Cretaceous Magmatic Rocks Research Articles

Intraoceanic Subduction System Within the Neo‐Tethys: Evidence From Late Cretaceous Arc Magmatic Rocks of the Eastern Himalaya

AbstractThe tectonic evolution of the Neo‐Tethys Ocean remains highly controversial, with several models existing in the community that conflict with each other. Here, we present new geochronologic and geochemical data for orthogneisses and amphibolites from the Greater Himalayan Sequence, eastern Himalayan orogen, which indicate that these rocks have Cenozoic metamorphic ages (∼52–3 Ma), but were derived from Late Cretaceous (∼89 Ma) magmas with arc‐like and depleted mantle geochemical signatures. Considering that northern India was a passive continental margin during the Mesozoic, and the previously reported Late Cretaceous magmatic rocks in the eastern Himalaya formed in a continental rifting setting, we suggest that the studied Late Cretaceous arc‐type magmatic rocks formed in an intraoceanic arc setting within the Neo‐Tethys, and accreted onto the passive margin of the Indian continent prior to the terminal continental collision. When combined with the existence of Late Mesozoic and intraoceanic arc‐type magmatic rocks in the western Himalaya, we suggest that a huge Late Cretaceous subduction system operated within the eastern Neo‐Tethys Ocean. This study supports two subduction zones having been responsible for the consumption and closure of the Neo‐Tethys basin, and a two‐stage collision history between India, Asia, and the intermediate island arc system. Our data therefore provide important constraints on the evolution of the Neo‐Tethys Ocean and India‐Asia collisional orogeny in southern Tibet.

A giant porphyry-skarn-epithermal Mo-W-Cu-Zn-Pb-Ag metallogenic system in the Nannihu ore field, southern North China Craton: Constraints from new geochronological and pyrite geochemical data

The southern North China Craton owns substantial Mo, W, Zn, Pb, Cu, and Ag resources, and most of them are hosted in hydrothermal deposits that may co-exist in the same ore field. However, the relationship between the different types of deposits is not well-constrained. To better understand this, here we present a comprehensive geological, geochronological, and pyrite geochemical study on the Nannihu ore field in the Luanchuan district. The Nannihu ore field is largely represented by the Nannihu Mo-W deposit, Luotuoshan Zn-Cu-W deposit, and Lengshuibeigou Pb-Zn-Ag deposit. Mo-W mineralization at Nannihu occurs in the contact zone between the Nannihu intrusion and the wall rocks, showing typical characteristics of the porphyry-type deposit. Three paragenetic stages are recognized at Nannihu, including quartz-potassium feldspar stage, quartz-sulfide stage, and quartz-carbonate stage. The Zn-Cu-W orebodies at Luotuoshan are mainly layered or lens-like, occurring within the skarn zones in the Luanchuan Group. Skarn stage, polymetallic sulfide stage, and carbonate stage are revealed at Luotuoshan. Pb-Zn-Ag mineralization at Lengshuibeigou is characterized by banded sulfide veins that are hosted in the NNE-trending fault zone. Three paragenetic stages are developed at Lengshuibeigou: quartz-pyrite stage, quartz-polymetallic sulfide stage, and quartz-carbonate stage. New geochronological data confirm that the Nannihu intrusion was formed by at least two phases of magmatism including the granite porphyry (zircon U-Pb age of 145 ± 2 Ma) and the porphyritic monzogranite (zircon U-Pb ages of 142 ± 1 Ma and 140 ± 1 Ma), which accompanied by two episodes of Mo mineralization at the Nannihu deposit identified by molybdenite Re-Os dating (144.9 ± 0.7 Ma and 142.8 ± 0.6 Ma). The Luotuoshan deposit and Lengshuibeigou deposit formed at 141 ± 1 Ma to 140 ± 1 Ma and 140 ± 3 Ma to 139 ± 3 Ma revealed by U-Pb dating of garnet and monazite, respectively, which are contemporaneous with the late episode of the Nannihu intrusion and Mo mineralization. Pyrite grains of the three deposits have Co/Ni ratios higher than 1, suggesting a unified magmatic-hydrothermal origin for them. Pyrite from Nannihu and Luotuoshan is enriched in Co, Ni, Se, Mo, Mn, and Bi, whereas pyrite from Lengshuibeigou contains high Zn, Pb, As, Cu, Ag, and Au. The regular elemental distribution of pyrite in the Nannihu ore field is consistent with that of typical porphyry-related systems worldwide.Collectively, the Early Cretaceous magmatic rocks and diverse deposits in the Nannihu ore field make up a giant magmatic-hydrothermal Mo-W-Cu-Zn-Pb-Ag metallogenic system, with the porphyry, skarn, and epithermal types of mineralization displaying a zonation from inside to outside. The multi-episode magmatism and mineralization could result in the remarkable enrichment of mineral resources. The close spatial relationship among different types of mineralization in the Nannihu ore field has a significant implication for porphyry-related mineral prospecting in the southern NCC.

