Mesozoic high-Ba–Sr granitoids from North China: geochemical
characteristics and geological implications
Qing Qian,
1Sun-Lin Chung,
2Tung-Yi Lee
3and Da-Jen Wen
21
Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, People’s Republic of China;2Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei, People’s Republic of China;3Department of
Earth Sciences, National Taiwan Normal University, Taipei, People’s Republic of China
Introduction
Granitic rocks have been classified into different types by various criteria, including field geology, major and trace element geochemistry, mineral-ogy, assumed origin and tectonic environment (Frost et al., 2001, and
references therein). Among these
types, three are particularly abundant, namely the I-type, S-type (Chappell and White, 1974) and A-type (White, 1979). Generally, the traditional I-, S-and A-type granites possess low Ba and Sr concentrations (hence, denom-inated together as low-Ba–Sr grani-toids). However, Tarney and Jones (1994) identified an additional type that, named high-Ba–Sr granitoids, exhibits many trace-element charac-teristics distinct from the low-Ba–Sr granitoids. The high-Ba–Sr granitoids are marked by high Ba and Sr, low Y
and heavy rare earth elements
(HREE), lack of apparent negative Eu anomaly and depletion of Nb in spidergrams.
The North China block (NCB), underlain by the Sino-Korean craton, was relatively stable from the Meso-proterozoic to Late Palaeozoic. It was
amalgamated with the Siberian block to the north in the Late Palaeozoic along the central Asian orogenic belt (Sengo¨r et al., 1993), and collided with the South China block (SCB) to the south during the Triassic (Li et al., 2000) along the Qinling–Dabie–Sulu orogenic belt. The latter resulted in deep (> 200 km) subduction of the SCB beneath the NCB along the Dabie–Sulu orogen (DSO) (Ye et al., 2000). Following the continental col-lision, a series of geological events (the
Yanshanian movements) occurred
in the eastern part of North China, of which the most prominent are: (1) replacement of a cold, thick (> 180 km) and refractory litho-spheric keel (Archaean in age, Gao et al., 2002) beneath the NCB by relatively hot, thin ( 80 km) and fertile mantle (Griffin et al., 1998; Menzies and Xu, 1998; Fan et al., 2000); and (2) widespread occurrence of magmatism (mainly 165–100 Ma in age) in the eastern part of the NCB and the DSO (Fig. 1) (Wang et al., 1996; Chen et al., 1997; Ma et al., 1998; Fan et al., 2001). Replacement of the lithospheric mantle probably occurred from Jurassic to Late Creta-ceous (Xu, 2001), largely contempora-neous with the magmatism. Hence the Yanshanian magmatism may be an important manifestation of mantle evolution beneath the NCB.
However, there is still controversy about the geochemistry and
petrogen-esis of the Yanshanian igneous rocks. Wang et al. (1996) proposed that these rocks are dominantly shosho-nitic and subordinately high-K
calc-alkaline in character, forming a
Ôshoshonite provinceÕ, while Xu et al. (1999) argued that they are mainly high-K calc-alkaline and subordinate-ly alkaline, with minor shoshonitic series. Zhang et al. (2001a,b) inter-preted most of them as adakites derived by high-pressure melting of lower continental crust (garnet-in and plagioclase-out). Based on a compil-ation of 192 analyses from the litera-ture (Appendix), plotted in Figs 2–4, we show that the intermediate to acidic rocks from the NCB and DSO
comprise dominantly high-Ba–Sr
granitoids and subordinately low-Ba–Sr granitoids. This identification leads to some geological implications that markedly differ from the previous views.
