Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yangtze Block)

TECTO-126257; No of Pages 19 Tectonophysics xxx (2014) xxx–xxx

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Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yangtze Block) Zilong Hu a, Xiaoyong Yang a,⁎, Liuan Duan a, Weidong Sun b a b

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China Guangzhou Institute of Geochemistry, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 13 October 2013 Received in revised form 31 March 2014 Accepted 4 April 2014 Available online xxxx Keywords: Geochemistry and geochronology Hf isotope Magmatic evolution High-Mg adakitic rocks Outang intrusion South Tan–Lu fault belt

a b s t r a c t The Outang intrusion was newly found near the South Tan–Lu fault belt (STLF) of the northeastern margin of Yangtze block due to the excavating construction of the Beijing–Shanghai high-speed railway. Observations show that lithology of the Outang intrusion is intermediate rock series, mainly composed of monzodiorite and quartz monzonite. Major and trace elements, zircon U–Pb dating and Hf isotope were analyzed. Two ages of 124.6 ± 2.9 Ma (MSWD = 0.60, n = 29) and 129.2 ± 4.1 Ma (MSWD = 1.5, n = 21) are obtained, showing that the regional igneous activity was in early Cretaceous, being consistent with the massive Yanshanian magmatic events in Eastern China. The quartz monzonite was identified as a high-Mg adakitic rocks, showing geochemical features of high Si, high K, and low Sr, which indicates that the igneous genesis may be results of partial melting of the delaminated lower crust. The negative εHf(t) values with much older tDM2 ages indicate that the original magma may derive from remelting of ancient continental crust. A tectonic model has been proposed to account for the massive subduction of the Pacific Plate beneath eastern China in Jurassic with effect of delamination and thinning, from which we conjecture that the Archean materials could compose the basement of the STLF caused by massive subduction of the Pacific Plate beneath the eastern China continent. © 2014 Published by Elsevier B.V.

1. Introduction The Outang region in Dingyuan County, northeast Anhui Province traditionally belongs to the tectonic unit between the Yangtze block and the North China block, where the Tan–Lu fault occurs across the whole region (BGMRA, 1987; Zhu et al., 2005). However, the poor development of outcrops has improved due to the excavation of the Beijing–Shanghai high-speed railway construction in 2009, as a large area of concealed intrusive rocks was revealed, made it possible to take a field trip and observation, thus we named this excavated igneous rock as the Outang intrusion. The Outang intrusion is located in the north Yangtze plate, distributing along the STLF, it is noticeable that this region is the tectonic joints relative to the North China Craton, Yangtze block and Qinling–Dabie– Sulu orogenic belt (BGMRA, 1987). The region has been a hot spot in the study of tectonics and magmatic activities in China (Dai et al., 2003; Hou et al., 2007; Xie et al., 2008, 2009). Using the accurate SHRIMP zircon U–Pb dating, Zi et al. (2008) once obtained the formation age of 131.5 ± 1.6 Ma on the Guandian intrusion nearby. Niu et al. (2008, 2010) who studied some of the granites sporadically distributed in the southern Feidong–Chaohu region in the Zhangbaling uplift with several small intrusions such as the Xixucunbei intrusion, Jianshan ⁎ Corresponding author. E-mail address: [email protected] (X. Yang).

intrusion, Xihuacun intrusion, Yongfeng intrusion and Jinzhangcun intrusion, obtained diagenetic ages between 103Ma and 126.9 Ma, indicating that these granite intrusions were formed in the early Cretaceous, but their emplacement history could be different. Ages of the four high-Mg adakitic rock intrusions named as Xiaolizhuang, Damaocun, Fangjiangzhuang and Qiaotouji intrusions nearby were measured at 125 Ma, 128 Ma, 129 Ma and 130 Ma, respectively (Liu et al., 2010), interpreted as the younger tendency away from the Tan– Lu fault belt. As a newly excavated intrusion, the Outang intrusion has not been studied so far. We made systematic studies, including detailed field work, sampling, measurements on major elements and trace elements, zircon U–Pb dating by LA-ICP-MS and zircon Hf isotopes, trying to present its lithological features, ages, geochemical properties and origin of magmatic source, so as to discuss its forming mechanism. 2. Regional geological background and samples The study region belongs to a part of STLF (Fig. 1), which is one of the main fault belt with a series of giant NE trend faults. In China, it extends more than 2400 km, throughout different tectonic units in eastern China, both its scale and structure are complicated. It is suggested that the Tan–Lu fault formed in Mesoproterozoic, and experienced multiphase tectonic process. As an active fault belt, the Tan–Lu fault is a “long-life” deep fault belt with shear movement primarily, inherited

http://dx.doi.org/10.1016/j.tecto.2014.04.004 0040-1951/© 2014 Published by Elsevier B.V.

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

Fig. 1. Tectonic sketch map showing location of study region (according to BGMRA, 1987).

the new tectonic movement (Hou et al., 2007; Wang et al., 2005; Zhu et al., 2001, 2004). We carried out geological observation and sample collections during the high-speed railway construction (Fig. 2a), where the Outang intrusion is composed of different igneous facies, mainly consisting of diorite and quartz monzonite (Fig. 2b). Consequently, we deduced that the intrusive rocks had experienced multiple magmatic activities. Part of this intrusion periphery has been mineralized with gossan on the surface.

Consulting from 1:200,000 scale regional geological map of the Dingyuan region, this stripped intrusion may belong to the concealed southward extending part of the Guandian intrusion. 3. Petrography The quartz monzonite mainly consists of plagioclase (~ 35%), K-feldspar (30%), quartz (~ 20%), and abundant biotite (~ 10%), with

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

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Fig. 2. Explosions of igneous rocks in the Outang pluton, Dingyuan (a, b). Microphotographs of quartz monzonite (c, d, e, f and g) and monzodiorite (h) in Outang intrusion. The images were taken by the microscope under plane and crossed polars. The scale bar for image c, d, e and f is500 μm and for image g and h is 250 μm. Kfs—K-feldspar; Pl—plagioclase; Qtz—quartz; Bi—biotite. c—quartz monzonite composed of K-feldspar, quartz, and biotite; K-feldspar has conspicuous Carlsbad twins. d—quartz and biotite, quartz has tiny crack. e—K-feldspar, showing a lot of regular streaks. d—K-feldspar with zonal extinction. g—plagioclase, showing strong sericite alteration, with many biotite fragment. h—plagioclase shows strong sericite alteration under plane polarized microscope, mineral boundary is unclear.

minor accessory phases, e.g., pyroxene, zircon, apatite (b5%). The Kfeldspar has conspicuous Carlsbad twins, and zonal structure, with sericite alteration in the central zones and unaltered in the margin,

which is called clean margin texture (Fig. 2c, f). Quartz appears as subhedral or anhedral crystals (Fig. 2c, d), with clear wavy extinction phenomenon under the crossed polarized microscope (Fig. 2d). The

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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Table 1 Results of the major chemical compositions of igneous rocks from Outang, Dingyuan (wt.%). Sample

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

Cr2O3

TiO2

MnO

P2O5

SrO

BaO

LOI

Total

090T04 090T05 090T06 090T07 090T08 090T09 090T10 090T11 090T12 090T13 090T14 090T15 090T16

55.70 55.26 54.75 65.33 64.11 67.01 67.25 67.49 64.75 66.46 6500 66.50 66.34

15.46 15.76 15.10 14.93 14.21 14.77 14.98 14.43 14.55 14.91 14.84 14.82 14.53

6.28 7.18 6.58 3.13 3.18 2.93 2.91 2.88 3.29 2.84 3.34 3.17 3.28

3.83 3.21 3.93 3.38 4.40 2.68 2.26 2.15 3.14 2.68 2.78 2.50 2.23

4.15 3.69 3.87 1.23 1.60 1.11 1.00 1.04 1.61 1.77 1.56 1.94 2.4

3.64 2.80 3.39 3.61 3.47 3.48 3.71 3.74 4.20 3.68 4.28 3.76 3.87

3.79 5.78 4.18 4.39 4.28 4.9 4.56 4.65 4.28 4.65 4.32 4.35 3.86

0.04 0.04 0.04 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

1.05 1.07 1.03 0.46 0.44 0.43 0.44 0.48 0.46 0.43 0.47 0.44 0.44

0.16 0.07 0.10 0.06 0.08 0.05 0.05 0.06 0.06 0.06 0.05 0.05 0.05

0.451 0.459 0.436 0.177 0.175 0.166 0.178 0.189 0.166 0.165 0.172 0.166 0.175

0.08 0.09 0.08 0.07 0.19 0.06 0.07 0.07 0.06 0.09 0.06 0.07 0.07

0.19 0.28 0.20 0.13 0.14 0.15 0.15 0.14 0.15 0.17 0.17 0.16 0.14

5.03 4.27 5.12 2.81 3.57 2.19 2.26 2.16 3.07 2.08 2.91 1.74 1.39

99.85 99.96 98.8 99.71 99.86 99.94 99.84 99.48 99.80 99.99 99.96 99.68 98.79

monzodiorite undergoes a grave alteration, mineral boundary is unclear and difficult to distinguish, the plagioclase shows strong sericite alteration under plane polarized microscope (Fig. 2h).

