Modelling the Petrogenesis of High Rb/Sr Silicic Magmas

( "heroical Geology, 92 ( 199 [ ) 107-114 107 Elsevier Science Publishers B.V., Amsterdam

Modelling the petrogenesis of high Rb/Sr silicic magmas

A.N. Halliday a, j.p. Davidson ~"", W. Hildreth b and P. Holden ~"~ ~' D~7~arll~tepll qt Geo/o~ica/ Sciences, The University q/,llichi~an, Inn. lrhor, UI 4S I ()~- I 0~ 3, (.S'I " ('S. Geological SI,'rVE')', MSglO, 345 )diddlefiehl Road, Menlo t'ark. (;,1 94025. ( ,S\-I " D~7~artment o/Earlh and Space Sciences, ~ )zivetwitr q/('alih;rnta. Los ..t n£,eh'~. (' I 90024, ~ ',S'. I (Received February 22, 1990: revised and accepted November 7, 1991))

ABSTRACT

Halliday, A.N., Davidson, J.P., Hildreth, W. and Holden, P., 1991. Modelling the petrogenesis o1 high Rh/Sr silicic magmas. In: A. Peccerillo (Guest-Editor), Geochemistry of Granitoid Rocks. Chem. Geol., 92: 107-114.

Rhyolites can be highly evolved with Sr contents as low as 0.1 ppm and Rb/Sr > 2,0(t0. In contrast, granite batholiths are commonly comprised of rocks with Rb/Sr < l 0 and only rarely > 100. Mass-balance modelling of source compositions, differentiation and contamination using the trace-element geochemistry of granites are therefi)re commonly in error because of the failure to account for evolved differentiates that may have been erupted from the system. Rhyolitic magmas with very low Sr concentrations ( ~< 1 ppm ) cannot be explained by any partial melting models involving typical crustal source compositions. The only plausible mechanism for the production of such rhyolites is Rayleigh fiactional crystalli- zalion involving substantial volumes of cumulates. A variety of methods ['or modelling the differentiation of magmas with extremely high Rb/Sr is discussed. In each case it is concluded that the bulk partition coefficients for Sr have to be largc. In the simplest models, the bulk D sr of the most evolved types is modelled as > 50. Evidence from phenocryst/glass/ whole-rock concentrations supports high Sr partition coefficients in feldspars from high silica rhyolites. However, the low modal abundance of plagioclase commonly observed in such rocks is difficult to reconcile with such simple fractionation models of the observed trace-element trends. In certain cases, this may be because the apparent trace-element trend defined by the suite of cogenelic rhyolites is the product of different batches of magma with separate differentiation histories accumulating in the magma chamber roof zone.

1. Introduction fractionated magmas with less than 1 ppm Sr (0.5 ppm at Yellowstone, 0.3 ppm at It has been known for a long time (e.g. La Primavera and 0.1 ppm at Glass Mountain, Nockolds and Mitchell, 1946) that most SiO2- Long Valley). In this paper we explain these rich igneous rocks have much higher Rb/Sr differentiated magmas in terms of extreme than SiO2-poor igneous rocks and that this ra- Rayleigh fractionation and discuss the con- tio can therefore be used as an index of differ- straints placed thereby on models of rhyolite entiation. This is obviously because Rb is more genesis. incompatible than Sr in nearly all magmatic systems. Accurate isotope dilution measure- 2. Rb and Sr concentration trends ments are now illustrating the remarkable ex- tent of this fractionation. Studies of rhyolitic Rb and Sr concentration data for some highly suites from Yellowstone (Leeman and Phelps, fractionated silicic magmas are shown in Fig. 1981, Hildreth et al., 1991 ), La Primavera 1 together with data for major granitic suites (Mahood and Halliday, 1988 ) and Long Val- from around the world. The individual Cale- ley (Halliday et al., 1989) have revealed highly donian granitic suites shown (from Donegal

.... Yellowstone ,Trawenagh Bay Rosses Complex Main Glass Mountain ..... Rhyolites / Pluton Donegal Rhyolites ~ ~ I ('~ ~ "G4 ~ P9 PI" ~ " °n 2,.e, i

