RENDlCONTl Soc. Ual/ana dl Mln~ralo9/a ~ PetroIO{,l/a, 32 (2) 669-680 (1976)

ALESSANDRA CESANA·, LAURA D EMONfE •• ,

MARIA FIORENTINI POTENZA •• , MARINA VENEGONI··

ORIGIN OF THE GRANITIC CORE OF THE CERVO PLUTON OCCORDING TO THE Th/U RATIO OF FRACfIONATION

ABSTRACT. . U and Th content has been determined by means of neutron activation analysis on the separated rock forming and accessory minerals coming from granite, syenite and monzonite of Valle Cervo pluton. The sequence of crystal fractionation of minerals, arranged according to an increasing Th/ U ratio, has been compared with the mintrogenetic sequence that can be deduced from the textural relationships at the microscope. There is a satisfactory agreement follow Ihe syenite one, but it runs almost parallel to the last. The feature plays in favour of a to be consecutive in the three rock.types of plulon. The granite r~nge of fractionation doesn't follow the syenite one but it runs almost paroJlel to the laSt. The feature plays in favour of a recessive and nOt residual character of the gronite showing thereby an indipendent trend of fractionation that can be related to an unmixed high-silica molten core. Chemical and modal (.'()mposition, euhedral quartz, and other features strongly sUPIX>rt such an interpretation together with the lack of disjunctive tectonic events in the host shell of syenite + monzonite.

RIASSUNTO. - II contenuto di U e Th e stato determinato mediante attivazione neutronica sui minerali accessori ed essenziali separati dal graniw, dalla sienite e daJla monzonite del plu­tone della Valle del Cervo. La sequenza di £razionamento cristallino ottenuta ordinando i mi­nerali secondo un rapIX>rtO crcscente Th/U e stata conirontata con la successione minerogenetica che si ricava dai rapporti strutturali al microscopio. Le due successioni concordano in modo soddisfacente nell'ambito di ciascuna fades, tuttavia i1 trend complessivo di frazionamento non appare (.'()nsecutivo nei tre differenziati principali del plutone. L'intervallo di frazionamento del graniw non consegue quello della sienite ma 10 ricopre parzialmente. Questo deIX>ne a favore di un carattere recessivo e non residuale del granito, che mostra un decorso indipendenle di frazionamento attribuibile ad un nudeo altamente si1iceo smescolato giJl a110 stato fuso dal magma monzonitico-sienitico. La composizione chimica e modale, I'abito esagonale idiomorfo e la d,s!ribuzione del quarzo suffragano fortemente questa interpretazione insieme a11a mancanza di eventi tettonici disgiuntivi nell'involucfo sienitico monzonitico.

Introduction

T he present paper is an attempt to apply a geochemical ratio, e.g. the ThfU one, to a geological problem: the origin of the granitic core of the Cervo pluton.

The pluton shows in fact a gradual boundary between peripheral monzonite and syenite, whereas the boundary between syenite (or monzonite) and the inner

* Centro Studi Nudeari "Enrico Fermi" (CeSNEF) del Politecnico di Milano. ** Isliluto di Mineralogia, Pelrografia e Geochimica delJ'Universid. di Milano.

670 A. C£SANA, L. DEMONTE, M. POTENZA F IORENT INI, ~1. V£NEOOS I

granite appears to be a very rapid but not discontinuous transition (FIORINTINI

POTf.NZA M., 1959 a)j therefore the granitic rock fo rrru; a very regular and well differentiated core marked by a wide ring of peraciditic rock.

Taking into account the extraordinary U and Th content of the pluton rocks ~F I OREl'o'TI N I M., 1958; FIORE~'TINI POTtNz.t. M., 1959 b) it has Ix:cn made the attempt to arrange the separated rock-forming minerals of the three main rocks according to an increasing ThjU ratio.

