Journal of the Geological Society, , Vol. 145, 1988, pp. 37-41, 1 fig., 5 tables. Printed in Northern Ireland

The distribution of ammonium in granites from South-West

A. HALL Department of Geology, Royal Holloway and Bedford New College, Egham, Surrey TW20 OEX, UK

Abstract: The ammonium contents of granites, pegmatites and hydrothermally altered rocks from SW England have been measured. Ammonium levels in the granites are generally high compared with those from other regions, averaging 36ppm,and they differ markedlybetween intrusions. The pegmatites show higherammonium contents than any other igneous rocks which have yet been investigated. Ammonium contents are strongly enriched in the hydrothermally altered rocks, includ- ing greisens and kaolinized granites. There is agood correlation between the average ammonium content of the intrusions in SW England and their initial "Sr/*'Sr ratios and peraluminosity. This relationship supports the hypothesis that the ammonium in the granites is derived from a sedimentary source, either in the magmatic source region or via contamination of the magma.

Introduction Results Ammonium is present as a trace constituent of granitic The granites rocks, in which it occurs in feldspars and micas substituting isomorphously for potassium (Honma & Itihara 1981). The The new analyses of Cornubian granites are given in Table amount of ammonium in granites varies from zero to over 1. They show a range of 3-179 parts per million NH:, with 100 parts per million, and it has been suggested that high the highest values being found in relatively small intrusions. concentrations may indicate the incorporation of organic- Taking the averagefor each of themajor intrusions,and rich sedimentary material into the magma, either from the weighting them according to their relative areas (see Table presence of such material in rhe magmatic source region or 4), the average ammonium contentof the Cornubian granites via the assimilation of organic-rich country rocks (Urano as a whole is 36 ppm. This is a somewhat higher level than in 1971; Itihara & Honma 1979; Hall 1987). The Variscan other granites that have been investigated: i.e. Cornubian granites of SW England show several indications of a granites, 36 ppm; all granitesand granodiorities (global substantialsedimentary contribution to theirparent magmas, forexample in their high initial H7Sr/s6Srratios, Table 1. Ammonium contents of Cornubian granites and they can therefore be used to test this hypothesis. These Specimen NH; granites also offer the opportunity of studying the influence Intrusionnumber Locality (Grid Ref.) (ppm) of post-magmatic hydrothermal phenomena on the distribu- tion of ammonium. P481 Dartmoor FernworthyForest (656836) 3 P487 Dartmoor Marytavy (545825) 6 P476 Dartmoor Merrivale (546753) 8 Analytical techniques P477 Dartmoor Dartmeet (680734) 9 P482 Dartmoor The granites which were analysed were chosen torepresent the. Chagford (713885) 11 P10 BurratorDartmoor (550680) 18 most widespread and common lithologies, with a principal aim of P547 Dartmoor BrentSouth (682631) 19 estimatingthe total ammonium content of theparental magmas. G11 HingstonG11Down Enrichment in ammonium associated with kaolinization was noticed Chilsworthy77 (410718) P453 BodminMoor early in thestudy, and thesuite of analysed rocks was therefore Cheesewring (262720) 84 P13 BodminMoor Hill (272705)87 restricted to those in the freshest possible condition. The majority P452 BodminMoor of the specimens were collected from quarry faces. Where outcrop Bolventor (177757) 111 P450 specimens were used they were obtained by breakingopen very (052588) 15 c7 Cligga Head Cligga Head (737538) 179 large blocks and their freshness was checked petrographically. The P512 Carnmenellis Polkanuggo50 (741347) criteria used to indicate freshness were minimal alteration of biotite P112 Carnmenellis Longdowns (738337) 50 to chlorite, and minimal presence of fine-grained alteration products P44 HernissCarnmenellis (735345) 58 in feldspar, although the feldspars in the granites of this region are P114 Carnmenellis Carnsew93 (759345) rarely completely clear. P113 Carnmenellis Mabe (743322) 102 The method of analysis was essentially that of Urano (1971). P23 Carnmenellis Pendarves (647383) 116 Samples were decomposed by an HF-H,S04attack, andafter P517 Carnmenellis Praze-an-Beeble (649353) 140 addition of NaOH the ammonia was separated by distillation and P49 Tregonning Wheal Reeth (589302) 161 measured colorimetrically ammoniumas mercuric iodide. P400 Lands End LamornaCove (450240) 16 Appropriate precautionswere takento minimize contamination P36 Lands End Geevor21 (374346) during the determination: deionized water was used throughout, all P404 Lands End(459342) Mill New 22 reagents were analysed for NH:, and reagent blanks were run with P4 1 Lands End Castle-an-Dinas (486346)37 every batch of samplesanalysed. Results are reproducible to P402 Lands End Sennen Cove (350263) 42 within 3 ppm for values below 30ppm, andto within 10% at P31 1 Lands End Porthmeor (425377) 53 higher levels. 37

