Late Cretaceous Volcanic Rocks Research Articles

Newly identified late Early Cretaceous volcanic rocks in the Beixiangshan area, Lower Yangtze River Belt, South China and its implications

Newly identified late Early Cretaceous volcanic rocks in the Beixiangshan area, Lower Yangtze River Belt, South China and its implications

The mechanism responsible for the geochemical diversity of the Putuoshan volcanic-plutonic complex and its implications for the late Mesozoic crustal evolution of the Cathaysia Block, Southeast China

Essential mechanisms for the geochemical diversity observed in a silicic igneous rock association, which is of paramount importance for understanding the differentiation and growth of continental crust, remain highly controversial. Here the issues are explored by comparing the Late Cretaceous intrusions (minor-phase miarolitic alkali-feldspar granite and main-phase alkali-feldspar granite) and coeval volcanic rocks (rhyolite and rhyodacite) within the Putuoshan (PTS) volcanic-plutonic complex in the coastal area of southeast (SE) China. Zircon UPb ages indicate that the rhyolite, rhyodacite, minor-phase granite and the previously studied main-phase granite are nearly contemporaneous (ca. 98–94 Ma), but the coexisting dacites are obviously earlier at ca. 160 Ma. The dacites show adakitic geochemical characteristics and extremely enriched NdHf isotopic compositions with Paleoproterozoic model ages, suggesting their derivation from remelting of Paleoproterozoic basement rocks in a thickened lower crust. In contrast, the coexisting Late Cretaceous volcanic and intrusive rocks have comparable and relatively depleted NdHf isotopic compositions. Our data, combined with previous studies suggest that parental magmas of these Late Cretaceous magmatic rocks were derived from partial melting of juvenile crust with varying degrees of involvement of mantle-derived melts, and the observed compositional variation are mainly controlled by crystal-melt segregation and rejuvenation of crystal mush facilitated by recharge of hot mafic magmas in the shallow magma reservoir. The Jurassic and Late Cretaceous magmatic rocks within the PTS complex were generated in an advancing subduction compressive regime and a retreating subduction extensional regime of the paleo-Pacific plate, respectively. The gradually increasing tectonic extension during the Cretaceous due to slab rollback and the induced underplating of depleted mantle-derived magmas not only gave rise to the continental rejuvenation of SE China but also are the main driving mechanism of this Late Cretaceous long-lived transcrustal magma system.

Beneficiation Of Artvin-Cerattepe copper-zinc ore by flotation

The Cerattepe (Artvin) mine contains volcanogenic massive sulfide (VMS) deposits in northeastern Turkey. This is a kuroko type deposit located in Late Cretaceous volcanic, intrusive and sedimentary rocks, and has a structure with dense alteration stages. In this study, batch flotation tests were carried out to determine the flotation behavior of a new sample (mix ore), that was the combination of two different ores with different flotation behaviors. The mixed ore contains 2.18% Cu, 1.46% Zn, (Cu/Zn ratio 1.49) 38.35% Fe, and 41.6% S content. An efficient separation of copper minerals from zinc minerals using the conventional selective flotation method could not be achieved due to complex mineralogy and very low liberation degree. Therefore, sequential selective flotation resulted in poor Cu recovery. The effects of flotation parameters such as collector type, collector amount, particle size, and pH conditions, were investigated to obtain a bulk copper-zinc concentrate. The best results were obtained using thionocarbamate (Aero 3894) and dithiophosphine (Aerophine 3418A) collector reagents at d<sub>80</sub>=40 µm particle size. Under optimal conditions (grain size, d<sub>80</sub>=40 µm, pH=11, amount of collector reagent 60 g/t (Aero 3894), frother (MIBC)=50 g/t, solid ratio=32%, flotation time=8 min), a bulk copper-zinc concentrate containing 13% Cu and 9.5% Zn was obtained with a copper yield of 84.4% and a zinc yield of 88.9%. The concentrate mass pull was 13%. After rougher and two-stage scavenger flotation, a concentrate (rougher concentrate + scavenger products) was obtained with a mass pull of 22%. Copper and zinc recoveries of this concentrate were 91.8% and 92.5%, respectively.

