Bimodal Volcanic Rocks Research Articles

New U–Pb age constraints on the geological history of the Ganderian Bras d’Or terrane, Cape Breton Island, Nova Scotia

The northern Appalachian orogen preserves evidence of a complex history of amalgamation of terranes with both Laurentian and Gondwanan affinities. The Ganderian Bras d’Or terrane of central Cape Breton Island is not well represented elsewhere in the orogen and its relationship to other Ganderian terranes is enigmatic, particularly with respect to its pre-Neoproterozoic history. The Boisdale Hills and Kellys Mountain areas contain the oldest metamorphic rocks in the Bras d’Or terrane. Quartzite units in the Boisdale Hills have detrital zircon populations with ages ranging from 3.2 to ca. 1 Ga. Paragneiss units from the Kellys Mountain area contain Meso- to Neoproterozoic detrital zircons, in which the youngest grains indicate that the maximum depositional age is <600 Ma. The detrital zircon populations of rocks from both areas are consistent with Gondwanan provenance for the protoliths, most likely the Amazonian craton. New U–Pb dates for subduction-related dioritic to granodioritic plutons in the Boisdale Hills yielded ages of 560 to ca. 540 Ma. Sedimentary, bimodal volcanic and plutonic rocks from the Bourinot belt in the Boisdale Hills and related plutons in the Kellys Mountain area have ages of ca. 510–490 Ma and are interpreted to have formed during extension related to separation of Ganderia from Gondwana. The southeastern Bras d’Or terrane preserves rocks formed in Pan-African subduction zones on a former passive margin of Gondwana as well as rocks formed during the initial stages of rifting of Ganderia from Gondwana and the subsequent opening of the Rheic Ocean.

Petrogenesis of the ca. 820–810 Ma felsic volcanic rocks in the Bikou Group: Implications for the tectonic setting of the western margin of the Yangtze Block

The petrogenesis of Neoproterozoic igneous rocks at the northwestern margin of the Yangtze Block and their tectonic setting are controversial. Here we present whole–rock major and trace elements and Nd–Hf isotopic data for felsic volcanic rocks from the Bikou Group. These ca. 820–810 Ma volcanic rocks consist of dacite and rhyolite with SiO2 ranging from 64.4 to 77.0 wt%. Most rocks show decreasing P2O5 with increasing SiO2 and Th and have I–type granite affinities (metaluminous volcanic rocks). However, four rhyolites from the lower part of the Bikou Group show A–type granite affinities (alkaline rhyolites), characterized by high 10,000*Ga/Al and FeOt/(FeOt + MgO) ratios and high (Zr + Nb + Ce + Y) contents. The felsic volcanic rocks commonly have relatively high Zr saturation temperature (up to 960 °C) and most of the felsic volcanic rocks have εNd(t) and εHf(t) values lower than the contemporaneous basalts in the Bikou Group, but higher than the basement rocks in the study area. Among them, four alkaline rhyolites have εNd(t) and εHf(t) values higher than the metaluminous volcanic rocks in the lower Bikou Group. We propose that the metaluminous volcanic rocks were likely produced by partial melting of newly underplated basaltic crust, with involvement of different proportions of basement rocks, whereas a two–stage model is proposed for the formation of the alkaline rhyolites, i.e. partial melting of newly underplated basaltic crust forming early granitoids, followed by low pressure partial melting of these granitoids to generate the alkaline rhyolitic magma. Combined with previous studies, the bimodal occurrence of volcanic rocks within the Bikou Group, and the A–type granite affinity of some rhyolites, as well as the presence of ocean island (OIB)–type basalts in the upper part of Bikou Group, suggest an extensional setting. This is consistent with opening of the Kangdian rift basin to the southwest at this time. We thus propose that the ca. 820–810 Ma Bikou bimodal volcanic rocks formed in response to the breakup of the Rodinia supercontinent.

Paleoproterozoic A-type granite from the southwestern margin of the North China block: high temperature melting of tonalitic crust in extensional setting

