Application of in situ titanite U–Pb geochronology to volcanic-hosted magnetite deposit: New constraints on the timing and genesis of the Zhibo deposit, Western Tianshan, NW China

Ore Geology Reviews 95 (2018) 325–341

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Application of in situ titanite U–Pb geochronology to volcanic-hosted magnetite deposit: New constraints on the timing and genesis of the Zhibo deposit, Western Tianshan, NW China

T

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Zongsheng Jianga, , Dachuan Wangb, Zuoheng Zhanga, Shigang Duana, Yongjian Kanga, Fengming Lic a b c

MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China Xinjiang Bureau of Geology and Mineral Resources, Urumqi 83000, China

A R T I C L E I N F O

A B S T R A C T

Keywords: U–Pb geochronology Titanite LA-ICP-MS Zhibo iron deposit Western Tianshan

The Awulale metallogenic belt within the Western Tianshan orogenic belt of northwestern China includes four large iron oxide deposits with a total resource of ∼1000 million metric tons (Mt) Fe. Among these, the Zhibo deposit is a large (337 Mt at 26–68 wt% Fe) volcanic-hosted magnetite deposit, where massive Ti-poor magnetite ores are hosted in the Carboniferous volcanic and volcaniclastic sequences. Here we use in situ U–Pb analyses of titanite and zircon by laser ablation ICP-MS to place tight constraints on the timing and genesis of iron mineralization at Zhibo. Titanite in the magnetite ore are closely associated with magnetite and Ca alteration assemblages consisting of actinolite, epidote, and calcite. The dated titanite exhibit strongly fractionated REE patterns with heavy REE enrichment, neutral to negative Eu anomalies, and have low Th and U concentrations, and low Th/U ratios. The textural and geochemical characteristics indicate that the titanite are hydrothermal in origin and coeval with magnetite in the paragenetic sequence. Titanite from three magnetite ores yield weighted mean 207Pb-corrected 206Pb/238U ages of 310.3 ± 1.8 Ma (MSWD = 0.17), 310.1 ± 1.8 Ma (MSWD = 0.30), and 315.3 ± 2.5 Ma (MSWD = 0.26), constraining the iron mineralization at Zhibo to a time interval between 315 Ma and 310 Ma. Magmatic zircon from a host andesite sample yield U–Pb age of 316.3 ± 3.4 Ma (MSWD = 0.079). The overlapping ages for magnetite ores and the host volcanic rocks confirm a genetic relationship between them, and are consistent with a magmatic contribution to the mineralization system, as also indicated by Fe and O isotope data of magnetite in previous work. These new U–Pb results are also consistent with age estimates for mineralization and igneous activity in other major magnetite deposits in the Awulale iron metallogenic belt, indicating a significant iron mineralization event related to the ca. 315–300 Ma volcanism. Combined with previous geological and geochemical evidence, we conclude that the Zhibo magnetite deposit was formed mainly by iron-rich fluids derived from a mafic to intermediate magma in a volcano-plutonic structure.

1. Introduction

et al., 1996; Zhang and Schärer, 1996) and metamorphic terranes (Essex and Gromet, 2000; Gao et al., 2012; Mezger et al., 1991; Scott and St-Onge, 1995; Verts et al., 1996). Recent advances in the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) technique permit the combined analyses of U–Pb isotope and trace element of titanite with petrographic context in thin sections. This method offers more accurate interpretations of the U–Pb data and has been increasingly applied to constrain the absolute age of mineralization in various types of ore deposits (Deng et al., 2015; Fu et al., 2016; Li et al., 2010; Seo et al., 2015; Smith et al., 2009; Storey et al., 2007;

Titanite (CaTiSiO5) is a common accessory mineral in igneous and metamorphic rocks (Enami et al., 1993; Higgins and Ribbe, 1976), and in a range of ore deposit types (Che et al., 2013; Li et al., 2010; Storey and Smith, 2017). Titanite commonly contains trace amounts of U and Th (usually 10’s to 100’s ppm) and has a high closure temperature to Pb diffusion (650–700 °C), making it a suitable mineral for U–Pb dating (cf. Frost et al., 2000). It has been widely used in U–Pb geochronology of magmatic intrusions (Corfu and Stone, 1998; Jiang et al., 2016; Pidgeon

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Corresponding author. E-mail address: [email protected] (Z. Jiang).

https://doi.org/10.1016/j.oregeorev.2018.03.001 Received 2 September 2017; Received in revised form 24 January 2018; Accepted 1 March 2018 Available online 02 March 2018 0169-1368/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Geological map of the Western Tianshan region in NW China showing the Awulale iron metallogenic belt and the major volcanic-hosted iron deposits (simplified from Gao et al., 2009; Zhang et al., 2012a). Inset map shows the location of the Western Tianshan within the Central Asian Orogenic Belt (modified from Jahn, 2004). (b) Geological map of the eastern part of the Awulale iron metallogenic belt showing the location of the Zhibo iron deposit (modified from Zhang et al., 2012a).

2016) and Southeast Missouri, USA (Day et al., 2016). Based on the close spatial association and geochemical similarities, previous studies proposed a genetic link between the magnetite ores and the host volcanic rocks that have been dated at 350–320 Ma (Feng et al., 2010; Jiang et al., 2014; Zhang et al., 2015). However, the absolute age of Zhibo magnetite ore remains unknown owing to the lack of suitable mineral for radiometric dating. Consequently, a temporal link between magnetite ore and the Carboniferous volcanism has not been clearly demonstrated and the genetic model for ore formation remains unresolved. In this study, we present new LA-ICP-MS U–Pb and trace element data for titanite in thin sections of magnetite ore from the Zhibo deposit. The titanite U–Pb data represent the first direct age

Zhu et al., 2017). The Awulale iron metallogenic belt (AIMB) within the Western Tianshan orogenic belt of northwestern China contains four large-tonnage iron oxide deposits and several minor deposits with a total resource of ∼1000 million metric tons (Mt) Fe (Fig. 1a; Dong et al., 2011). These deposits are primarily hosted in the Carboniferous basaltic to intermediate volcanic rocks with extensive development of sodiccalcic, and potassic alteration assemblages (Zhang et al., 2014). The Zhibo deposit, the focus of this study, is a major and most investigated iron deposit of the belt. It contains massive Ti-poor magnetite with variable amounts of diopside and actinolite, sharing some common features with volcanic-hosted iron oxide-apatite (IOA) deposits in, for example, the Chilean iron belt (Knipping et al., 2015a, b; Tornos et al., 326

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Fig. 2. (a) Geological map of the Zhibo iron deposit showing the locations of the drill hole ZK4003 and andesite sample ZB73 for zircon U–Pb dating. (b) Graphic log for drill hole ZK4003 showing the locations of ore samples for titanite U–Pb dating.

Fig. 3. Photographs of ore samples from the Zhibo iron deposit. (a) Coarse euhedral titanite crystals in epidote-quartz vein filling in the open space of magnetite ore. (b) Magnetite ore containing clots of quartz, epidote and pyrite (Sample ZB311). (c) Ore breccia with magnetite fragments and epidote-calcite cement (Sample ZB313). (d) Magnetite ore containing dendritic magnetite and actinolite with minor interstitial pyrite (Sample ZB334). Abbreviations: Act = actinolite, Cal = calcite, Ep = epidote, Mag = magnetite, Py = pyrite, Qtz = quartz, Ttn = titanite.

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Fig. 4. Representative backscattered electron images of the dated titanite in thin sections of magnetite ores from the Zhibo iron deposit. Circles with numbers show spot locations for LAICP-MS analyses. (a) Platy titanite crystals in quartz (Sample 311). (b) Aggregate of euhedral titanite hosted in epidote and calcite (Sample 313). (c, d) Platy titanite intergrown with magnetite in a groundmass of actinolite. Note the straight crystal boundary of titanite with magnetite (Sample 334). Abbreviations: Act = actinolite, Cal = calcite, Ep = epidote, Mag = magnetite, Py = pyrite, Qtz = quartz, Ttn = titanite.

Phanerozoic marine sedimentary and volcanic sequences and intruded by numerous Neoproterozoic to Permian granitoids (Gao et al., 2009). The closure of three Paleozoic oceans (the Terskey, South Tianshan, and North Tianshan oceans) during the Neoproterozoic to Paleozoic resulted in the amalgamation of the Yili Block with other terranes and the eventual construction of the Tianshan orogenic belt (e.g., Charvet et al., 2011; Gao et al., 2009; Kröner et al., 2014; Windley et al., 2007). During the prolonged accretionary orogeny, Late Devonian to Carboniferous arc magmatism produced voluminous volcanic and intrusive rocks in the Yili Block (Long et al., 2011; Wang et al., 2007; Zhu et al., 2009, 2011). The Dahalajunshan Formation is the most widespread volcanic sequence and consists predominantly of basalt, trachyte, andesite, rhyolite, and tuffaceous rocks with volcaniclastic sediments (Qian et al., 2006). This volcanic sequence has been dated at 363 Ma to 313 Ma (Zhai et al., 2006; Zhu et al., 2009), corresponding to a protracted volcanic event. The nature and tectonic affinity of the volcanic rock is controversial, with a rift-related origin (Che et al., 1996; Xia et al., 2004, 2008) and a subduction-related origin (Long et al., 2008; Wang et al., 2007; Zhu et al., 2009) have been proposed. Economic deposits associated with the volcanic to subvolcanic rocks (Fig. 1a) include porphyry Cu–(Mo) deposits (Tang et al., 2010; Zhang et al., 2010), epithermal Au deposits (Qin et al., 2002; Yang et al., 2009; Zhai et al., 2009) and volcanic-hosted iron oxide deposits (Dong et al., 2011; Zhang et al., 2012a, b).

determinations on the Zhibo magnetite ore. In addition, zircon U–Pb data for a host andesite is used to constrain the timing of volcanism related to the mineralization. In conjunction with previous geological and geochemical evidence, the geochronological data provide new insights into ore genesis and support a genetic link between iron mineralization and the host volcanic rocks. 2. Geological background 2.1. Geologic setting of the Western Tianshan The Western Tianshan region represents the western segment of the Tianshan orogenic belt in northwest China, which occupies the southwestern part of the Central Asian Orogenic Belt (CAOB). It is widely accepted as an accretionary orogenic belt that was formed by the closure of Paleozoic oceans and associated amalgamation of tectonic terranes during the late Paleozoic to early Mesozoic (e.g., Allen et al., 1992; Charvet et al., 2011; Gao et al., 1998; Wang et al., 2007; Windley et al., 2007; Xiao et al., 2008). The Western Tianshan is subdivided into four tectonic terranes: the North Tianshan Accretionary Complex, the Yili Block, the Central Tianshan Arc Terrane, and the South Tianshan Accretionary Complex, separated by major suture zones and regionalscale strike-slip faults (Fig. 1a; Gao et al., 2009, 2015; Klemd et al., 2015; Qian et al., 2009). The Yili Block is underlain by a Precambrian basement consisting of Proterozoic gneisses, schists, carbonates and clastic rocks, interpreted as part of the Paleo-Kazakhstan Plate (Gao et al., 2009). Granitic gneisses with U–Pb zircon age of 1609 ± 40 Ma (Li et al., 2009) and 919 ± 6 Ma (Hu et al., 2010) are exposed along the northern margin of the Yili Block. The basement rocks are overlain by Mesoproterozoic to

