Petrogenesis and metallogenesis of the Yaxi gabbroic intrusion associated with Fe-Ti-V-P ores in eastern Tianshan, NW China

Ore Geology Reviews 111 (2019) 103000

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Petrogenesis and metallogenesis of the Yaxi gabbroic intrusion associated with Fe-Ti-V-P ores in eastern Tianshan, NW China

T

Yu Shia,b, , Yu-Wang Wangb, Jing-Bin Wangb, De-Dong Lib, Ling-Li Longb, Guo-Chao Zhouc, Hong-Jing Xieb ⁎

a

State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China Technic Research Center for Deep Resources Exploration in Non-ferrous Metal Mines, Beijing Institute of Geology for Mineral Resources, Beijing 100012, China c School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China b

ARTICLE INFO

ABSTRACT

Keywords: Yaxi Fe-Ti-V-P deposit Zircon U-Pb age Mafic intrusion Eastern Tianshan Postcollisional extension

An Fe-Ti-V-P ore deposit hosted by the Yaxi layered gabbroic intrusion was recently discovered in eastern Tianshan of the Central Asian Orogenic Belt (CAOB). The Yaxi intrusion is composed of interlayered leucogabbro and melagabbro, and the Fe-Ti-V-P ore layers are mainly hosted at the bottom of the melagabbro layers. We have studied the geology, petrology, mineralogy and geochemistry of the Yaxi gabbroic rocks and associated ore. Igneous zircons separated from a gabbro sample yield a SIMS U–Pb age of 308.7 ± 1.4 Ma, indicating that the Yaxi ore-bearing intrusion was emplaced during the late Carboniferous postcollisional stage. The Yaxi gabbroic intrusion is characterized by enrichment in large ion lithophile elements and depletion of high field strength elements, positive zircon εHf(t) values (+2.5 to + 11.5), positive εNd(t) values (+1.69 to + 4.02), low initial 87 Sr/86Sr ratios (0.7054 to 0.7061), and low initial 206Pb/204Pb ratios (17.1 to 18.2). These results suggest that the parental magma was produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts at the early postcollisional stage. The parental magma experienced extensive fractional crystallization in a deep magma chamber, becoming oxidized and enriched in metallogenic elements and volatiles. During emplacement, extensive crystallization and accumulation of plagioclase from the highly evolved parental magma caused metallogenic elements and volatiles to become further enriched in the residual melts, leading to Fe-Ti-V-P mineralization. Therefore, the Yaxi Fe-Ti-V-P ore deposit benefited from extensive fractional crystallization in a deep magma chamber and accumulation of plagioclase during emplacement.

1. 1. Introduction

mainly clustered on the northern margin of the Central Tianshan Massif. Magmatic Fe-Ti-V-P deposits are commonly associated with Proterozoic massif-type anorthosite and related rocks (Charlier et al., 2015). Prior to this study, there were no precise geochronological data to constrain the age of this intrusion, although it has been proposed that this deposit was emplaced during the early Permian (Li et al., 2018). The chronology, mantle source, and petrogenesis and metallogenesis mechanisms of the Yaxi ore-bearing intrusion need to be further constrained for a better understanding of the Fe-Ti-V-P deposit genesis in the late Paleozoic orogenic belt. In this study, we present mineral compositions, geochronology data, whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotope data of the Yaxi layered gabbroic intrusion to address the following: (1) the emplacement age of the Yaxi gabbroic intrusion and its geological background; (2) the nature of the parental magma of the layered gabbroic intrusion by inference of the mantle source; and (3)

Magmatic Ni-Cu sulfide, Fe-Ti-V oxide and CuNi-VTiFe composite deposits are the three major types of orthomagmatic deposits related to postcollisional mantle-derived magma found in eastern Tianshan (Wang et al., 2008a). The formation age, magmatic characteristics, and temporal-spatial distribution of deposits associated with mafic-ultramafic intrusions in eastern Tianshan are important factors used to construct the metallogenic spectrum associated with mantle-derived magma during a postcollisional stage (Wang et al., 2008a). Compared with the mechanisms associated with the typical Ni-Cu sulfide deposits and the Xiangshanxi CuNi-VTiFe composite deposit, the petrogenesis and metallogenesis mechanisms of the Fe-Ti-V oxide deposits in eastern Tianshan remain poorly constrained. The Yaxi deposit is one of the typical apatite-enriched Fe-Ti-V deposits in eastern Tianshan and can be classified as a Fe-Ti-V-P deposit

⁎

Corresponding author at: Beijing Institute of Geology for Mineral Resources, Beijing 100012, China. E-mail address: [email protected] (Y. Shi).

https://doi.org/10.1016/j.oregeorev.2019.103000 Received 22 August 2018; Received in revised form 23 May 2019; Accepted 3 July 2019 Available online 04 July 2019 0169-1368/ © 2019 Published by Elsevier B.V.

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Fig. 1. (a) Geological map of the Central Asian Orogenic Belt and the location of the studied area (modified from Jahn et al., 2000) and (b) distribution of maficultramafic complexes in eastern Tianshan (modified from Su et al., 2011).

Ni-Cu sulfide deposits and comprise the Lubei, Baixintan, Tudun, Huangshandong, Xiangshan, Huangshan, Huangshannan, Hulu and Tulaergen deposits from west to east along the Kanggur Fault (Fig. 1b; Chen et al., 2018; Deng et al., 2015, 2017; Feng et al., 2018; Han et al., 2010, 2013; Mao et al., 2014, 2016; Qin et al., 2011; San et al., 2010; Shi et al., 2017b; Song et al., 2011; Sun et al., 2013; Tang et al., 2011; Zhou et al., 2004). The Xiangshanxi intrusion hosts a CuNi-VTiFe composite deposit in the Kanggur ductile belt (Shi et al., 2018c; Wang et al., 2010; Xiao et al., 2010a). The Central Tianshan Massif is composed of a Precambrian crystalline basement (Qin et al., 2002) and is bounded by the Aqikekuduk–Shaquanzi Fault to the north (Fig. 1b) and the Hongliuhe Fault to the south. The crystalline basement is represented by the Mesoproterozoic Xingxingxia and Kawabulake Groups and Neoproterozoic Tianhu Group and was metamorphosed mainly to upper greenschist or amphibolite facies. The lower Paleozoic unit is composed mainly of the Ordovician–Silurian unit and lies unconformably on or shows fault contact with the Precambrian basement. Two types of orthomagmatic deposits occur on the northern margin of the Central Tianshan Massif along the Aqikekuduke–Shaquanzi Fault. Mafic-ultramafic intrusions host Tianyu and Baishiquan Ni-Cu sulfide deposits (Chai et al., 2008; Tang et al., 2011), while gabbroic intrusions host Weiya, Yaxi, Shaxi and Shaxinan Fe-Ti-V-P deposits (Li et al., 2018; Shi et al., 2018a; Wang et al., 2005, 2006, 2008).

