Early Mesozoic Magmatism Research Articles

A Mid‐Crustal Channel of Positive Radial Anisotropy Beneath the Eastern South China Block From F‐J Multimodal Ambient Noise Tomography

AbstractWe investigated the crustal radial anisotropy in the eastern South China Block (ESCB) with the F‐J multimodal ambient noise tomography. Well corresponding to widespread mid‐crustal low‐velocity zones in the model, a pronounced mid‐crustal channel of positive radial anisotropy is revealed. In the Cathaysia Block, it may origin from sub‐horizontally aligned quartz induced by extension and correspond to the phase transition of to quartz. In other areas, while, it may be relevant to other well‐aligned minerals. This positive mid‐crustal radial anisotropy channel not only provides the solid evidence for dominative extensional deformation since late Mesozoic, but also may indicate an important detachment surface, contributing much to the early Mesozoic magmatism in the ESCB.

Transition from orogenic-like to anorogenic magmatism in the Southern Alps during the Early Mesozoic: Evidence from elemental and Nd-Sr-Hf-Pb isotope geochemistry of alkali-rich dykes from the Finero Phlogopite Peridotite, Ivrea–Verbano Zone

The Ivrea-Verbano Zone (IVZ) in the westernmost sector of the Southern Alps is an iconic upper mantle to lower continental crust sequence of the Adriatic Plate and provides a geological window into the tectono-magmatic events that occurred at the Gondwana–Laurussia boundary from Late Paleozoic to Early Mesozoic. In this work, we document new geochemical and Nd-Sr-Hf-Pb isotopic data for Early Mesozoic alkali-rich dyke swarms which intruded the Finero Phlogopite Peridotite (northern IVZ) to provide geological constraints on the nature, origin and evolution of Early Mesozoic magmatism in the Southern Alps. The studied dykes are amphibole-phlogopite-bearing and show geochemical features varying between two end-member groups. A dyke group is characterized by HFSE-poor, Al-rich amphibole (Al2O3 up to 16 wt%) with high LILE and LREE contents, high radiogenic 87Sr/86Sr(i) (0.704732 to 0.704934) and low radiogenic Nd isotopes (εNd(i) from –0.1 to –0.7), which support the occurrence of significant amounts of recycled continental crust components in the parental mantle melts and impart an overall “orogenic-like” affinity. This dyke group was largely derived from metasomatized lithospheric mantle sources. The second group is HFSE-rich with Al-poorer amphibole enriched in LILE and LREE, low radiogenic 87Sr/86Sr(i) (0.703761–0.704103) and higher radiogenic Nd isotopes (εNd(i) from +3.4 to +5.4) pointing to an “anorogenic” alkaline affinity and asthenospheric to deep lithospheric mantle sources. Some dykes show both orogenic and anorogenic affinities, providing evidence that the orogenic-like magmatism in the IVZ predates the alkaline anorogenic magmatism. The Finero dyke swarms therefore record a geochemical change of the Early Mesozoic magmatism of the Southern Alps from orogenic-like magmatism, typical of post-collisional settings, to anorogenic alkaline magmatism, common in intraplate to extensional settings, and places a temporal correlation of Early Mesozoic magmatism in the IVZ to those in the eastern and central sectors of the Southern Alps.

Origin of Early to Middle Triassic ultrapotassic syenites in South China: A record of block amalgamation in East Asia