2.2. Iron mineralization at the Western Tianshan In the eastern part of the Western Tianshan, numerous large to small iron oxide deposits occur in the Carboniferous volcanic rocks of the Dahalajunshan Formation, collectively defining the Awulale iron 328

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Fig. 5. Photographs and photomicrographs showing the features of the host andesite (sample ZB73) from the Zhibo iron deposit. (a) Hand sample photograph of sample ZB73. (b) Typical porphyritic texture in andesite with plagioclase phenocrysts in a fine-grained matrix of plagioclase and hornblende (Cross-polarized light). (c) Backscattered electron (BSE) image showing disseminated magnetite in matrix of plagioclase and hornblende. (d) BSE image showing rims of magnetite around plagioclase phenocrysts. Abbreviations: Ap = apatite, Ep = epidote, Hbl = hornblende, Mag = magnetite, Pl = Plagioclase.

related volcanism (Jiang et al., 2014; Zhang et al., 2015). Magmatic zircon from the host volcanic rocks yield U–Pb ages between 350 and 320 Ma (Feng et al., 2010; Jiang et al., 2014; Zhang et al., 2015). A large volume of dioritic and granodioritic intrusions were emplaced between 318.9 ± 1.5 Ma and 304.1 ± 1.8 Ma (Zhang et al., 2012a). Iron orebodies were locally intruded by small granite dikes with zircon ages of 320.3 ± 2.5 Ma and 294.5 ± 1.6 Ma (Zhang et al., 2012a). The main orebodies are composed of massive magnetite ores (> 50% Fe) enclosed within disseminated ores and occur as stacked arrays of tabular to lenticular bodies, ranging in thickness from one to tens of meters with strike extent of hundreds of meters. The contact between magnetite ores and the host volcanic rocks is commonly abrupt, but locally is characterized by ore veins and brecciation. Magnetite is the dominant iron oxide, occurring as anhedral to subhedral crystals or as fine-grained aggregate in the ores. Platy or dendritic magnetite crystals (up to several centimeters in length) are commonly present in massive ores. Minor hematite occurs along internal fractures and boundaries of magnetite. Pyrite occurs as disseminations, interstitial grains, or as coarse-grained veins that cut the ores. Gangue minerals are mainly diopside, actinolite, albite, K-feldspar, and epidote, reflecting pervasive Ca, Na and K alteration. The paragenetic sequence of the Zhibo deposit includes four stages (Jiang et al., 2014), where stages I and II are the main ore-forming stages. Stage I consists of massive magnetite, associated with diopside and minor albite. Stage II is characterized by disseminated magnetite in the groundmass of actinolite and K-feldspar, forming the disseminated and banded ores. Stage III is dominated by epidote, quartz, and pyrite, overprinting the main stage mineralization. Stage IV occurs chiefly as veins of calcite, quartz, and minor hematite.

metallogenic belt (Fig. 1a). Several volcano-hydrothermal centers have been recognized in this region as the first-order control on the spatial distribution of these deposits (Chen et al., 2008). The majority of iron deposits in this region (e.g., Zhibo, Beizhan, and Chagangnuoer; Fig. 1b) are dominated by Ti-poor magnetite, without economic Cu and Au mineralization. One exception is the Dunde deposit where Zn-Au veins occur in magnetite orebodies, with 1.49 Mt proven Zn resource grading 1.26% Zn, and 30 t inferred Au resource (Weimin Jiang, personal communication). Magnetite mineralization in these deposits was accompanied by extensive alteration mainly involving Ca, Na, and K metasomatism, but differ in terms of alteration assemblages (Duan et al., 2014; Hong et al., 2012; Jiang et al., 2014; Wang et al., 2017; Zhang et al., 2015), possibly related to the differences in chemical composition of host rocks and ore-forming fluids. 2.3. Geology of the Zhibo iron deposit The Zhibo iron deposit (Fig. 2a), located ca. 100 km west of the town of Baluntai, is one of the best characterized deposit in the Awulale iron metallogenic belt. The deposit geology, mineralogy, and mineral paragenesis have been described in detail by Jiang et al. (2014) and Zhang et al. (2015), and the salient features are briefly summarized here. Iron oxide orebodies are hosted in volcanic and volcaniclastic rocks of the Dahalajunshan Formation. The volcanic rocks range in composition from basalt to dacite with andesite as the dominant lithology. They mainly possess calc-alkaline affinities, with relative depletion in high field strength elements (e.g., Nb, Ta, P, Ti) and enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr), interpreted as products of arc329

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Table 1 Electron microprobe analyses (wt%) of titanite from magnetite ores of the Zhibo iron deposit. Sample No.

zb334-1.1

zb334-1.2

zb334-1.3

zb334-3.1

zb334-3.2

zb334-4.1

zb334-5.1

zb334-5.2

zb334-5.3

zb313-3.1

zb313-3.2

zb313-3.3

wt% SiO2 TiO2 Al2O3 FeO CaO F Cl Total

30.04 34.35 3.29 1.72 28.03 0.36 0.00 97.80

29.48 35.73 2.90 1.64 27.80 0.64 0.00 98.19

28.99 35.28 3.90 1.86 27.61 0.43 0.00 98.06

30.71 35.16 2.75 1.82 28.40 0.45 0.01 99.30

29.74 36.08 2.62 1.78 28.23 0.47 0.01 98.93

29.84 34.90 3.41 1.98 27.92 0.51 0.00 98.57

29.15 36.28 2.55 1.88 27.97 0.53 0.00 98.37

29.10 35.87 2.92 1.73 28.34 0.59 0.01 98.56

28.29 37.74 1.71 1.33 27.91 0.28 0.00 97.25

29.71 35.24 3.01 1.68 27.54 0.55 0.00 97.72

30.74 37.18 2.03 1.22 28.18 0.29 0.00 99.63

29.86 34.88 2.81 1.60 28.21 0.56 0.00 97.92

1.011 0.871 0.107 0.050 1.002 0.047 0.001 3.088 2.12 0.10

0.985 0.899 0.102 0.049 1.002 0.049 0.000 3.087 2.07 0.10

0.990 0.871 0.134 0.055 0.993 0.054 0.000 3.096 2.43 0.13

0.973 0.911 0.100 0.053 1.001 0.056 0.000 3.094 1.91 0.09

0.970 0.900 0.115 0.048 1.013 0.062 0.001 3.108 2.38 0.11

0.956 0.959 0.068 0.038 1.010 0.030 0.000 3.060 1.81 0.06

0.994 0.887 0.119 0.047 0.987 0.058 0.000 3.092 2.52 0.11

1.006 0.915 0.078 0.033 0.988 0.030 0.000 3.052 2.35 0.08

0.999 0.877 0.111 0.045 1.011 0.059 0.000 3.102 2.47 0.11

Normalization = 5 O and 3 cations per formula unit Si 1.003 0.984 0.967 Ti 0.862 0.897 0.885 Al 0.130 0.114 0.153 Fe3+ 0.048 0.046 0.052 Ca 1.002 0.994 0.987 F 0.038 0.067 0.045 Cl 0.000 0.000 0.000 Total 3.083 3.103 3.089 Al/Fe 2.70 2.49 2.96 XAl 0.12 0.11 0.14 Sample No.

zb313-2.1

zb313-2.2

zb313-1.1

zb313-1.3

zb311-1.1

zb311-1.2

zb311-1.3

zb311-1.4

zb311-1.5

zb311-2.1

zb311-2.2

wt% SiO2 TiO2 Al2O3 FeO CaO F Cl Total

29.54 35.02 3.23 1.85 28.66 0.59 0.00 98.90

28.62 34.60 3.15 1.74 28.22 0.65 0.00 96.98

30.04 35.73 3.24 1.79 28.21 0.62 0.01 99.63

29.34 35.91 2.71 1.75 28.12 0.53 0.00 98.37

28.98 37.25 2.49 1.23 28.24 0.72 0.00 98.91

29.96 35.76 3.02 1.70 28.42 0.80 0.01 99.67

30.60 36.10 2.22 1.47 28.00 0.58 0.00 98.96

28.93 33.73 3.70 1.93 27.76 0.94 0.00 96.99

30.63 35.57 2.44 1.84 27.74 0.47 0.02 98.71

28.02 39.04 1.17 0.93 27.53 0.17 0.00 96.87

29.23 37.08 1.60 1.27 27.77 0.15 0.01 97.12

Normalization = 5 O and 3 cations per formula unit Si 0.981 0.972 0.988 Ti 0.875 0.883 0.884 Al 0.126 0.126 0.125 Fe3+ 0.051 0.049 0.049 Ca 1.020 1.026 0.994 F 0.062 0.070 0.064 Cl 0.000 0.000 0.001 Total 3.115 3.127 3.104 Al/Fe 2.46 2.55 2.55 XAl 0.12 0.12 0.12

0.979 0.901 0.107 0.049 1.005 0.056 0.000 3.097 2.19 0.10

0.965 0.933 0.098 0.034 1.007 0.075 0.000 3.112 2.84 0.09

0.988 0.887 0.117 0.047 1.004 0.083 0.000 3.126 2.50 0.11

1.011 0.897 0.087 0.041 0.992 0.060 0.000 3.088 2.14 0.08

0.982 0.861 0.148 0.055 1.009 0.101 0.000 3.155 2.71 0.14

1.013 0.885 0.095 0.051 0.983 0.049 0.001 3.077 1.87 0.09

0.949 0.994 0.047 0.026 0.999 0.018 0.000 3.033 1.77 0.04

0.983 0.937 0.064 0.036 1.000 0.016 0.000 3.037 1.78 0.06

XAl = Al/(Al + Fe + Ti).

Fig. 6. (a) Positive correlation between (Al + Fe) and Ti and (b) negative correlation between (Al + Fe) and F of the analyzed titanite from the Zhibo iron deposit. Compositions are given in atoms per formula unit (a.p.f.u.) following normalization to 5 oxygen (Franz and Spear, 1985).

3. Samples and analytical methods

magnetite ores. It is readily recognizable by its brownish color and wedge-shaped form (Fig. 3a). Three ore samples for titanite U–Pb dating were collected from drill core (ZK4003) intersecting a massive tabular magnetite orebody in the eastern district (Fig. 2b). Sample ZB311 (depth 681.3 m) is a magnetite ore containing clots of quartz,

3.1. Samples Titanite is a minor but relatively common mineral in the Zhibo 330

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Fig. 7. Chondrite-normalized REE patterns (a-c) and plots of Th/U vs. total REE (d) for titanite from the Zhibo iron deposit. Chondrite values from Sun and McDonough (1989).

