MINERALOGICAL MAGAZINE Trace Element Variation in The
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MINERALOGICAL MAGAZINE VOLUME 52 NUMBER 365 APRIL 1988 Trace element variation in the leucogranites within the main Galway Granite, Connemara, Ireland M. FEELY Department of Geology, University College, Galway, Ireland AND J. S. MADDEN Applied Geophysics Unit, University College, Galway, Ireland Abstract Six hundred and forty four gamma-ray spectrometer assays of U and Th obtained within the late Caledonian Galway Granite western Ireland are presented. The data cover the range of granodiorites, adamellites and leucogranites (Murvey Granite) present in the batholith. There is an overall increase in U and Th abundances with petrological evolution. The broad scatter of values that characterizes the Murvey Granite reflects the geographically separate occurrences of this leucogranite. Rubidium and strontium data imply varying degrees of fractionation among these separate Murvey Granite occurrences. Y and HREE are notably enriched in the Murvey granite at Costelloe which also contains the highest Th levels amongst the leucogranites. Thorite, uraninite, monazite and Y-zircon are present in this leucogranite and are responsible for the observed enrichments in U, Th, Y and HREE. KEYWORDS: Galway Granite, Murvey Granite, trace elements, leucogranites, uranium, thorium, Rb/Sr data, yttrium, rare-earth elements. THE results of a quantitative gamma ray spectr- intrusion emplaced at approximately 400Ma ometer survey on the Galway Granite have been (Leggo et al., 1966; Pidgeon, 1969). Andalusite outlined by Feely and Madden (1986) but no and cordierite-bearing hornfelses were developed detailed description or interpretation of the U and in South Connemara Group rocks; minor deve- Th fractionation trends was attempted. This paper lopments of andalusite are observed along the presents an interpretative account of the behav- northern Dalradian-Granite contact. The batho- iour of these two radioelements during fractiona- lith is unconformably overlain to the south and tion. Other trace element data (Rb, Sr, Y and southeast by Carboniferous limestone (Coats and REE) from both published and unpublished Wilson, 1971; Max et al., 1978; and Figure 1). sources are also presented. This composite batholith is composed of four domes, i.e. Carna, Spiddal, Galway-Kilkieran, and Roundstone Domes (Max et al., 1978). Geology of the Galway Granite Wright (1964) and Leake (1974) have shown that The Galway Granite occupies approximately marginal zones of the batholith are occupied by 600 km2 along the northern shoreline of Galway a leucogranite--the Murvey Granite--which Bay in the west of Ireland. A southerly offshore passes inwards to less silicic adamellite termed extension beneath the bay is inferred from gravity the Errisbeg Townland Granite (E.T.G.). Mur- and aeromagnetic data (Murphy, 1952; Max et vey Granite also occurs in the central section of al., 1983). The batholith intrudes high-grade Dal- the batholith e.g. at Costelloe (M.G.1) and Kil- radian metamorphic rocks on its northern, eastern kieran (M.G.4)---see Fig. 1. and western margins (Leake and Leggo, 1963) The batholith is composed of granodiorites and lower grade rocks of the South Connemara (Carna and Spiddal Granites), adamellites Group (Ordovician?) to the south on the islands (E.T.G. and Roundstone Granite) and Murvey of Lettermullan and Gorumna (McKie and Granite (Max et al., 1978). Textural variants of Burke, 1955). The Granite is a late Caledonian these main varieties also occur. The area between Mineralogical Magazine, April 1988, Vol. 52, pp. 139-146 Copyright the Mineralogical Society 140 M. FEELY AND J. S. MADDEN ~E ~o Z(-- = 09 ~J co A 0 ~ ~ .- ~ # ~" ~ ~ ~.-~ ~ ~ir -~-~ ~.~ . ~ oo "o ~r~ c o | (D o -o o ~ m z~ ~ ~ ~, | - | 0,-.~ W ~ C..,§ + , ~..::.:.j > > ~7. ~d GO ~d W < I-- W | 0 < C~O ~ >-co 0 - e~,.o -r - ~.... W U) "r~ e- ~'- Z ,< TRACE ELEMENTS IN LEUCOGRANITES 141 the NE-SW trending Shannawona Fault and the TABLE 1 Mean eUppm and eThppm contents for the Main NW-SE trending Barna Fault is occupied by a granite types in the Galway Granite. mafic E.T.G. which has been partially mapped Granite Type (no. of samples) Mean moan by Plant (1968), Coats and Wilson (1971), and eUppm eThppm Feely (1982). Wright (1964), Leake (1974), Coats and Wilson Mafic Errisbeg Townland Granite (112) 4.O8 15.23 (1971), and Lawrence (1975) report major and Spiddal Granite (91) 4.68 16.79 trace element variations in the Galway Granite Carna Granite (47) 5.82 17.17 that are typical of a progressively fractionating calc-alkaline magma. Fractionation