Jurassic Granitoids Research Articles

Geochronology and Geochemical Characteristics of Granitoids in the Lesser Xing’an–Zhangguangcai Range: Petrogenesis and Implications for the Early Jurassic Tectonic Evolution of the Mudanjiang Ocean

This article focuses on zircon U-Pb isotope dating and a whole-rock elemental analysis of granodiorites, monzonitic granites, granodioritic porphyries, and alkali feldspar granites in the Yangmugang area of the Lesser Xing’an–Zhangguangcai Range. The zircon U-Pb isotope-dating results revealed that these granitic rocks formed during the late Early Jurassic period (182.9–177.2 Ma). Their geochemical characteristics and zircon saturation temperatures suggest that the granodiorites are moderately differentiated I-type granites and the monzonitic granite, granodioritic porphyries, and alkali feldspar granites are highly differentiated I-type granites. The degree of magma differentiation progressively increased from granodiorites to alkali feldspar granites. By combining the regional Nd and Hf isotope compositions, it was inferred that the magma source involved the melting of lower crustal material from the Mesoproterozoic to the Neoproterozoic eras. By integrating these findings with contemporaneous intrusive rock spatial variations, it was indicated that the late Early Jurassic granitoids in the Lesser Xing’an–Zhangguangcai Range formed within an extensional tectonic setting after the collision and closure of the Songnen–Zhangguangcai Range and Jiamusi blocks. Additionally, this study constrains the closure of the Mudanjiang Ocean to the late Early Jurassic period (177.2 Ma).

Two-stage Mesozoic oceanic subduction and related mantle metasomatism beneath the South Qiangtang terrane with implications for post-collisional magmatism

Subduction-related metasomatism at convergent margins is a crucial process for the lithosphere evolution of the overriding plate. In the central Tibetan Plateau, tectonic transitions and related mantle metasomatism history of the oceanic subduction beneath the South Qiangtang terrane (SQT) remain ambiguous, which impeded understanding the geological evolution of the SQT during the Mesozoic and interpreting the geodynamic process responsible for the Cenozoic post-collisional magmatism. This paper integrated the new and published geochronological and geochemical data from the Jurassic-Cretaceous magmatic rocks in the western part of the SQT to explore the variations of magmatism and processes of mantle modifications. The Jurassic arc magmatism is characterized by the southward younging migration with increasing zircon εHf (t) values and the east–west linear distribution of ca. 155–150 Ma slab-derived adakites. After a magmatic gap at ca. 145–130 Ma, the Early Cretaceous magmatism (130–100 Ma) with distinct juvenile isotopic compositions reinitiated firstly in the south of the Jurassic magmatic belt. In comparison, a younger phase of felsic rocks (120–100 Ma) spatially and geochemically overlaps the northern Jurassic granitoids with ancient crustal sources. These spatial–temporal-geochemical covariations of magmatism could be best explained by the southward transference of oceanic subduction at the earliest Cretaceous that was induced by the Jurassic accretion of an oceanic plateau onto the SQT. Under such subduction regimes, several high-Nb mafic rocks in the northern and southern magmatic belts which were formed by partial melting of the slab-melts-metasomatized lithospheric mantle respectively recorded the Jurassic and Cretaceous mantle metasomatism beneath the SQT, based on their spatial and geochemical relationships with two phases of the adakites. Combined with other geological evidence, these results contribute to elucidating the Mesozoic geodynamic evolution of the oceanic subduction beneath the SQT, which further has implications on the deep dynamic processes responsible for post-collisional magmatism.

In situ geochemistry of apatite: Petrogenetic and tectonic interpretations of Jurassic felsic magmatism in the Yanbian area, NE China

Geochemical analyses of individual minerals provide more detailed insights into the petrogenesis of igneous rocks than whole-rock analyses. This study conducted in situ geochemical and Nd isotope analyses of apatites from 10 Jurassic granitic plutons in the Yanbian area, NE China, to establish the petrogenesis and regional tectonic evolution. The results indicate that the apatite geochemistry of Jurassic granitoids in the Yanbian area was controlled primarily by the composition of parental melt. Post-magmatic alteration may lead to geochemical decoupling between apatite and parental melt, while Nd isotopes exhibit some resilience to such alterations. Apatites from Early Jurassic granitoids display characteristics that are consistent with an I-type origin, whereas those from Middle and Late Jurassic granitoids exhibit an adakitic affinity. Variations in apatite compositions indicate the fractional crystallization of other rare earth element (REE)-bearing minerals during magma evolution. The early crystallization of plagioclase and allanite led to decreases in Sr and Th contents in apatite, respectively, resulting in a negative Eu anomaly and light REE depletion. The fractional crystallization of titanite and hornblende resulted in the depletion of middle REE in apatite. Hornblende is regarded as the main residual phase in the magma source of Middle and Late Jurassic adakitic granitoids in the Yanbian area. Apatite Nd isotopic compositions suggest that the Jurassic granitoids in the Yanbian area originated from two crustal sources: the Central Asian Orogenic Belt and the North China Craton. Additionally, increasing trends in apatite Sr/Y and (La/Yb)N ratios from the Early to Late Jurassic suggest a gradual thickening of the regional crust, which is likely driven by the continentward migration of the subduction zone associated with the Paleo-Pacific Plate.