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average), 27 ppm; British Caledonian granites, 33 ppm. The volumetrically significant major rock types rather than worldwide average is based on the figure given by Wlotzka minor varieties, but bulksamples of severalpegmatites (1972) of 21 pprn expressed as total elemental nitrogen. The were analysed. The similarity in ionic size and chemical figure for BritishCaledonian granites is an unweighted properties between ammonium and rubidium suggested the average of specimens from 32 intrusions (Hall 1987). possibility that ammonium might resemble rubidium in its It is noticeable that there are different ammonium levels geochemical behaviour, and would therefore be enriched in in each of the intrusions in SW England (Fig. 1). This is in pegmatites in the same way as rubidium. This expectation marked contrast tothe lack of variability in theirmajor turnedout to bejustified, as the following NH: contents element compositions. For example,the average K,O were found in pegmatities from three localities: content of themajor intrusionsranges only from 5.0 to 5.3%, and yet the average ammonium contents vary from Knills Monument, St Ives (P129) NH: = 135 ppm 11 ppm in the Dartmoor granite to 94 ppm in the Sands,Praa easternend (P308) 163 ppm Moor granite. Megilligar Rocks,W of (P309) 332 ppm It would beinteresting to know whether there is any systematicvariation in ammonium levels within the Each of these pegmatites has a distinctive mineralogy. That individual plutons, but in practicethis is difficult to of Knills Monument contains minor tourmaline in addition establish. The largerplutons are undoubtedlycomposite, to quartz andfeldspars, but has a negligible mica (lepidolite) butthey are not sufficiently well exposed for detailed content.The Praa Sandspegmatite contains quartz, structuralsubdivision, although thebroad distribution of feldspars andabundant zinnwaldite. The pegmatite of petrographictypes hasbeen mapped out (Dangerfield & Megilligar Rocks is composed of quartz, feldspars anda Hawkes 1981). One particular type of granite predominates small amount of zinnwaldite. The zinnwaldite-bearing in all the intrusions, namely a coarse porphyritic rock with pegmatites are associated with microgranite offshoots of the K-feldspar megacrysts, and the majority of the specimens Tregonning granite, which appearsto be rather richin analysed here are of this type. The megacrystic facies varies ammonium,whereas the Knills Monumentpegmatite is in NH: content within as well asbetween individual intruded into the Lands End granite, in which ammonium intrusions, and there is no obvious relationship between the contents are generally more moderate. It may be surmised NH: content and the abundance of phenocrysts. thatthe ammonium contentof pegmatites is determined The variationin ammonium content within intrusions is partially by the availability of NH: in the main magma body greaterthan the variation in majorelements, especially from which they were derived, and partly by enrichment in potassium, and there must be a suspicion that some of this the process of pegmatite formation. Assuming thatthe variation is due to incipient hydrothermal alteration, despite pegmatitesformed by the crystallization of magma in the theapparent freshness of the analysed specimens. It is presence of exsolved water, according tothe widely noticeable thatthe minor graniteintrusions of Hingston accepted view of Jahns & Burnham (1969), this would Down, Cligga andTregonning, which show high NH: suggest that NH: (like both K' and Rb+) is preferentially levels, are foci of intense hydrothermal activity. Onthe partitioned into the exsolved aqueous phase compared with other hand, relatively high values of NH: were also found in its parent magma. The association between the ammonium very fresh granites from the eastern partof the Carnmenellis ion and water rather than silicate melt may perhaps be a granites(Longdowns, Carnsew, Mabe)where all types of reflection of the propensity of the NH: ion for hydrogen hydrothermal alteration are rare.Thus the major differences bonding.This is apparent from the crystal structures of in NH: content between the Dartmoor, Carnmenellis and various ammonium salts, and is held to be responsible for Lands End granites at least are thought to be of magmatic differences in co-ordination of the NH: and Rb' ions origin. despite their similarity in ionic size and charge (Baur 1974). Each of the pegmatite bodies is zoned or layered, and a The pegmatites moredetailed study would be neededto trace the The main emphasis in sampling for this study was on the distribution NH:of within the pegmatitesand their constituent minerals, but it is clear that the pegmatites as a

