Lithos 262 (2016) 44–57

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Petrology, phase equilibria and monazite geochronology ofgranulite-facies metapelites from deep drill cores in theOrdos Block of the North China Craton

Xiao-Fang He a,b, M. Santosh a,b,⁎, Kiara Bockmann b, David E. Kelsey b, Martin Hand b,Jianmin Hu c, Yusheng Wan d

a School of Earth Science and Resources, China University of Geosciences Beijing, No. 29 Xueyuan Road, Haidian District, Beijing 100083, Chinab Department of Earth Sciences, University of Adelaide, South Australia 5005, Australiac Institute of Geomechanics, Chinese Academy of Geological Science, Beijing 100081, Chinad Institute of Geology, Chinese Academy of Geological Sciences, No. 26 Baiwanzhuang Road, Beijing 100037, China

⁎ Corresponding author at: School of Earth Sciences andGeosciences Beijing, 29 Xueyuan Road, Beijing 100083, Ch

E-mail address: msantosh.gr@gmail.com (M. Santosh)

http://dx.doi.org/10.1016/j.lithos.2016.06.0220024-4937/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 January 2016Accepted 22 June 2016Available online 30 June 2016

Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the OrdosBlock remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtainedfromdeep-penetrating drill holes in theOrdos Block and report petrology, calculatedphase equilibria and in-situmonaziteLA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet–sillimanite–biotitemetapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MSdating of monazite in all three samples is in the range of 1930–1940 Ma. The (U + Pb)–Th chemical ages throughEPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated meta-morphic phase diagrams for the cordierite-bearing metapelite suggest peak P–T conditions ca. 7–9 kbar and 775–825 °C,followed by decompression and evolution along a clockwise P–T path. Our petrologic and age data are consistent withthose reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, sug-gesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.

© 2016 Elsevier B.V. All rights reserved.

Keywords:North China cratonOrdos blockLA-ICP-MS monazite geochronologyMetamorphic phase equilibriaTectonics

1. Introduction

Unraveling the growth, reworking and assembly history of continentsis central to our understanding of Earth evolution, plate tectonic theory,supercontinent cycles, and the complex interplay and feedback betweentectonic processes, the atmosphere and, importantly, the radiation ofcomplex life on Earth (e.g., Nance et al., 2014; Young, 2015). From an eco-nomic standpoint, the history of growth, reworking, and assembly of con-tinents provides key input to ideas and models of metallogenicprospectivity of Earth's crust (e.g., Groves and Santosh, 2015). TheArchean–Paleoproterozoic North China Craton (NCC) provides an excel-lent example of where reworking of the eastern part of the craton hasgreatly enhanced its metallogenic prospectivity (e.g., Li and Santosh,2014; Zhai and Santosh, 2011). However, models on metamorphic/ther-mal character to understand the geological and tectonic framework aresparse from the Ordos Block in the NCC, as it is largely buried beneathmore recent sediments (Guo and Jiao, 2002; Hu et al., 2013; Wan et al.,2013). Using samples from deep drill holes into the Ordos basement,

Resources, China University ofina. Tel.: +86 186 1836 7901..

this study presentsmetamorphic history and age data that provide betterconstraints on the geotectonic history of the NCC.

The objective of this study is to employ in situ U–Pbmonazite geochro-nology and metamorphic phase equilibria modeling to establish the dura-tion and thermal conditions of metamorphism in the Ordos Block. We usethree samples of paragneisses and metagranitoid from two drill cores inwidely spaced locations in the Ordos Basin. We performed texturally-controlled LA-ICP-MS and EPMA analysis of monazite from these drillcore samples to precisely constrain the timing of metamorphism. We re-evaluate the P–T conditions based on newly discovered assemblages andtextures with better constraints based on revised pseudosection computa-tions. The results are then discussed in the context of themetamorphic his-tory to provide a clearmetamorphic frameworkwithwhich to evaluate thegeodynamicmodels for theOrdosBlock and also gain insights on the extentof the Paleoproterozoic accretionary belt.

2. Geological background

2.1. Geological framework of the North China Craton

The NCC is one of the ancient Archean cratons of the world, preserv-ing vestiges of rocks as old as ca. 3850 Ma (Zhai and Santosh, 2011, and

45X.-F. He et al. / Lithos 262 (2016) 44–57

references therein). The craton is bound by the early PaleozoicQilianshanOrogen to thewest, the late Paleozoic Central AsianOrogenicBelt to the north, and the Paleozoic–Triassic Qinling–Dabie and Suluhigh–ultrahigh pressure metamorphic belt to the south and east, re-spectively (Fig. 1a) (Zhao and Zhai, 2013). The NCC has been tradition-ally divided into the Eastern and Western Blocks that were suturedalong the Trans-North China Orogen (e.g., Zhao and Zhai, 2013; Zhaoet al., 2005). However, recent studies have shown that the craton iscomposed of at least seven micro-blocks (Zhai and Santosh, 2011).The proposal and characterization of these blocks are based on the iden-tification of ancient tectonic boundaries represented by granite–greenstone belts. Amalgamation of the micro-blocks at the end ofArchean (Santosh et al., 2016; Yang et al., 2015; Zhai and Santosh,2011) resulted in closure of the intervening ocean basins by platetectonic processes including arc magmatism and collisional metamor-phism identical to those in Phanerozoic convergent margins(e.g., Santosh et al., 2016; Yang et al., 2015). The Western Block of theNCC is composed of two sub-blocks, the Yinshan and the Ordos whichwere welded along the Inner Mongolia Suture Zone (IMSZ, incorporat-ing the Khondalite Belt; Santosh, 2010; Santosh et al., 2013) (Fig. 1b).Among all these crustal blocks, only little is known on the Precambrianhistory of the Ordos Block as its basement is largely covered by a thickPaleozoic to Mesozoic sedimentary sequence. The boundary betweenthe Ordos Block and the Inner Mongolia Suture Zone also remainshidden. The ‘Khondalite Belt’within the IMSZ is considered to be an ac-cretionary belt generated during the collision of the Yinshan and OrdosBlocks (Santosh, 2010; Zhao and Zhai, 2013; Zhao et al., 1999). The beltis dominated by supracrustal rocks including high-grade, graphite-bearing, garnet–sillimanite, pelitic gneisses, garnetiferous quartzites,felsic paragneisses, calc-silicate gneisses, and marble, which have beencollectively termed as ‘Khondalite series’ in early studies (Condie et al.,1992; Dong et al., 2014; Wan et al., 2009; Xia et al., 2006, 2009). Theprotoliths of these rocks are considered to be the passive continentalmargin similar to the Khondalite belt in the Trivandrum Block in south-ern India (Santosh, 2010).

Fig. 1.Generalized geological and tectonic framework of the North China Craton showing them2005).

The Ordos Block makes up the southern part of the Western Block(Figs. 1 and 2) and occupies an area of 250,000 km2, with a lithosphericstructure significantly different from that of the Eastern Blockof theNCC(Huet al., 2013;Wanet al., 2013). However, since the Precambrian crys-talline rocks are covered by younger sedimentary rocks of the OrdosBasin, the structure, composition and evolution of this block remain ob-scure. Wu (2007) reported biotite gneiss beneath the Ordos Basin, andthe existence of granulite-facies rocks in the basement beneath theOrdos Basin, although this study did not provide thermobarometricconstraints or geochronology. Xia et al. (2006) suggested that theOrdos Block may have a ca. 2600 Ma lower crust, with significantcrustal growth at ca. 2000 Ma. This is different from the EasternBlock that has basement ranging from the early Archean (up to ca.3800 Ma), through the middle Archean (3400–2900 Ma), to lateArchean (2900–2500 Ma).

