[The Journal of Geology, 2004, volume 112, p. 593–605]䉷 2004 by The University of Chicago. All rights reserved. 0022-1376/2004/11205-0006$15.00 593
Western and Eastern Sino-Korean Craton: Constraints from
Geochemistry of Basalts and Mantle Xenoliths
Yi-Gang Xu, Sun-Lin Chung,
1Jinlong Ma, and Lanbin Shi
2Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China (e-mail: yigangxu@gig.ac.cn)
A B S T R A C T
Cenozoic basalts from both sides of the Daxin’anling-Taihang gravity lineament that separates the Sino-Korean craton (SKC) into western and eastern parts have been studied. In the western SKC, magmas evolved from xenolith-bearing alkali basalts of Late Eocene–Oligocene age to coexisting alkali and tholeiitic basalts of Late Miocene–Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and an increase in Yb content. Sr-Nd isotopic ratios and relative abundances on incompatible elements are consistent with a prevailing asthenospheric origin, although lithospheric mantle may have also contributed to these basalts. This temporal variation in basalt geochem-istry is interpreted as reflecting progressive lithospheric thinning in the western SKC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern SKC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magmatism in the Late Cretaceous–Early Tertiary. Such contrasting lithospheric processes may reflect diachronous extension in the SKC, with initial extension in the eastern part owing to the Late Mesozoic paleo-Pacific subduction and subsequent extension in the western SKC induced by the Early Tertiary Indian-Eurasian collision. An implication is that the lithospheric mantle in the western SKC is relatively old compared with that beneath the eastern SKC, which may be a mixture of old lithospheric relicts and newly accreted mantle. This predicted lithospheric architecture is con-sistent with Sr-Nd isotopic data and recent Re-Os age determinations from mantle xenoliths included in Cenozoic basalts.
Introduction
The Sino-Korean craton (SKC) in eastern China is an important natural laboratory for studying tem-poral change in the lithosphere because of the oc-currence of Ordovician diamondiferous kimber-lites, Mesozoic lamprophyre-basalt and Cenozoic basalts, and the presence of deep-seated xenoliths in these magmas (E and Zhao 1987; Liu et al. 1992b; Lu et al. 1995; Zhang et al. 2002). The SKC expe-rienced widespread thermotectonic reactivations during the Late Mesozoic and Cenozoic, which re-sulted in replacement of the old, cold, thick, and depleted lithospheric mantle by young, hot, thin,
Manuscript received November 18, 2003; accepted May 15, 2004.
1Department of Geosciences, National Taiwan University, Taipei, Taiwan.
2Institute of Geology, State Seismological Bureau, 100029 Beijing, China.
and fertile mantle (Menzies et al. 1993; Griffin et al. 1998).
Several studies further suggest a partial replace-ment and that the subcontinental lithospheric mantle (SCLM) in this region consists of old lith-osphere in the uppermost part and newly created “oceanic-type” lithosphere in the lower part (Fan et al. 2000; Xu 2001). These studies focused on the region east of the Daxin’anling-Taihang gravity lin-eament (DTGL), hereafter referred to as the eastern SKC (fig. 1). Despite studies of Miocene basalts and included xenoliths from Hannuoba (Song and Frey 1989; Song et al. 1990; Zhi et al. 1990; Basu et al. 1991; Tatsumoto et al. 1992; Gao et al. 2002; Xu 2002), little is known about the Cenozoic litho-spheric evolution in the region west of the DTGL (i.e., the western SKC). In fact, basaltic volcanism is widespread in the western SKC and began in the Late Eocene and Oligocene and continued into the
594 Y . - G . X U E T A L .
Figure 1. Simplified map showing distribution of Cenozoic rifting systems and basalts in the Sino-Korean craton (SKC), eastern China (modified after Xu 2002). Note that the SKC is cut by two major geological and geophysical linear zones: the Tan-Lu fault zone (TLFZ) to the east and the Daxin’anling-Taihang gravity lineament (DTGL) to the west. Two rift systems (Yinchuan-Hetao and Shaanxi-Shanxi) occur in the western SKC. The North China rift system is located in the eastern SKC. Two shaded and dashed lines outline the central zone that separates the western and eastern zones of the SKC (after Zhao et al. 2001).
Miocene and Quaternary (Fan and Hooper 1991; Liu et al. 1992b).
This article focuses on the western SKC basalts. Our objective is to use basalt geochemistry to de-fine melting conditions and temporal change in the lithospheric thickness in the western SKC. We use the lithospheric lid model described by Ellam (1992) and Fram and Lesher (1993) and also compare basaltic evolution trends in the western and eastern SKC. In conjunction with xenolith data, inferences are made regarding the heterogeneous composition and architecture of the lithospheric mantle beneath the SKC. We also discuss the cause and mechanism for thermotectonic destruction of the lithospheric root beneath eastern China during Late Mesozoic– Cenozoic time.
Geologic Background
The SKC, one of the oldest continental nuclei on earth (3.8–2.5 Ga; Jahn et al. 1987; Liu et al. 1992a), is crosscut by two large-scale geological and geo-physical linear zones. To the east, the craton is cut by the Tan-Lu Fault Zone (TLFZ), which is asso-ciated with significant Cenozoic and Mesozoic vol-canism. To the west, it is traversed by the
north-south-trending DTGL (fig. 1), which separates two topographically and tectonically different regions (Ma 1989). Lowlands that include the Huabei Plain, the Bohai Sea, the highlands of Shandong, and the Liaodong Peninsula dominate the eastern SKC. During Late Mesozoic and Cenozoic time, this re-gion experienced widespread lithospheric
exten-sion, producing the north-northeast-trending
North China rift system and widespread volcanism (fig. 1). The crust beneath this region is thin (!35
km) with a minimum crustal thickness of∼28 km beneath the Bohai Sea. The regional Bouguer grav-ity anomaly is weakly negative to positive, and heat flow is high, so the lithosphere is inferred to be thin (!80–100 km; Ma 1989).
