Depletion In High Field Strength Elements Research Articles

Petrogenesis, tectonic setting, and metallogenic significance of the Middle Permian volcanic rock system of the Miaoling Formation, Yanbian area, NE China: Constraints from geochronology, geochemistry, and Sr–Nd–Hf isotopes

The volcanic rock system of the Miaoling Formation contains the main ore-bearing rocks of two volcanogenic massive sulfide (VMS)-type deposits in the Yanbian area of NE China. Investigation of the VRSMF is needed to better understand the formation of these VMS-type deposits and the tectonic evolution of the Yanbian area. To determine the petrogenesis, material sources, and formation age of the VRSMF, and elucidate its late Paleozoic tectonic evolution and metallogenic significance, this paper presents new petrological, geochronological, geochemical, whole-rock Sr–Nd and in situ zircon Hf isotopic data for the VRSMF. The VRSMF is composed of marine carbonate, intermediate–felsic volcanic rocks (andesite–trachyandesite–dacite) and pyroclastic rocks. Laser-ablation–inductively coupled plasma–mass spectrometry zircon U–Pb dating gives an eruption age of ca. 265 Ma for the pyroclastic rocks in the VRSMF. These rocks are classified as low- to medium-K calc-alkaline series. They are characterized by enrichments in large-ion lithophile elements (e.g., K, Rb, and Ba) and light rare earth elements, and depletions in high field-strength elements (e.g., Nb, Ta, and Ti) and heavy rare earth elements, showing affinity to igneous rocks formed in arc-related tectonic settings. These features, together with homogeneous zircon εHf(t) values of 10.9–15.7 and depleted Sr–Nd isotopic compositions [εNd(t) values of 2.4–5.0], suggest that the parental magma was derived from the partial melting of depleted mantle that had been metasomatized by subduction-related fluids. These results, along with findings of regional geological investigations, suggest that the formation of the VRSMF was related to subduction of the Paleo-Asian oceanic plate during the middle Permian. The VMS-type mineralization in the Hongtaiping and Dongfengnanshan deposits is interpreted to have formed in a bimodal–felsic setting in a back-arc extensional tectonic environment.

40Ar–39 Ar dating, whole-rock and Sr-Nd isotope geochemistry of the Middle Eocene calc-alkaline volcanic rocks in the Bayburt area, Eastern Pontides (NE Turkey): Implications for magma evolution in an extension-related setting

Discussions continue about whether Middle Eocene magmatism in the Eastern Pontides is associated with collision or subduction. This paper presents new whole-rock geochemistry, Sr-Nd isotopic and 40Ar-39Ar age data for Middle Eocene volcanic rocks from the Bayburt area of the Eastern Pontides (NE Turkey) to investigate their sources and evolutionary history. The new 40Ar–39Ar ages reveal that these volcanic rocks erupted between 44.6 ± 0.1 Ma and 43.5 ± 0.1 Ma, within the Lutetian (Middle Eocene). The studied volcanic rocks are composed of basalt, andesite, basaltic andesite and minor dacite lava and pyroclastic rocks. These rocks consist of plagioclase, amphibole, pyroxene, olivine, biotite, sanidine and minor quartz phenocrysts with Fe-Ti oxides. They have microlithic, hyalo-microlithic, porphyritic and rarely glomeroporphyritic textures. The volcanic rocks have low to high-K calc-alkaline affinities. They display enrichment in large-ion lithophile elements and depletion in high-field strength elements with high Th/Yb ratios, which indicate that the magmas forming the volcanic rocks were derived from lithospheric mantle sources enriched by mostly slab-derived fluids in the spinel stability field. 87Sr/86Sr(i) values vary between 0.70485 and 0.70551 and 143Nd/144Nd(i) values vary between 0.51255 and 0.51267. These data correspond to the mantle array on the isotope ratio diagram. The main solidification processes consist of fractional crystallization with minor assimilation. In light of the data obtained in this study together with data from previous studies, petrogenetic character of the Middle Eocene magmas from the southern parts of the Eastern Pontides may be explained by melting of an enriched lithospheric mantle source initially metasomatized by subduction fluids in a post-collisional extensional-related tectonic setting.