the metallogenesis mechanism of the Yaxi Fe-Ti-V-P deposit. 2. Geological setting The Central Asian Orogenic Belt (CAOB) is the largest and most complex Phanerozoic orogenic belt in the world (Sengör et al., 1993; Xiao et al., 2008, 2010b). At more than 5000 km long, the CAOB extends west to east between the Siberian and North China-Tarim Cratons (Fig. 1a; Jahn et al., 2000, 2004). The CAOB was formed by the subduction of the Paleo-Asian Ocean crust and accretions of oceanic seamounts and plateaus, ophiolites and colliding ancient microcontinents, arc terranes, and successions of passive continental margins (Jahn et al., 2000, 2004; Safonova et al., 2004, 2011; Sengör et al., 1993; Xiao et al., 2008, 2010b). The southernmost part of the CAOB, the Tianshan Orogenic Belt, extends for ca. 2500 km from Xinjiang, China, to Kyrgyzstan and Kazakhstan (Gao et al., 1998). This part of the CAOB is separated from the Tarim Block by the North Tarim and Xingxingxia Faults. The eastern Tianshan, which forms the eastern part of the Tianshan Mountains, mainly consists of three tectonic units: the Bogeda–Harlik Belt in the north, Jueluotage Belt in the center and Central Tianshan Massif in the south (Fig. 1b). From north to south, the Jueluotage Belt can be subdivided into the Wutongwozi–Xiaorequanzi intra-arc basin, Dananhu–Tousuquan island arc, Kangguer–Huangshan ductile shear zone and Yamansu back-arc basin (Fig. 1b; Qin et al., 2002). The Bogeda–Harlik Belt is composed of well-developed Ordovician–Carboniferous volcanic rocks, granites and mafic-ultramafic complexes (Gu et al., 2001). Several mafic-ultramafic intrusions occur on the southern margin of the Harlik Belt (Fig. 1b). The Niumaoquan gabbroic intrusion formed during the late Carboniferous hosts a Fe-Ti-V oxide deposit (Shi et al., 2017a, 2018b; Wang et al., 2014), while the Sangong mafic-ultramafic complex formed during the early Permian is a Ni-Cu sulfide mineralized intrusion (Wang et al., 2016). The Jueluotage Belt is composed of Devonian and Carboniferous strata. The lower Devonian to lower Carboniferous rocks include sandstone and politic slate with interlayered conglomerate, siltstone, mudstone, pyrite-bearing mudstone and limestone. The middle to upper Carboniferous strata are composed of mafic to intermediate volcanic rocks with abundant chert and limestone that have undergone very low to low-grade metamorphism. The orthomagmatic deposits are mainly

3. General geology of the Yaxi intrusion and petrography of the samples The Yaxi Fe-Ti-V-P ore deposit has reserves of 108,500 tonnes, with a mean grade of ~26.30 wt% total Fe and ~5.79 wt% TiO2, and is hosted in a layered gabbroic intrusion. The Yaxi intrusion has an outcrop area of ~1 km2 and a sill-like body that dips 40–50° SSW and extends SE-NW along strike for approximately 1200 m (Fig. 2a). The ore-bearing gabbroic intrusion intruded the Xingxingxia Group, which is mainly Precambrian metamorphic rock with grades varying from greenschist to amphibole. The gabbroic intrusion is distinctly stratified with rhythmic layering evident in many places within the complex (Fig. 3a and b), and its southern part is cross-cut by syenite (Fig. 3c), which has not been dated by isotopic methods. 2

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Fig. 2. (a) Simplified geological map of the Yaxi layered gabbroic intrusion and (b) geological section showing the lithological variations and sampling locations.

Based on the mineralogy assemblages and lithologic textures, the Yaxi layered gabbroic intrusion is divided into a bottom leucogabbro layer, a middle melagabbro layer and an upper leucogabbro layer (Fig. 2a and b). The bottom leucogabbro layer is mainly composed of coarse-grained leucogabbro, the middle melagabbro layer is a finegrained melagabbro layer containing several ore layers (Fig. 2b), and the upper leucogabbro layer is mainly fine-grained leucogabbro. Leucogabbro layers are mainly composed of anorthositic gabbro, while the melagabbro layer is composed of olivine gabbro, Fe-Ti oxide gabbro and nelsonite. The gabbro is coarse-grained and contains 3–8% olivine, 50–55% plagioclase, 20–25% clinopyroxene, ~5% hornblende, and less than 5% Fe-Ti oxide and apatite. The plagioclase is mainly euhedral, displaying

a cumulate texture, and has minor occurrences of olivine inclusions (Fig. 4a). The granular olivine and clinopyroxene are subeuhedral and have reaction coronae of hornblende and biotite (Fig. 4a). The Fe-Ti oxides are interstitial and coexist with hornblende, which usually occurs on the rim of clinopyroxene (Fig. 4a). The Fe-Ti oxide gabbro is fine-grained and characterized by abundant Fe-Ti oxides and apatite (Fig. 4b). A typical mode consists of 30–35% plagioclase, 30–40% clinopyroxene, up to 15% Fe-Ti oxides, and approximately 5% apatite. The plagioclase is euhedral, while the clinopyroxene is subeuhedral and contains Fe-Ti oxide exsolution lamellae oriented parallel to the prismatic cleavage (Fig. 4b). Fe-Ti oxides and apatite mainly occur as interstitial phases, and minor apatite occurs as inclusions in clinopyroxene (Fig. 4b).

Fig. 3. Field photos of the Yaxi layered gabbroic intrusion. (a) Igneous layering of the Yaxi gabbroic intrusion; (b) banded-type ore from the melagabbro layer; and (c) syenite dike crosscutting the Yaxi layered intrusion. 3

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Fig. 4. Photomicrographs of the Yaxi gabbroic rocks and associated ore. (a) Olivine gabbro; (b) gabbro; (c) anorthosite; and (d) nelsonite. Figs. a, b and d were taken under polarized light, while Fig. c was taken under cross-polarized light. Ol: olivine; Cpx: clinopyroxene; Pl: plagioclase; Hb: hornblende; Bio: biotite; Ox: Fe-Ti oxides; Ap: apatite.

Fig. 5. (a) Cathodoluminescence images of zircon grains from the Yaxi gabbro; (b) concordia diagram of zircon grains from the Yaxi gabbro; and (c) zircon weighted 206 U/238Pb age diagram for the Yaxi gabbro. The U-Pb dating locations are denoted by white circles.

The anorthositic gabbro displays a cumulate texture (Fig. 4c) and contains 80–85% plagioclase, less than 10% clinopyroxene, and minor apatite. The cumulate plagioclase is relatively fresh, while the interstitial clinopyroxene is altered in the amphiboles (Fig. 4c). Nelsonite occurs on the bottom of the middle melagabbro layer and is fine-grained, containing approximately 50–65% Fe-Ti oxide and 20–25% apatite grains, with variable amounts of olivine, clinopyroxene and plagioclase. The sparse silicate minerals are completely surrounded by oxides (Fig. 4d).

were mounted in epoxy resin with the zircon standards TEMORA and Qinghu. The mounts were polished to expose the centers of the grains. All zircon grains were documented with transmitted and reflected light micrographs and cathodoluminescence (CL) images to examine their internal structures. U–Pb isotopes of the zircon crystals were determined at the analytical laboratory of the Beijing Research Institute of Uranium Geology. Measurements of U, Th and Pb were obtained by using the CAMECA IMS 1280 ion microprobe. The analytical protocols and procedures followed those from Li et al. (2009). The measured Pb isotopic compositions were corrected for common Pb by using nonradiogenic 204Pb. The corrections were adequately small, which indicated that the analysis was insensitive to the preference for common Pb composition. An average of the present-day crustal composition (Stacey and Kramers, 1975) was used for common Pb. The uncertainties of individual analyses were reported at a 1 sigma level; the mean ages for pooled U–Pb

4. Analytical procedures 4.1. Zircon U-Pb dating Zircon grains from the gabbro (Y6106-38) were separated by using standard density and magnetic separation techniques. These grains 4

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Table 1 U, Th and Pb concentrations and U-Pb isotopes of Zircon in gabbro from the Yaxi layered gabbroic intrusion. Sample

U

Th

Th/U

207

Pb/235U

±σ

206

Pb/238U

1034 1846 349 248 1874 968 678 306 332 922 413 654 178 262

0.80 1.14 0.52 0.52 1.13 0.84 0.84 0.53 0.49 0.75 0.65 0.67 0.47 0.59

0.35368 0.35628 0.35932 0.35516 0.36105 0.36161 0.35476 0.35659 0.35616 0.36086 0.35333 0.35399 0.35261 0.3575

0.00707 0.00684 0.00895 0.00977 0.00700 0.00723 0.00819 0.00827 0.00844 0.00808 0.00844 0.00729 0.00917 0.00865

0.0494 0.0495 0.0488 0.0491 0.0489 0.0498 0.049 0.0492 0.0486 0.0498 0.0492 0.0491 0.0488 0.0491

±σ

207

Pb/206Pb

±σ

t206/238Ma

±σ

0.0008 0.0009 0.0009 0.0010 0.0009 0.0009 0.0009 0.0009 0.0009 0.0010 0.0009 0.0009 0.0009 0.0009

0.0519 0.0522 0.0534 0.0525 0.0536 0.0527 0.0525 0.0526 0.0531 0.0526 0.0521 0.0523 0.0524 0.0528

0.0005 0.0004 0.0009 0.0009 0.0004 0.0005 0.0007 0.0007 0.0008 0.0005 0.0008 0.0006 0.0009 0.0008

310.9 311.5 307.1 308.8 307.6 313.0 308.2 309.5 306.1 313.3 309.8 308.9 307.1 309.1

2.0 1.9 1.7 6.3 3.4 2.0 5.6 2.1 1.8 2.7 5.6 2.3 2.0 5.6

ppm YX@01 YX@02 YX@03 YX@04 YX@05 YX@06 YX@07 YX@08 YX@09 YX@10 YX@11 YX@12 YX@13 YX@14

1287 1627 677 473 1661 1151 804 579 671 1224 638 979 376 444

Fig. 6. Harker diagrams for the Yaxi gabbroic rocks and associated nelsonite.

