The significance of geological and zircon age data derived from the wall rocks of the Ailao Shan–Red River Shear Zone, NW Vietnam

Journal of Geodynamics 69 (2013) 122–139

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The significance of geological and zircon age data derived from the wall rocks of the Ailao Shan–Red River Shear Zone, NW Vietnam a,∗ ˙ ´ Andrzej Zela zniewicz , Traˆ` n Tro.ng Hòa b , Alexander N. Larionov c a

Institute of Geological Sciences, Polish Academy of Sciences, Podwale 75, 50-449 Wrocław, Poland Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Hanoi, Viet Nam c Centre of Isotopic Research, All Russian Geological Research Institute, St. Petersburg, Russia b

a r t i c l e

i n f o

Article history: Received 10 August 2010 Received in revised form 29 March 2012 Accepted 3 April 2012 Available online 10 April 2012 Keywords: Indosinian orogeny Fan Si Pan complex Lo Gam complex South China Block Tectonics Zircon Ailao Shan–Red River Shear Zone

a b s t r a c t This paper offers new evidence on whether the Ailao Shan–Red River Shear Zone of NW Vietnam is part of a suture zone between two continental blocks (the IndoChina Block and the South China Block) or whether it is itself of intracontinental origin, developed within the South China margin. To help clarify the role that the Ailao Shan–Red River Shear Zone plays in South China tectonic reconstructions, we gathered new whole-rock geochemistry, structural field data, and zircon U–Pb (SHRIMP) ages from granites, rhyodacites, and migmatites that occur within geological units adjacent to both the SW and NE sides of the Red River Fault Zone, a segment of the larger shear zone. The new zircon ages show that both walls of the Red River Fault Zone contain metamorphic and intraplate A-type granitoid rocks of Late Permian–Early Triassic age (263–240 Ma) and are of Indosinian origin. In the SW wall, the Fan Si Pan complex is a Neoproterozoic basement of metagranites and metasediments that was intruded by Late Permian (∼260 Ma), peralkaline, A-type granites and by subalkaline, A-type, biotite granite of Eocene age (∼35 Ma), containing xenoliths of gneissified Permian granitoids. The two intrusive episodes were separated by regional tectonic deformations occurring within a transpressional regime of a NW/W-vergent thrusting with a left-lateral oblique component, that was associated with greenschist to amphibolite facies metamorphism, presumably also of Eocene age (∼50–35 Ma), and that may have been related to the left-lateral movement on the Ailao Shan–Red River Shear Zone. In the NE wall, the Lo Gam complex is a Neoproterozoic basement (∼767 Ma) that was repeatedly subjected to tectonothermal activity throughout the Palaeozoic (at ∼450–420 Ma, ∼350 Ma, ∼265 Ma), ending in the Early Triassic (∼248 Ma). There was no thermal overprint during the Cenozoic. In this wall, a significant part of the Permo-Triassic thermotectonism was ductile shearing that was concentrated along dextral, strike-slip NW-trending zones in the vicinity the Ailao Shan–Red River Shear Zone but that became a type of NE/N-ward extensional/contractional, regional movement further away of it. An early shearing on the Ailao Shan–Red River Shear Zone may date back to the Permo-Triassic and we consider that this probably originated in a continental fault zone initiated in the hinterland of the oblique Indosinian collisional zone. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction One of most prominent crustal-scale features in SE Asia is the Ailao Shan–Red River Shear Zone, sometimes abbreviated to the Red River Shear Zone or to the Red River Fault Zone. In this paper, the Red River Fault Zone is understood as the Vietnamese segment of this larger structure, which is exposed in the Day Nui Con Voi Massif, NW Vietnam, which carries multiple record of early ductile nad late brittle deformations. The Red River Fault Zone contains

∗ Corresponding author. ˙ ´ E-mail address: [email protected] (A. Zela zniewicz). 0264-3707/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jog.2012.04.002

abundant evidence of left lateral, strike-slip displacement during the Tertiary and has been interpreted as a suture zone separating the Indochina Block from the South China Block (Roger et al., 2000; Schärer et al., 1990; Tapponnier et al., 1990) (Fig. 1). However, the suture between the two blocks has also been inferred to lie along the more southerly Song Ma Zone because of the presence of serpentinite slivers, MORB-like metabasic rocks (Tran, 1979) and Triassic eclogites (Nakano et al., 2006, 2010). On this basis, the Song Ma Zone would be likely the suture zone between the Indochina and South China blocks, accomplished during the Indosinian orogeny (Chung et al., 1998; Lepvrier et al., 1997; Metcalfe, 1993), and the southeastern part of the Ailao Shan–Red River Shear Zone would be redefined as an intracontinental fault system that propagated along the South China continental margin (Chung et al., 1997). Whether

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Fig. 1. Geological sketch map of the Red River Fault Zone and its adjacent tectonic units, modified from Geological Map of Vietnam, 1:000000 (Nguyen Xuan Bao, 1989).

the Ailao Shan–Red River Shear Zone is a plate boundary or not, it is one of the largest fault zones in the region whose origin and evolution is a matter of ongoing debate (Anczkiewicz et al., 2007; Leloup et al., 2001; Schärer et al., 1990; Searle, 2006; Searle et al., 2010; Tapponnier et al., 1990). Although most authors interpret it as a localized high-strain zone which developed in the Tertiary (Cao et al., 2011; Chung et al., 1997), some authors point to the onset of the ductile shear deformation in this zone already in the Mesozoic ˙ ´ (Yeh et al., 2008; Zela zniewicz et al., 2005, 2008). However, neither the inception and timing of consecutive tectonothermal events in the zone, nor the amount of multiple strike-slip displacements that

have occurred on it from the late Oligocene to the late Holocene are unanimously constrained. Proposed estimates of Tertiary left-lateral horizontal movements along the Ailao Shan–Red River Shear Zone suggest displacement of some 500–1000 km (Chung et al., 1997; Leloup et al., 2001; Tapponnier et al., 1990), though this figure has been disputed (Searle, 2006). But considerable displacement is plausible: to the NE of the Ailao Shan–Red River Shear Zone are Lower Palaeozoic (mainly Cambrian) low-to-mid grade metasediments and (meta)igneous rocks, which are concealed by Devonian and Triassic successions; whereas to the SW are igneous and metamorphic

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complexes partly overlain by Devonian low-grade metasediments and Jurassic volcanogenic rocks (Tran, 1979). A problem has been that the protolith and metamorphic ages and the tectonics of the crystalline basement rocks that outcrop on either side of the Red River Fault Zone are poorly known. Such knowledge is vital for any interpretation of the entire Ailao Shan–Red River Shear Zone itself and for its role in the evolution of the SW part of the South China Block. To help clarify this situation, we investigated rock units either side of the Red River Fault Zone (the Vietnamese segment of the Ailao Shan–Red River Shear Zone) with a view to zircon-dating of some (meta)granites that occur to the SW of the fault zone and some (meta)rhyodacites and migmatites that occur to the NE of it (Fig. 1). The rocks themselves were structurally analysed in the field, petrographically examined in the lab, crushed and subjected to geochemical whole rock analyses and a selection of retrieved zircons were utilized for SHRIMP U–Pb analyses. 2. Geological setting The Ailao Shan–Red River Shear Zone cuts high-grade metamorphic massifs that stretch from Tibet (China) via Yunnan to Vietnam (Leloup et al., 2001; Tapponnier et al., 1990). In NW Vietnam, one such massif is the Day Nui Con Voi Massif, comprising mainly highgrade metasediments of unknown protolith age that had its last metamorphic event during the Neogene. The NW portion of the Day Nui Con Voi Massif overlaps, over the distance of some 100 km, with the SE termination of the Ailao Shan Massif, which extends from Yunnan (Fig. 1). But the two massifs cannot be taken as a simple continuation along strike of each other, and this may explain some significant differences between them in lithology, structure and the timing of thermotectonic events (Leloup et al., 2001, 2007; Searle, 2006). The Day Nui Con Voi Massif is bordered by the Song Hong (Red River) and Song Chay Faults (Fig. 1). Both faults are conspicuously associated with the narrow Neogene grabens that confine the Day Nui Con Voi Massif horst: these faults also define part of the Red River Fault Zone and dissect metamorphic rocks of this massif. Therefore the Red River Fault Zone is also understood as a Neogeneto-Holocene feature that shows evidence of brittle, right-lateral faulting (Harrison et al., 1992; Zuchiewicz and Cuong, 2009) and that terminates the presumably much longer and more complex history of the entire Ailao Shan–Red River Shear Zone (Lin et al., 2012; Searle et al., 2010). The Red River Fault Zone, in the area of this study, runs NE–SW. On its SW side lies the Fan Si Pan complex, dominated by Phanerozoic rocks, and the Cavinh complex, dominated by Archaean to Mesoproterozoic rocks (Lan et al., 2001; Nam et al., 1998; Nam et al., 2003). On its NE side lies the Palaeozoic Lo Gam complex (Tran, 1979), comprising Palaeozoic through Triassic, mainly low- to midgrade, metasedimentary successions that surround the Song Chay metagranite dome (Fig. 1): these metasediments and metagranite belong to the South China Block. 3. Characteristics of rocks either side of the Red River Fault Zone

