- Email: [email protected]
SHRIMP U-Pb Zircon Geochronology of Granitoids in the Kibaran Type-Area, Mitwaba-Central Katanga (Congo)
RODINIA, GONDWANA AND ASIA
661
~~
Fig. 2.
Occurrence of Permian to Triassic conodonts from the clastic rocks interbedded with Middle Triassic radiolarian bedded chert in the Hisuikyo area, Mino terrane. The conodonts were identified by Prof. T. Koike of Yokohama National University.
et al., 1999). The clasts are 0.2-0.5 mm in diameter, and are composed of chert, siliceous shale, volcanic rock, volcanic glass, apatite, dolomite, glauconite-like mineral, radiolarian and conodont remains, etc. Part of the clasts are interpreted to have been derived from shallow-marine environments, such as volcanic edifice covered by Permian to Triassic sediments. The conodont fossils, ranging in age from Permian to Middle Triassic (Fig. 2), are concentrated in the clastic rocks, although the cherts above and below include few conodonts. It means the conodonts were most probably reworked from shallow-marine environment, especially from the oceanic island. The conodont ages strongly indicate that the Permian limestones on the oceanic island were covered with Triassic carbonate rocks.
References Isozaki, Y (1997) Jurassic accretion tectonics of Japan. The Island Arc, v. 6, pp. 25-51.Kojima, S., Ando, H., Kida, M., Mizutani, S., Sakata, Y., Sugiyama, K. and Tsukamoto, H. (1999) Clastic rocks in Triassic bedded chert in the Mino terrane, central Japan: their petrographic properties and radiolarian ages. J. Geol. SOC. Japan, v. 105, pp. 421-434 (in Japanese with English abstract). Mizutani, S. (1990) Mino terrane. In: Ichikawa, K., Mizutani, S., Hara, I., Hada, S. and Yao, A. (Eds.), Pre-Cretaceous terranes of Japan. Publication of IGCP Project No. 224, pp.121-135. Sano, H. and Kojima, S. (2000) Carboniferous to Jurassic oceanic rocks of Mino-Tamba-Ashio terrane, southwest Japan. Mem. Geol. SOC.Japan, No. 55, pp.123-144 (in Japanese with English abstract).
SHRIMP U-Pb Zircon Geochronology of Granitoids in the Kibaran TypeArea, Mitwaba-Central Katanga (Congo) J. Kokonyangil, R. Armstrong2,A.B. Kampunzu3and M. Yoshida4
’ Department of Geosciences,Faculty of Science, Osah City University, Osaka 558-8585,Japan,E-mail:[email protected]
’ Research School of Ewth Sciences, The Australian National University, Canberra ACT 0200, Austrafia ’ Universit?]of Botswana, Private Bag 0022, Gaborone,Botswana * Gondwana Institute of Geology and Environment, 147-2 Hashiramoto, Hashimoto 648-0091, Japan Granitoids represent an important component in the Kibarides belt, the type-area of the Kibaran orogenic system ofAfrica. The accretion history of this belt is poorly constrained mainly because of the absence of reliable geochronologic data. This study provides the first precise U-Pb zircon age determinations on granitoids from Mitwaba, constraining the timing of the orogenic events in the belt. Gondwana Research, 1/: 4,No. 4,2001
Three main granitic bodies were mapped in the Mitwaba region: (1)the Kifinga-Kiselebatholith, (2) the Nyangwa Pluton and (3) the Kungwe-Kalumengongo Stock (Fig. 1). All the intrusive bodies are affected by the three deformation events (D1 to DJ identified in the supracrustal sedimentary country rocks and are elongated NE-SW, parallel to the D, compressional structures marking the climax of the Kibaran orogenesis in the
662
RODINIA, GONDWANA AND ASIA
-ate
lmer'ate
Fig. I .
(a). Location of the Mitwaba area in Congo and Africa. (b) Simplified geological map of Mitwaba (Lepersonne, 1974).