Petrogenesis of Early Cretaceous andesites and mafic dikes in central Tibet: Implications for the growth of continental crust in collision zones

Post-collisional magmatic rocks in central Tibet provide an opportunity to study the processes of continental crustal growth in a collision zone. Here, we report data for samples of andesites and coeval mafic dikes obtained from the Jiaqiong area of the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating shows that the andesites and mafic dikes were emplaced concurrently during the late Early Cretaceous (ca. 115 Ma). The andesites have high Mg# values (55.2–61.6) and initial 87Sr/86Sr ratios (0.7086–0.7088), negative whole-rock εNd(t) (−6.5 to − 6.1) and zircon εHf(t) (−5.1 to − 1.7) values, and elevated zircon δ18O (6.8‰–7.5‰) values, indicating that the andesites were probably derived by the partial melting of an ancient subduction-modified lithospheric mantle. The coeval mafic dikes have moderately high Mg# values (49.3–50.0) and initial 87Sr/86Sr ratios (0.7076–0.7082), and negative εNd(t) values (−5.4 to − 5.3), implying that they were derived from an enriched lithospheric mantle source. However, the zircon εHf(t) (−5.8 to + 1.6) and δ18O (5.2‰–6.3‰) values of these mafic dikes indicate the influence of depleted mantle components. Furthermore, results of Sr–Nd–Hf–O isotopic mixing modeling suggest that these mafic dikes could have been produced by interaction between asthenospheric and lithospheric mantle. Therefore, we suggest that the parental magmas of the mafic dikes were probably generated by interaction between asthenospheric and overlying metasomatized lithospheric mantle. Combined with data on nearby Cretaceous magmatic rocks and sedimentary strata, we propose that the studied magmatic rocks were formed in a post-collisional slab breakoff setting in central Tibet and have made a substantial contribution to the vertical growth of continental crust in this continental collision zone.

Late Cretaceous bimodal volcanic rocks in Shuanghu induced by lithospheric delamination beneath the Southern Qiangtang, Tibet

The southern Qiangtang (SQT) in central Tibet is characterized by sporadic Late Cretaceous magmatic rocks whose petrogenesis and tectonic implications remain unclear. Here, new zircon UPb ages and Hf isotope compositions, together with whole-rock element and isotope data have been collected for the Duoma volcanic rocks from the SQT, central Tibet. Zircon UPb dating indicates that the Duoma volcanic rocks crystallized at ca. 98 Ma. The basalts of the Duoma volcanic rocks have low SiO2 (51.49–53.36 wt%), high total alkalis (4.65–6.25 wt%), and display OIB-like isotopic signatures (87Sr/86Sr)i = 0.7051 to 0.7058, εNd(t) = +0.04 to +1.10), overall similar to intra-plate alkali basalts. Moreover, basalts exhibit slight depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti), but enrichment of light rare earth (LaN/YbN = 6.7–7.6) with no negative Eu anomalies (Eu/Eu* = 0.91–0.99). These geochemical features, together with their high La/Nb (1.56–1.70) and Zr/Hf (44.70–45.87) but low La/Ba (0.08–0.09) and Ca/Al (0.43–0.65) ratios, suggest that the Duoma basalts originated from partial melting of garnet clinopyroxenite that was metasomatized by depleted mantle-derived melts. The coexisting rhyolites have high SiO2 (69.78–69.92 wt%) and exhibit significant enrichment in large-ion lithophile elements (LILEs) and depletion in HFSEs (Nb, Ta, and Ti). Furthermore, the Duoma rhyolites are characterized by the decoupled Nd (εNd(t) values ranging from −4.4 to −11.1) and Hf (ɛHf(t) values ranging from −0.7 to +2.7) compositions, that character indicates that they were probably derived from the partial melting of the lower continental crust. The compositional bimodality and heterogeneous source of the Late Cretaceous volcanism indicate that they were formed in an extension setting, presumably triggered by lithospheric delamination beneath the SQT.

Zircon U-Pb chronology, geochemistry and geological significance of the Tongjiang-Fuyuan Mesozoic magmatic rocks, NE China

AbstractTypical ophiolitic rock assemblages such as siliciclastic rocks, basalts and gabbros, together with the subduction-related intermediate-acidic intrusive rocks, are newly discovered in the Tongjiang-Fuyuan area of the Heilongjiang Provence, NE China. To determine the formation age and genesis of the mafic rocks (basalts and gabbros) and intermediate-acidic intrusive rocks (granodiorites) in the area, as well as their geodynamic settings, the whole-rock geochemical analysis and zircon LA-ICP-MS U-Pb dating were carried out. Zircon U-Pb results suggest that the granodiorites are 93–95 Ma and gabbro is 95 Ma, respectively. Geochemical results show that the gabbros and basalts exhibit characteristics of ocean island basalt (OIB) affinity and are typically related to having originated from mantle plumes. While the granodiorites show the nature of the island-arc magmatic rocks and may originate from the lower crust. Based on the coeval igneous rock associations and regional tectonic evolution, we conclude that the late Cretaceous magmatic rocks in the Tongjiang-Fuyuan area are the product of continuous subduction of the Palaeo-Pacific plate and reflect the subduction rollback process of the Palaeo-Pacific plate.