I O0 - ,,'< / ' Bishop , '\ "~-...:~.."X ~New ~,~ La Primavere Tuff Peru - ~ , ~.~Brunswick E e-"v Rhyolites Coastal ~ 'k , i Mmnettes Batholith , ;,/[~ ch / A::) Kosciousko ',, ~ /~fi El2 BatholHh ~ln 5 ly~'-^e ' "~

I0 -~' Yei owstone ~,'' ' Ranges I Basalt '/ ,,. a Primaver~j © I B,)S<] It i i ~-'~ :* ,H , ' -t ,

1.0 C~l l O 16- ido " I000 Sr p pm

Fig. 1. Rb vs, Sr for rhyolites and associated basalts from western North America ( Halliday et al., 1984, 1989: Mahood and Halliday, 1988; Hildreth et el., 1991 ), and granitic suites from around the world (Halliday, unpublished data on Donegal; Hine et el., 1978: Bachinski and Scott, 1979; Atherton et al., 1979: Bateman and Chappell, 1979; Halliday el al., 1980: DePaolo, 1981 b ). The data for Rosses (Donegal) are divided into separate phases ES (early sheets ) and G l to ~;4. HI Sr concentrations < 50 ppm determined by isotope dilution. and S. Scotland) were selected specifically for which Rb and Sr are both incompatible, a pos- their relatively simple quasi-linear trends when itive slope should result in Fig. 1 (e.g., the New plotted in this manner. Certain key points can Brunswick minettes). These conditions would be made from examination of this diagram. be met in the lower crust or upper mantle at (1) In general, the rhyolite suites contain pressure-temperature conditions exceeding the highly fractionated magmas not normally in- stability of plagioclase. In basaltic systems Sr cluded in our granite sampling. There is there- is depleted (in response to plagioclase frac- fore a need for detailed studies of the relation- tionation and/or crustal assimilation) as Rb ship between what are commonly seen as late increases, corresponding to a more or less ver- stage differentiates (aplites and leucogranite tical trend of data on Fig. I (e.g., the Yellow- minor intrusives) on the one hand and granite stone basalts). In systems in which Sr deple- pluton geochemistry on the other. These mi- tion is more marked (many granites and nor intrusives are commonly regarded as eso- rhyolites, for example), a negative trend is an- teric and a minor aspect of the overall story. ticipated in Fig. 1. A cogenetic suite of com- ]'hey are, in fact, possibly of fundamental im- positions evolving from marie to felsic would portance to our understanding of the genesis of therefore be expected to display some kind of granite batholiths. Trying to mass balance an inverted "L" or recumbent "V" shape on crustal contamination, melt compositions and this diagram, which is seldom (if ever) ob- differentiation in granites using bulk geochem- served in perfection, but is possibly the reason istry is of limited value without this informa- for the less perfect inverted "L" shapes of the tion. Most large-volume high-level plutons may Peru Coastal Batholith and the Kosciusko be accumulative relative to the once associ- Batholith I-types (Lachlan foldbelt ). The trend ated, now lost, cogenetic volcanics. in Fig. 1 will be horizontal if bulk D R[' ~- 1 or if (2) In plagioclase-free differentiation in D sr 2>> D Rb. The former is only likely in assem- M()DELLING THE PETROGENESISOF HIGH Rb/Sr SILICIC MAGMAS 109 blages fractionating large amounts of biotite rocks with which this paper is mainly (Hanson, 1978 ). Many evolved granites such concerned. as the Trawenagh Bay pluton, Donegal (Fig. 1) have clearly undergone extensive K-feld- 3. Partial melting vs. fractional crystallization spar fractionation, as judged by their very high Rb/Sr, Rb/Ba, and Rb concentrations indica- Trace-element behavior as a result of sili- tive of bulk DRb< 1, bulk DS~>l and bulk cate-melt equilibria is described in a number DU~'> 1. of familiar equations (e.g., Shaw, 1970; Arth, ( 3 ) The fact that quasi-linear negative trends 1976 ). Partial melting is generally modelled by are observed in Fig. 1 for "evolved" granites an equilibrium batch melting equation such as: and rhyolites, suggests that there is some simple relationship between the change in Rb and C'/C~,=I/(F +D'-FD') (1) Sr in the magma. It will be shown below that whereas fractional crystallization is described this is indeed explicable with a manipulation by the Rayleigh equation: of the Rayleigh fractionation equation. We can take several approaches to the ques- (C'/C[,)=F ')' ' (2) tion of how evolved silicic magmas such as (in which C'= concentration of element i in the those of Long Valley (with more than three or- liquid, C~,=concentration of element i in the ders of magnitude variation in Rb/Sr ratio) are starting mineral-melt assemblage, F= fraction produced. In so doing we are also attempting of liquid). If the bulk crystal/liquid distribu- to place limited constraints on what kind of tion coefficient (D) is greater than unity, the parent magmas were involved in the genesis of trace element (i) is depleted much more rap- these volcanic rocks, although from the outset idly during crystal fractionation than in melt- we would admit our prejudice that many of the ing (Fig. 2). Even assuming a high D s', it is parent magmas were complex mixtures of almost impossible to achieve low Sr concentra- mantle- and crust-derived components with an tions (and correspondingly high Rb/Sr) overall intermediate to silicic composition through equilibrium partial melting. This is (A.N. Halliday in Leake et al., 1980; Halliday true whether modal or non-modal melting et al., 1980; DePaolo, 1981a, b; Hildreth, 1981; equations are considered (as F approaches Huppert and Sparks, 1988). Isotopic studies zero, approaches 1/D'). Large deple- prove that some of the arrays shown in Fig. 1 C'/C~, are not simply the result of Rayleigh fraction- 500 as ~ o2 ~r } s2 l ation. Processes such as crustal contamination I i C~R~ : ,b ppm ], can also play a role. Note that contamination 4oo~' aS' ~i L ..... cannot be responsible for the very low ob- ,2-' ,g" ] . , : 2 _ ' I E ,300 ~- i "fractional crystallization I served Sr concentrations. In fact, exceedingly / '1, © low Sr concentrations in a rhyolite are incon- / ' ,@" t par tia~ melting sistent with post-fractionation crustal contam- to ination (Halliday et al., 1989 ). ,oot , In this paper we evaluate and place limits upon the potential importance of Rayleigh o 20 40 60 80 Ioo crystal fractionation. High Rb/Sr ratios in Sr ppm some alkaline rocks have been explained in Fig. 2. Model Rb vs. Sr concentrations expected as a re- terms of sub-solidus fluid-rock interaction sult of differing degrees of partial melting as opposed to (Bonin et al., 1979 ). However, this is of little Rayleigh fractional crystallization for the same parent relevance to the (mostly glassy) fresh volcanic composition. I I 0 A.N. I|AI]AI)AY ET AL. tions in Sr could be achieved if an extreme rium conditions between minerals and melts. fractional melting model were used, but the For instance, the distribution coefficient for Sr physical plausibility of separating infinitesi- between plagioclase and basaltic melt may be mally small fractions of rhyolitic liquid from a very much less than that between plagioclase crystalline source is at best questionable (Arzi, and silicic melt (e.g., Korringa and Noble, 1978; McKenzie, 1985; Wickham, 1987). We 1971; Arth, 1976; Hanson, 1978). It has been are then drawn to the conclusion that the only shown that the mineral/melt distribution closed system model capable of generating the coefficients for most elements and minerals in- Rb-Sr characteristics of highly evolved rhyo- crease as the melt becomes more silicic and lites is fractional crystallization (Halliday, more polymerized and the concentration of the 1990 ). Furthermore, the existence of high Rb/ element becomes very low (Leeman and Sr rhyolites can be taken as evidence for a sig- Phelps, 1981; Mahood and Hildreth, 1983: nificant crustal magma chamber at the time of Watson, 1985). The effective distribution eruption. Many precaldera rhyolites with low coefficient would therefore be more realisti- Sr concentrations are therefore likely to reflect cally represented by an integration of D s' over the presence of a large magma system, rather the range ofF. Extremely high Ko-values for Sr than small pockets of unrelated melt (cf. Ba- in sanidine have been reported (Leeman and con et al., 1981; Metz and Mahood, 1985; Phelps, 1981; Halliday et al., 1989 ), and below Huppert and Sparks, 1988; Sparks et al., 1990 ). we present further evidence that high bulk D for Sr is required by a simple Rayleigh model. 4. Rayleigh fractional crystallization 5. Rayleigh fractionation and the To achieve three orders of magnitude varia- determination of bulk distribution coefficients tion in Sr concentration in a rhyolite system from Rb and Sr whole-rock concentrations such as the Long Valley caldera or the basalt- rhyolite system of La Primavera by fractional Another attempt to determine the bulk dis- crystallization alone reduces eq. 2 to: tribution coefficients and hence constrain a DSr= (-3/log F)+ 1 (3) Rayleigh fractionation crystallization model comes from inverting the problem. This requires a bulk D sr of 2.5 for an F of We can use the data shown in Fig. 1 to cal- 0.01. This translates into thousands of cubic culate bulk distribution coefficients for Rb and kilometers of cumulates for such systems (i.e., Sr as follows. By taking the logarithm of eq. 2 approximately 100 times the volume of erupted we obtain: rhyolite ). The calculated volume of cumulates would be reduced to hundreds of cubic kilo- logC' - logC[, = (D' - 1 ) log F ( 4 ) meters if the bulk D was 4. The highest min- We can therefore write: eral/melt Ko that would be suitable from the log Rb- log Rbo available literature would be 3.87 for K-feld- = slope on Fig. 1 spar (Philpotts and Schnetzler, 1970) 4.5-7.3 log Sr-log Sr~ for sanidine (Nash and Crecraft, 1985) and m Rb- 1 5.57 for anorthoclase (Sun and Hanson, 1976 ). -n= DSr_ 1 (5) These values are not very encouraging in terms of increasing bulk D and reducing the calcu- In Fig. 3 we show an array of lines of differ- lated mass of cumulates. D sr will change dur- ent slope (=n) which follow from this rela- ing fractionation both due to changing mineral tionship. The slopes that we observe in evolved assemblage and as a result of changing equilib- granitic (rhyolitic) systems vary from about MODELLING THE PETROGENESIS OF HIGH Rb/Sr SILICIC MAGMAS I I l

