447

Geochemical Journal, Vol. 40, pp. 447 to 461, 2006

*Corresponding author (e-mail: xuancewang@gig.ac.cn)

Copyright © 2006 by The Geochemical Society of Japan.

Mesozoic adakites in the Lingqiu Basin of the central North China Craton:Partial melting of underplated basaltic lower crust

XUAN-CE WANG,1,2,3,4* YONG-SHENG LIU2 and XIAO-MING LIU3

1Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640, China

2State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,Wuhan 430074, China

3Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China4Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

(Received June 14, 2005; Accepted February 16, 2006)

Intermediate to felsic volcanic rocks of the Baiqi formation from the Lingqiu basin in the central part of North ChinaCraton were studied. Single zircon U-Pb dating indicates that these volcanics formed at 125.8 ± 3.0 Ma. Their Sr and Ndisotopic compositions (143Nd/144Nd = 0.51180–0.51182, 87Sr/86Sr = 0.7062–0.7063) fall in the range of the nearby late-Mesozoic basaltic rocks. These volcanics share geochemical affinities to the adakites formed in the modern arcs, e.g.,high Na2O (>4.06%), Al2O3 (>15.4%) and Sr (645–1389 ppm) contents and Sr/Y ratios (55~103), and thus being termedas adakitic rocks. However, the Baiqi adakitic rocks were not temporospatially associated with active subduction. Further-more, their low Cr (2.19–47.4 ppm, with average of 25) and Ni (1.57–20.7 ppm, with average of 12) contents and Mg#(22–47, with average of 32) argue against interaction with the lithospheric mantle. Combined with the geological setting,we suggest that the Baiqi adakitic rocks resulted from partial melting of a thickened lower continental crust associatedtwo episodes of basaltic underplating events. We propose that enormous conductive heating from 80–140 Ma basalticunderplating resulted in partial melting of pre-existing mafic lower crust formed by ~150–160 Ma basaltic underplating.This study provides a case for partial melting of the thickened lower continental crust in association with basalticunderplating events.

Keywords: adakitic rocks, basaltic underplating, lower crust, Lingqiu, North China

Rapp et al., 1999), and thicken continental crust (Wang,Q. et al., 2005). Adakites in subduction zones have beenwidely investigated (Defant and Drummond, 1990; Mar-tin, 1999; Martin et al., 2005; Peacock et al., 1994; Sternand Kilian, 1996), whereas the adakitic rocks within thecontinent are yet to be fully understood (Gao et al., 2004;Liu et al., 2005; Wang, Q. et al., 2005).

Many adakitic rocks in the Eastern China were foundand studied in the last five years (e.g., Gao et al., 2004;Xiao et al., 2004; Xu et al., 2002; Zhang et al., 2001a, b).These adakitic rocks are predominately formed in lateJurassic to early Cretaceous (160–120 Ma, peak at 137–130 Ma). Zhang et al. (2001b) classified adakitic rocksinto two types: O-type adakites (typical adakites, relatedto the slab subduction) and C-type adakites (producedwithin intracontinent). Xiao et al. (2004) classified theC-adakites from North China Craton into type A and typeB. The type A rocks (most of C-type adakites, not all)have lower Mg# number and higher K contents than typi-cal subduction-related adakites, which could be producedby partial melting of underplated basaltic rocks (Xiao etal., 2004; Zhang et al., 2001a, b), whereas the type B

INTRODUCTION

Adakite is characterized by low K, Y, HREE contents(Y ≤ 18, Yb ≤ 1.9 ppm), and high Al, Na (Al2O3 >15% atthe 70% SiO2; Na2O/K2O >1), Sr (>400 ppm) contents,and high Sr/Y and La/Yb ratios (Defant and Drummond,1990). Partial melting of basaltic rocks can produceadakitic melt at pressures equivalent to a crustal thick-ness of >40 km (Rapp and Watson, 1995; Rapp et al.,1991). This rock attracts widespread attention due to itssignificance in revealing deep geodynamic processes, e.g.,oceanic crust subduction (Defant and Drummond, 1990;Martin et al., 2005; Martin, 1999; Peacock et al., 1994;Stern and Kilian, 1996), basalt underplating (Atherton andPetford, 1993; Petford and Atherton, 1996; Rapp andWatson, 1995; Xiong et al., 2003), recycling of lowercontinental crust (Gao et al., 2004; Kay and Kay, 1991;Xu et al., 2002), melt-peridotite reaction (Liu et al., 2005;

448 X.-C. Wang et al.

rocks featured by high Mg# were interpreted as founderinglower continental crust-derived melt interacted withperidotites in the mantle (Gao et al., 2004; Xiao et al.,2004). Despite these investigations, some questions re-main: how were these adakitic rocks related to thefoundering lower continental crust or basalt underplating?And what is the mechanism triggered the surge of theMesozoic adakites in the North China Craton? To addressthese questions, typical adakitic volcanics from the Trans-North China Orogen, central North China Craton werestudied in this paper. Except for some adakitic intrusiverocks, typical adakitic volcanic rocks have not been re-ported in the central of North China Craton.

GEOLOGICAL SETTING

The North China Craton is one of the oldest continen-tal nuclei in the world, with basement of mainly Archeanto Early Proterozoic gneisses (Jahn et al., 1988). Basedon isotopic age, lithological assemblage, tectonic evolu-tion and P-T-t paths, the North China Craton can be di-vided into the Eastern Block, the Western Block and theintervening Trans-North China Orogen (Zhao et al., 2000,2001) (Fig. 1B). This craton underwent a dramatic changefrom a Paleozoic cratonic mantle to a Cenozoic “oceanic”lithospheric mantle, accompanied by lithospheric thinning(Gao et al., 2002; Griffin et al., 1998; Menzies et al.,1993; Rudnick et al., 2004; Wu et al., 2003; Xu, 2001),lower crustal recycling (Gao et al., 2004) and widespread

Mesozoic magmatism (e.g., Davis et al., 1998; Chen etal., 2002, 2003, 2004; Chen and Zhai, 2003; Zhang et al.,2004).

