Garnet-rich granulite xenoliths from the Hannuoba basalts, North China: Petrogenesis and...

Journal of Earth Science, Vol. 21, No. 5, p. 669–691, October 2010 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-010-0125-x

Garnet-Rich Granulite Xenoliths from the Hannuoba Basalts, North China: Petrogenesis and

Implications for the Mesozoic Crust-Mantle Interaction

Liu Yongsheng* (刘勇胜)

State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; State Key Laboratory of Continental Dynamics,

Northwest University, Xi’an 710069, China Gao Shan (高山), Gao Changgui (高长贵), Zong Keqing (宗克清),

Hu Zhaochu (胡兆初), Ling Wenli (凌文黎) State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences,

China University of Geosciences, Wuhan 430074, China

ABSTRACT: Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China

craton (NCC), were studied to reveal the Mesozoic crust-mantle interaction. These xenoliths are char-

acterized by low SiO2 (37.7 wt.%–46.0 wt.%) and high Al2O3 (10.8 wt.%–17.9 wt.%) contents. Their

Mg# (60–75, Mg#=100×Mg/(Mg+Fe), atomic number) are relatively low for their low SiO2 contents.

They have low rare-earth element (REE) contents and LREE-rich REE patterns, and show remarkable

enrichments in Sr relative to the adjacent REE. Some of them exhibit convex REE patterns with a

maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode

(generally >30%), these features suggest that they are high-pressure metamorphic products of low-

This study was co-supported by the National Natural Science

Foundation of China (Nos. 90914007, 40821061, 90714010),

the State Administration of Foreign Expert Affairs of China (No.

B07039), the MOST Special Fund of State Key Laboratory of

Geological Processes and Mineral Resources and State Key

Laboratory of Continental Dynamics, and the Special Fund For

Basic Scientific Research of Central Colleges, China University

of Geosciences (Wuhan).

*Corresponding author: [email protected]; yshliu@hotmail.

com

© China University of Geosciences and Springer-Verlag Berlin

Heidelberg 2010

Manuscript received April 4, 2010.

Manuscript accepted May 10, 2010.

pressure cumulates (e.g., gabbro) after it had

been depressed into the garnet stability field.

They have evolved Nd and Sr isotopic composi-

tions (1 4 3Nd/1 4 4Nd=0.511 763–0.512 173, 87Sr/86Sr=0.705 34–0.706 99) and fall in the trend

defined by the >110 Ma Mesozoic basalts and

high-Mg# andesites from the NCC. Zircon U-Pb

dating by LA-ICP-MS shows a wide age range

from 83 to 2 581 Ma, most of which cluster in

83–134 Ma. CL images of some Mesozoic zircons

from the granulites show typical features of ig-

neous zircons, providing direct evidence for the

Mesozoic underplating event in this area. Nei-

ther peridotite-derived basaltic underplating

model nor residue model of ancient lower crust

after lithospheric thinning alone can reasonably

Liu Yongsheng, Gao Shan, Gao Changgui, Zong Keqing, Hu Zhaochu and Ling Wenli 670

explain the above features of the garnet-rich granulite xenoliths. Combined with the previous research,

we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Meso-

zoic magmatic underplating and mixing with the pre-existing lower crust (i.e., AFC process). However,

the melts could be mostly derived from partial melting of basaltic layers that were previously subducted

(a fossil oceanic slab) or underplated into the base of the lithospheric mantle, or from partial melting of

Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower

crustal eclogite, although it cannot be excluded that some of the melts were derived from depleted man-

tle peridotite. In other words, parent melts of most granulite xenoliths could share the same petrogene-

sis as the >110 Ma Mesozoic basalts from the NCC.

KEY WORDS: garnet-rich granulite xenolith, trace element, zircon U-Pb age, Nd and Sr isotopes,

crust-mantle interaction, North China craton.

INTRODUCTION

Granulite xenoliths brought to the surface by ba-saltic and kimberlitic magmas provide direct windows into the present and/or fossil lower crust (Rudnick, 1992). In most cases, mafic rocks dominate the lower crustal xenolith suites (Rudnick, 1992) and were in-terpreted as products of basaltic underplating at the base of the crust (e.g., Liu et al., 2004; Downes et al., 1990; Rudnick and Goldstein, 1990; Griffin and O’Reilly, 1986; Rudnick et al., 1986). Many xenoliths as portions of ancient lower crust were also carried by basalts or kimberlites (Liu et al., 2004; Davis et al., 2003; Schmitz and Bowring, 2001; Leeman et al., 1985), which are mainly found in the craton areas. To correctly constrain the age and petrogenesis of the granulite and co-existing pyroxenite xenoliths is criti-cal for our understanding of the evolution of the lower crust and crust-mantle interaction.

Abundant lower crustal and upper mantle xeno-liths are found in the Hannuoba basalts, the North China craton (NCC). It has been accepted that: (1) age spectrum of the granulite and pyroxenite xenoliths generally spread in a large range (Liu et al., 2010a, 2004, 2001; Zheng et al., 2009; Wilde et al., 2003), (2) granulite, gabbro and low-Mg# (<80) pyroxenite xeno-liths have mostly evolved Sr-Nd isotopic compositions (Liu et al., 2004; Xu, 2002; Zhou et al., 2002; Zhang and Sun, 1998; Chen et al., 1997, 1995), and (3) many granulite xenoliths are characterized by notably higher Mg# than most of the worldwide analogues and the terrain granulites from the NCC (Liu et al., 2005). Al-though a few of the granulite xenoliths sampled the pre-existing lower crust (Liu et al., 2004), Fan et al. (1998), Liu et al. (2004, 2001) and Zheng et al. (2009)

suggested that most of the granulite xenoliths were products of the Mesozoic and Paleogene basaltic un-derplating and mixing with the pre-existing lower crust. However, Wilde et al. (2003), Jiang and Guo (2010), Jiang et al. (2007) suggested that they are por-tions of residue of the ancient lower crust after litho-spheric thinning based on the evolved Sr-Nd isotopic compositions and presence of Precambrian zircons.

On the other hand, Sr-Nd isotopic compositions of the Hannuoba granulite and gabbro xenoliths mostly cluster in a small range and fall in the trend defined by the >110 Ma Mesozoic basalts that has been suggested to be derived from olivine-poor py-roxenite mantle formed by recycled ancient crust-derived melt-peridotite reaction (Gao et al., 2008; Liu et al., 2008a). Except for those Mesozoic igneous zircons found in the olivine pyroxenite xenoliths (clinopyroxene-rich cumulate) (Liu et al., 2004) and garnet pyroxenite xenoliths (products of melt- peridotite reaction) (Liu et al., 2010a), no Mesozoic zircon with clearly igneous CL image has been found in the granulite xenoliths so far (Liu et al., 2004; Wilde et al., 2003). Therefore, if the granulite and gabbro xenoliths are really products of basaltic under-plating, then what’s the mantle source of the basaltic melt (peridotite vs. pyroxenite)? And are there any ig-neous zircon records on the underplating event in the granulite xenoliths? Otherwise, if the granulite xeno-liths are portions of residue of the ancient lower crust after lithospheric thinning, how they got the relatively high-Mg#?

