Earth and Planetary Science Letters, 105 (1991) 149-169 149 Elsevier Science Publishers B.V., Amsterdam

[CL]

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications

for their origin from suboceanic-type mantle reservoirs

Asish R. Basu a, Wang Junwen b, H u a n g W a n k a n g b, Xie G u a n g h o n g b and Mi t sunob u T a t s u m o t o c

" Department of Geological Sciences, University of Rochester, Rochester, N Y 14627, USA Institute of Geochemistry, Academia Sinica, Guiyang, Guizhou, People's Republic of China

U.S. Geological Survey, MS 963, Box 25046, Denver, CO 80225, USA

Received October 30, 1990; revision accepted March 18, 1991

ABSTRACT

Major- and rare-earth-element (REE) concentrations and U-Th-Pb, Sm-Nd, and Rb-Sr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basahs, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China.

The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) Nd-Sr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) Sr-Pb also shows an anticorrelation, similar to that of Hawaiian and Walvis Ridge basalts, (3) Nd-Pb shows a positive correlation, and (4) the 2°7pb/2°4Pb vs 2°6pb/2°4pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 2°spb/2°4pb vs 2°rpb/Z°4Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components--a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.

1. Introduction

T h e g e o t e c t o n i c e v o l u t i o n of t h e e a s t e r n m a r g i n

of C h i n a , w h i c h c o n t a i n s C e n o z o i c v o l c a n i c r o c k s

o f t he c i r c u m - P a c i f i c v o l c a n i c be l t , h a s b e e n

s h a p e d s ince t h e l a t e M e s o z o i c b y t he i n t e r a c t i o n

of t h e w e s t e r n Pac i f i c a n d E u r a s i a n p la t e s . T h e

t e c t o n i c f e a t u r e s o f n o r t h e a s t e r n C h i n a in p a r t i c u -

l a r h a v e b e e n i n f l u e n c e d b y t he s e p a r a t i o n o f t h e

J a p a n e s e I s l a n d A r c f r o m t he A s i a t i c m a i n l a n d

during the Late Cretaceous as the back-arc basin of the Japan Sea formed. The tensional forces that formed the back-arc basin are also believed to have created the northeast China continental rift system [1]. This tectonism and volcanism con- verted northeast China into an epeiric region of two significantly different geotectonic uni t s - - the Sino-Korean paraplatform containing late A r c h e a n b a s e m e n t , a n d t h e s u r r o u n d i n g T i a n -

s h a n - X i n g ' a n g e o s y n c l i n a l f o ld be l t . S i n c e t he

0012-821X/91/$03.50 © 1991 - Elsevier Science Publishers B.V.

150 A.R. BASU ET AL.

Oligocene, the northeast coastal region of China has entered a new evolutionary stage, from a fairly passive continental margin to a rifting continental margin.

Several recent regional geochemical studies of Sr and Sr-Nd-Pb isotope systems, as well as of major- and trace-elemental characteristics of Cenozoic basalts of eastern China [2-5], have evaluated the petrogenesis of these basalts in rela-

tion to the nature of the crust and mantle through which they have erupted. These and a few other [6-9] studies attempted to characterize temporal and spatial variations in the source rock composi- tion by invoking a horizontally and vertically het- erogeneous mantle. We have initiated a petrologi- cal, geochemical, and Nd, Sr and Pb isotopic study of late Tertiary to Quaternary volcanic rocks of northeastern China, including their mafic and

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Fig. 1. Genera l i zed geologic sketch m a p of Cenozoic vo lcan i sm in eas tern China, modi f i ed af ter [1]. The volcanic centers chosen for the present s tudy are Wuda l ianch i , Changba i shan , Kuand ian , Hanoba r , Da tong and Mingxi . S L G = Songl iao graben; X L R =

Xia l iaohe rift; Y Y R = Y i t o n g - Y i l a n rift; F M R = Fushun Mishan rift. 1 = Qua te rna ry volcanic rocks; 2 = Neogene basal ts ; 3 = Eocene basal t ; 4 = ma in faul t and rift; 5 = h idden fault; 6 = out l ines of the Songl iao g raben and Xia l i aohe rift.

V O L C A N I C R O C K S O F E A S T E R N C H I N A : I S O T O P I C I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 151

ultramafic inclusions. In this paper, we report on concentrations of the major elements, REE, Rb, Sr, Pb, U and Th and the isotopic compositions of Nd, Sr and Pb in 26 volcanic rocks from north- eastern and eastern China. Our principal objec- tives in this study are to contribute to a better understanding of the petrogenesis and to evaluate the geochemistry of the source reservoirs of the volcanic rocks. We demonstrate here, using the isotopic data, the existence of suboceanic-type mantle domains beneath the northeastern Chinese continental block. We also demonstrate that the isotopic tracer data can be highly relevant in sup- port of models for either plate subduction or ridge submergence and back-arc spreading for the gen- eration and evolution of the continental rift sys- tem in northeast China.

2. Volcano-tectonic features of eastern and north- eastern China

The major volcano-tectonic features of eastern and northeastern China show characteristic north- east-southwest trending structures superimposed on the older basement rocks by the Cenozoic interaction of the western Pacific and Asian plates. A simplified tectonic map showing the distribu- tion of Cenozoic volcanism in eastern China is shown in Fig. 1 after Liu [1]. Figure 1 also shows the locations of the six volcanic areas in the pre- sent s tudy: Wuda l i anch i , C h a n g b a i s h a n , Kuandian, Hanobar, Datong and Mingxi. Most of these volcanic centers, with the exception of Datong and Hanobar to the west of Beijing (Fig. 1), are along the approximately nor th-south trending deep fault systems of the Tancheng- Lujiang fault zone. Because our study deals mainly with the Cenozoic volcanic rocks found in close association with the major faults parallel to the present continental margin of China, it is ap- propriate to summarize some of the salient fea- tures of these faults, which have recently been regarded as part of the Northeast China Continen- tal Rift System [8,10,11].

Based on geochronology, heat flow, gravity, magnetism, volcanism, nature of faulting, and the accumulation of red sedimentary formations in the fault-bounded basins, the following tectonic units in northeastern China have been recognized (Fig. 1): the Songliao Graben (SLG), the Yitong-

Yilan Rift (YYR), the Fushun-Mishan Rift (FMR), and the Xialiaohe Rift (XLR). This rift system contains about 690 volcanic cones and craters and 50,000 km 2 of basaltic lavas with minor amounts of alkali trachytes. One of the earliest known volcanic centers that became active in the Late Cretaceous (86 Ma) is in the SLG [9], al- though intense alkalic volcanism did not begin until the Early Tertiary in the central part of this graben around Shuangliao. It is apparent from the relative dispositions of the linear structural fea- tures in Fig. 1 that not only are SLG and XLR part of a single rift system, but they also link with the Tancheng Lujiang rift passing through the Bohai Sea. The volcanism of the Changbaishan region adjacent to FMR in the east (Fig. 1) began at the end of the Eocene and reached maximum intensity during the Pliocene, and has continued during the Quaternary. It has been established that the XLR, SLG, YYR and F MR (Fig. 1) are all part of the same interconnected rift system of northeast China and that this extensional tectonism accompanied by volcanism progressed from the SLG in the west to the eastern continen- tal margin [12]. The rift-related volcanic rocks can be classified into four principal types, including tholeiites, alkali basalts, potassic basalts, and al- kali trachytes and pantellerites. It is our intention in the present study to contribute to a better understanding of the origin and evolution of these principal volcanic rock types from northeastern China in the framework of the formation of the Northeast China Rift System. We strongly empha- size here that the volcanic rock association just listed contrasts markedly with the calc-alkaline basalt-andesite-dacite-rhyoli te suite of subduc- tion-related orogenic zones, which are so ubiquitous in the circum-Pacific volcanic belt.

