Interaction of adakitic melt-peridotite: Implications for the high-Mg# signature of Mesozoic adakitic rocks in the eastern North China Craton
Introduction
Several models for the origin of Archean tonalite–trondhjemite–granodiorite (TTG) suites and/or adakites have attributed their distinctive geochemical characteristics (e.g., high Sr/Y and La/Yb ratios) to their sources of eclogite or garnet-bearing amphibolite (Rapp et al., 1991, Martin, 1993, Rapp et al., 2003). Models proposing the partial melting of subducted oceanic crust (Martin et al., 2005) or thickened mafic continental lower crust (Condie, 2005) are usually employed to explain the origins of these rocks. However, melting of eclogite, amphibole eclogite or amphibolite source rocks will produce melts of low Mg# [Mg# < 45, where Mg# = molar 100 × Mg/(Mg + Fetot)] (Rapp and Watson, 1995, Rapp et al., 2003). Reaction between such melts and peridotite has been advocated to result in mantle refertilization and formation of high-Mg# continental crust (Kelemen et al., 1993, Rudnick et al., 1994, Kelemen, 1995, Kelemen, 1998, Liu et al., 2005) but could also explain the generation of high-Mg (Mg# > 45) TTG and adakitic rocks (Ringwood and Green, 1966; Ringwood, 1974, Kay, 1978, Myers et al., 1985, Kelemen, 1986, Kelemen et al., 1993, Yogodzinski et al., 1994, Kelemen, 1995, Yogodzinski and Kelemen, 1998, Gao et al., 2004, Xu et al., 2006, Scambelluri et al., 2006, Yogodzinski and Kelemen, 2007).
Although a genetic link between high-Mg TTG and adakitic rocks and reaction of melt with mantle peridotite has been suggested based on studies of mantle xenoliths and experimental grounds (Yaxley and Green, 1998, Rapp et al., 1999, Thompson and Gibson, 2000, Scambelluri et al., 2006), physical evidence is scarce (Liu et al., 2005, Xu et al., 2006). In this study we focus on a rare suite of mantle peridotite xenoliths entrained by Early Cretaceous high-Mg diorites that occur in the southern part of the eastern block of the North China Craton (NCC), in western Shandong. The petrography, mineral chemistry and bulk-rock compositions of these xenoliths provide compelling evidence for a genetic link between adakitic melt-peridotite interaction and formation of high-Mg dioritic magmas (Kelemen and Ghiorso, 1986).
Section snippets
Geological setting
Based on age, lithological assemblage, tectonic evolution, and P–T–t paths, the North China Craton (NCC) can be divided into the Eastern Block, the Western Block and the intervening Trans-North China Orogen (see insert of Fig. 1) (Zhao et al., 2001). Western Shandong is located in the Eastern Block of the NCC. Voluminous Mesozoic intermediate to felsic igneous rocks of adakitic affinity are widespread in the eastern NCC, including western Shandong (Fig. 1) (Zhang et al., 2001, Xu et al., 2002,
Petrography of the xenoliths
The peridotite xenoliths are generally rounded in shape and range in size from ∼ 8 × 6 × 4 cm3 to ∼ 3 × 2 × 1 cm3 in size (Fig. 2a). Based on the modal proportions of olivine, orthopyroxene, and clinopyroxene, they can be classified into chromite-bearing dunite and spinel-bearing harzburgite, with dunites constituting the dominant group.
The spinel-bearing harzburgites usually exhibit porphyroclastic and/or granoblastic textures. Veins of orthopyroxenite, or orthopyroxene + phlogopite, can be sometimes
Analytical methods
In order to avoid any potential contamination of the xenoliths, all traces of the host diorite, weathering surfaces and reaction rims between the xenoliths and their host rocks were physically removed using a diamond saw. The remaining xenolith fragment was then washed and crushed to 40 mesh in an alumina jaw crusher. About 30 g of the sample split were powdered in an agate ring mill to less than 200 mesh for whole rock analysis.
Major element compositions and BSE images of minerals were
Olivine
The Mg#'s of olivines from the xenoliths have a restricted range (∼ 88–95), whereas those associated with zoned orthopyroxene and/or orthopyroxene +phlogopite surrounding chromite range from 82 to 87 (Fig. 3, Fig. 2e, f). In both cases, olivine rims in contact with chromites have the highest Mg#. CaO contents of all olivines from the different types of xenoliths are less than 0.1 wt.%, averaging 0.04 wt.%. In the dunites, Group I and Group II olivines have similar chemical compositions (Table 3
Equilibration temperatures
Equilibration temperatures (T) for the harzburgite and dunite xenoliths have been estimated using selected cation exchange thermometers (Wells, 1977, Febries, 1979, Brey and Köhler, 1990, Köhler and Brey, 1990, Witt-Eickschen and Seck, 1991, Xu, 1993) and are given in Table 5 in the Appendix. Several features are apparent from the results (Table 5 in the Appendix). Firstly, the dunite and harzburgite xenoliths show low equilibrium temperatures (< 1000 °C), similar to ‘low temperature’ xenoliths
Major elements
The peridotite xenoliths have SiO2 = 39.71–44.47 wt.% and Mg# = 88.8–93.6 (Table 2). Harzburgite xenoliths have relatively higher Al2O3 and CaO contents than dunite xenoliths (Table 2). Compared with the mantle xenoliths from the Cenozoic basalts in eastern China (E and Zhao, 1987, Rudnick et al., 2004), dunites and harzburgites are poor in Al2O3 (0.22–1.48 wt.%) and CaO (0.16–0.62 wt.%), and similar to (spinel) peridotites from the Kaapvaal Craton (O'Reilly et al., 2001) and depleted dunites and
Petrogenesis of peridotite xenoliths
Whether peridotite xenoliths in the Mesozoic high-Mg diorite from western Shandong were derived from the lithospheric mantle (Xu et al., 1993, Xu et al., 2003) or cumulates at the crust–mantle transition (Chen and Zhou, 2005) remains controversial, largely owing to the low Mg#'s of the olivines in some xenoliths. Re–Os isotopic results indicate that three harzburgite xenoliths (LW8-44, LW8-56, T7-21) have the Archean Re-depletion model ages (TRD = 2.60–2.68 Ga) (Gao et al., 2007), implying a
Conclusions
Petrographic, mineralogical and geochemical data indicate that the harzburgite and dunite xenoliths entrained by Early Cretaceous adakitic rocks in the eastern North China Craton originated from the Archean lithospheric mantle formed by large degrees of partial melting prior to the introduction of adakitic melt. The vein or zoned orthopyroxene in dunite and some harzburgite xenoliths represents the product of adakitic metasomatism and implies that the previously depleted Archean lithospheric
Acknowledgements
We thank G. M. Shu for their technical support during the electron microprobe analysis. Comprehensive and constructive reviews of R. Rapp and P. Kelemen helped to improve the manuscript significantly. This research was financially supported by the Natural Science Foundation of China (Grants 40472033, 40673019, 40521001) and the Ministry of Education of China (20040183065, IRT0441, B07039, 306021).
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