Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen, a series of combined studies were carried out for UHP metamorphic rocks from the main hole (MH) at continuous depths of 100 to 5000 m. The results provide new insights into the chemical geodynamics of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as follows. (1) An O isotope profile of 100 to 5000 m is established for the UHP metamorphic minerals, with finding of ¹⁸O depletion as deep as 3300 m. Along with areal ¹⁸O depletion of over 30,000 km² along the Dabie–Sulu orogenic belt, three-dimensional ¹⁸O depletion of over 100,000 km³ occurs along the northern margin of the South China Block. (2) Changes in mineral O isotope, H isotope and water content occur in eclogite-gneiss transitions, concordant with petrographic changes. The contact between different lithologies is thus the most favorable place for fluid action; fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance, petrography and δ¹⁸O values of adjacent samples shows O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. (3) Studies of major and trace elements in the two continuous core segments indicate high mobility of LILE and LREE but immobility of HFSE and HREE. Some eclogites have andesitic compositions with high SiO₂, alkalis, LREE and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence for melt-induced element mobility in the UHP metamorphic rocks, and thus the possible presence of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO₂, LREE and LILE occur at the contact between eclogite and gneiss. This indicates their mobility between different slab components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the widespread retrogression, retrograde fluid was internally buffered in stable isotope compositions, and the retrograde fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals the episode of HP eclogite-facies recrystallization at 216 ± 3 Ma, with timescale of 1.9 to 9.3 Myr or less. Collectively, the Dabie–Sulu UHP terrenes underwent the protracted exhumation (2–3 mm/yr) in the HP-UHP regime. (6) Zircon U–Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780 Ma. The first type of bimodal magmatism was formed by reworking of juvenile Late Mesoproterozoic crust, whereas the second type of bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust. In conclusion, the geochemical studies of CCSD-MH core samples have placed important constraints on the nature and scale of fluid action and element mobility during the continental subduction and UHP metamorphism.     

大陆俯冲隧道中的应变不均一分布：来自大别山超高压变质岩的记录

俯冲隧道是俯冲板片与上覆板块之间的剪切带,也是高压—超高压变质岩折返和深部流/熔体活动的通道。大别山超高压变质岩分布广泛,变形程度差异很大,是研究大陆俯冲隧道中岩石变质- 变形过程的理想地区。本文系统总结了前人对中大别双河地区超高压变质岩的岩石学和年代学研究成果,在双河地区开展了地质填图、应变分析和三维构造重建。通过将超高压变质岩的变形特征与P- T- t轨迹结合,识别出超高压变质岩折返过程中的三期韧性变形。在双河北部发现了一个上盘向NW剪切的千米尺度的榴辉岩相鞘褶皱,枢纽向SE倾伏,倾伏角约20°,与榴辉岩、片岩和长英质片麻岩的拉伸线理平行,表明超高压变质岩初始折返阶段的流体活动使榴辉岩的强度显著降低,榴辉岩与围岩一起发生韧性变形。该期变形被角闪岩相退变质阶段上盘向NW的剪切叠加,此时应变集中于片麻岩、片岩、大理岩等非能干层,强度较高的榴辉岩成为构造透镜体。而绿片岩相变质阶段上盘向SE方向的剪切与早白垩世北大别花岗片麻岩穹隆的形成有关。对双河南部弱变形花岗片麻岩的锆石U- Pb定年揭示了757±14 Ma的原岩年龄和 240~216 Ma的变质年龄,与双河北部含柯石英强变形花岗片麻岩类似,暗示其也经历了三叠纪超高压变质作用及随后的角闪岩相退变质作用。通过计算长英质片麻岩的有效黏度,发现无水碱长花岗片麻岩的有效黏度高于黑云斜长片麻岩,折返阶段的流体活动使超高压变质岩的强度显著降低,当局部的流体活动不足以弱化碱长花岗岩体时,应变集中于黑云斜长片麻岩。因此,大陆俯冲隧道中的应变分布受矿物组成、流体活动和岩体规模的共同影响。     

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan,North Qaidam belt,NW China

abstract

An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.     

