact metamorphism of calcareous rocks from the Oslo Rift, Southern Norway BJùRN JAMTVEIT,1 SVEN DAHLGREN,2 AND HAAKON AUSTRHEIM3 1Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway 2The Norwegian Petroleum Directorate, P.O. Box 600, 4001 Stavanger, Norway 3Mineralogical-Geological Museum, Sars gt.1, N-0562 Oslo, Norway ABSTRACT Shallow-level plutons caused extensive contact metamorphism of Lower Paleozoic shale and carbonate sequences in the Permian Oslo Rift. A .500 m long and 100 m wide shale- limestone xenolith embedded within monzonites belonging to the Skrim plutonic complex experienced high-grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks. The peak metamorphic (Stage I) assemblages in calcite-saturated rocks include wollastonite, melilites, fassaitic pyroxenes, phlogopite, titanian grossular, kalsilite, nepheline, perovskite, cuspidine, baghdadite, pyr- rhotite, and occasional graphite. Mineral reactions involving detrital apatite produced a series of silicate apatites, including the new mineral species Ca3.5(Th,U)1.5Si3O12(OH). This assemblage equilibrated at T 5 820±870 8C with a C-rich, internally buffered pore-¯uid (20±40 mol% CO2 1 CH4). During cooling the shale-limestone xenolith experienced in®l- tration of C-poor (, 0.1 mol% CO2) ¯uids, triggering the formation of retrograde (Stage II) mineral assemblages comprising monticellite, tilleyite, vesuvianite, grandite garnets, diopside, and occasional hillebrandite. Rare potassium iron sul®des (rasvumite and djer- ®sherite) formed at the expense of primary pyrrhotite. These assemblages probably formed near 700 8C. Formation of diffuse sodalite-bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar. The sodalite-bearing Stage III assemblage formed by the in®ltration of saline brines at a max- imum temperature of 550 8C. Low-temperature (Stage IV) retrogression of the Stage I-III assemblage produced scawtite, giuseppettite, hydrogrossulars, phillipsite, thomsonite, and three hitherto undescribed mineral species. INTRODUCTION et al. 1992a, 1992b; Jamtveit and Andersen 1993; Sven- Mineral assemblages and mineral reactions occurring sen and Jamtveit 1998). Here, we present the ®rst de- during contact metamorphism of calcareous rocks have scription of high-grade mineral assemblages in contact been extensively studied and described since the pio- metamorphic carbonates from the Oslo rift. A large xe- neering work by Goldschmidt (1911) in the Oslo rift. Ob- nolith of Lower Paleozoic sedimentary rock within the served mineral assemblages re¯ect a wide range of tem- Permian Skrim monzonite complex in the Southern Oslo perature and ¯uid-composition space (Tracy and Frost rift experienced metamorphic temperatures exceeding the 1991). Equilibrium temperatures approaching and even solidus of the crystallizing monzonite. Mineral reactions exceeding 1000 8C are reported for limestone assem- involving carbonate and silicate minerals led to the pro- blages in aureoles around ma®c intrusions (e.g., Joesten duction of various of Si-poor silicates, including numer- 1983). Because most ma®c intrusions crystallize without ous phases rarely or never previously reported from meta- releasing large quantities of volatiles, most high-grade morphic rocks. Reactions between silicates and pyrrhotite contact metamorphic carbonate rocks have buffered the produced rare potassium iron sul®des, and reactions in- local pore-¯uid composition during heating and devola- volving detritial apatite produced LREE and Th-rich sil- tilization. High-grade assemblages re¯ecting equilibration icate apatites including a new mineral species [Ca3.5Th1.5Si3O12 (OH)]. with H2O-dominated ¯uids are rarely reported from shal- low level (P # 1000 bars) environments (see however, Williams-Jones 1981 and Inderst 1987). GEOLOGICAL SETTING In the Oslo area, contact metamorphism is related to The Permo-Carboniferous Oslo paleorift is situated in various shallow level intrusions. Frequently devolatiliza- the southwestern part of the Baltic shield. The exposed tion reactions in calcareous rocks are driven by in®ltra- parts of the rift are composed of two half grabens ®lled tion of externally derived H2O-dominated ¯uids (Jamtveit with Carboniferous sedimentary rocks, alkaline volcanic 0003±004X/97/1112±1241$05.00 1241 1242 JAMTVEIT ET AL.: CONTACT METAMORPHISM OF CALCAREOUS ROCKS [§] Lake Flekkeren Metamorphic Carboniferous • Permian Late Proterozoic • Faull [j) Fen Carbonatite Complex ' Ordovician Limestones r~~~l Various batholiths Permian Skrim Batholith Mesoproterozoic ~Major fault Mesoproterozoic D Volcanics l+t+tl Pegmatites li: \\~ Alkali syenite !SSJ Amphibolites and Metaleucogabbro Cambrian ·Silurian '\?.