NH,+ contents whole are rich in NH:. The Megilligar pegmatite has in fact

0 -20 pp". the highest ammonium content yetrecorded from an 0 20-50 pp" ,so pp". igneous rock. A very high concentration of ammonium was DARTMOOR also found in the so-called TrelavourDowns 'pegmatite' associated with the St Austell granite (Ussher et al. 1909). This consists of a dyke-like body 50 cm across, composed largely of extremelycoarse-grained biotite (lithian sidero- phyllite according tothe analysis of Cundy et al. 1960) with narrow margins of afiner-grained feldspar-biotite rock. The biotite-rich rock was found to contain 480 ppm of NH:. Although the TrelavourDowns body may well be pegmatitic in character, the biotitic core should probably be 50 km regarded as a mineral segregation. l

Fig. 1. The granites of SW England, showing the locationand NH: Hydrothermal alteration contents of analysed specimens. Theminor intrusions are:CH, The granites of SW England have been extensively affected Cligga Head; HD, Hingston Down; TR, Tregonning. by postmagmatichydrothermal alteration.Three types of

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alteration are particularly common: greisenization and Table 3. Ammonium contents of minerals from thegranite and tourmalinization which are localized in their distribution greben of Cligga and are of high-temperature origin, and kaolinization which affects much greater volumes of rock and, in the light of NH:/K K,O NH: isotopic and fluid inclusion studies, is considereda to be (PP4 (%l (x100) low-temperatureprocess, perhaps considerably post-dating magmatic emplacement. To illustrate the effects of eachRock Unaltered 179 0.46 4.66 451 0.40 13.51 of of Orthoclasegranite(C7) type alteration, examples freshand altered material Muscovite 349 0.42 9.92 were selected from three different intrusions and analysed Biotite 635 0.81 9.46 with the results shown in Table 2. In all the examples of greisenization there was found to RockGreisenized 340 5.22 0.78 be substantiala increase in ammonium. There is no granite(C203)Orthoclase 900 15.39 0.70 mineralogical reason to expect that greisenization would Muscovite 638 10.40 0.74 causea rock to becomemore of less accommodating to Biotite - absent - NH;, since the change involves areplacement by one Greisen(C221) Rock 287 0.79 4.39 potassic mineral (muscovite) at the expense of biotite and Orthoclase - absent - orthoclase, and the process does not involve a major loss or Muscovite 691 10.47 0.80 gain of K,O (Hall 1971). The increasein ammonium is Biotite - absent - thereforeattributed to a high NH: content in the metasomatizing fluid. The ultimatesource of this NH: cannot be determined, but in view of the large-scale nature altered, the only obvious difference from the fresh granite of the postmagmatichydrothermal convection systems being the disappearance of l-2% biotite. Two completely associated with the cooling granites (Durrance et al. 1982) a greisenized granites (C201, C221) gave values of239 and source in the country rocks or even at the ground surface is 287ppm.These are much more obviously altered, having perhaps more likely than metasomatic transfer from another lost all their feldspar as well as biotite. part of