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• The P–T regime of 6.8–8.0 kbar and 320–380 °C was constrained for the Aksu blueschist based on the phase equilibria modeling first. • An apparent thermal gradient of ∼470 °C/GPa was calculated for the peak stage. • The ages of blueschist facies metamorphism were limited to 750–805 Ma. The first occurrence of blueschist in the Neoproterozoic signifies a transition in thermal regime from a warmer Archean–Proterozoic Earth to modern colder subduction characterized by blueschist facies or ultrahigh pressure metamorphism. To evaluate the thermal regime of the transition, we apply phase equilibrium modeling to the Aksu blueschist in northwestern China to constrain the pressure (P) and temperature (T) conditions of the peak mineral assemblages and to calculate the thermal regime recorded by the blueschist. Phase equilibrium modeling in the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–O system indicates that the peak mineral assemblage (glaucophane/riebeckite + epidote + phengite + albite + quartz + chlorite) was developed at P–T conditions of 6.8–8.7 kbar and 320–410 °C for a stilpnomelane-bearing epidote blueschist (AK04) and of 6.5–8.0 kbar and 310–380 °C for an epidote blueschist (AK06). These P–T data define an apparent thermal gradient of ∼470 °C/GPa for the peak stage which is typical of the high-pressure intermediate type metamorphic facies series. In addition, we present new LA–ICP–MS U–Pb ages from detrital zircons from two blueschists (AK04 and AK06) and SHRIMP U–Pb ages from zircon from a cross-cutting post-metamorphic mafic dyke (AK134) to constrain the age of the metamorphism. The youngest concordant detrital zircon ages give a maximum depositional age for the blueschist protoliths of 806 ± 10 (AK04) and 803 ± 10 Ma (AK06), respectively, and the SHRIMP U–Pb ages of zircons from the mafic dyke yields a weighted mean age of 769 ± 19 Ma interpreted to date crystallization. These results constrain the age of Aksu blueschist facies metamorphism to between 769 ± 19 and c. 805 Ma. [ABSTRACT FROM AUTHOR]

The article presents a study that highlights geochronology constraints on the petrogenesis of gneissic alkali-granite in the Dabie-Sulu ultra-high pressure metamorphic terrain, Donghai County in eastern China. Topics include analysis of zircons in gneissic alkali-granites, the protolith of alkali granite genesis, and rock-forming minerals such as plagioclase, quartz and biotite found in the region.

Abstract: A paired metamorphic belt with ubiquitous migmatites and rare granulites in its high dT/dP part formed due to oceanic subduction in Chinese southwestern (SW) Tianshan. Although several geochronological studies have addressed it in recent years, the exact timing of the high-grade metamorphism and partial melting is still controversial. For this study, we selected nine samples obtained from three valleys transecting the SW Tianshan Migmatite Complex for U–Pb geochronology and Lu–Hf isotope analysis to provide constraints on partial melting and the mid-crustal evolution during oceanic subduction and subsequent continental collision. On the basis of internal morphology, Th/U ratios and REE patterns, three types of zircons are discriminated. Dating them reveals five age groups: (1) Inherited zircons either are detrital, with ages scattering from 432Ma to 3261Ma, or yield ages concentrated at 406–410Ma reflecting Early Devonian magmatism; (2) Recrystallized zircon has been transformed from pre-existing zircon under subsolidus conditions at ~400Ma and ~360Ma; (3) Metamorphic rims have grown on zircons at ~400Ma, ~290Ma and ~270Ma, corresponding to three phases of anatexis. The partial melting phase at ~400Ma is probably related to the emplacement of voluminous mafic to felsic magmas in a continental arc region above the subduction zone consuming the South Tianshan Paleo-Ocean that triggered the most extensive regional-scale thermal event. However, the two subsequent partial melting phases at ~290Ma and ~270Ma representing post-collisional stages were not penetrative. P–T conditions of the studied migmatites estimated by Hb–Pl thermobarometry and Ti-in-zircon thermometry are 682–763°C, 4.7–8.0kbar; together with composite Kfs+Q+Pl, Kfs+Q and Kfs+Q+Ab inclusions representing entrapped melt and Kfs inclusions with Ab exsolution lamellae in zircon rims, the P–T estimates support the occurrence of anatexis. Zircon Lu–Hf isotope analysis indicates that zircon rims formed under high-grade metamorphic conditions and during partial melting by dissolution–precipitation of pre-existing zircons in a close system. Zircon two-stage Hf model ages constrain the development of the protoliths of the migmatites at four major crustal formation periods (~1871Ma, 1368–1426Ma, 1099–1276Ma and 743Ma). Combined with previous researches in this area, we conclude that the main crustal formation of the Yili and Central Tianshan Plates has mostly occurred in Proterozoic times, but not or only subordinately during the Archean. [Copyright &y& Elsevier]