A- Strike and dip of F1 foliation Strike and dip of F2 foliation
g22Strike and plunge of lineation (L2)
region. The foliation is well developed at the margin of these granitoids but central portions are massive, strongly suggesting their syn-tectonic emplacement. The contact aureoles rimming these intrusive bodies include porphyroblasts of biotite and garnet (in metapelites) and hornblende (in mafic rocks). These porphyroblasts display an internal fabric pre-dating the main fabric-forming event (D2) in the country rocks, suggesting the early Kibaran (syn-D,) emplacement of these plutons. Ion Microprobe (SHRIMP) U-Pb zircon dating of representative samples from these intrusive bodies were performed at the Research School of Earth Sciences (RSES), The Australian National University. The samples were selected based on their field occurrence, geographic distribution and petrographic observations. Most of the granite samples processed for this study yielded good quality zircon suitable for U-Pb dating, with one sample producing the most complicated population. Inheritance, in the form of cores, is a major feature in the zircon grains from these granitoids. However, the use of cathodoluminescence imaging enables these xenocrystic components to be identified and avoided during analysis. The results of this investigation are summarized below: The Kifinga-Kisele batholith is the largest pluton in the region, extending over 75 in length and 60 km width in the eastern part of the study area. It intrudes into the Mitwaba group and the lower part of the Mulumbi Group (Fig. 1). It represents a composite Batholith made, from NW to SE, of three different
intrusions: Fwifwi leucogranite, Kisele monzogranite and Kabonvia granodiorite gneiss. The Kabonvia granodiorite gneiss is dark to light grey rich in mafic microgranular enclaves and contains xenoliths of country rocks. The Fwifwi white leucogranite is a silica-rich granite, with 45-51 vol.% quartz in the modal composition. The leucogranite contains enclaves of the Kisele monzogranite. Field relationship suggests that the Kisele monzogranite pre-dates the Kabonvia granodiorite. The only published geochronological data for this batholith are conventional lead ages determined almost half a century ago (Eberherdt et al., 1956; Ledent et al., 1956). This pioneers work yielded ages between 1050 k 50 Ma and 1144 k 30 Ma, enabling to establish the Mesoproterozoic age of the Kibarides belt. SHRIMP U-Pb zircon dating of three samples from this batholith yielded emplacement ages in the range of 1386.3 k 8.1 Ma and 1372 rt 9.6 Ma. The Nyangwa pluton (22x10 km) occurs in the northwest of the study area. It is elongated NE-SW, parallel to the orientation of the main Kibaran structures. It is intrusive into the supracrustal sedimentary rocks of the Mulumbi Group and is partly covered by Neoproterozoic and Phanerozoic sedimentary sequences to the NW and SE (Fig. 1). No previous age determination was done on this pluton. SHRIMP U-Pb zircon dating of one sample from this granite yield an emplacement age of 1383 f 4.8 Ma. The Kungwe-Kalumengongointrusive body is a small granitic Gondwana Research, V. 4, No. 4, 2001
RODINIA, GONDWANA AND ASIA
stock exposed on ca. 3 km2,intruding the Mitwaba Group sedimentary sequence. No geochronological data have been reported as yet for this granite. SHRIMP U-Pb zircon dating of one sample from this granite yield an emplacement age of 1377k 10Ma. Thus, the crystallization ages of these plutons are, within the margin of errors, similar. The data allow us to precisely constrain the age Of the Kibaran deformation event (Dl) at ca. 1380 Ma which mark the earliest Rodinia assembly event in Africa.
663
References Eberhardt, €?, Geiss, J., Von Gunten, H., Houtermans, EG. and Signer, I? (1956) Mesure ii l'age de l'yttrocrasite de Mitwaba par mkthode de plomb. 11: Mesures isotopiques. Bull. SOC.Belge de GCol., v. 65, pp. 251-255. Ledent, D., Picciotto, E.E. and Poulaert, G. (1956) Determination de l'8ge a l'yttrocrasite de Mitwaba (Katanga) par la mCthode au plomb. I: Mesures chimiques. Bull. SOC.belge de gCol., v. 65, pp. 233-250. Lepersonne, J. (1974) Carte gkologique du Zaire l'&helle du I/ 2.000.000 et notice explicative. SOC.GCol. du Zaire, Ministere des Mines, Kinshasa, Zaire.
