Paleo-Pacific Oceanic Plate Research Articles

Petrogenesis of the alkaline complex in the Weishan REE deposit, Luxi Block, eastern North China Craton: Implications for REE mineralization

The Weishan REE deposit is one of the largest LREE deposits in China, which is hosted by the Weishan alkaline complex (WSAC) and late Archean gneisses. Two types of mineralization including vein-type and disseminated-type occur in this deposit, of which the former is characterized by large quartz-sulfate-calcite-REE mineral veins cutting all kinds of wallrocks, while the latter is featured by REE minerals disseminated in the alkaline rocks. The vein-type mineralization commonly occurs at shallow depths and has been numerously studied. The disseminated-type mineralization typically occurs at deeper depths, which was newly discovered by drilling exploration and has been poorly studied. In this contribution, zircon U-Pb dating and Hf isotopic analysis as well as whole-rock geochemistry and Sr-Nd isotopic analyses were conducted on the deeply drilled alkaline rocks, combined with molybdenite Re–Os dating on the disseminated ore, with the aim to unravel the petrogenesis of the alkaline rocks and their contributions to the REE mineralization. Zircon U–Pb dating suggests an intrusive age of 119.9 ± 0.9 Ma for the WSAC, which is quite consistent the Re–Os age of 120.0 ± 1.8 Ma for the disseminated ore, corroborating their close genetic correlation. The studied alkaline rocks show enrichment of LILEs, depletion of HFSEs and strong fractionation between LREEs and HREEs, indicating a crustal involvement in the magmas or a metasomatized mantle source. They are characterized by higher Hf (εHf(t) = -6.7 to −12.2) and Nd (εNd(t) = -8.8 to −9.7) isotopic compositions compared with those of the contemporaneous subcontinental lithospheric mantle (SCLM). Based on the above features, it is inferred that the parental magmas of the WSAC were mainly derived from an enriched SCLM with some involvement of asthenospheric mantle-derived materials. The mantle source was likely metasomatized by the subducted Paleo-Pacific oceanic plate, leading to the enrichment of REE and carbonate. Rollback of the oceanic slab during the early Cretaceous resulted in an extensional setting in the study region, which was favorable for partial melting of the metasomatized mantle and thus generated the REE-rich alkaline magmas.

Early cretaceous multi-stage extension in the North Dabie complex in the eastern central China: insights from structural and geochronological data

A dominantly NW-SE directed extensional tectonics in the Early Cretaceous significantly reworked the Late Permian-Triassic orogenic framework of the Dabie orogenic belt. The North Dabie complex (NDC) is the principal domain recording this tectonic event. However, the precise structure-kinematic architectures, particularly those observed in the ductile regime, along with the respective time scales for different extensional stages, have not been adequately established. This significantly impedes our comprehensive understanding of the extensional style and deformation history in the North Dabie complex. To better address these issues, we conducted a systematic structural study and LA-ICP-MS zircon U-Pb dating of the pre-, syn-, and post-kinematic intrusions and syn-kinematically metamorphosed high-grade gneisses/migmatites of the NDC. Our results demonstrate that the extensional deformation in the NDC may initiate at ca. 144 Ma, which is characterized by a pervasive NW-SE oriented coaxial plastic flow in the ductile regime of the middle-lower crust. A large-scale detachment processing zone subsequently started activating at ca. 140 Ma at the upper-middle level of the middle crust, and concentratedly accommodated the extensional strain by top-to-NW ductile shearing. Locally, there was uprising of sub-magmatic flow in the atatexite-diatexite from the deeper lower crust taking place in the manner of top-to-outward shearing as early as ca. 137 Ma. This composite process of extension manifests vertical strain partitioning across the ductile middle-lower crusts and progressive strain localization during the lithospheric thinning. The NW-SE orientation dominated extensional tectonics was strongly driven by the westward subduction of the Paleo-Pacific oceanic plate during the Late Mesozoic.

Early Triassic S-Type Granitoids in the Qinzhou Bay Area, South China: Petrogenesis and Tectonic Implications

The influence of the paleo-Tethys or paleo-Pacific oceanic plate subduction on Early Triassic South China has long been debated. We have studied the zircon U-Th-Hf isotopes, trace elements, and whole-rock geochemistry of Early Triassic peraluminous granitoids in the Qinzhou Bay area, South China Block. LA–ICP–MS zircon U–Pb dating has revealed the Jiuzhou granodiorites and Dasi-Taima granite porphyries formed between 248.32 ± 0.98 and 246.6 ± 1.1 Ma. These rocks are characterized by high K2O and Al2O3, and low MgO, CaO, and P2O5 contents with A/CNK = 1.06–1.17, showing high-K calc-alkaline S-type affinities. The Early Triassic intrusive rocks and adjacent silicic volcanic rocks in the Qinzhou Bay area were found to be comagmatic and derived from a common magma pool, detached in an undifferentiated melt instead of indicating remarkable crystal—melt separation. Although the analyzed granitoids have highly enriched zircon Hf isotopic compositions (εHf(t) = −23.9 to −7.8), they cannot originate solely from metasedimentary protoliths. Source discrimination indicators have revealed enriched lithospheric mantle-derived magma was also an endmember component of the S-type silicic magma, which provided a heat source for the crustal anatectic melting as well. We inferred the studied Early Triassic granitoids formed under the paleo-Tethys tectonic regime before the collision of South China and Indochina blocks, as the oceanic plate subduction would have created an extensional setting which further caused the mantle-derived upwelling and volcanic eruption.