Alkaline intrusions such as ultrapotassic syenites are typically formed in extensional tectonic settings including continental rift, post-orogenic and post-collisional regimes. Here we present two early Mesozoic ultrapotassic syenite plutons (Tieshan and Yangfang) in South China that were formed in a continent collisional setting. LA-ICP-MS zircon UPb chronological, major and trace elemental, and SrNd isotopic data of these syenites are used to explore their origin and to further constrain the geodynamics of early Mesozoic magmatism in South China. Our new data indicate that the Tieshan and Yangfang plutons were formed in Early Triassic (249 Ma) and Middle Triassic (240 Ma), respectively. The Tieshan and Yangfang syenites have SiO2 contents of 50.84–56.95 wt% and 59.13–68.34 wt%, respectively, both are enriched in alkalis with very high K2O (5.60–9.68 wt%) and K2O/Na2O (1.8–15.7) and in LILE (Ba = 2464–8813 ppm; Sr = 652–3499 ppm), REE (498–1355 ppm) and HFSE (Zr = 264–766 ppm). They show enriched SrNd isotopic compositions with 87Sr/86Sr (t) and εNd (t) of 0.7096–0.7102 and − 5.2 to −6.5, respectively for Tieshan and of 0.7112–0.7119 and − 8.8 to −9.7, respectively for Yangfang. Geochemical data suggest that the studied ultrapotassic syenites were derived by partial melting of ancient subduction-modified lithospheric mantle including slab-derived melt hybridization and slab-derived carbonate fluid metasomatism, with the source depth of 60–90 km for Tieshan and at least 90 km for Yangfang. We further suggest that their origins are related to the transtension of the regional NE–trending strike-slip faults caused by continental collisions during the early Mesozoic amalgamation in East Asia.

Contrasting origins of A-type granites in the Late Triassic-Early Jurassic Pitou complex, southern Jiangxi province: Implications for Mesozoic tectonic evolution in South China

The origin of A-type granites is purely circumstantial. Granitic complexes are quite common and generally composed of different type granites. I- and A-type granite association in a granitic complex is often described in the literature. Here we present a granitic complex, i.e. the Pitou complex in southern Jiangxi province, South China that is composed of different varieties of A-type granites. Detailed zircon LA-ICP-MS U Pb chronology and in situ Hf isotope, mineral chemistry and whole-rock element and Sr Nd isotope data of this granitic complex are used to explore their origin and to further constrain the geodynamics of Mesozoic magmatism in South China. Our new data indicate that the Pitou complex was emplaced during the Late Triassic (237 Ma) (central to northern part) and Early Jurassic (188–185 Ma) (southern part), respectively and that both the Late Triassic and Early Jurassic granites belong to A-type but show contrasting origins. The Late Triassic granites show an association of alkali-feldspar granite-syenogranite-monzogranite and the rock type of the Early Jurassic granites is alkali-feldspar granite. All the granites are composed mainly of K-feldspar, quartz, plagioclase and biotite (ferri-biotite or annite) with biotite occurring as anhedral crystal, interstitial to quartz and feldspar. Both the Late Triassic and Early Jurassic granites have high SiO 2 (>70 wt%) and total alkalis (K 2 O + Na 2 O > 8 wt%) with high FeO T /(FeO T + MgO) and low Mg#. They are enriched in rare earth elements (except Eu) and depleted in Sr and Ba. They display Zr + Y + Ce + Nb >350 ppm, 10,000 × Ga/Al >2.6 and have high zircon saturation temperatures (>800 °C). Biotites in the granites are Fe-rich with very low Fe 3+ /Fe 2+ , indicating fairly reducing conditions. However, the Late Triassic and Early Jurassic granites also show some geochemical differences. The Late Triassic granites have lower FeO T /(FeO T + MgO) and higher Mg# than the Early Jurassic granites, with the Mg# of the former consistent with pure crustal melts and the Mg# of the latter much lower than pure crustal melts. In addition, the Late Triassic granites have lower zircon saturation temperatures (up to 843 °C, average 823 °C) than the Early Jurassic granites (up to 946 °C, average 875 °C). Furthermore, the Late Triassic granites have high 87 Sr/ 86 Sr (t) (0.7132–0.7226) and negative ε Nd (t) (−10.3 to −10.9) and ε Hf (t) (in situ zircon) (−11.5 to −11.6) that are similar to those of the early Paleozoic granitoids in the region whereas the Early Jurassic granites have low 87 Sr/ 86 Sr (t) (down to 0.7034) and positive ε Nd (t) (up to +3.0) and ε Hf (t) (in situ zircon) (up to +8.3) that are similar to those of the Early Jurassic gabbroic rocks in the region. Geochemical data and major element modeling suggest that the Late Triassic A-type granites were formed by shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas and the Early Jurassic A-type granites were produced by extensive fractionation (ca. 90% to 95%) from gabbroic magmas by removal of plagioclase, amphibole, K-feldspar, clinopyroxene and magnetite. We further suggest the origin of the Late Triassic A-type granites was related to the transtension of the regional NE–trending strike-slip faults caused by the early Indosinian continental collisions and the Early Jurassic A-type granites were emplaced in a back-arc extensional setting coupled with the slab rollback and subsequent slab break-off of the subducted Palaeo-Pacific plate. Our new data confirm that the early Mesozoic magmatism in South China was controlled by continental collisions during the Late Permian to Triassic amalgamation in Southeast Asia whereas the late Mesozoic magmatism was related to the northwestward subduction of the Palaeo-Pacific plate. A transition in the dominant tectonic regime from the Tethyan to the Pacific tectonic domain most likely took place during the Rhaetian. • The Pitou granitic complex emplaced during the Late Triassic and Early Jurassic. • Both Late Triassic and Early Jurassic granites are A-type but show contrasting origin. • Late Triassic granites formed by shallow dehydration melting of Paleozoic granitoids. • Early Jurassic granites formed by extensive fractionation from gabbroic magmas. • Origin of both A-type granites suggests different geodynamic mechanisms.

