Early Cretaceous continental delamination in the Yangtze Block: Evidence from high-Mg adakitic intrusions along the Tanlu fault, central Eastern China

Accepted Manuscript Early Cretaceous continental delamination in the Yangtze Block: Evidence from high-Mg adakitic intrusions along the Tanlu fault, central Eastern China Liqiong Jia, Xuanxue Mo, M. Santosh, Zhusen Yang, Dan Yang, Guochen Dong, Liang Wang, Xinchun Wang, Xuan Wu PII: DOI: Reference:

S1367-9120(16)30164-X http://dx.doi.org/10.1016/j.jseaes.2016.06.001 JAES 2720

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

7 December 2015 29 May 2016 4 June 2016

Please cite this article as: Jia, L., Mo, X., Santosh, M., Yang, Z., Yang, D., Dong, G., Wang, L., Wang, X., Wu, X., Early Cretaceous continental delamination in the Yangtze Block: Evidence from high-Mg adakitic intrusions along the Tanlu fault, central Eastern China, Journal of Asian Earth Sciences (2016), doi: http://dx.doi.org/10.1016/ j.jseaes.2016.06.001

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Early Cretaceous continental delamination in the Yangtze Block: Evidence from high-Mg adakitic intrusions along the Tanlu fault, central Eastern China Liqiong Jia1,2,3, Xuanxue Mo3, M. Santosh3,5, Zhusen Yang4, Dan Yang4, Guochen Dong3, Liang Wang3,6, Xinchun Wang1,2, Xuan Wu1,2

1

Development Research Center, China Geological Survey, Beijing, 100037, China

2

National Geological Archive of China, Beijing, 100037, China

3

School of Earth Science and Nature Resources, China University of Geosciences, Beijing,

100083, China 4

Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037,

China 5

Centre for Tectonics Resources and Exploration, Dept of Earth Sciences, University of

Adelaide, SA 5005, Australia 6

Gold Geology Institute of Chinese Armed Police Force, Langfang 065000, China

Corresponding Author: Liqiong Jia Development Research Center, China Geological Survey No. 45, Fuwai Street, Beijing 100037, P. R. China E-mail: [email protected]

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Abstract Early Cretaceous high-Mg adakitic rocks from central Eastern China provide important insights into the thinning mechanism of the over-thickened lithosphere in the Yangtze Block (YB) as well as the North China Block (NCB). The Tanlu fault (TLF), located between the North China and Yangtze Blocks, has been considered as a prominent pathway of magmas and fluids that resulted in lithosphere thinning of the YB during the Mesozoic–Cenozoic. Here we report the petrology, whole-rock geochemistry, zircon U–Pb geochronology, in situ Hf isotopes, and whole-rock Sr–Nd–Pb isotopes of four high-Mg adakitic intrusions along the TLF in northeastern Langdai. These adakitic intrusions consist of monzodiorite, quartz monzonite porphyry, and quartz monzodiorite. Zircon LA-MC-ICPMS analyses of five samples yield weighted mean 206Pb/238U ages of 127.58 ± 0.80, 126.90 ± 0.81, 120.71 ± 0.64, 122.75 ± 0.57, and 129.2 ± 1.1 Ma, indicating their emplacement during the Early Cretaceous. The intrusions have intermediate SiO2 (53.18–65.48 wt. %) and high potassium (K2O = 3.07–3.95 wt. %; Na2O/K2O = 1.02–1.26) and are classified as shoshonitic to high-K calc-alkaline series. They are characterized by high MgO (1.80–7.35 wt. %), Mg# (50–65), Sr (591–1183 ppm), Ni (20.3–143.0 ppm), and Cr (51.40–390.0 ppm) contents, high (La/Yb)N (11.60–28.33) and Sr/Y (27.9–113.5) ratios, and low Y (7.79–22.4 ppm) and Yb (0.60–2.01 ppm) contents, comparable with high-Mg adakites. The samples are enriched in light rare earth elements but depleted in heavy rare earth elements and high field strength elements with slightly negative to positive Eu anomalies (δEu = 0.81–1.30), resembling the features of high-Mg adakitic rocks. Their whole-rock εNd(t)= −16.2 to −15.0, initial (87Sr/86Sr)i = 0.7060−0.7074, low radiogenic Pb (206Pb/204Pb(t) = 16.208–16.509,

207

Pb/204Pb(t) =

3

15.331–15.410, and

208

Pb/204Pb(t) = 36.551–36.992), and zircon εHf(t) = −36.6 to −16.6

suggest magma derivation from a continental crustal source. The geochemical and isotopic features, in combination with existing geological data, suggest that the intrusions are high-Mg adakites and, by comparison with contemporaneous intrusions in Eastern China, were likely to have been formed by partial melting of over-thickened basaltic lower crust following the delamination of eclogitic lithosphere during the Late Jurassic to Early Cretaceous. It is possible that the TLF played a key role in lithospheric delamination and thinning and in the generation of the high-Mg adakitic rocks.

Keywords: High-Mg adakitic rocks; Delamination; Tanlu fault; Zircon U−Pb geochronology; Petrogenesis

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1. Introduction The Middle–Lower Yangtze River (MLYR) region is located in central Eastern China where many Early Cretaceous intermediate to felsic calc-alkaline intrusions occur (Fig. 1(b), Chang et al., 1991; Zhai et al., 1992; Pan and Dong, 1999). The tectonic setting, particularly the deep dynamics and mechanism of large-scale magmatic activity in the MLYR during the Mesozoic, has been hotly debated. Most geologists agree that the MLYR (or the whole of Eastern China) has experienced thickening, delamination, and thinning of the lithosphere, and basaltic underplating, although opinions are diverse on the causes and mechanisms for these processes, as well as on the relationship between large-scale magmatic activity and underplating (Deng et al., 1994, 1996; Wu and Sun, 1999; Wu et al., 2000, 2003; Xu et al., 2002; Gao et al., 2004; Wang et al., 2007a; Zhang et al., 2001a, 2002b; Kay and Kay, 1993). The results from receiver function studies along the Tanlu fault (TLF) suggest that the lithosphere in this region has been thinned to ca. 60–80 km from about 180 km thickness in the Paleozoic, providing compelling evidence for significant lithosphere thinning (Chen et al., 2006). Some geochemical studies have also emphasized that the TLF zone played an important role in the lithosphere thinning during the Mesozoic–Cenozoic (Xu, 2001; Xu et al., 2004; Huang et al., 2008; Guo et al., 2013). Recent investigations in the MLYR have revealed the occurrence of several intermediate-acid intrusive rocks ranging from the southeastern part of Hubei Daye–Yangxin (Teishan, Tonglüshan), through the southeast of Anhui Huaining–Lujiang–Tongling (Yueshan, Shaxi, Tongguanshan), to the Ningzhen (Anjishan) of Jiangsu. These intrusions share features similar to adakites and show high Al2O3, Sr, Sr/Y, and La/Yb ratios, Na

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enrichment, and low Y and Yb. Especially, the adakitic intrusive rocks from Ningzhen possess high MgO and Mg# [where Mg# = 100 Mg/(Mg + TFe2+)] (Xu et al., 2002). Along the south Tanlu fault (STLF) zone, adjacent to the MLYR, there are several Early Cretaceous dioritic intrusions that also display high-Mg adakitic geochemical signatures (Huang et al., 2008; Zi et al., 2008; He et al., 2009; Liu et al., 2010b; Xu et al., 2012). Detailed studies of these adakitic rocks are important for understanding the relationship between the Yanshanian magmatic activity in Eastern China and the TLF activity, as well as the processes of lithospheric thinning. The SinoProbe Program conducted a multidisciplinary transect (termed the Eastern Langdai) across the Nanjing–Wuhu ore district of the MLYR. The transect is about 300 km long, starting from Huzhou in Zhejiang Province in the northern Yangtze Block (YB), through the east-central part of the MLYR, and to the south STLF zone, culminating at Huainan in Anhui Province on the southeastern margin of the North China Block (NCB) (Fig. 1(c)) (Shi et al., 2013). In this paper, we present petrography, geochronology, geochemistry, and whole-rock Sr–Nd–Pb and zircon Hf isotopic compositions from four representative Early Cretaceous high-Mg adakitic intrusions in the north margin of the MLYR, adjacent to the STLF. Combined with regional geological and geochemical evidence, the results are further used to discuss the evidence for delamination of the lower continental crust in the northern margin of the MLYR close to the TLF and the important role of the TLF in formation of high-Mg adakitic rocks. We also evaluate the foundering mechanism of the lower continental crust and lithospheric thinning processes based on the distribution of the Mesozoic high-Mg adakitic rocks along the STLF.

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2. Geological background and samples The MLYR is located on the northern margin of the South China Block and runs along the southeastern margin of the NCB and Dabie ultrahigh-pressure (UHP) metamorphic belt (Fig. 1). It is bounded by the Xiangfan–Guangji fault (XGF) to the northwest, the Tancheng–Lujiang

regional

strike-slip

fault

(TLF)

to

the

northeast,

and

the

Yangxing–Changzhou fault (YCF) to the south (Fig. 1). The stratigraphic sequence in the MLYR is composed of Late Neoproterozoic low-grade metasedimentary rocks and carbonates, Late Triassic to Jurassic lake- and swamp-facies sediments and intercalated coal beds, and Cretaceous evaporates, red beds, and terrestrial volcanic rocks. Late Mesozoic igneous rocks are widespread in the MLYR and are mainly composed of Late Jurassic to Cretaceous calc-alkaline intrusive rocks, Early Cretaceous subalkaline to alkaline volcanic rocks (Pei and Hong, 1995; Chen et al., 2001; Xie et al., 2008; Jia et al., 2014), and A-type granitoids, which consist of quartz syenite, syenite, quartz monzonite, and alkaline granite (Xing, 1999). The magmatic rocks possessing geochemical features similar to adakites are widespread in the MLYR. These adakitic rocks likely played an important role in the polymetallic mineralization. The Dabie–Sulu UHP metamorphic belt was formed by continental collision between the YB and the NCB in the Triassic (Li et al., 1993; Hacker et al., 1998, 2000; Meng and Zhang, 2000; Wu and Zheng, 2013). On the eastern end of the Dabie orogen, the left-lateral movement of the TLF displaced the Sulu orogen northward by ~500 km during the Late Mesozoic time (Zhu et al., 2005). The TLF is a giant continental fault scale running through