3. Geologic setting

3.1. Datong and Hanobar

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1 5 4 A.R. BASU ET AL

amounts of lavas at the base. The lavas are Quaternary in age and usually contain normative olivine and nepheline, typical of alkali basalts. Some Tertiary alkali basalts and olivine tholeiites also outcrop 28 km north of Datong City. We analyzed two basalts from the Datong area, in- cluding one Quaternary alkali olivine basalt (BDTI-1) and a Late Tertiary olivine tholeiite (BDT-1). The Hanobar basalts occur along an east-west fault system covering up to 3000 km 2. These basalts can be divided into two magmatic cycles, a lower Early Miocene and an upper Mid- dle Miocene cycle. The upper cycle is composed essentially of a tholeiite series, while the lower cycle consists principally of alkali basalts with some olivine tholeiites.

3.2. Kuandian

The Kuandian volcanic field covers an area of 35 km × 15 km in eastern Liaoning Province. It is in the northeast part of the Sino-Korean block, which is composed of Proterozoic metamorphic rocks in the basement. The volcanic field consists of several cones that trend northeast-southwest parallel to the regional fault trend. One of the major volcanic cones in Kuandian , called Huangyishan, is a composite volcano composed of lavas in the lower strata and volcaniclastic rocks in the upper strata. The lavas represent five dis- tinctly different eruptions; the lowermost flow lies directly on top of the metamorphic basement. With the exception of the fifth and third flows from the bottom, all other flows contain mafic-ul- tramafic xenoliths and high-pressure megacrysts. K-Ar age determinations of Huangyishan basalts yielded a Pleistocene age (0.274 Ma). We analyzed one sample from each of these five lava flows in the Huangyishan volcanic cone of the Kuandian volcanic zone.

3.3. Changbaishan

The Changbaishan volcanic group represents the largest exposure (Fig. 1) of volcanic rocks in northeast China, and perhaps in all of east Asia. This volcanism covered an area of about 1500 km 2, spreading over the Ch ina -Korean border region. More than 100 volcanic cones are well preserved, including the largest crater lake in China

(Tianchi). Based on K-Ar geochronologic studies [13], volcanism in the Changbaishan region can be divided into seven episodes, from Early Miocene (20 Ma) volcanism, through the Middle Miocene (15 Ma) Naitoushan basalt phase of eruption, to Late Pleistocene Baitoushan alkali trachytes and the early Holocene trachyte, obsidian and pumice phase of eruption. In addition, historical eruptions in 1597, 1668 and 1702 A.D. are also well docu- mented. For this study, we concentrated on the 15-Ma-old Nai toushan basalts, the younger basalts, and the Baitoushan alkali trachytes. In the Changbaishan area, most of the Naitoushan basalts occur 25-30 km northeast of Tianchi in 300 to 400-m-thick lava terraces directly overlying Precambrian metamorphic rocks. Baitoushan al- kali trachytes are exposed around Tianchi crater, where they form a few layered volcanic cones covering approximately 200 km 2. The nine Changbaishan volcanic rocks of this study are from a 300-m-thick sequence composed from bot- tom to top of late Tertiary alkali basalt and olivine tholeiite followed by Quaternary trachytes, pumice and pantellerite.

3.4. Wudalianchi

The Wudalianchi volcanic group is char- acterized by the presence of the most potassic Cenozoic basalts in northeast China. This volcanic group is situated in the northern part of the Songliao Graben (SLG in Fig. 1). Frequent volcanism since the Quaternary has resulted in fourteen volcanic craters and prominent lava ter- races covering an area of about 600 km 2. The volcanic cones occur in three chains parallel to the trend of the SLG, suggesting tectonic control of this volcanism. K-Ar age determinations [14] documented the Middle Pleistocene as the major period of volcanism in this area. The last eruption probably took place between 1719 and 1721 A.D. The volcanic rocks in the region belong mostly to the phonotephrite and shoshonite clans, but more generally they are potassic alkali basalts. Three such basalts were chosen for this study; two of these samples, 82H-4 and 6 (Table 1), are from the 1719-1721 historical eruption and the other, 82H- 2, is from the Middle Pleistocene eruption of about 0.43 Ma.

V O L C A N I C R O C K S O F E A S T E R N C H I N A : I S O T O P I C I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 1 5 5

3.5. Mingxi

One sample of a limburgite from an intrusive Plio-Pleistocene plug [15] from Mingxi (Fig. 1) in western Fujian Province was analyzed. This rock is unique as it contains inclusions of originally deep-seated garnet lherzolites from the upper mantle. The location of Mingxi in the southern part of east China is far removed from the other volcanic rocks in the northeast. Since the middle Tertiary, basaltic volcanism in Fujian and Zhejiang provinces has been represented by alkali olivine basalt. Eruptions took place along three sets of volcanic belts following a northeasterly trend. Thus, the alkali basalts in Fujian and Zhejiang provinces are also related to the deep northeast- trending faults of eastern China, and it has been suggested [8] that the volcanism in this region is related to the island arc and continental margin tectonic environment of the western Pacific.

were less than 0.2 ng for Pb, Rb, Sm and Nd, 0.3 ng for Sr, and 0.01 ng for U and Th. Estimated errors in determining the concentrations of these elements are usually less than 0.7%.

The major-elemental compositions and the rare-earth-element concentrat ions of the 26 volcanic rocks were analyzed at the X-ray Assay Laboratories, Ontario, Canada, by X-ray fluores- cence and instrumental neutron activation (INA) methods, respectively. Estimated uncertainties for these methods are less than 2% for XRF and between 5 and 15% for INA. Independent mea- surements of Sm and Nd concentrations were performed by the isotope dilution technique on a separate split of the same rock sample. These concentrations are reported in Table 2 with the isotope ratios. By comparing Tables 1 and 2, good correspondence can be seen between the Sm and Nd concentrations measured by the two tech- niques.

4. Experimental procedures and samples 5. Analytical results

Analytical procedures and mass spectrometric techniques used in this study for Rb-Sr, Sm-Nd and U-Th-Pb isotopes are basically similar to those reported by Tatsumoto and Unruh [16], Nakamura et al. [17], and Tatsumoto et al. [18]. Each rock was prepared by ultrasonic washing in dilute HC1 and distilled water before being crushed to powder. Samples of 100 mg in weight were spiked with 87Rb, 848r, 1498m, 15°Nd, 233U, 236U, 23°Th and

2°spb tracer solution and were dissolved in HF and H N O 3 in a PFA teflon vial. For each sample, Pb, U, Th, Rb, Sr, Sm and Nd were isolated from the same solution by using ion-exchange resin columns. Pb isotopic compositions were measured by a silica gel-phosphate method using a NBS tandem-type mass spectrometer. Pb isotopic data were corrected for a fractionation of 0.12 _+ 0.03% per atomic mass unit based on analyses of NBS SMR-982. Sr and Nd isotopic compositions were measured with a V.G. Micromass 54R mass spec- trometer, normalizing to 86Sr/SSSr=0.1194 and 146Nd/144Nd = 0.7219. Measured values for the NBS SRM-987 Sr standard and La Jolla Nd standard were 87Sr/S6Sr = 0.710255 _+ 30 and 143Nd/144Nd = 0.511865 -+ 10, respectively. Un- certainties correspond to the last significant fig- ures and represent 20 of the mean. Total blanks

5.1. Major- and trace-element geochemistry

The major- and trace-element concentrations of 26 volcanic rocks from northeast China are given in Tables 1 and 2. These data also include the rare earth elements (REE) (Table 1). Similar data on northeast Chinese volcanic rocks have been con- tributed recently [e.g., 2,3]. However, the samples analyzed in our study were chosen from volcano- stratigraphic columns to cover temporal-spatial variations in chemical compositions in and among each of the volcanic centers (Tables 1 and 2). There are no volcanic rocks of calc-alkaline affin- ity in eastern or northeastern China; this observa- tion is supported by major oxide and mineral normative compositions derived from the chemical data (Table 1). In this section, the bulk of major element compositions and REE data are presented for each of the six centers of volcanism we have described.

