Whole-rock Geochemical Analyses Research Articles

Timing of magmatic crystallization and Sn–W–Mo greisen vein formation within the Mount Douglas Granite, New Brunswick, Canada

U–Pb geochronology was applied to a combination of magmatic and hydrothermal minerals to help constrain the timing of emplacement of three units in the Mount Douglas Granite (MDG) and reveal their association with a complex mineralized hydrothermal system containing endogranitic Sn–W–Mo–Zn–Bi–U-bearing greisen/sheeted veins within the pluton. Magmatic monazite and zircon U–Pb ages obtained by LA–ICP–MS overlap at 368 Ma, recording a Late Devonian crystallization age for the MDG. Although discrimination, outside analytical error, of sequential pulses of magmatism is beyond the resolution of LA–ICP–MS U–Pb geochronology, geochemical variations of monazite accompanied by previous whole-rock geochemical analyses support a progressive fractional crystallization process starting from a parental magma (Dmd1), leading to the generation of Dmd2, and finally Dmd3 as the most fractionated unit. Hydrothermal uraninite, cassiterite, and monazite, collected from endogranitic greisen/sheeted veins, reveal evidence for syn-magmatic-related mineralization and a longer-lived post-magmatic hydrothermal system. The first stage is recorded by concordant uraninite dates at 367 ± 3 Ma and by an inverse isochron lower intercept of 362 ± 8 Ma for cassiterite. In contrast, hydrothermal monazite crystallized over a wider range of ages from 368 to 344 Ma, demonstrating post-magmatic hydrothermal activity within the MDG. These magmatic and hydrothermal ages combined with the geochemical signature of the MDG are similar to those documented for the nearby Mount Pleasant Sn–W–Mo–Bi–In granite-related deposit, which suggests that the two mineralizing systems occur at different levels of the same magmatic system.

Processes of clastic sedimentation associated with Late Cretaceous ophiolite emplacement in the SW segment of the Antalya Complex (S Turkey)

Clastic sediments provide important clues to the timing and processes of ophiolite emplacement, when integrated into the regional temporal and tectonic framework. Four related settings of Late Cretaceous ophiolite-related clastic sediment deposition are exposed in the SW segment of the Antalya Complex, along the western limb of the Isparta Angle, an orocline in SW Turkey. The restored pre-emplacement paleogeography is indicative of a regional-scale Mesozoic carbonate platform, bordered to the east by a Late Triassic volcanic rift that partially infilled with deep-sea sediments, passing eastwards into several Mesozoic carbonate platforms (floored by pre-rift or syn-rift crust) and finally into the South Neotethys. First, emplacement-related sediments unconformably overlie exhumed ultramafic and gabbroic ophiolitic rocks and comprise locally variable mass-flow deposits (up to several-hundred m thick) that are dominated by subaqueous screes and debris-flow deposits. Fossil evidence (benthic and planktic foraminifera, radiolarians and macrofossils) indicates accumulation during the Campanian-Maastrichtian. Much of the coarse talus was derived from weathering and erosion of intrusive ophiolitic rocks (sheeted dykes, gabbro). Petrographic, XRD and whole-rock geochemical analysis of sandstones and igneous clasts indicate additional derivation from basaltic rocks, interpreted as the eroded extrusive layer of the ophiolite. Additional material was derived from Late Triassic-Cretaceous deep-water volcanics and sediments, and also from Late Cretaceous (and older) platform carbonates. The coarse clastics relate to ophiolite emplacement onto the continental margin, rather than forming in an oceanic setting (e.g., rifted ridge or oceanic fracture zone). The emplacement was associated with crustal extension that exposed ultramafic and gabbroic rocks on the seafloor, where they were covered by clastic debris. Secondly, Late Cretaceous calciturbidites and debris-flow deposits depositionally overlie Late Triassic-Late Cretaceous deep-water passive margin sediments. The clastic debris was derived from the emplacing ophiolite and from an adjacent Mesozoic carbonate platform. Thirdly, calciturbidites and debris-flow deposits also depositionally overlie the same carbonate platform. Fourthly, west of the same carbonate platform, ophiolite-derived mass-flow breccias accumulated locally and were then interleaved with previously emplaced ophiolitic plutonic rocks (mainly serpentinite and gabbro), typically at high angles. The four depositional settings relate to ongoing stages of Late Cretaceous ophiolite emplacement in a setting dominated by regional-scale thrusting and strike-slip (i.e., transpression). The latest Cretaceous deformation and related clastic deposition affected part of the northern, active continental margin of the Southern Neotethys, but without reaching more inboard continental crust (regional Tauride platform), a situation that contrasts with more orthogonally emplaced ophiolites.

