Nature And Evolution Of Lithospheric Mantle Beneath Northeastern China

The rigid,predominantly peridotitic keel,which forms the subcontinental lithospheric mantle?SCLM?in the cratonic regions,not only protects the overlying crust from the disruption by asthenospheric mantle convection,but also plays an important role in crust-mantle interactions and is a long-lived reservoir where ore metals can be pre-concentrated.In circum-cratonic areas?such as orogenic belts?and trans-lithospheric fault zones,the SCLM keels are usually strongly modified and partially or entirely destroyed,hence constraints on their response to tectonomagmatic processes are crucial for understanding crustal growth and the stabilization,preservation and transformation of continents.The mantle-derived xenoliths carried by volcanism are the most direct samples for the study of the deep mantle lithosphere,and they often retain the original petrographic characteristics and chemical compositional records,thus providing the most reliable information on the composition,architecture and genesis of the SCLM.The northeastern North China Craton?NCC?and the southeastern Xing'an-Mongolia Orogenic Belt?XMOB?are two distinct and adjacent tectonic units in Northeastern China,which is a collage area,thus representing an ideal place to study the difference in deep processes affecting the SCLM and potential genetic relationship between craton and circum-craton.In addition,a prominent trans-lithospheric fault,the Tan Lu Fault Belt?TLFB?,transects this collage area,which gave rise to intensified modification of the SCLM,thereby increasing the complexity of lithospheric evolution.This allows to decipher the metasomatic modification processes of the SCLM adjacent to the trans-lithospheric fault.Here,Northeastern China is selected as the study area,via the investigation of peridotite xenoliths carried by Mesozoic-Cenozoic basalts.Detailed petrographic observations and geochemical analyses,combined with similar data compiled from neighboring regions,are used to probe the nature and internal dynamic processes of the SCLM in different tectonic settings,including?1?A refined model for lithosphere evolution beneath the margin of the NCC;?2?An extraneous origin for the SCLM in the XMOB;?3?The metasomatic modification of the SCLM in the TLFB zone.The main observations and conclusions are as follows:?1?This study unveils the characteristic evolution of the SCLM in a craton margin setting.In the Huinan area,located in the northeastern NCC,the mantle xenoliths carried by Cretaceous basalts are spinel peridotites,which can be divided into two groups.Group 1peridotites show protogranular textures and consist of harzburgites and dunites.They have low Al₂O₃contents in whole-rock and orthopyroxene?Opx;0.53–1.06 wt.%and2.10–3.21 wt.%,respectively?,high olivine modes?79–96%?,whole-rock Mg O?44.8–47.9 wt.%?and Mg#?90.1–90.7?,suggesting that they were derived from moderately refractory SCLM.Clinopyroxenes?Cpxs?in this group of peridotites display LREE-enriched and convex-upward REE patterns.Moreover,spinel-pyroxene intergrowths indicative of garnet destabilization are ubiquitous in Group 1,consistent with variable Al₂O₃over a narrow range of Mg#in some Opx and low HREE in some Cpx.In contrast,Group 2 xenoliths display porphyroclastic to protogranular textures and consist of lherzolites and harzburgites with rare spinel-pyroxene intergrowths.They have overall higher Al₂O₃?1.48–3.23 wt.%and 3.02–4.65 wt.%,respectively?in whole-rock and Opx,lower olivine modes?64–83%?,Mg O?38.6–44.5 wt.%?and whole-rock Mg#values 87.6–90.1,and they may represent fertile SCLM.In addition,Cpxs in Group 2 mainly show LREE-depleted and spoon-shaped REE patterns,with minor LREE-enriched and convex-upward ones.These observations reflect that interaction of the fertile mantle with melts similar to the Cenozoic basalts at high melt-rock ratios eradicated most signatures of their origin in the garnet stability field.In contrast,the refractory peridotites,which reacted with residual melts or fluids at low melt/fluid-rock ratios,retained evidence for the former presence of garnet.However,peridotites of both groups have similar equilibration temperatures?i.e.,923–977°C and881–1110°C,respectively?,which are not correlated with Mg#in olivines,suggesting that they coexist over a range of depths.It is suggested that,combined,these observations are best reconciled if portions of ancient refractory lithosphere,which were partly delaminated during multiple subduction episodes affecting the eastern NCC,were re-accreted together with fertile mantle during asthenospheric upwelling driven by extension.?2?This study the extraneous origin of the SCLM beneath the XMOB in the circum-cratonic areaIn the Jiaohe area,located in the southeastern XMOB,the mantle xenoliths carried by Cretaceous basalts are spinel peridotites,which consist of predominantly lherzolites and subordinately harzburgites.Lherzolites have higher whole-rock Al₂O₃contents?2.27–3.46 wt.%?than harzburgites?0.84–1.02 wt.%?,whereas both exhibit bulk Fe O enrichment?Fe O up to 9.54 wt.