Discovery and Geological Significance of Neoproterozoic Metamorphic Granite in Jimo, Shandong Province, Eastern China

During the 1:50000 regional geological survey in Jimo,east Shandong Province,Paleoproterozoic metamorphic supracrustal rocks and Neoproterozoic metamorphic plutonite were newly discovered. These rocks displayed inclusions which had occurred in the Mesozoic granite,and the main lithologies are schist,granulite,marble,and granitic gneiss. Geochemical analyses suggest that Neoproterozoic metamorphic plutonite are characterized by high-K,metaluminous to weakly peraluminous. They are enriched in LILE and depleted in HFSE,with moderately enrichment of LREE,weak fractionation of LREE from HREE and negative Eu anomalies. The surface age of plutonic rocks in the survey area is 770.2±2.4 Ma,representing the age of magma crystallization,which is agreement with the the Neoproterozoic magmatic event after Rodinia supercontinent in the northern margin of Southern China continental block. In addition,the age of sporadic distribution(298 Ma and 269 Ma) is mixed zircon age,representing the rocks experienced metamorphism in Indosinian period. According to the associated mineral assemblages,and the characteristic metamorphic minerals and temperature pressure conditions,four metamorphic facies were identified,including amphibolitic,epidote amphibolite,greenschist,and mid-high pressure greenschist. Analysis of tectonic setting suggests that granitic gneiss is formed in an extensional environment and was involved from the continental margin magmatic arc to intraplate environment. Jimo is distributed in the east of Zhuwu fault,and has the same Spatial distribution location with the Weihai uplift UHP metamorphic belt rocks. The metamorphic rocks in Jimo area have similar geochemical characteristics of elements,tectonic setting and retrograde metamorphism with that in the Sulu UHP metamorphic belt. Therefore,Zhuwu fault may be the boundary fault of Sulu UHP metamorphic belt.     

Element mobility during continental collision: insights from polymineralic metamorphic vein within UHP eclogite in the Dabie orogen

Metamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction‐zone P–T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh‐pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite‐bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K‐feldspar ± plagioclase ± K‐bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute‐rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U–Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ¹⁸O and δD values, suggesting that the vein‐forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water‐soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2–5.7 times) and Nb–Ta (1.8–2.0 times) than those in the eclogite, indicating that these normally water‐insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein‐forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction‐zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein‐forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.     

南苏鲁超高压变质地体中罗迪尼亚超大陆裂解事件的记录

通过苏鲁超高压变质地体南部不同类型超高压变质岩石的原岩重塑．揭示超高压变质岩的原岩形成于由大陆玄武质岩石、辉长岩、表壳岩和花岗岩组成的被动陆缘拉伸构造环境。中国大陆科学钻探主孔中不同类型超高压变质岩石的锆石SHRIMP U-Pb定年表明。花岗质片麻岩原岩年龄为780～680Ma；榴辉岩、石榴角闪岩的原岩年龄为765～730Ma，副片麻岩中包含了730Ma、680Ma、621Ma和较年轻的继承性碎屑锆石和结晶锆石年龄。结合前人的研究成果表明，苏鲁超高压变质地体南部正片麻岩类和榴辉岩的原岩所代表的花岗岩浆和基性岩浆活动为罗迪尼亚超大陆形成后的新元古代裂解事件的产物．而副片麻岩的原岩为新元古代．古生代时期形成的扬子被动陆缘的沉积-火山表壳盖层，它们与结晶基底一起在240～220Ma期间经历了超高压变质作用。     

REE concentrations of garnet and omphacite in eclogites from the Dabie Mountain, central China

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.     

大别山北部榴辉岩和英云闪长质片麻岩锆石U-Pb年龄及多期变质增生

大别山北部榴辉岩及英云闪长质片麻岩的锆石U-Pb年龄分析表明：北部榴辉岩相峰期变质时代为226～230Ma左右;北部塔儿河一带英云闪长质片麻岩经历过印支期变质事件;大别山北部与南部超高压岩石中一致的（226～230Ma）高压或超高压变质年龄表明,北部镁铁-超镁铁质岩带中部分岩石也曾作为扬子俯冲陆壳的一部分,在印支期发生过高压或超高压变质作用;本区锆石发生过两期变质增生事件,一是印支期高压或超高压变质,另一期是燕山期热变质事件;榴辉岩及英云闪长质片麻岩的原岩形成时代为晚元古代;锆石U-Pb年龄可用多期变质增生模型来解释。     

