The Itifiba alkaline syenite massif, Bahia State

32 Pages·2002·2.17 MB·English

Precambrian Research, 51 ( 1991 ) 283-314 283
Elsevier Science Publishers B.V., Amsterdam
The Itifiba alkaline syenite massif, Bahia State (Brazil)"
mineralogical, geochemical and petrological constraints
relation to the genesis of rapakivi magmatism
Herbet Conceiqao a, Pierre Sabat6 a and Bernard Bonin b
a Departamento de Geoquirnica, UFBA, 40210 Salvador (Bahia), Brazil
b Dkpartement sed Sciences de al Terre, UPS, F-91405 Orsa y CPdex, France
(Received November 20, | 989; revised and accepted July 5, 1990 )
ABSTRACT
Conceiqao, H., Sabat6, P. and Bonin, B., 1991. The Iti6ba alkaline syenite massif, Bahia State (Brazil): mineralogical,
geochemical and petrological constraints--relation to the genesis of rapakivi magmatism. In: I. Haapala and K.C. Con-
die (Editors), Precambrian Granitoids--Petrogenesis, Geochemistry and Metallogeny. Precambrian Res., 15 : 283-314.
Numerous alkaline massifs occur throughout the state of Bahia (Brazil). Isotopic dates fall into two age groups: Brasi-
liano (0.45-0.70 Ga) and Trans-Amazon ( 1.8-2.1 Ga) ones. Brasiliano alkaline provinces comprise a silica-undersatur-
ated association, with related volcanic rocks; their emplacement is always controlled by fault zones.
Trans-Amazon alkaline massifs are characterized by large plutons (more than 100 km 2) of K-rich syenite and granite
associated with mafic cumulates and abundant dyke swarms. No associated volcanic rocks have been so far recorded. The
shape of the plutons varies as a function of their location within the Silo Francisco Craton. In its northeastern and southern
parts, syenite massifs are elongated, trending N-S, while in its western part, they are emplaced as circular bodies displaying
contact metamorphic aureoles. The presence of older (Archaean?) alkaline rocks cannot be ruled out, as some granulitic
facies display syenite compositions.
The Iti6ba massif provides a good example of Proterozoic alkaline syenite. Located in the northern part of the S~o
Francisco Craton, this 150-km-long pluton covers 1800 km 2 in area. Whole-rock Rb-Sr isotopic data yield a lower Proter-
ozoic age.
A N-S-trending foliation at the margins is gradually replaced by isotropic textures toward the core. Two sets of faults,
both of Trans-Amazonian age, have been defined: N-S-trending reverse faults, accompanied by intense mylonitization,
and younger NW-trending arcuate transcurrent faults related to a NE-SW compressive regime.
Alkaline syenites constitute 98% of the exposures and are medium- to coarse-grained clinopyroxene-amphibole, hyper-
solvus leucratic rocks. Cumulates are represented by mafic layers and clinopyroxene-apatite enclaves. Dykes are com-
posed of alkaline syenite, hypersolvus and transsolvus quartz-syenites and alkaline granites. Syenites are metaluminous
and rich in K, Mg, P, Ti and Ba, and their geochemical trends are controlled by alkali feldspar, clinopyroxene, apatite and
Fe-Ti oxide fractionation. Oxidizing conditions are reflected by the weak variation in the rag-ratio, the reverse Fe--.Mg
mineral zonation, the synchronous precipitation of oxides and clinopyroxene in mafic layers and the late development of
uralitic amphibole and low-Ti phlogopite. Temperatures for oxide equilibration of 930-880°C are compatible with the
hypersolvus feldspar mineralogy. The evolution of feldspar mineralogy is a good indicator of varying thermodynamical
conditions during late-stage crystallization of the pluton. Minimum values for initial crystallization temperature have been
evaluated at 950°C.
Structural and petrological results indicate that the presently exposed syenite massif represents the roof of a Proterozoic
magma chamber, emplaced in granulite-gneiss formations. Rapakivi magmatism may represent disrupted and floating
portions of the roof of earlier magma chambers refilled by new syenite-granite melts.