Early Jurassic A-type granitoids in the Xing'an block constrain on the spatial extent of the Paleo-Pacific tectonic regime

Northeast (NE) China underwent complex tectonic processes during the early Mesozoic era, influenced by both the Paleo-Pacific and Mongol-Okhotsk tectonic regimes. However, the dominant factor responsible for the early Jurassic magmatism in the Xing'an block, whether it was the Mongol-Okhotsk or the Paleo-Pacific tectonic regime, remains uncertain. This study aims to investigate the geochemical characteristics, petrogenesis, and geodynamic implications of the early Jurassic A-type granitoids in Heihe, Xing'an block, in order to shed light on the evolution and spatial extent of the Paleo-Pacific tectonic regime. The Heihe A-type granitoids were formed by partial melting of calc-alkaline meta-andesite/basalt under high-temperature and low-pressure conditions. During the later stages of magma evolution, fractional crystallization of K-feldspar played a significant role. The geochemical characteristics of the Heihe A-type granitoids resemble both A1-type and intraplate granite, similar to the early Jurassic A-type granites in the Songnen block. The Xing'an block, at least its southeastern margin, was mainly overprinted by the subduction of the Paleo-Pacific oceanic plate during the early Jurassic. This is evidenced by the north-south distribution pattern and the decreasing zirconium saturation temperature from east to west of the early Jurassic igneous rocks in the Xing'an block. The occurrence of early Jurassic various types of A-type granitoids and coeval OIB mafic rocks in NE China could be attributed to the back-arc extension and asthenosphere upwelling triggered by the Paleo-Pacific slab rollback.

Early Mesozoic accretionary complex scattered throughout NE China and its tectonic implications: Insights from the mylonitic meta-sedimentary rocks in Huanan, Heilongjiang Province

In NE Asia, the early stage (T-J) subduction history of the Paleo-Pacific oceanic plate was largely reconstructed by studies on the Heilongjiang complex, a blueschist-containing N-S trending high-pressure metamorphic belt, which is suggested to represent the “suture zone” between the Jiamusi Block and the rest of the Central Asian Orogenic Belt. However, recently more accretionary complexes with similar geochronological data were reported on both sides of the “suture zone”, making the tectonic affinities of the “suture zone” unclear. Here, we select the mylonitic meta-sedimentary rocks in Huanan, one of the potential accretionary complexes near the central Jiamusi Block to reveal its depositional and metamorphic history and discuss the tectonic implications. Our results show that these rocks were deposited in/after the Late Triassic (defined by the youngest detrital zircon group) and accepted clastics from both the Jiamusi and Songliao blocks. Before the Early Jurassic (c. 178 Ma by apatite UPb dating results in this study), they experienced high-P/low-T metamorphism (510–520 °C and 10.5–11 kbar), and then were heated to about 610 °C at 8.5–9.2 kbar during exhumation. The meta-sedimentary rocks in Huanan are components of the early Mesozoic accretionary complex in NE China. However, their present location is c. 85 km to the east of the previous proposed “suture zone” from Luobei to Mudanjiang. Considering the spatially scattering distribution of other similar accretionary complexes, such as the Zhangguangcai complex (c. 40 km to the west of the “suture zone”), we suggest that these complexes dispersedly exposed throughout NE China were either nappes transported by post-exhumation thrusting, or remnants of archipelagic oceanic crusts in the NW Paleo-Pacific region during the early Mesozoic.