ABSTRACTProterozoic igneous rocks in the southern margin of the North China Craton can place constraints on its crustal evolution. This paper reports geochemistry, zircon U-Pb ages, Sr-Nd isotopes and Lu-Hf isotopes for the Proterozoic Angou granite from the southwestern margin of the North China Craton (NCC). The Angou granite has zircon LA-ICP-MS U-Pb age of 1819 ± 14Ma (MSWD = 1.3, n = 37), which is synchronous with the Proterozoic Xiong’er Group in the southeastern margin of the NCC. The granite has A-type granite affinity: (1) high SiO2 (67.12% to 73.16%) and alkali (Na2O+K2O = 7.18% to 8.75%) contents, with K2O/Na2O ratios of 1.04 to 4.78; (2) high FeOT/MgO values (4 to 36); (3) significantly negative Eu anomalies (Eu*/Eu = 0.33 to 0.53) and negative Sr anomalies (38.1 to 102ppm); (4) high 104× Ga/Al (2.55 to 4.75, with an average of 3.43) values and Zr+Ce+Nb+Y (634 to 1022ppm) contents; (5) high zircon saturation temperatures (871 to 957°C). In combination with its slightly peraluminous (A/CNK = 1.02 to 1.26) nature, the Angou granite can be classified as slightly peraluminous A-type granite. It displays negative ɛNd(t) values of −2.0 to −3.9, variable εHf(t) values (−8.81 to +1.98) with Hf model ages of 2194 to 2721Ma, combined with its high Rb/Sr (1.05 to 3.26) and low Rb/Ba (0.08 to 0.18) ratios, suggesting that the Angou granite was derived from high temperature melting of tonalite in middle-lower crust. The formation of Angou A-type granite is consistent with the mafic dyke swarms and Xiong’er bimodal volcanic rocks in NCC, indicating a Paleoprotozoic extensional setting in the southwestern margin of the NCC.

Origin of Cretaceous aluminous and peralkaline A-type granitoids in northeastern Fujian, coastal region of southeastern China

In this work, we report the geochemical, whole-rock Sr-, Nd-, and Hf-isotopes, and zircon U–Pb age and Hf–O isotopic compositions for six granitic intrusions in northeastern Fujian, coastal region of southeastern China, to elucidate the petrogenesis of aluminous and peralkaline A-type granites in this region. Zircon U–Pb dating of these rocks (including porphyritic syenites, porphyritic granites, alkali-feldspar granites, and arfvedsonite granites) yielded Late Cretaceous ages of 94–98 Ma. Mineral assemblage and geochemical features suggested that the arfvedsonite granites were peralkaline A-type but the porphyritic syenites, porphyritic granites, and alkali-feldspar granites were aluminous A-type. Geochemical data, whole-rock Nd–Hf isotopes (εNd(t) = −5.5 to −4.0, εHf(t) = −3.6 to 0), and the zircon Hf–O isotopes (εHf(t) = −3.0 to +1.4, δ18O = 5.4‰–6.0‰) indicated that the porphyritic syenites were produced by partial melting of crustal rocks in the lower crust. The porphyritic granites and alkali-feldspar granites had similar geochemical features and zircon Hf–O isotopic composition with the porphyritic syenites, suggesting that they were formed by feldspar-dominant crystal fractionation of deep crust derived syenitic magma under shallow crustal level. Compared with the aluminous A-type granites, the peralkaline A-type granites had lower abundance of Al2O3, MgO, CaO, Ba, and Sr, higher abundance of SiO2 and HFSE, as well as higher ratio of total FeO/MgO, Ga/Al, and Rb/Sr. However, the whole-rock Nd-, Hf- and zircon Hf–O isotopic compositions of the peralkaline and aluminous A-type granites were similar. It could be inferred that the peralkaline A-type granites were formed by crystal fractionation of aluminous A-type granitic magmas. Since the studied A-type granitic intrusions were coeval with the bimodal volcanic rocks and the extensional basins in the coastal region of southeastern China, they were likely produced in an extensional setting, possibly during lithospheric thinning that resulted from the subduction of the paleo-Pacific plate beneath southeastern China.

An early Devonian intra-plate bimodal volcanic suite in the Kyrgyz North Tianshan belt, the central Asian orogenic belt

A suite of bimodal volcanic rocks in the Kyrgyz North Tianshan belt is composed of totally ∼100-m-thick basalt and basaltic andesite layers of the Karakastek Formation and totally ∼700-m-thick rhyolite layers of the Kastek Formation, with minor amounts of interlayered felsic to intermediate tuff/ignimbrite. Zircon grains from a rhyolite sample have a weighted mean 206Pb/238U age of 416 ± 8 Ma, which is considered to be the crystallization age of the rhyolite. The zircon grains have δ18O ranging from +6.5 to +7.3‰. The calculated zircon saturation temperatures (TZr) range from 850 to 900 °C. Rhyolites have high total alkaline, Zr, Y and Nb contents and high Ga/Al similar to A1-type granitoids formed in an intra-plate setting. Mafic volcanic rocks have SiO2 ranging from 49.4 to 55.3 wt% and FeOt/MgO from 1.4 to 2.4, showing a tholeiitic trend. They have negative Nb and Ta anomalies on the primitive mantle-normalized trace element patterns and have εNd(t) of −1.5 to 0. There is a positive correlation between εNd(t) and ΔεHf(t) of the mafic volcanic rocks, indicative of crustal contamination. However, one least evolved basalt sample has Mg# of 57 with low Zr and Hf concentrations, indicating insignificant crustal contamination. Modeling results indicate that the mafic volcanic rocks may have been derived from a dry, garnet-bearing lherzolite mantle source. They have εHf(t) and εNd(t) varying along the “Island arc array”, which is interpreted as the mantle source was metasomatized by slab-derived melts rather than fluids. In addition, the rhyolites have εNd(t) of −2.4 to −2.9, much lower than the mafic volcanic rocks, and were thus unlikely produced by the fractionation of basaltic magmas. The rhyolites show Nd-Hf isotope decoupling, which is interpreted as a result of the anataxis of Precambrian lower crust associated with the breakdown of garnet. Therefore, the early Devonian bimodal volcanic suite in the Kyrgyz North Tianshan belt was considered to have formed in an intra-plate setting.

