Journal of the Geological Society, London, Vol. 145, 1988, pp. 37-41, 1 fig., 5 tables. Printed in Northern Ireland The distribution of ammonium in granites from South-West England 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 the major 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 Caradon 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 St Austell specimens were used they were obtained by breakingopen very Luxulyan (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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/1/37/4900588/gsjgs.145.1.0037.pdf by guest on 30 September 2021 38 HALL A. 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 turned out 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 Bodmin Sands,Praa easternend (P308) 163 ppm Moor granite. Megilligar Rocks,W of Porthleven (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 and feldspars, 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 variation in 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