Timing of Left-Lateral Shearing Along the Ailao Shan-Red River

Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 DOI 10.1007/s00531-012-0831-y

ORIGINAL PAPER

Timing of left-lateral shearing along the Ailao Shan-Red River shear zone: constraints from zircon U–Pb ages from granitic rocks in the shear zone along the Ailao Shan Range, Western Yunnan, China

Yuan Tang • Junlai Liu • My-Dung Tran • Zhijie Song • Wenbin Wu • Zhaochong Zhang • Zhidan Zhao • Wen Chen

Received: 9 November 2011 / Accepted: 30 September 2012 / Published online: 7 November 2012 Ó Springer-Verlag Berlin Heidelberg 2012

Abstract As the boundary between the Indochina and the structural and microstructural analysis reveals that the South China blocks, the Ailao Shan-Red River (ASRR) granitic intrusions are ascribed to pre-, syn- and post- shear zone underwent a sinistral strike-slip shearing which shearing magmatisms. The zircon U–Pb ages of these is characterized by ductile deformation structures along the granites provide constraints on timing of the initiation (later Ailao Shan range. The timing issue of left-lateral shearing than 31 Ma from pre-shearing granitic plutons, but earlier along the ASRR shear zone is of first-order importance in than 27 Ma from syn-shearing granitic dykes) and termi- constraining the nature and regional significance of the nation (ca. 21 Ma from the post-shearing granitic dykes) of shear zone. It has been, therefore, focused on by many strong ductile left-lateral shearing, which is consistent with previous studies, but debates still exist on the age of ini- previous results on the Diancang Shan and Day Nui Con tiation and termination of shearing along the shear zone. In Voi massifs in the literature. We also conclude that the left- this paper, we dated 5 samples of granitic plutons (dykes) lateral shearing along the ASRR shear zone is the result of along the Ailao Shan shear zone. Zircon U–Pb ages of southeastward extrusion of the Indochina block during the four sheared or partly sheared granitic rocks give ages of Indian–Eurasian plate collision. Furthermore, the left-lat- 30.9 ± 0.7, 36.6 ± 0.1, 25.9 ± 1.0 and 27.2 ± 0.2 Ma, eral shearing was accompanied by the ridge jump, post- respectively. An undeformed granitic dyke intruding dating the opening, of the South China Sea. mylonitic foliation gives crystallization age of 21.8 ± 1Ma. The Th/U ratios of zircon grains from these rocks fall into Keywords Ailao Shan shear zone Granitic plutons two populations (0.17–1.01 and 0.07–0.08), reflecting (dykes) Structural and microstructural analysis magmatic and metamorphic origins of the zircons. Detailed Geochronological dating South China Sea

Y. Tang J. Liu (&) M.-D. Tran Z. Song W. Wu Introduction Z. Zhang Z. Zhao State Key Laboratory of Geological Processes and Mineral As the eastern boundary of the Indochina block, the Ailao Resources, China University of Geosciences, Shan-Red River (ASRR) shear zone has played important Haidian, Beijing 100083, China e-mail: [email protected] roles in accommodating the Indian–Eurasian plate collision and subsequent post-collisional evolution, and in shaping Y. Tang the present tectonic framework in the southeastern Tibet Chengdu Institute of Geology and Mineral Resources, (Molnar and Tapponnier 1975; Tapponnier et al. 1990; Chengdu 610081, China Zhong et al. 1990; Harrison et al. 1992, 1996; Leloup et al. M.-D. Tran 1995, 2001a, b, 2007; Chung et al. 1997, 2008; Maluski Hanoi University of Mining and Geology, Hanoi, Vietnam et al. 2001; Liu et al. 2006, 2007, 2010, 2012; Searle 2006; Anczkiewicz et al. 2007; Searle et al. 2010). Therefore, the W. Chen Institute of Geology, Chinese Academy of Geological Sciences, Cenozoic evolution of the ASRR has been extensively Beijing 100037, China studied in the past decades. A great amount of data about 123 606 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 the structural evolution, geochronology and exhumation Zhang and Scha¨rer (1999) argued that the left-lateral history of the ASRR shear zone (e.g. Scha¨rer et al. 1990, shearing occurred between 35 and 22 Ma, based on U–Pb 1994; Harrison et al. 1992, 1996; Leloup and Kienast 1993; dating of zircon, monazite, xenotime and other minerals Leloup et al. 1995, 2001a, b, 2007; Chung et al. 1997; from some granites in ASRR shear zone which were Wang et al. 1998, 2000a, 2001; Zhang and Scha¨rer 1999; believed to be syn-kinematic. According to the thermo- Sun et al. 2003; Gilley et al. 2003; Schoenbohm et al. 2004; chronological studies of hornblende, muscovite, biotite and Liu et al. 2006, 2007, 2010, 2012; Yeh et al. 2008; Searle k-feldspar from the Ailao Shan massif, Chen et al. (1992) et al. 2010) indicate that the shear zone underwent inten- and Harrison et al. (1992) suggested that the left-lateral sive sinistral strike-slip shearing in Cenozoic. The shearing shearing along the ASRR may have lasted to ca. 17 Ma. is accommodated by ductile deformation along the Ailao Gilley et al. (2003) reported in situ Th–Pb ion microprobe Shan range (Allen et al. 1984; Leloup et al. 1993, 2001a; analysis of monazite inclusions in garnet grains which were Replumaz et al. 2001). However, controversies still exist suggested to be of syntectonic and therefore may date the over the nature of the left-lateral shearing, for example, prograde metamorphism and left-lateral ductile deforma- timing, depth and total displacement of shearing (Searle tion within the ASRR shear zone (35–21 Ma). However, 2006; Leloup et al. 2007; Liang et al. 2007; Chung et al. from similar studies on minerals from the Day Nui Con Voi 2008; Liu et al. 2010; Searle et al. 2010). Tapponnier et al. metamorphic massif at the southeasternmost part of the (1982, 1986, 1990) stated that the strike-slip movement ASRR shear zone, Wang et al. (1998, 2000a, 2001) sug- along the ASRR shear zone is attributed to large-scale gested that rapid exhumation of the metamorphic massif southeastward extrusion of the Indochina Block along the due to left-lateral shearing along the ASRR zone started at ASRR shear zone during the Indian–Eurasian plate colli- ca. 27 Ma and lasted until ca. 22 Ma. A good correlation sion. The extrusion was also suggested to be responsible between location and cooling path for the samples along for the opening of the South China Sea (Tapponnier et al. the shear zone indicates that the transtensional deforma- 1982, 1986, 1990; Peltzer and Tapponnier 1988; Briais tion may have propagated northwestward at a rate of et al. 1993). In this ‘‘extrusion model’’ the strike-slip shear *6 cm years-1. Thus, Wang et al. (1998, 2000a, 2001) zone cut the entire lithosphere and had large-scale geo- proposed that the onset of the left-lateral shearing along the logical offsets ([500 km) and high slip rates (Tapponnier shear zone did not occur until ca. 27 Ma. However, Leloup et al. 1986, 1990, 2001; Peltzer and Tapponnier 1988; et al. (2001b, 2007) argued that the conclusions of Wang Leloup et al. 2001a). In contrast, England and Houseman et al. (1998, 2000a, 2001) were in contradiction with the (1985, 1989), Houseman and England (1986, 1993) pos- cooling histories and structural data from both the Xuelong tulated that crustal thickening and irrotational lithospheric Shan and the Fan Si Pan ranges, and also with the ages of shortening extensively developed within the Tibetan the oldest synkinematic leucocratic dykes and metamor- Plateau and adjacent areas after the Indian–Eurasian plate phic monazites elsewhere. Later, Searle (2006) interpreted collision. In the latter hypothesis, the ASRR shear zone and that the peak metamorphism of the Day Nui Con Voi other strike-slip faults in Indochina were purely of crustal metamorphic rocks in Vietnam predates shearing along the and had much reduced geological offsets and low slip ASRR shear zone. He concluded that the left-lateral rates (England and Houseman 1985, 1989; Houseman and shearing along the ASRR shear zone initiated from 21 Ma. England 1986, 1993; Searle 2006). Recently, Liang et al. (2007) reported zircon ages ranging Moreover, the following aspects have been discussed and from 34.0 to 36.3 Ma of potassic alkaline intrusions along debated in the literature: (a) the initiation, duration and the ASRR shear zone, of which emplacement was sug- termination of left-lateral shearing along the ASRR shear gested to be resulted from left-lateral shearing. Therefore, zone (Scha¨rer et al. 1994; Harrison et al. 1996; Chung et al. the onset age of the left-lateral movements along the ASRR 1997, 2008; Wang et al. 1998, 2000a; Searle 2006; Leloup shear zone is at or slightly older than 36 Ma. The move- et al. 2007; Liang et al. 2007; Searle et al. 2010), (b) ments lasted from C36 to 17 Ma (Liang et al. 2007). mechanisms of Cenozoic uplifting and exhumation of However, Chung et al. (2008) argued that all the calc- the metamorphic massifs along the ASRR shear zone alkaline and potassic alkaline granites and most leucogra- (Schoenbohm et al. 2004; Anczkiewicz et al. 2007; Zhu nites within the ASRR shear zone are pre-kinematic, and et al. 2009), (c) role of left-lateral shearing along the ASRR there is no spatial or temporal link between the alkaline shear zone in Cenozoic tectonic evolution of the south- igneous intrusions and the ASRR strike-slip shearing. eastern Tibet and Indochina area (Searle 2006; Anczkiewicz Thus, their U–Pb ages cannot serve as direct geochrono- et al. 2007; Leloup et al. 2007). logical constraints on the initiation of shearing. More Particularly, the timing of shearing is of first-order recently, Searle et al. (2010) proposed that the ductile importance in elucidating the nature of large-scale shearing shearing along the ASRR occurred after the growth of the along the ASRR shear zone. Scha¨rer et al. (1994) and rims of Oligocene metamorphic zircons and the formation 123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 607 of early folded leucograntic intrusions (31.9–24.2 Ma), but The Indochina block is a composite Tethyan tectonic prior to the small-scale cross-cutting dykes (21.7 Ma). domain bounded by the Red River fault and the Sagaing To help resolving controversies over the timing of the fault. In this domain, the Sibumasu, the Lincang-Sukhothai ASRR left-lateral shearing, this paper presents new geochro- and the Simao-Indochina blocks (Hutchison 1989; Mo et al. nological data of pre-, syn- and post-shearing granitic plutons 1993, 1998; Metcalfe 1996a, b, 1998; Lepvrier et al. 2004; (dykes) from the Ailao Shan massif along ASRR shear zone. Feng et al. 2005; Sone and Metcalfe, 2008) are welded by Combined with field structural and microstructural data, the several suture zones, for example, the Changning-Meng- present results allow us to constrain the temporal between lian, Chiangrai-Chiangmai, Chanthaburi-Raub sutures, the emplacement of granitic magma and left-lateral shearing Jinghong, Nan, Sra Kaeo sutures, and the Ailao Shan, Song along the ASRR shear zone in the Ailao Shan massif, and thus Ma sutures. The Changning-Menglian, Chiangrai-Chiangmai the timing of initiation and duration of the dominant left- and Chanthaburi-Raub sutures are supposed to be the relics lateral ductile shearing along the ASRR shear zone. of the major Paleo-Tethyan Ocean between the Cathaysia and the Gondwana continents (Liu et al. 1991, 2002; Zhang 2000; Zhong and Zhao 2000; Charusiri et al. 2002; Sone and Geological setting Metcalfe 2008). Continental blocks to the west of the suture zone, for example, the Sibumasu Block, has more Gondw- Regional tectonic framework ana affinity than the blocks to the east of the suture zone, for example, the Indochina and South China blocks. The former The over 1,000-km-long NW-trending ASRR belt is loca- are characterized by Lower Carboniferous hiatus, Upper ted in the southeastern Tibet and separates the South China Carboniferous to Lower Permian glaciogene diamictites Block to the east and Indochina Block to the west (Fig. 1a). with Gondwana-related fauna and flora, and Middle–Upper