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Eastern China for a length of more than 2400 km. The STLF, which constitutes a tectonic suture between the YB and the NCB, occurs in the present study area. The Early Cretaceous dioritic to granodioritic intrusions located in the eastern margin of the Dabie orogen in the southern segment of the TLF have been identified as high-Mg adakitic rocks and include the Chituling, Guanghui, Meichuan, Fangjiangzhuang, Damaocun, and Xiaolizhuang intrusions (Huang et al., 2008; He et al., 2009; Liu et al., 2010a, 2010b). In the northern section of the STLF, the Early Cretaceous Guandian (Zi et al., 2008), Wawuliu, and Wawuxue intrusions (Niu et al., 2002) were also categorized as high-Mg adakitic rocks. In this study, we collected 10 samples from four intrusions in the eastern segment of the STLF in the northern margin of the Yangtze Block: Fuxiao (FX-1), Qiaotouji (QTJ-3), Shangyaopu (SYP-1), and Guoying (GY-1) (Fig. 1). The samples are composed of monzodiorite, quartz monzonite porphyry, and quartz monzodiorite (Fig. 2). The quartz monzonite shows porphyritic textures with phenocrysts of plagioclase (20%–25%) and hornblende (5%) as well as subordinate quartz (1%–2%) and biotite (1%–3%), with a typical grain size of 0.5–2 mm. The matrix shows a fine-microcrystalline texture, consisting of K-feldspar (40%–45%), plagioclase (15%–20%), quartz (10%–15%), hornblende (1%–5%), and minor amounts of biotite. Accessory minerals include magnetite, zircon, and apatite. The monzodiorites show fine-grained or porphyritic textures. Rock-forming minerals of the fine-grained samples include plagioclase (65%–70%), K-feldspar (10%–15%), hornblende (15%–20%), biotite (5%–10%), and quartz (1%–5%); the accessory minerals include magnetite, titanite, zircon, and apatite. The fine-grained porphyritic monzodiorites consist of phenocryst and matrix. Phenocrysts are mainly plagioclase (2%–5%) and K-feldspar

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(1%–5%), with a typical grain size of 2–5 mm. The matrix consists of plagioclase (55%–60%), K-feldspar (5%–10%), hornblende (20%–25%), biotite (5%–10%), and minor amounts of quartz as well as accessory minerals including magnetite, titanite, zircon, and apatite. The quartz monzodiorites display medium- to fine-grained textures. Rock-forming minerals include plagioclase (75%–80%), K-feldspar (10%), quartz (5%–10%), hornblende (5%), biotite (2%–5%), and minor amounts of monoclinic pyroxene; the accessory minerals include magnetite, titanite, zircon, and apatite. These intrusions are not spatially associated with mafic igneous rocks, and all are ore-barren.

3. Analytical procedures 3.1. Zircon U–Pb isotopic dating Zircon separation was performed in the Geological Surveying and Mapping Institute of Hebei Province, Langfang City. Conventional heavy liquid and magnetic techniques were employed, followed by handpicking under the binoculars. The grains were mounted onto an epoxy resin disk, which were then polished to expose the cores of zircons. After examining and photographing under transmitted and reflected light, the internal structures were studied by cathodoluminescence (CL) technique at the Electron Microprobe Laboratory, Chinese Academy of Geological Sciences (CAGS) prior to zircon U–Pb dating. Zircon U–Pb analyses were conducted by laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and data reduction are the same as those

9

described by Hou et al. (2009). Laser sampling was performed using a Newwave UP 213 laser ablation system. A Thermo Finnigan Neptune MC-ICP-MS instrument was used to acquire ion-signal intensities. The array of four multi-ion-counters and three faraday cups allow for simultaneous detection of 207

202

Hg (on IC5),

204

Hg,

204

Pb (on IC4),

206

Pb (on IC3),

Pb (on IC2), 208Pb (on L4), 232Th (on H2), 238U (on H4) ion signals. Helium was applied as

a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. Each analysis incorporated a background acquisition of approximately 20–30 s (gas blank) followed by 30 s data acquisition from the sample. Off-line raw data selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for U–Pb dating was performed by ICPMSDataCal (Liu et al., 2010c). Zircon GJ1 was used as external standard for U–Pb dating, and was analyzed twice every 5–10 analyses. Time-dependent drifts of U–Th–Pb isotopic ratios were corrected using a linear interpolation (with time) for every 5–10 analyses according to the variations of GJ1 (i.e., 2 zircon GJ1 + 5-10 samples + 2 zircon GJ1) (Liu et al., 2010c). Preferred U–Th–Pb isotopic ratios used for GJ1 are from Jackson et al. (2004). Uncertainty of preferred values for the external standard GJ1 was propagated to the ultimate results of the samples. In all analyzed zircon grains the common Pb correction was not necessary due to the low signal of common 204Pb and high 206Pb/204Pb. U, Th and Pb concentration was calibrated by zircon M127 (with U:923 ppm; Th:439 ppm; Th/U: 0.475. Nasdala et al, 2008). The zircon Plesovice was dated as standard and yielded weighted mean 206Pb/238U age of 337 ± 2 Ma (2σ, n = 12), which is in good agreement with the recommended 206Pb/238U age of 337.13 ± 0.37 Ma (2σ) (Sláma et al., 2008).

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3.2. Zircon Hf isotope analyses The in situ Lu–Hf isotopic composition was measured on the dated zircon at the same domain were U–Pb analyses were conducted. Zircon Hf isotope analysis was carried out in-situ using a Newwave UP213 laser ablation microprobe, attached to a Neptune multi-collector ICP–MS at Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Instrumental conditions and data acquisition were comprehensively described by Hou et al. (2007) and Wu et al. (2006). The data were acquired from either 44 μm or 60 μm pits depending on the size of ablated domains. Helium was used as carrier gas to transport the ablated sample from the laser-ablation cell to the ICP–MS torch via a mixing chamber mixed with Argon. In order to correct the isobaric interferences of on

176

Hf,

176

Lu/175Lu = 0.02658 and

176

176

Lu and

176

Yb

Yb/173Yb = 0.796218 ratios were determined (Chu et

al, 2002). For instrumental mass bias correction Yb isotope ratios were normalized to 172

Yb/173Yb of 1.35274 (Chu et al, 2002) and Hf isotope ratios to

179

Hf/177Hf of 0.7325 using

an exponential law. The mass bias behavior of Lu was assumed to follow that of Yb, mass bias correction protocols details was described as Wu et al. (2006) and Hou et al. (2007). Zircon GJ1 was used as the reference standards during our routine analyses, with a weighted mean

176

Hf/177Hf ratio of 0.282003 ± 0.000014 (2σ, n = 11). It is not distinguishable from a

weighted mean

176

Hf/177Hf ratio of 0.282000 ± 0.000005 (2σ) using a solution analysis

method by Morel et al. (2008). The calculation of the Hf model age (single stage model age) (TDM) is based on a depleted mantle source with presentday176Hf/177Hf at 0.28325, using the 176Lu decay constant of 1.865

11

×10-11 year-1 (Scherer et al., 2001). The calculation of the “crust” (two stage) Hf model age (TDMC) is based on the assumption of a mean

176

Hf/177Hf value of 0.015 for the average

continental crust (Griffin et al., 2002). The calculation of the εHf(t) values was based on the zircon U–Pb ages and the chondritic values (176Hf/177Hf = 0.282772,

176

Lu/177Hf = 0.0332;

Blichert-Toftand Albarede, 1997).

3.3. Whole-rock major and trace element analyses A total of ten whole-rock samples were analyzed for geochemistry. Whole-rock samples were trimmed to remove weathered surfaces, cleaned with deionized water, crushed, and then pulverized to about 200–mesh size in an agate mill. Whole-rock major and trace elements were analyzed at the China National Nuclear Corporation (CNNC) Beijing Research Institute of Uranium Geology. Major element compositions were determined using an automatic X-ray fluorescence (XRF) spectrometer. Sample powders were put into glass disks after fusion with lithium metaborate. Flux materials and sample powders were mixed in a 1:10 ratio and fused at 1100 °C in a Pt–Au crucible for 20–40 min. The resultant melt was then poured into a preheated 34-mm-diameter pellet, in preparation for XRF analyses of the major element compositions. The XRF system was calibrated by international standard samples GSP-2 and JG-2, along with a national standard sample (GBW02103). The volatile components were obtained by measuring the weight loss after heating the sample at 1050 °C. The precision is 0.5% for major oxides, and the detailed analytical procedures are given in Liu et al. (2005). For trace element components, whole-rock powders (25 mg) were placed into Savillex

12

Teflon beakers within a high-pressure bomb. HF and HNO3 mixed at a 1:1 ratio were added and the beakers were heated for 24 h at 80 °C and then evaporated. HNO3 (1.5 ml) and HF (1.5 ml) accompanied by HClO4 (0.5 ml) were added after the evaporation, and the beakers were capped for digestion within a high-temperature oven at 180 °C for 48 h or longer, until the powders were completely dissolved. The solutions were diluted with 1% HNO3 into 50 ml for measurement. Trace elements, including rare earth elements (REEs), were measured using a high-resolution inductively-coupled plasma mass spectrometer (HR-ICP-MS, Element; Finnigan MAT Co., Germany) and the international standard GSR-15 (amphibolite) was used for analytical quality control.

3.4. Whole-rock Sr, Nd, and Pb isotopic analysis The Rb–Sr and Sm–Nd isotopic ratios were determined by isotope dilution methods. Isotopic measurement was performed on a Finnigan MAT thermal ionization mass spectrometer (Triton TI) at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Dissolution and chemical separation of Sm–Nd and Rb–Sr samples were accomplished using a procedure similar to that described in Pu et al. (2004, 2005). All Sr data are corrected for mass fractionation to 87

86

Sr/87Sr = 0.1194 and reported relative to a value of

Sr/86Sr = 0.705018  3 (2σ) for the BCR_2 standard. In the Nd isotopic analysis,

146

Nd/144Nd = 0.7219 was taken as the standard for mass-fractionation correction, and Nd

isotopic ratios were normalized to a value of 143Nd/144Nd = 0.511842  4 (2σ) for La Jolla. For Pb isotopic determination, whole-rock powder was analyzed by isotope dilution methods. Isotopic measurement was also performed on a Finnigan MAT thermal ionization

13

mass spectrometer (Triton TI) at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Samples were dissolved in concentrated HNO3 and HCl, and Pb was purified by using the conventional anion-exchange method (BioRadAG1X8, 200–400 resin) using HBr and eluant. The whole procedure blank for Pb is 0.05–0.1 ng. Fractionation of Pb isotopes during mass spectrometry analysis was calibrated against the standard NIST Pb-981. The NIST Pb-981 averages measured during the course of this study were 16.9339 ± 0.0005 (2σ),

207

Pb/204Pb = 15.4873 ± 0.0005 (2σ), and

208

206

Pb/204Pb =

Pb/204Pb = 36.6873 ±

0.0014 (2σ), respectively. The precision of Pb isotope data on the mass spectrometry is better than 0.05%.