Onset of the North-South Gravity Lineament, NE China: Constraints of Late Jurassic bimodal volcanic rocks

The North-South Gravity Lineament (NSGL) is an important geophysical boundary within NE China. Gravity anomaly, heat flow, crustal and lithospheric thickness, terrain and altitude all change dramatically across the NSGL. However, the timing of onset of the NSGL remains enigmatic, and the Mesozoic lithospheric thinning mechanism to the east of the NSGL is still unclear in NE China. Here we study a Late Jurassic bimodal volcanic suite located on the NSGL including N-MORB-like basalts, which constrains the initial formation of the NSGL. The bimodal volcanic rocks consist mainly of basalts and basaltic andesites with subordinate rhyolites. Our results show that the bimodal volcanic rocks are products of crust-asthenosphere interaction. Calculations indicate that lithospheric thicknesses in NE China have significantly changed from ~90–100 km in the west to 49–62 km in the east since the Late Mesozoic. The variation of lithospheric thicknesses on both sides of the NSGL could be ascribed to lithospheric delamination. Zircons from the bimodal volcanic rocks yield Late Jurassic ages of 147 Ma, nearly synchronous with the onset of the Songliao Basin and marking the start of extensional tectonics in NE China. Combined with previous studies, we confirm that the delamination induced by Paleo-Pacific subduction gave rise to the formation of the NSGL, which initiated at the Late Jurassic in NE China.

Contributions of basaltic underplating to crustal growth in island arc and extensional tectonic settings in the Chinese Tianshan Orogenic Belt, NW China

A mafic–ultramafic intrusive belt comprising Silurian arc gabbroic rocks and Early Permian mafic–ultramafic intrusions was recently identified in the western part of the East Tianshan, NW China. This paper discusses the petrogenesis of the mafic–ultramafic rocks in this belt and intends to understand Phanerozoic crust growth through basaltic magmatism occurring in an island arc and intraplate extensional tectonic setting in the Chinese Tianshan Orogenic Belt (CTOB). The Silurian gabbroic rocks comprise troctolite, olivine gabbro, and leucogabbro enclosed by Early Permian diorites. SHRIMP II U-Pb zircon dating yields a 427 ± 7.3 Ma age for the Silurian gabbroic rocks and a 280.9 ± 3.1 Ma age for the surrounding diorite. These gabbroic rocks are direct products of mantle basaltic magmas generated by flux melting of the hydrous mantle wedge over subduction zone during Silurian subduction in the CTOB. The arc signature of the basaltic magmas receives support from incompatible trace elements in olivine gabbro and leucogabbro, which display enrichment in large ion lithophile elements and prominent depletion in Nb and Ta with higher U/Th and lower Ce/Pb and Nb/Ta ratios than MORBs and OIBs. The hydrous nature of the arc magmas are corroborated by the Silurian gabbroic rocks with a cumulate texture comprising hornblende cumulates and extremely calcic plagioclase (An up to 99 mol%). Troctolite is a hybrid rock, and its formation is related to the reaction of the hydrous basaltic magmas with a former arc olivine-diallage matrix which suggests multiple arc basaltic magmatism in the Early Paleozoic. The Early Permian mafic–ultramafic intrusions in this belt comprise ultramafic rocks and evolved hornblende gabbro resulting from differentiation of a basaltic magma underplated in an intraplate extensional tectonic setting, and this model would apply to coeval mafic–ultramafic intrusions in the CTOB. Presence of Silurian gabbroic rocks as well as pervasively distributed arc felsic plutons in the CTOB suggest active crust-mantle magmatism in the Silurian, which has contributed to crustal growth by (1) serving as heat sources that remelted former arc crust to generate arc plutons, (2) addition of a mantle component to the arc plutons by magma mixing, and (3) transport of mantle materials to form new lower or middle crust. Mafic–ultramafic intrusions and their spatiotemporal A-type granites during Early Permian to Triassic intraplate extension are intrusive counterparts of the contemporaneous bimodal volcanic rocks in the CTOB. Basaltic underplating in this temporal interval contributed to crustal growth in a vertical form, including adding mantle materials to lower or middle crust by intracrustal differentiation and remelting Early-Paleozoic formed arc crust in the CTOB.

