Chinese Continental Scientific Drilling Main Hole Research Articles

Joint Inversion of Geochemical Data and Geophysical Logs for Lithology Identification in CCSD Main Hole

The Chinese Continental Scientific Drilling (CCSD) main hole is located in the Sulu ultrahigh-pressure metamorphic (UHPM) belt, providing significant opportunities for studying the metamorphic strata structure, kinetics process and tectonic evolution. Lithology identification is the primary and crucial stage for above geoscientific researches. To release the burden of log analyst and improve the efficiency of lithology interpretation, many algorithms have been developed to automate the process of lithology prediction. While traditional statistical techniques, such as discriminant analysis and K-nearest neighbors classifier, are incompetent in extracting nonlinear features of metamorphic rocks from complex geophysical log data; artificial intelligence algorithms are capable of solving nonlinear problems, but most of the algorithms suffer from tuning parameters to be global optimum to establish model rather than local optimum, and also encounter challenges in making the balance between training accuracy and generalization ability. Optimization methods have been applied extensively in the inversion of reservoir parameters of sedimentary formations using well logs. However, it is difficult to obtain accurate solution from the logging response equations of optimization method because of the strong overlapping of nonstationary log signals when applied in metamorphic formations. As oxide contents of each kinds of metamorphic rocks are relatively less overlapping, this study explores an approach, set in a metamorphic formation model and using the Broyden Fletcher Goldfarb Shanno (BFGS) optimization algorithm to identify lithology from oxide data. We first incorporate 11 geophysical logs and lab-collected geochemical data of 47 core samples to construct oxide profile of CCSD main hole by using backwards stepwise multiple regression method, which eliminates irrelevant input logs step by step for higher statistical significance and accuracy. Then we establish oxide response equations in accordance with the metamorphic formation model and employ BFGS algorithm to minimize the objective function. Finally, we identify lithology according to the composition content which accounts for the largest proportion. The results show that lithology identified by the method of this paper is consistent with core description. Moreover, this method demonstrates the benefits of using oxide content as an adhesive to connect logging data with lithology, can make the metamorphic formation model more understandable and accurate, and avoid selecting complex formation model and building nonlinear logging response equations.

Performance of the synergetic wavelet transform and modified K-means clustering in lithology classification using nuclear log

Accurate lithology identification is fundamentally crucial to reservoir evaluation from geophysical well logs. However, the traditional way of lithological identification is carried out in laboratory, which is not only expensive, but also time consuming in its interpretation. In this study, the synergetic wavelet transform and modified K-means clustering techniques are performed to classify metamorphic rocks from Chinese Continental Scientific Drilling Main Hole (CCSD-MH). At the beginning, different wavelet functions in different well logs are presented to detect lithologic interfaces. Meanwhile, the Haar wavelet and GR are determined to be the optimum wavelet function and well log, and the range of the optimum scales is about 8–15m in the reference well. After that, a fast and practical K-means clustering algorithm is employed to make a classification of stratigraphy into 5 groups, which are demarcated from the performance of wavelet transform. The results achieved are in accordance with the stratigraphic column and have a higher accuracy compared to the previous studies, indicating that the combination of the wavelet transform and modified K-means clustering can improve the accurate rate for the classification of metamorphic rocks in CCSD-MH.

The classification in metamorphic rocks using modified fuzzy cluster analysis from geophysical log data: evidence from Chinese Continental Scientific Drilling Main Hole

Lithology is one of the most important data in evaluating reservoir, and is mainly carried out by cores recovery in laboratory which is very expensive, and its interpretation is time consuming. Accurate identification of lithology is fundamentally crucial to evaluate reservoir from geophysical log data. Pattern recognition and statistical analysis have been proved to be the most powerful methods for constructing optimal model in lithology recognition. To address this issue, a fast and practical K-means clustering algorithm is proposed in order to better deal with lithology recognition from geophysical log data. Based on the traditional K-means clustering algorithm, Euclidean distance is replaced by Mahalanobis distance; the initial cluster centers are acquired from the average of characteristic values but not selected randomly, in addition, adding weight value in each characteristic value of the objective function, and thus a lithology recognition model named modified K-means clustering is established. The method is applied to identify the Chinese Continental Scientific Drilling Main Hole (CCSD-MH) metamorphic rocks. Compared with the traditional K-means clustering, the accurate rate of the modified K-means clustering in lithologic identification has improved for the same 45 samples, raised 11.11 %. According to the modified K-means cluster algorithm, nine kinds of lithology cluster centers are acquired from 45 samples. The classes of the samples can be determined by analyzing the hamming approach degree curves, which is calculated by the undetermined samples and 9 cluster centers. The predicted results and the core recovery are exactly the same by comparison. The hamming approach degree can identify the whole well of CCSD-MH lithology effectively and accurately. This model may be made applications to other areas.