Origin and mineralization processes of REE during magmatic-hydrothermal evolution: Insights from in-situ geochemistry of calcite from the Weishan REE deposit, eastern North China Craton

The Weishan REE deposit, hosting more than one million tons of light REE oxides (LREE2O3), is one of the largest LREE deposits in China. It has been considered to be a carbonatite-related REE deposit, but whether carbonatites occurred or not is still controversial, leading to the confusions of how the LREE originated and mineralized. Here, we conducted in-situ elemental and Sr-Nd isotopic analyses on different stages of calcite from the Weishan REE deposit, combined with petrographic and bulk C-O isotopic studies, with the aim to unravel the origin and mineralization processes of REE. Four stages of calcite (Cal-1, Cal-2, Cal-3 and Cal-4) were identified, of which the Cal-1 typically coexists with K-feldspar and quartz while the Cal-2 and Cal-3 coexist with sulfates and REE minerals (e.g., bastnäsite and parisite), respectively. The Cal-4 occurs in the post-ore veinlets and coexists with sulfides. The Cal-1 is characterized by exsolution of carbocernaite along its cleavage planes with highest Na, K, Sr, Ba and LREE contents. The other stages of calcite show continuous decrease of the above elements. Combined with petrographic features and elemental discrimination diagrams, it is concluded that the Cal-1 is magmatic while the others are hydrothermal. These calcites thus indicate a continuous magmatic-hydrothermal evolution. The four stages of calcite show similar isotopic compositions, which are consistent with those of the alkaline silicate rocks in the ore district, suggesting an uniform source or magma chamber for their derivation. Considering the magmatic features of Cal-1, it is inferred that carbonate melts coupled with abundant alkaline fluids were exsolved from the alkaline magmas. Alkalis (e. g., K and Na) were enriched in the magmatic-hydrothermal system with potassic metasomatism being more prevalent, which might play an important role in REE exsolution and transportation. Focused REE deposition occurred shortly after massive deposition of sulfates, indicating that REE-sulfate complexes might be the dominant transporting ligands of REE. The deposition of REE was mainly induced by fluid boiling, as indicated by the boiling/unmixing features of fluid inclusions. The Sr-Nd isotopic compositions of ores and associated alkaline rocks show affinities to the enriched lithospheric mantle (EM) beneath the study region, but are more depleted. Because subduction of the Paleo-Pacific oceanic plate beneath the eastern Asian occurred since the Jurassic, it is inferred that CO2- and REE-rich melts/fluids released from the subducted oceanic slab metasomatized the EM, leading to a REE-rich carbonated mantle. Rollback of the subducted slab occurred during the early Cretaceous, resulting in partial melting of the EM and thus generating the CO2- and REE-rich alkaline magmas. This deposit shows significant hydrothermal features with poorly-developed carbonatites, distinct from the typical carbonatite-related REE deposits in the other places. The REE mineralization is thus considered to be basically controlled by the alkaline magmas rather than the carbonatites.

Continental Arc Flare-Ups and Crustal Thickening Events in NE China: Insights from Detrital Zircon U-Pb Dating and Trace Elements from the Heilongjiang Complex

Continental arc is characterized by alternant magmatic flare-ups and lulls. From the Permian to the Middle Jurassic period, two flare-ups with a lull developed in NE China, but the tectonic controls that caused the flare-ups remain unclear. Sedimentary rocks of the Heilongjiang Complex were derived from these magmatic rocks; thus, we employed detrital zircon U-Pb dating and trace elements analyses to unravel the regional tectono-magmatic evolution. Eu anomaly, (Dy/Yb)N and Th/U ratios of the detrital zircons and Sr/Y and (La/Yb)N of the regional granitoids together indicate the occurrence of two episodes of crustal thickening during the two flare-ups, accompanied with a westward migration of magmatism. We propose that the Permian flare-up was caused by the shallowing subduction from the east, which thickened the upper plate and enhanced the deep crustal melting. During the Middle Triassic period, the mantle wedge was expelled by the flat slab and thickened crust, leading to the magmatic lull. However, the westward subduction of the back-arc oceanic plate occurred before the lull, gradually producing the Jurassic magmatic flare-up and crustal thickening. Closure of the back-arc ocean caused by the outboard Paleo-Pacific oceanic plate subduction was important in the formation of the episodic magmatic flare-ups and crustal thickening in NE China.

Late Mesozoic magmatism and gold metallogeny of the Jiaodong Peninsula, China: A response to the destruction of the North China Craton

The late Mesozoic destruction of the eastern North China Craton (NCC) resulted from intense crust-mantle interaction, culminating in the formation of world-class gold deposits in the Jiaodong Peninsula at ca. 120 Ma. We present zircon U-Pb geochronological data, Hf isotopic compositions, whole-rock major and trace element, and Sr-Nd-Pb isotopic compositions for two suites of granitoids: one (the Linglong suite) that hosts significant mineralization and the other (the Weideshan suite) that is largely unmineralized. The Linglong suite was emplaced in the Late Jurassic at 160 ± 3 Ma to 155 ± 3 Ma, whereas the Weideshan suite was emplaced in the Early Cretaceous at 121 ± 2 Ma to 118 ± 1 Ma, coeval with gold mineralization. Geochemical data indicate the Linglong suite was derived from the melting of thickened lower crust of the NCC, mixed with material from the Yangtze Craton (YC) and intervening Su-Lu Orogenic Belt, whereas the Weideshan suite was formed from the partial melting of lower continental crust, with the addition of enriched lithospheric mantle components. The Late Jurassic magmatism was associated with crustal growth resulting from the continental collision between the NCC and YC and subsequent subduction of the Paleo-Pacific Oceanic Plate (PPOP). In contrast, Early Cretaceous magmatism was triggered by rapid lithospheric thinning and asthenospheric upwelling resulting from slab roll-back and stagnation of the PPOP beneath the NCC. Lithospheric delamination induced the widespread development of secondary structures associated with the major Tan-Lu Fault, providing favorable loci for the accumulation of auriferous fluids.

