Isotopic Compositions Of The Peridotite Xenoliths And Age Of The Subcontinental Lithospheric Mantle In The Great Xing'an Range

The subcontinental lithospheric mantle (SCLM) is an important part of lithosphere, and serves as a bridge that connects the asthenosphere and the crust. Recently, understanding of the formation and evolution is largely depended on the cognition of the lithospheric mantle. However, for a long time, the most difficult problem is that how to dating the SCLM. The poor silicon and high temperature of the SCLM make the common isotopic methods useless to dating the SCLM. Fortunately, the newly developed Re-Os isotope dating technique provides a powerful method to determine the age of lithospheric mantle because of the unique geochemical behavior of the Re-Os system. Therefore, this work takes the advantage of the Re-Os isotopic dating technique, studying on the peridotite xenoliths contained by Cenozoic basalts in Great Xing'an Range, Xing'an-Mongolia Orogenic Belt (XMOB). Major, trace elements and Sr-Nd-Hf isotopes of clinopyroxene of the xenoliths were analyzed to determine the melting and metasomatism of the SCLM. Furthermore, the bulk-rock Re-Os isotopes were also used to constrain the forming age of the SCLM, and to discuss the relationship of crust-mantle and evolution histories of the lithospheric mantle in the XMOB.The Great Xing'an Range located in eastern part of the XMOB, is characterized by the widespread Mesozoic granite and volcanic rocks. During the Cenozoic, extensive basalt eruptions formed a series of volcanoes. Some of these basalts contain mantle xenoliths and xenocrysts. In Keluo area, the mantle xenoliths, hosted in potassic basalts, mainly comprise spinel lherzolite (some phlogopite-bearing) and harzburgite, with minor wehrlite and dunite. The xenoliths hosted in the Nuomin sub-potassic basalts, mainly comprise spinel lherzolite and harzburgite. Abundant mantle xenoliths have also been recovered from the sodic basalt in Chaihe volcanoes. They are mainly composed of spinel lherzolite, with minor spinel harzburgite. Garnets have been found in some peridotites, but they are broken down as fine-grained pyroxene and spinel aggregates. All the xenoliths display equigranular, protogranular to porphyroclastic textures, with weak deformation. The equilibrium temperatures of the xenoliths have been calculated using various geothermometers. The obtained equilibrium temperatures are 770～950℃for Keluo and 900～1070℃for Nuomin using the geothermometer of Brey and Kohler (1990), and 946-1279℃for Chaihe by Eichshen and Seek, (1991). There is little difference in temperatures between the lithologies in any area.The xenoliths have wide range of bulk-rock Al2O3 and CaO contents. However, in Al2O3-CaO diagram, they are all plotted along the oceanic trend. Of them, the refractory harzburgites fall into the area of the depleted cratonic mantle, indicating that these xenoliths are residues of mantle that underwent relatively high degrees of partial melting. The occurrence of phlogopite vein and wehrlite, low Fo dunites, and enriched LREE suggest that the mantle had been refertilized and metasomatised by carbonatite melts and silicate melt. Comparison of trace elements of the clinopyroxene and bulk-rock indicates that components introduced by refertilization and metasomatism mainly aggregated intergranularly, with little effect on major components and Os isotopes.187Os/188Os ratios of the Keluo samples range from 0.11458 to 0.13194, and that of the Nuomin xenoliths from 0.11852 to 0.12533. Most of them have 187Os/188Os ratios are in the range of estimated modern convecting upper mantle, indicating that the present lithospheric mantle is juvenile and formed during the Phanerozoic. However, two harzburgites in Keluo (KL3-40 and 08KL-02) which experienced high degrees of partial melting, show the 187Os/188Os radios of 0.1146 and 0.1160, with Re depleted model ages of 1.9-2.1Ga that agrees well with the proxy-isochron age. The same conclusion can obtain in Nuomin as well. Additionally, theεNd andf of xenoliths with Phanerozoic melt depletion ages are much higher than that of xenoliths with ancient melt depletion ages. So, it is concluded that the shallow lithospheric mantle (spinel face) beneath Keluo and Nuomin are mainly juvenile with minor ancient component. Previous studies had demonstrated that the host basalts of these xenoliths derived from the phlogopite-baring garnet peridotites which located beneath the spinel peridotites and synchronously metasomatised by the potassic silicate melt with spinel peridotites. The Sr-Nd isotopes of the basalts indicate they generated from an ancient EM-Ⅰtype SCLM source, which locates beneath the juvenile depleted mantle. Clearly, the stratigraphy in SCLM beneath Keluo and Nuomin is inverted, that is much different from the normal age structure of SCLM in most craton.The 187Os/188Os ratios of xenoliths in Chaihe range from 0.11059 to 0.12755. The most refractory harzburgite 03CH-10 with Fo value of 92.1, has the lowest 87Os/188Os ratios (0.11059), and less depleted Sr-Nd-Hf isotopes than other samples. This indicates the occurrence of residues of SCLM with ancient Re depleted model age (TRD=2.6Ga). The 187Os/188Os ratios of most of the remaining samples are the same as the modern convecting upper mantle. Sr-Nd-Hf isotopes of clinopyroxene from these samples show depleted signature. Contrasted Sr-Nd-Hf-Os results for peridotites from Dashihe, Yitong, Wangqing and Shuangliao, it likely that the present lithospheric mantle in there areas is mainly juvenile with minor ancient residues.Based on the Sm-Nd and Lu-Hf isotopic characteristics of the Phanerozoic granites and volcanic rocks in XMOB, it is the general consensus that the crust growth started at Mid-Proterozoic, with the peak at Neo-Proterozoic to Phanerozoic, which is decoupled with the age of SCLM. Therefore, the crust-mantle age decoupling and "inverted" mantle indicate the complicated evolution history of SCLM in XMOB.Usually, the SCLM and crust should be formed at the same time. The absence of Neo-Proterozoic and Phanerozoic mantle suggested that the XMOB had undergone the whole-scale lithosphere removal event. We believe that the SCLM fall into the asthenosphere by the delamination, then the asthenosphere upwelling and formed the depleted lithospheric mantle. According to geological evolution of XMOB, we inferred that the Neoarchean-Paleoperozoic lithospheric mantle should be exotic. The ancient lithosphere residues occurred in the juvenile mantle, e.g. Chaihe, Wangqing, Dashihe, may be originated from the asthenosphere with the depleted lithospheric mantle. However, the large-scale ancient SCLM beneath Keluo and Nuomin was extruded from elsewhere and impinged beneath the XMOB. We inferred that it may come from North China Craton or Yangtze Craton.