Geological Characteristics of the Suture Between West and East Gondwana

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Conclusion and Exploration Significance The Yangla copper deposit is a Sedex-type copper deposit, with a series of stacked, conformable lenses underlain by at least one stringer zone precipitated from hydrothermal solutions caused by the Jinshajiang PaleoTethys oceanic basin evolution in northern margin of Gondwanaland. The deposit was formed in the Carboniferous period, along with the host Gajinxueshan Group, indicating that earliest Jinshajiang Paleo-Tethysback arc basin would have formed at about 361.6+8.5Ma. The Jinshajiang ocean basin was closed giving rise to the collision suture aroud the boundary between the Middle and Late Triassic in association with tectonism, magmatism, dynamic metamorphism as well as mineralization characterized by copper-bearing stockwork veins. The ages of 227.08 i-1.38 Ma and 208.25k5.42 Ma from the Linong and Jiaren

intrusions probably indicate the time of the collision orogeny of Indo-China Plate with China Yangtze plate. No syngenetic sulphide deposit has previously been reported from Jinshajiang suture i n the northern margin of Gondwanaland, so this finding will provide an important avenue for exploration.

Acknowledgments This work is supported by Directive Grant of MGMR of China which is gratefully acknowledged. We also thank Yichang Institute of Geology and Mineral Resources for facilities provided,and thank Deqin Tibetan autonomous county and 3rdGeology Team of Yunnan province. Thanks go to Prof. Wang Xiaofeng and Prof.Xu Tao for constructive reviews of the paper.

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Geological Characteristics of the Suture Between West and East Gondwana Sospeter Muhongo Department of Geology, University of Dar Es Salaam, P.0. Box 35052, Dar E s Salaam, Tanzania

Introduction

Geological Characteristics

A suture zone can be simply defined as the tectonic expression of a collision zone, commonly consisting of a zone of ophiolitic or high-pressure metamorphic material (Howell, 1989). The most obvious indicators for the EastWest Gondwana suture might seem to be that it runs between the areas identifiable as parts of East and West Gondwana (Figl), and that it is the youngest of the sutures in that space (Shackleton, 1996). The transcontinental Mozambique belt which was first recognized and defined by Holmes (1951) is regarded as a megasuture between the eastern and the western Gondwana fragments (Fig.1); and that the supercontinent Gondwana was assembled along this suture during the Neoproterozoic (McWilliams, 1981; Shackleton, 1996and references therein).

Northern part of the Szitzire The Arabian Nubian Shield (ANS) of NE Africa (Fig.2 ) is considered to represent the northern extension of the Mozambique belt sensii stricto (Vail, 1976; Kroner, 1977; Stern, 1994). The region is made up of calc- alkaline volcanosedimentary greenschist assemblages associated with abundant batholithic granitoids and late anorogenic calcalkaline to alkaline plutons. Unequivocal ophiolites and juvenile arc assemblages are well documented and define the suture zone in this part of the belt (Stern, 1994; Shackleton, 1996 and references therein). Metasedimentary rocks from Sudan were metamorphosed to granulite-facies at about 720 Ma (Stern, 1994). Denkler et al. (1993) dated the Atmur - Delgo (northern

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plunging stretching lineations and NW - directed thrusts. This implies that the Archaean cra tonic margin was severally overprinted by the subsequent younger Proterozoic orogenies.

Cei7trnl pnrt of the Sirttrre

Fig.1. Location of the Mozambique belt in the supcrcontiiient Condwana (from Meert and Van der Voo, 1997).

Sudan) suture zone ophiolites at about 750 Ma (Sm - Nd method). This age is an estimation of the minimum age for the rifting event in this part of the suture. Dating of Neoproterozic ophiolites in Egypt and Sudan using single grain zircon evaporation technique has revealed ages ranging from 840 to 870 Ma (Kroner et al., 1992).These ages imply that a regional rifting event in this part of the suture occurred earlier than about 870 Ma. The ANS ophiolitic nappes are rooted somewhere east of the passive plate margin, and travelled, as much as 150 km or more (Stern, 1994; Shackleton, 1996 and references therein). Thus, the main suture zone is far away from the current ophiolitic nappes' geological and geographical positions. Isotope ages from the ANS indicate that arc evolution, collision, horizontal tectonic displacement, deformation, metamorphism and escape tectonics occurred from about 850 Ma to about 500 Ma, thus, this age bracket falls under the Pan-African tectono-thermal event of Kennedy (1964). The ophiolite - decorated greenschist assemblages of the ANS given way southwards through a zone of monotonous lithology which is multiply deformed and metamorphosed. These are gneisses, granulites, migmatites and marbles. This zone lies in southern Ethiopia, north - central Kenya and northeastern Uganda (Fig.3). This part of the suture represents deeply eroded Neoproterozoic amphibolite facies rocks which locally reached the granulite grade. The western boundary between the Archaean Craton and the Mozambique belt is very distinct and is a tectonic one. The structural trends in the cratonic and the foreland areas are identical e.g. NW- and NE- dipping foliations, SSE -

