Geochemical And Sr-Nd Isotopic Compositions Of Early Cretaceous Yixian And Sihetun Lavas From The North China Craton:Recycling Of Ancient Lower Continental Crust

Based on chemical and isotopic compositions of oceanic basalts and mantle xenoliths, the Earth’s mantle is highly heterogeneous. It has been suggested that this heterogeneity would be related to subduction of oceanic and sediments into the mantle source. However, recycling of the lower continental crust has also played an important role in the generation of mantle heterogeneity, especially for the origin of EMI mantle endmember (Enriched Mantle type 1) The Early Cretaceous (-125 Ma) Yixian and Sihetun primitive basalts (MgO>10 wt%) and intermediate-felsic lavas from Western Liaoning province in the Eastern Block of the North China craton (NCC) were studied for major and trace element and Sr-Nd isotopic compositions in this study. The following several conclusions were made based on this study:1. EMl-type mantle source beneath the North China craton during the Early Cretaceous and contribution from the lower continental crust.It is difficult or impossible to sample rocks from the deep mantle directly, because it is restricted by our limited drill technology. Geophysical technology can only reflect the present physical nature of the mantle and have a problem of multiple solutions. Therefore, primitive mantle-derived magma and mantle xenoliths in them become the best samples to reveal lithological and chemical compositions of the mantle sources, As peridotite xenoliths are rarely entrained by basaltic rocks during the Late Mesozoic, primitive basalt with high MgO is the best tool to study the nature of the Late Mesozoic lithospheric mantle of the NCC. This study reported geochemical and Sr-Nd isotopic compositions of the Early Cretaceous Yixian and Sihetun primitive basalts from the western Liaoning province in the Eastern Block of the NCC. These basalts show various and low d(t) (-1.5 to -11.8) and moderate 87Sr/86Sr (0.70622-0.70679). The low and concentrated 87Sr/86Sr ratios indicate insignificant crustal contamination. The high MgO (>10 wt%), Ni (<380 ppm) and Cr (<1467 ppm) of these basalts also negate remarkable fractional crystallization and crustal assimilation. Therefore, Sr-Nd isotopie compositions of these primitive basalts indicate their EMI-type mantle sources. Recycling of oceanic crust and sediments has been applied to explain EMI signatures of oceanic basalts, in which sediments play an important role in the generation of EMI isotopic signatures. However, Plank and Langmuir showed that addition of subducting sediments is expected to produce high 87Sr/86Sr ratio and radiogenic Pb isotopes (EM2) in the mantle source, which is inconsistent with signatures of EMI rocks. On the other hand, ancient sub-continental lithospheric mantle, represented by peridotite xenoliths in Paleozoic Fuxian and Mengyin kimberlites from the Eastern Block of the NCC, shows relative higher 87Sr/86Sr (0.70818-0.71398) and 206Pb/204Pb (19.532-19.627) ratios (features of EM2-type mantle), which are completely different from the EMl-type mantle sources reflected by the Yixian and Sihetun primitive basalts. In addition, ancient sub-continental lithosphere always have uniquely low 187Os/188Os ratios (<0.127) due to largely melt extraction at early time, as evidenced by peridotite xenoliths from the Western Block of the NCC. where ancient refractory lithospheric mantle is still preserved. In contrast, the Sihetun basalts exhibit elevated initial 187Os/188Os ratios (0.1643-0.2291) that are much higher than those of primitive mantle (0.127). These lines of evidence suggest that the Yixian and Sihetun primitive basalts were not significantly contributed from ancient lithospheric mantle of the NCC. The continent-like trace element (e.g. depletions of Nb and Ta, enrichments of LILE, low Ce/Pb and high Ba/Rb) and EMI isotopic compositions of these basalts argue strongly for great involvement of the lower continental crust components in the mantle sources.2. The hybridized pyroxenite mantle source beneath the North China craton during the Early CretaceousOlivines from the Yixian and Sihetun primitive basalts are euhedral in morphology and show high CaO (>0.10 wt%) in the core, indicating their magmatic origin. Calculated Mg-Fe partition coefficients (0.30-0.33) between the cores of the most forsteritic phenocrysts and whole rocks are consistent with experimental values (0.30±0.03). This indicates that these olivine phenocrysts are equilibrated with the basaltic magma. Therefore, the high MgO (and Mg#), Ni and Cr contents of these basalts are intrinsic features. Compared to peridotite-derived melt and primitive MORBs, the Yixian and Sihetun primitive basalts are characterized by high Fe/Mn (55-69), Zn/Mn (0.050-0.078) and Zn/Fe×104 (9-12) ratios. Correlations between MgO and other major oxides indicate predominant frantional crystallization of olivine with less or no clinopyroxene and garnet. Therefore, the high Fe/Mn, Zn/Mn and Zn/Fe ratios in these basalts were not resulted from frational crystallization of clinopyroxene and garnet, but suggest garnet-rich pyroxenite mantle sources. Such pyroxenite manlte source is also evidenced by major element compositions of these primitive basalts and olivine compositions. It has been suggested that the CaO content of initial peridotite-derived melt would be nearly constant at about 10 wt% over a wide range of initial melting pressures (3-7 GPa), although it will slightly increase as partial melting extent become greater. The low CaO content (<8.5 wt.