| The main hole of the Chinese Continental Scientific Drilling is located in Maobei, Donghaicounty, Jiangsu Province, at the eastern part of Dabie-Sulu ultra-high pressure (UHP)metamorphic belt which was formed by subduction and collision between the Sino-KoreanYangtze plates. The Dabie-Sulu UHP metamorphic belt is possibly the largest UHP terrane in theworld. The petrographical, mineralogical and geochemical characteristics of rocks in the mainhole (2000-3000m) were synthetically studied. The major conclusions are as follows:1. The main lithological types of the CCSD main hoe (2000～3000m) are orthogneiss,paragneiss, with minor interlayered eclogite, amphibolite and schist. Orthogneiss and paragneissconsist of plagioclase, K-feldspar, quartz, with minor and variable garnet, amphibole, biotite,epidote, titanite, magnetite and zircon. In most cases, orthogneisses have relatively lowferromagnesian mineral content (about 5％); and paragneisses have higher ferromagnesianminerals contents (about 20％). The eclogites contain garnet, omphacite, rutile, apatite, quartz(coesite), epidote, phengite, kyanite, titanite and zircon. The amphibolites include plagioclase,amphibole, biotite, garnet, epidote, and minor titanite, ilmenite.2. The mineral assemblages of eclogite include garnet+omphacite+phengite+quartz+rutile, garnet+omphacite+phengite+quartz+apatite, garnet+omphacite+phengite+quartz,and garnet+omphacite+quartz+futile. Retrograde metamorphism of the eclogites can bedivided into four stages by the replaced degree of garnet and omphacite: slightly retrograde stage,partly retrograde stage, retrograde stages and strongly retrograde stage. According toferromagnesian mineral contents, the paragneisses can be subdivided into six types as follows:amphibole-biotite-epidote-two feldspar gneiss; epidote-biotite-plagioclase gneiss; epidote-twomica-two feldspar gneiss; two mica- plagioclas gneiss. The orthogneisses can be subdivided intosix types as follows: garnet-muscovite-two feldspar gneiss; epidote-biotite-two feldspar gneiss;magnetite-bearing two feldspar gneiss; two-mica two-feldspar gneiss; epidote-amphiboletwo-feldspar gneiss; two-feldspar gneiss without ferromagnesian mineral. Amphibolites can besubdivided into garnet-biotite-epidote amphibolite, and biotite-epidote amphibolite. Schists can besubdivided into epidote-garnet two-mica schist, apatite bearing garnet-epidote-biotite schist, and garnet-two mica-quartz schist.3. Garnets of eclogites have rather uniform compositions: Pyr=19.2～37.7％, Aml+Spess=34.8～48.7％, And+Grss=26.1～40.9％. Garnets of orthogneisses have low pyrope compositions,Pyr=0.1～3.7％, Aml+Spess=43.3～85.7％, And+Grss=13.978～55.0％. According tomicroprobe analysis, some garnet porphyroblasts from gneisses show a distinctly growthcompositional zoning. All clinopyroxene in the investigated eclogites are omphacite, with 47～58％jadeite, 0～18％aegirine. The compositions of amphiboles have relationship with the replacedminerals. Amphiboles replaced garnets are mainly tschermakite and ferro-tschermakite,amphiboles; those replaced omphacites are mainly barroisite and ferro-barroitite. All white micasfrom eclogites are phengite. Some phengites show diffusional compositional zoning, indicatingthat the phengites with high Si contents have been replaced by the lower Si phengites during theretrograde metamorphism of the UHP eclogites.4. The orthogneisses from the CCSD main hole show rather uniform compositions, withrelatively high SiO2, and low MgO, CaO, Al2O3, FeO and MgO contents, Na2O and K2O contentsranging from 3.83 to 5.41％, and from 1.97％to 4.34％, respectively. Orthogneisses show stronglyfractionated REE patterns, and distinctly negative Eu anomalies (Eu/Eu*=0.11～0.58 with anaverage value of 0.36). In addition, orthogneisses are rich in large-ion lithophile elements, andslightly negative anomalies of Nb and Sr, and are characterized by evidently negative Baanomalies. Comparing with orthogneisses, paragneisses have lower SiO2, higher Al2O3, FeO,MgO and CaO contents. Paragneisses have fractionated REE patterns, and with negative Euanomalies, and riching in large-ion lithophile elements and negative Nb anomalies, positive Ba, Pband Gd anomalies. Schists have similar composition to the paragneisses, and show negative Euanomalies, positive Ba, Gd anomalies and evidently negative Hf and Nb anomalies. The eclogitesshow fractionated REE patterns with an enrichment of LRRE and depletion of HREE, evidentlynegative Hf anomalies, and have negative or positive Eu anomalies (Eu/Eu*=0.89～1.21).The Rb,Ba, La, Pb, Gd and Lu content of Schist are concentrated evidently and the Ce, Th, Nb, Sr and Hfcontent are depletion.5. The protoliths of the eclogites are basalt; the protoliths of orthogneisses belong to granite,Most rocks from the depth interval of 2000～3000 m of the CCSD main hole are subalkaline.6. Some eclogitic garnets contain quartz pseudomorph inclusions after coesite. The zircons ofthe eclogite and schist, orthogneiss and paragneiss have coesite, and other eclogite-facies mineralinclusions. These strongly indicate that all rocks from the main hole have been subjected to acoeval UHP metamorphism.7. The peak metamorphic P-T conditions of eclogites are estimated to be 780～834.6℃and＞2.8GPa; amphibolite-facies retrogression are 580～670℃and 0.9～1.1GPa. |
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