Per. Mineral. (2001), 70, 1, 111-129
: � PERIODICO di MINERALOGIA established in 1930
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An International Journal of MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on
Environment, Archaeometry and Cultural Heritage
Aluminium-phosphates and borosilicates in muscovite-kyanite metaquartzites near Diamantina (Minas Gerais, Brazil): Petrogenetic implications GIULIO MORTEANI 1 * DIETRICH ACKERMAND2 and ADOLF HEINRICH HORN3
1
3
Lehrstuhl fiir Angewandte Mineralogie und Geochemie, Technische Universitat Miinchen, Lichtenbergstrasse 4, 85747 Garching, Germany 2 Institut fiir Geowissenschaften, Christian-Albrechts-Universitat Kiel, Olshausenstrasse 40, 24105 Kiel, Germany Centro de Pesquisa Prof. Manuel Teixeira da Costa, Instituto de Geociencias, Universidade Federal de Minas Gerais, Av. Ant6nio Carlos 6277, 3 1000 Belo Horizonte MG, Brazil Submitted, November 2000- Accepted, Februm)' 2001
ABSTRACT.- Within the muscovite-dumortierite kyanite metaquartzites of the Middle Proterozoic B anderinha formation near Diamantina (S tate of Minas Gerais, Brazil), five rock types, Al through A4, and B are distinguished on the basis of whole rock composition. In addition to quartz, muscovite and Ti-bearing hematite (0.2 to 5.0 wt% Ti02), the five rock types show the following typical mineral assemblages: AI: lazulite, kyanite, andalusite; A2: lazulite, kyanite, augelite; A3: lazulite, amblygonite, b e rl i n i t e , t o u r m a l i n e ; A 4 : l a zu l i t e , g o y a z i t e , tourmaline, kyanite, and B : dumortierite, kyanite. From l i t h o s trati grap h i c a l c o n s i derati o n s , microscopic investigations and microprobe mineral a n a l y s e s , as w e l l as s t a b l e o x y g e n i s o t o p e thermo me t r y , t h e s e paragen e s e s for m e d at a maximum temperature of 440°C and pressure of at l e a s t 3.4 kbar during the mul ti s tage B ra s i l i ano tectonothermal epi sode between 680 and 450 Ma. Considerable solid solution series exist between lazul i te and scorzalite (Mg Fe), goyazite and svanbergite (P S) and goyazite and crandallite (Sr Ca). The MgO/FeO ratio of lazulite increases with the increasing MgO/FeO ratio of the bulk rock. A c ontact between lazulite and dumortierite was never observed. The protolith of the lazulite-bearing layer was probably a phosphorus- and l ocally also boron-rich clay-bearing sandstone deposited in a
*Corresponding author, E-mail:
[email protected]
s ab kh a - l i ke e n vi r o n me n t in o r i g i n al l y h i g h l y alkaline conditions. In the study area, the lazulite bearing muscovite-kyanite metaquartzites mark the strongly tectonised contact between the Middle P r o t ero z o i c Rio Paraun a a n d E s p i nh a z 0
Pl :::l 0..
>-
P+S=4 0.06
p 2:: 0 ::; tn
-
-
-
-
-
-
0.72
-
-
-
-
-
-
*: Fe calculated as FeO or Fe203; **: anhydrous total with F=O; (ss): solid solution; (small, large): grain size; -: below detection limit or not analysed. La203 in svanbergite- goyazite (ss) 0.02 wt%, Ce203 0.10 wt%.
[I: ::r:: 0 :;.:1 z
TABLE
2b
Representative microprobe analyses and mineral formulas of silicate minerals found in rock types AI to A4.
rock-type sample
tourmaline
kyanite
muscovite AI
A2
A3
5726
57 1 1+
5708+
5708
5717+
50.53
49.99 0.08 37.8 1
5 1.83 0.0 1 37.2 1
49.42 0. 11 36.73
49.62 0. 16 36.04
A4
[ �:
A4
AI
A3
57 17
5722
5726
5708
5708
57 17
57 16
5722
45.40
49.80 0.07 37.02
37.94
36.73 0. 19 32.28
37.40 0.35 33. 18
37.13 0. 19
37.66 0.02 35.9 1
38.00 0.09
g�::;
33. 13
�
�
Si02 TiO') AI263 Fe203** FeO** MgO CaO Na')O
K/)
P20s s o3 F anhydrous total
38.00 1.28 0.09 0.20 7.54 0.20
97.87
1.80 0.05 0.03 0.23 6.07
2.51 0. 16 0.02 0.20 5.0 1
0.03 0.45
0.02 0.02 0.04
96.35
97.0 1
34.28
0.05 62.84 0.37
2.49 0. 14 0.02
2.21 0. 13
2.4 1 0. 18
12.26 3.07 0.02
0.25
0.23 5.99 0.03 0.03
0.27 9.73 0.05
0.12
1.20 0.05 0.04
98.33
94.75
92.07
6.7 1 0.02
96.33
0.0 1 0.0 1 0.0 1
10 1.23
85.84*
10.74 3.63 0.0 1 1.15 0.03
10.63 3.60
3.28 6.65
1. 18 0.03
1.0 1 0.04 0.32
8.09 5.94 0.0 1 1.8 1 0.06 0.01
84.89*
87. 14*
0.02
0.04
86.50*
86.34*
K
p s
F
e �
