Late Paleozoic Volcanic Rocks Research Articles

Qia’erdunbasixi Fe–Cu Deposit in Sawur, Xinjiang: A Case Study of Skarn Deposit Hosted by Volcanic Rock

The Sawur Cu–Au belt, northern Xinjiang, China, is the eastward extension of the Zarma–Sawur Cu–Au belt in Kazakhstan, where Late Paleozoic volcanic rocks and intrusions are highly developed. The Qia’erdunbasixi Fe–Cu deposit in Sawur is a recently discovered deposit and is still under exploration. The intrusive rocks are syenite and diorite, and the wall rocks consist of andesite and minor basalt, lamprophyre, and tuff. The U–Pb SHRIMP zircon age of the Qia’erdunbasixi syenite intruding into the volcanic rocks is 345 ± 2.2 Ma (MSWD = 1.3), presenting as the lower limit of skarn Fe mineralization. The intrusives belong to the calc–alkaline to high-K calc-alkaline series with large ion lithophile element (LILE) enrichment, high LREE/HREE fractionation, and high field strength element (HFSE) depletion. The initial 87Sr/86Sr ratios of the Qia’erdunbasixi syenite range from 0.70403 to 0.70420, and the εNd(t) values are from +5.5 to +6.8, which are the typical characteristics of island arc igneous rocks. Diorites having similar REE features with syenite should share the same magma source. Magnetite and copper mineralization develop mostly along the contact zones of syenite and diorite, respectively. Fe mineralization develops along the contact zone of syenite, with typical skarn zonation. The metallogenesis event can be divided into the prograde skarn stage (diopside–augite–andradite–magnetite–calcite–quartz), retrograde skarn stage (epidote–chlorite–actinolite–K-feldspar–calcite–magnetite–quartz), and quartz–sulfide stage (quartz–magnetite–K-feldspar–calcite–sercite–chlorite–actinolite–prehnite–chalcopyrite–pyrite). The early–mid-stage magnetite with certain amounts of Ti and V was crystallized from magma, while the late-stage magnetite has the typical characteristics of hydrothermal calcic skarn magnetite. The temperature of mineralization is between 350 and 450°C based on mineral assemblages and phase diagrams. Copper mineralization is concentrated along the outer contact zone of the diorite. Paragenesis sequences of the four stages of mineralization could be identified for copper mineralization: 1) albite–quartz; 2) chalcopyrite–pyrite–gold–seriate–quartz; 3) chalcopyrite–pyrite–epidotic–reunite; and 4) sphalerite–galena–quartz–calcite. Qia’erdunbasixi is a composite deposit with skarn-type Fe mineralization and mesothermal Cu mineralization and has a genetic relationship with magmatism in an island arc setting.

Zircon U−Pb Ages and Tectonic Implications of Late Paleozoic Volcanic Rocks in the Western Tianshan, North Xinjiang, China

Zircon U−Pb Ages and Tectonic Implications of Late Paleozoic Volcanic Rocks in the Western Tianshan, North Xinjiang, China

Geochronology and Geochemistry of Late Paleozoic Volcanic Rocks and Their Relationship With Iron and Molybdenum Deposits in Xilekuduk Area, Northern Margin of Junggar

A large number of intermediate basic volcanic rocks and porphyry Cu-Mo deposits as well as volcanic-hosted magnetite deposit have been recently discovered in the Xilekuduk area. However, no reports concerning petrogenesis and age or their relationship with mineralization have been published to date. The purpose of this study was to make up for the absence of previous studies on Devonian volcanic activities in the area and to confirm the relationship between two stages of volcanic activities and mineralization so as to provide important theoretical basis for mineral exploration. Based on research results of zircon U-Pb geochronology and element geochemistry of volcanic rocks in the area, the ages of dacite, andesite, and stomatal andesite are considered as 375.2 ± 2.9 Ma, 386.5 ± 3.0 Ma, and 317.9 ± 2.9 Ma, respectively, corresponding to the Middle Devonian and Late Carboniferous Period. The Devonian volcanic rocks belong to the high-K calc-alkaline series and island arc volcanic rocks, which are enriched in LREE, strongly enriched in large ion lithophile elements Th, Rb, Ba, and K and relatively depleted in high-field strength elements (HFSEs) Nb, Ta, and Ti. The Carboniferous volcanic rocks are enriched in LREE, as well as the large ion lithophile elements Th, Rb, Ba, and K are strongly enriched, while depleted in the HFSEs Nb, Ta, and Ti; moreover, the contents of TiO2and V are 0.94–0.97% and 178–183×10–6, which are higher than those of island arc basalts. According to mineralogical typomorphic characteristics and geochemical analysis, magnetite mineralization is divided into two phases. The early stratiform magnetite ore MT1 has magmatic characteristics, forming a volcanic rock type magnetite deposit related to Devonian volcanic eruption and sedimentation (375–386 Ma). The magnetite MT2 in the magnetite-quartz vein is considered as hydrothermal genesis, which is a metal mineral in the early metallogenic stage of Carboniferous (317.1 ± 2.9 Ma) volcanic eruption and subvolcanism, and may be related to porphyry molybdenum mineralization. Therefore, the volcanism and Fe-Cu-Mo mineralization in this area is characterized by multistage superimposed mineralization.