Early Jurassic A-type granite and monzodiorite from the Baoji batholith: Implication for tectonic transition from post-collision to post-orogenic extension in the Qinling Orogenic Belt, China

Introduction: The early Jurassic granitoids in the Qinling Orogenic Belt (QOB) play a crucial role in understanding the tectonic implications for the geological evolution of China. To elucidate the early Jurassic tectonic setting of QOB, we performed a comprehensive analysis of zircon U-Pb ages, whole-rock geochemistry, and in situ zircon Lu-Hf isotopes from early Jurassic monzodiorite and Kfeldspar granite within the Baoji batholith in western QOB.Geochronology Method and Results: The intrusions yielded zircon U-Pb ages of 186 ± 2 Ma and 188 ± 2 Ma, respectively.Geochemistry Results: The monzodiorites are characterized by relatively high MgO, Rb, Th, U, and LREE contents, as well as low P, Ti, and HREE contents. They also exhibit high Nb/Ta ratios (20.6–23.4). The zircon εHf(t) values for the monzodiorite sample range from −4.36 to 6.47, indicating significant contributions from a fertile continental lithospheric mantle with the involvement of crustal components. The K-feldspar granites are enriched in K2O+ Na2O, Rb, Zr, Hf, and Nb, and lower Ba, Sr, Ti, and P. They exhibit high Nb/Ta and Ga/Al ratios but low Y/Nb and Yb/Ta ratios. Their geochemical characteristics reveal an A-type granite affinity with elevated zircon saturation temperatures (848°C–900 °C). Additionally, the K-feldspar granite exhibits REE and trace element patterns similar to those observed in the monzodiorite. However, a wide range of zircon εHf(t) values (−4.72 to 3.98), differing from those of the monzodiorite, indicate that the parental magma of the K-feldspar granite experienced magma mixing between a monzodioritic magma and a crustal-derived felsic magma.Discussion: These findings suggest that both A-type K-feldspar granite and monzodiorite likely formed during post-orogenic processes. Additionally, the QOB commenced its postorogenic evolution as an extensional tectonic environment during the early Jurassic period.

Late Jurassic granitoids in Mufushan complex and their significance for the Mesozoic tectonic evolution of eastern South China

The Mesozoic tectono-thermal evolution of eastern South China plays an important role in forming the abundant magmatic rocks and associated giant polymetallic deposits. The middle-late Mesozoic granitoids in Mufushan and adjacent regions (e.g., Lianyunshan, Wangxiang, Taohuahsan, etc.) represent the western front of the southeast China magmatic province. However, the tectonic regimes and petrogenesis of the magmatic rocks are still elusive. Here, we report zircon UPb ages, geochemistry, and Sr-Nd-Hf-Pb isotopic data for the granitic rocks in the Mufushan region, in combination with data available from the literatures, to discriminate the middle-late Mesozoic tectono–magmatic history of the Mufushan complex in north-central South China Block. New zircon UPb dating suggests that the Mufushan two-mica monzogranites were emplaced at 149–144 Ma, a short time after the 153–151 Ma granodiorites. These late Jurassic felsic magmas share broad similarities in geochemical-isotopic characteristics, with significant enrichment in LREEs and LILEs (e.g., Ba, Rb, Th, and K), depletion in some HFSEs (e.g., Nb, Ta, Ti, and P) and positive Pb anomalies, similar to those of arc-type rocks. The common existence of Neoproterozoic zircons within the magmatic rocks indicates a certain amount of ancient material involved in the genesis of the magma. The two-mica monzogranites have εHf(t) values ranging from −13.4 to +3.5, overlapping with those of the granodiorites (−11.3 to +6.6), but both are lower than contemporaneous diorites (−2.4 to +0.59), suggesting a greater incorporation of enriched materials into the source for the granitoids. The two-mica monzogranites and granodiorites have εNd(t) values ranging from −7.9 to −8.8 and − 10 to −7.9, with corresponding two-stage Nd model ages of 1.7–1.6 Ga and 1.6 Ga, respectively, falling within their two-stage Hf model ages of 2.0–1.4 Ga and 1.8–1.4 Ga, respectively. Compared with the geochemical and isotopic compositions of the coeval magmatic rocks in eastern South China, we favor that the late Jurassic Mufushan granodiorites evolved from variable mixing and differentiation of the diorites and granitoids, accompanied by a continuous magma assimilation. The presence of late Jurassic highly fractionated granites suggests an extensive magma differentiation in eastern South China and the well-developed early Cretaceous A-type granites in eastern South China reflect a dominant extensional tectonic regime induced by slab roll-back of the Izanagi plate. The involvement of subduction-related melts facilitated the underplating of mantle-derived magma and crustal heating, triggering intensive partial melting of the lithosphere and magma enrichment, as well as the polymetallic deposits in eastern South China.