- Gabbro and Amphibolite t~ ~ ~ ~ Larvikite t:::: ::j Quartzofeldspathic gneisses [IS! Shales, limestones ~ Various gneisses RLN%0904317 FIGURE I. Simplified geological maps of the Flekkeren area (left; Dahlgren 1998) and the Southern margin of the Oslo rift (right; Dahlgren 1998). and intrusive rocks, and Lower Paleozoic sedimentary 1992b; Svensen 1996), consistent with dominantly con­ rocks (Fig. 1). The latter are dominated by shales and ductive heating of the aureoles. limestones (Bjfllrlykke 1974). Extensive intrusive activity Recent stable isotope results have demonstrated wide­ in the time interval 300-250 Ma (Sundvoll et a!. 1990) spread pervasive infiltration of aqueous fluids with a included emplacement of various monzonitiG "to granitic magmatic 0 isotope signature around the various cooling rocks and caused widespread contact metamorphism and intrusions (Jamtveit et al. 1992a, 1992b, 1997). Decar­ hydrothermal activity within the originally unmetamor­ bonation reactions leading to formation of the contact phosed sedimentary rocks. metamorphic mineral assemblages are to a large extent The Lower Paleozoic shales are composed of illite, driven by this infiltration. Reaction-enhanced porosity chlorite, quartz, diagenetic feldspar, and occasional do­ and permeability at the shale-carbonate contacts was a lomite, whereas the limestones are dominated by calcite significant factor in controlling fluid release from these (Bjfllrlykke 1974). Both shales and limestones contain sig­ shallow intrusions (Jamtveit et a!. 1997). The current nificant but generally low contents of organic carbon ( < model for fluid flow during peak contact metamorphic I wt%). During contact metamorphism, the most notable conditions around granitoig intrusions in the Oslo rift in­ metamorphic reactions took place at the carbonate-shale cludes early pervasive and slow infiltration of magmatic contacts where metasomatic calc-silicate zones formed at fluids of low salinity in an overpressurized system (rela­ the expense of the carbonate and shale layers. Maximum tive to hydrostatic), interrupted by rapid and focused re­ temperature conditions near the intrusive contacts were lease of saline solutions with associated pressure drops in estimated to be in the range 400-550 oc (Jamtveit et a!. high permeability zones. JAMTVEIT ET AL.: CONTACT METAMORPHISM OF CALCAREOUS ROCKS 1243 TABLE 1. Abbreviations and formulas for observed minerals in limestones and calc-silicate rocks from Flekkeren Name Formula Petrogra®c status ab albite NaAlSi3O8 Stage IV ad andradite Ca3Fe2Si3O12 Stage IV ak akermanite Ca2MgSi2O7 Stage I al alabandite MnS Stage I? an anorthite CaAl2Si2O8 Stage I* ap apatite Ca5(PO4)3(OH) detrital au augite (fassaite) Ca(Mg,Fe,Al,Ti)(Si,Al)2O6 Stage I ba baddeleyite ZrO2 inclusions in zircon bg baghdadite Ca3ZrSi2O9 Stage I br britholite (LREE,Ca)5(SiO4,PO4)3(OH) Stage I cc calsite CaCO3 all assemblages cr chromite FeCr2O4 detrital cu cuspidine Ca4Si2O7(F,OH)2 Stage I di diopside CaMgSi2O6 Stage II dj djer®sherite K6(Fe,Ni)25S26Cl Stage II ga galena PbS detrital gp graphite C Stage I? gr grossular Ca3Al2Si3O12 Stage II gu giuseppettite (Na,K,Ca)7±8(Si,Al)12O24(SO4,Cl)1±2 Stage IV hb hillebrandite Ca2SiO3(OH)2 Stage II? hi hibschite Ca3Al2Si32xO12(OH)4x (x 5 0.2±1.5) Stage IV ka katoite Ca3Al2Si32xO12(OH)4x (x 5 1.5±3) Stage IV kfs K-feldspar KAlSi3O8 Stage II or III ki kimzeyite Ca3(Zr,Ti)2(Si,Al)3O12 Stage I ks kalsilite KAlSiO4 Stage I loÈloÈllingite FeAs2 detrital? ma marialite Na4Al3Si9O24Cl Stage I* mc marcasite FeS2 Stage IV mg magnetite Fe3O4 detrital mo monticellite CaMgSiO4 Stage II ne nepheline NaAlSiO4 Stage I op opal SiO2 Stage IV pe perovskite CaTiO3 Stage I ph phlogopite KMg3AlSi3O10(OH)2 Stage I pi phillipsite (K,Na,Ca)1±2(Si,Al)8O16´6 H2O Stage IV pt pentlandite (Fe,Ni)9S8 Stage I py pyrite FeS2 Stage IV pyh pyrrhotite Fe12xS Stage I qz quartz SiO2 Stage I* rs rasvumite KFe2S3 Stage II sa saponite (Ca/2,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2´4 H2O Stage IV sc scawtite Ca7Si6(CO3)O18´2 H2O Stage IV sd sodalite Na8Al6Si6O24Cl2 Stage III sh scheelite CaWO4 detrital so schorlomite Ca3Ti2(Fe,Si)3O12 Stage I sph sphalerite ZnS detrital ta thaumasite? Ca6Si2(CO3)2(SO4)2(OH)12´24 H2O Stage IV tb tobermorite Ca9Si12O30(OH)6´4 H2O Stage IV th thorite ThSiO4 detrital ti titanite CaTiSiO5 Stage I* tm thomsonite NaCa2Al5Si5O20´6 H2O Stage IV to thorianite ThO2 Stage IV ty tilleyite Ca5Si2O7(CO3)2 Stage II ur uraninite UO2 detrital uv uvarovite Ca3Cr2Si3O12 Stage I vs vesuvianite Ca10Mg2Al4(SiO4)5(Si2O7)2(OH)4 Stage II wo wollastonite CaSiO3 Stage I±III zrn zircon ZrSiO4 Stage I* X1 new species Ca3.5(Th,U)1.5(SiO4)3(OH)
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