n= slope = (DRb-I)/(DSr-I) 6. Rayleigh fractionation and the I0- determination of bulk distribution coefficients from whole-rock/glass data 09 081- Knowing the concentrations of Rb and Sr in glass separated from non-vesicular whole-rock rhyolite, we can determine bulk distribution coefficients provided we know the proportions DRb 05~ of crystalline phases. This approach does have the disadvantage that accurate modal propor- 04 L tions will be difficult to determine in crystal-

03P poor rhyolites. A simpler approach is as follows. The concentration of an element in the bulk crystals can be expressed as: Cmm=(Cw-Q,)M-' (6)

i i i__ 0(; 2 4 ; ; IO 12 14 16 18 where C= concentration, min=minerals, D sr wr=whole rock, gl=glass and M is the mass fraction of crystals. From this it follows that: Fig. 3. D Rb vs. D s' with slopes corresponding to (O Rb- l)/(D st- 1) (equivalent to those in Fig. 1). DRb-I=[Rbwr/(MRbg,)]-(I/M) (7) and that:

D Rb- 1 (Rbwr/Rbo) - 1 (8) D st- 1 - (Srwr/Sre,) - 1 which is the same as the slope (n) in Figs. 2 -0.4 (high temperature Bishop Tuff, or La and 3 and eq. 5. Whole rock and glass concen- Primavera) to -0.2 (Trawenagh Bay pluton ) trations of Rb and Sr have been measured by to near zero (Glass Mountain, Long Valley). isotope dilution for two samples at Glass From Fig. 3 we can see that these shallow slopes Mountain (Halliday et al., 1989) and for this require a bulk D for Sr that is about 2.5 or we determine values of n of -0.0122 and greater provided D Rb is around 0.2. This value -0.397 for glasses with 0.13 and 2.3 ppm Sr, will not change greatly in silicic melts in which respectively. The bulk D sr required by the first Rb is relatively concentrated (Hanson, 1978; value ofn (Fig. 3) is about 50, given D Rb <0.5, Mahood and Hildreth, 1983; Watson, 1985). confirming the conclusion based on Rayleigh Since sanidine is almost certainly the most im- modelling of Fig. 1. portant phase to control bulk D in rhyolitic liq- These indications of very high bulk distri- uids, bulk D Rb must be less than 0.5 (probably bution coefficients for Sr in highly evolved 0. l ). Rhyolitic suites such as Glass Mountain rhyolite systems can be supported in part by with a very gentle negative slope on Fig. 1 can some direct measurements data on pheno- clearly only be produced by Rayleigh fraction- crysts which is our final approach to the ation if bulk D s' is extremely high. For exam- problem. ple, if D Rb < 0.5, then D sr > 50! Such high values are virtually impossible on the basis of 7. Sanidine/melt partition coefficients for Sr observed assemblages. In nearly all high silica rhyolites, sanidine and quartz greatly exceed In Fig. 4 we show the concentration ratios of plagioclase in modal abundance. Sr in sanidine relative to coexisting whole rock 1 I 2 ;\.N. HALLII)AY ET A¿ .