Garnet-rich (>30%) granulite xenoliths are most likely the sample of ancient lower crust residue if any of the granulite xenoliths are portions of residue of the

Garnet-Rich Granulite Xenoliths from the Hannuoba Basalts, North China 671

ancient lower crust after lithospheric thinning. In this work, garnet-rich granulite xenoliths were studied in detail, and Mesozoic zircons with distinct igneous features were found in these xenoliths confirming the Mesozoic underplating event. We show that the “paradoxical” isotopic and chemical features and complex zircon U-Pb ages of the granulite xenoliths from the Hannuoba basalts could have resulted from combination of recycled ancient crust-derived melt-peridotite reaction and Mesozoic basalt-type melt underplating. GEOLOGICAL SETTING

The NCC can be divided into the eastern block, the western block and the intervening trans-North China orogen based on ages, lithological assemblages, tectonic evolution and P-T-t paths (insert of Fig. 1). Zhao et al. (2001) and Wilde et al. (2002) suggested that an ancient ocean existed between the eastern and western blocks, and the western block was subse-quently subducted beneath the eastern block, resulting in the final amalgamation of the two blocks along the central zone at ca. 1.85 Ga. In contrast, Li et al. (2000) and Kusky et al. (2001) argued that the western and eastern blocks were amalgamated during a ~2.5 Ga collision event. Zhai et al. (2000) suggested cratoniza-

tion of the NCC at ca. 2.5 Ga, followed by its in-volvement in a global pre-Rodinian supercontinent at ca. 2.0 Ga.

The NCC experienced widespread tectonother-mal reactivation during the Late Mesozoic and Ceno-zoic, as indicated by emplacement of voluminous Late Mesozoic granites and extensive Tertiary volcanism. The tectonothermal events resulted in replacement of the old, cold, thick depleted lithospheric mantle by young, hot, thin and fertile lithospheric mantle ac-companied by lithospheric thinning (Griffin et al., 1998; Menzies et al., 1993). However, a Re-Os erro-chron age of ~1.9 Ga preserved in spinel lherzolites from Damaping implies no significant lithospheric thinning in this area since the Mesoproterozoic (Gao et al., 2002), which differs significantly from the east part of the NCC where Phanerozoic replacement of the lithospheric mantle is well documented.

The Hannuoba basalts occur along the northern margin of the trans-North China orogen (Fig. 1) and comprise alternating layers of alkaline and tholeiitic basalts. They are dated at 14–27 Ma by the K-Ar method (Zhu, 1998). The alkaline basalts carried abundant lower crustal and upper mantle xenoliths. Granulite xenoliths vary compositionally from one locality to another (Liu et al., 2001). The equilibrium

Figure 1. Simplified geological map of the Hannuoba area. NCC. North China craton; YZ. Yangtze craton; SC. South China orogen. The division of the North China craton is after Zhao et al. (2000).


temperatures are estimated to be 768–835 ℃ for garnet-bearing mafic granulites (Liu et al., 2003) and 900–1 000 ℃ for two-pyroxene granulites (Fan and Liu, 1996). Pressure is much less constrained and var-ies from 13.5 to 18.3 kbar (Liu et al., 2003) or from 16.2 to 22 kbar (Chen et al., 2001) for garnet pyrox-enites and 9.8–10.7 kbar for garnet-bearing mafic granulites (Liu et al., 2003). SAMPLES AND ANALYTICAL TECHNIQUES

Ten garnet-rich granulite xenoliths from the Hannuoba basalt were selected for study. They are generally composed of clinopyroxene, garnet, or-thopyroxene and plagioclase (Appendix 1). A few of albites in the grain boundary and sulfide are found in some samples. Garnet mode is generally high up to >30%, and clinopyroxene is characterized by high Na2O content (up to 2.3 wt.%) (Liu et al., 2003).

Whole rock samples were crushed in a corundum jaw crusher (to 60 mesh). About 60 g was powdered in an agate ring mill to less than 200 mesh. Major ele-ment compositions of whole rocks were determined by XRF on fused glass disks at the State Key Labora-tory of Continental Dynamics (SLCD), Northwest University, Xi’an. Analysis of BCR-2 indicates preci-sion better than 6% and accuracy better than 4%. Trace elements and Sr-Nd isotopes were analyzed at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Trace elements were analyzed with an Agilent 7500a ICP-MS. The detailed sample- digestion procedure and analytical precision and ac-curacy for trace elements are the same as described by Liu et al. (2008c). Nd and Sr isotopic analyses were performed on a Finnigan Triton Ti TIMS at GPMR. The sample-digestion and separation procedure and analytical techniques for Sr-Nd isotopes are the same as described by Gao et al. (2004).

Zircon grains were picked from DMP141 and DMP146, and mounted in epoxy blocks and polished to obtain an even surface. Cathodoluminescence (CL) images of zircons were taken at SLCD and summa-rized in Figs. 2 and 3. U-Pb dating and trace element analyses were conducted synchronously by LA-ICP- MS at GPMR. Detailed operating conditions for LA-ICP-MS and calibration strategy are the same as

described by Liu et al. (2010a, b). Off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for trace element analyses and U-Pb dating were per-formed by ICPMSDataCal (Liu et al., 2010a, 2008b). Accuracy and precision of U-Pb isotopic dating and trace element analyses as monitored by analyses of zircon standards GJ-1 (Jackson et al., 2004), SK10-2 (Yuan et al., 2003) and 91500 (Wiedenbeck et al., 2004) are similar to the description of Liu et al. (2010a, b). Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003). RESULTS Major and Trace Elements

Major and trace element compositions of the garnet-rich granulite xenoliths are listed in Table 1 and shown in Fig. 4. These xenoliths are characterized by low SiO2 (37.7 wt.%–46.0 wt.%), high Al2O3 (10.8 wt.%–17.9 wt.%) and low K2O and Na2O contents. Their Mg# (60–75, Mg#=100×Mg/(Mg+Fe), atomic number) are relatively low for their SiO2 contents. At the same Mg# level, the garnet-rich granulite xenoliths have lower SiO2, Na2O and LREE contents than the co-existing garnet-free granulite xenoliths (Fig. 4). Na2O and Sr correlate positively with Al2O3 (correla-tion coefficients (r)=0.73 for Na2O-Al2O3 and 0.61 for Sr-Al2O3). Al2O3 and Sr contents show no obvious difference between the garnet-rich and garnet-free granulite xenoliths at the same Mg# level.

