WYLLIEITE REACTION CORONAS ON SCORZALITE IN...
Transcript of WYLLIEITE REACTION CORONAS ON SCORZALITE IN...
WYLLIEITE REACTION CORONAS ON SCORZALITE IN PEGMATITE DYKES - SERRA DE ARGA (PORTUGAL)
P. A. DIAS 1, C. LEAL GOMES 1, F. GUIMARÃES 2, F. HATERT 3
1 CIG-R, Univ. Minho, Portugal; [email protected]; [email protected]
2 LNEG, S. Mamede de Infesta, Portugal; [email protected]
3 Laboratory of Mineralogy, Univ. Liège, Belgium; [email protected]
In the region of Serra de Arga (Northern Portugal) pegmatite dykes with approximately 50 cm thick and 2 m long, affected by Variscan deformation, contain scorzalite that is partially replaced by wyllieite reaction coronas.
Mineral composition of the dykes consists of quartz, albite, potassium feldspar and muscovite. Accessory minerals include andalusite, Mn-rich fluorapatite, columbite-(Fe), gahnite, uraninite, montebrasite and brazilianite (Dias, 2012).
Scorzalite occur as disseminated bluish to greenish single crystals up to 3 mm in size. Inclusions of muscovite, gahnite and montebrasite (?) were identified. Scorzalite often displays complex alteration patterns corresponding to the development
of brownish to black Al-Fe-Mn rich products (gormanite or childrenite-eosphorite?). Other breakdown products include associations of crandallite-goyazite and variscite. Scorzalite electron-microprobe analysis showed the following average
composition: (Fe 0.90Mg0.05-0.07Mn0.02Zn0.0-0.01)=0.95-1.01Al 2.0-2.1(PO4)2(OH)2.
Wyllieite forms light blue corona-like overgrowths around primary scorzalite and also penetrate along fracture fillings of the scorzalite crystals, as revealed by transmitted light microscopy and EMP study. Electron-microprobe analysis provided
P2O5 = 45.5-47.2; Al2O3 = 8-8.6, MnO = 15.2-16.3, FeO = 23.5-24.6, MgO = 0.44-0.54; Na2O = 4.2-5.3 wt. %. The resulting formula, calculated on the basis of 12 O, is (Na0.64-0.79Ca0.02-0.03Mn0.30-0.39)=1.01-1.22 (Mn0.60-0.71 Fe 0.29-
0.40)=1(Fe 0.27-0.61Fe 0.34-0.67Mg0.05-0.06)=1(Al0.72-0.77Fe 0.23-0.28)=1(PO4)3. Some of these compositions correspond to wyllieite, while oxidized grains correspond to rosemaryite (Hatert , 2006).
Such unusual previously undescribed scorzalite breakdown was caused by post-magmatic, Na bearing fluids interacting with the pegmatite. Na could have become available by feldspar breakdown. Both albite and K-feldspar occur in the matrix
and reflect distinct high phosphorous contents. K-feldspar contains up to 3.6 wt% of P2O5 and coexisting albite up to 1.98 wt%. Distribution of P between Fk and Ab (PFk/Ab ) is 1.8. Textural relationships indicate albitization of the K-feldspar.
According to (Hatert , 2006), wyllieite could have formed at temperatures lower than 400ºC, considering a pressure of 0.1Kbar. These estimates are within the considered field for scorzalite collapse (475-560ºC, 1-3Kbar) (Schmid-Beurmann
, 2000).
2+
2+
2+ 3+ 3+et al.
et al.
et al.
3. SCORZALITE-WYLLIEITE INTERGROWTHS
2. SCORZALITE-BEARING PEGMATITES
5. CO-EXISTENT FELDSPARS - TEXTURES AND COMPOSITIONS
100 μm0.5 cm
Qz
Scz
Scz
Wyl
100 mμ
Abr Qz Scz Wyl. - quartz; - scorzalite; - wyllieite.
4. SELECTED COMPOSITIONS - ELECTRON MICROPROBE ANALYSIS
Mineral chemical analyses were performed on
representative sections using a five-channel
wavelength dispersion JEOL JXA 8500F
microprobe, routinely operated at an accelerating
voltage of 15 kV and a beam current intensity of 10
nA.The standards used were (K lines): Orthoclase
(Al, K), Albite (Na), MgO (Mg), Apatite (P, Ca),
Volastonite (Ca, Si), Fe2O3 (Fe), MnTiO3 (Mn, Ti),
Barite (Ba), Fluorite (F), Cr2O3 (Cr), SrTi O3 (Sr),
Sphalerite (Zn), FeS2 (S), AsGa (As), Vanadinite
(Cl), Yttrium Al Garnet (Y).