5

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Fig. 7. (a) Chondrite-normalized REE patterns and (b) N-MORB-normalized trace element diagrams for the Yaxi gabbro and associated nelsonite. Normalizing values are from Sun and McDonough (1989). Additional data for the mafic-ultramafic intrusions in the Tarim LIP are from Cao et al. (2014), Cao and Wang (2017), and Zhang et al. (2018).

analyses were quoted with a 95% confidence interval.

in 0.5 N HCl (for Sr and Nd separation) or 0.5 N HBr (for Pb separation). Sr and Nd fractions were separated following standard chromatographic techniques using AG50x8 and PTFE–HDEHP resins with HCl as the eluent, while the Pb fraction was separated using strong alkali anion exchange resin with HBr and HCl as the eluents. All isotopic measurements were made by using a Nu Plasma II multicollector mass spectrometer. 87Sr/86Sr isotope ratios were normalized to 86Sr/88Sr = 0.1194, and 143Nd/144Nd isotope ratios were normalized to 146 Nd/144Nd = 0.7219. The JNdi Nd standard yielded a 143Nd/144Nd ratio of 0.512117 ± 2 (reference value 0.512115 ± 7), and the NBS 987 Sr standard yielded an 87Sr/86Sr ratio of 0.710275 ± 4 (reference value 0.710250 ± 20). A factor of 1‰ per mass unit for instrumental mass fractionation was applied to the Pb analyses using NBS 981 as the reference material. Measurement of the common-lead standard NBS 981 gave average values of 208Pb/204Pb = 36.53072 ± 0.00763; 207 Pb/204Pb = 15.44012 ± 0.00324; and 206Pb/204Pb = 16.90046 ± 0.00339, with uncertainties of < 0.1% at the 95% confidence level. Lu–Hf isotopes were measured by using laser-ablation multicollector inductively coupled plasma mass spectrometry (LA-ICP-MS) at Beijing Createch Technology Co. Ltd. Lu–Hf isotopic analyses were obtained on the same zircon grains that were previously analyzed for U–Pb isotopes, with ablation pits that were 40–80 μm in diameter, an ablation time of 26 s and a repetition rate of 8 Hz. The analytical procedures are described in Yuan et al. (2008). The measured 176Hf/177Hf ratios were normalized to 179Hf/177Hf = 0.7325.

4.2. 4.2. Mineral chemistry Major element compositions were determined at the Institute of Geology and Geophysics, Chinese Academy of Sciences through wavelength-dispersive spectrometry using a JEOL JXA8100 electron probe microanalyzer. A 15 kV accelerating voltage, 10nA beam current, 5 μm beam spot and 10–30 s counting time on peak were employed for the analyses. The precisions of all analyzed elements were better than 98.5%. Natural (jadeite [NaAlSiO6] for Na, Al and Si, rhodonite [MnSiO3] for Mn, sanidine [KAlSi3O8] for K, garnet [Fe3Al2Si3O12] for Fe, Cr-diopside [(Mg, Cr)CaSi2O6] for Ca, and olivine [(Mg, Fe)2SiO4] for Mg) and synthetic (rutile for Ti, Cr2O3 for Cr, and Ni2Si for Ni) minerals were used for standard calibration, and the ZAF procedure was used for matrix corrections. 4.3. Whole-rock major and trace elements Whole-rock samples were taken from the best exposed and leastaltered outcrops considered representative of the major lithologies of the Yaxi intrusion. Fig. 2b shows the sampling location. All whole-rock analyses were performed at the Key Laboratory of Orogenic Belt and Crustal Evolution, Peking University. Major elements for whole-rock samples were determined using a Shimadzu XRF-1700/1500 X-ray fluorescence spectrometer after samples were fused with lithium tetraborate. Duplicate analyses of Chinese National References GSR-3 and GSR-15 showed that the precision was 1% for elements with a greater than 5 wt% concentration and 10% for element concentrations less than 5 wt%. Loss on ignition was measured for weight loss of the samples after baking for 1 h at 1000 °C. Trace element concentrations were determined using an inductively coupled plasma mass spectrometer (ICP-MS) after HNO3 + HF digestion of approximately 40 mg of sample powder in a Teflon vessel at 150℃. The precision was 5% of the quoted values for elements present at > 1 ppm and approximately 10% for elements present at < 1 ppm. Accuracy was estimated to be better than 5% for the reported values.

5. Analytical results 5.1. SIMS zircon U-Pb dating results Zircons from the sample vary from euhedral to anhedral, with most occurring as crystal fragments with rounded terminations from initially equant to short or long prismatic crystals (Fig. 5a). The lengths of the crystals range from 90 to 300 μm, with aspect ratios from 1:1 to 3:1. Most crystals display oscillatory or patchy linear zoning with variable luminescence in the CL images (Fig. 5a). The U and Th contents vary from 376 to 1661 ppm and 178 to 1874 ppm, respectively (Table 1). The Th/U ratios are ca. 0.47–1.14 (Table 1). All zircon grains exhibited concentric zoning and a well-developed crystal shape and had high Th/ U ratios (0.47–1.14) consisting of a mafic igneous origin (Fig. 5a; Table 1). The analyses were clustered on a U-Pb concordia curve (Fig. 5b). The analysis yielded a concordia age of 308.7 ± 1.4 Ma (MSWD = 0.34) and a weighted 206Pb/238U age of 309.3 ± 1.3 Ma (Fig. 5c, MSWD = 1.08), both of which are interpreted as the crystallization age of the Yaxi gabbroic intrusion.

4.4. Sr-Nd-Pb-Hf isotopes Sr, Nd, and Pb isotopic analyses were performed at the Key Laboratory of Orogenic Belt and Crustal Evolution, Peking University, and the procedures are briefly summarized below. Sample powders were dissolved in a HF + HNO3 + HClO4 mixture. The digested samples were dried and redissolved in 6 N HCl, dried again and redissolved 6

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Table 2 Major oxides and trace element abundances of the Yaxi intrusion. Rock

Anorthositicgabbro

Sample

Y6106-1

Y6106-32

Y6106-30

Y6106-38

Y6106-5

Y6106-27

Y6106-8

Y6106-33

Majoroxides(wt.%) SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total Mg#