Introduction alkali contents relatively to silica and alumina
(Sorensen, 1974; Bonin and Giret, 1985).
Alkaline plutonic rocks are defined by high They are emplaced at various structural levels
0301-9268/91/$03.50 © 1991 -- Elsevier Science Publishers B.V.
284 H. (7ONCEI(2~O EI M
( Bonin, 1986 ), but especially at shallow depths corresponds to the Pan-African orogenic cycle
(between 0.5 and 2 km), where they define in Africa, and Trans-Amazon correlates with
ring-complexes, these being roots of caldera the Eburnean.
volcanoes. However, petrological studies of al- Brasiliano alkaline formations are made up
kaline ring-complexes (Bowden and Turner, of small ring-complexes and dyke swarms.
1974; Bonin, 1980; Giret, 1983) have demon- Their emplacements were controlled by large-
strated that liquids forming ring-complexes scale faulting which occurred at the end of the
were produced by magma differentiation at an Brasiliano orogenic episode and during the
earlier stage in a reservoir emplaced at greater consolidation of the S~o Francisco Craton.
depths (8-20 km). Thus, geological and pe- Rock types are dominantly plutonic (gabbros,
trological studies of deep alkaline reservoirs diorites, nepheline syenites), with minor
will provide insights into liquid evolution and amounts of volcanic products (trachytes,
the acquisition of (per)alkaline characteristics. phonolites). Silica-undersaturated nepheline
Alkaline magmatic activity takes place in syenites are essentially metaluminous (mias-
within-plate settings. Thus, uplift and erosion kitic), and some examples of peralkaline (ag-
processes are weak, so that, if an erosion rate paiitic) nepheline syenites have been observed.
of 0.1 mm a- 1 is assumed, it takes at least 80- Trans-Amazon alkaline provinces consti-
200 Ma for 8 to 20 km deep magma reservoirs tute huge batholithic (> 100 km 2) syenite
to reach the surface. Thus, in any given alka- massifs (Barbosa and Costa, 1973; Inda and
line magmatic province, it is not possible to si- Barbosa, 1978; Marinho et al., 1979; Concei-
multaneously observe the shallow structural qao et al., 1989a, b), as well as subordinate
levels of caldera volcanoes and ring-com- small stocks (Tanner de Oliveira and Jesus,
plexes, and the deep magma reservoir. Pre- 1979; Silva et al., 1988; Leite and Fro6s, 1989 ).
cambrian alkaline provinces are therefore of All massifs intrude Trans-Amazon mobile belts
interest, as they are located in deeply eroded and their emplacement was controlled by large-
cratons. Significant examples studied include scale Archaean structural features. They are
the Pikes Peak Batholith (Colorado, U.S.A.) characterized by the presence of mafic cumu-
(Barker et al., 1975), Rogaland Province (S. lates and absence of volcanic rocks.
Norway ) (Duchesne, 1984) and the Rapakivi Among the Bahian Trans-Amazon alkaline
Magmatic Province (Finland and U.S.S.R.) massifs, the Itifiba syenite massif is the largest.
(Vorma, 1976; Haapala, 1988). We present Located in the north-central part of Bahia
here results from the alkaline syenite Itifiba State, it constitutes a north-south trending,
massif in Bahia State, Brazil. elongated body covering an area of 1,800 km .2
It has been mapped previously by Delgado and
Alkaline magmatic provinces in Bahia State Souza ( 1975 ), and by lnda et al. ( 1976 ). The
(Brazil) Rb-Sr whole-rock isochron method has yielded
an age of 2.1 Ga, which is interpreted as age of
Alkaline magmatism was widespread in emplacement (Conceiqho, 1990).
Bahia State (Brazil) during Precambrian times In this paper, we shall focus on petrographi-
(Conceiqao and Bonin, in prep.). Within the cal, mineralogical and geochemical aspects of
alkaline magmatic province, two age group- exposed rock types in the ltifiba massif\ Their
ings have been identified: Brasiliano (0.45- petrogenetic and tectonic significance will be
0.70 Ga) and Trans-Amazon (> 8.1 Ga). In discussed, with emphasis on their relation to
South America, the Brasiliano orogenic cycle the genesis of rapakivi magmatism.