%) of the Yixian and Sihetun primitive basalts may reveal pyroxenite mantle sources. Additionally, compared to olivines of peridotite-derived basalts, olivines of the Yixian and Sihetun primitive basalts have much higher Ni and lower Ca contents that are comparable to olivines of Hawaii basalts. Fractional crystallization model suggests that peridotite-derived melt can not produce olivines with such high Ni and low Ca contents in the Yixian and Sihetun and Hawaii basalts, even when a high Ni content (2360 ppm) was assumed for the peridotite mantle. We thus propose that the Yixian and Sihetun primitive basalts were derived from pyoroxenite mantle sources produced by interaction between Si-rich melt and peridotite, a mechanism was suggested for the origin of Hawaii basalts.3. Mantle heterogeneity and magma mixing in the deep mantleAlthogh both the Yixian and Sihetun basalts are suggested to derived from pyoroxenite mantle sources produced by interaction between Si-rich melt and peridotite and they share common features in major element and Sr isotopic compositions, and most trace element signatures (e.g. depletions of Nb, Ta and HREE, enrichments of LILE and LREE), there are also differences between the Yixian and Sihetun basalts. On one hand, the Sihetun basalts have much higher Rb, Ba, Th, U and LREE contents with similar Nb, Ta, Pb, Sr, Zr, Hf, Ti and HREE contents. As a result, the Sihetun basalts show more negative Nb, Ta and Ti anomalies, less or no positive Pb and Sr anomalies, higher La/Yb ratio, and remarkable negative Zr and Hf anomalies that are not present in the Yixian basalts. On the other hand, the Sihetun basalts have much higher εNd(t) values (-2.3 to -1.5) than the Yixian basalts (-11.8 to -10.3) with similar ininital 87Sr/86Sr ratios. These differences between the two suites of primitive basalts cann’t be interpreted by either fractional crystallization or various-degree partial melting of a homogeneous mantle source or heterogeneous mantle sources as a result of contribution from foundered eclogitic lower crust with various Nd isotopic compostions. The higher εNd(t) values (-2.3 to-1.5) of the Sihetun bssatls may imply contribution from partial melt of depleted mantle. Here we propose that the Sihetun basalts were produced by magma mixing between enriched pyroxenite-derived melt (e.g. the Yixian baslats) and depleted mantle-derived melt. The Sihetun basalts have greatly increased Th, U and LREE contents but faintly increased Nb, Ta, Zr, Hf, Ti and HREE contents compared to the Yixian basalts. Furthermore, the Sihetun basalts have superchondritic Zr/Hf ratios and negative correlation between Zr/Hf and Zr. These features suggest that the depleted matle-derived melt would be carbonatitic melt that were derived from a low-degree partial melting of the depleted asthenosphere. Trace element and Sr-Nd isotopic models both suggest that there were ～2-7% depleted mantle-derived carbonatitic melt involved in production of the Sihetun basalts.4. High-Mg andesite:Recycled lower continental crust-derived melt-peridotite reactionHigh Mg# andesites (HMAs) share several fundamental features in chemical composition with the continental crust, such as andesitic bulk composition with high MgO, Ni and Cr contents, depletions of Nb and Ta and enrichments of large ion lithophile elements (LILEs) (e.g. Rb, Ba, Pb, Sr) and light rare earth elements (LREEs) with low Ce/Pb and high Sr/Y ratios. It is therefore critical for understanding the andesitic bulk composition of the continental crust and its formation and evolution, which is one of the most important geosciencific issues. The Yixian andesites can be divided into the high-Mg and low-Mg andesites, in which some are primitive andesites with MgO>7wt.%, while the Sihetun andesites are all high-Mg andesites. In addition, some of the Yixian dacites and rhyolites have high MgO and Mg# at a given SiO2 content and thus are high-Mg dacites and rhyolites. Fractional crystallization model of the Yixian and Sihetun primitive basalts suggest that the high Ni content of the Yixian and Sihetun high-Mg andesites cann’t be derived from the Yixian and Sihetun primitive basalts by fractional crystallization. More important is that the Yixian primitive andesites and some high-Mg andesites have much depleted Sr and Nd isotopic compositions than the Yixian basalts. These lines of evidence suggest that the Yixian and Sihetun high-Mg andesites were not product of fractional crystallization of high-Mg basalts. Although partial melting of water-saturated peridotite could produce high-Mg andesite, the relative lower Sr and Sr/Y and reversely zoned orthopyroxenes indicate that the Yixian and Sihetun high-Mg andesites were partial melt of basaltic rocks followed by interaction with peridotite mantle. Variations of trace elements suggest that the Yixian and Sihetun high-Mg andesite-dacite-rhyolites of MgO<4.5 wt.% are derived from various-degree partial meling of rutile-bearing eclogite combined with slight interaction with peridotite mantle, whereas those of MgO>4.5 wt.% are resulted from intensive reaction between Si-rich melt and peridotite. The enriched Sr-Nd isotopic compositions of the Yixian and Sihetun high-Mg andesites imply significant contribution from ancient continental crust. Taken above observation together, we propose that the Yixian and Sihetun high-Mg andesites were derived from partial melting of foundered eclogitic lower crust followed by interaction with peridotite mantle. Based on Nb and Ta contents, the Yixian low-Mg andesite-dacite-rhyolites can be subdivided into high-Nb (Ta) and low-Nb (Ta) types, in which the former is suggested to be formed by fractional crystallization (ol+cpx+pl+ilm) of high-Mg andesites and the latter is suggested to be resulted from crust contamination of high-Mg andesites.