【作者】 王坤明；

【导师】 王宗起；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2014， 博士

【摘要】 陕西紫阳—岚皋地区广泛出露早古生代镁铁质岩脉,部分镁铁质岩脉产有岩浆型铁矿床。本文选取紫阳—岚皋地区五条路线剖面和两个与镁铁质岩相关铁矿床进行了研究,通过详细的野外地质考察、岩相鉴定、矿物电子探针分析、锆石U-Pb同位素年代测试、岩石地球化学测试以及Sr-Nd同位素分析,探讨研究区内早古生代镁铁质岩体的岩石成因以及成岩大地构造环境,进而探讨与镁铁质岩相关矿床的成矿作用研究。取得的认识和成果如下。紫阳—岚皋地区镁铁质岩岩石组合为—套深成—中深成—浅成岩石序列,包括辉绿岩、辉绿玢岩、辉长辉绿岩、辉长岩以及辉石闪长岩等岩石类型。主要矿物成分为辉石、长石、角闪石,副矿物有锆石、榍石、磷灰石和磁铁矿等,其中辉石种属主要为单斜辉石,长石种属为倍长石,部分剖面内部长石由于钠长石化表现为钠长石,岩体中辉石和角闪石压力计结果表明紫阳—岚皋地区镁铁质岩具有明显的深源特征。紫阳—岚皋地区镁铁质岩形成时代为早志留世,岩石地化显示高碱、高钛特征,在硅碱图中落于碱性岩区域指示镁铁质母岩浆为碱性岩系列。Mg#值变化于20～60之间,大多数介于33～49之间,指示母岩浆为高度演化的岩浆。区域镁铁质岩体的微量元素和稀土元素模式曲线类似典型OIB的模式曲线,轻重稀土分异显著,LREE/HREE介于5.45～8.94之间,(Gd/Lu)N介于1.84～5.43之间,无明显Eu异常,其中基性岩样品普遍富集Rb和Ba等大离子亲石元素,弱富集Ti,而辉石闪长岩样品明显亏损Sr、Ti和P等高场强元素。已发现柞木沟、朱溪河等多个与镁铁质岩相关的铁矿床(点),依据野外地质特征、矿物组成、铁钛氧化物产出状态、铁钛氧化物矿物化学特征指示其属于与镁铁质岩有关的岩浆型铁矿床。同时结合矿体产出位置、矿石结构构造、不同形成阶段造矿矿物及造岩矿物的特点、铁钛氧化物形成温度低于辉石等铁镁质矿物结晶温度,暗示成矿过程中存在着—期富铁质流体的注入,进而导致成矿金属大规模堆积,分布于辉石、角闪石和长石颗粒之间,后期无矿流体引起热液蚀变作用,随着黄铁矿、黄铜矿等低温硫化物结晶,最终完成该区的成矿作用。紫阳—岚皋地区镁铁质岩没有发生地壳混染,不同剖面岩体部分熔融程度较低(5%～10%),落于石榴子石和尖晶石过渡相,深度大约为75～80km,岩体的部分熔融深度较大,具有明显的深源特征。辉石闪长岩及已发表的镁铁质岩的初始878Sr/86Sr、143Nd/144Nd值和εNd(t)特征、不同剖面内部镁铁质岩的类OIB的岩石地球化学特征,与各剖面岩体所反映的地幔富集特征高度一致,进一步说明北大巴山地区镁铁质岩的地幔源区为HIMU、EMⅡ与EMI富集地幔混合源区。地幔动力学研究表明,岩体的富集地幔源区特征可能与俯冲作用存在着一定的成因联系。岩石组合、矿物学特征以及岩石地球化学特征共同表明紫阳—岚皋地区镁铁质岩体形成于与俯冲相关的弧后盆地环境,属弧后拉张作用的产物。随着早志留世一期向南俯冲作用导致弧后拉张作用,下部软流圈上涌,上涌的软流圈与上覆受地幔交代的富集岩石圈地幔混合,由于岩石圈减薄而发生减压熔融,最终在北大巴山地区形成大规模的镁铁质岩脉/岩床。更多还原

【Abstract】 Many Early paleozoic mafic rocks and some magma mineral deposit related to mafic rocks located in Ziyang-Langao area, Shaanxi. Five section and two magma mineral deposit related to mafic rocks in Ziyang-Langao area were selected to be systematic studied in this thesis. Basic on the field geologic investigation, petrographic examination, electron microprobe analysis, zircon U-Pb chronology, geochemical analysis and Sr-Nd isotopic analysis, the petrogenesis and petrogenic tectonic setting for these Early paleozoic mafic rocks were studied in this paper, and discussed the mineralization for mineral deposit related to mafic rocks.The main rock association of mafic rock in Ziyang-Langao area are deep-semi-deep-shallow sequence, which include diabase, diabase-porphyrite, gabbro-diabase, gabbro and pyroxene diorite. The minerals mostly contain pyroxene, plagioclase, hornblende and include accessory minerals like zircon, sphene, apatite, magnetite. The pyroxene is clinopyroxene, plagioclase include bytownite, and the albitite is the result of albitization for plagioclase. The barometer of pyroxene and hornblend indicate the mafic rocks in Ziyang-Langao area are deep-derived characteristics.The chronology of mafic rock in Ziyang-Langao area is Sillurian, and the geochemistry of mafic rock shows high alkaline and high-Ti characteristic and the parental magma of mafic rocks, which is plotted in the alkaline area for the Si-alkaline diagram, is an alkaline series. Mg#ranging from20to60(most between33-49), indicate a highly evolved magma. Regional mafic rock have primitive mantle-normalized multi-element and Chondrite-normalized REE patterns similar to that of OIB, LREE/HREE from5.45to8.94and (Gd/Lu)N from1.84to5.43, besides obvious Eu abnormality, the basic rocks enriched in large ion lithophile elements (such as Rb, Ba), slightly enriched in Ti, and pyroxene diorite depleted in high field strength elements(such as Sr, Ti and P).Magmatic origin of magnetite, mineral chemical composition and O isotope for the magnetite are all reveal that the regional iron deposit are typical magmatic deposits for Zhamugou and Zhuxihe iron mineral deposit related to mafic rocks. Location of ore body, texture and structure of ore, mineral for different stage and the temperature of Fe-Ti oxide which is lower than that of mafic silicate mineral all indicate that the large amount of Fe-Ti oxides crystallized after the deuteric injection of Fe-rich fluid, and eventually completed the mineralization of regional mineral deposit.Mafic rocks in Ziyang-Langao area are without crustal contamination and have partial melt in low degree (5%-10%), plotted in garnet and spinel transition facies with75-80km depth, indicating the depth of partial melt is high and have deep-derived characteristics. The initial87Sr/S6Sr,143Nd/144Nd and εNd(t) of pyroxene diorite and published mafic rocks in Daba mountains and OIB-like geochemistry of mafic rocks are consistent with enrichment characteristic of mafic rocks,which further indicate that the mantle source for mafic rocks in Daba mountains is a mixture source of HIMU, EMⅡ and EMI. Mantle geodynamic research indicate that the enrichment of mantle maybe related to the subduction.Rock association, mineral characteristic and geochemistry characteristic indicate that the mafic rocks formed in subduction related to the back-arc basin and the production of extention for back-arc basin. The asthenospheric upwelling mixed with the above enriched lithospheric mantle which is modified by the mantle mesomatism. The lithospheric thinning cause the decompressional melting and form large mafic dyke/bed in Daba Mountains.更多还原