THE AB1'IIT ALKALINE SYENITE ,FISSAM BRAZIL: MINERALOGICAL, GEOCHEMICAL AND PETROLOGICAL CONSTRAINTS 285
lacigoloeG setting tened syenite enclaves and parallelism of alkali
feldspar xenocrysts within the granite mass
Located in the northern-central part of the suggest that the granitic magma was emplaced
ot~S Francisco Craton, the Iti6ba syenite mas- while the already consolidated syenite was yet
sif is an elongate north-south trending, 150 km viscous (Conceiq~o et al., 1989a).
long and 21 km wide, complex (Fig. 1 ). The Syenitic rocks show a consistent north-south
massif intrudes Early Proterozoic gneiss-gran- trending foliation, marked by mafic mineral
ulite formations of the Salvador-Curagh ( > 2.1 alignment. The foliation is well developed at
Ga) mobile belt (Mascarenhas, 1979 ). the contacts and gradually passes into a core
The mobile belt was emplaced between the having an isotropic texture. Two groups of ma-
two cratonic nuclei of Remanso (also named jor fault patterns have been recognized (Do-
Len~ois) and Serrinha, which were consoli- mingues Alonso and Conceiq~o, 1986: Concei-
dated before the J6qui6 Archaean (> 2.5 Ga) ot~q et al., 1989b).
orogenic episode (Mascarenhas, 1979). Gran- The first major class trends north-south and
ulite and gneiss formation of the Salvador- corresponds to reverse faults with westward
Cura¢~ mobile belt have undergone several dip, implying an east-west compressive re-
phases of deformation (Delgado and Souza, gime. Within the syenitic rocks, the associated
1975; Inda et al., 1976; Hasui et al., 1982; Jar- deformation is marked by mylonitization and
dim de i~S et al., 1982), the major phase being development of a thick shear zone along the
characterized by a north-south trending folia- eastern boundary of the massif (Fig. 1 ). Syn-
tion (D'el R. Silva, 1985 ). According to these tectonic granitic dykes (Bastos Leal, 1986) and
authors, the mobile belt is made up of two dis- later pegmatites follow the same trend.
crete metamorphic units. The lower unit is A second class is represented by arcuate
composed of supracrustal rocks, with graphite- NW-SE trending faults which cut the N-S
and/or garnet-bearing quartz-feldspar parag- fractures, the granitic dykes and the pegma-
neisses, amphibolites and quartzites. The up- tires. The eastern boundary of the massif is
per unit is made up of orthogneisses and basic displaced by these faults, whereas along the
to ultrabasic massifs with Cu and Cr ore bod- western boundary, N-S reverse faults were re-
ies. All units are intruded by late basic dykes. juvenated by a dextral strike-slip movement.
In Bahia State, Brasiliano metamorphic and Within the syenitic rocks, the deformation is
folding events have not affected mineral or marked by vertical faults with a significant
whole-rock isotopic systems, implying that the horizontal component, inducing severe tex-
S~o Francisco Craton behaved as a rigid block tural transformations including narrow bands
during the Brasiliano orogeny, which is better of ultra-mylonite, and compatible with a NE-
documented in the surrounding states. SW compressive regime (Conceiq~o et al.,
The emplacement of the syenite massif is 1989b ). Since they displace intrusive contacts,
correlated with the last phase of deformation these faults post-date melt emplacement and
observed in the Curagzi Valley (D'el R. Silva, crystallization. They are characterized by solid-
1985). Sharp contacts against Early Protero- state deformation of rocks, but host late-stage,
zoic formations locally display features of highly differentiated syenite and granite dykes
magmatic stoping in the 300 m wide marginal and, therefore, are interpreted as having been
zone. The associated Pedra Solta granite active during the late stages of syenite
(Conceiq5o and Sabat6, 1986; Celino, 1986) emplacement.
displays lobate mutual contacts (Fig. 1 ), im- The N-S trending elongation of the Iti6ba
plying more or less synchronous time of em- syenite massif corresponds to a major fracture
placement: in the contact zone, abundant flat- zone, while the arcuate fault pattern is con-
286 H. CONCEI(,'~O ET AL
/'I ° ! ~2° •
./,*
2
~ % 4
%
,,,,' ' ' 5
++*++ ++* 6
++++÷~++.
