Abstract:Mg-Al granulite is a collective term for a class of granulite-facies metamorphic rocks that are enriched in magnesium and aluminum. Although its petrographic characteristics have been widely used to infer the peak metamorphic conditions and metamorphic evolution history of ultra-high temperature (UHT) metamorphism, our knowledge about the protolith and petrogenesis of Mg-Al granulite is still very limited. Taking the Huatugou UHT meta-morphic terrane of the western Qaidam Block as an example and based on the field observation, this study investigated the petrography and whole-rock geochemistry of Mg-Al granulites and pelitic gneisses. Mg-Al granulites and leucosome-bearing pelitic gneisses have similar contents of SiO₂, TiO₂, and P₂O₅, with overlapped variation ranges of ^TFe₂O₃, Al₂O₃, MnO, CaO and Na₂O. Meanwhile, Mg-Al granulites and pelitic gneisses exhibit similar distribution patterns of trace and rare earth elements. Combined with the similar mineral assemblages in parts of Mg-Al granulites and pelitic gneisses, we concluded that the protolith of the Huatugou Mg-Al granulite is similar to that of pelitic gneisses. From the lower-amphibolite facies metapelite to leucosome-bearing pelitic gneisses and Mg-Al granulites, the whole-rock compositions show descending trends in aluminum, calcium, potassium, and sodium, with increases in iron and magnesium. Mg-Al granulites have high X_Mg values (0.51~0.69), which distinguish them from other pelitic gneisses (X_Mg=0.34~0.43). According to the phase equilibrium modeling and related calculation on the metapelite, the partial melting and melt loss account for most of the chemical trends, but basically do not affect the whole-rock X_Mg value; only the removal of partial melts with garnet during prograde heating can transform the metapelite into Mg-Al granulite. In addition, compared with the nearby leucosome-bearing pelitic gneisses, the garnet-rich pelitic residue is depleted in silicon, sodium, and potassium, and enriched in aluminum, iron, magnesium, manganese, and calcium, with significantly higher contents of heavy rare earth elements. These geochemical features require the addition of garnet-bearing melts and then the removal of melts, which supports the movement of garnet-bearing melts observed in the field. Finally, we provide a hypothesis on why Mg-Al granulite in the Huatugou region only occurred as lenses. It is speculated that, because it is difficult for the melt to move with garnet for a long distance, only metapelite surrounded by felsic orthogneiss can get rid of garnet-bearing melts and transform into Mg-Al granulite during the prograde heating process. More studies are needed to verify this hypothesis, and we believe that evidence for the migration of garnet-bearing melts and the transformation of the Mg-Al granulite probably have been preserved within other HT-UHT metamorphic regions.

Abstract:The garnet-sillimanite-cordierite gneiss, which has been identified from the Chahanhe Group in the northern region of the Oulongbuluke block, consists of garnet, sillimanite, cordierite, biotite, plagioclase, ilmenite, and a small amount of K-feldspar. Petrographic observation shows that three stages (M1, M2 and M3) of mineral assemblage can be recognized. They are M1:Pl±Kfs+Grt+Sill+Qz, M2:Pl±Kfs+Grt+Sill+Qz+Ilm+Bt, and M3:Crd+Bt+Ilm+Qz+Grt+Pl±Kfs. Phase equilibrium modelling shows that the peak p-T conditions of the garnet-sillimanite-cordierite rock are p=0.92~1.08 GPa and t>790℃, and experienced a heating decompression during exhumation. The metamorphic ages acquired from zircon and monazite U-Pb dating by LA-ICP-MS are 1 133 ±14 Ma and 1 125 ±37 Ma, respectively, which represent the timing of granulite-facies metamorphism. Combining regional geology with our data, we suggested that the garnet-sillimanite-cordierite rock in the Chahanhe Group formed in an arc or back-arc system related to oceanic subduction. The magmatic-metamorphic complex belt in the northern region of the Oulongbuluke block underwent an evolutionary process from subduction-accretion to collision orogeny during late Mesoproterozoic to early Neoproterzoic, which is a response to the convergence process of Rodinia supercontinent.