Geometry and origin of supergiant gold deposits in the Jiaodong gold province, eastern China

Based on the new deep exploration results, the gold reserves of Jiaojia gold deposit and Sanshandao gold deposit of the Jiaodong gold province, eastern North China Craton (NCC), exceed 1300 and 1200 tons, respectively. Both deposits are distributed along adjacent faults of Jiaojia and Sanshandao, respectively, forming a globally significant supergiant deposit. The main orebodies are preserved along two low-angle steps on the faults, with Jiaojia orebodies deposited in left-stepping bends, and Sanshandao orebodies deposited in right-stepping bends. The above geometric distribution of orebodies of these two deposits is clearly related to the kinematics of the ore-controlling faults. Weideshan suite is the largest Cretaceous magmatic rocks in the Jiaodong peninsula, which is considered as a profound impact on the gold mineralization in Jiaodong. LA-ICP-MS U-Pb dating of magmatic zircons from the associated Weideshan suite granitoid yields ages of 121.3 ± 2.1 Ma and 116.7 ± 1.7 Ma, which are consistent with mineralization age of the gold deposits (ca. 120 Ma). On the basis of regional geological data, our study indicates that the large-scale magmatic activities in the Early Cretaceous of Weideshan suite provide thermodynamic conditions for gold mineralization. A metallogentic model suggests that the formation of Jiaodong type gold deposits may have involved a thermal doming-extension process during the Mesozoic decratonization of NCC. In the Early Cretaceous, mantle upwelling and crust-mantle interaction triggered magmatism and fluid activities. These activities were simultaneous with granitoid doming, crustal extension and detachment faulting, upgrading Au of gold deposits. Mantle-derived gold-bearing fluid mixed with crust-derived gold-bearing fluid precipitated from remelting lower crust, forming gold-rich fluid reservoir and producing gold-poor Weideshan suite granitoid. Subsequently, granitoid doming, crustal extension and detachment faulting occurred. This process provides both favorable channels for ore-forming fluid migration and potential locations for enriching and trapping fluids, forming supergiant ore deposits.

Early Cretaceous magmatism in Yunnan traces the tectonic transition from the Tengchong-Baoshan continental collision to Neo-Tethyan slab rollback

Early Cretaceous magmatic rocks of the Tengchong and Baoshan blocks provide insight into the late Mesozoic evolution of southeastern Tibet—Yunnan. We present LA-ICP-MS U-Pb zircon age, zircon εHf(t) isotope, and whole-rock geochemical and Sr-Nd isotopic data for dioritic porphyrite and rhyolite, outcropping in the Eastern Tengchong Belt and the Gaoligong Belt, respectively. The dioritic porphyrite has a 123.0 ± 1.3 Ma (2σ) age, negative whole-rock ℇNd(t) of −1.99 to −2.16, initial 87Sr/86Sr of 0.70690–0.70734, zircon ℇHf(t) of −1.42 to 4.11, and adakitic geochemical features, suggesting an origin from mixing of mantle- and crust-derived magmas; the rhyolite has a 122.2 ± 1.0 Ma age, low whole-rock ℇNd(t) of −9.69 to −8.83, initial 87Sr/86Sr of 0.71104–0.71246, and zircon ℇHf(t) of −8.55 to −2.59, suggesting crustal derivation. Our data—along with the published data—suggest that the Early Cretaceous rocks (140–109 Ma) comprise two magmatic phases with a geochemical and isotopic transition at ∼123 Ma. The 140–123 Ma phase—mainly outcropping in the Gaoligong Belt—is mainly peraluminous, highly silicic, and has negative zircon εHf(t) values, suggesting derivation from remelting of ancient crust in a postcollisional setting; the 123–109 Ma phase is mainly metaluminous, restricted to the Eastern Tengchong Belt, and formed in a continental-arc setting with mantle melt involved. We propose that the ∼123 Ma geochemical and isotopic change marks the transition from contraction due to the Tengchong-Baoshan collision to extension resulting from slab rollback of the Neo-Tethyan lithospheric slab.

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.