were used. Raw count rates for 172Yb, 173Yb, 175Lu, 176(Hf + Yb + Lu), 177 Hf, 178Hf, 179Hf, 180Hf and 182W were recorded simultaneously, and isobaric interference corrections for 176Lu and 176Yb on 176Hf were

4. Analytical methods 4.1. Major and trace elements Major elements of whole rock samples were analyzed by X-ray fluorescence spectrometry (XRF) and trace elements including rare earth elements were analyzed by Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) in the ALS Mineral Lab in Guangzhou. Accuracy and precision of the data are better than 10% on the basis of international standard reference material (SRM) analytical results and replicate analyses. 4.2. EPMA Major elements of biotite were determined by using a wavelength dispersive JEPL JXA-8800R electron microprobe analyses (EMPA) at the CAS Key Laboratory of Crust–Mantle Materials and Environments, the University of Science and Technology of China (USTC), Hefei, with the following operating conditions: 15 kV accelerating voltage, 20 nA beam current and b 5 μm beam diameter. The calibration was based on a suite of mineral standards and oxide standards from the American Standard Committee. 4.3. LA-ICP-MS zircon analytical methods Zircons were separated from samples 09OT07 and 09OT12, mounted on adhesive, enclosed in epoxy resin, polished, and then photographed in reflected light. The internal structure of the zircons was examined using the cathodoluminescence (CL) image technique in the Analytical Center of University of Science and Technology of China (USTC). The laser ablation ICP-MS (LA-ICP-MS) zircon U–Pb analyses were carried out at the Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, USTC. The GeoLas 200M laserablation system equipped with a 193 nm ArFexcimer laser was used in connection with ELAN6100 DRC ICP-MS. Helium was used as the carrier gas to enhance the transport efficiency of the ablated material. All measurements were performed using zircon 91500 as the external standard with a recommended 206Pb/238U age of 1065.4 ± 0.6 Ma (Wiedenbeck et al., 1995). Common Pb correction was carried out by using the EXCEL program of ComPbCorr#3_181 (Andersen et al., 2002). Ages were calculated using the ISOPLOT program (Ludwig, 2003). Zircon Hf isotope was in-situ measured on a Neptune multi-collector ICP-MS (with a 193 nm laser ablation microprobe attached) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an. Spot sizes of ca. 60 μm and a laser repetition rate of 10 Hz at 100 mJ

Fig. 3. Classifications of rock types of igneous rocks in the Outang pluton, Dingyuan, STLF (Data are from Table 1). a. Na2O + K2O–SiO2 diagram (after Middlemost, 1994). Ir—Irvine dividing line, above is alkaline, below is sub-alkaline (Irvine and Baragar, 1971). Plutonic: 1— peridotite gabbro; 2a—alkaline gabbro; 2b—sub-alkaline gabbro; 3—gabbro-diorite; 4—diorite; 5—granodiorite; 6—granite; 7—quartzolite; 8—monzogabbro; 9—monzodiorite; 10— monzonite; 11—quartz monzonite; 12—syenite; 13—nepheline gabbro; 14—nepheline monzodiorite; 15—nephelinemonzosyenite; 16—nephelinesyenite; 17—foidolite; 18— tawite/neapite/italite. Volcanic: 1—picro-basalt; 2—basalt; 3—basaltic andesite; 4—andesite; 5—dacite; 6—rhyolite; 7—silicate-rich rock; 8—trachy-basalt; 9—Basaltic trachyandesite; 10—trachyandesite; 11—trachydacite; 12—trachyte; 13—tephrite; 14— phonotephrite; 15—tephriphonolite; 16—phonolite; 17—Foidite; 18—sodalitite/nephelinite/leucitite. b. K2O–SiO2 diagram. Solid line is from Peccerillo and Taylor (1976), dashed line is from Middlemost (1985).

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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made based on precise determinations. The 176Lu was calibrated using the 175 Lu value, while 176Yb/172Yb ratio of 0.5887 and mean βYb value obtained on the same spot were employed for the interference correction of 176 Yb on 176Hf. Details of the analytical technique are published by Wu et al. (2006). During analyses, the 176Hf/177Hf and 176Lu/177Hf ratios of

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the standard zircon 91500 were 0.282307 ± 0.000035 (2σ, n = 29) and 0.00030, respectively. The measured average 176Hf/177Hf ratio agrees with those acquired by solution method (0.282302 ±0.000008 [2σ], (Goolaerts et al., 2004); 0.282306 ± 0.000008 [2σ],(Woodhead et al., 2004)).

Fig. 4. Harker diagrams showing the major element variations of the Outang monzodiorite and quartz monzonite, STLF.

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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5. Results 5.1. Major element compositions Major element results are shown in Table 1. Igneous rocks from the Outang intrusion are classified into monzodiorite and quartz monzonite (Fig. 3a), and belong to the shoshonitic series based on the classification of Irvine and Baragar (1971), where the quartz monzonite is close to the high K calc-alkaline series (Fig. 3b). The K2O and Na2O contents of all samples exhibit positive correlations with MgO, P2O5, TiO2, CaO and Fe2O3 contents, and negative correlations with SiO2 contents (Fig. 4), illustrating that fractional crystallization likely played an important role during igneous rock formation in the Outang region.

5.2. Trace elements Results of the REE and parameters and trace elements of igneous rocks in the Outang pluton are shown in Table 2, respectively, where it can be seen that the rare earth element parameters of monzodiorite and quartz monzonite are similar, their distribution patterns exhibit strong enrichments in LREEs relative to HREEs, showing features of flat HREE patterns (Fig. 5), indicating that fractionations between LREE and HREE are obvious. Monzodiorite samples have LREE/HREE ratios between 16.07 and 16.72 (average [a.v.] = 16.32), (La/Yb)N between 31.69 and 34.46 (a.v. = 33.30); and the quartz monzonite samples have LREE/ HREE ratios between14.81 and 17.65 (a.v. = 15.96), (La/Yb)N between 21.91 and 27.92 (a.v. = 25.13). But the ∑REE contents of monzodiorite (281.80 ppm–299.67 ppm, a.v. = 288.85 ppm) are considerably higher than those of quartz monzonite (114.56 ppm–150.91 ppm, a.v. =

135.09 ppm). Slightly Eu negative anomaly has been observed in both monzodiorite and quartz monzonite with average δEu = 0.93 and 0.88. They are enriched with large ion lithophile elements (LIIEs) and depleted of high field strength elements (HFSEs), with pronounced negative anomalies of Nb, Ta and positive anomalies of Pb (Fig. 6). The Harker diagram shows the trace element variations of the Outang monzodiorite and quartz monzonite (Fig. 7), where trends between Nb and other trace elements for monzodiorite and quartz monzonite are distinctly different (Fig. 7a, b, c,d), the La, Y, Zr and Hf exhibit positive correlation with Nb, U and Sr exhibit positive correlation with Nb (Fig. 7e,f). Interestingly, data in Fig. 7 for monzodiorite and quartz monzonite rarely overlap, indicating their different magma sources. Both the quartz monzonite and monzodiorite have high Sr (512 ppm–1525 ppm, a.v. = 659 ppm; 653 ppm–741 ppm, a.v. = 690 ppm), low Y (7.7 ppm–10.2 ppm, a.v. = 8.97 ppm; 15.8 ppm– 16.4 ppm, a.v. = 16.0 ppm) and HREE contents (e.g., Yb = 0.79– 1.04 ppm and 1.29–1.40 ppm). Consequently, they also resemble high ratios of Sr/Y and (La/Yb)N. According to these geochemical features, the Outang quartz monzonite and monzodiorite are identified as adakitic rocks (Fig. 8), Accordingly, these adakitic rocks are calculated with moderately high Mg# ranging from 40.50 to 59.17 (a.v. = 49.41) for monzonite and 50.45 to 56.69 (a.v. = 53.65) for monzodiorite, respectively.