Fig. 1 a Tectonic subdivision of the ASRR shear zone and nearby areas (revised after Sone and Metcalfe 2008); b structural geology of the Ailaoshan belt 123 608 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626

Permian platform carbonates (Metcalfe 1998; Zhong 1998). the eastern Paleo-Tethyan belt. The suture zone was dis- The latter can be subdivided into several secondary tectonic membered during Cenozoic left-lateral shearing along the units, for example, the Lincang-Sukhothai and the Simao- ASRR belt, so that there are also some mafic to ultramafic Indochina blocks separated by the Jinghong, Nan sutures. units in the highly sheared mylonites within the ASRR The over hundred-km-long and ten-km-wide N–S-striking shear zone. In the suture zone, tectonic me´langes are Lincang Granite belt is composed mainly of S-type constituted by fragments of ophiolites and exotic blocks of, monzogranite intruding Precambrian metamorphic rocks for example, limestones and greywacke (Shen et al. 1998; (Zhong 1998). The Simao block is characterized by the Zhong 1998; Jian et al. 2009). There are serpentinized development of upper Triassic to early Cretaceous marine peridotite (mostly plagioclase lherzolite and some spinel sediments, and late Cretaceous to Paleocene lacustrine harzburgite), gabbro, diabase, plagiogranite and basalt in sedimentation. the ophiolites. Locally chromite-bearing ultramafic units The South China Block to the east of the ASRR belt is occur as lensic bodies. The magmatic rocks were dated, characterized by Pre-Neoproterozoic metamorphic base- using zircon U–Pb dating, as 328 ± 16 Ma (plagiogranite, ment unconformably overlain by Neoproterozoic to Jian et al. 1998), 362 ± 42 Ma (gabbro, Jian et al. 1998), Paleozoic sediments. A regional tectono-thermal event at 375.9 ± 4.2 Ma (plagiogranite, Jian et al. 2009)and ca. 800 Ma affected the entire block that led to the crat- 382.9 ± 3.9 Ma (diabase, Jian et al. 2009). These units onization to form the Yangtze craton. The South China fold were highly sheared and slightly metamorphosed by sub- belt lies in the south of the craton. There is a sequence sequent events. The existence of a sequence of arc-type of early Paleozoic marine sedimentation in the fold belt. plutonic and volcanic rocks to the west of the suture zone They are supposed to be folded and metamorphosed in suggests that a westward subduction of oceanic plate is late Silurian. Devonian sedimentary rocks have obvi- responsible for the formation of the suture zone (Fan et al. ous unconformity with their underlying early Paleozoic 2010; Liu et al. 2011). sequences. The boundary between the South China Block and the Structural units along the ASRR shear zone Indochina Block lies actually along the Ailao Shan and Song Ma sutures (Zhang et al. 1994; Chung et al. 1997; The ASRR shear zone is a high-grade metamorphic massif Wang et al. 2000b; Yumul et al. 2008), although some bounded by two remarkable NW–SE trending faults, that literature cited that the Red River fault zone instead. The is, the Red River fault to the east of the massif and Ailao suture zone is mostly distributed to the west of the Ailao Shan fault to the west of it (BGMRY 1983, 1991). Three Shan fault (Fig. 1a) and is the easternmost component of basic units constitute the ASRR shear zone (Figs. 1b, 2):

Fig. 2 Geological map of the Yuanjiang area (location of the area are shown in Fig. 1b)