Silurian to Early Devonian arc magmatism in the western Sakarya Zone (NW Turkey), with inference to the closure of the Rheic Ocean

The Rheic Ocean is the most significant Paleozoic ocean that detached peri-Gondwana terranes from the northern Gondwana margin throughout the closure of the Iapetus Ocean. The suture of the Rheic Ocean spreads from Mexico to the Middle East, and the timing of its final closure is well-documented by the rocks formed in the Variscan-Alleghanian-Ouachita orogeny which led to the formation of the supercontinent Pangaea. However, as robust paleomagnetic and quantitative data are mostly lacking, the onset and evolution of the subduction of the Rheic Ocean are highly speculative, and they require further confirmation. Recently, the well-preserved metagranitoids along the western Sakarya Zone (SZ) in Anatolia have been identified, and they provide new data that improve our knowledge on the evolution of the Early Paleozoic Rheic Ocean along the northern Gondwana. Here, we present new geochronological, in situ zircon Hf isotope, and whole-rock geochemical analyses of these metagranitoids from the western SZ to enhance our understanding of the subduction processes of the Rheic Ocean. LA-ICP-MS zircon U-Pb dating demonstrated that the Bozüyük and Borçak metagranitoids from the western SZ were emplaced during the Silurian to Early Devonian (431 ± 2.7 to 403 ± 3.5 Ma). The both granitoids have medium- to high-K, calc-alkaline magmatic character, and exhibit peraluminous to slightly metaluminous geochemical signature. They show a typical arc pattern in terms of trace elements and have a uniform, moderate negative εHf (t) of −3.2 to −9.7, with Mesoproterozoic Hf depleted mantle model ages (TDM1 = 1.2 to 1.4 Ga). The geochemical and isotopic characteristics are not consistent with those of depleted mantle melts and melts derived from the crustal rocks in an intracontinental environment. Instead, the parental magma is likely generated from the partial melting of a homogeneous and enriched mantle wedge source. We propose that the Silurian to Early Devonian arc-related magmatism is associated with a northward subduction episode of the Rheic Oceanic lithosphere beneath the peri-Gondwana terranes. Hence, we consider that the opening of Paleotethys Ocean formed in a back-arc basin of subduction in Andean style to the north rather than a continental rift to the south in response to south-directed and short-lived supra-subduction zone (SSZ)-type subduction during the Silurian to Early Devonian.

Late Triassic extensional tectonics in the northern North China Craton, insights from a multidisciplinary study of the Wangtufang pluton

To better understand Late Triassic tectonic setting in the northern North China Craton (NCC), the emplacement mechanism of the Wangtufang pluton, which recorded the synmagmatic regional tectonic signature, has been investigated. Zircon U-Pb ages, and Hf isotopic data, and whole-rock geochemical analyses suggest that the Late Triassic Wangtufang pluton composed of syenogranite and diorite is derived from partial melting of lower crust with some depleted mantle components. Both the syenogranite and diorite appear isotropic. Anisotropy of magnetic susceptibility and gravity studies have been carried out to characterize internal fabrics and shape of the pluton. The diorite forms just thin remnants above the syenogranite. The syenogranite with a series of NW-SE trending dykes intruded into the diorite and its country rocks. In the syenogranite, the gently dipping magnetic foliations strike nearly parallel to the pluton border. The shallow plunging magnetic lineations mainly strike NE–SW. Combining NE-SW trending elongated subsurface shape with central root, unflat bottom, and moderate- to high- inward dipping sidewalls, the syenogranite could be considered as a lopolith-like intrusion. The syenogranite was likely emplaced by inflation of magma pulses from its central conduit and built up by floor depression. Emplacement of the syenogranite was in an extensional setting, considering: (1) the NE–SW striking magnetic lineation, (2) the NE–SW trending elongated subsurface pluton shape, and (3) the orthogonal NW–SE striking syenogranitic dykes considered as tension gashes during the NE–SW trending extension. The Wangtufang pluton provides reliable arguments to the Late Triassic intracontinental extensional setting already suggested in the northern NCC.

Discovery and identification of clay-sized tuffaceous rocks in Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, China

This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, which are rich in black organic matter (including oil and asphaltene), in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks (including tuff, sedimentary tuff and tuffaceous sedimentary rock) with high hydrocarbon generation capacities. Thus, these rocks can be defined as clay-sized tuffaceous source rocks. Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member. Through the macroscopic observation of drill cores and outcrop profiles, microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis (including major, trace and rare earth elements), this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks, especially clay-sized tuffs, have the following characteristics. First, the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white, yellowish brown and purplish grey, and mixed colors occur in areas with strong bentonite lithification. Second, the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization, forming abundant flaky aluminosilicate minerals with directional arrangement. In thin sections under a polarizing microscope, the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green. Third, the rock samples plot in the igneous rock field in the TiO2-SiO2 cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams, indicating that the samples are derived from the same source. The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin, China.