5.3. Zircon U–Pb dating Two samples of quartz monzonite (09OT07 and 09OT12) have been analyzed with the LA-ICP-MS method and their zircon U–Pb dating analysis result is listed in Table 3. The 206Pb/238U age of sample 09OT07 is 114 ± 6 Ma ~ 132 ± 8 Ma, and the weighted average is

Table 2 Trace element concentrations, the REE contents and the parameters of igneous rock samples from the Outang pluton, Dingyuan (ppm). Sample

090T04

090T05

090T06

090T07

090T08

090T09

090T10

090T11

090T12

090T13

090T14

090T15

090T16

Ba Rb Sr Y Co Cr Ni Zr Nb Th Pb V Hf Cs Ta U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ∑REE LREE HREE LREE/HREE (La/Yb)N δEu δCe

1625 136.0 653 15.8 28.1 290 142 276 13.2 7.01 34 105 6.3 5.48 0.8 2.04 64.6 124.5 14.40 50.8 8.54 2.45 7.78 0.96 3.68 0.63 1.78 0.20 1.29 0.19 281.80 265.29 16.51 16.07 0.18 0.92 0.98

2390 177.5 741 16.4 26.3 290 142 286 13.5 7.11 61 96 6.4 8.44 0.8 3.84 69.0 133.0 15.15 54.2 8.87 2.54 7.92 1.00 3.67 0.66 1.90 0.21 1.35 0.20 299.67 282.76 16.91 16.72 0.18 0.93 0.99

1740 138.5 676 15.9 26.7 290 139 284 13.2 7.29 46 107 6.4 6.74 0.7 2.83 65.8 126.5 14.50 50.7 8.49 2.49 7.57 0.98 3.70 0.66 1.88 0.22 1.40 0.19 285.08 268.48 16.60 16.17 0.18 0.95 0.99

1080 113.5 547 8.8 8.2 100 33 164 9.3 12.60 29 49 4.9 7.28 0.8 1.46 33.2 58.4 6.48 21.8 3.57 0.98 3.55 0.40 1.75 0.34 0.98 0.12 0.97 0.16 132.70 124.43 8.27 15.05 0.16 0.84 0.96

1195 105.0 1525 9.5 9.2 100 30 137 9.3 13.35 30 54 4.2 6.22 0.9 2.00 33.8 60.1 6.57 22.1 3.47 0.97 3.48 0.42 1.84 0.36 1.07 0.12 1.04 0.16 135.50 127.01 8.49 14.96 0.16 0.85 0.97

1255 123.0 512 7.7 7.5 90 28 121 8.5 13.05 28 47 3.7 8.13 0.7 1.39 28.7 50.1 5.61 19.2 2.98 0.88 3.00 0.35 1.51 0.28 0.92 0.11 0.79 0.13 114.56 107.47 7.09 15.16 0.17 0.90 0.95

1220 116.0 548 8.0 8.7 100 37 152 9.6 17.50 29 43 4.5 7.36 0.8 1.67 33.4 57.9 5.97 19.6 3.10 0.94 3.26 0.37 1.56 0.30 0.94 0.11 0.88 0.14 128.47 120.91 7.56 15.99 0.15 0.90 0.99

1165 123.5 518 8.1 7.3 100 25 188 9.6 14.85 24 47 5.5 5.38 0.8 1.33 31.0 56.7 5.88 19.7 3.16 0.93 3.19 0.38 1.67 0.31 1.01 0.12 0.86 0.15 125.06 117.37 7.69 15.26 0.16 0.90 1.01

1305 99.0 516 8.9 9.2 90 30 186 8.6 11.85 25 50 5.3 1.92 0.7 2.19 32.4 57.3 6.36 21.8 3.56 1.02 3.56 0.42 1.76 0.33 1.01 0.12 0.93 0.14 130.71 122.44 8.27 14.81 0.17 0.88 0.96

1465 96.7 642 8.5 9.2 90 34 144 8.6 11.80 22 48 4.2 2.18 0.7 1.75 35.5 63.0 6.61 22.5 3.38 1.02 3.67 0.40 1.74 0.31 0.99 0.11 0.90 0.14 140.27 132.01 8.26 15.98 0.16 0.89 0.99

1475 94.9 545 10.0 10.1 100 35 161 10.1 13.50 27 56 4.7 1.71 0.8 2.55 36.7 65.3 6.91 23.3 3.68 0.98 3.09 0.36 1.81 0.34 1.03 0.13 0.92 0.16 144.71 136.87 7.84 17.46 0.16 0.89 0.99

1425 91.3 634 10.0 10.1 100 32 164 9.6 14.05 26 53 4.8 2.22 0.8 2.43 38.1 67.0 7.03 23.2 3.77 1.01 3.15 0.39 1.83 0.33 1.04 0.13 0.92 0.15 148.05 140.11 7.94 17.65 0.16 0.90 0.99

1285 82.9 604 10.2 10.2 100 36 167 9.8 14.40 24 56 4.9 1.73 0.8 3.05 38.5 68.3 7.21 23.9 3.75 1.01 3.31 0.39 1.91 0.34 1.04 0.13 0.96 0.16 150.91 142.67 8.24 17.31 0.16 0.88 0.99

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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122.9 ± 1.6 Ma (MSWD = 0.78) (Fig. 9b), The zircon U–Pb concordant age is 124.6 ± 2.9 Ma (MSWD = 0.60, n = 29) (Fig. 9a); and the 206Pb/ 238 U age of sample 09OT12 is 116 ± 6 Ma–148 ± 8 Ma with weighted average of 129.8 ± 3.3 Ma (MSWD = 1.8) (Fig. 9d), and zircon U–Pb concordant age of 129.8 ± 3.3 Ma (MSWD = 1.8) (Fig. 9c). 5.4. Zircon Hf isotopic compositions Two samples of quartz monzonite (09OT07 and 09OT12) have been analyzed with the LA-ICP-MS method and their zircon Hf isotopic analysis result is listed in Table 4. The 176Hf/177Hf value of sample 09OT07 is 0.282040–0.282224, the corresponding εHf(t) is negative with the range from −19.37 to −25.88 (Fig. 10b), and the two-stage Hf isotope model age (tDM2) is 2235 Ma–2641 Ma (Fig. 10a); The 176Hf/177Hf value of sample 09OT12 is 0.282043–0.282407, the corresponding εHf(t) is negative with the range from −12.92 to −25.78 (Fig. 10d), and the two-stage Hf isotope model age (tDM2) is 1823 Ma–2629 Ma (Fig. 10c). 6. Discussion 6.1. Genesis of the Outang adakitic rocks 6.1.1. K2O contents and K2O/Na2O ratios The Outang quartz monzonite in the Table 5 shows distinctly high K2O contents (3.86–4.65 wt.%, a.v. = 3.78 wt.%) and K2O/Na2O (1.00– 1.41, a.v. = 1.17), the monzodiorite shows the same features with the K2O contents ranging from 3.79 to 5.78 wt.% (a.v.–4.58 wt.%) and

Fig. 5. Distribution patterns of REE of the monzodiorite and quartz monzonite from the Outang pluton, Dingyuan, STLF (a—monzodiorite; b—quartz monzonite) (Primative mantle normalization values are from Sun and McDonough, 1989).

7

K2O/Na2O = 1.04–2.06(a.v. = 1.40). These features are similar to those of low-Mg adakitic rocks from the Dabie orogen (Fig. 11), which were regarded to be derived from partial melting of eclogitic LCC with over-thickened mountain root (He et al., 2010; Wang et al., 2007). This phenomenon can be plausible with the explanation of amphibole absence in sources. As we know, amphibole is the main K-bearing mineral, having a very much higher K2O content than other rock-forming minerals during high-pressure melting, such as garnet and clinopyroxene. The presence of amphibole in the residual can relieve K in the melt, thus generating low-K siliceous melts. Conversely, residual assemblage without amphibole partial melting of dry mafic LCC rocks is likely to generate high K melts (Huang and He, 2010). The Y/Yb ratio and the distribution of HREE is associated with the composition of residual, and it has a difference in the distribution pattern between garnet and amphibole. Garnet is strongly enriched in HREE, while the amphibole is relatively enriched in MREE (Green, 1994). Therefore, melt would be distinguished by significant depletion in HREE with Y/Yb N 10 and (Ho/Yb)N N 1.2 when residual is dominated by garnet, otherwise, a flat distribution pattern of HREE (with Y/Yb ≈ 10 and (Ho/Yb)N ≈ 1) would suggest that amphibole is the major residual phase. The average Y/Yb ratio and (Ho/Yb)N of Outang monzodiorites are 11.91 and 1.45, respectively, whereas the Outang quartz monzonites are 9.78 and 1.06, indicating that the Outang monzodiorites and quartz monzonites may be derived from different sources, in which quartz monzonites have amphibole as residual. Regarding the K-bearing phlogopite in quartz monzonites, we suggest that the Outang quartz monzonites may have phlogopite presence in the source.

