Nature And Evolutionary Process Of The Lithospheric Mantle Beneath The Great Xing’an Range

The nature and evolutionary process of the subcontinental lithospheric mantle has long been one of the hottest issues of solid Earth science community. In the last twenty years, the studies on the evolution and modification of the lithospheric mantle underneath the eastern North China Craton (NCC) and eastern China have got considerable achievements. However, the lithosperic mantle beneath the Great Xing’an Range, which is located at the eastern Central Asian Orogenic Belt-a little west to the famous China Gravity Gradient Stabilization Line, has not been paid enough attentions. Compared with the lithopheric mantle beneath cratons, the orogenic lithospheric mantle may record informatons such as subduction and crust-mantle interaction. Therefore, we collected the peridotite xenoliths from the Xilinhot (XLHT) region and Ar Shan (AS) region, which is located in the southern and middle part of the Great Xing’an Range respectively. Based on the detailed petrologe and geochemistry studies, combined with previous studies and using comparison method, we tend to determine the nature of the lithospheric mantle beneath the Great Xing’an Range, discuss the mantle process such as partial melting and metasomatism and further explore the formation and evolution of the lithospheric mantle beneath the Great Xing’an Range.The XLHT peridotite xenoliths (in the southern part of the Great Xing’an Range) are mainly lherzolite with minor harzburgite (Cpx<5%). Compared with peridotite xenoliths in the XLHT region, peridotite xenoliths in the AS region contain more harzburgite. Xenoliths in the two locations are all spinel peridotite xenoliths, with mineral assemblage of olivine+orthopyroxene stone±clinopyroxene±spinel. No garnet was observed in the hand specimen and thin sections. Some peridotite xenoliths in the AS region contain small amount of modal metasomatic minerals such as amphibole and spondumene.Peridotite xenoliths from the XLHT region can be divided into three groups according to their clinopyroxene REE patterns. The Group1peridotite xenoliths consist of harzburgite or clinopyroxene-poor lherzolite, with clinopyroxene content less than7vol%. Group1xenoliths have high olivine Mg#, spinel Cr#values and low whole-rock basaltic components (CaO, TiO2, A12O3) level. The partial melting degrees of Group1xenoliths are14-18%through model calculating, showing the moderately refractory (less refractory than the Hebi but much more refractory than Shanwang) geochemical characteristics. The Group2xenoliths are lherzolites, with the clinopyroxene content more than9vol%. The Group2xenoliths have low olivine Mg#, spinel Cr#values and high CaO, TiO2, Al2O3bulk contents. The partial melting degrees of Group2xenoliths are no more than10%. The xenoliths of Group2show fertile (slightly more refractory than Shanwang) geochemical characteristics. The Group3xenoliths are composed of both harzburgites and lherzolites. Xenoliths of this Group contain clinopyroxene contents ranging between3-14vol%. The Group3xenoliths have large variations of major-elements, basically overlapping the Group1and Group2peridotite xenoliths. The partial melting degree of Group3xenoliths is5-18%. The Group1xenoliths from the XLHT region show LREE enrichment REE patterns. From the extended trace-element diagrams, the clinopyroxene of the Group1xenoliths have high large ion lithophile elements (LILE) levels and strong negative anomalies in high field strength elements (HFSE). However, relative to the degree of melt extraction, the clinopyroxenes of the Group1xenoliths have higher Nb contents, displaying clues of carbonate metasomatism. The clinopyroxenes in xenoliths of the Group3have sinusoid REE patterns with peak at Sm or Nd. They are moderately enriched in LILE and LREE content. Relative to the degree of partial melting, they have higher Nb, Zr, Ti concentrations and variable Ti/Eu values, displaying clues for the multi-stage metasomatism. The clinopyroxenes in the Group2xenoliths have LREE depleted patterns. There are no apparent characteristics for mantle metasomatism. For the Group3xenoliths, whole-rock REE patterns differ from the clinopyroxene REE patterns, possibly due to the melt/fluid in the mineral grain boudaries which are enriched in LREE.The peridotite xenoliths from AS region can be subdivided into three groups, according to their clinopyroxene REE distribution curves. Similar to the XLHT xenoliths, the Group1xenoliths from the AS region are all harzburgites, with clinopyroxene mode content not more than3vol%. They have high