Petrogenesis of the Lijiagou spodumene pegmatites in Songpan-Garze Fold Belt, West Sichuan, China: Evidence from geochemistry, zircon, cassiterite and coltan U-Pb geochronology and Hf isotopic compositions

Spodumene and other pegmatites in the Lijiagou area are located in the central Songpan-Garze Fold Belt (SGFB), West Sichuan. These pegmatites are hosted within the giant Songpan-Garze polymetallic belt. A lack of precise geochronological constraints on the magmatic crystallization of the pegmatite suite and nearby plutonic granitoids has precluded definitive statements on genetic links between spodumene ore formation and the magmatic suite. Systematic zircon, cassiterite and coltan U-Pb dating, Hf isotope and whole-rock geochemical analysis were undertaken. Two-mica granite and muscovite albite granite have S-type granite affinities and chemical compositions suggesting a post-orogenic setting, and the albite spodumene pegmatites belong to the LCT (Li-Cs-Ta) group of pegmatites. Zircon LA-MC-ICP-MS U-Pb dating of the two-mica granite, muscovite albite granite, albite pegmatite and albite spodumene pegmatite give crystallization ages of 219.2 ± 2.3 Ma (MSWD = 0.55), 217 ± 2.8 Ma (MSWD = 0.47), 202.8 ± 4.9 Ma (MSWD = 3.9) and 200.1 ± 4.6 Ma (MSWD = 3.1), respectively. Zircons from the same units yield εHf(t) values of −39.17 to 6.81, −22.73 to −2.83, −11.17 to 8.14, and −4.92 to −2.4, respectively, consistent with mixed crustal sources for the pegmatite and granite magmas. Cassiterite from albite spodumene pegmatite yields a concordia intercept age of 211.4 ± 3.3 Ma (MSWD = 2.9), while coltan yields a weighted mean age of 211.1 ± 1.0 Ma (MSWD = 0.97). The U-Pb zircon ages of the two-mica granite and muscovite albite granite are interpreted as ages of magmatic crystallization. Metamictization of zircon in both albite and albite spodumene pegmatites makes their U-Pb zircon ages liable to inaccuracy due to Pb loss. The coltan U-Pb age is regarded as an accurate measure of the magmatic crystallization age of the albite spodumene pegmatite. Given the differences in magmatic ages and εHf(t) ranges between albite spodumene pegmatite and both the two-mica granite and the muscovite albite granite, the albite spodumene pegmatites probably represent anatectic melts and not fractionation products of either of the granitic magmas. U-Pb coltan and U-Pb cassiterite dating are more likely to provide accurate crystallization ages of albite spodumene pegmatites than U-Pb zircon ages.

Petrology and geochemistry of early Carboniferous volcanic rocks in the Xinyuan region of western Tianshan: Implications for magma sources

Understanding the sources and processes that produce fossil continental arc magmas is still a challenging problem. To address this problem, here we perform detailed petrological investigations as well as whole-rock geochemical and SrNd isotopic analyses on the early Carboniferous volcanic rocks in the Xinyuan region. These volcanic rocks mainly consist of calc-alkaline basalt, trachy-andesite, and rhyolite, and show subduction-related geochemical and isotopic signatures. Petrographic observations and new geochemical data suggest that the rhyolite was generated by partial melting of juvenile crustal materials, while the basalt and trachy-andesite might undergo a series of open-system magmatic processes, such as fractional crystallization, crustal contamination, and magma replenishment and mixing. The primitive basaltic rocks with Mg# values of ~70 were least affected by crustal-level magmatic processes, and they could be used to constrain sub-arc mantle processes. The Nb/Yb, Zr/Yb, and Ta/Yb ratios of these basaltic rocks are similar to those of E-MORB, suggesting a relatively fertile mantle source prior to subduction, while their high Ba/Th, Sr/Nd, and Th/Nd ratios suggest a significant contribution of slab-derived components to the pre-subduction mantle wedge. Geochemical modelling based on trace elements and SrNd isotopic ratios indicates that the pre-subduction mantle wedge was modified by ~0.5% subducted sediment-derived fluids and ~2% bulk sediments. About 10% partial melting of such enriched mantle source could produce the early Carboniferous primitive magmas in the Xinyuan region.

A Plethora of Epigenetic Minerals Reveals a Multistage Metasomatic Overprint of a Mantle Orthopyroxenite from the Udachnaya Kimberlite

More than forty mineral species of epigenetic origin have been identified in an orthopyroxenite from the Udachnaya-East kimberlite pipe, Daldyn kimberlite field, Siberian platform. Epigenetic phases occur as: (1) Mineral inclusions in the rock-forming enstatite, (2) daughter minerals within large (up to 2 mm) crystallized melt inclusions (CMI) in the rock-forming enstatite, and (3) individual grains and intergrowths in the intergranular space of the xenolith. The studied minerals include silicates (olivine, clinopyroxene, phlogopite, tetraferriphlogopite, amphibole-supergroup minerals, serpentine-group minerals, talc), oxides (several generations of ilmenite and spinel, rutile, perovskite, rare titanates of the crichtonite, magnetoplumbite and hollandite groups), carbonates (calcite, dolomite), sulfides (pentlandite, djerfisherite, pyrrhotite), sulfate (barite), phosphates (apatite and phosphate with a suggested crystal-chemical formula Na2BaMg[PO4]2), oxyhydroxide (goethite), and hydroxyhalides (kuliginite, iowaite). The examined epigenetic minerals are interpreted to have crystallized at different time spans after the formation of the host rock. The genesis of minerals is ascribed to a series of processes metasomatically superimposed onto the orthopyroxenite, i.e., deep-seated mantle metasomatism, infiltration of a kimberlite-related melt and late post-emplacement hydrothermal alterations. The reaction of orthopyroxene with the kimberlite-related melt has led to orthopyroxene dissolution and formation of the CMI, the latter being surrounded by complex reaction zones and containing zoned olivine grains with extremely high-Mg# (up to 99) cores. This report highlights the utility of minerals present in minor volume proportions in deciphering the evolution and modification of mantle fragments sampled by kimberlitic and other deep-sourced magmas. The obtained results further imply that the whole-rock geochemical analyses of mantle-derived samples should be treated with care due to possible drastic contaminations from “hiding” minor phases of epigenetic origin.