Precambrian history of the Pacific Mantle Domain: New constraints from Woodsreef and Port Macquarie serpentinized spinel harzburgites of the New England Orogen, Australia

Abstract The present-day mantle is divided into the African and Pacific domains by the circum-Pacific subduction girdle. Very little is known about the mantle composition of the Pacific Domain before 120 Ma due to the scarcity of the oceanic record, having mostly been destroyed by subduction processes. Accreted oceanic lithosphere (ophiolites) in orogens along the Paleo-Pacific margins provide rare opportunities to partially fill this knowledge gap. The early Cambrian (530–505 Ma) Weraerai ophiolite mélange in the New England Orogen in Eastern Australia represents fragments of the now-consumed Paleo-Pacific oceanic plate, predecessor of the Panthalassa and Pacific oceanic plates, accreted to east Gondwana during ca. 410–277 Ma. Early work revealed the presence of an accreted volcanic island(s) of possible mantle plume origin. However, due to their heavy alteration and weathering, the geochemical signature of the mafic rocks in the Weraerai ophiolite cannot be used to certify their plume origin with confidence. Therefore, mantle rocks found in the ophiolitic belt offer an alternative way to decipher the origin of the oceanic lithospheric fragments in the Weraerai ophiolite mélange. Here we report the petrographic, major, and trace elements composition, including highly siderophile elements (HSE), and Re-Os isotope composition for 16 serpentinized spinel harzburgite samples from Woodsreef and Port Macquarie. The observed spinel-orthopyroxene symplectite intergrowth textures are interpreted as garnet-breakdown textures due to cooling from an initial high temperature of >1200°C. Silicon and Al contents and Mg# of serpentinized spinel harzburgite, as well as heavy rare earth element modelling results, suggest a high-degree of melt extraction of 20–30% in the garnet stability field and in an anhydrous environment, probably in ocean island or oceanic plateau environments. The samples in this study have HSE concentrations interpreted to have resulted from post-melting processes rather than a melt extraction feature, indicating that their Re-Os model ages need to be interpreted with caution. Nevertheless, the melt depletion ages obtained from the Woodsreef and Port Macquaries samples range between 1.4 and 1.1 Ga, consistent with previous studies on other Pacific-rim ophiolitic rocks. We argue that these ages might be related to a major depletion event during the transition between supercontinents Nuna and Rodinia. Such depletion events affected a large proportion of this section of the mantle before the incorporation of the peridotites into the oceanic lithosphere in the Paleo-Pacific associated with rapid ascent of mantle plumes. This interpretation is consistent with the occurrence of accreted volcanic islands in the Werearai ophiolite mélange, as shown by the OIB chemical signatures of some of the mafic rocks, and their association with shallow water limestones that formed in near-equatorial latitudes. The proposed ca. 530–510 Ma Paleo-Pacific Ocean mantle plume event coincides with a global peak of oceanic mantle plume events that may record the legacy of a circum-Rodinia subduction girdle driving antipodal mantle superplume episodes.

Polyphase deformation of the Youjiang fold-and-thrust belt during the Mesozoic: Implications for the tectonic transition of the South China block

The South China block (SCB) experienced the tectonic transition from the Paleo-Tethys to the Paleo-Pacific tectonic domains during the Mesozoic, but the transition process is hotly debated. The Youjiang fold-and-thrust belt (YFTB), in the interior of the Youjiang Basin in the southwestern SCB, is located in a junction of these two tectonic domains and thus witnessed their tectonic evolution. It also separates the northern thin-skinned structures from the southern thick-skinned structures in the basin. Therefore, the YFTB is an intriguing window into the Mesozoic evolution in the southwestern SCB and the Mesozoic tectonic transition of the SCB. In this study, we conduct structural analysis at the middle of the YFTB and discuss the Mesozoic tectonic transition of the SCB. Four phases of deformation are identified in the YFTB during the Mesozoic. The first phase of deformation (D1) is characterized by a series of conjugate joints, NW–SE trending thrust faults and folds resulting from NE–SW shortening that was related to the collision between the SCB and Indochina block. The second phase of deformation (D2) is manifested by the pre-existing NW–SE striking thrust faults transformed to normal faults and the Late Triassic mafic magmatism. Our structural observations, combined with previous geochronological data for mafic dykes, suggest the study area experienced post-orogenic extension in the Late Triassic. The third phase of deformation (D3), accompanied with the westward subduction of the Paleo-Pacific oceanic plate, is represented by a series of conjugate joints and NE–SW-striking faults formed by NW–SE compression. As a result of the continuous subduction of the Paleo-Pacific plate and its subsequent slab rollback, the fourth phase of deformation (D4) is featured with normal faults, magmatic-hydrothermal activities, and regional mineralization, which are associated with the nearly E–W extension setting. Our study results indicate that, in the Early Mesozoic, the Youjiang Basin was dominated by the Paleo-Tethys domain and then transitioned to the Paleo-Pacific domain. Together with the magmatic lull between the Triassic and the Early-Middle Jurassic identified in the SCB, we propose that the tectonic transition process manifested in the YFTB more likely initiated in the Early Jurassic.