4. Results 4.1. Geochronology Five representative samples were chosen for dating using the LA-MC-ICP-MS U–Pb method. The results of LA-MC-ICP-MS U–Pb data are presented in Supplementary Table A1. Representative CL images of the zircons and the age data from different samples in this study are shown in Fig. 3. Most of the zircons in sample QTJ-3 exhibit light-colored oval shapes with some euhedral grains. The zircons mostly lack well-defined internal structures and do not exhibit clear oscillatory zoning in CL images, suggesting the influence of fluids. They have a size range of 70–250 μm with length-to-width ratios of 1:1 to 2.5:1. Ninteen zircon spots were analyzed and the results yielded Th and U contents of 44–111 ppm and 39–206 ppm, respectively. The Th/U ratios vary from 0.28 to 1.58, indicating a magmatic origin for most

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of the grains (Hoskin and Schaltegger, 2003), but high U content in some cases possibly resulted from fluid-related processes. Excluding one spot (No. 7), 18 concordant or near-concordant data points yielded a weighted mean

206

Pb/238U age of 127.58 ± 0.80 Ma

(MSWD = 1.01) (Fig. 3(a)), which is interpreted as the emplacement age of the sample QTJ-3. Zircons from the sample FX-1a are light colored, transparent, and euhedral, exhibiting clear oscillatory zoning, and are 100–300 μm long with aspect ratios between 3:1 and 1:1. A total of 50 zircon spots were analyzed. The data show Th and U contents of 46–323 ppm and 60–274 ppm, respectively, with Th/U ratios ranging from 0.70 to 1.37, indicating a magmatic origin for all of the grains. Of the 50 analyses, 31 cluster as a coherent tight group on the concordia, yielding a weighted mean age of 126.90 ± 0.81 Ma and an MSWD of 0.54 (Fig. 3(b’)), which is interpreted as the timing of magmatic emplacement. Another two groups of 14 and 5 zircons yield younger and older ages at ca. 114.4 ± 1.5 Ma with an MSWD of 1.07 and 137.0 ± 4.7 with an MSWD of 0.112 (Fig. 3(b)), interpreted to be the result of post-crystallization Pb loss. Most zircons from sample FX-1b (adjacent to sample FX-1a described above) are colorless or light brown, transparent to translucent, and euhedral to subhedral. They exhibit long prismatic grain morphology with lengths varying from 80 to 300 μm and length-to-width ratios of 2:1 to 5:1. The zircons exhibit clear oscillatory zoning in CL images, typical of magmatic origin. Inherited zircon cores with discordant CL structures to the magmatic overgrowth were visible within a few grains. A total of 44 analyses of zircon grains were obtained during a single analytical session. These zircons have variable Th (63–989

15

ppm) and U (97–1109 ppm) contents. Th/U ratios vary between 0.50 and 2.25, indicating that all the zircons derived from magmatic protolith. Thirty-nine concordant spots yield a weighted mean

206

Pb/238U age of 120.71 ± 0.64 Ma (MSWD = 0.89) (Fig. 3(c)), which is

taken as the crystallization age of FX-1b. Five of the analyzed spots (Nos. 21, 23, 38, 43, and 49) are statistical outliers and are therefore not included in the concordia plot. The zircon grains of sample SYP-1a are mostly colorless, transparent, clear, and euhedral prisms with sharp terminations, indicating their igneous origin, which is further supported by their fine internal oscillatory zoning visible in the CL images. They exhibit long prismatic morphology with lengths varying from 100 to 300 μm and length-to-width ratios of 2:1 to 3:1. However, a few of the zircon grains have a core-rim structure with planar zoning. A total of 39 spots were analyzed, and the results show that the Th and U contents are 28–491 ppm and 42–1070 ppm, respectively, with Th/U ratios in the range of 0.20 to 4.11, indicating magmatic origin. Thirty-two spots cluster on the concordia and yield a weighted mean 206

Pb/238U age of 122.75 ± 0.57 Ma and an MSWD of 1.2 (Fig. 3(d)), which is interpreted as

the emplacement age of this rock. The remaining grains are inherited old zircons or of captured nature with a wide range of ages from ca. 1869 to 2256 Ma. Two of the analyzed spots (Nos. 9 and 22) are statistical outliers and are therefore not included in the concordia plot. Zircons separated from sample GY-1c are light gray in color, transparent to semitransparent, and euhedral to subhedral columnar or euhedral short columnar in shape, but most of them do not show clear oscillatory zoning. The zircon grains in this sample are very small (50–150 μm) with length-to-width ratios of 1:1 to 2:1. Twenty-six U–Pb analyses were

16

conducted on this sample. Zircon grains have Th contents of 22–434 ppm and U contents of 29–1067 ppm, with variable Th/U ratios of 0.04–1.48, suggesting that most of them were derived from magmatic crystallization and a few of the grains formed through recrystallization or overgrowth. Three spots show ages of less than 123 Ma (Nos. 2, 18, and 38) and another one shows an age of 2409 Ma (No. 19), both of which are statistical outliers. The older age (2409 Ma) might represent older inherited zircon. The other three analyses show younger ages (106.7, 111.6, and 122.2 Ma) probably resultting from late Pb loss. The remaining 22 zircons cluster as a coherent group on the Concordia, yielding a weighted mean age of 129.2 ± 1.1 Ma (MSWD = 1.7) (Fig. 3(e)), which is interpreted as the crystallization age of the rock.

4.2. Whole-rock major and trace element geochemistry Ten fresh samples were selected for whole-rock major and trace elements analyses and the data are compared with those from published literature for discussion (Table 1 and Supplementary Table A2). The rocks show intermediate composition with SiO2 contents ranging from 53.18 to 65.48 wt. %, and relatively high Na2O (3.57–4.42 wt. %) and K2O (3.07–3.95 wt. %) contents with corresponding high total alkali contents (Na2O+K2O) varying from 6.83 to 7.99 wt. %. On the TAS (K2O + Na2O vs. SiO2) and K2O vs. SiO2 diagrams (Figs. 4(a) and 4(b)), samples with SiO2 ≤ 61 wt. % plot in the monzodiorite and monzonite field of the shoshonitic series (Irvine and Baragar, 1971), whereas the other samples with SiO2 of 62%–65 wt. % plot in the quartz monzonite field of the high-K calc-alkaline series. The Na2O/K2O ratios of the

17

samples range from 1.02 to 1.26, similar to the Early Cretaceous adakitic rocks from the STLF (~0.7 to 1.5, Wang et al., 2007b; Huang et al., 2008; Liu et al., 2010b; Xu et al., 2012), but are lower than the values of the adakites related to subducted oceanic slab and liquid from melting experiments of basalts (Na2O/K2O > 2, Martin et al., 2005) (Fig. 4(c)). The samples exhibit high Al2O3 (14.27–15.86 wt. %), variable Fe2O3T (3.17–8.26 wt. %, where Fe2O3T = Fe2O3+1.1FeO), CaO (2.98–6.25 wt. %), and relatively consistent TiO2 (0.38–0.90 wt. %) and P2O5 (0.19–0.45 wt. %). They are metaluminous in composition [A/CNK = 0.68–0.95, molar Al2O3/(CaO + K2O + Na2O); A/NK = molar Al2O3/(Na2O+K2O)] (Fig. 4(d)). The MgO contents are variable and range from 1.80 to 7.35 wt. %, and the Mg# values vary from 50 to 65 (Figs. 4(e) and (f)). The intermediate intrusions have variable and moderate total REE abundances (100.41–207.55 ppm). On the chondrite-normalized REE plot (both chondrite and primitive mantle values are from Sun and McDonough, 1989), the rocks display enriched light rare earth element (LREE) patterns and depleted heavy rare earth element (HREE) patterns with (La/Yb)N ratios ranging from 11.60 to 28.33 (Fig. 5(a)). The (La/Yb)N ratios (average = 18.53, Table 1) is slightly higher than the values of upper (15.5) and lower continental crust (5.3) (Rudnick and Gao, 2003), indicating fractionation of the REEs in the intrusions. The δEu value shows slightly negative to positive anomalies, ranging from 0.81 to 1.30. The samples have variable Ni (20.3–143.0 ppm) and Cr (51.40–390.0 ppm) contents and high Sr (591–1183 ppm) and low Y (7.79–22.4 ppm) contents resulting in very high Sr/Y ratios (27.9–113.5). They also have low Yb (0.60–2.01ppm) contents. On the Sr/Y vs. Y and La vs. (La/Yb)N diagrams (Fig. 6), the majority of samples are plotted in the field of adakite,

18

indicating that the intrusive rocks studied here can be classified as adakitic rocks (Defant and Drummond, 1990). When normalized to primitive mantle (Fig. 5(b)), the samples show variable enrichment in Pb and large ion lithophile elements (LILEs, e.g., Ba, K, and Sr) but are depleted in high-field strength elements (HFSEs, e.g., Nb, Ta, Zr, and Ti). These features are similar to those of the high-Mg adakitic rocks from the STLF investigated in previous studies (Niu et al., 2002; Zi et al., 2008; Huang et al., 2008; Liu et al., 2010b; Xu et al., 2012, and reference therein) and also resemble the composition of the lower continental crust (Rudnick and Gao, 2003). However, they are different from the pattern of normal mid-ocean ridge basalt (N-MORB) (Sun and McDonough, 1989). The samples exhibit consistent Ce/Pb ratios of 2.62 to 5.85. The Mg# value of the intermediate intrusion from the STLF is higher than that of the experimental melts (Rapp and Watson, 1995) but similar to those of high-Mg# adakitic rocks in Eastern China (Fig. 4(e)), Therefore, given the high Mg# value, the STLF samples can be classified as high-Mg adakitic rocks.

4.3. Sr–Nd–Pb isotopic compositions The whole rock Sr–Nd–Pb isotopic data on representative samples of the high-Mg adakitic rocks from the STLF are listed in Tables 2 and 3. Initial isotopic values were calculated on the basis of LA-MC-ICP MS zircon U–Pb data for the samples. The whole-rock Nd model ages were calculated using the model of DePaolo (1981). The STLF intrusions studied here show (87Sr/86Sr)i of 0.7060 to 0.7074, low

143

Nd/144Nd

ratios of 0.511728 to 0.511795, εNd(t) values of −16.2 to −15.0, and Nd model ages of 1825 to

19

2051 Ma. On the εNd(t) vs. (87Sr/86Sr)i diagram (Fig. 7), the intrusions from the STLF display Sr–Nd isotopic compositions similar to the high-Mg adakites of the TLF in Eastern China (Niu et al., 2002; Zi et al., 2008; Huang et al., 2008; Liu et al., 2010b; Xu et al., 2012), but they are markedly different from the depleted mantle, MORB, and eclogites in the Dabie orogen (Jahn, 1998; Li et al., 2000; Hofmann, 2003). They are also different from the thickened lower-crust-derived low-Mg adakitic granites in the MLYR and Dabie orogen (Wang et al., 2004; Wang et al., 2007b). Samples from the STLF are characterized by low radiogenic Pb isotopes with 206

Pb/204Pb(t) = 16.208–16.509,

207

Pb/204Pb(t) = 15.331–15.410, and

208

Pb/204Pb(t) =

36.551–36.992. In 206Pb/204Pb–207Pb/204Pb and206Pb/204Pb–207Pb/204Pb diagrams (Figs. 8 (a) and (b)), they plot in the field of the lower crust and Dabie orogen Cretaceous granites, but are far from the field of the Early Cretaceous mafic igneous rocks in the MLYR. The Pb isotopic compositions of the STLF high-Mg adakitic rocks are similar to that of the Dabie-Sulu Mesozoic granite high-Mg adakitic intrusions (e.g. Chituling, Meichuan) from the STLF (Zhang, 1995; Zhang et al., 2002a, 2004; Huang et al., 2008; Liu et al., 2010b).

4.4. Zircon Hf-isotope compositions The zircon Lu–Hf isotope results are listed in Supplementary Table A4. The εHf(t) vs. age diagrams and the histograms of initial Hf isotope ratios are shown in Fig. 9. The data show that most of the

176

Lu/177Hf ratios are less than 0.002 and that the fLu/Hf values display a tight

range between −0.99 and −0.92. A total of 153 zircon grains were analyzed, and the results are listed in Supplementary Table A4.

