The Exhumation History of North Qaidam Thrust Belt Constrained by Apatite Fission Track Thermochronology:Implication for the Evolution of the Tibetan Plateau

Determining the spatio-temporal distribution of the deformation tied to the India-Eurasian convergence and the impact of pre-existing weaknesses on the Cenozoic crustal deformation is significant for understanding how the convergence between India and Eurasia contributed to the development of the Tibetan Plateau. The exhumation history of the northeastern Tibetan Plateau was addressed in this research using a new apatite fission track （AFT） study in the North Qaidam thrust belt （NQTB）. Three granite samples collected from the Qaidam Shan pluton in the north tied to the Qaidam Shan thrust, with AFT ages clustering in the Eocene to Miocene. The other thirteen samples obtained from the Luliang Shan and Yuka plutons in the south related to the Luliang Shan thrust and they have showed predominantly the Cretaceous AFT ages. Related thermal history modeling based on grain ages and track lengths indicates rapid cooling events during the Eocene-early Oligocene and since late Miocene within the Qaidam Shan, in contrast to those in the Cretaceous and since the Oligocene-Miocene in the Luliang Shan and Yuka region. The results, combined with published the Cretaceous thermochronological ages in the Qaidam Shan region, suggest that the NQTB had undergo rapid exhumation during the accretions along the southern Asian Andean-type margin prior to the India-Eurasian collision. The Cenozoic deformation initially took place in the North Qaidam thrust belt by the Eocene, which is consistent with the recent claim that the deformation of the northeastern Tibetan Plateau initiated in the Eocene as a response to continental collision between India and Eurasia. The immediate deformation responding to the collision is tentatively attributed to the preexisting weaknesses of the lithosphere, and therefore the deformation of the northeastern Tibetan Plateau should be regarded as a boundary-condition-dependent process.

Subduction channel fluid-rock interaction:Indications from rutile-quartz veins within eclogite from the Yuka terrane,North Qaidam orogen

High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431
3 Ma,which is consistent with the 432
2 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.

Grenvillian and early Paleozoic polyphase metamorphism recorded by eclogite and host garnet mica schist in the North Qaidam orogenic belt

The North Qaidam orogenic belt(NQOB) is generally considered to be an early Paleozoic ultrahigh pressure metamorphic belt,but increasing reports of the Neoproterozoic magmatic and metamorphic events indicate that the NQOB probably also experienced the assembly of the Rodinia.However,the Neoproterozoic evolution of the NQOB is not well constrained due to the sparse records and ambiguous nature of the Neoproterozoic metamorphism.In order to reveal the multi-orogenic history of the NQOB,an integrated study of petrology,phase equilibrium modelling and geochronology was conducted on an epidote eclogite and host garnet mica schist from the Yuka–Luofengpo terrane.New zircon and monazite U–Pb ages show that the protolith of the garnet mica schist was deposited during 994–920 Ma and experienced Neoproterozoic(920–915 Ma) and early Paleozoic(451–447 Ma) polyphase metamorphism together with the enclosed eclogite.Relic omphacite inclusions were first identified in garnet and early Paleozoic zircon domains from the garnet mica schist,which provide solid evidence for the early Paleozoic eclogite facies metamorphism of the mica schist.Similar early Paleozoic peak P–T conditions of >27.4 kbar/613–670 ℃ and 30.2–30.8 kbar/646–655 ℃ were obtained for the garnet mica schist and enclosed eclogite,respectively,indicating that eclogites and their host paragneisses in this region underwent continental deep subduction as a coherent metamorphic terrane in early Paleozoic.The peak P–T conditions of the Neoproterozoic metamorphism were roughly constrained at 7.7–12.0 kbar and 634–680 ℃ for the garnet mica schist,based on stability field of mineral inclusions in Neoproterozoic zircons domains in P–T pseudosection,the relic garnet core composition and Ti-in-zircon thermometer.The high thermal gradients(16–37 ℃/km) defined by presently our and previously reported P–T conditions indicate that the Neoproterozoic metamorphism likely occurred in continental collision setting at >945–890 Ma.Since the Grenvillian syn-orogenic granitic magmatism and metamorphism(ca.1.0–0.9 Ga) in the NQOB are much younger than the Grenvillian orogeny in the central part of Rodinia,the Qaidam Block was probably located at the north margin of Rodinia in Neoproterozoic.

