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Cumulate-like textures and chemical relationships in the Bressanone (Brixen) Granodiorite (Eastern Alps, Italy)— A new genetic approach
Chemical Geology, 40 (1983) 279--292 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
279
CUMULATE-LIKE TEXTURES AND CHEMICAL RELATIONSHIPS IN THE BRESSANONE (BRIXEN) GRANODIORITE (EASTERN ALPS, ITALY)-- A NEW GENETIC APPROACH DARIO VISONA' Istituto di Mineralogia e Petrologia, Universit~ di Padova, Padova (Italy) (Received July 14, 1982; revised and accepted November 26, 1982)
ABSTRACT Visona', D., 1983. Cumulate-like textures and chemical relationships in the Bressanone (Brixen) Granodiorite (Eastern Alps, Italy) -- A new genetic approach. Chem. Geol., 40: 279--292. The shallow level pluton of Bressanone is a Late Hercynian multiple intrusion into the South Alpine basement of the Eastern Alps. Most of this complex is composed of anatectic granodiorites and granites intruded in separate stocks 282 -+ 14 Ma ago; gabbros and leucogranites occur in smaller quantities. The chronological intrusion sequence is: layered gabbro, granodiorites and granites, two-mica cordierite leucogranite and fayalite leucogranites. The granodiorites and granites may contain hornblende or garnet. The hornblende and garnet rocks differ both in chemistry and (87Sr/8~Sr)l ratio, and may be identified as "It y p e " and "S-type", respectively, according to the Chappell--White classification. Textural and chemical patterns show that the granites may be linked to the granodiorites by cumulate-like processes. The granodiorite -~ granite transition, attributed to filter pressing, expresses an increase in the liquid/xenolith ratio in a magma where the liquid fraction was a m i n i m u m melt and the solid fraction was restitic material. INTRODUCTION
Recent geochemical studies (McCarthy and Hasty, 1976; McCarthy and R o b b , 1978; McCarthy and Groves, 1979; Tindle and Pearce, 1981) have shown that, as in basic plutons, fractionation processes may develop cumulate-like chemical trends in acid plutons. Textures characteristic of accumulation processes are only rarely reported in these rocks (McCarthy and Groves, 1979 and references therein), and consist of cumulate crystals with trapped interstitial melt. It is well known that the physical state of granitic magma is a mixture of crystals and melt with such high viscosity that no appreciable separation between the solid and melt phases is usually possible. In spite of this, fractionation processes due to filter pressing appear to have occurred, as reported for instance by Tindle and Pearce (1981) for the zoned pluton of Loch Doon. 0009-2541/83/$03.00
© 1983 Elsevier Science Publishers B.V.
280 So far, the problem of fractionation has been studied by considering plutons where cumulus minerals were considered as products of magmatic crystallization. On the other hand, anatectic melts frequently contain relicts of the rocks from which they originated (xenocrysts and/or xenoliths) (White and Chappell, 1977; Winkler and Breitbart, 1978). In some cases, these relicts may separate from the liquid due to fractionation processes, and give rise to cumulate-like structures. This work considers structures and chemistry of cumulate-like rocks originating from crustal anatexis, in which the "cumulus" is represented by relict fractions of the original rocks. In the light of new data presented here, a new petrogenetic model will be proposed for the granodiorites and granites, based on the possibility that some textures and chemical features may be linked to accumulative processes. FIELD RELATIONSHIPS The igneous complex of Bressanone (Brixen) is a composite shallow-level pluton of Late Hercynian age, intruded into the South Alpine basement of the Eastern Alps. It is mainly composed of granodiorites and granites, and associated garnet- or hornblende-bearing stocks, intruded by minor masses of two-mica cordierite leucogranites and by fayalite leucogranites. Near the southern margin (Fig. 1) the granodiorites intrude a small mass of layered gabbro with porphyritic dykes. The age of intrusion of the main body of rock is 282 -+ 14 Ma (Del Moro and Visona', 1982). The chronological intrusion sequence is: layered gabbro, granodiorite and granite, two-mica cordierite leucogranites and fayalite leucogranites. ~
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281
The layered gabbro and fayalite leucogranites seem to have derived from mantle melts, later contaminated by crustal material (Del Moro and Visona', 1982). On the other hand, the two-mica cordierite leucogranites, granodiorites and granites are of crustal anatectic origin (Bellieni et al., 1979; Visona', 1980): their mineralogical differences and (STSr/86Sr)i ratios reflect differences in composition of parent crustal materials and depth of crustal fusion (Del Moro and Visona', 1982). An initial chemical investigation (Visona', 1977) showed granites and granodiorites to be alighned over differentiation trends, while the leucogranites do not fit these trends. A N A L Y T I C A L METHODS
Microprobe analyses of the minerals (plagioclase, garnet, biotite and amphibole) were carried out using an EDS®--ORTEC ® system. Accelerating voltages of 15 kV and specimen current of 10 -9 A were employed. Natural crystals of known composition were used as standards. Accuracy of the method is better than 3% for major elements and 8% for minor ones. Whole-rock analysis was carried out using X-ray fluorescence (XRF) and atomic absorption spectrometry (AAS). Samples were prepared for analysis by the De Vecchi et al. (1968) technique; SiO2 and A1203 were determined (error 0.5%), together with Rb, Sr and Ba (error 8%). Other elements were determined by AAS, using the method proposed by Langmyhr and Paus (1968). Accuracy of determinations was checked by comparison with G1, G2, GSPI, AGV1 and BCR1 standard rocks and the values obtained were within 2% of the recommended values (Flanagan, 1973). PETROGRAPHIC DATA
The rocks studied are those described by Visona' (1977) as group-B rocks. They are medium- to coarse-grained rocks (1--6 mm), mostly equigranular, rarely porphyritic, with a hypidiomorphic structure. Their essential mineralogical composition is quartz, K-feldspar and plagioclase, classifying them as granodiorites and monzogranites (Fig. 2). Femic minerals are brown biotite (idiomorphic to sub-idiomorphic) and idiomorphic pargasitic hornblende. This last mineral appears both as an inclusion in plagioclase and as a phenocryst. A colourless amphibole (cummingtonite) also occurs occasionally, and may represent both relicts in the hornblende and irregular aggregates with dark-brown biotite and hornblende rims. Garnet, corundum and hercynite are less frequent and generally occur as small inclusions in the plagioclase of rocks with a cumulate texture. Accessory minerals are apatite, zircon, sphene and, less frequently, ilmenite and magnetite.