First identification of Early Cretaceous mafic dikes in the Baingoin area, central Tibet: Implications for crust-mantle interactions and magmatic flare-up

Mafic dikes are generally emplaced in extensional tectonic settings and provide key information regarding deep mantle processes and sources. The Bangong-Nujiang suture zone was formed by the collision of the Qiangtang and Lhasa terranes and experienced intense magmatism during the Early Cretaceous. However, the deep mantle processes and mechanisms involved in this magmatic flare-up (ca. 115 Ma) in the collisional belt remain controversial because of the lack of evidence for coeval mafic magmatism. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-O isotope data for the newly discovered hypersthene-bearing mafic dikes in the Baingoin area in the middle-eastern parts of the Bangong-Nujiang suture zone, central Tibet. Secondary ion mass spectroscopy (SIMS) zircon U-Pb dating showed that the mafic dikes were emplaced during 120−115 Ma. These mafic rocks are characterized by variable MgO contents (2.7−5.2 wt%) and Mg# values (38.5−52.8), slight enrichment in light rare earth elements (REEs; [La/Yb]N = 7.5−8.1), relatively flat heavy REE patterns ([Gd/Yb]N = 1.75−1.84), and negative Eu, Ta, Nb, and Ti anomalies. The dikes also have relatively low initial 87Sr/86Sr ratios (0.7060−0.7062) and negative εNd(t) (−2.2 to −1.6) and positive εHf(t) (+2.5 to +3.6) values, and variable zircon εHf(t) (−2.2 to +7.2) and slightly elevated zircon δ18O (5.6‰−7.0‰) values. These geochemical characteristics indicate that the mafic dikes were derived from an enriched lithospheric mantle source. However, compared with coeval magmatic rocks, the mafic dikes have relatively high εNd(t) and εHf(t) values, indicating that they contain a depleted mantle component. The mafic dikes contain clinopyroxene and orthopyroxene (i.e., hypersthene), indicative of derivation from a high-temperature magma source. Clinopyroxene-melt thermobarometry yielded a temperature range of 1167−1213 °C, further supporting the involvement of a high-temperature asthenospheric component. Therefore, we suggest that the parental magmas of the Nakoulai mafic dikes were probably generated by the interaction between the asthenospheric mantle and overlying metasomatized lithospheric mantle. Combined with data from nearby Cretaceous magmatic rocks and sedimentary rocks, we suggest that the mafic dikes were generated in a postcollisional setting caused by upwelling of asthenospheric mantle owing to slab breakoff beneath the Bangong-Nujiang suture zone. Slab breakoff played a key role in the crust-mantle interactions and the onset of the magmatic flare-up in the middle-eastern parts of the Bangong-Nujiang suture zone.

Petrogenesis of late Jurassic Mufushan high-Mg diorites and late Mesozoic tectonic evolution of the eastern South China Block

Abundant Mesozoic magmatic rocks are widely distributed in southeastern China, forming several remarkable magmatic-metallogenetic belts. However, the geodynamic mechanism for the Late Jurassic-Early Cretaceous magmatic processes and their relation to the rich polymetallic deposits of the region remain highly controversial. Here, we report zircon U-Pb ages, geochemistry, and Sr-Nd-Hf-Pb isotopic data for the high-Mg diorites in the Mufushan region, northeastern Hunan Province. The high-Mg diorites were emplaced in the late Jurassic (U-Pb ages of ∼151–155 Ma), contemporaneous with the surrounding granitic rocks which have ages ranging from 140 to 150 Ma. These high-Mg diorites are characterized by the enrichment of large-ion lithophile elements (LILEs, e.g. Ba, Pb, K and Rb) and depletion of high field strength elements (HFSEs, e.g., Nb, Ta, Ti), displaying typical geochemical features of continental-arc basalts (CAB). The relatively high Mg# values (71–76, average of 72), Cr contents (546–1562 ppm, average of 944 ppm) and Ni contents (211–519 ppm, average of 335 ppm) suggest that their derivation was likely from a relatively primitive magma, accompanied with weak crustal assimilation or fractional crystallization.The high-Mg diorites display isotopic characteristics similar to those of enriched mantle-derived mafic rocks, such as low 87Sr/86Sr(i) (0.70692–0.70789), εNd(t) and εHf(t) ranging from −2.4 to −4.3 and −3.8 to + 1.1 respectively, with single-stage Nd model ages (TDM1) of 1.3–1.5 Ga. In combination with a compilation of coeval magmatic rocks in eastern South China, a significant addition of ancient lithospheric mantle materials was likely involved in the Mesozoic magma genesis. The decreasing radiogenic Nd isotope compositions and negative Hf signatures suggest an increasing contribution of enriched components to the magma source from the Late Jurassic to Early Cretaceous. Our results suggest that the Mufushan high-Mg diorites were derived from mantle-derived magma, followed by the incorporation of slab-derived materials into mantle and subsequent sediment melting. Integrated with previous work, the widespread Late Jurassic-Early Cretaceous magmatic rocks in eastern South China formed within a continental back-arc setting that was formed by subduction-induced convection in the mantle wedge. The subsequent roll-back of subducted Paleo-Pacific slab (Izanagi plate) triggered extensive mantle upwelling and melt-mantle interaction, accounting for the production of contemporaneous felsic rocks and related polymetallic deposits in regions.

