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Pan-African pressure-temperature evolution of the Merelani area in the Mozambique Belt in northeast Tanzania
Journal
of African
Earth Sciences.
Vol.
29,
No. 2, pp. 353-365,
I 999
o 1333
Pergamon
PII:SO899-5382(99)00102-5
Elsewer Science Ltd All rights reserved. Printed in Great Brmam 0399~5362/99 s- see front matter
Pan-African pressure-temperature evolution of the Merelani area in the Mozambique Belt in northeast Tanzania S. MUHONGO’r*, P. TUISKU2 and Y. MTONV ‘Department of Geology, University of Dar es Salaam, PO Box 35052, Dar es Salaam, Tanzania ZDepartment of Geosciences and Astronomy, University of Oulu, FIN-90570, Oulu, Finland
ABSTRACT-The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and later the rocks suffered retrogression during isobaric cooling (IBC) at the amphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670-61 O°C and pressures of 6.5-6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite porphyroblasts at the expense of sillimanite in graphitic paragneisses as well as by garnet zoning in the same rock. Further retrogression was due to hydration along shear zones and faults. This led to the alteration of forsterite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X,l0=O.5 in calcsilicate rocks. These findings, combined with the textural and thermobarometric data from the Uluguru Mountains and the Furua area, suggest that the IBC P-Tpath was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Belt rocks in Tanzania. This was most probably caused by addition of igneous rocks (underplating) at the initial stage of the formation of the granulite-facies rocks in the belt; followed by their residence and cooling in the middle and upper crustal levels. The slight decompression during this stage could be explained by either extensional or transtensional deformation. Finally, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed. o 1999 Elsevier Science Limited. All rights reserved. RESUME-Le socle de la zone de Merelani de la Ceinture du Mozambique dans le nord-est de la Tanzanie a ete def& et metamorphise de multiplesfois dans les temps panafiicains (Nr%oproterozdique). Le premier evenement metamorphique a eu lieu dans le facies des granuliies et les roches ont subi plus tard une evolution retrograde par refroidissement isobare dans le facies des amphibolites. Les donnees thermobarom&riques des gneiss de Merelani indiquent des temperatures de I’ordre de 670-610°C et des pressions de 65(1600 MPa. Le refroidissement isobare est illustrt! par la croissance tardive posttectonique de porphyroblastes de disth&ne aux depens de la sillimanite dans les paragneiss graphiteux, ainsi que par la zonation du grenat dans les m8rnes roches. Une evolution retrograde supplementaire a ete due a I’hydratation le long de zones et de failles de cisaillement, conduisant a I’alteration de la forsteriie, I’enstatite et la pargasite en mineraux du groupe de la serpentine. Dans les roches calcosilicatees alt&es par hydrothermalisme, la cristallisation de zoisite vanadif&re ftanzanite) a eu lieu B ce stade pour X,,=O.5. Les resultats, combines aux donnees texturales et thermobarometriques des Monts Uluguru et de la zone de Furua, suggerent que le trajet pression-temperature de refroidissement isobare a 6te predominant dans la (les) evolution(s) metamorphiquefs) des roches de la Ceinture panafricaine
*Corresponding author [email protected]
Journal of African Earth Sciences 353
S. MUHONGO
et al.
du Mozambique de Tanzanie. II a et6 cause probablement par I’apport de roches magmatiques (sous-placage) au stade initial granulitique, suivi par leur installation et refroidissement dans les niveaux crustaux moyens et superieurs. La leg&e decompression a ce stade pourrait s’expliquer par une deformation soit distensive, soit transtensive. Enfin, les roches de Merelani, comme celles des Monts Uluguru, ont et6 rapidement exhumees. a 1999 Elsevier Science Limited. All rights reserved. (Received 412198: revised version received 4/l 2198: accepted 20/l O/98)
INTRODUCTION The Merelani area, which is famous for its gem-quality minerals, especially tanzanite (blue zoisite), and graphite deposits, lies between longitudes 36O 57’ and 37O 04’E, and latitudes 3O33’ and 3O37’S (Fig. 1). It is 65 km (via the Kilimanjaro International Airport, KIA) southeast of Arusha Town in northeast Tanzania (Fig. 1). It forms part of the discontinuous granulitegneiss complexes that make up the Neoproterozoic (Pan-African) Mozambique Belt in Tanzania (Muhongo and Lenoir, 1994) (Fig. 1). These mostly fault-bounded high-grade complexes are regarded as polymetamorphic nappes (Muhongo, 1994). Thermobarometric studies on high-grade rocks from one of these nappes in the Uluguru Mountains (central part of the belt, Fig. 1) have revealed isobaric cooling (IBC) with some decompression P-Tpath at about 9-l 2 kbar and about 800-900°C for the basic granulites, whilst mineral cores of the ultramafic granulites and meta-anorthosites recorded pressures of about 13-17 kbar and temperatures of about 950-l 100°C (Muhongo and Tuisku, 1996). In this paper, it is demonstrated, using textural and P-Tdata, that the Neoproterozoic Merelani metamorphic rocks in the northern part of the belt (Fig. 1) also experienced an IBC episode at about 6 kbar and about 6OOOC. Geological setting The Merelani area is underlain by strongly deformed high-grade rocks, which were severely retrograded from the granulite to the greenschist-facies. These rocks show a wide variation in colour, texture and mineral composition. Based on these characteristic features, seven major lithologies can be distinguished in the Merelani area (Fig. 2): i/ garnet-kyanite/sillimanite-biotite gneiss. This type of gneiss is reddish-brown in colour, medium to coarse-grained, and has well-developed northeastsouthwest striking schistosity. Kyanite-sillimanite coronae are common in these gneisses. iii kyanite/silimanite-graphite gneiss. This type is bluish- to dark grey, composed of blue and green kyanite; medium- to coarse-grained. It also strikes northeast-southwest.
354 Journal of African Earth Sciences
D flaggy biotite-graphite gneiss. This type is grey, fine- to coarse-grained. Clay (retrograde product) and carbonate minerals are developed in these gneisses. iv) talc-silicate rocks. These are whitish-grey, coarse-grained and are bounding the gneisses. v/ hydrothermally altered graphite gneiss: This type is dark grey, fine- to medium-grained. It is this type of gneiss which hosts most of the tanzanites, tsavorites, green tourmalines and other gemstones that are mined in the Merelani area. Gemstones are concentrated in the quartz veins, fold hinges and boudin structures (Malisa and Muhongo, 1990). Graphite of the highest grade is found in this type of gneiss (Davies and Chase, 1994). vii quartzo-feldspathic gneiss. This type of gneiss is whitish- to pinkish-grey and is coarse-grained. It is composed mostly of quartz, feldspar with minor amounts of mica, graphite and magnetite. viii pegmatites. Pegmatites are ubiquitous in the Merelani gneisses, but most of them are too small to be mapped. Some pegmatites are folded and are concordant with the host gneisses. Discordant pegmatite veins are also very common in the Merelani gneisses. Gemstones and micas are common mineral components of these pegmatites. Small bodies of ultramafic rocks are scattered in the Merelani gneisses. The Merelani gneisses lie in the western limb of a major fold structure, the Lelatema antiform fold (Malisa, 1987). The dominant strike direction of the schistosity in the gneisses is northeast-southwest, ranging from 20 to 70ONE. The prominent dip is to the northwest with dip angles ranging from 30-60°. The rocks in the Merelani area were isoclinally folded and the sequence of repeated rock units (Fig. 2) suggests a northeast-southwest striking recumbent fold to have deformed these rocks. It is very probable that this recumbent folding was associated with the prograde granulite-facies metamorphism. Numerous shear zones, with boudin structures trending northeast-southwest, can be seen in the gneisses. Open folds, with axes plunging northwest and west, refolded the recumbent folds into upright to reclined folds. This folding phase, with related shear zones
Pan-African pressure-temperature
evolution of the Merelani area in the Mozambique Belt
4
Pemba Is.