Capability of self-organizing map neural network in geophysical log data classification: Case study from the CCSD-MH

Well log interpretation is one of the prime sources of information for deep lithology in drilling research. Because of the complex geological features of the crystalline metamorphic rocks, more complex nonlinear functional behaviors exist for well log interpretation purposes. Hence, establishing a prediction technology that can accurately interpret/classify well log data in terms of lithology is of major significance. This study, for the first time, explores the application of self-organizing map neural network (SOM) in the classification of metamorphic rocks from Chinese Continental Scientific Drilling Main Hole (CCSD-MH) log data. For this purpose, a total of 33,326 data points derived from resistivity, P-wave velocity, bulk density, photoelectric absorption capture cross section, gamma ray, potassium content and neutron logs were used as an input pattern to a SOM to classify lithology in five categories: orthogneiss, paragneiss, eclogite, amphibolite and ultramafic rocks. Comparison of SOM results to those of feed-forward neural network (FFNN) was also carried out. The cross-validation method was used to investigate the robustness of the two neural networks in terms of classification accuracy in the context of lithology clustering tasks by sampling rotation. Statistical tests such as student paired samples t-test was carried out to guide in classification decision of the CCSD-MH data. The results of this study have proven that SOM appears to be comparable to FFNN in classifying lithology using geophysical log data from crystalline rocks. This proposed SOM approach can serve as practical alternative technology to be used in drilling research.

Prediction of porosity in crystalline rocks using artificial neural networks: An example from the Chinese Continental Scientific Drilling Main hole

Porosity plays an important part of understanding permeability and fluid flow within the continental, crystalline rocks. Geophysical well logs are presently the most consistent means of providing continuous information for porosity estimation. However, it is difficult to interpret geophysical well logs data in crystalline rocks due to their complex geological features and the difficulty in understanding and using the complex and intensive information content in these data. Motived by the successful prediction abilities of artificial neural networks (ANN) to solve different problems in geophysics, this study explore the applicability of using ANNs to predict porosity in continental, crystalline rocks. This ANN technique is calibrated on Chinese Continental Scientific Drilling Main Hole (CCSD-MH) data, which provides core porosity data combined with four geophysical well logs (density, neutron porosity, sonic and resistivity). The data from CCSD-MH is utilized to train feed-forward backpropagation (FFBP) neural network and radial basis function (RBF) neural network to derive a relationship between geophysical well logs and porosity, and hence predict porosity accurately. The findings demonstrate that ANNs provide better performances with sets of three geophysical well logs (density, sonic and resistivity) than regression technique. Comparison of FFBP to RBF showed that RBF reveals better stability and more accurate performances than FFBP. Based on the success achieved in this study, this intelligence artificial technique can be a very advantageous tool in facilitating the task of geophysicists in the framework of research drillings in continental crust.

Magnetic petrology of high Fe‐Ti eclogites from the CCSD main hole: Implications for subduction‐zone magnetism

The high Fe‐Ti eclogites with exsolved lamellar in 530–600 m depths from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu Ultra‐High Pressure (UHP) metamorphic belt, eastern China, record an anomalously high susceptibility (κ), natural remanent magnetization (NRM) and Köenigsberger ratio Q (NRM/Ji, Ji is induced magnetization). This provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks. In this paper, we have measured systematically magnetic properties and mineral assemblage and structure for three special samples (No. 83, No.86 and No.89). Results show that these Fe‐Ti‐rich eclogites are the result of the fine‐grained (titano)‐magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo‐ilmenite. We found that the dominant Fe‐bearing oxide minerals in samples studied are ilmenite, hematite (Hem + Ilm up to 25%), little (titano)magnetite and pyrite. The ferromagnetic susceptibility is mainly controlled by fine grained (titano)magnetite and NRM is closely related to the exsolved lamellar structure. We hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno‐hematite during cooling and decompression processes. These Fe‐Ti‐rich eclogites might be one of the sources of high‐magnetic anomalies observed in the Sulu subduction zone, eastern China.