1£ the assumption of an increasing T hfU ratio during the fractionation is virtually reliable, the obtai ned minerogenetic sequence would indicate probably if the T hjU ratio trend of fractionation in the granite continues the T h/U ratio trend of syenite and monzonite. otherwise if it belongs to an independent its own trend. This could help to answer to the q uestion: is an eutectic granite or a preexisti ng exolution of granitic melt.

T heoretical

T he crystallochemical behaviour of U and T h leads to believe that Th undergoes a concentration as regard as to U during the crystal fractionation and then it should tend to concentrate more than U in the last crystal fractions of the magma.

Electronegativity differences for T h-O and U.o bonds (1.9 and 1.8) suggests that the Th.o bond should have an higher percent of ionic bond - 52 %-54 % -(WEDEPOHL K. H ., 1969) than U.o (48 %-50 re).

Thereby according to T aylor (1965). the ThjU ratio should decrease during the crystal fractionation. Although the electronegativity of UII' is still unk nown, it may be expected to be higher than U· · and therefore the lower difference in electronegativity should indicate a more covalent Ue• -0 bond than U··.o; then the ThfU ratio should decrease even more.

O n the other hand, the ionization potelllial of T h results to be higher than U, although some incertanties still exist in these data. It leads to the opposi~e conclusion that U-O bond and therefore ThjU ratio should increase during the cryst:!,1 fractionation. Moreover the expected but not yet experimented higher ability to be leached of the Ue • ~McKELVEY v. E. et al., 1955) should favour its removal from the lattices as more as the mineral belongs to an high stage. Such a tendence like the above one should increase even more the T hjU ratio during the crystal fractionation.

T he comparison of ionic radii of U (0.97) and T h (1.02) eventually suggets that Th should tend to concentrate in the fina l mineral fractions (WITH FIELD J.M. et al., 1959).

From the above crystallochemical discussion, three suggestions rise in favour of an increasing ThfU ratio during the crystal fractionation against only one In favou r of a decreasing trend : that is the suggestion given by the differences of electronegativity.

OR IGIN OF THE CERVO GRANIT IC CORE OF THE CEIlVO PLlJTON ECC. 671

Ceochemical reliabilhy of an assumption of increasing Th /U ratio

In the light of the preceding discussion we believe it is theoretically correct to consider like more probable that ThfU ratio should increase during the igneous crystal fractionation.

No many experimental supports of this assumption can be drawn directly from the literature. There is in fact a general tendency to perform carefully U and Th determi nations on statistical collections of rock-forming minerals, coming from various and independent intrusive bodies.

This may be the reason, probably, why the natural ThfU trend of fractionation is still known with several uncenanties.

Experimental

All the rock-forming minerals of the granite, syenite, monzonite have been separeted one from the other by means of a Franz isodynamic magnetic separator.

TAilLE 1 Modal composition of th~ sd~cud jntrllsi/l~ rocks

lock. oM "",,u. • "-ml .... PI ... (-£0". -. ty . .... .1 .... 0-'" Or. od •• .......

_ .. od ..... h

roT"',.;.,. pi ... IT_It. )1 .0% )1.24 4.4' .. , a..H 1.01 O.H O . )~

(l1i,II .. el)

.."It ••••• 1 •• ), .n U.U 4. 46 0.4/ ".OS 0 . 14 0 . ' 1 0.24 (C.""I,II.)

1, •• 1<0 ( .. , .. ) • • J. T ... ~ .. " " " ••• U T ... ~ •• ~., ~., 11.' ••• ••• >.> > .•

,,.ml •• (lI.lloo11.) .I. T ...... " ".1 It.S ••• , >.> > UT .. , • • • U " 24.S ,.> , .. • •• __ It.

41.' H.' ••• '. ' ,.> LM •• > ••• (e. ... , ..... 110)

lIo_h. ~ .. x .• > .• ..> , .> > .• • •• (e._I)

~ .... _ ..... ito n .l 41 .' , .. , .. n.} L> ••• •• > (La Pi ..... d.)