~x ~ x x x
Fig. .1 Geographical location and geological sketch map of the Ititiba massif, after the Geological Map of Bahia State
(Inda and Barbosa, 1978): l=Itapicuru river; 2=main roads; 3=faults; 4=shear zones; 5=Quaternary sediments;
6 = Pedra Solta granite; 7= Itidba alkali-feldspar syenite; 8= Salvador Cura~l mobile belt; 9--- diatexites and metatexites.
Because of the scale, relatively small features such as dykes are not represented.
TIlE ITIIJBA ALKALINE SYENITE MASSIF, BRAZIL: MINERALOGICAL, GEOCHEMICAL AND PETROLOGICAL CONSTRAINTS 287
trolled by local conditions during the cooling classical alkaline provinces, such as Niger-Ni-
(and uplift? ) of the pluton. Other large syenite geria (Lameyre and Bowden, 1982), Oslo Rift
bodies may have been emplaced at the same (Oftedahl, 1978)and Corsica (Bonin, 1980).
time under the same regional stress field. Con-
ceiq~,o et al. (1989b) have suggested that the Syenites
syenite massifs follow older major discontinu-
ities initiated during Early Proterozoic or Ar- The alkali-feldspar syenites display a me-
chaean time. dium- to coarse-grained phaneritic texture,
with anisotropy marked by a N-S trending fol-
Petrographical, textural and mineralogical iation displayed by alkali feldspars. The dom-
aspects inant texture is allotriomorphic, with alkali
feldspar forming the major mineral phase,
More than 98% of the syenite massif con- while other minerals occupy spaces between
sists of hypersolvus leucocratic alkali-feldspar adjacent feldspar crystals (Fig. 3). This is in-
syenites with subordinate mesocratic and hol- terpreted as indicating magmatic accumula-
oleucocratic syenites. The remaining 2% are tion and compaction processes (Conceiqao,
mafic cumulates referred to as layers, micror- 1990). Moreover, contacts between adjacent
hythmites and enclaves, and (holo)leucocratic alkali-feldspar crystals are at angles of 100 to
dykes (Fig. 2). 120 °, implying a late-stage reequilibration
Modal data produce a trend following the (Hunter, 1987). As no Brasiliano-age meta-
evolution defined for the silica-saturated alka- morphism is recorded in the area, textural fea-
line suite by Lameyre and Bowden ( 1982 ) and tures are assigned to emplacement and consol-
Bowden et al. (1984). However, the range of idation processes.
modal compositions is more limited than in Major minerals in the paragenesis are, in ap-
Q
~3
o37 2 ~/~
A~
O y
Fig. 2. Q-A-P and Q-A+P-M plots (Streckeisen, 1976) for the alkali-feldspar syenite (open squares): Q=quartz:
A =alkali feldspar; P=plagioclase; M= mafic minerals. Note the restricted range of compositions within the alkali-feld-
spar syenite field (shaded), and the wide range of colour index (from 0 to 65 ).
288 .H O~,CIECNOC .IA-qE
I
0~ 1 2mm
' I
Fig. 3. Textural relationships within alkali-feldspar syenites (scale indicated by the 2-mm bar). Albite lamellae and oc-
casional cross-hatched twinning in alkali feldspar crystals are represented by dotted patterns; amphibole crystals can be
recognized by lines representing their preferred cleavage planes, cpx= clinopyroxene; A = apatite; pO = oxide minerals:
zQ = quartz; hps = titanite.
proximately decreasing order of abundance: proportions differ, suggesting magmatic accu-
alkali feldspar, clinopyroxene, amphibole, mulation of clinopyroxene, apatite and acces-
quartz and Fe-Ti oxides. Accessory minerals sory minerals in an amphibole-rich matrix
include: apatite, zircon, titanite, mica and (Concei~o et al. 1989a ).