New data on the geochemical and mineralogical features of the main manganese- and iron-bearing ore occurrences in the Central Srednogorie Zone, Bulgaria

The main goal of the present study is the characterization of the mineralogical and geochemical features of polymetallic (Mn- and Fe-bearing) nodules, lens- and layer-like bodies from different localities in the central part of the Late Cretaceous Srednogorie metallogenic zone, Bulgaria. The research is based on field studies, sampling and optical microscopy, followed by a combination of analytical techniques: XRD, SEM-EDS, ICP-OES and LA-ICP-MS methods. They define pyrolusite as the main ore mineral of the studied occurrences, while manganite, todorokite, bixbyite, sarkinite, hematite and hauerite are rarer. The most common gangue minerals are quartz, calcite and zeolites. Based on the MnO/SiO2 ratio, the established minerals are divided into two groups: manganese (i) and silica-manganese (ii) phases, respectively. Their trace element composition is dominated by a high content of V, Zn, Mo, W, Co, Ni, Cu, As, Tl and Sr, whereas some of them belong to the group of the critical raw materials for high-tech products. The measured values for Y and rare earth elements of the studied oxides and hydroxides are low compared to their concentrations in modern polymetallic nodules of the Pacific Ocean. Chondrite-normalized patterns indicated weak LREE enrichment with respect to MREEs and HREEs, which are slightly depleted. Common weak to strong negative Ce anomaly, accompanied by various Sm and Eu anomalies, is also observed. The close proximity of the Late Cretaceous volcanic rocks to the Mn- and Fe-bearing ore mineralization and some structural and textural features of the studied minerals suggest hydrothermal origin of the main Mn-Fe ore occurrences in the Panagyurishte area.

Evolution of the Garmab-e-Paein native copper-rich volcanogenic massive sulfide deposit from northeast Iran: Insights from sulfur isotope and chlorite chemistry

The Garmab-e-Paein Cu-Ag volcanogenic massive sulfide (VMS) deposit occurs as stratiform and stratabound orebody within a specific stratigraphic horizon in a Late Cretaceous volcano-sedimentary sequence of the Sabzevar zone, northeast Iran. The host rocks to mineralization are andesitic-trachyandesitic volcanic and volcaniclastic rocks. Based on textural and mineralogical studies, the VMS mineralization is comprised of four ore facies: 1) vein-veinlets (stringer) containing chalcopyrite, pyrite and minor magnetite, 2) massive ore, dominated by pyrite and minor chalcopyrite, 3) bedded ore containing laminated pyrite, and 4) exhalite containing Fe-Mn oxide-hydroxides such as hematite, psilomelane and pyrolusite. Wall rock alteration styles are chloritic, minor silicic and secondary argillic. There is a distinct metal zonation in the massive sulfide orebody; Au, Pb, As, and Ag contents and the Cu/Zn ratio increase vertically from the stringer to the massive facies, but decrease laterally from the stringer to the bedded facies. Zinc and Mn increase from the stringer to the bedded ore facies. Sulfur isotope values for pyrite range from −1.7 to +5.1‰, with average +1.4‰. The δ34S values of massive ore facies (−1.7 and +2.7‰) increase downward toward the stockwork (+2.2 to +4.3‰) and laterally outward toward the bedded (+5.1‰) zones. The near-zero sulfur isotope values indicate that most of the sulfur was derived from leaching of underlying host volcanic rocks by hydrothermal fluids circulating in the high temperature reaction zone.Extensive disseminations and vein-veinlets of native copper mineralization formed within the hanging wall Late Cretaceous volcanic rocks and younger Paleocene conglomerates. The native copper mineralization is accompanied by chlorite and zeolite alteration, and both formed after formation of the Garmab-e-Paein VMS deposit during diagenesis, burial metamorphism and uplift. The deposit is now potentially economic due to the native copper mineralization.Two generations of chlorite are observed in the deposit: Chl-I is accompanied pyrite and chalcopyrite, in the VMS mineralization system, and the second Chl-II occurs along with zeolite and native copper mineralization. The EPMA results show that Chl-I is more Fe-rich than Chl-II.

Geological characteristics, geodynamic setting of magmatism and metallogeny of Early Cretaceous Sn (W) deposits in southeastern coastal belt of China, and their implication for exploration