The studied xenoliths have low rare-earth ele-ment (LREE) contents and show mostly LREE-rich REE patterns. Some of them exhibit convex REE pat-terns with a maximum at Nd and remarkably positive Eu anomalies (Fig. 5a and Table 1). Th, U, K and Pb contents vary significantly. All the samples show re-markable enrichments in Sr relative to the adjacent REE (Fig. 5b). Whole Rock Nd and Sr Isotopes

Nd and Sr isotopic compositions are listed in Ta-ble 2. All the samples have lower 143Nd/144Nd ratios (0.511 763–0.512 173) than the host basalts (0.512 651–0.512 977; Song et al., 1990) and the spinel lherzolite and garnet-rich high-Mg# pyroxenite


xenoliths (Fig. 6a). Initial Sr isotopic compositions of the samples at 130 Ma are higher than the lherzolite xenoliths, but show no obvious difference from the high-Mg# pyroxenite formed by the melt-peridotite reaction. Although their Sr-Nd isotopic compositions overlap with the garnet-free pyroxenite, gabbro and granulite xenoliths (Liu et al., 2004; Xu, 2002; Zhou et al., 2002; Chen et al., 1997, 1995), they fall in the field of high 143Nd/144Nd and low 87Sr/86Sr ratios (Fig. 6a). Zircon Geochronology

Forty-nine spots on 42 zircon grains from

DMP141 and DMP146 were analyzed by LA-ICP-MS (Tables 3 and 4). The measurements of the extremely small amounts of radiogenic 207Pb in the young, low-U zircons are subject to large uncertainties due to counting statistics. Therefore, 207Pb/206Pb ages for the Precambrian zircons and 206Pb/238U ages for the Phan-erozoic zircons were used. Ages of zircons from DMP141 are dominated by 102–132 Ma. Zircons of 248 Ma and 2.05 Ga were also found (Fig. 7). The 102–132 Ma zircons are characterized by very low Pb contents (1 ppm–23 ppm), high Th/U (mostly >1) and low U/Yb ratios (<1). Some of the 102–132 Ma zir-cons show typical igneous zonation on CL images

Figure 2. CL images of the analyzed zircon grains from DMP141. The circles are analysis spots marked with ages.


Figure 3. CL images of the analyzed zircon grains from DMP146. The circles are analysis spots marked with ages.


Figure 4. Plots of major and trace elements versus Mg# and Na2O and Sr versus Al2O3 for the studied xeno-liths. Terrain granulites from the NCC (Liu et al., 1999), garnet pyroxenite formed by melt-peridotite reac-tion (Liu et al., 2005) and garnet-free granulite xenoliths (Liu et al., 2005, 2004, 2001) were shown for com-parison.


Table 1 Major and trace element compositions of the garnet-rich granulite xenoliths

Sample DMP08 DMP15 DMP133 DMP136 DMP137 DMP140 DMP141 DMP146 DMP157 DMP467SiO2 45.3 44.2 42.9 44.2 44.1 46.0 42.6 45.5 42.0 37.7 TiO2 0.55 0.79 1.59 0.81 0.65 0.71 1.65 0.54 1.67 2.66 Al2O3 17.2 13.8 10.8 14.0 15.4 12.5 15.0 16.3 14.8 17.9 Fe2O3 1.99 2.67 17.6 13.4 10.7 11.2 8.38 14.9 TFe2O3 9.86 12.4 17.6 13.4 10.7 11.2 13.6 8.38 14.9 17.1 MnO 0.12 0.14 0.11 0.14 0.12 0.17 0.12 0.12 0.13 0.11 MgO 12.4 14.7 17.5 13.2 13.1 15.8 10.5 12.5 11.5 8.46 CaO 12.1 11.8 7.46 11.7 11.4 10.4 11.4 11.0 10.1 11.5 Na2O 1.36 0.96 0.44 1.19 1.67 0.72 1.58 1.58 1.67 1.25 K2O 0.10 0.020 0.26 0.29 0.36 0.54 0.60 0.27 0.35 0.11 P2O5 0.040 0.030 0.030 0.040 0.10 0.060 0.060 0.050 0.060 0.030 LOI 1.59 1.80 0.95 0.89 2.17 2.11 1.63 3.35 3.00 1.26 Total 98.9 98.9 98.6 99.1 97.6 98.1 98.7 96.2 97.2 98.0 Mg# 71.4 70.0 66.4 66.2 70.8 73.7 75.3 74.7 60.4 66.3 Li 1.86 4.08 8.39 4.43 8.31 7.81 8.76 8.74 Be 0.060 0.060 0.21 0.090 0.37 0.25 0.34 0.29 Sc 16.2 20.0 36.4 60.7 30.9 39.7 13.7 31.5 41.2 31.2 V 116 193 505 41.1 160 217 446 156 402 569 Cr 128 156 161 28.1 161 301 219 113 82.8 149 Co 63.2 74.3 129 5.33 80.7 84.1 72.4 54.0 72.0 86.7 Ni 89.5 89.2 94.7 9.75 81.5 579 39.8 82.5 59.6 80.4 Cu 18.0 42.7 46.4 4.13 41.8 60.1 65.8 37.6 31.1 90.1 Zn 78.8 81.8 165 10.1 56.8 57.4 74.5 45.1 103 139 Ga 12.1 12.6 18.7 8.73 13.2 11.3 19.1 12.5 17.7 22.5 Rb 2.43 1.74 7.54 5.66 7.43 17.7 13.9 4.68 9.93 3.26 Sr 1108 477 400 475 1239 572 1262 876 777 760 Y 6.37 6.35 5.73 11.1 9.07 14.4 10.1 8.47 11.5 20.3 Zr 13.6 21.1 16.8 12.9 27.8 42.3 28.7 35.9 42.4 22.8 Nb 0.64 1.67 2.43 0.17 4.27 4.33 4.49 4.56 4.20 2.75 Cs 0.069 0.061 0.014 0.035 0.025 0.022 0.015 0.020 0.071 0.018 Ba 102 31.7 35.0 39.0 117 133 78.3 77.6 76.0 27.0 La 2.54 2.40 2.41 1.93 4.53 3.43 3.49 4.63 3.61 1.31 Ce 7.00 8.31 6.49 4.49 11.4 9.53 8.91 11.1 10.2 5.01 Pr 1.07 1.38 0.96 0.94 1.56 1.48 1.39 1.49 1.60 1.19 Nd 6.00 7.23 5.48 5.73 8.31 8.24 7.57 7.22 8.94 9.65 Sm 1.57 1.90 1.38 1.87 2.03 2.28 2.11 1.76 2.33 3.46 Eu 0.76 0.83 0.52 0.64 0.76 0.79 0.81 0.67 0.87 1.38 Gd 1.46 1.61 1.27 1.69 1.83 2.13 1.94 1.58 2.09 3.39 Tb 0.24 0.27 0.20 0.29 0.28 0.39 0.32 0.26 0.35 0.62 Dy 1.45 1.58 1.05 1.70 1.52 2.17 2.05 1.39 1.85 3.51 Ho 0.29 0.29 0.23 0.37 0.32 0.49 0.39 0.29 0.39 0.67 Er 0.70 0.73 0.58 0.94 0.80 1.22 1.02 0.73 0.96 1.77 Tm 0.098 0.10 0.080 0.13 0.12 0.18 0.16 0.11 0.14 0.27 Yb 0.58 0.59 0.51 0.84 0.75 1.13 0.86 0.66 0.84 1.33 Lu 0.088 0.092 0.076 0.13 0.11 0.17 0.11 0.10 0.13 0.20 Hf 0.59 0.85 0.65 0.20 0.84 1.33 0.90 1.09 1.08 0.77 Ta 0.011 0.053 0.16 0.017 0.26 0.28 0.30 0.25 0.24 0.14 Pb 4.72 2.44 0.56 0.24 0.59 0.51 1.58 1.00 0.43 1.96 Th 0.027 0.073 0.15 0.23 0.27 0.27 0.32 0.24 0.20 0.24 U 0.000 7 0.10 0.10 0.014 0.21 0.15 0.096 0.22 0.19 0.038 Eu/Eu* 1.53 1.45 1.20 1.11 1.20 1.09 1.22 1.22 1.20 1.23

Unit is wt.% for major elements and ppm for trace elements.