52.53 1.54 14.64 3.88 5.78 0.13 6.66 7.66 3.55 1.14 0.64 0.97 99.11 67.5

50.51 1.46 15.70 2.06 6.96 0.15 6.74 8.21 3.84 0.74 1.30 1.40 99.06 63.5

40.98 3.90 13.65 4.66 8.89 0.18 8.78 10.54 2.45 0.78 2.85 1.01 98.68 64.0

39.26 4.48 11.55 5.17 10.35 0.25 9.49 11.18 2.12 0.61 3.27 0.82 98.55 62.3

39.64 4.13 12.70 5.53 10.04 0.20 8.85 10.52 2.43 0.71 3.01 0.84 98.60 61.3

39.31 4.27 12.81 6.42 7.88 0.19 8.83 11.40 2.22 0.76 3.49 1.29 98.88 66.9

41.27 3.42 13.61 5.13 8.03 0.17 8.29 11.60 2.65 0.44 2.60 0.99 98.20 65.0

40.21 4.39 13.22 5.44 9.06 0.19 8.26 10.89 2.44 0.63 2.96 0.94 98.63 62.1

Traceelements(ppm) Sc 26.3 V 193 Cr 110.1 Co 27.9 Ni 21.3 Cu 32.3 Rb 112.0 Sr 446 Y 28.6 Zr 157 Nb 5.23 Ba 342 La 11.11 Ce 23.32 Pr 2.99 Nd 12.01 Sm 3.27 Eu 2.16 Gd 3.44 Tb 0.54 Dy 3.17 Ho 0.65 Er 1.75 Tm 0.27 Yb 1.88 Lu 0.30 Hf 3.65 Ta 0.94 Pb 40.34 Th 18.40 U 8.24 Rock Gabbro

35.3 160 32.17 18.5 4.79 19.3 41.8 524 39.3 97.1 5.39 532 14.52 33.88 4.71 19.86 5.22 3.16 5.45 0.80 4.46 0.86 2.24 0.32 1.98 0.30 2.02 0.55 16.93 2.77 1.61 Fe-Tioxidegabbro

46.0 377 2.92 43.5 6.61 36.8 62.5 431 53.8 68.9 4.91 430 15.81 41.67 6.58 30.95 8.56 5.26 9.14 1.29 6.75 1.26 3.02 0.38 2.19 0.31 1.52 0.57 6.13 1.52 1.52

62.5 325 7.07 36.6 2.84 25.2 37.4 386 70.9 85.7 7.28 696 20.59 54.71 8.68 40.15 11.15 7.25 11.82 1.68 9.02 1.72 4.08 0.53 3.02 0.43 1.89 0.78 5.97 1.71 3.11

48.5 431 15.12 47.4 20.5 43.5 62.4 453 53.7 64.4 2.91 456 14.97 40.07 6.44 30.59 8.72 5.62 9.32 1.30 6.83 1.26 2.99 0.37 2.12 0.30 1.45 0.48 12.74 2.63 2.08

48.5 426 0.26 44.9 5.59 48.1 60.9 438 60.7 75.7 4.03 463 17.66 47.15 7.72 36.52 10.12 5.90 10.73 1.48 7.66 1.41 3.31 0.41 2.27 0.31 1.64 0.49 7.61 1.17 3.38

45.8 326 4.62 42.5 3.54 26.5 23.7 465 48.6 126 4.93 475 13.56 35.26 5.48 25.37 7.20 4.02 7.66 1.11 5.98 1.12 2.75 0.36 2.11 0.30 2.69 0.59 24.21 1.60 1.94 Nelsonite

52.0 319 4.51 39.0 2.79 20.1 46.1 468 60.4 77.0 7.96 420 17.78 46.55 7.21 32.68 9.10 4.88 9.70 1.40 7.57 1.42 3.49 0.46 2.65 0.37 1.67 0.81 12.92 1.80 1.71

Sample

Y6106-29

Y6106-24

Y6106-3

Y6106-16

Y6106-14

Y6106-35

Y6106-12

Y3812-18

Majoroxides(wt.%) SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total Mg#

39.33 3.70 12.35 6.11 9.10 0.19 9.70 10.85 2.17 0.62 3.08 1.47 98.67 65.7

36.16 4.87 10.52 9.74 9.18 0.23 10.79 10.14 1.69 0.59 3.12 1.66 98.68 67.9

35.75 5.08 10.55 8.52 9.68 0.18 11.01 10.80 1.74 0.50 3.85 1.00 98.67 67.2

35.76 5.90 8.15 10.51 10.18 0.26 12.57 9.99 1.46 0.45 2.23 1.09 98.54 69.0

35.51 3.97 8.94 10.06 8.08 0.21 12.20 11.34 1.77 0.16 3.22 3.38 98.84 73.1

33.66 6.11 7.45 8.54 12.72 0.29 11.80 11.78 0.96 0.53 3.67 0.77 98.28 62.5

28.33 7.07 7.90 12.42 13.41 0.28 10.76 10.84 0.88 0.64 4.24 1.42 98.19 59.1

16.46 9.90 4.82 22.77 12.77 0.27 12.16 9.61 1.03 0.11 6.30 1.57 97.78 63.2

56.4 498 15.33 44.8

41.2 564 28.99 41.8

79.0 625 5.14 51.2

62.4 501 2.98 54.7

76.0 533 4.26 56.6

53.1 862 31.86 63.1

34.0 1422 12.50 42.2

Traceelements(ppm) Sc 48.3 V 416 Cr 7.67 Co 52.3

Gabbro

(continued on next page) 7

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Table 2 (continued) Rock

Anorthositicgabbro

Gabbro

Sample

Y6106-1

Y6106-32

Y6106-30

Y6106-38

Y6106-5

Y6106-27

Y6106-8

Y6106-33

Ni Cu Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

4.78 41.2 40.3 411 55.7 74.1 3.94 374 15.94 42.99 6.82 32.50 9.12 5.47 9.68 1.35 7.09 1.30 3.12 0.40 2.32 0.32 1.61 0.53 7.59 1.41 2.26

21.1 42.0 46.8 290 56.2 59.8 3.58 297 15.27 41.94 6.78 32.11 9.20 5.43 9.88 1.38 7.60 1.36 3.17 0.40 2.24 0.31 1.40 0.51 5.33 1.18 2.97

16.2 54.1 43.4 345 56.1 75.8 4.16 269 15.46 42.31 6.85 32.11 9.34 5.00 10.00 1.38 7.20 1.30 3.10 0.38 2.17 0.30 1.67 0.72 10.76 1.54 4.05

9.25 43.4 40.9 255 57.6 93.8 5.15 261 14.37 38.48 6.13 29.06 8.48 4.70 9.06 1.32 7.14 1.34 3.28 0.42 2.44 0.35 2.12 0.63 19.56 1.37 3.77

23.2 48.9 6.9 277 67.6 50.4 3.10 224 18.57 51.82 8.38 39.75 11.13 6.40 11.88 1.67 8.78 1.62 3.84 0.48 2.69 0.38 1.17 0.47 5.98 1.34 2.66

3.78 25.0 35.2 256 75.9 90.0 8.59 284 20.40 55.12 8.95 42.56 12.05 6.06 12.94 1.83 9.78 1.81 4.40 0.57 3.31 0.46 2.09 1.02 5.66 1.85 2.93

26.0 79.0 27.6 175 72.6 50.6 5.70 173 20.71 57.21 9.31 44.36 12.27 6.20 13.06 1.82 9.44 1.73 4.10 0.51 2.84 0.39 1.24 0.71 4.87 1.42 2.24

9.27 39.3 5.8 328 118.4 68.6 9.72 92 38.68 106.63 16.95 81.86 21.33 11.29 22.03 3.02 15.59 2.79 6.61 0.81 4.47 0.62 1.56 1.10 4.70 3.30 6.22

5.2. Mineral compositions

16.46% to 52.53%, with Fe-Ti oxide-enriched samples having generally lower SiO2 contents of < 28.33 wt% relative to other gabbroic rocks (Fig. 6). Al2O3 and total alkaline content (Na2O + K2O) increase with increasing SiO2 (Fig. 6a and f), whereas TiO2, Fe2O3T, MgO and P2O5 display a decreasing trend (Fig. 6b, c, e and h). MnO and CaO first increase with increasing SiO2, reaching a peak at approximately SiO2 at ~38 wt%, and then decrease with increasing SiO2 (Fig. 6d and g). The normalized trace element patterns are shown in Fig. 7. The Yaxi gabbroic rocks and Fe-Ti oxide ore display a uniform chondrite-normalized rare earth element (REE) pattern and normal mid-ocean ridge basalt (N-MORB)-normalized trace element pattern (Fig. 7a and b) and have trace elements increasing from anorthositic gabbro to nelsonite. All gabbroic samples and associated ore are enriched in light REE (LREE) and large ion lithophile elements (LILE) (Rb, Th, U) and depletion of high field strength elements (HFSE), such as Nb, Ta, Zr and Hf (Fig. 7b). Such trace element compositions are distinguished from the ocean island basalt (OIB) and the Tarim plume-related mafic-ultramafic intrusive rocks, which are relatively enriched in both LILE and HFSE (Fig. 7b). The gabbroic rocks and associated ores are characterized by