A comprehensive re‐look into Jurassic‐Cretaceous Neotethyan active margin in Iran: Evidence of deep mantle flow into subduction magma factory and formation of transitional basalts

Subduction zones display the most diverse magma compositions from various calc‐alkaline differentiates to alkaline mafic associations, but mechanisms generating Nb‐Enriched Basalts (NEB) and High‐Nb Basalts (HNB) in such settings have received less attention from the researchers. Occurrences of NEB and HNB magmas in the Mesozoic Neotethyan active margin in Iran, known as the Sanandaj‐Sirjan Zone, provide an excellent natural lab for examining this interesting petrologic puzzle. The Middle‐Jurassic typical calc‐alkaline arc plutons exhibit a NW‐SE subduction parallel trend. In the northwestern sector of the zone, the Late Jurassic–Early Cretaceous magmatic rocks (JCMR) define a chain that transects the Middle‐Jurassic arc trend. The two cross‐cutting magmatic chains are chemically and genetically different. The JCMR include three distinct compositional groups: (i) tholeiites that are depleted in high‐field‐strength elements (HFSE) and show subduction‐related geochemical fingerprints, (ii) NEB with higher abundances of Nb and other HFSE, and (iii) HNB with the highest abundances of Nb and other HFSE. The latter two groups do not display typical arc signatures and are transitional between arc and plume‐type magmas. These three groups of rocks show linear major and trace element, and Sr–Nd isotopic variation trends extending from Mid‐Ocean Ridge Basalts‐like tholeiites to Oceanic Island Basalts‐like HNB. Hence, a two‐component mixing process is suggested for explaining the compositional variation of the JCMR. We propose that a vertical window resulted from slab tear created the geodynamic environment for juxtaposition of magmas originated at different mantle depths and formation of the transitional basalts by magma mixing.

Episodic magmatism in the Lianhuashan tectonic belt: Implications for late Mesozoic crustal reworking in SE South China

The Lianhuashan tectonic belt preserves abundant evidence of episodic magmatism and deformation that is crucial for understanding the late Mesozoic crustal reworking in SE South China. Here, we report zircon U-Pb ages, geochemical, and Sr−Nd−Hf isotopic data for representative granitoids and volcanic rocks to clarify the episodic magmatism in the Lianhuashan tectonic belt and its association with regional tectonic evolution. Our results define three episodes of magmatism. The Late Jurassic (160−150 Ma) magmatic rocks show high CaO/Na2O and molar CaO/(MgO + FeOt) (CMF) ratios and enriched Sr−Nd−Hf isotopes that are consistent with high-temperature melting of psammitic sources. The Early Cretaceous (144−131 Ma) magmatic rocks display variable CaO/Na2O ratios and zircon εHf(t) values (−12.0 to −2.5), which implies binary mixing of melts from pelitic and basaltic sources. The Late Cretaceous (ca. 94 Ma) magmatic rocks show high A/CNK and low CaO/Na2O and CMF ratios, which are characteristics of pelite-derived melts. They have zircon Hf isotopes (−4.4 to −1.9) that are slightly higher than those of the Early Cretaceous rocks, which is consistent with partial melting of diverse sources with more involvement of juvenile crust. These magmatic records, coupled with regional structural observations, suggest that the Lianhuashan tectonic belt underwent alternating crustal shortening and extension during the Late Jurassic to Cretaceous time. The Late Jurassic magmatism possibly marks the initiation of crustal extension following Middle Jurassic retro-arc crustal shortening related to the advancing Paleo-Pacific subduction. In the Cretaceous, the two magmatic flare-ups were separated by a magmatic lull associated with crustal shortening. The magmatic flare-ups occurred coevally with two stages of regional extension, as evidenced by widespread generations of basins and extensional domes. The time consistency implies that the magmatism was associated with or facilitated crustal extension that occurred at a regional scale and may have resulted from slab rollback during subduction retreat. We suggest that the magmatism and extension provide essential evidence of the crustal reworking in SE South China governed by the Paleo-Pacific subduction. Changes in the slab dynamics (steepening and shallowing) may control the flare-up, lull, and resumption of magmatism, as well as alternating episodes of crustal extension and shortening.

Magmatic records of subduction and closure of the Meso-Tethys Ocean in the northern-central Tibetan Plateau

The evolution of the Tethys Ocean has received much research attention; however, the timing and subduction mechanisms involved in the closure of the Bangong-Nujiang Meso-Tethys Ocean remain poorly constrained. In this study, we present geological, geochronological, geochemical, and zircon isotopic data from Cretaceous magmatic rocks and combine these with previously published data within the central Bangong-Nujiang suture zone in the north-central Tibetan Plateau. The integrated data update the regional tectonic framework and enable a comprehensive geodynamic model to be developed for the subduction and ocean closure events. In detail, temporal and spatial variations in the Jurassic−Cretaceous magmatism reflect the influence of the northward subduction of the Dongqiao-Amdo oceanic basin and the bidirectional subduction of the Bangong-Nujiang Meso-Tethys oceanic crust. The evolution of the Bangong-Nujiang Meso-Tethys Ocean further involved initial intra-ocean subduction, slab rollback, and flat subduction, as evidenced by two phases of north-south migration of magmatism at ca. 190−160 and 160−130 Ma and a magmatic hiatus at 160−140 Ma. The ocean closed during a two-stage process, including the closure of the Dongqiao-Amdo oceanic basin to the north and the Bangong-Nujiang Meso-Tethys Ocean to the south. The Dongqiao-Amdo oceanic basin closed soon after ca. 180 Ma, accompanied by continent-continent collision between the Amdo microcontinent and the South Qiangtang terrane. The Bangong-Nujiang Meso-Tethys Ocean closed at 130−120 Ma, corresponding to a period of waning magmatism. This closure represented complete oceanic closure and caused an arc-continent collisional event involving the Baingoin magmatic arc, the Amdo microcontinent, and the Lhasa terrane. The Lower Cretaceous terrestrial strata and their basal unconformity mark the final closure of the ocean, and the Early Cretaceous ocean islands might have formed before ca. 130 Ma.