""~ ..... It. , ... it. JO.ll '1.11 U.,. 0 .12 7 . • ' loll (Cu.h .. "'Bboll,,)

_.".1 .• , .. nit. JI.a 1l . S. 1"11 , .. '.11 1.\) O.SS 0.71 (.'011>0110) _ .... , ..

ll.54 n.U " 0 . 49 Q.ll ... >., t.., ..... )

In this way it has lx:~n investigated th~ fractionation tr~nd in a single intrusion so as to lower as mor~ as possible the number of g~och~mical variabl~ affecting the solidification history (age, pr~ssure. temperature ... ).

672 ". CESANA, L. OEMONTE, M. POTENZ A FIORENTIN I, M. VENECON I

T ABLE 2

U and Th r:ont~nt of the rock-forming and acassory mina-ah

in the main rock types 0/ the pluton

~u, U ( ppoo) 11> (ppa) M

sp ... ,.. ~}Q . • o ( 2%) 491. 30 (2%) 1.10

Sio , ;« 12.50 (3%) U.'l<> ( 1) I . U

~-fol d.po< 3 . 28 (U) S.20 (U) ) .59

, . l dop .. o • a.h,d,,) I . n ( 41) u.,ou\ q uO' '' .. " (41) \. 69

1I.o,,,,,'Ho 21,0 . 21 (ll) \:;8 .80 (21 ) 2.2.

Aaphiboh 27 .18 (}1) 282 . 70 (21) 10.l.O

'i.tit. 17 .4 ! (2%) ~.~ (31) 0 .39

' .... - 620 .95 (21) 501.00 (21) 0 .11

.... "" ' i<o 02. 8 ' (2%) SI . 72 (3%) 1.21

• ~bi1>o1. . . 7S (41) 9 . /00 (41) 1.39

~-fold.p.r L .6\ (SI) I . n (SI) \ . 72

hId ........... " 1 . 28 (U ) 6 . 31 (41) \ .92

..... b;1>o 1 • • py, . .... 14 . 39 OIl 30 .9' (11) 2 . 1S

... , •• «1<0 20 . 2' nu S"loO (lI) ,. ro

lioti t . \ . 01 (41) IS .4Q (3%) ,.~

' dd.po ••• qun . .. lL ,49 OIl )6.81 (31) J . ll

., h _ plak ""rpbyd'l~ , <an", ( PlOTO) - b) h .,.. '7.~; ' . ( la l .. ) . , h .. """.on;'. (P1TO •. _ b . bu ,;.,)

Afterwards each mineral fraction of all the select~d rocks underwent further manual refinement by means of the stereomicroscope in order to improve the pllreness of the mineral.

Eventually X-ray diffraction patterns cheked the presence of impurities of masuyte in the biotite and of thorite in the hornblende fraction.

The modal percents of each mineral phase in the various kinds of granite, syenite and monzonite of the pluton have been carried out in the thin section by means of the point counter (see T able 1).

U and Th concentrations in the separated minerals were measured by neutron activation analysis, at CeSNEF-Politecnico Milano.

Without any else handling, the samples were sealed in lucite phials: their weight was from 40 to 100 mg according to the eventually separable quantity. The irradiations were performed at the surface of the core of LS4 reactor with a total avai lable Aux = 3.5 1011 n/cm2 x sec. fo r a time of about two hours.

OIlIGIN OF THE CEIIVO GIIIINITI C COIIE OF THE CEIlVO PI.UTON ECC. 673

The obtained y-spcctra were analysed by a coaxial 3S cm3 Ge-Li detector connected to a Laben 8192 channel pulse height analyser.

For each sample a c standard ., addi ng to it a known amount of Th and U in the fo rm of solution was prepared then it was dried (KRUCER P., 1972).

U content determination was made, beside y-ray counti ng, also by delayed neutron counting, following fission of 23~U (EcHO M. W. et al., 1957).

For irradiation the sample was pneumatically transferred in a c rabbit., to the reactor core surface. After two minutes, the sa mple was automatically taken back to the counting apparatus consisting of a BFs proportional counter in a paraffin block connected to a sca ler for a 100 sec. count.