Recent studies evaluated the metamorphic character of themetasediments from the Ordos Block (Gou et al., 2016; Wang et al.,2014). P–T conditions of ca. 6.3–7.8 kbar and 780–810 °Cwere retrievedfrom garnet–sillimanite and psammitic gneisses and correspondingmetamorphic U–Pb zircon ages of 1964 ± 21 Ma and 1960 ± 21 Mawere reported. Mineral reaction microstructures were not apparent inthe samples of Wang et al. (2014) and so a P–T path—and its potentialconstraint on tectonic models for the Ordos Block—was not proposedby those authors.

2.2. Sample localities and drill cores

Following the finding of thick Meso–Neoproterozoic sedimentarycover in the Ordos Basin from geophysical surveys, several holes weredrilled by Petrochina ranging in depth from 1 m to a few kms (Wanet al., 2013) to assess the petroleum potential of the Ordos Basin. Thedrill cores penetrated some of the basement rocks (Hu et al., 2013; Wanet al., 2013) thus offering the opportunity to study the crystalline base-ment. In this study, we employ three samples from drill core from twobasement-intersecting holes, Qitan 1 and Longtan1. Drill hole Qitan1

ajor crustal blocks and Paleoproterozoic collisional belts (after Yang et al., 2015; Zhao et al.,

Fig. 2. Locationmapof thedrill cores fromwhich sampleswere used in this study in relation to the crustal blocks of Yinshan andOrdos aswell as part of the Paleoproterozoic sutures (InnerMongolia Suture Zone and Trans-North China Orogen). (Modified after Hu et al., 2013; Wan et al., 2013).

46 X.-F. He et al. / Lithos 262 (2016) 44–57

penetrated to a depth of 5233 m in the eastern Ordos Basin, and samplesQT1-1 and QT1-12b used in this study are from a depth of 5135 m. Drillhole Longtan1 is located between the Huanxian–Hengshan and Huachi–Datong faults in the western Ordos Basin and has a total depth of3580 m. Sample LT1-1 used in this study comes from a depth of3495 m. The locations of the samples analyzed in this study are plottedin Fig. 2. The two drill holes show broadly identical stratigraphic se-quences, and it has been assumed that similar basement rocks represent-ed by the samples in this study are widespread across the Ordos Basin.

Drill hole Qitan1 penetrated to a depth of 5233 m, and the sampleused in this study represents a depth of 5135 m. In this drill hole, thethickness of the basement rocks is 233 m. The following sequencehave been identified from top to bottom based on core logging: lowerTriassic Liujiagou Formation, upper Permian Shiqianfeng Formation,middle Permian Shihezi Formation, the lower Permian Shanxi Forma-tion and Taiyuan Formation, the Carboniferous Benxi Formation, themiddle Ordovician Wulalike Formation, the lower Ordovician KelimoliFormation, Zhuozishan Formation and Sandaokan Formation, theupper Cambrian Sanshanzi Formation, the middle Cambrian ZhangxiaFormation and Xuzhuang Formation, and the PaleoproterozoicHelanshan Group. The cover sequences range in age from late Ordovi-cian to Devonian, and from the Mesoproterozoic to the early Cambrian.

Drill hole Longtan1 has a total depth of 3580m, and the sample usedin this study comes from a depth of 3495 m. The strata from top tobottom as identified through core logging include: middle TriassicZhifang Fm, lower Triassic Shanggou Fm and Liujiagou Fm, upperPermian Shiqianfeng Fm, middle Permian Shihezi Fm, lower PermianShanxi Fm and Taiyuan Fm, Carboniferous Benxi Fm, lower OrdovicianMajiagou Fm, upper Cambrian Sanshanzi Fm, middle CambrianZhangxia Fm and Xuzhuang Fm, and Paleoproterozoic basement.Middle Ordovician to Devonian and Mesoproterozoic to the earlyCambrian hiatus are also identified.

The drill core data show that the basement is composed of granu-lites, para- and orthogneiss, leptite, amphibolite, quartzite, schist, andmarble (Hu et al., 2013; Wan et al., 2013; Wang et al., 2014). Thegranulite-facies rocks mainly occur in the northern part.

3. Analytical methods

3.1. Petrography

Polished thin sections for petrographic studies were prepared fromrepresentative samples at the School of Earth and Space Sciences, Pe-king University, China.

47X.-F. He et al. / Lithos 262 (2016) 44–57

3.2. Whole rock analyses and mineral compositions

Whole-rock geochemical analyses of the Ordos Block samples wereobtained from Wang et al. (2014), and these were used as the basisfor the calculation of metamorphic phase equilibria. Mineral composi-tions were analyzed on polished thin sections using the Cameca SX5electron microprobe at Adelaide Microscopy, University of Adelaide,Australia. Mineral chemistry spot analyses as well as transects acrossgarnetwere obtained. For spot analysis, beamconditions included a cur-rent of 20 nA and an accelerating voltage of 15 kV (including transectsacross grains). The beam diameter was set to 1 μm. Representativemin-eral compositions are given in Table 1 and the full data set are availableupon request to the authors.

3.3. Monazite geochronology

3.3.1. Monazite U–Pb LA-ICP-MS geochronologyMonazite grains for geochronology were located and imaged in

thin section using backscatter electron (BSE) imaging on a PhilipsXL30 scanning electron microscope at Adelaide Microscopy, Universityof Adelaide.

U–Pb isotopic data were obtained by Laser Ablation InductivelyCoupled Plasma Mass Spectrometry (LA-ICP-MS) on in situ monazitegrains in thin section using an Agilent 7500cs ICPMS coupled with aNew Wave 213 nm Nd–YAG laser at the University of Adelaide,Australia. Ablation was performed in a He-ablation atmosphere, with abeam diameter of 15 μm and a repetition rate of 5 Hz. Total acquisitiontime for each analysis was 110 s, and involved 30 s of backgroundmea-surement, 10 s for beam and crystal stabilization with the shutterclosed, and 50 s of sample ablation. Dwell times for isotope measure-ments were 10 ms, 15 ms, 30 ms, and 15 ms for 204Pb, 206Pb, 207Pb

Table 1Representative electron probe micro-analyses (EPMA) of mineral compositions for drill core sa