allanite. Discontinuous mafic layers are present
throughout the massif, implying large-scale
cifaM setalumuc crystal settling. They constitute roughly paral-
lel units, with metre-scale lengths and centi-
Mafic layers display a medium-grained pha- metre-scale widths, some of them displaying
neritic texture, with the main paragenesis con- spindle shapes, and are sometimes accom-
sisting of perthitic alkali feldspar, clinopyrox- panied by minor rhythmites.
ene, amphibole and apatite. Minor minerals are Two types of mafic layers have been ob-
Fe-Ti oxides, titanite, zircon, pyrite and allan- served. The first is isomodal throughout the
ire. Mineral components and grain size are layer, while the second grades upward into the
similar to those in syenites, but the relative syenitic host rock. This upward gradational
THE ITIIJBA ALKALINE SYENITE MASSIF, BRAZIL: MINERALOGICAL, GEOCHEMICAL AND PETROLOGICAL CONSTRAINTS 289
second type of mafic layer generally dips gently (2) Pink to red dykes with sharp, rectilinear
to the west, suggesting that the most apical part contacts and chilled margins, and consisting of
of the intrusion is exposed in the western part. alkali-feldspar quartz-syenite and alkali-feld-
Small-scale layered structures are attributed spar granite; they are characterized by grains
to crystallization stages inside stagnant zones of perthitic K-feldspar and albite. The associ-
and may reflect thermal oscillations (Thy et al., ation of two discrete alkali feldspars suggests
1987). The presence of cumulus apatite pro- aqueous fluid enrichment of the residual
vides evidence for early phosphorus-satura- magma and consequent lowering of the solidus
tion of the melt (Watson, 1979, 1980). temperature. The result was a transition from
hypersolvus through transsolvus to subsolvus
Enclaves conditions (Tuttle and Bowen, 1958; Martin
and Bonin, 1976).
Three types of enclaves are distinguished on (3) Pegmatitic dykes with sharp rectilinear
the basis of mineralogy and geometry: contacts, often displaying zonation, with a
( 1 ) Basement rocks (granulites and quartz-rich core, and complex margins in
gneisses) form angular enclaves and are pres- which alkali-feldspar-rich zones (sometimes
ent in the marginal zones of the massif, indi- amazonitic) alternate with amphibole-bio-
cating local magmatic stoping. tite-rich zones. Mafic minerals and some al-
(2) Quartz-diorite to monzonite enclaves kali-feldspar crystals have crystallized perpen-
have a tabular shape and centimetre-scale dicular tot the contacts. Two discrete alkali
length. Decreasing grain size from core to rim feldspars, albite and microcline, characterize
reflects their primary intrusive characteristics the subsolvus type of crystallization.
and a strong thermal gradient between enclave The major mineral phases--alkali feldspar,
melts and host syenite at the time of mixing clinopyroxene, amphibole, mica, and Fe-Ti
(Didier, 1973). oxides--have been analyzed for major ele-
(3) Ellipsoidal, centimetre-scale mafic en- ments and Ba, rS with Camebax electron mi-
claves, with amphibole reaction rims. These croprobes at Laboratoire de Microanalyse
constitute apatite-clinopyroxene cumulates, Electronique, Universit6 Paris-Sud, Orsay, and
with amphibole, Fe-Ti oxides and biotite as at Camparis, Universit6 Pierre-et-Marie Curie,
intercumulus mineral phases. They have ex- Paris. The analytical details are given by Bonin
actly the same composition as the basal zone ( 1988 ), and Bonin and Platevoet ( 1988 ).
of the gradational layers and record mechani-
cal disruption of the early cumulative phase,
which formed prior to alkali-feldspar crystalli- Alkali feldspars
zation (Conceiq~o et al., 1989a ).
Dykes Some separated crystals were analyzed by
XRF techniques, and single crystals of alkali
Several generations of dyke swarms intrude feldspars have been studied by electron micro-
the syenites. They include: probe. Representative chemical data are listed
( 1 ) White granular dykes with centimetre- in Tables 1 and .2 Alkali feldspars record dif-
scale widths and indistinct contacts, composed ferent types of crystallization, ranging from
of leucocratic hypersolvus alkali-feldspar hypersolvus to transsolvus and subsolvus. The
syenites, similar to their syenitic host rocks; crystallographic structures of the alkali feld-
they may represent residual liquids extracted spars are triclinic, always with microcline and
by filter-pressing. low albite.
