Belt Of Rocks Research Articles

Petrogenetic evolution of early Paleozoic trachytic rocks in the South Qinling Belt, Central China: Insights from mineralogy, geochemistry, and thermodynamic modeling

Early Paleozoic volcanic–intrusive rocks in the South Qinling Belt, central China, consist of bimodal alkaline suites ranging in composition from basalt to trachyte, with an apparent scarcity of intermediate silicic components (the so-called “Daly gap”). A series of trachytic rocks have been identified in the Quanxi area of the South Qinling Belt. In this work, we combine mineralogical evidence with whole-rock major- and trace-element data, Sr–Nd–Pb isotope data, and the results of thermodynamic modeling to gain a better understanding of the petrogenesis and evolutionary process of the Quanxi trachytic rocks, as well as their compositional discontinuity with coeval mafic rocks. Our samples can be clearly classified into two lithological groups based on their phenocryst content: Group 1, phenocryst-poor trachytes (<5 vol% phenocrysts; SiO2 = 63.91–66.13 wt%) and Group 2, trachytic tufflavas (30–45 vol% pyroclasts; SiO2 = 53.34–59.03 wt%). All of the samples exhibit typical oceanic island basalt-like enrichment in light rare earth elements and high field strength elements, as well as marked depletion of Ba, Sr, and Ti. A cogenetic metasomatized lithosphere source for the Quanxi trachytic rocks and contemporaneous mafic rocks is indicated by their homogeneous isotopic features (εNd(t) = +2.3 to +3.4; initial 206Pb/204Pb = 17.92–18.69; initial 207Pb/204Pb = 15.53–15.57; initial 208Pb/204Pb = 37.96–38.99) and linear geochemical variations, with protracted fractional crystallization regarded as the key mechanism for the compositional variations among these cogenetic rocks. The results of Rhyolite-MELTS modeling predict 72%–80% crystallization involving plagioclase, pyroxene, biotite, apatite, and FeTi oxide, to yield the magmatic compositions from an evolved mafic progenitor to the Quanxi trachytes under conditions of low pressure (1.5–2 kbar), high H2O content (2–3 wt%), and high fO2 (FMQ–0.5). Some incompatible elements in trachytic tufflavas deviate markedly from the liquid lines of descent from coeval mafic rocks to the Quanxi trachytes: this deviation is attributed to a combination of the involvement of large proportions of alkali-feldspar pyroclasts and post-magmatic hydrothermal alteration. The presence of the “Daly gap” in early Paleozoic alkaline suites can be explained by an accelerated differentiation rate (i.e., dSiO2/dt) in the SiO2-intermediate field, which is driven by the simultaneous separation of a considerable scale of SiO2-poor phases.

U-Pb zircon and titanite age of the Christiansø granite, Ertholmene, Denmark, and correlation with other Bornholm granitoids

A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.

Late Neoarchean geodynamic regime of the northeastern North China Craton: Constraints from metamorphosed volcanic rocks of the Anshan-Benxi greenstone belt