Geochemical characteristics of the Yanshanian magmatism The Yanshanian igneous rocks from the NCB and DSO show a wide range of composition, with SiO2 varying
continuously from 46.5–77.5 wt%
(Fig. 2a). In the SiO2vs. K2O diagram
(Fig. 2b), the rocks plot dominantly in high-K calc-alkaline field, and subor-dinately in shoshonitic and rarely in medium-K calc-alkaline fields, show-ing an overall potassic character. A B S T R A C T
Mesozoic magmatism occurred extensively in the North China block (NCB) and the Dabie–Sulu orogen (DSO) post-dating the North–South China collision, resulting in abundant intrusive and volcanic rocks ranging from basic to acidic compositions. The intermediate-acidic intrusive rocks can be grouped into two types, namely high-Ba–Sr granitoids and low-Ba–Sr granitoids that both have distinct geochemical characteristics. The high-Ba–Sr granitoids are similar in most of the incompatible trace element systematics to the associated basic rocks, which probably originated from melting of subcontinental lithospheric
mantle, indicating significant mantle contributions to them. Geochemical similarities are observed between the basic rocks from the NCB and DSO, implying a regional-scale magma-generating mechanism and that mantle enrichment beneath the DSO was independent from the Triassic deep continental subduction in the region. We therefore interpret that the Mesozoic magmatism resulted from delamination of the ancient lithospheric mantle beneath the eastern part of North China. Terra Nova, 15, 272–278, 2003
Correspondence: Dr Qing Qian, Institute of Geology and Geophysics, Chinese Acad-emy of Sciences, PO Box 9825, Beijing 100029, China. Tel.: 86 10 62007825; fax: 86 10 62010846; e-mail: qianqing@ mail.igcas.ac.cn
Fig. 1 Sketch map of the distribution of the Yanshanian igneous rocks from North China and the Dabie–Sulu orogenic belt (modified after Chung, 1999).
Fig. 2 Geochemical classification diagrams for Mesozoic igneous rocks from the North China Block (NCB) and Dabie–Sulu orogen (DSO). (a) Na2O + K2O vs. SiO2diagram. Key to abbreviations: PB, picrobasalt; B, basalt; BA, basaltic andesite; A,
andesite; D, dacite; R, rhyolite; TB, trachybasalt; BTA, basaltic trachyandesite; TA, trachyandesite; TD, trachydacite; T, trachyte; TE, tephrite; PT, phonotephrite; TP, tephriphonolite; P, phonolite; F, foidite. (b) K2O vs. SiO2diagram. Note that rocks with SiO2
< 55 wt% and > 55 wt% are classified as mafic and felsic (granitoids if intrusive), respectively. Dashed line delineates the areas of the low-Ba–Sr granitoids. Data from the NCB are from Qin (1995), Chen et al. (1997), Xu et al. (1999), Qiu et al. (2000) and Guo et al. (2001). Data from the DSO are from Ma et al. (1998), Jahn et al. (1999) and Fan et al. (2001).
Rocks with SiO2 > 55 wt% and
< 55 wt% are termed felsic and mafic, respectively.
Based on Sr and Ba abundances, the granitoids can be divided into two types, namely, the high-Ba–Sr
grani-toids and low-Ba–Sr granitoids.
The former generally have Sr > 300 p.p.m. and Ba > 500 p.p.m., and the latter have Sr < 300 p.p.m. and Ba < 500 p.p.m. This criterion applies well for most of the samples, except a few samples with low Sr (< 300 p.p.m.) but high Ba (up to 1470 p.p.m.) concentrations (grouped as low-Ba–Sr granitoids). In Tarney and Jones’ (1994) Sr–Rb–Ba plot (Fig. 3a), the two types show appar-ently different trends, similar to the Scottish Caledonian granitoids. In addition, the high-Ba–Sr granitoids have high light rare earth elements (LREEs), low Y (4–30 p.p.m.) and heavy rare earth elements (HREEs), and depleted Nb (but not Eu) in
spidergrams (Fig. 4a,b).
Conse-quently, they have high Sr⁄ Y (16– 170) (Fig. 4d) and La⁄ Yb (10–115) ratios. The low-Ba–Sr granitoids are also depleted in Nb, but in contrast strongly depleted in Ba, Sr and Eu (Fig. 4a,b) and have lower Sr⁄ Y (1.8– 21) (Fig. 4d) and La⁄ Yb (6–56) ratios. It is notable that the high-Ba–Sr granitoids have incompatible element patterns similar to that of the associ-ated mafic rocks (which is also true of
many orogenic rocks worldwide,
Tarney and Jones, 1994), but are relatively lower in most of the moder-ate to slight incompatible elements (Fig. 4a,b).