我国华南是世界上最重要的锡矿资源省之一，其锡矿成矿时代从新元古代、古生代、早中生代至晚中生代均有发育，其中以晚中生代锡矿成矿作用最为重要，成矿集中在中晚侏罗世（160~150Ma）和晚白垩世（100~80Ma）。然而，最近在东南沿海成矿带新识别出一期早白垩世锡（钨）（145~130Ma）成矿事件，改变了以往华南锡（钨）矿床时空分布格局。本文在详细介绍东南沿海早白垩世锡（钨）矿床地质特征基础上，系统总结了其成岩成矿时空分布，并与南岭板内锡（钨）成矿作用进行对比，浅析了早白垩世锡（钨）矿的成岩成矿背景和找矿潜力。研究显示，早白垩世锡（钨）成矿主要发生在东南沿海成矿带的粤东和赣州会昌地区，矿床类型以斑岩型和锡石硫化物型为主。与成矿有关的岩石为黑云母花岗岩和花岗斑岩，成因上属于高分异I型或A型，与南岭板内锡（钨）矿相比，具有较高的锆石ε_Hf（t）值和较低δ¹⁸O值，以及较高成岩温度，表明成岩过程中有较多新生地壳或地幔物质加入。结合区内发育同时期基性岩脉，认为华南这期早白垩世锡（钨）矿形成于岩石圈伸展背景，与古太平洋板块俯冲后撤有关。最后指出覆盖大面积晚白垩世火山岩的东南沿海地区，火山岩盖层下部具有寻找该期锡（钨）矿床的找矿潜力。;South China is one of the most important tin provinces in the world, and the tin mineralization formed from Neoproterozoic, Early Paleozoic, to Early and Late Mesozoic are well developed in this area. In particular, the Late Mesozoic tin mineralization is the most important one, happened at two age peaks of the Middle-Late Jurassic (160~150Ma) and Late Cretaceous (100~80Ma), respectively. However, an Early-Cretaceous (145~130Ma) tin (tungsten) metallogenic event has also been newly recognized in southeastern coastal belt of China. This makes a change in temporal-spatial framework of tin (tungsten) deposits. In this paper, based on detailed observations of geological characteristics of the Early Cretaceous tin (tungsten) deposits in southeastern coastal belt, we summarize the temporal-spatial distribution of these deposits and their associated magmatisms, and compared them with those in the Nanling Range, in order to identify their metallogenic geodynamic setting and implications for metallogenic potential. The Early Cretaceous tin (tungsten) deposits are mainly developed in the coastal eastern Guangdong Province and Huichang district of the southern Jiangxi Province. Genetically, the tin mineralization mainly comprises porphyry and cassiterite-sulfide-vein types, and associated granitoids include biotite granite and granitic porphyry characterized by high-fractionated I- and A-type affinities. Compared with those in the Nanling Range, these granites display higher zircon ε_Hf(t) and δ¹⁸O values, as well as higher magma temperatures, indicating a contribution of more mantle or juvenile crustal components. Given the coeval mafic dikes in the region, we suggest that the Early Cretaceous tin (tungsten) deposits were formed in the setting of lithospheric extension, triggered by the retreat of the subducting paleo-Pacific plate. Finally, we suggest that there is potential for tin (tungsten) exploration under the large-scale cover of Late Cretaceous volcanic rocks in southeastern coastal belt of China.

Tectonic setting of Cretaceous porphyry copper deposits of northern Chile (28°-30° S) and its relations with magmatic evolution and metallogeny

Cretaceous porphyry copper deposits of northern Chile (28º-29º30’ S) are genetically related with dacitic to dioritic porphyries and they represent a still poorly-explored target for Cu resources. The porphyries correspond to stocks distributed into two separated discontinuous NS trending belts of different age. The location of these porphyries is generally adjacent to orogen-parallel major fault systems that extend along the studied segment and also have a marked temporal relationship with deformation events registered along these structures. A first episode of Cu-bearing porphyry emplacement took place between 116 and 104 Ma (Mina Unión or Frontera, Cachiyuyo, Punta Colorada, Dos Amigos, Tricolor porphyries). These Early Cretaceous dacite to diorite porphyries are spatially associated with the eastern segments of the Atacama Fault System, which records sinistral transpression that started at 121 Ma producing ground uplift, consequent denudation and exhumation of the Early Cretaceous magmatic arc. This resulted in a change from marine to continental deposition with an angular unconformity in the site of the back-arc basin after of eastward migration of the deformation around 112-110 Ma. At the scale of the continental margin, this deformation is correlated with early stage of the Mochica Orogenic event described in Perú. A second episode of Cu-bearing porphyry emplacement occurred between 92 and 87 Ma (Elisa, Johana, Las Campanas and La Verde deposits), which are spatially and temporally associated with the regional-scale Las Cañas-El Torito reverse fault, active between 89 and 84 Ma, during the Peruvian Orogenic Phase. This fault up thrust to the west part of the Chañarcillo Group rocks (Lower Cretaceous) over the younger upper levels of the Cerrillos Formation (Upper Cretaceous). The integrated geological mapping and geochemical data of the Early to Late Cretaceous volcanic rocks indicates that both Early Cretaceous sinistral transpression and Late Cretaceous east-west compression were not significant in promote changes in magma genesis, except for slight changes in trace element ratios (increase in Th/Ta, Nb/Ta and La/Yb) suggesting that the Late Cretaceous deformation event produced only slightly increase in crustal thickness (>40 km), but far from being comparable to major Cenozoic orogenic phases, at least along the magmatic arc to back-arc domains in the study area. Finally, our study give insights about regional geological parameters that can be used as a first order guide for exploration of Cu resources along Cretaceous magmatic belts of northern Chile, where both Early and Late Cretaceous Cu-bearing porphyry intrusions are restricted to a large structural block bounded to the west and east by Cretaceous fault systems.