Discovery of Early Mesozoic Magmatism in the Northern Muglad Basin (Sudan): Assessment of Its Impacts on Basement Reservoir

The crystalline basement in the Central African Rift System (CARS) has been confirmed as a good unconventional reservoir in the surrounding basins of the Muglad area, Sudan. This study has recognized at least five Early Mesozoic intrusions for the first time at the northern margin of the Muglad Basin. Field and petrographic investigations show that they are mainly granite, syenite, diorite, and monzonite, with dominant alkali feldspar, plagioclase, quartz, and minor hornblende, biotite, and magnetite. Zircon internal structures, Th/U ratios, and rare earth element (REE) patterns indicate their magmatic origin. A total of 210 zircons from five samples were analyzed using LA-ICP-MS and they yielded U-Pb ages from 195.0 ± 6 to 225.0 ± 3 Ma, belonging to the Late Triassic-Early Jurassic. These results suggest that the Muglad Basin basement is not universally Precambrian as previously considered. The older basement was intruded and modified by Early Mesozoic magmas as evidenced by field contacts and our dating results. These magmatic events produced diverse Mesozoic igneous rocks prior to the formation of the Muglad Basin (Early Cretaceous), becoming new members of the crystalline basement. This composite basement then experienced prolonged weathering and fracturing for 65–95 Ma, resulting in porous and permeable weathered layers or fractured zones, making the basement a good unconventional reservoir for oil and gas.

Early Mesozoic magmatism records the tectonic evolution from syn- to post-collisional setting in the Central Lhasa subterrane, Tibet