Abstract:The North Qilian is characterized by the early Paleozoic ophiolite and high-pressure/low-temperature metamorphic rocks, such as serpentinite, eclogite, blueschist and metasedimentary rocks. In this study, a detailed petrographic, mineral-chemical, and geochemical analysis is presented on the piemontite-bearing metacherts in Qingshuigou, North Qilian. The piemontite-bearing metacherts are mainly composed of quartz, phengite, piemontite, garnet, glaucophane, clinopyroxene, ardennite, and hematite. Based on the mineral assemblage, mineral chemistry, and the p-T conditions of the country rocks, the piemontite-bearing metacherts may have undergone low-temperature and high-pressure eclogite-facies metamorphism. Whole-rock geochemistry suggested that the protolith of the piemontite-bearing metacherts was argilliferous cherts deposited in the oceanic environment with the participation of hydrothermal activities which resulted in the deposition of Fe and Mn. They were involved in the subduction zone with the materials from the continental active margin or continental island arc, and then experienced low-temperature and high-pressure metamorphism. The piemontite, ardennite, spessartite, and many inclusions of hematite in garnet indicated that the piemontite-bearing metacherts experienced a condition of high oxygen fugacity, which also recorded by the decreases of the Fe³⁺ from core to rim of the garnet. Oxygen released in this process played a significant role in the exploration of oxygen fugacity in the lithosphere mantle, the generation of island arc magma, and the oxygen cycle in a subduction zone.

Abstract:The magmatic rocks formed in the forearc setting are considered to possibly record key evidences for subduction initiation. In this paper, the niobium-rich gabbro and plagiogranite are newly reported in the Corridor Nan-shan ophiolitic melange zone of the North Qilian Orogen. Based on field observation, combined with comprehensive studies of petrology, geochemistry, isotope geochemistry and chronology, the time of initial subduction of proto-Tethys Ocean is constrained and its evolution model and tectonic implication are discussed. Zircon U-Pb dating results reveal a weighted average ²⁰⁶Pb/²³⁸U zircon age at 512±4 Ma for a niobium-enriched gabbro sample, and 522±3 Ma and 519±1 Ma for two plagiogranite samples. The niobium-rich gabbro samples have Nb contents of 7.49×10^-6~10.80×10^-6, TiO₂ of 1.50%~2.08%, Nb/U values of 11.9~13.4 and (Nb/La)_N > 0.5, which are obviously higher than those of island-arc basalts. εNd(t) values of Nb-enriched grabbro are between +4.38 and +5.78. Plagiogranite samples have low K₂O content (0.31%~1.66%) and low ratios of K₂O/Na₂O (0.05~0.43). The chondrite-normalized rare earth element pattern is relatively flat, with weakly positive Eu or negative anomalies. The plagioclase samples are relatively enriched in large ion lithophile elements such as Ba, Sr and U, and depleted in high field strength elements such as Nb and Ta. Combing their positive εNd(t) value (+3.35) and highly positive εHf(t) ranging (mostly between 6.2 to 12.9), we suggest an oceanic plagiogranite. The new data, along with the regional geology, indicate the Baijingsi plagiogranites are resulted from partial melting of the young and hot subducting oceanic slab at shallow depths in relatively high temperature during subduction initiation stage. Subsequently, the partial melting of subducting slab led to product of adakitic melts, which metasomatized or interacted with the upper mantle wedge peridotites, then the partial melting of metasomatized mantle peridotites gave rise to Nb-enriched basaltic magma. These data indicate that initial stage of subduction of the Prototethyan ocean (Paleo-Qilian ocean) occurred in the Early Cambrian.

