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Fenitization of Some Mafic Igneous Rocks in the Seiland Province, Northern Norway

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Fenitization of Some Mafic Igneous Rocks in the Seiland Province, Northern Norway Fenitization of some mafic igneous rocks in the Seiland province, northern Norway BRIAN ROBI NS & MA GNE TYSSELA ND Robins, B. & Tysseland, M.: Fenitization of some mafic igneous rocks in the Seiland province, northem Norway. Norsk Geologisk Tidsskrift, Vol. 59, pp. 1-23. Oslo 1979. ISSN 0029-196X. Against alkaline pyroxenite dykes, metamorphosed picrites in gabbro are converted into amphibole-clinopyroxene fenites. Increases in Al, Ti, Fe', Ca, K and Mn accompanied fenitization, Mg fell, and Cu, Sr, Y, Zr, Nb, La, Ce, and Nd were augmented. Mafic rocks against litchfieldite pegmatites were fenitized to mafic carbonatite or ijolite, felsic rocks to aegirine-augite syenites. Carbonatitic fenites developed by the introduction of Fe', Ca, K, Mn, P, and C, and the abstraction of Si and Al. Zn, Rb, Y, Zr, Nb, and the light REE increased during fenitization. Ijolitization was caused by increases in Na, K, Al, and Fe', while Si, Fe2, Mg, and Ca decreased. Fenitization was probably achieved through the agency of strong chloride brines. B. Rabins & M. Tysse/and, Geologisk institutt, Avd. A, Universitetet i Bergen, 5014 Bergen, Norway. The chemical and mineralogical changes in­ nepheline syenite. Currie & Ferguson (1972) volved in the evolution of fenites from mafic and regard the Callander Bay fenites as typical in ultramafic igneous rocks are poorly known, composition and origin, but apparently disre­ particularly in comparison with the 'classic' fe­ garded the contrary data presented by Verwoerd nites developed from felsic palaeosomes. This is (1%6) for south African fenites of mafic principally a result of the limited number of parentage. alkaline complexes and carbonatites emplaced The textural development of mafic igneous into an environment of basic igneous activity rocks during fenitization is highly variable. In (Table l). general, existing minerals are replaced by al­ Two main hypotheses have been advanced kaline varieties, and few new phases are regarding major-element variations during the nucleated; either feldspar or the mafic minerals fenitization of mafic rocks. According to Ver­ may be attacked first (Verwoerd 1966). The woerd (1966), both acid and basic rocks undergo same minerals as in 'classic' fenites are typical of essentially the same process during fenitization, fenitized mafic rocks, viz: sodic amphiboles, involving enrichment in alkalies, especially aegirine-augite, aegirine, and alkali feldspars sodium, and depletion of silica. Iron and (Verwoerd 1966). Nepheline can appear if the magnesium are often removed from mafic rocks fenitization progresses beyond von Eckermann's while they are added to felsic rocks, the former 'saturation point'. Biotite, unlike that in salic in large amounts (McKie 1966). The behaviour of rocks which is commonly replaced by aegirine­ calcium and aluminium is variable both in south­ augite or alkali amphibole at an early stage of em African mafic fenites, and in salic fenites in fenitization, may be stable and increase in general, while the addition of large amounts of amount throughout fenitization (Johnson 1961). carbonate ions is said to be unique to the Even in those cases where C02 has been intro­ Goudini complex (Verwoerd 1966). Based on a duced, calcite seems to be a relatively uncom­ study of fenitized garnet amphibolites in the mon mineral in the southern African mafic fe­ aureole of the Callander Bay complex and a nites, the carbonate-bearing phases usually be­ comparison with other occurrences of fenitized ing cancrinite and scapolite (Verwoerd 1966). mafic rocks, Currie & Ferguson (1972) conclude However, calcite occurs in fenitized basic rocks that the content of Si, Al, Na, K and H in the associated with the Blue Mountain nepheline standard cell increases while Ti, Fe, Mg and Ca syenite (Payne 1968), and is a vein mineral in decrease. Since Si is depleted in the associated many fenites derived from mafic rocks (Johnson felsic rocks and Na, Al, Fe, Mg, and Ca are 1%1, Garson 1962, Verwoerd 1%6). augmented, the contrasting fenites tend to con­ Regarding the role of C02, Verwoerd (1966) verge towards the composition of a melanocratic makes the tentative suggestion that it may be l -Geologisk Tidsskr. l/79 2 B. Rabins & M. Tysse/and NORSK GEOLOGISK TIDSSKRIFT l (1979) Table l. Campilatian of report ed oc.currences of fenitized mafic rocks. Fenitized Source Complex of Gains Losses References _ ___ rocks _ ____ eniti at ____ __ _ _ __ _ _! __ _z _ _ic..on__________ _ Spitzi<op, Gabbro ljolite? No,Ca, K, Mg, Si, Strauss & E. Transvaal P, C Fe, Al Truter 1951, ----- --- --- ----- - �e�r w r __ ·-- - -V � _d__1 96_6 Messum, Basalt, Nepheline Na,AI, Si,Ca, Mothiao 1956, Afr <:_CJ____ ___ bb s enite K S. W. � d_c:!���_,JjO r� y _____:. H-"'c.: .:.__ Fe,C Verwoerd 1966 Nemegasenda Gabbro Alkaline rock/ no o no l ytical data Persons o961 Lake, On tari o sjlvite Lillebukt/ Metagobbro Carbonatite Hp. co2, Heier 1%1 Pollen, Stjernj\y, P205, ?K20 ? Oosterorr. 