Geochemical and zircon U-Pb geochronological constraints on late mesozoic Paleo-Pacific subduction-related volcanism in southern Vietnam

Late Mesozoic volcanic rocks comprising mainly basalt and basaltic-andesite to dacite occur in south-central Vietnam (Dalat zone) and to a lesser extent in southwestern Vietnam (Bay Nui area). Mineral and whole-rock chemistry indicate a calc-alkaline affinity for samples in the Dalat zone and a high-K calc-alkaline to shoshonitic affinity for rocks in the Bay Nui area. Mineral characteristics and variation diagrams of selected elements suggest that fractional crystallization dominated during magma differentiation. The Bay Nui volcanic rocks generally are more enriched in potassium and LILEs (large-ion lithophile elements) than volcanic rocks from the Dalat zone, which may indicate a more evolved nature or crustal assimilation. The similar chemical characteristics and eruption/emplacement age range of volcanic and plutonic rocks (ca. 90–110 Ma) of equal silica concentration indicate that the magma feeding the volcanic eruptions had the same source as that of the plutonic rocks. The observed mineral and whole-rock compositions with enrichment in LILEs, depletion in HFSEs (high field strength elements), and noticeably negative Nb, Ta, and Ti anomalies are characteristic for arc signatures. Zircon U-Pb geochronological data for the volcanic rocks indicate an age range of 95–105 Ma for the eruption. These geochemical and geochronological data link this Late Mesozoic volcanism with continental arc magmatism driven by the subduction of the Paleo-Pacific beneath eastern Indochina. Zircon xenocrysts with a likely magmatic origin cluster around 350 Ma and 250 Ma, indicating two earlier magmatic events most likely related to the subduction of the Paleo-Tethys beneath western Indochina and the subsequent Indosinian orogeny.

Two episodes of Eocene mafic magmatism in the southern Lhasa terrane imply an eastward propagation of slab breakoff

Past geodynamic events can be reconstructed using the spatial and temporal patterns of geochemical variability in igneous rocks. Here we apply this principle to define the timing and geometry of breakoff of the Neo-Tethyan slab after the India-Asia collision. Two episodes of mafic magmatism (∼53 Ma and ∼ 48 Ma) are preserved in the Quxu area of the southern Lhasa terrane (SL). We present their whole-rock geochemistry, zircon U-Pb ages, and in situ zircon Hf isotope ratios. The early gabbro (∼53 Ma) exhibits typical arc basalt characteristics, including enrichment of large-ion lithophile elements (LILEs), depletion of high-field strength elements (HFSEs), and radiogenic Nd and zircon Hf isotopes, implying that they were formed by partial melting of metasomatized lithospheric mantle. In comparison tothe early gabbro, the late hornblende gabbro (∼48 Ma) contains higher contents of HFSEs (e.g., Nb, Zr) and relatively less radiogenic Nd and zircon Hf isotopes, suggesting interaction between the enriched sub-continental lithospheric mantle and asthenospheric mantle. It is likely that upwelling asthenosphere reached the central SL around 50 Ma and that the Neo-Tethyan slab tore off under this area at this time. The spatial and temporal distribution of mafic rocks across the whole southern Lhasa terrane show that this geochemical transition and hence, presumably, the tearing off of the Neo-Tethyan slab, propagated eastward along the collision zone throughout Paleocene-Eocene time.

Late Paleozoic Shoshonitic Magmatism in the Southwestern Middle Tianshan (Tajikistan) of the Southwestern Altaids: Implications for Slab Roll-Back With Extensional Arc-Related Basins After Flat Subduction

Paleozoic magmatic rocks are widespread in the western Middle Tianshan. Their petrogenesis can provide important insights into the geodynamic evolution of the southwestern Altaids. Here, we present an integrated study of U–Pb zircon geochronology and geochemical and Lu–Hf isotopic compositions for the Late Paleozoic shoshonitic Chorukhdairon pluton and genetically and spatially related quartz porphyry in the southern Chatkal–Kurama terrane, western Middle Tianshan. The Chorukhdairon pluton mainly comprises monzodiorite and quartz monzodiorite (first phase), quartz monzonite (second and main phase), monzogranite (third phase), and leucomonzogranite (fourth phase). LA–ICP–MS zircon dating yielded magma crystallization ages of 294–291 Ma and 286 Ma for the Chorukhdairon pluton and quartz porphyry, respectively. All the rocks possess high K2O content (3.29–5.90 wt.%) and show an affinity with shoshonite series rocks. They display similar trace element compositions characterized by the enrichment of large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion of high-field strength elements (e.g., Nb, Ta, P, and Ti), compatible with typical arc magmatism. Combined with zircon Lu–Hf isotopic data, we suggest that the Chorukhdairon pluton was produced by partial melting of the enriched mantle, followed by fractional crystallization of pyroxenes, amphibole, plagioclase, biotite, and accessory Fe–Ti oxides, apatite, and zircon. The quartz porphyries are similar to highly fractionated I-type granitic rocks, and their parental magma could result from the mixing of different batches of mantle-derived magmas or magmas derived from the mantle and juvenile lower crust. Considering the continuousness of the Middle Carboniferous to Early Permian magmatism in the western Middle Tianshan and other regional geological data, we suggest that the Chorukhdairon pluton and related quartz porphyry probably formed in an oceanic subduction setting. Furthermore, the temporal and spatial evolution of the Paleozoic magmatism imply that the flat-slab subduction that was induced by the subduction/accretion of seamounts probably occurred beneath the Middle Tianshan during the Middle Devonian to Early Carboniferous, after which the southeastward slab roll-back occurred during the Middle Carboniferous to Early Permian. The late slab roll-back was responsible for the southeastward arc magmatism migration and magmatic flare-up in the Chatkal–Kurama terrane, western Tianshan, and led to the formation of arc-related extensional basins and significant crustal growth in the southwestern Altaids.