Figure 5. (a) CI-normalized REE patterns and (b) PM-normalized trace element patterns for the garnet- rich granulite xenoliths. CI. chondrite (Taylor and McLennan, 1985), PM. primitive mantle (McDonough and Sun, 1995).

Figure 6. (a) Sr-Nd isotopic compositions of the garnet-rich granulite and pyroxenite xenoliths. Garnet-free granulite, gabbro and spinel lherzolite xenoliths were from the Hannuoba basalts (Liu et al., 2004; Rudnick et al., 2004; Zhou et al., 2002; Chen et al., 1995), the Archean terrain granulite and the Mesozoic basalts (Gao et al., 2008; Zhang et al., 2003) and high-Mg# adakitic rocks (Gao et al., 2004) from the NCC were also shown for comparison. (b), (c) Plots of εNd(130 Ma) versus Ni and 87Sr/86Sr(130 Ma) versus Sr. Shadow ar-eas show the ranges of Ni and Sr contents in the Mesozoic basalts from the NCC (Liu et al., 2008a). (Fig. 2). Analytical signals of 207Pb of most zircon grains are close to the background of the instrument. They show heavy rare-earth element (HREE)-rich REE patterns with moderate positive Ce anomalies and negative Eu anomalies (Fig. 7). Compared to the 102–132 Ma zircons, zircons of 248 Ma and 2.05 Ga zircon core have higher U/Yb ratio (1.15–3.34). Zir-con of 248 Ma has a very high REE content and ex-

tremely large negative Eu anomaly, whereas the REE pattern of the 2.05 Ga zircon is featured by a low REE content and small negative Eu anomaly.

Zircons from DMP146 give generally concordant or near-concordant U-Pb ages spreading from ca. 2 581 to 83 Ma (Fig. 8). Zircons from DMP146 have relatively lower Th/U ratios (<0.9) and higher U/Yb ratios compared to those from DMP141 (Fig. 9).


Table 2 Nd and Sr isotopic compositions of the garnet-rich granulite xenoliths

Number SiO2 Mg# Sm Nd 147Sm/144Nd 143Nd/144Nd 2σ Rb Sr 87Rb/86Sr 87Sr/86Sr 2σ

Garnet-rich granulite

DMP08 45.3 71.4 1.57 6.00 0.164 6 0.511 911 17 2.43 1 108 0.005 5 0.706 890 4

DMP15 44.2 70.0 1.90 7.23 0.164 9 0.511 896 4 1.74 477 0.010 3 0.706 993 4

DMP133 42.9 66.4 1.38 5.48 0.157 9 0.512 044 2 7.54 400 0.053 1 0.706 794 3

DMP136 44.2 66.2 1.87 5.73 0.204 2 0.512 078 5 5.66 475 0.033 6 0.705 340 3

DMP137 44.1 70.8 2.03 8.31 0.153 2 0.511 763 123 7.43 1 239 0.016 9 0.705 808 3

DMP140 46.0 73.7 2.28 8.24 0.173 9 0.512 173 8 17.7 572 0.087 4 0.706 053 4

DMP141 42.6 75.3 2.11 7.57 0.175 1 0.512 028 12 13.9 1 262 0.031 1 0.705 614 4

DMP146 45.5 74.7 1.76 7.22 0.152 6 0.512 156 4 4.68 876 0.015 1 0.706 989 3

DMP157 42.0 60.4 2.33 8.94 0.163 9 0.512 056 4 9.93 777 0.036 0 0.706 230 3

DMP467 37.7 66.3 3.46 9.65 0.225 4 0.511 945 3 3.26 760 0.012 1 0.706 973 5

Garnet-rich pyroxenite

DMP254 46.0 86.7 1.25 3.71 0.203 4 0.513 030 6 9.34 224 0.120 4 0.706 823 15

DMP134 46.4 86.2 1.21 3.16 0.231 5 0.513 074 9 16.2 219 0.214 0 0.705 512 2

Unit is wt.% for SiO2 and ppm for Sm, Nd, Rb and Sr.

Zircons of 83–122 Ma show needle-shaped acicular crystals with elongation (length-to-width) ratios of 4–6 (Fig. 3) and have HREE-rich REE patterns. Zir-con of 134 Ma has the lowest Lu/Gd ratio (0.77) and ultrahigh-pressure (UHP) metamorphic zircon-type REE patterns (Zong et al., 2010). Although 206Pb/238U ages of other zircons vary greatly, they show similar REE patterns and have low Th/U ratios and high U/Yb and Lu/Gd (1.3–7.6) ratios (Table 4). DISCUSSION Petrogenesis of the Garnet-Rich Granulite Xeno-liths

The garnet-rich granulite xenoliths in this work do not have major-element compositions matching most known melt compositions. Most melts having low SiO2 contents should have higher MgO contents and Mg#, but this is not the case for the garnet-rich granulite xenoliths. On the other hand, the relatively low Mg# and Ni and Cr contents of the garnet-rich granulite xenoliths suggest that they are not likely to be the reaction products of silicic magmas with peri-dotite (Fig. 6b) (Liu et al., 2005; Kelemen et al., 1998). Furthermore, garnet-rich pyroxenites formed by melt-peridotite reaction have higher HREE and lower LREE contents than the garnet-rich granulite xenoliths (Fig. 5a).

The geochemistry and petrology of the studied xenoliths are similar to the Sierran low MgO pyrox-enites (Lee et al., 2006), which are characterized by high garnet mode (>50%). Based on the REE patterns and Ni, Cr and SiO2 contents, Lee et al. (2006) sug-gested that many of the Sierran low MgO garnet py-roxenites were probably high-pressure (>2–3 GPa) cumulates from an evolved basaltic to basaltic ande-sitic magma or represent the residues of melting pre-existing basaltic lower crust after it had been de-pressed into the garnet stability field and subsequently heated to partial melting conditions. Although garnet- rich granulite xenoliths have Nd and Sr isotopic com-positions overlapping with the Archaean terrane granulites, they all fall in the end field of high εNd(t) and low-radiogenic Sr (Fig. 6a). Furthermore, it is ex-pected that residues of melting pre-existing ancient lower crust at garnet stability field should have in-creased Y and HREE and decreased Sr contents. However, the garnet-rich granulite xenoliths have higher Sr contents and lower HREE contents than the exposed Archean granulite terrane (Fig. 4). These suggest that they are not simple residues of melting the ancient basaltic lower crust, although some Pre-cambrian zircons were found in these xenoliths. On the other hand, their LREE enrichments suggest that the garnets were formed during metamorphism rather

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1σ

Rat

io1σ

Age

(Ma)

1σA

ge (M

a)1σ

Age

(Ma)

1σA

ge (M

a)

1σ

Spot

No.

DM

P141

04

5.93

150

106

0.

190

1 0.

021

40.

375

80.

042

00.

015

90.

000

90.

005

80.

000

8

2 74

3 18

532

4 31

102

611

6 16

16

5.44

134

120

0.

211

3 0.

057

00.

303

20.

050

90.

016

80.

001

10.

007

00.

000

7

2 91

7 45

126

9 40

107

714

0 15

01

8.10

184

135

0.

111

6 0.

017

20.

215

30.

025

60.

016

80.

001

00.

007

80.