The Anshan-Benxi greenstone belt in the northeastern North China Craton preserves ∼ 3.88 Ga to ∼ 2.5 Ga lithologies that exhibit based secular trends in their geological signatures and hosts large amounts of valuable Neoarchean banded iron formations. However, whether plume or subduction mechanism responsible for those iron deposits remains confusing. This contribution focuses on the metamorphosed volcanic rocks and aims to constrain their geodynamic regime, based on temporal and spatial distributions, petrogenesises and tectonic characteristics. The late Neoarchean metamorphosed volcanic rocks of the Anshan-Benxi greenstone belt are prominently exposed in the Waitoushan-Gongchangling-Benxi area and consist of plagioclase amphibolites, amphibole/mica plagioclase gneisses and amphibole/mica two-feldspar gneisses. All 107 metamorphosed volcanic rock samples may be geochemically and petrographically classified as tholeiites, tholeiitic to calc-alkaline basalts, andesite, and dacite-rhyolite. The tholeiites are characterized by approximately flat REE patterns with negligible Eu anomalies and show negative Nb anomalies on primitive normalized multiple element diagrams, consistent with a back-arc basin basalt origin. The tholeiitic to calc-alkaline basalts have slightly heavy REE-depleted patterns and obvious negative Nb anomalies, which were likely produced by partial melting of mantle peridotites that were metasomatized by subducted slab-released fluids and melts. They may be comparable to island arc basalts. The andesites exhibit notably high MgO, Mg# and transition metal element contents, low FeOT/MgO ratios, fractionated REE patterns and evident negative Nb, Ta and Ti anomalies, which are akin to Phanerozoic high magnesian andesites and are melts of mantle peridotites that were previously metasomatized by subducted slab melts/fluids. The dacite-rhyolites have the highest SiO2 and lowest MgO, Cr and Ni contents and were derived from intracrustal remelting.The zircon U-Pb dating results reveal that the arc tholeiitic to calc-alkaline basalt-andesite-dacite-rhyolite assemblages erupted at ∼ 2.57 Ga – ∼2.52 Ga and back-arc basin basalt lithological association at ∼ 2.55–2.52 Ga. Furthermore, both the arc basalt-andesite-dacite-rhyolite and back-arc basin basalt associations are predominantly exposed within the Waitoushan-Gongchangling-Benxi range of the northeastern Anshan-Benxi greenstone belt. Therefore, we suggest that the Waitoushan-Gongchangling-Benxi area represents a rear-arc to back-arc zone, which was subsequently subjected to consistent NE-SW compression (σ1) and top-to-northeast kinematic features during back arc closure and underwent up to high amphibolite facies metamorphism. However, the Anshan area in the southwestern range of the Anshan-Benxi greenstone belt preserves large amounts of thick-bedded metasedimentary rocks instead of back-arc and arc volcanic rocks, which indicates a stable depositional environment in a typical back arc basin depositional zone. The supracrustal rocks within the back arc basin depositional domain underwent NE-SW compression (σ1) and top-to-southwest kinematic features and mostly experienced greenschist facies metamorphism.These crust-mantle interaction features inferred by metamorphosed volcanic rocks in the Anshan-Benxi greenstone belt, spatial and temporal variations in the lithological assemblages, together with the style of structural deformations, indicate a late Neoarchaean geodynamic evolution from SW-dipping subduction to arc-back-arc development and collisions between ancient continental blocks and arcs.

Provenance of the early Cretaceous sediments in the southern Alxa Area: Implications for tectonic evolution of the south Central Asian Orogenic Belt

The Central Asian orogenic belt is an accretionary orogenic belt that lies between the Siberian block and the Tarim and North China blocks. Since the central south of the Central Asian Orogenic Belt has been covered by desert and the Meso-Cenozoic Bayingobi Basin, its spatial configuration, evolution, closure time, and the Paleo-Asian ocean location have not been well described. Through zircon UPb dating, in-situ Hf isotope and rock geochemistry of the Lower Cretaceous Bayingobi Formation sandstones, the provenance of the Bayingobi Formation, and the central south boundary of the Central Asian orogenic belt were determined, the tectonic evolution process was discussed, and the tectonic evolution models were established. The provenance of the Bayingobi Formation mainly originated from Permian to Early Triassic granodiorite, diorite and potassium feldspar granite that cover a large area of the Zongnaishan-Shalazhashan tectonic belt. In the early Permian, this tectonic belt was mainly a continental island arc but gradually transitioned to a continental active margin, accompanied by intense tectonic magmatic activity. With the boundary as the Quagan Qulu ophiolite belt, the north is the Central Asian Orogenic Belt that mainly exhibits juvenile crustal materials, while the south is the Alxa block, which has an affinity with the North China block, and mainly exhibits older crustal materials. From the late Carboniferous to the early Permian, the Paleo-Asian Ocean plate was compressed, subducted along the Enger Us suture, and magmatic distributed along the Nuru-Langshan tectonic belt; from the late early Permian to late Permian, the Paleo-Asian Ocean plate rolled back, changing the subduction zone stress state from compression to extension. During this period, there were large-scale developments of magmatic rocks in the Zongnaishan-Shalazhashan tectonic belt. From early Early Triassic to Middle Triassic period, the Siberian active margin collided with the Alxa block, the Paleo-Asian Ocean was closed, and the overlying Alxa plate experienced lithospheric extension and thinning, accompanied by asthenospheric upwelling, partial melting and the addition of new crustal material. The Meso-Cenozoic basin sediments along the original suture belt are characterized by an enrichment of large ion lithosphere elements, a loss of high field strength elements, LREE enrichment and loss of HREE.