Geology and Petrogeochemistry of Lijiapuzi Nb-Ta Granitic Pegmatite Deposit: Implications for Ore Genesis and Prospecting

Most of the rare metal pegmatite deposits in China lie in West, Central and South China, and the Lijiapuzi Nb-Ta deposit is the only pegmatite-type Nb-Ta deposit in Northeast China, therefore a detailed study of the Lijiapuzi deposit is of great importance to the metallogeny and exploration of rare metal deposits in Northeast China. The Nb-Ta bearing pegmatites in Lijiapuzi district are composed of a microcline wall zone, a muscovite (lepidolite)-albite intermediate zone and a quartz core, and the rare metal mineralization mainly occurs in the intermediate zone and is intimately related to the albitization. The lithogeochemical features indicate that Lijiapuzi pegmatoids are rich in silicon and alkaline, poor in calcium, magnesium, titanium and phosphorus, and belong to peraluminous granitoid rocks. The enrichment of large ion lithophile elements (LILEs) and depletion of high field-strength elements (HFSEs), the distinct right-inclined REE pattern with obvious Eu negative anomaly, lower ratio of Nb/Ta and higher ratio of La/Nb and Rb/Sr indicate that the pegmatite was of a crustal origin and formed in relatively extensional tectonic setting of post-orogenesis. The pegmatites in Lijiapuzi area can be divided into two categories, i.e., the NW-trending pegmatites are shorter and thicker, with obvious zoning and localized in the upper part of the near-surface, while the NE-trending pegmatites are longer, thinner, devoid of zoning and localized in the lower part of the district. This probably means that excellent ore prospecting potential exists in the lower part of the Lijiapuzi district.

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

Geochronology, whole-rock geochemistry, Sr[sbnd]Nd isotopes, and biotite chemistry of the Deh-Bala intrusive rocks, Central Urumieh-Dokhtar Magmatic Arc (Iran): Implications for magmatic processes and copper mineralization

The Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran hosts a variety of volcano-plutonic rocks of different composition and age formed by subduction of Neo-Tethyan Ocean beneath Eurasian supercontinent and the subsequent processes of post-subduction magmatism. This study focuses on mineral chemistry of biotite and amphibole, zircon UPb geochronology, whole-rock geochemistry, and SrNd isotopic geochemistry in the Deh-Bala granitoid, in an attempt to investigate the porphyry-epithermal mineralization potential. The Deh-Bala intrusions in the central part of the UDMA consist mainly of granodiorite and minor tonalite, with microgranular enclaves (MEs) of gabbrodiorite, diorite, and monzodiorite composition. In addition, the main ore minerals associated with more altered and mineralized varieties of these intrusions include chalcopyrite, covellite, malachite, and pyrite. The silicic, argillic, phyllic (sericitic), and propylitic facies of alteration occur in parts of the Deh-Bala area; these types of alteration, which are typically considered the most important predictors of porphyry copper‑gold and (or) epithermal precious metal mineralization.LA-ICP-MS UPb dating of zircons reveals that the pluton was emplaced during Middle-Eocene (ca. 39.0 Ma). The Deh-Bala granitoids (DBG) are calc-alkaline in composition (Na2O + K2O: 4.8–7.3 wt%) and display SiO2 contents from ∼53 to ∼67 wt%. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with marked Eu anomalies. They are also characterized by an enrichment in large-ion lithophile elements (LILEs, such as Rb, K, and U) and a depletion in high-field strength elements (HFSE, such Nb, Y, Ti, and Zr), displaying obviously depletions with Ba, P, and Sc. The DBG shows whole-rock initial 87Sr/86Sr ratios (ISr) of 0.7048–0.7061, εNd(t) values of −1.79 to +1.64, and Nd two-stage depleted mantle model ages (TDM2) from 747 to 1004 Ma. These isotopic data, combined with the geochemical signatures, indicate that the DBG originated through mixing of mafic and felsic end-members, the former was derived from melting of the lithospheric mantle, and the latter derived from partial melting of Neoproterozoic crustal materials in a continental-arc.Biotite compositions analysed by electron microprobe are consistent with the Deh-Bala magmas being oxidized and chlorine-rich, similar to worldwide porphyry copper‑gold systems formed in arc-related settings associated with convergent margin subduction zones. Such a chlorine-rich magmatic-hydrothermal system is known to be effective in scavenging copper and (or) gold from melt and its transfer to shallow levels.