The Early Mesozoic (ca. 240–185 Ma) magmatic rocks in the central Lhasa subterrane (CL) provide crucial insight into the geodynamics of the Early Mesozoic orogenic assembly in the Lhasa Terrane. Here we present an integrated geochemical and geochronological study combining new and published data of the Early Mesozoic magmatism in the CL to reveal the geodynamic processes and magmatic responses involved in this orogeny. The here studied A-type granodiorites from the Luoza batholith display zircon U-Pb ages of 199 Ma, and zircon saturation temperatures of 833–857 °C. In addition, zircons have highly enriched radiogenic Hf isotope compositions (εHf(t) = −20.1 to −4.3). The Luoza granodiorite samples have high (87Sr/86Sr)t ratios between 0.7174 and 0.7209, and negative εNd(t) values between −9.5 and − 9.1. We propose the granodiorites were generated via partial melting of ancient basement rocks with minor inputs of basaltic melts (~10%). The here studied monzogranites and syenogranites from the Menba batholith are normal calc-alkaline and highly fractionated I-type granites that yield zircon U-Pb ages from 197 to 184 Ma and 187 Ma, respectively. The zircon grains of both rock types have relatively higher εHf(t) values (−13.8 to +0.5) than those of the Luoza granodiorites. The Menba monzogranites have (87Sr/86Sr)t ratios between 0.7062 and 0.7081, and εNd(t) values between −6.9 and − 3.4. The calculated εNd(t) values for the Menba syenogranites range from −4.5 to −4.0. The isotope data set indicates that the Menba granitoids were generated via mixing of ancient crustal melts and mantle-derived magmas (~50%). The high-temperature and low-pressure conditions indicated by the Early Jurassic Luoza A-type granitoids are geodynamically linked to a post-collisional setting. This evidence, in combination with Early Mesozoic magmatism identified within the CL, clearly displays a transitional conversion from ca. 240–210 Ma S-type-dominated to ca. 210–185 Ma I-type-dominated magmatism. This distinct change in magmatism reflects the geodynamic change from a syn-collisional to a post-collisional setting, involving the breakoff of the Sumdo Paleo-Tethys oceanic slab at ca. 210 Ma. The Early Mesozoic magmatic belt within the CL may record the orogenic assembly between the central and southern Lhasa subterranes, which represents the important evidence for closure of the Sumdo Paleo-Tethys Ocean. Consequently, the Early Mesozoic magmatic belt within the CL reveals that the Sumdo Paleo-Tethys Ocean extended at least from the Luoza area, central Lhasa Terrane, to the easternmost end of the Lhasa Terrane.

Early mesozoic arc–back-arc system in the leading edge of the Tibetan Plateau

The southern Lhasa subterrane is the leading edge of the Tibetan Plateau and preserves important records of the Neo-Tethyan oceanic subduction and India–Asia collision. However, the early tectonic evolutionary history of the Neo-Tethyan oceanic subduction remains unclear. Here we present results from a systematic study on the early Mesozoic (Middle Triassic–Late Jurassic) magmatic and sedimentary rocks in the southern Lhasa subterrane. These rocks are distributed zonally along the southern and northern parts of the subterrane, and have been classified into the north and south belts. The magmatic rocks in the north belt show a back-arc basin affinity, with accreted bimodal volcanic sequence and sediments from central and southern Lhasa subterranes, whereas the magmatic rocks in the south belt reveal typical magmatic arc characteristics. The sediments were derived from southern Lhasa subterrane, without the input of clastic components from the central Lhasa subterrane. Based on geochronological, geochemical, and sedimentary records from both belts, we conclude that the early Mesozoic magmatism was triggered by the northward subduction of the Neo-Tethyan oceanic slab, and its initial subduction likely occurred before the Middle Triassic (ca. 240 Ma). During the Early–Middle Jurassic (ca. 193–165 Ma), a short-lived episode of back-arc extension, we attribute to roll-back of the subducting Neo-Tethyan slab, resulted in the single magmatic arc that existed during the early stages (ca. 240–194 Ma) and evolved gradually into an arc–back-arc system. The north belt represents a back-arc basin setting associated with a continental arc, whereas an intra-oceanic and continental (island) arc simultaneously developed in the south belt, and both these belts were possibly generated under the same subduction system.

Early Mesozoic Rare-Metal Granites and Metasomatites of Mongolia: Mineral and Geochemical Features and Hosted Ore Mineralization (Baga Gazriin Chuluu Pluton)