0 o >-
91-14 66.45 20.47 0.03 0.65 I l.O0 0.11 0.00 0.39 99.10 2.941 1.068 0.001 0.031 0.944 0.006 0.000 0.010 5.000 3.14 96.22 0.63 3.14 96.22 0.63 0.00
91-13 61.09 19.17 0.00 0.00 0.27 14.27 2.02 0,44 98.75 2.913 1.077 0.000 0.000 0.117 0.898 0.038 0.012 5.056 0.00 11.56 88.44 0.00 11.05 85.27 3.58
57-12 67.20 19.97 0.13 0.12 t0.32 1.07 0.18 0.44 99.43 2.970 1.040 0.004 0.006 0.884 0.060 0.003 0.011 4.980 0,60 93.05 6.35 0.60 92.75 6.33 0.33
microprobe 57-11 62.29 18.77 0.00 0.00 0.62 15.74 0.46 0.11 98.99 2.948 1.047 0.000 0.000 0.057 0.950 0.027 0.003 5.032 0.00 5.65 94.35 0.00 5.50 91.88 2.62
electron 31-10 66.31 20.35 0.50 0.71 10.44 0.11 0.04 0.76 99.22 2.938 1.063 0.017 0.034 0.897 0.006 0.001 0.020 4.974 3.60 95.74 0.66 3.60 95.67 0.66 0.07
by
obtained 31-9 62.72 18.92 0.11 0.00 0.86 13.66 4.43 0.07 100.77 2.946 1.047 0.004 0.000 0.078 0.818 0.082 0.002 4.977 0.00 8.73 91.27 0.00 8.01 83.66 8.33
t2
massif, 3-8 67.44 20.01 0.00 0.97 11.48 0.19 0.16 0. 100.37 2.953 1.033 0.000 0.046 0.975 0.011 0.003 0.003 5.023 4.41 94.56 1.03 4.40 94.30 1.03 0.27
ltifiba
the 3-7 61.84 19.54 0.03 0.00 0.47 14.35 4.05 0.30 100.58 2.917 1.086 0.001 0.000 0.043 0.863 0.075 0.008 4.993 0.00 4.74 95.26 0.00 4.38 87.99 7.63
from
1
types 55-6 66.19 20.14 0.12 0.43 10.94 0.22 0.30 1.26 99.60 2.939 1.054 0.004 0.020 0.942 0.012 0.005 0.032 5.009 2.10 96.62 1.28 2.09 96.l 1.27 0.53
rock
55-5 62.99 18.97 0.15 0.02 0.65 13.28 3.26 0.01 100.33 2.946 1.046 0.005 0.001 0.150 0.792 0.060 0.000 5.000 0.11 15.87 84.03 0.10 t4.92 79.02 5.96
different
for 33-4 65.77 20.47 0.07 0.41 t0.97 0.17 0.00 0.99 98.85 2.930 1.075 0.002 0.020 0.948 0.010 0.000 0.026 5.010 2.00 97.01 0.99 2.00 97.01 0.99 0,00
compositions 33-3 56.63 20.08 0,18 0.74 0.67 11.99 8.07 0.08 99.44 2.790 0.166 0.044 0.039 0.064 0.753 0.156 0.002 5.014 4.56 7.47 87.97 3.86 6.32 74.43 15.39
17-2 67.26 20.59 0.13 0.06 11.04 0.16 0.29 0.40 99.93 2.952 1.065 0.004 0:003 0.939 0.009 0.005 0.010 4.988 0.30 98.76 0.94 0.30 98.25 0.94 0.52
alkali-feldspar
17-1 59.55 19.90 0.98 0.30 1.30 12.46 5.40 0.04 99.93 2.852 1.123 0.035 0.015 0.12l 0.761 0.101 0.001 5.010 1.72 13.45 84.83 1.54 12.09 76.22 10.15