The two types of granitoids also show differences in major element compositions. The high-Ba–Sr grani-toids have a relatively more mafic member and thus wider range of SiO2 (55.5–74.8 wt%), coupled with
higher Al2O3(11.5–18.4 wt%, mostly
> 14.0 wt%) and P2O5 (0.02–0.89
wt%, mostly > 0.20 wt%) contents, and lower K2O⁄ Na2O (0.6–1.6).
Cor-responding values for the low-Ba–Sr granitoids are: 66.0–77.5 wt%, 11.5–
16.8 wt% (mostly < 14.0 wt%),
0.02–0.25 wt% (mostly < 0.20 wt%) and 0.8–9.4, respectively.
The high-Ba–Sr granitoids from the NCB have a wide range in eNd(T) (T) () 3.8 to ) 20.2) but have a restricted range of initial 87Sr⁄86Sr
ratios (0.7051– 0.7068). Those from the DSO are more uniform in eNd(T) () 15.2 to ) 21.4), with relatively higher initial87Sr⁄86Sr ratios (0.7058– 0.7096). Moreover, isotope composi-tions of the high-Ba–Sr granitoids are largely identical with those of the associated mafic rocks in each region (Fig. 4c). Nd-depleted mantle model ages (TDM) of the high-Ba–Sr
grani-toids and mafic rocks mostly range between 1.50 and 2.20 Ga in both regions (Appendix). Limited Sr–Nd
isotope data available for the low-Ba–Sr granitoids are heterogeneous, with initial87Sr⁄86
Sr and eNd(T)
val-ues of 0.7043–0.7317 and ) 9.4 to
) 13.0, and 0.7086–0.7149 and ) 14.0 to) 20.5 for samples from the NCB and DSO, respectively (Appendix). Magma source and petrogenesis of high-Ba–Sr granitoids
Two competing hypotheses have
been proposed for the genesis of high-Ba–Sr granitoids. One is by
Tarney and Jones (1994) who
explored partial melting of three possible precursor magma sources: (1) subducted ocean islands or pla-teaus, (2) hydrous mafic underplates in the lower crust and (3) the lower continental lithosphere enriched by carbonatitic melts. The other is by Fowler and Henney (1996) and Fow-ler et al. (2001) who advocated the
Scottish Caledonian granitoids as
products of crystal fractionation
from associated shoshonitic mafic magmas that originated from an enriched mantle lithosphere. In the latter, it is considered that
crystal-lization of highly REE-conserved
accessory minerals such as apatite and titanite play a crucial role in decreasing the abundances of REE, P and Ti through differentiation.
In North China, the Badaling
complex (located 40 km north of
Fig. 3 (a) Sr–Rb–Ba plot (after Tarney and Jones, 1994), and (b) Na–K–Ca plot for Yanshanian intermediate-acidic rocks from the North China Block (NCB) and Dabie–Sulu orogen (DSO). Sample symbols are the same as that in Fig. 2. Fields of high-Ba–Sr and low-Ba–Sr granitoids are based on data from Fowler and Henney (1996) and Fowler et al. (2001). Fields of adakite are based on data from Kay (1978), Defant et al. (1991a,b), Sajona et al. (1993, 1994), Morris (1995), Stern and Kilian (1996) and Yogodzinski et al. (1994, 1995).
Mesozoic high-Br–Sr granitoids from North China • Q. Qian et al. Terra Nova, Vol 15, No. 4, 272–278
...
Beijing) serves as a good case exam-ple that comprises a whole series of magmas from mafic to intermediate and acidic compositions to study the magma source and petrogenesis of high-Ba–Sr granitoids. Major and trace element and Sr–Nd isotope data of the mafic rocks and high-Ba–Sr granitoids suggest that the Badaling high-Ba–Sr granitoids were generated by differentiation of the
associated mafic magmas derived
from an enriched subcontinental
lith-ospheric mantle, with substantial
crustal contamination (Qian et al., 2002). Similarly, Li et al. (2002)
postulated that Jurassic basaltic
rocks from western Liaoning in the NCB originated from an enriched lithospheric mantle, and the accom-panied andesitic and dacitic rocks
(also high in Ba and Sr) represent
differentiated products from the
basaltic magmas through a combined assimilation and fractional crystal-lization (AFC) process. Ma et al. (1998) proposed a similar interpret-ation for coeval mafic rocks and high Ba–Sr granitoids (mainly Group
II of Ma et al.’s classification)
from the northern Dabie Mountains.
