Ar±Ar and Fission-Track Ages in the Song Chay Massif

Journal of Asian Earth Sciences 19 (2001) 233±248 www.elsevier.nl/locate/jseaes Ar±Ar and ®ssion-track ages in the Song Chay Massif: Early Triassic and Cenozoic tectonics in northern Vietnam H. Maluskia,*, C. Lepvrierb, L. Jolivetb, A. Carterc, D. Roquesc, O. Beyssacd, Ta Trong Tange, Nguyen Duc Thangf, D. Avigadd aISTEM-CNRS, UniversiteÂ Montpellier 2, Place EugeÁne Bataillon, 34095, Montpellier, France bLaboratoire de Tectonique, UniversiteÂ Pierre et Marie Curie, 4 Place Jussieu, case 129, 75252 Paris cedex 05, France cLondon Fission Track Research Group, Department of Earth Sciences, Birkbeck and University College, Gower Street, London, WC1E 6BT, United Kingdom dLaboratoire de GeÂologie, Ecole Normale SupeÂrieure, 24 rue Lhomond, 75231 Paris cedex 05, France eNational University of Vietnam, Hanoi, 334 Nguyen Trai Str., Thanh Xuan, Hanoi, Viet Nam fGeological Survey, Hanoi, Viet Nam Received 14 October 1999; revised 9 May 2000; accepted 7 July 2000 Abstract The Song Chay Massif is the northeasternmost metamorphic complex in Vietnam, to the east of the Red River Shear Zone. It shows a large antiformal structure involving orthogneisses and migmatites overlain, on its northern ¯ank, by muscovite bearing marbles. An E±W striking fault bounds the dome to the South. Kinematic indicators along a S±N section reveal top-to-the-N shear sense along the interface between the orthogneissic core and the overlying metasediments. Radiometric ages were obtained by the 40Ar± 39Ar method using puri®ed mica separates. Across the dome ages range from 236 Ma at the southern edge to 160 Ma in the core, attesting to a strong imprint in the Early Triassic time. A clear difference is seen between these Mesozoic ages and the Eocene to Miocene ages (from 40 to 24 Ma) that obtained in the nearby Red River Shear Zone using the same method. These data show that the Song Chay Massif was already high in the crust when the high temperature deformation of the Red River Shear Zone took place. The ®nal exhumation of the Song Chay orthogneiss constrained by ®ssion-track analysis on samples along the same transect occurred during the Early Miocene and could be interpreted as the consequence of a ®rst normal sense of motion along the fault which bounds the massif to the south. Timing is similar to that of exhumation in the Red River Shear zone. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ar±Ar method; Fission-track ages; Song Chay Massif; Vietnam 1. Introduction studies (Jolivet et al., 1999) south of the Red River Shear Zone have identi®ed a large metamorphic core complex (the The Indochina peninsula, particularly northern Vietnam, Bu Khang Dome) and also evidence for extension during the is in a key-position for understanding the geodynamic Early Miocene. A number of structures in Vietnam are evolution of South Eastern Asia. Crossed by the southern known to date back to the Early Triassic (240 Ma, Lepvrier termination of the Red River Shear Zone it has been et al., 1997). Other thermotectonic episodes which may strongly affected by the India-Asia collision and by South have affected the region (e.g. during the Cretaceous, Lep- China Sea rifting. The precise role and extent of in¯uence of vrier et al., 1997; Lacassin et al., 1998) are more obscure, the Red River Shear Zone is not yet fully known and is the but this may be due to the current paucity of geochronolo- subject of ongoing debate (Tapponnier et al., 1982; 1986; gical and ®eld data. Thus, to decipher the geodynamic Briais et al., 1993; Leloup and Kienast, 1993; Leloup et al., evolution of Indochina it is essential that we understand 1995; Harrison et al., 1996; Dewey et al., 1989; Molnar and the timing and interaction between the different phases of Gipson, 1996; England and Molnar, 1990; Murphy et al., deformation and structures. In this context we have studied 1997; Rangin et al., 1995; Chung et al., 1997). The penin- the deformation and exhumation history of a large meta- sula is classically considered as a rigid block but recent morphic massif, close to the Red River Fault (RRF). The Song Chay Massif is located about 10 km north-east * Corresponding author. Tel.: 133-0467545926; fax: 133-0467547362. of the Day Nui Con Voi, east of the town of Lao Cai (Fig. 1). E-mail address: [email protected] (H. Maluski). It is a large domal structure which on ®rst examination 1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00038-9 234 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 Fig. 1. Location map and topography of northern Vietnam. The Song Chay Massif is close to the Day Nui Con Voi and the Red River. appears similar to the Bu Khang dome, and therefore, may Kontum