000

7

1 82

5 28

219

8 21

107

715

7 15

11

1.02

23.4

49.4

0.39

6 3

0.07

9 5

0.64

6 2

0.07

8 4

0.01

7 0

0.00

1 7

0.02

1 8

0.00

4 7

3

895

307

506

4810

9 11

435

94

14

11.4

331

265

0.

103

4 0.

018

60.

213

70.

033

10.

017

20.

000

80.

005

10.

000

4

1 68

7 33

819

7 28

110

510

3 9

25

17.0

401

246

0.

114

1 0.

011

80.

267

50.

027

70.

017

20.

000

90.

006

00.

000

5

1 86

5 18

924

1 22

110

612

1 11

20

14.7

628

549

0.

074

8 0.

014

70.

175

60.

029

50.

017

50.

000

80.

006

50.

000

5

1 06

5 40

616

4 26

112

513

0 9

18

7.00

314

263

0.

090

0 0.

016

00.

218

70.

041

60.

017

90.

001

20.

006

80.

000

5

1 42

5 34

420

1 35

114

713

6 11

03

5.41

133

98.0

0.17

6 7

0.03

2 9

0.41

5 6

0.07

4 5

0.01

7 9

0.00

1 0

0.00

5 9

0.00

0 7

2

622

315

353

5411

4 7

118

13

24

15.7

431

341

0.

094

4 0.

014

10.

225

50.

027

80.

018

10.

000

80.

004

80.

000

4

1 51

7 28

420

6 23

116

597

.5

7

19

22.9

501

436

0.

068

1 0.

007

40.

185

20.

020

50.

019

40.

000

90.

007

10.

000

5

870

226

173

1812

4 6

144

10

07

14.1

317

230

0.

134

3 0.

025

40.

338

60.

056

80.

019

60.

000

80.

007

00.

000

5

2 15

5 33

529

6 43

125

514

0 10

22

8.50

154

146

0.

151

8 0.

015

90.

435

50.

048

80.

020

10.

000

90.

007

60.

000

7

2 36

9 18

036

7 35

128

615

3 15

27

13.3

100

283

0.

049

6 0.

002

80.

138

60.

007

70.

020

20.

000

30.

006

40.

000

2

176

135

132

712

9 2

129

4

05

13.8

180

134

0.

328

6 0.

039

80.

830

90.

089

20.

020

40.

001

10.

012

90.

001

0

3 61

1 18

661

4 49

130

725

9 20

21

13.6

549

348

0.

163

2 0.

015

30.

474

70.

048

80.

020

60.

000

80.

009

40.

000

8

2 50

0 15

839

4 34

131

518

8 16

06

10.5

224

203

0.

091

4 0.

014

50.

254

30.

038

30.

020

60.

001

00.

006

70.

000

7

1 45

5 30

623

0 31

132

613

5 13

10

71.5

986

646

0.

051

6 0.

004

90.

278

30.

024

30.

039

20.

001

50.

011

80.

000

8

265

221

249

1924

8 9

238

16

08

222

156

370

0.

126

4 0.

006

06.

615

60.

340

30.

375

50.

008

80.

138

00.

007

0

2 04

8 84

2 06

1 45

2 05

5 41

2 61

3 12

4

Con

tinue

d

20

7 Pb/20

6 Pb

207 Pb

/235 U

20

6 Pb/23

8 U

208 Pb

/232 Th

207 Pb

/206 Pb

20

7 Pb/23

5 U

206 Pb

/238 U

20

8 Pb/23

2 Th

Pb

Th

U

R

atio

1σ

R

atio

1σ

R

atio

1σ

Rat

io1σ

Age

(Ma)

1σA

ge (M

a)1σ

Age

(Ma)

1σA

ge (M

a)

1σ

Spot

No.

DM

P146

07

58.6

809

1 06

4

0.05

1 7

0.00

7 4

0.09

1 3

0.01

2 3

0.01

2 9

0.00

0 4

0.00

3 8

0.00

0 3

33

3 23

589

11

83

377

5

01

57.7

183

3 03

1

0.05

5 6

0.00

2 6

0.13

1 2

0.00

5 9

0.01

7 0

0.00

0 3

0.00

8 4

0.00

0 7

43

5 13

612

5 5

109

216

9 14

22

29.7

306

349

0.

059

2 0.

012

10.

150

60.

032

70.

018

20.

000

90.

005

30.

000

5

572

458

142

2911

6 6

107

9

32

140

982

4 88

1

0.04

9 5

0.00

2 9

0.13

1 5

0.01

0 3

0.01

9 1

0.00

1 4

0.00

5 9

0.00

0 6

17

2 13

912

5 9

122

911

9 11

06

57.3

524

1 18

6

0.05

0 1

0.00

3 8

0.13

2 9

0.01

0 3

0.01

9 1

0.00

0 5

0.00

9 9

0.00

1 5

19

8 17

612

7 9

122

319

8 29

14

12.4

102

169

0.

091

8 0.

012

20.

252

70.

033

30.

019

80.

000

80.

006

00.

000

7

1 46

5 25

522

9 27

126

512

2 13

08

28.2

145

707

0.

048

8 0.

004

00.

142

60.

011

80.

021

00.

000

70.

007

70.

000

5

200

120

135

1013

4 4

156

11

31

269

1 38

11

676

0.

049

3 0.

002

90.

231

00.

013

60.

033

30.

000

60.

011

10.

000

4

161

140

211

1121

1 4

223

8

03

51.5

232

409

0.

049

4 0.

004

20.

244

00.

019

50.

035

80.

000

80.

012

00.

000

6

165

185

222

1622

7 5

240

12

17

47.5

146

462

0.

056

1 0.

004

50.

366

00.

028

40.

046

70.

001

10.

014

30.

000

9

457

150

317

2129

4 6

288

17

13

93.0

263

604

0.

066

0 0.

004

50.

445

60.

031

50.

048

40.

001

30.

017

40.

000

9

809

144

374

2230

5 8

348

17

30

47.6

165

255

0.

053

5 0.

004

30.

385

60.

030

70.

051

30.

001

20.

016

20.

000

8

350

179

331

2232

3 7

324

16

28

130

304

703

0.

053

9 0.

003

20.

398

40.

028

80.

051

90.

002

20.

023

30.

001

8

365

132

341

2132

6 13

466

36

18

752

1 66

54

789

0.

054

2 0.

002

40.

498

40.

021

20.

065

40.

001

00.

021

60.

000

8

389

9841

1 14

408

643

2 15

29

48.2

106

488

0.

058

2 0.

004

30.

536

60.

040

50.

066

20.

001

90.

017

10.

001

2

600

168

436

2741

3 11

343

23

10

175

286

1 40

4

0.05

7 5

0.00

3 3

0.54

7 5

0.03

1 5

0.06

8 2

0.00

1 4

0.02

4 1

0.00

1 3

50

9 11

944

3 21

425

948

0 25

26

226

377

713

0.

062

1 0.

003

00.

742

60.

037

70.

085

70.

002

50.

034

40.

001

7

680

108

564

2253

0 15

684

34

24

234

473

732

0.

055

9 0.

002

60.

684

60.

032

90.

087

20.

001

80.

028

00.

001

0

456

102

530

2053

9 11

559

20

23

1 90

296

32

817

0.

094

0 0.

003

93.

610

80.

152

60.

273

80.

005

70.

093

50.