Arc tempos of the Gangdese batholith, southern Tibet

The character of arcs varies over time with significant temporal fluctuations in the quantity and spatiotemporal patterns of magmatism. However, the driving mechanisms for this episodic behavior of arcs need more constraints. This paper analyzed the published data along with our new zircon U-Pb dating and Hf isotopic and whole-rock geochemical data of plutonic rocks in the Gangdese belt in southern Tibet to explore the features, potential drivers, and tectonic implications of episodic arc activity in the Gangdese arc. A comprehensive compilation of U-Pb ages and Lu-Hf isotopic analyses of zircon grains from igneous rocks in the Gangdese belt, sedimentary rocks in trench fill sequences, forearc basins and foreland basins, and sands from modern river reveals that: 1) Gangdese arc activity was episodic during Late Cretaceous to Middle Eocene, displaying two magmatic flare-ups (ca. 100–80 and 65–45 Ma) and one magmatic lull (ca. 80–65 Ma), and 2) both flare-up magmas show relatively positive εHf(t) values (+5 ~ +15) indicative of juvenile sources suggesting these magmas are dominated by contributions from the depleted mantle. In contrast, the magmatic lull between these two magmatic flare-ups could be caused by flat subduction of the Neotethyan slab beneath the southern margin of the Lhasa terrane. These flare-ups likely contributed greatly to the crustal thickening of the Gangdese belt. Constraints from paleo-elevation and geochemical proxies for crustal thickness showed that the ~100–80 Ma flare-up was accompanied by the formation of a thick arc root while the ~65–45 Ma flare-up likely developed in a thinner crust without an arc root.

Petrogenesis of Late Ordovician intrusive rocks in the West Kunlun orogenic belt: Constraints on the subduction process of the Proto-Tethys Ocean

Petrogenesis of Late Ordovician intrusive rocks in the West Kunlun orogenic belt: Constraints on the subduction process of the Proto-Tethys Ocean

Latitudinal land–sea distributions and global surface albedo since the Cretaceous

We estimate global surface albedo from the areal proportion of land to sea in climatically-significant latitudinal belts at ten million-year intervals for the Late Cretaceous and Cenozoic (120 million years ago to Present) using modern plate tectonic reconstructions and a composite apparent polar path designed to minimize known biases in the determination of paleolatitude. We find that global surface albedo stayed almost constant until it shifted 30% higher to the modern value of around 0.15 with the inception of the Late Cenozoic Ice Age 34 million years ago, reflecting polar ice-albedo amplification of global cooling resulting from the reduction of greenhouse gases below a critical threshold, most probably as the culmination of enhanced CO 2 weathering consumption of continental mafic rocks in the tropical humid belt. The contribution from cloud cover toward a planetary albedo is unclear in the absence of measurable proxies but might eventually be gauged from the role cloudiness evidently plays in maintaining radiative balance with the increasing land bias between northern and southern hemispheres over the Cenozoic. • Changes in land–sea area were small in equatorial and polar climate belts from 120 Ma • Land area decreased in temperate belts and increased in subtropical arid belts • Global surface albedo was low from 120 to 40 Ma and shifted to modern value by 30 Ma • Increase in surface albedo due to polar ice cover with Late Cenozoic Ice Age at 34 Ma • Ice age reflects polar amplification of global cooling triggered by drawdown of CO 2

Boudin Types, Their Morphology and Significance in Interpreting Deformational History in Proterozoic Rocks of North Delhi Fold Belt, North Western India

The boudins can be of symmetric as well as asymmetric type and can help in determining shear sense and help unfold the deformational history, hence are important structure for recognizing the history of deformation in an area; Once the rocks of an area undergo polyphase of deformation, the boudins get modified and can no longer help in determining the deformational evidences. The present study involves identification of various types of boudins their classification and morphology as per existing status of knowledge and their role in determining the type of deformation in Neem Ka Thana belt, Proterozoic rocks of North Delhi fold belt. The study of symmetric, asymmetric and modified boudins in the area enabled the author to deduce that the rocks in above belt have suffered a ployphase deformation marked with flattening type of progressive deformation during the first phase (DF1) of deformation and the second phase (DF2) with a time gap, has produced modified boudins and folded boudin structures with the largely same axis of deformation in both these deformations.

