摘要
中国大陆科学钻探工程主孔(CCSD-MH)2000.0-3000.0米深度范围内出露的岩心以正、副片麻岩为主,夹有薄层榴辉岩和斜长角闪岩等。地球化学研究结果表明,主孔20000.0-3000.0米之间的正片麻岩SiO2含量普遍偏高,为73.26%~78.17%之间,平均值76.40%;Al2O3含量为11.30%-13.66%。TiO2、Fe2O3、FeO、MnO和MgO含量则明显偏低,其中Fe2O3总量为0.39%-1.71%,FeO=0.20%-1.49%,MgO=0.01%-0.06%。CaO含量为0.19-1.41%,Na2O和K2O含量分别为3.38%-5.35%和1.31%-4.87%。正片麻岩的稀土元素和微量元素配分模式可分为三种类型。第一类表现出较强的轻、重稀土元素分馏,具有中等的负Eu异常,Eu/Eu*=0.39-0.64;洋脊玄武岩(MORB)标准化蛛网图表现出强烈富集大离子亲石元素(K、Rb、Ba、Th)的特点,显示明显的正Ba异常,Ba/Ba*=1.09-2.34,高场强元素Ti、Nb和Ta呈明显的负异常。第二类正片麻岩具有明显的负Eu异常,Eu/Eu*=0.39-0.41,稀土元素配分曲线具有明显右倾斜的特点,轻稀土元素明显富集,而重稀土元素明显亏损;洋脊玄武岩(MORB)标准化蛛网图与第一类正片麻岩比较相似,但却具有中等的负Ba异常,Ba/Ba*=0.57-0.67。第三类正片麻岩主要为含磁铁矿二长花岗质片麻岩,稀土元素球粒陨石标准化曲线呈“V”字型特点,具有异常强烈的负Eu异常,Eu/Eu*普遍低于0.11;洋脊玄武岩(MORB)标准化蛛网图显示出强烈富集大离子亲石元素(K、Rb、Th)的特点,具有异常强烈的负Ba异常,Ba/Ba*=0.03-0.21。2000.0-3000.O米深度范围内的正片麻岩具有多成因的特点,部分正片麻岩具有A型花岗岩的地球化学特征,反映它们有可能形成于板内的构造环境;而另一部分的原岩则可能形成于陆缘火山弧的构造环境。主孔2000.0-31000.0米深度范围内副片麻岩SiO2含量明显低于正片麻岩,Al2O3、Fe2O3 FeO、MgO和CaO含量则明显偏高,而Na2O和K2O含量则与正片麻岩大体相当。其中SiO2含量为64.21%-74.12%;Al2O3含量为13.06%-15.38%,Fe2O3 FeO含量为1.61%-4.92%;CaO含量为1.10%~3.27%,Na2O和K2O含量分别为3.68%-5.39%和2.46%-5.85%。副片麻岩稀土元素配分模式和洋脊玄武岩(MORB)标准化蛛网图与正片麻岩也存在明显差异。其中稀土元素配分模式表现出一定程度的轻、重稀土元素分馏,大多数样品具负Eu异常,Eu/Eu*=0.56-0.93之间,但远不及正片麻岩的明显;洋脊玄武岩(MORB)标准化蛛网图则显示出富集大离子亲石元素(K、Rb、Ba、Th)的特点,具有异常明显的正Ba异常,且变化范围较大,Ba/Ba*=1.02-4.83之间,高场强元素如Ti、Nb和Ta呈现负异常的特点。副片麻岩的原岩可能是形成于被动大陆边缘的一套典型的沉积岩或变沉积岩。SHRIMPU-Pb定年结果表明,主孔副片麻岩锆石微区记录了十分复杂的年代学信息。继承性碎屑锆石核部的年龄(206Pb/238U的年龄)为313-659Ma,表明原岩继承性碎屑锆石来源的复杂性,以及部分碎屑锆石在超高压变质过程中发生不完全重结晶,导致年龄变新;在含柯石英锆石微区记录的超高压变质年龄(206Pb/238U的年龄)为220-236Ma,加权平均值为227±5Ma;而锆石晶体边部所记录的退变质年龄(206Pb/238U的年龄)为209-219Ma,加权平均值为214±6Ma,上述含柯石英锆石微区和锆石边部的SHRIMPU-Pb定年结果分别与主孔CCSD-MH中的正片麻岩锆石微区获得的超高压变质年龄(227±2Ma)和角闪岩相退变质年龄(209±3Ma)十分接近,这进一步证明了中国大陆科学钻探工程主孔中的正、副片麻岩的原岩曾一起发生深俯冲,并经历了新三叠纪的超高压变质作用。
The main drill hole CCSD-MH, Chinese Continental Scientific Drilling Project ( CCSD) , with depth of 5000 m, is located in the Donghai area, southwestern Sulu terrane. The 2000. 0-3000. 0 m recovered cores in CCSD-MH are mainly comprised of orthogneiss and paragneiss, with thin layers of eclogite, retrogressive eclogite and amphibolite. The SiO2 and Al2O3 contents of analyzed orthogneiss samples range from 73. 26 % to 78. 17 % , and from 11. 30 % to 13. 66 % , respectively. All the analyses show lower TiO2, Fe2O3, FeO, MnO and MgO contents; Fe2O3=0. 39 %-1. 71 %, FeO = 0.20 %-1.49 %, MgO = 0.01 %- 0.06 % and CaO=0. 19 %-1.41 % . The Na2O and K2O contents range from 3. 38 % to 5. 35% and from 1.31 % to 4. 87% , respectively. According to the characteristics of REE patterns and MORB normalized multi-element variation diagrams, the analyzed samples could be divided into three groups. The REE patterns of the first orthogneiss group are characterized by midium negative Eu anomalies, with Eu/Eu* = 0. 39-0. 64. Light rare earth elements are concentrated evidently and heavy rare earth elements are deficient. In the MORB normalized multi-element variation diagram, multi-element variations show the relative concentrations of K, Rb, Ba and Th, evident depletions of Ta, Nb, P and Ti, and positive Ba anomalies, with Ba/Ba* = 1.09-2. 