Landscape inversion episodes in SE China during the Mesozoic−early Cenozoic: Constrained by trace-element contents, Nd isotope geochemistry, and detrital zircon U-Pb geochronology of sedimentary basins

The tectonics and landscape of SE China experienced significant changes throughout the Mesozoic and early Cenozoic, largely in response to variations in the slab dynamics of the paleo-Pacific plate, which was subducting beneath continental Asia. We investigated the Mesozoic Yong’an basin in western Fujian Province of SE China in comparison to the sedimentary records of coeval basins in the region to document how its clastic sediment types and their provenance varied through time during the Mesozoic and what regional geologic processes may have controlled these variations. The average εNd value of samples from the Middle Jurassic Zhangping Formation is −16.6, and its detrital zircons are dominated by 1800 Ma and 2000 Ma grains, sourced from the northern Wuyishan Mountains. These mountains underwent significant rock and surface uplift by the Middle Jurassic and became the main source of clastic sediments in SE China. The Lower Cretaceous Bantou Formation contains pyroclastic rocks and represents fluvial-lacustrine deposits with εNd values of −14.8 to −12.4 and abundant 160−120 Ma detrital zircons, sourced from Late Jurassic granitoid rocks, which were widely exposed at the surface in SE China by this time. The upper Lower and lower Upper Cretaceous Shaxian Formation contains coarse-grained and poorly sorted sandstones-conglomerates with volcanic and granitic rock fragments, and it rests unconformably on the Bantou Formation. The Shaxian Formation represents fluvial- to alluvial-fan deposits, and its formation marks the timing of a rapid uplift of the paleo−Coastal Mountains. The Upper Cretaceous Chong’an Formation (>2000 m thick) contains abundant volcanic and granitic rock clasts and represents alluvial-fan and fluvial deposits. The average εNd values of the Shaxian and Chong’an Formations range between −9.3 and −7.5, and their most abundant detrital zircon ages are between 120 Ma and 80 Ma. By the end of the Late Cretaceous, the paleo−Coastal Mountains constituted a nearly 4-km-high magmatic belt, with much of SE China situated in its rain shadow at a lower elevation to the north. Eocene−Oligocene sedimentary basin rocks in Taiwan have an average εNd value of −10.9 and abundant Phanerozoic detrital zircons. The sediment source for these rocks was the paleo−Coastal Mountains. The Miocene basinal strata in Taiwan have more negative εNd values (−13.0) and contain Jurassic−Cretaceous as well as abundant Paleoproterozoic and Neoproterozoic zircons, indicating that the Wuyishan Mountains were again the main sediment source later in the Cenozoic. Denudation rates in the SE margin of South China were high (0.12−0.10 km/yr) during the Cretaceous (140−60 Ma), while they were very low in SW China and in the interior of South China during the same period. These differences confirm the existence of high coastal mountains in SE China until the Late Cretaceous. Denudation rates in eastern South China, particularly the coastal areas, were very low (0.06−0.02 km/yr) during the late Cenozoic (30−0 Ma), whereas they were the fastest (0.14−0.16 km/yr) in the northern Nanling belt and the Yangtze block farther inland to the north, indicating the surface elevation became higher in the western part of South China but lower in its eastern part in the late Cenozoic. This dynamic landscape evolution of SE China through multiple and major shifts throughout the Mesozoic and early Cenozoic was driven by the subducting slab dynamics and the tectonics of the Tibetan Plateau.

Petrogenesis of Jurassic granitic plutons in the Yanbian area, NE China: Implications for the subduction history of the Paleo-Pacific Plate

It is widely accepted that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent began during the early Mesozoic; however, the details of the subduction process remain uncertain. This paper presents new zircon U–Pb–Hf and whole-rock geochemical data for Jurassic intrusive rocks from the Yanbian area of NE China to provide a better understanding of the subduction history of the Paleo-Pacific Plate. Zircon U–Pb dating results reveal that three stages of Jurassic magmatism are recorded in the Yanbian area, namely, Early Jurassic (188–179 Ma), Middle Jurassic (172–163 Ma), and Late Jurassic (158–156 Ma). Early Jurassic intrusive rocks from the Yanbian area are predominantly calc-alkaline gabbro, gabbro-diorite, quartz monzonite, granodiorite, and syenogranite. These geochemical features and rock associations are indicative of formation in an active continental margin setting related to the initial subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The Early Jurassic granitoids were formed mainly by the partial melting of heterogeneous lower crust. The Middle Jurassic granitoids are geochemically similar to adakites and have εHf(t) values of + 2.0 to + 10.1. These rocks are inferred to have been derived from magmas generated by the partial melting of thickened juvenile lower-crustal material. Their formation was related to the subduction of the Paleo-Pacific Plate. Late Jurassic adakites in the Yanbian area formed in a compressional tectonic setting. Their negative εHf(t) values (−17.2 to − 8.6) and TDM2 ages (2025–1552 Ma) indicate that the primary magma was derived from the partial melting of ancient continental crust. Combining our data with the distribution of Late Jurassic igneous rocks in the NE Asian continental margin and the northward migration of Jurassic accretionary complexes along the margin, we conclude that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent during the Late Jurassic–earliest Early Cretaceous had a low angle of obliquity.