The adakitic rocks in this study were collected fromthe Lingqiu volcanic basin in the northern part of theTrans-North China Orogen. The late Mesozoic volcanicsin the Hunyuan-Guangling-Lingqiu basins (Figs. 1A andC) are composed of intermediate to felsic volcanics withinterlayers of sandstones and mudstones. These volcanicswere classified into four formations (Fig. 1C): from bot-tom to top, Baiqi, Zhangjiakou, Dabeigou and Xiguayuan.The Baiqi formation was systematically studied in thiswork. The emplacement age of diorite-granite-rhyoliteand tuff lava of the Trans-North China Orogen has beendated at 127–138 Ma using U-Pb zircon and Rb-Sr whole-rock isochron methods (Cai et al., 2003; Davis et al.,1998; Peng et al., 2004). Several episodes of Mesozoicmafic igneous rocks in the Trans-North China Orogenhave also been identified at 150–160 Ma (e.g. ,Yunmengshan gabbro-diorite complex, Davis et al., 1998;Hanxing gabbro, Zhang et al., 2004), 135–145 Ma (e.g.,Laiyuan gabbro, Zhang et al., 2004) and 120–130 Ma(e.g., Linxian gabbro, Wang, Y. J. et al., 2005). Laiyuanlocate in the eastern of the Lingqiu basin with a distanceof ~80 km, and Linxian in the southeastern with a dis-tance of 300 km. Lower crustal xenoliths found in theNeogene Hannuoba basalts adjacent to the studied areaare dated at two age intervals of ~160–140 Ma and ~140–80 Ma (Liu et al., 2004; Wilde et al., 2003), and they

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Fig. 1. (a) Simplified geological map of the study area. (b) Tectonic division of the North China Craton (After Zhao et al., 2001).Locations of the Mesozoic gabbros and lower crustal xenoliths in the Trans-North China Orogen were also marked. 1 = Locationof the Baiqi adakitic rocks, 2 = gabbro from Laiyuan (Zhang et al., 2004), 3 = gabbro from Hanxing (Zhang et al., 2004), 4 =gabbro from Linxian (Wang, Y. J. et al., 2005), 5 = lower crustal xenolths from the Hannuoba, 6 = Xinglonggou high-Mg adakitesFrom Liaoxi (Gao et al., 2004). (c) Profile of the Mesozoic volcanics in Hunyuan-Guangling-Lingqiu basin.

Mesozoic adakites: Partial melting of underplated basalts 449

were interpreted as products of ~160–140 Ma basalticunderplating and subsequent ~140–80 Ma granulite-faciesmetamorphism (Liu et al., 2004). The xenoliths with con-vex-upward REE patterns had a cumulate origin (gabbroor pyroxenite) (Liu et al., 2001). The Mesozoic Hannuobagranulite xenoliths was products of basaltic underplatinghas been widely accepted (e.g., Chen et al., 2001; Fan etal., 1998, 2001; Liu et al., 2001, 2004; Zhang et al., 1998;Zhou et al., 2002).

The mafic igneous rocks from the Trans-North ChinaOrogen were derived from a special mantle metasomatisedby a SiO2-rich melt (Chen et al., 2004; Wang, Y. J. et al.,2005; Zhang et al., 2004). Metasomatism lower thesolidus significantly, triggered intensely melting of theenriched sub-continental lithospheric mantle. The SiO2-rich melt may be derived from the paleo-Pacific subductedslab (Chen et al., 2004), or the Paleoproterozoic subduc-tion slab during the collision between the Eastern andWestern Blocks of the North China Craton along theTrans-North China Orogen (Wang, Y. J. et al., 2005), orbasaltic layers that were previously subducted (a fossiloceanic slab) or underplated into the base of thelithospheric mantle (Liu et al., 2005).

SAMPLES AND ANALYTICAL METHODS

SamplesAdakitic rocks from the Baiqi formation were system-

atically sampled at the Ganhegou-Matou Mountain pro-file in the Lingqiu basin (Fig. 1A). This profile is com-posed of minor andesites and abundant dacites with mi-nor amount of rhyolites. Andesite contains phenocrystsof plagioclase, brown amphibole and pyroxene. In mostdacite specimens, plagioclase is the main phenocrysts andaccompanied by brown amphibole and mica set in agroundmass. The rhyolites contain quartz and plagioclasemicrophenocryst within perlitic groundmass. Fresh sam-ples were only selected for whole rock analyses based ondetailed petrological studies.

Analytical methodsHand picked zircon grains were mounted in epoxy

blocks, polished to obtain an even surface, and cleanedin an acid bath prior to LA-ICPMS analysis. Zircon U-Th-Pb measurements were made on 30–40 µm diameterspots of single grain using the Laser Ablation-InductivelyCoupled Plasma Mass Spectrometry (LA-ICPMS) at theKey Laboratory of Continental Dynamics, NorthwestUniversity in Xi’an, and the analytical procedures aresimilar to those described by Yuan et al. (2004). The datawere processed and plotted using Isoplot 3.0 (Ludwig,2003). Common Pb were corrected following the methodof Andersen (2002). Age uncertainties are quoted at the95% confidence level. Measurements of zircon standard

TEMORA 1 as an unknown in this LA-ICPMS over aperiod of 16 months yielded a weighted mean 206Pb/238Uage of 415 ± 4 Ma (MSWD = 0.112) (Gao et al., 2004;Yuan et al., 2004), which is in good agreement with therecommended ID-TIMS age of 416.75 ± 0.24 Ma (Blacket al., 2003).

Fresh chips of whole rock samples were powdered to200 meshes using a tungsten carbide ball mill. Major andtrace elements were analyzed using XRF (Rikagu RIX2100) and ICP-MS (PE 6100 DRC), respectively at theKey Laboratory of Continental Dynamics, NorthwestUniversity in Xi’an, China. Analyses of USGS and Chi-nese national rock standards (BCR-2, GSR-1 andGSR-3) indicate that both analytical precision and accu-racy for major elements are generally better than 5% (Ap-pendix 1). For trace element analysis, sample powderswere digested using HF + HNO3 mixture in high-pres-sure Teflon bombs at 190°C for 48 hours. Analytical pre-cision and accuracy are better than 2% and 10%, respec-tively, for most of the trace elements except for transi-tion metals (Appendix 2).