Datong volcanism is characterized by a change from nepheline-normative basanite in the Late Tertiary to alkali olivine basalt in the Quaternary. In the adjacent Hanobar area, all the volcanism was restricted to the Late Tertiary, although inter- rupted by a bed of lacustrine sediments that di- vides the Hanobar basalts into an upper and a

156 A.R. BASU E T AL.

T A B L E 2

Volcanic rocks of eastern China (Datong, Changbaishan, Wudal ianchi , Hanobar , Kuand ian and Mingxi) . Sin, Nd, Rb, Sr, Pb, U and Th concentra t ions and I,,c isotopic composit ions of Nd, Sr and Pb in 26 volcanic rocks from eas tern China. The uncertaint ies reported for the isotopic rat ios of N d and Sr correspond to the last significant figures and represent 2 a of the mean. Uncer ta in t ies for the Pb isotope ratios are l imited to 11, 14 and 49 of the last figures for 2°6pb/2°4pb, 2°7pb/204pb and 2°spb /2°4pb , respectively, because of correct ion uncertaint ies for mass f ract ionat ion

147Sm la3Nd 87Rb Sample Rock type Sequence Sm (ppm) Nd (ppm) ~ Nd Rb (ppm) Sr (ppm)

14nNd 144 Nd 86Sr

Datong BDT-1 Olivine

tholeiite B D T I - I Alkali

olivine basalt

Changbaishan X-1 Pantellerite~ K-1 Pumice [ X- 15 Alkal i |

/

t rachyte ~ X-17 Alkal i {

trachyte |

X-24 Alkal i l t rachyte ]

X-39 Tholei i te X-42 Alkali /

olivine

basalt X-40 Olivine

t h o l e i i t e | X-34 Alkal i |

olivine | basalt ]

Wudalianchi H-2 Leucite ~

ba san i t e | H-4 Basic \

tephri te I H-6 Leucite

tephri te

Kuandlan BC-5-2 Ol iv ine

tholeii te (5th) BC-4-1 Alkali

olivine basalt (4th)

BC-3-1 Olivine thole±ire (3rd)

BC-2-4 Olivine tholeii te (2nd)

BC-1-4 Alkal i olivine basal t (1st)

Hanobar DF-1 Quar tz ,

thole±ire D-37 Quar tz

tholei i te , D-a 1 Basani i te D-52 Olivine

thole±ire D-I Olivine

tholeiite DF-2 Alkali

olivine basalt

Mingxi MB-1 Limburgi te

host rock

Late 6.50 29.39 0.1335 0.51280 ± 1 + 3.2 16.49 723.4 0.0657 Ter t ia ry

Qua te rna ry 8.88 39.96 0.1342 0.51290 ± 1 + 5.1 48.26 701.0 0.2004

Quaternary

Late Ter t ia ry

Qua te rna ry

Upper

Lower

10.39 54.66 0.1147 0.51263 ± 1 --0.1 115.49 51.37 6.5099 26.17 124.07 0.1274 0.51259 ± 1 0.9 349.61 5.043 200.745

13.48 82.79 0.0981 0.51258 ± 1 -- 1.1 228.59 1.318 502.002

12.82 67.54 0.1147 0 .51264 ± 1 0.0 254.77 17.96 41.069

7.64 31.81 0.1450 0.51257 ± 1 1.3 30.82 406.0 0.2200 7.50 30.91 0.1465 0.51256 ± 1 -- 1.5 36.52 346.0 0.3074

3.80 14.64 0.1568 0 .51250 + 1 2.7 10.85 327.0 0.0961

4.78 21.33 0.1353 0.51277 + 1 + 2.6 81.92 626.6 0.3794

16.60 78.10 0.1283 0.51238 ± 1 - 5.0 64.34 1552.1 0.1208

10.31 61.32 0.1015 0.51242 ± 1 - 4.3 93.31 1273.3 0.2121

11.54 69.24 0.1006 0.51243 ± 2 - -4 .0 94.16 1463.1 0.1859

6.57 29.43 0.1348 0 .51279 ± 1 + 2.9 41.46 676.5 0.1769

6.07 27.62 0.1327 0.51278 ± 1 + 2.8 32.43 649.5 0.1448

6.19 27.73 0.1349 0.51278 ± 2 + 2.8 27.50 558.8 0.1425

5.72 24.57 0.1403 0.51265 ± l + 2.3 20.52 567.9 0.1049

7.28 33.37 0.1318 0.51283 ± 1 + 3 . 7 37.52 716.1 0.1486

2.90 11.60 0.1507 0.51276 ± 1 + 2 . 4 13.83 510.5 0.0783

3.27 13.28 0.1488 0.51284 ± 1 + 3.9 14.84 567.2 0.0746

8.45 40.15 0.1271 0.51289 ± 1 + 4 . 9 31.42 812.9 0.1171 4.06 15.36 0.1596 0.51268 ± 1 + 0 . 8 8.25 418. l 0.0571

4.49 18.09 0.1500 0.51275 ± 1 + 2.2 10.00 461.8 0.0625

10.60 53.28 0.1201 0.51294 ± 2 + 5.9 40.01 1297.9 0.0909

15.53 66.18 0.1143 0.51298 ± 1 + 6 . 7 90.476 1111.9 0.2355

V O L C A N I C R O C K S OF E A S T E R N C H I N A : ISOTOPIC I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 157