Farther south, into southern Kenya, Tanzania and down to northern Mozambique; the suture is more eroded down to 30 kms or more exposing its deepest portion (Fig.3).This portion constitutes the classical type area of the Mozambique belt of Holmes (1951). High grade rocks, especially enderbites, charnockites, gneisses, migmatites and meta anorthosites, metagabbros, amphibolites and marbles are ubiquitous in this part of the suture. The fault - bounded granulite - gneiss terranes, interpreted as thrust nappes (Muhongo, 1994),form a discontinous belt of over 1000 km long within this part of the suture. The high P-T garnet rich granulites, gneisses, granulite - facies meta-anorthosites and scattered mafic and ultramafic rocks define the suture line. Authentic ophiolites are yet to be discovered in this part of the suture. Thermo- barometric studies show ACWIBT P-T paths in the granulite - facies rocks from Tanzania with pressures ranging from about 8 to 15 kbars and temperatures ranging from 750" to 1100°C. They show evidence for rapid uplift, probably in response to the thrust - dominated crustal thickening process. U-Pb zircon ages from these rocks in Tanzania indicate diachronous peak granulite facies metamorphism, ranging from about 715 Ma to 645 Ma (Muhongo and Lenoir, 1994). Recent single zircon evaporation ages from some rocks of the same terranes reveal a persistent 640 Ma - old granulite facies metamorphic event (S. Muhongo and A. Kroner, unpubl. data). Sm - Nd and single grain zircon evaporation ages indicate that Archaean and Palaeoproterozoic crusts were involved in the evolution of these h g h P-T regional granulite terranes. Granulite-facies metamorphism in this part of the suture is a product of crustal thickening (e.g. through thrusting, folding and magmatic underplating) which occurred when the "Mozambique Ocean" was consumed during the Neoproterozoic continent - continent collision of the proto-Gondwana fragments (Muhongo, 1994; Stern, 1994; and Shackleton, 1996).

Soiithern part of the Stittire This part of the suture lies south of the ENE-trending Lurio thrust belt and the E-W trending Zambezi belt (Fig.3). There are two major schools of thought on the continuity of the Mozambique belt southwards beyond this major latitudinal "dislocation zone". The first one maintains the status quo (Holmes, 1951) and argues for the continuity of the belt down to SE South Africa, east of the Limpopo belt and the Kaapvaal craton (Fig.3). However, there are emerging ideas proposing the dual evolution nature of this Gondwana Research, V. 2, No. 4,1999

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1-1

D O M I N A N T YECTONISM Late Proterozoic I I

(0.75-0.55 Ga)

STRUCTURES LATE PROTEROZOIC (0.75-0.55Ga)

L a t e Proterozoic I (0.9-0.75 Ga) Mid -Prolerorolc ( 1 . 2 - 0 . 9 Ga) Early Proterozoic ( 2.0 Ga)

W]

Tectonic transport eireciion

/

Thrust Strike. slip tauti

4/Shear

I

MIn. R

Cratonic Archaean (inferred subsurface) Reworked Archaean or Early Proterozoic (inlerred where subsurface) Opt?ioIites ( L a t e Proterozoic)

zone i

ATC

PR%TgRcdF&C ( 2 0-0.75 Ga)

9 Tectonlc transport direclron 7Thrust fl

Strike-slip

fault

Fig.2. Map of the northern part of the suture (modified from Shackleton, 1996).