iopoo coefficients for Sr in sanidine (Fig. 4). Note the apparent KD of 18 observed for one Glass 3,000 Mountain sample. In order to produce bulk Ds,- 1,000 of 50 as suggested by the other lines of evi- o dence, we must also be fractionating plagio- 3 300 A clase with even higher KD. This does not seem ~PA unreasonable, although distribution coeffi- o LO0 0:: cients determined from high precision isotope o dilution measurements are needed to substan- Ju 30 tiate this. We note that Nash and Crecraft "6 I0 0 Gloss "~ Giqss Mounte,n ( 1985 ) record/~ur in plagioclases of 6.8 to 33. O-Whole Rock / n ~lm • Bishop Tuff WhoLe Rock 3 8. Conclusions Z~ GLass I • &-Whole Rock ~LO2 Primavero (l) Rhyolites exhibit enormous fractiona- tions in Rb/Sr ratios seldom reported from I granites. We conclude that our rationale for 0 5 I0 15 20 sampling of granites as representatives of si- (Srson/SrglQss) Or (Sr~n/Srwr) licic magma compositions is probably flawed and that the evolved portions of the granite Fig. 4. Srsa,,/Sr,~r) or Sr,an/Srglass against differentiation magma are lost from the system. It is therefore index (Rb/Sr) (note log scale) for volcanicsof the Long Valley caldera and La Primavera (Halliday et al., 1984, crucial to examine minor intrusives as clues to 1989: Mahood and Halliday, 1988). the more evolved liquid compositions. Fur- thermore, the notion that one should be able or glass concentrations, as a function of Rb/Sr to mass balance the trace-element or major- ratio of the whole rock or glass (a very suitable element chemistry of granitic batholiths to de- indicator of degree of fractionation). In an termine the characteristics of the magma ideal situation, Sr~a./Srglass= mineral/melt Kd, source is possibly erroneous. whereas Srs,n/Srwr < mineral/melt KD. Clearly, (2) First order trace-element modelling in- even minor amounts of crustal contamination dicates that the evolved compositions ob- of the rhyolitic liquid during eruption are likely served in high-silica rhyolites cannot be pro- to provide false apparent partition coeffi- duced realistically by crustal melting alone; cients. In short, trying to accurately measure considerable differentiation of the magma is extremely low concentrations of Sr is feasible, required. but the data may be suspect despite this. How- (3) Several different approaches to the ever, nature works in our favor in so far as the problem of generating high-silica rhyolites by effects observed (Halliday et al., 1984, 1989; fractional crystallization of a magma with low Mahood and Halliday, 1988; Halliday and Ba- Rb/Sr indicate that it can only be achieved con, unpublished) suggest preferential addi- with reasonably high bulk distribution coeffi- tion of Sr to the glass. We have no reason yet cients for Sr ( > 3 ) and/or large volumes of cu- to suspect a high apparent Kd in these very mulates. We do not see it as unreasonable to evolved compositions as being due to altera- produce evolved rhyolitic magma by fraction or contamination (i.e., loss of Sr in the tional crystallization of basalt on these trace- glass or liquid). element grounds alone. In certain cases (such Highly fractionated liquids do indeed ap- as alkaline rhyolite systems) this may be a bet- pear to have high mineral/melt distribution ter explanation than crustal melting, as dis- MODELLING THE PETROGENES1S OF HIGH Rb/Sr SILICIC MAGMAS l I 3 cussed by Mahood and Halliday ( 1988 ). Bacon, C.R., Macdonald, R., Smith, Ri. and Baedecker, (4) Attempts to