Resistivity logs of the Chinese Continental Scientific Drilling main hole: Implication for the crustal electrical structure of Dabie-Sulu Terrane, Central-Eastern China

To understand the electrical structure of the Sulu (苏鲁) terrane, we analyzed the resistivity logs of the Chinese Continental Scientific Drilling main hole (CCSD-MH) and obtained the statistical characteristics of the resistivity log of ultrahigh-pressure (UHP) metamorphic rocks. To check the logs’ quality, we compared the resistivity log data with the core test data. The two datasets show a good coherence. On the whole, the resistivity of the UHP metamorphic rocks from the CCSD-MH is very high (mostly higher than 1 000 Ω·m). From orthogneiss to paragneiss, amphibolite, retrograded eclogite, rutile eclogite, phengite eclogite, and ultramafic rock, the resistivity decreases gradually. The fracture zone, enrichment of metal minerals, and serpentinization of peridotite can probably cause the decrease of resistivity. Comparing the electrical structure of the crust of Dabie (大别) terrane and Sulu terrane, we can conclude that (1) the electrical structure of the crust of Dabie terrane and Sulu terrane shows a very big difference, (2) the high-resistivity zone of the north Sulu orogen extended to a very great depth (60 km), but the high-resistivity zone of Dabie orogen is over the depth of 25 km.

Well logging responses of UHP metamorphic rocks from CCSD main hole in Sulu terrane, eastern central China

The lithologies of the Chinese Continental Scientific Drilling main hole (CCSD-MH) are mainly comprised of orthogneiss, paragneiss, eclogite, amphibolite, and ultramafic rocks. The statistical results of logs of CCSD-MH indicate that ultramafic rocks are characterized by very high CNL (neutron log) and very low GR (gamma ray log) and RD (resistivity log); eclogites are characterized by high DEN (density), V P (P-wave velocity) and PE (photoelectric absorption capture cross section); orthogneiss and paragneiss are characterized by high GR, U (uranium content), Th (thorium content), K (potassium content) and RD, and low DEN, PE, and CNL; logging values of amphibolite are between the logging values of eclogites and paragneiss. In addition, the logs could reflect the degree of retrograde metamorphism of eclogites. The upper section (100–2 000 m) shows higher DEN, PE, V P, and lower GR, U, Th, K, RD than the lower section (2 000–5 000 m). Most logs of the upper section are more fluctuant than those of the lower section. This indicates that the upper section has more heterogeneities than the lower section. The cross plots of logs indicate that DEN, GR, K, and CNL are more powerful in identifying ultrahigh pressure metamorphic (UHPM) rocks at the CCSD-MH. GR value of the rocks from CCSD-MH shows obviously an increasing trend from ultramafic rock (the most mafic rocks at CCSD-MH) to orthogneiss (the most acid rocks at CCSD-MH). On the contrary, DEN value decreases from the ultramafic rock to the orthogneiss. CNL log is a good indicator of the content of structure water in crystalline rocks.

Magnetic properties of serpentinized garnet peridotites from the CCSD main hole in the Sulu ultrahigh‐pressure metamorphic belt, eastern China

Magnetic properties of 14 serpentinized garnet peridotites from the interval of 603.20–683.53 m of the Chinese Continental Scientific Drilling (CCSD) hole in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China, were investigated and compared with those of ultramafic rocks from other tectonic settings, to relate them to the UHP (post‐)metamorphic evolution. Serpentinized garnet peridotites are characterized by high susceptibility (χ), natural remanent magnetization (NRM), and Q (Köenigsberger ratio = (NRM/χ × H), where H is the intensity of the local geomagnetic field) values. The average χ, NRM, and Q values are 299.65 × 10−7 m3 kg−1, 19.15 × 10−3 Am2 kg−1, and 16.59, respectively, which are generally higher than those of other serpentinized peridotites in the world. The serpentine concentration for most samples ranges from 40% to 60%, suggesting that the peridotites are of obvious homogeneous serpentinization degree. Micro‐Raman and X‐ray diffraction spectra of serpentine for representative samples indicate that lizardite is the only serpentine mineral. Opaque minerals in samples are mainly nonstoichiometric magnetite and Cr‐spinel with or without minor ilmenite and Fe‐sulfide. Magnetite occurs as rims around olivine and garnet as well as in clusters. A chromian garnet further overgrew on the rims of the garnet. The grain size of magnetite in rims is in the pseudo‐single‐domain region and favorable for development of high NRM. High pressure‐UHP metamorphic processes possibly lead to enhanced concentrations of Fe‐Ti oxides in garnet peridotites and, thus, increase the magnetism of serpentinized garnet peridotites in the CCSD main hole. Integrated lizardite formation temperature (<450°C) and its relationships with the pressure‐temperature‐time evolution of the associated UHP rocks permit us to infer that serpentinization of garnet peridotites in the CCSD main hole occurred during late exhumation stages, ca. 200–215 Ma ago, under temperatures of 425°C. However, the exact relationship between magnetic properties of garnet peridotite and the UHP metamorphic process will require further studies.

