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The Mineralogical Magazine and Journal of the Mineralogical Society THE MINERALOGICAL MAGAZINE AND JOURNAL OF THE MINERALOGICAL SOCIETY No. 151 December, 1935 Vol. XXIV .Metasomatism associated wish the greenstone-hornfelses of Kenidjack and Botallack, Cornwall. (With Plates u and VII.) By C. E. TILLEY, Ph.D., B.Sc. Professor of Mineralogy and Petrology, University of Cambridge. [Read June 6, 1935.] N a paper entitled ' Hornfelses from Kenidjack, Cornwall' 1 the I nature and origin of the rock types characterized by anthophyllite and cummingtonite occurring in the aureole of the Land's End granite were discussed. Since these rocks were 'investigated in 1928 the writer has extended his study of them both in the field and in the laboratory, with the result that further light on the origin of these rock types has been obtained. It is the purpose of these notes to examine in more detail the evidence which is considered to add to their understanding. In the Kenidiack paper the petrography of the rocks was described at some length and in the present paper petrographic data will be given only in so far as they supplement the previous account, which may be consulted for a detailed statement. The contact altered yreenstones. The greenstones at Kenidiack are known only in their contact altered state. In the Kenidiack paper their mineralogical composi- tion was described but hitherto no chemical data on these rocks have 1 C. E. Tilley and J. S. Flett, Summ. Progr. Geol. Surv. Great Britain, 1930, for 1929, pt. 2, pp. 24-41, hereafter referred to as the Kenidjack paper. [M.A. 4-402.] P 182 c.E. TILLEY ON been presented. The account now given supplies this deficiency. The rocks are essentially hornblende-plagioclase types. Prior to con- tact metamorphism they had been subject to shearing and dynamic metamorphism so that banded types are prevalent among them. This fact had been realized long ago by Allport I in his description of these rocks in 1876, and has been emphasized both in the account in the Survey Memoir and in the Kenidjack paper. Though there is considerable variation in the relative proportions of the component minerals, the mineralogy is comparatively simple, the variety being dominated by the relative proportions of green hornblende and plagioclase. It is true that hornfelses occur among them containing grossular, epidote, diopside, and axinite, yet these types are com- paratively uncommon in the coastal region betweeri Kenidjack and Botallack dealt with in this paper. TABLE I. Analyses of hornfelses and cummingtonite. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Si0~ 40-25 49.10 58-15 48.87 47.07 33.58 52-60 51-15 53.55 53-12 TiO~ 2-65 1.20 1.85 2.71 2.12 2.96 1-85 2.20 0.40 0-21 A120a 17-22 10.54 12.95 16.71 20.20 11-79 20.64 16.42 8.40 2.78 Cr~Oa -- -- -- 0.01 0.02 ..... Fe~0a 1.88 0-87 trace 0.13 trace 11.27 0.20 4-42 0.74 0-25 FeO 11-16 10.27 10-76 12.85 12-73 17.50 7.66 11.91 26.75 22.46 MnO 0-24 0.17 0-12 -- 0.11 0-21 trace trace 0.25 0.27 Mg0 7.78 10.72 6.44 9.60 10.09 15.55 5-02 3.92 2.37 ]5.46 CaO 14-82 14.00 7.05 5.42 2.14 0.89 2-45 1-50 1.20 2-26 Na~O 1-20 1.22 1.02 1.61 1.48 0.60 2-28 3-57 0.26 -- K~O 0-97 0.10 0.14 0.27 1.97 0.32 4.73 2.83 2.94 -- H20+ 0-90 0.90 0.60 0.99 1-30 4.60 1.80 1.60 2-15 3.33 H~O - 0-20 0.10 0.50 0.07 0-16 0.13 0.40 0-30 0-65 -- P~O~ 0-65 0-78 0.49 0.50 0.12 0-36 0.40 0.04 0.06 -- C02 nil nil nil 0-07 -- nil nil nil nil -- 99-92 99.97 100.07 99.81 100.21 99-78 100.03 99.86 99.72 100.14 Sp. gr. 3-203 3.031 2.982 2.966 2-907 3.041 2.819 2.916 3.019 -- No. 5, also FeS$ 0.47, Li20 0.03. No. 6, also C1 0-02, F nil. 1. Greenstone-hornfels, Carn Kenidjack. Analyst, W. H. Herdsman. Green hornblende, small amounts of colourless augite, plagioc]ase (replaced by white mica), ilmenite, sphene, and apatite. 2. Greenstone-hornfels, Zawn a Bal. Analyst, W. H. Herdsman. Bands of green hornblende, alternating with plagioclase bands containing accessory ilmenite, sphene, and apatite. 3. Cummingtonite-plagioclase-hornfels, North Zawn quarry. Analyst, W. H. Herdsman. Plagioclase (AblAnl} with some anthophyllite, quartz, cordierite, biotite, ilmenite, and apatite. 1 S. Allport, Quart. Journ. Geol. Soc. London, 1876, vol. 32, p. 407. METASOMATISM OF GI4EENSTONE-HORNFELSES, CORNWALL 183 4. Cummingtonite-anthophyllite-cordieritc-plagioclase-hornfels, coast beiween North Za~;'n and Wheal Edward Zawn, Kenidjack (with ihnenite and biotite). Analyst, B. E. Dixon, ~umm. Progr. (~eol. Surv., 1930, for 1929, pt. 2, p. 32. 