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.

Petrogenesis and geochemical characteristics of Early Carboniferous sanukitic high‐Mg andesite from Atengtao Mountain, Yili Block: Implications for the tectonic setting during Late Palaeozoic in Chinese West Tianshan

As an important part of the Central Asian Orogenic Belt (CAOB), Chinese western Tianshan contains voluminous Late Palaeozoic volcanic rocks. Here, we present geochronological, geochemical, and in situ Lu–Hf isotopic data from the Atengtao sanukitic high‐Mg andesites (HMAs) of the western Yili Block. LA‐ICP‐MS zircon U–Pb analyses indicate that the Atengtao high‐Mg andesites were generated in the Early Carboniferous (346 ± 2 Ma). All the studied rock samples are characterized by high MgO (3.30–5.45 wt.%), SiO2 (53.25–60.10 wt.%), Cr (90.3–157 ppm), and Ni (31.0–46.7 ppm) contents, with geochemical characteristics analogous to those of sanukitoids of the Setouchi Volcanic Belt (SVB), SW Japan, and Bieluagaxi area, West Junggar. The Atengtao sanukitic HMAs are enriched in LILE and LREE but depleted in HFSE, similar to those of typical arc‐related magmatic rocks. They are also characterized by high Nb (11.4–15.8 ppm) concentrations, high Th/La (0.3) and Th/Yb (2.6–4.0) ratios, and varying zircon εHf(t) values (−2.3 to +4.3). These features are suggested to result from the partial melting of a relatively enriched mantle wedge metasomatized by subducting oceanic slab‐derived sediments. Considering the occurrence of sanukitic HMAs, combined with the contemporaneous special rock assemblages (such as slab‐derived adakite, Nb‐enrich basalt, A‐type granite/rhyolite, and bimodal volcanic rocks) found in the western part of Yili Block, we propose that the generation of the Carboniferous sanukitic HMAs in Atengtao Mountain were most likely related to the slab break‐off following the southern Tianshan Ocean slab northward subduction beneath the Yili–Central Tianshan Block. This mechanism not only produces intense magmatism with compositional diversity but also gives rise to a local extension in the western part of Yili Block. The slab break‐off model may play an important role in the crustal growth of the CAOB in Phanerozoic time.

Genesis of ore‐bearing volcanic rocks in the Derbur lead–zinc mining area of the Erguna Massif, western slope of the Great Xing'an Range, NE China: Geochemistry, Sr–Nd–Pb isotopes, and zircon U–Pb geochronology

The Derbur lead–zinc deposit is located in the Derbugan metallogenic belt in the north‐western portion of the Mesozoic Hailaer‐Genhe volcanic basin in the northern Great Xing'an Range. This deposit occurs in the Middle Jurassic intermediate–mafic volcanic rocks of the Tamulangou Formation and is spatially and temporally associated with acidic pyroclastic rocks. The host rocks are bedded trachyandesite and rhyolitic lithic‐crystal tuffs, and basaltic‐andesitic aplite veins are associated with the ore body. Because the deposit is a hypabyssal low‐temperature hydrothermal Pb–Zn deposit associated with volcanism, to determine the precise petrogenesis of the volcanic rocks, this study analysed the zircon U–Pb ages of the trachyandesite and rhyolitic lithic‐crystal tuffs, the Sr–Nd–Pb isotopic geochemistry of the trachyandesite and basaltic‐andesitic aplite, and the whole‐rock geochemistry of all three rock types. The results showed that the trachyandesite and basaltic‐andesitic aplite belong to the shoshonitic series and have similar whole‐rock and isotopic geochemical characteristics, including enrichments in large‐ion lithophile elements and light rare earth elements and depletions in high‐field‐strength elements. They also have low initial 87Sr/86Sr values of 0.705007–0.705240 and εNd(t) values of +0.6 to +1.7, with model ages (TDM) of 699–883 Ma. The ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb vary from 18.438 to 18.476, 15.570 to 15.577, and 38.254 to 38.320, respectively, with μ values of 9.40–9.41. In contrast, the rhyolitic lithic‐crystal tuffs are characterized by high SiO2 contents (73.44–79.48 wt.%); low Al2O3 (11.36–12.53 wt.%), TiO2 (0.14–0.18 wt.%), and K2O + Na2O (3.48–3.89 wt.%, Na2O ≤ K2O) contents; and low Mg# values (0.17–0.41), indicating that they belong to the calc–alkaline series. Additionally, they have relatively low REE contents and strong Sr depletions. LA‐ICP‐MS zircon U–Pb dating of the trachyandesite and rhyolitic lithic‐crystal tuffs indicates that their ages are 167.0 ± 2.0 Ma and 164.8 ± 1.6 Ma, respectively. We conclude that the trachyandesite and the basaltic‐andesitic aplite were derived from the partial melting of lower crustal material assimilated by a depleted lithospheric mantle that was subsequently metasomatized by subducted slab‐derived fluids. The rhyolitic lithic‐crystal tuffs likely originated from the partial melting of accreted lower crust. In summary, because the acidic volcanism was accompanied by increasingly felsic volcanism, bimodal volcanic rocks were produced. Among volcanogenic deposits, bimodal volcanic rocks are the most favourable ore‐hosting rocks, and ore‐forming materials were contributed by the magmatic system. These processes created conditions conducive to the formation of large‐scale Ag, Pb, and Zn mineralization in this area.