Fission track analysis and thermotectonic history of the main borehole of the Chinese Continental Scientific Drilling project

The Chinese Continental Scientific Drilling (CCSD) project, part of the International Continental Drilling Program (ICDP), has completed drilling a 5158 m hole in the eastern part of the Dabie–Sulu ultrahigh-pressure metamorphic belt. This study reports on an apatite fission track analysis of core samples from 0 to 4000 m depth in the CCSD main hole (CCSD-MH). We determined the fission track ages of 38 apatite samples from different depths. The ages range between 98.6 ± 17.0 and 3.2 ± 1.3 Ma, showing a general decreasing trend with depth, from 87.1 ± 11.2 Ma at the surface to 3.2 ± 1.3 Ma at 3899 m depth. As a first approximation, an average uplift rate of ~ 35 m/Ma is calculated for the period 90–30 Ma. The trend in ages within the borehole shows some fluctuations, and indicates movements along major faults. It is inferred that the highest-level major normal fault occurs at a depth of ~ 350 m, recording a vertical displacement of ~ 400 m. Movement along another prominent normal fault at a depth of ~ 2150 m occurred subsequent to ~ 25 Ma. Three major reverse faults occur at about 2450, 3050 and 3250 m depth. Testing geological constrains against the fission track data set indicated an agreement with a reheating of the area during the late Cretaceous and Eocene, followed by cooling to ~ 80 °C during the Eocene and a low cooling until the samples reached their present-day position in the Donghai area.

Petrological and geochronological constraints on the formation, subduction and exhumation of the continental crust in the southern Sulu orogen, eastern-central China

Detailed petrological, geochemical and geochronological studies were carried out for the core samples from the Chinese Continental Scientific Drilling Main Hole (CCSD-MH) with a final depth of 5158 m. This borehole has penetrated into an ultrahigh-pressure (UHP) metamorphic slice consisting mainly of eclogites, gneisses, garnet–pyroxenites and garnet–peridotites. Geochemical characteristics indicate that their protoliths are igneous rocks, and occur in a continental rifting tectonic setting. Quartz-, rutile- and ilmenite-rich eclogites from 0 to 710 m occur as alternating layers; the eclogites, together with interlayers of peridotites and gneisses form a layered ultramafic–mafic–acidic intrusion, which was formed by extensive fractional crystallization of basaltic magma in continental environments. The granitic gneisses from 1190 to 1505 m and 3460 to 5118 m show affinity to within-plate granite, whereas the granitic gneisses from 710 to 1190 m and 1505 to 3460 m exhibit characteristics of volcanic-arc granite. Zircon U–Pb dating demonstrates that the magmatic zircon cores, which have relatively high Th/U ratios (mostly > 0.4), from both eclogites and gneisses, yield the same age at c. 788.8 Ma, suggesting that the protoliths of UHP rocks were formed by bimodal magmatism in Neoproterozoic rifting tectonic zones along the northern margin of the Yangtze Plate, in response to the breakup of the supercontinent Rodinia. U–Pb dating of metamorphic zircons with coesite and other eclogite-facies mineral inclusions and with relatively low Th/U ratios (mostly < 0.14) gives similar Triassic ages, which define two main zircon-forming events at 221.1 Ma and 216.7 Ma. We suggest that the older weighted mean age represents the peak-UHP metamorphic event at a pressure of 5.0 GPa (corresponds to ∼ 165 km depth), whereas the younger mean age reflects the UHP/HP retrograde event at a pressure 2.8 GPa (∼ 92 km depth). Therefore, a maximum rate of vertical movement during early exhumation of the UHP rocks from the Sulu orogen would be 17 mm/year, which is quite similar to initial exhumation rates (16 to 35 mm/year) of many UHP terranes in the world.