Sample QT1-12b

Mineral pl pl ksp ksp cd cd bi bi ilm il

SiO2 62.08 61.71 64.21 64.77 48.66 48.71 36.66 36.74 0.00 0Al2O3 23.68 23.62 18.62 18.92 32.58 32.29 18.42 18.45 0.00 0TiO2 0.00 0.01 0.00 0.01 0.03 0.00 2.66 2.40 51.89 5Cr2O3 0.00 0.01 0.01 0.02 0.00 0.00 0.19 0.14 0.03 0FeO 0.00 0.00 0.00 0.05 8.01 8.38 17.65 17.78 46.70 4MnO 0.04 0.00 0.05 0.02 0.02 0.05 0.00 0.01 0.23 0MgO 0.00 0.00 0.00 0.04 8.74 8.45 10.11 9.74 0.05 0CaO 4.73 4.75 0.05 0.15 0.01 0.01 0.01 0.00 0.01 0Na2O 9.14 8.92 1.05 0.28 0.05 0.10 0.15 0.16 0.00 0K2O 0.17 0.18 14.24 14.21 0.01 0.00 9.23 9.12 0.01 0Totals 99.84 99.2 98.23 98.47 98.11 97.99 95.08 94.54 98.92 9No.Oxygens 8 8 8 8 18 18 11 11 3 3Si 2.757 2.757 2.994 3.001 5.005 5.025 2.762 2.782 0.000 0Al 1.240 1.244 1.024 1.033 3.950 3.927 1.636 1.647 0.000 0Ti 0.000 0.000 0.000 0.000 0.002 0.000 0.151 0.137 0.996 0Cr 0.000 0.000 0.000 0.001 0.000 0.000 0.011 0.008 0.001 0Fe3+ 0.000 0.000 0.000 0.002 0.047 0.043 0.000 0.000 0.009 0Fe2+ 0.000 0.000 0.000 0.000 0.642 0.680 1.112 1.126 0.988 0Mn 0.002 0.000 0.002 0.001 0.002 0.004 0.000 0.001 0.005 0Mg 0.000 0.000 0.000 0.003 1.340 1.299 1.135 1.099 0.002 0Ca 0.225 0.227 0.002 0.007 0.001 0.001 0.001 0.000 0.000 0Na 0.787 0.773 0.095 0.025 0.010 0.020 0.022 0.023 0.000 0K 0.010 0.010 0.847 0.840 0.001 0.000 0.887 0.881 0.000 0Sum 5.021 5.012 4.965 4.914 11.000 11.000 7.718 7.706 2.000 2XFe − − − − 0.324 0.344 0.495 0.506 0.998 0XNa 0.770 0.765 0.101 0.029 − − − − − −Xalm − − − − − − − − − −Xpy − − − − − − − − − −Xsps − − − − − − − − − −Xgrs − − − − − − − − − −

and 238U, respectively. Detailed procedure was completed as outlinedin Payne et al. (2008).

Data reduction was completed using GLITTER software (VanAchterbergh et al., 2001). Elemental fractionation and mass bias wascorrected using the monazite standard MAdel (TIMS normalizationdata: 207Pb/206Pb = 491.0 ± 2.7 Ma, 206Pb/238U = 518.37 ± 0.99 Ma,and 207Pb/235U = 513.13 ± 0.19 Ma: updated from Payne et al.(2008) with additional TIMS analyses). Data accuracy was monitoredusing monazite standard 222 (normalization data at ca. 450 Ma:Payne et al., 2008). Throughout the course of this study, the weightedaverage 207Pb/206Pb age for standard 222 is =462 ± 17 Ma (n = 14,MSWD = 0.28) and for MAdel is 490 ± 8 Ma (n = 50, MSWD =0.18). Conventional concordia plots were generated using Isoplot 4.15(Ludwig, 2008). Errors shown on the concordia diagrams and quotedin the data tables are at the 1σ level.

3.3.2. EPMA monazite (Th + U)–Pb geochronologyIn-situ electron microprobe-based (Th + U)–Pb chronology is also

used in this study to determine monazite ages. Electron Probe MicroAnalysis (EPMA) of monazites was undertaken on a CAMECA SX51Electron Probe Micro Analyzer at Adelaide Microscopy, University ofAdelaide. Th–U–Pb including an additional suite of elements (Ca, P, Y,La, Ce, Pr, Nd, Sm, Gd, Si) were analyzed under standard operating con-ditions using an accelerating voltage of 20 kV and beam current of60 nA. A PAP correction program was used to correct matrix effects(Pouchou and Pichoir, 1985). Total counting times for Pb, Th and Uwere 320, 160 and 80 s, respectively using X-ray lines PbMβ, ThMαand UMβ. Huttonite (ThSiO4), UO2 and NBS824 Pb standards wereused for calibrating Th, U and Pb, respectively. Background measure-ment positions were selected so as to minimize overlaps with otherelements, and the spectral interference of the second order Ce escape

mples QT1-12b and LT1-1 from the boreholes in Ordos basin, North China Craton.

LT1-1

m grt grt grt grt ksp ksp msc msc bi bi

.00 37.18 37.26 37.58 36.39 64.09 64.03 47.24 47.42 34.30 33.78

.02 20.91 21.37 21.09 20.57 18.49 18.62 37.55 36.94 18.36 18.311.67 0.01 0.02 0.02 0.02 0.00 0.00 0.76 0.85 2.91 2.29.07 0.03 0.01 0.03 0.04 0.02 0.02 0.00 0.01 0.02 0.096.50 36.55 34.51 33.74 36.38 0.00 0.00 1.03 1.03 21.25 23.76.31 0.25 0.24 0.31 0.35 0.01 0.00 0.00 0.05 0.14 0.07.10 3.87 6.04 6.03 3.53 0.01 0.00 0.66 0.62 8.56 9.11.01 0.74 0.78 0.80 0.67 0.00 0.00 0.00 0.00 0.00 0.01.00 0.01 0.00 0.00 0.00 1.06 1.18 0.48 0.33 0.09 0.08.02 0.00 0.00 0.00 0.02 14.46 14.16 7.20 7.34 9.16 7.378.7 99.55 100.23 99.60 97.97 98.14 98.01 94.92 94.59 94.79 94.87

12 12 12 12 8 8 11 11 11 11.000 2.998 2.940 2.982 2.988 2.995 2.992 3.071 3.094 2.656 2.593.001 1.988 1.988 1.973 1.992 1.019 1.026 2.877 2.842 1.676 1.657.993 0.001 0.001 0.001 0.001 0.000 0.000 0.037 0.042 0.169 0.132.001 0.002 0.001 0.002 0.003 0.001 0.001 0.000 0.001 0.001 0.005.013 0.015 0.128 0.058 0.028 0.000 0.000 0.039 0.039 0.000 0.229.981 2.449 2.149 2.181 2.470 0.000 0.000 0.017 0.017 1.376 1.297.007 0.017 0.016 0.021 0.024 0.000 0.000 0.000 0.003 0.009 0.005.004 0.465 0.710 0.713 0.432 0.001 0.000 0.064 0.060 0.988 1.042.000 0.064 0.066 0.068 0.059 0.000 0.000 0.000 0.000 0.000 0.001.000 0.002 0.000 0.000 0.000 0.096 0.107 0.060 0.042 0.014 0.012.001 0.000 0.000 0.000 0.002 0.862 0.844 0.597 0.611 0.905 0.722.000 8.000 8.000 8.000 8.000 4.975 4.971 6.763 6.750 7.795 7.696.996 0.840 0.752 0.754 0.851 − − 0.210 0.221 0.582 0.555

− − − − 0.100 0.113 − − − −0.818 0.731 0.731 0.827 − − − − − −0.155 0.241 0.239 0.145 − − − − − −0.006 0.005 0.007 0.008 − − − − − −0.021 0.022 0.023 0.020 − − − − − −

48 X.-F. He et al. / Lithos 262 (2016) 44–57

peak on PbMβ was corrected for by reducing Pb concentration by100 ppm (Kelsey et al., 2003).

Age calculations were made using the technique of Montel et al.(1996). Based on the assumptions that: (1) essentially all measuredPb is a product of the radiogenic breakdown of Th and U; and(2) there is no alteration of Th, U and Pb ratios by subsequent leadloss or thermal resetting (Montel et al., 1996; Parrish, 1990), an agecan be calculated for each analysis from the equation:

Pb ¼ Th232

� �eλ

232t−1� �

208þ U238:04

� �0:9928� eλ

238t−1� �

206

þ U238:04

� �0:9928� eλ

235t−1� �

207

(Pb, Th and U in ppm; λ232, λ238 and λ235 are decay constants of Th232,U238 and U235, respectively, t is age inMa). The age equation is solved it-eratively by entering age estimates into the equation with the knownconcentrations of Th, U, until the calculated Pb matches the measuredPb (Williams and Jercinovic, 2002).