123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 609 high-grade metamorphic rocks, mylonitic rocks and vari- The ASRR shear zone is evidenced by a wide mylonite ous granitic plutons (dykes). zone in which most rock types from the Proterozoic pro- High-grade metamorphic rocks occur locally as an un- toliths are mylonitized. Detailed ﬁeld observations of 3 sheared metamorphic belt located in the eastern part of the cross-sections—from the northern (i.e. Shuitang-Heping ASRR shear zone or as enclaves within the highly sheared section, Fig. 3a–a0), the middle (i.e. Honghe-Jiayin section, rocks in the ASRR shear zone. There are various gneisses, Fig. 3b–b0) to the southern (i.e. Manhao-Adebo section, schists, marbles and amphibolites. They were derived from Fig. 3c–c0) parts of the ASRR shear zone—were carried Proterozoic protoliths (i.e. the Ailao Shan group) which out to study the structural characteristics of the shear zone. underwent multi-phase deformation, metamorphism and A remarkable feature of the shear zone is the subhorizontal migmatization (BGMRY 1983, 1991). The metamorphism lineations, although foliations can be either of steeply or of such rocks grades from upper amphibolite facies at the gently dipping (Figs. 3, 4a, b). Lineations and foliations are southern end of the belt to lower amphibolite facies at the equally or unequally developed in the mylonites that the northern end (Zhai et al. 1990; Zhai and Cong 1993; Cong granitic mylonites can be grouped into L-tectonites, L–S- et al. 1993). Some elongated enclaves of weakly sheared tectonites or S-tectonites from the relative importance of metamorphic rocks within mylonites have similar mineral the lineation and foliation components. In particular, assemblages to those in the high-grade metamorphic rocks L-tectonites are widely distributed in the rocks close to the and are suggested to be the sheared relics of the high-grade Ailao Shan fault. Such rocks are characterized by weak rocks (Fig. 2; Liu et al. 2012). foliation and well-developed subhorizontal stretching

Fig. 3 Geological cross-sections at different segments of the ASRR S–P: Silurian–Permian; Pe: Permian basalt; T2–3: Mid-Upper Triassic; shear zone (locations of the sections are shown in Fig. 1b). Pzmd: the T3: Upper Triassic; N: Miocene E: Eocene; JAF: the Jiujia-Anding Madeng massif; T3x: Upper Triassic Xiangyun formation; T3g: Upper fault; ALF: the Ailao Shan fault; RRF: the Red River fault; *sample 2–3 Triassic Ganhaizi formation; c5 : Indosinian granite; D: Devonian; location 123 610 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 lineation. They are constituted by linear arrangement of progressively deformed to form asymmetrical folds, or metamorphic and deformed minerals, for example, quartz, commonly ‘‘a type’’ folds with hinges paralleling to the sillimanite, hornblende and biotite. Geometrically, mineral stretching lineations (Fig. 4e, f). aggregates in the L-tectonites parallel to the elongation Mylonitic rocks along the ASRR shear zone are either direction show rodding structures. Sigma- or delta-shaped derived from the high-grade metamorphic rocks of the feldspar porphyroclasts and shear bands are consistently Ailao Shan Group or from pre- and syn-shearing granitic indicating left-lateral shearing along the shear zone rocks. Microstructurally, banded felsic gneiss within the (Fig. 4c, d). Sigma-shaped garnet pressure shadows are shear zone experienced various deformation (Fig. 5a, b). observed in some garnet schist. Mylonitic foliations may be Amphibolites near the Ailao Shan fault have strong

Fig. 4 The structural geology of ASRR shear zone. a Sub-vertical amphibolitic rocks. f ‘‘a type’’ folds in mylonites from the contacts foliations in mylonitic rocks; b gently dipping mylonitic foliation; between granite and schist at outcrop scale (the red dashed line marks c, d left-lateral strike-slip shear indicators; c sigma and phi shaped the orientation of stretching lineation) feldspar porphyroclasts in mylonitic granite; e ‘‘a type’’ folds in

123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 611 lineation due to the alignment of amphibole grains grains in marbles are dominantly equigranular in grain (Fig. 5c), but foliations in such rocks are obscure. Silli- shapes, but weak orientations are also observed in some manite grains in garnet–sillimanite–mica schists are needle- cases (Fig. 5e). Mica grains in mica schists are mainly in the shaped and highly oriented (Fig. 5d). They are boudinaged form of banded micas or occur around feldspar porphyro- and retrograded into muscovite during progressive shearing clasts. Ribboned and kinked mica grains or mica ﬁsh are at relatively low-temperature conditions (Fig. 5d). Calcite also common in the mylonitic rocks (Fig. 5f).

Fig. 5 The microstructural geology of the Ailao Shan massif. garnet–sillimanite gneiss (muscovite recrystallize in the necked areas a Deformed banded felsic gneiss with the feldspar porphyroclast of sillimanite grains); e equant calcite grains in marble; f mica ﬁsh in and monomineralic quartz ribbons; b subgrain rotation dynamic mylonite. Fs feldspar, Qz quartz, Amp amphibole, Sil sillimanite, Cal recrystallization of quartz and bulging dynamic recrystallization of calcite, Ms muscovite. c Parallel polarization, a, b, d, e, f cross- feldspar in granite gneiss; c strong shape preferred orientations of polarization amphibole grains in amphibolite; d oriented and broken sillimanite in

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Granitic plutons (dykes) of calc-alkaline in composition, Fig. 6 The structural characteristics of granitic rocks (dykes) in c including biotite monzogranites, granitic pegmatites and ASRR shear zone (see text for explanations). a, b Pre-shearing biotite plagioclase granitic mylonites with stretching lineation (L-tectonite, granodiorites (Zhang and Scha¨rer 1999; Chung et al. 2008), sample AL08149-1); c–e syn-shearing granitic dyke intruding the are particularly abundant near the Ailao Shan fault. They mylonitic foliation (sample AL0814-2); e, f undeformed granitic dyke show complicated spatial relationships with the shear zone intruding the mylonitic foliation (sample AL0841-8) and shear zone foliations, that is, concordant and discor- dant (Fig. 6). Most of the granitic rocks were transformed LA-ICP-MS dating techniques into granitic mylonites or granitic protomylonites during the left-lateral shearing along ASRR shear zone (Fig. 6a–e). Five samples of the granitic plutons (dykes) in the ASRR Field observations reveal differences in structural rela- shear zone were chosen for LA-ICP-MS zircon U–Pb tionships between the dykes and their wall rocks. In dating. All zircon grains were separated from whole-rock Fig. 6a, the host granitic rocks, from which the Sample samples using conventional techniques. After crushing and AL08149-1 (Fig. 6b) was taken, are entirely mylonitized sieving of the samples, heavy minerals were concentrated by solid-state plastic deformation and are subsequently by panning and then by magnetic separation. Zircon grains intruded by dykes of various orientations. Figure 6c–e were handpicked, and then the grains were mounted in an present a concordant dyke with transitional relationship to epoxy disc with chips of standard zircons of 91500 zircon its host rocks (mylonitic gneiss). The former show typical age standards (91500–1063 ± 6 Ma, Wiedenbeck et al. mylonitic structures characteristic of ASRR left-lateral 1995) for LA-ICP-MS analyses. These are then carefully shearing. The latter, however, possesses the following polished until their cores were exposed. Cathodolumines- features. (a) The contacts between the dyke and its host cence (CL) images of zircons combined with reflected and rocks are transitional and are paralleling to the mylonitic transmitted light images were used to morphologically foliations in the host rocks. (b) The dyke is less deformed target distinct areas on the zircons for LA-ICP-MS analy- than its host rocks, indicated by a weak foliation in com- ses. CL images were obtained using a Mini CL attached to parison with the latter. (c) Mafic enclaves in the dyke are a scanning electron microscope (LEO1450VP) at the elongated in shapes and have long axes paralleling to both Electron Microprobe Laboratory at the Institute of Geology the dyke/host contacts and the foliations in the mylonites. and Geophysics (IGG), Chinese Academy of Sciences, In Fig. 6f and g, however, concordant (Fig. 6f) and dis- Beijing. The LA-ICP-MS analyses were finished at the cordant (Fig. 6g) dykes intrude the shear zone. These dykes same Institution. The analyses were conducted using the have obviously sharp contacts with their host rocks. Agilent ICP-MS equipped with a 193-nm laser ablation. Granitic mylonites develop typical mylonitic textures, The zircon standard 91500 was used as the external cali- with 10–90 % matrix and handful of small feldspar or bration standard. The collected data were then adjusted by hornblende porphyroclasts (Fig. 7a–c). In some samples, GLITTER, a data reduction software package for LA-ICP- quartz grains are elongated to form ribbons and oblique MS. The zircon U–Pb concordia plots diagram and foliation due to progressive deformation and subgrain weighted ages were calculated by Isoplot program. The recrystallization (Fig. 7a). In the other, coarse quartz grains detailed analytical procedure refers to Yuan et al. (2008)or are elongated and have serrated grain boundaries, an effect Xie et al. (2008). of grain boundary recrystallization (Fig. 7b, c). They are partly or completely recrystallized to constitute the fine- grained matrix. Microstructurally, feldspar porphyroclasts Results: deformation microstructures are surrounded by very fine grains of dynamically recrys- and geochronology tallized new grains of feldspar mostly from bulging recrystallization (Fig. 7a–c). The fine grains are homoge- Microstructural characteristics neously distributed in the matrix (Fig. 7a) or distributed along some separate zones (Fig. 7b, c). Biotite grains are The sample AL08149-1 was taken from the Manhao-Adebo strongly sheared to form fine-grained biotite banding section (E: 103°13028.200,N:22°55027.000, Figs. 1b, 3c–c0 , structure (Fig. 7a). Muscovite grains may form mica fished 6a). The sample is a typical granitic L-tectonite with well- in some rocks (Fig. 7b). developed subhorizontal stretching lineation (Fig. 6b). The In the undeformed granites, no evidences of ductile sample AL09213-1 is a granitic mylonite near the Ailao deformation due to left-lateral shearing are observed from Shan fault at the southern part of shear zone at southern the field or the microscope (Figs. 6f, g, 7d). At the outcrop Yuanyang (E: 102°4505.400,N:23°4030.400, Fig. 1b). It is scale, the undeformed granitic dykes intrude the sheared a typical L–S-tectonite with equally developed L and S host rocks and may cut across the mylonitic foliations of fabrics. Fig. 7a–c show a sequence of microstructures of the host rocks (Fig. 6g). mylonites with similar characteristics. Microscopically, 123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 613