Alkali-feldspar syenites from the Maomaogou area of Huili County in Panxi of Sichuan Province, SW China are sodium alkaline rocks, the minerals of which are mainly composed of calcic ferroaugite, ferroamphibole and biotite. Most of the zircons (>80%) selected for SHRIMP U-Pb dating are magmatic compound zircons and they have relict inner cores and growing outer shells. The age of zircon SHRIMP U-Pb dating is 224±8 Ma, suggesting that the Maomaogou alkali-feldspar syenite was formed in the late Triassic period. At the same time, two old age periods of relict cores of zircon are determined, they are the Archean age (2692–2818 Ma) and Neoproterozoic age (622–691 Ma), respectively. These periods indicate that the cores derived from the crystal basement. Furthermore, the Archean age of zircon provides the first reliable SHRIMP U-Pb zircon dating evidence of >2.8 Ga basement in the Xikang-Yunnan Axis of southwest China. [ABSTRACT FROM AUTHOR]

Detrital single‐grain zircon U–Pb geochronology is a powerful tool for provenance studies if information on the source rocks is available. This paper proposes a new source‐rock classification tool that uses the degree of annealing of radiation damage in detrital zircon; the annealing is expressed by the relationship between the width (full‐width at half‐maximum; FWHM) of the v3[SiO4] Raman band at ~1008 cm−1 and the calculated α‐dose. The host rocks of the zircons are classified into three types according to their emplacement process and/or thermal history: volcanic and rapidly cooled plutonic and high‐grade metamorphic rocks (type 1); rocks with hydrothermal zircons (type 2); slowly cooled igneous and metamorphic rocks (type 3). We construct a naive Bayes prediction model by training it with a collection of zircons of known types. The unknown zircons are assigned a probability of derivation from a specific host‐rock type. This classification scheme is best used as an accessory tool in provenance studies that apply detrital zircon U–Pb geochronology. [ABSTRACT FROM AUTHOR]

Yushugou granulite-peridotite complex, located at the east part of the northern margin of South Tianshan, may represent an ophiolitic slice subducted to 40–50 km depth with high-pressure granulite facies metamorphism. Although a lot of studies have been conducted on rocks in this belt, the rock association and tectonic background of the ophiolitic slice are still in dispute. A detailed study on petrology, phase equilibrium modeling and U-Pb zircon ages have been performed on the metagabbro vein in peridotite unit to constrain the tectonic evolution of the Yushugou granulite-peridotite complex. Three stages of mineral assemblage in the metagabbro were defined as stage I: CpxA+OpxA+PlA, which represents the original minerals of the metagabbro vein; stage II: CpxB+OpxB+PlB+Spl, which represents the mineral assemblage of granulite facies metamorphism with peak P-T conditions of 4.2–6.9 kbar and 940–1 070 °C; stage III is characterized by the existence of prehnite, thomsonite and amphibole in the matrix, indicating that the metagabbro vein may be influenced by fluids during retrograde metamorphism. Combined with the crosscut relationship, it can be deduced that the metagabbro vein, together with the peridotite in Yushugou granulite-peridotite complex has experienced similar high-temperature granulite facies metamorphism. The zircon chronological data shows that the protolith age of the metagabbro vein is 400.5±6.2 Ma, reflecting Devonian magmatism event and the granulite facies metamorphism occurred at ~270 Ma which may be related to the post-collisional magmatism. [ABSTRACT FROM AUTHOR]

Abstract: Zircon and apatite fission-track (FT) analyses were used to reveal the exhumation of Phanerozoic granitoid intrusions in the Dahinggan Mountains, northeastern China. Zircon FT ages ranging from 84±5 to 97±8Ma, and apatite FT ages from 47±3 to 67±4Ma with mean track lengths between 12.3±2.3 and 13.3±2.2μm, respectively, were obtained for the Phanerozoic granitoids sampled along a south–south-west to north–north-east transect across the Dahinggan Mountains. Three cooling stages from the Late Mesozoic to the Cenozoic have been identified in the studied region based on FT results and modeling. Stage one was a rapid cooling period from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110°C) during ∼90–57Ma. Stage two was a relatively slow cooling period (∼110–60°C) occurred between ∼57Ma and 20Ma, suggesting that the granitoids had been exhumed to depths of ∼1–2km. The final stage was a cooling period (60–20°C) occurred since the Miocene, which bring studied samples to the Earth’s surface at an accelerated exhumation rate. The estimated maximum exhumation is ∼5km assuming a steady-state geothermal gradient of 35°C/km. Our results indicate that this exhumation is possibly led by the Pacific plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling. [Copyright &y& Elsevier]