北秦岭榴辉岩及相关岩石年代学的进一步确定及其对板片俯冲属性的约束

在岩相学观察和锆石CL图像研究的基础上,利用LA ICP MS原位分析方法,对北秦岭官坡超高压榴辉岩和伴生的石榴石角闪岩(榴闪岩)进行了详细的锆石微区微量元素和U Th Pb同位素分析,在榴辉岩样品中得到变质年龄为(502±11)Ma,原岩结晶年龄>(657±18)Ma;在榴闪岩样品中得到原岩结晶年龄为(791±6)Ma,变质年龄为487~503 Ma,角闪岩相退变质年龄为(366±4)Ma。岩石地球化学研究显示,北秦岭官坡地区的榴闪岩具有低Si(SiO2质量分数为4916%~5078%),高Ti(TiO2质量分数为228%~283%)、富集LREE、LILE和大部分的HFSE元素,不显Nb、Ta负异常的板内玄武岩特征,与北秦岭超高压榴辉岩地球化学特征一致。结合两者的野外产状、岩相学特征、锆石形貌和年代学研究结果,表明本文研究的官坡地区的榴闪岩是超高压榴辉岩在抬升过程中在角闪岩相条件下退变质的产物。综合两者的年代学研究结果,得到北秦岭地区超高压榴辉岩的变质年龄为(502±11)Ma,原岩结晶年龄为(791±6)Ma,角闪岩相退变质年龄为(366±4)Ma。研究得到的(502±11)Ma的榴辉岩相变质年龄与前人得到的该榴辉岩围岩超高压泥质片麻岩的变质年龄(507±38)Ma以及北秦岭松树沟地区的超高压长英质片麻岩的变质年龄485~514 Ma一致,表明它们经历了同期超高压变质作用。而且,榴辉岩(502±11)Ma的变质年龄与其原岩的结晶年龄(791±6)Ma存在近300 Ma的时间间隔,表明原岩具有板内玄武岩性质的北秦岭官坡超高压榴辉岩不可能是秦岭古生代大洋板块深俯冲的产物,而可能是已构造就位的古洋壳或裂谷火山岩在古生代随陆壳一起发生大陆深俯冲作用的产物。     

Zircon U–Pb and Hf evidence for coupled subduction of oceanic and continental crust during the Carboniferous in the Huwan shear zone,western Dabie orogen,central China

Both oceanic and continental HP rocks are juxtaposed in the Huwan shear zone in the western Dabie orogen, and thus provide a window for testing the buoyancy‐driven exhumation of dense oceanic HP rocks. The HP metamorphic age of the continental rocks in this zone has not been well constrained, and hence it is not known if they are of the same age as the exhumation of the HP oceanic rocks. In situ laser ablation (multiple collector) inductively coupled plasma mass spectrometry (LA‐(MC‐)ICP‐MS), U–Pb, trace element and Hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the Huwan shear zone. U–Pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 ± 4 Ma. The zircon in this sample displays ε_Hf (t) values of +6.1 to +14.4. The positive ε_Hf (t) values up to +14.4 suggest that the protolith was derived from a relatively depleted mantle source, most likely Palaeotethyan oceanic crust. A granitic gneiss and the other eclogite yield protolith U–Pb ages of 738 ± 6 and 700 ± 14 Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the granitic gneiss has low ε_Hf (t) values of ?14.2 to ?10.5 and old T_DM2 ages of 2528–2298 Ma, suggesting reworking of Palaeoproterozoic crust during the Neoproterozoic. The zircon in the eclogite has ε_Hf (t) values of ?1.0 to +7.4 and T_DM1 ages of 1294–966 Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat heavy rare earth element patterns, weak negative Eu anomalies and low ¹⁷⁶Lu/¹⁷⁷Hf ratios. All these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar, and thus under eclogite facies conditions. The metamorphic zircon yields U–Pb ages of 310 ± 3 and 306 ± 7 Ma. Therefore, both the oceanic‐ and continental‐type eclogites share the same episode of Carboniferous eclogite facies metamorphism. This suggests that high‐pressure continental‐type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic‐type eclogites through buoyancy‐driven uplift.     

Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam,northern Tibet

A combined petrological, geochronological and geochemical study was carried out on felsic veins and their host rocks from the North Qaidam ultrahigh‐pressure (UHP) metamorphic terrane in northern Tibet. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrograpically recognized in metagranite, metapelite and metabasite. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspar with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in both the eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr–Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt derivation from hybrid sources. Felsic veins inside gneisses exhibit lithochemical compositions similar to experimental melts on the An–Ab–Or diagram. In trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during anatexis. Felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase‐rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in veins hosted by gneisses than veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U–Pb ages of c. 420 Ma, significantly younger than the peak UHP eclogite facies metamorphic event at c. 450–435 Ma. Combining the petrological observations with local P–T paths and experimentally constrained melting curves, it is inferred that anatexis of UHP gneisses was caused by muscovite breakdown while anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.     

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 H₂O concentrations of 522-1584 ppm for garnet and 1170-20745 ppm for omphacite, structural hydroxyl analysis yielded H₂O contents of 80-413 ppm for garnet and 228-412 ppm for omphacite. It appears that significant amounts of molecular H₂O 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 m³ 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 H₂O 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.     

Partial melting of ultrahigh-pressure metamorphic rocks at convergent continental margins: Evidences,melt compositions and physical effects

Ultrahigh-pressure(UHP) metamorphic rocks are distinctive products of crustal deep subduction,and are mainly exposed in continental subduction-collision terranes. UHP slices of continental crust are usually involved in multistage exhumation and partial melting, which has obvious influence on the rheological features of the rocks, and thus significantly affect the dynamic behavior of subducted slices. Moreover,partial melting of UHP rocks have significant influence on element mobility and related isotope behavior within continental subduction zones, which is in turn crucial to chemical differentiation of the continental crust and to crust-mantle interaction.Partial melting can occur before, during or after the peak metamorphism of UHP rocks. Post-peak decompression melting has been better constrained by remelting experiments; however, because of multiple stages of decompression, retrogression and deformation, evidence of former melts in UHP rocks is often erased. Field evidence is among the most reliable criteria to infer partial melting. Glass and nanogranitoid inclusions are generally considered conclusive petrographic evidence. The residual assemblages after melt extraction are also significant to indicate partial melting in some cases. Besides field and petrographic evidence, bulk-rock and zircon trace-element geochemical features are also effective tools for recognizing partial melting of UHP rocks. Phase equilibrium modeling is an important petrological tool that is becoming more and more popular in P-T estimation of the evolution of metamorphic rocks; by taking into account the activity model of silicate melt, it can predict when partial melting occurred if the P-T path of a given rock is provided.UHP silicate melt is commonly leucogranitic and peraluminous in composition with high SiO_2,low MgO, FeO, MnO, TiO_2 and CaO, and variable K_2 O and Na_2 O contents. Mineralogy of nanogranites found in UHP rocks mainly consists of plagioclase + K-feldspar + quartz, plagioclase being commonly albite-rich.Trace element pattern of the melt is characterized by significant enrichment of large ion lithophile elements(LILE), depletion of heavy rare earth elements(HREE) and high field strength elements(HFSE),indicating garnet and rutile stability in the residual assemblage. In eclogites, significant Mg-isotope fractionation occurs between garnet and phengite; therefore, Mg isotopes may become an effective indicator for partial melting of eclogites.     

Ultrahigh pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: I. Petrology and geochemistry of the main hole (0–2,050 m)

The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO₂, total Fe, V, Co and Sr, and very low SiO₂, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.     

Petrogenesis of UHP metamorphic rocks from Qinglongshan, southern Sulu, east-central China

Five kinds of UHP metamorphic rocks, including eclogite, orthogneiss, paragneiss, schist and quartzite are exposed in the Qinglongshan roadcut, southern Sulu orogenic belt of eastern central China. They comprise metamorphic supracrustal rocks with bimodal volcanic characteristics and continental affinity, and granitic intrusive associations. The preservation of coesite inclusions and/or its pseudomorphs in eclogite and other rocks indicate that they have been subjected to in-situ UHP metamorphism. Four stages of metamorphism were recognized by combining petrographic observations and compositions of minerals from various UHP rocks. Prograde epidote-amphibolite facies, UHP coesite–eclogite facies, post UHP quartz–eclogite facies, and retrograde amphibolite facies assemblages delineate an inferred P–T path with a clockwise trajectory and a retrograde event characterized by the coupling of decompression with a temperature decrease. Garnet porphyroblasts in UHP eclogites display a complex growth zoning and mineral distribution, and record a crucial segment of the prograde and retrograde metamorphic evolution. The preservation of growth zoning in eclogitic and gneissic garnets suggests that the UHP rocks had a short residence time before retrograde metamorphism and a very high uplift rate in order to preserve the prograde growth zoning.     