We sampled the Posen metagranite from the Bat Xat, Lao Cai and Tang Loong areas and determined that they can be classified as high-K, metaluminous (A/CNK 0.9–1.0), I-type, calc-alkaline, biotite–quartz monzodiorites to granites. These general characteristics essentially agree with the analyses of Lan et al. (2000). The Posen granite varies in composition (some with hornblende), colour and grain size (Fig. 3A), and the granite contains features such as mafic veins, migmatitic patches, grey granitic to aplitic veins, and occasional xenoliths of gneisses of undetermined Precambrian age. This granite evidently had a complex evolution. The Neoproterozoic Posen batholith is separated from the smaller Muong Hum granite, of unknown age, by the Sa Pa schist belt (Fig. 1). Published information on the igneous rocks are pretty scarce (Hung, 2010). Our study shows that the granite from the Muong Hum area itself is a fine-grained (ultrapotassic) alkali granite to quartz syenite (K2 O = 5–6%, Na2 O + K2 O = 10%, CaO = 0.2%; SiO2 = 67–74%) that was subsequently deformed and metamorphosed to a biotite–epidote gneiss (Table 1, Figs. 2 and 3B). The Muong Hum (meta)granites are peralkaline to peraluminous rocks of I/S type, with an A/CNK ratio of 0.85–1.05, that fall in the anorogenic field on the geotectonic R2–R1 plot and in the A1 field on all diagrams designed to discriminate between different anorogenic granites: there are geochemical hints of this granite being associated with either continental rifts, hotspots or mantle plumes (Eby, 1992). On a triangular Ba–Rb–Sr diagram they plot as matured normal granites. The Muong Hum granites are within plate (WPG) type (Pearce et al., 1984) and are characterized by low contents of Sr, P, Ti, Ba, Ca and high contents of Rb, Nb–Ta, Zr, Hf. Other significant geochemical characteristics include high REE contents amounting to 554.5–779.1 ppm, a K/Rb ratio of 170–260; K2 O/Na2 O > 1 (1.15–1.45) and Rb/Sr > 1 (1.5–15.0); Eu/Eu* = 02–0.6 with Eu negative anomaly; and La/Yb = 5.0–13.9. In the Fan Si Pan Massif, the youngest igneous unit is the Yeyensun granite of Palaeogene age (Leloup et al., 2001; Zhang and Schärer, 1999). Tran et al. (2002) distinguished two different types of granites in the Yeyensun batholith. A leucocratic, high-K, calcalkaline, I-type granite occuring to the west of the O Quy Ho Pass and a subalkaline, A-type, biotite granite occurring to the east of the pass. In view of the above, our samples come from the eastern type. The rocks we sampled were almost all leucocratic (biotite < 5%) alkali granites and quartz monzonites, which are peraluminous (A/CNK = 1.0–1.1), high-potassic (K2 O = 4–5%, CaO = 0.2–1.5%, SiO2 = 67–73%, K/Rb = 220–290) rocks of S-type. The Yeyensun granites also represent anorogenic or late orogenic rocks on the geotectonic R2–R1 plot, close to the border between the A1 and A2 fields on diagrams that discriminate A-type granites. On the Ba–Rb–Sr diagram they plot as anomalous granites and granodiorites (Fig. 2). Other characteristic features include a negative linear trend of Sr relative to SiO2 ; a relatively low content of P, Ti, Nb–Ta, Tb versus a rather high content of Ba–Rb, U, K, Pb, Zr–Hf; and a low REE total amounting to 50.2–171.0 ppm. In comparison with the Muong Hum granites, the Yeyensun granites are poorer in Nb–Ta and REE but are more strongly fractionated (La/Yb = 11.7–48.8), with Eu/Eu* = 0.58–1.16 and a weak Eu anomaly (Table 1, Figs. 2 and 3C).

3.1. Lithology and geochemistry 3.1.1. SW wall of the Red River Fault Zone The Fan Si Pan Massif is a composite unit built of several types of granites and metagranites separated by narrow intervening belts of Neoproterozoic metasediments, mainly mica schists and marbles. The Posen metagranite (Fig. 1) has yielded U–Pb zircon ages in the Neoproterozoic range of 760–640 Ma (Lan et al., 2000; Wang et al., 2011).

3.1.2. NE wall of the Red River Fault Zone The Lo Gam complex lies to the NE of the Day Nui Con Voi Massif and the Red River Fault Zone and is made up of Lower Palaeozoic mica schists and crystalline limestones intruded by the vast batholith of the Song Chay metagranite (Fig. 1). The granite has yielded both Ordovician (Tran et al., 2008a) and Silurian protolith ages (Carter et al., 2001; Roger et al., 2000). To the SE of the batholith, sheared metasediments contain ubiquitous leucocratic

1000 1 RbBa Th U K NbTa La CeSr Nd P Hf Zr Sm Ti Tb Y TmYb

Yb

Y

E

1000

2

tonalite

Peraluminous

Metaluminous

granodiorite

A/NK

granite quartz syenite

1

500

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Peralkaline

1.2 A/CNK

1.6

Rb Nb

Nb 10

10

ORG 1

1 1

10

100 Y+Nb

1000

1

10

Y

100

1000

2000

I

Continental arc and other non-A type granites

A1

3000

4000

J A-granites

Fractionated granites

A2

0.1

ORG

1000

M.-, I-, S-type granites

0.1

1

Rb

100

100

WPG

VAG

0

50

H

VAG+ syn-COLG

1.8

R1= 4Si - 11(Na + K) - 2(Fe + Ti)

500

WPG

1.4

CaO

1.0

K2O Na2O 5 10

0.8

1000

G syn-COLG

1.MH 2.MHe 3.Ye 4.TDr 5.TMd 6.TBm

0 0.6

R1 = 4Si - 11(Na + K) - 2(Fe + Ti)

2

3000

2000

1000

Syn-collision Postorogenic

Anorogenic

50 100

0

0.5 1

0

0

alkali granite

F

Postcollision Uplift Lateorogenic

100

Sm

20

Zr

1000

Hf

R = 6Ca + 2Mg + Al

Nb Ce

quartz monzonite

1000

100

0.1 0.01 Ta

D

syenite

C

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R = 6Ca + 2M g+ Al

Ba Th

Sample/REEchondrite

Sample/ Lower crust 100 1 10

10 Sample/ ORG 0.1 1

0.01

1.MH 2.MHe 3.Ye 4.TDr 5.TMd 6.TBm

K2O Rb

B

10

1000

100

A

0.5 1.0

5.0 10.0

Y Nb

50.0

1

5

10

50 100

500 1000

Zr Nb Ce Y

Fig. 2. Summaries of the geochemical characteristics of the wall rocks of the Red River Fault Zone. (A) ORG-normalized diagram, after Pearce et al. (1984). (B) Lower crust-normalized diagram, after Weaver and Tarney (1984). (C) Chondrite normalized diagram, after Sun et al. (1980). (D) R1–R2 classification plot, after de la Roche et al. (1980). (E) A/CNK–A/NK classification plot, after Shand (1943). (F) Geotectonic discrimination plot, after Batchelor and Bowden (1985). (G and H) Geotectonic discriminations plots, after Pearce et al. (1984). (I) Plot to discriminate between A-type (A1, A2) and non-A-type granites, after Eby (1992). (J) Plot to discriminate between A-type and other granite types, after Whalen et al. (1987). All plots were made using GCDkit program of Janouˇsek et al. (2005).

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126

Table 1 Geochemical characteristics of the zircon-sampled rocks. Element

Units

SiO2 Al2 O3 Fe2 O3 (T) MnO MgO CaO Na2 O K2 O TiO2 P2 O5 LOI Total Sc Be V Cr Co Ni Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo Ag In Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U

% % % % % % % % % % % % ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm

Detect limit 0.01 0.01 0.01 0.001 0.01 0.01 0.01 0.01 0.001 0.01 0.01 0.01 1 1 5 20 1 20 10 30 1 0.5 5 1 2 0.5 4 0.2 2 0.5 0.1 1 0.2 0.1 3 0.05 0.05 0.01 0.05 0.01 0.005 0.01 0.01 0.01 0.01 0.01 0.005 0.01 0.002 0.1 0.01 0.5 0.05 5 0.1 0.05 0.01

Sample MH

Sample MHe

Sample Ye

Sample TDr

Sample TMd

Sample TBm

73.86 11.62 2.6 0.028 0.14 0.3 4.42 5.16 0.172 <0.01 0.3 98.6 <1 8 <5 <20 91 <20 <10 180 33 2.4 <5 239 16 161 1134 217 <2 <0.5 0.1 13 <0.2 1.1 148 102 216 25.6 97.3 22 1.22 21.2 4.16 26.8 5.31 15.3 2.26 13.6 1.8 33.1 16.6 655 0.66 25 0.2 25.9 10.7

72.79 15.05 0.25 <0.001 0.03 0.19 4.55 5.36 0.046 0.02 0.83 99.12 <1 2 <5 <20 78 <20 <10 <30 26 2 <5 199 175 19.3 85 21.2 <2 <0.5 <0.1 <1 <0.2 2.1 745 26.6 6.36 5.38 19.8 4.22 0.788 4.15 0.58 3.14 0.61 1.7 0.246 1.52 0.245 3.2 1.36 493 0.95 34 <0.1 10.8 3.03

67.3 14.88 4.72 0.017 0.55 0.33 4.22 6.03 0.651 0.1 0.72 99.52 6 5 5 <20 89 <20 <10 <30 32 2.4 <5 191 160 86 1012 135 <2 <0.5 < 0.1 7 <0.2 3.3 1005 182 324 41.6 142 24.6 3.68 18.4 2.87 16.1 3.16 9.35 1.4 8.71 1.27 24.3 9.28 527 0.86 29 0.4 23.4 7.2

71.13 13.16 3.97 0.088 1.12 0.27 1.47 6.07 0.569 0.14 1.85 99.84 10 3 42 <20 57 <20 10 110 21 1.4 <5 285 58 52.1 295 13.8 <2 <0.5 <0.1 5 0.3 4.6 904 54.5 106 11.9 43.1 9.05 1.17 8.25 1.45 8.28 1.61 4.74 0.715 4.45 0.653 7.9 1.16 319 2 48 0.2 25.8 5.67

70.08 16.8 1.02 0.012 0.26 0.4 3.74 5.2 0.07 0.12 1.4 99.12 4 <1 <5 <20 88 <20 <10 40 20 3.8 <5 318 32 13 30 13.8 <2 <0.5 <0.1 10 <0.2 6.5 118 6.39 13 1.49 6.23 1.67 0.186 1.53 0.36 2.41 0.45 1.37 0.246 1.65 0.227 1.5 2.59 648 1.72 50 <0.1 5.27 3.12

74.8 13.98 0.52 0.004 0.4 2 5.18 1.58 0.098 0.04 1.1 99.72 <1 2 <5 <20 94 <20 <10 <30 16 1.7 <5 51 104 30.1 46 4.1 <2 <0.5 <0.1 <1 <0.2 1.2 236 56.5 106 11.8 40.1 7.84 0.904 6.5 1.09 5.71 1.01 2.91 0.457 2.8 0.401 2.1 0.55 613 0.22 28 <0.1 54.3 6.51

veins, there are orthogneisses, there is an extensive migmatitic gneiss suite in the Thác Bá area, and there is the Tam Ða o rhyolite complex (Fig. 1). All these units are probably linked to tectonothermal event(s) that controlled the evolution of the southern Cathaysia part of the South China Block (Lepvrier et al., 2011). Our study shows that the Song Chang granite is the twomica, high-K, peraluminous (A/CNK = 1.2), S-type, pre-collisional (R1–R2), matured granite (Ba–Rb–Sr plot of El Bouseily and El Sokkary, 1975). Other features include depletion in Ba, Nb, Ti, Sr; enrichment in K, Rb, U, Pb, Zr, Hf; moderate contents of REE = 126.1; negative Eu anomaly at Eu/Eu* = 0.37 and moderate fractionation of La/Yb = 7.16. Zonally the porphyritic granite protolith was sheared to form augen gneiss.