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Nineteen spots analyses were made for sample QTJ-3, which is from Qiaotouji monzonite. The initial 176Hf/177Hf ratios of this sample vary from 0.281918 to 0.282061. The determined negative εHf(t) values for these zircons vary between −27.4 and −22.4 with an average of −24.6. The corresponding Hf model ages (TDM) are in the range of 1643 to 1840 Ma. Their Hf crustal model age (TDMC) is between 2599 and 2916 Ma (average = 2742 Ma). Thirty-seven zircon grains from sample FX-1a show similar (176Hf/177Hf)i values ranging from 0.281857 to 0.282217. They are characterized by negative εHf(t) values varying from −29.6 to −16.8 with an average of −25.3. Their Hf model ages (TDM) are in the range of 1455 to 1957 Ma, and the Hf crustal model ages (TDMC) range from 2249 to 3050 Ma. Hf isotopes of 43 zircons in sample FX-1b show a range of (176Hf/177Hf)i values varying from 0.281856 to 0.282148 and negative εHf(t) values of −29.7 to −19.3. Their Hf model ages (TDM) are in the range of 1522 to 1963 Ma. The Hf crustal model ages (TDMC) vary from 2405 to 3055 Ma with an average of 2733 Ma. Thirty zircons were analyzed from sample SYP-1a for Hf isotopes. Twenty-eight magmatic zircon grains with concordant ages have variable ( 176Hf/177Hf)i ratios (0.281972 to 0.282225) and εHf(t) values (−25.6 to −16.6) at their crystallization ages. The Hf model ages (TDM) are between 1451 and 1771 Ma. Their Hf crustal model ages (TDMC) vary from 2236 to 2797 Ma with an average of 2448 Ma. The remaining two Paleoproterozoic inherited zircons with U–Pb ages of 2009 and 2366 Ma show low and similar (176Hf/177Hf)i values of 0.281269 and 0.281325 with negative and positive εHf(t) values of −8.3 and 1.9, respectively, close to those of chondrite mantle. The large variation in their Hf crustal model ages (TDMC) between 2781 and 3133 Ma corresponds well with their U−Pb age data.

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The 24 zircons from sample GY-1c have variable (176Hf/177Hf)i values (0.281454 to 0.281958) and εHf(t) values (−36.6 to 7.4) at the time of their crystallization, suggesting complex sources. The Hf model ages (TDM) are between 1794 and 2449 Ma. The Hf crustal model ages (TDMC) range from 2474 to 3486 Ma. Twenty-three Early Cretaceous zircons (106.7–136.2 Ma) are dominated by negative εHf(t) (−36.6 to −26.0) with (176Hf/177Hf)i ranging from 0.281658 to 0.281958. One Paleoproterozoic zircon with an age of 2410 Ma shows a significantly positive εHf(t) of 7.4 and a lower (176Hf/177Hf)i value (0.281454), the former of which is close to that of the depleted mantle at that time, and these yield a crustal model age (TDMC) of 2474 Ma.

5. Discussion 5.1. Zircon geochronology The five samples of high-Mg adakitic rocks in the STLF analyzed in this study show weighted mean ages of 127.58 ± 0.80, 126.90 ± 0.81, 120.71 ± 0.64, 122.75 ± 0.57, and 129.2 ± 1.1 Ma, indicating the emplacement of these rocks in the Early Cretaceous (Fig. 3). Wu et al. (2007) argued that the two-stage model age is much more precise than the single-stage model age for evaluating the time of source material extraction from the depleted mantle or the residence time of the source material in the crust for felsic igneous rocks. The discrepancy between the two-stage model age and the actual model age becomes larger when the ages of zircon are younger. The zircon Hf crustal model ages (TDMC) of STLF high-Mg adakitic rocks are within the range of 2236 to 3486 Ma, indicating that parental melts of high-Mg adakitic rocks originated from the ancient Paleoproterozoic to Paleoarchean crustal materials.

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Zircon U–Pb data indicate two age groups of 120–122 and 126–132 Ma for the high -Mg adakitic rocks (Supplementary Table A1, Fig. 10). These ages represent the emplacement of the intermediate intrusive rocks and two district magmatic pulses within 10 Ma in the STLF. During this time, Eastern China was experiencing delamination and extension and thinning of the lithosphere (Deng et al., 1994, 1996; Lü et al., 2005). The zircon U–Pb age data from these intrusions are comparable to those of previously reported high-Mg adakitic rocks from the STLF (Supplementary Table A5), which have isotopic ages of 132–120 Ma with a single-peak distribution that clusters around 132–126 Ma (Fig. 10). The MLYR is spatially adjacent to the DabieSulu UHP metamorphic belt and the TLF, suggesting a close relationship in the geodynamic context. Because the time series of magmatic rocks can be used to trace their tectonic environments, a comparison of the geochronology of high-Mg adakitic rocks and adakitic rocks from the STLF and the MLYR can offer information on the evolutionary history of these two areas. Jurassic–Cretaceous magmatism is widespread in the middle to lower region of the Yangtze River and the Dabie Belt. Isotopic ages for the adakitic rocks in the MLYR range from 100 to 147 Ma (Supplementary Table A5). These age data form a nearly bimodal distribution with a major peak at 132–138 Ma and a subordinate peak at 144–148 Ma (Fig. 10). On the basis of the current isotopic age dataset, two major adakitic magmatic events can be inferred for the MLYR. The Anjishan granodiorite porphyry formed in the late Early Cretaceous (100–109 Ma, Supplementary Table A5), suggesting that, in the Ningzhen fault-uplift area of the MLYR, the adakitic intermediate-felsic magma intrusion occurred at about 100 Ma (Zeng et al., 2013; Wang et al., 2014). The adakitic intermediate-felsic

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intrusions of the Ningzhen region marks another period of Mesozoic large-scale magmatic activity in the MLYR (Zeng et al., 2013). Notably, high-Mg adakitic intrusions from the STLF are slightly younger than the adakitic intrusions from the MLYR (Fig. 10).

5.2. Petrogenesis As described above, most intermediate intrusions from the STLF (Table 1 and Supplementary Table A2) can be classified as high-Mg adakitic rocks. Adakites or adakitic rocks were originally considered to be the result of partial melting of young and hot subducted oceanic crust (Defant and Drummond, 1990; Kay et al., 1993). Recent further studies indicate that intermediate to felsic rocks with adakitic affinity can be produced through other processes, such as assimilation and fractional crystallization (AFC) of basaltic magma (Castillo, 1999, 2006) and partial melting of thickened basaltic lower crust (Rudnick, 1995; Chung et al., 2003) or delaminated lower continental crust (Kay and Kay, 1993; Gao et al., 2004). The petrogenesis of the adakites in the Lower Yangtze River (LYR) remains quite controversial. In some studies it is argued that the adakites may be related to the melting and subduction of the oceanic crust (Ling et al., 2009; Liu et al., 2010b; Sun et al., 2012). Li et al. (2013) proposed a model of melting of an enriched mantle source metasomatized by subduction processes to explain the petrogenesis of the LYRB “C-type adakitic rocks”, and they argue that fractional crystallization played a major role in the magmatic processes experienced by these rocks. Another opinion is that the Cretaceous adakitic granites were generated by partial melting of thickened lower crust (Wang et al., 2004). Furthermore,

24

Huang et al. (2008) believe that the Chituling high-Mg adakities adjacent to the TLF were derived from partial melting of delaminated and foundered lower continental crust. The STLF adakitic rocks are characteristically potassium-rich, in contrast to the oceanic crust-derived melts, which are potassium-depleted (Li et al., 2013). The STLF adakitic rocks are also enriched in LILEs and LREEs but depleted in HFSEs and HREEs, making them clearly different from those of Cenozoic slab-derived adakites and adakitic rocks in MLYR (Fig. 5). The lower Na2O/K2O (1.02–1.26) of the samples than that of typical adakites from oceanic subduction zone settings (Martin et al., 2005) (Fig. 4(c)) may also be a common feature for the adakitic rocks developed within continental crust (Huang et al., 2008). Furthermore, the Ce/Pb ratios (2.62~5.85) of STLF high-Mg adakitic rocks indicate that adakitic melts were derived from continental crust since the continental crust has lower Ce/Pb ratios (~ 4–5; Taylor and McLennan, 1985; Rudnick and Gao, 2003) than that of oceanic crust (~ 24; Sun and McDonough, 1989). This view is further supported by the very low εNd(t) values (−16.2 to −15.0) of the adakitic rocks, because recycled oceanic crust is characterized by high εNd(t) values (Hofmann and Jochum, 1996; Gerya and Yuen, 2003). Given the typical “lower continental crust”–like Sr–Nd–Pb isotopic features (Fig. 7 and 8) and the fact that there was no clear evidence that subduction of oceanic crust occurred in or near the STLF during the Early Cretaceous (Tsai et al., 2000; Li et al., 2004; Wang et al., 2006a, 2006b, and references therein), the partial melting of oceanic slabs cannot form the STLF adakitic rocks. Although fractional crystallization processes could have occurred locally, this was not a dominant process in the present case. Castillo et al. (1999) proposed that the formation of the adakite in Camiguin Island, the Philippines, was associated with the assimilation and

25

contamination and fractional crystallization of basaltic magma. The source region of the adakite is the mantle wedge, which is dehydrated and metasomatized by the subducted slab. Low-pressure hornblende + plagioclase fractional crystallization was the major process. The adakite is characterized by “V”-shaped distribution patterns of REEs, and Rb, Ba and Zr increase with increasing SiO2; these features differ from those of the STLF adakitic rocks (Figs. 5(a) and 11). The STLF adakitic rocks exhibit either no Eu and Sr anomalies or positive anomalies (Fig. 5), arguing against a significant role for plagioclase fractional crystallization in their genesis (Wilson, 1989). Ma et al. (1998) suggested that AFC processes could only be responsible for Cretaceous Y- and Yb-depleted and Al2O3- and Sr-enriched granites near the Dabie area if amphibole fractionation had taken place. However, the adakitic rocks in the study area lack any obvious depletion in middle REEs (Fig. 5) and have discrete MgO content over a range of SiO2 contents (Fig. 4f), suggesting that amphibole fractionation did not play an important role in their petrogenesis (Gromet and Silver, 1987). The adakitic rocks in this study have relatively high Mg# (up to 65), Cr (up to 390 ppm), and Ni (up to 143 ppm), arguing against pronounced fractional crystallization (Li et al, 2013). The absence of spatial-temporal association with mafic rocks in the STLF area also confirms that the high-Mg adakitic rocks are not the products of fractional crystallization of basaltic magma (Castillo et al., 1999; Xu et al., 2006). Atherton and Petford (1993) argued that lower crustal melting induced by heating from a basaltic melt that underplates the continent could generate adakitic rocks. The melting of the lower crust cannot directly generate the high-Mg adakitic rocks, because such melt would be expected to have low Mg# (Rapp and Watson, 1995). Although experimental studies have