Zircon U–Pb ages and O–Hf isotopes of Quaternary trachytes from the East Sea: Implications for the genesis of low-δ18O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts, including Dokdo (DD), Ulleungdo (UD), Simheungtack (ST), Anyongbok, and Isabu in the southwest of the East Sea back-arc basin. In this study, we present whole-rock geochemical, zircon U–Pb age, and in situ O–Hf isotope data for the submerged volcanic rocks from DD, UD, and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes. All samples used in this study were trachytes and exhibited ferroan, alkalic, and metaluminous to weakly peraluminous characteristics. They showed light rare earth element (REE)-enriched patterns with (La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot. In addition, they were enriched in large-ion lithophile elements and high field strength elements; they exhibited positive Pb anomalies and strongly negative Ba, Sr, P, and Ti anomalies. The zircons yielded a weighted-mean 206Pb/238U age of 2.61, 0.348–0.704, and 2.76–2.94 Ma for the DD, UD, and ST trachytes, respectively. All zircons exhibited lower δ18O values than normal depleted mantle values, regardless of the crystallization age and spatial distribution of volcanoes. The δ18O values showed no correlation with U contents or Th/U ratios, indicating that the low δ18O signatures were of primary magmatic origin. The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive εHf values. Binary O–Hf mixing modeling revealed that low-δ18O rocks with positive εHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ18O and positive εHf signatures and an enriched mantle source with normal δ18O and negative εHf values. The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting. In contrast, the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks (Triassic–Jurassic granitoids), resulting in increased δ18O values and decreased εHf values relative to those of the UD and ST volcanoes. Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.

Early to Middle Jurassic (ca. 182–164 Ma) fractionated granitoids in the Korean Peninsula: Implication for the tectonomagmatic history of East Asia

The Jurassic granitoids (200–164 Ma) are distributed in the Korean Peninsula due to the Paleo-Pacific plate subduction. Early Jurassic (200–182 Ma) granitoids are mainly distributed in the southern Korean Peninsula. By contrast, Early to Middle Jurassic (182–164 Ma) granitoids are distributed in the central Korean Peninsula. In this study, we report detailed petrology, zircon U–Pb ages, and whole-rock geochemistry from the Seoul–Uijeongbu and Pocheon–Gimhwa pluton units in the central Korean Peninsula. The Seoul–Uijeongbu unit is dominated by biotite granite, with minor porphyritic biotite and garnet-biotite granite while the Pocheon–Gimhwa unit consists of biotite granite and porphyritic biotite granite, garnet-biotite granite, and two-mica granite. Zircon U–Pb age from those granites gives 180–167 Ma. The granitoids in the Pocheon-Gimhwa unit formed through fractional crystallization from biotite granite and porphyritic biotite granite to garnet-biotite granite, and two-mica granite based on gradually decreasing their Nb/Ta, Zr/Hf, and Eu/Eu* ratios. The strongly fractionated granitoids are garnet-biotite granite and two-mica granite. The LILE enrichment, Ta–Nb, Sr–P, and Eu–Ti troughs, and Ba depletion in most granitoids are similar to those of granitoids due to the subduction in the arc environment. Thus, these Jurassic granitoids (180–167 Ma) are mainly peraluminous granites with moderate crystal fractionation corresponding to I-type granite. Alkali feldspar granite associated with ore mineralization occurs in the Gwanaksan pluton from the southwestern Seoul–Uijeongbu unit. The alkali feldspar granite displays distinct negative Eu anomaly with high contents of Rb, Hf, Cs, and Nb compared with other granites. These characteristics imply that alkali feldspar granite experienced strong hydrothermal activity leading to feldspar ore mineralization compared to the other granites. The formation of a wide range of moderately evolved peraluminous granitoids is presumed to be related to rapid flat-subduction during 182–164 Ma, and the mineralization-related alkali feldspar granite indicates the termination of Jurassic granitoid magmatism in the central Korean Peninsula.

Petrogenesis and Tectonic Setting of the Early and Middle Jurassic Granitoids in the Chaihe Area, Central Great Xing’an Range, NE China

To ascertain the Early-to-Middle Jurassic tectonic setting in the central Great Xing’an Range, this study investigated the Early and Middle Jurassic granitoids exposed in the Chaihe area in the central Great Xing’an Range based on isotopic chronology and petrogeochemistry. The results of this study show that the Early and Middle Jurassic granitoids have emplacement ages of 179–172 Ma. Moreover, the Early and Middle Jurassic granitoids are high-K calc-alkaline unfractionated I-type granitoids and high-K calc-alkaline fractionated I-type granitoids, respectively. The magma sources of the Early and Middle Jurassic granitoids both originated from the partial melting of newly accreted lower crustal basaltic rocks. Meanwhile, the Middle Jurassic magma sources were mixed with mantle-derived materials or ocean-floor sediments formed by the dehydration and metasomatism of subducted slabs. The Early and Middle Jurassic granitoids in the study area were formed in the subduction environment of the oceanic crust, in which the Mongol-Okhotsk oceanic plate was subducted southward beneath the Eerguna and Xing’an blocks. Moreover, the Siberian plate began to collide and converge with northeast China during the Middle Jurassic.