The Tam Ða o Massif, which emerges from beneath Triassic and Tertriary cover some 90 km N of Hanoi, is a mountain ridge with a granite core and more hypabyssal, rhyolitic rims, this latter being in part zonally deformed under brittle/ductile conditions. Rhyodacites were sampled south of Tam Ða o health resort and in the Xuan Hoa area and were found to be peraluminous (A/CNK = 1.4), syn-collisional to post-orogenic rocks (R1–R2) that fall in the field of matured granites on a Ba–Rb–Sr plot (El Bouseily and El Sokkary, 1975). Although they are remarkably diverse in terms of major element composition (Na2 O + K2 O = 2–10%, K2 O = 1.5–6.0, CaO = 0.1–10%), which ranges from calc-alkaline to ultrapotassic, other geochemical characteristics are quite similar: a negative trend on the CaO–SiO2 plot, K/Rb = 110–200, Rb/Sr = 0.1–1.5,

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Fig. 3. Hand specimens of rocks from the Fan Si Pan and Lo Gam complexes that were sampled for zircons. (A) Posen granite. (B) Muong Hum metagranite. (C) Yeyensun

granite. (D) Tam Ða o rhyodacite. (E) Than Ming Son diatexite. (F) Thac Ba migmatitic gneiss, which contains leucosome segregations.

moderate contents of REE (235–291 ppm), moderate fractionation of La/Yb (8.16–9.40), and a Eu/Eu* of 0.42–0.61. The Tam Ða o Massif rhyodacites are also characterized by having relatively low contents of Nb, Ba, Sr, Ti, Ta, P but higher contents of Rb, Hf, Zr, U–Th (Table 1, Figs. 2 and 3D). The Tam Ða o Massif is further mantled by polydeformed gneisses whose metamorphic evolution culminated in the development of metatexite/diatexite migmatites (Figs. 1 and 3E). A diatexitic neosome was sampled at Than Ming Son and this rock type showed some geochemical affinities with some of the leucocratic veins and nests (Figs. 1 and 3F) that had developed in the sheared gneisses of the Thác Bá region. The similarities include moderate fractionation of REEs; low Ba, Nb, La–Ce, Sr, P, Ti, but relatively high contents of Rb, U, K, Pb, Zr; a linear correlation of Zr and Ti; and a significant negative Eu anomaly (Table 1 and Fig. 2). Despite the remarkable petrographic differences between the Thác Bá gneiss and the Than Ming Son migmatites, both rocks classify as alkali granites and both show a similar range in their major elements (Na2 O + K2 O = 6.5–10%; SiO2 = 63–75%; CaO = 2.0–2.7%; K2 O = 1.5–6.8%; K2 O/Na2 O = 0.5; Rb/Sr = 0.5; K/Rb = 150–250).

3.2. Structural characteristics 3.2.1. SW wall of the Red River Fault Zone The Neoproterozoic Posen metagranite does not show significant ductile deformation, despite being the oldest rock unit in the Fan Si Pan complex. In the three studied sections of Ban Vuoc, Lao Cai, and Tang Loong, there are domains of constrictional strain with a shallowly plunging (up to 30◦ to the NW or SE) rodding lineation, typical of an L > S tectonite. These constrictional domains dominate over a subordinate, but zonally dominant, L = S fabric, and a steep, NNW-trending and ENE- or WSW-dipping foliation and subhorizontal lineation. These fabrics developed in a sinistral strike-slip regime that had an oblique component (Fig. 4A). In the Ban Vuoc section, an ∼250 m thick marginal part of the batholith has been variably converted to a gneiss that steeply dips to the

ENE and locally can be tens of metres thick. These gneisses occur in intensely foliated zones and are brought into tectonic contact with paragneisses and mica schists that were also sheared in the same regime. In the Lao Cai section, further south, the batholith’s margin shows only incipient mylonitization on steeply dipping ENE planes. More prevalent here is evidence of a ductile, constrictional, planar fabric that has been overprinted by brittle–ductile, cataclastic deformation along two sets of shear planes oriented WNW–ESE and NNE–SSW. Both sets of shear planes record dextral kinematics of a similar type that was observed in the Bac Queo area (Figs. 4A and 5A). The timing of these deformations is unclear. In contrast to the Posen metagranites, the Muong Hum metagranites developed a planar, LS tectonite fabric with only occasional L > S constriction. In the Muong Hum area, the foliation planes dip at low angles to the SE and NE and contain a stretching lineation that is marked by quartz ribbons and epidote aggregates elongated in the NW–SE direction (Fig. 4B). The Muong Hum metagranites developed in a tectonic regime that was characteristically different from that which controlled the deformation of the Posen metagranite. In the vicinity of the Deo Hoang Lian Son Pass, possible enclaves of Muong Hum metagranite occur in the unfoliated Yeyensun granite. These enclaves possess a distinct gneissic foliation, expressed by a parallal arrangement of biotite flakes and quartz ribbons, which along with a biotite lineation testify to ductile deformation of the Muong Hum granite before the entrapment by the Yeyensun granite. The foliation dips shallowly to the NW, whereas the biotite lineation plunges to the WNW, thus roughly in the same direction as the mineral lineation observed in the Muong Hum metagranite body. Unlike the Posen and Muong Hum granitoids of the Fan Si Pan complex, theYeyensun granite is generally unfoliated except for a few narrow shear zones.

3.2.2. NE wall of the Red River Fault Zone Granites of the Song Chay Massif have been partly transformed into an augen gneiss. In the eastern part of the massif, Roger et al.

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128 N

Muong Hum

Posen

A

N

Tam Dao

B

N

C

N

Thac Ba

D Fig. 4. Stereoplot digrams showing the orientations of tectonic elements in the wall rocks of the Red River Fault Zone. Key: solid great circles (foliation), dots (stretching lineation), arrows (directions of movements on a given plane), stars (fold axes). (A) Posen granite cataclasite: dotted lines represent zones of cataclastic shearing (see Fig. 5A).

(B) Muong Hum gneissic granite. (C) Tam Ða o rhyodacite: dotted lines represent zones of overprinted cataclastic shearing; dash-dot line represents thrust planes (top-to-the NE) and axial planes to superposed open folds. (D) Thac Ba: stars represent fold axes in mylonitic foliation, dashed lines represent axial planes. Equal angle projection, lower hemisphere (Stereo 32, copyright: Röller K. and Trepmann C.A., 2003–2007).

(2000) observed that the foliation gently dipping to the SE, with a lineation and ␴-clasts showing a similar sense of tectonic transport. Such kinematics are characteristic for rock units located to the NE of the Red River Fault Zone and derive from the Indosinian event (Lepvrier et al., 2011), which, according to Ar–Ar dating of white mica, has an age of 245 Ma and so represents a passage by the Song Chay Massif through the 300–350 ◦ C isotherm during the Early Triassic (Maluski et al., 1999). Our study shows that in the western part of the massif, near Xin Man, a well developed foliation dips gently N-wards and an associated stretching lineation plunges toward the NE. Numerous feldspar porphyroclasts indicate top-to-the-NE kinematics in a clearly extensional regime (Fig. 5C). Further SE and as part of the Lo Gam complex are found the multiply deformed Thác Bá gneisses (Fig. 1). These gneisses have foliation planes discordantly cut by a strongly sheared metagranite that resembles the Song Chay augen gneiss. The foliation planes of the Thác Bá gneisses dip SW/W-ward at high to moderate angles. Early tight to isoclinal folds with subhorizontal WNW–ESE axes were co-axially refolded by younger open folds. Later, or perhaps

concurrent with the open folding, the NW–SE trending foliation planes were taken over or reactivated by ductile shearing with dextral strike-slip motion and the related formation of vertical folds. Subordinate strike-slip shears with sinistral kinematics seem to have occurred in a complementary manner along the steep, N–S oriented planes (Fig. 4C). Most of the shearing and folding occurred prior to migmatization and to the intrusions of leucocratic veins, which have remained undeformed. However, neosome pockets that occur systematically along the shear zones suggest that the time difference may be small and that the melting might have been actually triggered by ductile, relatively high-temperature, shearing. The biotite–garnet gneisses that grade to metatexites and diatexites (Than Ming Son), and which occur adjacent to the Tam Ða o rhyolite massif were multiply deformed before they became affected by migmatization and partial melting. Variable foliation attitudes testify to the presence of older vertical folds as well as later ones with suborizontal axes (SW or ENE plunges). The vertical folds point to a strike-slip regime characterized by changeable kinematics of shearing: sinistral along the N–S oriented planes, and

Fig. 5. Deformation structures from wall rocks of the Red River Fault Zone. (A) Posen granite showing cataclastic zones; (B) Song Chay augen gneiss showing ␴-clasts and S–C structures (top-to-the NE normal movement). (C) Tam Dao rhyodacite that has been mylonitised (dextral sense of shear). Scale bar in all photos is 3.5 cm.