Sr-Nd isotopes were determined at the China Univer-sity of Geosciences (Wuhan), following the proceduresdescribed by (Ling et al., 2003). A mixture solution of84Sr, 85Rb, and 145Nd and 149Sm isotope spikes wasweighted and added to each sample aliquots. Rb, Sr andREE were separated using cation columns; Sm and Ndfractions were further separated by HDEHP-coated Kefcolumns. The measured 143Nd/144Nd and 87Sr/86Sr ratioswere normalized to 146Nd/144Nd = 0.7219 and 88Sr/86Sr =8.375209, respectively. The La Jolla standard measuredduring the course of this study gives an average 143Nd/144Nd = 0.511862 ± 5 (2σ, n = 15), the BCR-2 gives 143Nd/144Nd = 0.512635 ± 4 (2σ, n = 6), Nd = 29.10 ppm andSm = 6.591 ppm; and the NBS-987 gives 87Sr/86Sr =0.710236 ± 16 (2σ, n = 6).

RESULTS

U-Pb Zircon ageZircons from a dacite sample GHG02 from the Baiqi

formation were separated for U-Pb dating. Zircon grains,together with a zircon U-Pb standard (TEMORA 1), werecast in an epoxy mount, and then were documented withCathodoluminescence (CL) images. In contrast to the pre-dominantly inherited zircons from those adakites derivedfrom melting of recycling lower continental crust (Gaoet al., 2004), most zircons from the Baiqi formation areeuhedral and show typical igneous oscillatory zonation(Fig. 6B). Thirteen zircons were analyzed, and they haverelatively high Th/U ratios of 0.65 to 2.5 (Table 3). Themeasured 206Pb/238U ratios are in good agreement withinanalytical precision, yielding a mean 206Pb/238U age of125.8 ± 3.0 Ma (95% confidence interval).

450 X.-C. Wang et al.

Major and trace elementsThe Baiqi volcanic rocks range from andesite to

rhyolite with SiO2 = 57–71% (Table 1). Their major ele-ment compositions are characterized by high Na2O (4.06–6.89%) and Al2O3 (≥15.4%) and high Na2O/K2O ratio(1.06–5.30), similar to those of the partial melts of meta-

morphic basaltic rocks (Fig. 2). SiO2 correlates positivelywith MgO and FeOT with correlation coefficients of 0.80and 0.92, respectively (Figs. 2B and C).

The Baiqi volcanic rocks have high Sr contents (645–1389 ppm), and high Sr/Y (55–103), LaN/YbN ratios (17–38; where subscript N denotes chondrite normalization),

Sample GHG16 GHG15 GHG03 GHG05 GHG14 GHG13 GHG06 GHG09

Major elements (%)

SiO2 64.60 63.30 65.00 64.20 59.70 57.30 60.30 70.50TiO2 0.79 0.71 0.71 0.86 0.90 0.97 0.84 0.23Al2O3 16.00 16.50 15.40 14.60 15.90 16.70 16.10 15.40Fe2O3

total 4.78 5.23 5.14 6.09 6.97 7.13 6.41 2.23MnO 0.03 0.05 0.07 0.06 0.07 0.10 0.07 0.04MgO 0.70 0.90 1.06 0.98 2.20 3.24 2.45 0.33CaO 3.02 2.97 3.21 3.45 3.76 4.30 3.72 1.43Na2O 5.63 4.97 5.78 5.68 4.12 6.89 4.06 5.63K2O 2.59 3.26 1.60 2.77 3.89 1.30 3.80 3.00P2O5 0.42 0.38 0.38 0.41 0.44 0.43 0.41 0.10Total 99.60 99.80 99.90 100.00 99.90 100.00 99.90 99.90Na2O/K2O 2.17 1.52 3.61 2.05 1.06 5.3 1.07 1.88Mg#(a) 22 25 29 24 38 47 43 23

Trace elements (ppm)

Cr 39.9 15.8 11.0 27.4 34.6 47.4 20.1 2.19Co 49.0 30.2 44.3 60.7 40.3 38.1 32.3 34.5Ni 14.0 8.85 7.05 12.5 18.6 20.7 12.1 1.57Rb 51.3 69.2 39.8 47.4 72.7 40.9 80.2 48.3Sr 1389 1129 1140 1205 1198 1077 1225 645Y 13.5 11.9 13.0 13.4 14.7 16.2 14.5 11.2Zr 178 163 157 156 157 155 164 176Nb 11.4 9.69 9.53 9.27 9.63 9.41 10.3 12.6Ba 1681 1699 770 1791 2015 470 2056 1488La 49.3 41.0 42.1 39.7 40.4 42.0 45.0 53.2Ce 87.3 70.6 76.4 74.8 74.8 77.2 80.6 92.4Pr 10.1 8.35 8.71 8.77 8.97 9.19 9.54 9.91Nd 38.2 32.4 33.2 34.6 35.5 37.3 37.4 34.2Sm 5.43 4.72 5.01 5.23 5.42 5.68 5.46 4.40Eu 1.71 1.53 1.50 1.70 1.82 1.71 1.80 1.24Gd 4.87 4.31 4.54 4.78 4.96 5.25 5.04 4.12Tb 0.52 0.48 0.51 0.53 0.55 0.60 0.55 0.42Dy 2.40 2.32 2.43 2.50 2.68 2.89 2.59 2.00Ho 0.39 0.40 0.42 0.43 0.47 0.51 0.44 0.34Er 1.02 1.05 1.12 1.11 1.23 1.34 1.16 0.98Tm 0.14 0.15 0.16 0.15 0.18 0.19 0.16 0.14Yb 0.88 1.02 1.00 0.96 1.14 1.25 1.04 0.98Lu 0.13 0.15 0.16 0.14 0.18 0.20 0.16 0.16Hf 4.29 3.93 3.99 3.73 3.76 3.99 4.04 4.60Ta 0.65 0.60 0.63 0.55 0.54 0.56 0.59 0.80Pb 18.6 15.6 16.3 8.44 13.1 11.5 14.1 19.5Th 5.51 5.00 4.84 4.44 4.33 4.27 5.08 6.84U 1.21 1.18 1.25 0.89 0.85 1.98 1.27 1.36Rb/Sr 0.04 0.06 0.03 0.04 0.06 0.04 0.07 0.07(La/Yb)N

(b) 38 27 29 28 24 23 30 37Sr/Y 103 95 88 90 81 66 85 58

Table 1. Chemical composition of the Baiqi adakitic rocks

(a)Mg# = 100∗Mg/(Mg + Fe), in atomic number.(b)Subscript N denotes chondrite normalization.