123 614 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 porphyroclastic feldspar grains surrounded by fine-grained N: 24°16041.700, Fig. 1b). The dyke extends along the counterparts in the matrix are indicative mylonitization foliations of the host rock (Fig. 6c–e). Well-developed through bulging dynamic recrystallization (Fig. 7a, c). foliation and mineral lineation are paralleling to the Elongated quartz (Fig. 7b), ribbon quartz (Fig. 7b) and banding in the host rocks. The dominant microstructural oblique foliation (Fig. 7a) are popular and are indicative the style of the mylonite is the existence of feldspar porphyr- dominant role of plastic deformation and dynamic recrys- oclasts and irregularly shaped but elongated quartz aggre- tallization. The serrated boundaries of these grains are gates (Fig. 8a–d). Exsolution microstructures, for example, ascribed to grain boundary migration recrystallization mymerkites, are observed along the boundaries of some of (Fig. 7c). Muscovite grains in the mylonites are sheared to the porphyroclasts. Their asymmetrical distribution around form fish bodies and S–C fabrics with C fabrics being the porphyroclasts, together with intragranular plastic mostly composed of fine grains of feldspar. The above deformation features, for example, undulose extinction and deformation characteristics of feldspar, quartz and musco- mechanical twinning, is indicative of high-temperature vite are indicative of shearing at medium temperatures solid-state deformation (Vernon 2000). Very fine grains of (*500 °C). The reduced grain sizes of biotite grains and feldspar surrounding their porphyroclastic hosts with con- their distribution along some distinctive zones (Fig. 7a) trasting grain sizes and weak intragranular plastic defor- suggest the dominant role of progressive shearing at low mation of the porphyroclasts suggest the dominant role of temperatures. bulging dynamic recrystallization during grain size reduc- Sample AL0814-2 is a mylonitic monzogranitic dyke near tion. Quartz grains in such rocks, however, form irregularly Shuitang at the northern ASRR shear zone (E: 101°22034.700, shaped aggregates in which quartz grains do not show

Fig. 7 Microstructures of pre-shearing (a–c) and post-shearing with bulging recrystallization of feldspar, elongated quartz with (d) granitic rocks (dykes) in ASRR shear zone. a Feldspar porphyr- irregular grain boundaries, and progressive deformation of quartz oclasts surrounded fine grains, dynamically recrystallized quartz grains, for example, undulose extinction (sample AL09213-1); ribbons with oblique foliation, zones of fine grains of biotite (sample d mosaic pattern of undeformed minerals in post-shearing granitic AL0655-1); b S–C fabrics with bulging recrystallization of feldspar, dyke (sample AL0841-8). a–d Cross-polarization; Fs feldspar, Qz elongated quartz with irregular grain boundaries and muscovite fishes quartz, Amp amphibole (sample AL08149-1); c shape preferred orientation of feldspar grains

123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 615 obvious plastic deformation (Fig. 8a–d). The long axes of crystallization. Simultaneously, they experienced solid- the aggregates are in parallel to the dimensional orientation state plastic deformation due to left-lateral shearing along of the feldspar porphyroclasts. These microstructural styles the shear zone. On the other hand, quartz grains in the suggest that the rocks experienced a transition of temper- sample are preferentially distributed in some speciﬁc grain atures from high (up to [700 °C) to medium (*500 °C) aggregates, which is especially obvious in the parallel light during shearing (Passchier and Trouw 2005). The temper- microphotographs (Fig. 8a). The aggregates are elongated ature variation is, however, interpreted as the result of and oriented, but do not show obvious crystal plastic cooling of syn-shearing magma. It is inferred that feldspar deformation. Sample AL09146-3 is from another concor- grains crystallize from the magma early during magmatic dant granitic dyke from east of Jiayin at the southern part of

Fig. 8 Microstructures of syn-shearing granite. a–d Microstructures by bulging recrystallization in b than in d; e, f magmatic ﬂow of sample AL0814-2. Feldspar porphyroclasts with bulging dynamic structure with shape preferred orientation of mineral grains with weak recrystallization and mymerkite around feldspar grains, quartz plastic deformation (sample AL09146-3). a Parallel polarization; b– aggregates without obvious plastic deformation, and less ﬁne grains f cross-polarization; Fs feldspar, Qz quartz, My mymerkite