3.4. Phase diagram methodology

Phase equilibria calculations were performed using the softwareprogram THERMOCALC (Holland and Powell, 2011; Powell andHolland, 1988) in the model chemical system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O, where ‘O’ a proxy for Fe2O3,using the latest internally-consistent thermodynamic dataset ‘ds6’(filename tc-ds62.txt; Holland and Powell, 2011) and activity–composition (a–x) models (Pownall et al., 2014; Wheller and Powell,2014; White et al., 2014).

Calculations in THERMOCALC are based on the user specifying thestable assemblage and calculating the diagram line by line (i.e. eachfield boundary), point by point (i.e. field boundary intersections),where lines (field boundaries) represent the zero abundance of aphase and points represent the zero abundance of two phases. The ini-tial stable assemblage is determined by performing a Gibbs energyminimisation calculation at a set pressure–temperature (P–T) condition.The diagram is built up and around from that initial assemblage and in-volves many trial and error calculations in order to determine whichphases appear or disappear as a function of pressure, temperature,and/or composition. In addition, the so-called ‘starting guesses’ (valuesfor compositional variables for phases with which THERMOCALC com-mences its iterative least-squares calculation for a line or point) requireregular updating as the pseudosection is calculated in different parts ofP–T–X space (X is composition). The most uncertain compositional var-iables in the bulk composition are Fe2O3 and H2O, commonly requiringthat these be constrained with T–M type diagrams (where M refersto amount of an oxide component) prior to the calculation of thepressure–temperature (P–T) pseudosection. The choice of pressureat which to calculate the T–M diagrams is based on broadly estimatingthe pressure at which the petrographically-determined peak meta-morphic assemblage is stable. For the Ordos Block samples thepressure used for the calculation of T–MO and T–MH2O diagrams was8 kbar. For the T–MO sections, the oxidation state in the bulk composi-tion across the x-axis varied from 100% Fe as FeO on the left-hand sideto 50% FeO and 50% Fe2O3 on the right-hand side. For the T–MH2O dia-grams the amount of H2O was varied along the x-axis from 0.01 mol%(un-normalized) on the left-hand side to the analyzed LOI amount(recast as mole%) on the right-hand side of the diagram. The amountof H2O for the P–T pseudosection was set so that the peak assemblageis stable just above the elevated solidus, interpreted to reflect the condi-tions where the peak assemblage would have been in equilibriumwiththe last vestiges of melt (e.g. Diener et al., 2008; Morrissey et al., 2011,2013, 2014, 2015; White et al., 2004). Phase modes and compositionsfor the P–T pseudosection were calculated and plotted usingTCInvestigator (Pearce et al., 2014).

4. Results

4.1. Petrography and mineral chemistry

Samples QT1-1, QT1-12b and LT1-1were used for petrographic anal-ysis and geochronology and QT1-12b was used for phase equilibria for-wardmodeling. In the petrographic analysis below, peak and retrogrademineral assemblages have been interpreted on the basis of grain sizeand microstructural context. Representative photographs of the handspecimens are given in Fig. 3 (Supplementary data). Chemical composi-tions of the rock forming minerals including garnet, cordierite, biotite,muscovite, plagioclase and K-feldspar were obtained from samplesQT1-12b and LT1-1 through electron microprobe analyses. Representa-tive results are given in Table 1.

4.1.1. Cordierite bearing garnet–sillimanite–biotite gneiss (aluminousmetapelite)

Samples QT1-1 and QT1-12b are of aluminous paragneiss from thewestern part of the Ordos Basin. In hand specimen, the rock is dark col-ored and coarse-grained, and displays a porphyroblastic texture(Fig. 4a-e). Thematrix shows medium to fine grained lepidoblastic tex-ture. Alternating biotite-rich and biotite-poor layers define the gneissicstructure with abundant sillimanite. The rocks are composed of garnet(5 vol.%), sillimanite (10 vol.%), cordierite (b5 vol.%), biotite (5 vol.%),plagioclase (40 vol.%), K-feldspar (5 vol.%) and quartz (35 vol.%), withminor rutile. Ilmenite, zircon, andmonazite occur as accessoryminerals.

Most of the garnet is coarse grained (3 mm to 1 cm), pink colored,subhedral to anhedral, and contain inclusions of quartz, sillimanite,and plagioclase (Fig. 4a). Coarse-grained garnet shows irregular androunded shapes, with oriented foliation. Smaller grains of garnet aresurrounded by cordierite. A compositional transect was obtained fromone of the large garnet grains in sample QT1-12b. Garnet is mostlyalmandine-rich, with XMg = 0.14–0.25. From rim to core, the garnetshows an obvious increase in the pyrope component and a decrease inthe almandine component, with a compositional range of Alm73–83

Prp14–24 Grs2–3 Sps0–1 (Table 1).Sillimanite is colorless and typically occurs as aggregates of fine-

grained (0.1–0.5 mm), acicular grains. Sillimanite occurs as grainsincluded in cordierite (Fig. 4b), as foliation-defining layers withilmenite ± biotite (Fig. 4c), as matrix grains with quartz and feldspar,and as abundant inclusions in some garnet poikiloblasts. Sillimanite ±ilmenite-rich layers may be separated from garnet porphyroblasts bycordierite. The sillimanite inclusions in cordierite show rotary textureand “S” shaped distribution. The cordierite composition does not showany variation with respect to its textural position. Recrystallized cordi-erite occurring in association with ilmenite and cordierite in direct con-tact with garnet shows an XMg of 0.68–0.64 (Table 1).

Biotite is dark brown, and occurs as euhedral laths with a size ofabout 0.1–0.5 mm. The mineral is distinctively oriented and shows atypical deformation texture with inhomogeneous extinction. It showsno obvious chemical variation between samples or among grainswithinthe samples and shows XMg ratio (XMg = Mg/(Fe2+ + Mg)) of 0.49–0.51, TiO2 contents vary between 2.40–3.48wt.%. Quartz occurs as irreg-ular grains and as elongated or ribbon-like grains with cuspate necks.The elongated quartz grains are characterized by undulose extinction,and coexist with biotite and sillimanite defining the foliation (Fig. 4c).Plagioclase also shows irregular granular or elongated tabulate grains(0.5–1 mm), most are albite rich (Ab76–78) without obvious composi-tional variations (Table 1). K-feldspar is uncommonly distributed inthe matrix. It is compositionally nearly homogeneous in all three sam-ples as Or88–96 (Table 1). Sample QT1-12b differs from QT1-1 in that itcontains fresh cordierite, less garnet, and more quartz. Rutile occurs aslarge elongate grains (up to 3 mm by 0.3 mm) that are parallel to thedominantly sillimanite-defined foliation. Typically, the rutile is accom-panied by ilmenite (Fig. 4d, e). In some instances, the rutile formssymplectitic intergrowths with sillimanite (Fig. 4d). While rutile and

Fig. 4.Photomicrographs showing assemblages and textures of drill coire samples from theOrdos Basin, North China Craton. (a) Pink colored garnetwith sillimanite occurs as aggregates offine-grained acicular grains (sample QT1-1); (b) sillimanite occurs as inclusions in cordierite in foliation-defining layers with biotite and ilmenite (sample QT1-1);(c) sillimanite + ilmenite-rich layers separated from garnet porphyroblasts by cordierite (sample QT1-12b); (d) inclusions of rutile within ilmenite, and the development of partial co-ronas of ilmenite on rutile and partial replacements of rutile-sillimanite symplectites by ilmenite (sample QT1-12b); (e) rutile occurs as large elongate grains parallel to the dominantlysillimanite-defined foliation accompanied by ilmenite (sample QT1-12b); (f) biotite and muscovite distributed in between the elongated quartz and feldspar grains defining the foliationwith an elongated graphite lath occurring alongwith biotite (sample LT1-1). (For interpretation of the references to color in this figure legend, the reader is referred to theweb version ofthis article.)