Fig. 6. Distribution patterns of trace element of the monzodiorite and quartz monzonite fromthe Outang pluton, Dingyuan (a—monzodiorite; b—quartz monzonite) (Primative mantle normalization values are from Sun and McDonough, 1989).

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

8

Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

Fig. 7. Harker diagrams showing the trace element variations of the Outang monzodiorite and quartz monzonite, STLF.

6.1.2. MgO and Mg# [Mg/(Mg + Fe)] The Outang adakitic rocks have high-MgO contents and Mg# than the experimental melts (Rapp et al., 1999) (Fig. 12). The MgO contents of the Outang quartz monzonite adakitic rocks range from 1.00 wt.% to 2.40 wt.% (a.v. = 1.53), the Mg# from 40.50 to 59.17, (a.v. = 49.41), whereas, the MgO contents of the Outang monzodiorites adakitic rocks range from 3.69 wt.% to 4.15 wt.% (a.v. = 3.90), and the Mg# from 50.45 to 56.69 (a.v. = 53.65). In fact, four possibilities may cause the differences in MgO contents and Mg#: (1) Mg# and MgO of the initial magmas, (2) degrees of interaction with the mantle (different extents of interaction with the mantle, a process by which silicic melts can dramatically elevate their MgO and Mg# (Prouteau et al., 2001; Rapp et al., 1999), (3) degrees of magma differentiation, and/or (4) extents of crustal contamination. Because this group of adakitic rocks was intruded into the Yangtze Block, more crustal contamination would produce higher contents of incompatible elements (e.g., potassium) (DePaolo, 1981), which corresponds to what is observed in Fig. 11. Thus, the high K contents of Outang adakitic rocks indicate that their high Mg# and MgO may be derived from crustal contaminations.

Fig. 8. The Sr/Y vs. Y correlation from igneous rocks in the Outang pluton, Dingyuan, STLF (after Defant and Drummond, 1990).

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

6.1.3. Trace elements Geochemical characteristics of the Outang adakitic rocks show enrichments in LREE and LILE, depletions in HFSE (Nb, Ti, and Ta), lacking positive Eu anomalies (Fig. 6), and these features are typical of adakitic rocks (Defant and Drummond, 1990). It is accepted that the positive correlation between Sr/Y and (La/ Yb)N of the low-Mg adakitic rocks in the Dabie orogen could be due to different degrees of partial melting of thickened LCC at high pressures, with absence of plagioclase in the residues (He et al., 2010). Given that the high values of Sr/Y and (La/Yb)N of an adakitic rock magma are produced by various levels of partial melting of an eclogite or garnet amphibolite, it should be a positive correlation between them, for both Sr and La are incompatible whereas both Y and Yb are compatible in the

9

garnet-bearing and plagioclase-free residues (Defant and Drummond, 1990; Moyen, 2009; Rapp and Watson, 1995). The consistent trend in the plot of Sr/Y vs. (La/Yb)N suggests that they both were derived from partial melting of the LCC of the Yangtze Block (Fig. 13). These data fit in the no-mineralization trend proposed by Liu et al. (2010), which is much different from those Cu–Au mineralized adakitic rock in the central-south Anhui Province formed at the similar period, the later is regarded as an oceanic slab melting origin related to Pacific plate subduction to east China (Deng et al., 2012; Liu et al., 2010; Xie et al., 2012; Yang et al., 2011). Aside from partial melting of delaminated lower crustal materials, there are several origins of these adakitic characteristic like high Sr low Y and high Sr/Y, such as (1) fractional crystallization of mafic magma

Table 3 Analytical data of zircon LA-ICP-MS U–Pb ages of quartz monzonite samples in the Outang pluton, Dingyuan. Analysis point

Th(×10−6)

U(×10−6)

Th/U

207

1σ

207

1σ

206

1σ

206

1σ

09OT07-01 09OT07-02 09OT07-03 09OT07-04 09OT07-05 09OT07-06 09OT07-07 09OT07-08 09OT07-09 09OT07-10 09OT07-11 09OT07-12 09OT07-13 09OT07-14 09OT07-15 09OT07-16 09OT07-17 09OT07-18 09OT07-19 09OT07-20 09OT07-21 09OT07-22 09OT07-23 09OT07-24 09OT07-25 09OT07-26 09OT07-27 09OT07-28 09OT07-29 09OT07-30 09OT12-01 09OT12-02 09OT12-03 09OT12-04 09OT12-05 09OT12-06 09OT12-07 09OT12-08 09OT12-09 09OT12-10 09OT12-11 09OT12-12 09OT12-13 09OT12-14 09OT12-15 09OT12-16 09OT12-17 09OT12-18 09OT12-19 09OT12-20 09OT12-21 09OT12-22 09OT12-23 09OT12-24 09OT12-25 09OT12-26 09OT12-27

153.26 408.14 104.80 147.58 97.08 233.41 301.03 101.95 173.35 184.28 224.58 214.84 122.19 63.05 123.19 295.82 202.27 158.11 119.01 89.21 124.34 175.81 393.30 174.86 49.91 204.10 131.83 113.19 109.54 190.15 143.06 150.51 110.86 153.45 2584.59 463.88 359.00 138.10 93.29 129.74 148.49 141.28 111.36 143.88 103.51 172.01 144.64 164.91 197.42 157.91 77.14 93.90 95.91 140.37 132.86 152.14 109.92

159.37 327.68 134.48 137.15 137.69 222.61 342.54 112.75 168.10 198.03 202.40 207.97 152.68 57.02 147.80 274.96 180.90 175.07 113.60 109.98 133.48 161.85 314.47 166.88 47.46 201.89 152.11 143.61 107.37 192.21 135.09 177.29 114.95 192.51 697.78 456.88 247.73 142.18 80.77 158.85 146.86 154.81 129.69 166.37 137.42 172.36 142.90 159.08 209.11 193.18 82.87 117.63 172.38 160.82 132.03 150.75 177.38

0.96 1.25 0.78 1.08 0.71 1.05 0.88 0.90 1.03 0.93 1.11 1.03 0.80 1.11 0.83 1.08 1.12 0.90 1.05 0.81 0.93 1.09 1.25 1.05 1.05 1.01 0.87 0.79 1.02 0.99 1.06 0.85 0.96 0.80 3.70 1.02 1.45 0.97 1.15 0.82 1.01 0.91 0.86 0.86 0.75 1.00 1.01 1.04 0.94 0.82 0.93 0.80 0.56 0.87 1.01 1.01 0.62

0.04551 0.04981 0.06349 0.04857 0.14760 0.04957 0.04809 0.05831 0.05571 0.05201 0.05197 0.04539 0.05040 0.10102 0.05260 0.05385 0.05642 0.06330 0.03761 0.09486 0.08345 0.00000 0.05503 0.04749 0.12562 0.05211 0.05014 0.05166 0.05099 0.05106 0.08899 0.04766 0.11448 0.05501 0.10467 0.08940 0.05424 0.06312 0.17900 0.12705 0.04808 0.04861 0.04450 0.04826 0.04414 0.07841 0.14124 0.05459 0.05726 0.05232 0.05789 0.04823 0.11582 0.04588 0.14347 0.01118 0.10065

0.01022 0.00606 0.01195 0.01339 0.02735 0.00904 0.00716 0.01300 0.01088 0.01225 0.01002 0.00831 0.01330 0.09085 0.01149 0.00932 0.01580 0.01252 0.01470 0.02222 0.01597 0.00000 0.00633 0.01527 0.00790 0.01150 0.01128 0.01591 0.01806 0.00888 0.02194 0.01556 0.06391 0.01571 0.01711 0.01548 0.01049 0.01030 0.04463 0.02083 0.01561 0.01434 0.01514 0.01523 0.02177 0.01319 0.00956 0.01767 0.01368 0.01694 0.02412 0.01935 0.01022 0.01187 0.02368 0.08085 0.00991