Early Paleozoic Ocean in the North Qaidam: Constraints from Kaipinggou Ophiolite

The North Qaidam orogenic belt(NQOB) lies at the northeast margin of Tibet Plateau, between the Qilian massif to the north and Qaidam massif to the south. It consists of predominantly gneisses and marbles with minor eclogite and garnet peridotite, and is known as an Early Paleozoic continental deep subduction ultrahigh pressure(UHP) metamorphic belt in northwest China. Recently some studies suggest that the NQOB experienced oceanic deep subduction prior to the continental deep subduction. However, the records of ocean events are rarely preserved because of the complex multi-stage transformation.Therefore, whether the Early Paleozoic ocean exists and the formation, nature and evolution of the Early Paleozoic ocean in North Qaidam are still in controversies. In this contribution, we report a new discovered SSZ-type ophiolite suite in Kaipinggou area, the western segment of the NQOB. The ophiolite is mainly composed of peridotite, gabbro,basalt, boninite, andesitic porphyrite and plagiogranite. In which,the basalt and homogeneous gabbro are both enriched in LILE and depleted in HFSE, and display flat or LREE-depleted REE patterns, similar to the MORB-like geochemical characteristics of fore-arc basalts;the boninite exhibits characteristics of forearc boninites with high Si and Mg, but low Ti and Ca contents;the andesitic porphyrite is enriched in LILE but depleted in HFSE, similar to island arc magmatic rocks. Rock assemblage and their geochemical characteristics suggest that this SSZ-type ophiolite was formed in fore-arc setting of intra-oceanic arc. The plagiogranite is a low-potassium tholeiite metaluminous granite with low trace elements contents, slight enrichment of LREE,depletion of HREE and positive Eu anomaly;combining with εHf(t) values(13.8–17.1) of zircons, the plagiogranite could be originated from partial melting of gabbros. LA–ICP–MS zircon U–Pb dating yielded the crystallization ages of 535±2 Ma, 513±3 Ma and 510±3 Ma for two cumulate gabbros and one homogeneous gabbro, respectively, and a formation age of 492±8 Ma for the plagiogranite. All these data suggest the existence of an Early Paleozoic ocean in the North Qaidam, and the initial subduction of the oceanic slab occurred earlier than 535 Ma.

An Early Paleozoic Ultramafic Complex in the North Wulan Metamorphic Complex,North Qaidam:Contraints on the Nature of the Alaskan-type Continental Arc Root

Orogenic peridotite is an important component of orogenic belts and retains crucial information on mantle magmatic activity,slab subduction,and melt or fluid metasomatism.To determine the source of the mantle-derived parental magma of the peridotite and to investigate the metasomatism that it experienced,we undertook an integrated study of the petrography,whole-rock major-and trace-element compositions,in situ zircon U-Pb geochronology,and mineral majorand trace-element compositions of an early Paleozoic ultramafic complex in the North Wulan area of North Qaidam.The Halihatu ultramafic-mafic complex is composed of dunite,pyroxene peridotite,and gabbro,which are characteristic of Alaskan-type complexes.The dunite yields a weighted mean^(206)Pb/^(238)U age of 479±5 Ma(MSWD=0.7),which reflects the age of the metasomatism rather than the crystallization age of the ultramafic magma.The peridotites have high Mg^(#)(89.8-91.8)and Cr contents(2419-5190 ppm),low Al_(2)O_(3)(0.20-1.68 wt%)and Ni(289-1012 ppm)contents,and high olivine Fo contents(87-91),suggesting a large degree(~15%-22%)of partial melting of lithospheric ultramafic rocks followed by variable degrees of fractional crystallization of olivine and pyroxene.This is consistent with estimates of 15%-22.3%partial melting calculated using the Cr#of spinel crystals and with the low Yb(0.04-0.33 ppm)and Y(0.72-1.29 ppm)contents of clinopyroxene crystals.Whole-rock trace-element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements,along with high Al_(2)O_(3)(2.10-6.47 wt%)and low TiO_(2)(0.01-0.21 wt%)contents of clinopyroxene crystals,suggesting an arc magma cumulate trend.These features,along with the high olivine Fo contents(87-91 ppm),imply that the Halihatu peridotite is an Alaskan-type crustal cumulates derived from Mgrich hydrous basaltic melts.The high estimated f O_(2)(FMQ+1.97 to FMQ+3.81)further supports the idea that they formed in an arc setting.The Ni/Co and Ni/Mn ratios and cumulate textures of the olivine,quenched boundaries between mafic and felsic melts,and the occurrence of tremolite and phlogopite reflect interactions between the Halihatu peridotite and injected silicate and carbonatitic melts in the lower crust.Therefore,we propose a new cumulate-infiltration model for the petrogenesis of Alaskan-type ultramafic complexes,which improves our understanding of the nature of Alaskan-type continental arc root.