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ilmenite both comprise foliation definingminerals, it is also evident thatilmenite growth outlasted rutile and occurred at its expense. The bestevidence for this are inclusions of rutile within ilmenite, the develop-ment of partial coronas of ilmenite on rutile and the partial replace-ments of rutile-sillimanite symplectites by ilmenite (Fig. 4d).

The peakmetamorphic assemblages in samples QT1-1 and QT1-12bare inferred as: garnet–sillimanite–biotite-rutile–Ilmenite–K-feldspar–plagioclase–quartz, followed by the formation of cordierite at the ex-pense of garnet, sillimanite, and biotite.

The interstitial quartz, with a cuspate neck and low dihedral angles,might suggest the presence of a former silicate melt (Holness andSawyer, 2008). The mineralogy distinguishes this rock as a probableparagneiss.

4.1.2. Graphite-bearing two-mica granitic gneiss (LT1-1)Sample LT1-1 from the eastern part of Ordos Basin is a two-mica gra-

nitic gneiss. In hand specimen, the rock is gray colored, coarse- tomedium-grained with an equigranular blastic texture (Fig. 4f) and hasa massive structure. The rock contains biotite (5 vol.%), muscovite(5 vol.%), plagioclase (30 vol.%), quartz (40 vol.%), and K-feldspar(20 vol.%) with minor graphite. Ilmenite, zircon, and monazite occuras accessory minerals.

Plagioclase is granular or elongated tabular in shape, mediumto coarse grained with grain size up to 3 mm, and minor sericitization.K-feldspar is more fresh compared to plagioclase, with a grain size ofabout 1–2 mm. The rock contains coarse-grained K-feldspar that has

been replaced by fine-grained muscovite (Fig. 4f). Biotite consists ofdark brown, euhedral laths with a size of about 0.1–0.5 mm, whichshows orientation and a typical deformation texture with moderatechloritization. It has Fe-rich (XMg= 0.42–0.45) composition and is rela-tively Ti-poor (TiO2 = 2.29–2.91 wt.%) (Table 1). Muscovite consists ofcolorless and subhedral laths with a size of about 0.1–0.5 mm, and oc-curs along with K-feldspar (Fig. 4f). Biotite and muscovite are distribut-ed in between the elongated quartz and feldspar grains and define thefoliation (Fig. 4f). Quartz is dominant and shows an elongated orienta-tion with a typical undulose extinction. Flake-like graphite is typicallyassociatedwith biotite and is elongated and oriented along the foliation.The mineralogy of this rock suggests that it was probably magmatic inorigin.

4.2. Phase equilibria forward modeling

The calculated P–T pseudosection for sample QT1-12b is presentedas Fig. 5, and diagrams showing the calculated abundance and phasecompositions are in Fig. 6 (Supplementary data). The peak assemblageof garnet–sillimanite–biotite–rutile–ilmenite–plagioclase–K-feldspar–quartz (+melt) occurs over the P–T range ca. 7–9 kbar and775–825 °C. The low-temperature limit is defined by the (elevated) sol-idus, appropriate for granulite-facies rocks that have lost melt, and theupper temperature limit is defined by the disappearance of plagioclase.Down-pressure from the peak assemblage field, cordierite becomes sta-ble. The P–T evolution of the Ordos rocks corresponds to the light gray

Fig. 5. The calculated P–T pseudosection and P–T path for sample QT1-12b. g—garnet, pl—plagioclase, ksp—K-feldspar, bi—biotite, ru—rutile, ky—kyanite, q—quartz, cd—cordierite,ilm—ilmenite, sill—sillimanite, als—aluminosilicate, liq—liquid.

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arrow shown in Fig. 5. The path is characterized by a decrease in themodal abundance of garnet and sillimanite, and increases in cordieriteand biotite (Fig. 7, Supplementary data).

The calculated T–MO and T–MH2O pseudosections are presented asFigs. 8 and 9 respectively, respectively. The composition, correspondingto that at MO = 0.1, was chosen for the calculation of the T–MH2O dia-gram, as this composition corresponds to the peak assemblageconstrained from petrographic analysis. On the T–MH2O diagram(Fig. 9), the composition corresponding to that atMH2O = 0.7 was cho-sen to be used for calculation of the P–T pseudosection due to the ob-served peak assemblage occurring at this bulk H2O content.

4.3. Monazite U–Pb LA-ICP-MS geochronology

4.3.1. Monazite textual setting in thin sectionsIn situ Laser Ablation-Inductively Coupled-Mass Spectrometry

(LA-ICP-MS) was conducted on monazites from all the three samplesQT1-1, QT1-12b and LT1-1. The analytical results are presented inSupplementary Table 2 and representative BSE images of the in situgrains are given in Fig. 10.

Monazite is abundant in the garnet–cordierite-bearing samples(QT1-1 andQT1-12b); however, it is rare in the twomica granitic gneisssample LT1-1. In sample LT1-1 monazite usually occurs along grainboundaries of muscovite and biotite. In some cases monazite is withinquartz, cordierite, or garnet porphyroblasts. Monazite in garnet occursas inclusions isolated from micro-fractures, or along micro-fractures. Ithas an average size of 30–50 μm, with some grains up to 150 μm. Theyare usually rounded, with rare, blocky, angular shapes. Occasional elon-gate monazite grains are oriented parallel to the foliation. Weighted

mean ages were also calculated for samples plotting on or within theuncertainty of concordance.

4.3.1.1. Sample QT1-1.Monazite grains in theQT1-1 occur in three differ-entmicrostructural settings: (1) as inclusions in porphyroblastic garnet;(2) outside the garnet porphyroblasts in associationwith cordierite andbiotite; and (3) in the quartz–feldspar matrix. The majority of analyzedgrains are located in the cordierite-bearing coronas, mainly along withbiotite, where they occur as irregularly-shaped, 30–50 μm in diameter.In back scattered electron (BSE) images, the monazites inclusions ingarnet occur as small (up to 150 μm), rounded to sub-rounded grains.

A total of 45 analyses were collected from 45 grains, out of whichtwenty two analyses are considered to be outliers from the main popu-lation andwere excluded. The remaining 23 analyses are highly concor-dant within analytical error with a Gaussian-style distribution patterndefining one single peak (Fig. 11a, b), and yield a weighted mean207Pb/206Pb age of 1927 ± 9 Ma (MSWD = 0.71). A discordia linewith an upper intercept 207Pb/206Pb age of 1924 ± 15 Ma (MSWD =3.9) is defined on the concordia plot.

4.3.1.2. Sample QT1-12b. Monazite grains in sample QT1-12b also occurin three different textural settings: (1) as early inclusions inporphyroblastic garnet; (2) outside the garnet porphyroblasts in associ-ation with biotite in the matrix; and (3) as inclusions in matrix quartz.Themajority of analyzed grains are located in the biotite-bearingmatrixwhere they occur as irregularly shaped yellow crystals 30–50 μm in di-ameter. Four monazite grains hosted in garnet were analyzed. In BSEimages, the monazites inclusions in garnet occur as small rounded to

Fig. 8. The calculated T–MO pseudosections for sample QT1-12b. g—garnet, pl—plagioclase, ksp—K-feldspar, bi—biotite, ru—rutile, ky—kyanite, q—quartz, cd—cordierite, ilm—ilmenite,sill—sillimanite, als—aluminosilicate, mt—magnetite, liq—liquid.