ANALYTICAL PROCEDURES
FORMULAS
Wyllieite-rosemaryite
Scorzalite
(Na Ca Mn ) (Mn
Fe ) (Fe Fe Mg
) (Al Fe ) (PO ) ;
(Fe Mg Mn Zn ) Al
(PO ) (OH) ; .
0.64-0.79 0.02-0.03 0.30-0.39 =1.01-1.22 0.60-
0.71 0.29-0.40 =1 0.27-0.61 0.34-0.67 0.05-
0.06 =1 0.72-0.77 0.23-0.28 =1 4 3
0.90 0.05-0.07 0.02 0.0-0.01 =0.95-1.01 =2.0-2.1
4 2 2
2+ 2+ 3+
3+
2+
N=5.
N=5
SiO2 Na2O Al2O3 K2O P2O5 MnO SrO CaO BaO Total
Fk 61,36 0,58 21,29 12,48 3,60 0,02 0,23 - 0,07 99,64
Ab 67,07 8,43 21,68 0,08 1,98 0,01 0,30 0,13 0,09 99,77
3Sta.
Cristina
1. GEOLOGICAL SETTING OF SERRA DE ARGA PEGMATITES
CUMIEIRA
COUTO DE OURO
CABEÇO DO MEIO DIA
300
40
0
500
200
100
600
700
10
0
400
200
300
600
Coura
Arga de Cima
56
7063
80
70
84
46
70
80
80 82
82
56
7654
18 84
80
15
8478
78
75
70
74
7630
66
80 70
84
62
8186
82
6840
74
76
20
28
7030
72
70 74
5070
38
63
52
30
30
12
1826 18 26
24
8030
38
2122
32
10
38
36
30
7070
1220
60
6220
50
45
84
3074
44
35
35
6010
10
6242
48
26
28
72
78
80
28
4450
50
7451
45
30
28
30
60
525000 526000 527000 528000 529000 530000 531000
4632000
4633000
4634000
4635000
4636000
D E T A I L E D G E O L O G I C A L M A P1C sheet - revision proposal (CIG-R, 2008; Dias, 2012)
coarse grained granite (Arga)
medium to fine grained gneissic granite (Romarigães - Sabariz)
medium grained granite (Covas E)
coarse grained granite (Covas W)
Syn-tectonic two mica granites
porphyritic granodiorite
Syn- to late-tectonic biotite granites
VARISCAN GRANITOIDS
�´´1
�g
�m
�mz
�g
Sapardos a Gandrachão - metapsammopelites interstratified withampelites, quartzites and lidites
Formigoso - metapsammopelites interstratified with black-shists, lidites,quartzites and tourmalinites
Minho Central Unit - Silurian (Wenlock, Landovery)
PARAUTOCHTHONOUS AND ALLOCHTHONOUS UNITS
UMC
UMC
Rib. Pombas
polygenic metapsammopelites (mica-schists and quartzphyllites)interstratified with quartzites, black-schists, lidites and exotic psammiticlithologies with volcanogenic-exhalative affiliation (+), volcanogenic-carbonated-exhalative affinity (++) and metassomatic derived products(+++).