282
Q
• "
3a
A
~ 2
t
3b
P
Fig. 2. Modal classification according to Streckeisen (1967). Filled triangles indicate the most common rock types. Minor bodies of leucogranite are generalized by the d o t t e d area (Visona', 1980).
Based on the occurrence of hornblende and/or garnet and on textures, these rocks may be classified into two groups: (1) Amphibole-bearinggranodiorites and granites: these rocks show a hypidiomorphic texture and contain rounded aggregates of quartz, plagioclase and biotite. These aggregates are essentially composed of quartz, surrounded by small idiomorphs of plagioclase and biotite. A distinct type of aggregate is composed of plagioclase and biotite, the plagioclase being widely saussuritized and showing a thin rim of oligoclase. Biotite is brown, deformed, and partly chloritized. These aggregates sometimes show a rectangular section so that, under the microscope with parallel polars, they may appear as single crystals. However, with crossed polars, they show their complex structure. Biotite forms small crystals, all or partly associated with chlorite, pumpellyite, prehnite, hornblende, apatite and ore minerals. Equally frequent are plagioclase aggregates in which large flakes of deformed, partially or totally choritized biotite occupy the intergranular spaces between plagioclase crystals, enclosing the smaller ones. In these rocks, dark-grey microgranular inclusions frequently occur. They are 1--10 cm long, rarely schistose, with an ophitic structure, and are composed of plagioclase and biotite with lesser quantities of quartz and sometimes pargasitic hornblende; ovoids of pumpellyite and prehnite occur frequently in the biotite. Worthy of note is the fact that the minerals included in the plagioclase of the aggregates are the same as those found in the larger inclusions and in their plagioclases. This suggests that the above aggregates may have the same origin as the larger ones, from which they differ only in size. The mediumsized plagioclase crystals (44% An; with only a few saussuritic p a t c h e s a n d
283
rare inclusions) which are frequently found in the granites, may represent extreme reorganization of the smaller inclusions. (2) Garnet-bearing granodiorites and granites: these are rocks with a special texture caused by large crystals of quartz or perthitic K-feldspar containing small inclusions of plagioclase, quartz and biotite: the plagioclase is idiomorphic, with a calcic core (86--92% An), frequently saussuritized with strongly zoned and broad rims (32% An); the biotite is pinkish-brown, often deformed and partly chloritized; the quartz occurs in rounded granules, sometimes with curved contours (Fig. 3). Garnet, corundum and hercynite are found exclusively included in the plagioclase. This texture, c o m m o n in the basic cumulitic rocks, suggests the occurrence of cumulus crystals (plagioclase, biotite and quartz) and interstitial phases (K-feldspar and quartz), crystallized from an intercumulus liquid. Finally, both hornblende and garnet rocks show clear microtextural evidence for the presence of numerous restitic phases. (a) In the case of the hornblende rocks, the plagioclase of the variously sized inclusions (from mm to cm dimensions) and the single crystals contain the same inclusions (biotite, pumpellyite, etc.). This relationship presupposes a c o m m o n source or, in any case, fine disaggregation of the larger inclusions to single xenocrysts. (b) As for the garnet rocks, the identification of the relict phases is based
Fig. 3. Garnet-bearing rocks. Cumulate-like texture caused by large pecilitic areas of orthoclase and quartz.
28,1
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A
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113 F i g . 4. P l o t
t
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115 (A = g r a n o d i o r i t e
110 and granite;
A[203 °/o 117
~ = xenoliths;
,, = r o c k s u s e d
for mass-balance calculation). E x t r e m e rock c o m p o s i t i o n s f r o m t h e same s t o c k are tielined. Inset: field o f rocks s t u d i e d in relation to their mineral c o m p o s i t i o n s .
on different criteria, since the inclusions are missing. Garnet and corundum are always included in plagioclase with hercynite, and may show reabsorption borders and plagioclase + biotite rims (Bellieni et al., 1979). The extremely calcic composition of the plagioclase core (up to 92% An) and its strong zonations reveal the restitic nature of its inner crystal sections (White and Chappell, 1977). In the quartz (not the large pecilites) the rounded and sometimes curved contours are significant. As for the biotite, the deformations -- shown only by this mineral -- indicate magma movement before crystallization of the larger pecilites of K-feldspar and quartz, giving the rock its cumulate-like texture. The presence of significant quantities of restitic phases poses the problem of their influence on the chemistry of the rocks containing them. Different quantities of the same restitic phases dispersed in a melt of constant composition will influence the chemistry of the rocks in the same way that quantities of magmatic cumulus crystals determine the characteristic trends of the cumulitic rocks. CHEMICAL DATA
Both new analyses and those carried out by Visona' {1977} are presented: Ba, Rb and Sr were redetermined using the same methodology as that adopted for the new samples*. * F o r reasons o f space, t h e tables " C h e m i c a l microanalysis o f m i n e r a l s " and " C h e m i c a l analysis o f w h o l e r o c k s " have n o t b e e n included. R e a d e r s w h o require these tables are req u e s t e d to write to t h e a u t h o r .
285
The aim of the chemical study was to establish that the accumulation of phases extraneous to the melt, as suggested by textural relationships, was consistent with the variation trends of the rocks. With this aim in mind, the variation diagrams of some major elements (Figs. 4--7) and trace elements (Figs. 8 and 9) were considered. The tie-lined open triangles represent rocks of extreme composition (granites and grano-
Kz0%
.A"
A~
O A
31
41 Naz0%
Fig. 5. Plot of K=O vs. Na=O for rocks of the Bressanone pluton. Symbols as in Fig. 4.
Mg0 ~wt.% )
1,
21
31
41 C a O ( w t % )
Fig. 6. Plot of MgO vs. CaO. Symbols as in Fig. 4.
MgO(wt °/o)
• Hnb • Biot
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= .~ i ~ ¢~=rtz K-SPar
Fig. 7. Plot of MgO vs. SiO=. Symbols as in Fig. 4.