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sub-rounded grains. One monazite is up to 300 μm in diameter(Fig. 10d).

A total of thirty analyses were collected from 27 grains, out of whichnine analyses are considered to be outliers from the main populationand were excluded. The remaining 21 analyses are concordantwithin analytical error with a Gaussian-style distribution patterndefining one single peak (Fig. 11c, d), and yield a weighted mean207Pb/206Pb age of 1935 ± 9 Ma (MSWD = 0.46). A discordia linewith an upper intercept 207Pb/206Pb age of 1933 ± 7 Ma (MSWD =3.2) is defined in the concordia plot. Monazite inclusions in garnetshow different ages associated with their locations in the host mineral,with two monazite grains yielding ages of 1915.6 ± 23.41 and1937.9 ± 22.84 Ma, respectively.

4.3.1.3. Sample LT1-1. Monazite is rare in this sample. The majorityof analyzed grains in this sample are located either along grain bound-aries or as inclusions of plagioclase, where they occur as irregularlyshaped yellow crystals 30–50 μm in diameter. One grain is up to200 μm in size. In BSE images, monazite occurs as small, rounded tosub-rounded grains.

A total of eight analyses were collected from 8 grains, out of which 6analyses are concordant within analytical error with a Gaussian-styledistribution pattern defining one peak (Fig. 11e, f), and yield aweightedmean 207Pb/206Pb age of 1926 ± 16 Ma (MSWD = 0.22). A discordialine with upper intercept 207Pb/206Pb age of 1926 ± 11 Ma(MSWD = 3.6) is defined in the concordia plot. The upper interceptage and mean age are similar.

4.4. EPMA monazite (Th + U)–Pb geochronology

The monazite (U + Th)–Pb ages are reported in SupplementaryTable 3 which are based on calculated age with the suspected outliersremoved.

4.4.1. Sample QT1-1Seventeenmonazite grains from sampleQT1-1were chosen for EPMA

dating. Fiftyfive spot analyseswere analyzed on individual grains, amongwhich thirty two were excluded from the age calculation because theyare suspected outliers that give low total contents. The remaining twentythree spots have ages ranging from 1701 to 2183 Ma (SupplementaryTable 3). Seven analyses of monazite in garnet yield an average age of1952 Ma. Sixteen spot analyses made on the monazite in the matrixyield an average age of 1887 ± 31 Ma. The data indicate that monazitegrains hosted by garnet are older than monazite grains from the matrix.As seen in Supplementary Table 3, Y content of monazite grains in garnetis distinctively higher than those in thematrix. There is no correlation be-tween the spot ages and other chemical composition.

4.4.2. Sample QT1-12bSeventeen monazite grains from sample QT1-12b were chosen for

EPMA dating. Fifty seven spot analyses were analyzed on individualgrains, among which fifteen spots were excluded from the age calcula-tion as these are suspected outliers that give low total contents. The re-maining forty two spots show ages ranging from 1701 to 2078 Ma

Fig. 9. The calculated T–MH2O pseudosections for sample QT1-12b. g—garnet, pl—plagioclase, ksp—K-feldspar, bi—biotite, ru—rutile, ky—kyanite, q—quartz, cd—cordierite, ilm—ilmenite,sill—sillimanite, als—aluminosilicate, liq—liquid.

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(Supplementary Table 3). Fourteen analyses of monazite in garnet yieldan average age of 1888 Ma. Twenty eight spots on monazite in the ma-trix yield an average age of 1878 Ma. The data indicate that monazitegrains hosted by garnet are on an average older than monazite grainsfrom thematrix. The Y content of monazite grains in garnet is obviouslyhigher than those in thematrix. The spot ages do not display correlationwith any other chemical composition.

5. Discussion

5.1. Interpretation of monazite U–Pb age data and correlations

Many studies have demonstrated monazite to be a reliable geo-chronometer for granulite-facies metamorphism as it remains closedto Pb diffusion up to temperatures of around 900 °C (Cherniak et al.,2004). Moreover, monazite has also been shown to be highly resistantto Pb loss during metamictization (Meldrum et al., 1997). In dry rocks,the closure temperature for Pb diffusion in monazite is similar to thatof zircon and may exceed temperatures of 900 °C (Cherniak, 2010;Cherniak et al., 2004; Kelsey and Hand, 2015). The growth and stabilityof monazite is related to the presence of fluid and melt (Kelsey et al.,2008; Williams et al., 2011) and monazite has been demonstrated asmore reactive and responsive than zircon to metamorphism (e.g.Williams, 2001). Therefore, compared to zircon, monazite has the po-tential to offer a much greater insight into the metamorphic history ofterranes than zircon. Monazite solubility in a melt is a function of thetemperature and the melt composition, and majority of monazitegrowth occurs near the solidus (Kelsey et al., 2008). Althoughmonazitegrowth may occur after peak metamorphism (e.g. Kelsey et al., 2008;

Reno et al., 2012), Kelsey and Hand (2015), predict that monazite cansurvive to high granulite-facies temperatureswithout complete dissolu-tion into melt. Consequently, monazite from granulite-facies rocks re-cording peak temperatures of ca. 800 °C potentially records prograde,peak, and retrograde ages.

The locations of the three samples analyzed in this study arefrom drill holes at the east and west domains of the Ordos Basin andthus widely separated from each other. However, the U–Pb LA-ICP-MSages of monazite from the three samples yielded a statistically identicalupper intercept andweightedmean 207Pb/206Pb ages of 1926–1935Ma.Inmost cases, a grain that yielded anolder age also yielded younger agesin other parts of the grain. In sample QT1-12b, Monazite inclusions ingarnet show different ages associated with their locations in the hostmineral.

Themonazite grains that we analyzed are mostly located away frommicrofractures, although a few grains occur along microfractures. Thegrains themselves are undeformed. In sample QT 1-1 and QT1-12b,monazite inclusion in garnet show similar ages (on average by isotopicand by total-Pb EPMA-based chemical method) no matter whetherthe monazite is isolated from microfractures or located alongmicrofractures. The ages from the two settings in garnet are more orless indistinguishable, suggesting that the microfractures play no rolein controlling the age of the monazite, and are later features. Indeed, ifmonazite is shielded from chemical communication with the melt-bearing matrix within a porphyroblastic mineral such as garnet, thosemonazite grains are known to preserve prograde ages (e.g. Montelet al., 2000). The capability of monazite to retain information aboutthe prograde, peak and retrograde segments of the P–Twas also report-ed from HP migmatites (e.g. Langone et al., 2011).

Fig. 10. Representative BSE images showing the textural positions of themonazite grains, monazite LA-ICP-MS U–Pb analysis numbers, and ages for drill core sample QT1-1 and QT1-12bfrom boreholes in Ordos Basin, North China Craton. (a) Monazite grains occur as inclusions in porphyroblastic garnet and outside the quartz–feldspar matrix (sample QT1-12b);(b) Monazite grain occurs as the largest euhedral grain in association with cordierite and biotite matrix; (c) Monazite grain occurs as inclusions in porphyroblastic garnet, along themicrofractures and in contact with the quartz–feldspar matrix; (d) Representative monazite grain with analytical spots showing age variation at different domains.