0.12595 0.13291 0.16950 0.12926 0.35668 0.12573 0.14029 0.13627 0.13066 0.12559 0.12612 0.12934 0.13247 0.15129 0.13231 0.13599 0.13260 0.16486 0.12269 0.19802 0.19899 0.13095 0.14525 0.12350 6.16225 0.12491 0.13677 0.14195 0.14647 0.13439 0.25586 0.14201 0.12071 0.14773 0.31798 0.27033 0.14456 0.40173 0.37431 0.39906 0.12317 0.14829 0.14005 0.12578 0.14273 0.19610 7.04470 0.13238 0.13813 0.13880 0.16890 0.12137 4.63769 0.13229 0.29356 0.11808 3.83630

0.02762 0.01554 0.03019 0.03685 0.06378 0.02241 0.02112 0.03012 0.02311 0.02667 0.02339 0.02368 0.03430 0.08577 0.02695 0.02375 0.03939 0.03668 0.04296 0.04190 0.03672 0.03750 0.01677 0.03838 0.41335 0.02688 0.03113 0.04079 0.04447 0.02354 0.05909 0.04472 0.07360 0.04365 0.05619 0.05091 0.02833 0.05880 0.09692 0.06968 0.03699 0.04411 0.04351 0.03329 0.05625 0.03472 0.49581 0.04175 0.03298 0.04710 0.06779 0.04606 0.42877 0.03366 0.04653 0.07189 0.41458

0.01869 0.01997 0.02007 0.01916 0.01912 0.01863 0.02049 0.01930 0.01936 0.01867 0.01882 0.01936 0.01947 0.02066 0.01973 0.01898 0.01947 0.01983 0.01855 0.01784 0.01925 0.01939 0.01891 0.01856 0.35116 0.01865 0.02042 0.01909 0.01869 0.01952 0.02127 0.02094 0.01949 0.02130 0.02075 0.02071 0.02166 0.04753 0.02096 0.02116 0.01864 0.02202 0.02020 0.01902 0.02145 0.02018 0.35489 0.01972 0.02079 0.02063 0.02329 0.01823 0.28406 0.01964 0.01783 0.01855 0.27935

0.00062 0.00059 0.00074 0.00076 0.00074 0.00063 0.00057 0.00085 0.00083 0.00059 0.00069 0.00059 0.00063 0.00128 0.00067 0.00054 0.00070 0.00077 0.00076 0.00091 0.00080 0.00078 0.00054 0.00068 0.00889 0.00071 0.00076 0.00069 0.00075 0.00067 0.00090 0.00078 0.00119 0.00090 0.00076 0.00065 0.00108 0.00329 0.00126 0.00104 0.00076 0.00092 0.00092 0.00079 0.00103 0.00091 0.01002 0.00074 0.00079 0.00071 0.00121 0.00093 0.01216 0.00079 0.00092 0.00094 0.01082

119 127 128 122 122 119 131 123 124 119 120 124 124 132 126 121 124 127 118 114 123 124 121 119 1940 119 130 122 119 125 136 134 124 136 132 132 138 299 134 135 119 140 129 121 137 129 1958 126 133 132 148 116 1612 125 114 118 1588

4 4 5 5 5 4 4 5 5 4 4 4 4 8 4 3 4 5 5 6 5 5 3 4 42 4 5 4 5 4 6 5 8 6 5 4 7 20 8 7 5 6 6 5 7 6 48 5 5 4 8 6 61 5 6 6 55

Pb/206Pb

Pb/235U

Pb/238U

Pb/238U (Ma)

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

derived from melting mantle (Li et al., 2009; Macpherson et al., 2006), (2) partial melting of the metasomatized mantle (Martin et al., 2005), and (3) magma mixing (Guo et al., 2007). Firstly, if those adakitic magma generated through partial melting of such metasomatized mantle underwent fractional crystallization, the high Sr low Y and high Sr/Y features must be formed by the fractional crystallization with relatively enriched HREE (like garnet), instead of the property of the initial magma, in other words, the Sr and Sr/Y of these adakitic rocks magma must increase as the Mg# decreases. However, Fig. 15 does not show these features, thus we deduce that Outang adakitic rocks could not be a product of fractional crystallization of mafic magma. Secondly, a group of adakites which have been formed by melting of a peridotitic mantle wedge whose composition has been modified by reaction with felsic slab-melts was described by Martin et al. (2005), as “low-silica adakites” (LSA). Low-silica adakites have 50% b SiO2 b 60%, and correlatively high contents in compatible elements. Sr concentrations vary in a wide range from 1000 ppm to 3000 ppm in LSA. As a contrast, Outang adakitic rocks have higher SiO2 (~55% to ~ 67%) and obvious lower Sr concentrations (almost c.a. 500–600 ppm). Thus Outang adakitic rocks are not generated by melting of the metasomatized mantle. At last, magma mixing could not be a possible mechanism of Outang adakitic rocks, as no coeval mafic rocks coexist with Outang adakitic rocks observed in the surrounding area. (See Fig. 14.) We collected published geochemical data from adjacent intrusions, such as Guandian (Zi et al., 2008), Wawuliu and Wawuxue (Niu et al., 2002), Fangjiangzhuang, Damaocun, Xiaolizhuang and Qiaotouji (Liu

et al., 2010), and Chuzhou and Shangyaopu (Zi et al., 2007), in view of the significant resemblance in geochronology (Guandian, 131.5 ± 1.6 Ma; Wawuliu, 127.87 ± 0.46 Ma; Wawuxue, 120.00 ± 0.50 Ma; Fangjiangzhuang, 129.1 ± 1.1 Ma; Damaocun, 128.1 ± 1.2 Ma; Xiaolizhuang, 125.1 ± 1.3 Ma; Qiaotouji, 131.7 ± 1.8 Ma; Chuzhou, 127.17 ± 0.40 Ma; and Shangyaopu, 129.90 ± 0.23 Ma), we suggest that the prediction of these STLF adakites for the origin of Outang adakitic rocks is quite reasonable. Some thickened Yangtze lower crust-drived adakites in Dabie orogen, Meichuan and Guanghui (He et al., 2010), Chituling (Huang et al., 2008), and several adakites from the lower Yangtze River Belt (LYRB) (Li et al., 2009; Liu et al., 2010; Wang et al., 2003, 2004a, 2004b, 2006; Xu et al., 2002) are also shown for comparison (Fig. 15), which are in trending towards the typical EM-1 end member, with similarity to those of Yangtze LCC-derived adakitic rocks (Fig. 15), the STLF adakites could be derived from partial melting of the Yangtze LCC, so could be the Outang adakitic rocks. 6.1.4. Zircon CL, U–Pb age and Hf isotope compositions The CL images display the zircon surface contents of some trace elements (such as U, Y, Dy, Tb) and/or the difference of lattice defects, general zircon U, REE and Th contents of trace elements zircon cathodoluminescence intensity is weaker. CL images reflect the internal structure of zircon as the most clear, commonly used and most effective method in zircon internal structure study (Wu and Zheng, 2004). Sample 09OT07 and sample 09OT12 are quartz monzonite, and most of their selected zircons show idiomorphic crystals, particle sizes range

Fig. 9. Ages of quartz monzonite samples in the Outang pluton, Dingyuan.