Quadrupole ICP-MS instrument equipped with a GeoLas Pro 193 nm ArF excimer laser, at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Analytical and data reduction procedures followed methods described in Sun et al. (2012a). The ICP-MS measurement was performed using time-resolved analyses and peak hopping at one point per mass. Laser ablation was operated at 6 Hz ablation frequency with laser energy density of 12 J/cm2, using a spot diameter of 60 μm. The dwell time for the isotopes was 10 ms for 29Si, 43Ca, 232Th and 238U, 15 ms for 204Pb, 206Pb and 208Pb, and 30 ms for 207Pb. Each measurement consisted of 30 s background acquisition followed by 60 s data acquisition from sample and 60 s for cleaning the sample cell and plumbing lines. The matrix-matched BLR-1 reference titanite (1047.1 ± 0.4 Ma; Aleinikoff et al., 2007) was used as a calibration standard for mass discrimination and U–Pb isotope fractionation. The OLT-1 titanite (1015 ± 2 Ma; Kennedy et al., 2010) was employed as monitor standard to verify the procedure. Data reduction, including selective integration of signals and conversion of integrated signals to element concentrations, was performed using GLITTER (Griffin et al., 2008). Trace element analyses of titanite were calibrated using Ca contents determined by electron microprobe as an internal standard, using NIST SRM 610 glass as an external standard. Measured 206Pb/238U ratios were normalized relative to a 206* Pb/238U value of 0.17636 corresponding to an age of 1047.1 Ma of standard BLR-1 (Aleinikoff et al., 2007). The common Pb correction was done using a linear regression through the raw data on a Tera-Wasserburg diagram to determine the common Pb component (y-intercept) on the 207Pb/206Pb axis (Chew et al., 2014; Simonetti et al., 2006). A 207 Pb-based common Pb correction was then applied to calculate the weighted mean 207Pb-corrected 206Pb/238U ages. Concordia plots and weighted averages were derived using Isoplot 4.15 (Ludwig, 2012). Individual analyses in the data table and concordia plots are presented with 1σ and uncertainties for weighted mean ages are quoted at the 95% confidence level.

epidote and pyrite in matrix of magnetite (Fig. 3b). Titanite in this sample are associated with epidote and quartz, generally occurring as platy crystals or as fine-grained aggregate, up to 2.5 mm in length (Fig. 4a). Sample ZB313 (depth 687.2 m) is a brecciated ore containing fragments of magnetite cemented by veins of epidote and calcite (Fig. 3c). Titanite in this sample are associated with epidote and calcite where they occur predominantly as euhedral platy-grained aggregate between 0.5 and 3 mm in length (Fig. 4b). Sample ZB334 (depth 748.6 m) consists primarily of dendritic magnetite and actinolite with minor interstitial pyrite (Fig. 3d). Titanite occur in this sample as 0.2–1 mm platy crystals in close association with magnetite and actinolite (Fig. 4c, d). In addition, an andesite sample (ZB73; Fig. 5a) was collected from an outcrop of the immediate host rocks of magnetite ores. It contains minor plagioclase phenocrysts (ca. 1–2 mm diameter) set in a finegrained matrix composed of plagioclase and hornblende (Fig. 5b). Magnetite (up to ∼5%) is commonly found as disseminated crystals or as rims around plagioclase phenocrysts (Fig. 5c, d). 3.2. Electron microprobe analyses Backscattered electron (BSE) imaging and major element analyses of titanite were completed using a JEOL JXA-8230 electron microprobe at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. The analytical conditions involved an acceleration voltage of 15 kV, a beam current of 20 nA, and a defocused electron beam diameter of 5 μm. Natural and synthetic minerals were used as calibration standards. Data reduction was performed using the ZAF correction of JEOL. 3.3. U–Pb dating and trace element analyses of titanite In situ U–Pb isotope and trace element analyses of titanite in thin sections were simultaneously accomplished using an Agilent 7500a 331

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Table 2 LA-ICP-MS U–Pb data for titanite from magnetite ores of the Zhibo iron deposit. Sample No.

Th/U

207

Pb/206Pb

207

Pb/235U

206

Pb/238U

208

Pb/232Th

Pb

Th

U

(ppm)

(ppm)

(ppm)

ZB311 ZB311-1 ZB311-2 ZB311-3 ZB311-4 ZB311-5 ZB311-6 ZB311-7 ZB311-8 ZB311-9 ZB311-10 ZB311-11 ZB311-12 ZB311-13 ZB311-14 ZB311-15 ZB311-16 ZB311-17 ZB311-18 ZB311-19 ZB311-20 ZB311-21 ZB311-22 ZB311-23 ZB311-24 ZB311-25 ZB311-26 ZB311-27 ZB311-28 ZB311-29 ZB311-30 ZB311-31 ZB311-32 ZB311-33 ZB311-34 ZB311-35 ZB311-36 ZB311-37 ZB311-38

2.4 129.9 6.2 4.1 4.1 5.8 6.0 7.9 4.1 5.0 2.3 2.5 16.9 3.5 1.8 2.3 5.6 1.9 3.5 1.2 2.1 3.9 3.5 4.2 3.2 5.4 4.3 22.3 2.9 2.7 5.4 5.6 7.7 21.6 25.8 3.4 8.0 3.3

0.9 3.1 1.0 1.0 1.6 1.7 4.0 3.5 1.3 1.6 0.9 0.8 1.6 0.7 1.4 1.5 5.9 1.3 2.3 0.5 2.5 4.2 0.5 0.6 0.5 1.4 0.7 1.6 0.9 0.4 0.9 1.7 4.3 1.9 2.9 1.4 3.0 0.5

5.2 671.5 20.0 9.0 11.4 11.8 27.5 24.4 9.3 10.7 6.2 5.4 46.4 5.9 7.8 8.6 30.8 9.8 16.5 4.1 6.3 13.5 7.6 9.0 4.8 10.9 6.1 115.7 6.9 3.9 20.0 23.3 22.5 107.0 142.7 9.7 25.8 6.5

0.18 0.00 0.05 0.11 0.14 0.14 0.15 0.14 0.14 0.15 0.15 0.14 0.03 0.12 0.18 0.17 0.19 0.14 0.14 0.13 0.39 0.31 0.06 0.07 0.10 0.13 0.11 0.01 0.14 0.10 0.04 0.07 0.19 0.02 0.02 0.15 0.12 0.08

0.34355 0.07978 0.20879 0.30795 0.23053 0.31405 0.10861 0.18189 0.28318 0.26597 0.28762 0.27533 0.22608 0.34589 0.12565 0.15535 0.05831 0.11212 0.08728 0.19661 0.15933 0.172 0.29345 0.30317 0.35598 0.31155 0.37963 0.08544 0.26268 0.36125 0.13175 0.12188 0.19411 0.06436 0.05657 0.17039 0.17932 0.35612

0.00902 0.00057 0.0029 0.00621 0.00371 0.00437 0.00224 0.00243 0.00622 0.00505 0.00711 0.00686 0.00345 0.00729 0.00267 0.00252 0.00174 0.00707 0.00159 0.00449 0.0037 0.00253 0.0049 0.00635 0.00982 0.00457 0.00782 0.00172 0.00428 0.01292 0.00254 0.00177 0.00238 0.00075 0.00121 0.00348 0.00289 0.01641

3.6782 0.56343 1.75081 3.0307 1.98993 3.15683 0.79026 1.46763 2.66063 2.44494 2.78659 2.57819 1.92855 3.65912 0.93176 1.20673 0.40342 0.83043 0.62939 1.61612 1.25015 1.3706 2.83733 2.98272 3.96607 3.09702 4.39903 0.6073 2.38272 4.04882 0.99234 0.90569 1.59663 0.44706 0.3824 1.34735 1.43425 3.92469

0.07029 0.00388 0.01965 0.04513 0.02524 0.0335 0.01413 0.01625 0.04356 0.03531 0.05132 0.0481 0.02325 0.05554 0.0171 0.01656 0.01111 0.04471 0.01038 0.03023 0.02438 0.01681 0.03598 0.04653 0.07804 0.03458 0.06495 0.01085 0.03014 0.10243 0.01626 0.01164 0.01642 0.00487 0.00756 0.02287 0.01915 0.12728

0.07761 0.0512 0.06079 0.07135 0.06258 0.07287 0.05275 0.0585 0.06812 0.06665 0.07024 0.06789 0.06185 0.0767 0.05377 0.05632 0.05016 0.05371 0.05229 0.0596 0.0569 0.05778 0.07012 0.07135 0.0808 0.07209 0.08403 0.05155 0.06578 0.08128 0.05463 0.05389 0.05965 0.05038 0.04903 0.05735 0.05801 0.07993

0.00176 0.00055 0.00084 0.00131 0.00094 0.00105 0.00081 0.00078 0.00131 0.00114 0.00148 0.00142 0.0009 0.00148 0.00084 0.00079 0.00082 0.00199 0.0007 0.00105 0.00098 0.00079 0.00112 0.00134 0.00195 0.00107 0.00162 0.00074 0.00101 0.00246 0.00083 0.00069 0.00076 0.00058 0.00068 0.00093 0.00084 0.00305

0.31354 0.71934 0.48948 0.46772 0.23495 0.37768 0.06965 0.15552 0.32829 0.25488 0.34037 0.31116 0.83345 0.52569 0.06543 0.10688 0.02538 0.09014 0.05625 0.18229 0.05965 0.08133 0.69488 0.73451 0.66819 0.37928 0.72222 0.25562 0.2845 0.77267 0.37705 0.15196 0.13117 0.10509 0.048 0.13473 0.18377 0.77615

ZB313 ZB313-1 ZB313-2 ZB313-3 ZB313-4 ZB313-5 ZB313-6 ZB313-7 ZB313-8 ZB313-9 ZB313-10 ZB313-11 ZB313-12 ZB313-13 ZB313-14 ZB313-15 ZB313-16 ZB313-17 ZB313-18 ZB313-19 ZB313-20 ZB313-21 ZB313-22 ZB313-23 ZB313-24 ZB313-25 ZB313-26 ZB313-27 ZB313-28 ZB313-29 ZB313-30 ZB313-31 ZB313-32

5.1 2.1 1.9 5.0 3.7 1.9 19.2 5.4 2.2 9.4 3.8 4.0 5.1 89.4 24.4 40.4 13.7 4.9 3.2 7.3 3.9 2.0 3.6 8.2 14.3 2.4 2.9 3.7 30.1 23.0 3.2 2.0

1.3 1.3 1.7 0.4 0.5 0.7 2.5 4.8 2.5 1.1 2.1 2.9 3.0 1.0 2.9 4.3 2.8 2.1 0.8 2.7 0.9 0.6 0.7 4.4 5.0 1.3 1.2 0.9 10.0 1.5 0.4 1.4