Multiphase Late Devonian to Carboniferous volcanic events in the west of Oyu Tolgoi, southeastern Mongolia: New geochronological, geochemical, and isotopic constraints on tectonic history

The Late Paleozoic volcanic rocks distributed in southeastern Mongolia are key to reconstructing the tectonic processes along the south margin of the Central Asian Orogenic Belt during this period. In this study, representative volcanic samples were collected in the west of Oyu Tolgoi near the eastern border of the South Gobi and Trans-Altai Zones. The geochronological results reveal intermittent Late Devonian to Early Carboniferous volcanic belts with zircon UPb ages of 360, 320, and 340 Ma for the south, center, and north belts, respectively. Geochemical and isotopic results imply comparable petrogenesis for the samples from different belts. The basic samples are primarily derived from the different degrees of partial melting of a mantle source, and their compositions are slightly affected by the assimilation of crustal materials with different maturities. The intermediate to acidic samples were more likely produced by partial melting of a crust-derived source triggered by underplating of the mantle magma. The low Mg# and apparent Eu anomalies of the extremely high-Si rhyolites sampled from a bimodal-like volcanic sequence in the south belt indicate strong fractional crystallization after derivation from the partial melting of the juvenile crust. In conjunction with the spatiotemporal distribution pattern of magma and the observations recorded in previous studies, we propose that the Late Devonian to Early Carboniferous volcanic rocks from the south and north belts are a continental arc formed from the northward subduction of the Tsagaan Uul terrane under the Mongolian microcontinent and that the Late Carboniferous volcanism in the center belt was produced under a post-collision extensional setting. The subsequent increasingly intense extension was also responsible for the local Late Carboniferous to Early Permian calc-alkaline to alkaline granitic plutons.

Dike Complexes of the Gromadnen–Vurguveem Ophiolite Massif, West Chukotka: Composition and Geodynamic Setting

The Middle (?) to Late Paleozoic ophiolite association of West Chukotka comprises the large Aluchin, Atamanov, and Gromadnen–Vurguveem mafic–ultramafic massifs, which are spatially associated with Late Paleozoic volcanic rocks of the Yarakvaam terrane. This paper is devoted to the dikes of the Gromadnen–Vurguveem massif, which form two spatially isolated complexes: “dikes 1” and “dikes 2.” Both complexes intrude the plutonic gabbroids of the massif. The “dikes 1” complex forms a dike body nappe structurally linked with Paleozoic island-arc volcanics. The rocks of the complex are differentiated from basalts to dacites and chemically correspond to island-arc tholeiites. The “dikes 2” complex consists of separate basaltic dikes, some of which are similar to the “dike 1” rocks. The Ar/Ar dating of these samples gave 264–266 Ma. The other part of the rocks of this complex shows mixed signatures of island-arc tholeiites and N-MORB. It was concluded that the studied dike complexes, together with the earlier studied dikes of the Aluchin massif, marks the early (Late Permian) stage of the back-arc basin evolution. The Late Triassic dikes of the Atamanov massif reflect a more mature stage in the evolution of this basin.

Geochronology and geochemistry of late Paleozoic volcanic rocks from eastern Inner Mongolia, NE China: Implications for igneous petrogenesis, tectonic setting, and geodynamic evolution of the south-eastern Central Asian Orogenic Belt

Here we report new petrography, geochemistry, UPb zircon geochronology, and Hf isotope results for volcanic rocks of the Dashizhai Formation in the Dashizhai area, Keerqin Right Wing Front Banner, Inner Mongolia. The volcanic rocks have mafic to felsic bulk-rock compositions and include basalt, rhyolite, basaltic andesite, andesite, dacite, and rhyolitic tuff. Two dacite and rhyolitic tuff samples yield laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 206Pb/238U zircon ages of 332 ± 1 Ma and 300 ± 1 Ma, respectively, suggesting that the timing of felsic volcanism ranged from the early to late Carboniferous. Mafic volcanic rock samples are enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE; La/Nb = 1.58–4.84), and are depleted in high-field-strength elements (HFSE; Th/Yb ratios <2.2) and heavy rare earth elements [HREE; (Tb/Yb)N ratios 1.27–1.91]. Based on these trace element characteristics, we propose that the mafic volcanic rocks represent fractionated, high-degree partial melts that were sourced from the subduction-modified and melt-depleted mantle wedge. The felsic volcanic rocks yield LILE- and LREE-rich compositions and arc-like HFSE- and HREE-poor signatures. Magmatic zircons recovered from the dated rhyolitic tuff and dacite samples yield positive εHf(t) values (+1.21 to +14.85) and two-stage Hf model ages of 1236 and 391 Ma, suggesting that arc-like felsic volcanism was sourced from, and/or interacted with, the older juvenile lower-crust underlying the Dashizhai Formation. Therefore, the mafic to felsic volcanic rocks from the Dashizhai area likely formed in a Carboniferous magmatic arc at a continental margin. Based on the isotopic and geochemical signatures of the Carboniferous–Permian volcanic rocks in the Dashizhai area, we preliminarily divide central and eastern Inner Mongolia into three zones: (i) North Zone, which formed at the magmatically active continental margin; (ii) Middle Zone, which formed in a magmatic arc peripheral to the continental marginal; and (iii) South Zone, which formed in an oceanic island arc. The age and geochemical signature of the Dashizhai Formation points to active, northward subduction during the early Carboniferous to the early Permian and suggests that the Paleo-Asian Ocean closed after the early Permian.