Batholith recorded mesozoic multistage tectonic evolution of the South china block: A case study of the guandimiao intrusions

South China is a well-known grand felsic igneous rocks province. However, it is still controversial and not well understood whether the Mesozoic tectono-magmatic pattern is dominated by the subduction of the paleo-Pacific oceanic plate. In this study, we address this question by concentrating on the long-term evolutionary Guandimiao batholith, which has complex lithofacies with different formation ages and can be a superb record of the Mesozoic tectonic evolution in South China. Geochronologically, four stages of magmatism can be identified combined with previous reports: granodiorite (G1, 239 Ma), biotite monzogranite (G2-1) and two-mica monzogranite (G2-2) (230–203 Ma), granite porphyry (G3, 211–190 Ma), and lamprophyre (L4, 121 Ma). G1 and G2-1 have an affinity with I-type granite and were derived from metabasaltic to metatonalitic sources, whereas G2 and G3 show S-type granite characteristics and were derived from the para-metamorphic basement of the Cathaysia block. The L4 was derived from partial melting of garnet and spinel lherzolite and underwent mixing between Mesoproterozoic pelagic and/or terrigenous sediments and the subcontinental lithosphere mantle (SCLM) of South China. The granitoids of the Guandimiao batholith underwent intensely fractional crystallization of feldspar, Ti-bearing minerals, allanite and monazite. The zircon U–Pb dating of L4 in the Guandimiao batholith completely records the six stages of pre-Mesozoic tectonic events in the SCB. During the Mesozoic, the main body of the Guandimiao batholith (G1, G2-1 and G2-2) recorded the closure of the paleo-Tethys Ocean in the Triassic and the subsequent regional extension of the postcollision. G-3 and L4 of the Guandimiao batholith documented the transition of tectonic and dynamic regimes in the early Yanshanian and the rollback and steep subduction of the paleo-Pacific Ocean in the late Yanshanian.

Age, petrogenesis, and tectonic setting of granitic gneiss and amphibolite from the Weizigou gold deposit, Heilongjiang Province, NE China: Constraints from geochronology and geochemistry

The Weizigou gold deposit is located in the western Jiamusi Massif, Northeast China. Gold mineralization is hosted in the amphibolite, which intruded the granitic gneiss. Although the deposit shows similarities to iron-oxide–copper–gold deposits, the detailed ore-forming process remains uncertain. To determine the formation age, petrogenesis, and tectonic setting of the granitic gneiss and amphibolite, LA–ICP–MS zircon, titanite, and monazite UPb dating, whole-rock major- and trace-element analyses, and LA–ICP–MS in situ zircon Hf isotope analyses were conducted on samples from these rocks. The granitic gneiss yielded two age populations of 951–882 Ma, and ca. 500 Ma, with a monazite UPb concordia age of 501.5 ± 5.1 Ma. The amphibolite yielded a crystallization age of 292 Ma, consistent with the results for magmatic titanite UPb dating, and a metamorphic age of 272–258 Ma. The granitic gneiss contains typical aluminum-rich minerals, such as garnet and muscovite, mean SiO2 = 73.31 wt%, and molar ratio Al2O3/(CaO + K2O + Na2O) values of 1.02–1.07, indicating an S-type granite protolith. The amphibolite belongs to the tholeiitic basalt series and has low SiO2 and high MnO contents. These results, together with εHf(t) values and two-stage model ages ranging from −9.5 to 2.3 and − 0.3 to 5.7, and from 2010 to 1659 Ma and from 1331 to 947 Ma, respectively, allow us to infer that the parental magmas of the granitic gneiss and amphibolite were derived from the partial melting of Paleoproterozoic lower crust and the partial melting of metasomatized depleted mantle, respectively. The granitic gneiss is characterized by positive Th and Hf anomalies, and negative Nb, Ta, Sr and Ti anomalies, whereas the amphibolite is enriched in K, Rb, and depleted in Ba, Nb, Ti, and Zr. These geochemical features suggest that the S-type granite was formed in an active continental margin during the Neoproterozoic and underwent granulite-facies metamorphism during the early Paleozoic. The protolith of the amphibolite was gabbro that formed in an extensional setting (e.g., a backarc basin) associated with westward subduction of the Paleo-Pacific oceanic plate beneath the eastern Jiamusi Massif during the early Permian. The gold mineralization can most likely be attributed to contact metasomatic metamorphism of gabbro during the middle–late Permian.