Provenance and tectonic setting of the Gudongjing Group in Beishan Orogen

增生造山带中陆源碎屑岩物源区特征的研究可为解剖造山带结构甚至大陆地壳的形成和演化提供关键证据。北山造山带中部的古硐井群被认为是前寒武纪稳定沉积盖层，是北山造山带存在微陆块的重要依据。本文围绕古硐井群的物源区特征，进行了碎屑颗粒和重矿物统计、全岩主微量元素地球化学分析、碎屑锆石U-Pb年代学和Hf同位素测试。古硐井群碎屑岩的碎屑颗粒多呈棱角状，主要为石英、长石，同时含大量硅质岩碎屑和一定数量的火山岩碎屑；重矿物组成以褐铁矿、锆石、白钛石、尖晶石为主，角闪石、电气石、辉石次之，暗示物源区可能存在蛇绿岩、增生杂岩。全岩主量元素以高硅、高铝、富碱、低锰为特征，结合REE、Cr、Co、Sc和Th等惰性元素含量特征共同指示了长英质的物源区。最年轻的碎屑锆石年龄为443.9±13Ma，表明古硐井群最大沉积时限为晚奥陶世。碎屑锆石的年龄高度集中于470Ma附近，且该区间锆石ε_Hf（t）值多为正值，指示物源区存在大量新生地壳物质。本文推测古硐井群可能形成于增生楔楔顶盆地；研究结果支持北山造山带是古生代持续增生造山的产物这一认识。;The clastic sedimentary rocks in an accretionary orogenic belt may preserve key evidence for understanding the architecture of orogenic belts and even the crustal evolution. The clastic sedimentary rocks of the Gudongjing Group in the middle of the Beishan Orogen is considered to be a set of stable sedimentary caprocks of the Precambrian, and it is also an important basis for the existence of micro-continents in the Beishan Orogen. In this paper, statistics on clastic particle grain sizes and heavy mineral compositions, the whole rock geochemical composition analyses, detrital zircon U-Pb dating, and Hf isotope testing are carried out on the clastic rocks of the Gudongjing Group to reveal their prevenances and tectonic settings. The clastic components of the rocks are mainly composed of quartz, feldspar and rock fragments which are mainly siliceous rock fragments together with a certain amount of volcanic ones. Heavy mineral analysis reveals they mainly consist of limonite, zircon, ilmenite and spinel, followed by hornblende, tourmaline and pyroxene, which indicates the contribution of the accretionary complex materials and ophiolites to their provenance. The whole rock major element compositions are characterized by high contents of silicon, aluminum and alkali, while low in manganese. The contents of the inert elements, such as REE, Cr, Co, Sc, Th, etc., show characteristics of a felsic source area. The youngest detrital zircon age obtained in this study is 443.9±13Ma, indicating that the maximum deposition age of the Gudongjing Group is Late Ordovician. The age of detrital zircons is highly concentrated around 470Ma, and the corresponding zircon ε_Hf(t) values are mostly positive, indicating a large number of new crustal materials have developed in their provenance area. Based on the comparative analysis, it is suggested that these clastic rocks are possiblely formed in a slope basin on the top of an accretionary wedge. The results support the point that the Beishan orogenic belt is the product of a continuous accretion of a Paleozoic orogenic belt.

The transition of the Late Paleozoic tectonic regime in eastern Jilin and Heilongjiang provinces: Constraints from the double accretionary complex and island arc magmatic rock belts in the Jiamusi-Khanka Block.

The transition of the Late Paleozoic tectonic regime in eastern Jilin and Heilongjiang provinces: Constraints from the double accretionary complex and island arc magmatic rock belts in the Jiamusi-Khanka Block.

The transition of the Late Paleozoic tectonic regime in eastern Jilin and Heilongjiang provinces: Constraints from the double accretionary complex and island arc magmatic rock belts in the Jiamusi-Khanka Block

The transition of the Late Paleozoic tectonic regime in eastern Jilin and Heilongjiang provinces: Constraints from the double accretionary complex and island arc magmatic rock belts in the Jiamusi-Khanka Block

Regional geology and tectonic framework of the Southern Indian domain, Trans-Hudson orogen, Manitoba

The collaborative federal–provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic, and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper, we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba.