˙ A. Zela´ zniewicz et al. / Journal of Geodynamics 69 (2013) 122–139

dextral along the NW–SE planes, thus similar to those observed in the Thác Bá gneisses. Subvolcanic rocks of the Tam Ða o Massif locally have a NWtrending, weak magmatic foliation. In the SE tip of the massif (near the village of Xuan Hoa), the rhyodacites become distinctly deformed. They were turned into an L > S tectonite and zonally sheared, locally even down to an ultramylonite. The mylonitic fabric comprises subvertical foliation and subhorizontal lineation, both trending NW–SE. Quartz ribbons and ␴-type and ␦-type feldspar porphyroclasts have asymmetries showing dextral kinematics (Figs. 4C and 5B). Ultramylonites are locally seen that must have had a fluid-assisted development, as shown by the presence of quartz veins, and which suggests that temperature of the deformed rocks was still high enough to ensure dynamic recrystallization of the feldspars. In some places, the steeply dipping foliation was bent into open folds that are associated with the SW-dipping brittle–ductile cleavage; and there are signs of topto-the-NE thrusting (Fig. 4C). Later still, there was a cataclastic overprint that produced a dense network of fractures, one set of which cuts the subvertical foliation at low angles. 4. Zircon age data (U–Pb SHRIMP analyses) In order to get a good initial set of age data to help better constrain the complex history of the Red River Fault Zone we analysed zircons from rocks on both sides of the fault zone. From the SW side we analysed the Muong Hum metagranite and the Yeyensun granite; from the NE side we analysed the Tam Ða o rhyodacite, the Than Ming Son diatexite, and the Thác Bá gneiss. We did not use zircons to date the Posen metagranite as this granite had already been dated (Lan et al., 2000; Wang et al., 2008). Zircons from the relevant samples were retrieved by conventional crushing, sieveing, heavy-liquid and paramagnetic techniques, then hand picked under a microscope, mounted in epoxy resin and polished. Transmitted and reflected light photomicrographs and cathodoluminescence (CL) images were made in order to select grains and choose sites for analyses, omitting cracks and inclusions. The SHRIMP II at the Center of Isotopic Research (CIR) of the All-Russian Geological Research Institute (VSEGEI), St. Petersburg, was used to perform in situ U–Pb analyses following the procedure described in Williams (1998) and Larionov et al. (2004). The zircon U–Pb standard TEMORA (Black et al., 2003) with an accepted 206 Pb/238 U age of 416.75 ± 0.24 Ma was used. The collected results were processed with SQUID v. 1.12 (Ludwig, 2005a) and ISOPLOT/Ex 3.22 (Ludwig, 2005b) software, using the decay constants of Steiger and Jager (1977). The common lead correction was done using measured 204 Pb according to the model of Stacey and Kramers (1975). Some examples of the analyzed zircons are shown on Fig. 6, the analytical data are presented in Table 2 and plotted as an Ahrens–Wetherill U–Pb concordia diagram on Fig. 7. For the Muong Hum sample (MH; 22◦ 32.21 , 103◦ 43.64 ), eighteen analyses were made on 15 different zircon grains. These revealed two distinct age groups (Fig. 6A). An older, dominant, population with a mean age of 260 ± 3 Ma (Late Permian); and a younger population with a mean age of 50.2 ± 0.7/1.1 Ma (Early Eocene) at the lower end of the age spectrum. The Late Permian zircons are short prismatic subhedral crystals showing oscillatory zoning, usually face-parallel, or sector zoning. These zircons are up to 250 ␮m in maximum dimension and correspond to zircons of the S5 type and P1 type of Pupin (1980). This also correlates with zircons originating from alkaline to hyperalkaline granitic magmas that may have been derived from mantle melts and crystallized below 650 ◦ C (Pupin and Turco, 1972). Their U content is ∼200–600 ppm and they have a Th/U ratio of 0.44–0.73 (Table 2 and Fig. 6A). Some of the older zircon crystals became irregularly embayed later (grains 4 and 6).

129

The Muong Hum Early Eocene zircons are anhedral to subhedral, CL-dark crystals but which contain subordinate and irregular CLbright domains that form rims, deep embayments or belts across the crystals (Fig. 6A). Thin, CL-bright outgrowths develop around both the Permian and Eocene grains. The dark parts have moderate U (most are <200 ppm) but they are significantly richer in Th, which can lead to the Th/U ratio going up to 2.14. In the CL-bright parts, the U content is quite low (<100 ppm). In the younger zircons, both the CL-dark and bright parts belong to the same age cluster of ∼50 Ma (Table 2, Fig. 7A and B). In general, the irregular shapes of the CL-bright domains and the discrete mobility of U and Th that is unevenly distributed throughout the zircon grains seem to indicate that the Palaeogene zircons were likely formed via recrystallization of older zircons. The process may have proceeded under relatively ‘dry’ conditions without magmatic melt, which offers an explanation for the absence of oscillatory growth zoning. Some of the zircons became deformed and the strained portions were bleached with U-removal (Fig. 6A, grain 2); therefore, the recrystallization was associated with significant deformation of the host rock and accompanied by active fluids. Fluid activity may also explain the strongly corroded habit of some of the grains (grains 4 and 6). And the field evidence for the Muong Hum granites indicates that they were metamorphosed and changed to gneisses (Fig. 3B). Our data suggests that Muong Hum peralkaline granite intruded in the latest Permian and then yielded to deformation and metamorphism. In view of the above, the presence of two suggests that this event plausibly occurred in the Early Eocene. In one grain (MH, spot 8), however, the CLbright rim yielded a distinctly older age of ∼155 Ma, but this can be explained as an incomplete U–Pb systematics reset. In the unfoliated Yeyensun granite, there are occasional enclaves of older gneisses that resemble the Muong Hum metagranite. A sample from one of the enclaves (MHe; 22◦ 21.15 , 103◦ 46.35 ) contained zircons similar to the Permian zircons from sample MH above. They are short prismatic crystals of the S5 type of Pupin (1980). Most are CL-dark and only show faint sector zoning. Some crystals show distinct subhedral to anhedral, equant to prismatic, inner cores (Fig. 6B). Their internal growth structure is a faint broad sector zoning or oscillatory zoning, suggestive of magmatic crystallization. The U-content of the Yeyensun zircons is generally < 600 ppm; Th/U ratios vary from 0.53 to 0.86 (Table 2). The different thickness of outgrowths are zoned in similar fashion. Both the cores and the outgrowths that were analysed belong to the same age cluster of concordant analyses, namely 263.7 ± 1.5 Ma (Fig. 7C). Notably, a dated gneiss enclave from within the Yeyensun granite is, within error limits, of the same age as the Muong Hum metagranite. For the Yeyensun granite itself (sample Ye, 22◦ 21.15 , 103◦ 46.35 ), three types of zircons were distinguished. First, complex grains with sector- or broad oscillatory zoned cores that were distinctly discordant to the face-parallel zoned overgrowths (Fig. 6D: grain 8). Second, grains with similar cores to the first group but with thick, CL-mottled, cauliflower-like rims (grains 11 and 7). Third, grains having CL-mottled or CL-dark inner parts but with brighter outer parts showing weak sector zoning (Fig. 6D: grains 6 and 9). In general, the zircons are mostly short, prismatic and subhedral; euhedral or anhedral grains are more unusual. The Yeyensun granite zircons are more complex than the Permian zircons from the gneiss enclaves. The types 1 and 2 cores in three of the analysed grains (grains 7, 9 and 11) have 206 Pb/238 U ages between ∼264Ma and 242 Ma and Th/U ratios of ∼0.5–0.7 (Table 2, Figs. 6D and 7D). The ‘old’ ages and the structures of the grains suggest that the cores most likely came from the entrapped, and almost entirely digested, Muong Hum metagranite. The type 3 grains and their patchy outgrowths have a high-U content and a Th/U of 0.15–1.93, and they yielded two groups of concordant ages: 41.4 ± 0.6 Ma (e.g. core in grain 6) and 34.1 ± 0.6 Ma (e.g. rim

130

Table 2 Ages (determined by the U–Pb SHRIMP method) of igneous and metamorphic rocks from the Fan Si Pan and Lo Gam complexes. Sample. grain. spot

U (ppm)

Th (ppm)

Th/U

206

Pb* (ppm)

206

Pbc %

Total 238

Radiogenic 206

U/

Pb

±

207

206

Pb/

Pb

±

206

238

Pb/

U

Age (Ma) ±

206

Pb/238 U

±

0.0773 0.0606 0.0495 0.0589 0.0515 0.0550 0.0527 0.0585 0.0557 0.0574 0.0533 0.0524 0.0521 0.0548 0.0517 0.1052 0.0501 0.5876

0.0059 0.0030 0.0014 0.0024 0.0013 0.0015 0.0005 0.0014 0.0007 0.0015 0.0005 0.0007 0.0005 0.0013 0.0005 0.0116 0.0014 0.0313

0.0081 0.0072 0.0078 0.0074 0.0079 0.0399 0.0407 0.0400 0.0407 0.0243 0.0403 0.0411 0.0410 0.0398 0.0412 0.0056 0.0079 0.0079

0.0002 0.0001 0.0001 0.0001 0.0001 0.0005 0.0004 0.0006 0.0005 0.0003 0.0004 0.0005 0.0004 0.0005 0.0004 0.0002 0.0001 0.0023

1322 1099 0.86 47.5 0.01 MHe.1.1 564 287 0.53 20.6 – 2.1 413 315 0.79 14.8 0.03 3.1 137 97 0.73 5.52 8.39 4.1 97 52 0.55 3.49 0.96 4.2 5.1 116 63 0.56 4.35 0.46 135 25 0.19 2.28 3.93 6.1 81 47 0.60 2.88 1.19 6.2 121 73 0.62 4.26 – 6.3 169 120 0.73 5.67 0.64 7.1 Errors are 1-␴. Error in Standard calibration was 0.18%. Common Pb corrected using the measured 204 Pb

23.908 23.53 23.9 21.32 23.93 23.02 50.8 24.33 24.42 25.57

0.35 0.44 0.49 0.81 1.7 1.6 2.1 1.8 1.6 1.6

0.0508 0.05225 0.05372 0.1132 0.0583 0.0549 0.0523 0.0559 0.0493 0.0527

0.99 1.4 1.5 1.8 5.2 5 7.5 5.9 6.5 4.5

0.04181 0.04248 0.04181 0.04311 0.04155 0.04319 0.01949 0.0411 0.04119 0.03911

0.36 0.45 0.5 1.4 1.8 2 2.2 1.8 1.6 1.6

305 76 0.26 1.42 3.42 Ye.1.1 10606 3456 0.34 59.2 0.33 2.1 3.2 2823 5284 1.93 15.4 0.45 3.1 466 69 0.15 2.11 0.01 401 356 0.92 2.01 7.58 6.1 6.2 223 79 0.37 1.09 – 529 373 0.73 18.9 – 7.1 8.1 456 363 0.82 2.15 2.16 771 2399 3.22 25.7 1.40 9.1 739 154 0.22 3.93 – 10.1 198 103 0.54 6.96 1.02 11.1 244 45 0.19 2.79 3.27 11.2 Errors are 1-␴. Error in Standard calibration was 0.44%. Common Pb corrected using the measured 204 Pb

184.6 153.88 157.3 190.3 171.7 175.3 23.98 182.4 25.75 161.6 24.51 75.2

2.7 0.42 0.73 2.3 1.8 3.0 0.83 1.9 0.77 1.4 1.3 1.8

0.0610 0.04954 0.0561 0.0522 0.0601 0.0538 0.0501 0.0538 0.0661 0.0497 0.0515 0.0593

0.00523 0.006477 0.006328 0.00525 0.00538 0.00611 0.04177 0.00536 0.03829 0.00635 0.04039 0.01287