Magnetic study of the UHP eclogites from the Chinese Continental Scientific Drilling (CCSD) Project

To establish the relationships between rock magnetism of eclogites and the corresponding retrograded metamorphic processes, this study integrated both rock magnetism and metamorphic petrology of 171 eclogite samples with different degree of retrograded metamorphism at the depth range of 100–2050 m of the main hole of the Chinese Continental Scientific Drilling (CCSD) Project, located in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China. Results show that the average density is a suitable indicator for the degree of the retrograded metamorphism. Opaque minerals in variable eclogites are mainly magnetite, ilmenite, pyrite, and hematite. With increasing the metamorphic retrogression, the concentration of opaque minerals generally increases; however, magnetic properties of eclogites first increase from well‐preserved eclogite to partially retrogressed eclogites because of the neoformation of coarse grained magnetite particles, and then decrease for the completely retrogressed eclogites because of the consumption of the magnetite particles and the formation of weakly magnetic minerals (e.g., hematite). These processes can be summarized as garnet + omphacite + rutile + SiO2 + H2O → amphibole + plagioclase + magnetite + ilmenite → amphibole + epidote + hematite + ilmenite. The retrograded metamorphic eclogites with elevated magnetic properties take up 25% of all rocks and thus are one of major magnetic rocks in the CCSD main hole located in the Sulu UHP area. Therefore the significant volumes of retrograded eclogites may account for the magnetic anomalies flanking the northeastern part of the Sulu UHP metamorphic belt.

Integrated Log Interpretation in the Chinese Continental Scientific Drilling Main Hole (Eastern China): Lithology and Mineralization

Delineation of lithology and mineralization of different rock units of the Chinese Continental Scientific Drilling Main Hole (CCSD-MH), Eastern China, is the main objective of this study. Different logging operations were conducted in the studied area. Subduction and metamorphism lead to high compressional wave velocity (Vp) and low neutron density (CNL) values of different rocks.

Zircon U–Pb age and Hf isotope evidence for contrasting origin of bimodal protoliths for ultrahigh‐pressure metamorphic rocks from the Chinese Continental Scientific Drilling project

AbstractA combined study of zircon morphology, U–Pb ages and Hf isotopes as well as whole‐rock major and trace elements was carried out for ultrahigh‐pressure (UHP) eclogite and felsic gneiss from the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The results show contrasting Hf isotope compositions for bimodal UHP metaigneous rocks, pointing to contrasting origins for their protoliths (thus dual‐bimodal compositions). The samples of interest were from two continuous core segments from CCSD MH at depths of 734.21–737.16 m (I) and 929.67–932.86 m (II) respectively. Zircon U–Pb dating for four samples from the two core segments yields two groups of ages at 784 ± 17 and 222 ± 3 Ma, respectively, corresponding to protolith formation during supercontinental rifting and metamorphic growth during continental collision. Although the Triassic UHP metamorphism significantly reset the zircon U–Pb system of UHP rocks, the Hf isotope compositions of igneous zircon can be used to trace their protolith origin. Contrasting types of initial Hf isotope ratios are, respectively, correlated with segments I and II, regardless of their lithochemistry. The first type shows positive ɛHf(t) values of 7.8 ± 3.1 to 6.0 ± 3.0, with young Hf model age of 1.03 and 1.11 Ga. The second type exhibits negative ɛHf(t) values of −6.9 ± 1.6 to −9.1 ± 1.1, with old Hf model ages of 2.11 and 2.25 Ga. It appears that the UHP rocks from the two segments have protoliths of contrasting origin. Consistent results are also obtained from their trace element compositions suggesting that mid‐Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the South China Block. Thus, the first type of bimodal magmatism formed by rapid reworking of juvenile crust, whereas the second type of bimodal magmatism was principally generated by rift anatexis of Paleoproterozoic crust. Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins. Because arc–continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc–continent collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at c. 780 Ma.

