The Nature Of The Mesozoic Lithospheric Mantle Beneath The Western Liaoning Province

The Archean North China Craton（NCC）is a natural laboratory on Earth for investigating the secular evolution of the subcontinental lithospheric mantle（SCLM）,as the NCC experienced extensive lithospheric thinning and transformation in response to widespread thermo-tectonic activities during the Mesozoic and early Cenozoic from cold,refractory and thick lithospheric mantle to hot,fertile and thin lithospheric mantle with oceanic affinities.Upper mantle peridotites are predominantly composed of olivine,clinopyroxene and orthopyroxene,which are of significant importance to deduce the compositional characteristics and secular evolution of the Earth’s upper mantle.Thus,this paper focuses on the olivine and clinopyroxene hosted by the Early Cretaceous olivine basalts from the western Liaoning Province and reports the mineralogy,major element geochemistry and O isotopic features of zoned olivines,as well as major and trace element geochemistry of zoned clinopyroxenes from the Yixian Formation basalts.After recognizing the xenocrystic origin of the olivine and clinopyroxene,we have discussed the nature of the Mesozoic lithospheric mantle beneath the northern margin of the NCC and identify the important role of the peridotite-melt reaction played in the evolution of lithospheric mantle in terms of the in-situ O isotopic composition of olivines.Abundant zoned olivine xenocrysts from Early Cretaceous basalts of the Yixian Formation in western Liaoning Province contain critical information about the nature and evolution of the lithospheric mantle of the northern NCC.These olivine xenocrysts tend to be large（600–1600μm）,usually rounded and embayed,with well-developed cracks.Their cores have high and uniform forsterite（Fo）contents（88–91）,similar to the peridotitic olivine entrained by regional Cenozoic basalts.Their rims have much lower Fo contents（74–82）,comparable to phenocrysts（72–81）in the host basalts.These characteristics reveal that the zoned olivine has been disaggregated from mantle peridotites and thus can be used to trace the underlying lithospheric mantle at the time of basaltic magmatism.The olivine cores have high oxygen isotope compositions(?¹⁸O_SMOW=5.9–7.0‰)relative to the normal mantle value,suggesting that the Early Cretaceous lithospheric mantle was enriched and metasomatized mainly by melts/fluids released from subducted oceanic crust that had experienced low-temperature hydrothermal alteration.Preservation of zoned olivine xenocrysts in the Early Cretaceous basalts indicates that olivine-melt/fluid reaction could have been prevalent in the lithospheric mantle as an important mechanism for the transformation from old refractory（high-Mg）peridotitic mantle to young,fertile（low-Mg）and enriched lithospheric mantle during the early Mesozoic.Two populations of clinopyroxenes are present in the Yixian high-Mg basalts including the normally-zoned and subordinate reversely-zoned ones.The reversely-zoned clinopyroxenes are generally large,mostly in the range of 0.8-1.6 mm,and have erosion textures.They have variable and low Mg#（65.5-79.7）in the core,compositionally close to crustal clinopyroxenes from the NCC,whereas their mantles have uniform and much higher Mg#（85.1-89.6）.Normalized REE patterns of the cores are relatively flat,and they exhibit prominently negative anomaly of Eu and Sr,suggesting an xenocrystic origin from crustal metamorphic rocks in equilibrium with plagioclase.The mantles are much richer in Mg O and Cr₂O₃ but poorer in Fe O,Al₂O₃and Na₂O than cores.Their LREE and HREE are highly fractionated,indicating that they are magmatic pyroxenes formed by overgrowth on the eroded cores.Normally-zoned clinopyroxene grains are collectively small（<500?m）and possess erosion textures with cracks and fractures.They have high and uniform Mg#（85.9-89.6）in the core,similar to Mesozoic olivine pyroxenites in the NCC,whereas their rims have much lower Mg#（65.1-83.9）.In addition,Mg O and Cr₂O₃ contents decrease from core to rim whereas Fe O,Ti O₂,Al₂O₃ and Ca O increase in normally-zoned clinopyroxenes.The cores and rims both have relatively low REE abundance and highly fractionated REE patterns.The cores of normally-zoned clinopyroxenes share similar REE patterns with Mesozoic olivine pyroxenites in the NCC,suggesting that they might be directly disaggregated from olivine pyroxenites and the rimes are the reaction results between the core and host basaltic melt.Some subhedral rims are magmatic origin.Previous studies on the petrology and Sr–Nd isotope compositions of the widely distributed late Mesozoic basaltic rocks through the eastern NCC show that the Paleozoic cratonic lithospheric mantle evolved into the Mesozoic fertile and enriched mantle as a result of interactions with crust-derived melts.Mesozoic enriched mantle did not survive for a long period and then was transformed into Cenozoic depleted lithospheric mantle through asthenosphere-lithosphere interactions,as suggested by the Sr–Nd isotope compositions of Late Cretaceous and Cenozoic mantle xenoliths and basalts.The in-situ O isotope analysis of zoned olivine in olivine basalts from the northern margin of the NCC in this study further demonstrates the importance of oceanic crust-derived melts to the metasomatism of the lithosphere mantle in the Early Cretaceous.The discovery of abundant zoned olivine and clinopyroxene indicates that peridotite-melt reaction might be the main mechanism for the transformation and destruction of the NCC.