3.9 0.44 0.81 2.9 2.7 4.4 0.83 2.6 0.83 1.9 1.3 2.2

33.6 41.62 40.66 33.78 34.61 39.3 263.8 34.49 242.2 40.80 255.3 82.4

1.3 0.18 0.33 0.96 0.95 1.7 2.2 0.88 2.0 0.77 3.3 1.8

25.44 26.45 25.76

0.75 1.5 1.2

0.0539 0.0562 0.0536

0.03931 0.03781 0.03882

0.75 1.5 1.2

248.5 239.2 245.5

1.8 3.5 2.8

TDr.1.1 2.1 3.1

709 162 268

119 60 103

0.17 0.38 0.40

24.0 5.29 8.91

0.14 0.33 –

10 1.5 6.0 8.1 11 12 2.7 7.6 2.2 5.9 4.3 6.4 2.3 4.7 3.8

52.1 46.3 50.1 47.3 50.5 252.3 257.3 252.8 257.4 154.7 254.9 259.6 259.0 251.8 260.5 36.2 50.5 50.4

1.5 0.8 0.7 0.8 0.6 3.4 2.8 3.4 2.9 2.1 2.7 2.8 2.7 2.9 2.7 1.4 0.6 15.0

264.02 268.2 264 272.1 262.4 272.6 124.4 259.6 260.2 247.3

0.92 1.2 1.3 3.6 4.6 5.3 2.7 4.7 4.2 3.8

˙ A. Zela´ zniewicz et al. / Journal of Geodynamics 69 (2013) 122–139

MH.1.1 48 12 0.25 0.4 3.82 118.62 3.18 2.1 158 169 1.07 1.0 1.73 136.28 2.37 580 798 1.37 3.9 0.31 127.69 1.85 2.2 3.1 202 322 1.59 1.3 1.50 133.65 2.12 3.2 539 1151 2.14 3.7 0.56 126.45 1.59 4.1 118 52 0.44 4.1 0.47 24.93 0.33 631 310 0.49 22.1 0.16 24.52 0.27 5.1 110 49 0.45 3.8 0.91 24.77 0.33 6.1 387 204 0.53 13.6 0.54 24.41 0.27 7.1 8.1 143 55 0.39 3.0 1.04 40.75 0.56 9.1 675 493 0.73 23.4 0.25 24.73 0.26 502 277 0.55 17.8 0.12 24.31 0.27 10.1 11.1 695 450 0.65 24.5 0.09 24.37 0.26 1.1 387 212 0.55 13.3 0.45 24.99 0.29 13.1 735 509 0.69 26.1 0.04 24.24 0.26 41 19 0.45 0.2 7.39 164.49 5.39 14.1 15.1 771 1391 1.80 5.2 0.39 126.68 1.52 83 53 0.65 1.8 68.21 40.46 0.69 15.2 Errors are 1-␴. Error in standard calibration was 0.52%. Common Pb corrected using the measured 238 U/206 Pb and 207 Pb/206 Pb ratios

Table 2 (Continued) Sample. grain. spot

U (ppm)

Th (ppm)

Th/U

206

Pb* (ppm)

206

Pbc %

Total 238

Radiogenic 206

U/

Pb

±

207

206

Pb/

Pb

±

206

238

Pb/

U

Age (Ma) ±

206

248.1 247.9 248.2 655.1 475.2 1938.9 2364 2119 508.4

Pb/238 U

±

3.2 1351 1553 1.19 45.6 0.17 4.1 343 113 0.34 11.5 0.00 5.1 501 106 0.22 16.9 – 1108 5.2 137 0.13 102 0.08 6.1 309 75 0.25 20.3 0.29 7.1 1180 83 0.07 356 0.01 8.1 271 115 0.44 104 0.42 9.1 272 113 0.43 91.1 0.05 10.1 308 89 0.30 21.7 – Errors are 1-␴. Error in Standard calibration was 0.44%. Common Pb corrected using the measured 204 Pb

25.49 25.50 25.48 9.348 13.07 2.850 2.258 2.570 12.19

0.54 1.5 0.84 0.68 0.93 0.42 0.93 0.79 1.0

0.05160 0.0515 0.0511 0.0649 0.0675 0.11941 0.1388 0.1377 0.0853

1.7 3.3 2.7 5.0 2.2 0.47 0.72 0.77 4.0

0.03923 0.03921 0.03925 0.10697 0.07650 0.3509 0.4429 0.3891 0.08206

0.54 1.5 0.84 0.68 0.93 0.42 0.93 0.79 1.0

TMd.1.1 843 91 0.11 47.5 1.04 1.2 84 31 0.38 9.1 0.13 10.1 3822 19 0.01 224 0.01 11.1 4552 22 0.01 155 0.01 3415 12.1 24 0.01 111 0.01 12.2 3298 41 0.01 161 – 13.1 2430 19 0.01 118 0.10 2598 14.1 33 0.01 129 0.07 88 103 1.21 9.66 – 2.1 1070 11 0.01 51 2.2 0.02 3.1 39 32 0.86 4.2 1.23 1332 147 0.11 87.6 0.06 4.1 5.1 2779 589 0.22 175 0.01 469 35 6.1 0.08 25.7 0.08 6.2 1874 469 0.26 116 0.06 542 57 7.1 0.11 32.3 0.13 8.1 963 52 0.06 60.5 – 8.2 573 37 0.07 35 0.24 5968 18 0.00 214 0.07 9.1 9.2 2637 48 0.02 95.8 0.13 Errors are 1-␴. Error in Standard calibration was 0.18%. Common Pb corrected using the measured 204 Pb

15.243 7.97 14.659 25.264 26.407 17.553 17.762 17.244 7.82 18.007 7.92 13.058 13.655 15.72 13.832 14.401 13.672 14.055 23.936 23.65

0.57 1.5 0.3 0.32 0.35 0.38 0.43 0.41 1.4 0.53 2.2 0.54 0.4 2 0.39 0.66 0.52 0.64 0.29 0.43

0.0589 0.0711 0.05575 0.05148 0.05113 0.05465 0.05465 0.05477 0.0633 0.05375 0.0708 0.05609 0.05612 0.0567 0.05545 0.05661 0.0561 0.05902 0.05123 0.0562

1.4 3.1 0.68 0.84 0.99 1.1 0.95 0.89 3 1.5 4.5 1.1 0.8 1.9 0.94 1.8 1.3 1.7 0.72 2.3

0.06537 0.1247 0.06821 0.03958 0.03787 0.05693 0.05628 0.05796 0.1281 0.05546 0.1256 0.07664 0.07321 0.0635 0.07231 0.06938 0.07311 0.07094 0.04178 0.04207

0.58 1.5 0.31 0.32 0.35 0.38 0.43 0.41 1.4 0.54 2.2 0.54 0.4 2 0.39 0.66 0.52 0.66 0.29 0.44

408.2 757 425.3 250.2 239.62 357 353 363.2 777 348 763 476 455.4 396.8 450.1 432.4 454.8 441.8 263.86 265.7

2.3 11 1.3 0.79 0.82 1.3 1.5 1.5 10 1.8 16 2.5 1.8 7.7 1.7 2.8 2.3 2.8 0.74 1.1

TBm.6.1.1 4223 516 0.13 148 – 1.2 586 621 1.10 85.1 0.04 8377 1944 0.24 295 – 2.1 329 50 0.16 18.4 – 2.2 1288 41 3.1 0.03 62.8 0.19 4.1 6137 1111 0.19 223 – 405 177 4.2 0.45 58.2 0.00 5.1 3214 114 0.04 114 – 5.2 429 282 0.68 27.1 0.44 7162 863 0.12 260 – 6.1 102 57 0.58 8.59 0.49 6.2 6782 7.1 1386 0.21 247 0.00 7.2 1230 62 0.05 146 0.05 8.1 2491 144 0.06 88.0 – 8.2 407 157 0.40 209 0.02 Errors are 1-␴. Error in Standard calibration was 0.49%. Common Pb corrected using measured 204 Pb.

24.443 5.911 24.392 15.34 17.61 23.672 5.979 24.21 13.62 23.697 10.18 23.629 7.230 24.31 1.670

0.38 0.70 0.38 1.0 0.65 0.33 0.82 0.42 0.90 0.32 1.7 0.34 0.53 0.46 0.73

0.05022 0.07446 0.05129 0.0547 0.05418 0.05138 0.0707 0.05325 0.0588 0.05095 0.0591 0.05134 0.06714 0.05062 0.2854

1.1 1.2 0.83 2.9 1.8 0.87 1.8 1.2 3.3 0.81 4.1 0.88 0.97 1.4 0.79

0.04092 0.1691 0.04101 0.06527 0.05667 0.04225 0.1672 0.04130 0.07309 0.04221 0.0977 0.04232 0.13825 0.04115 0.5988

0.39 0.71 0.38 1.0 0.66 0.33 0.82 0.42 0.90 0.32 1.7 0.34 0.53 0.46 0.73

258.56 1007.2 259.07 407.6 355.3 266.79 996.9 260.9 454.7 266.51 601.0 267.20 834.8 260.0 3025

0.98 6.6 0.97 4.1 2.3 0.87 7.6 1.1 4.0 0.83 9.9 0.89 4.1 1.2 18

1.3 3.6 2.1 4.2 4.3 7.0 18 14 5.1

˙ A. Zela´ zniewicz et al. / Journal of Geodynamics 69 (2013) 122–139

In all cases Pbc and Pb* indicate the common and radiogenic portions, respectively.

131

132

˙ A. Zela´ zniewicz et al. / Journal of Geodynamics 69 (2013) 122–139

Fig. 6. Some cathodoluminescence images of zircons that were dated using the U–Pb SHRIMP method. (A) MH (Muong Hum metagranite). (B) MHe (Muong Hum enclave).