Geological Evolution of the Kibarides Belt, Mitwaba, Democratic Republic of Congo, Central Africa J. Kokonyangil, T. Okudaira', A.B. Kampunzu2and M. Yoshida3 Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558-8585,Japan,E-mail:[email protected] University of Botswana, Private Bag 0022, Gaborone, Botswana Gondwana Institute of Geology and Environment, 147-2 Hashiramoto, Hashimoto 648-0091, Japan The Kibaran orogenic system of Africa is of wide importance in evaluating the assembly and tectonics of the Rodinia supercontinent. It represents one of the few well-preserved Mesoproterozoic belts worldwide. The type area of the Kibaran orogenic system of Africa is within the Kibarides belt (700 long and 300 km width) in Katanga, southeast of Democratic Republic of Congo (D.R.C.). The study area is located between longitudes 27" and 28O East and latitudes 8" and 9" South (Mitwaba degree sheet). Excellent exposures of supracrustal metasedimentary sequences and granitoids from the Kibaran Mountains in Mitwaba were documented. The work included mapping (1/50000), structural and petrological analyses and geotectonic setting reconstruction. The data from this study allow constraining the processes involved during the orogenic evolution of this Mesoproterozoic belt and thus are important for Rodinia tectonics. The sedimentary sequence exposed in the Mitwaba area belongs to two informal tectonostratigraphic units: the Mitwaba group (oldest) and the Mulumbi group (youngest). The Mitwaba group includes four formations: Makanda, Mitwaba, Kananda and Kalumengongo. These formations are metapelitic, with minor intercalations of quartzites. The Mulumbi group is composed of five formations: Konga, Kabelo, Lwende, Nambia, Tambo and Kataba. The Kataba formation is a conglomerate located at the bottom of the Mulumbi group (Fig. 1).The Mulumbi and Kibara formations are predominantly quartzitic whereas Lwende, Nambia and Tambo formations are metapelitic units. Three deformation events were identified in the Mitwaba area. The earliest ( D l ) is shown by a poorly preserved ENE-WSWtrending flow foliation, associated to Z-shape mesoscopic folds. D1 fabrics are better preserved in the high-grade metamorphic rocks exposed in the southeastern part of the study area. However, generally these early Kibaran structures are strongly Gondwana Research, V. 4, No. 4,2001
overprinted by major NE-SW-trending isoclinal folds and related planar structures marking the regional tectonic grain of the belt (Fig. 1).These regional compressional structures were formed during the second Kibaran deformation event (D2). The third deformation event (D3) is characterised by S-shape folds and a mylonitic foliation sub-parallel to D2 structures. These structures were formed during a late orogenic extensional event. The Mitwaba group is intruded by three plutonic bodies: the Kifinga-Kiselebatholith, the Kungwe-Kalumengongo stock and the Mitwaba tin granite. The Mulumbi group is intersected by the Nyangwa and Kisandji plutons. All the granitoids are affected by the three Kibaran deformation events. However, the maximum strain is mainly recorded in the marginal rim zone of the intrusive bodies. Petrographic studies of fresh samples of supracrustal country rocks (mafic lithologies and metapelites) indicate that the contact aureole surrounding the Kifinga-Kisele batholith includes biotitegarnet hornfels and hornblende hornfels parageneses. The porphyroblasts formed during this contact metamorphism record an internal fabrics predating D2 and inferred to be syn-D1. These observations are consistent with the emplacement of this pluton before D2, most probably during D1. Furthermore, garnet, biotite and hornblende formed during contact metamorphism pre-date growth, along D2 structures, of mineral parageneses formed during low-grade (greenschist facies) orogenic metamorphism. The three prograde metamorphic zones mapped in the northwestern part of the study area, within this low-grade metamorphic terrane, include: Chlorite zone (metapelites): Chl + Ms + Ep + Qz + P1 + graphite Biotite zone (metapelites): Bt + Chl + Ms + Ep/Czo P1+ Qz Garnet zone (metapelites): Grt + Bt + Chl + Ms + P1 + Qz
+
661
~~
Fig. 2.