olivine Mg#, spinel Cr#values, and have low whole-rock CaO, TiO2, Al2O3levels. Through model calculating, the degree of partial melting of Group1xenoliths are14-20%, showing moderate refractory (less refractory than Hebi but more refractory than Shanwang) geochemical characteristics. The Group2xenoliths are Iherzolite, with clinopyroxene mode content of12vol%. The Group2xenoliths have low olivine Mg#, spinel Cr#values and high whole-rock CaO, TiO2, Al2O3levels. Its melt extraction degree is about1%, showing very fertile features. The Group3xenoliths contain both harzburgite and lherzolite, with clinopyroxene mode contents no more than7vol%. The Group3xenoliths have a large range of major-elements, with the partial melting degrees from5%to20%. Part of the Group3xenoliths from AS region may contain garnet. However, after model calculating, the garnet contents of the Group3xenoliths, if have, can not be very high. The clinopyroxene in the Group1xenoliths from AS region have flat REE patterns. But they has significantly negative (Nb, Ta, Zr, Hf, Ti) HFSE anomalies, positive Sr anomaly from the trace-elements extended diagrams. They aslo show no enrichment of Nb contents relative to its degree of partial melting. Combined with thin section observations (the presence of hornblende), the Group1xenoliths show characteristics of aqueous fluid metasomatism. The Group3xenoliths have sinusoid clinopyroxene REE patterns. Some of the Group3xenoliths have Sr positive anomaly and significantly Ti anomalies, reflecting the features of multi-stage metasomatism. No HFSE negative anormalies of the rest Group3xenoliths shows the characteristics of silicate metasomatism. The Group2xenoliths from the AS region have LREE depleted patterns, showing no apparent metasomatic features. The Group3xenoliths from the XLHT and AS region experienced melt/rock reaction by some degree. They have higher levels of orthopyroxene modes, higher clinopyroxene Cr and spinel Ti contents, ande whole-rock Ti enrichment and slight Fe enrichment. The reactions may dissolve clinopyroxenes and precipitate orthopyroxenes and/or olivines.Compared with the North China Craton xenoliths (Hebi and Shanwang), peritotite xenoliths from the Great Xing’an Range show moderately refractory to fertile features. The Group1xenoliths in the XLHT and AS region have higher equilibrium temperatures than the Group2xenoliths. However, taking account for recent geochemical data, the peridotite xenoliths from the XLHT and AS region show no regular variations between spinel Cr#values and equilibrium temperatures. For the XLHT and AS xenoliths, the Sr-Nd data and187Os/188Os values also show no regular change with equilibrium temperatures. However, for the Wudalianchi-Keluo (WEK) xenoliths, the187Os/188Os values seem to decrease with increasing of the equilibrium temperatures. The xenoliths from the Great Xing’an Range show similar Sr-Nd, Re-Os isotopic compositions with other places in the northeast China peridotite xenoliths. The lithospheric mantle beneath the northeast China is mainly fertile and new, with local presence of the remnants of ancient refractory mantle. We interpreted that the Central Asian orogenic belt is composed of a variety of tectonic units (oceanic island, island arcs, accretionary wedge and Precambrian microcontinental). The Great Xing’an Range is located in the eastern Central Asian Orogenic Belt. During the the Paleozoic and Mesozoic, the Great Xing’an Range was governed by Paleo-Asian Ocean and Paleo-Pacific ocean domain. Long term slab subduction and magmatic activity maintain a high heat budge and positive buoyancy of the lithosphere relative to the asthenosphere. After the end of the subduction and magmatism, new accreted lithospheric gradually cooled and became instability on gravity. Finally, the new accreted lithosphere delaminated and the upwelled asthenosphere cooled to form newly accreted lithospheric mantle. Lithospheric mantle underneath the ancient continent fragments may be (partly) preserved due to theric gravity stability. Today, the lithospheric mantle beneath the Great Xing’an Range, even northeastern China are mainly new accreted and fertile, with local presence of ancient refractory lithospheric mantle relict. For the lithospheric mantle beneath the WEK region, previous studies on the host potassic basalts suggested that the garnet-face lithospheric mantle is formed before Mesoproterozoic. Thus the lithospheric mantle beneath the WEK region is featured by younger mantle at shallow levels and older mantle at deep levels. The lithospheric mantle beneath the WEK region need to be further investigated.