塔里木大火成岩省方解霞黄煌岩的成因及其对大陆板内深部碳循环的启示

Petrogenesis of Aillikites in the Tarim Large Igneous Province,Northwest China:Implications for the Deep Carbon Cycle in the Continental Interior

作为典型深部地幔来源的岩石,碱性超镁铁质煌斑岩的源区特征和岩浆演化为揭示再循环壳源物质对地幔的改造及深部碳-水-硫等循环提供重要信息。本次研究通过对塔里木大火成岩省瓦吉里塔格方解霞黄煌岩岩石学、矿物学和全岩地球化学研究,揭示方解霞黄煌岩的岩石成因及其相关的大陆板内深部碳循环过程。瓦吉里塔格方解霞黄煌岩显示出碱性(Na_(2)O+K_(2)O=1.12%~2.18%)和超镁铁质特征(SiO_(2)=28.67%~31.20%;Mg^(#)=69~73),同时富集Fe_(2)O_(3)^(T)(13.57%~15.32%)、TiO_(2)(1.89%~2.11%)和地幔相容元素(Ni、Cr)。在微量元素上,样品富集轻稀土元素、大离子亲石元素但亏损高场强元素,具有强烈的K、P、Ti、Nb-Ta、Zr-Hf负异常,类似火山弧玄武岩的特征。(^(87)Sr/^(86)Sr)i变化于0.703765~0.703914之间,εNd(t)值变化于4.48~4.75之间,(^(206)Pb/^(204)Pb)i值为18.4563~18.7527,均与FOZO地幔端元特征相似。全岩微量元素和同位素组成表明,瓦吉里塔格方解霞黄煌岩可能是交代富集的岩石圈地幔和地幔柱共同作用的结果。瓦吉里塔格方解霞黄煌岩中斑晶橄榄石具有高的100×Mn/Fe(1.02~3.29)和10000×Zn/Fe值(10.6~23.0),但其Mn/Zn值为11.1~16.4,暗示源区中含有辉石岩组分,而不是均一的橄榄岩地幔。这些斑晶橄榄石中Sc与Ni呈正相关关系,但Co与Ni之间没有明显的相关性,进一步表明源区中的辉石岩组分可能是俯冲洋壳加入的结果。这些岩石富碳酸盐,结合橄榄石中Ca和Ti同时富集趋势,表明其地幔源区存在碳酸盐交代作用。据此,推测受交代的含PIC型脉体的岩石圈地幔在碳酸盐化地幔柱作用下发生低程度部分熔融产生方解霞黄煌岩岩浆。本次研究为板内岩浆作用相关的深部碳循环过程研究提供了借鉴和指示意义。

As typical deep-mantle-derived melts,alkaline ultramafic lamprophyre provide important clues to mantle metasomatism by recycled crustal materials and the deep-earth carbon-water-sulfur cycle.To provide new insights into the petrogenesis of aillikites as well as their mantle source,more broadly,the genetic link between intraplate magmatism and the deep carbon cycle in continental interiors,we examined the Wajilitag aillikites in the Tarim Large Igneous Province.New mineral and bulk-rock geochemistry along with Sr-Nd-Pb isotope results were obtained.The Wajilitag aillikites are nearly primitive(Mg^(#)=69-73),silica-undersaturated(SiO_(2)=28.67%-31.20%),alkaline in nature(Na_(2)O+K_(2)O=1.12%-2.18%),and enriched in FeO^(T)(13.57%-15.32%),TiO_(2)(1.89%-2.11%),and compatible elements(i.e.,Ni and Cr).They are also characterized by enrichment of incompatible elements,including light rare earth element(LREE),and depletion of high field strength elements(HFSE),and possess negative K,P,Ti,Nb-Ta,and Zr-Hf anomalies,similar to those of volcanic arc basalts.Bulk-rock(^(87)Sr/^(86)Sr)i(0.703765-0.703914),ε_(Nd)(t)(4.48–4.75),and(^(206)Pb/^(204)Pb)i(18.4563-18.7527)of the Wajilitag aillikites indicate derivations from slightly depleted mantle sources similar to those of FOZO-type mantle.The trace element and isotopic compositions therefore indicate that the aillikites are integrated results of metasomatic enriched lithospheric mantle and mantle plume.In addition,high 100×Mn/Fe(1.02-3.29)and 10000×Zn/Fe ratios(10.6-23.0)in phenocrystic olivines from the Wajilitag aillikites,coupled with their Mn/Zn ratios of 11.1-16.4,are indicative of a source containing pyroxenite rather than a homogeneous peridotite source.These phenocrystic olivines also show a positive correlation between Sc and Ni as well as no obvious correlation between Co and Ni,further indicating that the addition of recycled oceanic crust to the mantle peridotite produced pyroxenite components in the source of the Wajilitag aillikites.The coupled enrichment of Ca and Ti in the phenocrystic olivines and the occurrence of carbonate minerals in the aillikites point to a carbonate metasomatized mantle source.Hence,it can be inferred that low-degree partial melting of the metasomatized lithospheric mantle containing PIC-type veins under the impact of a carbonated mantle plume produced the aillikitic magmas.Our results provide solid evidence of the deep carbon cycle in the continental interior.