Geochemistry and Petrogenesis of Lower Jurassic Mafic Rock Suites in the External Rif Belt, and Chemical Geodynamics of the Central Atlantic Magmatic Province (CAMP) in Northwest Morocco

We present new field evidence, geochemical and isotopic data, and age constraints on Lower Jurassic mafic rock suites within a >200-km-long curvilinear belt in the Rif orogenic belt in northern Morocco and show that these rock assemblages formed as part of the Central Atlantic Magmatic Province (CAMP). The CAMP represents a large igneous province that straddles the edges of the modern peri-Atlantic continents. It developed ~200 Ma, following the initiation of the breakup of Pangea. Main magmatic rocks in the Rif External Zone include basaltic lavas, massive dolerite, and isotropic and cumulate gabbros, all intruded by dolerite and trondhjemite dikes and sills. Available U-Pb zircon ages from dolerite, gabbro, and trondhjemite dike rocks are , , and , respectively. Based on their geochemical affinities and isotopic compositions, the analyzed rocks define basalt-dolerite and gabbro-cumulate gabbro-trondhjemite groups. The basalt-dolerite group samples are subalkaline in nature and have low TiO2 contents, whereas the gabbro-cumulate gabbro-trondhjemite group samples are alkaline and display high TiO2 values. Most samples are tholeiitic in character and show large-ion lithophile and light rare earth element enrichment and high field strength element depletion compared with normal mid-ocean ridge basalt (MORB). Samples of both groups display low 143Nd/144Nd201 Ma (0.51182–0.51262) and high 87Sr/86Sri ratios with ɛNd values ranging from −1.51 to +4.85. The basalt-dolerite group rocks have enriched MORB compositions, compatible with the low-Ti CAMP suites, whereas the gabbro-cumulate gabbro-trondhjemite group rocks have oceanic island basalt compositions reminiscent of high-Ti CAMP suites in other continents. Magmas of the gabbro-cumulate gabbro-trondhjemite group underwent differentiation through tholeiitic fractionation. Magmas of the rocks of both groups included melt components, originated from partial melting of a previously subduction-modified subcontinental lithospheric mantle. Our results indicate that the Early Jurassic CAMP magmatism in northern Morocco marked a major episode of continental magmatism before the opening of the Maghrebian Tethys between Africa and Iberia in the latest Jurassic.

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

Geochemistry of late paleoproterozoic Anjana and Amet granites of the Aravalli craton with affinities to sanukitoid series granitoids: Implications for petrogenetic and geodynamic processes

The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO2, high MgO, Mg#, K2O, Ba, and low Na2O/K2O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO2 content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.

Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China

Lamprophyre dykes that crosscut different types of ultrahigh pressure (UHP) metamorphic rocks are widely distributed in the Triassic Sulu UHP orogenic belt. Although abundant studies have been performed on these dykes, their origin and petrogenesis remain topics of controversy. This study presents the results of a detailed field-based study of petrology, whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes on lamprophyre dykes exposed in the central Sulu UHP zone, aiming at shedding lights on their petrogenesis and providing clues on the geological evolution of eastern continental China during the Cretaceous. The lamprophyres are typically porphyritic, with phenocrysts dominantly composed of amphibole and clinopyroxene set in a lamprophyric matrix. The dykes have moderate SiO2 (47.70 wt.%–60.44 wt.%), variably high MgO (2.58 wt.%–8.28 wt.%) and Fe2O3T (4.88 wt.%–9.26 wt.%) contents with high Mg# of 49–66. Geochemically, they have enriched light rare earth element (REE) and flat heavy REE patterns ((La/Gd)N=5.14–10.56; (Dy/Yb)N=1.43–1.54) with negligible Eu anomalies (Eu/Eu*=0.83–1.10), and they show enrichment in large ion lithophile elements (e.g., Ba and K) but depletion in high-field strength elements (e.g., Nb, Ti and P). In-situ zircon U-Pb geochronology reveals that the lamprophyres have concordant ages of 120–115 Ma, demonstrating that the dykes emplaced in the Early Cretaceous. These zircons have eHf(t) values ranging from −26.0 to −11.0. Inherited zircons that occur in the dykes are dated to be Neoproterozoic, in line with the protolith ages of their host (i.e., the UHP rocks). An integration of these data allows us to propose that the lamprophyres were generated during the Cretaceous, by melting of subcontinental lithospheric mantle-derived metasomatite with enriched chemical compositions underneath the North China Craton. The metasomatite was formed mainly by peridotite-fluid/melt reactions, with the fluids/melts mainly liberated from subducted Yangtze continental crust during the Triassic. Regional extension, lithospheric thinning and mantle upwelling caused by rollback of the subducted paleo-Pacific plate is considered to account for the generation of the lamprophyres as well as the extensive arc-like magmatic rocks in eastern continental China during the Early Cretaceous.