1
TABLE Representative SiO2 A1203 Fe203 CaO Na20 K20 BaO SrO Total Si A1 Fe Ca Na K Ba Sr Total An Ab Or An Ab Or Ce
t" t" zr rn 7z N v- zr 7~ t-- © V o o m t-" Z © o t-J
83-24 67.97 19.69 0.09 0.02 11.03 0.21 0.00 0.35 99.36 2.989 1.021 0.003 0.001 0.940 0.012 0.000 0.009 4.975 0.10 98.67 1.24 0.10 98.67 1.24 0.00 been
have
grains
83-23 63.78 18.92 0.03 0.01 0.39 15.83 0.27 0.00 99.23 2.973 1.039 0.001 0.000 0.035 0.941 0.005 0.000 4.995 0.05 3.61 96.34 0.05 3.59 95.86 0.50
discrete
70-22 66.19 20.17 0.03 0.21 11.61 0.01 0.03 0.89 99.14 2.940 1.056 0.001 0.010 1.000 0.001 0.001 0.023 5.031 0.99 98.95 0.06 0.99 98.90 0.06 0.05 types,
subsolvus
70-21 62.04 19.26 0.16 0.00 1.04 14.60 0.79 0.16 99.05 2.929 1.072 0.006 0.000 0.095 0.879 0.033 0.004 5.019 0.00 9.77 90.23 0.00 9.45 87.27 3.29 and dykes.
83,
and
transsolvus 70
64-20 68.52 19.22 0.19 0.23 10.31 1.19 0.19 0.76 100.61 3.000 0.992 0.006 0.011 0.875 0.066 0.003 0.019 4.972 1.13 91.89 6.98 1.13 91.58 6.95 0.34 in anions. 64,
23,
grain;
oxygen 10,
8
64-19 62.35 18.78 0.24 0.01 0.59 15.84 1.02 0.26 99.09 2.944 1.045 0.009 0.001 0.054 0.954 0.019 0.007 5.033 0.05 5.36 94.59 0.05 5.26 92.86 1.84 single of
one basis syenites;
the
within on
23-18 63.83 21.95 0.13 2.78 9.55 0.29 0.01 0.33 98.87 2.849 0.155 0.004 0.133 0.826 0.017 0.000 0.009 4.993 13.62 84.69 1.69 13.62 84.67 1.69 0.02
analysed computed 91, alkali-feldspar
23-17 62.81 18.20 0.21 0.00 0.37 16.77 1.02 0.05 99.43 2.964 0.012 0.007 0.000 0.034 1.010 0.019 0.001 5.048 0.00 3.24 96.76 0.00 3.19 95.04 1.78 been been 57 and
have have 31,
3,
2
!
10-16 65.77 21.02 0.02 1.05 10.88 0. 0.01 0.23 99.10 2.913 1.097 0.001 0.050 0.934 0.007 0.000 0.006 5.008 5.03 94.29 0.68 5.03 94.27 0.68 0.02 Na-phases formulae layers;
and
mafic
K-
Structural 55,
1 (continued) 10-15 64.03 18.39 0.00 0.00 0.47 15.15 1.82 0.10 99.96 2.988 1.011 0.000 0.000 0.043 0.902 0.033 0.003 4.979 0.00 4.50 95.50 0.00 4.35 92.25 3.40 types, (see text). 17, 33 and
hypersolvus
TABLE SiO2 A1203 Fe203 CaO Na20 K20 BaO SrO Total Si A1 Fe Ca Na K Ba Sr Total An Ab Or An Ab Or Ce In analysed Samples:
292 .H O.~,),(IECNO'( EF At.