榴辉岩退变质过程中的微量元素地球化学行为：对CCSD主孔退变质榴辉岩的研究

对中国大陆科学钻探工程主孔榴辉岩退变质过程中的微量元素地球化学行为进行了研究。对退变质程度连续变化样品的不同部分的对比研究表明，流体作用下的退变质过程中大离子亲石元素（Cs、Rb、Ba、Sr、K、Th、U）和轻稀土元素表现出较大的活动性，重稀土元素和高场强元素变化相对较小。退变质后大离子亲石元素的显著增加和高场强元素、重稀土元素的轻微变化（甚至相对降低），表明与退变质作用有关的流体中的络阴离子含量很少，并不富集高场强元素和重稀土元素。退变质后总体上表现出的Si、大离子亲石元素和轻稀土元素的明显变化，表明外来流体参与了榴辉岩的退变质过程，带入和带出了一些元素。结合榴辉岩中单矿物微量元素组成以及前人对D^Mineral/Fluid的研究成果，对流体-榴辉岩作用形成的退变质分带（富石英条带→角闪岩→退变质榴辉岩→新鲜榴辉岩）的微量元素组成变化进行了详细研究。结果表明在流体作用下的榴辉岩退变质过程中，大离子亲石元素、轻稀土元素和高场强元素含量的变化除了受退变质流体性质的影响外，更大程度上取决于退变质过程中的矿物相（尤其是副矿物）的变化。     

安徽省潜山县韩长冲地区的地质构造特征

韩长冲地区主要由超高压变质岩、碱长花岗片麻岩、二长花岗片麻岩组成。前者包括超高压片麻岩、大理岩、榴辉岩和石英硬玉岩等。其中存在由黑云斜长片麻岩、石英硬玉岩、大理岩及层状榴辉岩组成的超高压变质表壳岩组合。超高压变质岩先后经过柯石英榴辉岩相、石英榴辉岩相、角闪岩相及绿片岩相多阶段的变质作用;经过榴辉岩相、角闪岩相、角闪岩相后及脆性等四期变形。其中角闪岩相变形形成本区的主要构造要素,以发育大量Ａ型褶皱及普遍的糜棱岩化为特征。碱长花岗片麻岩及二长花岗片麻岩只经过角闪岩相及其后的变质和变形作用。超高压变质岩与碱长花岗片麻岩呈侵入接触,可能沿缓倾向ＳＳＥ的韧性剪切带逆掩于二长花岗片麻岩之上。     

大别山北大别杂岩的大地构造属性

北大别杂岩主要由花岗质片麻岩及斜长角闪岩组成 ,含有不同类型、大小不等的麻粒岩岩块和变质超镁铁质岩块 ,侵入有大量白垩纪花岗岩和辉石 -辉长岩类。其中的花岗质片麻岩、斜长角闪岩具有岛弧环境的岩石地球化学特征 ,代表拼贴于扬子陆块北缘的新元古代古岛弧。北大别杂岩北可与庐镇关群相连 ,南俯于超高压变质岩之下 ,在三叠纪扬子陆块与华北陆块的碰撞过程中 ,曾与超高压变质岩一起俯冲到地幔深度并经受榴辉岩相变质作用 ,然后在折返过程中叠加了麻粒岩相及角闪岩相变质作用 ,是扬子陆块北缘陆壳俯冲基底的一部分     

大别—苏鲁超高压变质带内的块状榴辉岩及其构造意义

大别—苏鲁超高压(> 27× 108Pa) 变质带内的榴辉岩, 在大陆深俯冲、碰撞和折返剥露过程中, 大都遭受了强烈的变形和变质作用的重置与再造.但是, 大型榴辉岩体核部以及包裹于大理岩和石榴橄榄岩体内部的块状榴辉岩, 往往保留其初始简单的矿物组合、中-细粒状变晶结构和块状构造.详细地分析了块状榴辉岩的几何学、岩相学及变质作用特征, 指出它们是超高压榴辉岩递进及多期变质变形分解作用的残留体, 位于尺度不同的弱应变域内, 是大陆深俯冲及碰撞作用的真正记录.      

SHRIMP Dating and Recrystallization of Metamorphic Zircons from a Granitic Gneiss in the Sulu UHP Terrane

An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.