Arfvedsonite in Basalt Dvkes. Buchans. Newfoundland

American Mineralogist, Volume 63, pages 413414, 1978

Arfvedsonitein basaltdvkes. Buchans. Newfoundland

R. W. Hexleyl M emorial Uniuersity of N ewfoundland St. John's, Newfoundland, Canada

Abstract

Arfvedsonite,an uncommonsodic amphibole, occurs in basaltdykes at Buchans,New- foundland.Stability and texturalrelations suggest that the mineralgrew during subsolidus coolingof basaltwithin the submarinegeothermal system which persisted during contempo- raneousmassive sulfide ore formation.It is only sporadicallypreserved, readily altering to chlorjteduring prehnite-pumpellyite facies metamorphism or associatedhydrothermal alteration. Sodicamphiboles may occurmore widely in similarparageneses than haspreviously beenrecosnized.

Arfvedsonite,the uncommon sodic amphibole,has al. (1974) and on the basisof optical properties.The been recognizedin prehnite-pumpellyite facies me- high Ti contents are similar to those reported in the tabasalts from the Buchans area, Central New- literature, and entry of Ti into tetrahedral sites is foundland. The mineral occurs sporadically within suspected through low tetrahedral cation totals. submarine vesicular basalt dykes cutting vesicular Single grains are heterogenous,but no systematic basalticandesite breccias of the Lower Silurian Red pattern of zoning could be detectedby scanningor Indian Formation ("Footwall Andesite") in drillcore point analyses; magnesium varies sympathetically 800 feet beneath the Buchans polymetallic massive with ferrous iron and to a lesserextent with calcium. sulphidedeposit. The geologyofthe areaand ofthese Compositionsvary markedly betweengrains. depositsis describedby Thurlow et al. (1975). The amphibole forms sharply bounded crystals The amphibole in this occurrence is strongly and crystal fragments (up to 0.15 mm long), occa- pleochroic from straw yellow to deep blue-green, sionally in ophitic contact with primary titanaugite with length-sloworientation, strong dispersion,and a and plagioclase. Isolated grains, often extensively maximum extinction angle of 22o. Absorption colors corroded, occur within patchesof secondarymatrix and extinction are variablewithin singlegrains, prob- or amygdular chlorite, and rarely the amphibole ably reflecting the compositional heterogeneityre- mantles quartz (?) or quartz-chlorite aggregates. vealedby electronmicroprobe analyses.The average These grains develop a distinct blue-green rim in of four partial electron probe analysesgave the fol- contact with matrix chlorite and become inter- lowing composition (weight percent):Sior 49.8,TiO, fingered with it through preferential replacement 2.14, Al2Os0.78, FeO 33.9, MnO 0.70, MgO 1.65, along amphibole cleavages(Fig. I ). Definitive micro- CaO 2.09, Na,O 6.86, K,O 0.90; semi-quantitative probe analysescould not be obtained in this region- analysisshowed the order of 0.057oCl but no detect- low analytical totals suggesthydration and partial able F. The structural formula estimatedby the pro- alteration to a chlorite intergrowth. Chlorite itself is cedure of Finger (1972), based on 23 oxygens,is Si an early-phasealteration product of matrix and an 7.74, Altv 0.14, Ti 0.25, Fe3+0.93, Fe2+3.48, Mn amygdular phase clearly postdating the amphibole. 0.09,Mg 0.38,Ca'0.35, Na (M4) 1.65,Na(A) 0.42, Pervasivelow-pressure (< I kbar) partial alterationof K(A) 0.18,OH 2.0.The amphibolewas diagnosedas the host basaltsto prehnite-pumpellyitefacies will be arfvedsonite,following the classificationof Papike et describedelsewhere (Henley and Thornley, in Swan- son, in preparation), and togetherwith extensivehy- I Presentaddress: Chemistry Division, D.S.I.R., Wairakei, dration and oxidation may have resulted from sub- PrivateBag, Taupo, New Zealand. sea-floorgeothermal activity associatedwith massive

0003-004x/78/0304-04I 3$02.00 413 HENLEY.'ARFVEDSONITE IN BASALT

some petrographicaffinity betweenthe arfvedsonite- bearing dykes and local alkali gabbro and granite intrusions. The upper stability limit of arfvedsoniteis depen- dent upon oxygen fugacity but is of the order of 700- 750'C (Ernst, 1968,p.77-l0l). This precludesa liq- uidus paragenesisfor the amphibole in these dykes but suggeststhat the arfvedsoniteis deuteric, nucle- ating and growing during submarinesubsolidus cool- ing. This parallelsthe deutericparagenesis of similar alkalic amphiboles previously described only from alkaline granites (reviewed by Deer et al., 1963). Black (1970) has documented a somewhat similar occurrence of riebeckite-arfvedsonite in glauco- phane-lawsonite-bearingbasalts from New Cale- donia.The amphiboleat Buchansis only sporadically preserved,having beenreadily alteredto chlorite during metamorphism,so that elsewherealkalic amphiboles may more commonly occur in this paragenesis than previously recognized. Recent studies have shown that amphiboles,generally of actinolitic affinity, are developed in some present-daygeothermal systems(P. R. L. Browne, New Zealand Geological Survey,personal communication).

Acknowledgments I wish to thank R. Jamieson,A. Thompson,J. Malpas,and V. S. Papezikfor discussionin preparing this note.

References

Black, P. M. (1970) Coexisting glaucophanti and riebeckite- arfvedsonite from New Caledonia, Am. Mineral., JJ, l06l-1064. Deer, W. A., R A. Howie and J. Zussman (1963) Rock-Forming Minerals, vol.2, Chain Silicates. Longmans, London. Ernst, W. G (1968) Amphiboles. Springer-Verlag, New York Finger, L. W. (1972) The uncertainty in the calculated ferric iron content of a microprobe analysis. Carnegie Inst. Wash Year Book. 71. 600-602. Papike,J J., K. L. Cameron and K. Baldwin (1974)Amphiboles (a) Fig. l. Arfvedsonite altering to chlorite along cleavages and pyroxenes'. characterization of other than quadrilateral (sample 542-952).(b) Large arfvedsonite grain, altering to chlorite components and estimatesof ferric iron from microprobe data. : and in sharp contact with plagioclase (sample 542-816). py Geol. Soc Am Abstracts with Programs,6, 1053-1054. pyroxene, pl : plagioclase, : = : chl chlorite, ep epidote, calc Thurlow, J G., E. A. Swansonand D. F. Strong (1975)Geology calcite. and litho-geochemistry of the Buchans polymetallic sulfide de- sulfideore formation. The basaltsand overlyingfelsic posits.Ecoz. Geol,70, 130-144. volcanicshave been ascribedan island-arccalc-alkaline affinity by Strong (in preparation), and Thurlow Manuscript receiued,April 13,1977; accepted (personal communication, 1977) has speculatedon for publication, October 25, 1977.