26

shown that fluid-absent melting of basaltic rocks would generate melts with slightly high MgO and Mg# (Rapp and Watson, 1995; Prouteau et al., 2001; Pertermann and Hirschmann, 2003), the high Mg# values (up to 65) of STLF adakitic rocks are higher than those (most of them being <45) of experimental melts from hydrous basalt in the garnet stability field (Rapp and Watson, 1995; Rapp et al., 1999). Their Sr–Nd isotopic compositions are distinct from that of the thickened lower crust-derived adakitic granites in the MLYR and Dabie orogen (Fig. 7). These features suggest the rocks were not derived from the partial melting of thickened lower crust (Atherton and Petford, 1993, Wang et al., 2006b). Instead, different extents of interaction with the mantle material can dramatically elevate MgO and Mg# (Rapp et al., 1999; Prouteau et al., 2001). The high-Mg adakitic rocks of this study exhibit higher Cr and Ni contents relative to the lower crust. Mantle rocks have much higher Ni and Cr contents than felsic magmas and, hence, even the addition of minor mantle materials will significantly raise the levels of Cr, Ni, and Mg# with a simultaneous decrease in SiO2 (Figs. 11). Zircon Hf isotope data and whole-rock Sr–Nd–Pb isotope data further provide strong evidence for the origin of the STLF adakitic rocks. The low radiogenic Pb of the adakitic rocks (Fig. 8) are typical features of the ancient lower continental crust, being consistent with their derivation from partial melting of the lower crust. The extreme negative εHf(t) values (−16.6 to −36.6) (Fig. 9) of most magmatic zircons from these high-Mg adakitic rocks imply a source in the ancient lower continental crust. In addition, the zircons of STLF high-Mg adakitic intrusions show a large variation in Lu–Hf isotopic composition, with the majority of zircons possessing negative εHf(t) values of −8.3 to −36.6 (Fig. 9) and crustal model ages (TDMC) of 2236 to 3486 Ma (Supplementary Table A4 and Fig.9). Wide variation

27

in Hf isotopic composition is generally considered to be the result of either insufficient mixing during the melting process or a heterogeneous source region (Zheng et al., 2006, 2008). In combination with the high Mg# values of the STLF adakitic rocks, we consider that their wide variation in Lu−Hf isotopic composition reflects the interaction between crustal-derived melts and mantle rock. The εNd(t) values range from −16.2 to −15.0 and are plotted between the enrichment mantle I (EMI) and lower continental crust (LCC) sources (Fig.7), suggesting mixing between the EMI and LCC components, which is similar to the other high-Mg adakitic rocks from the STLF (Niu et al., 2002; Zi et al., 2008; Huang et al., 2008; Liu et al., 2010b; Xu et al., 2012). In summary, the genesis of the STLF adakitic intrusions can be best interpreted as the interaction of melts derived from delaminated lower crust with mantle material during ascent upward. An origin via partial melting of delaminated lower crust, leaving an eclogitic residue (Kay and Kay, 1993), can also readily account for the elevated MgO contents or Mg # values of the STLF adakitic rocks (Rapp et al., 1999). A similar mechanism has also been proposed for the Early Cretaceous adakitic intrusions within Eastern China (Gao et al., 2004; Huang et al., 2008; Wang et al., 2007b; Zi et al., 2008; Xu et al., 2002; Xu et al., 2006; Xu et al., 2012; Liu et al., 2010b).

5.3. Geodynamic model In this study, the geochemical characteristics of STLF adakitic intrusions show that they were derived from a (garnet ± amphibolite)-bearing lower crust in the area. Experimental studies have also shown that mafic crustal rocks can melt to produce adakitic liquids at

28

sufficient depths (>40 km, ~1.2 GPa) for garnet to be stable within the residual assemblage (garnet-amphibolite, amphibole-bearing eclogite, and/or eclogite) (Rapp et al., 2003 and references therein). The depth of the source region should be more than 40 km, suggesting a relatively thick crust during the Early Cretaceous. The geophysical data from the SinoProbe Program show that, along the Eastern Langdai transect, crustal thickness changes between 29 and 35 km, with the thinnest crust in the Ningwu volcanic basin at 29 km, ranging up to 33 km to the southeast and up to 35 km under the STLF (Shi et al., 2013). The data suggest that, from the Cretaceous (about 120 Ma) to now, the STLF region has experienced significant thinning of the crust, from more than 40–50 km to 35 km. Several possible mechanisms have been invoked to explain the thinning of the lithosphere, including de-rooting of the lithospheric keel (Deng et al., 1994); thermal-tectonic destruction and chemical erosion (Xu, 1999); upwelling, erosion, and replacement (Lu et al., 2000); and delamination (Xue et al., 2010); among other models (Santosh, 2010; Li and Santosh, 2014). Our results favor delamination as the thinning mechanism of lithosphere in this region. The spatial distribution of the STLF high-Mg adakitic rocks (Fig. 12) suggests a prominent role of the deep-seated movements along the TLF as the trigger for delamination of the dense eclogitic lower continental crust. The intracontinental subduction between the NCB and the YB would have started in the Late Permian, with the peak UHP metamorphism in the Middle Triassic (Faure et al., 2008; Zheng et al., 2009, 2012; Liu and Liou, 2011; Wu and Zheng 2013). The Triassic continental collision between the YB and the NCB resulted in the Dabie–Sulu orogenic belt, forming one of the largest UHP metamorphic zones in the world (Li et al., 1993; Hacker et al., 1998, 2000;

29

Meng and Zhang, 2000; Zheng et al., 2009, 2012; Liu and Liou, 2011; Wu and Zheng 2013). Followed by subduction of the continental crust of the YB under the NCB, the collision between the NCB and the YB began in the Late Permian or Early Triassic with a north-dipping subduction zone (Li et al., 1993; Zhang, 1997, 2012, and references therein; Ye et al., 2000). Many geologists agree that the Dabie UHP metamorphism occurred between 240 and 210 Ma and was followed by “rapid cooling” between 210 and 170 Ma (Hacker et al., 1998, 2000; Li et al., 2000; Ayers et al., 2002). These ages can be best explained by the diachronous exhumation of Dabie UHP metamorphic belts in response to Triassic subduction (Hacker et al., 2000; Wang et al., 2007b). In the early stage of subduction at about 240–210 Ma, the subducted slices of the Yangtze continental crust melted with concomitant uplift of older overlying basement, and the continental lithosphere ceased to descend further owing to its buoyancy (Fig. 13(a)).. The Yangtze continental crust was dehydrated and began to be converted into eclogite (Davis and von Blanckenburg, 1995; Jahn et al., 1999; Wang et al., 2007b). Subsequently (210–170 Ma), previously exhumed UHP slices thrust southward into the YB (Fig. 13(b)). The combination of southward thrusting and coeval normal sense shear from beneath the NCB led to an upward extrusion of the subducted slices onto the YB. (Wang et al., 2007b; Fig. 13(b)). In these two stages, the crust and lithospheric mantle beneath the Dabie orogen were probably thickened (Fig. 13(a) and (b)). Paleomagnetic data show that the NCB and the YB did not converge together until the Late Jurassic (Gilder et al., 1999). This suggests that, even after subduction and slab break-off, the YB and the NB continued converging until the Middle to Late Jurassic (~150 Ma). The Early Cretaceous adakitic rocks formed from the lower crust also indicate that the subduction of the two blocks

30

and eclogitic source remained in the lower crust beneath the Dabie Orogen until that time (Wang et al., 2007b). As a giant fault belt in Eastern China, the TLF experienced a long-term, complex evolution since the Mesozoic. Several previous studies have indicated that the development of the TLF during the Late Jurassic to Early Cretaceous marks one of the most important geological events in Eastern China (Xu and Zhu, 1994; Zhu et al., 2005, 2010, 2012; Wu et al., 2005; Wang et al., 2006c; Zhang et al., 2003). The TLF has been correlated with lithosphere weakening and thinning of the North China Block in the Mesozoic, with associated magmatism and metallogeny (Ratschbacher et al., 2000; Menzies et al., 2007; Guo et al., 2013; Goldfarb and Santosh, 2014; Yang and Santosh, 2015). Based on geochronological studies on zircons and mylonite, some geologists proposed that the middle-south section of TLF initiated large-scale sinistral strike-slip movement during the Early Cretaceous (~140 Ma) (Zhu et al., 2005, 2010; Wang, 2006; Zhang and Dong, 2008). The sinistral strike-slip movement of the TLF has a displacement of more than 500 km in Anhui (Okay and Sengör, 1992; Xu et al., 1987). In addition, from the middle stage of the Early Cretaceous, large-scale magmatic activity commenced with a widespread fault basin in the middle-south section of the TLF, which shows that the TLF witnessed a transformation from sinistral strike-slip to extension (Zhu et al., 2010, 2012; Zhang et al., 2003). The Late Mesozoic high-Mg adakitic rocks define a magmatic belt along the TLF and the eastern Dabie orogen to Sulu orogen along the eastern boundary of the North China Block. In this study, the adakitic rocks from the STLF and the MLYR show ages in the range of 120–140 Ma (Supplementary Table A5, Figs. 10, and 12). The coupling between the emplacement

31

ages of the adakitic rocks and the timing of large-scale strike-slip of the TL, together with the spatial association of the high-Mg adakitic rocks along the TLF, suggests the key role of the trans-lithospheric fault in lithospheric delamination and thinning processes and the petrogenesis of the high-Mg adakitic rocks. As one of the major foundering mechanisms of the lower continental crust (Fig. 13(c)), the strike-slip motion of the TLF also destroyed the lithosphere within and around this fault. The upwelling asthenospheric materials likely intruded into the lower crust along the TLF and the destructed lithosphere. The density increase associated with asthenospheric upwelling led to gravitational instability of an over-thickened lithospheric keel. A combination of the deep-seated, large-scale strike-slip movement and the upwelling of the asthenosphere triggered the delamination or foundering. The dense garnet-rich lower crustal restite might have promoted the decoupling of the lithospheric mantle from the upper lithosphere and its sinking into the asthenosphere (Lustrino, 2005). This is also confirmed by the geophysical data of the SinoProbe Program. The tomographic results show that the lithosphere with high-velocity anomalies is separated into two parts: one above a depth of 100 km and the other at depths between 250 and 400 km; the asthenosphere with low-velocity anomalies is located between these two domains, indicating that the lithosphere delaminated together with the asthenospheric upwelling (Jiang et al., 2013). At this stage, the heat input from upwelling asthenosphere caused a partial melting of the thickened lower continental crust, which generated the low-Mg adakitic rocks with ages ranging between 148 and 130 Ma in the Dabie orogen and in the MLYR (Fig. 13(c) melt 1, Wang et al., 2007b; Xu et al., 2007, 2013; and Supplementary Table A5). Some high-Mg adakitic melts (>130 Ma, melt 2) and

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mafic–ultramafic magmas might have been generated by interactions between the partial melting of the delaminated eclogitic melts (adakitic) and the mantle. Although there is a minor overlap between the ages of the low-Mg adakitic rocks (148–130 Ma) and the high-Mg adakitic rocks (132–130 Ma) (Fig. 10 and Supplementary Table A5) in the Dabie orogen and the MLYR, the earliest low-Mg adakitic rocks are significantly older than the high-Mg adakitic rocks (130–120 Ma, Jahn et al., 1999; Zhao et al., 2005; Liu et al., 2010b; This study). This suggests that the large-scale partial melting of the upper mantle and mantle upwelling might have occurred later than the earliest melting of thickened lower crust. Liu et al. (2012) suggested that anatexis of the preexisting thickened LCC that attenuated the LCC itself and left the residues denser as a result of felsic (adakitic) melt extraction had resulted in gravitational instability and foundering of the lithosphere. Partial melting of the lower crust could further weaken the over-thickened lithosphere and produce eclogite restite, consequently resulting in the delamination of lithospheric mantle and removal of the orogenic root by convective mantle flow, followed by large-scale asthenosphere mantle upwelling and foundering of the eclogite restite after the extraction of low-Mg adakitic magma (Wang et al., 2007b; Huang et al., 2008; Xu et al., 2012). These processes caused extensive magmatism (both intrusive and extrusive; Yang and Santosh, 2015) in the region surrounding the TLF (Fig. 13(d)). During this period (130–120 Ma), the upwelling of hot asthenosphere caused partial melting of eclogitic lower crust and the enriched upper lithospheric mantle generated high-Mg adakitic rocks (130–120 Ma, melt 2’). Furthermore, the interaction of mafic lower-crust-derived melt with mantle peridotites could have resulted in enriched mantle with pyroxenite veins or a modally and cryptically