Geochronology, petrogenesis and tectonic implications of Late Triassic and Late Jurassic granitoids in the South China Block

Geochronology, petrogenesis and tectonic implications of Late Triassic and Late Jurassic granitoids in the South China Block

Zircon U–Pb ages and Lu–Hf isotopes of the Jurassic Granites on the east coast of the Korean Peninsula and Southwest Japan: Petrogenesis and tectonic correlation between the Korean Peninsula and Japanese Islands

Jurassic granitoids in the Korean Peninsula and Japanese Islands are key for deciphering the tectonic evolution of East Asia and the tectonic correlation between them. Zircon U–Pb age dating of seven granitoids in Southwest Japan gives 247.3 ± 1.8 and 192–191 Ma for the Ebi Granites and 200.8 ± 2.1 and 184–183 Ma for the Hida Granites. On the east coast of the Korean Peninsula, 188–186 Ma, 183–177 Ma, 167.7 ± 1.5 Ma, and 173–166 Ma are obtained for the granitoids in the Gyeongsang Basin, Yeongnam Massif, Taebaeksan Basin, and Gyeonggi Massif, respectively. All granitoids show depletions in Nb, Ta, P, and Ti in whole-rock compositions, suggesting an arc environment. The studied granitoids can be divided into two types. The early-stage (∼201–183 Ma) granitoids from Southwest Japan and the Gyeongsang Basin are characterized by the absence of Precambrian inherited zircons, high εHf(t) values of +13.0 to −0.8, and higher crystallization temperatures of ∼750–830 °C. The later-stage (∼183–166 Ma) granitoids from the remaining areas have abundant Paleoproterozoic inherited zircons, low εHf(t) values of −13.9 to −25.0, and lower crystallization temperatures of ∼680–750 °C. The early-stage granitoids formed during a steep subduction regime under higher temperatures due to the heat supply from the asthenospheric mantle. In contrast, later-stage granitoids mostly have an adakitic nature and formed by the partial melting of garnet- and hornblende-bearing thickened Precambrian lower crust under lower temperature conditions during flat subduction. The Ebi and Hida Granites in Southwest Japan formed along the continental arc passing through the Gyeongsang Basin to the Dumangang Belt of the Korean Peninsula, indicating their paleopositions. A change in the subduction angle from steep to flat caused the migration of magmatism to the northwest or west throughout Northeast Asia, including Japan, Korea, and China.

LateTriassic A‐type granite boulders in Lower Cretaceous conglomerate of theHida belt, Japan: Their origin and bearing on theYamatotectonic line inFar East Asia

AbstractTo identify the origin of the Hida belt in central Japan, geochemistry and U–Pb age of zircons were analyzed for the extra‐large granitoid boulders in the Lower Cretaceous fluvial conglomerate of the Jinzu Group. This study clarified that the boulders of granitoids have geochemistry of typical A‐type granite, as characterized by high Nb + Y and high Ta + Yb values. U–Pb ages of igneous zircons from three individual granite boulders are concentrated at ca. 220 Ma (Late Triassic). As to Late Triassic A‐type granites, there is no corresponding body previously recognized within Japan, whereas identical A‐type granites occur in the eastern Songliao block on the immediate west of the Jiamusi block in NE China. The large size of boulders and the fluvial facies of the hosting conglomerate indicate their origin in the Hida belt per se, suggesting the cryptic and/or past occurrence of A‐type granite, rather than in NE China. Together with the eastern Songliao block (China), Laoelin‐Grodekov (LG) belt (Primorye, Russia), and Yamato Ridge (Japan Sea), the Hida belt forms a unique domain on the immediate west of GSC in Far East Asia, which is characterized commonly by the co‐occurrence of Permo‐Triassic and Jurassic granitoids, and probably with Late Triassic A‐type granite. These confirm that Hida belt represents an allochthonous unit tectonically emplaced onto the rest of Japan, which is composed of the Phanerozoic subduction‐related orogenic belt (Nipponides) developed along the Pacific side of Greater South China (GSC) since the Cambrian. For emphasizing a major geotectonic boundary of the Mesozoic granitoid provinces in Far East Asia, we propose the Yamato tectonic line between the easternmost Central Asian orogenic belt and GSC/Nipponides, which is traced for up to 3000 km from the Russia/China/North Korea border to SW Japan.

Lithospheric thinning and ignition of a Cordilleran magmatic flare-up: Geochemical and O-Hf isotopic constraints from Cretaceous plutons in southern Korea

Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the (paleo-)Pacific plate. As observed in many Cordilleran arcs, magmatic evolution in this area was punctuated by high-volume pulses amid background periods. The present study investigates the early evolution of the Cretaceous magmatic flare-up using new and published geochronological, geochemical, and O-Hf isotope data from plutonic rocks in the southern Korean Peninsula. After a long (∼50 m.y.) magmatic hiatus and the development of the Honam Shear Zone through flat-slab subduction, the Cretaceous flare-up began with the intrusion of monzonites, granodiorites, and granites in the inboard Gyeonggi Massif and the intervening Okcheon Belt. Compared to Jurassic granitoids formed during the former flare-up, Albian (∼111 Ma) monzonites found in the Eopyeong area of the Okcheon Belt have distinctly higher zircon εHf(t) (−7.5 ± 1.3) and δ18O (7.78‰ ± 0.25‰) values and lower whole-rock La/Yb and Sr/Y ratios. The voluminous coeval granodiorite and granite plutons in the Gyeonggi Massif are further reduced in Sr/Y and to a lesser extent, in La/Yb, and have higher zircon εHf(t) values (−13 to −19) than the Precambrian basement (ca. −30). These chemical and isotopic features indicate that Early Cretaceous lithospheric thinning, most likely resulting from delamination of tectonically and magmatically overthickened lithospheric keel that was metasomatized during prior subduction episodes, and consequent asthenospheric upwelling played vital roles in igniting the magmatic flare-up. The O-Hf isotopic ranges of synmagmatic zircons from the Albian plutons and their Paleoproterozoic and Jurassic inheritance attest to the involvement of lithospheric mantle and crustal basement in magma generation during this decratonization event. Arc magmatism then migrated trenchward and culminated in the Late Cretaceous, yielding widespread granitoid rocks emplaced at shallow crustal levels. The early Late Cretaceous (94–85 Ma) granites now prevalent in Seoraksan-Woraksan-Sokrisan National Parks are highly silicic and display flat chondrite-normalized rare earth element patterns with deep Eu anomalies. Synmagmatic zircons in these granites mimic their host rock’s chemistry. Delamination-related rejuvenation of crustal protoliths is indicated by zircon εHf(t) values of granites (−6 to −20) that are consistently higher than the Precambrian basement value. Concomitant core-to-rim variation in zircon O-Hf isotopic compositions reflects a typical sequence of crustal assimilation and fresh input into the magma chamber.

Subduction initiation of the Bangong–Nujiang Tethys Ocean, Tibetan Plateau

• The northward subduction initiation of the meso -Tethys Ocean was diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). • The collision and slab breakoff during the closure of the Paleo-Tethys Ocean were vital to the northward subduction initiation. • The southward subduction of the meso -Tethys Ocean was initiated no later than the Middle Triassic . • The southward subduction was attributed to the closure of the Paleo-Tethys Ocean and the spreading of the NeoTethys Ocean. The initiation of subduction is widely recognized as a critical process associated with the evolution of the meso -Tethys Ocean. Here, we present zircon U–Pb, Lu–Hf isotope, and whole-rock geochemical data for a suite of Triassic–Jurassic magmatic rocks from the central Tibetan Plateau, with the aim of gaining insights into the subduction initiation of the Bangong–Nujiang Tethys Ocean (BNTO). Early Triassic trachyandesite (248 Ma) from the South Qiangtang terrane resemble intraplate magma and probably resulted from magmatic activity at the passive continental margin during the BNTO opening. Late Triassic S-type granodiorite (220 Ma) and Middle Jurassic SSZ-type gabbro (∼165 Ma) from the South Qiangtang terrane may result from the northward subduction of the BNTO oceanic lithosphere. Jurassic granitoids (I and S type; 193–177 Ma) and metamorphic rocks (179–174 Ma) from the Amdo and Jiayuqiao subterranes are related to the northward subduction of the BNTO oceanic lithosphere. Middle–Late Jurassic diorites (164–162 Ma) from the Central Lhasa subterrane have geochemical features of continental margin arc magma with a sediment component, and these are derived from the southward subduction of the BNTO oceanic lithosphere. Finally, this Triassic–Jurassic magmatic suite of various geochemical features was combined with the results of previous studies, and we suggest that the initiation of northward BNTO subduction was probably diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). This initiation resulted both from collision and slab breakoff during the closure of the Longmuco–Shuanghu–Lancangjiang Paleo-Tethys Ocean and considerable density and rheology contrasts between the thick oceanic lithosphere and continental lithosphere. The southward subduction was initiated no later than the Middle Triassic and was attributed to the closure of the Sumdo Paleo-Tethys Ocean and the spreading of the Indus–Yarlung Zangbo NeoTethys Ocean.

Contrasting Sources and Related Metallogeny of the Triassic and Jurassic Granitoids in the Chifeng–Chaoyang District, Northern Margin of the North China Craton: A Review with New Data