Petrogenesis of post-collisional Late Paleozoic volcanic rocks of the Bohemian Massif (Central Europe): Isotopic variations of the lithospheric mantle related to Variscan orogeny

Post-collisional volcanic rocks associated with the Variscan orogeny are preserved in relicts of Late-Paleozoic volcano-sedimentary basins in the Bohemian Massif (central Europe). They include both the Late Carboniferous calc-alkaline and the Early Permian tholeiitic suites, which were emplaced in an extensional intracontinental setting. Both suites were derived from an enriched continental lithospheric mantle and underwent early fractionation of mafic silicate minerals. A second period of fractionation occurred within the crust. Modeling indicates that the calc-alkaline rocks fractionated at a greater depth (0.55 GPa), under oxidizing (ΔFMQ +1) and hydrous conditions whereas the tholeiitic rocks fractionated at shallow depth (0.1 GPa), under moderately oxidizing (ΔFMQ +0.7) and drier conditions. During the Variscan orogeny, the lithospheric mantle under the Bohemian Massif underwent wholesale modification, where the mantle was contaminated by subducted Precambrian sediment admixture. The Carboniferous-Permian mantle-derived rocks typically have negative ɛNd(t) and Precambrian depleted mantle model ages (TDM) compared to the equivalent Early Paleozoic rocks of the Bohemian Massif, which have positive ɛNd(t) and younger TDM. Subsequently, prior to the Cenozoic, the lithospheric mantle under the whole Bohemian Massif was rejuvenated as the Cenozoic mantle-derived rocks have positive ɛNd(t) and young TDM.

Soft collision and polyphasic tectonic evolution of Wuxia foreland thrust belt: Evidence from geochemistry and geophysics at the northwestern margin of the Junggar Basin

A key unit of the Paleozoic Central Asian Orogenic Belt (CAOB), the northwestern margin of the Junggar Basin is located between the Kazakhstan, Siberian and Tarim plates with widespread late Paleozoic volcanic rocks and molasse deposits. In recent years, there has been intense debate about how the Carboniferous-Permian soft-collision geodynamic setting was developed and how long-distance detachment nappes were formed. In this study, on the basis of seismic reflection data, magnetotelluric profiles, and geochronological and geochemical data, we established the superimposed tectonic framework of the Wuxia thrust belt and Hala’alat Mountain and reconstructed the polyphasic tectonic evolution since the Late Carboniferous period. The petrographic and geochemical data suggest that the basaltic rocks of the Late Carboniferous were derived from a metasomatized depleted mantle source in a subduction-related island arc setting, while the Early Permian magmas derived from subduction-modified lithosphere mantle sources in a post-collision intraplate environment. Analysis of the Rittmann Index, Gottini Index and DI frequency curve suggested that the study area had experienced a key tectonic transition from a Late Carboniferous (i.e. end of Carboniferous) subduction-orogen setting to an Early Permian post-collisional extension setting. The Hala’alat Mountain and Zhayier Mountain exhibited a similar tectonostratigraphic framework and structure in space, such as double-layer nappes, long-distance slab detachment and unilateral transection by the northern Darbut fault. Both of these mountain ranges could be called soft-collision foreland thrust belts. On the basis of described four regional unconformities, the tectonic evolution of the soft-collision Wuxia thrust belt and adjacent regions could be divided into four crucial tectonic stages. As a result, thick autochthonous Lower Permian strata are buried under long-distance Hala’alat nappe and extended to the northern Darbut fault, which showed the huge oil and gas exploration potential in the northwestern margin of the Junggar Basin.