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Ages obtained from monazite hosted in garnet preserve relativelyolder U–Pb and EPMA-based total-Pb chemical ages (SupplementaryTable 3). As thesemonazite grains are located inside amineral that com-prises part of the peak metamorphic mineral assemblage, these agesmost likely represent prograde ages. Therefore, monazite in the studiedsamples records prograde as well as retrograde ages, implying that themetamorphic cycle—at least the high temperature part of it—was ofthe order of ca. 10 Myr.

Wang et al. (2014) reported SIMS zirconU–Pb data fromOrdos Blockdrill core samples,which show ages of 1964±21 and 1960±21Ma forone pelitic granulite (QT1–2) and one psammitic gneiss (LT1–2), re-spectively. The weighted mean 207Pb/206Pb ages (1953 ± 10 Ma forQT1–2 and 1954± 11Ma for LT1–2) obtained in their study are consis-tent within uncertainty of the older monazite ages in this study, sug-gesting peak metamorphism at 1964–1960 Ma. The ages frommonazite in garnet are likely to be prograde ages, rather than coolingages, because they are hosted within a peak metamorphic mineral. Al-though their zircons were not from the same samples we have studied,this trend of older zircon ages and younger monazite ages fromgranulite-facies metamorphic rocks is common and could be attributedto the different temperature atwhich zircon commences growing, com-pared to monazite, as the rock cools (e.g. Kelsey et al., 2008). In anotherrecent study, Gou et al. (2016) used petrography, phase equilibriamodeling and in situ (U–Th)–Pb monazite geochronology on peliticgneiss from the Qitan1 borehole. Based on P–T pseudosectionmodeling,they defined a clockwise path involving slight heating during near iso-thermal decompression ca. 8.8 kbar at ca. 769 °C to 5.5 kbar at 785 °Cfollowed by close-to-isobaric cooling across the solidus. Monazite agesof 1.96–1.94 Ga in their study were considered to represent theprograde-to-peak metamorphism. Another group of monazites with1.90–1.88 Ga were also identified, correlated to post decompressioncooling. Taking the monazite geochronology from this study, togetherwith existing metamorphic geochronology from the Ordos Block drill

holes, we interpret that the age of metamorphism probably spans (atleast) the interval ca. 1964 Ma to ca. 1926 Ma.

Late Paleoproterozoic metamorphic and magmatic ages have beenwidely recognized in the Khondalite Belt that occurs between theOrdos and Yinshan blocks, as well as from the Lüliang Complex(Santosh et al., 2007, 2015). In the Lüliang Complex, themetasedimentary rocks underwent metamorphism at 1890–1830 Ma(Liu et al., 2006; Xia et al., 2009). In the Jining Complex, meta-maficdykes record metamorphic ages of 1920–1860 Ma (Peng et al., 2010).

Many Paleoproterozoic intrusions of gabbro and dolerite in theDaqingshan area of the Khondalite Belt have metamorphic ages of1950–1830 Ma (Wan et al., 2013). The metamorphic zircons in thesupracrustal rocks from the Daqingshan area have an age range of1960–1830 Ma (Dong et al., 2014; Wan et al., 2009). The supracrustalrocks in the Helanshan–Qianlishan area of the Khondalite Belt yieldmetamorphic ages of ca. 1950 and 1920 Ma, the former beinginterpreted as the timing of the collision between the Yinshan andOrdos blocks to form the Western Block (Santosh, 2010; Santosh et al.,2007), and the latter as the age of subsequent post-orogenic extension(Yin et al., 2009).

5.2. Potential correlations of Ordos block to other parts of NCC

Wan et al. (2013) reported zircon 207Pb/206Pb ages of 2003 ± 24 to2118 ± 14 Ma from cordierite bearing garnet-sillimanite-biotite gneissand an upper intercept age of 2015± 13Ma graphite bearing two-micagranitic gneiss (LT1-1), which is slightly younger than the SHRIMP agesdated byHu et al. (2013). However, only imprecisemetamorphic zirconages of 1947 ± 74 and 1882 ± 45 Ma were obtained from the samplesbecause of strong Pb loss from the metamorphic domains in the zircon.

Wang et al. (2014) also obtained SHRIMP zircon U–Pb data for thedetrital zircons in the basement rocks of northernOrdos Basinwith con-cordant or nearly concordant 207Pb/206Pb ages. The sample QT1 yielded

Fig. 11.Monazite LA-ICP-MS U–Pb concordia plots and age data histograms showing probability curves for drill core samples from boreholes in Ordos Basin, North China Craton. (a) and(b) are from a cordierite-bearing garnet–sillimanite–biotite gneiss (sample QT1-1). (c) and (d) are from a cordierite bearing garnet–sillimanite–biotite gneiss (sample QT1-12b). (e) and(f) are from a graphite-bearing two-mica granitic gneiss (sample QT1-1).

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ages between 2162±9 and 1982±11Ma and sample LT1was dated at2151±11 and 2019±9Ma. Based on these results, they suggested thatthe sedimentary protoliths of the basement rocks below the northernOrdos Basin came predominantly from middle to late Paleoproterozoicmaterial.

Zhao et al. (2010) reported two groups of zircon ages for graphite-garnet-sillimanite gneiss in Huai'an complex in the north part of theKhondalite belt and the Trans-North China Orogen. They obtained twometamorphic ages at about 1950 and 1850 Ma. The former one is con-sidered to represent the age of collision between the Ordos basement

and the Yinshan block, whereas the ca. 1850 Ma age represents the col-lision time between the western and the eastern blocks of the NorthChina Craton.

Yin et al. (2009) reported 2275–1999 Ma igneous cores (most sam-ples are at around 2030Ma) from the Qianlishan Khondalite detrital zir-cons, and got 1941–1955Ma ages frommetamorphic units based on themetamorphic rim of the detrital zircons. Also, similar magmatic agesand metamorphic detrital zircon ages were reported by Wan et al.(2006) from the Jining complex in the east of the Khondalite belt. Wecompile all the ages in an age data histogram figure with probability

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curve (Fig. 12), where the monazite age data and ages from the meta-morphic zircon rims have a similar age range.

In summary, the age data obtained from zircon and monazite aresimilar, in both rock association and metamorphic history, to rocks sur-rounding theOrdos Basin, including the Lüliang, Jining, Daqingshan, andQianlishan complexes (Dong et al., 2012; Wan et al., 2006, 2009; Xiaet al., 2009), and exhibit ages similar to the metamorphic ages of theKhondalite Belt in the Western Block. This furthers the notion thatthe Paleoproterozoic metasedimentary rocks are widely distributedacross the Ordos basement with metamorphism at the end of thePaleoproterozoic, at around 1930 Ma.

5.3. Pressure–temperature evolution

Combining petrography andmineral chemistrywith the phase equi-libria calculations allows for a P–T path to be proposed for the OrdosBlock metapelites. Ideally, measured mineral chemistry can be used tomore tightly constrain peak P–T conditions within the peak assemblagefield (e.g. Kelsey, 2008). Indeed, the measured composition of plagio-clase, XCa (= Ca/(Ca + Na + K) = 0.215–0.227, plots within thepeak assemblage field towards the higher-P, lower-T portion ofthe field. However, measured biotite compositions (XFe = Fe2+/(Fe2+ + Mg) = 0.488–0.514) are slightly too Fe-rich to occur in thepeak assemblage field. The measured garnet compositions (x(g) =Fe2+/(Fe2+ + Mg) = 0.856 (core)–0.749 (rim) and z(g) = Xgrs = Ca/(Fe2+ + Mn + Mg + Ca) = 0.027 (core)–0.019 (rim)) mostly do notoccur inside the peak assemblage field. The rim x(g) compositionsoccur on the lower-temperature side of the peak assemblage field butthis result is not considered as a robust constraint on peak conditionssince garnet is resorbed and shows the classic signs of Fe–Mg resettingthat occurs upon cooling (i.e. higher Fe at rims compared to core). In-stead, themeasured garnet compositions typically correspond to condi-tions at lower P and/or T than the peak assemblage field. This result isinterpreted to be the consequence of the rock cooling sufficiently slowlyto allow resetting of Fe–Mg as well as Ca chemistry in garnet to lowertemperatures (e.g. Pattison et al., 2003). The cordierite-bearing retro-grade mineral assemblages possibly developed as a consequence ofnear isothermal decompression. The cordierite formation at the ex-pense of garnet may have resulted in the release REE that was incorpo-rated by the younger generation of monazites.

