摘要
发育于长江中下游构造-岩浆-成矿带最东端宁镇地区的晚中生代中酸性-酸性侵入岩以富钠、高Sr和Ba、富集轻稀土元素,强烈亏损重稀土元素为特征,高场强元素Ti、Nb、Ta等表现出显著的负异常,岩石地球化学的总体特征类似于高Sr/Y的埃达克岩。但与长江中下游其他地区的高Sr/Y岩浆岩相比,宁镇地区的这些侵入岩体还表现出低的放射性成因Pb、更低的重稀土元素含量、相对高的MgO、Cr和Ni含量及Mg#值等特征,推测它们的形成过程是由(再)富集的上地幔物质部分熔融形成的岩浆与古老的大陆下地壳物质部分熔融形成的埃达克质岩浆混合,混合岩浆在演化过程中还经历过较强的矿物分离结晶作用以及一定程度的上地壳物质混染。该再富集事件的发生可能与俯冲的古太平洋板块部分熔融所析出的熔体(或流体)交代上覆的地幔楔有关,导致原已富集的陆下岩石圈地幔更进一步富集挥发分以及不相容元素,古老的富集地幔中所累积的放射性成因Sr、Nd同位素被"中和",该作用在长江中下游地区可能仅影响到最东段的宁镇地区,并没有波及到更靠内陆的其他地区。宁镇地区的这些埃达克岩侵位于100~108Ma的早白垩世末期,晚于长江中下游其他地区高Sr/Y岩体长达30Ma左右,其间存在着较长时间的岩浆活动静止期,意味着它们可能形成于不同的动力学背景。后者的形成可能与区内印支期—早燕山期陆内挤压造山后的应力松池,加厚的大陆下地壳及其富集的岩石圈地幔发生拆沉和部分熔融有关,这种发生在长江中下游广大地区的岩石圈减薄事件可能并没有波及到最东段的宁镇地区。而前者可能与早白垩世末期—晚白垩世初期,太平洋板块相对亚洲大陆板块运动方向发生了改变,中国东部大陆边缘进入拉张-裂解的构造环境有关,但其效应在长江中下游地区可能主要影响到了最东段的宁镇地区,其他地区对这次构造变动并没有产生明显的岩浆活动。
The late Mesozoicintermediate-acid intrusive bodies developed in Ningzhen district,the easternmost of the Middle-Lower Yangtze River tectonic-magmatic-metallogenic belt,are characterized by high-Sr and Ba,and enriched in Na,strong incompatible elements such as Rb,Th,U as well as light rare earth elements(LREE),depleted in high-field-strong-elements(HFSE)such as Ti,Nb and Ta,similar to the high Sr/Y adakitic rocks in geochemistry.The high Sr/Y bodies in Ningzhen district exhibit low radiogenic Pb isotopic compositions,lower HREE,higher Mg,Mg#and Cr-Ni contents for a given SiO2 contents compared to those high Sr/Y bodies developed in other area of the Middle-Lower Yangtze River reaches,inferring their parent magma was formed by magma mixing between the melt from a re-enriched mantle wedge and the melt from an ancient lower continental crust.The mixed magma had undergone strong fractional crystallization,and may be some assimilation of upper crust.The re-enriching event may be related with the mantle wedge metasomatized by melts and/or fluids from the paleo-Pacific plate,which result in more enriched in incompatible elements and fluids,but the radiogenic Sr and Nd isotope ratios cumulated in ancient enriched subcontinental lithospheric mantle were "neutralized".The event only affected Ningzhen district,the easternmost of the Middle-Lower Yangtze River reaches,may not influenced other area more inland.The emplacement ages of the adakitic bodies in Ningzhen district are about 100 to 108 Ma,later about 30 Ma than those high Sr/Y intrusive bodies developed in other districts in the Middle-Lower Yangtze River reaches,may imply different dynamic setting between them.The latter may be related with the partial melting of thickened lower crust under the stress relaxation after intracontinental compressional orogeny in about 200~160 Ma.This event of lithospheric thinning was accompanied by compression stress relaxation-extension developed in the Lower Yangtze River reaches may not affected the easternmost(Ningzhen district).But the former may be related with change of the Pacific plate affect the Continental plate in late period of early cretaceous-late cretaceous,in this period,the tectonic setting of eastern margin of China continent entry into extension-dissociation,but only effect the easternmost part(Ningzhen district),no obvious magmatic response for this tectonic event in other area of the Lower Yangtze River reaches.
作者
薛怀民
XUE Huaimin(Institute of Geology,Chinese Academy of GeologicalSciences,Beijing,100037)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2019年第1期147-169,共23页
Acta Geologica Sinica
基金
中国地质调查局项目(编号DD20160123)资助成果
关键词
埃达克岩
晚中生代
宁镇地区
再富集
板块俯冲
长江中下游
Adakitic rock
Late Mesozoic
Ningzhen district
re-enrich
subduction
Middle-Lower Yangtze River reaches