Mesozoic adakites: Partial melting of underplated basalts 451

and low heavy rare-earth element (HREE; Yb ≤ 1.8 ppm),Y contents (≤18 ppm) (Table 1), resembling those of thetypical adakite (Defant and Drummond, 1990). In theprimitive mantle-normalized trace element spidergram,they display clear depletion in Nb-Ta and pronouncedenrichment in Pb relative to neighboring elements. Inaddition, they exhibit clear positive Sr anomaly (Fig. 3B).Their rare earth element (REE) patterns are characterizedby LREE enrichment, relatively flat HREE distribution

and insignificant Eu anomaly (Fig. 3A). Their very lowRb/Sr ratios (0.03 to 0.07) suggest little, if any, fractionalcrystallization of plagioclase (Xiong et al., 2003). In theSr/Y-Y diagram, the Baiqi volcanic rocks fall into the fieldof typical adakites and TTG, contrasting the arc andesiteand dacites (Fig. 4B). All these indicate that the Baiqivolcanics belong to adakite in terms of geochemical fea-tures (Defant and Drummond, 1990). Thus, we term theBaiqi volcanics as adakitc rocks in the following discus-

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Fig. 2. Geochemical comparison of the Baiqi adakitic rocks with partial melts of basalts. R1 = Partial melts of alkali basalt at12~38 kbar (Rapp, 1995; Rapp et al., 1991, 1999; Rapp and Watson, 1995); R2–4SW = Partial melts of basalts compositionallyclose to N-MORB (Rapp and Watson, 1995; Rapp et al., 1991; Sen and Dunn, 1994; Winther, 1996); WW = Partial melts of low-K and Na, high-Mg and Ca basalt (Wolf and Wyllie, 1994). The arc-related adakites (grey field) following Xiong et al. (2003).

452 X.-C. Wang et al.

sion in order to distinguish them from the typical adakitesformed by slab melting in modern arc settings.

Whole rock Sr-Nd isotopic compositionsThe Baiqi adakitic rocks are characterized by constant

initial Nd and Sr isotopic compositions at T = 126 Ma:εNd (T) = –14.4 to –14.8 and (87Sr/86Sr)i = 0.7060~0.7061(Table 2). We noticed that their Nd and Sr isotopic com-positions are consistent with those of the MesozoicHannuoba granulites (87Sr/86Sr = 0.706–0.707, εNd (130Ma) = –18 to –12, Liu et al., 2004) and the late-Mesozoicbasaltic magmatism from North China Craton ((87Sr/86Sr)i= 0.704~0.708, εNd (T) = –17 to –12) (Cai et al., 2003;Chen et al., 2004; Wang, Y. J. et al., 2005; Zhang et al.,2004) (Fig. 5), and clearly differ from the MORB-likeisotopic compositions of the modern arc-related adakites(e.g., Kay and Kay, 1993; Stern and Kilian, 1996).

DISCUSSIONS

The case of thickened lower continental crust-derivedadakitic rocks

The significances of adakitic rocks rest with itsgeochemical indicator for high-pressure processes at sta-bility field of garnet, most probably under eclogite-faciesconditions. Various hypotheses have been put forward toexplain the origin of the adakitic magmas. These includeformation by partial melting of subducted oceanic slab(e.g., Defant and Drummond, 1990; Martin, 1999; Mar-tin et al., 2005), partial melting of thickened lower conti-nental crust (Atherton and Petford, 1993; Petford andAtherton, 1996; Rapp and Watson, 1995; Xiong et al.,2003; Wang, Q. et al., 2005) or partial melting of recy-

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Fig. 3. Chondrite-normalized REE patterns and Primitive man-tle (PM)-normalized spider diagrams for the Baiqi adakiticrocks. The mean values of adakites are from Condie (2005).The Xinglonggou high-Mg adakites (Gao et al., 2004) were alsoshown for comparison. Chondrite and PM values are from Sunand McDonough (1989).

Fig. 4. (La/Yb)N-YbN and Sr/Y-Y plots (following Defant andDrummond, 1990) of the Baiqi adakitic rocks. The solid linesrepresent batch melting model assuming 150–160 Ma gabbrosfrom Hanxing (Zhang, H. F., 2005, unpublished data) as start-ing material, and numbers show the partial melting degrees(%). Parameters used for calculations are listed in Table 4. Aand B represent melts in equilibrium with 50%Cpx + 50%Grtand 20%Cpx + 80%Grt, respectively.

Mesozoic adakites: Partial melting of underplated basalts 453

Table 2. Sr-Nd isotopic compositions of the Baiqi adakitic rocks

143Nd/144Nd ratios have been adjusted relative to the La Jolla standard = 0.511860.Dupl. = duplicate sample.

Sample Nd Sm 143Nd/144Nd 143Nd/144Nd ±2σ 147Sm/144Nd εNd

(ppm) (ppm) (126 Ma) (126 Ma)

GHG03 34.8 5.48 0.511799 0.51172 5 0.09517 –14.6GHG03_Dupl. 34.5 5.43 0.511794 0.51171 5 0.09508 –14.7GHG16 38.5 5.69 0.511796 0.51172 5 0.09519 –14.7GHG07 43.8 7.54 0.511817 0.51173 5 0.10396 –14.4GHG13 39.4 6.34 0.511794 0.51171 5 0.09831 –14.8

Sample Sr Rb 87Sr/86Sr 87Sr/86Sr ±2σ 87Rb/86Sr

(ppm) (ppm) (126 Ma)

GHG03 1194 37.9 0.70619 0.70603 11 0.0918GHG03_Dupl. 1192 37.9 0.70620 0.70603 9 0.0920GHG16 1390 50.9 0.70620 0.70603 12 0.0916GHG07 1299 34.6 0.70620 0.70606 8 0.0769GHG13 1090 36.5 0.70631 0.70613 11 0.0967

Spots Th/U *Pb 207Pb/206Pb±1σ

207Pb/235U±1σ

206Pb/238U 208Pb/232Th 207Pb/235U±1σ

206Pb/238U±1σ

208Pb/232Th±1σ

(%) age (Ma) age (Ma) age (Ma)