Rapid early Permian tectonic reorganization of Laurentia’s plate margins: Evidence from volcanic tuffs in the Permian Basin, USA

Early Permian arc magmatism is critical to understanding the final assembly of the supercontinent Pangea and subsequent plate reorganization. Here we report the geochronology and geochemistry of Cisuralian and Guadalupian volcanic tuffs in southwestern Laurentia and infer the magmatic sources and plate reorganization related to Laurentia-Gondwana collision. Zircon CA-ID-TIMS U-Pb dates of tuffs in the Wolfcamp B, Wolfcamp A and lower Spraberry units in the Permian Basin provide the first set of absolute ages of Cisuralian deposits in this basin. Zircon geochemistry data further show that the parent melts of these tuffs were granitic melts that formed in continental arcs. Syn-depositional zircons from the Wolfcamp B tuff beds (288.2 ± 1.7 Ma) have an average εHf value of +5.5 and TDM2 model ages between 675 and 1207 Ma, reflecting a mixture of juvenile mantle melts and Precambrian crust. The isotopic signature is consistent with Mississippian Laurentian tuffs derived from a northern Gondwana arc formed by the subduction of the Rheic oceanic plate. However, zircons from a Wolfcamp A tuff bed (287.2 ± 0.5 Ma) show an average εHf value of −4.5, much more evolved than the inferred northern Gondwana arc, but consistent with granitoids of similar or younger age in the Oaxaquia terrane. This change in εHf most likely reflects magmatism related to subduction of a paleo-Pacific oceanic plate beneath western Pangea. Our interpretation suggests a late Cisuralian plate reorganization that was caused by plate reorganization following Pangea assembly led to rapid (∼1 Myr) initiation of subduction beneath western Pangea. This study also compares tuff dates from LA-ICPMS and CA-ID-TIMS U-Pb dating and concludes that the weighted mean date of the youngest dominant KDE mode (WMYDM) is the best way to approximate LA-ICPMS dates to true depositional ages.

Early Cretaceous rapid exhumation processes in the southern Great Xing’an Range, NE China: implications for extensional geodynamics

ABSTRACT The Mesozoic-Cenozoic geological evolution of the southern Great Xing’an Range (GXR) has long been controversial since it is located in the superposed region of different tectonic regimes and involves complex interactions. Here, we present low-temperature thermochronological data from granitoids emplaced in multiple magmatic stages to delineate the tectono-thermal history and further understand the geodynamic process. Most zircon and apatite (U-Th)/He ages, within error, indicate Early Cretaceous: the zircon average (U-Th)/He ages range from 146.7 ± 8.7 to 120.0 ± 7.1 Ma, except for the oldest data (162.5 ± 9.6 Ma) and the apatite (U-Th)/He ages vary from 138.7 ± 8.2 to 94.8 ± 7.5 Ma, except for the youngest age (70.8 ± 4.3 Ma). Systematic numerical modelling reveals that a rapid cooling and exhumation process with rates of 2.5-10 oC/Ma and 0.07-0.29 mm/yr occurred in the southern GXR during the early Cretaceous (145–110 Ma). We propose that the rapid uplift and exhumation of intrusive rocks during the early Cretaceous is also a manifestation of the extensional regime in the southern GXR, which is dominated by the rollback of the subducted Paleo-Pacific oceanic plate superposed by the post-orogenic extensional collapse of the Mongol-Okhotsk belt. In addition, the slow exhumation process since the late Cretaceous (~0.01 mm/yr) suggests that the Paleo-Pacific Ocean tectonic regime has a weak impact on the southern GXR area.

Multiple-Stage Mineralization in the Huayangchuan U\u2212REE\u2212Mo\u2212Cu\u2212Fe Ore Belt of the Qinling Orogen, Central China: Geological and Re\u2212Os Geochronological Constraints