Neoproterozoic bimodal volcanic rocks and granites in the western Dabie area, northern margin of Yangtze block, China: implications for extension during the break-up of Rodinia

ABSTRACTTo better understand the Neoproterozoic tectonic evolution along the northern margin of Yangtze Block, we have determined the geochronological and geochemical compositions of newly recognized bimodal volcanic suite and coeval granites from the western Dabie terrain. LA-ICP-MS zircon U-Pb dating reveals that the felsic and mafic volcanics from the Hong’an unit have crystallization ages of 730 ± 4Ma and 735 ± 5Ma, respectively, indicating that the bimodal suite was erupted during the Neoproterozoic. The Xuantan, Xiaoluoshan, and Wuchenhe granites yield U-Pb ages of 742 ± 4 Ma, 738 ± 4 Ma, and 736 ± 4 Ma, respectively. The felsic volcanic rocks show peraluminous characteristics, and have a close affinity to S-type granite. The mafic volcanic rocks are basalt in compositions, and are likely generated from a depleted mantle source. The granites belong to high-K calc-alkaline and calc-alkaline series, display metaluminous to peraluminous, and are mainly highly fractionated I-type and A-type granite. The granites and felsic volcanics have zircon εHf(t) values of −16.4 to + 5.6 and two-stage Hf model ages (TDM2) of 1.28 to 2.40 Ga, suggesting that they were partial melting of varying Mesoproterozoic–early-Neoproterozoic crust. The granites have εNd(t) of -14.7 to -1.5, and the two-stage Nd model ages (TDM2) values of 1.54 to 2.61 Ga, also implying the Yangtze crustal contribution. These Neoproterozoic bimodal suite and coeval granites were most likely generated in a rifting extensional setting, triggered by the mantle upwelling, associated with crust–mantle interaction. Intensive magmatic rocks are widespread throughout the South Qingling, Suizhao, western Dabie and eastern Dabie areas during 810–720 Ma, and show peak ages at ~ 740 Ma. Combining regional geology, we support a continental rifting extensional setting for the north margin of the Yangtze Block during the break-up of the supercontinent Rodinia.

Late Permian Bimodal Volcanic Rocks in the Northern Qiangtang Terrane, Central Tibet: Evidence for Interaction Between the Emeishan Plume and the Paleo‐Tethyan Subduction System

AbstractMany studies provide compelling evidence for a fossil mantle plume beneath the Late Permian Emeishan large igneous province located near the southeastern margin of the central Tibetan Plateau, a region associated with the Paleo‐Tethyan subduction system. However, little is known about whether direct plume‐subduction interaction occurred during the Late Permian. Here we report geochronological, mineralogical, geochemical, and Sr‐Nd‐Hf‐O isotopic data for the Late Permian (~259–256 Ma) bimodal volcanic rocks (BVRs) in the northern Qiangtang Terrane (NQT), central Tibet. These BVRs consist mainly of basalts, rhyolites, and rhyolitic tuffs. The rhyolites with A‐type affinity were generated by partial melting of newly underplated basalts. Geochemical and isotopic data suggest that two components (ocean island basalt‐type mantle and subducted sediment‐derived fluid) were involved in the generation of the basalts in an extensional back‐arc region. Considering the Permian tectonic evolution of the NQT, we propose that this enriched ocean island basalt‐type mantle was distinct from the Paleo‐Tethyan depleted upper mantle. It most likely originated from Emeishan‐plume material that flowed westward from the South China Block to the nearby NQT, driven by slab rollback‐induced counterflow. The presence of high‐temperature A‐type rhyolites in bimodal back‐arc magmatism also indicates a special subduction setting influenced by a nearby plume. Thus, we propose that the Late Permian BVRs were the result of interaction between the Emeishan plume and the Paleo‐Tethyan subduction system.