286 1
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100
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soa I
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Fig. 8. Plot o f Sr vs. R b o n log axes. S y m b o l s as in Fig. 4. D - - E a n d A - - B are q u a r t z + plagioclase + b i o t i t e c o n t r o l lines. F - - G a n d B--C are q u a r t z + plagioclase + b i o t i t e + alkali-feldspar c o n t r o l lines ( M c C a r t h y a n d R o b b , 1978). D i a g r a m : z = g a r n e t - b e a r i n g rocks; • = h o r n b l e n d e - b e a r i n g rocks; A = c o r d i e r i t e l e u c o g r a n i t e s ( V i s o n a ' , 1980). D o t s are arr a n g e d along t h e solidus--liquidus mixing-line. I n s e t : p r o j e c t i o n o f c o m p o s i t i o n o f all r o c k s studied. F
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Fig. 9. Plot o f Ba vs. Rb on log axes. Symbols as in Fig. 8.
diorites) from three different stocks, two of
are hornblende-bearing
which rocks and one a garnet-bearing rock. The negative correlation between A1203 and SiO2 (Fig. 4) may reflect the variation of the ratio between quartz and feldspars and the reduction in the
287
An c o n t e n t in the plagioclases during magmatic crystallization. However, the tendency of the dots to scatter along the ordinate may also be indicative of a concentration of xenocrysts of calcic plagioclase, biotite and hornblende (Fig. 4). The great variability in the Na20/K20 ratio (Fig. 5) is due to several subtrends with negative correlation which, according to McCarthy and R o b b (1978), represent mixing-lines between liquid and cumulus phases (plagioclase, quartz and biotite). The inversion of this correlation with higher K20 content indicates crystallization of the magma without cumulates (xenocrysts). In Fig. 6 it is still possible to note strong scattering of the dots along the abscissa. The presence of sub-trends with positive correlation is due to the extensive solid solution of MgO and CaO in biotite and plagioclase, respectively. But the sub-trends are also parallel in increasing CaO contents, which implies the presence of phases rich in CaO and only subordinately in MgO. The same observations may be made for the diagram in Fig. 7 (MgO vs. SiO2), in which scattering along the abscissa may be caused by the presence of phases relatively low in SiO: and small amounts of MgO (i.e. plagioclase with a little biotite). This pattern is similar to that described by White and Chappell (1977) for the granitoids of the Moruya suite (SE Australia) and interpreted as progressive separation of restite from a minimum melting liquid. These observations on the major elements are reinforced for the trace elements with large ion radius -- Ba, Rb and Sr. The Sr/Rb and B a / R b ratios are characteristic of granitic rocks with cumulate-like chemistry, as proposed by McCarthy and Hasty (1976). In effect, the rocks studied here may be plotted along parallel lines which intersect the compositions of the liquid (line B--C in Figs. 8 and 9) and that of the solid (line F--G in Figs. 8 and 9). The t w o groups of rocks --hornblende-bearing granodiorites and granites, and garnet-bearing granodiorites and granites, differing in mineralogy and structure -- are arranged in the Sr/Rb diagram along mixing-lines with different Sr contents. They intersect both the line of expected liquid composition and the trend supplied by the more differentiated rocks (garnet granites and two-mica cordierite leucogranites). The latter represent the composition of the liquid in which variable quantities of xenoliths and xenocrysts are dispersed. Their composition is very near the minimum melt (Visona', 1977,
1980). In general, the chemical variation observed is similar to that o f typical cumulitic rocks and is thus consistent with the cumulitic hypothesis advanced on petrographic evidence. Finally, a c o m m o n characteristic of all diagrams above is the presence of several trends which generally reflect chemical variations within single stocks. The chemical differences between these seem to be related to the t y p e of restitic material dispersed in the liquid. Inside each stock, the granodiorite -* granite differentiation may be explained as due to the accumulation of restitic phases (essentially plagioclase, biotite, quartz and amphibole).
-100 --
GD229" GD264 GD510
*Garnet-bearing rocks.
371
Granodiorite
-. 84.27
246A
.
Parg GD510 -. 5.47
Biot. GD229
38.75 . -. --
41.54 .
PL GD229
PL GD510
10.26
--
--
19.71
Qz
62.26 0.83 16.55 1.25 5.50 0.16 1.55 4.61 3.50 3.39 0.41
66.46 0.81 15.67 0.62 4.85 0.10 0.87 3.83 3.03 3.50 0.25
75.84 57.49 55.89
Parg.
G274" G262 G213
Granite
37.90 48.44 2.51 0.82 18.29 7.46 . . . 23.64 21.62 0.26 0.53 8.14 8.94 -10.60 0.16 0.91 9.11 0.67 . . . .
%
Xenocrysts
71.63 0.35 14.42 0.41 2.61 0.08 0.75 2.54 2.69 4.42 0.10
Xenoliths
67.26 0.56 15.99 0.22 3.99 0.08 1.42 3.98 3.01 3.34 0.16 Residual liquid
73.30 0.26 13.78 0.30 2.07 0.06 0.56 2.28 2.86 4.43 0.09
Biot.
Cumulating minerals recalculated to 100%
68.59 0.46 15.31 0.60 3.05 0.08 1.34 3.29 3.30 3.84 0.13
246A
Parent-rocks
73.99 0.13 13.74 0.09 1.41 0.06 0.58 0.63 3.41 5.00 0.95
G213
69.17 0.42 15.90 0.18 3.34 0.08 1.42 2.09 2.72 4.56 0.12
GD510
Xenocrysts
SiO: TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K~O P2Os
G262
371
GD264
GD229
G274
Xenoliths
Rocks
Results of least-squares model using methods of Wright and Docherty (1970) and Stormer and Nicholls (1978)
TABLE I
0.5554 0.5561 0.4537
ER 2
46.21 -34.74 . 0.20 --18.01 0.82 0.02 . .