Given the fact that high-Ba–Sr
granitoids from both the NCB and the DSO exhibit incompatible
ele-mental characteristics and Sr–Nd
isotope compositions almost indistin-guishable from those of the
associ-ated mafic rocks (Fig. 4a–c), we
argue that the high-Ba–Sr granitoids contain a significant mantle compo-nent. Similarities in the multi-ele-ment distribution patterns between
the high-Ba–Sr granitoids and the associated mafic rocks have been
observed in Scottish Caledonides
and many other orogens worldwide (Tarney and Jones, 1994), interpreted to indicate large mantle contribu-tions to the generation of high-Ba–Sr granitoids (Tarney and Jones, 1994; Fowler et al., 2001). Enrichment of the lithospheric mantle beneath the NCB may have been caused by interactions with small-volume melts
from the lithosphere,
astheno-sphere or recycled crustal materials (Menzies, 1990). A similarly enriched old mantle lithosphere may also exist beneath the DSO (Fan et al., 2001). In contrast, the low-Ba–Sr granitoids were possibly derived from melting of the lower–middle continental crust (Qian et al., 2002).
Fig. 4 (a,b) Primitive mantle-normalized multi-element distribution diagrams for Yanshanian igneous rocks from the North China Block (NCB) and Dabie–Sulu orogen (DSO), respectively. Normalizing data are after Sun and McDonough (1989). High-Ba–Sr granitoids from the Scottish Caledonian Orogen (Fowler et al., 2001) are shown for comparison. (c) Initial87Sr⁄86Sr vs. eNd(T)
plot. Fields of Sino-Korean lower and upper crust are after Jahn et al. (1999). (d) Y vs. Sr⁄ Y plot for Yanshanian igneous rocks from the NCB and DSO. Fields of Archaean TTG (trondhjemite, tonalite and granodiorite) and adakites, and arc andesite–dacite– rhyolite are after Martin (1999).
Geological implications
Tarney and Jones (1994) pointed out that high-Ba–Sr granitoids dominate in the Archaean, and also occur extensively as plutons and volcanics in many Late Cretaceous and Tertiary orogenic belts worldwide. Our obser-vations from North China lead to identification of an important high-Ba–Sr granitoid province of Mesozoic age. The high-Ba–Sr granitoids and their extrusive associates from the NCB and DSO show elemental char-acteristics (e.g. high Sr, Ba and LREE,
low Y and HREE, elevated La⁄ Yb
and Sr⁄ Y ratios, and lack of negative Eu anomaly) that are typical of ada-kites and Archaean TTG (trondhje-mite, tonalite and granodiorite) suites
(Martin, 1986; Drummond and
Defant, 1990; Martin, 1999, and ref-erences therein). Therefore, they have been envisaged by some workers as adakites derived from lower crustal melting under high pressure with
eclogite residue (Zhang et al.,
2001a,b; Defant et al., 2002). Zhang et al. (2001a) further argued that the continental crust of the NCB was > 50 km thick during the Mesozoic so that there was an ÔEast China PlateauÕ comparable in height to the present Tibet. However, we note that these high-Ba–Sr granitoids are differ-ent from adakites by possessing higher K2O⁄ Na2O (0.65–1.1), more variable
Al2O3 (11.5–19.5 wt%) and much
more enriched Sr and Nd isotopes. Furthermore, they delineate a calc-alkaline trend, rather than showing the trondhjemitic affinities of adakites in the Na–K–Ca plot (Fig. 3b), and
are associated with Nb-depleted
potassic mafic rocks (Ma et al., 1998; Chen et al., 2002; Li et al., 2002; Qian et al., 2002), in contrast to adakites that are occasionally associated with Nb-enriched basalts (Aguillo´n-Robles et al., 2001, and references therein). The adakite-like geochemical features of the high-Ba–Sr granitoids can be explained as reflecting the enriched mantle source signatures, followed by
differentiation of highly
REE-conserved accessory phases (Li et al., 2002; Qian et al., 2002), similar to the case reported in the Scottish Caledo-nides (Fowler and Henney, 1996; Fowler et al., 2001). Hence we argue
that the adakite-like elemental
characteristics of the high-Ba–Sr
granitoids do not witness high-pres-sure melting of the lower crust.