Abstract:The Qinling Complex is a major tectonic unit in the Northern of the Qinling Orogenic Belt, which is the key to understand the tectonic evolution of the Qinling Orogenic Belt during the early Paleozoic. The migmatites of the Qinling Complex in the Weiziping area, were formed by strong anatexis of meta-sedimentary rocks. In this paper, a combined study of petrology, zircon U-Pb dating and phase equilibrium modeling was carried out on these rocks, in order to better constrain their metamorphic p-T evolution, and tectonic implications. The mesosomes are composed of garnet-bearing hornblende-biotite-plagioclase gneiss, as well as garnet-bearing biotite-hornblende-plagioclase gneiss. The garnet-bearing hornblende-biotite-plagioclase gneiss records only peak mineral assemblage of cummingtonite+garnet+plagioclase+biotite+quartz+ilmenite+melt. However, the garnet-bearing biotite-hornblende-plagioclase gneiss displays three metamorphic mineral assemblages, including biotite+plagioclase+quartz (prograde metamorphic stage, M1), cummingtonite+garnet+plagioclase+biotite+quartz+ilmenite+melt (peak metamorphic stage, M2), and hornblende+biotite+plagioclase+quartz+melt (retrograde metamorphic stage, M3). Phase equilibrium modelling constrained the peak p-T conditions of the garnet-bearing hornblende-biotite-plagioclase gneiss and the garnet-bearing biotite-hornblende-plagioclase gneiss to be 790~810℃/990~1 040 MPa, and 840~862℃/1 000~1 190 MPa, respectively. In addition, the retrograde metamorphic p-T conditions were constrained to be 735~814℃/400~810 MPa, 721~794℃/430~700 MPa, and 740~810℃/470~780 MPa, using p-T pseudosections calculated with local effective composition, for the garnet-bearing biotite-hornblende-plagioclase gneiss. As a result, the meta-sedimentary rocks from the Qinling Complex in the Weiziping area underwent a nearly isothermal decompression p-T path. LA-ICP-MS zircon U-Pb analyses obtain the weighted mean ²⁰⁶Pb/²³⁸U ages of 383.2±7.0 Ma, 400±3.6 Ma, and 406.7±7.8 Ma for metamorphic zircons from garnet-bearing hornblende-biotite-plagioclase gneiss and two leucosome samples, respectively. Integrated with previously published data, the timing of peak metamorphism and strong migmatization of the meta-sedimentary rocks in the Weiziping area is constrained to be ca. 410~390 Ma, whereas the age of ca. 380 Ma may represent the timing of retrograde cooling to the solidus. The nearly isothermal decompression path indicates that the granulite-facies meta-sedimentary rocks in the Weiziping area underwent rapid exhumation, which is interpreted to be resulted from the crustal thickening associated with continental collision and subsequent extension.

Abstract:Oceanic plagioganites refers to felsic intrusions which can be generated in different tectonic setting including midocean ridge and subduction zone. Plagiogranite is volumetrically minor component of oceanic crust and ophiolites, but it is crucial to probe the evolution of oceanic lithosphere, subduction initiation and mechanism of oceanic-continental transformation. In this paper, we carried out an integrated study of petrology, geochemistry and zircon U-Pb chronology for the newly identified plagiogranite in the Tianshui area of the western part of the Shangdan suture zone. The whole rock geochemistry indicate that the plagiogranite have high content of SiO₂ and Al₂O₃ and low content of K₂O and TiO₂, and show characteristics of low potassium subalkaline, metaluminous to weak peraluminous. In terms of trace element composition, they are relatively enriched in Sr and depleted in Nb, Ta and Ti, and show a characteristic of REE trends is relatively flat. Generally, it has geochemical affinities to oceanic plagiogranite which formed via partial melting of subducted oceanic crust slabs in shallow level and developed in the forearc area. Zircon U-Pb datings of plagiogranite samples yield age of 526±4 Ma, 515±4 Ma and 517±6 Ma, respectively. Combined with previous studies data, we suggest that the Shangdan suture zone of the West Qinling records similar forearc magmatic rock associations as that of the Izu-Bonin-Mariana (IBM) subduction zone. The boninites, High-Mg andesites and plagiogranites generated almost simultaneously in forearc setting, reflecting the initial subduction of the Shangdan ocean at Early Cambrian. Combining the present study with regional geological data reveals that the northern Central China Orogenic system experienced a simultaneous subduction initiation of Proto-Tethys at Cambrian, similar to the scale in the IBM subduction zone.

Abstract:Granite plays an important role in exploring the growth and evolution of continental crust as well as the geodynamics of orogenic belts. The Liqiao granite in the Tianshui area, West Qinling Orogen, is mainly composed of plagioclase, quartz, K-feldspar, and biotite. Zircon U-Pb dating shows that the crystallization age of the granite is 439 ±3 Ma. Geochemistry suggests that the granite contains high silicon and alkali, and it belongs to the peraluminous to high potassium calc-alkaline series. The granite is rich in large ion lithophilic elements (e.g. K and Ba), light rare earth elements (e.g. La and Ce), and depleted in high field strength elements such as Nb, Ta, and Ti, which shows the geochemical characteristics of island arc magmatic rocks. Furthermore, the granite in the Liqiao area shows low Mg^# values (25~41), and the La/Yb-La and La/Sm-La diagrams show that the formation of the granite is related to partial melting. In addition, the existence of microgranular enclaves in the host rock mass and the positive εHf(t) values of zircons indicate that the formation of these granites is influenced by mantle-derived materials. Combined with the regional geological background and the results in this study, we believe that the Liqiao granite was formed by partial melting of the juvenile basaltic lower crust before the arc-continental collision. Certainly, it cannot be ruled out that it was the product of partial melting of the thickened of the continental marginal arc.

Abstract:The systematic investigations and studies of the back-arc basin magmatism can provide important information for exploring the crust-mantle interaction at the converging plate margin, plate subduction dynamics, and the formation-evolution mechanism of back-arc basins. This paper presents a comprehensive study of petrography, zircon U-Pb chronology, rock geochemistry and isotope geology for the Permian mafic rocks recently discovered in the northern Yushu area, and the data could offer key clues to understand the formation mechanism of continental back-arc basins and the origin of mafic magmas. The zircon LA-ICP-MS U-Pb dating results show that the crystallization age of the gabbro is 260±1 Ma.The basalt samples have relatively low TiO₂ content and Nb/Y ratio and relatively high Ce/Nb ratio. Compared with the primitive mantle, those samples are significantly enriched in elements such as light rare earth elements and Th, and depleted of Nb, Ta, Ti, and P, etc., which are similar to the composition characteristics of typical back-arc basin basalts (such as the Okinawa Trough BABBs) in the world. The gabbro samples have relatively high TiO₂ content and Nb/Y ratio and relatively low Ce/Nb ratio. Those samples are significantly enriched in elements such as light rare earth elements and Th, and slightly depleted in Nb, Ta, and Ti, which could be comparable to those of OIB. The basalt samples have variable I_Sr (0.706~0.709) and εNd(t) (-1.8~+1.3), while the gabbros samples have relatively uniform I_Sr (0.709) and εNd(t) (-1.8~-1.5). Based on the results of this study, it can be concluded that the basalts should be derived from a lithospheric mantle modified by subducted components, while the gabbros might be originated from a mantle plume-related source. Combining with new regional studies, we propose that the mafic rocks in the study area should be formed in a continental back-arc basin environment, and the formation of the back-arc basin could be attributed to the slab-rollback caused by the deep oceanic subduction and the activity of the Emeishan mantle plume.