10# 0.65 1 0.052 ± 0.004 0.144 ± 0.011 0.020 ± 0.00031 0.006 ± 0.00019 137 ± 10 129 ± 2 128 ± 1

16# 0.71 0.047 ± 0.006 0.129 ± 0.015 0.020 ± 0.00043 0.006 ± 0.00028 124 ± 14 128 ± 3 128 ± 6

17# 1.22 0.051 ± 0.003 0.133 ± 0.007 0.019 ± 0.00027 0.007 ± 0.00011 127 ± 6 121 ± 2 131 ± 2

18# 1.27 0.052 ± 0.002 0.143 ± 0.006 0.020 ± 0.00026 0.006 ± 0.00010 135 ± 5 127 ± 2 125 ± 2

21# 1.85 0.049 ± 0.001 0.137 ± 0.004 0.020 ± 0.00022 0.006 ± 0.00006 131 ± 3 130 ± 1 123 ± 1

22# 1.23 0.17 0.053 ± 0.002 0.149 ± 0.006 0.018 ± 0.00027 0.006 ± 0.00011 141 ± 6 130 ± 2 131 ± 2

27# 2.5 0.046 ± 0.001 0.129 ± 0.003 0.019 ± 0.00021 0.006 ± 0.00005 123 ± 3 130 ± 1 121 ± 1

29# 1.85 0.047 ± 0.001 0.128 ± 0.004 0.020 ± 0.00022 0.006 ± 0.00006 122 ± 3 126 ± 1 111 ± 1

34# 1.47 0.047 ± 0.002 0.128 ± 0.006 0.020 ± 0.00027 0.006 ± 0.00009 122 ± 6 125 ± 2 114 ± 2

37# 2.08 0.047 ± 0.001 0.134 ± 0.004 0.021 ± 0.00023 0.006 ± 0.00006 127 ± 4 131 ± 1 119 ± 1

41# 1.36 0.05 ± 0.003 0.133 ± 0.007 0.019 ± 0.00030 0.005 ± 0.00015 127 ± 7 124 ± 2 109 ± 3

42# 1.11 0.051 ± 0.003 0.131 ± 0.008 0.019 ± 0.00030 0.005 ± 0.00013 125 ± 7 120 ± 2 110 ± 3

43# 1.45 0.047 ± 0.002 0.126 ± 0.005 0.019 ± 0.00024 0.006 ± 0.00007 120 ± 4 123 ± 2 112 ± 1

Table 3. U-Pb isotopic compositions of zircons from the Baiqi formation

*Pb indicate common Pb (corrected by ComPbCorr#3_151, Andersen, 2002).

cling lower continental crust in the mantle (e.g., Gao etal . , 2004; Xu et al . , 2002), and/or assimilation-fractionation-crystallization (AFC) process (e.g., Feeleyand Hacker, 1995). Either partial melting of subductedoceanic slab or AFC of a basaltic magma is unlikely forthe Baiqi adakitic rocks based on the following observa-tions.

First, zircon U-Pb dating indicates that the Baiqiadakitic rocks formed at about 126 Ma (Fig. 6A), whenthere was no active subduction zone at the northern mar-gin of the North China Craton. Furthermore, the Baiqi

formation locates in the interior of the North ChinaCraton, >1500 km away from the subduction zone of Pa-cific oceanic crust, and thus no significant contributionfrom the subducted Pacific oceanic slab. Although geo-physical data implies a possible old large flat subductedslab beneath the studied area (Chen et al., 2005; Pei etal., 2005), these adakitic rocks present no melt-peridotiteinteraction feature (e.g., high Mg#) of slab-derived meltstraversed part of the mantle peridotite. Second, fractionalcrystallziation of basaltic magma is an inefficient mecha-nism for felsic rocks (Green and Ringwood, 1968). Frac-

454 X.-C. Wang et al.

petrology demonstrate that partial melting of dryperidotite produces basaltic melt (e.g., Falloon and Green,1987; Falloon et al., 1988; Hirose and Kushiro, 1993;Kushiro, 2001; Walter, 1998), and partial melting of hy-drous peridotite can produce high Mg# andesitic melt(Hirose, 1997). On the other hand, dehydration meltingof mafic lower crust is known to be capable of givingrise to felsic melts. It remains to resolve whether or notadakitic rocks can be produced by this kind of anatexis inpost-orogenic tectonic settings. Nevertheless, the remark-ably homogeneous Sr-Nd isotopic compositions for theBaiqi adakitic rocks preclude the possibility that they wereproduced by the melting of orogenic lithospheric keel thathas sandwich-layers composed of felsic and mafic-ultramafic rocks like those in the Dabie orogen (Zhao etal., 2004, 2005).

As suggested by experiments (Rapp and Watson,

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Fig. 5. 87Sr/86Sr (125 Ma) – εNd(125 Ma) plot for the Baiqiadakitic rocks. Gabbros from Laiyuan, Hanxing, Linxian andXishu (Cai et al., 2003; Chen et al., 2004; Wang, Y. J. et al.,2005; Zhang et al., 2004), lower crustal xenoliths from Nushan(Huang et al., 2004; Yu et al., 2003), granulite xenoliths fromHannuoba (Liu et al., 2004; Zhang et al., 1998; Zhou et al.,2002), Archean TTG (Jahn et al., 1988; Jahn and Zhang, 1984;Sun et al., 1992), peridotite xenoliths from Hannuoba (Rudnicket al., 2004) are shown for comparison.

tional crystallization degree of >70% is required to pro-duce the trondhjemitic liquids (Spulber and Rutherford,1983) or rhyolite (Meijer, 1983). Furthermore, fractionalcrystallization of garnet will produce liquid with not onlyhigh Sr/Y and La/Yb ratios, but also negative Al2O3-SiO2correlations, differing from the trends of the Baiqi adakiticrocks (Fig. 2A). Third, although the Baiqi adakitic rockshave various SiO2 contents from 57 to 71%, their Sr andNd isotopic compositions are remarkably homogeneous(Table 2). This is the most important, because the homo-geneous Sr and Nd isotopic compositions suggest that theBaiqi formation derived from one magma chamber, andwere not contaminated significantly by evolved old base-ment rocks such as 1.9 Ga or 2.5 Ga lower crustalxenoliths from Hannuoba (Liu et al., 2004) and Nushan(Huang et al., 2004; Yu et al., 2003) or Archean terranegranulites (Jahn et al., 1988; Jahn and Zhang, 1984; Liuet al., 2004; Sun et al., 1992) (Fig. 5).