The Huayangchuan ore belt is located in the western segment of Xiaoqinling Orogen in the southern margin of the North China Craton (NCC), and hosts voluminous magmatism and significant U−REE−Mo−Cu−Fe polymetallic mineralization. However, geochronological framework of the various mineralization phases in this region is poorly understood. Here, we present new Re−Os isochron ages on magnetite from the Caotan Fe deposit (2 675 ± 410 Ma, MSWD = 0.55), and on pyrite from the Jialu REE deposit (2 127 ± 280 Ma, MSWD = 1.9) and Yuejiawa Cu deposit (418 ± 23 Ma, MSWD = 11.5), and Re−Os weighted average model age on pyrite from the Taoyuan Mo−U deposit (235 ± 14 Ma, MSWD = 0.17). These ages, combined with regional geology and mineralization ages from other deposits, suggest that mineralization in the Huayangchuan ore belt lasted from the Neoarchean to the Late Mesozoic. The mineralization corresponds to regional tectono-magmatic events, including the Neoar-chean alkali magmatism (REE mineralization), Paleoproterozoic plagioclase-amphibolite emplacement (Fe mineralization), Paleoproterozoic pegmatite magmatism (U mineralization), Paleozoic Shangdan oceanic slab subduction-related arc magmatism (Cu mineralization), Early Mesozoic Paleo-Tethys Ocean subduction-related arc magmatism (Mo−U mineralization), and Late Mesozoic Paleo-Pacific oceanic plate subduction direction change-related Mo(-Pb) mineralization. We proposed that the Huayang-chuan ore belt has undergone prolonged metallogenic evolution, and the magmatism and associated mineralization were controlled by regional geodynamic events.

Fault-controlled carbonate-hosted barite-fluorite mineral systems: The Shuanghe deposit, Yangtze Block, South China

Fault-controlled carbonate-hosted Ba-F deposits occur in many major orogenic belts of the world and are important repositories of fluorine production required in various fields of chemical, metallurgical glass, ceramic, and cement industries. Here we investigate the poorly documented Shuanghe barite-fluorite deposit in the Yangtze Block of South China. This deposit offers an excellent case for studying the processes of formation of fault-controlled carbonate-hosted barite-fluorite mineral system. The orebodies in Shuanghe are hosted in carbonate rocks belonging to the early Ordovician Honghuayuan Formation and are dominantly controlled by NE-trending faults. Fluorite and barite exhibit similar rare earth element patterns with positive Eu anomalies and negative Ce anomalies. We report fluorite Sm–Nd isotopic data which yielded an isochron age of 103.1 ± 1.3 Ma. The δ18OV‐SMOW values range from +0.2‰ to +1.1‰ and the δDV‐SMOW values show a range of –33.1‰ to −26.3‰. The δ34SV-CDT values for barite range from 31.1‰ to 38.2‰. The bulk and in-situ Sr values for fluorite and barite show a range of 0.709081 to 0.712258. The petrological and geochemical data reported in this study suggest the following processes: (1) derivation of the ore-forming fluid from basinal water mixed with meteoric water; and (2) origin of the ore-forming components from Cambrian carbonate rocks, black shale, and Ordovician limestone. We propose a model that correlates the tectonic background of mineralisation with the Late Cretaceous subduction of the Paleo-Pacific Oceanic plate.

Geochemical evidence for the Paleo-Pacific plate subduction at ~125 Ma in Eastern China

The interaction of Paleo-Pacific slab-derived recycled materials with mantle peridotites is an important mechanism for causing the intensive lithospheric thinning in Eastern China during the Early Cretaceous. However, the definitive geochemical observation recording the Paleo-Pacific plate subduction at the peak time (~125 Ma) of lithospheric thinning has not been clearly identified so far. Here, we report an Early Cretaceous (~123 Ma) mafic dyke in Eastern China, the Daixi diabase, which is characterized by OIB-type elemental affinities and depleted Nd–Hf isotopes [εNd(t) = 5.4–6.9; εHf(t) = 9.6–12.1]. Its exceptionally high Nb/Nb*, Ti/Ti*, Eu/Eu* and Sr/Sr* ratios are well correlated with Hf/Sm, Th/LREE, Ce/Pb, and εHf(t) values, indicating that recycled basaltic, gabbroic oceanic crusts and terrigenous sediments were involved as source components. These recycled oceanic crustal materials (as eclogites) have contributed to the formation of lithologically distinct pyroxenite within the mantle peridotite matrix by melt-rock interaction. This is revealed by the elevated δ56Fe value (0.21‰ ~ 0.35‰) of the diabase and its good correlations with Fe/Mn, FC3MS (FeO/CaO-3*MgO/SiO2, all in wt%), Nb/Nb*, Sr/Sr*, εHf(t), and (206Pb/204Pb)i ratios. High U/Pb (μ = 25.0–33.4) and Th/Pb (ω = 85.6–118) ratios but unexpectedly unradiogenic Pb isotopic compositions [(206Pb/204Pb)i = 17.863–18.084, (207Pb/204Pb)i = 15.499–15.517, (208Pb/204Pb)i = 37.605–37.904] suggest that the subducted young oceanic crust is possibly recycled into the source ca. 180 Ma ago and most likely derived from the Paleo-Pacific oceanic plate. This is consistent with the conclusion drawn from the coupled NdHf isotopic systematics. Our study thus provides compelling geochemical evidence for the Paleo-Pacific subduction at the peak time (125 Ma) of the lithospheric thinning in Eastern China.