Abstract:The Himalayan orogenic belt represents one of the youngest continent-continent collisional orogenic belts and the collision is still an ongoing process. Eclogites in the Greater Himalayan Crystalline complex contain important information about subduction and exhumation processes of the Indian plate. In this paper, we investigate the petrology, whole-rock geochemistry and zircon chronology of retrograde eclogites in the Nyönno Ri region, Central Himalaya. The objective of our study is elucidate the metamorphic evolution, nature of protolith and tectonic setting of these rocks. The eclogites are enclosed within granitic gneisses and occur as lens or bands. Three metamorphic stages are identified with different mineral assemblages:① peak eclogite-facies:garnet, omphacite, polysilicate muscovite, rutile, and quartz; ② high-pressure granulite-facies:symplectite of clinopyroxene and plagioclase, and symplectite of biotite and plagioclase; and ③ amphibolite-facies:amphibole in the matrix. Zircon grains of the peak eclogite-facies stage has an age of ~15.5~14 Ma and have chondrite-normalized REE patterns with flat HREE and without negative Eu anomalies, whereas zircon grains of the granulite-facies stage have an age of ~12 Ma and show significantly enriched HREE and negative Eu anomalies, indicative of garnet breakdown and plagioclase growth. From the mineral thermobarometry, the peak metamorphic conditions are estimated to be ~730℃ and 1.9~2.1 GPa. The eclogites have a clockwise metamorphic p-T path with the peak eclogite facies metamorphism followed by the near-isothermal rapid decompression, then the granulite-facies metamorphism. The final metamorphism was cooling and decompressing under the amphibolite-facies conditions. The eclogites in the Nyönno Ri region were formed from protoliths of alkaline basalts geochemically similar to mid-ocean ridge basalts (MORBs). Relict magmatic core of zircon grains has a protolith age of 890~850 Ma. In summary, we suggest that the eclogites in the Nyönno Ri region formed from Neoproterozoic MORBs that were subsequently subducted and metamorphosed under the eclogite-facies condition beneath the Eurasian continent at ~15.5 Ma, followed by exhumation.

Abstract:The Dahongshan Group in Xinping County, Yunnan Province, is located in the southwestern Yangtze Block. It mainly consists of volcanic-sedimentary rocks with lower greenschist-amphibolite facies. The lack of systematic research on the petrology, chronological framework and tectonic setting of the Dahongshan Group has hampered a comprehensive understanding of the tectonic evolution history in the southwestern Yangtze Block at <~1.75 Ga. In this paper, meta-sedimentary rocks and intercalated meta-volcanic rocks from the Laochanghe Formation in the bottom of the Dahongshan Group were selected for an integrated study including petrographic observation, whole-rock geochemistry and zircon U-Pb dating. The geochemical data reveal that the meta-sedimentary rocks are chemically close to that of the upper crust sediments. The meta-sedimentary rocks are derived from mudstone/shale that was formed in the tectonic setting of the passive continental margins with high maturity and low degree of sedimentary recirculation. Chemical compositions of the meta-volcanic protolith equates to a calc-alkaline peraluminous A-type rhyolite which formed in the post-orogenic continental rift environment. Zircon U-Pb age data show that the detrital zircons from meta-sedimentary rocks of the Laochanghe Formation document two main age peaks of 2.3~2.2 Ga and 1.9~1.75 Ga and a secondary age peak of 2.7~2.6 Ga. Combined with the previous studies, we proposed that the sources of the Dahongshan Group mainly from the Paleoproterozoic and Archean basement rocks in the southwestern Yangtze Block. The zircon cores of the meta-volcanic rocks define the protolith formation age of the Laochanghe Formation as 1 713~1 711 Ma. Zircon rims of the meta-volcanic rocks constrain the timing of peak metamorphism at 843±5 Ma. Combined with the previous and studies, we conclude that the Dahongshan Group underwent the Neoproterozoic metamorphic event of 849~837 Ma. In summary, the Dahongshan Group well preserved an anorogenic magmatism is related to Columbia supercontinent Cracking event in the southwestern Yangtze Block. The Neoproterozoic metamorphic event may be closely related to the assemblage and break-up of Rodinia supercontinent.