Occurrence of Permian to Triassic conodonts from the clastic rocks interbedded with Middle Triassic radiolarian bedded chert in the Hisuikyo area, Mino terrane. The conodonts were identified by Prof. T. Koike of Yokohama National University.
et al., 1999). The clasts are 0.2-0.5 mm in diameter, and are composed of chert, siliceous shale, volcanic rock, volcanic glass, apatite, dolomite, glauconite-like mineral, radiolarian and conodont remains, etc. Part of the clasts are interpreted to have been derived from shallow-marine environments, such as volcanic edifice covered by Permian to Triassic sediments. The conodont fossils, ranging in age from Permian to Middle Triassic (Fig. 2), are concentrated in the clastic rocks, although the cherts above and below include few conodonts. It means the conodonts were most probably reworked from shallow-marine environment, especially from the oceanic island. The conodont ages strongly indicate that the Permian limestones on the oceanic island were covered with Triassic carbonate rocks.
References Isozaki, Y (1997) Jurassic accretion tectonics of Japan. The Island Arc, v. 6, pp. 25-51.Kojima, S., Ando, H., Kida, M., Mizutani, S., Sakata, Y., Sugiyama, K. and Tsukamoto, H. (1999) Clastic rocks in Triassic bedded chert in the Mino terrane, central Japan: their petrographic properties and radiolarian ages. J. Geol. SOC. Japan, v. 105, pp. 421-434 (in Japanese with English abstract). Mizutani, S. (1990) Mino terrane. In: Ichikawa, K., Mizutani, S., Hara, I., Hada, S. and Yao, A. (Eds.), Pre-Cretaceous terranes of Japan. Publication of IGCP Project No. 224, pp.121-135. Sano, H. and Kojima, S. (2000) Carboniferous to Jurassic oceanic rocks of Mino-Tamba-Ashio terrane, southwest Japan. Mem. Geol. SOC.Japan, No. 55, pp.123-144 (in Japanese with English abstract).
SHRIMP U-Pb Zircon Geochronology of Granitoids in the Kibaran TypeArea, Mitwaba-Central Katanga (Congo) J. Kokonyangil, R. Armstrong2,A.B. Kampunzu3and M. Yoshida4
’ Department of Geosciences,Faculty of Science, Osah City University, Osaka 558-8585,Japan,E-mail:[email protected]
’ Research School of Ewth Sciences, The Australian National University, Canberra ACT 0200, Austrafia ’ Universit?]of Botswana, Private Bag 0022, Gaborone,Botswana * Gondwana Institute of Geology and Environment, 147-2 Hashiramoto, Hashimoto 648-0091, Japan Granitoids represent an important component in the Kibarides belt, the type-area of the Kibaran orogenic system ofAfrica. The accretion history of this belt is poorly constrained mainly because of the absence of reliable geochronologic data. This study provides the first precise U-Pb zircon age determinations on granitoids from Mitwaba, constraining the timing of the orogenic events in the belt. Gondwana Research, 1/: 4,No. 4,2001
Three main granitic bodies were mapped in the Mitwaba region: (1)the Kifinga-Kiselebatholith, (2) the Nyangwa Pluton and (3) the Kungwe-Kalumengongo Stock (Fig. 1). All the intrusive bodies are affected by the three deformation events (D1 to DJ identified in the supracrustal sedimentary country rocks and are elongated NE-SW, parallel to the D, compressional structures marking the climax of the Kibaran orogenesis in the
662
RODINIA, GONDWANA AND ASIA
-ate
lmer'ate
Fig. I .
(a). Location of the Mitwaba area in Congo and Africa. (b) Simplified geological map of Mitwaba (Lepersonne, 1974).