Early Cretaceous magmatism is also developed exten-sively along the Dabie-Sulu orogenic belt that formed byTriassic collision between the North China and YangtzeCratons (Cong, 1996; Jahn et al., 2003; Zheng et al.,2003). Anatexis of subducted continent and thus result-ant orogenic lithospheric keel has been suggested to in-terpret the petrogenesis of post-collisional intrusives (in-cluding adakitic ones) in the Dabie orogen (Zhang et al.,2002; Zhao et al., 2004, 2005). Studies of experimental

Mesozoic adakites: Partial melting of underplated basalts 455

1995), the Baiqi adakitic rocks could have been formedby partial melting of mafic lower crust. Melts derived frompartial melting of recycled lower continental crust in themantle will obtain mantle signature (e.g., high Mg# num-bers and Cr, Ni contents) due to reaction with the mantleperidotite (Liu et al., 2005; Rapp et al., 1999). However,low Mg# number and Cr and Ni contents of the Baiqiadakitic rocks (Table 1), differing from the Mesozoicvolcanics from Xinglonggou (Gao et al., 2004), are atvariance with their derivation from partial melting of re-cycled lower continental crust in the mantle, but consist-ent with the conclusion that no significant Mesozoiclithospheric thinning took place within the Trans-NorthChina Orogenic belt based on Re-Os isotopic system ofthe peridotite from the Hannuoba basalt (Gao et al., 2002).Thus, partial melting of a thickened lower continentalcrust could be responsible for the formation of the Baiqiadakitic rocks, as suggested by Wang, Q. et al. (2005) forthose from Tibet. This recognition is very important, be-cause (1) it suggests that partial melting of the crustalrocks were not attributed to recycling of lower continen-tal crust into asthenospheric mantle as demonstrated bythose adakitic volcanics from Xinglonggou (Gao et al.,2004); and (2) it implies a crust with thickness >40 km(>1.2 GPa) (Petford and Atherton, 1996; Rapp andWatson, 1995) and a strong Mesozoic thermal event (e.g.,

basaltic underplating) triggering partial melting of thepreexisting lower crust in the Trans-North China Orogen.If the lower crust would be thickened by collisionalorogeny, the petrogenetic model for the Dabie post-collisional magmatism (Zhao et al., 2004, 2005) couldbe also applicable to the Baiqi adakitic rocks. In the nextsection, we discuss possible sources of the Baiqi adakiticrocks and processes triggering the partial melting of thick-ened lower crust.

Links between partial melting of thickened lower crustand basaltic underplating

If the Baiqi adakitic rocks were produced by partialmelting of thickened lower crust, what were their parentrocks and what did drive the partial melting of the thick-ened lower crust? The Baiqi adakitic rocks have remark-ably different 87Sr/86Sr ratios and εNd(t) values from thatof the Precambrian lower crustal xenoliths (Huang et al.,2004; Liu et al., 2004; Yu et al., 2003) and granulite-TTG-khondalite exposed on the Earth’s surface (Jahn et al.,1988; Jahn and Zhang, 1984; Liu et al., 2004; Sun et al.,1992) at 125 Ma (Fig. 5), which demonstrate clearly thatthe Baiqi adakitc rocks were not formed by anatexis ofthe Precambrian lower continental crust.

However, the Baiqi adakitic rocks have Sr-Nd isotopiccompositions overlapping with the adjacent Mesozoic

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Fig. 7. Primitive mantle-normalized spider diagrams of melts equilibrated with garnet-rich pyroxenite/eclogite. Based on batchmelting model (Shaw, 1970), the melts were calculated assuming the Mesozoic gabbros from Hanxing (Zhang, H. F., 2005, unpub-lished data) and mafic granulite xenolith (DMP-72) from Hannuoba (Liu et al., 2001) as starting material. The Parameters usedfor calculations are listed in Table 4. The results indicate that gabbro-derived melts agree well with the Baiqi adakitic rocks.

456 X.-C. Wang et al.

lower crustal xenoliths from the Hannuoba basalts (Liuet al., 2004; Zhou et al., 2002) and gabbros from theTrans-North China Orogen (Zhang et al., 2004) (Fig. 5),which implies that the Baiqi adakitic rocks could be ge-netically related with the Mesozoic lower crustal xenolithsand/or gabbros. Zircon cathodoluminescence (CL)imaging and U-Pb dating for granulite and olivinepyroxenite demonstrate two episode of basalticunderplating and subsequent granulite-facies metamor-phism at 160–140 Ma and 140–80 Ma, respectively. Themost important is that the overlapping timing for basalticunderplating and granulite-facies metamorphism indicatesthat the latter was induced by the former (Liu et al., 2004).This agrees well with the two episodes of gabbros fromLaiyuang-Linxian and Yunmengshan-Hanxing, whichwere dated at 125–145 Ma and 150–160 Ma, respectively(Davis et al., 1998; Wang, Y. J. et al., 2005; Zhang et al.,2004). The typical igneous oscillatory zonation of the zir-cons (Fig. 6B) in this work indicates that the mean 206Pb/

238U age of 125.8 ± 3.0 Ma should record the formationage of the Baiqi adakitic rocks. Furthermore, the ages ofintermediate-felsic igneous rocks around the study areacluster in the range of 138–125 Ma (Cai et al., 2003; Chenet al., 2004; Davis et al., 1998; Peng et al., 2004). Allthese ages make it reasonable to speculate that the ~150–160 Ma gabbro or granulite xenoliths formed by basalticunderplating could be the parent rocks of the Baiqi adakitcvolcanics, consistent with their similar Sr-Nd isotopiccompositions (Fig. 5). Heating of the subsequent ~140–120 Ma basaltic underplating caused granulite-faciesmetamorphism and partial melting of the mafic lower crustformed at ~150–160 Ma. This coincides with the EarlyCretaceous giant igneous event in eastern China (132~120Ma, peaked at 125 Ma; Wu et al., 2005). While the west-ward subduction of the Pacific plate beneath the Eura-sian continent may provide a geodynamic setting for de-velopment of Mesozoic magmatism along the eastern edgeof the China continent (Wu et al., 2005), the intensive