Understanding of the mechanism between magma sources and metallogeny is still vague. As an important gold and molybdenum producing area, the Chifeng–Chaoyang district, located at the northern margin of the North China Craton (NCC), is a key place for this issue. New geochemical data relating to Taijiying gold-deposit-related granites are presented. These data, coupled with previous studies, are used to explore the relationship between magma sources and mineralization processes. Two major magmatic periods, the Middle Triassic (220–230 Ma) and Late Jurassic (150–160 Ma), are identified based on the compiled data. The Triassic magmatic rocks are mostly fractionated I-type and A-type granites, including monzogranite, biotite granite, and syenogranite. They have low initial 87Sr/86Sr values (0.7050), moderately enriched εNd(t)–εHf(t) values (−8.5 and −5.6), and relatively young Nd–Hf model ages (TDM2-TDMC) (1.47–1.57 Ga). These features indicate that more Archean–Paleoproterozoic mantle-derived materials were involved in their sources. On the other hand, Jurassic granites are high-K calc-alkaline of the calc-alkaline series and mainly consist of granite, monzogranite, leucogranite, and granodiorite. They have high Na2O/K2O, Sr/Y, and La/Yb ratios and low Y and Yb contents. The adakitic features suggest the existence of a thickened lower crust. Their significant negative εNd(t)–εHf(t) values (−15.0 and −12.8) and older Nd–Hf model ages (TDM2–TDMC) (2.17–2.11 Ga) are consistent with their derivation from thickened ancient lower crust, indicating the initial activation of NCC. It is proposed that the change in the main source resulted from the tectonic transition during the early Mesozoic initial decratonization, that is, from the post-collisional extension to the subduction of the Paleo-Pacific plate beneath the East Asia plate from the Triassic to the Jurassic. Comparative analysis suggests that the medium–large-scale gold deposits with a high grade are closely related to the Triassic granites; however, most molybdenum deposits formed in the Jurassic. The decratonization of the NCC in the early Mesozoic experienced tectonic transition and controlled the gold and molybdenum mineralizations in the different stages by the changing magma sources. This pattern is beneficial to understanding the metallogenesis in the Chifeng–Chaoyang district.

Petrogenesis of the Late Jurassic Granodiorite and Its Implications for Tectonomagmatic Evolution in the Nuocang District, Western Gangdeses

The Gangdese magmatic rocks of the southern Lhasa terrane, are generally thought to be an important window to witness the formation and evolution of the Neo-Tethys oceanic opening, subduction, and closure, and India-Eurasian continental collision. We investigated a new occurrence of granodiorite in the Nuocang district of western Gangdese, southern Lhasa terrane, and conducted a series of analyses on their petrology, chronology, and geochemistry. The Nuocang granodiorites have the zircon U-Pb ages of 151–154 Ma, which suggest that Late Jurassic granitoids are present in the western Gangdese of southern Lhasa terrane. They are relatively high in SiO2, Al2O3, low K2O, Na2O, and Sr/Y ratios, enrichments of LILE and LREE, and depletion of HFSE, with the positive correlation between Rb and Th, and negative correlations between SiO2 and P2O5, Rb, and Y, showing the features of I-type granites. The relatively high (87Sr/86Sr)i values from 0.712231 to 0.712619, low εNd(t) values from −9.56 to −8.99, together with the negative εHf(t) values from −10.8 to −5.0 (mean value −8.9) suggested that the Nuocang granodiorites probably sourced from the partial melting of the ancient Lhasa terrane, with parts of mantle materials involving in. Combined with the previous geochronology and geochemical data of Mesozoic magmas in the Gangdese belt, as well as the Late Jurassic granodiorite, in this paper, we propose that the Nuocang granodiorites formed in a continental margin arc environment triggered by the northward subduction of Neo−Tethys oceanic crust.

Jurassic subduction of the Paleo-Pacific plate in Southeast Asia: New insights from the igneous and sedimentary rocks in West Borneo

The tectonic affinity of West Borneo and its relationship with the East Asia continental margin are key issues for unraveling SE Asia tectonics. New zircon U-Pb and Hf-O isotopes and whole-rock elemental and Sr-Nd-Pb isotopic data of the igneous and associated sedimentary rocks in West Borneo document the development of Early (∼200–185 Ma) and Late (∼160–145 Ma) Jurassic arc-like igneous rocks. In the Kuching Zone and South Schwaner Mountains, the Jurassic mafic-intermediate igneous rocks are characterized by enrichment in LILEs, depletion in HFSEs, εHf(t) = +0.6 – +11.6, δ18O = 4.97–5.61‰, εNd(t) = −2.5∼+2.4, (206Pb/204Pb)i = 18.71–19.37 and (208Pb/204Pb)i = 38.75–39.58. They originated from a sediment-modified mantle wedge source in a supra-subduction zone setting. The Jurassic granitoids shared consistent elemental and Sr-Nd-Pb-Hf-O isotopic signatures with εHf(t) = +1.7∼ +15.6, δ18O = 4.97–6.01‰, εNd(t) = −1.1 ∼ +4.2, with Δ8/4 = 22.5–51.9 and Δ7/4 = 5.8–14.3, arguing for a juvenile crust-dominated origin. Detritus from the associated sedimentary rocks in the Kuching Zone and South Schwaner Mountains was largely derived from the nearby Jurassic magmatic arc. Their detrital zircon age patterns are comparable to those from the East Malaysia-Indochina and Sibu zone, but distinctive from those in SE Borneo. Integrating all data with those from regional surveys, the igneous and sedimentary rocks in the South Schwaner Mountains are suggested to commence in the earliest Jurassic or even Triassic. West Borneo might be the eastward extension of the Triassic Sundaland. The western Kuching Zone and western South Schwaner Mountains represent the southmost part of the active Jurassic Andean-type East Asian continental margin. The westward subduction of the paleo-Pacific plate was initiated around the earliest Jurassic at ∼200 Ma or earlier.