Genesis of Cretaceous igneous rocks and its related large scale porphyry Cu-Au mineralization in Chating, the Middle-Lower Yangtze River Metallogenic Belt: The geochemical constrains

The newly discovered large Chating porphyry Cu–Au deposit is associated with early Cretaceous intrusive rocks in the Middle-Lower Yangtze River Belt (MLYRB), eastern China. Two types of intrusive rocks in the Chating area have been identified, i.e., the quartz-diorite porphyry, related to the Cu–Au mineralization, and the diorite porphyry that is ore-barren. Zircon LA–ICP–MS U–Pb dating yields 145–136 Ma for the Chating intrusions. They both belong to High-K calc-alkaline series, with enrichments of large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), depletions of high field-strength elements (HFSEs) and heavy rare earth elements (HREEs), and no or slightly negative Eu anomalies. The quartz-diorite porphyry shows high Sr contents and Sr/Y ratios; low MgO, Cr and Ni contents; medium (La/Yb)N and Mg# (34.6–54.6) and low K2O/Na2O (<1) values; decoupling of Sr/Y and (La/Yb)N; and low Th/U (3.49–6.08) values, thus exhibiting characteristics of slab-derived adakitic rocks. In addition, it has slightly enriched Sr–Nd–Hf isotopes ((87Sr/86Sr)i = 0.70655–0.70717, εNd(t) = −6.9 to −8.8, and εHf(t) = −7.6 to −10.6) and high radiogenic Pb isotopes ((206Pb/204Pb)i = 18.22–18.70, (207Pb/204Pb)i = 15.63–15.66 and (208Pb/204Pb)i = 38.49–38.97), reflecting the involvement of subducted sediments in the magma source. Therefore, the fertile quartz-diorite porphyry was most likely generated by the partial melting of the slab-derived, subducted sediments and a partial degree of metasomatized mantle-derived magma with crustal assimilation. As to the barren diorite porphyry, it has similar Sr–Nd–Pb isotope and trace element features as the fertile adakitic samples. However, it shows relatively lower SiO2 contents; high contents of MgO (3.89–4.06 wt%), Cr and Ni (avg. 47.6 ppm and avg. 15.01 ppm); high Mg# (51.2–69.8); and more enriched Hf isotope characteristics (−8.5 to −12.9), which imply that this barren diorite porphyry could be generated from the partial melting of enriched metasomatised lithospheric mantle assimilated with old continental crust materials. The δ34S values of the pyrite and chalcopyrite of the ores in Chating indicate a magmatic-derived source, resembling other porphyry-skarn Cu–Au deposits in the MLYRB. The fertile Qtz-diorite porphyry adakitic rocks are characterized by higher oxygen fugacities than the barren diorite porphyry, which are closely related with the partial melting of the subducted oceanic slab in an oxidizing environment, thus making them favorable for Cu–Au mineralization.

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

AbstractThe Chisana Formation consists of Lower Cretaceous volcanic rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that the volcanic succession is &gt;2 km thick at the Bonanza Creek type section. We present stratigraphic, geochemical, Sr‐Nd‐Pb isotope, and U‐Pb age data from samples collected from various stratigraphic levels of the Chisana Formation. We demonstrate that the Chisana Formation can be divided into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites. Trace element geochemistry shows high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc‐alkaline to tholeiitic chemistries, all consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. Volcanism occurred between ~131 and 117 Ma judging from previously reported lava ages and new U‐Pb ages of detrital zircons recovered from sandstones that conformably underlie the lowermost Chisana Formation lavas. Our results support existing tectonic models in which an east dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake Formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of Wrangellia against inboard terranes.