87Sr 2°~pb 2OTpb ZoSpb 238 U 232Th

86Sr ~sr Pb (ppm) U (ppm) Th (ppm) 2O4p b 2O4p b 2Cutpb 2O4p b 238U

0.70473 5 :2 + 3.3 2.91 0.65 3.02 17.403 15.417 37.292 13.7 4.82

0.70380 5 :2 - 9.9 3.24 1.24 4.89 17.948 15.410 37.938 23.9 4.09

0.70514 + 4 + 9.1 10.49 2.18 9.97 17.473 15.471 37.682 12.9 4.72

0.70540 5 :2 + 12.8 36.44 9.27 41.48 17.298 15.325 37.565 15.7 4.63

- - 17.460 15.447 37.606 -

0.71039 5 :9 + 83.6 28.26 4.43 14.99 17.711 15.510 37.938 9.8 3.50

0.70513 5 :8 + 8 . 9 13.11 2.68 12.50 17.464 15.465 37.739 12.7 4.82

0.70520 5 :7 + 9 . 9 4.21 0.56 2.75 17.511 15.521 37.759 8.3 5.04

0.70513 5- 2 + 8.9 4.39 0.71 2.29 17.486 15.303 37.574 10.0 3.32

0.70495 + 6 + 6.4 2.32 0.38 1.86 17.534 15.460 37.704 10.2 5.04

0.70495 5- 4 + 6.4 4.91 0.78 2.53 18.082 15.438 37.954 10.0 3.34

0.70544 5 :3 + 13.3 13.93 1.38 6.07 16.723 15.382 36.542 6.0 4.55

0.70527 5 :3 + 10.9 12.92 1.47 6.78 16.874 15.407 36.761 6.9 4.78

0.70510 + 2 + 8.5 12.46 1.70 7.92 16.996 15.382 36.948 8.3 4.83

0.70443 5- 3 - 1 . 0 3.95 1.24 4.80 17.626 15.432 37.691 19.5 4.00

0.70474 5- 4 + 3.4 4.12 1.35 5.10 17.668 15.411 37.645 20.4 3.90

0.70453 5 :5 + 0.4 3.77 1.71 4.29 17.628 15.433 37.711 19.4 3.79

0.70470 5- 3 + 2.8 3.26 0.76 3.69 17.599 15.421 37.716 14.6 5.00

0.70417 5 :3 4.7 4.26 1.50 5.79 17.671 15.404 37.621 21.8 4.00

0.70417 5- 5 - 4.7 2.05 0.31 1.29 17.471 15.407 37.512 9.2 4.37

0.70382 ± 3 -- 9.6 1.76 2.58 1.46 17.400 15.365 37.442 90.3 0.59

0.70417 5 :6 - 3 . 3 3.10 0.98 4.41 17.655 15.391 37.510 19.5 4.67

0.70497 5 :4 +6 .7 1.83 0.04 1.14 17.173 15.322 37.111 11.9 3.29

0.70494 5 :3 + 6.2 1.97 0.45 1.58 17.414 15.341 37.333 14.2 3.69

0.70408 5:1 5.9 4.35 1.84 6.6198 17.849 15.439 37.769 26.4 3.72

0.70357 5- 2 -- 13.2 3.41 1.96 9.0066 18.243 15.437 38.092 36.3 4.74

lower cycle. In contrast to the Datong basalts, the Hanobar basalts include both typical alkali basalts, including basanite, and typical tholeiite, mainly quartz tholeiite. The upper cycle Hanobar basalts are composed of quartz tholeiites (samples D-37

I 0 O0

and DF-1), whereas the lower cycle consists of essentially basanitic alkali olivine basalts and olivine tholeiite.

The REE patterns of the six Hanobar and two Datong basalts are plotted in Fig. 2. The

100

[] Datong

A Hanobar

10

1000

~') 100

t- o

..c

o ¢r

158 A.R. BASU ET AL.

J J i l t i I i ~ t ~ i i l [ T I Wudallanchi

[] Minxi • Kuandian

1000

Changbaishan

100

10

1

La Co Nd Sm Eu Gd" "['b Yb Lu

Fig. 2. Chondrite-normalized REE abundances of Cenozoic volcanic rocks of eastern China. Note complementary positive and negative Eu-anomalies between the basalts and the trachytes of Changbaishan. Gd* is the interpolated ratio.

V O L C A N I C R O C K S O F E A S T E R N C H I N A : I S O T O P I C I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 159

chondrite-normalized patterns for eight basalts show similar slopes in light REE enrichment de- spite significant differences in concentration. The patterns of the Neogene basalt north of Datong (BDT-1) and the Datong Quaternary basalt (BDTI-1) are similar to those of Hanobar basalts. However, within the Hanobar basalts, there are significant variations between rock types and stra- tigraphic positions. First, the stratigraphically lower basalts (DF-2, D-l , D-52 and D-11) have higher REE concentrations than the quartz tholei- ites of the top (DF-1 and D-37); the tholeiites make up the last phase of eruption in Hanobar. The tholeiites display a slight positive Eu-anomaly and a (La /Yb)N of 5.8-8.4, whereas the alkali basalts show ( L a / Y b ) N of 8-49. The lowermost flow of alkali olivine basalt (DF-2) in the Hanobar region demonstrates a distinctively different REE pattern relative to the other rocks (Fig. 2). The light portion of this rock's pattern is similar to and parallel to those of the other rocks of the region, but the heavier REE part of the pattern of Tb to Lu intersects at a high angle those of all other rocks. Clearly, this alkali olivine basalt has a different genetic history. Finally, there is a very good positive correlation of L a / S m ratios and the La contents between Datong and Hanobar basalts. This correlation and range of 2-3.7 in (La /Sm)N ratios implies equilibrium partial melting of the source region, rather than fractional crystallization in the same magma chamber, to produce each of the basaltic flows of Datong and Hanobar.

Five Kuandian basaltic rocks collected from a sequence that becomes progressively younger to- ward the top (Tables 1 and 2) are undersaturated with respect to SiO 2, and rich in TiO 2 and alkalis. All five rocks contain normative nepheline, olivine, and diopside; these lavas can all be termed basanites. REE abundances for these lavas (Table 2) show variations in their total concentrations of between 111 and 158 ppm, but their (La /Lu)N are restricted between 14 and 17. The chondrite- normalized REE patterns (Fig. 2) dip sharply to- wards the heavy REE, but in a parallel fashion. The narrow range in L a / S m values (4.6-5.3) for all five basalts is noteworthy as La abundance varies between 26 and 38 ppm. These REE pat- terns are consistent with the hypothesis that these five lavas may have originated by variable frac- tional crystallization in a magma chamber beneath

the Huangyishan volcanic cone in Kuandian. However, the Nd, Sr and Pb isotopic data of these samples to be discussed in a later section require a heterogeneous source for some of these basalts.

All the basaltic rocks studied from the Changbaishan area are alkali basalts except X-39, which is a quartz-normative tholeiitic basalt (Ta- ble 1). The basalts contain 2.5-3.5% N a 2 0 and 0.56 to 3.4% K20 and represent both potassic and sodic varieties. The REE data (Fig. 2) of nine whole-rock samples, including four lower basalts and five upper trachytes, show the trachytes hav- ing much higher REE concentrations, with the exception of Eu, than the basalts. The (La /Yb)N ratio in basalts and trachytes ranges from 4.90 to 8.6 and 10.2 to 17.9, respectively. Positive Eu- anomalies of different degrees are evident in the basalts, and strongly negative anomalies are char- acteristic of each of the five trachytes analyzed (Fig. 2), suggesting that the Eu-depletion in the trachytes is a result of plagioclase-crystal fractionation in the basaltic parent magma and that these two suites of rock are probably com- agmatic, related by assimilation-fractional crys- tallization processes.

The Wudalianchi volcanic rocks are char- acterized by their high N a 2 0 + K 2 0 contents and values for the K z O / N a 2 ° ratios higher than unity. Petrographically, these rocks are distinguishable from the alkali basalts by the presence of leucite and the absence of plagioclase. We prefer the generalized name of potassic alkali basalts for these rocks. The REE analyses of three basalts from Wudalianchi (Table 1 and Fig. 2) are very similar, showing a light REE-fractionated pattern with (La /Yb)N values of 27.9-33.5. In terms of their total REE contents and the high (La /Yb)N, the Wudalianchi basalts are the most conspicuous among all of the Cenozoic basalts of east China. The Mingxi limburgite of southeastern China is a basanite with strong silica undersaturation and highly fractionated REE enrichment pattern that has a (La /Yb)N ratio of 24.9. The REE pattern of this rock is strikingly similar to that of the Wudalianchi potassic volcanic rocks (Fig. 2).

Sm and Nd concentrations and the S m / N d ratios of all the volcanic rocks mentioned vary as a function of rock type. In general, the tholeiitic basalts have the lower concentrations of these elements and higher S m / N d ratios, followed by

160 A.R. BASU ET AL.

the alkal basalts, basanites, the limburgite, and the trachytes and the pumice. Rb and Sr concentra- tions of these rocks vary considerably. The highest concentrations of Rb and highest 87Rb/86Sr ratios are shown by the trachytes; the 87Rb/S6Sr ratio of one sample from Changbaishan exceeds 500. As the Rb and Sr contents systematically decrease from the basanites and the limburgites, through the alkali olivine basalts to the olivine tholeiites, the R b / S r ratios also decrease accordingly. In terms of U, Th, and Pb concentrations, the trachytes show the highest concentrations, fol- lowed by the Wudalianchi basanites, the Mingxi limburgite, and last, the alkali basalts and the olivine tholeiites. The 238U/2°4pb ratios are con- spicuously low in the Wudalianchi potassic volcanic rocks (6-8.3); all the other suites show ratios of 10-23. The limburgite and a Hanobar quartz tholeiite show much higher values of 36.3 and 90.3, respectively.