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metasomatized mantle. Melting of such enriched mantle and lower crust by heat input from the upwelling mantle produced abundant basaltic rocks (melt 3) and granites (melt 4) slightly later, respectively (Huang et al., 2008 and Xu et al., 2012 reference therein)

6. Conclusions (1) Zircon LA-MC-ICPMS dating of five samples from monzodiorite, quartz monzonite porphyry, and quartz monzodiorite yield U–Pb ages of 127.58 ± 0.80, 126.90 ± 0.81, 120.71 ± 0.64, 122.75 ± 0.57, and 129.2 ± 1.1 Ma, indicating their emplacement in the Early Cretaceous. (2) All the four intermediate intrusions have similar geochemical features with high Al2O3 and Sr contents, high (La/Yb)N and Sr/Y ratios, but low Y and Yb contents, resembling the features of adakitic rocks. The high Mg# value and high Cr and Ni contents indicate that the STLF intrusions are high-Mg adakitic rocks. They show continental-crust signatures, such as enrichment in LREEs but depletion in HREEs and HFSEs with slight negative to positive Eu anomalies, moderately enriched initial (87Sr/86Sr)i, very low εNd(t), low radiogenic Pb isotopes, and consistent (176Hf/177Hf)i and low εHf(t). These features do not favor magma generation by the melting of a young and hot oceanic slab, AFC of basaltic magma, or partial melting of thickened basaltic lower crust. (3) In combination with the geochemical and isotopic features and existing geological data, the high-Mg adakitic intrusions in the STLF are considered to have resulted from mixing of mantle-derived magmas with partial melts of delaminated eclogitic lower continental crust under a lithospheric extension and thinning regime in the North China

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Block. (4) The large-scale strike-slip movement along the TLF might have triggered the lithospheric delamination and subsequent crust–mantle interaction.

Acknowledgments Prof. Faure, editor of Journal of Asian Earth Sciences, and Dr. Sheng-Ao Liu are thanked for their helpful reviews and comments which greatly improved our manuscript. We also appreciate Dr. Kejun Hou, Dr. Chunli Guo, Xin Xiong, Jun Li, Qian Wang, Xuejing Gong for their assistance in zircon U-Pb and Lu-Hf analysis and other experiment. This work is supported by the Ministry of Land and Resources of China under the Project SinoProbe-03-02 and the SinoProbe-03-07, and the National Sci-tech Support Plan (2009BAB43B03; 2011BAB04B03).

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Wu, F.Y., Li, X.H., Zheng, Y.F., Gao, S., 2007. Lu–Hf isotopic systematic and their applications in petrology. Acta Petrologica Sinica 23 (2), 185–220 (in Chinese with English abstract). Wu, Y.B., Zheng, Y.F., 2013. Tectonic evolution of a composite collision orogen: an overview on the Qinling–Tongbai–Hong'an–Dabie–Sulu orogenic belt in central China. Gondwana Research 23, 1402–1428. Wang, Y., 2006. The onset of the Tan-Lu fault movement in eastern China: constraints from zircon (SHRIMP) and 40Ar/39Ar dating. Terra. Nova. 18(6), 423–431. Xie, G.Q., Mao, J.W., Li, R.L., Frank, P.B., 2008. Geochemistry and Nd–Sr isotopic studies of late Mesozoic granitoids in the southeastern Hubei Province, middle-lower Yangtze River belt, Eastern China: petrogenesis and tectonic setting. Lithos 104, 216–230. Xing, F.M., 1999. The magmatic metallogenetic belt around the Yangtze River in Anhui. Geology of Anhui 9, 272–279 (in Chinese). Xu, H.J., Ma, C.Q., Ye, K., 2007. Early Cretaceous granitoids and their implications for collapse of the Dabie orogen, eastern China: SHRIMP zircon U–Pb dating and geochemistry. Chem. Geol. 240, 238–259. Xu, H.J., Ma, C.Q., Zhang, J.F., 2012. Generation of Early Cretaceous high-Mg adakitic host and enclaves by magma mixing, Dabie orogen, Eastern China. Lithos 142–143, 182–200. Xu, H.J., Ma, C.Q., Zhang, J.F., Ye, K., 2013. Early Cretaceous low-Mg adakitic granites from the Dabie orogen, eastern China: petrogenesis and implications for destruction of the over thickened lower continental crust. Gondwana Research 23(1), 190–207. Xu, J.F., Shinjo, R., Defant, M.J., Wang, Q., Rapp, P.T., 2002. Origin of Mesozoic adakitic intrusive rocks in the Ningzhen area of east China: partial melting of delaminated lower continental crust?

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52

Figure captions Fig.1. (a) The location of the middle and lower Yangtze River metallogenic belt. (b) Geological sketch map of the middle and lower Yangtze River metallogenic belt (modified from Lü et al., 2013). (c) Geology of the Eastern Langdai transect across the Ning-Wu ore-district and Tan-Lu fault . TLF: Tancheng–Lujiang fault, XGF: Xiangfan–Guangjifault,

YCF:

Yangxing–Changzhou

fault,

XT-MZTF:

Xiaotian-Mozitan fault, CHF: Chuhe fault. Fig. 2. Petrographic characteristics of the investigated intrusions from the STLF. (a) the hand specimens of the monzodiorite; (a’) petrographic characteristics of the monzodiorite (b) the hand specimens of the quartz monzodiorite; (b’) petrographic characteristics of the quartz monzodiorite; (c)

the hand specimens of the quartz

monzonite porphyry; (c’) petrographic characteristics of the quartz monzonite porphyry. Q: quartz, Kf: K-feldspar, Pl: plagioclase, Bt: biotite, Hbl: hornblende. Fig. 3. Zircon U-Pb concordia diagrams and cathodoluminescence (CL) images of representative zircons for (a) Qiaotouji, (b) and (c) Fuxiao, (d) Shangyaopu, and (e) Guoying intrusions. Fig. 4. (a) Total alkalis (Na2O + K2O) versus SiO2 diagram for investigated intrusions from the STLF. (b) K2O (wt.%) versus SiO2 (wt.%) diagram. (c)Na2O/K2Oversus SiO2 (wt.%) diagrams for investigated intrusions from the STLF. Adakites derived from partial melting of oceanic slab, Martin et al. (2005). (d) Plots of A/NK versus A/CNK (after Maniar and Piccoli, 1989) for investigated intrusions from the STLF. (e) Mg# versus SiO2 (wt.%) diagram for investigated intrusions from the STLF. The field

53

for experimental melts at 1–4 GPa are after Rapp et al. (1999). (f) MgO versus SiO2 diagram. The field for adakites derived from slab melting is from Defant and Kepezhinskas (2001), for experimental melts is from Rapp et al.(1999), and for arc xenolith glass inclusion is from Schiano et al.(1995). Fig.5. Chondrite normalized rare earth element patterns (a) and primitive mantle normalized trace element patterns (b) for investigated intrusions from the STLF. The normalizing values are from Sun and McDonough (1989). LCC is from Rudnick and Gao (2003) and N-MORB is from Sun and McDonough (1989). Fig. 6. Plots of (a) Sr/Y vs. Y and (b) (La/Yb)N vs. YbN for investigated intrusions from the STLF (modified after Defant and Drummond (1990), and Atherton and Petford (1993)). Symbols are the same as in Fig. 4. Fig. 7. Comparison of the initial Sr-Nd isotopic compositions of the investigated intrusions from the STLF. Cenozoic slab-derived adakites, Defant and Kepezhinskas (2001); the UHPM eclogite, Jahn (1998); thickened lower crust-derived low Mg adakites in Dabie, Wang et al. (2007b); adakitic rock in MLYR, Liu et al., (2010b); Low-Mg adakites in the Yueshan-Hongzhen area of east China, Wang et al., (2004); MORB, Hofmann (2003); upper-lower crust, Jahn et al. (1999). Data are from this study (Table 2) and the literature. Symbols are the same as in Fig. 4 Fig. 8. (a)

207

Pb/204Pb vs.

206

Pb/204Pb and (b)

208

Pb/204Pb vs.

206

Pb/204Pb diagrams

(modified from Rollinson, 1993) for investigated intrusions from the STLF. NHRL (northern hemisphere reference line); BSE (bulk silicate Earth value); MORB (mid-ocean ridge basalt); DM (depleted mantle); EMI and EMII (enriched mantle);

54

PREMA (frequently observed prevalent mantle composition) (Zinder and Hart, 1986). Cretaceous granites from the Dabie Orogen, Wang et al. (2006a, and reference therein) and early Cretaceous mafic igneous rocks in the MLYR, Yan et al. (2008) are shown for comparison. Fig. 9. Plots of εHf(t) vs. U-Pb ages and histograms of initial Hf isotope ratios for the investigated intrusions from the STLF. Fig. 10. Histograms of isotopic ages for adakitic rocks from the MLYR and the STLF. Fig. 11. Harker-type chemical variation diagrams for investigated intrusions from the STLF. Data sources are the same as the Fig. 4. Fig. 12. Sketch map showing the spatial and temporal correlations between the adakitic rocks and the TLF (modified after Gu et al., 2013; age data from this study, Huang et al., 2008; Wang et al., 2007b; Xu et al., 2004; Zi et al., 2008; Liu et al., 2010b; Xu et al., 2012; Zhang et al., 2013; Gu et al., 2013). Fig. 13. Cartoons showing the geodynamic processes that generated the Cretaceous magmatic rocks including high Mg adakitic intrusion via partial melting of thickened and delaminated lower crust (modified after Wang et al., 2007b; Huang et al., 2008). (a) The early stage of collision between the Yangtze Block and Northern China Block (modified after Wang et al., 2007b). (b) Extrusion and thrusting of subducted slices onto the Yangtze Block during the Jurassic (210–170 Ma), convergence, and exhumation of the HP/UHP metamorphic terrane (modified after Wang et al., 2007b). (c) The foundering mechanism of the lower continental crust (modified after Huang et al., 2008). (d) Partial melting of eclogitic lower crust and changed asthenospheric

55

mantle, resulting in large-scale delamination of lithospheric mantle.