Slab break-off origin of 105 Ma A-type porphyritic granites in the Asa area of Tibet

AbstractThe study of the petrogenesis of some magmatic rocks with special geochemical attributes provides effective information for us to explore the deep geodynamic background of their formation. A series of granitic porphyry dykes have been found in the mélange zone of the Asa region in southern Tibet, whose genesis may be closely related to the evolution of the Meso-Tethyan Ocean. Regional geodynamic evolution is investigated by whole-rock geochemical analysis, zircon U–Pb dating and Lu–Hf isotopic analysis of two porphyritic granites. The Asa porphyritic granites have high SiO2(74.29–78.65 wt %) and alkalis (Na2O + K2O = 6.51–9.35 wt %) contents, and low Al2O3(11.60–14.51 wt %), CaO (0.04–0.19 wt MgO (0.01–0.10 wt %) contents. They are enriched in Zr, Nb, Ce, Y and Hf and depleted in Ti, Ba, Sr and P, consistent with A-type granites. The samples are relatively rich in LREEs, with LREE/HREE ratios of 1.73–3.04. They display negative Eu anomalies (Eu/Eu* = 0.24–0.28) and obvious Ce anomalies in some samples. Zircon U–Pb analyses show that the porphyritic granites formed in late Early Cretaceous time, 107.4 to 105.5 Ma. Zircon εHf(t) values are in the range of 6.9 to 12.0. These data indicate that the porphyritic granites were sourced from interaction between mantle-derived and juvenile lower crust-derived melts, with the addition of oceanic sediment-derived melts. This occurred when the subducting Bangong–Nujiang oceanic crust split to create a slab window. Rising asthenosphere triggered re-melting of lower crust basalts, resulting in the formation of the late Early Cretaceous A-type granites around Asa.

Closure of India–Asia collision margin along the Shyok Suture Zone in the eastern Karakoram: new geochemical and zircon U–Pb geochronological observations

AbstractNew whole-rock geochemical analyses along with laser ablation multi-collector inductively coupled plasma mass spectrometry U–Pb zircon ages of the granite–rhyolite from the Karakoram Batholith, exposed along the Shyok Valley, NW India, have been performed to understand the timing and geochemical evolution of these magmatic bodies and their implications for the geodynamic evolution of the Karakoram Batholith. New geochronological data on granites and rhyolites along with previously published geochronological data indicate that the Karakoram Batholith evolved during Albian time (~110–100 Ma) owing to the subduction of Tethys oceanic lithosphere along the Shyok Suture Zone. This region witnessed a period of no magmatism during ~99–85 Ma. Following this, the Kohistan–Ladakh arc and Karakoram Batholith evolved as a single entity in Late Cretaceous and early Palaeogene times. Late Cretaceous (~85 Ma) rhyolite intrusions within the Karakoram Batholith show calc-alkaline subduction-related signatures with a highly peraluminous nature (molar A/CNK = 1.42–1.81). These intrusions may have resulted fromc. ~13.8 % to ~34.5 % assimilation of pre-existing granites accompanied by fractional crystallization during the ascent of the magma. The contamination of mantle wedge-derived melts with crust of the active continental margin of the Karakoram most likely enhanced the high peraluminous nature of the rhyolite magma, as has been constrained by assimilation fractional crystallization modelling. Two granite samples from the contact of the Shyok Metamorphic Complex and Karakoram Batholith indicate that the post-collisional Miocene magmatism was not only confined along the Karakoram Fault zone but also extends ~30 km beyond the Shyok–Muglib strand.