In contrast, the part of the western SKC that in-cludes the Yan and Taihang Mountains and the Loess Plateau is characterized by thicker crust (140
km), negative Bouguer gravity anomalies, and lower heat flow, all of which reflect a thick (1
100-km) lithosphere (Ma 1989). Although widespread extension is absent, two isolated Cenozoic rift sys-tems, the Yinchuan-Hetao rift and the Shaanxi-Shanxi rift, occur in the western SKC (fig. 1). The Shaanxi-Shanxi rift, a series of en echelon grabens, extends from the southern margin of the Loess
Pla-Table 1. Petrographic Characteristics of Cenozoic Basalts from the Western Sino-Korean Craton
Locality and rock type
Phenocryst Composition of groundmass phases
Alteration Xenolith and xenocryst Mineral % (Mm) Ol Plag Cpx Opaque Glass
Datong:
Alkali basalts Ol 10–20 .2–1.0 5–10 50–60 5–15 5–10 … … Rare small peridotite
Ol 1–10 180 Tholeiites Ol 5–15 .2–1.0 !1 170 !1 !1 … … Plag 5–15 .2–.5 Cpx 5–20 .2–1.0 Fangzhen: Ol basalts Ol 5–10 .2–2.0 5–15 40–70 15–25 !5 5–20 Idd,!5% … Plag 5–10 .2–1.0 Cpx 1–5 .5–1.5 Yangyuan:
Alkali basalts Ol ∼10 .3–.7 5–10 60 5–10 !5 20 … Abundant peridotite
and pyroxenite
Plag 1–10 .2–.5 Feldspar megacrysts
Yinxian:
Alkali basalts Ol ∼10 .2–.5 ∼5 150 10–20 ∼5 10–20 Idd,!10% …
Plag 5–10 .2–.5
Cpx ∼5 .2–1.0
Fanshi:
Ol basalts Ol ∼5 .1–.7 … 55 25–30 !2 … Idd, 10% Peridotite and rare
pyroxenite xenoliths
Plag 20–30 .5–1.0 Vesicules
Note. Ol p olivine Cpx p clinopyroxene Plag p plagioclase Idd p iddingsite; ; ; .
teau north-northeastward across the plateau, delin-eating a roughly S-shape form. Although rifting started in the southern part of the Shaanxi-Shanxi rift system in the Late Eocene or Early Oligocene, major extension occurred in the Neogene and Qua-ternary (Ye et al. 1987; Ren et al. 2002). Rift activity apparently migrated from southwest to northeast during the Oligocene to Pliocene epochs (Ye et al. 1987). In contrast to the other parts of the North China rift system, most grabens in the Shaanxi-Shanxi rift lack volcanism, except in the Datong graben, where a series of volcanic rocks were erupted during the Late Eocene to Quaternary (fig. 1). Some of these mantle-derived rocks carry deep-seated peridotite and pyroxenite xenoliths.
For this study, basalts were collected from five localities in the Datong graben and its adjacent regions, namely the Late Eocene Fanshi and Yinx-ian (38–40 Ma), the Oligocene Yangyuan (30–35 Ma), the Miocene Fengzhen (7–14 Ma), and the Quaternary Datong (!1 Ma) regions (Liu et al.
1992b). A brief summary of the petrographic char-acteristics of lavas from each locality, including phenocryst assemblages and grain sizes, degrees of alteration, and presence or absence of xenoliths, is presented in table 1. Most of the lavas are char-acterized by a phyric texture. Olivine is an almost ubiquitous phenocryst phase, often occurring with clinopyroxene (augite) and plagioclase (labradoritic composition). The groundmass ranges from glassy
to cryptocrystalline. The identifiable components of the groundmass are olivine, plagioclase, clino-pyroxene, and opaque minerals, mostly magnetite. The rocks are generally fresh with the only indi-cation of altering being iddingsitization of olivine.
Methodology
Theoretical modeling suggests that only a small amount of melt can be produced from the litho-spheric mantle by conduction of heat from the as-thenosphere (McKenzie and Bickle 1988; Arndt and Christensen 1992). However, hydrous or veined lithospheric mantle may be a source of extension-induced magmas (Gallagher and Hawkesworth 1992; Leeman and Harry 1993), whereas dry and refractory lithospheric mantle is likely to be a me-chanical barrier that halts the upward progression of the asthenospheric melting column (Ellam 1992; Langmuir et al. 1992). Experimental studies also demonstrate that more SiO2-undersaturated (i.e., alkaline) magmas are produced at higher pressures than tholeiitic lavas (e.g., Falloon et al. 1988). For example, Nohda et al. (1991) reported that Ceno-zoic tholeiites in North China were generated at pressures of 15–20 kbar (50–60 km) and alkali ba-salts at 25–30 kbar (180 km). Given that the
lith-ospheric mantle and asthenosphere generally have different geochemical and isotopic signatures (Menzies 1990; DePaolo and Daley 2000), it can be
596 Y . - G . X U E T A L .
inferred that the lithosphere is180 km thick if the
alkali basalts have a SCLM isotopic signature. On the other hand, if the tholeiitic basalts have an as-thenospheric signature, the lithosphere is inferred to be !60 km thick. Because typical continental
lithosphere extends to about a 100-km depth and the crust is about 40 km thick, the depth for gen-erating most basaltic magmas corresponds to the depth interval of typical SCLM prior to extension (DePaolo and Daley 2000). In the eastern SKC, stud-ies have already shown that Mesozoic basalts in-deed have a SCLM signature (negativeNdand low Nb/La), in contrast to the dominant asthenospheric signature (positiveNdand high Nb/La) observed in Cenozoic basalts (Xu 2001; Zhang et al. 2002). This temporal change in magma source has been inter-preted as a result of loss of the lithospheric keel in Late Cretaceous time (Xu 2001; Xu et al. 2004).