Late Paleozoic arc magmatism in the southern Yili Block (NW China): Insights to the geodynamic evolution of the Balkhash – Yili continental margin, Central Asian Orogenic Belt

The Yili Block represents the easternmost segment of the Kazakhstan–Yili microcontinent. Late Paleozoic magmatic rocks are extensively exposed in the Yili Block and are essential in understanding the tectonic development of the SW Central Asia Orogenic Belt (CAOB). We conducted field investigations, zircon U–Pb dating, whole-rock geochemical and Sr–Nd isotopic studies on the magmatic rocks from the Wusun Range, southern Yili Block. The Late Paleozoic volcanic and plutonic rocks of the Wusun Range show geochemical features consistent with that of calc-alkaline series, and they have positive εNd(t) values (0.9 to 5.6), Neo- to Mesoproterozoic single-stage Nd model ages (0.80~1.33Ga), and variable initial 87Sr/86Sr ratios (0.7041–0.7059), suggesting that these magmatic rocks are likely derived from partial melting of a moderately depleted mantle wedge with minor involvement of a Precambrian continental crust. These new data, combined with previously published results put constraints on the Late Paleozoic magmatic and geodynamic evolution of the Balkhash–Yili active continental margin. The Late Paleozoic arc magmatism initiated from ~386Ma along the northern Yili Block, and migrated southward to have occurred since ~340Ma along its southern margin. Subsequently, the arc-related magmatism moved back northward and terminated at 310Ma in the south and at ~295Ma in the north. The OIB-like geochemical features of some volcanic rocks indicate incipient marginal rifting occurred along the southern Yili Block during Late Carboniferous. Taking into account the regional structural and tectonic data, the Carboniferous Yili magmatic arc may be laterally correlated with the Balkhash–Yili active margin of the Kazakhstan microcontinent, which was likely resulted from the southward subduction of the Junggar – North Tianshan Ocean.

Carboniferous bimodal volcanic rocks in the Eastern Tianshan, NW China: Evidence for arc rifting

Detailed geochronological, geochemical, and Sr–Nd isotopic data are presented for late Paleozoic volcanic rocks in the Dashitou area from the Eastern Tianshan, NW China, aiming to constrain their petrogenesis and tectonic implications. The Dashitou volcanic rocks show a bimodal distribution in composition, with dominant rhyolite and subordinate basalt. LA–ICPMS zircon U–Pb age indicates that the volcanic rocks were erupted at 330±3Ma. The basaltic rocks are medium-K calc-alkaline, enriched in light rare earth element (LREE) and large ion lithophile element (LILE; Ba and U), and depleted in high field strength element (HFSE; Nb, Ta, and Ti). These features, together with their depleted isotopic signature (initial 87Sr/86Sr=0.7035 to 0.7039, εNd(t)=6.8 to 7.1), suggest that they were likely derived from a depleted mantle source metasomatized by subduction-related fluids. The rhyolitic rocks are high-K calc-alkaline and exhibit characteristics of A2-type granite, with typical enrichment in alkalis, Zr, and Nb and depletion in Sr, P, and Ti. They are characterized by positive εNd(t) values (6.2–6.5) with young Nd model ages (0.57–0.58 Ga). Results of MELTS modeling imply that they were unlikely produced by fractional crystallization of coeval basaltic rocks. Instead, they were probably generated by partial melting of juvenile basaltic crust as a result of magma underplating. The geochemical and Sr–Nd isotopic characteristics, coupled with regional geology, indicate that the formation of the Dashitou bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the Dananhu–Harlik arc in response to the retreat of the subducting Northern Tianshan oceanic lithosphere may account for the Dashitou bimodal volcanic rocks.

Geochronology, petrogenesis and tectonic implication of Late Paleozoic volcanic rocks from the Dashizhai Formation in Inner Mongolia, NE China

In this paper we present geochemical, zircon U–Pb and Hf isotopic data on the late Paleozoic volcanic rocks of the Dashizhai Formation, which are exposed along the northwestern margin of the Songnen terrane in eastern Inner Mongolia. Our aim is to constrain the petrogenesis and tectonic setting of the volcanic rocks and to unravel the late Paleozoic tectonic evolution of the northwestern part of the Songnen terrane, along the eastern segment of the Central Asian Orogenic Belt. Lithologically, the Dashizhai Formation is composed mainly of rhyolitic tuff, rhyolite, dacite, andesite, basaltic andesite and basalt, with minor basaltic trachyandesite. The zircons separated from these rocks are euhedral–subhedral, have high Th/U ratios (0.2–1.6), and display broad oscillatory growth zoning, indicating a magmatic origin. The results of zircon U–Pb dating indicate the volcanic rocks formed during the early Permian (295–283Ma). Geochemically, these volcanic rocks belong to the mid-K to high-K calc-alkaline series and are characterized by an enrichment in large ion lithophile elements (LILEs) and a depletion in high field strength elements (HFSEs, such as Nb, Ta, and Ti), similar to igneous rocks that form in active continental margin settings. Most magmatic zircons of the rhyolites show positive εHf(t) values (+3.65 to +13.0) and two-stage model ages (TDM2) of 1396–551Ma. These geochemical characteristics indicate that the acidic volcanic rocks of the Dashizhai Formation were most likely derived from the partial melting of dominantly juvenile crustal components with a possible addition of “old” materials. In contrast, the basic to intermediate volcanic rocks were derived from the partial melting of a depleted lithospheric mantle that had been metasomatized by fluids derived from a subducted slab. These data, together with regional geological investigations, suggest that the generation of the early Permian volcanic rocks of the Dashizhai Formation was related to the southward subduction of the Paleo–Asian oceanic plate beneath the Songnen terrane. This also implies that the terminal collision between the Songnen and Xing'an terranes did not occur before the early Permian.