5.2. Nd and Sr isotopes

The 26 volcanic rocks show a wide range in their 143Nd/144Nd and S7Sr/86Sr ratios (Table 2 and Fig. 3). 143Nd/144Nd varies from 0.51298 in the Mingxi l imburgite to 0.51238 in the Wudalianchi leucite basanite. The corresponding

878r/86Sr ratios for these two rock types are 0.70357 for Mingxi and 0.70544 for the Wuda- lianchi basanite. These two values are the ex- tremes and, in general, define the range of the isotopic compositions of Nd and Sr for all the volcanic rocks. An exception is sample X-17 (Ta- ble 2), an alkali trachyte from Changbaishan with an unusually high 87Rb/86Sr ratio of 502 and 87Sr/86Sr ratio of 0.71039. Clearly an age correc- tion for the radiogenic growth of 87Sr is needed. Similar corrections also apply to samples X-24 up to X-1 in the Changbaishan Quaternary lava sec- tion of trachytic rocks (Table 2). Uncertainties in the ages of these samples, however, make the necessary correction rather tentative, and we opted not to correct the measured 87Sr/86Sr ratios using hypothetical ages of eruption. We suggest that for many of the trachytes from the Changbaishan area, the initial 87Sr/86Sr ratios may be lowered by as much as 0.003 if we assume that the Quaternary lava section is 0.6 Ma old at the bottom (sample X-24) and is becoming progressively younger, re- aching an age of 0.1 Ma (sample X-01) near the top. Assuming an age of 0.4 Ma for sample X-17 (Table 2), the initial 87Sr/86Sr ratio would be 0.70753, still much higher than any of the samples analyzed in this study. Clearly this rock shows the effect of crustal contamination, falling away from the main negative trend defined by all the other

0.513~ ~ ~ C ~ G B ~ o DATONG !

/ o I o o o 5131 I - - I

" ' ~ 0 C F.AR

0.5125 - - " ~ , '~

0 5123 I J I ] ~ ' ~ J I I l I

0 , 7 0 2 0 0. '7030 0.704-0 0 . 7 0 5 0 0.7060 0.7070 0.7080 87Sr/88Sr

Fig. 3. Correlated Nd and Sr isotopic ratios in volcanic rocks of eastern China. Mantle reservoirs of DMM, EMI, EMII and H1MU are from [19, 24, 25]. The fields of ocean island basalt data are from [19,20,21,22,23,24].

V O L C A N I C R O C K S O F E A S T E R N C H I N A : I S O T O P I C I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 161

volcanic rocks of Table 2 in the Nd-Sr isotope correlation diagram (Fig. 3).

A striking resemblance in Sr-Nd isotopic data between the Cenozoic volcanic rocks of eastern China and those from ocean island basalts is evident in Fig. 3, which also shows the limits of the present-day ocean island basalt fields [19-24]. This figure also shows an idealized field of mid- ocean-ridge basalt (MORB) and four hypothetical mantle components, DMM, EMI, EMII and HIMU, as defined [24, 25] from oceanic basalt data. In the framework of Fig. 3, all the analyzed samples, except the Changbaishan trachyte (X-17), are contained within the field of ocean island basMts. Further, all the Nd-Sr data show a well- defined trend between the MORB field and the EMI component. The Wudalianchi potassic basalt data coincide with the EMI field in Fig. 3. Among the marie lavas, individual rock types tend to show distinct isotopic ratios. This observation has been noted before, particularly for the Hanobar basalts by Song et al. [5].

5.3. Sr and Pb isotopes

A clear negative correlation is displayed be- tween the 87Sr//86Sr and 2°6pb/2°4pb ratios of the

volcanic rocks (Fig. 4). This relationship was also recognized, although as a weak negative correla- tion, by Peng et al. [3] for the volcanic rocks of eastern China; however, no clear relationship was discernible within individual suites in the data provided by Peng et al. The results from our study indicate a negative correlation for most of the individual suites studied. In particular, the Wudalianchi data are conspicuous because they show the least radiogenic 2°6pb/2°4pb (16.723) and most radiogenic 87Sr/S6Sr (0.70544) ratios. It is also interesting that most of the Changbaishan volcanic rocks, as well as some Hanobar and Datong basalts, plot near the Wudalianchi data in the region of the EMI field. The other end of this trend is defined by the Mingxi limburgite with 8VSr/86Sr and 2°6pb//2°4pb ratios of 0.70357 and 18.243 respectively. This overall negative correla- tion of the Chinese volcanic rocks (Fig. 4) con- verges at a sharp angle with the positive trend in 87Sr/86Sr versus 2°6pb/2°4pb space for MORB. Although a negative Sr-Pb isotopic trend is not restricted to basalts of east China and has been reported for other continental volcanic rocks, such as in northwestern U.S. basalts, the East African Rift Valley [26-28], and from the Deccan, Parana and some Karoo basalts, the present data are also

0.700

0 .707 .

0 .706-

~0.705

0.701-.

0.703.

0.702 -

m

°o +

[ ]

[] o

,,'~ oc~~ k MORB ./~7 /

i i i

17 16 19 20

me Pb/ ~4Pb

21 22

Fig. 4. 87Sr/arsr versus 2°rpb/2°4pb in Chinese volcanic rocks of this study. Field of Indian Ocean MORB from Mahoney et al. [22].

162 A.R. B A S U E T AL.

'-d

0.5134

0.5132

0.5130

0.5128

0.5126

0.5124

0.5122

16

O O

O

o

o DATONG

• KUANDIAN

o "g'UDI~..~CHI

%

i I I I

17 18 19 2O

2°6pb / 2°4pb

21

Fig. 5. 143Nd/144Nd against 2°6pb/Z°4Pb in the volcanic rocks showing a positive correlation between the MORB and EMI fields. Note the proximity of the Wudalianchi data to the EMI component, as in Figs. 3 and 4. The hypothetical mantle and MORB fields

are from [22,24,25].

similar to the observed negative trends in the Sr-Pb isotopic data from oceanic basalts of the Walvis Ridge [29] and the Hawaiian islands [30]. In addition, it is noteworthy that the low 2°6pb/2°aPb ratios of the Chinese lavas are similar to the P-Ti tholeiites of the Parana flood basalt province [31].

5.4. Nd and Pb isotopes

A plot of 14SNd/144Nd versus 2°6pb/2°4Pb for the Chinese volcanic rocks (Fig. 5) mimics the relationship shown in the plot (Fig. 4) of STSr/86Sr and 2°6pb/Z°4pb. As expected from the behavior of Sr and Pb isotopes in Fig. 4, a strong positive correlation is seen for all the 26 volcanic rocks of eastern China in Fig. 5. The trend begins with low 14SNd/1~Nd and 2°6pb/Z°4pb ratios of 0.51238 and 16.723 respectively for the Wudalianchi basanite, and terminates with high ratios of 0.51298 and 18.243 respectively for the Mingxi limburgite. The similarity of the Wudalianchi data with the hypothetical field of EMI in Fig. 5 is consistent with the observations made earlier in connection with the 87Sr/86Sr versus 2°6pb/2°4pb plot of Fig. 4. It is worth noting that the general field of 143Nd/144Nd and 2°6pb/2°4Pb space de-

fined by the 26 volcanic rocks covers the field of data in the same space for basalts of the Hawaiian islands and the Walvis Ridge. A small scatter in the STSr/S6Sr ratios of the Changbaishan volcanics is also due to variable R b / S r ratios in the trachytes, for which no correction for radiogenic growth in SVSr/S6Sr ratios could be made due to uncertainties in their ages of eruption.