Geochemistry of the asthenosphere-derived lavas is dependent on the melting interval between the depth at which the adiabatic geotherm crosses the mantle solidus and the depth of the lithosphere-asthenosphere boundary (Langmuir et al. 1992). For example, the mean degree of partial melting is rel-atively smaller and melting depth is higher in regions with thick lithosphere. Accordingly, the geochemistry of basalts can be used to monitor var-iation in lithospheric thickness through time (El-lam 1992; Fram and Lesher 1993; DePaolo and Daley 2000; Wang et al. 2002). Assuming a lith-ospheric lid effect, alkali basalts are likely gener-ated when the lithosphere is relatively thick (180
km), while the eruption of tholeiites is largely con-fined to zones of thinned lithosphere (!60 km). In
a given region, a temporal transition from alkali to tholeiitic basalt may be accompanied by progres-sive lithospheric thinning due to convective re-moval or replacement of the lithosphere by the as-thenosphere (Perry et al. 1987; Ellam 1992; Fram and Lesher 1993; Kerr 1994; DePaolo and Daley 2000). The opposite transition, from tholeiitic to alkali basalt, may be indicative of lithospheric thickening as the asthenosphere cools to form lithosphere.
Cenozoic basalts from the SKC are mostly as-thenosphere derived with negligible crustal con-tamination (Zhou and Armstrong 1982; Peng et al. 1986; Song et al. 1990; Basu et al. 1991). Although some experienced interaction with the lithospheric mantle (see “Basaltic Evolution in the Western SKC from Progressive Lithospheric Thinning”), we en-visage that the lithospheric lid effect would be, in the first order, significant in their genesis.
Basaltic Evolution in the Western SKC from Progressive Lithospheric Thinning
As seen from the total alkalis versus SiO2plot (fig. 2a), Late Eocene–Oligocene lavas from the western SKC are mainly alkali basalts with a few samples straddling the alkali-tholeiitic boundary. Among these samples, normative Ne is common, norma-tive Hy is generally !10% (with the exception of
two analyses), and normative Q is absent (table 2). In contrast, the Miocene-Quaternary lavas are mainly tholeiitic, although alkali lavas are also found at Datong (fig. 2a). Among these younger la-vas, normative Q is common, normative Hy is gen-erally 110% (mostly 115%), and normative Ne is
absent. An overall subalkaline feature for the younger lavas is also clear from a normative Ne-Ol-Q plot, in contrast to the alkaline feature of the older basalts (fig. 2b). This is consistent with rel-atively higher SiO2 contents in the Miocene-Quaternary magmas at a given MgO compared to the pre-Miocene lavas. In addition, abundant pe-ridotite and pyroxenite xenoliths occur in the Fan-shi and Yangyuan basalts, but only rare peridotite xenoliths of small size (!2 cm) have been
docu-mented in the Datong lavas.
The Late Eocene–Oligocene lavas are character-ized by enrichment in light rare earth elements (LREE), with La/Yb ranging from 10 to 57 (fig. 3a). Specifically, lavas from Yangyuan show the strong-est REE fractionation and the lowstrong-est heavy rare earth elements (HREE) among the samples ana-lyzed (table 2). Despite some overlap, the Miocene-Quaternary basalts tend to be lower in La/Yb and higher in Yb content compared with the older lavas (fig. 3a). Given the similar MgO contents (6%–9%) for the studied samples, such differences in trace element composition cannot be related to differ-entiation but are mostly related to the variable ex-tent of partial melting or to heterogeneous sources. This is seen in figure 3, where melting trajectories of an enriched peridotite with different clinopyrox-eme/garnet (Cpx/Gt) ratios are compared with the observed data. The observed trace element varia-tion can be accounted for by variable degrees of partial melting of a source with the Cpx/Gt ratio of 19 : 1 (fig. 3a). Specifically, samples with high Yb contents and low La/Yb ratios are generated by a higher degree of melting compared with those with low Yb contents and high La/Yb ratios. Alterna-tively, at a fixed melting degree, the former can be formed by melting a source containing a low pro-portion of garnet, whereas the latter are derived from a garnet-rich source. Although distinguishing between these alternatives is not straightforward,
Figure 2. Basalt classification plots in terms of total alkalis (Na2O⫹ K2O) versus SiO2(a, c; both in wt%; Le Bas
et al. 1986) and normative Ne-Ol-Q (b, d; Poldervaart 1966). In the western SKC, most Late Eocene–Oligocene lavas are within the alkali field, but some points straddle the dividing line, whereas the post-Miocene lavas are dominantly tholeiitic basalts with subordinate alkali basalts. However, the opposite trend is noted for Cenozoic basalts from the eastern SKC. Data for the eastern SKC basalts: E and Zhao (1987), Dostal et al. (1988, 1991), Chen et al. (1990), Fan and Hooper (1991), Liu et al. (1994), and Chung (1999).
both appear to be consistent with variable depth of melt segregation. If the degree of melting is the main factor, the samples with high La/Yb ratios and low Yb contents are likely generated under a thick lithosphere because the degree of melting is in-versely proportional to the lithospheric thickness (see discussion in “Methodology”). Accordingly, the different La/Yb ratios and HREE contents in the Early Tertiary and Miocene-Quarternary ba-salts (fig. 3) are related to different lithospheric thicknesses. This conclusion holds if the variable Cpx/Gt ratio is the controlling factor because in the upper mantle, this ratio increases with depth.