(C) Ye (Yeyensun granite). (D) 4TDr (Tam Ða o rhyodacite). (E) TMd (Tam Ðao diatexite). (F) TBm (Thac Ba migmatitic leucosome). Scale bars given in each photo.

in grain 6). Taking into account the ages and textures shown by grains 6 and 8, the younger age (Early Oligocene) is taken as the time of final emplacement of the Yeyensun granite. In one grain (11), a weakly sector-zoned core gave an age of 255 Ma, and this core had a markedly discordant boundary against the corona-like rim that itself showed a secondary patchy texture resembling the effects of metamorphic recrystallization (Corfu et al., 2003; Hoskin and Schaltegger, 2003). The overgrowth to the core in grain 11 gave an age of 82.4 Ma, a date which hints either at some late Cretaceous event or to an incomplete U–Pb reset. Such highly irregular internal patchy texture may indicate variable degrees of recrystallization (Bulle et al., 2010) or it can be attributed to secondary requilibration caused by self-irradiation and diffusion–reaction processes (Geisler et al., 2007). In contrast, the zircons from the units NE of the Red River Fault Zone and the Dai Nui Con Voi Massif are remarkably different. In

the Tam Ða o rhyodacite (sample TDr; 21◦ 17.45 , 105◦ 43.70 ), the zircons are bigger, 200–300 ␮m long, euhedral, have a 1:2–1:3.5 aspect ratio and are commonly oscillatory zoned. The rhyodacite zircons are mainly P3–P4 type grains of Pupin (1980), indicative of having alkaline granitoids as hosts (Fig. 6C). Fifty-percent of the analysed zircons (e.g. grains 3 and 4) had U-contents of 162–1351 ppm, Th/U ratios of 0.17–1.19, and yielded a concordant age of 248 ± 2 Ma (Table 2, Figs. 6C and 7E). This age is interpreted as the intrusive age of the rhyodacite. The other 50% of the analysed zircons (e.g. grains 5, 6) appear to be mostly xenocrystic zircons (grains/cores) derived from Palaeoproterozoic (2.0–2.3 Ga) and Neoproterozoic (655 Ma) sources with a few weakly defined Cambro-Ordovician inherited components shown by discordant analyses. The xenocrysts have both ‘magmatic’ and ‘metamorphic’ Th/U ratios (0.07–0.44), suggestive of diverse protoliths subjected to crustal melting during Early Triassic magmatism.

˙ A. Zela´ zniewicz et al. / Journal of Geodynamics 69 (2013) 122–139 0,070

sample MH

0,066

Concordia age = 260 ±3 Ma

0,064

N = 4, MSWD = 7.6, Probability = 0.006

B

0,12

0,062 0,060 0,058

207

207

MH

0,14

A Pb/206Pb

Pb/206Pb

0,068

133

0,056

N=4, Intercept 50.19 +0.72/-1.1 Ma MSWD = 0.059

0,10

0,08

0,054

0,06

0,052 270 0,050 0,048

266

262

258

254

250

246

200

data-point error ellipses are 2 σ

data-point error ellipses are 2 σ

0,04

23,2

23,6

24,0

24,4

24,8

25,2

25,6

0

26,0

40

0,08

sample Mhe

C

0,16

0,07

260

220

180

Pb/206Pb

Pb/206Pb

0,06

140

Concordia age 263.7±1.5 Ma

0,04

207

207

Concordia age 41.4±0.6 Ma

Ye

200

N = 5, MSWD = 0.35 Probability = 0.55

D

N = 4, MSWD = 0.59 Probability = 0.44

0,14

340 300

160

U/206Pb

U/206Pb

0,05

120

80

238

238

0,12

Concordia age 34.1±1.2 Ma

0,10

N = 3, MSWD = 0.21 Probability = 0.64

0,08 0,06 200

0,04

0,03 0,02 data-point error ellipses are 2σ

data-point error ellipses are 2 σ

0,00

0,02 15

25

35

45

55

0

40

80

238

U/206Pb

U/

sample TDr

0,14

E

0,068

200

240

Pb

F

Concordia age 248±2 Ma N = 6, MSWD = 0.40 Probability = 0.53

Pb/206Pb

0,060 0,10

0,056

1400

Concordia age 248±2 Ma

0,08

207

Pb/206Pb

160

206

sample TDr

0,064

0,12

207

120

238

N = 6, MSWD = 0.40 Probability = 0.53

1000

0,06

0,052

260

250

240

230

0,048

600

data-point error ellipses are 2σ

200

0,044

data-point error ellipses are 2σ

0,04 0

10

20

238

U/206Pb

30

24

25

26

27

28

238

U/206Pb

Fig. 7. Tera–Wasserburg plots for zircons analysed in this study. (A) MH (Muong Hum older generation). (B) MH, (Muong Hum younger generation). (C) MHe (Muong

Hum enclave). (D) Ye (Yeyensun granite). (E) 4TDr (Tam Dao rhyodacite). (F) Than Ming Son diatexite. (G) Frequency distribution plot for the Tam Ða o diatexite (TMd). (H) Tera–Wasserburg plot for all analysed zircons from the Thac Ba leucosome. (I) Tera–Wasserburg plot for the younger zircons from the Thac Ba migmatite leucosome (TBm).

˙ A. Zela´ zniewicz et al. / Journal of Geodynamics 69 (2013) 122–139

134

sample TMd Concordia age 1120

767±13 Ma

3

Pb/206Pb

N = 3, MSWD = 0.11 Probability = 0.74

Concordia age 453.1±2.2 Ma

960

N = 3, MSWD = 0.72 Probability = 0.40

0,068

sample TMd

H Bins are 5 m.y.

2

Relative probability

G

Number

0,078

Concordia age 351.1±2.3 Ma

800

207

N = 2, MSWD = 0.14 Probability = 0.24

Concordia age 264.4±1.3 Ma

640

1

N = 2, MSWD = 0.31 Probability = 0.58

0,058

480

0 220

320

260

300

340

380

420

460

500

540

580

620

660

700

740

780

206 0,048

Pb/ 238U, Ma

data-point error ellipses are 2 σ

2

6

10

238

14

18

U/206Pb

22

26

30

0,28

0,062

sample TBm

sample TBm

I

0,24

J

0,060

Concordia age 260±1 Ma 0,058

Pb/206Pb

0,056

207

0,16

207

540

Pb/206Pb

0,20

Concordia age 1003±10 Ma

0,12

Concordia age 267±1 Ma

500

N = 3, MSWD = 0.76 Probability = 0.38

460 420

0,054

380 340

N = 2, MSWD = 0.11 Probability = 0.74

0,052

300

1800

0,08

260

1400

0,050 data-point error ellipses are 2σ

1000

0,04

600 0

N = 4, MSWD = 0.30 Probability = 0.58

4

8

data-point error ellipses are 2σ

238

12

16

U/206Pb

20

24

28

0,048 10

14

238

18

U/ 206Pb

22

26

Fig. 7. (continued)

The zircons retrieved from the Than Ming Son diatexite (sample TMd; 21◦ 28.52 , 105◦ 26.38 ) are far more diverse and complexely structured than those in the rhyodacite and can be divided into four types. Type I zircons: long prismatic, euhdral crystals with rather uniform cores, often with oscillatory zoned rims and distinct overgrowths that show a CL-mottled, cauliflower-like texture (Fig. 6E: grains 7 and 14). Type II zircons: cores have a patchy texture, and high-U outgrowths appear uniformly dark and unzoned (Fig. 6 E: grain 9). The zircons of types I and II have high U-contents, sometimes in excess of 600 ppm, but very low Th contents, and this results in low ‘metamorphic’ Th/U ratios between 0.01 and 0.11 (Table 2). Type III zircons: apparently unaltered, transparent crystals that are structured in a core–rim manner, with cores usually displaying sector zoning and overgrowths that show face-parallel, dense, oscillatory zoning suggestive of crystallization from a melt (Fig. 6E: grain 5). Type III zircons have a very variable U content (540 up to 3300 ppm) and have Th/U ratios in the range of 0.01–0.26, which is often taken as an indicator of metamorphic origin (Hoskin and Schaltegger, 2003). Type IV zircons: subhedral/euhedral with

sector zoned cores that are truncated and/or embayed by unzoned, CL-dark domains (Fig. 6E: grain 3) or patchy texture (grain 1). All the types I–IV zircons have U-contents <100 ppm and Th/U ratios of 0.38–1.21, suggestive of a magmatic origin. Somewhat unexpectedly, the SHRIMP U–Pb analyses fell into four age groups that coincided with the four zircon types identified using cathodoluminescence (Table 2, Figs. 6E and 7G–H). Zircon type IV cores yielded a concordant age of 767 ± 13 Ma. The relatively common type III zircons yielded 238 U/206 Pb ages between ∼475 and ∼400 Ma. A concordant age of 453.1 ± 2.2 Ma was computed for 3 type III grains that are either uniform crystals or that have oscillatory zoned overgrowths developed around Neoproterozoic cores and signs of magmatic resorption. Zircons of types I and II yielded 238 U/206 Pb ages that form age clusters around either the Devonian–Early Carboniferous or Late Permian–Early Triassic. Ages of 363–348 Ma with a concordant age of 351.1 ± 2.3 Ma were obtained exclusively from the re-textured parts of zircons, irrespective of whether these parts occured in cores or in rims on the older grains (Figs. 6E: grains 14 and 2, and 7G). These secondary zircon

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textures suggests periods of variable metamorphic recrystallization. In the Permian–Triassic cluster of 265–240 Ma, a concordant age of 264.4 ± 1.3 Ma was computed for zircons with cauliflowerlike textured cores and CL-dark uniform overgrowths, which had developed on both Permian and Carboniferous, or even older, cores (Fig. 6E: grain 9). Combining the cathodoluminescene images with the age data, one may infer that the domains of patchy texture are associated with a Carboniferous episode, whereas dark rims are associated with a Permian one. Given that the host rock is a diatexite, the two youngest episodes probably indicate metamorphic recrystallization. The leucocratic neosome veins that dissect the sheared and mylonitized Thác Bá gneisses (sample TBm; 21◦ 44.85 , 105◦ 02.58 ) also contain a composite zircon population. Individual grains are 150–250 ␮m long, mostly euhedral, with aspect rations of 1:2–1:2.5. In their complex internal structure, conspicuous cores are either oscillatory or sector zoned. In contrast to the zircons from the diatexite above, patchy textures are rare in these gneisses. Most grains have thick overgrowths that are either oscillatory zoned (with no continuity between the core and rim) or are CL-dark owing, to a high U-content (Table 2, Figs. 6F and 7I–J), and almost unzoned. A distinct cluster of late Permian ages gave two concordant groups at ∼267 Ma and 260 Ma, with a weighted mean of 262.9 Ma. These ages invariably come from CL-dark, unzoned or poorly zoned, overgrowths with high U-contents (1230–8377 ppm) and low, ‘metamorphic’, Th/U ratios of 0.03–0.21. The older, and distinct, cores display oscillatory or sector zonation with no continuation across to the rims. The cores, with U-contents of 102–586 ppm and Th/U ratios of 0.16–1.10 (‘magmatic’), yielded highly scattered concordant ages spread between the Carboniferous, down to the Neoproterozoic, and included a genuine Archaean age of 3.02 Ga (Fig. 6F: grain 8). Such inherited components are compatible with those found in the Than Ming Son migmatite. 5. Discussion 5.1. The SW wall In the SW wall of the Red River Fault Zone, the Fan Si Pan complex forms the SE tip of the Ailao Shan Massif. It follows from our study that the latter is a polygenic unit which comprises Neoproterozoic (Posen), upper Permian (Muong Hum) and Eocene (Yeyensun) granites, with narrow belts of Neoproterozoic–(?)Lower Palaeozoic metasediments. The unfoliated to poorly foliated Posen granite and the unfoliated Yeyensun granite clearly indicate that the southeasternmost portion of the Ailao Shan Massif has remained beyond the main course of the Ailao Shan–Red River Shear Zone. Thermal evolution of this part of the massif was terminated in the Eocene with the intrusion of the Yeyensun granite at ∼35 Ma, which was followed by fast uplift and cooling (Viola and Anczkiewicz, 2008), thus significantly before the termination of ductile deformation in the shear zone itself. Similar result was recently obtained for the Posen granite (Wang et al., 2011). 5.1.1. Permian peralkaline, A-type granite intrusion and Palaeogene shearing Zircons from the Muong Hum metagranite (sample MH) showed zoning styles and a Pupin typology that indicates crystallization from a melt. Thus, the concordant mean age of 260 ± 3 Ma is interpreted as the intrusion age of the original peralkaline, A-type granite magma. Enclaves of the Muong Hum metagranite, which were entrapped by the Eocene Yeyensun granite, contain similar zircons, which have yielded a mean age of 263.7 ± 1.5 Ma. Zircons of ∼260 Ma were also found as an inherited component in the