003

7

1 50

9 79

1 55

2 34

1 56

0 29

1 80

7 68

21

577

217

1 65

7

0.08

9 9

0.00

3 5

3.51

5 8

0.20

6 7

0.27

7 5

0.01

2 5

0.07

1 9

0.00

2 7

1

433

751

531

471

579

631

403

50

16

219

88.5

282

0.

113

6 0.

005

55.

415

70.

276

40.

338

60.

008

00.

104

60.

004

8

1 85

8 88

1 88

7 44

1 88

0 38

2 01

0 87

11

791

307

720

0.

152

8 0.

006

68.

175

30.

369

20.

383

20.

007

40.

113

20.

004

9

2 37

7 74

2 25

1 41

2 09

1 35

2 16

8 88

25

203

72.2

101

0.

172

4 0.

007

312

.537

20.

548

10.

517

00.

008

60.

141

90.

005

1

2 58

1 71

2 64

6 41

2 68

6 36

2 68

2 91


Table 4 Trace element compositions (ppm) of zircons from DMP141 and DMP146

Element Y Nb La Ce Pr Nd Sm Eu Gd Tb Dy

Isotope 89 93 139 140 141 143 147 151 155 159 163

Spot No. DMP141

04 1 043 0.60 0.13 19.3 0.76 11.2 11.2 1.80 33.8 9.50 102 16 607 0.88 0.025 17.4 0.35 4.44 4.08 0.75 18.9 5.15 56.0 01 1 081 0.96 0.27 22.2 0.82 11.2 12.8 1.74 36.8 10.0 106 11 593 0.54 0.046 12.8 0.076 1.18 3.32 1.18 11.2 3.37 45.0 14 1 243 1.28 0.000 30.2 0.22 3.06 8.57 1.00 37.6 9.69 120 25 1 764 1.73 0.86 24.0 0.92 11.6 11.2 1.90 52.0 14.9 160 20 1 367 1.31 0.10 28.8 0.78 8.47 11.7 1.34 38.5 11.6 124 18 1 385 1.14 0.12 30.6 0.96 14.8 13.5 1.41 41.3 12.8 126 03 869 0.41 0.14 15.7 0.79 11.2 8.43 1.24 34.5 9.09 89.5 24 1 501 1.40 0.17 28.4 0.87 10.1 11.3 0.97 41.3 11.5 140 19 1 324 1.48 0.049 38.0 0.33 5.20 9.59 0.84 34.8 10.2 120 07 1 447 1.29 0.59 24.8 1.09 10.9 12.0 1.79 44.6 11.9 135 22 790 0.99 0.058 21.6 0.22 3.77 5.86 0.61 23.8 6.66 74.4 27 0.022 21.0 0.067 1.09 2.30 1.38 14.5 5.27 70.9 05 1 073 1.06 0.095 20.9 0.72 11.7 14.9 1.43 36.4 9.59 108 21 2 044 2.45 0.17 31.4 0.74 11.9 16.2 1.89 59.1 17.7 192 06 651 1.72 0.20 23.5 0.27 4.27 5.01 0.60 18.2 5.22 61.1 10 2 146 18.0 0.049 64.2 0.22 9.57 15.2 0.17 73.6 21.5 246 08 309 1.19 0.046 14.4 0.053 0.79 1.03 0.24 5.52 1.64 21.9

Spot No. DMP146

07 4 664 4.33 0.037 19.1 0.64 12.0 22.8 1.08 117 37.6 450 01 2 715 1.57 0.007 0.97 0.100 1.76 4.59 0.21 37.8 15.4 221 22 1 816 2.31 0.021 41.0 0.32 7.73 15.2 8.00 70.7 17.7 186 32 2 378 6.17 0.31 8.97 0.33 3.17 5.22 0.61 30.0 13.0 168 06 3 200 3.42 0.28 5.99 0.81 8.44 11.8 0.88 63.4 22.6 292 14 900 3.50 0.12 37.2 0.085 1.07 2.22 1.09 13.1 4.50 61.1 08 1 257 0.96 0.014 0.96 0.16 4.11 9.76 0.36 49.0 13.5 132 31 2 776 13.2 0.029 32.8 0.19 2.22 7.79 0.78 53.7 18.3 241 03 1 338 1.55 1.43 14.5 0.50 4.55 7.13 1.51 30.0 9.37 111 17 810 1.24 0.19 6.23 0.12 0.93 3.00 0.30 11.5 4.51 58.5 13 1 762 4.72 0.029 12.0 0.096 1.80 4.51 0.21 30.7 11.2 148 30 2 420 1.52 2.87 9.90 1.05 9.20 10.7 0.61 52.7 17.6 215 28 873 3.57 2.27 15.0 0.60 3.63 2.20 0.29 10.5 3.80 57.3 18 2 767 16.5 23.5 0.23 3.39 9.35 1.56 55.9 19.3 239 29 1 894 1.01 0.022 2.11 0.18 2.46 6.04 0.31 34.6 12.3 158 10 2 645 5.17 0.078 14.6 0.088 1.49 3.63 0.89 31.4 14.2 204 26 685 1.86 0.095 24.6 0.088 1.27 1.38 0.56 7.76 3.02 41.8 24 1 492 2.20 0.17 22.2 0.21 3.07 5.31 1.48 28.4 9.42 121 23 1 985 4.34 39.6 150 19.8 122 50.1 12.0 78.9 15.9 161 21 1 147 2.76 3.79 37.6 2.22 13.8 11.6 2.42 26.3 7.09 82.6 16 1 232 1.47 0.001 4.15 0.062 0.94 1.90 0.16 16.9 6.86 96.1 11 560 1.70 0.15 27.3 0.12 1.68 1.39 0.93 9.62 2.92 37.4 25 1 070 0.73 0.90 15.4 0.34 2.88 2.90 1.37 21.2 6.61 85.2