The zircon U–Pb and Hf isotope constraints on the basement nature and Paleozoic evolution in northern margin of Yili Block, NW China

The northern margin of Yili Block in Xinjiang, China, consists of Precambrian basement, Early Paleozoic sedimentary rocks intersected with bimodal volcanic rocks, conglomerate and Late Paleozoic volcanic and sedimentary rocks. Five magmatic events are identified: ~2.5Ga, 1.5–1.4Ga, 1.1 to 0.9Ga, 480 to 450Ma and 410 to 350Ma based on SHRIMP zircon U–Pb ages of volcanic rocks and conglomerate in Tulasu Basin in west part of Northern Yili Block margin. The Precambrian ages record the evolution of the basement under Yili Block. The age population of 480 to 450Ma corresponds to the Early Paleozoic bimodal volcanic rock, that of 410 to 350Ma is documented by Late Paleozoic volcanic rocks.The εHf(t) value of ~2.5Ga zircon is 1.0, corresponding to TDM2 of 2.9Ga, indicating the old crust of Yili Block formed during 3.0 to 2.5Ga, which is also evidenced by TDM2 (3.2 to 2.6Ga) of 1.5–1.4Ga aged zircons. The old crust reworked at 1.5–1.4Ga, proved by εHf(t) (−16.3 to −1.3) value of the zircons. The 1.1 to 0.9Ga zircons show εHf(t) value of −7.6 to 2.7 and TDM2 of 2.3 to 1.7Ga, might be the remelting products of old crust with some contribution of mantle. The Precambrian age groups of Yili Block are coeval with Chinese Central Tianshan, Tarim Craton and Kyrgyz North Tianshan, suggesting that they may represent fragments of a single continent characterized by magmatic events at 1.5 to 1.4Ga and 1.1 to 0.9Ga.Bimodal volcanic rocks erupted at 480 to 450Ma, thus a rift-related tectonic setting was proposed in combination with reported geochemical characteristic. The Basaltic rocks with age of ~477Ma have εHf(t) value of 3.4 to 13.9, showing a magma source of depleted mantle with involvement of little crustal material. The felsic rocks with age of ~450Ma have εHf(t) value of −20.4 to 1.2, indicative of a continental crust source and little contribution of mantle magma. Their magma sources also show rift-related magma property. The volcanic rocks erupted during 410 to 350Ma were the products of subduction of the North Tianshan oceanic crust, which made contribution to crustal growth of northern Yili Block. The magma source of the Paleozoic volcanic rocks is depleted mantle contaminated by crust, constraint by their positive εHf(t) value (−0.2~12.8).

Zircon age and geochemistry of the Tost bimodal volcanic rocks: Constraints on the Early Carboniferous tectonic evolution of the South Mongolia

SIMS zircon U–Pb dating, geochemical and Sr–Nd isotopic data are presented for the Late Paleozoic volcanic rocks from Tost area in Mongolia, the southern portion of the Central Asian Orogenic Belt (CAOB). The Tost volcanic rocks show a bimodal feature characterized by a mafic member of basalt and a felsic component of rhyolite, which are temporally and spatially related each other, implying a genetic relationship. Zircon U–Pb isotopic data of the rhyolite constrain the Tost bimodal magmatism occurring from 355Ma to 320Ma. The Tost basalt is characterized by high abundances in Th, U and Pb, slightly enriched LREE patterns and low HFSE/LREE ratios. These features, together with their OIB-like isotopic signature ((87Sr/86Sr)i=0.7039378–0.704397, εNd(t)=3.55–5.02), suggest that they were likely derived from low-degree partial melting of a metasomatized asthenospheric mantle source with subordinate input of subduction components. The Tost rhyolite, which displays an intimate affinity to Tost basalt, with enrichment in Th, U and Pb, depleted in Nb, Ta and Ti, and gently right-tilted REE patterns, is inferred to be generated by partial melting of a juvenile lower crustal source heated by underplating mafic magmas which rise from asthenosphere during continued rifting. The Tost bimodal volcanic rocks are comparable both in age and composition with those in the East Tianshan, which together constitute an E–W-oriented belt of bimodal volcanic rocks, marking an Early Carboniferous rifting event. Considering regional geology, we propose that the rifting took place in a back-arc extensional setting, probably induced by the subduction of the Dzungaria Ocean between the East Tianshan and Junggar-Kazakhstan plate during the Early Carboniferous.