New constraints on the timing and character of the Laramide Orogeny and associated gold mineralization in SE California, USA

Abstract The timing of deformation and associated gold mineralization in SE California, USA, is contentious, partly due to the challenges involved with dating ductile deformation. We therefore combine modern geo- and thermochronology with field and microscopic observations to show that the Cargo Muchacho Mountains preserve evidence of northward thrusting in a kilometer-scale ductile shear zone during the Late Cretaceous Laramide Orogeny, accompanied by hydrothermal fluid flow, gold mineralization, and pegmatite emplacement. Penetrative strain was largely accommodated within the Jurassic metavolcaniclastic Tumco Formation, whereas intrusive Jurassic granitoids behaved as competent bodies. Quartz microstructures suggest deformation at ~500 °C, which is consistent with fabrics defined by amphibolite facies minerals. The timing of thrusting is constrained by dynamically recrystallized titanite with a U-Pb age of 68 ± 1 Ma and late syn-kinematic pegmatites that yield U-Pb zircon ages of 65.0 ± 4.2–63.2 ± 4.8 Ma. Syn-kinematic fluid flow was focused into a lateral thrust ramp where the shear zone foliation was deflected around a relatively rigid pluton, creating zones rich in magnetite-quartz veins and epidote, and precipitating gold associated with pyrite and chalcopyrite. Dating of these sulfides via Re-Os yields an age of 64.7 ± 0.8 Ma, which confirms a Laramide age for the gold mineralization. Together, apatite from the pegmatites and a nearby Jurassic granite yields a U-Pb age of 60.4 ± 3.5 Ma, reflecting cooling to below 530–450 °C. Comparison with published studies suggests that thick-skinned deformation in the Cargo Muchacho Mountains was driven by flat-slab subduction of the conjugate Hess Plateau, which occurred several million years after and to the south of flat-slab subduction of the conjugate Shatsky Rise. This suggests that the conjugate Hess Plateau may have been subducted up to several hundred kilometers farther north than previously thought. Metamorphic devolatilization of underplated Orocopia Schist likely generated the gold-bearing hydrothermal fluids, and anatexis of the schist formed the peraluminous pegmatites, which highlights the importance of schist underplating and devolatilization along much of the Californian and Mexican cordillera.

Partial melting caused by subduction of young, hot oceanic crust in shallow high-temperature and low-pressure environments: Indications from Middle and Late Jurassic oceanic plagiogranite in Shiquanhe, Central Tibet

The oceanic plagiogranites that intruded into the ophiolite suite have received a great deal of attention from researchers. Some of these plagiogranites were controlled by mid-ocean ridge spreading and were formed via crystallization of basaltic magma or partial melting of the mafic gabbroic oceanic crust. The other plagiogranites were controlled by subducted oceanic crust slabs, and their formation mechanism is similar to that of the oceanic island arcs. The identification of different types of oceanic plagiogranites is of great significance for understanding the evolution of ancient ocean basins. In the Shiquanhe–Namtso mélange zone in central Tibet, scholars have recently identified many rock sequences related to intra-oceanic arcs, including MORB-like basalts, boninites, high-Mg andesites, and multiple series of granitic rocks. To further explore the formation mechanism of felsic rocks in oceanic island arcs and their role in the growth of continental crustal material, we sampled a series of Middle and Late Jurassic granitoids exposed in the ophiolitic mélange belt in the Shiquanhe area. Using zircon U-Pb dating, zircon Lu-Hf isotopic testing, whole-rock Sr-Nd isotopic testing and whole-rock major and trace geochemical testing, we determined the magmatic genesis and tectonic history of these samples. According to the test results and our field survey, we divided our samples into three groups. The first group consists of low-K plagiogranites (LKGs). These samples have ages that fall between 163 and 162 Ma. With their low K2O/Na2O ratios, low La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, weak negative Eu anomalies and their significantly depleted zircon isotopic compositions (εHf(t) = 10.0–12.5; εNd(t) = 0.64–1.10), we propose that these samples are the products of partial melting of oceanic gabbro and turbidite at shallow depths in low-pressure and high-temperature conditions. The second group consists of samples from dioritic enclaves within the LKGs. These samples, which are slightly older than the LKG samples (175–173 Ma), have isotopic characteristics that are similar to those of the LKG samples (εHf(t) = 10.4–13.5; εNd(t) = 0.51–0.83). The samples in the second group may be homologous captives that formed during the early crystallization of LKG homologous magmas. The third group consists of samples with compositions of high-K granodiorites (HKGs) and ages similar to those of the LKG group (~163 Ma). With high K2O/Na2O ratios, relatively high La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, negative Eu anomalies, significantly enriched isotopic compositions (εHf(t) = −11.1 to −15.4; εNd(t) = −13.29 to −13.43). Considering the basement exposure and known magmatic characteristics in the Shiquanhe area, we propose that HKGs samples are the result of interaction between continental subduction sediment melts and heterogeneous mantle material in high-temperature and low-pressure conditions.By comparing the oceanic plagiogranites in the Shiquanhe–Namtso mélange zone with those developed in the young, hot intra-oceanic subduction zone, we determined that the oceanic plagiogranites in the Shiquanhe–Lhaguotso–Asa area are forearc granites developed in the forearc area. These plagiogranites were controlled by the young, hot subducting plate and were formed by partial melting of a multi-component source area in a high-temperature and low-pressure environment.