The olivine from the olivine gabbro in the Yaxi intrusion has SiO2, MgO, FeO and NiO contents of 36.7–37.5, 31.0–32.9, 30.5–30.7 and 0.00–0.04, respectively (Appendix A). The Fo values [molar 100 × Mg/ (Mg + Fe)] in the olivine gabbro range from 63.1 to 66.0. The clinopyroxene has end members of En42.8–45.6Fs13.3–15.0Wo39.3–43.3, with Mg-rich augite (Appendix B). The clinopyroxene from the olivine gabbro in the Yaxi intrusion has Cr2O3 and TiO2 contents of 0.01–0.02 and 0.81–0.95, respectively, and the Mg# [defined as molar 100 × Mg/ (Mg + Fe)] varies from 75.4 to 76.4 (Appendix B) in the clinopyroxene. The plagioclase from the Yaxi intrusion has An contents ranging from 50.5 to 51.5 (Appendix C). No significant zoning (i.e., > 2 mol.% in An content) is observed within individual grains. 5.3. Major and trace elements The gabbroic rocks and Fe-Ti oxide ore of the Yaxi intrusion show systematic variations in major elements. The SiO2 contents vary from Table 3 Hf isotopic compositions of zircon crystals separated from the Yaxi gabbro. No

176

Yb/177Hf

YX@01 YX@02 YX@03 YX@04 YX@05 YX@06 YX@07 YX@08 YX@09 YX@10 YX@11 YX@12 YX@13 YX@14

0.021407 0.010024 0.018991 0.020332 0.010803 0.026680 0.008545 0.030362 0.015239 0.022709 0.023899 0.021742 0.023201 0.020629

176

Lu/177Hf

0.000912 0.000403 0.000824 0.000867 0.000452 0.001104 0.000361 0.001265 0.000633 0.000929 0.001026 0.000941 0.000996 0.000897

176

Hf/177Hf

0.282688 0.282762 0.282909 0.282669 0.282831 0.282657 0.282840 0.282688 0.282857 0.282721 0.282698 0.282697 0.282709 0.282707

εHf(t) = [176Hf/177Hfsample(t)/176Hf/177HfCHUR(t) − 1] × 10,000; t = 308.7 Ma.

176

2σ

(176Hf/177Hf)i

εHf(0)

εHf(t)

TDM1

fLu/Hf

0.000008 0.000001 0.000010 0.000004 0.000005 0.000007 0.000008 0.000013 0.000009 0.000011 0.000018 0.000007 0.000014 0.000010

0.282682 0.282760 0.282904 0.282664 0.282828 0.282651 0.282838 0.282681 0.282853 0.282716 0.282692 0.282692 0.282703 0.282702

−3.0 −0.3 4.8 −3.6 2.1 −4.1 2.4 −3.0 3.0 −1.8 −2.6 −2.7 −2.2 −2.3

3.6 6.4 11.5 3.0 8.8 2.5 9.1 3.6 9.7 4.8 4.0 3.9 4.3 4.3

798 684 484 824 589 845 575 805 555 752 787 786 771 771

−1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00 −1.00

Hf/177Hf(t) = 176Hf/177Hf(0) − 176Lu/177Hf × (eλt − 1); 8

λ = 1.867 × 10−11

a−1;

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from 0.282675 to 282909, εHf(t) values from + 2.5 to + 11.5 (average = 5.7) and young Hf model ages (845 to 484 Ma), indicating that these zircon grains were derived from a depleted mantle source that involved crustal materials or an enriched mantle source. The Sr, Nd and Pb isotope compositions of the selected Yaxi gabbros are presented in Table 4 and summarized in Fig. 8. The samples have a wide range of initial 87Sr/86Sr values varying from 0.7054 to 0.7061, 147 Sm/144Nd ratios between 0.1588 and 0.1764, and initial 143 Nd/144Nd ratios between 0.512648 and 0.512798, corresponding to initial εNd(t) values between +1.69 and +4.20 (Table 4). Such highly radiogenic Nd isotopic compositions suggest that the parental magmas of the Yaxi gabbros were derived from a time-integrated depleted mantle source. The calculated initial 206Pb/204Pb, 207Pb/204Pb and 208 Pb/204Pb values vary from 17.1 to 18.2, 15.5 to 15.6 and 38.2 to 38.3, respectively. The Sr-Nd-Pb isotope compositions of the Yaxi gabbroic rocks mainly plot on the mixing trends between the fields enriched mantle I (EMI) and enriched mantle II (EMII) (Fig. 8), showing a derivation from enriched mantle sources.

Table 4 Isotopic compositions of Sr, Nd and Pb isotopes. Rock type

Fe-Ti oxide gabbro

Anorthositic gabbro

Sample

y6106-16

y6106-14

y6106-32

Rb(ppm) Sr(ppm) 87 Rb/86Sr 87 Sr/86Sr ± 2σ (87Sr/86Sr)i Sm(ppm) Nd(ppm) 147 Sm/144Nd 143 Nd /144Nd ± 2σ (143Nd/144Nd)i εNd(t) 206 Pb/204Pb 207 Pb/204Pb 208 Pb/204Pb (206Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i

40.9 254.6 0.4787 0.707520 0.000007 0.7054 8.48 29.06 0.1764 0.512752 0.000003 0.5123949 3.03 18.5 15.6 38.3 17.8 15.6 38.2

6.86 277.4 0.0737 0.706438 0.000006 0.7061 11.13 39.75 0.1694 0.512798 0.000006 0.5124551 4.20 18.7 15.6 38.5 17.1 15.5 38.3

41.8 524.5 0.2378 0.706451 0.000003 0.7054 5.22 19.86 0.1588 0.512648 0.000005 0.5123261 1.69 18.6 15.6 38.4 18.2 15.6 38.2

6. Discussion 6.1. Nature of the mantle source

Note: The initial isotopic ratios were calculated at 308.7 Ma. Calculation parameters: λ(Sr) = 1.42 × 10−11 a−1, λ(Nd) = 0.654 × 10−11 a−1, (87Sr/86Sr)CHUR = 0.7045, (143Nd/144Nd)CHUR = 0.512638, t = 308.7 Ma.

Crustal contamination of continental basaltic magma occurs when the magma rises from its source in the mantle through the continental crust (Watson, 1982), and this contamination could modify elemental and isotopic compositions during magma evolution. There are no correlations between the Sr-Nd isotope values and trace element ratios, such as Th/Nb or Ce/Nb (not shown), that would suggest crustal contamination. Thus, the trace element and radioactive isotope compositions of the Yaxi layered gabbro have not been significantly modified by crustal contamination. The relatively positive whole-rock εNd(t) values (+1.69 to +4.02) suggest that a depleted mantle end-member was involved in the formation of the Yaxi gabbro (Fig. 8), while the variable εHf(t) values (2.5–11.5) indicate a somewhat heterogeneous mantle source. The SrNd-Pb isotopic data show enriched mantle source affinities lying on the

positive Eu anomalies (Fig. 7a) and P anomalies (Fig. 7b), suggesting that abundant plagioclase and apatite crystallized and accumulated during emplacement. 5.4. Hf-Sr-Nd-Pb isotopes The Hf isotope compositions of 15 zircon crystals separated from a Yaxi intrusion Fe-Ti oxide gabbro are listed in Table 3. All initial isotopic ratios were corrected to 308.7 Ma using the zircon concordia age obtained in this study. The zircon grains from gabbro sample Y6106-38 have variable Hf isotopic compositions, with 176Hf/177Hf ratios ranging

Fig. 8. Plots of Sr-Nd-Pb isotopes for the Yaxi gabbro (modified after Zindler and Hart, 1986). 9

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Fig. 9. Plots of (a) Th/Yb vs. Nb/Yb (after Pearce, 2008), (b) Nb/U vs. Nb (after Chung et al., 2001), (c) Nb/Th vs. La/Nb (after Zhang et al., 2012a), and (d) (Hf/Sm)PM vs. (Ta/La)PM (after Laflèche et al., 1998) for whole-rock analysis of the Yaxi gabbroic intrusion in comparison with mafic-ultramafic intrusions associated with Fe-Ti oxide deposits in the Tarim LIP. Additional data for the mafic-ultramafic intrusions in the Tarim LIP are from Cao et al. (2014), Cao and Wang (2017), and Zhang et al. (2018).