belt: Mesoproterozoic crustal growth and major collision tectonics; and Neoproterozoic transpressive intracontinental tectonics (Pinna, 1995; Pinna et al., 1993).The argument here is that the Lurio thrust belt (Fig.3) controlled the geometry and stress field between the northern portion of the belt ("Pan-African", Neoproterozoic Mozambique belt) and the southern one ("Kibaran", Mesoproterozoic Mozambique belt). Jacobs et al. (1998) made litho-chronological correlation between Dronning Maud Land (DML)of East Antarctica and southern part of the Mozambique belt (Fig.3),and found out that there are strong geological similarities between the two orogenic domains. They proposed that the DML represents the southern continuation of the Mozambique belt. The second school of thought is marshalled by Unrug (1996) who argues for bifurcation of the belt around the Gondzvana Research, V. 2, No. 4, 1999

Zambezi belt (Fig.3) continuing to SW Africa, thus, the Damara belt which lies between the Congo and the Kalahari cratons (Fig.l), represents the southwest continuationof the Mozambique belt. This scenario places the Kaapvaal craton into East Gondwana during the Neoproterozoic times. The ongoing geological research in southern Tanzania and Mozambique will solve this problem. In whatever scenario, the southern part of the collisional suture is defined by high P-T granulites and gneisses; and associated mafic and ultramafic rocks.

Conclusions A suture zone, the tectonic expression of a collision zone, can be defined by ophiolites or high-pressure metamorphic rocks. The transcontinental Mozambique belt is the

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LOCAL1 TIES DS Dar es Salaam F Furua T Tete SR Sor Rondane hfH Muhlig Hoflman htI.3 sv Sverdrupfjetla KV Kirwanveggen A Annandagstoppane n Helme/rontljel/a 0

Manica bell Vohibory belt Lurio bell Zambezi belt Highland C0mPtex Eastern G h a t s Limpopo bell

CD

Cenfral Dronnlng Maud Land betI

-

Lh.t

Lake Malawi

Fig.3. Map of the central and southern part of the suture (modified from Sliackletoii, 1996)

megasuture between the western and eastern Gondwana fragments. The northern part of this collisional suture is definedby authentic ophiolites and juvenile arc assemblages. The central part of the suture is defined by deeply eroded high P-T garnet-rich granulite-facies rocks (e.g. gneisses, gr?ndiles, meta-anorthosites) and associated bodies of mafic and ultramafic rocks. These same rock assemblages define the southern part of the Mozambique belt. Neoproterozoic high P-T regional granulite terranes which form the most significant part of this megasuture are a product of crustal thickening (e.g. through thrusting, folding and magma tic underplating) that occurred when the ”Mozambique Ocean” w a s consumed during the Neoproterozoic continent-continent collision of the protoGondwana fragments (Muhongo, 1994; Stern, 1994; and Shackleton, 1996).

References Denkler, T., Harms, U., Franz, G., Darbyshire, D.P.F., Pilot, J. and Schandelmeier, H. (1993) Evolution of the south-

western (Northern Province, Sudan). In: Thorweihe, U. and Schandelmeier, H. (Eds.) Geoscientific Research Northeast Africa, Balkema, Holland, pp. 171-175. Holmes, A. (1951)The sequence of Precambrian orogenic belts in southern and central Africa. Proceedings of the 18“’ International Geological Congress, London, v. 14, pp. 254-269. Howell, D.G. (1989) Tectonics of Suspect Terranes: Mountain Building and Continetal Growth. Topics in Earth Sciences, 3, Chapman and Hall, UK, 232p. Jacobs, J., Fanning, C.M., Henjes - Kunst, F., Olesch, M. and Paech, H-J. (1998) Continuation of the Mozambique Belt into East Antarctica: Grenville-age metamorphism and polyphase Pan-African high grade events in central Dronning Maud Land. J. Geol., v. 106, pp. 385-406. Kennedy, W.Q. (1964) The structural differentiation of Africa in the Pan-African (2500 m.y.) tectonic episode. Research Institute of African Geology, 81i1Annual Report, Leeds University, pp. 48-49. Kriiner, A. (1977) The Precambrian geotectonic evolution of Africa: plate tectonic accretion versus plate destruction. Precamb. Res., v.4, pp. 164-213. Kroner, A,, Pallister, J.S. and Fleck, R.J. (1992) Age of initial Gondwana Research, V. 2, No.4, 1999

