368 in 1997*

Gondwctnci Resetrrch, V I , No. 2, pp. 299-301. 0 1998 Interncitioncrl Associcitionji)r Gonciwann Resecirch, Japan. ISSN: I342 - 93 7 X

Gond wana Research CONFERENCE REPORT

Proterozoic Geology of Madagascar: International Field Workshop of IGCP-348/368 in 1997* Masaru Yoshida Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558, Japan (*Translated and revised version of a report in Japanese by the same author in J. Geol. SOC.Japan, 1997) Madagascar was situated at t h e central part of the Gondwanaland in the geologic past, and therefore is a key area for thc study of Gondwanaland tectonics. Recent discussions on the dispersion of Rodinia and assembly of Gondwanaland further highlighted the importance of this world’s 4th largest island. Because of the above, field and laboratory studies have been conductcd on the geological aspects of Madagascar by scveral groups of scientists from various parts of the world recently, and new and exciting findings have come out. The international symposium and field workshop was planned under this circumstancc by the IGCP-368, in collaboration with IGCP-348 and South African and Madagascaran organizations. The bascment geology of Madagascar is composed of metamorphic and igncous rocks ranging from early Precambrian to early Paleozoic, which are covered in the western part of the island by the dominant Gondwana Sediments ranging in age from late Paleozoic to Cretaceous. The basement geology is divided into that of the northern and southcrn regions by thc Ranotsara Shear Zone (also called the Bongolova-Ranotsara Shear Zone) which the major tectonic divide in Madagascar. The northern region is mostly rcprcsentcd by Archaean rocks, while the southern region is occupied largely by Proterozoic rocks. The field workshop consisted of pre- and post-symposium lours during the period August 17-29, 1997, with two days of symposium in Antananarivo on 21st and 22nd. The presymposium tour aimed mainly to observe the Maebatanana Greenstone Belt, one of the Archaean greenstone belts recently identified by Ashwal and others in the northern region. Field leaders were M. Le Grange, L.D. Ashwal, R.D. Tucker and R.A. Rambeloson. Participants included 42 scientists from 19 countries. The countries represented include South Africa, other African countries, Madagascar, France, Germany, United States, among several other countries. South Africa’s representation was the largest, with 13 participants, including experts from mining companies. 17th August was the departure from Antananarivo for the pre-symposium tour. Two buses in which the participants

travelled were followed by 3-4 Jeeps carrying the camping equipments and other logistics from Gillers Co. Several representative outcrops of basement rocks along the National Road No. 4, northward from Antananarivo, were visited. The abundance of gneissic granites of early Pan-African age (ca. 700-800 Ma) were impressive. Sillimanite gneiss which is cut by the early Pan-African granite carry zircons of late Archacan and Pan-African ages. Field guides for the Maebatanana Greenstone Belt were Grange and Tucker. Many stepwise dissolution ages have been obtained from the greenstone belts in the northern region by Tucker. This geochronologic work is supported by detailed geologic survey by Ashwal and other South African scientists. Some late Pan-African granites cross cut rocks of the greenstone belt, and thus provide good constraints on the time of the Pan-African tectonism in this area. In the night, several tents, each accomodating two persons and one large tent for the logistic centre, were set up on the school ground, the school being on vacation. Heated discussions and debates, and interesting observations continued into the late night and very early morning, encouraged by the cheap but excellent Madagascar wine. On the second day (18th), we observed the meta-acid volcanics (termed quartz mylonite), biotite gneiss originated from pelitic to psammitic rocks, amphibolite, and ultramafic intrusive(?) rocks (Fig. 1). In all the rocks are developed dominant N-W foliation with westerly dip and two lineations with NW and SW plunges. NW lineation is the “stretching lineation” shown by the elongation of quartz grains and mafic mineral clots. The SW lineation is the hinge of microfolds. At one locality, a sinistral drag fold with a SW hinge was associated with the axial plane foliation of mylonitic signature, which is just the same as the dominant mylonitic foliation in the area. A granodioritic mylonitic dyke with a ca. 800 Ma zircon age also carried the mylonitic foliation and stretching lineation, indicating that both the early and late deformations and metamorphic events in this area are of PanAfrican age.