Extensive mineralization in the eastern segment of the Xingmeng orogenic belt, NE China: A regional view

• There are four types of EXOB deposits: porphyry , skarn , hydrothermal vein, and epithermal types. • There are five stage mineralizations in EXOB: 480–440, 300–220, 200–165, 155–130, and 125–100 Ma. • The five metallogenic periods correspond to different geological setting. The eastern segment of the Xingmeng orogenic belt (EXOB) hosts numerous Mo, polymetallic Ag, Pb–Zn , Sn, Cu, Au (Ag), and rare-earth element (REE) deposits and is one of the most important polymetallic-ore regions globally. The mineralization type includes i) porphyry Cu (Mo, Au), Mo (Cu, Pb–Zn, W), Sn (Cu–Ag) deposits, ii) skarn Cu–Au, polymetallic Pb–Zn and Fe–Sn deposit, and iii) veined polymetallic Ag, Pb–Zn, Sn, Cu deposits and W, Mo deposits, with an REE deposit related to magmatic hydrothermal fluid; and iv) epithermal Ag and Au (Ag, Te) deposits, respectively. The mineralization-related rocks in the EXOB are mainly high-K calc-alkaline granites together with minor alkaline granites that were derived from the melting of juvenile crust. The mineralizations in the EXOB are typically related to magmatic-hydrothermal events. In this study we attempt a systematic analysis of the temporospatial distribution of ore deposits in the EXOB, in order to constrain the geodynamic setting of mineralization. Based on available geochronological data, we suggest five mineralization stages for the EXOB at 480–440, 300–220, 200–165, 155–130, and 125–100 Ma, respectively. The porphyry Cu–Mo mineralization occurred during the early Paleozoic (480–440 Ma). The extensive Mo (W, Sn), and Ag (Pb–Zn) polymetallic mineralization was formed during the Mesozoic. The Ordovician–Triassic mineralization (480–220 Ma) is related to the Paleo-Asian Ocean tectonism and developed in an island arc setting. The Permian–Triassic mineralization (300–220) was formed in a post-collisional tectonic setting related to the development of Hegenshan suture. The early–middle Jurassic deposits (200–165 Ma) are related to the subduction of the Paleo-Pacific oceanic plate on the East Asian continental margin. The Late Jurassic–Early Cretaceous deposits (155–130 Ma) were formed in an extensional setting related to rollback of the Paleo-Pacific Plate. The Jurassic–Cretaceous mineralization (200–100 Ma) in the northern segment of the Da Hinggan Mountains is associated with the subduction and post-collisional tectonic setting of the Mongol–Okhotsk oceanic plate. The latest stage of epithermal deposits (125–100 Ma) were developed in a continental margin back-arc setting associated with subduction of the Paleo-Pacific Plate beneath the Eurasia.

Chau Thoi and Nui Gio volcanic rocks: A remnant of the Loei Phetchabun volcanic belt in Viet Nam

Introduction: Extrusive volcanic rocks, such as dacite, andesite, basalto-andesite, basalt… of Chau Thoi and Nui Gio hills in Bien Hoa and Binh Phuoc provinces, southern Viet Nam, characterize volcanic island arc rocks. These rock suites formed as the convergent tectonic between the Indochina and Sibumasu geological blocks.
 Methods: Geochemical data of rock samples collected on the field were examined and analyzed by the Academia Sinica I E S (Institute of Earth Science, Taiwan and processed with a GCD kit (Geochemical Data Toolkit) to ascertain their characteristics and geotectonic setting. Result: Geochemical data both in major elements and trace elements of the Chau Thoi – Nui Gioshow a specific characteristic of a volcanic island arc environment.
 Discussion: Chau Thoi and Nui Gio rocks are suitable to correlate to the Permian Thailand Loei Phetchabun volcanic belt. However, at the current time, Chau Thoi and Nui Gio rocks have been classified as Deo Bao Loc formation - late Jurassic to early Cretaceous in ages - belong Truong Son magmatic belt. This magmatic belt resulted from the Yanshanian orogeny by the subduction of the Paleo-Pacific oceanic plate beneath the Eurasia (Indochina) continental plate. More studies needed to be performed, specially geochronological data to support the study.
 Conclusion: Chau Thoi and Nui Gio rock suites characterize volcanic island arc rocks, products of a convergence tectonic between Indochina and Sibumasu blocks. They are remnants of the Thailand Loei Phetchabun volcanic belt, the first time reported in Vietnam.

Constraints on the Formation of the Giant Daheishan Porphyry Mo Deposit (NE China) from Whole-Rock and Accessory Mineral Geochemistry

AbstractThere are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from –1·1 to 1·4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. >5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical.