中国大陆科学钻探工程主孔及周边地区花岗质片麻岩的地球化学性质和超高压变质作用标志的识别

Geochemical characteristics and UHP metamorphism of granitic gneisses in the main drilling hole of Chinese Continental Scientific Drilling Project and its adjacent area

花岗质片麻岩是中国大陆科学钻探工程主孔0-2000m深度范围内出露的主要岩石类型之一,集中分布于1113.14-1596.22m之间,而在0-1113.14m和1596.22-2000m深度范围内主要以“夹层”形式赋存于副片麻岩和(退变)榴辉岩中,夹层的厚度为0.54-5.82m左右。花岗质片麻岩累计厚度430.98m,占2000m岩心的21.55％左右。地球化学研究结果表明,主孔中花岗质片麻岩SiO2含量普遍偏高,为71.55％-77.18％之间,Al2O3含量为11.54％-13.57％。TiO2、Fe2O3、FeO、MnO和MgO含量则明显偏低,其中Fe2O3+FeO总量为1.05％-2.94％,MgO=0.06％-0.59％。CaO含量为0.30％-2.65％。Na2O和K2O含量变化相对较大,分别为0.29％-4.06％和2.90％-6.67％之间,且大多数样品K2O含量高于Na2O含量。稀土元素配分模式具有右倾式的特点,轻稀土相对富集,而重稀土相对亏损,具有强烈的负Eu异常,Eu/Eu=0.21-0.26之间。在标准化蛛网图上,则显示Ta、Nb、P和Ti的明显亏损以及中等-强烈的负Ba异常,Ba/Ba=0.25-O.64之间,平均值为0.45左右。上述主元素、稀土元素和微量元素特征与中国大陆科学钻探预先导孔CCSD-PP1岩心以及东海及其邻区地表露头的花岗质片麻岩的化学成分十分相似,具有A型花岗岩的地球化学特征。采用激光拉曼技术,配备阴极发光测试。

The main drilling hole CCSD-MH, Chinese Continental Scientific Drilling Project (CCSD), with depth of 5000 m, is located in the Donghai area in southwestern Sulu terrane. The 0 similar to 2000 m recovered cores in CCSD-MH are mainly comprised of fresh eclogite, retrogressive eclogite, aniphibolite, paragneiss, granitic gneiss (orthogneiss) and ultramafic rock. The granitic gneiss constitute about 21.55 % of the 2000 to recovered lithology and as a granitic body mainly occur at the depth interval of 1113. 14 similar to 1596. 22 m, and as thin layers with thickness ranging from 0.54 m to 5.82 m are intercalated within paragneiss and fresh and retrogressive eclogite at the depth intervals of 0 similar to 1113. 14 m and 1596. 22 similar to 2000 m. The SiO(2) and Al(2)O(3) contents of analyzed granitic gneiss samples range from 71.55 % to 77.18 %, and from 11.54 % to 13.57 % respectively. All the analyses show lower TiO(2), Fe(2)O(3), FeO, MnO and MgO contents; Fe(2)O(3) + FeO = 1.05 % similar to 2.94 %, MgO = 0.06 % similar to 0.59 % and CaO = 0.30 % similar to 2.65 %. However, the Na(2)O and K(2)O contents show various extents ranging from 0.29 % to 4.06 % and from 2.90 % to 6.67 %, respectively, with an obvious characteristic of K(2)O > Na(2)O. The REE patterns of granitic gneiss samples are characterized by evidently negative Eu anomalies, with Eu/Eu(*) = 0.21 similar to 0.26. 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 depletions of Ta, Nb, P and Ti, Laid medium-obviously negative Ba anomalies. Ba/Ba(*) = 0.25 similar to 0.64, with an average value of 0.45. These major and trace element compositions are characterized by A-type granite, and identical to those of the typical Country rock granitic gneisses at outcrops in Donghai and adjacent areas, aid in the prepilot drilling hole CCSD-PP1, Chinese Continental Scientific Drilling Project. Coesite and coesite-bearing ultrahigh-pressure (UHP) minerals are identified rising Ranam spectroscopy as inclusions in zircons separated from granitic gneiss, amphibolite and paragneiss in the main drilling hole CCSD-MH, the prepilot drilling hole CCSD-PP1 and -PP2, and from 3800 km(2) outcrops around Donghai area. These evidences suggest that voluminous continental materials, consisting mainly of protoliths of eclogite and its country rocks were commonly subjected to deep subduetion and ultrahigh-pressure (UHP) metamorphism. Combined study of laser Ramam, cathodoluminescence (CL) image and SHRIMP U-Pb dating reveals that zircons separated from the granitic gneiss in the main drilling hole CCSD-MH recorded complicated geochronological traces. All the analyzed zircons retain inherited magmatic crystallization cores with inherited ages of 663 similar to 750 Ma, implying partial loss of Pb from zircon in the protolith, therefore the protolith age should be older than 750 Ma. Coesite-bearing domains of zircons recorded 226 similar to 230 Ma for the UHP metamorphic condition, with an average age of 228 +/- 5 Ma, and quartz- and albite-bearing rims of zircons recorded 203 similar to 214 Ma for the late amphibolite facies thermal event, with an average age of 209 +/- 5 Ma. These data indicate that UHP metamorphism in the Sulu terrane occurred in the Indosinian time. The exhumation rate of the Sulu terrane is approximately 5.3 km/Ma calculated based on the SHRIMP data and relative P-T conditions. The large exhumation rate of the massif is in the same range as other UHP metamorphic terranes (e.g. Kokchetav massif, northern Kazakhstan) in the world, and suggests that slab decoupling may be a driving force for the exhumation of UHP metamorphic rocks from mantle depth.