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oceanic magmatism in the Late Proterozoic Arabian Shield. Geol., v. 20, pp. 803-806. McWilliams, M.O. (1981) Palaeomagnetism and Precambrian tectonic evolution of Gondwana. In: Kroner, A. (Ed.) Precambrian plate tectonics, Elsevier, Holland, pp.649-687. Meert, J.G. a n d Van der Voo, R. (1997) The assembly of Gondwana 800-500 Ma. J. Geodynam., v. 23, pp. 223-235. Muhongo, S. (1994) Neoproterozoic collision tectonics in the Mozambique belt of east Africa: evidence from the U~LI~LIIII Mountains, Tanzania. J. African Earth Sci., v. 19, pp. 153-168. Muhongo, S. and Lenoir, J.L. (1994)Pan-African granulite-facies metamorphism in the Mozambique Belt of Tanzania: U-Pb zircon geochronology. J. Geol. Soc. London, v. 151, pp. 343-347. Pinna, P. (1995) On the dual nature of the Mozambique Belt, Mozambique to Kenya. J. African Earth Sci., v. 21, pp. 477-480.

Pinna, P., Jourde, J., Calvez, J.Y., Mroz, J.P. and Marques, J.M. (1993) The Mozambique Belt in Northern Mozambique: Late Proterozoic (1.1 - 8.85Ga) crustal growth and tectogenesis and superimposed Pan- African (0.8 - 0.55 Ga) tectonics. Precamb. Res., v.62, pp. 1-59. Shackleton, R.M. (1996) The final collision zone between East and West Gondwana: where is it? J. African Earth xi., v. 23, pp. 271-287. Stern, R.J. (1994)Arc assembly and continental collision in the Neoproterozoic East African Orogen: implications for the consolidation of Gondwanaland. Ann. Rev. Earth Planet. Sci., v.22, pp. 319-351. Unrug, R. (1996) The assembly of Gondwanaland, Episodes, v.19, pp. 11-20. Vail, J.R. (1976) Outline of the geochronology and tectonic units of the basement complex of northeast Africa. Proceedings of the Royal SOC.London, A.350, py. 127-141.

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The Higo High-Grade Metamorphic Rocks in Japan as a Part of the Collisional Terrane Between the Sino-Korean and Yangtze Craton Yasuhito Osanail, Takuji Hamamoto2,Hiroo Kagami2, Kazuhiro Suzuki3,Masaaki Owada4and Atsushi Kamei4 ‘Department of Earth Sciences, Oknynnia University, Oknyama, 700-8530, Japan *Graduate Scliool of Science nnd Teclzizology,Niiptn University, Niignta, 950-2181, Japan 3Department of Earth and Planetary Sciences, Nagoya University, Nagoya, 464-8602, Japan 4Department of Earth Scierices, Ynnzagticlzi University, Yanzagtichi, 753-8512, Japan

The Higo metamorphic terrane, located in the western part of central Kyushu island, south-west Japan, extends for about 40 km in length and has a maximum width of 20 kin. This metamorphic terrane can be divided into three distinct units (e.g. Karakida, 1992) from north to south. These are the Manotani metamorphic unit, the Higo metamorphic unit and the Ryuhozan metamorphic unit. The Manotani metamorphic unit consists of high-P/T metamorphic rocks including alkali-amphibole and lawsonite (Karakida et al., 1989). The Ryuhozan metamorphic unit is characterized by intense mylonitization and low-P low-T metamorphism (Yainamoto, 1962). In contrast the Higo metamorphic unit consists of high-T and

metamorphic rocks up to ultrahigh-temperature granulitefacies with anatexite (Yamamoto, 1983; Obata et al., 1994; Osanai et al., 1996, 1998). The boundary between the Manotani and the Higo metamorphic units is a thrust fault accompanied by serpentinite intrusion while those of the Higo and the Ryuhozan metamorphic units are bounded by intrusions of granitic rocks (Higo plutonic complex) of the Miyanohara tonalite (c.210 Ma; Kamei et al., 1998) and the Shiraishino granodiorite (c.120 Ma; Kamei et al., 1997). In the southern e n d of the Higo metamorphic unit hornblende gabbro and hornblende-clinopyroxene gabbro (c. 257 Ma) without any deformation aIso occur as small masses. In the north-western part of the Higo metamorphic