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

11

Table 4 Results of Hf isotopes of zircon in the Outang pluton, Dingyuan. Analysis point

176

Lu/177Hf

176

Hf/177Hf

2σ

t* (Ma)

ε(0)

ε(t)

tDM1 (Ma)

tDM2 (Ma)

07_01 07_02 07_03 07_04 07_05 07_06 07_07 07_08 07_09 07_10 07_11 07_12 07_13 07_14 07_15 07_16 07_17 07_18 07_19 07_20 07_21 07_23 07_24 12_01 12_02 12_03 12_04 12_06 12_07 12_08 12_09 12_10 12_11 12_12 12_13 12_14 12_15 12_16 12_17 12_18 12_19 12_20 12_21 12_22 12_23 12_24

0.000769 0.000512 0.000649 0.001002 0.000743 0.000831 0.000837 0.000676 0.001003 0.001001 0.000657 0.000608 0.000372 0.000775 0.000722 0.000356 0.000533 0.000867 0.000637 0.000767 0.000621 0.000666 0.000420 0.000520 0.000596 0.000747 0.000770 0.000683 0.000622 0.000605 0.000784 0.000585 0.000630 0.000768 0.000682 0.000796 0.000505 0.000639 0.000673 0.000779 0.000791 0.000691 0.000967 0.000673 0.000653 0.000866

0.282117 0.282152 0.282139 0.282123 0.282177 0.282139 0.282096 0.282139 0.282224 0.282190 0.282100 0.282082 0.282126 0.282138 0.282192 0.282040 0.282062 0.282164 0.282141 0.282169 0.282120 0.282173 0.282112 0.282046 0.282068 0.282086 0.282160 0.282105 0.282046 0.282075 0.282094 0.282094 0.282100 0.282144 0.282121 0.282209 0.282043 0.282407 0.282088 0.282145 0.282191 0.282058 0.282182 0.282055 0.282107 0.282223

0.000011 0.000017 0.000012 0.000013 0.000011 0.000012 0.000015 0.000013 0.000012 0.000014 0.000010 0.000012 0.000012 0.000010 0.000013 0.000012 0.000011 0.000010 0.000010 0.000012 0.000011 0.000010 0.000015 0.000015 0.000012 0.000022 0.000016 0.000014 0.000016 0.000014 0.000013 0.000017 0.000014 0.000012 0.000012 0.000016 0.000009 0.000013 0.000012 0.000011 0.000013 0.000009 0.000012 0.000012 0.000012 0.000015

119.4 128.1 124.6 125.7 130.8 119.0 124.6 123.2 120.2 123.6 119.2 124.3 131.8 126.0 114.0 118.5 122.9 120.7 123.8 118.6 130.3 122.0 119.4

−2.32 −2.19 −2.24 −2.29 −2.10 −2.24 −2.39 −2.24 −1.94 −2.06 −2.38 −2.44 −2.29 −2.24 −2.05 −2.59 −2.51 −2.15 −2.23 −2.13 −2.30 −2.12 −2.33 −2.57 −2.49 −2.43 −2.16 −2.36 −2.57 −2.47 −2.40 −2.40 −2.37 −2.22 −2.30 −1.99 −2.58 −1.29 −2.42 −2.22 −2.06 −2.53 −2.08 −2.54 −2.35 −1.94

−23.16 −21.94 −22.40 −22.95 −21.03 −22.39 −23.92 −22.40 −19.37 −20.60 −23.76 −24.39 −22.86 −22.42 −20.51 −25.88 −25.13 −21.52 −22.31 −21.31 −23.05 −21.19 −23.33 −25.68 −24.88 −24.26 −21.65 −23.57 −25.66 −24.66 −23.99 −23.96 −23.75 −22.21 −23.02 −19.92 −25.78 −12.92 −24.20 −22.18 −20.56 −25.26 −20.85 −25.36 −23.51 −19.43

1589 1531 1554 1590 1505 1561 1621 1555 1449 1498 1608 1630 1561 1560 1483 1677 1655 1528 1550 1516 1578 1508 1581 1676 1649 1631 1530 1601 1680 1640 1622 1612 1606 1552 1580 1463 1680 1183 1625 1551 1488 1667 1506 1670 1597 1447

2472 2389 2421 2456 2332 2424 2517 2422 2235 2310 2509 2545 2444 2422 2309 2641 2592 2369 2416 2357 2458 2347 2481 2623 2573 2535 2371 2492 2622 2559 2518 2516 2502 2407 2457 2263 2629 1823 2531 2405 2303 2597 2322 2603 2487 2233

from 50 μm to 200 μm, and form as graininess to columnar with the length–width ratio from 1 to 4. The CL images also show oscillatoryzones which is a feature of typical magmatogenic zircon (Fig. 16). Some researchers have shown that the width of oscillatory-zones may be related to the temperature of magma when zircon crystallized. Trace elements diffuse faster at higher temperature, so the oscillatory-zones are wider, on the contrary, the oscillatory-zones are narrow at low temperature because the trace elements diffuse more slowly (Wu and Zheng, 2004). The character of narrow oscillatory-zones observed in Fig. 16 indicates that the zircon may be crystallized at a low temperature. A large number of studies show that zircons with different geneses have different Th, U contents and Th/U ratio: magmatic zircon have higher Th and U contents with Th/U generally more than 0.4; in contrast, metamorphic zircons have lower Th, U contents with most Th/U ratios less than 0.1. The Th/U ratio of magmatic zircon is related to the Th and U contents of magma and distribution coefficient between zircon and magma, it can be represented in Eq. (1):   Th U ðTh=UÞzircon ≌ D =D

zircon=melt

 ðTh=UÞmelt :

ð1Þ

Usually, (DTh/DU)zircon/melt approximately equals to 0.2, and Th/U ratio in an average crustal material is about 4, as a result, magmatic

zircon has a Th/U ratio of c.a. 1 (Wu and Zheng, 2004). As we can see in Fig. 17, most datum points are close to the Th/U = 1 line, indicating that the zircons in this study are magmatic origin. The zircon U–Pb dating for the two samples of Outang adakitic rocks shows that their ages are 124.6 ± 2.9 Ma and 129.8 ± 3.3 Ma, which correspond to other studies in the nearby region, the age of quartz monzonites of Wawuliu intrusion and Wawuxue intrusion in the Zhangbaling tectonic region is 128 Ma (Niu et al., 2002); The SHRIMP zircon U–Pb age of Mesozoic Guandian intrusion in the Zhangbaling region is 131.5 ± 1.6 Ma (Zi et al., 2008); Liu et al. (2010) carried out a systematic study on four intrusions along STLF named Fangjiangzhuang, Qiaotuoji, Damaocun and Xiaolizhuang nearby, zircon U–Pb dating results show that they have ages of 129.1 ± 1.1 Ma, 131.7 ± 1.3 Ma, 128.1 ± 1.2 Ma, 125.1 ± 3 Ma, with a tendency of age gradually becoming young as far away from the south Tan–Lu Fault belt. The adakitic rocks we identified in this study, with four high-Mg adakites nearby and three high-Mg adakites in the eastern margin of the Dabie orogen, compose a high-Mg adakitic rock belt along the Tan–Lu Fault belt, indicate that the large-scale activity of Tan–Lu Fault in early Cretaceous may play an important role in inducing the delamination of thickened LCC. However, being different from the four adakites in (Liu et al., 2010), we found some Late Proterozoic inherited zircon (206Pb/238U age =

Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004

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Z. Hu et al. / Tectonophysics xxx (2014) xxx–xxx

Fig. 10. Diagram of tDM2 v.s. εHf(t) of zircons from igneous rocks in the Outang pluton, Dingyuan, STLF (a—tDM2 distribution histogram of sample 09OT07; b—εHf(t) distribution histogram of sample 09OT07; c—tDM2 distribution histogram of sample 09OT12; d—εHf(t) distribution histogram of sample 09OT12).

1600 ~ 2000 Ma), which is regarded as low-Mg adakitic rocks in Dabie orogen (He et al., 2011; Wang et al., 2001, 2007; Xu et al., 2007) and high-Mg adakitic rocks in other regions (Gao et al., 2004; Xu et al., 2006) with lots of inherited zircon. This observation indicates that the magma may have undergone significant crustal contamination during ascent, or the magma formed in a low temperature (coincide with the narrow oscillatory-zones in zircon CL image), caused the unfinished fusion of inherited zircon. Although it is generally considered that the Yangtze Craton contains ancient Archean components with inherited zircons (ages N 2.5 Ga), they are mostly identified in a small area near Kongling, northwest of the Yangtze Craton (Jiao et al., 2009; Liu et al., 2008; Qiu et al., 2000; Zhang et al., 2006), indicating that either the crustal block was small or that the Yangtze Craton might have been extensively destroyed and recycled (Zheng and Zhang, 2007). However, results of U–Pb ages and Hf isotope manifest that of the xenocrystic zircons were brought to surface by lamproites within Proterozoic outcrops, suggesting a widespread unexposed Archean basement, with zircon age populations of 2900– 800 Ma and 2600–2500 Ma and Hf model ages of 2.6 to ca. 3.5 Ga or older, indicating that Archean materials might be located in the deep crust (Zheng et al., 2006b). In the north Yangtze River region, the oldest zircon identified from paragenesis in the Dongling Group is 2372 ± 10 Ma (Grimmer et al., 2003), indicating that Paleoproterozoic basement might be present in the region, although the whole rock data from the Hongzhen batholith give a Nd model age of 1.8–2.0 Ga (Wang et al., 2004a). In analyzed samples, quartz monzonites from the Outang batholith show a large variation of Hf isotopic compositions, with εHf(t) values ranging from −19.37 to −25.88 and −12.92 to −25.78, corresponding to Hf model ages of 2235 Ma–2641 Ma and 1823 Ma–2629 Ma,