123 616 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626

123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 617 b Fig. 9 a, c, e Cathodoluminescence (CL) images of representative concordant and yield 206Pb/238U ages ranging between 23 zircons from the dated samples: a AL08149-1, c AL0814-2, and 30 Ma (Table 1; Fig. 9d) defining a weighted mean e AL0841-8, Circles with enclosed data indicate spots and ages of 206 238 LA-ICP-MS dating. The diameters of the circles are 60 lm. b, d, Pb/ U age of 25.9 ± 1.0 Ma (MSWD = 3.7). Such an f LA-ICP-MS concordia diagrams of the dated samples: b AL08149-1, age is interpreted as the age of metamorphism related to the d AL0814-2, f AL0841-8 left-lateral shearing, which is account for the crystallization of zircons from melts by anataxis (Liati and Gebauer 1999; the ASRR shear zone (E: 102°29032.300,N:23°15040.200, Wu et al. 2007; Zeh et al. 2010). Fig. 1b). Microscopically, magmatic flow structures are Zircons from sample AL0841-8 are euhedral to subhe- clearly observed in this sample. Minerals, for example, dral, range from 100 to 200 lm in lengths and have length mica, quartz and feldspar, show obvious shape preferred to width ratios between 1:1 and 3:1. Most Zircons are col- orientations, but rare evidences for solid-state plastic orless. CL images of zircons show fine oscillatory zoning deformation, suggesting that crystallization occurred late (Fig. 9e). Twenty-five laser spots were conducted on during shearing (Fig. 8e, f). twenty-five zircon grains for this sample. All analyzed Sample AL0841-8 is from an undeformed biotite pla- zircons have Th 38-46460 ppm and U 135-12523 ppm. The gioclase granitic dyke from the Honghe-Jiayin section at the dated 25 zircon grains have an average Th/U ratio of 1.01, middle part of ASRR shear zone (E: 102°25031.800,N: among which 18 grains have Th/U ratios ranging from 0.1 23°16039.900, Figs. 1b, 3b–b’). According to field observa- to 1.0 (Table 1). Most analyses are concordant or nearly tions, the granitic intrusion cut across the foliations of the concordant and yield concordia 206Pb/238U ages ranging strongly sheared host rocks (the Ailao Shan massif, Fig. 6g) from 19 to 24 Ma (Table 1; Fig. 9f). Weighted mean of the and has not experienced any shear deformation, implying 16 concordant data yields a 206Pb/238U age of 21.8 ± 1Ma that the granitic dyke is post-shearing intrusion. Micro- (MSWD = 3.5). The age is interpreted as that of crystalli- scopic observation is consistent with the above conclusion zation of the post-shearing biotite plagioclase granitic dyke. in that equant grains of quartz, feldspar and micas are Zircons from sample AL09213-1 are mostly long randomly distributed and do not show any preferred columnar or acicular, ranging from 150 to 300 lm in lengths dimensional or crystallographic orientation (Fig. 7d). with length to width ratios between 1.5:1 and 3:1. Most Zircons are pale yellow or colorless. CL images of zircons Zircon U–Pb results show fine oscillatory zoning (Fig. 10a). Twenty laser spots were conducted on 20 zircon grains for this sample. All Zircons from sample AL08149-1 are mostly long columnar analyzed zircons have content of Th 386-3714 ppm and U or acicular in shapes. The grains range from 100 to 200 lm 906-10282 ppm. The 20 dated zircon grains have an average in lengths and have length to width ratios between 3:1 and Th/U ratio of 0.56 (Table 1). Most analyses are concordant 4:1. Most Zircons are pale yellow or colorless. CL images or nearly concordant and yield 206Pb/238U ages ranging of zircons show fine oscillatory zoning and irregular core between 36.2 and 36.9 Ma (Table 1; Fig. 10b). Seventeen (Fig. 9a). Twenty-four laser spots were conducted on 24 concordant data yield a weighted mean 206Pb/238U age of zircon grains for this sample. All analyzed zircons have 36.6 ± 0.1 Ma (MSWD = 0.26). The age is interpreted as content of Th 17-1139 ppm and U 251-10458 ppm. The 24 the age of crystallization of the granitic rocks. dated zircon grains have an average Th/U ratio of 0.17 Zircons from sample AL09146-3 generally have regular (Table 1). On the concordia diagram, most analyses are shape, and the appearance of prismatic faces. The lengths concordant or nearly concordant and yield 206Pb/238U ages of zircons range from 200 to 300 lm with length to width ranging between 28 and 35 Ma (Table 1; Fig. 9b) with ratios between 2:1 and 4:1. In CL image, some zircons weighted mean age of 30.9 ± 0.7 Ma (MSWD = 4.2). display no or weakly zoned internal structure with low CL The weighted mean age is interpreted as the age of crys- intensity, and the other zircons have metamorphic over- tallization of the granitic rocks. growth of rims around inherited cores (Fig. 10c). Twenty Zircons from sample AL0814-2 are euhedral to subhe- analyses were obtained on twenty different zircon grains dral, range from 100 to 200 lm in lengths and have length for this sample. All analyzed zircons have Th 70-574 ppm to width ratios between 1.5:1 and 3:1. In CL image, most and U 1393-4480 ppm. The dated 20 zircon grains have an zircons have metamorphic overgrowth of rim around average Th/U ratio of 0.07 (Table 1), indicating that the inherited core with strong CL brightness (Fig. 9c). Twenty- zircons grew during the metamorphism due to left-lateral five analyzed zircons have relatively low Th (8-135 ppm) shearing along the ASRR shear zone. Twenty analyses on and U (124-883 ppm) content (Table 1) as well as average the zircons represent our data set, 10 being concordant or Th/U ratio (0.08), which show the characteristics of meta- nearly concordant, with 206Pb/238U ages ranging between morphic zircons (Wu and Zheng 2004). On the concor- 26.7 and 28.5 Ma (Table 1; Fig. 10d). The 10 analyses dia diagram, seventeen analyses are concordant or nearly gave a weighted mean 206Pb/238U age of 27.2 ± 0.2 Ma 123 618 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626

Table 1 Zircon LA-MC-ICPMS U–Pb isotopic data of magmatic rocks from the Ailao Shan-Red River shear zone Spot number Content Th/U Isotopic ratio Age/Ma