Figure 1 A r 3- P J t1

K

Є

N

E 1

K

FX-1 Pt 3

Ar3

0

20

βμ

25°N

E

a

J

F

Pz

Pz

J T

n Ya

D-C P-T T

ξο S γδο

ξο

K Tongling

Pz E

J ηο 117°00'

ze

E

ξο 117°30'

T

r

Wuhu J P-T

Q S

S J

K

gt

e riv

γ

δ

K 31°00'

Pz Trias

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J

S

γ

γδηο J T

K γδ

K

ηο

P-T K D-C E

E K E S Xuancheng K

E K

T Pz 00' 118°

granitoids Cretaceous Volcanic and subvolcanic rocks

N

50km

High-Mg adakites

120°E

F

Pt 3

K

Є 30°N K Late Jurassic-Early Cretaceous

Pz

K δ J

E

D-C

P-T S

F

Q K Changzhou J E K Neogene Cretaceous Jurassic Quaternary K

K γ

Pz

31°30'

YC

South China Block

Q

J Chao lake

Pz

T

115°E

S P-T

Є-O S Chaohu

Pt

XT-MZTF

0

b

Є-O

Ma’anshan

3

J

C

Є

Pt

3

Pt

QTJ-3 γ

3

Ar P t 3- P t 1 3

YCF

F H Є-O

31°N

Zhenjiang ηο δ

T γδο

G

Є

32°N

Yangtze Block

Yangzhou

TL

K

K Z K Nanjing

Taizhou

block

X

K

32° 00'

Hefei

Nanjing

Chuzhou

Z Є

TLF

Beijing

North China γδ b ie Block o ro gen ic e lt δ bγδ E Cathaysian

g -D

1000km

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g li n

E Dabie UHP metamorphic belt

SYP-1

40km Q

c Q in

0

γδ J

K ηγ

North China Block

120°E

45°N

E

γδ ηγ E βμGY-1 Z K δ

ηγ

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80°E

δ

E

-Pt

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Pz 118°30'

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Є

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c

S

Pz T

S P-T T PermianPz Devonian— Silurian —Trias Carboniferous D-C Є Z Pz

Є-O J Сambrian— Cambrian Paleozoic γ Sinian Ordovician γ T Tai lake γδ Pt 3 Ar 3 -Pt 1 γδο Pz S Pz Neoarchaean Neoprotγδ Tonalite S Granodiorite γ—Palaeoerozoic S proterozoic Pz P T ηο γ/ηγ P-T ξο δ D-C Granite/ Quartz— Quartz— Pz Monzonite syenite monzonite Diorite γ granite K

βμ

S Diabase γδ Fault

119°00'

119°30'

D-C Sampled Langdai S J position transect 120°00'

Figure 2

(a)

(b)

(a’)

(b’)

(c)

(c’)

Hbl

Bt

Bt

Hbl

Hbl

Q Q Pl

Pl 200μm

Kf

Pl

200μm

Kf

Bt

200μm

Figure 3

(a)

0.026

0.024

170

0.025

(b)

150 0.023

0.024

100μm

0.021

150

130

0.019

140

0.022 206Pb/238U

206 Pb/238U

0.022

0.020

0.017 0.06

110 0.10

0.14

0.18

0.22

Mean=137.0±4. 7 MSWD= 0.112,n=5

0.020

130

0.018

Mean=114.4±1. 5 MSWD=1.07，n= 14

110

120

0.022

0.018

0.018

Mean=127.58±0.80 MSWD= 1.01，n=18

QTJ-3 0.016 0.08

0.10

0.12

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0.16

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0.020

0.016

110

90

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0.016

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0 .025

(b’)

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206 Pb/238U

206 Pb/238U

0.021

0.019

115

0 .021

130

0 .019

0.017

110

FX-1b 0.015 0.05

0 .017

Mean=120.71±0.64 MSWD=0.89, n=39

0.07

0.09

0.11

0.13 .

0.15

0.17

Mean=126.90±0.81 MSWD=0.54，n=31

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0.19

0 .06

0 .10

0 .14

0 .18

0 .22

207Pb/235 U

0.025

(d)

0.5

(e) 150 0.023

206 Pb/238U

206 Pb/238U

1800 0.3 0.023

1400

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0.021

0.2

100μm

2200

100μm

0.4

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0.16

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0.20

0.24

0.28

Figure 4

7

14

(a)

This study Literature

(b) 6

Syenite

10

5

Quartz monzonite Granite

Monzonite

8 Monzoniticdiorite

6

Shoshonitic

K 2 O(wt.%)

K 2 O+Na 2 O (wt.%)

12

High-K calc-alkaline

4 3

Calc-alkaline

2

4 2

Gabbro

Granitic Diorite diorite

1

Granodiorite

Tholcitic

0

0 40

50

45

55

60

65

70

40

75

45

50

SiO 2 (wt.%)

55

60

65

70

75

80

SiO 2 (wt.%)

3

3.0

(c)

(d)

Oceanic slab-derived adakite

Metaluminous

2.5

Peraluminous

Na 2 O/K 2 O

2

A/NK

2.0

1.5 1 1.0 Peralkaline Continental crust-derived adakite 0.5 0.5

0 50

55

60

65

70

75

1.0

SiO 2 (wt.%)

1.5

2.0

A/CNK

90

8

(f)

(e)

80

7

70

Adakites derived from oceanic slab melting in subduction zones

6

60

MgO (wt.%)

Mg

#

5

50 40 30

4 3 Experimental metabasalt and eclogite melts (1-4GPa)

2

20 Experimental melts at 1-4GPa

10

1

Arc xenolith glass inclusion

0

0 50

55

60

65

SiO 2 (wt.%)

70

75

50

55

60

65

70

SiO 2 (wt.%)

75

80

Figure 5

1000

1000

(a) High Mg adakitic rock from STLF 100

(b)

High Mg adakitic rock from STLF

QTJ-3

FX-1a

QTJ-3

FX-1a

FX-1b

FX-1c

FX-1b

FX-1c

FX-1d

FX-1e

FX-1d

FX-1e

SYP-1a

SYP-1b

SYP-1a

SYP-1b

GY-1c

GY-1g

GY-1c

GY-1g

100

LCC 10

10

LCC

N-MORB

N-MORB 1

1 La

Ce

Pr

Nd

Sm Eu Gd Tb Dy Ho

Er Tm Yb Lu

Rb Ba Th U K Ta Nb La Ce Pb Pr Sr P Nd Zr Hf SmEu Ti Dy Y Ho Yb Lu

Figure 6

200

120

(a)

(b) 100

150

(La/Yb) N

Sr/Y

Adakite

100

80 60

Adakite

40

Normal andesite-dacite-rhyolite

50 20 Normal andesite-dacite-rhyolite 0

0 0

5

10

15

20

25

Y(ppm)

30

35

40

45

0

5

10

Yb N

15

20

Figure 7

10

This study Literature

Cenozoic slab-derived adakites

Low-Mg adakites in Yueshan-Hongzhen area of east China

MORB

0

Adakitic rocks in MLYR Thickened lower crust-derived low Mg adakite in Dabie

εNd(t)

-10

-20

STLF high Mg adakitic rocks

-30

UHPM eclogite

EM1 Lower Crust

Upper Crust

-40 0.702

0.705

0.708

0.711

87 Sr/ 86 Sr

i

0.714

0.717

Figure 8

(b) t us cr

15.5

RL

77

Pb/

NH

. (1

204

EMⅡ

) Ga

MORB

208

207

EMⅠ PREMA

Cretaceous granites from the Dabie Orogen

39

MO

RB

NH

38

RL

DM

15.3

15

Pb

pp

er

40

U

Cretaceous granites from the BSE Dabie Orogen Lower crust

Pb/

204

Pb

15.7

41 (a)

Geo chr on (4.5 5Ga )

15.9

Early Cretaceous mafic igneous rocks in the MLYR

16

19

18

17 206

Pb/

204

Pb

37

20

21

36

Early Cretaceous mafic igneous rocks in the MLYR

16

17

19

18 206

Pb/

204

Pb

20

Figure 9

20

D ep le te d M an tle 10

FX -1a

GY-1c

FX -1b

QTJ -3

D e p le te

2.5

SYP -1a

Ga

CHUR

CHUR

0

3.0

Crust 1.5Ga

Ga

-10 40

Crust 2.1Ga

35

QTJ-3 FX-1a FX-1b SYP-1a GY-1c

30

-20

25

Number

εHf(t)

d M a n tl e

Crust 2.5Ga

20 15 10

-30

5 0 -40 -36 -32 -28 -24 -20 -16 -12 -8 -4 0 4 8 εHf(t)

-40 100

110

120

130

Age(Ma)

140 2000

2500

3000

Figure 10

8 STFL MLYR

7 6

Number

5 4 3 2 1 0 100

108

116

124

132 140 Age(Ma)

148

156

164

172

Figure 11

160

3000

350

140

300 2500

120

60

Zr(ppm)

Ba(ppm)

80

2000

1500

200 150 100

40 1000

50

20 0 55

60

65

70

500 50

75

0 55

SiO 2 (wt.%)

60

65

70

75

50

55

SiO 2 (wt.%)

450

160

400

140

350

120

300

Ni(ppm)

50

Cr(ppm)

Rb(ppm)

250 100

250 200

100 80

40

100

20

50

0

0 50

55

60

65

SiO 2 (wt.%)

70

75

50

65

SiO 2 (wt.%)

60

150

60

55

60

65

SiO 2 (wt.%)

70

75

70

75

Figure 12

113°

115°

119°

117°

Dabie-Sulu orogen belts

132Ma

Faults

Laiwu

High Mg adakitic rocks

134Ma

Low Mg adakitic rocks 35° 131Ma Zhengzhou 132Ma

Nanyang

N

0

100km Wuhan

128Ma 131Ma 132Ma

Haiyang

Qingdao 124Ma 127Ma 124Ma Rizhao 123Ma

129Ma 128Ma 125Ma 128Ma 129Ma 122Ma

131Ma 126Ma Yangzhou 120Ma 142-129Ma Chuzhou Nanjing Hefei 132Ma 127Ma Shangcheng Chaohu 127Ma Macheng

31°

Weifang

Xuzhou

Benbu 33°

153Ma

132Ma

Fault

Jinan

The T an-Lu

Felsic plutons

121° Weihai

123Ma 141Ma

Intermediate plutons

37°

Penglai

131Ma

Figure 13

S

suture

Yangtze Block

N North China Block

S

UCC

UCC

UCC

UCC

LCC

LCC

LCC

LCC

Ec Lithospheric mantle

log

itic

LC

C

E c lo

Lithospheric mantle

Lithospheric mantle

Asthenospheric mantle

Asthenospheric mantle

Asthenospheric mantle (a) 240-210 Ma W

N North China Block

Yangtze Block

Dabie orogen

g it ic

LCC

Lithospheric mantle

Asthenospheric mantle

(b) 210-170 Ma 1

2 TLF

Yangtze Block

E

W

Dabie orogen

UCC

UCC

UCC

LCC

LCC

LCC

3

4

2'

TLF

Pyroxenite vein

LCC Pyroxenite vein

Lithospheric mantle Eclogite resitite

Lithospheric mantle

Lithospheric mantle

Lithospheric mantle

Eclogite Asthenosphere Foundering Mantle upwelling (c) >130 Ma

Asthenosphere

E

UCC

Eclogite resitite

Enriched mantle

Yangtze Block

Convective removal (d) 130-120 Ma

Foundering

56

Tables Table 1. Major (wt.%) and trace elements (ppm) of investigated intrusions from the STLF, central Eastern China Table 2. Whole-rock Sr and Nd isotopic data for investigated intrusions from the STLF, central Eastern China Table 3. Pb isotopic data for the investigated intrusions from the STLF, central Eastern China