A- Strike and dip of F1 foliation Strike and dip of F2 foliation
g22Strike and plunge of lineation (L2)
region. The foliation is well developed at the margin of these granitoids but central portions are massive, strongly suggesting their syn-tectonic emplacement. The contact aureoles rimming these intrusive bodies include porphyroblasts of biotite and garnet (in metapelites) and hornblende (in mafic rocks). These porphyroblasts display an internal fabric pre-dating the main fabric-forming event (D2) in the country rocks, suggesting the early Kibaran (syn-D,) emplacement of these plutons. Ion Microprobe (SHRIMP) U-Pb zircon dating of representative samples from these intrusive bodies were performed at the Research School of Earth Sciences (RSES), The Australian National University. The samples were selected based on their field occurrence, geographic distribution and petrographic observations. Most of the granite samples processed for this study yielded good quality zircon suitable for U-Pb dating, with one sample producing the most complicated population. Inheritance, in the form of cores, is a major feature in the zircon grains from these granitoids. However, the use of cathodoluminescence imaging enables these xenocrystic components to be identified and avoided during analysis. The results of this investigation are summarized below: The Kifinga-Kisele batholith is the largest pluton in the region, extending over 75 in length and 60 km width in the eastern part of the study area. It intrudes into the Mitwaba group and the lower part of the Mulumbi Group (Fig. 1). It represents a composite Batholith made, from NW to SE, of three different
intrusions: Fwifwi leucogranite, Kisele monzogranite and Kabonvia granodiorite gneiss. The Kabonvia granodiorite gneiss is dark to light grey rich in mafic microgranular enclaves and contains xenoliths of country rocks. The Fwifwi white leucogranite is a silica-rich granite, with 45-51 vol.% quartz in the modal composition. The leucogranite contains enclaves of the Kisele monzogranite. Field relationship suggests that the Kisele monzogranite pre-dates the Kabonvia granodiorite. The only published geochronological data for this batholith are conventional lead ages determined almost half a century ago (Eberherdt et al., 1956; Ledent et al., 1956). This pioneers work yielded ages between 1050 k 50 Ma and 1144 k 30 Ma, enabling to establish the Mesoproterozoic age of the Kibarides belt. SHRIMP U-Pb zircon dating of three samples from this batholith yielded emplacement ages in the range of 1386.3 k 8.1 Ma and 1372 rt 9.6 Ma. The Nyangwa pluton (22x10 km) occurs in the northwest of the study area. It is elongated NE-SW, parallel to the orientation of the main Kibaran structures. It is intrusive into the supracrustal sedimentary rocks of the Mulumbi Group and is partly covered by Neoproterozoic and Phanerozoic sedimentary sequences to the NW and SE (Fig. 1). No previous age determination was done on this pluton. SHRIMP U-Pb zircon dating of one sample from this granite yield an emplacement age of 1383 f 4.8 Ma. The Kungwe-Kalumengongointrusive body is a small granitic Gondwana Research, V. 4, No. 4, 2001
RODINIA, GONDWANA AND ASIA
stock exposed on ca. 3 km2,intruding the Mitwaba Group sedimentary sequence. No geochronological data have been reported as yet for this granite. SHRIMP U-Pb zircon dating of one sample from this granite yield an emplacement age of 1377k 10Ma. Thus, the crystallization ages of these plutons are, within the margin of errors, similar. The data allow us to precisely constrain the age Of the Kibaran deformation event (Dl) at ca. 1380 Ma which mark the earliest Rodinia assembly event in Africa.
663
References Eberhardt, €?, Geiss, J., Von Gunten, H., Houtermans, EG. and Signer, I? (1956) Mesure ii l'age de l'yttrocrasite de Mitwaba par mkthode de plomb. 11: Mesures isotopiques. Bull. SOC.Belge de GCol., v. 65, pp. 251-255. Ledent, D., Picciotto, E.E. and Poulaert, G. (1956) Determination de l'8ge a l'yttrocrasite de Mitwaba (Katanga) par la mCthode au plomb. I: Mesures chimiques. Bull. SOC.belge de gCol., v. 65, pp. 233-250. Lepersonne, J. (1974) Carte gkologique du Zaire l'&helle du I/ 2.000.000 et notice explicative. SOC.GCol. du Zaire, Ministere des Mines, Kinshasa, Zaire.