PL229
0.16 --16.79 1.42 --
47.43 -34.15
PL510
0¢
t~
289 APPROACH TO THE P E T R O L O G I C A L MODEL
The two preceding sections have shown that the rocks studied may be products of mixing between a granitic melt and restites (xenocrysts and variously sized inclusions). Where the chemistry of the two extreme components (granitic melt and restites) and the hypothetical mixing products {granodiorites) are known, it is possible, by means of mass-balance calculations, to evaluate the reliability of the relationship granodiorite (rock) ~ xenoliths/xenocrysts (restite) + granite This calculation firstly deals with the total composition of the two chemical systems considered (major elements: Wright and Docherty, 1970; Stormer and Nicholls, 1978), and determines the relative proportions of restite and melt for the formation of granodiorite. As the whole plutonic complex is composed of many minor stocks, the above relationship must be applied to rocks belonging to the same stock. For three of these, the chemistry of the extreme compositions (granites and granodiorites; open triangles in Figs. 4--6) is considered, together with the associated restitic material. In the hornblende rocks, this is represented only by dark microgranular inclusions or by inclusions plus plagioclase and restitic amphibole. In the garnet rocks, the restitic portion considered for calculation purposes is given by plagioclase, biotite and quartz. Table I shows the results of the calculation and indicates that, both for the hornblende and garnet rocks, the granites may have been derived from the respective granodiorites by subtraction of the phases listed in the table. Lastly, the congruency of the cumulitic model may also be seen through the mass-balance calculation. CONCLUDING REMARKS
The intrusive complex of Bressanone is composed almost entirely of granodiorites and associated granites. New textural and chemical data demonstrate that these rocks may be referred to variable mixtures of restitic material (xenocrysts + xenoliths) and liquid, presumably having a minimum melt composition. As for the type of fractionation, filter pressing is the most probable mechanism in granitoid rocks (Tindle and Pearce, 1981). In effect, the viscosity of the silicatic melt controlled the location of the xenoliths, both hindering their separation by gravity (White and Chappell, 1977) and preserving their possible lack of distributional homogeneity. This second consideration may be explained by the melting behaviour of rocks of different composition or of similar rocks under different conditions of P, T and H20. Indeed, different rocks produce minimum melt of equal composition under identical conditions (Winkler et al., 1975; Wyllie et al., 1976; Thompson and Algor, 1977; MacRae and Nesbitt, 1980), but the closer the rock composition approaches
290
the granitic minimum, the greater is the melt/restite ratio+ On the other hand, the same rock, under different P--T--H20 conditions, may produce different melts (Brown and Fyfe, 1970; Hall, 1971; Fyfe, 1973; Wyllie, 1977; Win kler an d Breitbart, 1978). Thus, extensive anatexis of a chemically heterogeneous crystalline basement necessarily leads to the formation of masses coexisting with different restitic fractions and characterized by different total chemistry (Brown and Fyfe, 1970; Fyfe, 1973; White and Chappell, 1977). As for the source materials of the rocks studied, it is worth considering that, because of their mineralogy, chemistry (see also Fig. 10) and (STSr/ S6Sr)i (0.07101 -+ 0.0002 for the garnet rocks; slightly less than 0.7096 -+ 0.0006 for the amphibole rocks), they may be identified as "S type" (garnet rocks) and "I type" (hornblende rocks) according to the classification of Chappell and White (1974). The differentiation granodiorite + granite reflects an increase in the liquid/xenolith ratio in the melts produced by two types of parent-rocks: (1) pelitic rocks, which gave rise to "S type" magmas, including the garnet rocks, and (2) metasediments containing basic igneous rocks, which produced "I type" magmas, including the hornblende rocks. At -
Pt
_
Na
- K
_
' Mu
Cor
....
d
ot
/ Ca
~.::+ ,
2+
Fe+Mg
Fig. 10. C a - - ( F e 2÷ + M g ) - - ( A 1 - - N a - - K ) v a r i a t i o n diagram (+ = h o r n b l e n d e rocks; o = granet rocks; -~ = " S t y p e " m i n i m u m m e l t ; * = " I t y p e " m i n i m u m melt). A r e a s b e t w e e n c o n t i n u o u s lines c o n t a i n c o m p o s i t i o n s o f " S t y p e " r o c k s ( t o w a r d s right) and " I t y p e " ( t o w a r d s Ca apex) o f White and Chappell (1977); d o t t e d line s h o w s field o f r o c k s s t u d i e d here.
291 ACKNOWLEDGEMENTS
The author is indebted to Profs. B. Zanettin and F.P. Sassi for their critical reading of the manuscript. Many thanks are due to Prof. A. Cundari for his significant contribution to the final arrangement of the text. Chemical analyses were carried o u t in the laboratories of the Institute of Mineralogy and Petrology of the University of Padova, with the collaboration of P. Da Roit, A. Giarretta and G. Mezzacasa. This work forms part of the programmes of the Centro di Studio per i Problemi dell'Orogeno delle Alpi Orientali (Padova) (Director: Prof. B. Zanettin) of the Italian National Council for Research.
REFERENCES Bellieni, G., Molin, G.M. and Visona', D., 1979. The petrogenetic significance of the garnets in the intrusive massifs of Bressanone and Vedrette di Ries (Eastern Alps, Italy). Neues Jahrb. Mineral., Abh., 136: 238--253. Brown, G.C. and Fyfe, W.S., 1970. The production of granitic melts during ultrametamorphism. Contrib. Mineral. Petrol., 28: 310--318. Chappell, B.W. and White, A.J.R., 1974. Two contrasting granite types. Pac. Geol., 8: 173 -174. Del Moro, A. and Visona', D., 1982. The epiplutonic Hercynian complex of Bressanone (Brixen, Eastern Alps, Italy). Petrographic and radiometric data. Neues Jahrb. Mineral., Abh., 145: 66--85. De Vecchi, Gp., Piccirillo, E.M. and Quareni, S,, 1968. Contributo all'analisi rapida di rocce mediante fluorescenza ai raggi X, Rend. Soc. Ital. Mineral. Petrol., 24: 3--14. Flanagan, F.J., 1973. 1972 values for international geochemical reference samples. Geochim. Cosmochim. Acta, 37: 1189--1200. Fyfe, W.S., 1973. Granites past and present. Spec. Publ. Geol. Soc. S. Afr., 3: 13--16. Hall, A., 1971. The relationship between geothermal gradient and the composition of granitic magmas in orogenic belts. Contrib. Mineral. Petrol., 32: 186--192. Langmyhr, F.J. and Paus, P.E., 1968. The analysis of inorganic siliceous materials by atomic absorption spectrophotometry and the hydrofluoric acid decomposition technique. Anal. Chim. Acta, 43: 397--408. MacRae, N.D. and Nesbitt, H.W., 1980. Partial melting of common metasedimentary rocks: a mass balance approach. Contrib. Mineral. Petrol., 75: 21--26. McCarthy, T.S. and Hasty, R.A., 1976. Trace element distribution patterns and their relationship to the crystallization of granitic melts. Geochim. Cosmochim. Acta, 40: 1351--1358. McCarthy, T.S. and Groves, D.I., 1979. The Blue Ties Batholith, northeastern Tasmania. A cumulate-like product of fractional crystallization. Contrib. Mineral. Petrol., 71: 193--209. McCarthy, T.S. and Robb, L.J., 1978. On the relationship between cumulus mineralogy and trace and alkali-element chemistry in an Archaean granite from the Barberton region, South Africa. Geochim. Cosmochim. Acta, 42: 21--26. Stormer, J.C. and Nicholls, J., 1978. XLFAAC: A program for the interactive testing of magmatic differentiation models.Comput.--Geosci., 4: 143--159. Streckeisen, A., 1967. Classification and nomenclature of igneous rocks, Neues Jahrb. Mineral., Abh., 107: 144--240.