Origin of retrograde fluid in ultrahigh-pressure metamorphic rocks: Constraints from mineral hydrogen isotope and water content changes in eclogite–gneiss transitions in the Sulu orogen

By taking advantage of having depth profiles between contrasting lithologies from core samples of the Chinese Continental Scientific Drilling (CCSD) project, a combined study was carried out to examine changes in mineral H isotope, total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh-pressure (UHP) eclogite and gneiss in the Sulu orogen, east-central China. The samples of interest were from two continuous core segments from the CCSD main hole at depths of 734.21–737.16 and 929.67–932.86 m, respectively. The results show δD values of −116‰ to − 64‰ for garnet and −104‰ to −82‰ for omphacite, consistent with incorporation of meteoric water into protoliths of UHP metamorphic rocks by high-T alteration. Both equilibrium and disequilibrium H isotope fractionations were observed between garnet and omphacite, suggesting fluid-assisted H isotope exchange at local scales during amphibolite-facies retrogression. While bulk water analysis gave total H 2O concentrations of 522–1584 ppm for garnet and 1170–20745 ppm for omphacite, structural hydroxyl analysis yielded H 2O contents of 80–413 ppm for garnet and 228–412 ppm for omphacite. It appears that significant amounts of molecular H 2O are present in the minerals, pointing to enhanced capacity of water storage in the UHP eclogite minerals. Hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents. Petrographically, the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite–gneiss boundary. Thus, retrograde metamorphism results in mineral reactions and H isotope variation. Because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure, significant amounts of water are expected to be released from the minerals during decompression exhumation. Decompression exsolution of structural hydroxyl from 1 m 3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3.07–3.44 kg water that can form 140–156 kg amphibole during exhumation. Therefore, it is concluded that fluid for retrogression of the eclogites away from the eclogite–gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular H 2O in nominally anhydrous minerals. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water. Consequently, both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic UHP rocks.

Petrologic Study of Ultrahigh-Pressure Metamorphic Cores from 100 to 2000 m Depth in the Main Hole of the Chinese Continental Scientific Drilling Project, Eastern China

The Chinese Continental Scientific Drilling (CCSD) Project in Donghai recovered more than 1000 m of eclogite and garnet peridotite cores for study. Examination of rocks from 100-2000 m of the main borehole has identified five major lithological types: (1) eclogite and garnet pyroxenite; (2) eclogitic gneiss; (3) garnet peridotite; (4) biotite (hornblende) two-feldspar gneiss; and (5) fault breccia and mylonite. The eclogite was further subdivided into two types: crustal eclogite and mantle-derived eclogite. Crustal eclogites are ubiquitous as layers of various thickness in gneissic rocks, and contain low-Prp (<40 mol%) garnet and omphacite. Mantle-derived eclogites are spatially associated with ultramafic cores composed mainly of garnet wehrlite, and have higher Prp-bearing (>40 mol%) garnet and low Jd-bearing clinopyroxene. Chemically, the crustal eclogites are relatively low in MgO and high in SiO2, but have high, variable contents of Al2O3 and rare-earth elements. Most crustal eclogites range in SiO2 content from 49 to 60 wt%, whereas mantle-derived eclogites are rich in MgO, and have SiO2 content less than 49 wt%. Garnet peridotites consist of olivine (Fo = 85-91), enstatite, Mg-rich garnet, and diopsidic clinopyroxene; Ti-clinohumite is also widespread. Mineral paragenesis indicates that the garnet peridotites together with other lithologies underwent in situ ultrahigh-P metamorphism (UHPM). Based on the differences in rock association, structural kinematics, and seismic characteristics, we have identified two different rock slices separated by a fault zone at 1600 m depth, where breccia and mylonite developed. Rutile eclogites are dominant in the upper slice, and phengite eclogites are layered with deformed tonalite and paragneiss in the lower slice. These UHPM rocks underwent variable retrograde metamorphism; eclogite is replaced by symplectite-bearing garnet amphibolite, and eclogitic gneiss is retrograded to biotite (hornblende) plagioclase gneiss. Late-stage crustal extension resulted in local cataclasis, forming tectonic breccia with the development of chlorite, calcite, hematite, and epidote under epidote amphibolite-to greenschist-facies conditions. Nearly 2000 m of recovered UHP core from the CCSD main hole reveals that voluminous crustal materials were subducted to mantle depths and rapidly returned to the surface. UHPM cores record subduction and exhumation processes of the continental crust and provide information for the study of continental subduction/collision and mantle dynamics.