has resulted Errisbeg Townland Granite (310) 6.01 20.87 in a comagmatic suite of granite types that display Murvey Granite (84) IO.16 33.55 a progressive increase in silica and alkalis with concomitant decrease in Fe, Mg, Ti, Ca, A1, Mn and P, ranging from diorite xenoliths to grano- diorites and adamellites and finally to the leucoc- ratic Murvey Granite. Leake (1974) showed, from There is an overall increase in mean radioele- a traverse of chemically analysed rocks and bio- ment (U and Th) abundances from the least tites from the western end of the batholith that evolved mafic E.T.G. through the Spiddal and the Carna Granite crystallized from the outside Carna Granites towards the E.T.G. and Murvey inwards. Furthermore the marginal Murvey Granite (Figs. 2A, B). Furthermore, there are sta- Granite (M.G.6--Fig. 1) formed from a residuum tistically significant differences in mean U and Th in the E.T.G. which was drawn out of the latter levels between the geographically separate Mur- into 'zones of rarefaction' (Leake, 1974) left vey Granite outcrops, e.g.M.G.1 and M.G.2. behind by the foundering of stoped blocks of Mean Th/U ratios remain relatively constant country rock. through the series of granites. The Murvey granite Finally, Leake (1978) notes that the Galway on the islands of Lettermullan and Gorumna Granite is spatially associated with the extension (M.G.2) has a high mean Th/U ratio, while the of the Southern Uplands Fault in Ireland, a major Carna Granite possesses the lowest ratio splay of which probably passed through the zone (Fig. 2C). now occupied by the batholith to emerge in the The fields defined by all major granite types Skerd Rocks (Fig. 1). There, major pre-granite in Fig. 3A show that, despite the scatter of values faulting separates South Connemara Group rocks within any one granite type, there is a general from the Connemara Dalradian. Leake links the trend of increasing U and Th contents with petro- generation and emplacement of the Galway Gran- logical evolution. The greatest degree of scatter ite to this major deep fault. is evident in the Murvey Granite data. Fig. 3B shows that the large Murvey Granite field is com- Uranium and thorium variation in the Galway posed of four discrete smaller fields defined by Granite M.G.1, M.G.2, M.G.6 and M.G.3-5. A fractio- nation trend is drawn from the granodiorites to A Geometrics GR410A four-channel gamma- the adamellite towards the Murvey Granite. ray spectrometer was used to determine 644 Uranium enrichment trends in the M.G.1 and assays of eUppm I and eThppm 2 on the main M.G.6 Fields (Fig. 3B) are based upon field and granite types (excluding the Roundstone Granite) spatial distribution studies (Feely and Madden, in the batholith (Feely and Madden, 1986). The 1987). Spatial distribution studies on the circular spectrometer was calibrated at the Riso National Murvey granite body at Costelloe (M.G.1) show Laboratory, Roskilde, Denmark, using large that U increases in abundance from its margin doped concrete pad sources simulating outcrops to its centre while Th levels decrease slightly in of known radioelement concentrations. The over- the same direction. Leake (1974) showed that the all error of a field spectrometer assay with known Murvey granite at the western end of the batholith calibration constants ranges between 5 and 10% (M.G.6) becomes more fractionated and garneti- on the U and Th channels (Lovborg, 1984). A ferous towards the margin of the batholith. The summary of the U and Th contents in the main trend in the M.G.6 field reflects a marked enrich- granite types is presented in Table 1. ment in U from the non-garnetiferous M.G.6 to 1 I ppm eU = I ppm U in radioactive equilbrium with the marginal garnet-bearing leucogranite. The 7-emitting daughters. field defined by the Murvey granite on Letter- 2 lppm eTh = lppm Th in radioactive equilibrium muUan and Gorumna islands (M.G.2) shows that with 3,-emitting daughters. this leucogranite is poorer in U relative 142 M. FEELY AND J. S. MADDEN 55 A A M.G.1 '& "" .... "'" M.G 50 " "-. 84 %.. 11.5 45 "... 40 E 35 30 E 7.5 25 t- [] O i-- 20 M.G.2 A 15 10 3.5 66 71 76 5 MefiG E.T.G 112 i i i i i i i J i 0 B M.G.1A 0 2 4 6 8 10 12 14 16 18 20 39 55 B . "'*" "... a3 5O .. ".. M.G.1 ./40 E 45 44 ~ Q..ett%~ " n.,,~" ~. M.G. 2& 40 27 Depleted . - ......... " i--- [ f ~" ' " "" M.G.3-5 b'- 35 ~t, ~" "" " ;.~" ~,,~16 E 30 21 Q. I'~ ~O.t " a 25 1:3 M.G-2 ~'t / ~ 2o 23 /O~ 15 - - 66 71 76 15 e,m 10 Fractionation Trend C 5 M.G.2& i i i i i i 0 5.5 2 4 6 8 10 12 14 16 18 20 eU ppm r- 4.5 F]6.3A. Plot of eU ppm v. eTh ppm showing the indivi- b- dual granite fields.