the intrusion.A magmatic source is nevertheless The effect of tourmalinization on the ammonium contents possible, since the pegmatities show that N&* can be is clearly relatedto the mineralogy of the tourmalinized enriched in water exsolved from the crystallizing magma. rocks. The products of tourmalinization are highly variable, Someadditional specimens which were analysed from and each of the examples examined has a different mineral Cligga Head show thatthe enrichment in NH: is nota assemblage. In the example from the Lands End granite, the linear one. The NH: content of the unaltered granite (C7) is tourmalinized rock is the variety known as ‘trevalganite’, 179 ppm.Two partially greisenized granites (C203, C204) and it consists of quartz, feldspar and tourmaline.It is yielded values of 340 and 333 ppm. These are not severely considerably richer in NH: than the unaltered granites of this intrusion. Inthe examplefrom StAustell, the tourmalinized rock is ‘luxulyanite’ which consists mainly of feldspar and tourmaline, with some mica. It is very strongly Table 2. Ammonium contents of hydrothermally altered granites enriched in ammonium. The example from the Hingston from SW England Downgranite is a ‘quartz-schorl rock’ consisting only of quartzand tourmaline, and as neither of these minerals NH; contains potassium as an essential constituent the depletion Specimen (ppm) Locality (ppm) Specimen in ammonium is to be expected. ~~ 1. Greisenization The kaolinizedgranites show astriking enrichment in ammonium compared with unaltered granites from adjacent 179 Cligga Head CliggaUnaltered C7 Head areas.Unlike the freshgranites, the kaolinized rocks C221 Greisen 287 Cligga Head HingstonDown G11Unaltered 77 Chilsworthy containsecondary phyllosilicate minerals which can (coarsefacies) GreisenG9 166 Chilsworthy accommodateexchangeable cations such as NH: in a HingstonDown UnalteredG5 137 Chilsworthy loosely held form. Of the kaolinite-groupminerals, (fine facies) Greisen G7 198 Chilsworthy disorderedkaolinite and halloysite have a large cation exchange capacity, although well-crystallized kaolinite does 2. Kaolinization not. However, china clays from SW England have also been Lands End P36 Fresh 21 Geevor found to contain smectitesas well as minerals of the P146 Kaolinized 106 Balleswidden kaolinite group (Exley 1976; Talibudeen & Goulding 1983), St Austell P450 Fresh 15 Luxulyan andthese minerals have an evengreater ability to hold P73Kaolinized 138 Luxulyan exchangeable cations. As in the case of greisenization, the Tregonning P49Fresh 161 Wheal Reeth source of the NH: taken up by the kaolinized granites could P140 Kaolinized 250 Tolmennor lie in the granite or the country rocks, or in the soil horizon 3. Tourmaliniration overlying bothat the time of kaolinization. Additional Lands End P404 Unaltered 22 New Mill ammonium may even have been absorbed from present-day P221Tourmalinized 77 Trevalgan groundwaters. St Austell P450 Unaltered 15 Luxulyan P184Tourmalinized 216 Luxulyan Distribution of ammonium in minerals Hingston Down G11Unaltered 77 Chilsworthy G21Tourmalinized 24 Chilsworthy Table3 shows the distribution of ammonium in three - specimens from the Cligga Head granite. These analyses