%?,indicating that these samples have experienced various degrees of refertilization,which is similar to peridotite xenoliths from the northeastern NCC?Huinan,Changbaishan and Kuandian?and from the northeastern XMOB?Shuangliao,Yitong and Wangqing?,but different from the SCLM beneath the northwestern XMOB?Abaga,Halaha,Nuomin and Keluo?.In addition,the lherzolitic Cpxs have higher HREE levels?2.79–5.11 ppm?,Ti/Eu?3882–6864?and V/Sc?3.8–4.6?,coupled with wider variation of ⁸⁷Sr/⁸⁶Sr?0.7021–0.7040?and lower average oxygen fugacity??O₂=FMQ-1.90;relative to the fayalite–magnetite–quartz buffer?,in contrast with those in the harzburgites(HREE:0.94–2.11 ppm;Ti/Eu:163–2044;V/Sc:2.6–3.5;⁸⁷Sr/⁸⁶Sr:0.7032–0.7036;?O₂=FMQ-1.24).The differences between?O₂of lherzolites and harzburgites are further confirmed by Opx-olivine inter-mineral V partitioning when combined with the two-pyroxene REE thermometer,suggesting that this combination may yield quantitative?O₂estimates.These observations suggest that harzburgites may have originated from a remnant,highly depleted mantle domain that interacted with silica-undersaturated oxidizing melts,whereas the lherzolites were produced by adding various proportions of Cpx±spinel to the protolith from a reduced silicate melt.Furthermore,Jiaohe harzburgites have Ni-Cu-rich sulfide assemblages that occur mainly interstitially,distinct from the Cu-poor sulfides in Huinan peridotites?Group 1?dominantly occurring as inclusions,reflecting different processes affecting the regional SCLM in each area.Nevertheless,the harzburgites from Jiaohe have Re depletion(T_RD)ages up to 1.76 Ga,similar to the oldest T_RDage?1.92 Ga?determined for Huinan and elsewhere across the XMOB.Together with the unique collisional tectonic setting,in this thesis,it is suggested that the lithospheric mantle beneath Jiaohe is genetically related to reworked cratonic mantle sampled at Huinan and was migrated to its current position during Phanerozoic plate convergence.The survival of old,buoyant and variably reworked cratonic lithospheric mantle in younger collisional settings,as in the XMOB,is a global phenomenon.Such settings may be metallogenic hotspots,by juxtaposing suitable pathways?lithosphere-scale discontinuities?for the transport and focusing of metals into ore deposits with long-lived,multiply?subduction-?metasomatised lithospheric mantle sources.?3?This study identifies an important metasomatic feature of the SCLM in the trans-lithospheric fault zone across different tectonic unitsIn the Liaoyuan area,near the TLFB,the mantle xenoliths carried by Cretaceous basalts are feldspar-bearing wehrlites.Most of the studied samples show strong deformation and recrystallization features,and some retain evidence for replacement of Opx by Cpx.Wehrlitic Cpxs have low Ti/Eu?692–4425?,along with ratios of Ca/Al,?La/Yb?_Nand Zr/Hf in Cpxs display increasing trends with decreasing Ti/Eu.These signatures suggest metasomatism by carbonated Si-undersaturated melts,whereby the Mg#[100×Mg/(Mg+Fe^total)molar]variations of mineral pairs?olivine,Cpx±Opx?in these wehrlites can be reproduced by interaction at various melt-rock ratios.Additionally,K-rich?K₂O up to 9.84 wt.%?feldspar is ultimately produced by decomposition of hydrous metasomatic minerals such as amphibole and mica.By contrast,a late metasomatic event is supported by unequilibrated microstructures?sieve textures,corroded Cpx-mantled Opx grains?as well as compositional heterogeneities preserved in the samples?e.g.,large variation of Mg#in olivine and Cpx?.Integrated with data from peridotite xenoliths in Cenozoic basalts,these observations may indicate deep carbonated melt metasomatism,probably occurring during a known Late Cretaceous to Early Tertiary extensional stage,which is suggested to represent a precursor to basaltic magmatism in the north-south direction along the TLFB zone.Thus,wehrlitization beneath the TLFB-and elsewhere-monitors the passage and mobility of carbonated melts linked to lithospheric thinning up to decratonization.In summary,the peridotite xenoliths in the northeastern margin of the NCC reflect the coexistence of moderately refractory and fertile mantle in the same depth range,and further reveal that the SCLM has undergone a series of complex deep processes,including partial delamination,re-accretion and metasomatic modification,all of which were probably related to the closure of the Paleo-Asian Ocean and the slab rollback of the subducting Paleo-Pacific plate.In contrast,the SCLM beneath the XMOB in the circum-cratonic area is largely of extraneous origin,whereby the neighboring cratonic SCLM was migrated into the orogenic belt during collision and thin-skinned tectonics.Finally,the SCLM in the trans-lithospheric fault zone across the craton and surrounding circum-cratonic area represents the most intensely modified example.It is shown that such a trans-lithospheric fault provides a pathway for the penetration of deep melts/fluids,which may lead to the Fe-enrichment of the SCLM and metasomatism by carbonated melts.Thus,the SCLM underlying the craton and circum-craton in Northeastern China,while showing different evolution processes in detail,have close genetic links,whereas the SCLM associated with the trans-lithospheric fault across different tectonic units shares similar metasomatic features.