The Sr-Nd isotopic variation in the western SKC basalts (fig. 4) reflects that of mantle sources in-volved in basaltic genesis because the presence of mantle xenoliths and oceanic island basalt (OIB)-like Nb/La and U/Nb ratios preclude significant crustal contamination. Although the isotopic range in the Cenozoic basalts overlaps that of the mantle xenoliths (fig. 5), the melting of the mantle similar to that of xenoliths (lithospheric mantle) is not a viable mechanism to generate the Cenozoic basalts from the SKC. This is because of the following three reasons: (1) Sm-Nd model ages (Song and Frey 1989) and Re-Os isotopes (Gao et al. 2002) suggest
Table 2. Representative Analyses of Cenozoic Basalts from the Shanxi Graben, Western Sino-Korean Craton
Datong (!1 Ma) Fengzhen (7–14 Ma) Yangyuan (15–30 Ma) Yinxian (38–40 Ma) Fanshi (38–40 Ma)
DT-10 (Q-Th) DT-19 (Tran) DT-22 (AOB) FZ-4 (Q-Th) YSZ-1 (Q-Th) YSZ-8 (AOB) YY-4 (AB) YY-7 (AB) YY-8 (AOB) HHL-8 (AOB) HHL-11 (AB) TXG-6 (AB) TXG-10 (AB) TXGB-5 (AB) Q 7.8 1.5 0 4.3 5.9 0 0 0 0 0 0 0 0 0 Ne 0 0 0 0 0 0 9.7 11.7 0 0 3.3 6.9 1.2 7.4 Ol 0 0 6.6 0 0 5.3 12.3 12.0 6.2 11.5 15.2 16.5 18.7 17.6 Hy 21.1 18.0 9.9 14.8 17.3 9.3 0 0 5.6 7.9 0 0 0 0 SiO2 55.06 52.65 50.5 52.67 52.73 49.14 46.18 45.76 48.94 48.34 44.77 43.35 46.81 43.67 TiO2 1.71 2.22 2.36 2.39 2.22 2.99 2.69 2.70 2.06 1.85 2.45 2.72 1.95 2.56 Al2O3 13.16 13.12 13.75 13.08 13.40 12.79 14.38 14.38 14.90 14.48 13.71 14.70 14.70 14.83 Fe2O3 11.99 12.78 13.34 12.24 12.21 13.57 13.44 13.52 12.05 13.69 13.35 13.21 13.32 13.22 MgO 7.30 6.49 6.72 6.25 6.77 6.99 6.65 6.64 5.96 7.79 8.59 8.88 9.26 9.67 MnO .15 .16 .17 .14 .14 .15 .15 .15 .13 .15 .17 .17 .17 .19 CaO 7.73 7.10 7.30 7.89 8.34 7.56 7.26 7.32 9.29 8.56 9.54 8.13 9.11 8.99 Na2O 3.03 3.80 3.88 3.23 2.92 3.41 5.43 5.64 3.35 3.20 3.36 4.02 2.93 3.66 P2O5 .19 .57 .63 .41 .37 .80 1.09 1.08 .38 .28 .80 .72 .33 .68 K2O .52 1.46 1.60 1.45 .89 2.11 1.46 1.47 1.36 .93 1.03 1.04 1.31 1.16 LOI .80 .36 .56 .18 .06 .45 1.02 1.09 1.59 .66 1.98 2.85 .13 1.33 Total 100.02 99.98 99.70 99.93 99.93 99.94 99.78 99.76 100.00 99.95 99.76 99.78 99.77 99.95 Cr 244.7 202.9 197.1 188.9 195.6 125.9 150.2 140.3 272. 247.9 193.8 134.1 257.8 142.5 Ni 137.1 142.6 137.5 105.4 131.3 139.6 118.7 114.3 117.9 165.4 147.0 155.8 168.7 185.9 Rb 8.42 24.78 26.91 20.65 11.56 30.08 60.15 75.24 11.67 10.17 33.76 5.68 13.18 10.33 Sr 277. 632. 708. 459. 445. 835. 1185. 1175. 591. 428. 793. 886. 537. 1132. Y 23.84 28.63 29.09 25.12 21.12 23.71 22.73 23.01 19.65 21.04 21.91 20.69 20.25 24.38 Zr 110.5 259.4 269. 179.3 154.1 291.7 376.7 375.2 147.7 131.9 214. 295.3 149.9 278.1 Nb 10.08 42.52 45.68 29.24 23.21 57.72 85.63 84.37 26.55 13.41 43.78 56.67 21.07 58.75 Ba 157.2 348.4 378.2 294.5 278.5 473.1 589.9 601.6 271. 306.7 741.4 445. 