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show not only the distribution of ammonium between the Table 4. Average ammonium contents of the major granite intrusiorrr various host minerals but also the effects of postmagmatic of SW England, compared withtheir initial 8%/86sr ratiosand alterationonthe ammoniumdistribution. The rocks average contents of normative corundum (C) represent asequence from fresh granite to greisen inan outcrop which is characterized by the presence of a swarm Mean NH: 87Sr/86Sr Normative Outcrop of greisen-bordered mineralized quartz veins. (ppm) (initial)’ C (%) area (km’) Inthe unaltered granite (C7) the most ammonium-rich 11 0.7094 1.44 688 mineral is biotite, which in this particular locality is a rather Dartmoor St Austell 15 0.7095 1.63 94 iron-rich lithian variety (Hall 1971). Orthoclase has the next Lands End0.7133 32 2.31 206 highest NH: content, followed by muscovite. The sequence Cammenellis0.7130 87 2.55 144 of decreasing NH: contents,i.e. biotite-orthoclase-mus- 94 0.7166 2.43 214 covite, differs from that found by Honma & Itihara (1981) in granitic gneisses fromJapan, i.e. biotite-muscovite- ‘The initial87Sr/86Sr ratios and analyses used to calculate orthoclase, but is in agreement with the finding by Wlotzka normative corundum are from Darbyshire & Shepherd (1985). (1972) of higher NH: inorthoclase than muscovite in a pegmatite from West Germany.The analyses of the greisenized granite (C203) confirm that orthoclase is richer countryrocks fromthe whole of theUpper Palaeozoic in ammonium than the muscovite. Cornubian massif collected foranother project. The The weight ratio lOONH:/K which is given in Table 3 is sampling plan was to collect one specimen from each of the an indication of the enrichment of eachmineral in NH: 10 X 10 km squares of the National Grid to represent the relative to K’ for which it is presumed to substitute. In both principal lithostratigraphic unit in each square. All of these the unaltered and greisenized granites the muscovite does in specimens weresandstones, shales or slates. Thear- fact have a slightly greater substitution of NH; for K+ than gillaceous rocks were divided into two categories according the orthoclase, although the difference is not great, and the to their colour. Supplementary specimens were collected to biotite is clearly strongly favoured as a host for ammonium. representother less abundant lithologies such as volcanic The ionic weight of NH: is less than half that of K+ (18.04 and related intrusive rocks, and limestones.Composite compared with 39.10) so the NH:/K ratiosin Table 3 samples of each of the country rock lithologies were then represent ionic substitutions ofNH: for K+ of between 0.8 prepared by mixing 1g portions of the individual powered and 1.7%. rocks. Thesecomposite samples were analysed with the results shown in Table 5. Unlike the granites, in which all the combined nitrogen Discussion can be assumed to be present in the form of the ammonium The main features of the ammoniumdistribution in ion, either fixed or exchangeable, the sedimentary rocks Cornubian granites are the generally high levels of NH:, may containorganic nitrogen compounds in addition to and the large difference in NH: content between individual ammonium-bearing clay minerals. These compounds could intrusions. include some that are readily hydrolysed to yield ammonia It is generally believed that the trace ammonium contents and others that are present in acid-resistant kerogen. In the of granitic rocks ultimately have a sedimentary origin, either method used to analyse these rocks, the former compounds through the partial melting of a metasedimentary protolith are measuredas ammonia along with that extractedfrom orthrough contamination of the magma (Urano 1971; the silicates, while the kerogenremains undissolved. The Itihara & Honma 1979). The high ammonium contents of amount of ammoniarecovered is therefore only an the Cornishgranites arequite consistent with this view, approximateguide tothe amount of ammoniumion that since thereare several lines of evidence to suggest a might become available to the magma if these rocks were substantial pelitic contribution to the parent magmas. The partially melted or assimilated. Table 5 gives the actual Cornubian graniteshave many ‘S-type’ features, including amounts of ammoniaextracted fromthe country rock high initial 87Sr/86Srratios of 0.709-0.717 (Darbyshire & samples,from which the equivalentammonium concen- Shepherd 1985), generallyperaluminous major element trations have been calculated. chemistry, high K,O contents(and hence abundant The country rock materials can be seen to have ample K-feldsparmegacrysts), and high levels of trace elements ammonium from which that in the magmas could have been which are most abundant in pelitic sediments (B, As, Li, Sn). derived,even allowing for some to be lost during Table 4 compares the average NH: content of each of the metamorphismbefore it could beincorporated in the main intrusions with its initial 87Sr/86Srratio and normative magma. The country rocks are also a potential source for corundum.There is a definitecorrelation between the ammonium content of each intrusion and its initial 87Sr/86Sr Table 5. Extractable ammonia andequivalent ammonium contents ratio and also with its peraluminosity. The Dartmoor granite of SW England country rocks has the lowest value of both indices and the lowest NH: content, whilst the other granites have a more pronounced No. of specimens Extractable Equivalent ‘S-type’ characterand higherammonium contents. These in composite NH3 (ppm) NH: (ppm) relationships provide further evidence that the ammonium in the granite is of sedimentary origin. 330 Sandstones312 25 As a guide to the possible amounts of ammonium likely to Grey 973shales 49 1031 be available in the country rocks, composite samples of1345 the Black shales1270 23 major types of Cornubian country rock have been analysed. Limestones 30 4 28 Volcanic rocks The specimenswere drawn from largea 60collection of 57 20