Table 4. Parameters used for modeling calculations

Partition coefficient Initial material Grt:Cpx = 1:1 Grt:Cpx = 4:1 Baiqi adakite

Dcpx/melt Dgrt/melts Gabbro(ppm)

average 1(ppm)

average 2(ppm)

average(n = 8)(ppm)

K 0.007 0.0003 9841 35164 35368 23047Sr 0.08 0.005 300 1454 1629 1126Y 0.58 3.10 22.0 13.9 10.4 13.6Nb 0.021 0.008 4.58 15.9 16.0 10.2Ta 0.012 0.004 0.26 0.92 0.92 0.62Rb 0.0035 0.0007 31.3 112 113 56.2La 0.054 0.01 11.2 36.2 37.5 44.1Ce 0.098 0.021 27.5 84.9 89.9 79.3Pr 0.15 0.054 3.86 10.8 11.5 9.19Nd 0.21 0.087 19.1 48.6 52.4 35.3Sm 0.26 0.217 4.63 10.1 10.3 5.17Eu 0.31 0.32 1.42 2.77 2.76 1.63Gd 0.3 0.498 4.05 7.04 6.56 4.73Tb 0.31 0.75 0.57 0.85 0.75 0.52Dy 0.33 1.06 3.21 4.08 3.41 2.48Ho 0.31 1.53 0.6 0.63 0.50 0.43Er 0.30 2.00 1.55 1.43 1.06 1.13Tm 0.29 3.00 0.2 0.14 0.13 0.16Yb 0.28 4.03 1.25 0.69 0.48 1.03Lu 0.28 5.50 0.17 0.07 0.05 0.16Th 0.007 0.0015 1.03 3.67 3.69 5.04U 0.008 0.006 0.35 1.23 1.23 1.25Pb 0.13 0.18 6.78 17.1 16.6 14.7Ba 0.0019 0 429 1545 1547 1496Zr 0.093 0.40 55.1 119 104 163Hf 0.17 0.31 2.06 4.35 4.08 4.04Sr/Y 13.6 105 156 83.2La/Yb 8.76 52.3 77.5 43.3

The initial material is a gabbro sample from Hanxing area (Zhang, H. F., 2005, unpublished data). The 1 and 2 are average calculation meltswho are derived from batch melting of Hanxing gabbro with partial melting degree F = 0.2, 0.3, 0.4. The REE partition coefficients data are fromMcKenzie and O’Nions (1991) and the other partition coefficients from Klemme et al. (2002).

Mesozoic adakites: Partial melting of underplated basalts 457

occurrence of Early Cretaceous magmatism in this widearea points to a thermal event that may be associated withthe Pacific superplume activity as advocated by Zhao etal. (2004, 2005) for the post-collisional magmatism inthe Dabie orogen. Furthermore, the estimated tempera-tures of pyroxenite and granulite xenoliths from theHannuoba basalts could be high up to 900–1100°C (Chenet al., 2001; Liu et al., 2003). This implies that the basal-tic underplating can provide the necessary heat to inducepartial melting of the preexisting mafic lower crust(Guffanti et al., 1996; Petford and Gallagher, 2001;Rushmer, 1991).

If the above model is correct, the Baiqi adakitic rocksshould match the partial melts of ~150–160 Ma gabbrosor mafic granulites. Batch melting models were calcu-lated assuming ~150–160 Ma gabbros (Zhang, H. F, 2005,unpublished data) and mafic granulite xenoliths from theHannuoba basalts (Liu et al., 2001) as starting materials,respectively (Fig. 7). The results indicate that the Baiqiadakitic rocks match well the partial melts formed by 20–40% partial melting of gabbros (Table 4) leaving therestite of 50% Grt + 50% Cpx (Figs. 4, 7C and 7D). How-ever, partial melts of mafic granulites present lower Ba,Th, U, Nb, Ta and LREE (Figs. 7A and B). This diver-gence indicates that the mafic granulites could suffer fromgranulite-facies metamorphism/partial melting at 80–140Ma resulted in decrease in these elements. As a result, wesuggest that the source rock of the Baiqi adakitic rockscould be represented by the ~150–160 Ma gabbro.

The above observations and discussions outline sucha process that the thickened lower crust generated by ba-saltic underplating at ~150–160 Ma experienced subse-quent granulite-facies metamorphism and thus partialmelting, which could have been caused by heating of an-other phase of congenetic basaltic underplating at ~80–140 Ma. However, where was the basaltic melt derivedfrom remains enigmatic due to its evolved Sr-Nd isotopiccompositions compared with the mantle peridotites(Rudnick et al., 2004) (Fig. 5).

Implications for the Mesozoic deep geodynamic processin the Trans-North China Orogen differing from the eastpart of the East China

Gao et al. (2004) argued that partial melting ofdelaminated lower crust in the mantle from Jurassic vol-canic rocks at Liaoxi indicates that lithospheric founderingin the east part of the North China Craton could haveserved as the heat engine. During the delamination proc-ess, the upwelling of asthenospheric mantle heated theoverlying crust and resulted in the formation of juvenilecrust by underplating of mantle-derived basaltic magma.However, neither Mesozoic high-Mg andesite nor high-Mg adakitic rock has been identified in the Trans-NorthChina Orogen (Li et al., 2001; Chen et al., 2002, 2003,

2004; Chen and Zhai, 2003; Cai et al., 2003; Davis, 2003;Peng et al., 2004; Li and Li, 2004). This implies that thelithosphere of the Trans-North China Orogen might havenot thinned as the east part of the East China suggestedby Gao et al . (2002). We speculate that basalticunderplating, and possibly the resultant chemical erosion,could dominate over much of the Trans-North ChinaOrogen. SiO2-rich melt-mantle peridotite interaction (Liuet al., 2005) and the resultant chemical erosion (Xu, 2001)within the gradually upward moving lithosphere-asthenosphere interface played an important role in gen-erating a thickened lower continental crust due to basal-tic underplating.