Mesozoic-Cenozoic rocks
.1. .
‘CJ
??
m
. . . .
+_‘~LAAM . . ‘,
,_,
. ........ . . . . . . .
Granulite-gneiss complexe in the Mozambique Belt Mozambique Belt (?Late Proterozoic) U bendian-Usagaran Belt (Early Proterozoic) 8
..........
Tanzania craton (Archaeal .
. ... . . .... ...... ..... ....... ....... . ......... . . ....... ......... ......... ......... ......... ......... ........ ........
.
1 OOd . ....
.
0
4,O
$0
I?0
lf0
:OOkm
Figure 7. Geological map indicating the granulite-gneiss complexes in the Mozambique Belt in Tanzania. 1: Loliondo; 2: Longido; 3: Loibor Serrit; 4: Pare Mountains; 5: Usambara Mountains; 6: Handeni; 7: Magubike; 8: Nguru Mountains; 9: 14: Tunduru; 15: Songea (from Muhongo Wami River; 10: Uluguru Mountains; 11: lfakara; 12: Furua; 13: Nachingwea; and Lenoir, 19941.
Journalof African Earth Sciences 355
S. MUHONGO
et al.
9400N
200 m
Legend Garnet-kyanite/sillimanite-biotite-graphite
gneiss
Calc sillicate rocks Kyanite? sillimanite-graphite
gneiss
Pegmatite Hydrothermally
altered graphite gneiss (tanzanite mineralization)
Flaggy-biotite-graphite Quartz-feldspathic
gneiss
unit
Disturbed ground (Artisanal mining activity) Faults l?gun? 2. Geological map of the the Merelani
356 Journal of African Earth Sciences
area showing
lithological units.
Pan-African pressure-temperature
evolution of the Merelani area in the Mozambique Belt
Figure 3. Si fabric in kyanite grains is parallel to the external sillimanite; Si : internal foliation; $ : external schistosityl.
and strike-slip faults, was associated with the extensive retrogressive metamorphism, which affected the Merelani high-grade rocks down to the greenschist-facies.
PETROGRAPHY AND MINERAL CHEMISTRY Graphitic paragneisses Graphitic paragneisses in the Merelani area are very aluminous and are strongly foliated. Up to 1 cm large prophyroblasts of garnet and 7 mm long porphyroblasts of kyanite occur in medium-grained foliated gneisses. Quartz, plagioclase, garnet, orthoclase, graphite flakes, kyanite and needles of prismatic silimanite make up the major component of these gneisses. Accessory minerals include rutile, ilmenite, sulphides (pyrite and marcasite), monazite, zircon, apatite and muscovite, whose amount may be large in some retrogressed rock samples. Some gneisses contain conformable layers of orthoclase granite and/ or pegmatite leucosomes. Blastomylonitic segregation layering with alternating quartz and sillimanite, and graphite-rich
schistosity
of the graphitic
gneiss.
(KY: Kyanite;
Sil:
bands, is a prominent metamorphic feature in most of the Merelani gneisses. Garnet porphyroblasts are deformed into lenses that lie parallel to the S-fabric. On the contrary, kyanite porphyroblasts are subidioblastic and usually show little or no signs of deformation. Kyanites grew up to 2 cm long prisms, cross-cutting the schistosity in some gneisses, but in some cases they are poikiloblastic with a clear internal S-fabric parallel to that of the matrix (Fig. 3). In many graphitic gneiss samples, kyanite is seen to be replacing sillimanite in their schistosity planes (Fig. 4). llmenite is found in some garnet-bearing samples only as inclusions in garnets, whereas rutile does exist both as inclusions in garnets and as part of the rock matrix. In a few samples, feldspar and other aluminosilicate minerals are intensely altered to muscovite. Garnets in graphitic gneisses are almandine-rich with about two thirds being almandine and one third pyrope component (Table 1). The rims of these garnets may show considerable increase in grossular content. Plagioclases are albite-rich with a composition from An,, to An19.5.
Journal of African Earth Sciences35 7
S. MUHONGO
Figure 4. Kyanite poikiloblast as in Fig. 3.
replacing sillimanite
in the schistosity plane of the graphitic gneiss. Symbols are the same
Calc-silicate rocks Calc-silicate rocks are composed of poikiloblasts of diopside and anorthite of sizes of over 1 cm. Pyroxene in talc-silicate rocks is Al-bearing diopside (Table 2, No.Tl /l Dll) and plagioclase is anorthite (An,,). They contain inclusions of calcite, graphite and ilmenite. Carbonate is 99% pure calcite. The talc-silicates are the major host rocks for the tanzanite (blue zoisite), especially where late shear zones and faults crosscut them. Ultramafic rocks Ultramafic rocks are relatively finegrained and massive. They are composed of olivine, ortho-pyroxene, clinopyroxene, hornblende, spinel, serpentine and opaque alteration products. Serpentine-rich parts show a typical mesh texture of altered ultramafites. Olivine as well as ortho-pyroxene and clinopyroxene are Mg-rich (Fo,,,, Table 3). These two pyroxenes have Mg/(Mg + Fe) values of 0.90 and 0.93-0.94, respectively (Table 2). They are both Al-bearing and the clinopyroxene contains about a 4% jadeite component. Honblende is pargasite, as usual, in ultramafic rocks (Table 41, whereas spine1 contains
358 Journalof African Earth Sciences
et al.
about 14% hercynite and 11% chromite components (Table 5).