292 Thompson, A.B. and Algor, J.R., 1977. Model system for anatexis of pelitic rocks. Contrib. Mineral. Petrol., 63: 247--269. Tindle, A.G. and Pearce, J.A., 1981. Petrogenetic modelling of in situ fractional crystallization in the zoned Loch Doon Ptuton, Scotland. Contrib. Mineral. Petrol., 78: 196--207. Visona', D., 1977. Contributo alla conoscenza del massiccio di Bressanone (Alpi Orientati), Rend. Soc. Ital. Mineral. Petrol., 33: 642--665. Visona', D., 1980. Lo stock granitico a cordierite del M. Sella nel massiccio di Bressanone. Rend. Soc. Ital. Mineral. Petrol., 36: 91--106. White, A.J.R. and Chappel], B.W., 1977. Ultrametamorphism and granitoid genesis. Tectonophysics, 43: 7--22. Winkler, H.G.F. and Breitbart, R., 1978. New aspects of granitic magmas. Neues Jahrb. Mineral., Monatsh., 10: 463--480. Winkler, H.G.F., Boese, M. and Marcopulos, T., 1975. Low temperature granitic melts. Neues Jahrb. Mineral., Monatsh., 6: 245--268. Wright, T.L. and Docherty, P.C., 1970. A linear programming and least-squares computer method for solving petrologic mixing problems. Geol. Soc. Am. Bull., 81: 1995-2008. Wyllie, P.J., 1977. Crustal anatexis: an experimental review. Tectonophysics, 43: 41--71. Wyllie, P.J., Huang, W.L., Stern, C.R. and MaalCe, S., 1976. Granitic magmas: possible and impossible sources, water contents and crystallization sequences. Can. J. Earth Sci., 13: 1007--1019.
279
CUMULATE-LIKE TEXTURES AND CHEMICAL RELATIONSHIPS IN THE BRESSANONE (BRIXEN) GRANODIORITE (EASTERN ALPS, ITALY)-- A NEW GENETIC APPROACH DARIO VISONA' Istituto di Mineralogia e Petrologia, Universit~ di Padova, Padova (Italy) (Received July 14, 1982; revised and accepted November 26, 1982)
ABSTRACT Visona', D., 1983. Cumulate-like textures and chemical relationships in the Bressanone (Brixen) Granodiorite (Eastern Alps, Italy) -- A new genetic approach. Chem. Geol., 40: 279--292. The shallow level pluton of Bressanone is a Late Hercynian multiple intrusion into the South Alpine basement of the Eastern Alps. Most of this complex is composed of anatectic granodiorites and granites intruded in separate stocks 282 -+ 14 Ma ago; gabbros and leucogranites occur in smaller quantities. The chronological intrusion sequence is: layered gabbro, granodiorites and granites, two-mica cordierite leucogranite and fayalite leucogranites. The granodiorites and granites may contain hornblende or garnet. The hornblende and garnet rocks differ both in chemistry and (87Sr/8~Sr)l ratio, and may be identified as "It y p e " and "S-type", respectively, according to the Chappell--White classification. Textural and chemical patterns show that the granites may be linked to the granodiorites by cumulate-like processes. The granodiorite -~ granite transition, attributed to filter pressing, expresses an increase in the liquid/xenolith ratio in a magma where the liquid fraction was a m i n i m u m melt and the solid fraction was restitic material. INTRODUCTION
Recent geochemical studies (McCarthy and Hasty, 1976; McCarthy and R o b b , 1978; McCarthy and Groves, 1979; Tindle and Pearce, 1981) have shown that, as in basic plutons, fractionation processes may develop cumulate-like chemical trends in acid plutons. Textures characteristic of accumulation processes are only rarely reported in these rocks (McCarthy and Groves, 1979 and references therein), and consist of cumulate crystals with trapped interstitial melt. It is well known that the physical state of granitic magma is a mixture of crystals and melt with such high viscosity that no appreciable separation between the solid and melt phases is usually possible. In spite of this, fractionation processes due to filter pressing appear to have occurred, as reported for instance by Tindle and Pearce (1981) for the zoned pluton of Loch Doon. 0009-2541/83/$03.00
© 1983 Elsevier Science Publishers B.V.