CONCLUSION

The Baiqi volcanics possess the same geochemicalcharacteristics as those of typical adakites in modern arcs.However, they have relatively lower Cr and Ni contentsand Mg#, suggesting that their parental magma did notinteract with the lithospheric mantle. Combined with thegeological setting of the studying area, these geochemicalfeatures suggest that the Baiqi adakitic rocks formed bypartial melting of a thickened lower continental crust at adepth of >40 km in central North China. More impor-tantly, the formation of the Baiqi adakitic volcanics im-plies that partial melting of the underplated mafic lowercrust of ~150–160 Ma, rather than the Precambrian base-ment rocks, was caused by enormous conductive heatingfrom a subsequent basaltic underplating event at 80–140Ma.

Acknowledgments—We thank Dr. S. Gao for the assistance infield work, and J. Q. Wang for XRF whole rock analyses. Con-structive comments by Dr. X. H. Li and Dr. S. Gao have sub-stantially improved the manuscript. We also thank Dr. H. F.Zhang for provide their unpublished data. Two anonymous re-viewers are thanked for their helpful comments that improvedthe manuscript significantly. This work was financially sup-ported by National Nature Science Foundation of China (Nos.40521001, 40473013) and the Program for Changjiang Schol-ars and Innovative Research Team in University (IRT0441).

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Mesozoic adakites: Partial melting of underplated basalts 461

(a)n = number of analysis; Meas. = measured value; Rec. = recommended value (http://minerals.cr.usgs.gov/geo_chem_stand); Units are ppmfor all elements.

BCR-2(n = 4) GSR-3(n = 3) GSR-1(n = 4)

Meas. 1σ Rec. Meas. 1σ Rec. Meas. 1σ Rec.

SiO2 54.01 0.24 54.10 44.60 0.02 44.64 72.81 0.02 72.83TiO2 2.26 0.02 2.26 2.40 0.03 2.37 0.30 0.03 0.29Al2O3 13.37 0.08 13.50 13.86 0.10 13.83 13.50 0.20 13.40Fe2O3* 13.85 0.04 13.80 13.29 0.08 13.4 2.15 0.07 2.14MnO 0.18 0.01 0.19 0.16 0.01 0.17 0.06 0.01 0.06MgO 3.69 0.01 3.59 7.79 0.04 7.77 0.44 0.04 0.42CaO 7.17 0.02 7.12 8.81 0.03 8.81 1.54 0.03 1.55Na2O 3.10 0.10 3.16 3.47 0.09 3.38 3.07 0.08 3.13K2O 1.80 0.01 1.79 2.31 0.02 2.32 5.06 0.06 5.01P2O5 0.35 0.00 0.35 0.94 0.01 0.95 0.09 0.00 0.09

Appendix 1. Major element analyses of the international rock standards(a)

a)n = number of analysis; Meas. = measured value; Rec. = recommended value (http://minerals.cr.usgs.gov/geo_chem_stand); Units are wt% forall elements.

Appendix 2. Analyses of international rock standards BHVO-1, AGV-1 and GSR-1 by ICP-MS(a)

BHVO-1 (n = 5) AGV-1 (n = 5) GSR-1 (n = 3)

Meas. 1σ Rec. Meas. 1σ Rec. Meas. 1σ Rec.

Li 5.08 0.17 4.60 10.5 0.34 12.0 147.4 5.00 131Be 0.99 0.01 1.10 2.21 0.06 2.10 13.5 1.80 12.4Sc 31.8 0.07 31.8 12.0 0.06 12.2 6.00 0.80 6.10V 314 0.17 317 122 0.38 121 25.3 1.20 24.0Cr 285 5.00 289 12.0 0.13 10.1 9.80 2.30 5.00Co 45.0 0.29 45.0 15.0 0.08 15.3 3.20 0.80 3.40Ni 121 3.35 121 15.0 1.68 16.0 5.60 3.20 2.30Cu 138 0.65 136 56.0 0.48 60.0 3.54 0.40 3.20Zn 110 0.68 105 80.0 0.25 88.0 25.5 1.60 28.0Ga 21.0 0.06 21.0 20.3 0.15 20.0 19.9 1.20 19.0Ge 1.65 0.03 1.64 1.28 0.04 1.25 2.10 0.08 2.00Rb 9.60 0.13 11.0 66.0 0.18 67.3 454 4.60 466Sr 399 2.06 403 662 1.33 662 114 4.50 106Y 27.3 0.05 27.6 21.0 0.09 20.0 65.8 1.00 62.0Zr 173 0.67 179 233 0.87 227 170 1.35 167Nb 19.3 0.07 19.0 15.0 0.10 15.0 43.6 0.96 40.0Cs 0.11 0.01 0.13 1.34 0.01 1.28 36.6 0.10 38.4Ba 138 0.32 139 1234 8.00 1226 353 16.0 343La 15.6 0.05 15.8 38.4 0.11 38.0 52.6 1.90 54.0Ce 38.3 0.01 39.0 68.4 0.10 67.0 100 1.90 108Pr 5.44 0.02 5.70 8.40 0.03 7.60 11.9 0.41 12.7Nd 25.6 0.10 25.2 32.8 0.07 33.0 44.5 1.10 47.0Sm 6.24 0.03 6.20 5.90 0.04 5.90 9.10 0.80 9.70Eu 2.01 0.01 2.06 1.69 0.02 1.64 0.80 0.06 0.85Gd 6.17 0.06 6.40 5.40 0.05 5.00 9.10 1.60 9.30Tb 0.96 0.003 0.96 0.70 0.00 0.70 1.56 0.03 1.65Dy 5.20 0.01 5.20 3.63 0.01 3.60 9.79 0.12 10.2Ho 0.98 0.00 0.99 0.68 0.01 0.67 2.11 0.15 2.05Er 2.36 0.01 2.40 1.75 0.03 1.70 6.18 0.20 6.50Tm 0.32 0.00 0.33 0.25 0.00 0.34 1.10 0.02 1.10Yb 2.03 0.02 2.02 1.70 0.02 1.72 7.40 0.16 7.40Lu 0.30 0.00 0.29 0.26 0.01 0.27 1.20 0.01 1.20Hf 4.41 0.03 4.38 5.10 0.03 5.10 5.70 0.12 6.30Ta 1.23 0.005 1.23 0.90 0.01 0.90 6.90 0.04 7.20Pb 2.33 0.11 2.60 36.3 0.70 36.0 32.2 0.02 31.0Th 1.25 0.02 1.08 6.39 0.05 6.50 51.6 0.02 54.0U 0.42 0.01 0.42 1.88 0.02 1.92 18.2 0.04 18.8