Continued

Element Ho Er Tm Yb Lu Hf Ta Pb Th U Isotope 165 166 169 173 175 179 181 Total 232 238 Spot No. DMP141 04 32.7 132 25.8 257 43.9 8 369 0.36 5.93 150 106 16 20.9 85.2 18.1 186 33.1 8 297 0.40 5.44 134 120 01 33.6 143 29.3 298 45.5 8 889 0.34 8.08 184 135 11 19.2 85.7 19.6 217 40.6 8 658 0.24 1.02 23.4 49.4 14 38.3 166 35.2 376 56.9 11 477 0.85 11.4 331 265 25 57.1 228 45.5 453 71.6 7 902 0.96 17.0 401 246 20 40.5 187 39.8 387 64.7 9 517 1.03 16.8 409 354 18 45.6 202 41.4 438 64.5 10 311 0.85 14.8 345 302 03 29.3 118 24.7 248 39.4 7 872 0.42 5.41 133 98.0 24 48.3 205 42.7 429 69.2 10 060 1.01 15.7 431 341 19 41.8 181 40.2 407 66.2 12 618 0.96 22.9 501 436 07 45.6 194 40.7 400 67.4 8 797 0.79 14.1 317 230 22 24.2 112 22.9 241 40.1 10 574 0.59 8.54 154 146 27 29.0 160 37.2 445 90.9 13.3 100 283 05 35.7 139 28.8 292 46.9 9 003 0.44 13.8 180 134 21 65.3 276 56.9 570 90.9 9 527 1.20 13.6 549 348 06 21.1 89.8 20.4 210 33.3 9 403 0.64 10.5 224 203 10 79.1 314 62.1 561 83.9 7 340 4.11 71.5 986 646 08 7.79 40.7 9.66 111 18.8 10 112 1.14 222 156 370 Spot No. DMP146 07 149 644 120 1 168 173 9 540 1.83 58.6 809 1 064 01 77.7 386 86.4 970 149 13 876 1.94 57.7 183 3 031 22 53.4 222 40.8 449 75.8 8 810 0.34 29.7 306 349 32 65.5 344 77.8 900 149 12 612 8.70 140 982 4 881 06 94.9 417 79.9 789 116 10 413 2.22 57.3 524 1 186 14 24.1 130 29.7 357 64.2 9 333 0.92 12.4 102 169 08 35.3 143 25.7 248 37.6 9 467 0.39 28.2 145 707 31 85.1 390 77.1 798 121 10 380 3.81 269 1 381 1 676 03 37.7 178 34.4 373 62.2 8 117 0.78 51.5 232 409 17 23.1 116 25.2 283 49.4 9 722 0.95 47.5 146 462 13 53.4 254 47.7 487 72.4 10 470 2.14 93.0 263 604 30 73.1 331 64.3 660 98.4 10 016 0.79 47.6 165 255 28 22.9 128 29.7 360 64.5 8 895 4.65 130 304 703 18 78.3 359 71.6 762 121 9 645 5.58 752 1 665 4 789 29 57.1 278 58.1 649 101 11 169 0.96 48.2 106 488 10 77.4 395 86.5 958 149 11 230 3.95 175 286 1 404 26 17.0 98.9 24.4 307 58.8 8 783 0.85 226 377 713 24 41.4 206 42.8 476 79.3 9 559 1.37 234 473 732 23 53.4 264 56.0 629 106 10 454 3.67 1 902 963 2 817 21 30.9 157 35.6 406 66.9 14 326 4.49 577 217 1 657 16 36.6 186 38.9 411 66.0 10 366 1.01 219 88.5 282 11 14.0 76.4 18.2 225 41.4 10 244 0.74 791 307 720 25 31.5 154 31.6 350 58.3 8 178 0.30 203 72.2 101


Figure 7. Diagrams of 206Pb/238U-207Pb/235U and CI-normalized REE patterns of zircons from DMP141. Data-point error ellipses in U-Pb diagram are 2σ. CI. chondrite (Taylor and McLennan, 1985). than crystal accumulation (Fig. 5), which would oth-erwise lead to HREE enrichments. Their extremely high Sr contents and positive Eu anomalies suggest an origin of plagioclase-rich cumulate (Table 1, Fig. 6c).

The above observations together indicate that the garnet-rich granulite xenoliths are not high-pressure (>2–3 GPa) cumulates at the garnet stability field, but could be high-pressure metamorphic products of low-pressure cumulates (e.g., gabbro) after it had been depressed into the garnet stability field. The gabbroic cumulates could be products of magmatic underplat-

ing and mixing with the pre-existing lower crust at the Mesozoic, as discussed in the following. Constraints on the Time of Lithospheric Thicken-ing and Magmatic Underplating

Zhang et al. (2001a) proposed that the eastern part of the China continent was probably a plateau in the middle–late Yanshanian periods, which was up-lifted in the Middle–Late Jurassic and thinned after the Early Cretaceous. It has been suggested that the litho-spheric thinning in eastern China mainly took place


Figure 8. Diagrams of 206Pb/238U-207Pb/235U and CI-normalized REE patterns of zircons from DMP146. Data-point error ellipses in U-Pb diagram are 2σ. CI. chondrite (Taylor and McLennan, 1985).

Figure 9. Plots of U/Yb-Hf and Th/U-Th of zircons from DMP141 and DMP146. Dark and dashed lines in (a) are ranges of zircons from continental crust and oceanic crust (Grimes et al., 2007). during 131±2 to 117±7 Ma constrained by the giant igneous event (Wu et al., 2005). Our previous work demonstrates that the garnet-rich granulite xenoliths from the Hannuoba basalts record an increasing pressure-temperature trend (Liu et al., 2003), which could have resulted from the Mesozoic lithospheric

thickening event. Although the studied xenoliths contain abundant

garnet (>30%), most zircons from these xenoliths show no chemical features of zircons formed in the UHP metamorphism of eclogite from the Sulu area (e.g., low HREE contents and Lu/Gd ratio) (Gao et al.,


2010; Zong et al., 2010). Rubatto and Hermann (2007) demonstrated that zircon contains significantly more HREE than garnet at temperatures of 800–850 ℃. The HREE-rich REE patterns of most zircons are consis-tent with the relatively low temperatures estimated for the garnet-rich mafic granulites (768–835 ℃; Liu et al., 2003). The zircon of 134±4 Ma (1σ) in DMP146 has flat HREE patterns and relatively lower HREE contents (Fig. 8), a chemical feature of zircon being equilibrated with garnet at a relatively high tempera-ture. The rare occurrence of zircon with chemical fea-tures of high-temperature and high-pressure implies that (1) abundant garnets could have been formed be-fore 134 Ma and (2) overprint of a short high- temperature event at ~134 Ma (e.g., mantle-derived magmatic underplating) on the pre-existing garnet- rich granulite. In other words, the lithosphere could have been thickened before ~134 Ma in the Hannuoba area, and a thermal event caused by magmatic under-plating took place at ~134 Ma. The ~134-Ma mag-matic underplating event was confirmed by the pres-ence of igneous zircon of 131 Ma in DMP141 (Fig. 2). The bloom and co-existence of igneous zircons and metamorphic zircons of 102–134 Ma in the garnet- rich granulite xenoliths (Figs. 2, 3, 7 and 8) confirm our previous conclusion that granulite facies meta-morphism was induced by multiple-episodic mag-matic underplating based on igneous zircons from oli-vine pyroxenite (Liu et al., 2004). The consistent Mesozoic age range of magmatic underplating events in the central zone of the NCC and the occurrence of igneous rocks with adakite features (118–136 Ma) (Zhang et al., 2001b) and giant igneous event (131±2 to 117±7 Ma) in the east part of the NCC (Wu et al., 2005) suggest that, although there is no significant lithospheric thinning in the central zone of the NCC since the Mesoproterozoic as suggested by Re-Os iso-topes of spinel lherzolite xenoliths (Gao et al., 2002), the asthenosphere mantle in this area was disturbed by the Mesozoic lithospheric thinning in the east part of the NCC.