Reservoir characteristics, formation mechanisms and petroleum exploration potential of volcanic rocks in China

Characterized by complex lithology and strong heterogeneity, volcanic reservoirs in China developed three reservoir space types: primary pores, secondary pores and fractures. The formation of reservoir space went through the cooling and solidification stage (including blast fragmentation, crystallization differentiation and solidification) and the epidiagenesis stage (including metasomatism, filling, weathering and leaching, formation fluid dissolution and tectonism). Primary pores were formed at the solidification stage, which laid the foundation for the development and transformation of effective reservoirs. Secondary pores were formed at the epidiagenesis stage, with key factors as weathering and leaching, formation fluid dissolution and tectonism. In China, Mesozoic–Cenozoic volcanic rocks developed in the Songliao Basin and Bohai Bay Basin in the east and Late Paleozoic volcanic rocks developed in the Junggar Basin, Santanghu Basin and Tarim Basin in the west. There are primary volcanic reservoirs and secondary volcanic reservoirs in these volcanic rocks, which have good accumulation conditions and great exploration potential.

Geochronology and geochemistry of late Paleozoic volcanic rocks on the western margin of the Songnen–Zhangguangcai Range Massif, NE China: Implications for the amalgamation history of the Xing'an and Songnen–Zhangguangcai Range massifs

Abstract We here elucidate the tectonic evolution of the Xing'an and Songnen–Zhangguangcai Range massifs during the early Carboniferous–early Permian, based on zircon U–Pb dating and whole-rock geochemical analyses of volcanic rocks of the Songnen–Zhangguangcai Range Massif in the Sunwu area, Heilongjiang Province, NE China. Euhedral–subeuhedral zircons from three rhyolites and one dacite from the study area display fine-scale oscillatory growth zoning, indicating a magmatic origin. Zircon U–Pb dating by LA–ICP–MS indicates that these acidic volcanic rocks formed in the early Carboniferous–early Permian; i.e., early Carboniferous (~ 351 Ma), early late Carboniferous (~ 319 Ma), and early Permian (295–293 Ma). The early Carboniferous rhyolites exhibit chemical affinities to A-type rhyolites, implying an extensional environment. Their positive eHf(t) values (+ 8.67 to + 13.4 except for one spot of + 1.63) and Hf two-stage model ages (T DM2 = 562–988 Ma) indicate that the primary magma was possibly derived from partial melting of newly accreted continental crust. The early late Carboniferous rhyolites and dacites (~ 319 Ma) exhibit calc-alkaline peraluminous signature [molar Al 2 O 3 /(CaO + K 2 O + Na 2 O) ratio, or A/CNK = 1.04–1.22]. The eHf(t) values and T DM2 ages of zircons from the 319 Ma dacites are in the range of + 5.33 to + 9.32 and 907–1268 Ma, respectively, suggesting that the primary magma was derived from partial melting of newly accreted crust. The early Permian rhyolites (295–293 Ma) show chemical affinities to A-type rhyolites, implying an extensional tectonic environment; their positive eHf(t) values (+ 8.82 to + 13.8) and Hf two-stage model ages (484–743 Ma) indicate that the primary magma was derived from partial melting of newly accreted crust. Combined with the geochemical features of coeval igneous rocks from the eastern margin of the Xing'an Massif, these data reveal the late Paleozoic tectonic history and relationships of the Xing'an and Songnen–Zhangguangcai Range massifs, i.e., early Carboniferous westward subduction of the Paleo-Asian oceanic plate beneath the Xing'an Massif, followed by early late Carboniferous collision and amalgamation of microcontinental blocks, and early Permian post-collisional extension.

Petrogenesis of late Paleozoic volcanic rocks from the Daheshen Formation in central Jilin Province, NE China, and its tectonic implications: Constraints from geochronology, geochemistry and Sr–Nd–Hf isotopes