280 So far, the problem of fractionation has been studied by considering plutons where cumulus minerals were considered as products of magmatic crystallization. On the other hand, anatectic melts frequently contain relicts of the rocks from which they originated (xenocrysts and/or xenoliths) (White and Chappell, 1977; Winkler and Breitbart, 1978). In some cases, these relicts may separate from the liquid due to fractionation processes, and give rise to cumulate-like structures. This work considers structures and chemistry of cumulate-like rocks originating from crustal anatexis, in which the "cumulus" is represented by relict fractions of the original rocks. In the light of new data presented here, a new petrogenetic model will be proposed for the granodiorites and granites, based on the possibility that some textures and chemical features may be linked to accumulative processes. FIELD RELATIONSHIPS The igneous complex of Bressanone (Brixen) is a composite shallow-level pluton of Late Hercynian age, intruded into the South Alpine basement of the Eastern Alps. It is mainly composed of granodiorites and granites, and associated garnet- or hornblende-bearing stocks, intruded by minor masses of two-mica cordierite leucogranites and by fayalite leucogranites. Near the southern margin (Fig. 1) the granodiorites intrude a small mass of layered gabbro with porphyritic dykes. The age of intrusion of the main body of rock is 282 -+ 14 Ma (Del Moro and Visona', 1982). The chronological intrusion sequence is: layered gabbro, granodiorite and granite, two-mica cordierite leucogranites and fayalite leucogranites. ~
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281
The layered gabbro and fayalite leucogranites seem to have derived from mantle melts, later contaminated by crustal material (Del Moro and Visona', 1982). On the other hand, the two-mica cordierite leucogranites, granodiorites and granites are of crustal anatectic origin (Bellieni et al., 1979; Visona', 1980): their mineralogical differences and (STSr/86Sr)i ratios reflect differences in composition of parent crustal materials and depth of crustal fusion (Del Moro and Visona', 1982). An initial chemical investigation (Visona', 1977) showed granites and granodiorites to be alighned over differentiation trends, while the leucogranites do not fit these trends. A N A L Y T I C A L METHODS
Microprobe analyses of the minerals (plagioclase, garnet, biotite and amphibole) were carried out using an EDS®--ORTEC ® system. Accelerating voltages of 15 kV and specimen current of 10 -9 A were employed. Natural crystals of known composition were used as standards. Accuracy of the method is better than 3% for major elements and 8% for minor ones. Whole-rock analysis was carried out using X-ray fluorescence (XRF) and atomic absorption spectrometry (AAS). Samples were prepared for analysis by the De Vecchi et al. (1968) technique; SiO2 and A1203 were determined (error 0.5%), together with Rb, Sr and Ba (error 8%). Other elements were determined by AAS, using the method proposed by Langmyhr and Paus (1968). Accuracy of determinations was checked by comparison with G1, G2, GSPI, AGV1 and BCR1 standard rocks and the values obtained were within 2% of the recommended values (Flanagan, 1973). PETROGRAPHIC DATA
The rocks studied are those described by Visona' (1977) as group-B rocks. They are medium- to coarse-grained rocks (1--6 mm), mostly equigranular, rarely porphyritic, with a hypidiomorphic structure. Their essential mineralogical composition is quartz, K-feldspar and plagioclase, classifying them as granodiorites and monzogranites (Fig. 2). Femic minerals are brown biotite (idiomorphic to sub-idiomorphic) and idiomorphic pargasitic hornblende. This last mineral appears both as an inclusion in plagioclase and as a phenocryst. A colourless amphibole (cummingtonite) also occurs occasionally, and may represent both relicts in the hornblende and irregular aggregates with dark-brown biotite and hornblende rims. Garnet, corundum and hercynite are less frequent and generally occur as small inclusions in the plagioclase of rocks with a cumulate texture. Accessory minerals are apatite, zircon, sphene and, less frequently, ilmenite and magnetite.
282
Q
• "
3a
A
~ 2
t
3b
P
Fig. 2. Modal classification according to Streckeisen (1967). Filled triangles indicate the most common rock types. Minor bodies of leucogranite are generalized by the d o t t e d area (Visona', 1980).
Based on the occurrence of hornblende and/or garnet and on textures, these rocks may be classified into two groups: (1) Amphibole-bearinggranodiorites and granites: these rocks show a hypidiomorphic texture and contain rounded aggregates of quartz, plagioclase and biotite. These aggregates are essentially composed of quartz, surrounded by small idiomorphs of plagioclase and biotite. A distinct type of aggregate is composed of plagioclase and biotite, the plagioclase being widely saussuritized and showing a thin rim of oligoclase. Biotite is brown, deformed, and partly chloritized. These aggregates sometimes show a rectangular section so that, under the microscope with parallel polars, they may appear as single crystals. However, with crossed polars, they show their complex structure. Biotite forms small crystals, all or partly associated with chlorite, pumpellyite, prehnite, hornblende, apatite and ore minerals. Equally frequent are plagioclase aggregates in which large flakes of deformed, partially or totally choritized biotite occupy the intergranular spaces between plagioclase crystals, enclosing the smaller ones. In these rocks, dark-grey microgranular inclusions frequently occur. They are 1--10 cm long, rarely schistose, with an ophitic structure, and are composed of plagioclase and biotite with lesser quantities of quartz and sometimes pargasitic hornblende; ovoids of pumpellyite and prehnite occur frequently in the biotite. Worthy of note is the fact that the minerals included in the plagioclase of the aggregates are the same as those found in the larger inclusions and in their plagioclases. This suggests that the above aggregates may have the same origin as the larger ones, from which they differ only in size. The mediumsized plagioclase crystals (44% An; with only a few saussuritic p a t c h e s a n d
283
rare inclusions) which are frequently found in the granites, may represent extreme reorganization of the smaller inclusions. (2) Garnet-bearing granodiorites and granites: these are rocks with a special texture caused by large crystals of quartz or perthitic K-feldspar containing small inclusions of plagioclase, quartz and biotite: the plagioclase is idiomorphic, with a calcic core (86--92% An), frequently saussuritized with strongly zoned and broad rims (32% An); the biotite is pinkish-brown, often deformed and partly chloritized; the quartz occurs in rounded granules, sometimes with curved contours (Fig. 3). Garnet, corundum and hercynite are found exclusively included in the plagioclase. This texture, c o m m o n in the basic cumulitic rocks, suggests the occurrence of cumulus crystals (plagioclase, biotite and quartz) and interstitial phases (K-feldspar and quartz), crystallized from an intercumulus liquid. Finally, both hornblende and garnet rocks show clear microtextural evidence for the presence of numerous restitic phases. (a) In the case of the hornblende rocks, the plagioclase of the variously sized inclusions (from mm to cm dimensions) and the single crystals contain the same inclusions (biotite, pumpellyite, etc.). This relationship presupposes a c o m m o n source or, in any case, fine disaggregation of the larger inclusions to single xenocrysts. (b) As for the garnet rocks, the identification of the relict phases is based
Fig. 3. Garnet-bearing rocks. Cumulate-like texture caused by large pecilitic areas of orthoclase and quartz.