Abstract: Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6±4.7 Ma and 87.3±4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9±3.6 and 37.3±2.3 Ma with mean track lengths between 13.26±0.16 µm and 13.95±0.19 µm whilst AHe ages are marginally younger 47.5±1.9–15.3±0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin. [Copyright &y& Elsevier]

The volcanic rocks of the Xiong'er Group occur widely in the southern part of the North China Craton, which mark the beginning of the cover in the southern part of the North China Craton. The age of the volcanic rocks is thus crucial to understand the tectonic regime and evolutionary history of the North China Craton in the Proterozoic age. Zircons from five volcanic rocks and intrusions were dated by U-Pb SHRIMP method. The results indicate that the Xiong'er Group formed in 1.80–1.75 Ga of Paleo-Proterozoic. Since the Xiong'er Group formed earlier than the Changcheng System, the earliest rocks in the Changcheng System is therefore assumed to be formed in 1.75 Ga. A thermal-tectonic event of ca. 1.84 Ga is indicated by new zircon U-Pb SHRIMP ages in the southern part of the North China Craton. The volcanic rocks of the Xiong'er Group thus represent the initial magmatism of the Paleo-Proterozoic breakup of the North China Craton. Numerous inherited zircons in the volcanic rocks mainly formed in ∼2.20 Ga, indicating that the source magma of the volcanic rocks may be derived from the ∼2.20 Ga crust, or from a mantle magma with significant contamination of the ∼2.20 Ga crust. [ABSTRACT FROM AUTHOR]

High‐pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging‐wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution. [ABSTRACT FROM AUTHOR]

We present new detrital zircon U–Pb chronological and whole‐rock geochemical data for the newly discovered black rock units in West Kunlun Orogen in an attempt to constrain the provenance variations of the siliciclastic rocks and the tectonic history of NW China. Geochemical data indicate these black rock units could have formed in an active continental margin to continental arc setting during the Late Ordovician and source rocks could have undergone modest chemical weathering under a relatively warm and humid climate during the deposition of the black rock units. U–Pb dating of 170 detrital zircons defines five major age populations at 459–545 Ma, 710–900 Ma, 1,300–900 Ma, 1,814–1,641 Ma, and 2,153–2,845 Ma, demonstrating that the black rock units deposited during the Late Ordovician and the tectonic events that occurred in the West Kunlun Orogen were related to the assembly and breakup of the supercontinents of Columbia and Rodinia. Prevalence of Early Palaeozoic magmatic zircons further indicates the black rock units were derived from a proximal source, most likely from the South Kunlun arc. [ABSTRACT FROM AUTHOR]

The Mesozoic Xigaze ophiolite is a key to understanding the tectonic evolution of the Yarlung Zangbo suture zone. Although many studies have been reported, the formation age and petrogenesis of the Xigaze ophiolite remain controversial. In this paper, new geochronological and geochemical data for mafic dikes (diabase, dolerite), lavas, and gabbros of the Xigaze ophiolite are provided to constrain the origin of the Xigaze ophiolite. Combined with previous studies, three new zircon U-Pb ages of samples from two gabbro and one dolerite samples show that the Xigaze ophiolite was produced at two distinct stages of 174-149 Ma and 137-123 Ma. Whole-rock geochemical data indicate that these rocks exhibit N-MORB-like features, but the gabbros are more depleted in trace elements and belong to cumulates. Geochemical characters, combined with their positive εNd(t) values (+3.2 to +9.6), suggest that these samples originated from depleted mantle sources with minor influence of slab-derived fluids. Considering the previous studies on the Yarlung Zangbo suture zone, the Xigaze ophiolite was likely generated in an active continental margin fore-arc basin with a multistage model associated with the northward subduction of the Yarlung Zangbo Neo-Tethys Ocean beneath the Lhasa terrane. The Middle-Late Jurassic ophiolitic massifs (174-149 Ma) were produced as the result of slab rollback and were followed by subsequent slab break-off at ~ 150 Ma. The fore-arc lithosphere may be frozen at ~150-137 Ma, consistent with the termination of the Gangdese arc magmatism during this period. The Early Cretaceous ophiolitic massifs (137-123 Ma) were developed in relation to the reinitiation of the Neo-Tethyan oceanic lithosphere subduction, the retreat of the subduction zone, and the creation of a fore-arc basin with strong hyperextension in a new cycle. [ABSTRACT FROM AUTHOR]