Post-collisional interactions of the subducted crust (i.e. part of the SCB) with the asthenosphere (Jahn et al., 1999) or the North China mantle lithosphere (Li et al., 1998; Fan et al., 2001) have been envisaged to explain some of the arc-type geochemical characteristics (e.g. enriched LILE, depleted Nb, and enriched Sr and Nd isotopes) of the mafic rocks from the DSO. These scenarios, however, could not apply further northward to account for the widespread occurrence of contemporaneous magmatism with similar elemental and isotopic charac-teristics (Fig. 4a–c) in the northern part of the NCB (Fig. 1). Taking the occurrence of Ordovician kimberlites into account, the suggested presence of an ancient (Griffin et al., 1998; O’Reilly et al., 2001) and enriched (Xu, 2001) mantle source in the NCB is considered vital, and consistent with the Nd TDMdata (Appendix)
domin-ated by Palaeo- to Meso-proterozoic ages. Such a tempero-spatial associ-ation and petrochemical similarities of the Yanshanian magmas from the NCB and DSO indicate that mantle source enrichment beneath the DSO does not require continental subduc-tion, but does require a larger-scale mechanism for the magma generation in the regions.
Basalt- and kimberlite-borne mantle xenoliths and geophysical data have consistently documented that delami-nation of c. 80–140 km of Archaean lithospheric mantle took place beneath the NCB during the Mesozoic (Griffin et al., 1998; Menzies and Xu, 1998; Xu, 2001), coeval with the Yanshanian magmatism. The identification of the high-Ba–Sr granitoids provides further information that links the magmatism and the mantle evolution. Considering the significant contribution of the lith-ospheric mantle to the high-Ba–Sr granitoids and making reference to other continental collision zones where removal of the lower part of thickened lithospheric mantle has been widely accepted for causing post-collisional magmatism (Platt and England, 1994), we invoke delamination of the ancient Sino-Korean cratonic mantle litho-sphere and upward movement of hot-ter asthenosphere to account for the post-collisional magmatism in the NCB and DSO. The geotherm thus
raised may have triggered partial melt-ing of veined peridotite (Foley, 1992) within the remnant lithospheric mantle to form potassic mafic magmas, some of which later differentiated into the high-Ba–Sr granitoids. The resultant extension and basin formation lasted into the Cenozoic and, after a sub-stantial amount of lithospheric thin-ning was achieved, led to a new episode of magmatism consisting exclusively of intraplate basalts (Chung, 1999) resulting from decompression melting of the ascended asthenosphere. Fur-thermore, detailed studies, in partic-ular precise dating of the Yanshanian magmatism, are required to unravel the complete scenario of tectonic evo-lution from stable craton to extension in North China.
Concluding remarks
The geochemical data presented here suggest that in the NCB and DSO the intermediate-acidic rocks of the Yan-shanian post-collisional magmatism
consist dominantly of high-Ba–Sr
granitoids and subordinately of low-Ba–Sr granitoids. The geochemical and Sr–Nd isotope characteristics of the high-Ba–Sr granitoids are similar to those of the associated mafic lavas, suggesting significant contributions from the enriched lithospheric mantle to this type of granitoid. Geochemical similarities of the Yanshanian mag-matism from the NCB and DSO require a regional-scale magma-gen-erating mechanism that we ascribe to delamination of the Sino-Korean cratonic lithospheric mantle.
Supplementary material
The following supplementary material is available from http://www.black wellpublishing.com/products/journals/ suppmat/ter/ter491/ter491sm.htm Appendix
Compiled data of Yanshanian igneous rocks from the North China Block
(NCB) and Dabie–Sulu Orogen
(DSO).
Acknowledgments
Comments from two anonymous reviewers that led to significant improvement of this paper are highly appreciated. This Mesozoic high-Br–Sr granitoids from North China • Q. Qian et al. Terra Nova, Vol 15, No. 4, 272–278
...
study has been supported by Major State Basic Research Development Pro-gram (G1999075502) and National Natural Science Foundation (No. 40002006) of China.
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Received 18 July 2002; revised version accepted 30 April 2003
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