5.5. Pb-Pb isotopes

The Pb isotopic data of the 26 volcanic rocks from northeastern China are compared in Fig. 6 and 7. These ratios are similar and are nearly contained within the ranges in Pb isotope ratios previously reported by Peng et al. [3]; only for 2°8pb/2°4pb ratios are our measured values dis- tinctly lower than the range of 36.98-38.91 re- ported by Peng et al. The least radiogenic samples measured in our study, w i t h 2°6pb/Z°4pb ratios of 16.723-16.996, are from Wudalianchi. In the 2°7pb/2°4pb versus 2°6pb/2°4pb correlation plot (Fig. 6), a fairly linear array is observed. This array is subparallel to the 1.77-Ga general trend for MORB, first recognized by Tatsumoto [32] and later referred to as the Northern Hemisphere Reference Line (NHRL) by Hart et al. [33]. Most

V O L C A N I C R O C K S O F E A S T E R N C H I N A : I S O T O P I C I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 163

16.0

-~ CHANGBAESHAN <> DATONG /

1 5 . 8 - [] mNm O ~JUm,mNC~I ~.~,/~"

/ ~ 1 5 . 6 -

= I = ~ - ~ % - - ~ D ~15.4- o o o ~ . . ~ , ~ e ' ° " - ~ 1 . - J " ~

15.2-

15.0 I I I I I

16.6 17.0 17.4 17.8 18.2 18.6 19.0

208 204 Pb/ Pb

Fig. 6. A plot of 2°Tpb/2~pb versus 2°sPb/Z°4Pb in the eastern Chinese volcanic rocks shown in relation to the 1.77-Ga NHRL [32,33] and the 4.55 Ga Geoehron. EMI and DMM fields are from [24,25]. Note the distribution of the data array on both sides of

the geochron.

(60%) of the d a t a p o i n t s fal l to the left o f the 4.55

G a G e o c h r o n on this d i a g r a m (Fig. 6), and, t aken

toge ther , all the d a t a e x t e n d to the lef t o f the E M I

and D M M c o m p o n e n t s as a pos s ib l e m i x i n g e n d

m e m b e r , i f the l inear a r r ay r ep re sen t s m i x i n g be-

tween the M O R B and E M I . M o s t i m p o r t a n t l y , the

38.6

38.2 --

37.8--

n. 37.4--

37.0 --

36.6 -

36.2 -

35.8

16.~

,,." ~ ~ / ,/ ~ . ~ o/

/ ; + ~>~ ,/

o" /

0 . . /

+ ceaN~smH~ [] H~L-~OBAR

/ /

I

D

/

/

o DATONC $ KUANDIAN

O ~ J D ~

i I I I I

17.0 17.4 17.8 18.2 10.6 10.0 19.4 208 204

Pb/ Pb

Fig. 7. 2°sPb/2°4pb against 2°6Pb/=°4Pb for the volcanic rocks compared with the 1.77-0a NHRL and EMI and DMM. This data array is comparable to that of ocean island basahs showing a Dupal signature and corresponds to the EMI branch of the ocean basalt

array [24]. Field of Indian Ocean MORB from [22].

164 A.R. B A S U ET AL.

similarity of the data from the Changhaishan, Hanobar and Wudalianchi volcanic rocks with the hypothetical end members EMI and DMM in Fig. 6 is noteworthy.

The 2°spb/2°4pb versus 2°6pb/Z°4pb correla-

tion of the volcanic rocks of this study is shown in Fig. 7. Most of the relationships displayed in Fig. 6 are also evident here, except that the linear array defined by the data points is displaced consider- ably above the N H R L and parallel to it, and pointing to lower 2°8pb/2°4pb and 2°6pb/Z°4pb

ratios and to the Indian Ocean-type MORB [22] at the high end. The relative uniformity of the 2°7pb/2°4pb ratios of all 26 volcanic rocks can be contrasted here with their varying 2°8pb/2°4pb ratios. The highest 2°8pb/Z°4pb and 2°6pb/2°4pb

ratios are in the Mingxi limburgite, and the lowest values are in the Wudalianchi leucite basanite at 16.723 and 36.542, respectively (Table 2). The data array of Fig. 7 shows a clustering between the EMI and DMM fields. In this cluster, the Changbaishan suite of rocks and the Kuandian basalts are characterized by their small variations in Pb isotopic compositions. This relationship, which is not discernible in the Nd and Sr isotopic data, may have implications regarding any com- agmatic relationship between the late Tertiary basalts and the Qua te rna ry trachytes of Changbaishan, as well as for Kuandian basalt petrogenesis.

Also note that the variations of Pb isotopic ratios within any given suite are, generally, sub- parallel to the NHRL. Vertical deviations in 207pb / 2o4 Pb and 208 Pb/2°4pb ratios of the volcanic

rocks from the two lines marked N H R L in Figs. 6 and 7 can be calculated using the approach of Hart [34]. This deviation, called the Dupal anomaly [34], is conspicuous in the present data set and measures over 100 for A8/4Pb and over 6 for A7/4Pb. The magnitude of these anomalies is comparable to the EMI branch of the anomaly observed in oceanic basaltic rocks [19]. Thus, the entire range of 208 Pb/204 Pb and 206 Pb/204 Pb ratios observed (Fig. 7) falls mostly within the range found in ocean island basalts constituting the Dupal anomaly. This overlap and the similarity of the trends in the Pb isotopic ratios are prominent at the low 2°spb/Z°4pb and 2°6pb/2°4pb end of the Dupal trend; the Wudalianchi data (Fig. 7) pull this trend even below the low end of the

oceanic basalt data. The two Dupal anomaly Pb reference lines for the Chinese volcanic rocks (Figs. 6 and 7) may be described as A7/4 Pb = 6 and A8/4 Pb = 70.

6. Discussion

6.1. Petrogenesis of volcanic rocks of eastern China

Although the geochemical data set presented here does not provide unique models of petrogene- sis for the six volcanic suites studied, some general conclusions can be drawn about how the heteroge- neous magma sources, fractional crystallization, and crustal contamination may be principal causes of major- and minor-element and isotopic varia- tions among the volcanic rocks. Previous workers [2-4] have observed the general absence of mixing relationships in incompatible trace-element abun- dances, isotope ratios of Pb, Nd and Sr, and bulk-rock major-element chemical compositions. These observations were used to rule out continen- tal crustal contamination as a primary cause of petrogenetic variations. The analytical results pre- sented here strongly support these conclusions. Except for one alkali trachyte (X-17 in Table 2) from Changbaishan, we rarely observed initial 87Sr/86Sr ratios greater than 0.7050 as implying significant crustal assimilation.