Petrology and LA-ICP-MS zircon geochronology for Late Cretaceous felsic dikes and intermediate volcanic rocks hosted in Mersin ophiolite, South Turkey and its implications

LA-ICP-MS U-Pb geochronology of zircons, whole-rock (main, trace, and REE) geochemistry, and mineral chemistry (feldspars) are constrained for felsic dikes and intermediate volcanic rocks of the Mersin ophiolite in the Inner Tauride Suture Zone (Southern Turkey). The geochronological research, dependent on LA-ICP-MS U-Pb zircon analyses from examined rocks, has yielded a concordia age of 87.38 ± 0.32 Ma for a microgranular felsic dike, stating a Coniacian (Late Cretaceous) age. The felsic dikes show mainly hemicrystalline, aphanitic, porphyritic textures and petrographic analyses from the felsic dikes, and volcanic rocks exhibit that they are mainly dacite, rhyodacite, andesite and basaltic andesite, which are poor in mafic minerals. The mineral chemistry of the dacite and rhyodacite dikes present that plagioclases have a wide range of contents, from andesine (An31.1–497) to albite (An03–28.15) and K-feldspars represented by sanidine (Or42.5–93.3). Dacite/aplite dikes cutting mainly andesitic volcanic rocks present island arc tholeiite-like geochemistry. These felsic intrusive rocks are depleted in HFSE (high-field-strength elements), enriched in LILE (large-ion lithophile elements) and present flat-type REE trends; these patterns furthermore verify their supra-subduction zone geodynamic setting. The initial phase of arc-type volcanics formed above, as can be observed in most of the ophiolites in the Inner Tauride Suture Zone. The arctype volcanics and SSZ-type slab (of metamorphic rock and ophiolite) are cut by dacitic rocks (Coniacian). From the geochronology, field relations, and geochemical evidence from the felsic dikes and volcanics, it can be concluded that the Mersin ophiolite occurred in a tectonic environment (subduction zone) in the southern branch of the Neo-Tethys, located in modern Southern Anatolia.

Evidence for two stages of back-arc compression in the late Cretaceous fold-and-thrust belt in the Precordillera of northern Chile (24°30′S–25°30′S)

In the Andean forearc of northern Chile, the interplay between deformation and magmatism is not fully understood. Outcrops in the Precordillera of northern Chile record a long and complex history of magmatism and overprinting deformational events. The growth of the Precordillera is predominantly associated with the Late Eocene-Early Oligocene transpressive Domeyko fault system (Incaic Phase), however, a series of compressive late Cretaceous folds and faults have also been recognized. In this study, we combined structural mapping, stereographic fold analysis, and geochronologic data of both deformed rocks and post-tectonic hypabyssal bodies to reveal the distribution and kinematics of the late Cretaceous thrust system exposed in the Precordillera between 24°30′S and 25°30′S. We identified an approximately NS-striking belt of reverse faults and folds along the Precordillera, which was cut and displaced by the Late Eocene-Early Oligocene Domeyko fault system. This deformation corresponds to thick-skinned structures affecting Paleozoic basement and thin-skinned structures which include folding with a general sub-horizontal NNW-SSE shortening direction with westward and eastward vergence. The ages of post-tectonic hypabyssal bodies cutting the deformed rocks indicate that the timing of the contractional structures occurred before 86 Ma taking place in the westernmost margin of Precordillera and continued between 72 Ma and 65 Ma. Our results agree with the timing proposed for regional Peruvian and ‘K-T’ deformations. We also used the distribution of the syn-kinematic late Cretaceous volcanic rocks to identify late Cretaceous contraction that evolved into a back-arc fold-and-thrust belt. Its structures are currently distributed along the northern Precordillera and were obliterated by younger strike-slip Incaic deformation. Moreover, this deformation must have forced the topography upward, creating an early late Cretaceous topographic relief related to the tectonic activity along these structures. These results indicate that late Cretaceous deformation had a role in the early history of the proto-Precordillera of northern Chile, prior to development of the strike-slip Domeyko fault system in the Paleogene.