10.6 7.4 8.8 3.1 8.1 4.8 114.6 27.4 9.5 45.3 8.4 16.8 19.0 519.4 123.8 234.5 80.3 13.1 13.3 28.8 6.7 3.7 4.6 32.6 50.8 7.0 6.7 6.8 163.9 118.8 8.0 8.2

0.12 0.17 0.19 0.11 0.06 0.14 0.02 0.18 0.27 0.02 0.25 0.17 0.16 0.00 0.02 0.02 0.04 0.16 0.06 0.09 0.13 0.15 0.15 0.13 0.10 0.19 0.17 0.14 0.06 0.01 0.05 0.18

0.3215 0.1273 0.1159 0.4560 0.2850 0.3047 0.1014 0.1197 0.2489 0.2032 0.4199 0.1429 0.2133 0.1041 0.0714 0.0867 0.1661 0.2059 0.1188 0.1126 0.3340 0.3270 0.4396 0.1427 0.1076 0.2595 0.2888 0.3622 0.1357 0.0923 0.2475 0.1514

0.0101 0.0059 0.0042 0.0137 0.0055 0.0056 0.0015 0.0024 0.0048 0.0034 0.0086 0.0024 0.0045 0.0009 0.0006 0.0011 0.0032 0.0052 0.0024 0.0027 0.0076 0.0105 0.0088 0.0026 0.0013 0.0060 0.0055 0.0061 0.0014 0.0014 0.0055 0.0054

3.2968 0.9507 0.8483 6.1681 2.5743 2.9563 0.7278 0.8910 2.1963 1.6760 5.1379 1.0881 1.7997 0.7538 0.5001 0.6160 1.3154 1.7165 0.8855 0.8295 3.4502 3.3096 5.7417 1.0868 0.7886 2.3226 2.7302 4.0106 1.0158 0.6500 2.1437 1.1831

0.0751 0.0381 0.0270 0.1271 0.0376 0.0414 0.0095 0.0151 0.0326 0.0227 0.0740 0.0152 0.0297 0.0060 0.0040 0.0071 0.0206 0.0340 0.0156 0.0174 0.0567 0.0764 0.0810 0.0169 0.0087 0.0407 0.0396 0.0499 0.0094 0.0089 0.0369 0.0353

0.0743 0.0541 0.0531 0.0980 0.0655 0.0703 0.0520 0.0540 0.0640 0.0598 0.0888 0.0553 0.0612 0.0525 0.0508 0.0516 0.0575 0.0605 0.0541 0.0535 0.0750 0.0735 0.0949 0.0554 0.0533 0.0651 0.0687 0.0805 0.0544 0.0512 0.0630 0.0568

0.0020 0.0015 0.0012 0.0027 0.0012 0.0012 0.0007 0.0008 0.0011 0.0009 0.0017 0.0008 0.0011 0.0006 0.0006 0.0006 0.0009 0.0012 0.0008 0.0009 0.0015 0.0020 0.0019 0.0009 0.0007 0.0013 0.0012 0.0014 0.0007 0.0007 0.0012 0.0014

0.4580 0.0753 0.0599 0.9390 0.6637 0.3482 0.2833 0.0744 0.1555 1.2934 0.3384 0.0958 0.1803 3.6214 0.1108 0.2062 0.1563 0.1556 0.1624 0.1069 0.4178 0.3930 0.6281 0.1052 0.1122 0.1968 0.2668 0.4932 0.2275 0.4735 0.6495 0.0858

1σ

1σ

1σ

1σ

207

Pb-Corrected ages (Ma) Pb/238U

332

206

1σ

0.00923 0.01205 0.00979 0.01134 0.00469 0.00648 0.00195 0.00273 0.00838 0.00577 0.0093 0.00881 0.01998 0.01319 0.00196 0.00223 0.00104 0.00722 0.00143 0.00517 0.00149 0.00146 0.01601 0.02056 0.0219 0.00699 0.0181 0.00991 0.0057 0.03296 0.01149 0.00323 0.00209 0.00335 0.0034 0.00345 0.00385 0.04297

313.8 311.4 309.2 308.2 307.8 311.3 309.1 309.7 307.3 309.5 314.4 310.4 306.3 308.8 308.0 309.7 313.4 313.2 314.9 308.7 311.1 310.3 310.7 310.8 318.9 309.4 316.3 311.3 307.2 317.5 310.3 310.2 310.1 312.4 307.1 308.7 308.3 315.4

10.7 3.3 5.1 8.2 5.7 7.2 4.8 4.7 7.9 7.0 8.8 8.4 5.5 9.5 5.0 4.7 5.1 11.8 4.2 6.2 5.8 4.7 7.2 8.3 11.7 7.2 10.8 4.5 6.3 14.1 4.9 4.1 4.6 3.6 4.2 5.5 5.0 16.8

0.0166 0.0043 0.0028 0.0349 0.0174 0.0075 0.0072 0.0020 0.0034 0.0392 0.0075 0.0020 0.0046 0.0981 0.0020 0.0050 0.0039 0.0047 0.0052 0.0037 0.0113 0.0139 0.0147 0.0026 0.0022 0.0052 0.0058 0.0101 0.0038 0.0138 0.0198 0.0040

315.7 11.1 309.8 8.8 308.4 6.8 316.4 16.3 296.6 6.9 307.9 7.4 308.1 4.0 312.1 4.8 307.4 6.4 308.1 5.4 310.9 11.4 309.8 4.6 310.6 6.2 310.0 3.5 312.3 3.4 311.1 3.8 312.1 5.3 310.4 6.9 313.3 4.9 312.4 5.5 311.7 9.1 309.5 11.1 318.0 12.5 310.4 5.0 312.9 3.9 307.2 7.4 309.2 7.2 317.2 8.8 308.1 3.8 306.8 4.0 303.1 6.9 314.9 7.8 (continued on next page)

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Table 2 (continued) Sample No.

Th/U

207

Pb/206Pb

207

Pb/235U

206

Pb/238U

208

Pb/232Th

Pb

Th

U

(ppm)

(ppm)

(ppm)

ZB313-33 ZB313-34 ZB313-35 ZB313-36 ZB313-37 ZB313-38 ZB313-39

3.0 4.3 4.5 1.5 3.4 3.6 5.5

5.0 3.4 1.1 0.6 0.6 0.9 2.2

12.9 17.4 9.5 4.0 5.1 6.7 20.2

0.38 0.19 0.12 0.14 0.12 0.13 0.11

0.1331 0.1462 0.1867 0.2884 0.4292 0.3435 0.1831

0.0027 0.0027 0.0026 0.0076 0.0086 0.0101 0.0027

0.9997 1.1203 1.5157 2.6768 5.2676 3.6064 1.4784

0.0171 0.0177 0.0173 0.0523 0.0746 0.0760 0.0178

0.0546 0.0558 0.0591 0.0675 0.0893 0.0764 0.0588

0.0009 0.0009 0.0008 0.0015 0.0017 0.0019 0.0008

0.0446 0.0801 0.3885 0.2934 0.7483 0.4499 0.2036

ZB334 ZB334-1 ZB334-2 ZB334-3 ZB334-4 ZB334-5 ZB334-6 ZB334-7 ZB334-8 ZB334-9 ZB334-10 ZB334-11 ZB334-12 ZB334-13 ZB334-14 ZB334-15 ZB334-16 ZB334-17 ZB334-18 ZB334-19 ZB334-20 ZB334-21 ZB334-22 ZB334-23 ZB334-24

11.3 32.2 40.8 192.1 12.3 63.3 51.4 6.2 34.2 6.2 159.3 32.0 79.7 103.8 48.3 18.7 12.3 12.6 16.9 113.9 25.7 7.4 117.8 18.7

4.2 14.7 11.9 46.2 20.6 27.4 27.9 20.7 19.7 14.5 30.3 7.4 29.5 27.1 15.5 15.0 5.0 9.7 7.3 9.5 12.6 20.7 15.6 18.0

30.1 46.4 90.9 283.6 30.6 189.2 118.8 24.9 114.1 23.9 178.2 89.1 99.8 155.6 63.4 49.3 63.2 36.0 53.3 193.5 65.9 25.3 270.3 51.6

0.14 0.32 0.13 0.16 0.67 0.14 0.23 0.83 0.17 0.61 0.17 0.08 0.30 0.17 0.24 0.30 0.08 0.27 0.14 0.05 0.19 0.82 0.06 0.35

0.1803 0.4068 0.3759 0.4455 0.2036 0.2556 0.3272 0.1219 0.3468 0.0946 0.3620 0.2057 0.4286 0.3514 0.4436 0.2281 0.1053 0.3897 0.1898 0.3560 0.3010 0.1127 0.2914 0.2189

0.0043 0.0037 0.0059 0.0048 0.0040 0.0033 0.0036 0.0028 0.0033 0.0026 0.0042 0.0022 0.0052 0.0028 0.0046 0.0043 0.0021 0.0050 0.0023 0.0031 0.0039 0.0044 0.0038 0.0041

1.4859 4.9123 4.2242 5.8162 1.7134 2.3610 3.3580 0.9118 3.6556 0.6849 3.8647 1.7534 5.3943 3.7212 5.7542 2.0444 0.7712 4.5106 1.5530 3.8321 2.9600 0.8441 2.7922 1.8987

0.0285 0.0369 0.0491 0.0487 0.0268 0.0244 0.0291 0.0182 0.0282 0.0170 0.0348 0.0157 0.0495 0.0255 0.0464 0.0301 0.0132 0.0434 0.0158 0.0279 0.0298 0.0288 0.0284 0.0278

0.0598 0.0876 0.0815 0.0947 0.0610 0.0670 0.0744 0.0542 0.0765 0.0525 0.0774 0.0618 0.0913 0.0768 0.0941 0.0650 0.0531 0.0840 0.0594 0.0781 0.0713 0.0543 0.0695 0.0629

0.0011 0.0011 0.0013 0.0013 0.0010 0.0009 0.0010 0.0009 0.0009 0.0009 0.0011 0.0008 0.0013 0.0009 0.0012 0.0011 0.0008 0.0012 0.0008 0.0009 0.0010 0.0013 0.0010 0.0010

0.1368 0.2901 0.4079 0.5265 0.0533 0.2434 0.3028 0.0281 0.4232 0.0282 0.2571 0.2961 0.3666 0.3889 0.4107 0.1178 0.0834 0.4100 0.1653 1.2477 0.2849 0.0275 0.7985 0.0917

1σ

1σ

1σ

1σ

207

Pb-Corrected ages (Ma)

206

Pb/238U

1σ

0.0011 0.0020 0.0077 0.0088 0.0181 0.0152 0.0039

310.3 311.3 311.4 304.1 306.5 312.0 311.5

5.0 5.0 4.8 8.6 11.6 11.1 4.9

0.0039 0.0037 0.0078 0.0075 0.0011 0.0041 0.0043 0.0006 0.0056 0.0008 0.0037 0.0046 0.0055 0.0047 0.0055 0.0024 0.0027 0.0065 0.0027 0.0180 0.0045 0.0010 0.0155 0.0019