Each volcanic suite studied here, particularly Changbaishan, Datong, Wudalianchi, Hanobar and Kuandian, has a close similarity in isotopic composition and in the REE concentration pat- terns, in spite of the multiple periods of eruption spanning much of the late Tertiary to Quaternary. In evaluating the petrogenesis, the role of crustal contamination in causing the observed geochem- ical variation can be ruled out. This is because the Rb, Sr and several other incompatible trace ele- ment abundance patterns of the basic lavas are indicative of mantle sources. The striking resemb- lance of the correlated Nd-Sr isotopic patterns (Fig. 3) of these lavas with those of the ocean island basalts also reflects mantle source char- acteristics. These observations lead us to suggest that at each volcanic center, magmas originated from a common source by partial melting of the upper mantle. This mantle retained some inherent isotopic heterogeneity due to its complex evolu- tionary history. Different degrees of partial melt-

V O L C A N I C R O C K S O F E A S T E R N C H I N A : ISOTOPIC I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N I C - T Y P E M A N T L E R E S E R V O I R S 165

ing of this source may have caused the observed variation in REE patterns. Partial melting at dif- ferent depths and at variable pressures could be responsible for the variations in the major-element compositions of the basaltic rocks. For example, in the Hanobar suite, typical alkali basalts and quartz tholeiites are found in the lava sequence along with gradational alkali olivine basalts and olivine tholeiites (Tables 1 and 2). Experimental phase-equilibria studies at high pressures [35] demonstrate that these magma types cannot evolve from each other at shallow levels by magmatic differentiation, although crystal fractionation at high pressures allows crossing of the composi- tional barriers. Because the Nd, Sr and Pb isotopic ratios of the different basaltic rock types at H a n o b a r are not identical, high-pressure fractionation of a homogeneous magma chamber at depth can be ruled out. However, a hypothesis involving partial melting at different depths in a vertically heterogeneous mantle will be more sui- table for explaining the major- and trace-element and isotopic variations of the Hanobar basalts. The steep, intersecting heavy REE-depleted pat- tern (Fig. 2) of Hanobar alkali basalt DF-2 (Table 1) clearly suggests the contribution of garnet dur- ing partial melting in the deeper mantle, an ob- servation consistent with a vertically heteroge- neous mantle in Hanobar basalt petrogenesis. Al- though this interpretation is similar to the ob- servations of Nakamura et al. [9] for northeast Chinese basalts, Zhi et al. [4] prefer clinopyroxene and garnet fractionation in the Hanobar alkali basalts for producing the observed geochemical trends.

Many of the volcanic rocks of the present study, such as those of Hanobar, Datong, Kuandian and Mingxi, carry mantle-derived, garnet- and spinel- peridotite xenoliths, and various types of mega- crysts. These inclusions support the interpretation that these basaltic rocks are derived from the upper mantle without considerable interaction with the crust. Thus, the Pb, Nd, and Sr isotopic sig- natures of these basalts can be directly correlated with their upper mantle source regions. For the Changbaishan volcanic rocks, there is some indi- cation of crustal contamination, particularly in the upper trachytes, which show a trend of more radiogenic 87Sr/86Sr ratios with decreasing Sr con- tents. However, the overall REE patterns of all the

different rock types, including the alkali basalts of the lower sequence, and the uniformity of the Pb isotopic signature throughout the entire suite, strongly suggest that the Changbaishan lavas be- long mostly to a comagmatic suite, derived prin- cipally from the upper mantle. Similarly, the REE patterns of Wudalianchi potassic basalts suggest a comagmatic character. Their overall similarity in bulk major- and minor-element, REE, and Pb, Nd and Sr isotopic geochemistry with other potassic basalts worldwide [36-38] imply a common origin in the upper mantle.

6.2. Implications for mantle source reservoirs

Previous workers [2,3] recognized that a hori- zontally and vertically heterogeneous mantle, with a "continental-like" mantle to the west, offered the most reasonable explanation for the geochem- ical data of the basalts. Peng et al. [3], in particu- lar, recognized a distinctive continental litho- spher e component (keel to the continent) with time-integrated, low U / P b ratios involved in the generation of these basalts. The data presented here and compared in the framework of the hypo- thetical mantle reservoirs (Figs. 3-7) further elucidate these source characteristics. The negative correlation observed between 143Nd/ l~Nd and 87Sr/86Sr ratios (Fig. 3) strongly indicates two mantle reservoirs in the origin of these rocks: a MORB-like source with a fairly high 143Nd/144Nd and low 87Sr/~6Sr, and another component, the EMI source of Zindler and Hart [25]. Clearly, the Wudalianchi data are similar to the EMI compo- nent (Fig. 7). It has been recognized that the EMI signature is missing in mantle xenolith-bearing continental alkali basalts [24], perhaps due to the storage of this component in the continental litho- sphere, which is not sampled by the alkali basalts originating in a deeper source. In our opinion the coherent trends shown by the different volcanic suites of eastern China, particularly in plots in- volving different isotope ratios, require mixing between mantle components without significant crustal contribution. It may not be too surprising that we have discovered one of the missing com- ponents required to describe the oceanic basalt data-set beneath the northeastern continent of China, because the concept of delamination of continental lithosphere and its entrainment in the

166 A.R. B A S U E T AL.

upper mantle convection pattern has found some acceptance among the current researchers [e.g., 24,39].

It is our contention that the Nd, Sr and Pb isotopic systematics of the Cenozoic volcanic rocks of eastern China can be attributed to two compo- nents: a MORB-like reservoir and an EMI compo- nent. This conclusion clearly follows from the 87Sr/86Sr versus 2°6pb/2°4pb (Fig. 4) and the 143Nd/144Nd versus 2°6pb/2°4Pb (Fig. 5) correla- tions of the volcanic rocks. In the Sr-Pb diagram (Fig. 4), the negative correlation between the two end-members, EMI and MORB, is consistent with the radiogenic 87Sr/s6Sr and unradiogenic 2°6pb/Z°4pb character of the EMI. Similarly, the Nd-Pb plot of Fig. 5 shows a strong positive correlation of the data set; EMI has both low 2°6pb/2°4pb and 143Nd/ln4Nd ratios. In both fig-

ures, the Wudalianchi and the Changbaishan data sets are similar to the EMI component. The Pb-Pb plots of Fig. 6 and 7 show two linear arrays parallel to the N H R L , and the mixing relationship between EMI and D M M is not discernible be- cause MORB plots parallel to NHRL. Also, note that there is no evidence of a data array in the Pb-Pb plots (Figs. 6 and 7) between EMI and the lowest 2°6pb/2°4pb ratio of normal MORB (N- MORB). It is noteworthy from Figs. 4 -7 that the EMI component, as defined by Hart [24], does not accurately serve as the end-member for eastern Chinese lavas; this disparity was also noted by Song et al. [5]. Possibly the EMI component is yet to be constrained in the Pb-Pb plot, and our data may point to slightly lower T h / U and U / P b ratios and a growth history in the subcontinetal mantle reservoir for the EMI component.

6.3. Dupal anomaly in eastern Chinese basalts

As mentioned previously, the range of 2°spb/ 2°4pb and 2°6pb/2°4pb ratios observed in the east- ern China volcanic rocks falls within that observed in ocean island basalts comprising the Dupal anomaly, particularly at the low end of the Dupal trend. Initially, Har t [34] defined the Dupal anomaly using the EM components characterized by high 87Sr/86Sr and high A8/4Pb. Later, values of 87Sr/86Sr > 0.705 and A8/4 Pb > +60 were considered by Hart [24] as Dupal values. Both Dupal and EM components were considered char-

acteristic of southern hemisphere oceanic basalts. However, the rocks of our study area also docu- ment the EM (Dupal) character, particularly the volcanic rocks of Wudalianchi and Changbaishan,