368.5 509. La 9.38 29.24 31.23 19.7 16.32 34.33 53.66 53.97 17.71 14.81 33.43 38.74 18.29 42.06 Ce 20.23 58.57 63.14 39.96 35.61 70.14 106.10 106.40 36.34 31.52 71.97 75.87 39.14 81.04 Pr 2.79 7.40 7.99 5.30 4.82 9.29 13.58 13.41 4.84 4.41 9.52 9.48 5.15 9.85 Nd 13.17 31.26 32.95 22.75 21.84 39.06 55.50 55.41 21.15 19.02 39.28 39.60 22.19 39.82 Sm 3.96 7.24 7.63 5.77 5.37 8.83 11.14 11.33 5.24 4.51 7.66 8.02 4.86 7.52 Eu 1.51 2.51 2.55 1.98 1.88 2.89 3.59 3.57 1.82 1.58 2.43 2.56 1.61 2.35 Gd 5.31 7.84 7.83 6.56 5.85 8.38 9.71 9.51 5.56 5.08 6.83 7.19 5.38 7.04 Tb .86 1.14 1.17 .98 .86 1.12 1.22 1.24 .80 .77 .91 1.00 .77 .99 Dy 4.78 6.10 6.21 5.37 4.59 5.57 5.84 5.83 4.20 4.24 4.79 4.82 4.24 5.10 Ho .90 1.09 1.11 .97 .81 .91 .88 .89 .75 .80 .84 .77 .76 .88 Er 2.29 2.65 2.65 2.34 1.95 2.03 1.88 1.90 1.80 2.02 2.05 1.91 2.13 2.39 Tm .33 .38 .37 .34 .27 .25 .21 .22 .25 .29 .28 .24 .30 .32 Yb 2.07 2.19 2.20 2.01 1.63 1.44 1.17 1.17 1.49 1.81 1.67 1.41 1.87 1.90 Lu .30 .32 .33 .30 .24 .20 .15 .16 .22 .28 .25 .20 .28 .29 Hf 3.16 6.20 6.46 4.58 3.95 6.79 8.45 8.49 3.62 3.25 4.67 6.45 3.61 5.95 Ta .64 2.67 2.90 1.80 1.42 3.64 5.43 5.40 1.63 .87 2.60 3.31 1.19 3.32 Pb 3.49 3.83 2.57 3.36 3.39 3.37 4.50 4.58 1.65 2.34 3.53 3.02 2.88 3.46 Th 1.03 3.82 4.10 2.25 1.64 3.98 6.63 6.63 1.80 1.17 3.26 3.70 1.65 4.23 U .28 .96 .94 .66 .41 1.09 1.74 1.74 .46 32 .87 1.23 .45 1.20 Nb/La 1.1 1.5 1.5 1.5 1.4 1.7 1.6 1.6 1.5 .9 1.3 1.5 1.2 1.4 La/Yb 4.5 13.3 14.2 9.8 10.0 23.8 45.8 46.3 11.9 8.2 20.0 27.6 9.8 22.1 87Sr/86Sr .704393 .703650 .704170 .704940 .704692 .703822 .703805 .703788 .704716 .704615 .704000 .703768 .704947 .705085 (87Sr/86Sr) i .704393 .703650 .704169 .704922 .704681 .703808 .703742 .703709 .704692 .704576 .703932 .703757 .704907 .705070 143Nd/144Nd .512723 .512939 .512958 .512698 .512783 .512930 .512924 .512909 .512746 .512460 .512760 .512843 .512513 .512725 (t)Nd 1.66 5.87 6.24 1.42 3.08 5.95 6.34 6.05 2.87 –2.48 3.38 4.99 –1.44 2.69
Note. The whole-rock major and trace element compositions were obtained using inductively coupled plasma mass spectrometry (ICP-MS) and ICP-AES at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). Precision is better than 1 for major elements, 5% for REE, and 5%–10% for other incompatible elements. Sr-Nd isotopic ratios have been measured using a micromass isoprobe multicollector ICP-MS at GIGCAS. Analyses of standards during the period of analysis: NBS987 gave87Sr/ Sr p 0.71024386 Ⳳ 14(2j); Shin Etou gave143Nd/ Nd p 0.512124144 Ⳳ (2j). (87 86 )iand are the initial isotope ratios calculated using published K-Ar ages (Liu et al. 1992b). CIPW calculation was performed with adjustment of FeO and Fe2O3contents following
11 Sr/ Sr (t)Nd
Figure 3. Plots of La/Yb versus Yb. Data sources for basalts from the eastern SKC: Fan and Hooper (1991), Dostal et al. (1988, 1991), Chen et al. (1990), Liu et al. (1994), and Chung (1999). Symbols are as in figure 2a. Partial melting lines are calculated using the batch melt-ing model of Shaw (1970) with different clinopyroxene/ garnet (Cpx/Gt) ratios in residual mineralogy (olivine 60%; orthopyroxene 20%). The source composition is that of an enriched lherzolite xenolith from Yangyuan with 0.72 ppm La and 0.2 ppm Yb (Y.-G. Xu, J. L. Ma, F. A. Frey, M. D. Feigenson, and J. F. Liu, unpub. data). A constant melting mode of Gt (40%) and Cpx (40%) was assumed. Numbers mark the degree of partial melting. Partition coefficients are from those of Johnson (1998).