Archean Boninite-like Rocks of the Northwestern Youanmi Terrane, Yilgarn Craton: Geochemistry and Genesis

Abstract Rocks with chemical compositions similar to Cenozoic boninites occur in many Archean cratons (boninite-like rocks), but they are rarely well-preserved, well-sampled, or presented within chrono- and chemo-stratigraphic context. This study provides a detailed description of the most extensive and well-preserved Archean boninite-like rocks reported to date. Within the 2820 to 2740 Ma magmatic suites of the northwest Youanmi Terrane, Yilgarn Craton, boninite-like rocks occur as two distinct units. The first boninite-like unit is thinner (several 10 s of m thick), occurs close to the base of the 2820–2800 Ma Norie Group and includes both volcanic flows and subvolcanic intrusions. The second boninite-like unit is thicker (locally several 100 s m), occurs near the base of the 2800–2740 Ma Polelle Group and consists of mainly fine-grained volcanic flows with local cumulate units. On average, major and trace element compositions for Youanmi Terrane boninite-like rocks are marginal between basalt, picrite and boninite and they have asymmetrically concave REE patterns, and Th–, Zr–Hf enrichments, similar to many Phanerozoic low-Si boninite suites, but at generally higher MREE–HREE contents. We report over 300 new whole-rock geochemical analyses, and 16 new Sm–Nd isotopic analyses, and associated petrographic evidence, including representative mineral compositions, which we support with published geochemical analyses and several decades of fieldwork in our study area. Comparison between Archean boninite-like rocks and Cenozoic boninites shows that most Archean examples had less depleted sources. We consider two possible petrogenetic models for the Youanmi Terrain examples: (1) they reflect variably contaminated komatiites, or (2) they reflect melts of metasomatised refractory mantle, analogous to Phanerozoic boninites. Trace element modelling indicates that crustal contamination could potentially produce rocks with boninite-like compositions, but requires an Al-enriched komatiitic parent liquid, for which there is no field evidence in our study area. Initial εNdT values in pre-2800 Ma rocks (εNdT -0·4 to +1·2) are on average slightly higher than those in 2800–2733 Ma examples (εNdT -3·2 to +1·2), compatible with increasing mantle metasomatism involving recycling of ≥ 2950 Ma crust. Integration of trace element and Nd isotopic data demonstrates that significant direct crustal assimilation was restricted to felsic magmas. The Th–Nb and Ba–Th systematics of mafic-intermediate rocks reflect fluid- and sediment-derived processes in the mantle, with boninite-like examples being linked primarily to fluid metasomatism. We compare the well-preserved igneous textures and mineralogy of Youanmi Terrane boninite-like rocks with those of their Phanerozoic counterparts, and based on studies of the latter, suggest that former had similarly hot, H2O-rich parent magmas. The association of boninite-like rocks in the Norie and Polelle Groups with coeval high-Mg andesites, sanukitoids and hydrous mafic intrusions of the Narndee Igneous Complex strongly suggests a metasomatised mantle source and subduction operating in the Yilgarn between 2820 and 2730 Ma.

Li–Nb–Ta mineralization in the Jurassic Yifeng granite-aplite intrusion within the Neoproterozoic Jiuling batholith, south China: A fluid-rich and quenching ore-forming process

The Neoproterozoic Jiuling batholith in northwestern Jiangxi Province, south China, is one of the largest batholiths in the Nanling Range, and it is intruded by Mesozoic granitic intrusions associated with mineralization. The Jurassic Yifeng granitic pluton intrudes to the southern part of the Jiuling batholith and comprises Li-rich muscovite (Li-Mus) granite, trilithionite (Tri) granite, and aplite dikes. The granitic rocks are enriched in rare metals (Li–Rb–Cs–Nb–Ta). We conducted whole-rock geochemical analyses, undertook columbite U–Pb dating, and examined the textures and chemistry of feldspars, micas and the accessory minerals montebrasite, columbite-group minerals (CGMs), microlite, cassiterite, and zircon. All three granitic rock types have high Li contents (up to ~6200 ppm), and the δ7Li values of the Li-Mus granite and aplite are −3.55 ± 0.14‰ and −5.34 ± 0.04‰, respectively. The isolated grains of CGMs and cassiterite in the aplite are magmatic products, but intense fluid activity in the later stages and quenching resulted in unique mineral aggregates of cassiterite + CGMs + zircon. The Nb–Ta mineralization of the Li-Mus granite took place at 144 ± 5 Ma according to columbite U–Pb dating, and this age is similar to that of the neighbouring Yichun granitic pluton to the south. Moreover, both the Yifeng and Yichun plutons contain P-rich rare-metal-bearing Li–F granitic rocks that contain P-rich feldspars, montebrasites (amblygonites), Cs-rich lepidolites, CGMs close to the Mn endmember, and Hf-rich zircons. It is inferred that a metallogenic district characterized by Li–Nb–Ta mineralization exists in northwestern Jiangxi Province, thus providing a reliable guide for further exploration.