1963 Norway _______________.___ _ --------- --·-- ------- Dorawa, Rhodesia Dolerite K 0,Na 0, Johnson ljolite/ 2 2 Si02 1961, corbonotite CaO,Fe, Verwoerd 1966 Tundulu, Daleri te Nepheline no analytical data Garson 1962 Nyasaland syenite/carbona­ tite 3 . Metagabb o Breivikbotn, 1 r Syenite/ Fe ,Mn, Sturt & Sl\rl'>y, Norway carbonatite Y,Zr,Nb Romsay 1965 2 Metagabbro Nepheline S 0 ,AI Fe0 ,Fe0, . i 2 2o3 3 syenite? Na20,K20 MgO,CaO Goudini, Transvaal Norite Carbonatite Na,C0 , Si,AI,Co, Verv.oerd 3 1966 OH, Fe Mg Swortbooisdrif, Anorthosi te Nepheline Na,C0 Verwoerd 3 Si 1966 S.W. Africa syenite/ OH carbonoti te Vishnevogorsk, Greenschist Alkali ne AI,K,Na, Si,Fe,Ca, Borodin & Urals, Soviet 11solutions" Sr,Ba,Nb, Mg, Ti,Mn Pavlenko 1974 Zr,RE,P Khibina,Kola, Diobose Khibinite K,Na,Mg, Ca,Si Borodin & Sov iet Fe, Al, Ti Pavlenko 1974 followed by: K,Na,AI,Si Ca,Mg,Fe,Ti M>oi:i,S.W. Gabbro Nepheline K,Na,AI, Co,Mg,Fe, Brock 1968 Tanzania syenite Si,P Ti Blue Mountain, Amphibolite Nepheline K20,Na20 CaO,MgO Payne 1968 O Ontaria syenite AI203, Fe H20,C02 Callander Bay, Garnet alkaline rock/ Si,AI,Na, Ti,F, Currie & 2 Ontario omphibo lite carbonatite K,H Mg,Ca Ferguson 197 NORSK GEOLOGI SK TI DSSKR!Ff l (1979) Fenitization of mafic rocks 3 necessary to distinguish between fenites as­ The present account describes the textural and sociated with alkaline intrusives alone, and those chemical evolution of fenites deri ved from mafic in conjunction with a carbonatitic phase. Many dykes and gabbroic rocks crosscut by composite other authors have discussed the relative im­ alkaline pyroxenite/melanocratic syenite pegma­ portance of alkali ne rocks versus carbonatites as tites and litchfieldite pegmatites. Both of these the source of fenitizing solutions (King & Suther­ groups of alkaline intrusives embrace a wide land 1960, Dawson 1964, Woolley 1969). range of chemical variation in individual dykes; It has been known for a considerable time that typical analyses are listed in Table 2. One alkaline intrusives and carbonatites were em­ characteristic of the felspathoidal syenite dykes placed in the syn-Caledonian Seiland province not illustrated in Table 2, and which distin­ towards the dose of an extended period of basic guishes them from the alkaline pyroxenites, must and ultrabasic magmatism (Hoel & Schetelig be emphasised since it is almost certainly of 1916, Barth 1927, 1953, Strand 1952). However, major importance in the interpretation of the fenitization of mafic host rocks was not de­ respective fenites: The syenites are typically rich scribed until quite recently when an aegirine­ in calcite, either as patches representing a augite pyroxenite from the Breivikbotn alkaline carbonatitic mesostasis, or as large segregations complex was recognised as a metasomatized containing albite, apatite, and biotite, which are metagabbro (Sturt & Ramsay 1965). Field evi­ viewed as the result of the co-existence of im­ dence from the same complex, as well as from miscible syenitic and carbonatitic fluids (Robins the Dønnesfjord area (Appleyard 1965), sug­ 1972). The pyroxenite/melanocratic syenite gested the fenitic derivation of gneissic dykes, however, never contain calcite in more nepheline syenites (litchfieldites) from both than accessory amounts (Robins 1974). psammitic metasediments and metagabbros, By collecting from fenitized mafic dykes, one consequent upon the emplacement of volatile­ of the major problems in petrochemical studies rich nepheline syenite magmas. Heier (1961) of fenitization, viz. initial homogeneity of com­ attributed an as sumed absence of fenitization of position, is as effectively guaranteed as is possi­ the gabbros surrounding the major Lillebukt ble. The changes in chemistry recorded in alkaline complex to a lack of alkalies in the fenitized dykes are compared in certain cases metasomatizing agent. The mafic rocks were with changes which occurred in gabbroic rocks believed to have been converted to whose initial composition was considerably less 'hornblendite' when gaseous H20 and C02 were uniform. A limited amount of data referring to introduced during carbonatite emplacement. the fenitization of almandine-amphibolite facies However, it was suggested that the biotite-rich psammitic metasediments are also presented for hands in the central carbonatite originated by comparison. reaction of gabbro with potassium-rich carbona­ The method employed has a further tite (Heier 1961: !50). A similar conclusion was advantage: There can be no doubt of the identity reached by Oosterom (1963) for both the Lil­ of the source of the fenitizing agents. The fenitic lebukt and Pollen complexes; metasomatic intro­ aureoles are relatively narrow with respect to the duction of H20, C02, and P205 was held to be associated intrusions, which are themselves responsible for the ongm of both the minor and, in all cases, widely separated by hornblendites and carbonatites. unfenitized rocks. An unusual aspect of the Seiland province is that fenitization occurred during the course of orogenesis and Barrovian metamorphism.
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