Neoarchean seafloor hydrothermal metamorphism of basalts in the Zanhuang ophiolitic mélange, North China Craton

Epidosites are suggested to form in up-flow zones located at the base of ore-forming oceanic hydrothermal systems, and are commonly preserved in ophiolitic sequences. However, occurrences of epidosites in Archean tectonic mélanges are rarely documented. In this study, we report the presence of late Neoarchean epidosites in the Zanhuang ophiolitic mélange in the North China Craton using meter-scale litho-structural and petrologic analysis, zircon U-Pb dating, major and trace element, and oxygen and strontium isotopic geochemistry. Field studies show that the epidosites are mainly preserved as the cores of meta-pillow lavas, and are distributed as blocks in different matrices within the tectonic mélange. The pillow rims are used as protolith to estimate the element mobility during the alteration process. The epidosites display strong depletion in Na2O (0.01–0.27 wt%), MgO (0.43–3.01 wt%), depletion in large ion lithophile elements (LILE) (e.g. Rb, Ba, Cs, U) and transition metals (e.g., Co, Sc, Cu, Ni, Zn), and show enrichment of CaO (13.35–21.0 wt%), Sr and Pb. The high field strength elements (HFSE) (e.g. Zr, Hf, Ta, Th) of epidosites are varied but rare earth elements (REE) are unchanged. The amphibolitic pillow rims are characterized by enrichment of LILE, flat heavy rare earth elements (HREE) patterns, and depletion in HFSE with negative Ti anomalies and positive Pb and Ba anomalies. Oxygen and strontium isotopic ratios of epidosites are 5.0–8.6‰ and 0.70284–0.70551, respectively, both of which are greater than those of fresh basalts. The major and trace elements, oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence. Magmatic zircons from two samples of epidosites yield 207Pb/206Pb ages of 2512 ± 13 Ma (MSWD = 0.51) and 2542 ± 13 Ma (MSWD = 1.4), confirming that the epidosites were generated at ca. 2.51–2.54 Ga. Based on geochemical data and field observations, we suggest that the protolith of the epidosites formed in an arc-related tectonic setting during the late Neoarchean. This study shows that hydrothermal activity operating in the late Neoarchean was similar to that of the modern ocean floor.

Early Cretaceous Wulong intermediate-mafic dike swarms in the Liaodong Peninsula: Implications for rapid lithospheric delamination of the North China Craton

The North China Craton (NCC) underwent a dramatic change in the lithospheric architecture, and more than 100 km of the ancient and refractory lithospheric mantle was removed. However, the timing and mechanism of the lithospheric thinning remain controversial. New zircon UPb ages and whole-rock/in-situ geochemical and isotopic data of the Early Cretaceous intermediate-mafic dike swarms in the Liaodong Peninsula have been carried out to decipher the timing and mechanism of lithospheric thinning of the NCC. The dike swarms are chemically subdivided into the ca. 126 Ma diorite porphyry and coeval (ca. 119 Ma) low-Ti (TiO2 < 1.20 wt%, Ti/Y < 375) and high-Ti (TiO2 > 1.90 wt%, Ti/Y > 580) diabase dikes. The diorite porphyries exhibit a moderate silica (56.71–61.13 wt%) and high-K calc-alkalic signature, with low TiO2 and total Fe2O3 and high Cr and Ni compositions. They are enriched in large ion lithophile elements (LILEs) (e.g., Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, Ti, and P). They have high radiogenic Sr isotopes (whole-rock (87Sr/86Sr)i = 0.711553–0.714284, in-situ (87Sr/86Sr)i of plagioclase = 0.71392), low εNd(t) values (−19.9 to −14.5), and old Nd model ages (1.81–2.25 Ga). Meanwhile, some plagioclase phenocrysts have reverse zonings with Na-rich cores (An: 35–48) and Ca-rich rims (An: 48–57). These features imply that the diorite porphyries were generated by mixing between magmas from the Archean–Paleoproterozoic lower crust and the lithospheric mantle. The low-Ti diabase dikes are characterized by medium to high-K calc-alkaline series with high Al2O3 but low TiO2 contents. They show arc-like trace element signatures, such as enrichment in LILEs (Rb, K, Pb, and Sr) and depletion in HFSEs (Nb, Ta, Zr, Hf, Ti, and P). They also have relatively lower initial 87Sr/86Sr ratios (0.706331–0.708315 and 0.70837 for the whole-rock and in-situ Sr isotopes, respectively) compared to the porphyries and negative εNd(t) values (−13.6 to −5.0), indicating that they were derived from partial melting of an enriched lithospheric mantle. Their variable Th/Yb and Th/Nb ratios with constant Sr/Nd and U/Th values further suggest that the mantle source might have been previously modified by the subduction-related melt. In contrast, the high-Ti diabase dikes belong to the high-K calc-alkaline to shoshonitic series, with higher TiO2 and total Fe2O3 but lower Al2O3 contents. They have OIB-like trace elements (e.g., slight enrichments of Nb and Ta with no HFSE depletion, high Nb/U values (42 ± 5)) and SrNd isotopic compositions (e.g., positive whole-rock εNd(t) values (+3.1 to +4.3)), suggesting that they were generated from an asthenospheric mantle source. The coexisting Wulong low-Ti and high-Ti mafic dikes with similar formation ages (ca. 119 Ma) may indicate a transition from lithospheric to asthenospheric mantle source. In combination with previous studies on the Mesozoic–Cenozoic mafic rocks in the Eastern Block of the NCC, we propose that the lithospheric thinning beneath the Liaodong Peninsula initiated at ca. 119 Ma and this process beneath the NCC lasted for a narrow duration (ca. 13 Ma). The unsteady mantle flows due to the rapid subduction and rollback of the Paleo-Pacific plate might trigger the thickened, modified lithosphere to lose its gravitational stability, eventually leading to lithospheric delamination and thinning of the NCC.