Block, in the South (Fromaget, 1941). These extend have had a similar history. To understand the temporal rela- into the metamorphic ranges of Burma, Thailand, eastern tionship between this structure, the Red River Shear Zone Laos and Vietnam, as well as the extreme south-western part and Miocene extension found in the Bu Khang Dome south of China. The northern region is occupied by a complex of the fault (Jolivet et al., 1999) we have used a combination realm (Figs. 1 and 2), in which the NW±SE RRF zone is of ®eld observation, 40Ar± 39Ar mica dating (Maluski et al., central. Parallel to the active RRF is the Cenozoic Red River 1999), and apatite ®ssion-track analysis. The results are Shear Zone. The elongate Day Nui Con Voi Dome is compared with those from the Red River Shear Zone in bounded by the RRF to the west and by the Song Chay the Dai Nui Con Voi. Fault to the east. To the west of the RRF, alkaline granites intrude the gneissic Phang Si Pan Massif. Our main study area, the Song Chay Massif, is located on 2. Geology the eastern side of the Red River and extends into China. It has a dome-like shape, roughly trending in a NE±SW direc- The major structures within the Indochina peninsula are tion and is bounded on its western ¯ank by the Song Chay the Truong Song belt (CordillieÁre Anamitique of the early Fault and on its southern ¯ank by an E±W trending mylo- French authors), in North to Central Vietnam, and the nite zone, which on geological maps appears to be H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 235 Fig. 2. Geological map (from Geological map Vietnam, 1/200,000) and cross-section of the studied area showing the major structures, small scale structures in the Song Chay Massif as well as the location of samples and the 40Ar± 39Ar ages and ®ssion track ages. terminated by the Song Chay Fault. The eastern and south- lain by Devonian conglomerates, slates and limestones. The eastern limits of the dome correspond to the Lo river valley Permo-Carboniferous is represented by carbonates. which also occupies a major fault. Sample collection and observations of the structural and deformational history were made along the single road that crosses the dome, 3. Deformation in the Song Chay dome from the city of Bac Quang to the villages of Hoang Su Phi and Xin Man. Terranes surrounding the dome, to the We describe a cross-section of the dome from the SE to south and east, consist of greywackes and micaschists to the NW (Fig. 2). The southern limit of the dome is a narrow slaty schists overlain by a karstic formation of Cambrian EW trending fault, which cuts strongly lineated quartzites, limestones. The Ordovician and Silurian are represented micaschists and marbles. by limestones and quartzite, and are unconformably over- The foliation is folded into a broad antiform with an axis 236 H. Maluski et al. / Journal of Asian Earth Sciences 19 (2001) 233±248 Fig. 3. Photographs of outcrops in the Song Chay Massif showing top-to-the-north kinematic indicators. All sections are parallel to the lineation and perpendicular to the foliation. (a) Orthogneiss near Huang Xu Phi in the northern part of the section. (b)±(d) Orthogneiss from the southern side of the dome. Photograph (c) shows a high strain zone slightly oblique on the foliation in the less strained gneiss (lower). The button with the star gives the scale (2 cm). trending NE-SW and is steeper in the southern rim. Hori- stretching lineation and a weak planar anisotropy. These zontally foliated orthogneisses and migmatites are found orthogneisses are not ubiquitously deformed and locally near the core of the antiform, as shown on the cross section, occur in an unfoliated facies with large feldspars in an Fig. 2. To the North, upper levels of the core are made of undeformed groundmass. This rock has been considered biotite and muscovite-bearing orthogneisses containing K- to be an intrusive granite, but its occurrence suggests to feldspars several centimeters in length. Close to the village us that it is simply the undeformed equivalent of the of Xin Man, horizontally sheared micaschists are directly orthogneiss. Gradients of strain are seen at the scale of overlain by muscovite-bearing marbles that alternate with tens of meters and a general increase in deformation is pelitic schists, considered as Cambrian (Geological Survey observed from the undeformed granite toward the north of Vietnam, 1999 (Geological map 1/200,000); Tran Van and south. The most intense deformation is observed in Tri, 1977; Phan Cu Tien et al., 1989). the northern part of the section between Xin Man and A conspicuous NW- or N-trending stretching lineation is HuangSuPhi.

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