Geological Evolution of the Kibarides Belt, Mitwaba, Democratic Republic of Congo, Central Africa J. Kokonyangil, T. Okudaira', A.B. Kampunzu2and M. Yoshida3 Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558-8585,Japan,E-mail:[email protected] University of Botswana, Private Bag 0022, Gaborone, Botswana Gondwana Institute of Geology and Environment, 147-2 Hashiramoto, Hashimoto 648-0091, Japan The Kibaran orogenic system of Africa is of wide importance in evaluating the assembly and tectonics of the Rodinia supercontinent. It represents one of the few well-preserved Mesoproterozoic belts worldwide. The type area of the Kibaran orogenic system of Africa is within the Kibarides belt (700 long and 300 km width) in Katanga, southeast of Democratic Republic of Congo (D.R.C.). The study area is located between longitudes 27" and 28O East and latitudes 8" and 9" South (Mitwaba degree sheet). Excellent exposures of supracrustal metasedimentary sequences and granitoids from the Kibaran Mountains in Mitwaba were documented. The work included mapping (1/50000), structural and petrological analyses and geotectonic setting reconstruction. The data from this study allow constraining the processes involved during the orogenic evolution of this Mesoproterozoic belt and thus are important for Rodinia tectonics. The sedimentary sequence exposed in the Mitwaba area belongs to two informal tectonostratigraphic units: the Mitwaba group (oldest) and the Mulumbi group (youngest). The Mitwaba group includes four formations: Makanda, Mitwaba, Kananda and Kalumengongo. These formations are metapelitic, with minor intercalations of quartzites. The Mulumbi group is composed of five formations: Konga, Kabelo, Lwende, Nambia, Tambo and Kataba. The Kataba formation is a conglomerate located at the bottom of the Mulumbi group (Fig. 1).The Mulumbi and Kibara formations are predominantly quartzitic whereas Lwende, Nambia and Tambo formations are metapelitic units. Three deformation events were identified in the Mitwaba area. The earliest ( D l ) is shown by a poorly preserved ENE-WSWtrending flow foliation, associated to Z-shape mesoscopic folds. D1 fabrics are better preserved in the high-grade metamorphic rocks exposed in the southeastern part of the study area. However, generally these early Kibaran structures are strongly Gondwana Research, V. 4, No. 4,2001
overprinted by major NE-SW-trending isoclinal folds and related planar structures marking the regional tectonic grain of the belt (Fig. 1).These regional compressional structures were formed during the second Kibaran deformation event (D2). The third deformation event (D3) is characterised by S-shape folds and a mylonitic foliation sub-parallel to D2 structures. These structures were formed during a late orogenic extensional event. The Mitwaba group is intruded by three plutonic bodies: the Kifinga-Kiselebatholith, the Kungwe-Kalumengongo stock and the Mitwaba tin granite. The Mulumbi group is intersected by the Nyangwa and Kisandji plutons. All the granitoids are affected by the three Kibaran deformation events. However, the maximum strain is mainly recorded in the marginal rim zone of the intrusive bodies. Petrographic studies of fresh samples of supracrustal country rocks (mafic lithologies and metapelites) indicate that the contact aureole surrounding the Kifinga-Kisele batholith includes biotitegarnet hornfels and hornblende hornfels parageneses. The porphyroblasts formed during this contact metamorphism record an internal fabrics predating D2 and inferred to be syn-D1. These observations are consistent with the emplacement of this pluton before D2, most probably during D1. Furthermore, garnet, biotite and hornblende formed during contact metamorphism pre-date growth, along D2 structures, of mineral parageneses formed during low-grade (greenschist facies) orogenic metamorphism. The three prograde metamorphic zones mapped in the northwestern part of the study area, within this low-grade metamorphic terrane, include: Chlorite zone (metapelites): Chl + Ms + Ep + Qz + P1 + graphite Biotite zone (metapelites): Bt + Chl + Ms + Ep/Czo P1+ Qz Garnet zone (metapelites): Grt + Bt + Chl + Ms + P1 + Qz
+