Elsevier

Lithos

Volume 77, Issues 1–4, September 2004, Pages 609-637
Lithos

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

https://doi.org/10.1016/j.lithos.2004.03.033Get rights and content

Abstract

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.

Introduction

Archean cratons are underlain by mantle lithosphere that is thick, cold and refractory Jordan, 1975, Jordan, 1988, van der Hilst and McDonough, 1999. Such lithosphere has a high viscosity because it is cold and nearly anhydrous, and thus contributes significantly to craton stability Pollack, 1986, Hirth et al., 2000. However, not all regions of Archean-aged crust are underlain by such refractory mantle lithosphere, and these regions are characterized by a more protracted history of tectonism and magmatism than their cratonic counterparts. There are at least two possible reasons for the absence of thick mantle keels beneath Archean-aged crust: (1) they may have never formed, or (2) they may have formed but were subsequently removed.

An example of the first possibility is the Mojave terrain in SW U.S. Here, the crust has Paleoproterozoic to late Archean Nd model ages Bennett and DePaolo, 1987, Raymo and Calzia, 1998 but middle Proterozoic crystallization ages (Wooden and Miller, 1990). Mojavia is underlain by late Archean lithospheric mantle that is considerably more fertile and dense than typical cratonic mantle (Lee et al., 2001). Hence, this mantle lithosphere did not grow to the same thickness as that beneath Archean cratons. Lee et al. (2001) proposed that the thinner lithosphere beneath Mojavia failed to shield this small fragment of Archean lithosphere from tectonic reworking. This study demonstrated that thick lithospheric keels do not always form beneath Archean crust.

An example of the second possibility is the North China craton, where multiple lines of evidence (surface geology, xenolith studies, seismic and heat flow data) show that this craton formed with a thick lithospheric keel in the Archean that was subsequently removed (e.g., Menzies et al., 1993, Griffin et al., 1998). The timing and mechanisms of lithospheric mantle removal beneath the North China craton are yet to be fully understood.

The present paper reports the petrography, mineral chemistry, thermometry and major element compositions of spinel peridotite xenoliths from the Hannuoba and Qixia localities for which Os data have previously been reported (Gao et al., 2002). In addition, for the Hannuoba peridotites, we present trace element and Sr and Nd isotope geochemistry. We show that the Sr and Nd isotopes reflect recent metasomatic overprinting not related to the Tertiary hosts and that both Hannuoba and Qixia peridotites are indistinguishable in terms of their bulk compositions, despite the fact that their formation ages differ by nearly 2 billion years.

Section snippets

Geologic setting

The North China craton is divided into three regions based on geology, tectonic evolution and PTt paths of metamorphic rocks (Fig. 1; Kusky et al., 2001, Zhao et al., 2000, Zhao et al., 2001). The western block forms a stable platform composed of late Archean to Paleoproterozoic metasedimentary belts that unconformably overly Archean basement Wu et al., 1998, Li et al., 2000, Zhao et al., 2000. The latter consists of granulite facies tonalite–trondhjemite–granodiorite (TTG) gneiss and

Samples and previous work

The samples investigated here are spinel-facies peridotites from the Hannuoba and Qixia xenolith localities (Fig. 1). In this section, we review the xenolith associations at each locality and some results from previous investigations. A more complete discussion of previous results for the peridotite xenoliths, in the context of our new data, is provided in 6 Results, 7 Discussion.

Analytical methods

The xenoliths were sawn from their lava hosts and the cut surfaces were abraded with quartz in a sand blaster to remove any possible contamination from the saw blade. The samples were then disaggregated between thick plastic sheets with a rock hammer and reduced to powder using first an alumina disk mill followed by an alumina ring mill. A portion of the crushed fraction was sieved and clinopyroxene separates were handpicked under a binocular microscope to a purity of >98%. They were cleaned in

Hannuoba

The Hannuoba spinel-facies peridotites range from coarse- to medium-grained and have granuloblastic textures (Fig. 2). Most samples are massive, but a few show foliation defined by aligned spinel grains. In general, both clinopyroxene and orthopyroxenes are homogenous, showing no exsolution lamellae. All xenoliths show alteration along grain boundaries and fractures (typically pale brown serpentine), but the degree of alteration is highly variable from one sample to the next (Fig. 2). A

Whole rock major and trace element data

Major and trace element analyses of whole rock samples are reported in Table 3 and plotted in Fig. 3, Fig. 4, Fig. 5, Fig. 6. Both Hannuoba and Qixia peridotites show a considerable spread in major element compositions, ranging from fertile compositions approaching primitive mantle to refractory harzburgites with up to 45% MgO. These refractory compositions slightly overlap the compositional field of cratonic peridotites, as exemplified by samples from the Tanzanian craton Rudnick et al., 1994,

Origin of the peridotites and major element systematics

The major element, compatible and moderately incompatible trace element and mineral chemical data for the Hannuoba and Qixia spinel peridotite xenoliths reflect their origin as residues from variable degrees of partial melting of a primitive mantle composition Fig. 3, Fig. 4, Fig. 7. Based on major element systematics (Fig. 3; Table 3) and published experimental melting studies, these peridotites may reflect between 0% to 25% removal of a batch melt from a primitive mantle composition at

Conclusions

The data presented here, coupled with the previously published Os isotopic results for these same samples provide insights into the nature of the mantle lithosphere underlying the North China craton. The main conclusions are:

  • (1)

    The spinel peridoitite xenolith suites from Hannuoba and Qixia are compositionally indistinguishable from each other and yet markedly different from cratonic mantle lithosphere found beneath other Archean cratons.

  • (2)

    The lithospheric mantle beneath Hannuoba and Qixia formed by

Acknowledgements

We thank Mike Rhodes for providing some of the XRF whole rock analyses. David Lange and Phil Piccoli provided guidance in electron mircroprobe analyses. Trisha Fiore helped with electron microprobe analyses at Harvard and Xiao-min Liu with XRF and ICP–MS analyses in Xi'an. We thank Stephanie Schmidberger and an anonymous reviewer for very helpful comments. The EPMA used in this study at the University of Maryland was purchased with grants from Department of Defense-Army/ARO (DAAG 559710383) and

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