Mantle Response to Slab Breakoff in the North Qaidam Tectonic Belt: Geochemical Constraints from Syn-subduction Mafic Igneous Rocks

Slab breakoff originally denotes the detachment of dense subducted oceanic slab from the light subducted continental slab, which is driven by opposing buoyancy forces during continental collision(Davies and von Blanckenburg, 1995;von Blanckenburg and Davies, 1995). The breakoff of subducted oceanic slab can induce the upwelling of sub-slab asthenosphere through the slab window, and then heat the overriding lithospheric mantle resulting in the melting of its fertile layer within the metasomatic mantle wedge. The decompression partial melting of uprising asthenospheric mantle commonly produce mafic magma with depleted MORB-like geochemical signatures(Davies and von Blanckenburg, 1995;Cole et al., 2006;Wang et al., 2018), whereas the partial melting of enriched lithospheric mantle will produce mafic magma with alkaline, calc-alkaline or ultrapotassic features(von Blanckenburg and Davies, 1995). These mafic magmas rise into overlying lower crust and trigger crustal melting to generate the granitic magma. The North Qaidam tectonic belt(NQTB) records the evolutionary process of the South Qilian Ocean from subduction to closure. The subduction of oceanic and continental lithosphere to mantle depths is proven by the identification of oceanic-type and continental-type eclogites enclosed in crustal metapelite and gneiss from the North Qaidam tectonic belt(Song et al., 2006;Zhang et al., 2008;Zhang et al., 2010;Zhang et al., 2017). However, details of this process are not exactly constrained, in particularly, the closure timing of South Qilian Ocean. The study of characteristic mafic magmatism, combined with the previous studies of ultra-high pressure metamorphism, give us an excellent opportunity to trace the detailed processes associated with the transition from oceanic subduction to continental subduction, and assess the feasibility of slab breakoff in the North Qaidam tectonic belt. In this contribution, an integrated study of petrology, geochemistry, geochronology and Sr-Nd-Hf isotopes is performed on the mafic igneous rocks from Chahanhe area in the North Wulan gneiss complex. These mafic igneous rocks can be divided into two groups, namely, arc-like type and E-MORB type based on their trace element patterns. Arc-like mafic rocks(441–428 Ma) were characterized by enrichment of light rare earth elements(LREEs), large ion lithophile elements(LILEs) and depletion of heavy rare earth elements(HREEs), high field strength elements(HFSEs). Combined with variable zircon εHf(t) values of-6.17 to +1.58, it is suggested that arc-like mafic rocks are predominantly derived from the partial melting of the enriched lithospheric mantle, and minor juvenile materials have contributed to their sources. The E-MORB mafic rocks(440 Ma) exhibit relatively flatted REE patterns and positive εNd(t) values of +1.63 to +4.28, but high(87Sr/86Sr)i ratios of 0.706825 to 0.708979, indicting a derivation from partial melting of asthenospheric mantle, with involvement of enriched components probably derived from ambient lithospheric mantle or stagnant subducted oceanic crust. Collectively, it is proposed that the break-off of the subducted South Qilian oceanic slab triggered the decompression melting of asthenospheric mantle, and the upwelling of asthenosphere provided heat and induced partial melting of the enriched lithospheric mantle and preexisting crust, resulting in generation of arc-like mafic rocks and widespread granites.

He Grenville Orogenesis Recorded by Monazite from the Paragneiss of North Qaidam UHP Metamorphic Belt, Western China

The poly-phase orogeny information included in one orogenic belt is the key for studying the regional tectonic evolution at different time period.It also has important significance of understanding the rock association and

THE ALTUN—NORTH QAIDAM ECLOGITE BELT IN WESTERN CHINA—ANOTHER HP-UHP METAMORPHIC BELT TRUNCATED BY LARGE SCALE STRIKE-SLIP FAULT IN CHINA