THERMOBAROMETRY
AND P-T EVOLUTION
TWEEQU software of Berman (1991) with the thermodynamic data set of Berman (1988, 1990) was used for thermobarometric calculations. Useful mineral equilibria in the Merelani graphitic gneisses are: Grossular + 2AI Silicate + Quartz = 3Anorthite (1) (This equation represents the GASP barometer.) 2Almandine + Grossular + 6Rutile = 3Quartz + Gllmenite + 3Anorthite
(2)
(This equation represents the GRIPS barometer.) Figure 5 shows results from a gneiss sample containing zoned garnets with rutile and ilmenite inclusions. Kyanite is used as an Al-silicate phase
Pan-African pressure-temperature Table 1. Garnet analyses
in graphitic paragneiss
T14/1 GRTl
T14/1 .l
Core SiOn TiOz
evolution of the Merelani area in the Mozambique Belt
GRTl.2 Near
core
T14/1 GRTl.6 Near
rim
T14/1 GRTl.9 Rim
38.74
39.01
38.60
38.49
0.00 22.09
0.00 22.19
0.00
A1203
0.00 22.23
Cr203 Fe0 MnO M9O CaO
0.06 30.43 0.48 8.82 0.71
0.02 30.15 0.34 8.80 0.75
0.06 29.80 0.43 a.41 1.27
Total
101.47
101.15
100.76
21.86 0.05 30.51 0.37 7.25 1.26
99.80
T14/3 GRTl
T14/1 .l
GRTl.3
Core
Between
38.85 0.03 22.12 0.02 29.14 0.38 a.55 1.42
100.52
Si” AI’”
5.937
5.981
5.950
6.010
5.983
0.063
0.019
0.050
0.000
0.017
T site
6.000
6.000
6.000
6.010
6.000
AI”’ Ti”’ Cr 0 site
3.952 0.000 0.007
3.959
3.974 0.000 0.003
3.977
3.982 0.000 0.008
3.989
4.024 0.000 0.006
4.031
3.998 0.003 0.003
4.004
38.55 0.00 22.01 0.00 31.16 0.34 8.68 0.56
101.30
T14/3 GRT2.5 Rim 38.85 0.00 21.91 0.05 30.01 0.42 7.92 1 .oo
100.16
5.934 0.066
6.023 0.000
6.000
6.023
3.927 0.000 0.000
4.003 0.000 0.006
3.927
4.010
Fe2+ Mn2’ Mg Ca
3.900 0.062 2.016 0.116
3.666 0.044 2.012 0.122
3.842 0.056 1.933 0.210
3.985 0.049 1.689 0.211
3.754 0.049 1.963 0.235
4.012 0.044 1.993 0.093
3.890 0.056 i .a29 0.166
A site
6.093
6.044
6.041
5.934
6.001
6.142
5.941
in its stability field (two uppermost curves) and sillimanite in higher temperature zones. The error in temperature calculation, diamond shaped (Fig. 51, for the core of the garnet is larger than that obtained in its pressure calculation. The bold arrow is drawn through the calculated garnet’s core to rim intersection points. The plausible interpretation for the P-Tpath indicated by this arrow is that the initial growth of the garnet took place during isobaric heating and, subsequently, its rim was retrogressively equilibrated during IBC. Petrographic studies give evidence for the replacement of sillimanite by kyanite in this gneiss sample. Thus, the P-T path would intersect the sillimanite-kyanite equilibria at about 67OOC. Olivine-orthopyroxene-clinopyroxene-pargasiteferropargasite-bearing equilibria in the ultramafic rocks give a P-Tintersection at 6.4 kbar and 610°C (Fig. 6). Although the error is large, the P-Tvalues are approximately the same as those obtained from the rim of the garnet in the Merelani paragneiss (Fig. 5). Thus, it seems that the above mineral
assemblage in the ultramafic rocks also reached equilibrium during the latest stage of the P-T IBC path. Calculations indicate that the late hydration event, which produced serpentine minerals, took place at temperatures less than 52OOC. It is during this late hydration event that tanzanite (blue zoisite) was produced. In talc-silicate rocks the limiting assemblage is diopside-anorthite-calcite (Fig. 6, XHZO= 0.5). If the abundant occurrence of kyanite is considered in the adjacent gneisses, then all these mineral assemblages would together confine the conditions of metamorphism in the ruled area of Fig. 6. The asterisk (-610°C and 6.4 kbar, Fig. 6) shows the results calculated from both mineral assemblages in the ultramafic rocks and the garnet’s rim of the graphitic gneiss. All three rock types give consistent results for the P-T conditions of the last stage of the regional amphibolite-facies metamorphism in the Merelani area. Petrographic (e.g. replacement of sillimanite by kyanite) and thermobarometic studies do not contradict one another and both indicate an IBC P-T
Journal of African Earth Sciences 359
S. MUHONGO Table 2. Pyroxene analyses in talc-silicate
Cab-silicate rock Tl/l Dll SiOz TiOz Al203 Fe0 MnO NiO MgO CaO Nan0 K20
ULlOPXl
99.60
Total
ULlCPXl
56.72 0.05 2.74 0.25 6.28 0.12 0.09 33.47 0.33 0.00 0.00
100.06
1 CPX1.2
52.74 0.06 3.36 0.48 2.00 0.00 0.06 16.45 23.14 0.59 0.00
53.19 0.15 3.14 0.48 2.41 0.11 0.07 16.60 22.93 0.54 0.00
98.88
99.60
1.979 0.021
1.952 0.048
1.933 0.067
1.938 0.062
T site
2.000
2.000
2.000
2.000
AI”’ Ti Cr Fe2+ Mn2+ Ni Mg Ca Na K
0.048 0.002 0.000 0.057 0.016 0.000 0.886 0.971 0.011 0.000
0.063 0.001 0.007 0.181 0.004 0.002 1.717 0.012 0.000 0.000
0.078 0.002 0.014 0.061 0.000 0.002 0.899 0.909 0.042 0.000
0.072 0.004 0.014 0.073 0.003 0.002 0.901 0.895 0.038 0.000
Ml,M2
1.991
1.988
2.006
2.003
+ Enstatite
+ 4H,O
and
360Journal
rock
Si” AI’”
path in the Merelani graphitic paragneisses, ultramafic and talc-silicate rocks. The late hydration event, which postdated the kyanite-forming event, was associated with shearing and faulting of the Merelani high-grade rocks. This hydration brought about sericitisation of the feldspars in the paragneisses, serpentinisation of ultramafic rocks, and formation of vanadiferous zoisite (tanzanite). The shaded area in Fig. 7 shows the possible P-T field for tanzanite generation at XHZ,,= 0.5 in talc-silicate rocks. This event signifies that after the formation of the kyanite (asterisk in Fig. 7) IBC continued to lower temperatures (e.g. about 45OOC). The reaction curves (Fig. 7): 2Forsterite
and ultramafic rocks
Ultramafic
54.23 0.09 1.60 0.00 1.87 0.51 0.00 16.29 24.84 0.16 0.00
cr203
et al.
of African Earth Sciences
= 2 Chrysotile (3)
14Forsterite
+ 10Enstatite
+ 31 H,O = Antigorite (4)
show the first possible appearance of serpentine minerals (chrysotile and antigorite) when Ptotal= PIWS CONCLUSIONS The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply-deformed and metamorphosed in the Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade, at about 800°C, but at lower pressures (6-7 kbar) and shallower depth (2025 km), than other granulite-facies rocks in the Mozambique Belt of Tanzania, e.g. about 9-l 2 kbar
Pan-African pressure-temperature Table 3. Olivine analyses
in ultramafic
ULlOll
rock
L1011.2
MgO
41 .Ol 0.05 9.41 0.16 0.47 48.62
Total
99.72
101.81
Si4+ Cr3+ Fe*+ Mn*+ Ni*+ Mg*+
1.007 0.001 0.193 0.003 0.009 1.779
0.997 0.000 0.220 0.002 0.008 1.776
Fo
90.21
89.00
SiOz CrzO3 Fe0 MnO NiO
evolution of the Merelani area in the Mozambique Belt
41.22 0.00 10.85 0.10 0.41 49.23
Table 4. Pargasite analyses ULl HBLl SiOn TiOz
MgO ZnO CaO NazO K20 H20calc
42.63 1.09 14.98 0.66 3.70 0.02 17.94 0.01 12.19 2.75 0.93 2.09
Total
98.17
98.99
6.204 1.796 0.000
6.115 1.885 0.000
T site
8.000
8.000
AI”’ Ti Cr Mg Fe*+ Zn Mn Ca
0.678 0.097 0.084 3.790 0.352 0.000 0.000 0.000
0.647 0.118 0.075 3.836 0.325 0.000 0.000 0.000
Ml ,2,3
5.000
5.000
Mg Fe*+ Zn Mn Ca Na
0.000 0.051 0.002 0.003 1.872 0.072
0.000 0.119 0.001 0.003 1.874 0.004
M4 site
2.000
2.000
Ca Na K
0.000 0.672 0.240
0.000 0.761 0.170
A site
0.912
0.931
O,_,C”‘C
2.000
2.000
A1203
Table 5. Spine1 analyses
in ultramafic
ULl SPLl SiO2 TiOn A1203 Cr203 Fe203 Fe0 MnO
rock
ULl SPL3
MgO CaO NiO ZnO
0.00 0.05 55.78 10.41 0.69 11.84 0.17 18.04 0.00 0.20 0.29
0.00 0.03 56.35 10.04 1.36 11.49 0.12 18.51 0.00 0.40 0.14
Total
97.47
Si Ti Al Cr Fe3+ Fez+ Mn
0.000
0.000
0.001
Mg Ca Ni Zn
1.763 0.221 0.014 0.266 0.004 0.721 0.000 0.004 0.006
0.001 1.761 0.210 0.027 0.255 0.003 0.732 0.000 0.009 0.003
Cations 0
3.000 4.000
3.000 4.000
98.43
Si’” AI’” Ti”’
rock
ULl HBL2
42.93 0.90 14.52 0.73 3.33 0.02 17.59 0.02 12.09 2.66 1.30 2.08
Cr203 Fe0 MnO
I
in ultramafic
Journal of African Earth Sciences 36 1
S. MUHONGO
et al.