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Yeyensun granite. Although the three Muong Hum and Yeyensun samples represent very different settings, the data point to Late Permian peralkaline, I/S-type and A-type, intraplate magmatism in the Fan Si Pan complex. An augen orthogneiss from the Van Ban area in the Fan Si Pan complex (Fig. 2) was dated at 240 Ma (U–Pb zircon; Lan et al., 2000), thus the granite magmatism may have continued into the Early Triassic. As no inherited zircon was found in the Permian granites of this study, we speculate that Neoproterozoic basement contributed little to their formation and that they may have developed from a juvenile magma derived from sublithospheric mantle. The peralkaline to peraluminous, I/S-type geochemistry of the Permian granites suggests a possible mixing between lower crustal rocks and underplated basaltic magma, though arc accretion cannot be excluded. The extensive presence of granites that intruded into an active margin setting between 270 and 247 Ma was reported by Tran et al. (2008b) from the Truong Son fold belt further south, in the eastern part of the Indochina Block, as a response to Palaeotethys subduction, which terminated with the Indosinian orogeny around 240 Ma. A similar situation was recognized further NW in the Jinshajing–Ailao Shan suture zone, where crustal fragments that had rifted off South China during the Devonian were re-accreted during the Triassic as part of the Indosinian orogeny (Tran et al., 2008a). The evidence from this paper when combined with the published evidence to date suggests that the Late Permian–Early Triassic magmatism, now recognized in the Fan Si Pan Massif, may have been connected with intraplate extension in a back-arc setting probably combined with complex melting of the subducted crust. In the Muong Hum sample, the younger zircon population that occurs both as overgrowths and as new grains yielded a mean age of ∼50 Ma, and the very zircon population also showed deformation features (Fig. 6A: grain 2). We propose to tentatively interpret this Early Eocene age as that of metamorphic recrystallization and deformation and the time when the Muong Hum granites were converted to gneisses. The gneissification could not be later than the Eocene because the Muong Hum metagranite appears as enclaves in the ∼35 Ma unfoliated Yeyensun granite. Several small bodies of the Permian Muong Hum metagranite were observed in the field (Fig. 1) and the outcrop pattern suggests that this was once a larger igneous body that suffered shearing, large-scale boudinage and disruption either during the ∼50 Ma tectonometamorphic event or during the intrusion of the ∼35 Ma Yeyensun granite at the latest. At the time of deformation, the relatively finer grained Muong Hum granite must have been rheologically weaker than the much coarser Posen granite, thus strain was mainly localized in the former and in the metasediments. An alternative option is that the deformation occurred already in the Triassic, during the Indosinian events, but this would be inconsistent with the above zircon data. Summarizing all our observations, we suggest that the Fan Si Pan Massif was affected in the Eocene by the transpressional, left-lateral shearing which was localized in the Muong Hum granite and in the adjacent metasediments (Fig. 2). 5.1.2. Eocene peraluminous A-type magmatism and termination of ductile shearing in the SE tip of the Ailo Shan Massif: the Yeyensun granite Our new data for the Muong Hum metagranite suggest that the Eocene peraluminous magmatism in the Fan Si Pan Massif may have overlapped with, or followed, the regional metamorphism and the left-lateral wrench dominated thrusting. The Eocene magmatism apparently coincided with partial melting at deeper crustal levels from that enclaves may have been derived. The geochemistry of the A-type Yeyensun granite may reflect an inheritance from an older peralkaline/peraluminous granitic crust (Muong Hum type) and/or sourcing from LREE- and LILE-enriched sublithospheric mantle (Zhang and Schärer, 1999). However, Tran et al. (2002) reported

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a more complex compositional range for the Yeyensun pluton, one that comprises not only an A-type, biotite granite but also a high-K, calc-alkaline, I-type granite. The geochemical similarities between the Permian and Palaeogene A-type granites indicate that partial remelting of the lower crust in an extensional setting seems plausible. The zircon populations found in the unfoliated Yeyensun granite, and most notably these zircon’s core–rim relationships, point to two magmatic pulses having occurred, the older of which took place around 41 Ma and the younger at around 35 Ma. The younger pulse is taken as the age of final granite emplacement that also constrains the upper time limit of penetrative ductile deformation in the Fan Si Pan Massif and, thus, in the Vietnamese part of the Ailao Shan Massif. Previous estimates, based on a variety of techniques (Gilley et al., 2003; Leloup et al., 2001; Viola and Anczkiewicz, 2008; Zhang and Schärer, 1999), of the timing of the thermal events that affected the Yeyensun granite agree with the data in this paper. Leloup et al. (2001) proposed that the granite was produced by heat provided by left-lateral shearing in the adjacent Red River Fault Zone; Viola and Anczkiewicz (2008) pointed to the exhumation of the granite before mylonitization occurred in this zone. The planar fabric observed in the Van Ban augen orthogneiss may also have developed during the tectonothermal event(s) of the Eocene. In summary, the Fan Si Pan Massif was apparently tectonothermally quiet between the late Neoprotorozoic and the Late Permian. The new U–Pb zircon data highlights two Phanerozoic magmatic episodes in this region, one during the Late Permian–Early Triassic and a second during the Eocene. The Eocene magmatic episode was preceded by a tectonometamorphic event at around 50 Ma that included a transpressional regime: a thrust regime with a sinistral oblique component that may have been related to the left-lateral movement on the Ailao Shan–Red River Shear Zone. 5.2. The NE wall 5.2.1. Silurian, high-K, peraluminous granite intrusion and its Triassic deformation The high-K, peraluminous Song Chay granite has been dated by U–Pb zircon at 428–424 Ma (Carter et al., 2001; Roger et al., 2000) and represents Silurian magmatism of a type that has also been reported from elsewhere in the South China Block (Charvet et al., 2010). Russian geologists recognized four “series” among the Song Chay granitoids (sodium, K-sodium, potassium, and highpotassium) of which the oldest is Ordovician (Tran et al., 2008a). The massif within which the granitoids occur has been interpreted as a metamorphic core complex (Jolivet et al., 2001) containing a flatlying fabric that probably developed at 245 Ma (Maluski et al., 1999) during the N-ward thrusting that was related to the Indosinian orogeny (Lepvrier et al., 2011; Roger et al., 2000, 2012). Our structural observations in the western part of the massif indicate that the foliation planes in the augen orthogneisses dip to the NE, which is not quite consistent with the expected domal structure, and point to NE-ward extensional movements (sigma clasts, S–C structures: Fig. 5B) that are incompatible with a model assuming only on N/NEvergent thrusting (Lepvrier et al., 2011). 5.2.2. Late Permian–Early Triassic metamorphism and magmatism In the Than Ming Son diatexite mantle of the Triassic Tam Ða o Massif, four types of concordant zircons were encountered. The type IV zircons (oscillatory zoned structure, low-U and a Th/U ‘magmatic’ ratio of 0.38–1.21) yielded a concordant age of 767 ± 13 Ma. They are single grains with younger overgrowths, interpreted here as xenocrysts inherited from a Neoproterozoic igneous rock. Nearly half of the analysed type III grains from the diatexite yielded Late

Ordovician–Early Devonian ages. These zircons are sector zoned and can develop oscillatory zoned overgrowths, and, despite having ‘metamorphic’ Th/U ratios (0.01–0.11), they are interpreted to have grown from a melt, presumably of migmatitic origin. However, some grains yielded a concordant age of 453.1 ± 2.2 Ma and these have higher Th/U ratios (up to 0.26) and are interpreted to have originated in a Late Ordovician migmatitic/magmatic episode that affected, or produced, a protolith of the present-day Than Ming Son migmatite. The types I and II zircons show secondary cauliflowerlike textures, have CL-dark domains, and have 238 U/206 Pb ages between 363–348 Ma and 265–240 Ma, respectively. They are interpreted as recording two episodes of metamorphic recrystallization; the younger cluster is taken to record widespread Permian–Triassic thermotectonic activity in the Lo Gam complex, as expressed by the production of diatexites and A-type granites. We interpret the Than Ming Son diatexite as being part of a rock unit that developed at the expense of Neoproterozoic granitoid crust during Late Ordovician–Late Silurian metamorphism and migmatization. These events were followed by two further (tectono)thermal overprints, both clearly metamorphic, that took place during the Devonian–Early Carboniferous and then during the Late Permian–Early Triassic. The Early Palaeozoic tectonothermal event in the Lo Gam region also concurs with the emplacement of the Song Chay granite. An alternative explanation for the concordia between ∼780 Ma and 240 Ma would have to assume that the Than Ming Son diatexite comes from a mid-Triassic or younger sedimentary source and that the zircons are all detrital from a heterogeneously aged source region. This possibilty is, however, highly unlikely: it conflicts with the diatexitic nature of the host rocks and is inconsistent with the regional geology in which the mid-Triassic and younger sedimentary strata overlap the massif (Fig. 1) and remain unmetamorphosed. The geology itself confirms the validity of Maluski et al. (1999) who argued for fast uplift and unroofing in the region. In the Thác Bá gneisses, the presence of leucosome nests and pegmatitic intrusions that accompany the associated ductile shear zones suggests the development of a relatively high-temperature, right-lateral wrench regime for the Early Triassic migmatites. The younger population of zircons from the leucosomes and pegmatites of the Thác Bá gneisses yield an age of 245 Ma. This fits perfectly with both the estimated time of metamorphism of the Song Chay porphyrytic granite into augen gneisses (Maluski et al., 1999) and with the intrusion of the Tam Ða o rhyodacites. The geochemical characteristics of the Song Chay granite are very similar to those of the Thác Bá leucocratic neosomes and the Than Ming Son diatexitic neosome. Furthermore, the Tam Ða o rhyodacite is also geochemically similar to these neosomes and to the Song Chay granite. These sets of similarities suggest that the Permian–Triassic migmatites and magmatic rocks of the Lo Gam complex were developed via partial melting of affine rocks in deeper levels of Lower Palaeozoic crust. In the Tam Ða o Massif, a concordant zircon age of 248 ± 2 Ma may be the time of intrusion of the rhyodacite. The rhyodacite only locally shows a weak, NW-trending magmatic foliation; the role of tectonic control over emplacement of the acid magma is, therefore, uncertain. However, in the SE tip of the massif (E of the village of Xuan Hoa), a discrete planar fabric with asymmetric feldspar porphyroclasts of ␴-type and ␦-type clearly points to solid-state ductile deformation along NW–SE zones. Local fluid-assisted ultramylonitisation suggests that temperatures in the rhyodacite during deformation were high enough to ensure dynamic recrystallization of the feldspars. This implies that the ultramylonites probably developed soon after the intrusion and solidification of the parent rhyodacite. The mylonitisation event may have been coeval with the 245 Ma fabric formation, similar to that observed in the Song Chay granite further NW (Maluski et al., 1999; Roger et al., 2000).