318.9 318.7 315.4 317.1 314.9 319.5 315.2 313.1 312.6 313.8 307.8 317.9 317.3 311.4 316.5 323.4 313.3 316.3 312.5 313.8 316.0 317.1 312.9 317.4

6.4 8.4 8.6 10.0 5.9 5.6 6.5 5.4 6.6 5.6 7.2 4.6 9.6 6.5 9.8 6.3 4.7 8.4 4.5 6.8 6.3 7.7 6.0 5.9

The analyzed titanite grains show no apparent compositional zoning in BSE images (Fig. 4) but have little variations in major elements. They contain variable SiO2 (28.02–30.74 wt%), TiO2 (33.73–39.04 wt%), CaO (27.53–28.66 wt%), Al2O3 (1.17–3.9 wt%) and FeO (0.93–1.98 wt %). Fluorine contents are from 0.15 to 0.94 wt% and chlorine is below detection limits of ∼0.02 wt%. The (Al + Fe) contents show negative correlation with Ti and positive correlation with F (Fig. 6), consistent with the coupled substitution of (Al, Fe)3+ + (F, OH)− ↔ Ti4+ + O2− in titanite (e.g., Oberti et al., 1991). Titanite commonly incorporates trace amounts of REE, which replace Ca by coupled substitution of 2Ca2+ + Ti4+ ↔ 2(REE, Y)3+ + (Al, Fe)3+, enhanced by additional Na + Ca2+ exchange (Tiepolo et al., 2002). Titanite from samples ZB311 and ZB313 have total REE contents of 83.9 to 411.4 ppm and 46.9 to 473.2 ppm, respectively (Appendix A). The REE patterns are relatively fractionated with HREE being strongly enriched over the LREE ((La/ Yb)N = 0.001–0.021 and 0.002–0.015, respectively) and no significant Eu anomalies (Eu/Eu∗ = 0.64–1.39 and 0.75–1.38, respectively; Fig. 7a, b). Titanite from sample ZB334 have relatively higher total REE contents of 322.7–1387.1 ppm (Fig. 7d; Appendix A). They also show enrichment in HREE relative to LREE ((La/Yb)N = 0.013–0.137; Fig. 7c), but differ from those in samples ZB311 and ZB313 in having flat HREE patterns and marked negative Eu anomalies (Eu/ Eu∗ = 0.50–1.03).

3.4. U–Pb dating of zircon Zircon grains were separated from the andesite sample using heavy liquid and magnetic separation, followed by handpicking under a binocular microscope. The grains were mounted in epoxy and polished to expose their interiors. All grains were studied by optical and cathodoluminescence (CL) imaging to select locations for U–Pb analyses. U–Pb isotope were analyzed using an Agilent 7500a Quadrupole ICPMS with a New Wave UP-193 laser ablation system at the Geological Lab Center, China University of Geosciences (Beijing). The laser spot size was 36 μm, with a laser frequency of 10 Hz and energy density of 8.5 J/cm2. Ablation was conducted using high-purity He (0.8 L/min) as the carrier gas, which was subsequently mixed with Ar (1.13 L/min) prior to introduction into the ICP-MS. The counting time were 20 ms for U, Th, 204Pb, 206Pb, 207Pb and 208Pb, and 10 ms for other elements. Data reduction was performed with GLITTER (Griffin et al., 2008), where isotope ratios were corrected for mass bias and isotopic fractionation using zircon 91500 (1065.4 ± 0.6 Ma; Wiedenbeck et al., 1995) as the primary standard. The zircon TEMORA (417 Ma; Black et al., 2003) was used as a secondary monitor of data quality. Concordia plots and weighted averages were derived using Isoplot 4.15 (Ludwig, 2012). Individual analyses are presented with errors of 1σ, and uncertainties for the weighted mean ages are quoted at the 95% confidence level.

4. Results 4.2. Titanite U–Pb age 4.1. Chemical composition of titanite The U–Pb data of titanite are summarized in Table 2, and presented in Tera-Wasserburg (T-W) Concordia diagrams (Tera and Wasserburg, 1972) and weighted average plots (Fig. 8). Thirty-eight spot analyses were performed on titanite from sample ZB311. The analyzed titanite have variable U (3.9–671.5 ppm; mostly < 50 ppm) and Th

Major oxide contents and calculated cations of titanite are given in Table 1. Mineral formulae were calculated based on 5 oxygens and all Fe was calculated as Fe3+. Rare earth element (REE) compositions of titanite are presented in Appendix A. 333

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Fig. 8. Tera-Wasserburg concordia diagrams and weighted mean plots of 207Pb-corrected 206Pb/238U age for titanite from the Zhibo iron deposit. (a, b) Sample ZB311. (c, d) Sample ZB313. (e, f) Sample ZB334. Age uncertainties are quoted as 95% confidence level (2σ), individual precision ellipses are 1σ.

conducted using the 207Pb-based method. All analyses yield a weighted 207 206 mean Pb-corrected Pb/238U age of 310.1 ± 1.8 Ma (MSWD = 0.30; Fig. 8d), which is within error to the age obtained for titanite of sample ZB313. Twenty-four spot analyses were performed on titanite in sample ZB334. Uranium and Th contents are in the range of 23.9–283.6 ppm and 4.2–46.2 ppm, respectively, with Th/U ratios ranging from 0.05 to 0.83 (Table 2). The common Pb-uncorrected data lie on a regression line on the T-W diagram, yielding a lower intercept age at 315.3 ± 4.4 Ma, and a Y-intercept of 207Pb/206Pb ratio at 0.893 ± 0.026 (Fig. 8e). After common Pb correction using the Y-intercept, all analyses yield a weighted mean 207Pb-corrected 206Pb/238U age of 315.3 ± 2.5 Ma (MSWD = 0.26; Fig. 8f).

(0.4–5.9 ppm) contents, and low Th/U ratios (< 0.38) (Table 2). All spot analyses form a linear array in the T-W diagram (Fig. 8a), reflecting the presence of common Pb. Regression of the analyses yield a lower intercept age at 310.3 ± 2.7 Ma, and a Y-intercept of 207 Pb/206Pb ratio at 0.867 ± 0.029 (Fig. 8a), which was used for the 207 Pb-based common Pb correction of the U–Pb data. A weighted mean 207 Pb-corrected 206Pb/238U age of 310.3 ± 1.8 Ma (MSWD = 0.17; Fig. 8b) was obtained for titanite in sample ZB311. For sample ZB313, a total of 39 spot analyses on titanite were performed. Uranium and Th contents are also variable (3.1–591.4 ppm and 0.4–10.0 ppm, respectively), with Th/U ratios < 0.38 (Table 2). All the 39 analyses define a linear array on the T-W diagram, with a lower intercept age at 310.3 ± 2.5 Ma, and a Y-intercept of 207Pb/206Pb ratio at 0.881 ± 0.027 (Fig. 8c). With this value, common Pb correction was 334

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Fig. 9. Representative CL images (a) and U–Pb Concordia diagrams (b) for zircon in andesite sample ZB73 from the Zhibo iron deposit. Circles with numbers represent spot locations for LA-ICP-MS analyses. Weighted mean 206Pb/238U ages are presented as inset.

Table 3 LA-ICP-MS U–Pb data for zircon from andesite of the Zhibo iron deposit. Sample No.

Th (ppm)

U (ppm)

Th/U

207

Pb/206Pb

ZB73-01 ZB73-02 ZB73-03 ZB73-04 ZB73-05 ZB73-06 ZB73-07 ZB73-08 ZB73-09

81 127 447 118 82 102 92 141 107

212 215 456 176 138 197 175 282 207

0.38 0.59 0.98 0.67 0.60 0.52 0.53 0.50 0.51

0.0517 0.0522 0.0567 0.0559 0.0526 0.0553 0.0525 0.0525 0.0527

1σ

207

Pb/235U

1σ

206

Pb/238U

1σ

207 Pb/206Pb age (Ma)

1σ

207 Pb/235U age (Ma)

1σ

206 Pb/238U age (Ma)

1σ

0.0028 0.0025 0.0021 0.0033 0.0032 0.0028 0.0029 0.0022 0.0028

0.3594 0.3588 0.3948 0.3862 0.3645 0.3855 0.3656 0.3645 0.3638

0.0190 0.0167 0.0138 0.0225 0.0219 0.0191 0.0196 0.0151 0.0187

0.0504 0.0499 0.0505 0.0501 0.0502 0.0506 0.0505 0.0504 0.0500

0.0009 0.0008 0.0008 0.0008 0.0009 0.0008 0.0009 0.0008 0.0008

271 292 478 448 312 423 308 306 317

89 77 50 100 105 81 88 66 86

312 311 338 332 316 331 316 316 315

14 13 10 16 16 14 15 11 14

317 314 318 315 316 318 317 317 315

6 5 5 5 5 5 6 5 5

316.3 ± 3.4 Ma (MSWD = 0.079; Fig. 9b). The U and Th contents are variable (138–456 ppm and 81–447 ppm, respectively), with Th/U ratios of 0.38 to 0.98 (Table 3), a typical range for magmatic zircon (Hoskin and Schaltegger, 2003). The weighted mean 206Pb/238U age of 316.3 ± 3.4 Ma is interpreted as the crystallization age of the host andesite, and is in good agreement with the U–Pb titanite age of sample ZB334. 5. Discussion 5.1. Paragenesis of titanite The origin of titanite and its paragenetic relationship with magnetite is critical to the interpretation of U–Pb data. The majority of titanite crystals in Zhibo magnetite ore are closely associated with the earlystage Ca alteration assemblage and magnetite. The high Ca activities and actinolite contents would control the presence of titanite in the Ca alteration assemblage, as noted in previous studies (Frost et al., 2000; Thieblemont et al., 1988). Moreover, the presence of straight crystal boundary and lack of replacement texture with magnetite (Fig. 4c, d) indicate that titanite is in textural equilibrium and coeval with magnetite in the paragenetic sequence. Titanite is also observed as platy aggregates or discrete crystals in veins of calcite-epidote-quartz that crosscut the magnetite ore (Fig. 3a, c), implying that titanite in latestage veins postdate the precipitation of magnetite. The homogeneous textures without any internal zoning and the absence of inheritance indicate a hydrothermal origin for the dated titanite. They have Al/Fe ratios from 1.77 to 2.96, and mostly plot between the fields for metamorphic and magmatic titanite in the plot of Al versus Fe (Fig. 10). The Al/Fe ratios of Zhibo titanite are somewhat

Fig. 10. Plot of Fe vs. Al (a.p.f.u.) of the analyzed titanite from the Zhibo iron deposit. Fields for metamorphic titanite and magmatic titanite from Aleinikoff et al. (2002).