Tracing exhumation record in high-pressure micaschists: A new tectonometamorphic model of the evolution of the eastern part of the Fore Sudetic Block, Kamieniec Metamorphic Belt, NE Bohemian Massif, SW Poland

The Kamieniec Metamorphic Belt comprises a volcano-sedimentary succession exposed within a collision zone between the Saxothuringian and Brunovistulian crustal domains of the European Variscides. The studied rocks recorded two metamorphic episodes. The first episode, M1, occurred at conditions of c. 485 ± 25 °C and 18 ± 1.8 kbar related to burial within a subduction zone. The subsequent episode, M2, was linked to the final phases of exhumation to mid-crustal level, associated with pressure and temperature (P–T) conditions ranging from c. 520 ± 26 °C and 6 ± 0.6 kbar through 555 ± 28 °C and 7 kbar ± 0.7 to ~590 ± 30 °C and 3–4 ± 0.4 kbar. The documented deformation record is ascribed to three events, D1 to D3, interpreted as related to the burial and subsequent exhumation of the Kamieniec Metamorphic Belt. The D1 event must have witnessed the subduction of the Kamieniec Metamorphic Belt rock succession whereas the D2 event was associated with the exhumation and folding of the Kamieniec Metamorphic Belt in an E-W-directed shortening regime. A subsequent folding related to the D2 event was initiated at HP conditions, however, the planar fabric produced during a late stage of the D2 event, defined by a low-pressure mineral assemblage M2, indicates that the D2 final stage was synchronous with the onset of the M2 episode. Consequently, the entire D2 event seems to have been associated with the exhumation of the Kamieniec Metamorphic Belt to mid crustal level. The third deformation event D3, synchronous with the M2 episode, marked the last stage of the exhumation, and was linked to emplacement of granitoid veins and lenses. The latter resulted in heating and rheological weakening of the entire rock succession and in the formation of non-coaxial shear zones.

Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

AbstractThis study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

Tracking the Origin and Evolution of Diagenetic Fluids of Upper Jurassic Carbonate Rocks in the Zagros Thrust Fold Belt, NE-Iraq

Utilizing sophisticated tools in carbonate rocks is crucial to interpretating the origin and evolution of diagenetic fluids from the Upper Jurassic carbonate rocks along the Zagros thrust-fold Belt. The origin and evolution of the paleofluids utilizing in-situ strontium isotope ratios by high resolution laser ablation ICP-MS, integrated with stable isotopes, petrography and fieldwork are constrained. Due to the lack of information on the origin of the chemistry of the fluids, the cements that filled the Jurassic carbonate rocks were analysed from the fractures and pores. This allowed us to trace the origin of fluids along a diagenetic sequence, which is defined at the beginning from the sediment deposition (pristine facies). Based on petrography and geochemistry (oxygen-, carbon- and strontium-isotope compositions) two major diagenetic stages involving the fluids were identified. The initial stage, characterized by negative δ13CVPDB values (reaching −10.67‰), involved evaporated seawater deposited with the sediments, mixed with the input of freshwater. The second stage involved a mixture of meteoric water and hot fluids that precipitated as late diagenetic cements. The late diagenetic cements have higher depleted O–C isotope compositions compared to seawater. The diagenetic cements display a positive covariance and were associated with extra- δ13CVPDB and δ18OVPDB values (−12.87‰ to −0.82‰ for δ18OVPDB and −11.66‰ to −1.40‰ for δ13CVPDB respectively). The distinction between seawater and the secondary fluids is also evident in the 87Sr/86Sr of the host limestone versus cements. The limestones have 87Sr/86Sr up to 0.72859, indicative of riverine input, while the cements have 87Sr/86Sr of (0.70772), indicative of hot fluid circulation interacting with meteoric water during late diagenesis.