We present geochronological, geochemical, whole-rock Sr–Nd and zircon Hf–isotopic data for late Paleozoic volcanic rocks from the Daheshen Formation in central Jilin Province, northeastern China, and constrain the petrogenesis of the volcanic rocks and late Paleozoic tectonic evolution of the northern margin of the Northern China Craton, which is regarded as the eastern segment of the Central Asian Orogenic Belt (CAOB). Lithologically, the Daheshen Formation is composed mainly of rhyolite, rhyolitic tuff, dacite and andesite, with minor basalt. The zircons from three rhyolites, two dacites, one rhyolitic tuff and one basalt are euhedral–subhedral, display oscillatory zoning and have high Th/U ratios (0.50–2.28), implying a magmatic origin. LA–ICP-MS zircon U–Pb age data indicate that the volcanic rocks from the Daheshen Formation formed during Late Carboniferous–Early Permian time (302–299Ma). Geochemically, late Paleozoic volcanic rocks have SiO2=52.13–81.77wt.% and K2O=0.86–6.88wt.%, belonging to mid-K to high-K calc-alkaline series. These rocks are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, such as Nb, Ta, and Ti) and heavy rare earth elements (HREEs), with affinities to igneous rocks forming in an active continental margin setting. All volcanic rocks have depleted Nd isotopic compositions (εNd(t)=+2.4 to +2.5 for the basalts and +5.8 to +7.1 for the andesites and dacites, respectively). In situ Hf isotopic results of zircon from the rhyolites show that they have εHf(t)=−1.1 to +10.6. All these geochemical features indicate that the andesites, dacites, and rhyolites likely originated from the partial melting of Meso-Neoproterozoic accreted lower crust (Hf and Nd model ages (TDM2) of 1384–662Ma and 1061–800Ma, respectively). In contrast, the basalts were derived from the partial fusion of a depleted lithospheric mantle that had subsequently been metasomatized by subducted slab-derived fluids. These data, along with the regional geological investigations, suggest that the generation of late Paleozoic volcanic rocks from the Daheshen Formation was related to southward subduction of the Paleo-Asian oceanic plate beneath the northern margin of the North China Craton. This also indicates that the Paleo-Asian Ocean may have not closed before the Early Permian.

An Early Devonian to Early Carboniferous volcanic arc in North Tianshan, NW China: Geochronological and geochemical evidence from volcanic rocks

The Late Paleozoic volcanic and sedimentary rocks are widespread in the North Tianshan along the north margin of the Yili block. They consist of basalt, basaltic andesite, andesite, trachyandesite, dacite, rhyolite, tuff, and tuffaceous sandstone. According to zircon sensitive high-resolution ion microprobe (SHRIMP) dating, the age of the Late Paleozoic volcanic rocks in Tulasu basin in western part of North Tianshan is constrained to be Early Devonian to Early Carboniferous (417–356Ma), rather than Early Carboniferous as accepted previously. Geochemical characteristics of the Early Devonian to Early Carboniferous volcanic rocks are similar to those of arc volcanic rocks, which suggest that these volcanic rocks could be the major constituents of a continental arc formed by the southward subduction of North Tianshan Oceanic lithosphere. Geochemical studies indicate that the magma source of the volcanic rocks might be the mantle wedge mixed with subduction fluid, which is geochemically enriched than primitive mantle but depleted than E-MORB. The calculation shows that the basalt could be formed by ∼10% partial melting of subduction fluid modified mantle wedge. Andesites with high initial 87Sr/86Sr (0.7094–0.7104) and negative εNd(t) (−4.45 to −4.79) values reveal the contribution of continental crust to its source. The calculation of assimilation–fractional crystallization (AFC) shows that the fractional crystallization process of the basaltic magma, which was accompanied with assimilation by different degree of continental crust, produced andesite (7–9%), dacite (∼12%) and rhyolite (>20%).

Carboniferous–Permian volcanic evolution in Central Europe—U/Pb ages of volcanic rocks in Saxony (Germany) and northern Bohemia (Czech Republic)

Nine SHRIMP U/Pb ages on zircon and two Pb/Pb single zircon ages have been determined from Late Paleozoic volcanic rocks from Saxony and northern Bohemia. Samples came from the Teplice-Altenberg Volcanic Complex, the Meissen Volcanic Complex, the Chemnitz Basin, the Dohlen Basin, the Brandov-Olbernhau Basin, and the North Saxon Volcanic Complex. The Teplice-Altenberg Volcanic Complex is subdivided into an early Namurian phase (Mikulov Ignimbrite, 326.8 ± 4.3 Ma), thus older than assumed by previous studies, and a late caldera-forming phase (Teplice Ignimbrite, 308.8 ± 4.9 Ma). The age of the latter, however, is not well constrained due to a large population of inherited zircon and possible hydrothermal overprint. The Leutewitz Ignimbrite, product of an early explosive volcanic episode of the Meissen Volcanic Complex yielded an age of 302.9 ± 2.5 Ma (Stephanian A). Volcanic rocks intercalated in the Brandov-Olbernhau Basin (BOB, 302 ± 2.8 Ma), Chemnitz Basin (CB, 296.6 ± 3.0 Ma), Dohlen Basin (DB, 296 ± 3.0 Ma), and the North Saxon Volcanic Complex (NSVC, c. 300–290 Ma) yielded well-constrained Stephanian to Sakmarian ages. The largest Late Paleozoic ignimbrite-forming eruption in Central Europe, the Rochlitz Ignimbrite, has a well-defined middle Asselian age of 294.4 ± 1.8 Ma. Ages of palingenic zircon revealed that the Namurian-Westphalian magmatism assimilated larger amounts of crystalline basement that formed during previous Paleozoic geodynamic phases. The Precambrian inherited ages support the chronostratigraphic structure assumed for the Saxo-Thuringian Zone of the Variscan Orogen. The present results help to improve the chronostratigraphic allocation of the Late Paleozoic volcanic zones in Central Europe. At the same time, the radiometric ages have implications for the interbasinal correlation and for the geodynamic evolution of the Variscan Orogeny.