Origin of Mesozoic ophiolitic mélanges in the western Yarlung Zangbo suture zone, SW Tibet

The petrogenesis and tectonic evolution of the Mesozoic ophiolitic mélanges in the western section of the Yarlung Zangbo suture zone (YZSZ) remain controversial. In this paper, we present the results of whole-rock geochemical and SrNd isotope analyses, zircon UPb ages and in situ LuHf isotopic data obtained from mafic rocks of the northern and southern sub-belts of the western YZSZ Mesozoic ophiolitic mélanges to help us understand these controversial issues. Diabases and dolerites from the northern sub-belt and gabbros from the southern sub-belt exhibit variable fore-arc basalt (FAB)-like geochemical compositions and have zircon UPb ages of ∼126.4–120.3 Ma. In addition, gabbro-diabases from the northern sub-belt have boninite series affinities and yield a zircon UPb age of ∼125.7 Ma. These results, along with previous studies on the YZSZ Mesozoic ophiolitic mélanges and the Gangdese arc, reveal that the western YZSZ Mesozoic ophiolites were likely generated over multiple stages in the epicontinental Gangdese fore-arc basin as the Yarlung Zangbo Neo-Tethyan Ocean subducted northward in front of the Lhasa terrane. The Early Cretaceous FAB-like and boninite series mafic rocks were formed by the reinitiation of subduction, which was followed by a retreat of the subduction zone and the creation of the fore-arc basin and strong hyperextension, accompanied by asthenosphere upwelling at ∼130–120 Ma. During this process, the upwelling asthenosphere underwent decompressional melting with limited penetration of slab-derived fluids and gave rise to the N-MORB (normal mid-ocean ridge basalt)-like basaltic magmas that intruded the overlying, previously generated depleted mantle as FAB-like gabbro, diabase and dolerite sills or dykes. Then, boninitic magmas represented by boninitic gabbro-diabases were generated by remelting the extremely depleted residual mantle source, which was metasomatized by a small amount of slab-derived fluids, following previous extractions of FAB-like magma.

Petrogenesis and tectonic implications of the late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain, western Tianshan

The widespread late Carboniferous calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain provide crucial constraints on the tectonic evolution of the western Tianshan. Here, we perform detailed petrological investigations as well as zircon U-Pb chronological, whole-rock geochemical and Sr-Nd isotopic analyses on these magmatic rocks from two geological sections along the Duku road. Magmatic rocks in the section I with zircon SHRIMP U-Pb ages of 306.8 Ma and 306.4 Ma are composed of medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte, while those in the section II consist of shoshonitic trachy-andesite, trachyte with a U-Pb age of 308.1 Ma, and monzonite with a U-Pb age of 309.6 Ma. All these magmatic rocks are characterized by strong enrichments in large iron lithophile elements with depletions of Nb, Ta and Ti, indicating the origination from subduction-modified lithospheric mantle. The εNd(t) values of the rock samples collected from the section I (2.80–5.45) and section II (3.34–5.37) are generally higher than those of the Devonian to early Carboniferous arc-type magmatic rocks in the Yili-central Tianshan, suggesting that depleted asthenosphere might also be involved in their generation. Based on these geochemical data and petrological observations, we suggest that the early-stage (308.1–309.6 Ma) shoshonitic monzonite, trachy-andesite and trachyte in the section II were generated by mixing between mafic magmas and trachytic melts, while the late-stage (306.4–306.8 Ma) medium-K calc-alkaline to shoshonitic basalt, trachy-andesite and trachyte in the section I were produced by partial melting of depleted asthenospheric and metasomatized lithospheric mantle, followed by the processes of fractional crystallization and crustal contamination. Taking into account the available regional geological data, the subduction of south Tianshan ocean was probably ceased at ∼310 Ma, and these calc-alkaline and shoshonitic magmatic rocks in the Awulale mountain formed in a post-collisional setting subsequent to slab break-off.

Mixed local and ultra-distal volcanic ash deposition within the Upper Cretaceous Kanguk Formation, Sverdrup Basin, Canadian Arctic Islands

AbstractThe Upper Cretaceous Kanguk Formation of the Sverdrup Basin, Canadian Arctic Islands, contains numerous diagenetically altered volcanic ash layers (bentonites). Eleven bentonites were sampled from an outcrop section on Ellesmere Island for U–Pb zircon secondary ion mass spectrometry dating and whole-rock geochemical analysis. Two distinct types of bentonite are identified from the geochemical data. Relatively thick (0.1 to 5 m) peralkaline rhyolitic to trachytic bentonites erupted in an intraplate tectonic setting. These occur throughout the upper Turonian to lower Campanian (c. 92–83 Ma) outcrop section and are likely associated with the alkaline phase of the High Arctic Large Igneous Province. Two thinner (<5 cm) subalkaline dacitic to rhyolitic bentonites of late Turonian to early Coniacian age (c. 90–88 Ma) are also identified. The geochemistry of these bentonites is consistent with derivation from volcanoes within an active continental margin tectonic setting. The lack of nearby potential sources of subalkaline magmatism, together with the thinner bed thickness of the subalkaline bentonites and the small size of zircon phenocrysts therein (typically 50–80 μm in length) are consistent with a more distal source area. The zircon U–Pb age and whole-rock geochemistry of these two subalkaline bentonites correlate with an interval of intense volcanism in the Okhotsk–Chukotka Volcanic Belt, Russia. It is proposed that during late Turonian to early Coniacian times intense volcanism within the Okhotsk–Chukotka Volcanic Belt resulted in widespread volcanic ash dispersal across Arctic Alaska and Canada, reaching as far east as the Sverdrup Basin, more than 3000 km away.

New age constraints on the Lan Sang gneiss complex, Thailand, and the timing of activity of the Mae Ping shear zone from in-situ and depth-profile zircon and monazite U-Th-Pb geochronology

The Mae Ping shear zone (MPSZ) is one of four major strike-slip systems in Thailand and a key structural element in some models of the Cenozoic tectonic evolution of Southeast Asia. In northwestern Thailand, the MPSZ exposes a ∼5-km-wide core of intensely deformed high-grade metamorphic and minor undeformed magmatic rocks, the Lan Sang gneiss complex (LSGC). Despite previous efforts to determine the chronology of the LSGC and, ultimately, the MPSZ, the evolution of this gneiss complex remains incompletely resolved. Here we present new zircon and monazite U-(Th-)Pb LA-(MC-)ICP-MS ages and whole-rock geochemical analyses from three peraluminous, high-K (meta)igneous granitic rocks (augen gneiss, sheared granodiorite, undeformed subvolcanic dike) of the LSGC. Our zircon U-Pb dating suggests that the granitic protolith of the augen gneiss intruded metasedimentary basement of Sibumasu as early as ∼235–220 Ma. Zircon rim and monazite U-(Th-)Pb ages from the same sample attest to periods of metamorphic recrystallization and overgrowth in the Late Triassic-Early Jurassic, Early Cretaceous and Eocene. The latter event had started by ∼45 Ma and culminated in partial melting of the basement gneisses and the intrusion of granodiorite magma into the LSGC at ∼35 Ma, contemporaneous with ductile deformation along the MPSZ. A late undeformed subvolcanic dike that crosscuts the mylonitic fabric of the calc-silicate and marble core of the MPSZ, dated in this study at ∼22 Ma, may indicate that ductile shearing had ceased by that time. Thus, our study provides the first evidence for Late Eocene-Early Miocene magmatism in the LSGC.

Mesozoic felsic dikes in the Jiaobei Terrane, southeastern North China Craton: Constraints from zircon geochronology and geochemistry, and implications for gold metallogeny

The Jiaobei Terrane in the southeastern North China Craton (NCC) witnessed intense magmatic pulses during Mesozoic in response to lithospheric thinning. These include sporadic felsic-porphyritic dikes which preserve important information on the magmatic processes and of its implications on lithospheric thinning. Here we present a detailed investigation of representative felsic-porphyritic dikes based on whole-rock geochemical analysis, zircon LA-ICPMS U-Pb geochronology, trace elements and hafnium isotopic compositions. Our results identify stages of felsic dikes in the Jiaobei Terrane with ages in the range of 131–127 Ma and 123–116 Ma. The early stage felsic dikes (ESFDs) have negative zircon ɛHf(t) (−22.4 to −11.1), high zircon EuN/EuN* (0.41–0.92) and Ce4+/Ce3+ (62–944), low Ti-in-zircon crystallization temperatures (524–738 °C), and are geochemically comparable to the coexisting Guojialing granodiorite. Abundant inherited zircons and scattered zircon Hf, Yb/Gd, Th/U, and Ce/Sm values suggest that the magmas from which the dikes formed involved crustal contamination. They are interpreted to be the hypabyssal phase of the Guojialing suite sourced from the Paleoproterozoic lower crust of the eastern NCC with input of mantle components, associated with extensive crust-mantle interaction during the peak lithospheric thinning. The late stage felsic dikes (LSFDs) are characterized by variable zircon ɛHf(t), EuN/EuN* and Ce4+/Ce3+ values (−20.1 to −12.2, 0.38–0.88 and 64–956 for the granodiorite- and quartz-porphyritic dikes; −27.6 to −16.2, 0.17–0.75 and 22–503 for the monzogranite-porphyritic dikes), low Ti-in-zircon crystallization temperatures (562 to 774 °C), and display geochemical similarity with the Aishan granitoids. Systematic variations of zircon Hf, Yb/Gd, Th/U and Ce/Sm suggest that fractional crystallization of hornblende played a significant role in the formation of the late dikes. The LSFDs originated from the Neoarchean-Paleoproterozoic lower crust of the NCC with heterogeneous involvement of lesser amounts of mantle materials than the early dikes, consistent with weak crust-mantle interaction during the waning stage of lithospheric thinning. In the LSFDs, the granodiorite and monzogranite porphyries are broadly coeval (123–118 Ma) with large scale gold mineralization in the area as also favored by the high zircon Ce4+/Ce3+ ratios. Our study also provides new insights into the formation of gold deposits.