Abstract:The simple mineral assemblage of serpentinite and the extensive thermobaric stability range of its primary mineral, serpentine, pose challenges in determining the thermobaric conditions of the serpentinite. This paper addresses this issue by utilizing high-pressure experiments, establishing that Ti-chondrodite serves as a diagnostic mineral for serpentinite undergoing ultra-high pressure metamorphism. Based on experimental observations, Ti-chondrodite is found to form within the temperature range of 600~700℃, reaching pressures of 3.0~3.5 GPa, signifying its stability at pressures exceeding 3.0~3.5 GPa. However, temperatures above 750℃ lead to a gradual reduction and eventual disappearance of Ti-chondrodite. The experimental conditions define the determination of the upper pressure limit for Ti-chondrodite. Ti-clinohumite typically forms earlier than Ti-chondrodite and persists longer, exhibiting a broader range of stability. In this experiment, a noteworthy observation is the intergrowth of Ti-chondrodite, Ti-clinohumite, and olivine. The observed phenomena in this experiment align with natural occurrences. Under the conditions of low temperature and high pressure, due to the similarity of the structure of these three types of minerals, Ti-clinohumite and olivine usually appear each other's nuclear edge. With the increase of temperature, Ti-clinohumite appears in olivine as patches or lamellae until it finally decomposes. The appearance of Ti-chondrodite depends on the temperature and pressure:under the conditions of low temperatures and low pressures, Ti-chondrodite is unstable and decomposes into Ti-clinohumite, then the phenomenon of Ti-chondrodite coated with Ti-clinohumite appeares; under the conditions of low temperatures and high pressures, the Ti-chondrodite becomes stable, and the Ti-chondrodite develops on the edge of Ti-clinohumite; at high temperatures, Ti-chondrodite is unstable and exists in the form of lamellae in olivine or Ti-clinohumite. This study establishes Ti-chondrodite as an indicator of ultrahigh-pressure metamorphism for serpentinites in cold subduction zones. Detailed petrographic investigations elucidate the behavior of titanium-humite minerals during the metamorphic process, offering a theoretical foundation for the examination of ultramafic rock metamorphism.

Abstract:Zircon is a common accessory mineral in magmatic rocks, sedimentary rocks and metamorphic rocks. Due to its characteristics of high Th and U, zircon has become one of the ideal minerals in the study of geochronology. The principle of the microscopic laser Raman spectroscopy zircon dating method is that the spontaneous α decay of U and Th atoms in zircon causes damage to their own crystal lattice.The accumulation of lattice damage in zircon is positively correlated with the time, and also is correlated with the full width at half maximum Γ of zircon Raman spectrum. The cumulative radiation damage of zircon can be calculated by measuring the full width at half maximum of zircon characteristic peaks with zircon micro laser Raman spectrometer. Thus, the cumulative time of radiation damage can be calculated. Zircon radiation damage dating has the advantages of the high spatial resolution, simple sample preparation and testing methods. However, this method is also subject to various influencing factors, such as the heterogeneity of zircon structure, the rate of thermal annealing, the recrystallization of zircon and radiation damage saturation, all of which would affect the final dating results. The establishment of micro-laser Raman spectroscopy zircon dating method could help to determine the source area of detrital zircon, to reveal the thermal evolution history of rock mass and to identify the later metamorphic thermal events experienced by the inherited zircon. This paper introduces the basic principle, calculation process and age influencing factors of micro-laser Raman spectroscopy zircon dating method. This method is applied for the study of zircon chronology in the metapelites of the Sumdo high pressure metamorphic belt. It provides a new technical method for the zircon chronology.

Abstract:The combination of non-traditional stable isotope (Fe-Cu-Zn-Mo) theory and data has enhanced the understanding of redox processes in geological systems. This paper provides a comprehensive review of this relatively new field, including theoretical and experimental constraints on isotope fractionation behavior related to redox processes, oxygen fugacity at different spatial and temporal scales, and the use of isotopic tracers to study redox processes. Stable isotope theory predicts that Fe-Cu-Zn-Mo isotopes should respond to changes in redox states. The results indicate that Fe isotopes have promising applications as "oxybarometer" for magmatic processes, surface processes, and fluid properties in subduction zones. Cu isotopes can effectively trace redox processes in magmatic, hydrothermal, and terrestrial systems. Zn isotopes, due to their fractionation during complex chelation processes, have been used as sensitive tracers for the migration of sulfur/carbon-bearing fluids in various environments. Mo isotopes serve as paleo-oxybarometer and can be used to reconstruct the ancient ocean-atmosphere redox state effectively.