Fig. 10. Binary plots of (a) K2O vs. SiO2 and (b) TiO2 vs. Fe2O3T/MgO for the Yaxi gabbroic rocks and associated ores.

EMI and EMII mixing trend (Fig. 8). Usually, the EMI component is related to the lithospheric mantle, whereas the EMII component is associated with crustal material (Hawkesworth et al., 1990; Menzies, 1990). The Yaxi rocks are characterized by relative enrichment in LILE, LREE and U and depletion in HFSE (e.g., Nb, Ta, Zr and Hf), indicative of arc geochemical affinities (Fig. 7b). In the plots of Th/Yb vs. Nb/Yb, Nb/U vs. Nb and Nb/Th vs. La/Nb (Fig. 9a–c), the Yaxi gabbroic rocks and associated ores generally plot in the area of subduction-modified continental lithospheric mantle (SCLM), which possibly experienced carbonatite metasomatism (Fig. 9d), showing a significant difference from the mafic-ultramafic intrusions hosting Fe-Ti-V oxide deposits in the Tarim large igneous province (LIP) (Fig. 9). This finding conforms well to the derivation of a lithospheric mantle metasomatized by slabderived hydrous fluids or sediment-derived melts (Laflèche et al., 1998; Chung et al., 2001; Pearce, 2008). The Yaxi layered gabbroic rocks and associated ores are characterized by low K2O contents and a positive correlation between TiO2 and (Fe2O3)T/MgO (Fig. 10a and b) and demonstrate a tholeiitic affinity (Ewart, 1982; Wilson, 1989). Upwelling of the asthenosphere facilitates the generation of tholeiitic magmas at high temperatures (Davies and

von Blanckenburg, 1995; Gvirtzman and Nur, 1999), whereas the predominant magmas produced by melting of the mantle wedge are commonly calc-alkaline. Thus, the geochemistry of the Yaxi gabbroic rocks displays a mixed derivation of asthenospheric melt and lithospheric mantle, possibly produced by upwelling partial melts from asthenospheric mantle that reacted with the enriched lithosphere. 6.2. Geodynamic setting Layered mafic-ultramafic intrusions usually form in an intracontinental, extensional or rift environment (von Gruenewaldt and Harmer, 1992; Wilson, 1996). However, the cause of such extension for eastern Tianshan during the late Carboniferous remains a matter of debate. Existing models mainly include postcollisional extension (Wang et al., 2008a; Wang and Xu, 2006; Yuan et al., 2007; Zhou et al., 2010), back-arc extension associated with slab roll-back of the subducted northern Tianshan lithosphere (Jiang et al., 2017; Zhang et al., 2016; Lu et al., 2017), and mantle plumes (Xia et al., 2004, 2008). Late Paleozoic Fe-Ti-V oxide deposits in China are generally associated with LIPs, such as the large-scale deposits in the Tarim LIP (Cao 10

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during the late Carboniferous. The following observations led us to conclude that the late Carboniferous layered gabbroic intrusion in eastern Tianshan was the result of regional extension during postcollision. 1. Late Carboniferous A2-type granites. The late Carboniferous gabbros associated with the Fe-Ti-V-P ore deposits are generally coeval with A2type granites, such as the Yaxi gabbro, Shaxi gabbro, Shaxinan gabbro (Shi et al., 2018a), and Sujishn gabbro (Lei et al., 2016b). Although the Yaxi syenite, which is spatially associated with the Yaxi ore-bearing gabbro, has not been dated by isotopic methods, the counterparts from the Shaxinan gabbro-syenite complex yielded a zircon SHRIMP U-Pb age of 306.4 ± 3.4 Ma (Chen and Shang, 2015), identical to the Shaxinan Fe-TiV-P ore-bearing gabbro, which yielded a SIMS U-Pb age of 307.2 ± 1.5 Ma (Shi et al., 2018a). The late Carboniferous syenites of eastern Tianshan display the geochemical characteristics of A2-type granites (Chen and Shang, 2015; Lei et al., 2016a), which are significantly different from those of A1-type granites in LIPs, and were possibly formed in a postcollisional extensional environment. At the same time, plumerelated underplating can give rise to A1-type granites by fusing crustal protoliths under high-temperature conditions (Shellnutt and Zhou, 2007; Zhong et al., 2007). Therefore, high temperature is one of the features discriminating plume-related A1-type granite from others. The zircon saturation temperatures of the A2-type granite of the Sujishan late Carboniferous gabbro-syenite complex (Tzr = 841–883 °C, Lei et al., 2016a) are significantly lower than those of the A1-type granites related to the Emeishan plume (Tzr = 934–1053 °C, Xu et al., 2001). This finding, together with the geochemical characteristics of the late Carboniferous A2type granites, suggests that the late Carboniferous magmatism of eastern Tianshan was possibly the result of postcollisional extension rather than mantle plume activities. 2. Voluminous late Carboniferous basalt. The Yaxi gabbroic intrusion has a crystallization age of ~309 Ma based on SIMS zircon analyses (Fig. 5b and c), and these ages are identical to those of basalts erupted in the southern margin of the Jueluotage Belt along the crustal-scale Aqikekuduke–Shaquanzi Fault (Yuan et al., 2007). The Yaxi gabbro and basalt in the Shilipo area are temporally and spatially associated and have similar Sr-Nd-Pb isotopic compositions (Yuan et al., 2010; Zhang et al., 2012b), possibly representing the intrusive and extrusive phases, respectively, of late Carboniferous mantle-derived magmatism. The geochemical characteristics of the late Carboniferous Shilipo basalt are similar to those of continental flood basalt (CFB) and dissimilar from those of island arc basalt (IAB), OIB and ocean flood basalt (OFB), possibly indicating that the Shilipo basalt was produced in a postcollisional extensional setting (Yuan et al., 2010; Zhang et al., 2012b, 2013). The absence of OIB-like late Carboniferous basalts in the eastern Tianshan again suggests that the

Fig. 11. CaO + Al2O3–FeO + TiO2–MgO diagram for the Yaxi gabbroic rocks and associated nelsonite. Additionally, plagioclase (Pl), clinopyroxene (Cpx) orthopyroxene (Opx), amphibole (Amp) and Fe-Ti oxide are plotted according to their chemical compositions. The arrow indicates an Fe enrichment trend of the silicate rocks of the Yaxi intrusion in comparison with typical gabbros, which follow the line between plagioclase and clinopyroxene.

and Wang, 2017), and the giant deposits in the Emeishan LIP. However, the Yaxi gabbroic rocks and associated ores display significantly different geochemical characteristics from the LIP counterparts. Compared to the mafic-ultramafic rocks hosting Fe-Ti-V oxide deposits in the Tarim LIP (Cao et al., 2014; Cao and Wang, 2017; Zhang et al., 2018), the Yaxi gabbroic rocks are characterized by pronounced negative Nb, Ta, Zr and Hf anomalies and positive Pb and P anomalies in MORBnormalized trace element diagrams (Fig. 7b). The trace element characteristics of the Yaxi intrusion are similar to the Jinbulake intrusion in Central Tianshan, which has been argued to be linked to the melting of mantle sources metasomatized by subduction-related processes (Zhang et al., 2017). Both the Tarim LIP suites and the Yaxi magma were emplaced through similar Proterozoic metamorphic rocks, so it is difficult to attribute the negative HFSE anomalies of the Yaxi gabbroic rocks to crustal contamination of magmas that were initially compositionally akin to mantle plume primary magma. This conclusion is reinforced by the fact that there is a lack of compelling evidence (e.g., large volume of basaltic magma, radiating dike swarms, or domal uplift of the surface of the earth) supporting the mantle plume hypothesis