Fig. 12. Compiled age data histograms and probability curves of zircon and monazite grains in(a) cordierite bearing garnet–sillimanite–biotite gneiss from the Qitan drill core (QT). (b) grapfrom cordierite bearing garnet–sillimanite–biotite gneiss from the Qitan drill core (QT). Data fr

Plagioclase compositions may not have reset with cooling due to thecoupled exchange required for plagioclase to change composition.Therefore, in summary, the measured mineral compositions do nottightly constrain the P–T conditions within the peak assemblage fieldbeyond the plagioclase compositions suggesting a shift towards thehigh-P, lower-T part of the field.

Petrographic observation clearly shows the garnet porphyroblasts tobemantled by coronae of cordierite. In addition, sillimanite occurs as in-clusions in cordierite. Both these observations suggest that cordieritegrew at the expense of garnet and sillimanite. In other words, the rockadjusted its modal abundance of garnet and sillimanite downwardswith a corresponding increase in the modal abundance of cordierite.Cordierite does not occur as porphyroblasts in the sample/s, stronglysuggesting that cordierite was not an early-formed (prograde) mineralthat was stable at the peak of metamorphism. All cordierite in thesample is interpreted as having developed during post-peak evolution.A P–T evolution, corresponding to the light gray arrow shown in Fig. 5,involves modal decreases in garnet and sillimanite abundances, andmodal increases in cordierite and biotite abundance. Along this down-pressure evolution from peak metamorphic conditions there is also anincreased stability in ilmenite, largely at the expense of rutile, andpost-peak biotite growth. The results of geochronology in this studysuggest that changes in the modal abundance of the peak assemblagephases, and the growth of cordierite, occurred during a single eventrather than during later, temporally unrelated polymetamorphism.However, the timescale over which the rocks remained above the clo-sure temperature for diffusion of Fe, Mg, and Ca in garnet as the rockcooled must have been sufficiently long for the peak composition ofthe garnet to not be recorded in this sample. The approximate 30 Myrdifference between our monazite age data and the existing metamor-phic zircon age data (albeit from different samples) from the OrdosBlock drill holes suggests that metamorphism spanned at least a30 Myr time interval.

The mineralogy of the granitic sample LT1, specifically muscovite-bearing but not garnet-bearing, suggests that the pressure at whichthe rock crystallized and/or was metamorphosed was low enough todisallow the stabilization of garnet. If the P–T path, established for thewestern part of the Ordos Block, is applicable to the eastern part of theOrdos Block, then the aluminous mineralogy of the granitic sampleLT1may reflectmagmatic crystallization and/or metamorphism at rela-tively lower pressures—perhaps below about 4–5 kbar—compared to

corporating previous studies from the same drill cores in Ordos Bock, North China Cratonhite-bearing two-mica granitic gneiss from Longtan drill core (LT). (c) monazite age dataom Wang et al. (2014), Wan et al. (2013), Hu et al. (2013) and Gou et al. (2016).

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the metapelitic samples. In this way, granitic sample LT1 may reflect apart of the history along a slightly different (shallower) part of the ret-rograde P–T path than the (deeper)metapelitic samples from this study.

5.4. Constraints on tectonic setting and implications for the extent of theOrdos Block

The apparent thermal gradients corresponding to the peakassemblage stability field in the P–T pseudosection (Fig. 5) are ca.85–120 °C/kbar. These thermal gradients correspond to the hotter partof the high T/P or Barrovian spectrum (Brown, 2007; Kelsey and Hand,2015). Such thermal gradients are not particularly diagnostic of a singletectonic setting. However, this range of apparent thermal gradientsmay be used in concert with the proposed P–T path to argue thatdecompression-style P–T paths are common to terranes undergoingcrustal thickening and erosion-driven exhumation. As the thermal gra-dients are elevated above the ‘normal’ thermal gradient for Earth's crust,it may be that the crustwas enriched in radiogenic heat production (e.g.Clark et al., 2011; Kelsey and Hand, 2015) and/or the crust was thinnerand hot prior to thickening (e.g. Collins, 2002; Hyndman et al., 2005)such that the Paleoproterozoic age of the tectonism, documented bythis study, reflects a hotter Earth.

Previously, the clockwise P–T–t paths involving near-isothermal de-compression of the high-pressure granulites from the Khondalite Belthave been related to a setting that resulted from the collision betweenthe Eastern and Western Blocks of the NCC at ca. 1850 Ma to form toTrans-North China Orogen (Zhao and Zhai, 2013). However, thismodel cannot explain the metamorphism of the Khondalites inour study and other similar complexes (Qianlishanx, Daqingshan,Wulashan, and Helanshan Complexes) that occur far away from theTrans-North China Orogen with different metamorphic ages.

Regardless of whether the samples in this study are a part of theKhondalite Belt or from a separate basin of the Ordos Block, it can beproposed that the Ordos Basin does not represent a whole piece of theArchean block, but is composed of Paleoproterozoic units, which amal-gamatedwith the YinshanBlock at about 1940Mawith the formation ofthe Inner Mongolia Suture Zone. In addition, the similarity and consis-tency of the Ordos basement and the Inner Mongolia Suture Zone(IMSZ) Khondalite belt could indicate that the southern boundary ofIMSZ is further southward than currently defined, suggesting the largestPaleoproterozoic accretionary belt in the NCC.

6. Conclusions

The Ordos Basement is a critical part of the North China Craton, andtherefore of importance in continent and supercontinent assemblymodels. A rare opportunity to study the mostly non-outcroppingOrdos Basement is afforded by deep-penetrating drill holes that haveintersected the basement rock. LA-ICP-MSU–Pbmonazite geochronologyand thermobarometry conducted on metapelitic and granitic rocks fromthe eastern and western Ordos Basement reveal metamorphic ages of ca.1926–1934Ma and a clockwise P–T evolution. The (U+Pb)–Th chemicalages through EPMA dating reveals that monazite occurring as inclusionsin garnet are older than those in thematrix.We interpret that the similar-ity in metamorphic age with the previously described Khondalite Belt tothe north, the clockwise P–Thistory, and the aluminousmetasedimentarynature of the studied samples may indicate a large Paleoproterozoic ac-cretionary belt in the NCC extending beneath the Ordos Block.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2016.06.022.

Acknowledgments

We thank Editor in-Chief Professor Marco Scambelluri and two ref-erees Professor Daniel Harlov and Professor Antonio Langone for theirconstructive and helpful suggestions which greatly improved our

manuscript. Santosh thanks University of Adelaide, Australia andChina University of Geosciences Beijing, China for support. We thankDr. Ben Wade and Mr. Ken Neubauer from Adelaide Microscopy fortheir much-appreciated assistance with the SEM and LA-ICP-MS.

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