Possible source of Au in the Jiaodong area from lower crustal sulfide cumulates: Evidence from oxygen states and chalcophile elements contents of Mesozoic magmatic suites

The genesis of giant Jiaodong gold province remains perplexing although many ore-forming models have been proposed in the past decades. The key issues are what caused the burst of gold around 120 Ma and possible sources of gold. Here we report gold and platinum group elements (PGE) data and magmatic oxidation states of the pre-mineralization Mesozoic plutonic rocks and some mafic dykes to discuss their potential effects on gold mineralization. Gold concentrations of the Late Jurassic Linglong granite are 0.015–0.278 ppb. The Early Cretaceous plutonic rocks (129–127 Ma) have the same levels of Au concentrations (Au = 0.031–0.047 ppb). Both of the Jurassic and Cretaceous magmatic rocks are depleted in Au compared to normal granitic rocks worldwide (∼1 ppb). Palladium and platinum concentrations in the Linglong granite are 0.160–0.385 ppb and 0.046–0.116 ppb, respectively. Meanwhile, Pd and Pt contents of Cretaceous magmatic rocks are 0.069–0.363 ppb and 0.063–0.465 ppb, respectively.Oxygen fugacities of the Late Jurassic Linglong granite calculated by biotite oxybarometer are FMQ + 0.5 to FMQ + 1.2 (where FMQ refers to fayalite-magnetite-quartz buffer). Oxidation states of the Early Cretaceous magmatic rocks estimated via amphibole oxybarometer are from FMQ + 0.3 to FMQ + 1.1 for Guojialing granodiorite. Zircon trace element data yield overall higher fO2 values than those estimated with biotite and amphibole compositions. Oxygen fugacities are from FMQ – 0.6 to FMQ + 1.9 (sample CJ-3) and FMQ – 0.7 to FMQ + 1.1 (sample LL-1) for the two Linglong granite samples. Zircons from the Early Cretaceous Guojialing pluton yield oxidation states between FMQ – 0.8 and FMQ + 1.9 (DCW-1), which are about one log unit higher than those of the Beijie pluton (BJ-1: FMQ – 1.0 to FMQ + 0.7). However, oxidation states from nearly syn-mineralization mafic dyke reach FMQ + 2.0 to + 3.0 when estimated via zircon oxybarometer. Oxidation states given by whole rock Fe3+/Fe2+ ratios also support the results calculated by mineral compositions.There is no fractionation crystallization of redox-sensitive minerals, like magnetite or garnet, as indicated by geochemical data, which implies that the oxidation states of magmas may reflect their source regions. However, the depleted nature of Au and PGE must imply that these elements are missing. We propose that sulfide saturation at the deep crust can be invoked to explain the extremely low gold and PGE contents of these magmatic rocks. The sulfide cumulates act as sinks for sulfide-loving elements (e.g., Au), which are pre-enriched source for later large-scale gold mineralization in Jiaodong area. The underplated water-rich mafic magma from enriched mantle source ponded at the lower crust, continuously crystallized anhydrous minerals (e.g., olivine and clinopyroxene) and reached volatile saturation at the very moment. This volatile, probably carrying gold, resorbed the sulfide cumulate and escaped to shallow levels to form the giant Jiaodong gold deposits.

Petrogenesis of Early Cretaceous granitoids in Liyang volcanic basin, eastern China: Geodynamic implications for Paleo-Pacific slab rollback