respectively. Therefore, it can be concluded that some Archean materials may be added to the magmatic systems in the Outang region. The εHf(t) values are negative, and two-stage Hf model ages (close to 2.5 Ga) of these two intrusive rock types are far greater than the zircon U–Pb age, indicating that these intrusive rocks were probably derived from ancient crustal, as re-melted result of Paleoproterozoic continental crust materials. When primitive mantle differentiated to crust and depleted mantle, the crust had obviously lower Lu/Hf ratio than depleted mantle. Furthermore, the 176Hf/177Hf values of crust increase slower relatively during evolution, so the εHf values become more and more negative. On the contrary, the εHf of depleted mantle get more and more positive. In consequence, if zircon initial εHf N 0, then there were more mantle source or new crustal material in it when metamorphic rock sources formed. Otherwise, the crustal material played a dominant role (Wu et al., 2007). The initial εHf of both two samples in this study are negative, ranging from −2.59 ~ −1.94 (a.v. = −2.25) and −2.58 ~ −1.29 (a.v. = −2.29), respectively, indicating that the crustal material played a dominant role during formation of quartz monzonite. 6.2. Temperature and petrogenesis Hydrothermal experiments in the temperature range of 750– 1020 °C have defined the saturation behavior of zircon in crustal anatectic melts as a function of both temperature and composition (Watson and Harrison, 1983). The results provide a model of zircon solubility given by: zircon=melt

lnDZr

¼ f−3:80−½0:85ðM−1Þg þ 12900=T

ð2Þ

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Table 5 Chemical compositions of the Outang quartz monzonite. 09OT071

09OT072

09OT073

09OT074

09OT075

09OT091

09OT092

09OT093

09OT094

09OT095

09OT131

09OT132

09OT133

09OT135

37.52 3.76 13.30 17.81 0.11 14.25 0.00 0.05 9.88 0.00 0.04 0.05 96.76

36.30 3.70 13.25 19.10 0.33 12.91 0.00 0.08 9.46 0.09 0.05 0.00 95.26

36.88 4.06 13.29 19.10 0.20 12.81 0.00 0.06 9.57 0.09 0.02 0.16 96.23

37.12 3.72 13.05 19.04 0.26 13.45 0.00 0.08 9.47 0.13 0.15 0.06 96.53

36.87 3.94 13.39 19.65 0.26 13.64 0.00 0.11 9.25 0.09 0.12 0.11 97.40

37.31 3.70 13.07 19.11 0.22 13.10 0.00 0.06 9.51 0.07 0.07 0.19 96.39

36.29 3.47 13.25 19.42 0.25 12.71 0.00 0.08 9.39 0.03 0.07 0.03 94.99

36.99 3.29 13.04 19.65 0.22 13.59 0.00 0.06 9.31 0.00 0.14 0.12 96.40

36.45 4.07 13.19 18.59 0.25 12.74 0.00 0.09 9.61 0.06 0.11 0.00 95.15

35.03 3.62 13.71 19.89 0.27 13.77 0.00 0.07 9.17 0.07 0.05 0.15 95.80

39.73 2.94 14.35 17.34 0.16 12.89 0.00 0.09 7.90 0.08 0.05 0.38 95.90

35.89 3.50 13.20 19.03 0.35 13.31 0.00 0.03 9.00 0.00 0.10 0.23 94.64

36.18 3.64 13.38 19.76 0.23 14.51 0.00 0.08 8.54 0.06 0.19 0.08 96.65

Mineral formulae based on O = 22 Na 0.03 0.02 K 1.84 1.89 Si 5.70 5.62 Al 2.36 2.35 Mg 2.95 3.18 Fe 2.35 2.24 Ni 0.00 0.00 Cr 0.00 0.00 Ti 0.40 0.42 Mn 0.02 0.01 Total 15.65 15.73 Mg/Mg + Fe 0.41 0.44 T/°C 694.24 708.21 P/Mpa 61.52 57.72 H/km 1.86 1.75

0.02 1.85 5.57 2.39 2.95 2.46 0.01 0.01 0.43 0.04 15.74 0.40 703.62 72.16 2.19

0.02 1.86 5.60 2.38 2.90 2.43 0.01 0.00 0.46 0.03 15.68 0.40 715.16 66.63 2.02

0.03 1.83 5.61 2.32 3.03 2.41 0.02 0.02 0.42 0.03 15.72 0.41 703.63 50.66 1.54

0.03 1.77 5.54 2.37 3.05 2.47 0.01 0.01 0.44 0.03 15.73 0.41 710.40 63.64 1.93

0.02 1.84 5.65 2.33 2.96 2.43 0.01 0.01 0.42 0.03 15.69 0.41 702.32 52.69 1.60

0.03 1.85 5.59 2.40 2.92 2.51 0.00 0.01 0.40 0.03 15.74 0.40 693.94 74.67 2.26

0.02 1.80 5.61 2.33 3.07 2.50 0.00 0.02 0.37 0.03 15.75 0.41 685.12 52.49 1.59

0.03 1.88 5.59 2.38 2.91 2.39 0.01 0.01 0.47 0.03 15.70 0.41 717.64 68.11 2.06

0.02 1.80 5.38 2.48 3.15 2.56 0.01 0.01 0.42 0.04 15.86 0.41 701.52 98.05 2.97

0.03 1.50 5.89 2.50 2.85 2.16 0.01 0.01 0.33 0.02 15.28 0.43 666.46 105.60 3.20

0.01 1.78 5.55 2.40 3.06 2.47 0.00 0.01 0.41 0.05 15.73 0.41 697.63 74.78 2.27

0.02 1.65 5.47 2.38 3.27 2.50 0.01 0.02 0.41 0.03 15.76 0.42 701.60 67.73 2.05

Component oxides (wt.%) SiO2 37.43 TiO2 3.46 Al2O3 13.15 FeO* 18.41 MnO 0.18 MgO 13.01 CaO 0.00 0.10 Na2O K2O 9.44 NiO 0.00 Cr2O3 0.03 F 0.15 Total 95.36

where Dzircon/melt is the concentration ratio of Zr in the stoichiometric Zr zircon to that in the melt, T is the absolute temperature, and M is the cation ratio (Na + K + 2Ca)/(Al·Si). According to Eq. (2), zircon saturation temperature of Outang monzodiorite is calculated as the range from 785–795 °C, and Outang quartz monzonite from 726–784 °C. The temperature is the critical control factor for the Ti content in biotite (Henry et al., 2005; Patiño Douce, 1993; René et al., 2008), thus the Ti concentration could be used as a biotite geothermometer. Using the empirical formula provided by Henry et al. (2005):  T¼

 3 0:333 ln ðTiÞ−a−c XMg

ð3Þ

where T is temperature in °C, Ti is the apfu normalized to 22 O atoms, XMg is Mg/(Mg + Fe), and the a, b, and c parameters are given, the crystallization temperature of Outang quartz monzonite is calculated as the range from 666 to 718 °C, suggesting that the intrusive magma has relatively higher temperature. Our estimated temperature based on biotite geothermometer is consistent with the result inferred from Ti–Mg/(Mg + Fe) diagram (Fig. 18b). The total cations of Al in biotite has a positive correlation with the consolidation pressure of granite(Uchida et al., 2007), following the equation as: T

Pð100MPaÞ ¼ 3:03  Al−6:53ð0:33Þ

ð4Þ

In Eq. (4), TAl represents the total cations of Al in biotite with assumed 22 oxygen atoms. The crystallization pressure of biotite in Outang quartz monzonite intrusion is from 51 MPa to 106 MPa, equivalent to a depth of1.54 km to 3.20 km. The composition of biotite can reflect the magmatic material source (Abdel-Rahman, 1994; Batchelor, 2003; Shabani et al., 2003; Wones and Eugster, 1965; Zhang, 1982; Zhou, 1988). The FeO/(FeO + MgO) vs. MgO diagram (Fig. 18c) shows the Outang quartz monzonite was derived from crust–mantle mixed source. 6.3. Tectonic implications from the high-Mg adakites in eastern China

Fig. 11. K2O/Na2O vs. Al2O3 diagram of Outang adakitic rocks, STLF, thickened lower crustderived low-Mg adakitic rocks from the Dabie orogen (He et al., 2010; Wang et al., 2007), and oceanic slab-derived adakites (after Kamei et al., 2009).