18-
&8 a$@ d*’
14-
Q5
P kbar
& RIM OF GARNET WITH &Q
RUTILE BUT WITHOUT
”
,’
ILMENITE
I+@
IO,-
6-
ZONE
BETWEEN RIM AND
CORE OF GARNET WITH ,.
’
ILMENITE AND RUTILE CORE OF GARNET
2-l
INCLUSIONS
,’ WITH ILMENITE AND RUTlLE INCLUSIONS I
400
I
I
I
600
I
8bO
T F&m
I
do0
’ 1200
OC
5. P-T path for the graphit& pamgneiss calculated using the Berman (1988,
and 35-45 km (Coolen, 1980; Muhongo and Lenoir, 1994; Muhongo and Tuisku, 1996; Appel, 1996). At this stage, the C-bearing alumino-silicate sediments were transformed into the graphitic garnet-sillimanite gneiss. The gneiss was intensely deformed developing well-defined S-f fabrics and a blastomylonitic texture. A quiet period followed, thus, rocks were left in the middle crust where they underwent IBC at pressures of about 6-6.5 kbar (about 20-25 kms of depth). At this stage, -610°C, sillimanite in the alumino-silicate rocks was partly transformed into large kyanite poikiloblasts, which escaped deformation. Simultaneously, rims of garnet porphyroblasts in the gneisses reached equilibrium with minerals in the matrix; and ilmenite in the matrix disappeared after reacting with anorthite to form garnet and rutile f kyanite. Petrographic and thermobarometric studies on talc-silicate and ultramafic rocks from the same locality show a similar IBC P-Tpath. The Merelani high-grade rocks underwent further cooling and, subsequently, brittle deformation and influx of hydrous fluids took place along shear zones and faults. Sericitisation of paragneisses, serpentinisation of ultramafic rocks and crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks, took place at this stage. The metamorphic history of the Merelani rocks is similar to
362 Journal of African Earth Sciences
1991) data set.
that of the Uluguru Mountains (Muhongo and Tuisku, 1996) about 500 km away, and of the Furua area (Coolen, 1980) about 700 km away from the Merelani area. It is increasingly becoming evident that the metamorphic evolution of the high-grade rocks in the Pan-African Mozambique Belt of Tanzania was characterised by the IBC P-Tpath. This was probably caused by addition (underplating) of igneous rocks at the initial stage of the formation of the granulite-facies rocks in the belt (in the middle and lower crustal levels at about 20-45 km depth), followed by their residence and cooling in the middle and upper crustal levels before they were finally rapidly exhumed. The Merelani area apparently represents a middle crustal section during this IBC event, whereas both the Uluguru Mountains and the Furua area granulites (Fig. 1) represent a deeper crustal section. Thus, the P-Tand petrographic data from the Neoproterozoic granulite-gneiss complexes in the belt in Tanzania demonstrate the prominence of the IBC P-T path in granulite-facies rocks from two different crustal levels within the Mozambique Belt.
ACKNOWLEDGEMENTS The authors wish to thank the University of Dar es Salaam, Faculty of Science, and the University of Oulu for material and financial support. Graphtan
Pan-African pressure-temperature
I
400
!
!
evolution of the Merelaniarea
I
I
I
I
I
I
I
500
600
700
800
in the Mozambique Belt
I
I
900
-
~___
.__
I
1000
1100
1200
T°C figure 6. Possible area (ruled) for the co-existence of the diopside-anorthite-calciteand kyanite-bearing rocks from the Merelani area. The asterisk shows the P-T conditions calculated for the ultramafic rocks and the garnet’s rim of the graphitic paragneiss from the same area.
Joumel of African Earth Sciences 363
S. MUHONGO
I
20
I
I
I
I
!
et al.
I
I
/
t-
1
18
16
14
_
+ f! .
12
w’ + I
s 9
n
n 10
8
6
i
400
I
500
I
600
I
700
I
I
800
900
I
1000
I
1100
I
1200
T°C Figure 7. Possible area (shaded) for blue zoisite (tanzanite) generation from the diopside-anorthite-calcite assemblage at The XHfl =0.5. Reaction curves for serpentine generation in ultramafic rocks at P,,,,, =PHlo are given for comparison. asterisk shows the calculated P-T conditions for kyanite paragneiss.
364 Journal of African Earth Sciences
Pan-African pressure-temperature
evolution of the Merelani area in the Mozambique Belt
Co. Ltd (Tanzania) is acknowledged for field support. Comments by the anonymous reviewers are greatly appreciated. This paper is a contribution to IGCP 368.
REFERENCE6 Hockdruckgranulite und Eklogite in Appel, P.. 1996. Mosambique Belt von Tansania: eine geochemische und petrologische Studie. Ph.D. dissertation, Christian Albrechts - Universitaet zur Kiel, Germany, 208~. Berman, R.G., 1988. Internally-consistent thermodynamic data for stoichiometric minerals in the system Na,OK,O-Ca0-Mg0-Fe0-Fe,0,-Al,O,-SiO,-Tio,-H,O-CO~, Journal of Petrology 29, 445-522. Berman, R.G., 1990. Mixing properties of Ca-Mg-Fe-Mn garnets. American Mineralogist 75, 328-344. Berman, R.G., 1991 . Thermobarometry using multiequilibrium calculations: a new technique with petrologic applications. Canadian Mineralogist 29, 833-855. Coolen, J.J.M.M.M., 1980 Chemical petrology of the Furua Granulite Complex, southern Tanzania. GUA Papers Geology, Series 1 (13). 258~.
Davies, C., Chase R.J., 1994. The Merelani-graphitetanzanite deposit: an exploration case history. Exploration, Mining Geology 3, 372-392. Malisa, E. 1987. Geology of the tanzanite gemstone deposits in the Lelatema area, NE Tanzania.Ph.D. dissertation, University of Helsinki, Finland, 160~. Malisa, E., Muhongo, S., 1990. Tectonic setting of gemstone mineralization in the Proterozoic metamorphic terrane of the Mozambique Belt in Tanzania. Precambrian Research 46, 167-l 76. Muhongo, S., 1994. Neoproterozoic collision tectonics in the Mozambique Belt of East Africa: evidence from the Uluguru Mountains (Tanzania). Journal of African Earth Sciences 19, 153-I 68. Muhongo S., Lenoir, J.C., 1994. Pan-African granulite facies metamorphism in the Mozambique Belt of Tanzania: UPb zircon geochronology, Journal of the Geological Society of London 151, 343-347. Muhongo, S., Tuisku, P., 1996. Pan-African high pressure isobaric cooling: evidence from the mineralogy and thermobarometry of the granulite-facies rocks from the Uluguru Mountains, eastern Tanzania. Journal of African Earth Sciences 23, 433-463.