CumieiraCerdeirinha
Domo de Covas Unit - Silurian (Landovery)
AC
++ +
Peraluminous anatectic pegmatites (abyssal and rare-elementmuscovite classes?) and metamorphic segregation veins(see )Dias, 2012
Hydrothermal quartz veins and breccias
Rare-element aplite-pegmatites and pegmatites - REL-Li LCT typespredominate: beryl, spodumene (after petalite), petalite, lepidolite andelbaite subtypes (see ).Leal Gomes, 1994
INTRUSIVE DYKES
+++
Scorzalite bearingpegmatites1
1
2
2
3
3Fisga Lousado
Sta.Cristina
Orbacém Thrust
Argagranite
Sto. Ovídeogranite
Vigo-Régua Shear Zone
FDC
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a
a
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UMC
UMC
UMC
UMC
UMC
UMC
UMC
UMC
Domode
Covas
Serro
Porto Morais
Bragança
CIZ
GTMZ
SERRADE ARGA
1
2
2
1
Recent sedimentary deposits - Pliocenic to Quaternary
GTMZ
CIZ
Galiza-Trás-os-Montes Zone
Central Iberian Zone ( Terrain).Autocthonous
( andTerrains).Parautocthonous
Allochthonous
Variscan syn- to late-tectonic granitoids
Cambrian and Ordovician Autocthonous Formations
Intrusive dykes: pegmatites and hydrothermal quartzSilurian Parautochthonous Formations
Polygenic Silurian Allochthonous (?) Formations
Minho Central Unit (W of the Vigo-Régua shear)
Vilar de Mouros
Domo de Covas Unit
Major thrusts
FDC
Minho Central Unit (E of the Vigo-Régua shear)Continuous shear structures
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a
UMC
UMC
VM
S I M P L I F I E D G E O L O G I C A L M A P
SPAIN
AT
LA
NT
IC O
CE
AN
AT
LA
NT
IC O
CE
AN
PORTUG
AL
Geotectonic Units:
Dias & Leal Gomes (2013)
The photomicrographs were made from a thin
section of the sample illustrated above and show
PPL, XPL and BSE views of scorzalite crystals.
: a section of irregularly shaped scorzalite
which has been replaced by gormanite or
childrenite-eosphorite (brownish to black in PPL)
show inclusions of muscovite, gahnite (Zn Fe
Mn ) Al O ) and montebrasite (?). The
brownish yellow color around the margin of
scorzalite is a narrow mantle of rosemaryite which
is also clearly identified in the BSE view. : another
scorzalite crystal less altered to gormanite showing
a deep blue absorption color and a corona of
wyllieite and rosemaryite (developed
indiscriminately between scorzalite and the various
groundmass minerals).
A-C
D
0.85 0.08
0.002 =0.94 2.04 4
E-G: High magnification views of parts of the field
of view shown in fig. D. In the first view (E) the
brownish yellow color of rosemaryite serve to
distinguish it from the wyllieite which shows a light
blue absorption color and high relief. In the second
view (F) wyllieite appears to form an inner mantle
surrounded by rosemaryite. Photopgraph G shows
scorzalite relics and a predominant slightly more
hydrated and Mn-rich scorzalite blue to greenish in
color (Scz-OH). The dark patches which are lighter
in the BSE view are mainly of gormanite/childrenite-
eosphorite. The narrow brown rim is of rosemaryite,
with anomalous interference colors in the XPL view.
H-K: scorzalite crystals altered to aggregates of
variscite and crandallite. Scorzalite relics can be
recognized and wyllieite-rosemaryite coronas are
absent.
The photomicrographs show the equigranular
groundmass consisting of quartz, plagioclase,
potassium feldspar and muscovite ( ). Plagioclase
feldspar and potassium feldspar are mostly subhedral
and have slightly interdigitating boundaries. It
appears that two generations of plagioclase are
present: small subhedral to euhedral crystals showing
multiple twinning (Pl1) and large subhedral crystals
most easily identified by a turbid brownish
appearance (Ab) resulting from very small inclusions
of mica. The strong alteration of the plagioclase ( ) is
the first characteristic that enables to distinguish it
from potassium feldspar, recognized here by the lack
of alteration and sometimes the presence of simple
twins ( ). Evidence for potassium feldspar dissolution
and replacement by albite + muscovite is shown in .
Apatite microinclusions are widely distributed within
the albite and are clearly seen in the BSE view ( ). The
compositions of both albite and potassium feldspar
are exceptionally rich in phosphorous.
A
B
A
C
D
The polished specimen is of a fine-grained portion of a pegmatite dyke from Santa Crisitina type locality (see Fig.2), consisting essentially of quartz, plagioclase, potassium feldspar and muscovite in a subhedral granular texture.
Scorzalite crystals ranging in size from 0.1-3mm are scattered throughout the rock and may be recognized by their deep blue color. The photomicrographs show PPL views of one scorzalite crystal surrounded by a 0.02-0.05 mm
wide rim of wyllieite.