Table 1 SampleNo

QTJ-3

FX-1a

FX-1b

FX-1c

FX-1d

FX-1e

SYP-1a

SYP-1b

GY-1c

GY-1g

SiO2

56.37

64.09

53.18

53.85

53.62

62.67

65.48

65.24

62.39

63.04

TiO2

0.77

0.62

0.90

0.86

0.85

0.56

0.38

0.41

0.63

0.55

14.87

14.66

14.70

14.39

14.27

14.95

15.76

15.71

15.86

15.85

6.72

4.89

8.26

7.89

7.78

4.70

3.17

3.55

5.40

4.91

MnO

0.10

0.07

0.14

0.14

0.14

0.07

0.03

0.04

0.09

0.09

MgO

6.47

3.66

7.33

7.35

7.34

3.42

1.80

2.19

2.69

2.51

CaO

5.35

3.72

6.25

6.16

6.11

3.85

3.44

2.98

3.95

3.93

Na2O

3.80

3.77

3.73

3.57

3.67

3.86

4.36

4.42

4.15

4.04

K2O

3.07

3.45

3.10

3.46

3.49

3.55

3.62

3.52

3.73

3.95

P2O5

0.45

0.27

0.41

0.39

0.39

0.25

0.19

0.19

0.34

0.30

LOI

1.91

0.68

1.91

1.84

2.25

2.02

1.68

1.63

0.67

0.73

Total

99.88

99.88

99.91

99.90

99.91

99.91

99.91

99.88

99.90

99.90

Mg

65

60

64

65

65

59

53

55

50

50

Na2O/K2O

1.24

1.09

1.20

1.03

1.05

1.09

1.20

1.26

1.11

1.02

7.22

6.83

7.03

7.16

7.41

7.98

7.94

7.88

7.99

Al2O3 Fe2O3

T

#

K2O+Na2O

6.87

A/CNK

0.77

0.88

0.70

0.69

0.68

0.87

0.91

0.95

0.88

0.88

Be

1.56

1.79

2.49

2.77

2.20

1.97

1.45

2.17

2.14

1.47

Sc

16.3

10.3

20.3

21.1

20.6

10.4

6.54

7.17

10.9

10.1

V

144

103

157

160

152

94.8

68.5

79.8

105

86.2

Cr

276

149

353

390

375

132

70.5

80.3

51.6

51.4

Co

31.8

20.2

32.5

33.9

33.0

17.0

11.5

12.4

15.6

14.9

Ni

143

66.1

133

140

141

58.7

31.1

38.9

21.8

20.3

Cu

27.2

12.8

62.5

97.5

103

7.85

25.3

39.9

32.5

12.7

Zn

83.9

52.8

79.7

89.3

78.7

42.6

27.6

35.0

70.7

66.2

57

Ga

18.8

19.1

19.7

19.9

19.3

17.5

18.3

19.7

19.5

18.3

Rb

66.6

77.8

72.2

74.3

69.6

68.0

81.5

85.8

101

103

Sr

1183

781

591

624

603

724

884

912

795

795

Zr

20.8

15.5

23.8

24.1

22.5

15.1

125

144

13.2

21.2

Nb

8.17

9.60

12.0

12.4

11.6

9.87

6.37

6.82

12.4

11.3

Cs

1.21

1.14

1.16

1.05

0.962

1.15

2.37

1.54

3.51

3.89

Ba

2414

1661

1135

1398

1402

1498

1559

1579

1419

1471

Hf

1.36

0.98

1.32

1.45

1.21

0.92

3.54

4.17

0.72

0.98

Ta

0.33

0.54

0.44

0.48

0.48

0.74

0.46

0.52

0.82

0.71

Pb

15.7

17.1

12.2

13.2

13.2

14.7

15.2

14.7

22.4

24.6

Th

4.32

6.05

2.25

2.48

2.47

14.9

4.98

5.25

9.25

9.38

U

0.912

1.21

0.983

1.12

1.02

1.75

1.57

1.67

1.92

2.43

La

46.9

23.1

31.7

36.9

35.6

36.0

23.7

24.8

46.5

43.8

Ce

86.0

44.8

69.9

77.2

73.1

66.8

43.1

45.2

82.7

78.9

Pr

10.5

5.68

9.04

9.75

9.39

7.41

4.84

5.24

9.65

8.70

Nd

41.0

23.7

37.4

38.6

37.0

27.3

17.8

19.7

36.1

32.8

Sm

6.97

3.86

6.81

6.62

6.07

4.47

3.27

3.17

6.37

5.61

Eu

2.27

1.54

1.68

1.77

1.80

1.59

1.22

1.33

1.92

1.93

Gd

5.38

3.56

5.51

5.56

5.37

3.99

2.54

3.00

5.22

4.78

Tb

0.71

0.51

0.85

0.85

0.83

0.59

0.35

0.38

0.68

0.64

Dy

3.62

2.60

4.53

4.60

4.37

2.85

1.69

1.90

3.26

3.15

Ho

0.63

0.46

0.75

0.77

0.71

0.51

0.29

0.31

0.59

0.55

Er

1.72

1.22

2.13

2.14

2.09

1.38

0.786

0.818

1.66

1.36

Tm

0.25

0.17

0.33

0.37

0.35

0.25

0.11

0.13

0.25

0.22

Yb

1.39

1.13

1.96

2.01

1.87

1.45

0.60

0.77

1.46

1.37

Lu

0.22

0.16

0.29

0.33

0.32

0.21

0.12

0.11

0.23

0.21

Y

16.5

12.8

21.2

22.4

21.1

14.1

7.79

8.66

16.8

15.4

∑REE

207.55

112.49

172.87

187.47

178.86

154.79

100.41

106.86

196.59

184.01

13.92

10.46

9.58

10.27

10.25

12.80

14.50

13.40

13.73

14.00

LREE/HREE Eu/Eu*

1.09

1.25

0.81

0.87

0.94

1.13

1.25

1.30

0.99

1.11

(La/Yb)N

24.20

14.66

11.60

13.17

13.66

17.81

28.33

23.01

22.85

22.93

Sr/Y

71.70

61.02

27.88

27.86

28.58

51.35

113.48

105.31

47.32

51.62

Yb/Lu

6.32

6.93

6.88

6.18

5.92

7.07

5.08

6.84

6.40

6.52

Dy/Yb

2.60

2.30

2.31

2.29

2.34

1.97

2.82

2.46

2.23

2.30

(Ho/Yb)N

0.47

0.42

0.40

0.40

0.39

0.37

0.50

0.41

0.42

0.41

Ce/Pb

5.48

2.62

5.73

5.85

5.54

4.54

2.84

3.07

3.69

3.21

1/2

Eu/Eu*=EuN/(SmN+GdN) , N denotes the chondrite normalization (Sun and McDonough, 1989)

58

Table 2 Sample

Age

Rb

Sr

87

No.

(Ma)

(ppm)

QTJ-3

128

67

1183

FX-1a

127

78

781

FX-1b

121

72

591

FX-1c

121

74

624

FX-1d

121

70

603

FX-1e

127

68

724

123

82

884

123

86

912

GY-1c

129

101

795

GY-1g

129

103

795

SYP-1 a SYP-1 b

Rb/

(ppm

86

Sr

)

87

Sr/8

(87Sr/8

2σ

6

Sr

6

Sr)i

0.159

0.706

0.000

0

263

007

0.281

0.706

0.000

4

550

006

0.345

0.706

0.000

1

781

007

0.336

0.706

0.000

3

809

007

0.326

0.706

0.000

0

761

008

0.265

0.706

0.000

3

581

003

0.260

0.706

0.000

4

726

006

0.265

0.706

0.000

7

738

004

0.358

0.707

0.000

8

881

009

0.365

0.708

0.000

9

017

006

0.70 60 0.70 60 0.70 62 0.70 62 0.70 62 0.70 61 0.70 63 0.70 63 0.70 72 0.70 74

Sm

147

Sm/1

Nd

44

(ppm)

(ppm)

Nd

6.97

41.0

0.1028

3.86

23.7

0.0984

6.81

37.4

0.1101

6.62

38.6

0.1037

6.07

37.0

0.0992

4.47

27.3

0.0990

3.27

17.8

0.1110

3.17

19.7

0.0973

6.37

36.1

0.1067

5.61

32.8

0.1034

143

Nd/1

44

Nd

2σ

εN

TDM

d(t)

(Ga)

0.5117

0.000

-16

28

004

.2

0.5117

0.000

-15

46

005

.8

0.5117

0.000

-15

57

006

.9

0.5117

0.000

-15

55

004

.8

0.5117

0.000

-15

53

004

.8

0.5117

0.000

-15

40

006

.9

0.5117

0.000

-15

66

006

.7

0.5117

0.000

-15

54

004

.7

0.5117

0.000

-15

95

005

.0

0.5117

0.000

-16

41

006

.0

1950

1855

2045

1930

1857

1871

2051

1825

1927

1943

Table 3 Sample No.

206

Pb/20

4

Pb

207

2σ

Pb/20

4

Pb

208

2σ

Pb/20

4

Pb

2σ

Pb

Th

(ppm)

(ppm)

U

t

(ppm

(Ma

)

)

(206Pb/20 4

Pb)t

(207Pb/20 4

Pb)t

(208Pb/20 4

Pb)t

59

QTJ-3 FX-1a

FX-1b

FX-1c

FX-1d

FX-1e

SYP-1a

SYP-1b

GY-1c

GY-1g

16.552

0.00

15.334

0.00

37.081

0.00

3

07

8

08

0

18

16.442

0.00

15.339

0.00

37.056

0.00

9

03

8

03

7

10

16.396

0.00

15.353

0.00

37.065

0.00

5

03

5

04

6

09

16.401

0.00

15.355

0.00

37.070

0.00

6

04

8

05

5

13

16.399

0.00

15.354

0.00

37.061

0.00

5

05

9

06

1

16

16.498

0.00

15.349

0.00

37.337

0.00

2

05

7

05

2

13

16.647

0.00

15.417

0.00

36.982

0.00

1

03

1

03

6

10

16.655

0.00

15.413

0.00

36.987

0.00

2

08

6

09

4

18

16.362

0.00

15.361

0.00

36.748

0.00

9

04

7

04

4

11

16.345

0.00

15.358

0.00

36.720

0.00

7

03

5

03

8

11

15.7

4.32

0.91

17.1

6.05

1.21

12.2

2.25

0.98

13.2

2.48

1.12

13.2

2.47

1.02

14.7

14.9

1.75

15.2

4.98

1.57

14.7

5.25

1.67

22.4

9.25

1.92

24.6

9.38

2.43

128

16.471

15.331

36.959

127

16.345

15.335

36.901

121

16.291

15.348

36.988

121

16.290

15.350

36.992

121

16.298

15.350

36.983

127

16.333

15.342

36.889

123

16.509

15.410

36.843

123

16.503

15.406

36.835

129

16.243

15.356

36.565

129

16.208

15.352

36.551

60

Graphic abstract

Cartoons showing the geodynamic processes that generated the Cretaceous magmatic rocks including high Mg adakitic intrusion via partial melting of thickened and delaminated lower crust.

61

Research Highlights

 Zircon

206

Pb/238U ages of high Mg adakitic rocks show emplacement during

120-129 Ma  Whole-rock Nd, Sr and Pb isotopes and zircon Lu-Hf data indicate magma derivation from continental crust  Partial melting of over-thickened basaltic lower crust following delamination