Figure 4. 143
Nd/144
Nd versus 87
Sr/86
Sr of the Cenozoic basalts from the western SKC. Symbols are the same as in figure 2a. The field for Cenozoic basalts from the Shan-dong and Bohai areas in the eastern SKC is outlined, based on data from Zhou and Armstrong (1982), Peng et al. (1986), Basu et al. (1991), and Liu et al. (1995).
that the mantle lithosphere beneath the western SKC was subjected to melting during the Protero-zoic. This considerably predates the Cenozoic melting events. (2) Most of the studied basalts dis-play OIB-like trace element compositions, imply-ing an enriched mantle source. In contrast, mantle xenoliths are either LREE depleted or LREE en-riched. Furthermore, some enriched xenoliths are depleted in Nb and Ta and therefore cannot be the source of OIB-like basalts. (3) In general, the lith-ospheric mantle is too cold to melt. The thermo-mechanical model suggests that only a small
amount of melt can be produced from the litho-spheric mantle by conduction of heat from a mantle plume (McKenzie and Bickle 1988; Arndt and Christensen 1992). Generation of intraplate basalts is likely confined to the convective asthenosphere or plume.
An asthenospheric origin of the Cenozoic basalts in the western SKC is supported by the fact that all samples have positive Nb and Ta anomalies, with Nb/La (1.1–1.5) within the range of OIB (Sun and McDonough 1989; table 2). Although the an-alyzed samples display a range of Sr and Nd isotopic
ratios ( –0.7051 and Nd(t) p
87 86
( Sr/ Sr) p 0.7036i
⫹ 6.3–⫺2.5), there is no systematic difference be-tween the Early Tertiary and the Miocene-Quater-nary magmas. For instance, all but one sample from Yangyuan (Early Tertiary) show a homogeneous Sr-Nd isotopic composition with (87Sr/86Sr)
i ranging
from 0.70371 to 0.70374 and (t)Nd of⫹6.0–6.3, de-spite an SiO2 range of 48%–52%. Similar isotopic ratios are observed in some Miocene-Quaternary basalts from Datong, Fengzhen, and Hannuoba (Song et al. 1990; Basu et al. 1991; table 1). This suggests that the Cenozoic lavas share a common end member (i.e., the asthenosphere).
Lavas from individual localities, except Fanshi, define roughly negative correlations in Sr-Nd iso-topic space (fig. 4). Specifically, tholeiites generally have lower and higher (87Sr/86Sr)
i than alkali
(t)Nd
600 Y . - G . X U E T A L .
Figure 5. Variation of 143Nd/144Nd versus 87Sr/86Sr of
mantle xenoliths in Cenozoic basalts from the SKC. Data sources for xenoliths: Song and Frey (1989), Tatsumoto et al. (1992), Xu et al. (1998, unpub. data), Fan et al. (2000), and Xu (2002). The field for Cenozoic basalts is outlined according to Zhou and Armstrong (1982), Peng et al. (1986), Basu et al. (1991), Liu et al. (1995), and this study.
resulted from interaction between the astheno-sphere and lithospheric mantle (Song et al. 1990; Basu et al. 1991; Y.-G. Xu, J. L. Ma, F. A. Frey, M. D. Feigenson, and J. F. Liu, unpub. data). There is no significant correlation between (t)Nd and La/Yb. For example, for samples with relatively low La/ Yb (!15), their (t)Nd range from ⫺2.5 to ⫹6 (table
2). It seems that the lithosphere-asthenosphere in-teraction, if any, did not significantly modify the La/Yb ratio. Lack of Sr-Nd isotopic correlation in the Fanshi basalts may be related to surface alter-ation processes that affected the Rb-Sr systematics because these samples are the oldest of all those studied. Also, there is no correlation between Nb (immobile element) and Rb.
According to the rationale described in “Meth-odology,” the temporal decrease in alkaline content (fig. 2) and La/Yb (fig. 3) probably mirrors a pro-gressive thinning of the lithosphere beneath the Shanxi graben during the Cenozoic. Compared with experimental data, the predominance of alkali basalts in Late Eocene time suggests a lithosphere thicker than 80 km at that time. This inference is consistent with the fact that some Ne-normative laves from Fanshi (e.g., TXG-10) have negative (table 2; see also DePaolo and Daley 2000). (t)Nd
Under such a thick lithosphere, the generation of basalts was largely confined to the garnet stability field; thus, the samples have relatively low Yb con-tents and high La/Yb ratios (fig. 3a) because the HREE are buffered by residual garnet in the source. Extremely low HREE contents and strongly
frac-tionated REE in the Yangyuan basalts further sug-gest a very low degree of partial melting. Relatively low (t)Nd values (⫹5 to ⫺2.5) in the pre-Miocene lavas, compared with those of the asthenospheric end member, may indicate significant involvement of SCLM components in basaltic generation, prob-ably in a “plum pudding” fashion as envisaged by Perry et al. (1987) during lithospheric thinning. In contrast, predominant SiO2-saturated and oversa-turated features observed in the Miocene-Quater-nary basalts suggest a shallower depth (spinel-garnet stability field) of magma generation. This is consistent with relatively lower La/Yb and higher Yb contents in these lavas. Specifically, the Qua-ternary lavas from Datong have higher SiO2 and HREE contents and Na/Ti ratios compared with other samples at comparable MgO. Modeling in-volving REE and Na/Ti for these lavas (Y.-G. Xu, J. L. Ma, F. A. Frey, M. D. Feigenson, and J. F. Liu, unpub. data) implies an upwelling of the astheno-sphere to a depth of around 70 km, that is, a locally very thinned lithosphere in the western SKC at the present time.