Ore Formation During Jurassic Subduction of the Tethys Along the Eurasian Margin: Constraints from the Kapan District, Lesser Caucasus, Southern Armenia

AbstractThe Kapan mining district in the southernmost Lesser Caucasus is one of the few locations along the central Tethyan metallogenic belt where ore-forming processes were associated with magmatic arc growth during Jurassic Tethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study: Centralni West, Centralni East, and Shahumyan. The ore deposits are hosted by Middle Jurassic andesitic to dacitic volcanic and volcaniclastic rocks of tholeiitic to transitional affinities below a late Oxfordian unconformity, which is covered by calc-alkaline to transitional Late Jurassic-Early Cretaceous volcanic rocks interlayered with sedimentary rocks.The mineralization consists of veins, subsidiary stockwork, and partial matrix replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite, and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote, and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulfidation mineral assemblages with enargite and luzonite. Dickite, sericite, and diaspore prevail in altered host rocks in the Centralni East deposit. At the Shahumyan deposit, phyllic to argillic alteration with sericite, quartz, pyrite, and dickite is dominant with polymetallic veins, and advanced argillic alteration with quartz-alunite ± kaolinite and dickite is locally developed.The lead isotope composition of sulfides and alunite (206Pb/204Pb = 18.17–18.32, 207Pb/204Pb = 15.57–15.61, 208Pb/204Pb = 38.17–38.41) indicates a common metal source for the three deposits and suggests that metals were derived from magmatic fluids that were exsolved upon crystallization of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The δ18O values of hydrothermal quartz (8.3–16.4‰) and the δ34S values of sulfides (2.0–6.5‰) reveal a dominantly magmatic source at all three deposits. Combined oxygen, carbon, and strontium isotope compositions of hydrothermal calcite (δ18O = 7.7–15.4‰, δ13C = −3.4−+0.7‰, 87Sr/86Sr = 0.70537–0.70586) support mixing of magmatic-derived fluids with seawater during the last stages of ore formation at Shahumyan and Centralni West.40Ar/39Ar dating of hydrothermal muscovite at Centralni West and of magmatic-hydrothermal alunite at Shahumyan yield, respectively, a robust plateau age of 161.78 ± 0.79 Ma and a disturbed plateau age of 156.14 ± 0.79 Ma. Re-Os dating of pyrite from the Centralni East deposit yields an isochron age of 144.7 ± 4.2 Ma and a weighted average age of the model dates of 146.2 ± 3.4 Ma, which are younger than the age of the immediate host rocks. Two different models are offered, depending on the reliability attributed to the disturbed 40Ar/39Ar alunite age and the young Re-Os age. The preferred interpretation is that the Centralni West Cu deposit is a volcanogenic massive sulfide deposit and the Shahumyan and Centralni East deposits are parts of porphyryepithermal systems, with the three deposits being broadly coeval or formed within a short time interval in a nascent magmatic arc setting, before the late Oxfordian. Alternatively, but less likely, the three deposits could represent different mineralization styles successively emplaced during evolution and growth of a magmatic arc during a longer time frame between the Middle and Late Jurassic.

Process of lithospheric delamination beneath the Lhasa–Qiangtang collision orogen: Constraints from the geochronology and geochemistry of Late Cretaceous volcanic rocks in the Lhasa terrane, central Tibet

The geochemistry of post-collisional granitoids can be used to reconstruct crustal thickness and constrain deep geodynamic processes. Two series of volcanic rocks of the Jiangba Formation in the Xiongmei area, central Tibet, record the process of lithospheric delamination in the Lhasa–Qiangtang collisional orogen. The andesites and dacites crystallized at c. 87 and 85 Ma, respectively, and have positive zircon εHf(t) values (2.7–5.4 and 2.7–7.1, respectively). They have calc-alkaline compositions, with relatively high Mg# (100 × molar Mg2+/[Mg2+ + Fe2+]; 39–66), Sr contents, and Sr/Y and (La/Yb)N ratios (subscript N signifies normalization to chondrite), low Y and Yb contents, and no clear negative Eu, Sr, or Ti anomalies, similar to adakites. The andesite–dacite series likely formed by partial melting of foundering juvenile mafic lower crust with residual garnet, followed by the interaction of this melt with peridotite. The rhyolite series was emplaced at c. 81 Ma, and has positive zircon εHf(t) values (1.0–3.5). The rhyolites are characterized by non-adakitic signatures and have high-K calc-alkaline to shoshonite compositions, with low MgO contents (Mg# = 21–34) and Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and marked negative Eu, Sr, and Ti anomalies. Geochemical data indicate that these rhyolites were generated by partial melting of a much shallower crustal root, with a plagioclase-rich residue. Zircon U–Pb ages and different compositional trends for the two series indicate that the lithospheric mantle keel was partially removed by small-scale mantle convection that triggered the Late Cretaceous high-Mg# adakitic magmatism in the Lhasa–Qiangtang collisional orogen. Partial lithospheric delamination increased heat flow at the base of the crust and caused the thinned crust to melt, producing the low-Mg# non-adakitic rocks.