Th U 207Pb/206Pb 1r 207Pb/235U1r 206Pb/238U1r 207Pb/235U1r 206Pb/238U1r

AL08149-1 Biotite granitic mylonite (pre-shearing granitic rocks) Al08149-1 01 264 2096 0.13 0.0480 0.0026 0.0352 0.0017 0.0053 0.00014 35 2 34.2 0.9 Al08149-1 02 134 1494 0.09 0.0472 0.0019 0.0341 0.0013 0.0052 0.00014 34 1 33.6 0.9 Al08149-1 03 279 6045 0.05 0.0494 0.0012 0.0327 0.0007 0.0048 0.00011 32.7 0.7 30.8 0.7 Al08149-1 04 195 2643 0.07 0.0443 0.0014 0.0296 0.0009 0.0049 0.00012 29.6 0.9 31.2 0.8 Al08149-1 05 743 9559 0.08 0.0443 0.0008 0.0308 0.0006 0.0050 0.00011 30.8 0.5 32.4 0.7 Al08149-1 06 445 2291 0.19 0.0481 0.0016 0.0331 0.0010 0.0050 0.00012 33 1 32.1 0.8 Al08149-1 08 374 4174 0.09 0.0461 0.0018 0.0315 0.0010 0.0050 0.00012 31.5 0.9 31.9 0.7 Al08149-1 10 429 3734 0.11 0.0438 0.0010 0.0288 0.0006 0.0048 0.00010 28.8 0.6 30.6 0.6 Al08149-1 11 320 2497 0.13 0.0506 0.0017 0.0327 0.0010 0.0047 0.00011 32.7 1 30.2 0.7 Al08149-1 12 863 10458 0.08 0.0464 0.0007 0.0317 0.0005 0.0050 0.00010 31.6 0.4 31.8 0.6 Al08149-1 14 133 1630 0.08 0.0536 0.0020 0.0377 0.0013 0.0051 0.00013 38 1 32.9 0.8 Al08149-1 15 739 2381 0.31 0.0487 0.0014 0.0318 0.0008 0.0048 0.00011 31.8 0.8 30.5 0.7 Al08149-1 16 1139 4486 0.25 0.0441 0.0009 0.0274 0.0005 0.0045 0.00009 27.4 0.5 29 0.6 Al08149-1 18 534 2911 0.18 0.0493 0.0016 0.0305 0.0009 0.0045 0.00011 30.5 0.9 28.9 0.7 Al08149-1 19 582 1792 0.32 0.0509 0.0017 0.0328 0.0010 0.0047 0.00011 32.7 1 30 0.7 Al08149-1 20 426 2274 0.19 0.0447 0.0013 0.0282 0.0008 0.0046 0.00010 28.2 0.8 29.5 0.6 Al08149-1 21 370 2422 0.15 0.0480 0.0021 0.0337 0.0013 0.0051 0.00013 34 1 32.8 0.8 Al08149-1 22 684 5861 0.12 0.0475 0.0012 0.0306 0.0008 0.0047 0.00010 30.6 0.7 30 0.6 Al08149-1 23 307 3629 0.08 0.0472 0.0020 0.0303 0.0011 0.0047 0.00011 30 1 29.9 0.7 Al08149-1 24 449 2667 0.17 0.0477 0.0027 0.0300 0.0015 0.0046 0.00012 30 2 29.4 0.8 AL09213-1 Granitic mylonite (pre-shearing granitic rocks) AL09213-1 01 1537 907 1.69 0.0511 0.0025 0.0398 0.0019 0.0057 0.00008 40 2 36.6 0.5 AL09213-1 02 820 3332 0.25 0.0483 0.0013 0.0375 0.0010 0.0056 0.00004 37.4 0.9 36.2 0.3 AL09213-1 03 1111 3540 0.31 0.0481 0.0013 0.0380 0.0010 0.0057 0.00005 37.9 1 36.8 0.3 AL09213-1 04 1000 2295 0.44 0.0490 0.0015 0.0383 0.0011 0.0057 0.00005 38 1 36.5 0.3 AL09213-1 05 1111 1407 0.79 0.0459 0.0020 0.0361 0.0016 0.0057 0.00006 36 2 36.9 0.4 AL09213-1 06 3714 10282 0.36 0.0463 0.0008 0.0365 0.0007 0.0057 0.00005 36.4 0.7 36.6 0.3 AL09213-1 07 2021 3337 0.61 0.0465 0.0014 0.0366 0.0011 0.0057 0.00004 37 1 36.6 0.2 AL09213-1 08 788 2064 0.38 0.0462 0.0018 0.0363 0.0015 0.0057 0.00006 36 1 36.5 0.4 AL09213-1 10 1243 2051 0.61 0.0461 0.0018 0.0359 0.0013 0.0057 0.00006 36 1 36.4 0.4 AL09213-1 11 1633 2036 0.80 0.0455 0.0019 0.0358 0.0015 0.0057 0.00006 36 1 36.7 0.4 AL09213-1 12 1300 2515 0.52 0.0476 0.0015 0.0378 0.0012 0.0057 0.00005 38 1 36.8 0.3 AL09213-1 13 1051 2410 0.44 0.0448 0.0015 0.0353 0.0012 0.0057 0.00005 35 1 36.6 0.3 AL09213-1 14 2199 3493 0.63 0.0461 0.0012 0.0362 0.0010 0.0057 0.00004 36.1 1 36.6 0.3 AL09213-1 15 871 1739 0.50 0.0440 0.0023 0.0342 0.0017 0.0057 0.00007 34 2 36.6 0.5 AL09213-1 16 642 1659 0.39 0.0455 0.0019 0.0353 0.0014 0.0057 0.00005 35 1 36.4 0.3 AL09213-1 17 2560 4007 0.64 0.0441 0.0010 0.0347 0.0008 0.0057 0.00004 34.6 0.8 36.6 0.2 AL09213-1-18 387 2508 0.15 0.0455 0.0017 0.0356 0.0014 0.0057 0.00008 36 1 36.6 0.5 AL0814-2 Augen amphibole monzogranitic mylonite (syn-shearing granitic rocks) AL0814-2 01 9.8 182 0.05 0.0544 0.0070 0.0319 0.0038 0.0043 0.00021 32 4 27 1 AL0814-2 08 10.5 248 0.04 0.0632 0.0055 0.0374 0.0029 0.0043 0.00020 37 3 28 1 AL0814-2 09 13.2 208 0.06 0.0534 0.0054 0.0315 0.0030 0.0043 0.00020 32 3 28 1 AL0814-2 10 12.5 240 0.05 0.0503 0.0059 0.0272 0.0029 0.0039 0.00022 27 3 25 1 AL0814-2 13 18.9 349 0.05 0.0558 0.0047 0.0293 0.0022 0.0038 0.00015 29 2 24.5 0.9 AL0814-2 14 11.6 205 0.06 0.0470 0.0056 0.0249 0.0028 0.0038 0.00017 25 3 25 1 AL0814-2 15 9.1 148 0.06 0.0581 0.0086 0.0304 0.0042 0.0038 0.00023 30 4 24 1 AL0814-2 16 9.8 208 0.05 0.0497 0.0061 0.0255 0.0030 0.0037 0.00019 26 3 24 1 AL0814-2 17 14.0 258 0.05 0.0629 0.0053 0.0388 0.0030 0.0045 0.00020 39 3 29 1

123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 619

Table 1 continued

Spot number Content Th/U Isotopic ratio Age/Ma

Th U 207Pb/206Pb 1r 207Pb/235U1r 206Pb/238U1r 207Pb/235U1r 206Pb/238U1r

AL0814-2 18 35.1 406 0.09 0.0554 0.0051 0.0343 0.0029 0.0045 0.00021 34 3 29 1 AL0814-2 19 26.6 305 0.09 0.0498 0.0044 0.0302 0.0025 0.0044 0.00017 30 2 28 1 AL0814-2 20 10.6 198 0.05 0.0675 0.0055 0.0352 0.0025 0.0038 0.00017 35 2 24 1 AL0814-2 21 11.7 217 0.05 0.0432 0.0061 0.0263 0.0035 0.0044 0.00024 26 3 28 2 AL0814-2 22 10.7 179 0.06 0.0590 0.0096 0.0359 0.0053 0.0044 0.00031 36 5 28 2 AL0814-2 23 9.5 185 0.05 0.0527 0.0052 0.0267 0.0024 0.0037 0.00018 27 2 24 1 AL0814-2 24 12.8 204 0.06 0.0489 0.0072 0.0260 0.0036 0.0039 0.00021 26 4 25 1 AL0814-2 25 8.7 124 0.07 0.0590 0.0081 0.0302 0.0038 0.0037 0.00020 30 4 24 1 AL09146-3 Biotite monzo-granitic mylonite (syn-shearing granitic rocks) AL09146-3-06 116 2342 0.05 0.0446 0.0017 0.0255 0.0010 0.0042 0.00004 25.6 1 26.7 0.3 AL09146-3-07 196 3549 0.06 0.0452 0.0013 0.0262 0.0007 0.0042 0.00003 26.3 0.7 27.1 0.2 AL09146-3-09 155 2019 0.08 0.0439 0.0020 0.0261 0.0018 0.0042 0.00015 26 2 27.2 1 AL09146-3-10 115 2698 0.04 0.0444 0.0021 0.0262 0.0013 0.0043 0.00008 26 1 27.5 0.5 AL09146-3-12 146 2489 0.06 0.0487 0.0018 0.0285 0.0011 0.0043 0.00008 29 1 27.4 0.5 AL09146-3-13 176 2551 0.07 0.0473 0.0020 0.0279 0.0012 0.0043 0.00008 28 1 27.5 0.5 AL09146-3-14 87 2532 0.03 0.0490 0.0021 0.0291 0.0013 0.0044 0.00015 29 1 28.2 1 AL09146-3-15 171 2611 0.07 0.0479 0.0021 0.0290 0.0013 0.0044 0.00010 29 1 28.5 0.6 AL09146-3-16 213 2672 0.08 0.0442 0.0017 0.0261 0.0011 0.0043 0.00008 26 1 27.6 0.5 AL09146-3-19 195 2141 0.09 0.0465 0.0020 0.0271 0.0011 0.0043 0.00010 27 1 27.5 0.6 AL0841-8 Biotite plagioclase granitic dyke (post-shearing granitic rocks) AL0841-8 01 1641 9265 0.18 0.0443 0.0009 0.0218 0.0004 0.0036 0.00010 21.9 0.4 23 0.6 AL0841-8 02 990 11789 0.08 0.0442 0.0013 0.0205 0.0006 0.0034 0.00009 20.6 0.5 21.7 0.6 AL0841-8 03 629 5993 0.11 0.0431 0.0010 0.0204 0.0004 0.0034 0.00009 20.5 0.4 22.1 0.6 AL0841-8 06 46459 3784 12.28 0.0483 0.0028 0.0230 0.0012 0.0035 0.00012 23 1 22.3 0.8 AL0841-8 07 513 7653 0.07 0.0450 0.0010 0.0215 0.0005 0.0035 0.00009 21.6 0.5 22.2 0.6 AL0841-8 08 9665 6802 1.42 0.0419 0.0011 0.0200 0.0005 0.0035 0.00009 20.1 0.5 22.2 0.6 AL0841-8 10 6010 8263 0.73 0.0480 0.0014 0.0232 0.0006 0.0035 0.00010 23.3 0.6 22.5 0.6 AL0841-8 11 732 4031 0.18 0.0401 0.0015 0.0192 0.0007 0.0035 0.00010 19.3 0.7 22.3 0.6 AL0841-8 12 557 3580 0.16 0.0487 0.0027 0.0233 0.0012 0.0035 0.00012 23 1 22.3 0.8 AL0841-8 13 1796 4357 0.41 0.0471 0.0014 0.0235 0.0007 0.0036 0.00010 23.6 0.6 23.3 0.6 AL0841-8 14 922 11488 0.08 0.0453 0.0009 0.0206 0.0004 0.0033 0.00009 20.7 0.4 21.2 0.6 AL0841-8 15 5697 10396 0.55 0.0469 0.0010 0.0209 0.0004 0.0032 0.00008 21 0.4 20.7 0.5 AL0841-8 16 3043 8299 0.37 0.0477 0.0010 0.0210 0.0004 0.0032 0.00008 21.1 0.4 20.5 0.5 AL0841-8 17 1180 6346 0.19 0.0446 0.0010 0.0219 0.0005 0.0036 0.00009 22 0.5 22.9 0.6 AL0841-8 18 2949 12522 0.24 0.0475 0.0011 0.0200 0.0005 0.0030 0.00008 20.1 0.4 19.6 0.5 AL0841-8 21 938 5437 0.17 0.0455 0.0013 0.0224 0.0006 0.0036 0.00009 22.5 0.6 22.9 0.6