34. The REE patterns of the second orthogneiss group are characterized by medium-obviously negative Eu anomalies, with Eu/Eu* = 0.39-0.41, and medium-obviously negative Ba anomalies. Ba/Ba* ratio ranges from 0. 57 to 0. 67 , with an average value of 0. 63. The REE patterns of the third orthogneiss group, however, are characterized by obviously negative Eu anomalies, with Eu/Eu* < 0. 11. In the MORB normalized multi-element variation diagram, multi-element variations show the relative concentrations of K, Rb, and Th, and obviously negative Ba anomalies, with Ba/Ba* = 0. 03-0. 21. These major and trace element compositions indicate that some orthogneiss samples are characterized by A-type granite protolith, and formed in the intraplate tectonic surroundings. Others are characterized by volcanic arc granites, and formed in the volcanic arc tectonic surroundings. Compared with the compositions of the orthogneisses, the analyzed paragniess samples show lower SiO2 content, higher A12O3, Fe2O3 FeO, MgO and CaO contents, and analogous Na2O and K2O contents. The SiO2 and Al2O3 contents of analyzed paragneiss samples range from 64. 21 % to 74. 12 % , and from 13.06 % to 15. 38 % , respectively. Fe2O3 FeO=1. 61 %-4. 92 % and CaO = 1.10% -3.27 %. The Na2O and K2O contents range from 3.68 % to 5. 39% and from 2.46% to 5. 85% , respectively. The REE patterns of the analyzed paragneiss samples are characterized by negative Eu anomalies, with Eu/Eu* = 0.56-0.93. Light rare earth elements are concentrated and heavy rare earth elements are deficient. In the MORB normalized multi-element variation diagram, multi-element variations show the relative concentrations of K, Rb, Ba and Th, relative depletions of Ta, Nb, P and Ti, and obviously positive Ba anomalies, with Ba/Ba*=1. 02-4. 83. These major and trace element compositions indicate that the protoliths of the paragneisses came from the sedimentary rocks (or metamorphic sedimentary rocks) on the passive continental margins. Combined study of Laser Raman, cathodoluminescence (CL) image and SHRIMP U-Pb dating reveals that zircons separated from the paragneiss in the main drill hole CCSD-PH recorded complicated geochronological traces. All the analyzed zircons retain inherited detrital cores with inherited ages (206Pb/238U age) of 313 -659 Ma, indicating that the detrital zircons of the protolith have a variety of sources, and partial loss of Pb from some zircons in the protolith. Coesite-bearing domains of zircons recorded 220-236 Ma (206Pb/238U age) for the UHP metamorphic condition, with an weighted mean age of 227±5 Ma, and quartz-and albite-bearing rims of zircons recorded 209-219 Ma (206Pb/238U age) for the late amphibolite facies thermal event, with an weighted mean age of 214±6 Ma. These UHP and retrograde ages are similar to that (227±2 Ma) of coesite-bearing zircon domains and that (209±3 Ma) of quartz- and albite-bearing zircon rims in the orthogneiss from CCSD-MH, respectively. These data indicate that UHP metamorphism in both orthogneiss and paragneiss from Sulu terrane occurred in the Late Triassic time, rather than Middle-Late Proterozoic or Early Paleozoic time.