Geochemical comparison of late Mesozoic and early Cenozoic volcanic rocks in South Mongolia

The Mesozoic-Cenozoic volcanic rocks are widely distributed in the interior of the East Asia and document the tectonic transition of East Asia. We present new geochronology and geochemistry data of late Cretaceous-early Cenozoic basalts in Bayantsagaan and Han-Uul volcanic provinces in South Mongolia, in order to explore their petrogenesis and geodynamic settings. The volcanic rocks in the Bayantsagaan and Han-Uul field yielded K-Ar ages of 90.55±1.93 Ma and 55.49±1.49 Ma, respectively. The volcanic rocks in South Mongolia can be subdivided into to alkaline basalts and tholeiitic series, and are characterized by ocean island basalts (OIB) trace elements features, such as enrichment of light REE relative to heavy REE and enrichment in large ion lithophile elements (LILE) with positive K anomaly. Compared with the late Cretaceous, the early Cenozoic basalts show a decrease in the contents of HREE and an increase of Nb and Ta. Crustal contamination and fractional crystallization are insignificant in the genesis of late Cretaceous-early Cenozoic basalts South Mongolia. The available Sr-Nd isotope results indicate that a mixing depleted (DM) and enriched mantle (EM) signature characterize in late Cretaceous volcanic rocks, which derived from magmas from the asthenosphere with some contributions of metasomatized subcontinent lithospheric mantle, whereas the early Cenozoic basalts are ascribed to contributions from the asthenospheric mantle. We propose that the generation of the late Cretaceous-early Cenozoic volcanism (90-40 Ma) in Mongolia is probably related to the shallow mantle upwelling (asthenosphere) induced by the edge convection along the northern margin of the North China Craton (NCC), triggered by a far-field effect of Indo-Asian collision.

Episodic bimodal magmatism at an active continental margin due to Paleo-Pacific Plate subduction: A case study from the southern segment of the Tan-Lu Fault Zone, eastern China

Volcanic rocks in the southern part of the Tan-Lu Fault Zone (TLFZ), eastern China, provide key evidence for the connection between spatially and temporally related pulses of Cretaceous magmatism, activity on the TLFZ, and the dynamics of the Paleo-Pacific Plate. We identify two stages of bimodal volcanism in the Chaohu–Lujiang segment, southern TLFZ, based on lithological assemblages and precise isotopic ages. The Early Cretaceous (125–121 Ma) volcanic rocks are fractionated aluminous A-type rhyolites and basalts, whereas the Late Cretaceous volcanic rocks (97–83 Ma) range in composition from porphyritic andesites and trachydacites to rhyolites. The Early Cretaceous rhyolites display strongly negative Nd-Hf isotopic compositions (εNd(t) = −16.3 to −16.1, εHf(t) = −24.1 to −17.9). The most likely source rocks of the fractionated aluminous A-type rhyolites are metaigneous rocks of the Feidong Complex. The Early Cretaceous potassic basalts (K2O/Na2O = 1.5–3.4, εNd(t) = −9.0) exhibit more enriched isotopic compositions than the Late Cretaceous sodic porphyritic andesites (K2O/Na2O = 0.6–0.8, εNd(t) = −5.6, εHf(t) = −15.1 to −6.8), indicating derivation from heterogeneously enriched lithospheric mantle. This heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures. Modeling of selected elements reveals that the Late Cretaceous trachydacites (εNd(t) = −12.2 to −8.2, εHf(t) = −16.5 to −6.4) originated by mixing between coeval mafic and felsic magmas. The most negative (εNd(t) = −18.8 to −17.1, εHf(t) = −20.5 to −4.7) isotopic compositions, along with Neoproterozoic inherited zircons within the Late Cretaceous rhyolites, indicate derivation from lower crust of the South China Block. We suggest that the Early Cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting Izanagi slab. However, the TLFZ became a focus of Late Cretaceous bimodal volcanism due to weak far-field extension caused by the slow northward motion of the Paleo-Pacific Plate. Moreover, the high velocity and low angle of subduction of the Izanagi slab were the main driving mechanisms behind an Early Cretaceous transpressional event that separated the two stages of bimodal volcanism.

Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction

Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon UPb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon UPb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns to E-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock εNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock εNd(t) (+2.9 to +3.4) and zircon εHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.

Reliable paleointensity determinations from Late Cretaceous volcanic rocks in Korea with constraint of thermochemical alteration

Paleointensity determinations were carried out from Late Cretaceous (∼77 Ma) volcanic rocks in Korea using a Thellier-type IZZI experimental protocol with systematic partial thermal remanent magnetization (pTRM) checks. Various data selection criteria were used to estimate reliable paleointensities. We set stringent threshold values for each parameter to ensure that there was: (1) a linear relationship between natural remanent magnetization (NRM) lost and TRM gained; (2) negligible thermal alteration of magnetic minerals; and (3) uni-vectorial decay of NRM towards the origin. From the 336 samples, ∼88% were rejected because of an insufficient extrapolated NRM fraction in the best-fit line (fvds < 0.6), highlighting that fvds is the most stringent selection criterion in this study. For the 31 accepted samples, paleointensities range from 6.4 to 30.4 µT. Among the 31 samples, eight samples yielded extremely low paleointensities. Although single-domain (titano)magnetite was identified as the stable paleointensity recorder, oxidation of superparamagnetic fractions upon repeated heating probably caused enhancement of pTRM acquisition and yielded a low paleointensity estimate. Apart from these low paleointensities (n = 8) as screened by a newly proposed data selection criterion of Δk < 0.2, the remaining 23 samples have a mean paleointensity of 23.1 ± 4.8 µT, corresponding to a virtual axial dipole moment (VADM) of 40.8 ± 8.5 ZAm2, which is ∼50% of the present-day VADM.

Removal of deep lithosphere in ancient continental collisional orogens: A case study from central Tibet, China

AbstractWidespread but small‐volume Late Cretaceous volcanic rocks in central Tibet contain important information on the Lhasa‐Qiangtang collision process. In this contribution, we focus on Late Cretaceous volcanics in the southern Qiangtang subterrane and present zircon LA‐ICP‐MS U‐Pb ages, whole‐rock major and trace element compositions, and Sr‐Nd isotopic data. Zircon LA‐ICP‐MS U‐Pb dating yielded a concordant age of 80 Ma, which postdates the Early Cretaceous collision of the Qiangtang and Lhasa terranes. The volcanic rocks are potassium‐rich alkaline andesites with high contents of K2O (3.45–5.11 wt %) and Th (13.39–25.02 ppm), as well as high K2O/Na2O ratios (0.6–0.9). They have higher REE and HFSE contents than coeval Mg‐rich and adakite‐like magmatic rocks that can be related to partial melting of a thickened lower crust. Moreover, they have higher values of Mg# and lower contents of SiO2 than lower continental crust‐derived rocks in central Tibet and experimental data of mafic rocks. We argue that the andesites were generated after the removal of thickened lithospheric mantle and subsequent to the final Lhasa‐Qiangtang amalgamation in a postcollisional setting. The high‐K characteristics can be explained by producing the primitive andesite magmas from partial melting of the residual and shallow metasomatized lithospheric mantle (the K‐rich layer) during heating by upwelling asthenosphere; subsequently, these primitive andesite magmas were subjected to fractional crystallization to generate the Amdo andesites. The way in which these andesites were formed provides evidence for the lithospheric thickening and uplifting of central Tibet during the Late Cretaceous prior to India‐Asia collision.

Widespread Paleoproterozoic basement in the eastern Cathaysia Block: Evidence from metasedimentary rocks of the Pingtan–Dongshan metamorphic belt, in southeastern China