TABLE 2 with high K20/Na20 ratios from about 3 to .5
Alkali feldspar compositions obtained on mineral separates
Low P205 and L.O.I. values are due to apatite
by XRF techniques
inclusions. K-rich alkali feldspars range from
224 75 204 20.01 27 813 Or7o in mafic layer 224 to Or63 in syenite 204,
and from Or61 tO Or78 in pegmatite dykes. Ba,
SiO2 63.87 65.03 66.34 65.05 64.70 65.46
Rb and Sr display a compositional trend from
TiO2 0.02 nd 0.06 nd 0.02 nd
3021A 18.71 19.01 0.34 18.95 18.53 18.60 Sr- and Ba-rich, and Rb-poor compositions
302eF nd nd 0.05 0.22 0.27 nd (mafic layer 224) to Sr- and Ba-depleted and
OgM nd nd 0.44 nd 0.04 nd
Rb-enriched compositions (amazonite dyke
CaO 0.20 0.06 3.79 0.10 0.17 nd
Na20 21.3 3.54 10.46 4.20 19.2 2.42 318 ). Enrichment and depletion ratios are, re-
K20 11.30 11.10 0.19 10.23 11.85 12.82 spectively, about 51 for Rb, 120 for St. Ba be-
sO2P 0.34 0.17 0.40 0.12 0.14 0.80
haves more erratically, with high values (2800-
LOI 0.53 0.32 100.45 0.42 0.49 14.0
7000 ppm ) in the mafic layer, syenites and two
Total 98.09 99.25 99.29 99.29 99.12 99.79 pegmatite dykes, and a very low value (125
ppm) in the amazonite dyke. Fractionation of
aB 5000 5000 4342 7020 2843 421
major and minor element contents is well illus-
Sr 5000 5000 1346 5871 754 43
Rb 373 286 364 490 215 4995 trated in the field by the two pegmatite dykes
10.02 and 72, for which cross-cutting relations
Or 69.7 1.76 1.36 72.2 61.3 7.77
indicate the succession: first 10.02, then 72; K
Ab 29.3 5.23 7.43 26.9 38.2 22.3
An 0.1 0.4 2.2 0.9 5.0 0.0 and Rb increase, while Na, Sr and Ba decrease.
Within single crystals, major and trace ele-
Structural formulae have been calculated on the basis of 8
ments are partitioned between the K-rich and
oxygen anions.
Samples: 224, mafic layer; 75 and 204, alkali-feldspar syen- the Na-rich phases. The K-rich phase, ranging
ite; 10.02 and ,27 pink dyke; 318, amazonite dyke.
from Or74 (in mafic layers) to Or96 (in dykes ),
is always anorthite-poor (less than 3.9% in
suvlosrepyH facies mafic layers, near 0.0% in other samples), and
In the hypersolvus syenites, pink to red per- Ba-rich (up to 8. I wt% BaO corresponding to
thitic alkali-feldspar crystals are generally in- 15.4% celsian molecule in the mafic layers, and
clusion-poor, while in the mafic layers, alkali- between 0.33 and 4.4 wt% BaO corresponding
felspar crystals are white and inclusion-rich. In to 0.6 to 8.3 wt% of celsian molecule in syen-
film-perthites, the albite is always white and ires). An evolution from Ba- and Ca-rich com-
seldom twinned. Optically homogeneous crys- positions toward Ba- and Ca-depleted compo-
tals are consistently cryptoperthitic. In some sitions can be observed from sample to sample,
cases, exsolved albite comprises the dominant as well as from crystal to crystal within a sam-
phase (antiperthite). ple, thus substantiating the role of K-rich al-
In order to obtain the average compositions, kali feldspar in Ba-Ca fractionation. Iron,
we have separated some crystals from a mafic analyzed as total Fe203, is also present: in mafic
layer (sample 224), two syenites (samples 57 layers, white alkali feldspars can contain up to
and 204), and three pegmatite dykes (large 2.1 wt% Fe203, while in syenites, pink to red
pink crystals: samples 72 and 10.2; and green alkali feldspars contain only 0.0 to 0.5 wt%
amazonitic crystals: sample 318 ) (Table 2). In
Fe203.
addition, chemical analyses of the different The Na-rich phase is chemically pure low al-
phases inside single grains have been per- bite with less than 2% anorthite, %1 K end-
formed (Table l ). member, and 0.5% celsian molecule. However,
Separated alkali-feldspar crystals yield a SrO contents can reach 3.1 wt% in albite of
fairly constant major element composition, mafic layers and 0.8 wt% in albite of syenites.