Abstract: As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite‐peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure (HP) granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative felsic granulite samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism (Stage I) with P‐T conditions of 9.8–10.4 kbar at 895–920°C was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism (Stage II) shows P‐T conditions of 13.2–13.5 kbar at 845–860°C, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anticlockwise P‐T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA‐ICP‐MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are ∼430 Ma, with two age groups of ∼390 Ma and 340–350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of ∼390 Ma, which may be related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism (340–350 Ma) may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism. [ABSTRACT FROM AUTHOR]

We present geological, whole-rock geochemical, Sr–Nd isotopic and U–Pb geochronological data for the mafic dykes from the Purang ultramafic massif, South Tibet (China). The mafic dykes are composed of dolerite within a mylonitized harzburgite body. These mafic dykes are dated at 138.0–138.5 Ma using zircon and apatite U–Pb methods, and are characterized by enrichments in large-ion lithophile elements (LILEs) and high-field-strength elements (HFSEs), resembling those of typical ocean island basalts (OIB). They have initial 87Sr/86Sr ratios of 0.70612 to 0.70670, initial 143Nd/144Nd ratios of 0.51231 to 0.51233 and high ε Nd (t) values of −3.1 to −2.6. Geochemical modeling suggests that the parental melts of dolerites were derived by ~50% melting of garnet pyroxenite or eclogite veins in the lithospheric mantle. The formation of the dyke-filling melts can thus be linked to melting of a thin mantle layer containing old low buoyancy plume (LPB)-components in the form of garnet-bearing veins. Our results provide a scenario for the tectono-magmatic evolution of the oceanic lithosphere beneath the Neo-Tethys in the South Tibet. We conclude that the existence of a thermal plume might not be essential to the petrogenesis of the Yarlung-Zangbo Suture Zone (YZSZ) ophiolites. • Dating of dolerite intrusions within the South Purang harzburgites at 138.5 ± 1.8 Ma. • Derivation of dyke-filling melts from a lithopsheric mantle source. • Genesis of dyke-filling magmas by ~50% melting of ancient garnet-bearing veins. • A plume may not be necessary for the petrogenesis of the YZSZ ophiolites. [ABSTRACT FROM AUTHOR]

Abstract: The Dulong-Song Chay tectonic dome lies on the border of China (SE Yunnan Province) and northern Vietnam, and consists of two tectonic and lithologic units: a core complex and a cover sequence, separated by an extensional detachment fault. These two units are overlain unconformably by Late Triassic strata. The core complex is composed of gneiss, schist and amphibolite. SHRIMP zircon U–Pb dating results for the orthogneiss yield an age of 799±10Ma, which is considered to be the crystallization age of its igneous protolith formed in an arc-related environment. A granitic intrusion within the core complex occurred with an age of 436–402Ma, which probably formed during partial closure of Paleotethys. Within the core complex, metamorphic grades change sharply from upper greenschist-low amphibolite facies in the core to low greenschist facies in the cover sequence. There are two arrays of foliation within the core complex, detachment fault and the cover sequence: S1 and S2. The pervasive S1 is the axial plane of intrafolial S0 folds. D1 deformation related to this foliation is characterized by extensional structures. The strata were structurally thinned or selectively removed along the detachment faults, indicating exhumation of the Dulong-Song Chay tectonic dome. The major extension occurred at 237Ma, determined by SHRIMP zircon U–Pb and 39Ar/40Ar isotopic dating techniques. Regionally, simultaneous tectonic extension was associated with pre-Indosinian collision between the South China and Indochina Blocks. The S2 foliation appears as the axial plane of NW-striking S1 buckling folds formed during a compressional regime of D2. D2 is associated with collision between the South China and Indochina Blocks along the Jinshajiang-Ailao Shan suture zone, and represents the Indosinian deformation. The Dulong granites intruded the Dulong-Song Chay dome at 144±2, 140±2 and 116±10Ma based on 39Ar/40Ar measurement on muscovite and biotite. The dome was later overprinted by a conjugate strike-slip fault and related thrust fault, which formed a vortex structure, contemporaneously with late Cenozoic sinistral movement on the Ailao Shan-Red River fault. [Copyright &y& Elsevier]

The article is a study on the Yushugou high-pressure granulite-peridotite complex from the Southern Tianshan mountains in China. They contain a rare association of granulite and peridotite and the article discusses a study proposing a subduction model for the granulites based on the U-Pb zircon dating and petrological study. The high-pressure mafic granulites are grouped into two types namely the hypersthene-bearing and the hypersthene-free granulite.