Petrography, geochemistry, and geochronology of Lower Jurassic sedimentary rocks from the Northern Tianshan (West Bogda area), Northwest China: Implications for provenance and tectonic evolution

The Bogda Mountains, situated in the southern part of the Central Asian Orogenic Belt (CAOB), is an important intraplate orogenic belt for studying the Mesozoic tectonic evolution mechanism of the CAOB. The detailed palaeogeographic evolution process of the Bogda area during Early Jurassic times remains poorly understood. The sandstone petrography, sedimentary rock geochemistry, and detrital zircon U–Pb geochronology were integrated to research the provenance of the Lower Jurassic Badaowan and Sangonghe formations clastic rocks from the northern flank of the Bogda Mountains and further constrain the palaeogeographic evolution of Bogda Mountains. The petrological and geochemical data in this study indicate that the Lower Jurassic sediments were derived from a felsic source area. Compared with the Badaowan Formation, the source rocks for the Sangonghe Formation show more mafic characteristics. U–Pb age patterns of the Badaowan Formation have multiple age groups, and most of the zircon ages are concentrated in the Carboniferous and Late Permian–Early Triassic ages. The Late Permian–Early Triassic age group disappears in the U–Pb age spectrum of the Sangonghe Formation, and every peak age is 20–50 Ma younger than those of the Badaowan Formation. Combined with the compiled sedimentary facies and paleocurrent data, the felsic‐intermediate volcanic rocks and granitoid intrusions in the Harlik arc and the east of the Jueluotage belt were the predominant sources of the Badaowan Formation, and then the dominant source changed to the bimodal volcanic rocks in the Eastern Bogda Mountains during the Sangonghe Formation deposition period. Early Mesozoic palaeogeographic evolution of the Northern Tianshan belt is speculated: (1) the paleotopography of the Jueluotage belt had a differential uplift along its strike, transforming from high in the west during Triassic times to high in the east during early Early Jurassic times and (2) the Eastern Bogda Mountains initiated uplift during late Early Jurassic times.

Carboniferous tectonic configuration of the Yining Massif in Western Tianshan, NW China

The tectonic evolution and configuration of the Carboniferous trench‐arc‐basin system (TABS) in the Yining Massif (Western Tianshan, NW China) have long been controversial. The question of the existence of an ancient ocean basin and its subduction polarity is crucial to address this issue. In this study, we suggest that the Early Carboniferous calc‐alkaline volcano‐sedimentary rocks were formed in a fore‐arc/back‐arc basin system, whereas the Late Carboniferous bimodal volcanic rocks and their associated sedimentary rocks were formed under a continental rift setting after the Junggar‐Tarim collision. We propose that the Southern Tianshan ocean basin may have closed in the Late Early Carboniferous, terminating the TABS at Yining. Spatial–temporal distribution of volcanic‐sedimentary rock assemblages suggests that the Early Carboniferous Southern Tianshan ancient ocean basin was similar to the TABS system of the present‐day Japanese island arc.

Petrogenesis of Mesozoic volcanic rocks in the Zha'ertai area, Inner Mongolia: Constraints from trace elements and Sr–Nd–Pb isotopes

The Mesozoic acid volcanic rocks of the Zha'ertai area, located in Inner Mongolia, were formed in the late Early Cretaceous and resulted in polymetallic deposits of gold, silver, and other precious metals with huge commercial value. The more developed acid volcanic rocks include subvolcanic and pyroclastic rocks and volcanic lava, with SiO2 of 73.18–83.82%, Na2O + K2O of 6.17–9.03%, and K2O/Na2O of 1.28–70.27%. These rocks are enriched in large‐ion lithophile elements and high‐field strength elements. They are also enriched in light rare earth elements (REEs), with light to heavy REE ratios of 2.91–14.30; light to heavy REE fractionation is significant, and there are obvious negative Eu anomalies (δEu = 0.02–0.25). The acid volcanic rocks have extremely low TiO2, MgO, P2O5, and TFe2O3 contents, exhibit significant enrichment in Th, U, and Pb, and are strongly depleted in Ba and Sr, showing the trace elements feature of A‐type rhyolite. The rocks have smaller (87Sr/86Sr)i, lower εNd (t), younger TDM2, lower (206Pb/204Pb)i,(207Pb/204Pb)i, and (208Pb/204Pb)i. The acid volcanic rocks differ considerably from the intermediate to basic volcanic rocks in geochemical composition, and there is a compositional hiatus present in the succession. This indicates that neither endmember of these Mesozoic bimodal volcanic rocks is cogenetic and that rhyolitic rocks might not be a product of basaltic magma crystallization differentiation. Instead, they may result from partial melting of crust‐derived rock, stemming from partial melting of rocks in the pre‐existing crust underplated by mantle‐derived magma. This suggests that the acid volcanic rocks were formed in a lithospheric extensional setting during the Early Cretaceous.