Cessation of collisional tectonism and rapid crustal uplift recorded by 430–420 Ma igneous rocks in the South Kunlun belt, northwest China

ABSTRACT The Wanbaogou granitic plutons are exposed along with coeval volcanic rocks, and provide an opportunity to investigate Silurian tectonism in the South Kunlun belt, northwest China. This study presents new whole-rock geochemical, and zircon U–Pb geochronological and Lu–Hf isotope data for Silurian intrusive rocks of the Wanbaogou area. We analysed four samples: a porphyritic monzogranite and dioritic dike from the Wanbaogou pluton and a granodiorite and monzogranite from the Wudaogou pluton. Zircon U–Pb dating yielded similar ages of 423.8 ± 1.9, 422.9 ± 1.9, 424.9 ± 2.0, and 425.3 ± 2.6 Ma, respectively. indicating that the intrusions were coeval. The granitic samples are mostly transitional between the high-K and shoshonite series. The Wanbaogou monzogranite is metaluminous and has features typical of A-type granites in terms of trace element geochemistry, whereas the Wudaogou granitoids are metaluminous to weakly peraluminous I-type granites. The Wanbaogou dikes are intermediate to mafic in composition and belong to the low- to medium-K series. The four intrusive samples have εHf(t) values of −10.62 to +0.99 (average = −2.52), similar to the εHf(t) values of intraplate volcanic rocks (−6.35 to +2.89; average = −1.82) and, combined with the geochemical data, indicate the 430–420 Ma magmatism could have been generated by crustal melting with input from mantle-derived materials, followed by crustal contamination. Data for the Wanbaogou and Wudaogou plutons, dikes, and volcanic rocks from the same area suggest rapid orogenic uplift at 430–420 Ma. Our data and the results of previous research on the Kunlun high- to ultra-high-pressure metamorphic belt and coeval volcanic and intrusive rocks elsewhere in the region suggest that collisional tectonism ceased abruptly at 430–420 Ma. Slab break-off may be responsible for this termination of tectonism and rapid uplift.

Petrogenesis and tectonic significance of Paleoproterozoic metavolcanic rocks in the Khondalite Belt, North China Craton

As a typical continent–continent collision belt, the Khondalite Belt (KB) of the North China Craton (NCC) represents a Paleoproterozoic orogenic belt that underwent subduction and collision. Although many high-grade metamorphic rocks have been well studied, the low to medium grade metamorphic rocks well preserved in KB are less studied, which hinders a better understanding of the initiation of Paleoproterozoic subduction and collision processes. The Erdaowa Group is a representative of such a metamorphic rock series. Zircon U-Pb dating of the metavolcanic rocks in the Erdaowa Group yields the eruption ages of 2465 ± 24 Ma and 2350 ± 34 Ma, and the metamorphic ages of 1933 ± 32 Ma and 1844 ± 30 Ma. The geochemical results show that the metavolcanic rocks of the Erdaowa Group belong to tholeiitic basalts series. The enrichment of large ion lithophile elements (LILEs) such as Rb and Ba, and the depletion of high field strength elements (HFSEs) such as Ta, Nb and Ti indicate that the metavolcanic rocks have arc geochemical affinities and may have been formed at an active continental margin associated with subduction. Zircon Lu-Hf isotopic analysis shows that εHf(t) values are −0.15 – + 8.72, and the single-stage depleted mantle model age is 2448 – 2707 Ma, indicating that the primary magma originated from partial melting of the depleted lithospheric mantle and experienced slight crustal contamination. The results further prove that the low and medium grade rocks were directly involved in the subduction and collision between the Yinshan Block and the Ordos Block.

Early mesozoic arc–back-arc system in the leading edge of the Tibetan Plateau

The southern Lhasa subterrane is the leading edge of the Tibetan Plateau and preserves important records of the Neo-Tethyan oceanic subduction and India–Asia collision. However, the early tectonic evolutionary history of the Neo-Tethyan oceanic subduction remains unclear. Here we present results from a systematic study on the early Mesozoic (Middle Triassic–Late Jurassic) magmatic and sedimentary rocks in the southern Lhasa subterrane. These rocks are distributed zonally along the southern and northern parts of the subterrane, and have been classified into the north and south belts. The magmatic rocks in the north belt show a back-arc basin affinity, with accreted bimodal volcanic sequence and sediments from central and southern Lhasa subterranes, whereas the magmatic rocks in the south belt reveal typical magmatic arc characteristics. The sediments were derived from southern Lhasa subterrane, without the input of clastic components from the central Lhasa subterrane. Based on geochronological, geochemical, and sedimentary records from both belts, we conclude that the early Mesozoic magmatism was triggered by the northward subduction of the Neo-Tethyan oceanic slab, and its initial subduction likely occurred before the Middle Triassic (ca. 240 Ma). During the Early–Middle Jurassic (ca. 193–165 Ma), a short-lived episode of back-arc extension, we attribute to roll-back of the subducting Neo-Tethyan slab, resulted in the single magmatic arc that existed during the early stages (ca. 240–194 Ma) and evolved gradually into an arc–back-arc system. The north belt represents a back-arc basin setting associated with a continental arc, whereas an intra-oceanic and continental (island) arc simultaneously developed in the south belt, and both these belts were possibly generated under the same subduction system.