For the Wudal ianchi and Changba ishan volcanic suites, we have argued that crustal assimi- lation was minimal and that the isotopic sig- natures of these volcanic suites were inherited from the subcontinental mantle. It is instructive to compare the available Pb isotopic data of lower crustal xenoliths and granulites because they may reflect the isotopic signatures of subcontinental mantle. Two suites of mafic xenoliths from Lashaine [40] in eastern Africa and Lesotho [41] in southern Africa have unradiogenic Pb isotopic compositions and plot to the left of the geochron in Fig. 6, covering the entire range of our volcanic data to the left of the geochron. Similarly, the xenolith data plot along the trend of the Wudalianchi volcanic rocks at lower values of Z°6pb/2°4pb in Fig. 7, above and parallel to the NHRL. These two suites of mafic rocks, if present as a source component of the Chinese volcanic rocks, may represent remelted basaltic underplates that assimilated unradiogenic crust, or, alterna- tively, they may represent older basaltic under- plates that were melted and intruded with or without accompanying assimilation. Although, the overall isotopic characteristics of these lower crustal mafic granulites match very well the pro- posed EMI component of this study, we prefer a subcontinental mantle source for the EMI compo- nent because: (1) the correlative isotopic ratio trends and the geochemical variations of the vari- ous suites do not support crustal assimilat ion- fractionation processes in the evolution of these lavas, (2) some of the alkali lavas contain mantle xenoliths, and (3) the ranges of the isotopic ratios are well within the field of ocean island basalts. In our interpretation, the complete data set of Pb- Nd-Sr isotope correlation diagrams represents a mixture of two components, a MORB source of depleted upper mantle beneath the continents and an EMI component (low U / P b and S m / N d , and fairly high and aged T h / U and R b / S r ) of subcon- tinental lithospheric mantle. The array resulting from combination of these two sources is similar to the ranges of isotopic compositions observed in ocean island basalts that have the Dupal anomaly. Thus, the Dupal signature as observed in ocean

V O L C A N I C R O C K S O F E A S T E R N C H I N A : I S O T O P I C I M P L I C A T I O N S F O R O R I G I N F R O M S U B O C E A N 1 C - T Y P E M A N T L E R E S E R V O I R S 167

Tan-Lu Fault Sea of ~ - ~ Wvdnll~w~ C ~ Jatmll ~ J ~ u a

EURASIAN pLATE KULA-PACIFIC

Wide Zone of volc~mimn as ridge migzat~ west Sea of

Japan Japan PACIFIC PLATE

Fig. 8. Schematic diagram showing the effects of the Pacific Plate subduction (A) causing arc volcanism, back-arc spread- ing and rifting in the continental margin. (B) and (C) show an alternative model of the submergence of the Kula-Pacific Ridge beneath the continental plate of Asia around (B) 90 and (C) 70 Ma, after Uyeda and Miyashiro [45]. The submergence of the ridge produced thinning, and, ultimately, the breaking of the plate to form the Sea of Japan. This tectonism also in- flicted tensional forces on the eastern continental margin, producing the rift system and the accompanying volcanism. The volcanism shows isotopic signatures partly inherited from the submerged ridge and from the overlying subcontinental

lithospheric mantle.

island basalts of the southern hemisphere is not a unique feature restricted to the suboceanic mantle. Using the present data set, we infer that the Dupal characteristic of ocean island basalts may be due to subducted or otherwise delaminated continental lithospheric fragments in the source regions of these basalts.

6.4. Geochemical evidence for the plate tectonic evolution of eastern China

The general interpretation that the Cenozoic eastern Chinese volcanic rocks were produced by partial melting of mantle sources characteristic of a MORB-like end-member and a subcontinental enriched mantle (EMI) signature may need further exploration. This interpretation is particularly rel- evant with regard to the post-Jurassic, plate tectonic evolution (Fig. 8A) of the Asian conti- nent, Japan, the western Pacif ic-Kula plate, and

of the Kula-Pacif ic Ridge [e.g. 42,43,44]. Here, we evaluate our geochemical data in the light of a study by Uyeda and Miyashiro [45] which sug- gested the submergence of the Kula-Paci f ic Ridge beneath the Asiatic plate (Figs. 8B and C) in the Late Cretaceous, imposing strongly tensional tectonic stresses along the continental margin of Asia.

According to Uyeda and Miyashiro [45], the Kula-Pacif ic Ridge approached the Asian conti- nent at 120 Ma with rapid underthrusting of the Kula plate. At 90 Ma, the ridge was closer to the continental margin and the thermal effects from the gently dipping Kula plate caused thinning of the continental lithosphere, producing volcanism. At 70 Ma, the ridge was submerged beneath the continental plate of Asia, producing further thin- ning, and, ultimately, breaking the continental lithospheric plate to form the Sea of Japan. The same tensional forces have continued to operate, since the Cenozoic, on the eastern continental margin of China, producing the rift system of eastern China and accompanying volcanism.

The Cenozoic volcano-tectonic evolution of northeastern China is compatible with the scenario just discussed. The thermal effect of ridge subduc- tion reduces the thickness of the subcontinental lithospheric plate, which is eventually broken by tensional forces. Magmatism results from two sources: (1) a source initially belonging to the suboceanic asthenosphere and now emanating from the subducted ridge beneath the continent, and (2) the heterogeneous source of the mantle part of the subcontinental lithosphere, which par- tially melts due to heating by the subducted ridge and due to thinning. The first source component is clearly the MORB source and the other is the EMI component, equated here with the subcontinental lithosphere of northeastern China. The end result is that the volcanism shows isotopic signatures which reflect mixtures of MORB mantle inherited from the ridge, and of the pre-existing EMI-like subcontinental lithospheric mantle above this ridge. Mixing between these two sources, in our view, may have produced the Pb-Nd-Sr isotopic mixing array observed in the Cenozoic volcanic rocks of eastern China. Alternatively, the MORB- source may have originated from the pre-existing subcontinental asthenosphere (Fig. 8A) in which flow was perturbed due to continued subduction

168 A.R. BASU ET AL.

to the east (Nakamura and Tatsumoto, in prep.). These perturbations may have caused thermal up- welling resulting in the back-arc spreading of the Japan Sea and in the development of the North- eastern China Continental Rift System.

7. Conclusions

The geochemical and isotopic data of the Cenozoic volcanic rocks of eastern China show some consistent patterns, which can be explained in the context of mantle reservoirs based on the oceanic basalt data. The covariation of 143Nd/ 144Nd vs 87Sr/86Sr, 87Sr/86Sr vs 2 ° 6 p b / 2 ° n p b , a n d

143Nd/144Nd vs 2 ° 6 p b / 2 ° n p b in t hese vo l can i c

rocks indicates two distinct end-members: (1) a depleted MORB source and (2) an EMI-like en- riched mantle reservoir, usually described as a restrictive source component associated with southern hemisphere oceanic basalts. In the Pb-Pb isotopic variation diagrams, our volcanic samples describe linear arrays parallel to the N H R L that are based on oceanic basalt data. Furthermore, the present data demonstrate a time-integrated high T h / U ratio for the EMI-like component similar in magnitude to that found in ocean island basalts belonging to the Dupal anomaly. Thus, the con- tinental volcanic rocks of eastern China show the EMI (Dupal) anomaly.

The two end-members inherent in the isotopic data from all our eastern China volcanic rocks are consistent with the subduction of the Pacific plate to the east. They are also consistent with the proposed model of plate subduction followed by ridge submergence beneath the eastern Asian con- tinental margin during the Late Cretaceous. These two end-members are the M O R B component, de- rived from the subducted ridge, and the EMI component, which was already present as the sub- continental mantle lithosphere of eastern China.

Acknowledgements

This research was partially supported by the United States National Science Foundation ( INT- 8312284), jointly with Academica Sinica in China. Professors Tu Guangchi and Chen Yu-Wei, among others, are thanked for their generous hospitality during A.R. Basu's and M. Tatsumoto 's visit to China. We also thank our colleagues at the U.S.

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Geological Survey in Denver, Colorado, and at the University of Rochester in New York for hospital- ity extended to J. Wang, W. Huang and G. Xie. This paper has benefited from thorough reviews by F. Frey, J. Mahoney, W.R. Premo and R.E. Zartman.

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