The U content is si mply determined from the relationship:

Weight of uranium = c.-c"

c." x weight of uranium in the standard

were: C. is the neutron count due to the sample; Cb is the mean neutron count of background; C.ld is the mean neutron cou nt for standard sa mples.

The resul ts of analysis are shown in Table 2: for uranium the concentration IS glVen as weight mean value of the concentrations measured in the different methods.

Results

The U and Th determination, in the rock-forming and accessory minerals of the rocks of the pluton, shows that sphene and magnetite are the most enriched phases in U and Th.

The statement agrees with the total radioactivity that has been evalueted by Longin'elli (1958) who investigated however by autoradiographic method only one sample of the syenite fades of the pluton. More particularly Table 2 displays that the sphene, drawn from syenite, has an higher U content than the sphene drawn from granite. However the former turns to be out the only mineral phase which is able to concentrate U more than Th (ThfU < I).

All the remai ning mineral phases of granite, syenite and monzonite show a somewhat wide range of U and Th content.

Then the analyzed minerals can be arranged into a paragenetic sequence according to the geochemical considerations that allowed us to postulate an increasing trend of Th/U ratio during the igneous mineral fractionation.

The paragenetic sequence resulting from the geochemical arrangemt:nt are the following (see ~ig. 1):

- III the granite: sphene-biotite-K/feldspar- (feldspars+quartz)-magnctite-horn­blende;

674 A. CESANA, L. DEMONTE, M . POTENZA FlORENTINI, M. VENECONI

- • . , , , ~

,< 0

SPM[~( • 8[DTlTE • 0 • > M + 'HOSP_ ~ • •

~ fEL05PARh EUMEOUL • EXAGONU QUAMTl • MAGN£TIH • AMPNIBOH •

'" . ~ BIOTIT~

SPNEN£ • • < MAGNET ITE • • • AMPNlaOLE • ~ • K- fElOSPlR • FElOSI'ARS. QUA~ll •

lMPNIBOLE" PYRQX[N[ • HHH[f ITE • BIOTIT[ < • 0 • FELDSPlRS" o.UAATZ N • ~

~ •

Fig. I.

III the syenite: biotite-sphene-magnetite-hornblende-K/feldspar-(fe1dspars + +quartz); in the monzonite: (hornblende + dyopside}-magnetite-biotite-(feldspart + quartz).

Di8Cu ssion

T he mineral fractionation of monzonite, as indicated by increasing Th/U ratio, seems to continue the mineral fractionation of the syenite. On the contrary. the mineral fractionation of granite runs almost parallel to the syenite one.

In this light the granite mineral assemblage seems to represent the product of an indipendent its own trend of fractionation rather than a fi nal product of fractionation of the pluton magma.

ORIGIN OF THE CERVO CRANIT IC CORE OF T HE CERVO PLUTON ECC. 675

TABLE 3

CompariJion b~lw~~n th~ Ir~nd of Th / V ratio in th~ diO"~nJial~d rocks 0/ C~rvo and Wh;t~ MOllntain pllltons

l""k. g (,pool n (,pool M ...... ,. ,,~ ........ , .. , . • ... TO '

O,..,h. u." ~. U .61 12-7<) 1.9J

".~It. lO.n 19-40 tl.H .~ 1.71

__ 1<0 )2 .76 D-lI 62." 71--97 1.)2

"0.1 •• , .. "I<. ' . 14 1-14 ~ .• ,.~ '.~

"1t1 ... I<o ••• D.S' '.01 ".0.'<0

Qu.a'''-.yeol •• .. ~ 0.I-Z.1 ••• HO •• • 1 __ I ..

1 . lS :1-1.1 \1.0 U-:!O S.OI

1Oo, .. O<II.rI .. , ., ,., ... Cabbro 0.' M

.1 Y.II. c .... " .I~''''' " .. « ... u,_o'. - b) .... \t. _'.'0 1 ...... 1 ...