28,1
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A
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for mass-balance calculation). E x t r e m e rock c o m p o s i t i o n s f r o m t h e same s t o c k are tielined. Inset: field o f rocks s t u d i e d in relation to their mineral c o m p o s i t i o n s .
on different criteria, since the inclusions are missing. Garnet and corundum are always included in plagioclase with hercynite, and may show reabsorption borders and plagioclase + biotite rims (Bellieni et al., 1979). The extremely calcic composition of the plagioclase core (up to 92% An) and its strong zonations reveal the restitic nature of its inner crystal sections (White and Chappell, 1977). In the quartz (not the large pecilites) the rounded and sometimes curved contours are significant. As for the biotite, the deformations -- shown only by this mineral -- indicate magma movement before crystallization of the larger pecilites of K-feldspar and quartz, giving the rock its cumulate-like texture. The presence of significant quantities of restitic phases poses the problem of their influence on the chemistry of the rocks containing them. Different quantities of the same restitic phases dispersed in a melt of constant composition will influence the chemistry of the rocks in the same way that quantities of magmatic cumulus crystals determine the characteristic trends of the cumulitic rocks. CHEMICAL DATA
Both new analyses and those carried out by Visona' {1977} are presented: Ba, Rb and Sr were redetermined using the same methodology as that adopted for the new samples*. * F o r reasons o f space, t h e tables " C h e m i c a l microanalysis o f m i n e r a l s " and " C h e m i c a l analysis o f w h o l e r o c k s " have n o t b e e n included. R e a d e r s w h o require these tables are req u e s t e d to write to t h e a u t h o r .
285
The aim of the chemical study was to establish that the accumulation of phases extraneous to the melt, as suggested by textural relationships, was consistent with the variation trends of the rocks. With this aim in mind, the variation diagrams of some major elements (Figs. 4--7) and trace elements (Figs. 8 and 9) were considered. The tie-lined open triangles represent rocks of extreme composition (granites and grano-
Kz0%
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Fig. 5. Plot of K=O vs. Na=O for rocks of the Bressanone pluton. Symbols as in Fig. 4.
Mg0 ~wt.% )
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41 C a O ( w t % )
Fig. 6. Plot of MgO vs. CaO. Symbols as in Fig. 4.
MgO(wt °/o)
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Fig. 7. Plot of MgO vs. SiO=. Symbols as in Fig. 4.
286 1
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Fig. 8. Plot o f Sr vs. R b o n log axes. S y m b o l s as in Fig. 4. D - - E a n d A - - B are q u a r t z + plagioclase + b i o t i t e c o n t r o l lines. F - - G a n d B--C are q u a r t z + plagioclase + b i o t i t e + alkali-feldspar c o n t r o l lines ( M c C a r t h y a n d R o b b , 1978). D i a g r a m : z = g a r n e t - b e a r i n g rocks; • = h o r n b l e n d e - b e a r i n g rocks; A = c o r d i e r i t e l e u c o g r a n i t e s ( V i s o n a ' , 1980). D o t s are arr a n g e d along t h e solidus--liquidus mixing-line. I n s e t : p r o j e c t i o n o f c o m p o s i t i o n o f all r o c k s studied. F
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diorites) from three different stocks, two of
are hornblende-bearing
which rocks and one a garnet-bearing rock. The negative correlation between A1203 and SiO2 (Fig. 4) may reflect the variation of the ratio between quartz and feldspars and the reduction in the
287
An c o n t e n t in the plagioclases during magmatic crystallization. However, the tendency of the dots to scatter along the ordinate may also be indicative of a concentration of xenocrysts of calcic plagioclase, biotite and hornblende (Fig. 4). The great variability in the Na20/K20 ratio (Fig. 5) is due to several subtrends with negative correlation which, according to McCarthy and R o b b (1978), represent mixing-lines between liquid and cumulus phases (plagioclase, quartz and biotite). The inversion of this correlation with higher K20 content indicates crystallization of the magma without cumulates (xenocrysts). In Fig. 6 it is still possible to note strong scattering of the dots along the abscissa. The presence of sub-trends with positive correlation is due to the extensive solid solution of MgO and CaO in biotite and plagioclase, respectively. But the sub-trends are also parallel in increasing CaO contents, which implies the presence of phases rich in CaO and only subordinately in MgO. The same observations may be made for the diagram in Fig. 7 (MgO vs. SiO2), in which scattering along the abscissa may be caused by the presence of phases relatively low in SiO: and small amounts of MgO (i.e. plagioclase with a little biotite). This pattern is similar to that described by White and Chappell (1977) for the granitoids of the Moruya suite (SE Australia) and interpreted as progressive separation of restite from a minimum melting liquid. These observations on the major elements are reinforced for the trace elements with large ion radius -- Ba, Rb and Sr. The Sr/Rb and B a / R b ratios are characteristic of granitic rocks with cumulate-like chemistry, as proposed by McCarthy and Hasty (1976). In effect, the rocks studied here may be plotted along parallel lines which intersect the compositions of the liquid (line B--C in Figs. 8 and 9) and that of the solid (line F--G in Figs. 8 and 9). The t w o groups of rocks --hornblende-bearing granodiorites and granites, and garnet-bearing granodiorites and granites, differing in mineralogy and structure -- are arranged in the Sr/Rb diagram along mixing-lines with different Sr contents. They intersect both the line of expected liquid composition and the trend supplied by the more differentiated rocks (garnet granites and two-mica cordierite leucogranites). The latter represent the composition of the liquid in which variable quantities of xenoliths and xenocrysts are dispersed. Their composition is very near the minimum melt (Visona', 1977,
1980). In general, the chemical variation observed is similar to that o f typical cumulitic rocks and is thus consistent with the cumulitic hypothesis advanced on petrographic evidence. Finally, a c o m m o n characteristic of all diagrams above is the presence of several trends which generally reflect chemical variations within single stocks. The chemical differences between these seem to be related to the t y p e of restitic material dispersed in the liquid. Inside each stock, the granodiorite -* granite differentiation may be explained as due to the accumulation of restitic phases (essentially plagioclase, biotite, quartz and amphibole).
-100 --
GD229" GD264 GD510
*Garnet-bearing rocks.
371
Granodiorite
-. 84.27
246A
.
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Biot. GD229
38.75 . -. --
41.54 .
PL GD229
PL GD510
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--
--
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66.46 0.81 15.67 0.62 4.85 0.10 0.87 3.83 3.03 3.50 0.25
75.84 57.49 55.89
Parg.