Journal of African Earth Sciences 365
of African
Earth Sciences.
Vol.
29,
No. 2, pp. 353-365,
I 999
o 1333
Pergamon
PII:SO899-5382(99)00102-5
Elsewer Science Ltd All rights reserved. Printed in Great Brmam 0399~5362/99 s- see front matter
Pan-African pressure-temperature evolution of the Merelani area in the Mozambique Belt in northeast Tanzania S. MUHONGO’r*, P. TUISKU2 and Y. MTONV ‘Department of Geology, University of Dar es Salaam, PO Box 35052, Dar es Salaam, Tanzania ZDepartment of Geosciences and Astronomy, University of Oulu, FIN-90570, Oulu, Finland
ABSTRACT-The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and later the rocks suffered retrogression during isobaric cooling (IBC) at the amphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670-61 O°C and pressures of 6.5-6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite porphyroblasts at the expense of sillimanite in graphitic paragneisses as well as by garnet zoning in the same rock. Further retrogression was due to hydration along shear zones and faults. This led to the alteration of forsterite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X,l0=O.5 in calcsilicate rocks. These findings, combined with the textural and thermobarometric data from the Uluguru Mountains and the Furua area, suggest that the IBC P-Tpath was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Belt rocks in Tanzania. This was most probably caused by addition of igneous rocks (underplating) at the initial stage of the formation of the granulite-facies rocks in the belt; followed by their residence and cooling in the middle and upper crustal levels. The slight decompression during this stage could be explained by either extensional or transtensional deformation. Finally, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed. o 1999 Elsevier Science Limited. All rights reserved. RESUME-Le socle de la zone de Merelani de la Ceinture du Mozambique dans le nord-est de la Tanzanie a ete def& et metamorphise de multiplesfois dans les temps panafiicains (Nr%oproterozdique). Le premier evenement metamorphique a eu lieu dans le facies des granuliies et les roches ont subi plus tard une evolution retrograde par refroidissement isobare dans le facies des amphibolites. Les donnees thermobarom&riques des gneiss de Merelani indiquent des temperatures de I’ordre de 670-610°C et des pressions de 65(1600 MPa. Le refroidissement isobare est illustrt! par la croissance tardive posttectonique de porphyroblastes de disth&ne aux depens de la sillimanite dans les paragneiss graphiteux, ainsi que par la zonation du grenat dans les m8rnes roches. Une evolution retrograde supplementaire a ete due a I’hydratation le long de zones et de failles de cisaillement, conduisant a I’alteration de la forsteriie, I’enstatite et la pargasite en mineraux du groupe de la serpentine. Dans les roches calcosilicatees alt&es par hydrothermalisme, la cristallisation de zoisite vanadif&re ftanzanite) a eu lieu B ce stade pour X,,=O.5. Les resultats, combines aux donnees texturales et thermobarometriques des Monts Uluguru et de la zone de Furua, suggerent que le trajet pression-temperature de refroidissement isobare a 6te predominant dans la (les) evolution(s) metamorphiquefs) des roches de la Ceinture panafricaine
*Corresponding author [email protected]
Journal of African Earth Sciences 353
S. MUHONGO
et al.
du Mozambique de Tanzanie. II a et6 cause probablement par I’apport de roches magmatiques (sous-placage) au stade initial granulitique, suivi par leur installation et refroidissement dans les niveaux crustaux moyens et superieurs. La leg&e decompression a ce stade pourrait s’expliquer par une deformation soit distensive, soit transtensive. Enfin, les roches de Merelani, comme celles des Monts Uluguru, ont et6 rapidement exhumees. a 1999 Elsevier Science Limited. All rights reserved. (Received 412198: revised version received 4/l 2198: accepted 20/l O/98)
INTRODUCTION The Merelani area, which is famous for its gem-quality minerals, especially tanzanite (blue zoisite), and graphite deposits, lies between longitudes 36O 57’ and 37O 04’E, and latitudes 3O33’ and 3O37’S (Fig. 1). It is 65 km (via the Kilimanjaro International Airport, KIA) southeast of Arusha Town in northeast Tanzania (Fig. 1). It forms part of the discontinuous granulitegneiss complexes that make up the Neoproterozoic (Pan-African) Mozambique Belt in Tanzania (Muhongo and Lenoir, 1994) (Fig. 1). These mostly fault-bounded high-grade complexes are regarded as polymetamorphic nappes (Muhongo, 1994). Thermobarometric studies on high-grade rocks from one of these nappes in the Uluguru Mountains (central part of the belt, Fig. 1) have revealed isobaric cooling (IBC) with some decompression P-Tpath at about 9-l 2 kbar and about 800-900°C for the basic granulites, whilst mineral cores of the ultramafic granulites and meta-anorthosites recorded pressures of about 13-17 kbar and temperatures of about 950-l 100°C (Muhongo and Tuisku, 1996). In this paper, it is demonstrated, using textural and P-Tdata, that the Neoproterozoic Merelani metamorphic rocks in the northern part of the belt (Fig. 1) also experienced an IBC episode at about 6 kbar and about 6OOOC. Geological setting The Merelani area is underlain by strongly deformed high-grade rocks, which were severely retrograded from the granulite to the greenschist-facies. These rocks show a wide variation in colour, texture and mineral composition. Based on these characteristic features, seven major lithologies can be distinguished in the Merelani area (Fig. 2): i/ garnet-kyanite/sillimanite-biotite gneiss. This type of gneiss is reddish-brown in colour, medium to coarse-grained, and has well-developed northeastsouthwest striking schistosity. Kyanite-sillimanite coronae are common in these gneisses. iii kyanite/silimanite-graphite gneiss. This type is bluish- to dark grey, composed of blue and green kyanite; medium- to coarse-grained. It also strikes northeast-southwest.
354 Journal of African Earth Sciences
D flaggy biotite-graphite gneiss. This type is grey, fine- to coarse-grained. Clay (retrograde product) and carbonate minerals are developed in these gneisses. iv) talc-silicate rocks. These are whitish-grey, coarse-grained and are bounding the gneisses. v/ hydrothermally altered graphite gneiss: This type is dark grey, fine- to medium-grained. It is this type of gneiss which hosts most of the tanzanites, tsavorites, green tourmalines and other gemstones that are mined in the Merelani area. Gemstones are concentrated in the quartz veins, fold hinges and boudin structures (Malisa and Muhongo, 1990). Graphite of the highest grade is found in this type of gneiss (Davies and Chase, 1994). vii quartzo-feldspathic gneiss. This type of gneiss is whitish- to pinkish-grey and is coarse-grained. It is composed mostly of quartz, feldspar with minor amounts of mica, graphite and magnetite. viii pegmatites. Pegmatites are ubiquitous in the Merelani gneisses, but most of them are too small to be mapped. Some pegmatites are folded and are concordant with the host gneisses. Discordant pegmatite veins are also very common in the Merelani gneisses. Gemstones and micas are common mineral components of these pegmatites. Small bodies of ultramafic rocks are scattered in the Merelani gneisses. The Merelani gneisses lie in the western limb of a major fold structure, the Lelatema antiform fold (Malisa, 1987). The dominant strike direction of the schistosity in the gneisses is northeast-southwest, ranging from 20 to 70ONE. The prominent dip is to the northwest with dip angles ranging from 30-60°. The rocks in the Merelani area were isoclinally folded and the sequence of repeated rock units (Fig. 2) suggests a northeast-southwest striking recumbent fold to have deformed these rocks. It is very probable that this recumbent folding was associated with the prograde granulite-facies metamorphism. Numerous shear zones, with boudin structures trending northeast-southwest, can be seen in the gneisses. Open folds, with axes plunging northwest and west, refolded the recumbent folds into upright to reclined folds. This folding phase, with related shear zones
Pan-African pressure-temperature
evolution of the Merelani area in the Mozambique Belt
4
Pemba Is.