Permian bimodal volcanism in the Zhangguangcai Range of eastern Heilongjiang Province, NE China: Zircon U–Pb–Hf isotopes and geochemical evidence

In this paper, we report on zircon U–Pb dating, Hf isotopes, major and trace elements, and Sr–Nd isotope data, with the aim of constraining the petrogenesis and regional tectonic evolution of late Paleozoic volcanic rocks in the Zhangguangcai Range of eastern Heilongjiang Province, NE China. Located in the eastern segment of the Central Asian Orogenic Belt (CAOB), between the Siberian and North China cratons, the late Paleozoic volcanic rocks in the Zhangguangcai Range are composed mainly of basalt, basaltic andesite, rhyolite, and minor dacite. The results of LA–ICP–MS zircon U–Pb dating for two basaltic rocks, three rhyolites, and one dacite indicate that they formed in the early Permian (ca. 292 Ma). The mafic rocks have SiO 2 = 50.13–53.80 wt.%, K 2O = 0.98–2.28 wt.%, Mg# = 0.51–0.71, Cr = 144–541 ppm, Ni = 74–260 ppm, ( 87Sr/ 86Sr) i = 0.7044, and ε Nd ( t) = +4.28, whereas the felsic rocks have SiO 2 = 69.12–77.98 wt.%, K 2O = 3.09–5.33 wt.%, Mg# = 0.17–0.36, ( 87Sr/ 86Sr) i = 0.7032, and ε Nd ( t) = +4.32. These data are typical of bimodal volcanism. The mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements (LILEs) such as Rb, Ba, Sr, and Pb, depletion in high field-strength elements (HFSEs) such as Nb, Ta, and Ti, depletion in heavy rare-earth elements (HREEs), and weak negative Eu anomalies (Eu/Eu * = 0.88–0.94). On the other hand, the felsic rocks show a strong depletion in Nb, Ta, Sr, P, and Ti, enrichment in Th, U, and K, and relatively large negative Eu anomalies (Eu/Eu * = 0.28–0.95). Taken together, these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle, modified by subducted slab-derived fluids, and that the felsic magma originated by partial melting of newly accreted crust. The early Permian bimodal volcanic rocks, together with the coeval A-type granites, indicate an extensional environment, similar to a back-arc basin. Such a setting was possibly related to subduction of the Paleo-Asian oceanic plate beneath the Jiamusi and Songnen–Zhangguangcai Range massifs.

Heavy mineral-based provenance analysis of Mesozoic continental-marine sediments at the western edge of the Bohemian Massif, SE Germany: with special reference to Fe–Ti minerals and the crystal morphology of heavy minerals

During the Mesozoic, the epicontinental Germanic Basin and the Regensburg Strait the latter being an embayment of the Tethys Ocean that had subsided into the Moldanubian Zone of the Central European Variscides were filled with terrigenous continental-marine sediments. Both sediments’ heavy mineral (HM) grains and aggregates have been studied in a drill section in the Wackersdorf area, SE Germany. The majority of them belong to the (semi)opaque group of Fe–Ti minerals. In Wackersdorf, the entire stratigraphy of the basin fill, which occurred between the Triassic and the Late Cretaceous, is well exposed. In addition to the chemical composition of HM, the morphology and texture of zircon, apatite and Fe–Ti compounds have been studied in a provenance-related mineral classification. Provenance analysis has yielded five discrete source rock lithologies: (1) Moldanubian H-T-metamorphics, (2) late Paleozoic (sub)volcanic rocks, (3) gneisses of the Tepla-Barrandian unit, (4) ophiolites of the Tepla-Barrandian unit, (5) silicified shear zones and quartz cores of pegmatites. The detrital minerals include zircon, tourmaline (dravite-schoerl), apatite, monazite (Ce–Th–La–Nd), xenotime, biotite, rutile, ilmenite, “nigrine” (ilmenite-rutile intergrowth), sphene, amphibole, staurolite, garnet and spinel (Cr–Mg–Al). Based on the allogenic Ti and Fe minerals, a magnetite-type source area (Eh > 0, near-surface felsic to intermediate (sub)volcanic rocks) was distinguished from an ilmenite-type source area (Eh < 0), deeply eroded crystalline basement rocks (gneiss, granite, shear zones). The latter may be subdivided into “nigrine –I” (deep) and “nigrine-II” (intermediate) subtypes, according to the level of erosion in the source area. At the Jurassic–Cretaceous transition, extrabasinal erosion provoked a noticeable variation of allogenic heavy minerals with the incisions of rivers into source rock lithologies (4) and (5). Uplift and erosion along the western edge of the Bohemian Massif took place contemporaneously with spreading and closure in the central parts of the adjacent Tethys Ocean.