4.3. Zircon U–Pb age Only a few zircon grains were separated from the andesite sample ZB73. The grains are euhedral to subhedral prismatic, clear, and range in length from 60 to 100 μm, with aspect ratios of 1:1–2:1. The majority of zircon grains exhibit oscillatory zoning in CL images (Fig. 9a), typical of magmatic zircon (e.g., Corfu et al., 2003). Nine spot analyses on 9 zircon grains yielded concordant 206Pb/238U ages ranging from 314 ± 5 to 318 ± 5 Ma (Table 3), with a weighted mean of 335

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for Ca2+) into the titanite crystal structure (Horie et al., 2008). Thus, positive Eu anomalies in titanite reflect a relatively low oxygen fugacity during crystallization (Storey et al., 2007). The late (310 Ma) titanite have higher Eu/Eu∗ compared to the earlier (315 Ma) titanite and apparently crystallized under low oxidation state that decreased the Eu2+/Eu3+ ratio of the fluid. 5.2. Timing of mineralization The age of Zhibo magnetite ore was previously constrained through inference by its close association with the igneous rocks. Zhang et al. (2012a) reported zircon U–Pb ages of 320.3 ± 2.5 Ma and 294.5 ± 1.6 Ma for granite dikes that cut across the No. Fe15 orebody, and constrained the mineralization at ca. 320 Ma. Jiang et al. (2014) interpreted zircon U–Pb age of 329.9 Ma for a disseminated ore sample to represent the crystallization age of the host volcanic rock, thereby providing a maximum age constraint on the mineralization. Recently, Zhang et al. (2015) dated igneous zircon from a magnetite lava flow at 350 ± 2 Ma, which they interpreted as record of ortho-magmatic, high-temperature iron oxide mineralization event. Our petrographic observations demonstrate that titanite precipitation at Zhibo is contemporaneous with to slightly late relative to magnetite deposition. In situ U–Pb analyses of titanite intergrown with magnetite in sample ZB334, yield a weighted mean 206Pb/238U age of 315.3 ± 2.5 Ma (MSWD = 0.26). Given the fact that the dated titanite are from the volumetrically dominant magnetite ore, the age of 315.3 ± 2.5 Ma is interpreted as a robust age determination on the main stage of iron mineralization. Titanite in association with epidote and calcite in paragenetically late veins of samples ZB311 and ZB313, yield identical weighted mean 206Pb/238U ages of 310.3 ± 1.8 Ma (MSWD = 0.17) and 310.1 ± 1.8 Ma (MSWD = 0.30), slightly younger than the main mineralization age. We interpret these ages to represent a subsequent hydrothermal event that affected the massive magnetite ores. These new radiometric age data, combined with mineral paragenesis (Jiang et al., 2014), define an episodic mineralization event that covered a time span of 5 m.y. between 315 and 310 Ma, from the magnetite-actinolite mineralization stage to the late-stage veins of epidote and calcite. In a regional context, the absolute age of the Zhibo iron ore is similar to the Sm–Nd garnet ages of 316.8 ± 6.7 Ma (Hong et al., 2012) and 313 ± 7 Ma (Zhang et al., 2015) for the Chagangnuoer deposit, located only a few kilometers west of the Zhibo deposit. Muscovite from muscovite-rich ores gave 40Ar/39Ar plateau ages of 304.5 ± 1.9 to 308.1 ± 1.9 Ma and pyrite from pyrite-rich ores yielded a Re–Os isochron age of 302.5 ± 8.2 Ma (95%, MSWD = 2.2, N = 5) for the Beizhan deposit (Duan et al., 2017). These ages are comparable to our U–Pb titanite ages for the late-stage epidote alteration at Zhibo. It is therefore likely that iron mineralization and alteration in the three major magnetite deposits occurred broadly during the same time from ca. 315 to ca. 302 Ma (Fig. 11). In addition, the mineralization ages of these deposits fall within or close to the span of zircon U–Pb ages for the host volcanic rocks (Fig. 11). Coupled with their similarity in oreforming environments, these deposits might belong to a single iron mineralization event related to the Carboniferous volcanism in the Awulale iron metallogenic belt. The diverse mineralization styles in these iron oxide deposits are different expressions of the significant mineralization event. Considering analytical uncertainty between different dating techniques, further accurate geochronology is required to verify this interpretation, and to demonstrate their genetic link with the Carboniferous arc volcanism in the Awulale iron metallogenic belt. This may be achieved by U–Pb analyses of titanite, which has proven to be an ideal chronometer to date iron oxide mineralization affected by latestage hydrothermal alteration because of its chemical resistance and high closure temperature to Pb diffusion.

Fig. 11. Summary of geochronological data showing the timing of host volcanic rocks, intrusions, and iron mineralization in the major magnetite deposits of the Awulale iron metallogenic belt, including the Zhibo, Chagangnuoer, Beizhan, and Dunde deposits. The shaded area represents the approximate age range of the iron mineralization. Data compiled from Duan et al. (2014, 2017), Feng et al. (2010), Han et al. (2013), Jiang et al., (2012, 2014), Li et al. (2013, 2015b), Sun et al., (2012b), Zhang et al. (2012a, b, 2015), and this study.

lower than those of other hydrothermal titanite (Aleinikoff et al., 2002), perhaps due to the low Al/Fe ratio of the ore-forming fluid from which titanite precipitated. The XAl values [XAl = Al/(Al + Fe + Ti)] are from 0.04 to 0.14, within the range of low-Al titanite (XAl < 0.25; Oberti et al., 1991). The trace element characteristics including low REE contents with notable LREE depletion and low Th/U ratios (< 0.83) of the Zhibo titanite are similar to those of hydrothermal titanite (Aleinikoff et al., 2002). Depletion of LREE in titanite may be explained by the coprecipitation of LREE-enriched epidote or by a source fluid which was deficient in LREE (Smith et al., 2009). Despite the variations in Th/U ratios and REE contents, the dated titanite exhibit generally similar REE patterns, indicating a common source. The gradual decrease of total REE contents in titanite from 315 Ma to 310 Ma may reflect the consumption of REE by precipitation of titanite, apatite, and allanite. The extent of the Eu anomaly in titanite is largely a function of oxygen fugacity because of the preferential incorporation of Eu2+ (substituting

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(Li et al., 2015b). In fact, the iron-rich igneous rocks are also known elsewhere in the Awulale iron metallogenic belt and has been traced by magnetic anomalies. These include, for example, the magnetite-rich gabbro from drill cores near Xinyuan county (Li et al., 2015a) and in the Wuling iron deposit (Yan et al., 2015), and particular the magnetiterich trachyandesite and diorite in the Taertage iron deposit, which have enrichment of magnetite up to economic significance (22–29% TFe). The emplacement of these iron-rich igneous rocks indicates the likely presence of a ferrobasaltic magma chamber, which contributed to the extraordinary iron endowment of the Awulale area during the Late Carboniferous.

5.3. Implications for ore genesis Iron mineralization in the Zhibo deposit is largely confined to the Carboniferous volcanic and volcaniclastic sequences. The close spatial association of magnetite ores with the host volcanic rocks, together with their similar REE patterns and Pb isotope compositions, suggest that they are related to a single parental magma (Feng et al., 2010; Zhang et al., 2015). The reliable titanite U–Pb age of 315.3 ± 2.5 Ma for magnetite ores reported in this study provides evidence for the iron mineralization to have a close temporal association with the mafic to intermediate volcanic suite, represented by the host andesite with U–Pb zircon age of 316.3 ± 3.4 Ma. It is noteworthy that magnetite is commonly found in this volcanic suit with a high proportion (up to 5–8 modal%), mainly occurring as interstitial phase in the matrix and around primary plagioclase phenocrysts (Fig. 5b–d). We interpret this feature to indicate the exsolution of Fe-rich fluid from the crystallizing parental magma that percolated into the host andesite to precipitate magnetite. Combined with the good match of ages between iron ores and the host andesite, we infer that the Fe-rich fluid was probably responsible for the genesis of the Zhibo orebodies. The dendritic-like and platy magnetite crystals in the Zhibo magnetite ores were interpreted as evidence for a magmatic origin of the magnetite (Jiang et al., 2014; Zhang et al., 2015), analogous to those observed at El Laco (Henríquez and Martin, 1978) and Kiruna deposits (Nyström, 1985; Nyström and Henríquez, 1989, 1994). This is also supported by the Fe-O isotope data of Zhibo magnetite (Günther et al., 2017), which overlap those reported for ortho-magmatic magnetite from IOA deposits in the Kiruna and Grängesberg districts, Sweden (Weis, 2013), the Chilean iron belt (Bilenker et al., 2016; Knipping et al., 2015a, b) and the Pea Ridge, USA (Childress et al., 2016). Günther et al. (2017) recently proposed that a high-temperature orthomagmatic fluid that separated from an intermediate melt could be responsible for the magnetite mineralization at Zhibo, similar to the interpretations that have been presented in recent studies on other IOA deposits (e.g., Childress et al., 2016; Knipping et al., 2015a, b; Tornos et al., 2016). In combination with the geological and geochemical evidence reported in previous studies (Günther et al., 2017; Jiang et al., 2014; Zhang et al., 2015), our geochronological data support a genetic link between the ore deposit and the time-equivalent volcanism. We infer that magnetite in the Zhibo deposit was preferentially precipitated from iron-rich fluids derived from the ca. 316 Ma mafic to intermediate magma. The iron-rich fluids ascended upward into the shallow levels within a volcanic structure where magnetite precipitated in open pore space to form massive ores. The changes in pressure during eruption were likely one of the cause of magnetite precipitation as evidenced by breccia textures and partially altered volcanic clasts in brecciated ores. This is consistent with experimental data that with decompression the iron solubility slightly decreases whereas the solubility of Na and K increase greatly (Simon et al., 2004). Precipitation of magnetite would then promote the exsolution of large amounts of highly saline fluids, which subsequently interacted with the host volcanic rocks to produce Ca-Na-K alteration assemblages. On a regional scale, a close spatial and temporal association of magnetite ores with volcanic rocks has been documented in the Chagangnuoer deposit, where ferrobasalts (316.3 ± 3.4 Ma) were considered as the source magma for the overlying magnetite orebodies

6. Conclusions In situ U–Pb analyses of titanite and zircon by laser ablation ICP-MS were applied to place tight constraints on the timing and genesis of the Zhibo magnetite ores in the Western Tianshan, NW China. The dated titanite are hydrothermal in origin as indicated by chemical characteristics including strongly fractionated REE patterns with heavy REE enrichment, low Th and U concentrations, and low Th/U ratios. The textural features indicate that titanite are coeval with magnetite in the paragenetic sequence, and therefore provide a reliable estimate for the age of iron mineralization. Titanite from three magnetite ores yield weighted mean 207Pb-corrected 206Pb/238U ages of 310.3 ± 1.8 Ma (MSWD = 0.17), 310.1 ± 1.8 Ma (MSWD = 0.30), and 315.3 ± 2.5 Ma (MSWD = 0.26), constraining the iron mineralization at Zhibo to a time interval between 315 Ma and 310 Ma. Magmatic zircon from the host andesite yield U–Pb age of 316.3 ± 3.4 Ma (MSWD = 0.079), which is within error to the mineralization ages. Our results support a genetic link between magnetite ores and the host volcanic rocks, and are consistent with a magmatic contribution to the mineralization system. The absolute ages of the Zhibo magnetite ore are also consistent with the age estimates reported for other iron orebodies in the Awulale iron metallogenic belt, indicating that they were formed broadly at the same time. These deposits are assigned to a significant iron mineralization event related to the Carboniferous volcanism in the Awulale iron metallogenic belt. The new geochronological data, together with previous geological and geochemical evidence, demonstrate that the Zhibo magnetite deposit was formed mainly by iron-rich fluids derived from a mafic to intermediate magma system in a volcano-plutonic structure. Acknowledgments We would like to thank Dr. Yueheng Yang and Dr. Yanbin Zhang for the assistance with LA ICP-MS analyses of titanite and data reduction, and Dr. Zhenyu Chen for assistance with the EPMA analyses. We also appreciate the support of the Third Geological Team of XBGMR during fieldwork. We are grateful to the associate editor, Prof. Huayong Chen, and two anonymous reviewers for their thorough and constructive comments, which greatly improved the manuscript. This research was funded by the National Natural Science Foundation of China (grants 41502088), the National Nonprofit Institute Research Grant of CAGSIMR (K1608), the National Basic Research Program of China (973 Program) (2012CB416803), the National Scientific and Technological Supporting Key Project (2011BAB06B02).