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redistribution by circulating waterduring postmagmatic DANGERFIELD,J. & HAWKES,J. R. 1981. The Variscan granites of south-west alteration. The argillaceous rocks are the richest potential England: additional information. Proceedings of the Ussher Society, S, 116-20. source of ammonium, but even the volcanic rocks contain a DAREIYSHIRE,D. P. F. & SHEPHERD,T. J. 1985. Chronology of granite significant amount. These volcanic rocks are spilitized magmatism and associated mineralization, SW England. Journal of the basalts, diabasic minorintrusions and some tuffs, and Geological Society, London, 142, 1159-77. contain or may have containeda variety of secondary DURRANCE,E. M,, BROMLEY, A. V., BRISTOW,C. M., HEATH, M. J. & PENMAN,J. M. 1982. Hydrothermal circulation and post-magmatic minerals with a cation exchange potential. changes in granites of south-west England. Proceedings of the Ussher The exact route by which the ammonium has entered the Society, S, 304-20. magma isdifficult to identify. The ammonium-bearing EXLEY, C. S. 1976. Observations on theformation of kaolinite in theSt. sedimentary source materials may have been present in the Austell granite, . Clay Minerak, 11, 51-63. HALL, A. 1971. Greisenisation in thegranite of Cligga Head, Cornwall. magmatic sourceregion, or they may have been Proceedings of the Geologists’ Association, 82, 209-30. incorporated as xenoliths during high-level contamination of - 1987. The ammonium content of Caledonian granites. Journal of the the magma. One may also envisage continuous interaction Geological Society, London, 144, 671-4. between country rocks and magma by zone melting at levels HONMA,H. & ITIHARA,Y. 1981. Distribution of ammonium in the minerals between the formation of the magma and its emplacement. of metamorphic and granitic rocks. Geochimica et Cosmochimica Acta, 45, 983-8. Onthe whole, high-level contaminationseems the most InHARA, Y. & HONMA,H. 1979. Ammonium in biotitefrom metamorphic likely, because high-level contaminants would have had less and granitic rocks of Japan. Geochimica et Cosmochimica Acta, 43, opportunity to lose their ammonium in the course of high 503-9. grade metamorphism. JAHNS, R. H. & BURNHAM, C.W. 1969. Experimental studies of pegmatite genesis. I. A model for thederivation and crystallization of granitic pegmatites. Economic Geology, 64, 843-64. TALIBUDEEN,0. & GOULDING,K. W. T. 1983. Apparent charge heterogeneity I thank David Alderton and Chris Ward for helpful comments on in kaolins in relation to their 21 phyllosilicate content. Clays and Clay the manuscript of this paper. Collection of specimens was aided by Minerals, 31, 137-42. a grant from the Central Research Fund of London University. URANO,H. 1971. Geochemicaland petrological study on the origins of metamorphic rocks and granitic rocks by determination offixed ammoniacal nitrogen. Journal of Earth Science, Nagoya University, 19, 1-24. References USHER, W. A. E., BARROW, G.& MACALISTER,D. A. 1909. The geology of BAUR,W. H. 1974. Nitrogen. In: WEDEPOHL,K. H. (ed.) Handbook of the country around Bodmin and St. Aurtell. Memoir of the Geological Geochemistry, Vol. 11. Springer-Verlag, Berlin, Chap. 7A. Survey of England and Wales. CUNDY,E. K., WINDLE,W. & WARREN,I. H. 1960. The occurrence of WLOTZKA,F. 1972. Nitrogen. In: WEDEPOHL,K. H. (ed.) Handbook of zinnwaldite in Cornwall. Clay Minerals Bulletin, 4, 151-6. Geochemistry, Vol. 11. Springer-Verlag, Berlin, Chap. 7B-0.

Received 16 March 1987; revised typescript accepted 8 May 1987

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