Mesozoic-Cenozoic rocks
.1. .
‘CJ
??
m
. . . .
+_‘~LAAM . . ‘,
,_,
. ........ . . . . . . .
Granulite-gneiss complexe in the Mozambique Belt Mozambique Belt (?Late Proterozoic) U bendian-Usagaran Belt (Early Proterozoic) 8
..........
Tanzania craton (Archaeal .
. ... . . .... ...... ..... ....... ....... . ......... . . ....... ......... ......... ......... ......... ......... ........ ........
.
1 OOd . ....
.
0
4,O
$0
I?0
lf0
:OOkm
Figure 7. Geological map indicating the granulite-gneiss complexes in the Mozambique Belt in Tanzania. 1: Loliondo; 2: Longido; 3: Loibor Serrit; 4: Pare Mountains; 5: Usambara Mountains; 6: Handeni; 7: Magubike; 8: Nguru Mountains; 9: 14: Tunduru; 15: Songea (from Muhongo Wami River; 10: Uluguru Mountains; 11: lfakara; 12: Furua; 13: Nachingwea; and Lenoir, 19941.
Journalof African Earth Sciences 355
S. MUHONGO
et al.
9400N
200 m
Legend Garnet-kyanite/sillimanite-biotite-graphite
gneiss
Calc sillicate rocks Kyanite? sillimanite-graphite
gneiss
Pegmatite Hydrothermally
altered graphite gneiss (tanzanite mineralization)
Flaggy-biotite-graphite Quartz-feldspathic
gneiss
unit
Disturbed ground (Artisanal mining activity) Faults l?gun? 2. Geological map of the the Merelani
356 Journal of African Earth Sciences
area showing
lithological units.
Pan-African pressure-temperature
evolution of the Merelani area in the Mozambique Belt
Figure 3. Si fabric in kyanite grains is parallel to the external sillimanite; Si : internal foliation; $ : external schistosityl.
and strike-slip faults, was associated with the extensive retrogressive metamorphism, which affected the Merelani high-grade rocks down to the greenschist-facies.
PETROGRAPHY AND MINERAL CHEMISTRY Graphitic paragneisses Graphitic paragneisses in the Merelani area are very aluminous and are strongly foliated. Up to 1 cm large prophyroblasts of garnet and 7 mm long porphyroblasts of kyanite occur in medium-grained foliated gneisses. Quartz, plagioclase, garnet, orthoclase, graphite flakes, kyanite and needles of prismatic silimanite make up the major component of these gneisses. Accessory minerals include rutile, ilmenite, sulphides (pyrite and marcasite), monazite, zircon, apatite and muscovite, whose amount may be large in some retrogressed rock samples. Some gneisses contain conformable layers of orthoclase granite and/ or pegmatite leucosomes. Blastomylonitic segregation layering with alternating quartz and sillimanite, and graphite-rich
schistosity
of the graphitic
gneiss.
(KY: Kyanite;
Sil:
bands, is a prominent metamorphic feature in most of the Merelani gneisses. Garnet porphyroblasts are deformed into lenses that lie parallel to the S-fabric. On the contrary, kyanite porphyroblasts are subidioblastic and usually show little or no signs of deformation. Kyanites grew up to 2 cm long prisms, cross-cutting the schistosity in some gneisses, but in some cases they are poikiloblastic with a clear internal S-fabric parallel to that of the matrix (Fig. 3). In many graphitic gneiss samples, kyanite is seen to be replacing sillimanite in their schistosity planes (Fig. 4). llmenite is found in some garnet-bearing samples only as inclusions in garnets, whereas rutile does exist both as inclusions in garnets and as part of the rock matrix. In a few samples, feldspar and other aluminosilicate minerals are intensely altered to muscovite. Garnets in graphitic gneisses are almandine-rich with about two thirds being almandine and one third pyrope component (Table 1). The rims of these garnets may show considerable increase in grossular content. Plagioclases are albite-rich with a composition from An,, to An19.5.
Journal of African Earth Sciences35 7
S. MUHONGO
Figure 4. Kyanite poikiloblast as in Fig. 3.
replacing sillimanite
in the schistosity plane of the graphitic gneiss. Symbols are the same
Calc-silicate rocks Calc-silicate rocks are composed of poikiloblasts of diopside and anorthite of sizes of over 1 cm. Pyroxene in talc-silicate rocks is Al-bearing diopside (Table 2, No.Tl /l Dll) and plagioclase is anorthite (An,,). They contain inclusions of calcite, graphite and ilmenite. Carbonate is 99% pure calcite. The talc-silicates are the major host rocks for the tanzanite (blue zoisite), especially where late shear zones and faults crosscut them. Ultramafic rocks Ultramafic rocks are relatively finegrained and massive. They are composed of olivine, ortho-pyroxene, clinopyroxene, hornblende, spinel, serpentine and opaque alteration products. Serpentine-rich parts show a typical mesh texture of altered ultramafites. Olivine as well as ortho-pyroxene and clinopyroxene are Mg-rich (Fo,,,, Table 3). These two pyroxenes have Mg/(Mg + Fe) values of 0.90 and 0.93-0.94, respectively (Table 2). They are both Al-bearing and the clinopyroxene contains about a 4% jadeite component. Honblende is pargasite, as usual, in ultramafic rocks (Table 41, whereas spine1 contains
358 Journalof African Earth Sciences
et al.
about 14% hercynite and 11% chromite components (Table 5).