The Serra de Arga pegmatite field consists of a swarm of granite-related aplite-pegmatite sills and dykes (Leal Gomes, 1994) and earlier highly peraluminous anatectic
pegmatites (Dias, 2012), mostly emplaced in metassedimentary and metavolcanosedimentary-exhalative Silurian series (Minho Central and Domo de Covas Units). The
first group, developed around the Arga granite plutonite (S-type peraluminous granite, ± 318 Ma), comprises evolved Li-bearing pegmatites with a layered structure,
belonging to the beryl, petalite, lepidolite and elbaite subtypes of the rare-element class; the pegmatites are mineralized with cassiterite and Nb-Ta oxides. The anatectic
pegmatites consist of thin stroma and vein-like irregular bodies, derived from low-degree hydrated partial melts in conditions of intermediate P-T (2.9-4.2 kbar, 650-710 º
C). The composition is significantly enriched in muscovite and andalusite (or albite) and depleted in potassium feldspar. They are characterized by a more or less simple
structure although an internal zonation is commonly observed and inward fractionation is noticeable. A classification as abyssal or muscovite types is proposed, although a
remarkable feature is the occurrence of tantalian rutile, ferrocolumbite and tapiolite in some of the vein deposits.
Abr Scz Wyl Ros Gm/Ch Scz (OH) Cra/GoyVar Qz Ab Ms Fk Mbr Ghn
. - scorzalite; - wyllieite; - Rosemaryite, - gormanite or childrenite-eosphorite; - more hydrated and Mn-rich scorzalite; -crandallite- goyazite; - variscite; - quartz; - albite; - muscovite; - potassium feldspar; - montebrasite; - gahnite.
S3
S3
S2
S2, S3 , L2, L3
39
c/s53º
43º
40º
70º
50º
53º
28º35º
L3
S3
S2
S2
D2
D3
D3/D´3
S3
49º
50º
50º
L3
L2
L2
Qz
Qz
30 cm
N
N
10 cm
S2
S3
1cm
2 mm
1cm
Fisga type
(*1)
- Thin, highly
deformed, fine-grained pegmatite
veins hosted in metaquartzphyllite
and lidite layers, possibly anatectic
in origin. Although scorzalite is
disseminated across the veins, is
more persistent along a 3 cm thick
border zone. Sillimanite is a
widespread mineral mostly
replacing scorzalite, and is related
to a late-tectonic increase of
regional P conditions.
Lousado type - Thicker (up to 1 m),
deformed, fine-grained pegmatite
lenses with simple structure hosted
in a poly-deformed micaschist. The
lenses are subconcordant and the
contact with the host rock is not
easily traced. Scorzalite occur as
millimeter disseminated grains.
(*2)
1
2
Fisga
Lousado
quartzphyllite
50 cm
N
B
B
A A
A
1
123
ScorzaliteMuscovite fractionPotassium feldspar with
fractures filled with scorzalite 32
3
Sta.Cristina
80
andalusitemicaschist
Sorzalite salband
micaschist
Pegmatite contour
Peg. contour
Metamorphic foliation
B
20cm
2.5 cm
20cm
Metamorphicfoliations andlineations
Shear structures
Legend
Legend
Legend
A B
Aplitic facies
Pegmatiticfacies
W12
G11
S17
Ms
Ghn
Scz(OH)
Mbr?
Scz
Scz
Gm/Ch
Gm/Ch
Gm/Ch
Cra/GoyVar
Var
Scz
Ghn
Ros
hole
Ros
R4
R9
S´8
G1
S6
S23
C25
Wyl
Wyl
Ros
Ros
200 m� 200 m�
100 m�
200 m�
200 m�
QzQz
Qz
Ms
Fk
Fk
Pl
Pl(I)
Qz
Qz
Qz
Qz
QzPl
Fk
Pl
Ms
MsQzQz
MsMs
Ms
Ms
Ms
Fk
200 m�
100 m�
Ros
10 m�
Qz Qz
Qz
Qz
Qz
Ab
Ab
Ab
Ab
Ab
Ab
Pl1Ms
Ms
Fk
Fk
Fk
Fk
Ap
Fk
200 m�200 m� 200 m� BSEXPLXPLXPL
PPL
A B C D
Poster Session: Pegmatites, and pegmatite mineralogy
ABSTRACT
A B C D
E F
H I J K
BSE
BSE
BSE PPL
PPL PPLPPL XPL
PPL XPL BSE BSE
G
BSE images in Fig. 3 (B, C, G, J and K) indicate the spots chosen to perform the analyses.
- Below detection limit.