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Granite
37.90 48.44 2.51 0.82 18.29 7.46 . . . 23.64 21.62 0.26 0.53 8.14 8.94 -10.60 0.16 0.91 9.11 0.67 . . . .
%
Xenocrysts
71.63 0.35 14.42 0.41 2.61 0.08 0.75 2.54 2.69 4.42 0.10
Xenoliths
67.26 0.56 15.99 0.22 3.99 0.08 1.42 3.98 3.01 3.34 0.16 Residual liquid
73.30 0.26 13.78 0.30 2.07 0.06 0.56 2.28 2.86 4.43 0.09
Biot.
Cumulating minerals recalculated to 100%
68.59 0.46 15.31 0.60 3.05 0.08 1.34 3.29 3.30 3.84 0.13
246A
Parent-rocks
73.99 0.13 13.74 0.09 1.41 0.06 0.58 0.63 3.41 5.00 0.95
G213
69.17 0.42 15.90 0.18 3.34 0.08 1.42 2.09 2.72 4.56 0.12
GD510
Xenocrysts
SiO: TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K~O P2Os
G262
371
GD264
GD229
G274
Xenoliths
Rocks
Results of least-squares model using methods of Wright and Docherty (1970) and Stormer and Nicholls (1978)
TABLE I
0.5554 0.5561 0.4537
ER 2
46.21 -34.74 . 0.20 --18.01 0.82 0.02 . .
PL229
0.16 --16.79 1.42 --
47.43 -34.15
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0¢
t~
289 APPROACH TO THE P E T R O L O G I C A L MODEL
The two preceding sections have shown that the rocks studied may be products of mixing between a granitic melt and restites (xenocrysts and variously sized inclusions). Where the chemistry of the two extreme components (granitic melt and restites) and the hypothetical mixing products {granodiorites) are known, it is possible, by means of mass-balance calculations, to evaluate the reliability of the relationship granodiorite (rock) ~ xenoliths/xenocrysts (restite) + granite This calculation firstly deals with the total composition of the two chemical systems considered (major elements: Wright and Docherty, 1970; Stormer and Nicholls, 1978), and determines the relative proportions of restite and melt for the formation of granodiorite. As the whole plutonic complex is composed of many minor stocks, the above relationship must be applied to rocks belonging to the same stock. For three of these, the chemistry of the extreme compositions (granites and granodiorites; open triangles in Figs. 4--6) is considered, together with the associated restitic material. In the hornblende rocks, this is represented only by dark microgranular inclusions or by inclusions plus plagioclase and restitic amphibole. In the garnet rocks, the restitic portion considered for calculation purposes is given by plagioclase, biotite and quartz. Table I shows the results of the calculation and indicates that, both for the hornblende and garnet rocks, the granites may have been derived from the respective granodiorites by subtraction of the phases listed in the table. Lastly, the congruency of the cumulitic model may also be seen through the mass-balance calculation. CONCLUDING REMARKS
The intrusive complex of Bressanone is composed almost entirely of granodiorites and associated granites. New textural and chemical data demonstrate that these rocks may be referred to variable mixtures of restitic material (xenocrysts + xenoliths) and liquid, presumably having a minimum melt composition. As for the type of fractionation, filter pressing is the most probable mechanism in granitoid rocks (Tindle and Pearce, 1981). In effect, the viscosity of the silicatic melt controlled the location of the xenoliths, both hindering their separation by gravity (White and Chappell, 1977) and preserving their possible lack of distributional homogeneity. This second consideration may be explained by the melting behaviour of rocks of different composition or of similar rocks under different conditions of P, T and H20. Indeed, different rocks produce minimum melt of equal composition under identical conditions (Winkler et al., 1975; Wyllie et al., 1976; Thompson and Algor, 1977; MacRae and Nesbitt, 1980), but the closer the rock composition approaches
290
the granitic minimum, the greater is the melt/restite ratio+ On the other hand, the same rock, under different P--T--H20 conditions, may produce different melts (Brown and Fyfe, 1970; Hall, 1971; Fyfe, 1973; Wyllie, 1977; Win kler an d Breitbart, 1978). Thus, extensive anatexis of a chemically heterogeneous crystalline basement necessarily leads to the formation of masses coexisting with different restitic fractions and characterized by different total chemistry (Brown and Fyfe, 1970; Fyfe, 1973; White and Chappell, 1977). As for the source materials of the rocks studied, it is worth considering that, because of their mineralogy, chemistry (see also Fig. 10) and (STSr/ S6Sr)i (0.07101 -+ 0.0002 for the garnet rocks; slightly less than 0.7096 -+ 0.0006 for the amphibole rocks), they may be identified as "S type" (garnet rocks) and "I type" (hornblende rocks) according to the classification of Chappell and White (1974). The differentiation granodiorite + granite reflects an increase in the liquid/xenolith ratio in the melts produced by two types of parent-rocks: (1) pelitic rocks, which gave rise to "S type" magmas, including the garnet rocks, and (2) metasediments containing basic igneous rocks, which produced "I type" magmas, including the hornblende rocks. At -
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2+
Fe+Mg
Fig. 10. C a - - ( F e 2÷ + M g ) - - ( A 1 - - N a - - K ) v a r i a t i o n diagram (+ = h o r n b l e n d e rocks; o = granet rocks; -~ = " S t y p e " m i n i m u m m e l t ; * = " I t y p e " m i n i m u m melt). A r e a s b e t w e e n c o n t i n u o u s lines c o n t a i n c o m p o s i t i o n s o f " S t y p e " r o c k s ( t o w a r d s right) and " I t y p e " ( t o w a r d s Ca apex) o f White and Chappell (1977); d o t t e d line s h o w s field o f r o c k s s t u d i e d here.
291 ACKNOWLEDGEMENTS
The author is indebted to Profs. B. Zanettin and F.P. Sassi for their critical reading of the manuscript. Many thanks are due to Prof. A. Cundari for his significant contribution to the final arrangement of the text. Chemical analyses were carried o u t in the laboratories of the Institute of Mineralogy and Petrology of the University of Padova, with the collaboration of P. Da Roit, A. Giarretta and G. Mezzacasa. This work forms part of the programmes of the Centro di Studio per i Problemi dell'Orogeno delle Alpi Orientali (Padova) (Director: Prof. B. Zanettin) of the Italian National Council for Research.
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