Petrochemical and isotopic evidences for multiple sources involving in the generation of post-collisional granitoid complex: A case study of the Mangling complex from the eastern Qinling orogen, China

Granitoids widely occur in collisional orogens, which are a critical target for probing the nature of deep crust and mantle-crust interaction. In this study, we present an integrated study of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock geochemistry, for the Mangling complex in the eastern Qinling orogen. The Mangling complex is composed mainly of monzogranites with some mafic microgranular enclaves (MMEs) and subordinate diorites. Zircon U–Pb dating yields identical crystallization ages of 141–143 Ma for the diorites and monzogranites, suggesting that they are formed simultaneously and belong to post-collisional magmatism. The Mangling diorites have relatively low SiO2 but high MgO, Cr and Ni contents and Mg# values. They are characterized by enrichment of large ion lithophile elements (LILEs) but depletion of high field-strength elements (HFSEs). They show initial Sr isotope (ISr) of 0.706491 to0.706622, εNd(t) of −10.82 to −10.38, and zircon εHf(t) of −10.8 to −7.9 and δ18O values of 6.48–7.18‰. These geochemical features indicate that the diorites were derived from an enriched lithospheric mantle source. In contrast, the Mangling monzogranites have high silica and low Mg# values. They exhibit more variable isotopic compositions: Isr = 0.707191–0.708310, εNd(t) = −13.74 to −8.10, and zircon εHf(t) = −17.2 to −7.0 and δ18O = 5.83–7.08‰. Some inherited zircon cores have been observed in the monzogranite samples with U–Pb ages of 1773–1791 Ma. These inherited cores have δ18O values of 5.42–5.69‰ and εHf(t) values of −7.8 to −5.9. Their U–Pb ages and Hf–O isotopic compositions are similar to those of the Xiong'er Group in the southern margin of the North China Craton (NCC), which have underthrusted the juvenile (in terms of isotopes) Kuanping unit where the Mangling complex has been emplaced. Isotopic modeling shows that the monzogranites were likely produced by fusion of the Kuanping unit and Xiong'er Group, with the involvement of enriched mantle-derived materials. The crustal rocks as well as the enriched sub-lithospheric mantle were both contributed to the Mangling complex, and some of the crustal components show more juvenile isotopic fingerprints. Therefore, it is not a mandatory to involve a depleted juvenile mantle-derived component to recognize mantle-crust interaction during granitoid generation, especially in which the preceded juvenile arcs were superimposed on ancient crust and/or enriched mantle. The ‘inverse’ isotopic compositions in crust and mantle components imply that a detailed petrochemical and isotopic study should be conducted to address the issue about crust-mantle interaction during granitoid generation.

Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet

Andesites in the Yangla district of the Sanjiang Tethys, eastern Tibet, are characterized by relatively high MgO and with rich large-ion lithophile elements (LILEs), indicating similarities to typical high-Mg andesites formed in continental collisional zones. Zircon U-Pb age of 232 ± 1 Ma for these andesites shows that they occurred synchronously with the regionally widespread late Middle Triassic granitoids and bimodal volcanic rocks. Primitive mantle-normalized spidergrams for these andesites show depletion in high field-strength elements (HFSEs), with Sr-Nd-Pb isotopic compositions that are comparable to both arc- and MORB-like high-Mg rocks generated from subduction-modified sources. Modelling results suggest that an input of ~20% sediment-derived melts into mantle-derived melts could be possibly responsible for the Sr-Nd isotopic characteristic of these andesites. MORB- and arc-like compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the Paleo-Tethys Ocean and subsequent continental collision. It is speculated that the late Middle to Late Triassic intra-continental re-activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction-modified mantle, leading to the formation of the high-Mg andesites in the Yangla district.