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In conclusion, dominant mylonitization in the Maebatanana Creenstone Belt is later than ca. 800 Ma, and shows the thrusting of the overlying layers from WNW to ESE under the transpressive shearing tectonic regime. The upper limit of age is constrained by the undeformed granite of ca. 500 Ma: Thus, the deformation is evidently of Pan-African age. The post-symposium tour was conducted for 7 days during 23-29 August. 47 scientists from 18 countries participated. They were mostly the same members as the pre-symposium tour, with the addition of a French group of scientists. L.D. Ashwal, R.D. Tucker, R. Cox and B.G.J. Muller took the role of the field leaders. This tour aimed to observe the typical lithotypes of the Precambrian basement rocks of central Madagascar, major lithology of the Proterozoic cover sequence, “Itremo Group” distributed widely in the central to southern part of the northern region. Relationship between Itremo Group and the basement gneisses, and the Ranotsara Shear Zone dividing the island into the northern and the southern regions were also investigated. The tour, routed along the national road No. 7 southward from Ihosi, payed a visit to Itremo, deviating several kilometers westward. Total distance covered is nearly 750 km, with 15 observation points. Four campings and 2 hotel stays were included in the tour which lasted 7 days. The initial plan to reach Turear on the southwestern corner of the island and return to Antananarivo by air was abandoned due to thc failure of booking seats. Thus we werc obliged to return from Ihosi to Antananarivo by road, driving about 550 km. On the first day (23rd) the team drove up to Itremo, about 270 km south of Antananarivo and camped there. First stop was weakly gneissose granite with amphibolite xenoliths. Zircon geochronology gave age clusters of ca. 500, 800 and 2500-2600 Ma for these rocks, among which the ca. 800 Ma event was thought to indicate the time of intrusion. Other stops on this day were mostly similar to the first stop. An unexpected stop which took much time to visit was said to be the PanAfrican granite with xenolithic blocks of Itremo Group rock. However, the “xenolithic block” appeared to be something like aplite-mylonite cutting across the weak foliation of the granite, and not xenolith. We passed two stops on this day. On 24th and 25th, we drove rugged road in the mountains of the Itremo Group. The first outcrop was sheared granite with xenolithic calc-silicate rock. This outcrop is said to be vcry critical in giving the upper age limit of the Itremo Group based on the zircon age (790-795 Ma) of the granite. However, at this particular outcrop, the boundary between the green rock which appeared to include calc-silicate rocks apparently cuts across the shear foliation of the granite, and therefore, it seemed that the granite was intruded by the green rock which carried the calc-silicate rock. However, it was difficult to ascertain the relationship within the limited time spent at this outcrop. The next stop was hornfelsic phyllite of marl origin with distinct andalusite (and locally cordierite?) porphyroblasts. According to Tucker, ca. 800 Ma granite crops out near this

locality, suggesting that it caused the contact effect on the marl, thus providing the upper limit of the age of the Itremo Group. Distinct graded bedding develops here indicating an apparent westward younging, indicating the reverse structure of strata at this outcrop. The last stop was the deformed stomatolitic marble, where we could reach after a 30 minutes walk in the wild grassy field. The stromatolite structure shows beautiful features, with the quartz lamina often having a complete closure. Enthusiastic discussions were held, such as: is the structure definitely of stromatolitic origin, or reflects only the deformational structure of bedded limestone, either the closure shows sheeth fold or only the daigenetic structure, is the intensity of the deformation strong or weak, which could be the movement direction of the deformtion, etc. The participants were truly excited. According to Tucker, the Itremo Group was formed sometime during 1900-800 Ma on the continental shelf environment, and may be compared with the Cuddapah of India. However, A.B. Kampunzu pointed out a strong similarity in lithology and stratigraphy with weakly metamorphosed Neoproterozoic sediments in East Africa. On 26th, we drove about 90 km afrom the hotel in Flanarantosa to the camping site in Ihosy. The latter half of the road, although still the No. 7 national road, was so rugged that we were once obliged to get out of the bus and push it ahead. Four stops along the road included weakly gneissose granite, leptynite and biotite gneiss, with their foliation mostly dipping west. Migmatization develops to various degrees in these rocks, and the upper limit of the time of migmatization is given by the zircon age of 530 Ma of the granite cutting across the migmatite. In the late evening, after dark at about 8:30, we arrived at the camping area east of Ihosy situated within a wild forest environment. On 27th, we visited the outcrops of sillimanite gneiss which were the only typical metasediments that we could see during this tour. Foliation is dominantly N-W, which cuts across the earlier NNW foliation. Two kind of lineations have developed. The stretching lineation plunges about 20 degrees south. The former lineation parallels the hinge of sheeth fold which develops sporadically in some restricted quartz-rich layers. The area of our observations lies either at the southern margin or within the Ranotsara Shear Zone itself, and the rocks belong either to the Ihosy Group (Besarie, 1967, In: Rankama, H., Editor, The Precambrian) south of the Ranotsara Shear Zone are the mylonites (Windley, 1994, Geol. Rundsch., v. 83). In published structural maps, the N-S structure north and south of central and southern Madagascar is cut across by the sinistral Ranotsara Shear Zone and bent severely southeastward in the north and northwestward i n the south. However, our observations indicate that the NNW structure is cut by the NW structure. Earlier foliation mostly shows flattening characteristics, and only some planes and quartz grains show distinct shearing structure involving sheeth fold. The later NS structure is mostly sinistral shear planes, but some dextral Gondwana Research, V1, No. 2, 1998

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Fig. 1. Metasupracrustal rocks of the Maebatanana Greenstone Belt. Mylonitic granite, magnetite quartzite, biotite gneiss and quartz mylonite occur here.

Fig. 2 . The Ranotsara Plade in the Ranotsara Shear Zone near Ihosy

shears were also observed. It is possible that the deformation of the Ranotsara Shear Zone changed N-S at our survey area; however not much details are known. lineament Of the the In the late “Ranotsara Plane”, reflecting the Ranotsara tectonic line was observed from a hill about 10 km east of Ihosy (Fig. 2). In the morning of 28th, we departed from the Ihosy camp Gonciwana Research, V 1 , No.2, I998

site to Antananarivo, when Ashwal and a group separated to go further westward up to Turea where they planned to conduct a detailed field survey, and J.G. Ghosh and B.G.J. Muller stayed back for field surveys surrounding Ihosy. On 29th, after one night’s comfortabie stay in a hotel in Fanarantzao,an 8 hours’ drive took us to Antananarivo.