MTI., •••• fr_ (k.l .. 10.' .. \tU) (!!!lOT. J . J .W .... 1. . IHI)

More particulary the paragenetic sequence of rock-forming and accessory mi­nerals of the granite, arranged according to an increasing ThfU ratio, agrees with the eutectic considerations that can be deduced from the textural relationships among the mineral shapes in the th in section of the granite. The arrangement is the following: sphene-biotite-K/fcldspar-(feldspars+ q uartz}-magneti te-hornblende. T he hornblende appears to be the most eteromorphic mineral phase also in the thin section. Its low modal percent (4-7 %) agrees with its role of late crystal phase.

The IMe role of magneti te on the contrary is more difficult to explai n; the ThfU ratio results to remai n constant in different fractions of magnetite in spite of the observed great variability of its U and Th content ; moreover in the thin section the magnetite appears to give constantly typical association of sphene+ + zircon + black-oxides. These features lead to believe that the magnetite with black­oxides of the granite could raprescnt a late stage segregation product of the above association of the fir st stage radioactive minerals.

Nevertheless some compa rison and furthe r geological-pet rographic considerations need to support more satisfactory the choise we have to made between the two hypotheses: the granitic core is a post<rystalline intrusion of a residua system (eutectic granite) or a preexisti ng exsolutioo of high.silica granitic melt.

Taking into considera~ion the t rend of the average of the ThfU ratio in the total rock, we can observe that in the selected rocks of the pluton the ratio increases from monzonites to syenites but it appears to decrease again in the granite_

676 A. CESANA, L. OEMONTE, M. POT ENZ A fIORENTIN1, M. VEN£GONI

In the T able 3 the above trend of variation of the ThjU ratio in the CervQ pluton is compared with the analogous trend in the serie of igneous rocks of the White Mountain pillton (BOTLER A. P., 1961).

In these rocks the mean T hjU ratio appears to increase regularly in the sequence gabbro-monzodiorite-monzonite-quartz syenite-hastingsite gra nite; passing to the last term-biotite granite-Thj U ratio still increases showi ng however a discontinuity towards high values.

T hen it appears that both the compared igneous sequences of plutonic rocks show a break in the Increasing trend of the T hfU ratio passing to the last granitic 'errn.

However while In the White Mountain pluton there is a break with rising of T hfU ratio passing to the final granitic term, in the Cervo pluton wc observe that the T hfU ratio undergoes to a recessive break.

T hereby the final biotite-granite of White Mountain pluton shows late character of final system (eutectic e.g.) while the central granite of the Cervo pluton shows the mentioned recessive character suggesti ng a primitive molten core of silice-rich unmixed melt. No other differentiated plutons are known unfortunately in regard to the trend of ThfU ratio in total rocks.

Besides the modal composition of the granite inth e core indicates a truly quartz bearing rock contrasting with the surrounding syenite typically void of quartz.

T he following petrologic features have to be pointed out :

I ) a band of quartz rich rock forms the ring of separation between the granitic core and the syenite, containing more than 70 % of modal quartz. In the system [Si02] [ FeO+Mg+CaO ] [Na~+ K20+Ab03] the quartz-rich facies, as well as the common granitic facies, close the field of exsolved silica-

A) Hypothetic initial stage (Oligoccne): in the deep igneous body a layer of high·silica unmixed granitic liquid floats on the molten mon:z:onite: high silica unmix~d liquid is mor~ viKOus and mOf~ light than the und~rlying molten monzonite. M.G. = high silica molten gra nit~.

M.M. = mol ten monzonite.

B) Hypothetic intermediate stage: under basal compreuive conditions hotte. and more fl uid monzonite: melt begins to ri ... , overlapping by conve<:lion the: lay~r of viscous high·silica unmixed pha&e (molten granile:). According to the «fluid me<:hanic. the following conditions must be verified : that the rising thrust does continue and that the ~mplaccment process is able: to make way, by jointi ng, cracking and roof digestion, to let the molten monzonite overlaps the granite: melt .