Fig. 12. Plots of (a) plagioclase An vs. olivine Fo, (b) clinopyroxene Mg# vs. olivine Fo, and (c) clinopyroxene Mg# vs. plagioclase An for the Yaxi gabbro in comparison with Fe-Ti oxide ore-bearing intrusions in the Emeishan LIP and Tarim LIP (data for Panzhihua are from Pang et al., 2009; data for Baima are from Shellnutt and Pang, 2012; Zhang et al., 2012c; data for Taihe are from Hou et al., 2012; data for Wajilitag are from Li et al., 2012b; Zhang et al., 2018; and data for Mazaertag are from Cao and Wang, 2017). 11

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Fig. 13. Pearce element ratio diagrams for the Yaxi gabbroic rocks and associated ores.

mantle plume model is not necessary for the formation of the late Carboniferous apatite-rich Fe-Ti-V oxide-mineralized gabbroic intrusions at the northern margin of the Central Tianshan Massif. 3. Linear distribution of the late Carboniferous gabbroic intrusions in eastern Tianshan. A series of late Carboniferous layered gabbroic intrusions in eastern Tianshan have been reported in recent studies (Lei et al., 2016b; Li et al., 2012a; Lu et al., 2017; Shi et al., 2018a), and the majority occur on the northern margin of the Central Tianshan Massif (Fig. 1b). The late Carboniferous gabbroic intrusions on the northern margin of the Central Tianshan Massif comprise the Honglianzi, Shaquanzi, Yaxi, Shaxi and Shaxinan intrusions from east to west (Fig. 1b), and these intrusions were emplaced at 309.7 ± 1.5 Ma (Li et al., 2012a), 307.2 ± 1.5 Ma (Lu et al., 2017), 308.4 ± 1.4 Ma (this study) and 307.2 ± 1.5 Ma (Shi et al., 2018a), respectively. The linear distribution of mantle-derived magmatism is usually caused by detachment of oceanic lithosphere. Once a subducted slab begins to detach, the detachment will rapidly propagate laterally to form a linear gap in a very short period (van de Zedde and Wortel, 2001; van Hunen and Allen, 2011), leading to an upwelling of hot asthenosphere from the gap and resulting in a narrow, linear zone of mantle-derived magmatism. Thus, the linear distribution of these late Carboniferous gabbroic intrusions does not support a geodynamic setting for the mantle plume but postcollisional extension. Thus, eastern Tianshan likely stepped into a postcollisional stage, and upwelling of the asthenosphere induced by regional extension could account for the generation of the late Carboniferous syenitegabbro complexes, the intrusive-extrusive associations, and the linear distribution of the late Carboniferous gabbroic intrusions.

V, but also in phosphorus elements. The Yaxi intrusion displays strong Fe enrichment in the CaO + Al2O3–FeO + TiO2–MgO diagram (Fig. 11), suggesting that the intrusive rocks are silicate-oxide mineral cumulates. In addition, magnetite-ilmenite occurs as exsolution lamellae in clinopyroxene (Fig. 4b), suggesting that the cumulus minerals crystallized from Fe-Ti enriched melts. All gabbroic rocks and associated ores of the Yaxi deposit display significant positive P anomalies (Fig. 7b), indicating that the parental magma was oversaturated in phosphorus. In general, mantle-derived primary melts have Ni > 400 ppm and Cr > 1000 ppm (Wilson, 1989), and Mg# = 73–81 (Sharma, 1997). The Yaxi gabbroic samples show large variations in Mg# (59.1–73.1; Table 2) and in compatible elements such as Cr (0.26–110.1 ppm) and Ni (2.79–25.98 ppm), suggesting that these samples underwent fractional crystallization to varying degrees. The magnetite ores and gabbroic rocks in the Yaxi intrusion are relatively enrich in Cu, resulting in high Cu/Ni ratios (0.11–0.66), a feature characteristic of high degrees of fractionation (Zhou et al., 2005). The depletion of Ni relative to Cu can be explained by olivine fractionation, which preferentially concentrates Ni (Barnes et al., 1985). The Fo contents of olivine, Mg# of clinopyroxene and An values of plagioclase are significantly lower than their counterparts in the Fe-Ti-V oxide deposits in the Emeishan LIP and Tarim LIP (Fig. 12a–c). These features, together with very low Mg#, low Cr and Co contents, and LREE enrichment, are consistent with highly evolved Yaxi parental magmas. Thus, the parental magma of the Yaxi deposit possibly experienced extensive fractional crystallization in a deep magma chamber, leading to enrichment in metallogenic elements and volatiles in the residual melts.

6.3. Characteristics of the parental magma

6.4. Fractional crystallization in the shallow magma chamber

The parental magma composition for the Yaxi gabbro is difficult to estimate due to the lack of chilled margins that would represent the original liquid composition. The parental magma of the Yaxi gabbroic intrusion is enriched not only in metallogenic elements, e.g., Fe, Ti and

In petrology, Pearce element ratio (PER) diagrams have been used to identify the minerals involved in differentiation processes and to evaluate the extent to which those minerals are involved. PERs have a denominator constituent that is conserved by processes affecting the 12

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material quantities and chemical evolution of a system (Russell and Nicholls, 1988; Stanley and Russell, 1989). Cu is an incompatible element of oxidized basaltic magma and is thus a conserved constituent during magma differentiation. Thus, Cu is chosen to be used as a denominator to transform concentration data into a PER diagram. In the Si/Cu vs [Mg + Fe]/Cu diagram, the variation in different rocks is slightly scattered (Fig. 13a), suggesting that crystallization or accumulation of magnesium minerals was not the dominant mechanism during the emplacement of the intrusion. In the other PER diagrams (Fig. 13b–d), the differentiation trends of the Yaxi gabbroic rocks and associated ores were controlled by plagioclase, suggesting that crystallization/accumulation of plagioclase dominated the differentiation and solidification in the shallow magma chamber. This proposal was supported by the ubiquitous positive Eu anomalies of the Yaxi gabbroic rocks and associated ores (Fig. 7a).