One of the most important geological events occurring in eastern China from late Jurassic to early Cretaceous is the development of the Tan–Lu fault (Xu and Zhu, 1994; Xu et al., 1987; Zhu et al., 2005). The Tan–Lu fault was recently suggested to be critically associated with lithospheric weakening and thinning of the North China Craton in the Mesozoic (Menzies et al., 2007). Adakites observed inside the Dabie orogen have low-Mg content (Wang et al., 2007; Xu et al., 2007), while high-Mg adakite has only been reported from Chituling site (Huang et al., 2008),

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Fig. 14. The Sr and Sr/Y vs. Mg# diagrams for Outang High-Mg adakities.

Fig. 12. MgO (a) and Mg# (b) vs. SiO2 (wt.%) diagrams for Outang adakitic rocks, STLF. Mantle AFC curves, with proportions of assimilated peridotite indicated, are after Stern and Kilian (1996) (Curve 1) and Rapp et al. (1999) (Curve 2). The field of adakites inferred to be of subduction oceanic crust origin is after Defant and Kepezhinskas (2001). The data for metabasaltic and eclogite experimental melts (1–4.0 GPa), and peridotite-hybridized equivalents, are from Rapp et al. (1999).

Fig. 13. Sr/Y vs. (La/Yb)N diagram for Outang adakitic rocks, STLF Adakites related to slab melting in subduction zones (Aguillón-Robles et al., 2001; Beate et al., 2001; Defant and Drummond, 1990; Defant et al., 1991; Morris, 1995; Puig et al., 1984; Stern and Kilian, 1996) and thickened lower crust-derived low-Mg adakitic rocks in the Dabie orogen (He et al., 2010; Wang et al., 2007) are also shown for comparison. The positive correlation between Sr/Y and (La/Yb)N in the Dabie adakitic rocks was interpreted to reflect partial melting of the eclogitic lower crust at various degrees (He et al., 2010).

which presents the similar geochemical features to our samples and other high-Mg adakites from South Tan–Lu Fault belt (Liu et al., 2010). Distributions of the late Mesozoic high-Mg adakitic rocks generally follow the Tan–Lu fault, forming a high-Mg adakitic rock belt from the eastern Dabie orogen to Sulu orogen along the eastern boundary of the NCB. Notably, the high-Mg adakitic rock belt and the Tan–Lu fault are also spatially coincident with the region where the thinnest

Fig. 15. Sr–Nd initial isotopic composition of STLF high-Mg adakities (Liu et al., 2010; Niu et al., 2002; Zi et al., 2007, 2008), Dabie orogen thickened Yangtze LCC-drived adakites (He et al., 2010; Huang et al., 2008) and LYRB adakites (Li et al., 2009; Liu et al., 2010; Wang et al., 2003, 2004a, 2004b, 2006; Xu et al., 2002), Cenozoic slab-derived adakites (Defant and Kepezhinskas, 2001) and depleted Cenozoic basalts (Nushan from Anhui Province, Fangshan from Jiangsu Province)(Zou et al., 2000) are shown for comparison.

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Fig. 16. CL imagines of zircon grains from igneous rocks in the Outang pluton, Dingyuan, STLF (a,b—sample 09OT07; c—sample 09OT12).

lithosphere–asthenosphere boundary (LAB) in the NCB has recently been revealed by seismic tomographic study in (Chen et al., 2008). The coupling between the age of adakitic rocks and the large-scale strike-slip of the Tan–Lu fault may also suggest the key role of the trans-lithospheric fault in lithospheric delamination and thinning processes (Huang et al., 2008; Liu et al., 2010). In Early Cretaceous, the rapid northward movement of the Izanagi plate (20.7 cm/year) continued and the resultant highly oblique subduction beneath Asia terminated the formation of accretionary complex along the Japan margin. Instead, a transform margin appeared between

Fig. 17. Th vs. U diagram of zircons from igneous rocks in the Outang pluton, Dingyuan, STLF.

Asia and the Izanagi Plate (Sun et al., 2007). Development of relevant sinistral strike-slip tectonics is suggested for the inland part of Asia, for example, the Tan–Lu fault in China (Maruyama et al., 1997). Subduction of the West Pacific plate cause the presence of active continental margin magmatic arc settings in the east of China, result in the extensive magmatic activity along Tan–lu fault zone and flanking fracture system at the same term (Zhu et al., 2004). Consequently, samples from Outang pluton fall into VAG region in Y vs. Nb and Yb vs. Ta diagram (Fig. 19). The evolution of the Tan–Lu fault and magmatic activities, suggesting that the major geological events in eastern China in the Cretaceous were mainly controlled by the subduction of the Pacific plate, and that plate interactions during subduction are important driving forces for geological evolution in eastern China and intraplate tectonics in general (Sun et al., 2007). Due to the massive subduction of the Pacific Plate beneath eastern China in Jurassic, the NCC lithosphere was extruded and became thickened, and leads to the transformation of LCC basalt to eclogite, which has greater density (Fig. 20a). The gravitational instability of eclogite can cause delamination of the lower crust, resulting in large-scale lithospheric thinning (Qiao et al., 2012; Wu et al., 2005a, 2005b, 2008) (Fig. 20b). After the delamination and thinning in Early Cretaceous, lithospheric mantle and part of LCC descended into the asthenosphere and participated in the mantle circulation, and still a minor part of them melted. At the same time, the upwelling asthenosphere made it possible to contact directly with LCC, thus heat conduction caused the partial melting of LCC, and produced large amounts of eclogite magma along giant fault such as Tan–Lu Fault (Xu et al., 2008; Zheng et al., 2006a, 2007) (Fig. 20c), performanced as a wide magmatism in Eastern China and culminated at ~ 125 Ma (Wu et al., 2005a), corresponding to the Outang magmatic event.

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Fig. 18. Results of mineral compositions by EPMA. a—BSE images of biotite in Outang quartz monzonite. b—diagram of Ti–Mg/(Mg + Fe) for Outang quartz monzonite (after Henry et al., 2005). c—diagram of FeO/FeO + MgO vs. MgO for Outang quartz monzonite (after Zhou, 1988).

may be the partial melting of delaminated lower crust, which experienced mantle contamination during magma ascent.

7. Conclusions From the results of elements, zircon U–Pb age determination and Hf isotopic analyses for the Mesozoic igneous rocks in the Outang region, we draw the following conclusions: 1) Igneous rocks in the Outang region can be identified as monzodiorite and quartz monzonite in petrology with geochemical features of high Sr contents, high Sr/Y and La/Yb, but low Y and Yb contents. The high Mg# and high Cr and Ni contents indicate that the Outang igneous rocks are high-Mg adakitic rocks, and their igneous genesis

2) Zircon U–Pb ages of the Outang intrusion show that magma activities were between 124 and 129 Ma, corresponding to the massive magmatic activity in eastern China in Early Cretaceous. Differences between the two ages imply that the magmatism in the Outang region might last for a relatively short period of time. 3) The negative initial εHf values of quartz monzonites indicate that the crustal materials played a dominant role when metamorphic rock source formed. The two-stage Hf model age are close to 2.5 Ga,

Fig. 19. Y vs. Nb (a) and Yb vs. Ta (b) diagram of igneous rocks samples in the Outang pluton, Dingyuan, STLF (after Pearce et al., 1984).

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Fig. 20. Model figure of the North China Craton delamination with the subduction of the Pacific Plate.

suggesting that the origin of these igneous rocks was probably derived from old crustal, as a result of Archean continental crust remelting. 4) A tectonic model has been proposed to account for the massive subduction of the Pacific Plate beneath eastern China in the Jurassic with effect of delamination and thinning, from which we conjecture that the Archean materials could compose the basement of the STLF caused by massive subduction of the Pacific Plate beneath eastern China continent.

Acknowledgment This study was supported by the Natural Science Foundation of China (Grant Nos. 41090372, 41372087, 41173057 and 90814008). Especially, we express our sincere thanks to the referees for such detailed proof-reading, which improved the manuscript greatly.

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Please cite this article as: Hu, Z., et al., Geochronological and geochemical constraints on genesis of the adakitic rocks in Outang, South Tan–Lu Fault Belt (Northeastern Yan..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.04.004