THERMOBAROMETRY
AND P-T EVOLUTION
TWEEQU software of Berman (1991) with the thermodynamic data set of Berman (1988, 1990) was used for thermobarometric calculations. Useful mineral equilibria in the Merelani graphitic gneisses are: Grossular + 2AI Silicate + Quartz = 3Anorthite (1) (This equation represents the GASP barometer.) 2Almandine + Grossular + 6Rutile = 3Quartz + Gllmenite + 3Anorthite
(2)
(This equation represents the GRIPS barometer.) Figure 5 shows results from a gneiss sample containing zoned garnets with rutile and ilmenite inclusions. Kyanite is used as an Al-silicate phase
Pan-African pressure-temperature Table 1. Garnet analyses
in graphitic paragneiss
T14/1 GRTl
T14/1 .l
Core SiOn TiOz
evolution of the Merelani area in the Mozambique Belt
GRTl.2 Near
core
T14/1 GRTl.6 Near
rim
T14/1 GRTl.9 Rim
38.74
39.01
38.60
38.49
0.00 22.09
0.00 22.19
0.00
A1203
0.00 22.23
Cr203 Fe0 MnO M9O CaO
0.06 30.43 0.48 8.82 0.71
0.02 30.15 0.34 8.80 0.75
0.06 29.80 0.43 a.41 1.27
Total
101.47
101.15
100.76
21.86 0.05 30.51 0.37 7.25 1.26
99.80
T14/3 GRTl
T14/1 .l
GRTl.3
Core
Between
38.85 0.03 22.12 0.02 29.14 0.38 a.55 1.42
100.52
Si” AI’”
5.937
5.981
5.950
6.010
5.983
0.063
0.019
0.050
0.000
0.017
T site
6.000
6.000
6.000
6.010
6.000
AI”’ Ti”’ Cr 0 site
3.952 0.000 0.007
3.959
3.974 0.000 0.003
3.977
3.982 0.000 0.008
3.989
4.024 0.000 0.006
4.031
3.998 0.003 0.003
4.004
38.55 0.00 22.01 0.00 31.16 0.34 8.68 0.56
101.30
T14/3 GRT2.5 Rim 38.85 0.00 21.91 0.05 30.01 0.42 7.92 1 .oo
100.16
5.934 0.066
6.023 0.000
6.000
6.023
3.927 0.000 0.000
4.003 0.000 0.006
3.927
4.010
Fe2+ Mn2’ Mg Ca
3.900 0.062 2.016 0.116
3.666 0.044 2.012 0.122
3.842 0.056 1.933 0.210
3.985 0.049 1.689 0.211
3.754 0.049 1.963 0.235
4.012 0.044 1.993 0.093
3.890 0.056 i .a29 0.166
A site
6.093
6.044
6.041
5.934
6.001
6.142
5.941
in its stability field (two uppermost curves) and sillimanite in higher temperature zones. The error in temperature calculation, diamond shaped (Fig. 51, for the core of the garnet is larger than that obtained in its pressure calculation. The bold arrow is drawn through the calculated garnet’s core to rim intersection points. The plausible interpretation for the P-Tpath indicated by this arrow is that the initial growth of the garnet took place during isobaric heating and, subsequently, its rim was retrogressively equilibrated during IBC. Petrographic studies give evidence for the replacement of sillimanite by kyanite in this gneiss sample. Thus, the P-T path would intersect the sillimanite-kyanite equilibria at about 67OOC. Olivine-orthopyroxene-clinopyroxene-pargasiteferropargasite-bearing equilibria in the ultramafic rocks give a P-Tintersection at 6.4 kbar and 610°C (Fig. 6). Although the error is large, the P-Tvalues are approximately the same as those obtained from the rim of the garnet in the Merelani paragneiss (Fig. 5). Thus, it seems that the above mineral
assemblage in the ultramafic rocks also reached equilibrium during the latest stage of the P-T IBC path. Calculations indicate that the late hydration event, which produced serpentine minerals, took place at temperatures less than 52OOC. It is during this late hydration event that tanzanite (blue zoisite) was produced. In talc-silicate rocks the limiting assemblage is diopside-anorthite-calcite (Fig. 6, XHZO= 0.5). If the abundant occurrence of kyanite is considered in the adjacent gneisses, then all these mineral assemblages would together confine the conditions of metamorphism in the ruled area of Fig. 6. The asterisk (-610°C and 6.4 kbar, Fig. 6) shows the results calculated from both mineral assemblages in the ultramafic rocks and the garnet’s rim of the graphitic gneiss. All three rock types give consistent results for the P-T conditions of the last stage of the regional amphibolite-facies metamorphism in the Merelani area. Petrographic (e.g. replacement of sillimanite by kyanite) and thermobarometic studies do not contradict one another and both indicate an IBC P-T
Journal of African Earth Sciences 359
S. MUHONGO Table 2. Pyroxene analyses in talc-silicate
Cab-silicate rock Tl/l Dll SiOz TiOz Al203 Fe0 MnO NiO MgO CaO Nan0 K20
ULlOPXl
99.60
Total
ULlCPXl
56.72 0.05 2.74 0.25 6.28 0.12 0.09 33.47 0.33 0.00 0.00
100.06
1 CPX1.2
52.74 0.06 3.36 0.48 2.00 0.00 0.06 16.45 23.14 0.59 0.00
53.19 0.15 3.14 0.48 2.41 0.11 0.07 16.60 22.93 0.54 0.00
98.88
99.60
1.979 0.021
1.952 0.048
1.933 0.067
1.938 0.062
T site
2.000
2.000
2.000
2.000
AI”’ Ti Cr Fe2+ Mn2+ Ni Mg Ca Na K
0.048 0.002 0.000 0.057 0.016 0.000 0.886 0.971 0.011 0.000
0.063 0.001 0.007 0.181 0.004 0.002 1.717 0.012 0.000 0.000
0.078 0.002 0.014 0.061 0.000 0.002 0.899 0.909 0.042 0.000
0.072 0.004 0.014 0.073 0.003 0.002 0.901 0.895 0.038 0.000
Ml,M2
1.991
1.988
2.006
2.003
+ Enstatite
+ 4H,O
and
360Journal
rock
Si” AI’”
path in the Merelani graphitic paragneisses, ultramafic and talc-silicate rocks. The late hydration event, which postdated the kyanite-forming event, was associated with shearing and faulting of the Merelani high-grade rocks. This hydration brought about sericitisation of the feldspars in the paragneisses, serpentinisation of ultramafic rocks, and formation of vanadiferous zoisite (tanzanite). The shaded area in Fig. 7 shows the possible P-T field for tanzanite generation at XHZ,,= 0.5 in talc-silicate rocks. This event signifies that after the formation of the kyanite (asterisk in Fig. 7) IBC continued to lower temperatures (e.g. about 45OOC). The reaction curves (Fig. 7): 2Forsterite
and ultramafic rocks
Ultramafic
54.23 0.09 1.60 0.00 1.87 0.51 0.00 16.29 24.84 0.16 0.00
cr203
et al.
of African Earth Sciences
= 2 Chrysotile (3)
14Forsterite
+ 10Enstatite
+ 31 H,O = Antigorite (4)
show the first possible appearance of serpentine minerals (chrysotile and antigorite) when Ptotal= PIWS CONCLUSIONS The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply-deformed and metamorphosed in the Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade, at about 800°C, but at lower pressures (6-7 kbar) and shallower depth (2025 km), than other granulite-facies rocks in the Mozambique Belt of Tanzania, e.g. about 9-l 2 kbar
Pan-African pressure-temperature Table 3. Olivine analyses
in ultramafic
ULlOll
rock
L1011.2
MgO
41 .Ol 0.05 9.41 0.16 0.47 48.62
Total
99.72
101.81
Si4+ Cr3+ Fe*+ Mn*+ Ni*+ Mg*+
1.007 0.001 0.193 0.003 0.009 1.779
0.997 0.000 0.220 0.002 0.008 1.776
Fo
90.21
89.00
SiOz CrzO3 Fe0 MnO NiO
evolution of the Merelani area in the Mozambique Belt
41.22 0.00 10.85 0.10 0.41 49.23
Table 4. Pargasite analyses ULl HBLl SiOn TiOz
MgO ZnO CaO NazO K20 H20calc
42.63 1.09 14.98 0.66 3.70 0.02 17.94 0.01 12.19 2.75 0.93 2.09
Total
98.17
98.99
6.204 1.796 0.000
6.115 1.885 0.000
T site
8.000
8.000
AI”’ Ti Cr Mg Fe*+ Zn Mn Ca
0.678 0.097 0.084 3.790 0.352 0.000 0.000 0.000
0.647 0.118 0.075 3.836 0.325 0.000 0.000 0.000
Ml ,2,3
5.000
5.000
Mg Fe*+ Zn Mn Ca Na
0.000 0.051 0.002 0.003 1.872 0.072
0.000 0.119 0.001 0.003 1.874 0.004
M4 site
2.000
2.000
Ca Na K
0.000 0.672 0.240
0.000 0.761 0.170
A site
0.912
0.931
O,_,C”‘C
2.000
2.000
A1203
Table 5. Spine1 analyses
in ultramafic
ULl SPLl SiO2 TiOn A1203 Cr203 Fe203 Fe0 MnO
rock
ULl SPL3
MgO CaO NiO ZnO
0.00 0.05 55.78 10.41 0.69 11.84 0.17 18.04 0.00 0.20 0.29
0.00 0.03 56.35 10.04 1.36 11.49 0.12 18.51 0.00 0.40 0.14
Total
97.47
Si Ti Al Cr Fe3+ Fez+ Mn
0.000
0.000
0.001
Mg Ca Ni Zn
1.763 0.221 0.014 0.266 0.004 0.721 0.000 0.004 0.006
0.001 1.761 0.210 0.027 0.255 0.003 0.732 0.000 0.009 0.003
Cations 0
3.000 4.000
3.000 4.000
98.43
Si’” AI’” Ti”’
rock
ULl HBL2
42.93 0.90 14.52 0.73 3.33 0.02 17.59 0.02 12.09 2.66 1.30 2.08
Cr203 Fe0 MnO
I
in ultramafic
Journal of African Earth Sciences 36 1
S. MUHONGO
et al.