oxide deposits (Song et al., 2013; Luan et al., 2014). Magnesium minerals, e.g., olivine, orthopyroxene and clinopyroxene, are Fe2+bearing silicates, and fractional crystallization of these minerals would elevate Fe2O3T contents as well as the Fe2O3/FeO ratio, inducing saturation and crystallization of Fe-Ti oxides. Volatiles including H2O and phosphorous would also be gradually enriched in the residual melts after extensive anhydrous silicate fraction in a closed system. Many researchers have suggested that H2O and phosphorous play important roles in the formation of magmatic Fe-Ti oxide ore deposits (Gwinn and Hess, 1993; Toplis et al., 1994; Thompson et al., 2007). It has been suggested that an increase in H2O in a basaltic melt partially oxidizes FeO into Fe2O3 in the melt, leading to increased fO2. Additionally, it has been suggested that P5+ can react with Fe3+ to form the Fe3+(PO4)3− complex in silicate liquids and thus may increase the solubility of P and Fe to inhibit the crystallization of Fe-Ti oxides (Toplis et al., 1994). On the other hand, once apatite or Fe-Ti oxides are present on the liquidus, the other one will also crystallize substantially (Tollari et al., 2006). This proposal is supported by the positive correlations between P2O5 and metallogenic elements (Fig. 14a–c). Therefore, extensive fractional crystallization in deep magma chambers not only oxidized the parental magma but also enriched the magma with metallogenic elements and volatiles. Plagioclase crystallization/accumulation was the dominant mechanism during the emplacement of the Yaxi intrusion. Since metallogenic elements and volatiles are incompatible in plagioclase, extensive fractional crystallization and accumulation of plagioclase would not only drive the residual melt to be enriched in metallogenic elements and volatiles but also promote ore mineral-enriched melts separated from the gangue minerals. Metallogenic elements and volatiles increase significantly with decreasing δEu (Fig. 15a–d), and a similar variation trend is observed in the enormous Taihe Fe-Ti-V oxide deposit (Fig. 15a–d). Therefore, in the shallower magma chamber, even if magnetite crystallizes slightly later than silicate minerals, it can still be winnowed out from the plagioclase-olivine-clinopyroxene-magnetitemagma mixture and settle on the floor during slurry flow along the base of the magma chamber. However, plagioclase will float in the magma because of its low density, and thus, the massive oxides will contain very minor plagioclase. Therefore, fractionation of plagioclase from a basaltic parental magma not only plays a key role in the formation of the Yaxi deposit but also an important mechanism in producing Fe-Ti-V oxide deposits. The parental magma experienced extensive fractional crystallization in a deep magma chamber and was not only enriched in metallogenic elements and volatiles but also oxidized. Abundant plagioclase crystallized and accumulated during emplacement, inducing metallogenic elements and volatiles to be further enriched in the residual melt and saturation of Fe-Ti oxides and apatite. Plagioclase crystals tend to float in shallow magma chambers, while heavy minerals such as olivine and clinopyroxene, together with Fe-Ti-V-P enriched residual melts, sink to the bottom. This mechanism could explain both the formation of the upper anorthositic gabbro layer and the genesis of the lower Fe-Ti-V-P ore-bearing layers in the lower melagabbro layers. Based on field observations and petrology studies, the formation of the Yaxi ore-bearing gabbro requires at least two major pulses of magma injections similar to other layered intrusions. Each cyclic injection of magma formed a lower ore-bearing melagabbro layer and an upper anorthositic leucogabbro layer.

6.5. Metallogenesis mechanism

7. Conclusions

Precipitation of Fe-Ti-V oxide from silicate magmas depends largely on the Fe2O3/FeO ratio of the melt, which is a function of the fO2, temperature, and H2O content of the magma (Hill and Roeder, 1974; Reynolds, 1985; Kress and Carmichael, 1991). Extensive fractional crystallization of basaltic magma in deep magma chambers plays an important role in the formation of Fe-Ti-V

The Yaxi Fe-Ti-V-P ore-bearing gabbroic intrusion was emplaced at 308.7 ± 1.3 Ma, possibly during the postcollisional stage. The primary magma was derived from partial melting of subcontinental lithospheric mantle induced by upwelling of asthenospheric melts, and the parental magma became enriched in metallogenic elements and volatiles caused by extensive fractional crystallization in a deep magma chamber.

Fig. 14. Binary diagrams for the Yaxi gabbroic intrusion.

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Fig. 15. Binary diagrams for the Yaxi gabbroic intrusion (data for the Taihe deposit are from She et al., 2014).

During emplacement, extensive crystallization and accumulation of plagioclase from the highly evolved parental magma induce metallogenic elements and volatiles to become further enriched in the residual melts, leading to Fe-Ti-V-P mineralization.

and Development Program of China (Grant No. 2017YFC0601204). Constructive reviews from Prof. Evgenii V. Sharkov and anonymous reviewers are appreciated. The authors sincerely thank Baoling Huang at the Key Laboratory of Orogenic Belt and Crustal Evolution, Peking University; Sheng He and Zengwei Fan at the analytical laboratory of Beijing Research Institute of Uranium Geology; and Professor Qian Mao, Yuguang Ma and Di Zhang at the Institute of Geology and Geophysics, Chinese Academy of Sciences, for laboratory analyses.

Acknowledgments This study is financially supported by the National Key Research Appendix A

Appendix A Compositions of olivine of the Yaxi olivine gabbro. Sample

YX003-31

YX003-33

No.

1

2

3

4

5

1

2

l4

5

SiO2 wt% MgO FeO CaO NiO MnO Total Si (O = 4) Mg Fe Ca Ni Mn Total Fo Fs Ac Ni ppm

36.7 31.8 31.8 0 0.02 0.67 101 0.99 1.28 0.72 0.00 0.00 0.01 2.99 64 35.8 0.3 174

37 32.1 31.4 0.05 0.04 0.66 101.2 0.99 1.29 0.70 0.00 0.00 0.01 2.99 64.4 35.2 0.4 365

37.4 32.9 30.5 0.05 0.01 0.59 101.5 1.00 1.31 0.68 0.00 0.00 0.00 2.99 65.6 34.1 0.3 58

37.1 32.6 31.7 0.02 0.01 0.62 101.9 0.99 1.29 0.70 0.00 0.00 0.01 2.99 64.6 35.1 0.3 116

36.8 32.4 31.6 0.03 0.03 0.59 101.5 0.99 1.30 0.71 0.00 0.00 0.00 2.99 64.5 35.2 0.3 274

36.7 32.2 31.7 0.03 0.01 0.63 101.3 0.99 1.29 0.71 0.00 0.00 0.01 2.99 64.3 35.5 0.3 50

37.2 31.7 31.5 0.02 0.01 0.64 101.1 1.00 1.28 0.71 0.00 0.00 0.01 2.99 64.1 35.6 0.3 100

36.8 31 32.7 0.01 0 0.66 101.1 0.99 1.25 0.74 0.00 0.00 0.01 2.99 62.7 37 0.3 0

37.5 32.3 31.5 0 0.02 0.61 101.9 1.00 1.29 0.70 0.00 0.00 0.01 2.99 64.5 35.2 0.3 174

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Appendix B

Appendix B Compositions of clinopyroxene of the Yaxi olivine gabbro. Sample

Yx003-31

Yx003-33

No.

1

1

2

SiO2 wt% TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO Total Si (O = 6) Ti Al Cr Fe Mn Mg Ca Na K Ni Total Wo En Fs Mg#

51.6 0.95 3.17 0.01 8.16 0.30 14.7 21.0 0.50 0.01 0.00 100.5 1.90 0.03 0.14 0.00 0.25 0.01 0.81 0.83 0.04 0.00 0.00 4.00 43.9 42.8 13.3 76.3

51.1 0.81 2.63 0.02 9.02 0.34 15.5 19.2 0.45 0.00 0.00 99.1 1.91 0.02 0.12 0.00 0.28 0.01 0.86 0.77 0.03 0.00 0.00 4.00 40.2 45.1 14.7 75.4

51.4 0.83 2.69 0.01 9.27 0.34 15.8 18.9 0.43 0.00 0.01 99.7 1.91 0.02 0.12 0.00 0.29 0.01 0.87 0.75 0.03 0.00 0.00 4.00 39.3 45.6 15.0 75.2

Appendix C

Appendix C Compositions of plagioclase of the Yaxi olivine gabbro. Sample

Yx003-31

Yx003-33

No.

1

2

3

1

2

3

4

SiO2 wt% Al2O3 CaO Na2O K2O Total Si (O = 8) Al Ca Na K Total An Ab Or

54.7 28.4 10.3 5.49 0.19 99.1 2.48 1.52 0.50 0.48 0.01 5.00 50.5 48.4 1.1

55 28.6 10.7 5.45 0.2 99.9 2.48 1.52 0.52 0.48 0.01 5.00 51.5 47.4 1.1

55.2 28.9 10.6 5.53 0.18 100.3 2.48 1.53 0.51 0.48 0.01 5.00 50.8 48.2 1

55.7 28.7 10.5 5.58 0.14 100.6 2.49 1.51 0.51 0.48 0.01 5.00 50.7 48.5 0.8

55.2 29 10.7 5.59 0.13 100.5 2.47 1.53 0.51 0.48 0.01 5.00 51 48.3 0.7

54.9 28.5 10.5 5.58 0.18 99.6 2.48 1.51 0.51 0.49 0.01 5.00 50.5 48.5 1.1

55.6 28.5 10.5 5.47 0.18 100.1 2.50 1.51 0.50 0.48 0.01 5.00 50.9 48.1 1.1

Appendix D. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.oregeorev.2019.103000.

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