The Altun and North Qaidam Mountains at the northern margin of Qinghai\|Tibet plateau are separated by the Altyn Tagh sinistral strike\|slip fault, which is one of the largest strike\|slip fault systems in the world and was considered as the key element in the escape tectonics model for Euraisa\|India continent\|continent collision.Recently,the eclogites within quratzifeldspathic gneisses or pelitic gneisses characterized by amphibolite\|facies paragenesis were discovered in the Altun and the North Qaidam Mountains(Fig.1). They occur as lens or boundins within the Altun Group and Dakendaban Group respectively which previously were considered as metamorphic basement of Tarim block and Qaidam block. Our studies indicate that the eclogites outcrop in both the Altun and North Qaidam Mountains show similar occurrences, associated country rocks, rock and mineral assemblages, p\|T\% estimates, geochemistryand protolith feature and ages of peak metamorphism (see table) . The garnet\|omphacite\|phengite geothermobarometer gave equilibrium condition of \%p\%=2 8～3 0GPa and t =820～850℃ for the Altun eclogite and p =2 8GPa and \%t\%=730℃ for North Qaidam eclogite respectively(Fig..2). These p\|T conditions are in the coesite stability field. Moreover, Po lycrystalline quartz pseudomorphs after coesite have been identified in the Dulan area, North Qaidam Mountains (Song et al, in review). Therefore, these features suggest that both eclogites of Altun and North Qaidam Mountains probably are a same HP\|UHP metamorphic belt formed from the same of Early Paleozoic age deep subduction and collision, and subsequently displaced by the Altyn Tagh fault.The case is similar to the Dabie\|Sulu HP\|UHP metamorphic zone which was truncated by the Tanlu sinistral strike\|slip fault and splitted it into two distincts, the Dabie region and Sulu region. These correlations support an about 350～400km displacement of the Altyn Tagh sinistral strike\|slip fault (Fig.1).

Zircon Record of Ocean-Continent Subduction Transition Process of Dulan UHPM Belt, North Qaidam

The North Qaidam UHPM（ultra-high pressure metamorphism） belt is a typical continental subduction-collision belt containing continental crust deep subduction metamorphic products and oceanic crust relics, And it is an ideal region to study the ocean-continent transition and exhumation mechanism of oceanic UHP rocks during continental deep subduction process. In this paper, we report integrated in situ U-Pb, Lu-Hf and O isotope analyses of zircons from a serpentinized harzburgite as well as U-Pb dating for zircons from a kyanite eclogite from the North Qaidam Dulan UHPM terrane, and use these data to discuss the ocean-continent transition and exhumation mechanisms of oceanic UHP rocks during continental deep subduction. The serpentinized harzburgite was dated at 448±9 Ma, consistent with 455±5 Ma age for the kyanite eclogite within analytical errors. Zircons from the serpentinized harzburgite have uniform 176Hf/177 Hf values ranging from 0.282 842 to 0.282 883 and εHf（t） values from 11.6 to 13.3. Zircon δ^18O values of the serpentinized harzburgite vary from 4.47‰ to 5.29‰, slightly lower than the value of 5.3‰±0.6‰ for the normal mantle zircon. These Hf-O isotopic features indicate that the protolith of the serpentinized harzburgite was derived from depleted-mantle source, and might have experienced high-temperature rock-water interaction. Therefore, the serpentinized harzburgite was possibly located in the lower part of an oceanic section. The serpentinized harzburgite and kyanite eclogite were both formed due to the subduction of oceanic crust. The UHP metamorphism occurred successively from the oceanic crust to continental crust rocks of the North Qaidam UHP terrane. Low-density serpentinized peridotite and continental rocks possibly have negative buoyancy and play a key effect on preservation and exhumation of high-density oceanic eclogite.

Geochemistry and zircon SHRIMP U-Pb dating of granitoids from the west segment of the North Qaidam

Granitoid intrusives such as Saishitenshan, Tuanyushan, Aolaohe and Sanchagou occur widely in the western segment of North Qaidam. All these bodies trend NW, roughly parallel to the regional structure. Zircon SHRIMP dating for these granites show that they range in age from Ordovician to Permian; 465.4±3.5 Ma for Saishitenshan, 469.7±4.6 Ma and 443.5±3.6 Ma for Tuanyushan, 372.1±2.6 Ma for Aolaohe, and 271.2±1.5 Ma and 259.9±1.2 Ma for Sanchagou. Both the Tuanyshan and Aolaohe plutons record two distinct intrusive events. Geochemically, the early Paleozoic granites have an island arc or active continental margin affinity, and their protolith may have been Mesoproterozoic oceanic crust derived from depleted mantle. The protolith of the late Paleozoic granites may have been Mesoproterozoic lower crust from the root of an island arc with the magmas reflecting a mixture of mantle and crustal material.