(MSWD = 1.16). These ages are interpreted as the age of constrain the timing of shearing along the shear zone and zircon crystallization from anatectic melts during left-lat- reveal the related lower crustal and mantle processes dur- eral shearing along ASRR shear zone. ing the middle or upper crustal shearing. Such distinctions, especially between the pre- and syn-shearing dykes in the shear zones, however, are often difﬁcult because both types Discussions of intrusions are strongly transformed to form concordant dykes. They are distributed along the shear zone and show Pre-, syn- and post-shearing intrusions structures parallel to the structures in the wall rocks. Sev- eral studies, for example, Vernon et al. (1989), Paterson Granitic intrusions in shear zones are pre-, syn- or post- et al. (1989), Searle (2006), Cao et al. (2011), have con- shearing from their temporal relationships with shearing. tributed to the buildup of criteria which are applied to Distinctions between the different types of dykes may help studies of granitic intrusions in shear zones (Searle 2006;

123 620 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626

Fig. 10 a, c Cathodoluminescence (CL) images of representative diameters of the circles are 40 lm. b, d LA-ICP-MS concordia zircons from the dated samples: a AL09213-1, c AL09146-1, Circles diagrams of the dated samples: b AL09213-1, d AL09146-3 with enclosed data indicate spots and ages of LA-ICP-MS dating. The

Cao et al. 2011). In general, the structural relationships Samples AL0655-1 (Fig. 7a, not dated), AL08149-1 between the intrusions and the wall-rock foliations, the (Fig. 7b) and AL09213-1 (Fig. 7c) are characteristic pre- internal structural and microstructural characteristics, and shearing intrusions. These intrusions were emplaced prior deformation of minerals are the important aspects in elu- to shearing and may possess some or all of the following cidating the relative timing of shearing and emplacement of features: (a) The granitic rocks are cut across by the ductile the intrusions. In addition, zircon grains from pre-shearing shear zone. Rocks in the shear zone are intensely sheared, intrusions are mostly of magmatic origin and those from but the counterparts outside the shear zone remain unde- syn-shearing intrusions may be of metamorphic origin formed. (b) Foliation and/or lineation are highly developed related to anatexis during shearing. In such cases, the Th/U throughout the entire rock bodies. They are consistent with isotope systematics may also be applied to distinguish the the foliation in the host rocks. In most cases, the plutonic pre- and syn-shearing origin of the magmatism if the iso- rocks are equally strained with their host rocks. (c) Rocks tope systematics in the pre-shearing zircons are not or only are characterized by solid-state plastic deformation partly reset during shearing. throughout the entire intrusion. Early high-temperature