C) Hypothetic last stage : the molten monwnite rising the border becomes more viscous by cooling and eventually can transmit the compressive deformation to the viscous high-si lica melt and can enclose it.

D) Pr~scnt stage: the Cervo valley erosion .hows the pluton imernal structure with ,he inner grani,ic core. (M = mon:ronite; G = granite).

OR IGIN 01' THE CERVO GRANITIC CORE 01' THE CE RVO PLUTON liCC. 677

rich melts: e.g. in the Campiglia granite we have (FIORENTINI PQTENZA M., 1959 a; PEYRONEL P AGLlANI G., 1961 a) [Si02] = 67.9; [FeO+Mg+CaO] = 7; [Na20+ KzO+ Ab03] = 25.1. These values lead in the range of immiscibility of the above system;

678 A. CESANA, L. DEMONTI>, M. POTENZA FIORENTlNI, M. VENEGONI

2) in the pink granite the quartz is pres<:nt with two populations of grains: a first population of grains showi ng well.sha~d exagonal habit and a late population of heteromorphic grains; this suggests that in its history of crystallization the granite melt stayed temporary in [he quartz-phase field of the system silica­K/feldspar-plagioclase. Both points one and two lead still the primitive molten core into the range of immiscibility of the high silica melts. Eventually other features caoRiet with a postcrystaJline intrusion of a molten core made of eutectic granite in the Cervo pluton;

3) the boundary lx:twttn the granite core and its endosing shell of syenite+mon­wnite appears to be a very rapid but not discontinuous transition; that is typic of a surface of separation between mc:lts of different density and not of a solid­liquid sepa ration;

4) the outcropping shape of the granitic core IS that of a regular dome-shaped cupola;

5) the shell of syenite + monzanite, surrounding th!': granite core, shows a rigorous lack of a plastic and rigid radial ddormations (PEYRONEL PACLIANI G., 1961 b) such as it should be caused by an hypothetieal late stage intrusion of granite melt in a rigid or plastic rock. In the granitic core there is besides a typical lack of fragments of syenite or monzonite while in the monzonite there are many fragments of metamorphic surrounding rocks disrupted by the intrusion. Keeping in mind the above features we lean towards the second hypothesis thinking of an emplacement of the molten pluton together with its unmixed granitic liquid core. The sequence of Fig. 2 sketchs a possible mechanism of emplacement of the Cervo pluton, we take into consideration.

Conclu8ion

The assumption of a general increasing trend of Th/U ratio during the crystal fract ionation has been postulated theoretically on geochemical bases.

The result of the present attempt of application shows that the increasing trend of Th/U ratio seems to suggest mineral fractionation sequences that appear to be verified satisfactory within each selected petrographic facies of the Cervo Teniary pluton as a whole. The observed fractionation trend in the monzonite mineral assemblage results in fact to continue the fractionation trend of the syenite one. From this point of view, the syenite should represent the differentiation product of the highest temperature in the pluton. The granite of the core shows, on the

ORIGIN 01' THE CERVO CRANITIC CORE OF THE CERVO PL11TON ECC. 679

contrary, a recessive position of its range of Th/V ratio of fractionation. This fact plays in favour of an independent its trend of mineral fractionation in respect to the remaining part of the pluton.

Such a recessive character suggests a primitive unmixed molten core of silica­rich exsolved melt.

Vnmixing of granite melt is supported also by its relative poorness in large cations in respect to the syenite (CESANA A. et aI., 1976), by its modal composition, by euhedral exagonal quartz, by lack of plastic/rigid radial deformations in the enclosi ng syenites and monzonites and by other featu res melllioned in the discussion.

The richness in large cations showed by the syenite faci es is apparently coo­flicti ng with its high thermal stage features and it asks for less common and less simple process to be satisfactorily explained.

Acltmowltdgmmts - To Prof. lvo Uus for discussion and aids in the Duid-dynamic of molten magma.

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