5. Anthophyllite-cordicrite-hurnfels, Carn Kenidiack (with biotite, plagioclasc, ilmenite, a,ld some eummingtonite). Analyst, B. E. Dixon. Summ. Progr. Geol. Surv., 1930, for 1929, pt. 2, p. 28. 6. Anthophyllite-cordierite-(clinochlore)-spincl-diaspore-hornfels, Innerquarry, Kenidjack (with magnc!ite, ilmenite, apatite, and biotite). Analyst, R. Sutcliffc, Summ. Progr. Geol. Surv., 1930. for 1929, pt. 2, p. 33. 7. Cordieritc-biotite-horafels, Zawn ~ Bal, Botallaek. Analyst, W. H. Herdsman. Cordierite, biotite, quartz, oligoclase-albite, ilmenite, rutile, apatite, muscovite. 8. Cordierite-biotite-hornfels, Zawn a Bal, Botallack. Analyst, W. 14. Herdsman. Cordierite, biotite, quartz, albite-oligoclase, ilmenite, magnetite, apatite. 9. Gruncrite-biotite-hornfels, Zawn a Bal, Botallack. Analyst, W. 14. Herdsman. Grunerite, biotite, quartz. 10. Cummingtonite from cummingtonitc-biotite-cordierite-hornfels, Rifle Ranges, Kenidjack. Analyst, R. Sutcliffe. Water below 105~ not separately determined. Sign and elongation positive, optic axial plane parallel to (010). Extinction ~, : c = 19~ refractive indices a 1.643, ~ 1.670. Analysis corresponds to (OH)3.2(Ca,Fe,Mg,Mn,Ti)6.~(Si,A1)sO22. Summ. Progr. Geol. Surv., 1939, for 1929, pt. 2, p. 31. To determine the range of chemical composition amongst these greenstones, two varieties have been chosen for analysis. In the one, the dominant mineral is grce 9 hornblende ; in the other, hornblende and plagioclase form alternating bands. These two types may be taken as representing the prevalent varieties among the greenstones of the area. A hornblende-rich type from Cam Kenidjack chosen for analysis gave the result under l, table I. Green hornblende is the dominant mineral forming more than 80 ~ of the rock. The mineral is optically negative, ~:c = 19 ~ and has fl 1.678. The remaining constituents are granules of colourless augite, ilmenite, sphene, and small amounts of plagioclase now partly replaced by white mica (? 1.586). The analysis makes it clear that this hornblende must be a highly aluminous type. A hornblende with high AI.~O3 (16-75~ has already been recorded by J. A. Phillips 1 from the greenstones of the Botallack cliffs. The second rock (analysis 2, table I) comes from the promontory on the north side of Zawn a Bal. In this, bands rich in green horn- blende alternate with bands composed of plagioclase and ilmenite (fig. 1). Granular sphene in part replaces the ilmenite. The horn- blende is optically negative, ~ : c = 19 ~ and has fl 1.650. The plagio- i j. A. Phillips, Quart. Journ. Geol. Soc. London, 1876, vol. 32, p. 167. 184 C.E. TILLEY ON clase bands contain a minutely granular plagioclase of composition AblAn3, fl 1.569, with streams of ilmenite partly replaced by sphene, together with minute granules of apatite. The modal composition of the rock is green hornblende 64, plagioclase 30.5, ilmenite, magnetite, and sphene 3.5, apatite 2%. In contrast with the hornblende of the first rock the hornblende of this greenstone-hornfcls must be practi- cally flee from alumina, as the rock itself contains only 10.54% Al20 a. The low silica percentage of the Cam Kenidjack greenstone-hornfals is accounted for by the replacement of silicon by aluminium in the constitution of its dominant amphibole. In table II the analyses of these two hornfelses are set down together with comparative analyses. The principal distinguishing feature of the analyses is the high lime percentage and low soda figures. Com- parable figures can be found amongst existing analyses of basic igneous rocks but it seems probable that these greenstones may have suffered some enrichment in lime during the shearing (and segregation) to which they have been subject prior to contact metamorphism. TA.BLE II. Analyses of type varieties of greenstone, with comparative analyses. 1. la. 2. 2a. 2b. 2c. 3. Si02 ... 40.25 42.08 49-10 48.72 49-75 48.70 44.67 TiOu ... 2-65 2.26 1.20 1-04 4.60 2.41 1.92 A120 s ... 17.22 16-04 10.54 11-75 12-18 12.61 13.88 Fe20 s ... 1.88 5.93 0.87 1.44 0.03 3.26 1.37 FeO ... 11-16 8-75 10.27 6.63 9"88 5.65 10.72 MnO ... 0.24 0.32 0-17 0.45 0.48 0-03 0.20 MgO ... 7.78 6.95 10.72 10-92 9.54 8-49 9.25 CaO ..
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