determine the bulk distri- P.R., 1981. Pleistocene high-silica rhyolites of the Coso volcanic field, Inyo County, California. J. Geophys. bution coefficients for Sr in highly evolved Res., 86: 10223-10241. rhyolites yield similar information. The bulk Bateman, P.C. and Chappell, B.W., 1979. Crystatlisation distribution coefficients appear to be high fractionation and solidification of the Tuolumne In- (>2.5) to very high (as high as 50 in ex- trusive Series, Yosemite National Park, California. Bull. Geol. Soc. Am., 90: 465-482. tremely evolved liquids). Bulk distribution Bonin, B., Bowdem P. and Vialette, Y., 1979. Le com- coefficients this high are difficult to reconcile partement des eldments Rb el Sr au cours des phases with observed phenocryst assemblages which de mineralisation: L'example de Ririwai (Liruei), N i- are dominated by quartz and sanidine. A pos- geria. C. R. Acad. Sci., Paris, 89 (Ser. D): 707-710. sible explanation for this is that the modelling DePaolo, D.J., 1981 a. Trace element and isotopic effects of combined wallrock assimilation and fractional crys- is in error, because DRb is higher than generally tallisation. Earth Planet. Sci. Lett., 53:189-202. appreciated in very evolved rhyolites. How- DePaolo, D.J., 1981b. A neodymium and strontium iso- ever, this is not supported by observed data. topic study of the Mesozoic calc-alkaline granitic bath- Another possible explanation is that the shal- oliths of the Sierra Nevada and Peninsular Ranges, California. J. Geophys. Res., 86: 10470-10488. low sloping trends for data from evolved rhy- Halliday, A.N., 1984. Coupled Sm-Nd and U-Pb system- olites shown in Fig. 1 are not the product of alics in late Caledonian granites and the basement un- single-stage differentiation. Rather they are an der northern Britain. Nature (London), 307: 229-233. artefact of combining a variety of highly Halliday, A.N., 1990. A reply to comments by R. Stephen evolved, genetically related, but separate dif- J. Sparks, Herbert E. Hupperl, and Colin J.N. Wilson on "'Evidence for long residence times of rhyolitic ferentiates, which accumulate near the roof of magma in the Long Valley magmatic system: the iso- the same magma chamber. topic record in prccaldera lavas of Glass Mountain". Earth Planet. Sci. Lett., 99: 390-394. Acknowledgements Halliday, A.N., Stephens, W.E. and Harmon, R.S., 1980. Rb-Sr and O isotopic relationships in three zoned (a- ledonian granitic plutons. Southern Uplands, Scot- This research was supported by NSF grants land: evidence for varying sources and hydridisalion EAR-8720564 and EAR-9004133 to ANH. We of magmas. J. Geol. Soc. London, 137: 329-348. are very grateful to Gail Mahood for her thor- Halliday, A.N., Fallick, A.E., Hutchinson, J. and Hil- ough and useful criticism of an early version of dreth, W., 1984. ANd, Sr and O isotopic investigation into the causes of chemical and isotopic zonation in the text and Klaus Mezger and Gareth Davies the Bishop Tuff; California. Earth Planet. Sci. Lelt., 68: for discussion. 379-391. Halliday, A.N., Mahood, G., Holden, P., Metz, J.M., References Dempster, T.J. and Davidson, J.P., 1989. Evidence fiw long residence times of rhyolite magma in the Long Arth, J.G., 1976. Behavior of trace elements during mag- Valley magma system; the isotopic record in the pre- matte processes--a summary of theoretical models and caldera rhyolite lavas of (}lass Mountain. Earth Planet. their applications. 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