Coesite in eclogite from the North Qaidam Mountains and its implications

Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a pali-sade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7―3.25 GPa, 670―730℃). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism.

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

The geological characteristics of ultrahigh-pressure (UHP) metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithic assemblage of typical oceanic crust, the peak metamorphic temperature <600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, respectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler subducted into the mantle depth and then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior and cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alps and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.

Zircon U-Pb SHRIMP dating of the Yematan batholith in Dulan, North Qaidam, NW China

The Yematan batholith crops out over 120 km2 in the North Qaidam ultrahigh pressure (UHP) metamorphic belt. It consists of granodiorite, monzogranite and biotite granite and forms an irregular intrusion into Neoproterozoic gneiss that has undergone Caledonian UHP metamorphism. Zircons from the Yematan granodiorite yield a SHRIMP U-Pb age of 397? Ma. These granitic rocks have geochemi- cal characteristics intermediate between I- and S-type gran- ites, and are post-collisional. We suggest that the Yematan granitic rocks were formed during the last exhumation event of the North Qaidam UHP belt.

Genesis of the mega-crystal zircons in the dunite veins of North Qaidam Mountains,northwestern China

A megacrystal zircon occurs in serpentinized dunite veins from North Qaidam, northwestern China. It is red-brown and 3―4 mm long and 2 mm wide in size. BSE imaging shows the presence of micro-grains (30―50 μm) of zircon between olivine and serpentine, part of them gathering into clusters (100―300 μm). Thus we infer that these zircon crystals were precipitated from hydrothermal fluid. Inclusions of baddeleyite (white) in the shape of isolated island locally occur in the large zircon (grey), implying that the former formed earlier than the latter. The mirco-baddeleyite (50―100 μm) aggregate consists of baddeleyite (3―5 μm) and olivine (<5 μm), indicating the zircon formation through interaction between baddeleyite and silica-bearing fluid. Therefore, we consider a genetic link between the formation of the large zircon and the serpentinization of dunite. SHIRIMP U-Pb dating of the large zircon gave two groups of concordant ages at 414 ± 13 Ma and 371 ± 9 Ma, respectively. This might reflect two episodes of zircon growth in response to two episodes of fluid action.

Metamorphic Evolution and Tectonic Implications of the Granulitized Eclogites from the Luliangshan Terrane in the North Qaidam Ultrahigh Pressure Metamorphic Belt,NW China: New Constraints from Phase Equilibrium Modeling

The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites:(1) an earlier eclogite facies metamorphism(P>18.5 kbar, T> 830℃) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz;(2) the early stage of high pressure granulite facies assemblages(garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 ℃, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages(garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 ℃;(3) the later medium pressure granulite facies assemblage(garnet+ orthopyroxene+clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 ℃;(4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of <5 kbar and <650 ℃. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade "Barrovian" metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a "hot" HP-UHP terrane.

Reconstruction the Process of Partial Melting of the Retrograde Eclogite from the North Qaidam,Western China: Constraints from Titanite U-Pb Dating and Mineral Chemistry

Retrograde eclogite and garnet amphibolite of the Lüliangshan unit of the Shenglikou area,North Qaidam,were studied with emphasis on rutile and titanite.A special focus is on the formation of rutile and its corona of titanite(Ttn1)in retrograde eclogite and on coarse-grained titanite(Ttn2)from the garnet amphibolite.Using zirconium(Zr)-in-rutile and Zr-in-titanite thermometers,the temperatures estimated for the formation of an early generation of rutile are 823–884℃at 2.5–2.8 GPa,while 812–894℃at 1.3–1.5 GPa are derived for the formation of coronitic Ttn1 in the retrograde eclogite.Therefore,isothermal decompression must have occurred during exhumation,which also has triggered the partial melting of the retrograde eclogite.Ttn2 of the garnet amphibolite has high REE contents and high Th/U ratios,indicating that it is newly grown from a Ti,Ca,and LREE enriched anatectic melt derived from the partial melting of retrograde eclogite.LA-ICP MS U-Pb dating yields a lower intercept age of 423±4 Ma for Ttn2,which is consistent with the granulite-facies metamorphic age of the retrograde eclogite.Moreover,a temperature of 781–823℃at 1.0–1.2 GPa is obtained for Ttn2,which fits the P-T conditions of the HP granulite-facies metamorphic stage(P=1.07–1.24 GPa and T=774–814℃),and documents that the crystallization of the melt occurred at the granulite-facies stage at 423 Ma.The high amount of REE of the garnet amphibolite is a consequence of the formation of Ttn2 from the melt.The contents and ratios of Zr and Hf in rutile and Ttn2 differ from those in the garnet amphibolite,and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios in rutile and Ttn2,suggesting that rutile and titanite cannot be the major carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.