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Appendix A. See Appendix A

Table A1 LA-ICP-MS trace element data (ppm) for titanite from magnetite ores of the Zhibo iron deposit. Sample no.

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Total REE

La/YbN

Eu/Eu*

ZB311 ZB311-1 ZB311-2 ZB311-3 ZB311-4 ZB311-5 ZB311-6 ZB311-7 ZB311-8 ZB311-9 ZB311-10 ZB311-11 ZB311-12 ZB311-13 ZB311-14 ZB311-15 ZB311-16 ZB311-17 ZB311-18 ZB311-19 ZB311-20 ZB311-21 ZB311-22 ZB311-23 ZB311-24 ZB311-25 ZB311-26 ZB311-27 ZB311-28 ZB311-29 ZB311-30 ZB311-31 ZB311-32 ZB311-33 ZB311-34 ZB311-35 ZB311-36 ZB311-37 ZB311-38

127.6 433.7 203.6 167.5 301.9 214.7 513.5 406.6 176.3 213.7 147.4 143.1 239.4 193.7 340.6 345.1 611.7 309.6 359.3 140.3 284.8 378.0 137.4 173.7 123.0 180.2 169.5 163.5 282.4 117.5 259.9 429.7 371.9 166.8 236.3 398.6 527.9 166.0

0.2 1.2 0.3 0.3 0.3 0.4 0.8 0.7 0.3 0.4 0.2 0.2 0.4 0.3 0.6 0.6 1.6 0.5 0.5 0.2 0.7 0.9 0.1 0.2 0.2 0.3 0.2 1.4 0.3 0.2 0.2 0.6 0.9 0.1 0.4 0.8 0.8 0.2

1.2 4.3 2.0 2.0 2.1 2.5 5.6 4.8 2.0 2.5 1.7 2.7 2.8 1.7 3.8 4.4 11.1 3.4 3.4 1.3 4.7 6.4 0.8 2.2 1.3 2.1 1.3 2.9 2.2 1.2 1.9 4.3 6.2 0.5 2.5 5.4 5.8 1.6

0.3 0.9 0.5 0.5 0.6 0.6 1.5 1.2 0.5 0.6 0.4 0.4 0.7 0.5 1.0 1.1 2.8 0.9 0.9 0.4 1.1 1.4 0.2 0.4 0.3 0.5 0.3 0.4 0.6 0.3 0.5 1.1 1.5 0.1 0.6 1.3 1.5 0.4

2.1 5.7 3.5 3.3 4.3 4.3 11.5 9.3 3.7 4.4 2.9 2.8 5.0 3.4 7.2 8.0 20.0 6.3 6.8 2.3 7.1 9.5 1.7 3.0 2.2 3.6 2.4 2.6 4.4 2.2 4.0 8.6 10.5 1.0 4.2 9.7 12.1 2.9

1.3 4.5 2.5 2.2 3.8 2.8 8.8 7.0 2.4 3.0 1.8 1.9 3.3 2.7 5.7 5.9 13.3 4.9 5.3 1.7 4.0 5.3 1.4 2.2 1.6 2.3 2.3 1.5 3.7 1.5 3.1 7.1 6.4 1.1 2.8 7.0 9.4 2.0

1.0 2.9 1.4 1.3 2.2 1.7 4.3 3.5 1.4 1.6 1.1 1.1 1.7 1.4 2.9 3.1 6.4 2.5 2.8 1.0 1.8 2.4 0.7 1.1 0.9 1.3 1.2 0.9 2.1 0.8 1.9 3.5 2.5 0.4 1.6 3.7 4.1 1.1

3.2 10.5 5.9 5.4 10.7 6.1 20.3 15.6 5.3 6.6 4.2 4.1 7.8 6.4 13.3 13.7 28.1 11.4 13.0 4.1 8.6 11.2 3.6 5.1 3.6 5.3 5.5 4.0 9.7 3.5 7.3 16.9 12.9 3.3 6.3 16.1 21.2 4.9

1.1 4.0 2.1 1.8 3.5 2.2 6.4 4.8 1.8 2.2 1.5 1.4 2.5 2.1 4.2 4.3 8.3 3.6 4.2 1.4 2.8 3.8 1.3 1.8 1.2 1.7 1.8 1.4 3.1 1.2 2.6 5.4 3.9 1.3 2.2 5.0 6.4 1.7

13.5 48.2 23.7 19.5 36.3 24.3 63.7 49.1 20.1 24.1 15.9 15.9 28.1 22.6 42.3 42.7 78.3 37.1 42.7 15.4 31.1 40.9 14.8 19.4 13.5 19.3 18.9 17.0 33.3 12.9 29.5 53.6 41.5 15.4 24.6 49.4 63.1 18.3

4.8 16.3 7.7 6.4 11.1 8.1 18.9 15.2 6.7 8.0 5.6 5.4 9.2 7.2 12.7 12.9 22.7 11.3 13.2 5.1 10.4 13.6 4.9 6.4 4.6 6.6 6.2 6.0 10.5 4.3 9.7 16.0 13.3 5.6 8.5 15.0 18.8 6.1

21.8 71.8 32.6 26.3 42.6 34.6 71.6 58.0 28.1 34.0 24.6 23.5 37.7 28.4 46.6 48.3 82.4 43.4 51.1 22.3 40.6 53.8 21.6 26.9 20.0 28.6 24.6 27.0 39.7 18.3 39.6 59.1 52.7 25.8 38.2 54.8 71.8 25.6

4.4 14.9 6.4 5.2 7.8 6.8 12.6 10.4 5.5 6.6 5.0 4.6 7.4 5.3 8.1 8.5 14.1 7.8 9.2 4.5 7.3 9.7 4.3 5.2 3.9 5.7 4.6 5.7 7.2 3.7 7.6 10.5 9.7 5.9 8.0 9.4 12.9 5.2

37.2 127.5 53.3 41.8 59.6 56.2 96.6 80.6 44.8 54.3 41.1 39.2 59.9 42.0 62.6 65.0 108.1 61.3 71.6 36.9 58.3 77.7 36.1 42.6 32.3 45.7 36.8 47.9 56.1 30.0 58.4 78.3 79.4 56.5 69.2 73.0 100.7 43.4

4.9 16.4 6.9 5.3 7.4 7.3 12.1 10.1 5.7 7.1 5.6 5.2 7.8 5.4 8.3 8.8 14.5 8.0 9.3 5.0 8.3 11.1 4.8 5.6 4.1 5.8 4.7 6.4 7.3 3.9 7.5 9.9 11.4 8.1 9.5 10.2 13.3 6.0

96.8 328.9 148.8 121.1 192.2 157.7 334.5 270.2 128.2 155.3 111.4 108.3 174.3 129.4 219.1 227.3 411.4 202.5 233.8 101.3 186.7 247.8 96.4 122.2 89.7 128.7 110.8 125.0 180.0 83.9 173.8 274.9 253.0 125.1 178.5 260.6 341.8 119.5

0.003 0.007 0.004 0.005 0.003 0.004 0.006 0.006 0.005 0.005 0.004 0.004 0.005 0.004 0.006 0.007 0.010 0.006 0.005 0.004 0.009 0.009 0.002 0.004 0.004 0.005 0.004 0.021 0.004 0.005 0.003 0.006 0.008 0.001 0.004 0.007 0.006 0.004

1.39 1.23 1.10 1.08 0.97 1.21 0.96 0.98 1.12 1.09 1.16 1.17 1.00 1.00 0.98 1.01 0.98 0.98 0.98 1.09 0.91 0.93 0.87 1.00 1.07 1.12 0.97 1.11 1.02 1.05 1.16 0.94 0.83 0.64 1.14 1.03 0.85 1.02

ZB313 ZB313-1 ZB313-2 ZB313-3 ZB313-4 ZB313-5 ZB313-6 ZB313-7 ZB313-8 ZB313-9 ZB313-10 ZB313-11 ZB313-12 ZB313-13 ZB313-14 ZB313-15 ZB313-16 ZB313-17 ZB313-18 ZB313-19 ZB313-20 ZB313-21 ZB313-22 ZB313-23 ZB313-24 ZB313-25 ZB313-26

244.2 146.6 365.9 113.5 62.9 196.8 215.8 506.4 347.8 201.9 354.0 366.8 479.4 730.4 333.0 323.8 165.1 432.6 207.6 394.0 289.5 122.1 116.9 535.0 443.0 292.3

0.4 0.3 0.5 0.2 0.3 0.2 0.3 0.8 0.5 0.4 0.7 0.6 0.8 0.8 0.4 0.4 0.2 0.5 0.2 0.7 0.3 0.2 0.2 1.0 0.9 0.9

2.7 1.4 3.7 1.0 0.9 1.2 1.9 5.8 3.3 1.3 4.6 4.4 5.9 5.6 2.8 3.0 1.4 3.2 1.8 5.2 2.0 1.5 1.2 7.2 6.5 5.8

0.7 0.4 1.0 0.3 0.2 0.3 0.5 1.5 0.9 0.5 1.0 1.1 1.5 1.5 0.7 0.7 0.4 0.9 0.5 1.3 0.5 0.4 0.3 1.8 1.7 1.3

4.8 2.5 7.4 1.8 1.3 3.4 3.2 11.6 7.0 3.2 7.5 7.8 11.5 12.0 4.9 4.9 2.4 6.5 3.4 9.4 4.5 2.4 2.0 13.4 11.8 8.7