The Late Ordovician granitoids in the East Kunlun Orogenic Belt, Northwestern China: petrogenesis and constraints for tectonic evolution of the Proto-Tethys Ocean

Petrology, whole-rock geochemistry, in situ zircon U–Pb ages, and Lu–Hf isotopic data are reported for granodiorite and monzonitic granite samples from the Harizha Region in the eastern section of the East Kunlun Orogenic Belt. The results of the LA-ICP-MS zircon dating indicate that the granodiorite and monzonitic granite were emplaced at 447.9 ± 3.0 Ma and 443.8 ± 3.5 Ma, respectively, i.e., in the Late Ordovician–Early Paleozoic periods. The granodiorite is characterized by relatively high MgO (4.52–5.91 wt%), Sr (541–712 ppm), Cr (218–357 ppm), Ni (56.7–80.7 ppm) and V (66.8–94.7 ppm), with low Y (10.40–13.60 ppm) and Yb (1.05–1.45) contents. This results in elevated Mg# (65–68), Sr/Y (39.78–58.84), and (La/Yb)N (10.21–18.30) ratios, common features for adakitic high-Mg andesite (HMA). The granodiorite is metaluminous to weakly peraluminous and high-K calc-alkaline in composition. In addition, the granodiorite is enriched in light rare-earth elements (LREE) and large ion lithophile elements (LILEs, Rb, K, Sr, and Pb) U, and Nd; and depleted in high-field strength elements (HFSEs, Nb, Ta, Zr, Hf, Ti) and P. The zircon grains from the granodiorites have eHf(t) values of − 5.5 to + 4.7, with two-stage Hf model ages (tDM2) ranging from 0.92 to 1.71 Ga. These features suggest that the granodiorites were formed by mixing processes of crustal melts and magma from a metasomatized mantle in a subduction setting. The monzonitic granite possesses high normative corundum values (2.34–4.10 wt%), is strongly peraluminous (A/CNK ratios of 1.16–1.32, with minor mineral muscovite), with high-K calc-alkaline affinity. Furthermore, enrichments in large ion lithophile elements (Rb, K, and Pb), U, Th, and Nd, depletions in high-field strength elements (Nb, Ta, and Ti) and Ba and P, and mainly right-inclined REE patterns with negative Eu anomalies, exhibits an S-type granite affinity. The magmatic zircons from the monzonitic granite yielded eHf(t) values between − 4.9 and − 2.7, corresponding to two-stage Hf model ages of 1.60–1.73 Ga. Therefore, this study suggests that the monzonitic granite magma was derived from crustal material (metagreywacke) in a syn-collision setting. Subsequently, by combining these findings with the previous research results, this study propose that a transverse diachronism closure model which entailed an “earlier on the sides–later in the middle” in an East–West direction in the East Kunlun Orogenic Belt for the Proto-Tethys Ocean’s evolution process.

Origin of mafic intrusions in the Micangshan Massif, Central China: Implications for the Neoproterozoic tectonic evolution of the northwestern Yangtze Block

The mafic intrusions exposed in the Micangshan Massif preserve important information pertaining to the Neoproterozoic tectonic evolution along the northwestern margin of the Yangtze Block. In this paper, we report new LA-ICP-MS zircon U–Pb ages, Lu–Hf isotopes and whole-rock compositions for mafic sills in the Micangshan Massif to understand their petrogenesis and supplement the regional tectonic evolution. Xenocrystic zircon grains from two mafic sill samples yield weighted mean ages of 860 ± 4 Ma and 858 ± 5 Ma, respectively, suggesting the maximum emplaced age of the mafic sills was at ca. 860 Ma. A granitic dyke sample which intruded into the mafic sills, yields a crystallization age of 799 ± 4 Ma, implying the minimum emplacement of the mafic sills was at ca. 800 Ma. These results indicate that the mafic sills were formed within the span of ca. 860–800 Ma. Geochemical results show that most of these mafic sills are tholeiitic in composition and are characterized by low to moderate FeOtotal and TiO2 contents, as well as moderate to high MgO contents and Mg# values. They display distinct enriched light rare earth element (LREE) patterns, variable remarkable enrichment in large-ion lithophile elements (LILEs; e.g., Cs and Ba), and evident depletion of high-field strength elements (HFSEs; e.g., Nb, Ta and P), resembling signatures of typical subduction-related basaltic rocks. In addition, these sills exhibit comparatively high Nb/Ta, moderate Nb/Yb, as well as low Nb/La, Th/Yb, and Nb/Y ratios, indicating that they probably originated from partial melting of a previously metasomatized mantle wedge source. These observations, together with regional tectonics, suggest that these mafic sills in the Micangshan Massif might be formed in a subduction-related continental arc setting. Therefore, combining our new data with available results from previous studies in the adjacent regions, our study supports the dominant subduction-related tectonics along the northwestern margin of the Yangtze Block during the Early Neoproterozoic time.