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Common field evidence of dextral strike-slip in the Thác Bá gneisses, Tam Ða o rhyodacites and Than Ming Son migmatites (where there are also subvertical folds) suggest that the ductile, right-lateral shearing along the NW/NNW-trending planes played an important role in the Permian–Early Triassic tectonothermal event that affected the Lo Gam complex (southernmost South China Block). The neosome pockets systematically occuring along the shear zones indicate that the melting might have been triggered by ductile and relatively high-temperature shearing. Furthermore, because the Lo Gam complex is adjacent to the Dai Nui Con Voi Massif, it might be reasonable to expect that this massif was also affected by the deformations. By this logic, multiple movements along this part of the Ailao Shan–Red River Shear Zone may have occurred during the Late Permian–Early Triassic. Finally, the NW-trending ductile fault zones were predisposed to subsequent brittle reactivation throughout the region during the Oligocene–Miocene. These reactivation processes brought about conspicuous geomorphic features, such as the grabens that bound the Red River Fault Zone, and the SW marginal fault that borders the Tam Ða o Massif (Fig. 1). 5.2.3. Significance of inherited zircon xenocrysts The rocks of the Lo Gam complex were affected by Permo–Triassic tectonothermal events and the xenocrystic zircons from the rocks in this complex contain zircons of widely different ages. In the Tam Ða o rhyodacite, the zircon xenocrysts were derived from Neoproterozoic (655 Ma) and Paleoproterozoic (2.0–2.3 Ga) sources, and these xenocrysts carry evidence of inherited components having both ‘magmatic’ and ‘metamorphic’ Th/U ratios (0.07–0.44). To explain these, variable and/or complex protoliths were possibly subjected to crustal melting that eventually contributed to the Early Triassic magmatism. In the Thác Bá leucocratic vein that was examined, single concordant analyses point to diverse Palaeozoic (355, 407, 454 Ma), Early Neoproterozoic (834, 997–1007 Ma) and even Archaean (3025 Ma) sources. The Than Ming Son diatexite contains zircons that grew or recrystallized during the Carboniferous (348–357 Ma), Devonian (363–408 Ma), Silurian–Ordovician (425–476 Ma) and Neoproterozoic (a mean age of 767 ± 14 Ma). These peak ages suggest four major periods of protolith formation that were subsequently incorporated into the diatexite. Although the data are difficult to interpret in terms of palaeogeography and specific provenance of these components, their concordant nature seems to indicate a multiple reworking of strongly diversified crust, with the last legible thermal event being that of the Late Permian–Early Triassic. Reports have been made of Triassic, Carboniferous and Neoproterozoic ages from the Diancang and Ailao Shan segments of the Ailao Shan–Red River Shear Zone, Yunnan, where early amphibolite facies rocks were intruded by Triassic granites (Lin et al., 2012; Searle et al., 2010). Carboniferous ages were also found in the Jinshajing–Ailao Shan suture further to the NW (Wang et al., 2000). Inherited and detrital zircons with ages ranging from 1.0 to 0.7 Ga, combined with high grade metamorphism during the early Palaeozoic (458–425 Ma) have also been reported from the Cathaysia Block (Wan et al., 2007). The results of our study and of previously published work confirm that the process of crustal accretion in the South China Block was indeed complex, and it emphasizes the importance of the Permian–Triassic event in this block‘s evolution. 5.3. Indosinian event Although the units in both the SW and the NE walls of the Ailao Shan–Red River Shear Zone were intruded by peralkaline to peraluminous A-type granites between 265 Ma and 240 Ma, intrusions of gabbro–syenite occur in the Lo Gam complex that gave

137

Ar–Ar (hornblende, plagioclase) ages of 250–233 Ma. These ages are interpreted to represent post-collisional intrusive processes in a supra-subduction setting (Tran et al., 2004). Furthermore, Sr–Nd characteristics suggest arc magmatism was taking place in northern Vietnam (Lan et al., 2003), But our data cannot confirm or discard a role for the Song Ma Zone being the Indosinian collisional suture. If Song Ma is the subduction-related Indosinian suture (Bunopas and Vella, 1978; Metcalfe, 1993; Nakano et al., 2006, 2010; Tran, 1979), the Ailao Shan–Red River Shear Zone wall rock units may be interpreted as the back-arc region that was discretely affected by dextral strike-slip shearing, presumably due to the obliquity of the Indochina–South China collision. The Song Ma belt is probably continuous with the Jingshajiang–Ailao Shan Suture Zone, which itself resulted from the collision of the South China Block with crustal fragments of different age and origin that had been rifted off South China and then re-accreted, or from the collision with other Gondwanaderived fragments that separated from eastern Gondwana during the Middle Devonian (Golonka et al., 2006; Metcalfe, 1996, 2002). This terrane assembly underwent several diachronous episodes of metamorphism, deformation, synorogenic granite magmatism and post-orogenic molass formation (Liang and Li, 2005; Wang et al., 2000). Further mixing of rock bodies and their complicated evolution occurred during subsequent Mesozoic through Cenozoic deformation events that became localized in NW–SE trending crustal-scale faults such as the Song Da Zone and the Ailao Shan–Red River Shear Zone. These large faults were driven by successive accretions of the terranes of Simao, Sibumasu–Qiangtang, West Burma, Lhasa and, lastly, by the subcontinent of India itself to the South Asia continent, which, by that time, already included South China and Indochina (Wang et al., 2000). Nevertheless, Findlay (1997) and Findlay and Trinh (1997) have questioned the subduction affinity of the Song Ma belt for which they preferred an arc-related origin and concluded that parts of the South China Block exist south of the Ailao Shan–Red River Shear Zone. Carter and Clift (2008) did not find convincing evidence for the claim that the Indosinian was a major mountain building event and instead suggested a widespread tectonothermal reactivation resulting from the collision of the Sibumasu and Indochina blocks at ∼250–220 Ma (Fig. 1). Another explanation was put forward by Lepvrier et al. (2008) who explained the Indosinian event as the collision between the Kontum and Truong Son parts of the Indochina Block during the Early Triassic and synchronous with contemporaneous and extensive tectonothermal activity throughout Vietnam.

6. Conclusions Our results, combined with earlier data, show that both walls of the Vietnamese segment of the Ailao Shan–Red River Shear Zone (the Red River Fault Zone) contain metamorphic and intraplate A-type granitoid rocks of Late Permian–Early Triassic age (263–240 Ma), which emphasizes the importance of a PermoTriassic tectonothermal event in the evolution of the entire region. Despite such similarity, the two sides of the Red River Fault Zone differ significantly in their Phanerozoic history, which is consistent with the notion that large-scale displacements occurred along this zone. The SW wall of the Red River Fault Zone exposes both the Cavinh and Fan Si Pan complexes, which are Archaean to Neoproterozoic basement that were intruded first by peraluminous A-type granites of Late Permian–Early Triassic age (263–240 Ma) and, second, by subalkaline, A-type, biotite granite of Eocene age (∼35 Ma), accompanied by calc-alkaline, I-type granite. The two intrusive episodes were separated by regional tectonic deformations occurring within a transpressional regime of a NW/W-vergent thrusting with a

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left-lateral oblique component, that was associated with greenschist to amphibolite facies metamorphism, presumably also of Eocene age (∼50–35 Ma), and that may have been related to the left-lateral movement on the Ailao Shan–Red River Shear Zone. The NE wall of the Red River Fault Zone exposes the Lo Gam complex, which is Neoproterozoic basement that had been repeatedly reworked during the course of several Palaeozoic events, culminating with Triassic deformation, metamorphism and magmatism, and showing no Cenozoic thermal overprint. In this wall, a significant part of the Permo-Triassic thermotectonism was ductile shearing that was concentrated along dextral, strike-slip NW-trending zones in the vicinity the Ailao Shan–Red River Shear Zone but that became a type of NE/N-ward extensional/contractional, regional movement further away of it. The onset of shearing along the Ailao Shan–Red River Shear Zone could date back to the Permo-Triassic. Although our data do not directly contribute to the question of a Triassic collision between the Indochina Block and the South China Block, the new geological, geochemical and isotopic evidence suggests that the Ailao Shan–Red River Shear Zone is a major continental fault zone that probably initiated in the hinterland of the Indosinian oblique collisional zone during the Permo-Triassic and remarkably contributed to a complex history of crustal accretion in the South China Block.

Acknowledgements This research was conducted with the financial support of the Polish State Committee for Scientific Research (KBN Grant no. 3 P04D 052 25) within the framework of a bilateral cooperation agreement between the Polish Academy of Sciences and the Vietnamese Academy of Sciences and Technology. Dr. Bui An Nien is cordially thanked for invaluable assistance in the field. Helpful comments by an anonymous reviewer, Nguyen Hoang, Patrick Roycroft and Randell Stephenson are greatly appreciated.

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