Geochronology and geochemistry of metasedimentary rocks from the Dongnancha Formation in the Huadian area, central Jilin Province, Northeast (NE) China: Implications for the tectonic evolution of the eastern segment of the Paleo-Asian Ocean

To constrain the tectonic evolution of the eastern segment of the Paleo-Asian Ocean, we conducted zircon U–Pb-Hf dating and whole-rock geochemical analyses for metasedimentary rocks from the Dongnancha Formation in the Huadian area in central Jilin Province, Northeastern (NE) China. Most detrital zircons from the metasedimentary rocks display clear oscillatory zoning and striped absorption in cathodoluminescence (CL) images and have Th/U ratios of 0.1–1.8, thus indicating a magmatic origin. U–Pb isotopic dating using LA-ICP-MS method for zircon samples from the metasedimentary rocks reveals that the depositional age can be constrained to the period between 250 and 222 Ma. Geochemical data reveal low to intermediate degrees of weathering of the source material and compositionally low to intermediate maturity. Detailed analyses of detrital zircon U–Pb-Hf geochronology and geochemistry show that these metasedimentary rocks are derived from a bidirectional provenance. The predominant derivation is from Permian–Early Triassic felsic-intermediate igneous rocks of central Jilin Province and adjacent regions in the northern margin of the North China Craton, although felsic-intermediate igneous rocks and continental material in the eastern segment of the Central Asian Orogenic Belt from the Cambrian–Carboniferous represent additional sources and minor amounts of Paleoproterozoic–Neoproterozoic material have been input from the North China Craton. A number of geochemical indicators and tectonic discrimination diagrams collectively indicate a continental island arc-active continental margin setting for the deposition of the protoliths of the metasedimentary rocks. The results of geochemical and geochronological analyses of the provenance and tectonic setting of the metasedimentary rocks indicate that the Dongnancha Formation was likely deposited in an intermountain basin in a post-orogenic fast uplift setting, suggesting that the final closure of the eastern segment of the Paleo-Asian Ocean in the Huadian area of central Jinlin Province likely occurred between the Early Triassic and Middle Triassic.

Detrital zircon U–Pb geochronology and geochemistry of late Neoproterozoic – early Cambrian sedimentary rocks in the Cathaysia Block: constraint on its palaeo-position in Gondwana supercontinent

AbstractWe present updated U–Pb ages and Hf isotopic compositions of detrital zircons and whole-rock geochemical data to investigate the provenance and tectonic setting of late Neoproterozoic and early Cambrian sandstones from the Cathaysia Block, in order to offer new constraints on its tectonic evolution and its palaeo-position within the supercontinent. The source rocks for the studied sandstones were dominated by felsic–intermediate materials with moderate weathering history. U–Pb dating results show major populations atc. 2500 Ma, 1000–900 Ma and 870–716 Ma with subordinate peaks at 655–532 Ma, consistent with the global Neoarchean continental crust growth, assembly and break-up of Rodinia, and Pan-African Event associated with the formation of Gondwana. Zircon U–Pb ages and Hf isotopic data suggest that most derived from exotic terranes once connected to the Cathaysia Block. Using whole-rock geochemical analysis, it was determined that the studied sedimentary rocks were deposited in a passive continental margin and the Cathaysia and Yangtze blocks were part of the same continent; no Cambrian ocean existed between them. Compiling a detrital zircon dataset from Qiangtang, northern India, the Lhasa Terrane and Western Australia, the Cathaysia Block seems to be more similar to the Qiangtang and western part of the northern India margin, instead of having a direct connection with the Lhasa Terrane and Western Australia in the Gondwana reconstruction during the late Neoproterozoic and Cambrian eons.

New insights into multiple provenances evolution of the Jurassic from heavy minerals characteristics in southern Junggar Basin, NW China

Abstract The method of random forest was used to classify the heavy mineral assemblages of 2 418 Jurassic samples in the southern Junggar Basin, and determine the distribution of the heavy mineral assemblages from the same provenance systems. Based on the analysis of heavy minerals assemblages, ZTR index, sedimentary characteristics, U-Pb zircon ages, whole-rock geochemical and paleocurrent direction analysis, the study reveals that five important provenances were providing sediments to the southern Junggar Basin in the Jurassic period: The North Tianshan (NTS), Central Tianshan (CTS), Bogda Mountains, Zhayier Mountains and Kalamaili Mountains. During the Early Jurassic, NTS-CTS, Kalamaili Mountains and Zhayier Mountains are primary provenances, Bogda Mountains started to uplift and supply clastic materials in the Middle Jurassic. There are three sedimentary area in the Jurassic of southern Junggar Basin: the western part, the central part and the eastern part. In the western part, the clastic materials of the Early Jurassic was mainly from NTS blocks and Zhayier Mountains, and the sediments were dominantly derived from the Zhayier Mountains during the Middle–Late Jurassic. In the central part, the main provenance of the Early Jurassic switched from NTS to CTS. In the Xishanyao Formation, the main source went back to NTS again. The NTS was the primary provenance during the sedimentary periods of Toutunhe Formation and Qigu Formation. In the eastern part, the contribution of CTS and Kalamaili Mountains were considered as major provenances in the Early Jurassic−Xishanyao Formation, small proportion of sediments were from NTS. The Bogda mountains uplifted and started to provide sediments to the Junggar Basin in the sedimentary period of Xishanyao Formation, and became the major source during the Toutunhe Formation period, with small amount of sediments from CTS. The provenance from CTS was hindered during the sedimentary period of Qigu Formation owing to the uplifting of the Bogda mountains, and the sediments were mainly from the Bogda mountains and NTS.