Geochronology and geochemistry of Mesozoic intrusive rocks in the Xing'an Massif of NE China: Implications for the evolution and spatial extent of the Mongol–Okhotsk tectonic regime

This study presents new zircon U–Pb–Hf and whole-rock geochemical data for intrusive rocks in the Xing'an Massif of NE China, with the aim of furthering our understanding of the evolution and spatial influence of the Mongol–Okhotsk tectonic regime. Zircon U–Pb dating indicates that five stages of Mesozoic magmatism are recorded in the Xing'an Massif, namely during the Middle Triassic (~237 Ma), the Late Triassic (~225 Ma), the Early Jurassic (~178 Ma), the Middle Jurassic (~168 Ma), and the late Early Cretaceous (~130 Ma). The Middle Triassic–Early Jurassic intrusive rocks in the Xing'an Massif are dominantly granodiorites, monzogranites, and syenogranites that formed from magma generated by partial melting of newly accreted continental crust. Geochemistry of the Middle Triassic–Early Jurassic granitoid suites of the Xing'an Massif indicates their formation at an active continental margin setting, related to the southwards subduction of the Mongol–Okhotsk oceanic plate. The Middle Jurassic monzogranites in the Xing'an Massif are geochemically similar to adakites and have εHf(t) values (+3.8 to +5.8) and Hf two-stage model ages (TDM2; 979–850 Ma) that are indicative of derivation from magma generated by partial melting of thickened juvenile lower crust. The Middle Jurassic monzogranites formed in a compressional setting related to the closure of the Mongol–Okhotsk Ocean. The late Early Cretaceous intrusive rocks in the Xing'an Massif are dominated by A-type granitoids that are associated with bimodal volcanic rocks, suggesting their formation in an extensional environment related to either (i) delamination of a previously thickened region of the crust, associated with the Mongol–Okhotsk tectonic regime; (ii) the subduction of the Paleo-Pacific Plate; or (iii) the combined influence of these two tectonic regimes.

Identifying Early Carboniferous bimodal volcanic rocks and geochemical characteristics in the Atengtao Mountain, Yili Block (Chinese western Tianshan)

The Early Carboniferous bimodal volcanic rocks are firstly recognized in the Atengtao Mountain, Yili Block (Chinese Western Tianshan). The bimodal volcanic rocks are composed of dominant rhyolite and subordinate basaltic andesite. The SiO2 contents of basaltic andesites span a range of 52.08 to 60.44 wt.%, whereas those of the rhyolites range from 72.50 to 76.78 wt.%, showing a sharp gap between 60.44 and 72.50 wt.% for SiO2 content. Zircon U–Pb dating of rhyolitic sample yielded crystallization age of 340 ± 6 Ma, which is interpreted as the extrusive age of the bimodal volcanic rocks. Basaltic andesite samples belong to the calc‐alkaline series, whereas rhyolites are peraluminous. The basaltic andesite samples display enrichment in LREE ((La/Yb)N = 3.16–6.27), and nearly no obvious Eu anomalies (δEu = 0.86–1.21), with relative enrichment in large ion lithophile elements (Rb, K, Sr, U) and depletion in high field strength elements (Nb, Ta). Compared to the basaltic andesites, the rhyolites show enriched LREE patterns ((La/Yb)N = 7.05–9.27) and significantly negative Eu anomalies (δEu = 0.36–0.62), with remarkably negative Nb, Ta, P, Ti, and Sr anomalies, which is consistent with A‐type granites. The rhyolites have positive εHf(t) values from 1.5 to 4.8 and TDM2(Hf) values from 1,036 to 1,247 Ma. The basaltic andesites are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction‐related components, whereas the rhyolites were derived from basaltic melt‐induced reworking of Proterozoic juvenile crustal material. Based on our data, and taking into account the regional geology, a model of back‐arc setting was proposed for the Early Carboniferous formation of the bimodal volcanic rocks in the Atengtao Mountain. Therefore, we suggest that the volcanic rocks were likely induced by the northward subduction of the South Tianshan Ocean. The subduction resulted in opening of a back‐arc basin in Atengtao Mountain, leading to upwelling of mantle that had produced the basaltic andesites. Moreover, this process provided heat to remelt the crustal materials and form the rhyolites.

Devonian post-orogenic extension-related volcano-sedimentary rocks in the northern margin of the Tibetan Plateau, NW China: Implications for the Paleozoic tectonic transition in the North Qaidam Orogen