Constraints of rare detrital V-rich tourmaline and rutile on late Devonian palaeogeographic reconstruction in the Azarbaijan district, NW Iran

• New geochemical data on detrital tourmaline and rutile from the Devonian sedimentary rocks in NW Iran. • Specific V-bearing tourmaline and rutile compositions indicate metaultramafic source rocks. • Evidence for far-distant transport of sediment from the Gondwanan-East African Belt to the distal margin of Gondwana . This paper presents the first geochemical data on detrital tourmaline and rutile from sedimentary rocks of the upper Devonian Ilanqareh Formation (Fm.) which is extended in the Azarbaijan area in NW Iran. The deposits have typically mature character with ultra-stable heavy minerals predominantly derived from the metamorphic rocks. The detrital tourmaline has mainly dravitic, uvitic and Mg-foititic composition, thus indicating metaultramafic source rocks and common metapelites and metapsammites. Detrital rutile geochemistry indicates the presence of lower-up-to-upper amphibolite facies metapelitic source rocks, and part of this rutile most likely underwent granulite facies metamorphism. Unique detrital V-bearing tourmaline with high V 2 O 3 content up to 8.93 wt% indicates its primary derivation from far-distant source rocks in the East African Belt and adjacent areas. This is confirmed by both the exceptional geochemistry and the shape and roundness of the tourmaline which indicate recycled detrital origin and multiple reworking. The less frequent V-bearing rutile also indicates the same sources. These results confirm the wide circulation of sediments distributed by the Gondwana super-fan system from the inner parts of this super-continent to the Gondwana and peri-Gondwanan terrain continental margins, and also their later re-distribution during the Devonian era. The results highlight the significant contribution from Gondwanan sources to the supply of upper Devonian siliciclastic formations in NW Iran compared to those from the Iranian plate.

Carboniferous high-Mg andesitic and dioritic rocks in the Aqishan-Yamansu belt: Implications for mantle metasomatism and tectonic setting of the Eastern Tianshan

The Aqishan-Yamansu belt, Eastern Tianshan is characterized by voluminous Late Paleozoic intermediate–felsic igneous rocks and magmatic–hydrothermal Fe (–Cu) deposits. This study presents zircon U–Pb ages and whole-rock geochemistry of newly identified high-Mg andesitic and dioritic rocks (HMAs and HMDs) within the Hongshanliang copper district in the Aqishan-Yamansu belt, Eastern Tianshan to investigate their petrogenesis and constrain the Carboniferous tectonic setting of the Eastern Tianshan. Zircon U–Pb dating of the HMAs and HMDs yielded ages between 336.9 ± 1.9 Ma and 329.0 ± 4.2 Ma. The HMAs and HMDs show calc-alkaline affinities, and exhibit weak Rb, Ba, U, and Pb enrichment and Nb and Ta depletion, with minor negative Eu anomalies. Moreover, the positive εNd(t) values (+6.4 to +6.6), low Th and (La/Sm)N contents/ratios, as well as relatively low and variable εHf(t) values (−3.71 to +1.55) indicate that they were probably sourced from a depleted mantle wedge with negligible to minor crustal contamination. They have low Ba/Th ratios (10.8–93.3), minor negative Eu anomalies, high and variable (La/Sm)N and Th/Yb ratios, which are comparable to the well-known sanukites of the Setouchi volcanic belt, suggesting involvement of sediment-derived magmas during their formation. Integrating published regional mafic–intermediate rocks from the Aqishan-Yamansu belt and this study, we propose that the Hongshanliang high-Mg rocks formed in a continental arc-related setting in the Carboniferous and the belt underwent a sharp tectonic transition event (ca. 330–322 Ma) from the Kangguer oceanic slab steepening to breakoff, corresponding to the Late Carboniferous basin inversion.