Wyl Wyl Wyl Ros Ros Scz Scz Scz(OH) Grm/Ch Cra/Goy
W7 W12 W13 R9 R4 S6 S23 S´8 G11 C25
TiO2 0,02 - - - 0,02 - -0,008 0,0057 -
Al2O3 8,07 8,59 8,41 8,29 8,43 30,61 30,63 28,10 25,72 31,99
FeO 24,64 23,88 23,49 23,81 24,12 19,38 19,04 21,51 22,86 1,11
MnO 16,16 16,43 16,27 15,92 15,20 0,43 0,37 2,80 5,42 0,15
MgO 0,52 0,47 0,51 0,54 0,44 0,57 0,80 0,42 0,24 0,008
CaO 0,22 0,28 0,34 0,28 0,35 - 0,03 0,07 0,09 7,99
Na2O 4,98 5,20 5,31 4,69 4,24 0,008 - 0,04 0,21 0,01
K2O 0,02 0,02 0,00 0,01 0,03 0,061 - 0,01 0,03 0,06
BaO 0,08 0,12 0,01 0,11 - 0,061 0,13 0,02 0,77
ZnO - - - - 0,09 0,04 0,33 0,06 0,06 -
F 0,15 0,08 - 0,15 0,07 - - 0,04 - 0,35
Cl 0,01 0,00 - - - - 0,01 0,00 0,01 0,01
Cr2O3 0,07 - 0,02 0,06 - - 0,01 0,04 0,08 0,01
P2O5 46,56 46,30 45,93 47,18 45,46 41,16 41,7 39,15 33,4 30,26
Y2O3 - - - 0,04 0,02 - - -
SrO 0,10 - - 0,04 0,03 - 0,006 0,03 0,03 6,97
SO3 - 0,04 0,05 - - - 0,05 0,06 0,002 -
As2O5 - 0,04 - 0,04 - - - 0,01 -
Total 101,59 101,44 100,35 101,17 98,47 92,33 93,11 92,32 88,18 79,53
-
-
-
OTHER PHOSPHATESWYLLIEITE - ROSEMARYITE SCORZALITE
Electron microprobe analysis; Rb O not quantified; - below detection limit.2
CIG-R (2008)Dias, P.A.; Leal Gomes, C. (2013).
Dias, P. A. (2012)
Hatert, F.; Hermann, R.; Fransolet, A.; Long, G.; Grandjean, F. (2006)Leal Gomes, C. (1994)
. Carta Geológica de Portugal 1/50 000.
. In: Actas da
9ª Conferência anual do GGET: Geodinâmica e Tectónica global; a Importância da Cartografia Geológica, pp. 117-121.
.
. Ph.D Thesis, Univ. Minho, 464 p.
. . Eur. J. Mineral., 18, 775-785.
. . Ph.D Thesis, Univ. Minho, 695 p.
Minutas de revisão da folha 1C – Caminha.
Considerações sobre a natureza vulcanogénica a exalativa de algumas formações metamórficas da Serra de Arga – Minho – Portugal – consequências para a cartografia
Análise estrutural e paragenética de produtos litológicos e mineralizações de segregação metamórfica – Estudo de veios hiperaluminosos e protólitos poligénicos Silúricos da região da Serra de Arga
(Minho)
A structural, infrared, and Mossbauer spectral study of rosemaryite, NaMnFe Al(PO )
Estudo estrutural e paragenético de um sistema pegmatóide granítico - O campo filoniano de Arga - Minho (Portugal)
3+
4 3
REFERENCES
Three types of scorzalite bearing pegmatites were identified:
*1 and 2
*3
- The composition of the veins is 45% quartz, 26% muscovite, 13% andalusite+sillimanite, 6% albite, accessory minerals (scorzalite, apatite, monazite,
chrysoberyl, columbite-tantalite (?) and gahnite).
- The composition of the aplitic facies is: 41% quartz, 7.5% muscovite, 15.2% potassium feldspar, 29.6% albite, accessory minerals (scorzalite, wyllieite,
rosemaryite, crandallite group phosphates, variscite, Mn-rich fluorapatite, montebrasite, brasilianite, andalusite, columbite-Fe, gahnite, uraninite).
Sta. Cristina type
(*3)
- Dikes up to 50
cm thick, ranging from aplite (A) to
pegmatite (B), concordant to the
metamorphic foliation of the
andalusite micaschist host. The
pegmatitic part display a muscovite
enriched border zone followed
inward by an intermediate zone
composed of quartz, potassium
feldspar, muscovite, albite and
scorzalite. In the core zone
potassium feldspar exhibits thin
fractures filled with scorzalite.
wt.%