The Cathaysia Block in South China Block (SCB) is famous for its polyphase granitic magmatism and world-class mineralization, especially W, Sn and REE. Although isotopic data suggest that widespread igneous rocks were derived from Precambrian basement, outcrops of the basement rocks are rare in the Cathaysia Block. Therefore, the distribution of the Archean-Paleoproterozoic basement beneath Cathaysia Block is still unclear.The northeastern-striking Pingtan–Dongshan metamorphic belt (PDMB) is located along the coastal region of southeastern China, and consists mainly of voluminous granites and volcanic rocks with minor metasedimentary rocks. These metasedimentary rocks were intruded or captured by 160–143Ma granites. U–Pb dating of detrital and metamorphic zircons from seven metasedimentary rocks indicates that their protoliths were deposited between 183Ma and 160Ma, rather than pre-Devonian as previously proposed. Dating of metamorphic zircons suggests that the most important metamorphism took place at ca 97Ma, consistent with emplacement of A-type granites and mafic magmatism in the coastal region, and implying an extensional setting.Age spectra of detrital zircons in samples from the middle and southern PDMB exhibit a bimodal distribution with abundant Late Paleoproterozoic and Early Mesozoic zircons. In contrast, age spectra of detrital zircons from the northern PDMB display pronounced Neoproterozoic and Early Paleozoic age peaks as well as Late Paleoproterozoic and Early Mesozoic ones, suggesting a different provenance. The comparisons of age and Hf-isotope with the exposed rocks of the SCB suggest that the terrigenous material in these sediments was sourced from the Wuyi terrane, in the eastern Cathaysia Block.The age spectra of detrital zircons from Early Paleozoic to Late Mesozoic sedimentary rocks in the SE SCB show that late Paleoproterozoic (1.9–1.8Ga) detritus increases markedly in Middle Permian to Early Cretaceous sedimentary rocks. Comparison with zircon U–Pb–Hf isotopic data from exposed Paleoproterozoic rocks suggests that a Paleoproterozoic basement existed extensively beneath the eastern Cathaysia Block. It has been uplifted and eroded since Middle Permian time, and then covered by Late Cretaceous volcanic rocks. The uplift of the Paleoproterozoic basement was probably caused by the Late Paleozoic orogeny in the eastern Cathaysia Block.

Stratigraphy of the Late Cretaceous volcanic rocks from the Banshu-Ako District (Quadrangle Series 1: 50,000)

Stratigraphy of the Late Cretaceous volcanic rocks from the Banshu-Ako District (Quadrangle Series 1: 50,000)

兵庫県姫路市，家島諸島に分布する後期白亜紀火山岩類のジルコン U-Pb 及び FT 年代

The Ieshima Islands, in the eastern Seto Inland Sea of southwest Japan, are dominated by sedimentary complex in the Tamba Belt, Late Cretaceous volcanic and plutonic rocks, and Quaternary sediments. The Late Cretaceous volcanic rocks consist mainly of dacite pumiceous lapilli tuff, rhyolite welded crystal tuff, and rhyolite tuff breccia. Here, they are described for the first time as the Ieshima Formation. Rhyolite welded crystal tuff overlies the dacite pumiceous lapilli tuff, and both are intruded by plutonic rocks. I present laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb and fission-track (FT) ages of the rhyolite welded crystal tuff. U-Pb ages of individual zircons range from 86.0 to 104.5 Ma, and show a bimodal distribution with peaks at ca. 90 and 100 Ma. The younger cluster (89.8 ± 0.9 Ma, n = 19) corresponds the eruption age of the felsic volcanic rocks of the Ieshima Formation, while the older grains may be inherited from the basement rocks. The FT age (76.1 ± 2.5 Ma) is significantly younger than the U-Pb dating age, and represents thermal resetting by concealed plutonic rocks.

Timing of the Late Cretaceous ignimbrite flare-up at the eastern margin of the Eurasian Plate: New zircon U–Pb ages from the Aioi–Arima–Koto region of SW Japan

The Kinki district in the Inner Zone of southwest Japan is characterized by Late Cretaceous volcanic rocks of the Aioi and Arima groups and the Koto Rhyolites (from west to east). These rocks are dominated by pyroclastic flow deposits related to caldera-forming events. Here, we present new laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb ages for the Aioi Group (four isolated caldera-filling deposits); the lower, middle, and upper parts of the Arima Group and one intrusive rock (the Kashihara Quartz Gabbro); and the lower and upper parts of the Koto Rhyolites, including the oldest volcanic unit within these rhyolites. These analyses yielded ages of ca. 86–82Ma for the Aioi Group, ca. 83–81Ma for the Arima Group, 78.6Ma for the Kashihara Quartz Gabbro, and ca. 74–73Ma for the Koto Rhyolites. The Aioi Group represents a cluster of calderas and the ages obtained for individual units in this group differ from the ages of adjacent units. The Arima Group and the Koto Rhyolites both consist of pyroclastic flow deposits associated with caldera-forming events, and the ages of these rocks are all the same within error. This suggests that the Aioi Group represents a series of individual caldera-forming eruptions that are distinct from the Arima Group and the Koto Rhyolites, which formed during a single stage of caldera formation. The U–Pb ages presented here indicate that the Late Cretaceous caldera-forming eruptions in the study area occurred at intervals of >1Myr and represent individual events that lasted for <1Myr. The oldest volcanic unit within the Kinki district is similar in age to the oldest volcanic unit within the Chubu district, suggesting that caldera-forming eruptions in southwest Japan commenced at ca. 90Ma.