Geochemical Features and Tectonic Settings of Upper Ordovician Marine Volcanic Rocks on North Margin of Qaidam

The geochemical and petrological features show that the Upper Ordovician marine volcanic rocks on north margin of Qaidam can be classified into three main kinds of petrotectonic assemblages. The three assemblages exhibit features of tectonic melange. And they indicate that there exists an ancient ocean basin on north margin of Qaidam during Late Ordovician period. At the last stage of Ordovician period, the ocean basin on north margin of Qaidam closed gradually, then Qaidam continental block subducted continually and Qilian continental block thrusted onto Qaidam continental block in the direction from north to south, formed the orogenic belt of continent to continent collision type. As a result, a three unit tectonic model is proposed. The three units are: the overlying Qilian crustal wedge, the collision tectonic melange belt on the north margin of Qaidam and the Qaidam underthrusted wedge.

柴北缘孤山一带侵入岩U-Pb年代学约束及构造环境特征

研究区位于祁连山脉南西部,岩体呈不规则的残块状、条带状混杂侵入于早元古代达肯大坂群中—深变质岩中,现已变质为中细粒二云花岗片麻岩。岩石中TiO_(2)质量分数为0.61%,Al_(2)O_(3)质量分数为12.5%,Na_(2)O和K_(2)O质量分数共7.43%,Fe_(2)O_(3)质量分数为0.94%,具有高Ti、Al、Fe和富钾的特征。锆石U-Pb加权平均值为766.1 Ma±5.4 Ma,侵位于新元古代南华纪。侵位方式为被动的岩墙扩张,为火山弧型花岗岩,形成于同碰撞构造环境,可能为I-S型花岗岩。

柴北缘中酸性侵入岩成因及其构造意义

研究区位于柴北缘缝合带与磨拉石前陆盆地的结合部,区内构造经历了多期演化,岩浆岩广泛发育。岩体侵入于下元古界金水口岩群和寒武—奥陶纪滩间山群中,岩性为闪长岩、石英闪长岩、花岗闪长岩,属次铝质钙碱性岩系。岩石地球化学特征表明,该岩体具有火山弧花岗岩的特征,物源具有地壳和地幔的成分,并具同源岩浆演化特征。研究区主体属加里东期构造岩浆岩带,形成于板块碰撞前的构造环境,花岗岩结晶温压条件为640~720℃、0.1~0.3 GPa。

柴北缘西段落凤坡南榴辉岩的冷却史:锆石和金红石年代学及微量元素温度计制约

金红石是一种重要的战略矿产资源,榴辉岩型金红石矿床是其重要来源之一。柴北缘西段鱼卡-落凤坡高压-超高压变质地体是榴辉岩型金红石的远景区之一,但该类型金红石的保存条件仍不清楚。本文选择在落凤坡以南沙梁沟地区新发现的榴辉岩透镜体,对榴辉岩和榴闪岩进行了LA-ICP-MS锆石和金红石U-Pb定年及微量元素分析。榴闪岩中锆石给出了435±2Ma(MSWD=0.26)的加权平均年龄,锆石内部结构和较为平坦的重稀土配分曲线((Lu/Gd)_(N)=2.08~8.56)以及无明显的Eu负异常(δEu=0.59~0.81)表明该年龄代表榴辉岩相变质时间,锆石Ti含量温度计给出了643±27℃的峰期变质温度。榴辉岩中金红石给出了410±8Ma的回归下交点年龄,这代表了榴辉岩从峰期变质时代冷却到金红石封闭温度约570℃的时代,对应的金红石Zr温度计计算结果给出618±1℃的峰期变质温度。由此可以得出榴辉岩在折返过程中的冷却速率为2~3℃/Myr,低于鱼卡榴辉岩的冷却速率(~4℃/Myr)。榴辉岩规模,矿体品位等证据显示落凤坡南榴辉岩比鱼卡榴辉岩中的金红石退变强,形成钛铁矿或榍石。这表明了较缓慢冷却不利于榴辉岩中金红石的保存。