Basaltic Evolution in the Eastern SKC from Lithospheric Thickening
Data available for Cenozoic basalts in the eastern SKC show a temporal variation in petrochemistry generally opposite to that observed in the western SKC (fig. 2c, 2d). The Early Tertiary basalts, which occur voluminously in extensional basins, are mainly quartz- and olivine-normative tholeiites, with subordinate subalkali basalts (olivine basalts) and minor alkali basalts (Fan and Hooper 1991; Liu et al. 1992b). It has been documented that the erup-tion of tholeiitic basalt was concomitant with a peak period of lithospheric extension that created several basins (Ren et al. 2002). Although limited, available isotopic data show a predominant de-pleted mantle signature in the Eocene tholeiites (Zhou and Armstrong 1982; Peng et al. 1986; Xu 2001). This suggests that the asthenosphere was uplifted to a very shallow level, the lithosphere was probably!60 km thick (Xu 2001), and there was a
very high thermal gradient (Menzies and Xu 1998). Subalkaline rocks continued to occur but dimin-ished in quantity in the Late Tertiary and Quater-nary, while alkali and strongly alkali basalts (bas-anite and nephelinite) progressively became the dominant rock types (fig. 2c, 2d). As expected, the Miocene-Quaternary lavas generally have higher La/Yb than the Eocene basalts (fig. 3b). Also, man-tle xenoliths are only found in post-Miocene basalts and are absent in Eocene-Oligocene magmas (E and
Zhao 1987). The trend of increasing alkalinity with time in Cenozoic basalts from the eastern SKC has been recognized by several authors (Zhou and Armstrong 1982; Fan and Hooper 1991; Xu et al. 1995; Xu 2001) and can be attributed to declining degrees of partial melting and increasing depth of the source region. Thermal decay since Miocene time had been observed in the Bohai basin (Hu et al. 2001), consistent with a weaker lithospheric ex-tension during the Neogene than the Early Tertiary (Deng 1988). It is envisaged that upwelled asthen-osphere converts thermally and rheologically to lithosphere as a response to decreasing temperature (Jordan 1988).
Discussion and Conclusions
Diachronous Lithospheric Extension in the Western and Eastern SKC. Basalt compositions are consis-tent with a dominant asthenospheric source for the Cenozoic basalts from both sides of the DTGL. However, there is marked temporal variation in depth and degree of mantle melting in the two regions. While basalts from the western SKC were generated by progressive lithospheric thinning, those from the eastern SKC likely reflect litho-spheric thickening/accretion. Such contrasting lithospheric processes may be related to diachron-ous lithospheric extension in the SKC. In a recent review, Ren et al. (2002) showed that widespread continental rift systems and continental basins in the eastern SKC were mainly formed during the Late Cretaceous-Paleogene. These Early Tertiary basins then entered a postrifting phase during Neo-gene time, thus forming regional downwarping. The thermal subsidence is also revealed by thermal modeling in the Bohai area, which showed a de-crease in heat flow from 75–90 mW/m2in the Early
Miocene to 55–65 mW/m2 nowadays (Hu et al.
2001). In the western SKC, although rifting started in the Hetao-Yinchuan graben system and the southern part of the Shanxi graben in the Early Ol-igocene or Late Eocene, major extension and rapid subsidence occurred in the Neogene and Quater-nary (Ye et al. 1987); thus, a delayed extension in the western SKC relative to the eastern SKC is im-plied. In this context, a lithospheric thinning stage, similar to that observed in the Shanxi graben, should have occurred in the eastern SKC prior to the Cenozoic because Cenozoic basalts record only a state of lithospheric thickening/accretion. Such a lithospheric thinning stage has been proposed on the basis of the compositions of Mesozoic to Ce-nozoic basalts (Xu 2001). Late Mesozoic alkali ba-salts and intrusive mafic rocks in the Shandong
Province have low (t)Nd (⫺10 to ⫺20) and Nb/La (!0.5; Guo et al. 2001; Zhang et al. 2002; Yang et
al. 2004), in contrast with a positive (t)Nd and high Nb/La (11) in the Early Tertiary tholeiites (Zhou
and Armstrong 1982; Peng et al. 1986; Chung 1999). The transition from Cretaceous SCLM-derived al-kali basalts to Paleogene asthenosphere-derived tholeiites in the eastern SKC may have resulted from the replacement of lithospheric keel by up-welling asthenosphere during the Late Cretaceous (Xu 2001; Xu et al. 2004).
Diachronous lithospheric extension in the west-ern and eastwest-ern SKC may be related to interactions between different tectonic regimes. While the ex-tension in the Shanxi graben was likely related to the Indo-Eurasian collision (Molnar and Tappon-nier 1977), the north-northeast-trending basins in the eastern SKC may have been induced by sub-duction of the Pacific plate underneath the Asian continent, which began at the end of the Mesozoic (Ye et al. 1987). Nevertheless, the north-south-trending extension in some Late Mesozoic basins in the southern SKC (Ren et al. 2002) may reflect subduction of the Yangtze Block beneath the SKC.
Heterogeneous Lithospheric Composition and Struc-ture beneath the SKC. An important implication of the diachronous lithospheric evolution is that lithospheric mantle beneath the eastern SKC is stratified with old lithosphere overlying newly ac-creted lithosphere, whereas the lithospheric mantle beneath the western SKC may consist mainly of old lithospheric relicts after thermomechanical erosion (fig. 6). This prediction is supported by iso-topic data for mantle xenoliths, which are direct samples of the lithospheric mantle. As shown in figure 5, mantle xenoliths from the eastern SKC isotopically resemble oceanic basalts, in contrast to those from the western SKC that encompass both depleted and enriched mantle fields. More-over, a recent Re-Os isotopic investigation reveals remarkably different stabilization ages for the lith-ospheric mantle beneath the eastern and western SKC (Gao et al. 2002). Although the peridotite xe-noliths from Hannuoba (western SKC) define a Me-soproterozoic Re-Os isochron age of 1.9 Ga, the Qixia peridotites from the eastern SKC have mod-ern TRDand TMAages (Gao et al. 2002).