Mesozoic Crust-Mantle Interaction in the Central Zone of the NCC

Based mainly on zircon U-Pb ages of the granu-lite xenoliths, Fan et al. (1998), Liu et al. (2004, 2001)

and Zheng et al. (2009) suggested that the granulite xenoliths were products of the Mesozoic and Paleo-gene basaltic underplating and mixing with the pre-existing lower crust. However, Wilde et al. (2003), Jiang and Guo (2010), and Jiang et al. (2007) sug-gested that they are portions of residue of the ancient lower crust or cumulates from the ancient lower crust-derived granitic melt based on the presence of the Precambrian zircons and evolved Sr-Nd isotopic compositions of the xenoliths. Each of the two proc-esses alone cannot well explain the “paradoxical” iso-topic and chemical features and complex zircon U-Pb ages of the granulite xenoliths (Zheng et al., 2009; Liu et al., 2005, 2004; Wilde et al., 2003; Zhou et al., 2002). Although the residue model can explain the presence of Precambrian zircons in some xenoliths, it cannot explain the following observations: (1) 87Sr/86Sr ratios of the granulite xenoliths are much lower than those of most of the Archean terrain granu-lites from the NCC (Fig. 6a), (2) Sr-Nd isotopic com-positions of most granulite xenoliths are completely different from those of the granulite xenoliths formed at the Precambrian (Fig. 6) (Liu et al., 2004) and (3) many granulite xenoliths have remarkably higher Mg# than those of the terrain granulites at the same SiO2 level (Fig. 10) (Liu et al., 2005). Based on the experi-mental results of Pertermann and Hirschmann (2003), mass balance calculations indicate that, although res-tites left after partial melting of the ancient lower crust would have increased Mg# compared to the parent rocks, the increments of Mg# in the restites (average= 1.92±0.92 (2σ) for melting degree of 10%–40%) are too small to account for the high Mg# of the granulite xenoliths unless the parent rocks have high Mg# (Fig. 10b). Underplating model requires either a high pro-portion of contamination by ancient crust if the parent melt was derived from depleted mantle peridotite as represented by the peridotite xenoliths (Rudnick et al., 2004), or a Mesozoic basalt-type parent melt with relatively evolved Sr-Nd isotopic compositions and high Mg# (Gao et al., 2008; Liu et al., 2008a).

More and more evidence shows that ancient re-cycled crust in the mantle could have played an im-portant role in yielding the enriched mantle compo-nents (Gao et al., 2008; Liu et al., 2008a; Xu et al., 2008; Ishikawa et al., 2007). The relatively high


Figure 10. Plots of Mg#-SiO2 of granulite xenoliths and garnet pyroxenites formed by peridotite-melt reac-tion. Worldwide granulite xenoliths (http://www.EarthRef.org), Archean terrain granulites were from NCC (Liu et al., 1999), eclogite residue of MORB (Pertermann and Hirschmann, 2003) and >110 Ma Mesozoic basalts (Liu et al., 2008a) and high-Mg andesites (Gao et al., 2004) were shown for comparison. The Han-nuoba granulite xenoliths include data of this work and Zhang et al. (1998), Liu et al. (2001) and Zhou et al. (2002), and garnet pyroxenites are from Liu et al. (2005).

87Sr/86Sr ratios of some garnet pyroxenite xenoliths (Xu, 2002) and variable Li isotopes of peridotite xenoliths (Tang et al., 2007) from the Hannuoba ba-salts suggest contributions from the Late Jurassic– Early Cretaceous subduction of altered oceanic crust of the Mongol-Okhotsk plate. To explain the “para-doxical” high-Mg# and high-SiO2 features of many granulite xenoliths, Liu et al. (2005) speculated that parent melts of the granulite xenoliths are partial melts of basaltic layers that were previously subducted (a fossil oceanic slab) or underplated into the base of the lithospheric mantle, and their high-Mg# inherited from interaction with the mantle peridotite. Recently, con-tinental crust-derived Precambrian zircons were found in garnet/spinel pyroxenite veins hosted in lherzolite (Liu et al., 2010a). All these works demonstrate that the ancient recycled crustal rocks have played an im-portant role in yielding the enriched mantle compo-nents beneath the Hannuoba area.

The partial melt of the ancient recycled crust in the mantle would have evolved Sr-Nd isotopic com-position and has been used to explain the evolved Sr-Nd isotopic compositions of the >110 Ma Meso-

zoic basalts (Gao et al., 2008; Liu et al., 2008a) and high-Mg# adakitic rocks (Gao et al., 2004) from the NCC. The Hannuoba granulite and gabbro xenoliths fall in the trend defined by the >110 Ma Mesozoic ba-salts and high-Mg# adakitic rocks in the plot of εNd-87Sr/86Sr, but show lower εNd(130 Ma) values (Fig. 6). They overlap with the >110 Ma Mesozoic basalts and high-Mg# adakitic rocks in the plot of Mg#-SiO2 (Fig. 10), but show much larger range of CaO (4.5 wt.%–20 wt.%), Al2O3 (6.4 wt.%–19 wt.%) and SiO2 contents (42 wt.%–62 wt.%) (Liu et al., 2005, 2001). These features of the granulite xenoliths can be well explained by assimilation and fractionation crystalli-zation (AFC) of recycled ancient crust-associated high-Mg# melt (e.g., the >110 Ma Mesozoic high-Mg# basalts and andesites) underplated at the base of the lower crust. The wide ranges of SiO2, CaO, Al2O3 and Sr contents and positive Eu anomalies (Figs. 4 and 5) are consistent with variations from plagioclase- dominated cumulates to clinopyroxene-dominated cumulates, as suggested by Liu et al. (2001). Further-more, the wide age spectrum of the granulite and py-roxenite xenoliths (Zheng et al., 2009; Liu et al., 2004,


2001; Wilde et al., 2003) can be interpreted by recy-cled ancient crust-associated melt as well, as demon-strated by those zircon xenocrysts found in the garnet pyroxenite veins hosted in spinel lherzolites (Liu et al., 2010a).

Thus, based on previous and this work, we pro-pose that some granulite xenoliths were ever portions of residue of the ancient lower crust after lithospheric thinning (e.g., G1 and G2 granulite xenoliths de-scribed by Liu et al., 2004); the others are products of high-Mg# magmatic underplating and mixing with the pre-existing lower crust at the Mesozoic and Paleo-gene. Although it cannot be excluded that some of the melts were derived from peridotite, most of them could be partial melts of Archean lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite (Gao et al., 2008), or partial melts of basaltic layers that were previously subducted (a fossil oceanic slab) or underplated into the base of the lithospheric mantle (Liu et al., 2005). CONCLUSION

This work shows that garnet-rich granulite xeno-liths from the Neogene Hannuoba basalt are high-pressure metamorphic products of low-pressure cumulates (e.g., gabbro) after it had been depressed into the garnet stability field rather than portions of ancient lower crust. A few of the Mesozoic igneous zircons from the garnet-rich granulite xenoliths pro-vide direct evidence for the Mesozoic underplating event under the Hannuoba area.

However, neither peridotite-derived basaltic un-derplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonably explain the complicated petrologic and chemical fea-tures of the granulite xenoliths. Combined with the previous research, we suggest that most of the granu-lite xenoliths from the Hannuoba basalts are products of high-Mg# magmatic underplating and mixing with the pre-existing lower crust at the Mesozoic. The par-ent melts could be mostly derived from ancient litho-spheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite, or from basaltic layers that were previously subducted or un-derplated into the base of the lithospheric mantle.

ACKNOWLEDGMENTS Prof. Wenliang Xu and Dr. Neng Jiang are highly

thanked for the constructive comments and sugges-tions to improve this manuscript. We are grateful to Dr. Hujun Gong for the assistance with CL imaging.

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Appendix 1. Photographs showing the rocks microstructure and mineral assemblage of from Damaping

The brown-black grains are kelyphite of garnet, the grey white grains are plagioclase, the others are clinopyroxene

and orthopyroxene.

Garnet-rich granulite xenoliths from the Hannuoba basalts, North China: Petrogenesis and...

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