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

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 - 2000 m recovered cores in CCSD-MH are mainly comprised of fresh eclogite, retrogressive eclogite, amphibolite, paragneiss, granitic gneiss ( orthogneiss) and ultramafic rock. The granitic gneiss constitute about 21. 55 % of the 2000 m recovered lithology and as a granitic body mainly occur at the depth interval of 1113. 14 -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 0-1113. 14 m and 1596. 22 -2000 m. The SiO2 and A12O3 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 TiO2, Fe2O3, FeO, MnO and MgO contents; Fe2O3 + FeO = 1. 05 % -2. 94 % , MgO = 0.06 % -0.59 % and CaO = 0. 30 % -2.65 %. However, the Na2 O and K2 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 K2 O Na2O. The REE patterns of granitic gneiss samples are characterized by evidently negative Eu anomalies, with Eu/Eu = 0. 21 - 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, and medium-obviously negative Ba anomalies. Ba/Ba = 0. 25 - 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, and in the prepilot drilling hole CCSD-PP1, Chinese Continental Scientific Drilling Project. Coesite and coesite-bearing ultrahigh-pressure (UHP) minerals are identified using Raman 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 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 subduction and ultrahigh-pressure (UHP) metamorphism. Combined study of Laser Raman, 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 -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 -230 Ma for the UHP metamorphic condition, with an average age of 228 ? Ma, and quartz- and albite-bearing rims of zircons recorded 203 -214 Ma for the late amphibolite facies thermal event, with an average age of 209 ? 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.