Heterogeneous lithospheric mantle composition beneath the SKC is also reflected by the different Sr-Nd isotopic ranges of Cenozoic lavas from the western and eastern parts (fig. 4). Specifically, the western SKC lavas display a larger Sr-Nd isotopic range and tend to have lowerNdthan those from the eastern SKC (fig. 4). This is understandable within the lithosphere-asthenosphere interaction
602 Y . - G . X U E T A L .
Figure 6. Schematic illustration of heterogeneous composition and structure of the present-day lithospheric mantle beneath the SKC. Left, Western SKC. The lithosphere is progressively thinned as a result of continuous upwelling of asthenosphere. The lithospheric mantle consists of old lithospheric relicts after thermomechanical erosion. Right, Eastern SKC. The lithosphere becomes progressively thickened through an asthenospheric accretion due to thermal loss. As a consequence, the lithospheric mantle is stratified with old lithosphere overlying newly accreted lithosphere.
scheme. In the western SKC, where Proterozoic lithospheric mantle is in contact with the convec-tive asthenosphere (fig. 6, left), basalts partly in-herit the enriched isotopic character of old SCLM. In contrast, in the eastern SKC, the lower
litho-spheric mantle accreted recently from the
asthenosphere (fig. 6, right) and therefore has a de-pleted isotopic signature. So, even if lithosphere-asthenosphere interaction were operative, the inherited asthenospheric signature of the newly accreted lithosphere would not cause any signifi-cant isotopic variation in the basalts formed. In summary, Sr-Nd isotopic compositions of Cenozoic basalts provide additional support for the contrast-ing composition of the lithospheric mantle beneath the western and eastern SKC.
Implication for the Mechanism of Lithospheric De-struction in the SKC. Lithospheric thinning in east-ern China probably proceeded diachronously, with that in the eastern SKC occurring later than that in the western part. This diachronous lithospheric extension (or thinning) is most likely related to
in-teractions of different tectonic regimes. There could, therefore, be multiple causes for the desta-bilization of the cratonic lithosphere beneath the SKC (Menzies et al. 1993; Griffin et al. 1998; Fan et al. 2000; Xu 2001); at least the lithospheric thin-ning in the western and eastern SKC should be con-sidered separately. In the eastern SKC, the destruc-tion of the lithospheric keel was probably related to a combined effect of (1) the Triassic collision between the North China and Yangtze Blocks and (2) the Mesozoic subduction of the Pacific plate un-derneath the Asian continent. The former may have destroyed the physical integrity of the litho-sphere beneath the SKC, resulting in the formation and/or reactivation of major shear zones/strike-slip faults (Xu 2001) and east-west-trending Mesozoic extensional basins in the southern SKC (Ren et al. 2002). The latter governed the north-south-trend-ing extensional basins (Ye et al. 1987). The influ-ence of the western Pacific subduction in the west-ern SKC must have been very limited because there
is little evidence of faulting in the Eocene, and the Oligocene graben-type sediments are generally thin (Ye et al. 1987). Lithospheric thinning in the west-ern SKC is most likely related to mantle flow and convection along the lithospheric weakness in-duced by the Indo-Eurasian collision (Molnar and Tapponnier 1977). The northeastward propagation of rifting and the stress patterns in the Shanxi gra-ben can be accounted for by the east-northeastward movement of the Tibetan Plateau (Ye et al. 1987). It is also important to note that Cenozoic rift systems in the SKC were developed along major translithospheric faults and/or boundaries between craton and fold belts. For instance, the North China rift system is bounded to the east by the Tanlu fault and to the west by the boundary between the cen-tral and eastern zones of the SKC (see the new tec-tonic subdivision of the SKC in Zhao et al. 2001). The Shanxi graben roughly coincides with the boundary between the central and western zones (fig. 1). Lithospheric extension and thinning there-fore likely was initiated along these major struc-tural-tectonic boundaries where the lithosphere was weakest and mantle flows became focused. Mantle flow could be further enhanced by thermal anomalies resulting from secondary convection in-duced by topographic differences between different blocks and lithospheric extension along lines of weakness (King and Anderson 1995). This require-ment of topographic difference is met in the case of the SKC, where the thickness of lithospheric
keel near the DTGL appears to be different (Men-zies et al. 1993; Griffin et al. 1998). It is therefore possible that convective removal of the litho-spheric keel beneath the SKC started along the lith-ospheric boundaries and faults and propagated to-ward the interior of the rigid lithosphere. The lithospheric thinning probably proceeded through heat transport into the lithosphere by small-scale asthenospheric convection induced by extension (Buck 1986). Once lithospheric mantle is thermally converted to asthenosphere, it can convectively mix with, and eventually be replaced by, astheno-sphere. Progressive lithospheric thinning triggered further upwelling of asthenosphere, which may have led to widespread Cenozoic volcanism in the SKC.
A C K N O W L E D G M E N T S
We gratefully acknowledge the financial support of the National Natural Science Foundation of China (49925308) and the Chinese Academy of Sciences (the Baren Project). S.-S. Sun and an anonymous reviewer are thanked for their constructive criti-cism, which helped to improve the manuscript. F. Frey provided critical reading on the revised ver-sion. This article was completed while Y.-G. Xu was a visiting scientist at the National Taiwan Uni-versity, which is supported by the National Science Council of Taiwan.
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