123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 621 magmatic structures are superimposed by late intermediate (AL0814-2) are surrounded by various amount of fine to low-temperature shearing structures (Fig. 7a–c). Various feldspar grains due to bulging recrystallization (Fig. 8b, d), mylonitic rocks from protomylonites to ultramylonites are which is due to solid-state plastic deformation. The quartz widely distributed in the sheared intrusion. All mineral grains, however, do not bear much information for the phases are deformed or dynamically recrystallized. Feld- plastic deformation, although their aggregates are elon- spar grains are dominated by bulging recrystallization and gated and in overall parallel to the mylonitic foliation do not show obvious intragranular plastic deformation. (Fig. 8a–d). Taking the outcrop and microscopic observa- Quartz grains are elongated (Fig. 7b, c) and recrystallized tions into consideration, the occurrence of such aggregates via grain boundary migration or subgrain rotation recrys- is interpreted as the results of segregation of SiO2-enriched tallization (Fig. 7a). The recrystallized grains also form residual magma and crystallization of quartz grains along oblique foliation due to progressive shearing. Biotite grains oriented boundaries between preexisting feldspar grains, in the sample AL0655-1 are reduced to very small grains preferentially at the triple junctions due to elongation forming biotite bands (Fig. 7a), but muscovite in the during grain growth. sample AL08149-1 tends to develop into porphyroclastic Post-shearing dykes intrude the left-lateral shear zone ‘‘fish’’ (Fig. 7b). (d) Feldspar and amphibole grains form after ductile shearing: (a) They are generally small-scale porphyroclasts of either lens-shaped or fish-shaped r, d and leucocratic dykes of meter to centimeter scale in width S–C fabrics. (e) Owing to a long-lasting progressive (Fig. 6f, g). (b) The contacts between the intrusions and shearing history, the sheared rocks may possess micro- wall rocks are generally sharp and straight, which may structures formed at different stages of shearing, but all the indicate emplacement along brittle fractures. (c) The microstructural styles from high to medium temperatures intrusions are mostly confined to the shear zone, but they consistently document a left-lateral shearing. may cut across the mylonitic rocks or in some cases Samples AL09146 and AL0814-2 are from syn-shearing metamorphic foliations in the wall rocks. (d) Rocks in the dykes which intruded during the left-lateral shearing. Such dykes generally have massive structures, in which equi- dykes are often rootless and are distributed locally within granular quartz, feldspar and micas are randomly distrib- the shear zone (Fig. 6c–e). Rocks in the dykes are weakly uted without any preferred dimensional or crystallographic to strongly sheared, relying on relative timing of magma orientations (Fig. 7d). Quartz and feldspar grains rarely emplacement during progressive deformation. Therefore, show evidences for plastic deformation. there is a progressive microstructural variation from The Th/U isotope systematics of zircon grains from the the dykes intruded early during shearing to those emplaced plutons (dykes) are related to the origins of the zircons. In late during shearing. Late dykes often experienced weak the case of our present study, there is an obvious difference intragranular plastic deformation, for example, sample in Th/U ratios between zircons from different types of AL09146, although minerals in such rocks show strong rocks. The pre-shearing granitic mylonites (samples preferred shape orientation due to magmatic flow (Fig. 8e). AL08149-1 and AL09213-1) and undeformed post-shear- In contrast, early dykes may show structural evidences for ing dyke (sample AL0841-8) have zircons with high Th/U magmatic flow, late magmatic hydrothermal metasomatism ratios (averaging 0.17, 0.56 and 1.01, respectively), or high- to medium-temperature solid-state deformation which suggest that the zircons are originated from mag- after magmatic crystallization. Solid-state ductile defor- matic sources. The syn-shearing samples (AL0814-2 and mation structures may predominate over the magmatic AL09146-3) have zircons with very low Th/U ratios structures in some cases. The sample AL0814-2 is a typical (averaging 0.08 and 0.07, respectively). Such Th/U ratios example of such a dyke. Macroscopically (Fig. 6c–e), the are similar to those of zircons of metamorphic origin. We granitic dyke shows weak deformation in comparison with interpret the origin of such granitic dykes as the conse- the host mylonitic rocks. Foliation and lineation in the dyke quence of crystallization of anatectic magma formed dur- become more obviously developed near the contacts to the ing ductile shearing. Therefore, the two populations of host mylonites than in the central parts of the dyke. They Th/U ratios from various plutons (dykes) can help distinguish are constituted mostly by crystallized minerals in its central dykes of different origins in relation to ductile shearing. parts and some deformed minerals near the contacts to the host mylonites. On the other hand, the contacts between the The initiation and termination of left-lateral shearing dyke and the host rocks are transitional, form more granitic along the ASRR shear zone components in the dyke to more mylonitic components in the host. Furthermore, mafic enclaves in the dyke are From the above descriptions, the samples AL08149-1 elongated and parallel to the dyke/host contacts and foli- (L-tectonites) and AL09213-1 (granitic L-S-tectonites) ations in the mylonites. In the microscope, the subhedral posses characteristics of pre-shearing granitic intrusions. to anhedral crystals of feldspar from the granitic sample The crystallization ages of zircons from the two samples, 123 622 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 that is, 30.9 ± 0.7 Ma and 36.6 ± 0.1 Ma, predate the Abundant fission track ages of the apatites, zircons and initiation of ductile left-lateral shearing, suggesting that the titanites from the southeastern Tibet and Three River area left-lateral shearing has not started before 30.9 ± 0.7 Ma. range from 26 to 20 Ma (Fig. 11, Zhong et al. 2001), Meanwhile, structures and microstructures of the samples revealing an intensive cooling from about 26 Ma. This AL0814-2 and AL09146-3 are resulted from syn-shearing cooling event is well related to tectonic deformation granitic emplacement and progressive ductile shearing. (faulting) along the ASRR and other shear zones (Fig. 11). Th/U ratios from zircons support the above conclusions. At the southernmost end of the ASRR shear zone, the The U–Pb ages of zircons from these samples (25.9 ± initiation of this intense left-lateral shearing along the shear 1 Ma and 27.2 ± 0.2 Ma, respectively), therefore, may zone may have accompanied with a major ridge jump from suggest that a strong left-lateral shearing was already active an E–W direction to a NE–SW major spreading ridge in the between 27 and 26 Ma. In addition, undeformed granitic South China Sea, instead the onset of opening of the Sea dyke (sample AL0841-8) is a reliable indicator of post- since 35 Ma (Briais et al. 1993, Fig. 11). Marine magnetic shearing magmatic activities, which may suggest that profile interpretation showed that seafloor spreading in the ductile shearing may have locally terminated. The zircon South China Sea was asymmetric and confirmed that there U–Pb age of 21.8 ± 1 Ma may constrain the local termi- was at least one ridge jump (Briais et al. 1993). The ridge nation timing of the left-lateral shearing. However, dating jump started from 27 Ma correlated with a variation of the of post-shearing dykes at other localities across and along ridge orientation, from nearly E–W to NE–SW, and with a the shear zone is necessary to definitely constrain the variation in the spreading rate. Their results also indicated timing of termination of ductile shearing. that the propagation of the spreading system in the South The above data bracket the timing of a dominant left- China Sea stopped at ca. 20 Ma, when a steady-state lateral shearing in the range from [ca. 27 to ca. 21 Ma. connection was finally reached between the ASRR left- Taking the Ar–Ar dating results by Wang et al. (1998, lateral strike-slip shear zone and the South China Sea 2000a, 2001) on the Day Nui Con Voi massif and U–Pb spreading center. dating results by Chung et al. (1997) on the Fan Si Pan Based on the litho-biostratigraphic data from the seis- massif along the southern extension of the Ailao Shan belt, mic profile through Ocean Drilling Program (ODP) Site and recent U–Pb dating of magmatic zircons by Cao et al. 1148, there is an unconformity and a lot of slump sedi- (2011) from the Diancang Shan massif into consideration, mentation in a *3 Ma period at ca 28.5–26 Ma in the we would suggest that the left-lateral shearing along the South China Sea (Wang et al. 2003; Li et al. 2005). At ASRR shear zone initiated at about ca. 27 Ma and termi- the same time, the regional sea level fluctuated abruptly nated at ca. 21 Ma. Therefore, the age of initiation of since about *27 Ma (Fig. 11, Wang et al. 2003; Li et al. dominant left-lateral shearing (ca. 27 Ma) in the Ailao 2005). Li et al. (2005) has also noted that there is an Shan area is much later than those (ca. 35 Ma) proposed in abrupt change in Nd isotopes of sediments at the ODP the literature (e.g. Scha¨rer et al. 1990, 1994; Chen et al. 1148 site in the South China Sea at ca. 26–23 Ma. Such a 1992; Harrison et al. 1992, 1996; Leloup and Kienast 1993; change coincided with a major sedimentation disconti- Leloup et al. 1995, 2001a, b, 2007; Zhang and Scha¨rer nuity and a dominant variation of physical properties of 1999; Gilley et al. 2003). The zircon U–Pb age of the the sediments, which implies a drastic change in sedi- undeformed post-shearing granitic dyke (sample AL0841- mentation provenance in South China Sea (Li et al. 2003). 8, 21.83 ± 1.1 Ma) is also much older than the termination Comparison of the Nd isotopes of sediments from major age of 17 Ma that proposed in the literature (Chen et al. rivers flowing into the South China Sea suggests that 1992; Harrison et al. 1992, 1996; Gilley et al. 2003). pre-27 Ma sediments were dominantly derived from a Meanwhile, our results do not support the conclusion that southwestern provenance (Indochina-Sunda Shelf and the left-lateral shearing started after ca. 21 Ma (Searle possibly northwestern Borneo), whereas post-23 Ma sed- 2006) as well. Moreover, the initiation of left-lateral iments were derived from a northern provenance (South shearing (about 27 Ma) in this paper is much more accurate China) (Li et al. 2003). A similar provenance change of than the data (31.9–24.2 Ma) recently proposed by Searle sediments is also recorded in the Pearl River Basin (Shao et al. (2010). et al. 2007). The continental margin basins in the northern South Regional responses to the left-lateral shearing China Sea have also recorded the effects of the intense left- along the ASRR shear zone lateral shearing. There was a rapid tectonic subsidence starting at ca. 26 Ma (Gong et al. 1997). The sedimentation The left-lateral shearing along the ASRR shear zone has rate was up to 500 m/Ma with tectonic subsidence rate up strong sedimentary, tectonic and magmatic responses in the to 150 m/Ma in Yinggehai Basin, and the sedimentation southeastern Tibet. rate up to 450 m/Ma with tectonic subsidence rate up to 123 Int J Earth Sci (Geol Rundsch) (2013) 102:605–626 623

Fig. 11 Regional Cenozoic sedimentation–tectonic–magmatic events Leloup et al. 1993, 2001a; Harrison et al. 1996; Wang et al. 1998, in the southeast Tibet, Three River area and South China Sea 2000a; Zhang and Scha¨rer 1999; Zhang et al. 2006; Liang et al. 2007; (Revised after Briais et al. 1993; Wang et al. 2000c; Zhong et al. Liu et al. 2010, 2012) 2001; Li et al. 2005; Data source of the histograms: Chen et al. 1992;

150 m/ma in Pearl River Basin (Fig. 11, Gong et al. 1997; jump in the South China Sea, which further induced Zhong et al. 2001; Zhu et al. 2009). the formation of sedimentary unconformity and change of sedimentation provenance in the South China Sea, and increases in sedimentation rates in the Yinggehai Conclusions and Pearl River basins.

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