3.5 1.5 5.5 1.3 0.9 2.4 2.1 9.0 5.4 2.3 4.8 5.4 8.8 11.1 3.3 3.5 1.7 5.5 2.5 6.7 3.8 1.6 1.3 9.8 7.8 4.8

1.8 1.1 2.9 0.7 0.5 1.3 1.5 4.2 2.6 1.1 2.3 2.6 4.1 5.2 2.3 2.0 0.9 2.8 1.4 3.7 2.0 0.9 0.8 4.8 4.2 2.5

7.8 3.7 13.1 3.3 1.8 6.4 5.1 20.5 12.9 5.2 11.2 12.5 20.1 29.0 7.8 7.7 4.0 13.5 6.0 15.1 10.1 3.5 3.0 22.2 18.0 10.1

2.6 1.2 4.1 1.1 0.6 2.1 1.7 6.3 4.1 1.8 3.6 3.8 6.1 9.5 2.8 2.7 1.3 4.6 2.0 4.8 3.4 1.2 1.0 7.0 5.6 3.1

27.9 15.2 43.6 12.6 7.0 22.1 21.4 62.8 41.8 21.2 38.7 40.9 59.8 96.9 34.5 32.6 15.7 48.5 22.6 48.6 34.2 13.4 12.0 67.6 55.4 33.5

9.2 5.5 13.6 4.1 2.3 7.1 7.8 18.6 12.6 7.4 12.9 13.3 17.7 28.2 12.2 11.6 6.0 15.6 7.7 14.4 10.6 4.6 4.3 19.9 16.3 10.7

37.3 24.8 53.2 17.9 9.9 28.3 36.1 70.3 49.2 32.8 52.4 53.5 65.8 102.9 53.7 52.0 29.4 63.0 32.1 53.7 40.8 20.1 19.7 72.5 59.4 40.9

7.0 5.1 9.6 3.6 2.0 5.2 7.7 12.2 8.9 6.6 9.7 10.0 11.5 17.9 10.6 10.5 6.6 12.2 6.3 9.3 7.4 4.1 4.1 12.3 10.1 7.2

56.9 42.4 76.6 29.7 16.8 41.5 66.1 92.5 69.3 55.5 78.6 80.6 88.1 136.1 83.8 85.8 58.9 100.7 51.9 74.7 58.1 35.6 34.8 94.4 79.0 56.6

7.4 6.1 10.6 4.1 2.2 5.4 9.2 11.9 9.4 7.4 11.9 11.1 12.1 16.5 10.6 11.0 8.1 13.5 6.6 10.1 7.5 5.0 4.8 11.8 10.4 8.1

170.0 111.0 245.4 81.6 46.9 126.9 164.6 327.9 227.9 146.6 239.8 247.5 313.6 473.2 230.3 228.5 136.7 291.1 144.9 257.6 185.4 94.3 89.3 345.6 287.1 194.0

0.005 0.004 0.005 0.004 0.015 0.003 0.003 0.006 0.005 0.005 0.007 0.006 0.006 0.004 0.003 0.004 0.002 0.003 0.003 0.007 0.004 0.004 0.003 0.007 0.008 0.012 (continued on

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1.02 1.37 0.98 0.99 1.06 0.98 1.38 0.91 0.92 0.97 0.90 0.95 0.92 0.84 1.31 1.16 0.99 0.95 1.05 1.08 0.95 1.19 1.20 0.96 1.04 1.05 next page)

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Table A1 (continued) Sample no.

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Total REE

La/YbN

Eu/Eu*

ZB313-27 ZB313-28 ZB313-29 ZB313-30 ZB313-31 ZB313-32 ZB313-33 ZB313-34 ZB313-35 ZB313-36 ZB313-37 ZB313-38 ZB313-39

194.3 159.8 480.4 173.1 166.9 378.9 350.3 402.2 168.6 144.7 112.2 200.1 437.0

0.2 0.3 1.3 0.2 0.1 1.0 1.0 1.1 0.2 0.2 0.2 0.5 0.7

1.4 2.2 8.3 1.3 0.9 6.4 6.7 7.2 1.6 1.5 1.0 2.1 4.8

0.3 0.5 1.9 0.3 0.2 1.5 1.6 1.6 0.4 0.3 0.2 0.5 1.3

2.8 3.8 12.9 2.2 1.6 10.1 10.2 10.7 2.7 2.6 1.7 3.5 9.5

2.4 2.3 7.8 1.6 1.4 6.5 5.4 6.0 1.9 1.8 1.2 2.5 7.9

1.3 1.3 2.8 1.0 0.8 3.6 2.1 3.0 1.2 1.0 0.8 1.3 3.8

6.3 5.1 16.3 4.1 3.8 14.5 10.5 12.4 4.7 4.2 2.8 5.9 18.7

2.0 1.7 5.0 1.6 1.4 4.5 3.4 3.9 1.6 1.4 1.0 2.0 5.7

21.3 18.5 52.4 18.2 17.5 46.1 36.2 43.0 17.9 15.8 11.4 21.9 56.0

7.0 6.0 17.0 6.4 6.2 14.0 12.5 14.2 6.3 5.4 4.2 7.3 16.2

28.5 24.7 67.7 28.2 27.7 52.2 51.6 56.7 27.1 23.0 18.8 30.6 58.2

5.5 4.7 12.4 5.8 5.8 9.2 9.5 10.1 5.3 4.5 3.8 6.0 9.9

45.0 38.9 101.2 48.8 50.3 71.5 77.5 80.5 44.2 36.0 32.0 49.2 74.5

5.7 5.2 15.3 6.4 6.8 10.4 11.9 11.9 6.1 5.0 4.5 6.9 9.3

129.6 115.2 322.1 126.0 124.5 251.5 240.0 262.3 121.0 102.8 83.3 140.2 276.4

0.003 0.005 0.009 0.003 0.002 0.010 0.009 0.010 0.004 0.004 0.003 0.008 0.006

0.95 1.09 0.75 1.11 1.01 1.10 0.85 1.02 1.15 1.09 1.25 1.03 0.91

ZB334 ZB334-1 ZB334-2 ZB334-3 ZB334-4 ZB334-5 ZB334-6 ZB334-7 ZB334-8 ZB334-9 ZB334-10 ZB334-11 ZB334-12 ZB334-13 ZB334-14 ZB334-15 ZB334-16 ZB334-17 ZB334-18 ZB334-19 ZB334-20 ZB334-21 ZB334-22 ZB334-23 ZB334-24

463.1 858.4 765.6 1267.5 1101.6 913.8 966.8 948.9 749.9 979.1 1383.8 473.8 1292.1 961.7 628.5 666.9 604.9 729.3 451.8 473.3 601.3 955.6 586.1 892.6

1.8 6.1 6.5 29.9 14.8 9.7 15.3 11.1 7.6 11.4 25.8 2.6 19.1 16.7 7.7 5.7 2.0 4.6 2.6 7.6 4.6 9.5 7.9 12.4

11.9 38.7 32.9 125.1 100.4 53.1 92.2 75.1 43.7 77.0 140.3 12.9 115.3 89.9 44.3 38.8 12.7 28.2 16.6 23.2 30.1 62.6 22.9 80.0

3.0 9.5 7.8 27.9 24.5 12.5 22.2 18.3 10.7 19.2 32.8 2.9 28.0 21.1 10.5 9.4 2.7 6.9 4.0 4.6 7.6 15.1 4.5 19.9

20.9 69.3 55.0 191.0 171.0 90.4 155.2 129.5 75.2 135.3 226.5 20.0 200.2 147.1 75.1 66.7 19.2 50.5 29.8 30.9 54.7 106.8 28.7 138.9

13.4 42.1 33.2 96.9 87.6 52.0 77.9 66.6 42.9 71.0 113.0 13.2 103.7 73.5 41.0 37.1 11.3 32.1 18.5 18.5 32.6 58.3 16.8 71.2

5.7 12.1 10.4 19.8 18.5 13.5 17.1 16.6 10.9 15.5 21.7 5.6 22.1 16.4 9.5 10.3 5.5 9.5 5.7 5.5 7.5 16.1 7.0 15.3

26.5 71.2 56.6 139.1 126.3 83.9 109.6 101.3 69.7 104.0 156.7 26.8 148.3 106.3 62.1 60.1 22.7 56.2 31.8 33.3 52.9 93.4 33.3 101.8

7.1 16.4 13.9 28.8 25.6 18.6 22.4 21.3 15.3 21.6 32.2 7.2 30.2 21.8 13.4 13.8 7.2 13.4 7.9 8.4 12.4 20.3 9.3 20.5

63.7 127.3 114.2 207.5 182.1 144.2 160.8 152.2 114.5 157.3 229.3 64.5 215.2 154.8 98.0 102.6 73.4 106.8 64.5 68.0 93.7 150.5 82.4 143.5

17.1 31.7 28.1 47.1 40.9 33.9 36.5 35.0 27.4 36.3 51.9 17.3 48.3 35.4 22.9 24.6 21.8 26.5 16.5 17.3 22.1 35.4 21.5 32.5

58.6 98.9 90.0 141.5 121.1 103.7 107.6 104.3 85.1 108.3 152.6 59.7 142.3 104.6 69.0 76.0 81.8 85.5 54.7 56.0 69.5 107.6 71.2 96.4

9.6 15.3 14.2 21.5 17.6 15.7 15.7 15.5 12.9 16.3 22.7 9.6 21.4 15.4 10.5 11.6 14.1 13.5 8.8 9.0 10.6 16.2 11.5 14.5

73.9 111.7 102.7 156.6 127.4 115.8 114.1 111.1 95.6 118.1 161.7 73.8 153.2 111.2 78.1 85.8 111.1 98.5 65.1 66.8 78.8 117.0 86.0 102.7

9.6 14.3 12.8 20.0 16.1 14.7 14.5 14.7 12.2 15.2 19.9 9.8 19.3 14.4 10.5 11.3 14.6 12.7 8.3 8.4 10.1 14.8 11.4 12.8

322.7 664.7 578.3 1252.7 1073.7 761.7 961.1 872.7 623.6 906.3 1387.1 325.9 1266.4 928.5 552.5 553.8 400.0 544.8 334.9 357.5 487.1 823.6 414.1 862.5

0.018 0.039 0.046 0.137 0.083 0.060 0.096 0.071 0.057 0.069 0.115 0.025 0.090 0.108 0.070 0.048 0.013 0.034 0.028 0.082 0.042 0.058 0.066 0.086

0.91 0.67 0.73 0.52 0.54 0.62 0.57 0.62 0.61 0.55 0.50 0.89 0.55 0.57 0.57 0.66 1.03 0.68 0.72 0.67 0.55 0.66 0.89 0.55

Note: Eu/Eu* = EuN/(SmN × GdN)^1/2. N = normalized to chondrite values of Sun and McDonough (1989).

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