Late Mesozoic magmatism in eastern China was extensive and closely related to the subduction of the Paleo-Pacific plate, but genetic relationship between magmatism and subduction remains unclear. Cretaceous magmatic rocks distributed along the eastern coast of China can provide an opportunity to evaluate the influence of subduction on the evolution of the magmatism. Here, a combined study of whole-rock major-trace elements and Sr-Nd-Pb-Hf isotopes as well as zircon U-Pb ages was carried out for the Cretaceous granitoids in Liyang volcanic basin, eastern China. Zircon U-Pb dating results indicate that the Liyang granitoids were emplaced in the Early Cretaceous (ca. 130–125 Ma). These samples (SiO2 = 70.8–75.3 wt%) are mainly composed of high-K calc-alkaline granite and granite porphyry. Their geochemical characteristics are similar to I-type granites, with enrichment in LREEs, Rb, Th and U, and negative Eu (δEu = 0.04–0.51), Ba, Sr, P and Ti anomalies. All samples show negative whole-rock εNd(t) (−9.8 to −5.7) and zircon εHf(t) (−15.7 to −3.60) values (corresponding crustal Hf model ages of 1.40–2.17 Ga), and high radiogenic Pb isotopic compositions. The integrated interpretation of geochemical data reveal that rollback of the Paleo-Pacific subducted slab resulted in lithospheric extension and thinning, and the upwelling of hot asthenosphere in eastern China, which induced more intensive underplating of the mantle-derived mafic magma. The ascent of heat and mafic magma, intruded in Meso-Neoproterozoic crust, and triggered partial melting of the Meso-Neoproterozoic crust. The Liyang I-type granitoids were the result of the Meso-Neoproterozoic crustal components mixing with a small number of mantle-derived magmas in the extensional tectonic setting, which was response to slab rollback of the Paleo-Pacific plate.

Terminal stage of divergent double subduction: Insights from Early Cretaceous magmatic rocks in the Gerze area, central Tibet

Divergent double subduction (DDS) plays an important role in facilitating tectonic processes such as ocean closure, accretion and amalgamation of magmatic arcs, and continental growth. However, DDS geodynamics remain poorly constrained, especially its evolution and the accompanying magmatism at the terminal stage. According to geological and geophysical data, the Meso-Tethys Ocean (also known as the Bangong–Nujiang Tethyan Ocean) in central Tibet has been considered a Mesozoic example of DDS, which provides a rare opportunity to evaluate the geodynamic processes of the terminal stage of a DDS system. Here, we present new geochronological, geochemical, and isotopic data of volcanic rocks from the Gerze area in the western segment of the Bangong–Nujiang suture zone (BNSZ). Combined with previous data, we found that the ca. 105 Ma magmatic rocks in the Gerze area are distributed not only along the two sides of the BNSZ but also as manifold types. On the northern side, the magmatism ca. 105 Ma varied, including A-type rhyolites, bajaitic latites, high-Nb basalts, N-MORB-type basalts, adakites and bimodal volcanic rocks, which originated from sources including a sinking oceanic slab, asthenospheric mantle, metasomatized mantle and overlying continental crust. The lithology on the southern side only includes A-type rhyolite and magnesia-rich andesite and adakitic intrusions. Spatial and compositional variations in ca. 105 Ma magmatism should have been the response to slab breakoff, which may have occurred only in the northern branch of the Meso-Tethys oceanic slab after ocean closure. Our results not only verify the existence of slab breakoff in the classical DDS model but also emphasize that breakoff can cause upwelling of the subslab asthenosphere, which leads to a complex magmatic response. • Manifold magmatism from the Gerze area, central Tibet, is active at ca. 105 Ma. • The ca. 105 Ma Gerze magmatic rocks record a slab breakoff event. • Slab breakoff plays an important role in the divergent double subduction system.

The Early Cretaceous tectonic evolution of the Neo-Tethys: constraints from zircon U–Pb geochronology and geochemistry of the Liuqiong adakite, Gongga, Tibet

AbstractThe subduction model of the Neo-Tethys during the Early Cretaceous has always been a controversial topic, and the scarcity of Early Cretaceous magmatic rocks in the southern part of the Gangdese batholith is the main cause of this debate. To address this issue, this article presents new zircon U–Pb chronology, zircon Hf isotope, whole-rock geochemistry and Sr–Nd isotope data for the Early Cretaceous quartz diorite dykes with adakite affinity in Liuqiong, Gongga. Zircon U–Pb dating of three samples yielded ages of c. 141–137 Ma, indicating that the Liuqiong quartz diorite was emplaced in the Early Cretaceous. The whole-rock geochemical analysis shows that the Liuqiong quartz diorite is enriched in large-ion lithophile elements (LILEs) and light rare-earth elements (LREEs) and is depleted in high-field-strength elements (HFSEs), which are related to slab subduction. Additionally, the Liuqiong quartz diorite has high SiO2, Al2O3 and Sr contents, high Sr/Y ratios and low heavy rare-earth element (HREE) and Y contents, which are compatible with typical adakite signatures. The initial 87Sr/86Sr values of the Liuqiong adakite range from 0.705617 to 0.705853, and the whole-rock ϵNd(t) values vary between +5.78 and +6.24. The zircon ϵHf(t) values vary from +11.5 to +16.4. Our results show that the Liuqiong adakite magma was derived from partial melting of the Neo-Tethyan oceanic plate (mid-ocean ridge basalt (MORB) + sediment + fluid), with some degree of subsequent peridotite interaction within the overlying mantle wedge. Combining regional data, we favour the interpretation that the Neo-Tethyan oceanic crust was subducted at a low angle beneath the Gangdese during the Early Cretaceous.