The Maoniushan Formation in the northern part of the North Qaidam Orogen (NQO), NW China, contains key information on a Paleozoic change in tectonic setting of the NQO from compression to extension. Here, new zircon U–Pb, petrological, and sedimentological data for the lower molasse sequence of the Maoniushan Formation are used to constrain the timing of this tectonic transition. Detrital zircons yield U–Pb ages of 3.3–0.4 Ga with major populations at 0.53–0.4, 1.0–0.56, 2.5–1.0, and 3.3–2.5 Ga. The maximum depositional age of the Maoniushan Formation is well constrained by a youngest detrital zircon age of ∼409 Ma. Comparing these dates with geochronological data for the region indicates that Proterozoic–Paleozoic zircons were derived mainly from the NQO as well as the Oulongbuluk and Qaidam blocks, whereas Archean zircons were probably derived from the Oulongbuluk Block and the Tarim Craton. The ∼924, ∼463, and ∼439 Ma tectonothermal events recorded in this region indicate that the NQO was involved in the early Neoproterozoic assembly of Rodinia and early Paleozoic microcontinental convergence. A regional angular unconformity between Devonian and pre-Devonian strata within the NQO suggests a period of strong mountain building between the Oulongbuluk and Qaidam blocks during the Silurian, whereas an Early Devonian post-orogenic molasse, evidence of extensional collapse, and Middle to Late Devonian bimodal volcanic rocks and Carboniferous marine carbonate rocks clearly reflect long-lived tectonic extension. Based on these results and the regional geology, we suggest that the Devonian volcano-sedimentary rocks within the NQO were formed in a post-orogenic extensional setting similar to that of the East Kunlun Orogen, indicating that a major tectonic transition from compression to extension in these two orogens probably commenced in the Early Devonian.

Geochemistry of metamorphosed volcanic rocks in the Neoarchean Qingyuan greenstone belt, North China Craton: Implications for geodynamic evolution and VMS mineralization

The Qingyuan greenstone belt (QGB) situated in the northeastern section of the North China Craton (NCC) is considered to be the unique metallogenic province in China that hosts both banded iron formations (BIFs) and volcanogenic massive sulfide (VMS) deposits. Traditionally, the QGB is divided into the Hunbei and Hunnan terranes separated by the Hunhe fault and grouped into the lower Shipengzi, middle Hongtoushan, and upper Nantianmen formations, which have undergone amphibolite- to granulite-facies metamorphism. Exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of BIF, marble, and schist. In this study, detailed field and petrographic observations, lithogeochemical, U-Pb geochronological, as well as whole-rock Sm-Nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks (i.e. plagioclase amphibolite and biotite plagioclase gneiss, respectively) in the QGB in order to discuss implications for tectonic evolution and comitant VMS mineralization.The supracrustal rocks of both Hunbei and Hunnan regions were formed synchronously from 2.57 to 2.53 Ga and then had undergone 2.52–2.48 Ga regional metamorphism. Specifically, the lower section of the Shipengzi Formation is composed dominantly of transitional-tholeiitic basalts with negative Nb anomalies, suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab-derived fluids or melts. In contrast, the lithologic assemblages grade upward into interlayered bimodal volcanic rocks (i.e. N-MORB-type tholeiitic basalts and FI- to FII-type dacites of transitional to calc-alkalic affinities), representing a change in tectonic regime related to subduction from compression to extension. Among them all the basalts are characterized by consistently positive initial Nd compositions (εNd(t) = +2.7 to +3.4) further indicate a depleted mantle source; whereas those dacites possess negative to positive initial Nd values (εNd(t) = −4.6 to +3.0), suggesting they were formed by partial melting of depleted mafic source (likely lower mafic crust) and accompanied with variable addition of old crustal material. Significantly, VMS deposits are genetically linked to this extensional event and closely associated with the FII-type dacites that were generated by higher melting degree at high-temperature conditions and shallow crustal level (likely 10–15 km). In all, geochemical features (e.g., negative Nb anomalies and εNd value), together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks, imply a continental arc setting where the QGB was evolved.

Early Cretaceous MORB-type basalt and A-type rhyolite in northern Tibet: Evidence for ridge subduction in the Bangong–Nujiang Tethyan Ocean

New zircon U–Pb ages, major- and trace-element data, and Hf isotopic compositions are presented for bimodal volcanic rocks of the Zhaga Formation (ZF) in the western–middle segment of the Bangong–Nujiang suture zone (BNSZ), northern Tibet. The genesis of these rocks is described, and implications for late-stage evolution of the Bangong–Nujiang Tethyan Ocean (BNTO) are considered. Detailed studies show that the ZF bimodal rocks, which occur as layers within a typical bathyal to abyssal flysch deposit, comprise MORB-type basalt that formed at a mid-ocean ridge, and low-K calc-alkaline A-type rhyolite derived from juvenile crust. The combination of MORB-type basalt, calc-alkaline A-type rhyolite, and bathyal to abyssal flysch deposits in the ZF leads us to propose that they formed as a result of ridge subduction. The A-type ZF rhyolites yield LA–ICP–MS zircon U–Pb ages of 118–112 Ma, indicating formation during the Early Cretaceous. Data from the present study, combined with regional geological data, indicate that the BNTO underwent conversion from ocean opening to ocean closure during the Late Jurassic–Early Cretaceous. The eastern segment of the BNTO closed during this period, while the western and western–middle segments were still at least partially open and active during the Early Cretaceous, accompanied by ridge subduction within the Bangong–Nujiang Tethyan Ocean.