18-
&8 a$@ d*’
14-
Q5
P kbar
& RIM OF GARNET WITH &Q
RUTILE BUT WITHOUT
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BETWEEN RIM AND
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ILMENITE AND RUTILE CORE OF GARNET
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INCLUSIONS
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OC
5. P-T path for the graphit& pamgneiss calculated using the Berman (1988,
and 35-45 km (Coolen, 1980; Muhongo and Lenoir, 1994; Muhongo and Tuisku, 1996; Appel, 1996). At this stage, the C-bearing alumino-silicate sediments were transformed into the graphitic garnet-sillimanite gneiss. The gneiss was intensely deformed developing well-defined S-f fabrics and a blastomylonitic texture. A quiet period followed, thus, rocks were left in the middle crust where they underwent IBC at pressures of about 6-6.5 kbar (about 20-25 kms of depth). At this stage, -610°C, sillimanite in the alumino-silicate rocks was partly transformed into large kyanite poikiloblasts, which escaped deformation. Simultaneously, rims of garnet porphyroblasts in the gneisses reached equilibrium with minerals in the matrix; and ilmenite in the matrix disappeared after reacting with anorthite to form garnet and rutile f kyanite. Petrographic and thermobarometric studies on talc-silicate and ultramafic rocks from the same locality show a similar IBC P-Tpath. The Merelani high-grade rocks underwent further cooling and, subsequently, brittle deformation and influx of hydrous fluids took place along shear zones and faults. Sericitisation of paragneisses, serpentinisation of ultramafic rocks and crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks, took place at this stage. The metamorphic history of the Merelani rocks is similar to
362 Journal of African Earth Sciences
1991) data set.
that of the Uluguru Mountains (Muhongo and Tuisku, 1996) about 500 km away, and of the Furua area (Coolen, 1980) about 700 km away from the Merelani area. It is increasingly becoming evident that the metamorphic evolution of the high-grade rocks in the Pan-African Mozambique Belt of Tanzania was characterised by the IBC P-Tpath. This was probably caused by addition (underplating) of igneous rocks at the initial stage of the formation of the granulite-facies rocks in the belt (in the middle and lower crustal levels at about 20-45 km depth), followed by their residence and cooling in the middle and upper crustal levels before they were finally rapidly exhumed. The Merelani area apparently represents a middle crustal section during this IBC event, whereas both the Uluguru Mountains and the Furua area granulites (Fig. 1) represent a deeper crustal section. Thus, the P-Tand petrographic data from the Neoproterozoic granulite-gneiss complexes in the belt in Tanzania demonstrate the prominence of the IBC P-T path in granulite-facies rocks from two different crustal levels within the Mozambique Belt.
ACKNOWLEDGEMENTS The authors wish to thank the University of Dar es Salaam, Faculty of Science, and the University of Oulu for material and financial support. Graphtan
Pan-African pressure-temperature
I
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!
!
evolution of the Merelaniarea
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I
I
I
500
600
700
800
in the Mozambique Belt
I
I
900
-
~___
.__
I
1000
1100
1200
T°C figure 6. Possible area (ruled) for the co-existence of the diopside-anorthite-calciteand kyanite-bearing rocks from the Merelani area. The asterisk shows the P-T conditions calculated for the ultramafic rocks and the garnet’s rim of the graphitic paragneiss from the same area.
Joumel of African Earth Sciences 363
S. MUHONGO
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et al.
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T°C Figure 7. Possible area (shaded) for blue zoisite (tanzanite) generation from the diopside-anorthite-calcite assemblage at The XHfl =0.5. Reaction curves for serpentine generation in ultramafic rocks at P,,,,, =PHlo are given for comparison. asterisk shows the calculated P-T conditions for kyanite paragneiss.
364 Journal of African Earth Sciences
Pan-African pressure-temperature
evolution of the Merelani area in the Mozambique Belt
Co. Ltd (Tanzania) is acknowledged for field support. Comments by the anonymous reviewers are greatly appreciated. This paper is a contribution to IGCP 368.
REFERENCE6 Hockdruckgranulite und Eklogite in Appel, P.. 1996. Mosambique Belt von Tansania: eine geochemische und petrologische Studie. Ph.D. dissertation, Christian Albrechts - Universitaet zur Kiel, Germany, 208~. Berman, R.G., 1988. Internally-consistent thermodynamic data for stoichiometric minerals in the system Na,OK,O-Ca0-Mg0-Fe0-Fe,0,-Al,O,-SiO,-Tio,-H,O-CO~, Journal of Petrology 29, 445-522. Berman, R.G., 1990. Mixing properties of Ca-Mg-Fe-Mn garnets. American Mineralogist 75, 328-344. Berman, R.G., 1991 . Thermobarometry using multiequilibrium calculations: a new technique with petrologic applications. Canadian Mineralogist 29, 833-855. Coolen, J.J.M.M.M., 1980 Chemical petrology of the Furua Granulite Complex, southern Tanzania. GUA Papers Geology, Series 1 (13). 258~.
Davies, C., Chase R.J., 1994. The Merelani-graphitetanzanite deposit: an exploration case history. Exploration, Mining Geology 3, 372-392. Malisa, E. 1987. Geology of the tanzanite gemstone deposits in the Lelatema area, NE Tanzania.Ph.D. dissertation, University of Helsinki, Finland, 160~. Malisa, E., Muhongo, S., 1990. Tectonic setting of gemstone mineralization in the Proterozoic metamorphic terrane of the Mozambique Belt in Tanzania. Precambrian Research 46, 167-l 76. Muhongo, S., 1994. Neoproterozoic collision tectonics in the Mozambique Belt of East Africa: evidence from the Uluguru Mountains (Tanzania). Journal of African Earth Sciences 19, 153-I 68. Muhongo S., Lenoir, J.C., 1994. Pan-African granulite facies metamorphism in the Mozambique Belt of Tanzania: UPb zircon geochronology, Journal of the Geological Society of London 151, 343-347. Muhongo, S., Tuisku, P., 1996. Pan-African high pressure isobaric cooling: evidence from the mineralogy and thermobarometry of the granulite-facies rocks from the Uluguru Mountains, eastern Tanzania. Journal of African Earth Sciences 23, 433-463.
Journal of African Earth Sciences 365