Elemental and Sr-Nd-Pb-Hf Isotope Signatures of Early Cretaceous Magmatic Rocks in the Wulian Area, Eastern Shandong: Implications for Crust-Mantle Interaction at the Edge of the Sulu Collisional Orogenic Belt

Abstract We present new whole-rock geochemical, Sr-Nd-Pb isotopic, and zircon U-Pb and Hf isotopic data for Early Cretaceous magmatic rocks (trachy-andesite, pyroxene diorite, diorite porphyrite, and dolerite) in the Wulian area of Shandong, eastern North China. These data are used to constrain the mechanism of crust-mantle interaction at the edge of the Sulu orogenic belt. The belt formed by the collision of the Yangtze Craton (YC) and the North China Craton (NCC) during the Mesozoic. New zircon U-Pb dating indicates that the magmatic rocks were formed during the Early Cretaceous (123-124 Ma). These rocks are characterized by moderate contents of SiO2 (50.0-60.0 wt.%), MgO (3.3-5.6 wt.%), Cr and Ni with low Nb/U ratios (0.8-11.8), and high La/Nb (5.2-24.5) and Ba/La ratios (12.3-38.1). They are enriched in large ion lithophile elements, depleted in high field strength elements, and are characterized by high initial 87Sr/86Sr values (0.7079-0.7088) and low εNdt (-20.6 to -14.6). The samples have relatively low initial 206Pb/204Pb (16.38-17.18), 207Pb/204Pb (15.38-15.48), and 208Pb/204Pb (37.24-37.83) values. The Sr-Nd-Pb-Hf isotopic characteristics of the samples are similar to those of mafic rocks in the Sulu orogenic belt, suggesting that they might have similar sources. It is clear that the magma source of the samples involves both crustal and mantle materials and so we propose a model for crust-mantle interaction at the edge of Sulu orogenic belt. In this model, the Yangtze plate subducted deep below the northwestern NCC during the Triassic and was trapped in the lithospheric mantle. In the early Cretaceous, lithospheric extension in combination with asthenospheric upwelling resulted in partial melting of the overlying lithospheric mantle. The magmatic rocks in the Wulian area were generated by mixing between melts of both the lithospheric mantle of the NCC and the residual lower crust of the YC. This study therefore provides significant information on crust-mantle interaction at a continental subduction zone.

Chronology and Geochemistry of Early Cretaceous Magmatism in the Northwestern Erguna Block, Northeast China

This study was conducted to define the background structure and petrogenetic significance of the Early Cretaceous magmatic rocks in the Badaguan area of northern Daxing’anling and to explore the Late Paleozoic tectonic evolution of the Mongol-Okhotsk suture zone. The Early Cretaceous magmatic rock was systematically investigated using zircon U-Pb dating and geochemical and petrological analyses. The results show that the rock: mainly consists of granites and rhyolites; has an age of 125–140 Ma; has a strong MgO, Al2O3, and total alkali content; has a SiO2 content of 61.68 wt% to 77.41 wt%; and contains Rb, Th, U, and light REEs with depleted levels of of Sr, P, Ti, and heavy REEs. When combined with the Hf isotopic characteristics of the Early Cretaceous magmatic rock from the Erguna Massif, these results suggest that the magma originated from the partial melting of basal crustal materials during the Neoproterozoic–Phanerozoic period and that various mineral forms (including hornblende, plagioclase, and apatite) underwent fractional crystallization processes during the evolution of the magma. The Early Cretaceous magmatic rock from the Badaguan area recorded the extensional environment of the lithosphere after the closure of the Mongol-Okhotsk Ocean, and this hypothesis is consistent with the results of previous studies on the tectono-magmatic activities in Northeast China during the same period.

Zircon xenocrysts in Late Cretaceous magmatic rocks in the Kermanshah Ophiolite: link to Iran continental crust supports the subduction initiation model

ABSTRACT Geochronological and geochemical data show that Late Cretaceous Zagros ophiolites were generated by forearc seafloor spreading during a subduction initiation event along the southwestern margin of Iran. These ophiolites define a ~ 3000 km long belt passing from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. Ten samples of plagiogranites, amphibole gabbros and diorites of the Kermanshah ophiolite yield U-Pb zircon magmatic ages indicating ophiolite formation at 98 to 96 Ma. Some Kermanshah rocks also contain abundant zircon xenocrysts with age peaks (in order of decreasing size): Triassic (~247 Ma), mid-Paleoproterozoic (~2150 Ma), Late Paleoproterozoic (~1850 Ma), Late Ediacaran (~560 Ma), Early Cretaceous (~140 Ma), and Tonian (~850 Ma). Similar age peaks are seen in magmatic and detrital rocks of Iranian crust to the north. The xenocrysts have εHf(t) values from +6.3 to – 21.6, corresponding to depleted mantle Hf model ages (TDM) of 0.6 to 3.6 Ga. The xenocryst age peaks are similar to those of zircons in Iranian continental crust. The occurrence of zircon xenocrysts in the Kermanshah ophiolite suggests that this formed adjacent to the Iranian continent, as expected for Late Cretaceous subduction initiation.