Abstract —We present results of petrographic, mineralogical, and geochemical study of all types of rocks of a multiphase pluton and consider the chemical evolution of igneous and metasomatic rocks of the Baga Gazriin Chuluu pluton, based on new precise analytical data. At the early stage of their formation, the pluton granites were already enriched in many trace elements (Li, Rb, Cs, Be, Nb, Ta, Th, and U), F, and HREE relative to the upper continental crust. They show strong negative Ba, Sr, La, and Eu anomalies, which is typical of rare-metal Li–F granites. The geochemical evolution of the Baga Gazriin Chuluu multiphase pluton at the postmagmatic stage was marked by the most intense enrichment of greisens and microclinites with lithophile and ore elements (Sn, W, and Zn) and the formation of ore mineralization. In the permeable rift zone where the Baga Gazriin Chuluu pluton is located, the fluid–magma interaction took place under the impact of a mantle plume. High-temperature mantle fluids caused melting of the crustal substratum, which determined the geochemical specifics of Li–F granite intrusions. Genesis of granitic magma enriched in Li, F, Rb, Sn, and Ta is possible at the low degrees of melting of the lower crustal substratum. The Baga Gazriin Chuluu pluton formed in the upper horizons of the Earth’s crust, where magma undergoes strong differentiation and the saturation of fluids with volatiles can lead to the postmagmatic formation of metasomatites of varying alkalinity (zwitters (greisens), microclinites, and albitites) producing rare-metal mineralization. By the example of the early Mesozoic magmatism area of Mongolia, it is shown that the formation of granites and associated rare-metal minerals is due to the interaction of mantle fluids with the crustal material and the subsequent evolution of granitic magmas.

Late Permian–Triassic tectonic nature of the eastern Central Asian Orogenic Belt: Constraints from the geochronology and geochemistry of igneous rocks in the Bureya Massif

The late Paleozoic–early Mesozoic tectonic nature of the eastern Central Asian Orogenic Belt (CAOB) has been subject to debate, and the late Paleozoic–early Mesozoic magmatism in the Bureya Massif provides an opportunity to address this issue. We report new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data for late Paleozoic–early Mesozoic igneous rocks from the south of the Bureya Massif. These data provide insight into the petrogenesis of the igneous rocks, and constrain the late Paleozoic–early Mesozoic tectonic nature of the eastern CAOB. Zircon UPb ages indicate that the late Permian (ca. 255 Ma) and Middle–Late Triassic (ca. 216 Ma) magmatic events are identified in the south of the Bureya Massif. The late Permian igneous rocks consist mainly of bimodal rock suites, which include quartz monzodiorites, gneissic monzogranites, and A-type gneissic syenogranites. The Middle–Late Triassic igneous rocks are dominated by porphyritic syenogranites, and A-type granite porphyries, syenogranites, and gneissic macroporphyritic quartz syenites. The quartz monzodiorites have low SiO2 contents (53.9–56.1 wt%), high Mg# (56–57), and Cr (160–175 ppm), Co (25.2–27.8 ppm), and Ni (61.2–72.3 ppm) contents, and are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; e.g., K, Rb, Sr, and Ba). These characteristics, along with enriched zircon Hf compositions [εHf(t) = −8.31 to −0.44], suggest that their primary magma was derived from the partial melting of lithospheric mantle that had been metasomatized by fluids released from a fossil subducted slab. The granitoids have high SiO2, and low MgO contents, and are enriched in LILE, depleted in high field strength elements (HFSE), and yield εHf(t) values of −11.5 to +2.87 and two-stage model ages (TDM2) ages of 1968–1024 Ma. These characteristics suggest that they were derived from the partial melting of Paleoproterozoic–Mesoproterozoic accreted crustal material; however, these granitoids have distinct, heterogeneous geochemical compositions, precluding simple fractional crystallization of a single parental melt, and strongly suggesting that they were originated from distinct petrogenetic processes. The late Permian–Triassic bimodal igneous rock suites and A-type granites imply an extensional environment in the eastern CAOB.

Genesis of Middle Triassic high-Mg# quartz diorites from the Xiahe area, West Qinling Orogen, Central China, and their geodynamic implications

The West Qinling Orogen (WQO) features widespread Triassic granitic rocks, which are crucial to constrain the tectonic evolution of the WQO. Here, we conducted an integrated petrological, geochronological, geochemical, and Sr-Nd-Hf isotope study on quartz diorites and mafic microgranular enclaves (MMEs) from the Ren’ai and Daerzang plutons, Hezuo area, WQO. The quartz diorites and MMEs have crystallization ages between 244 and 241 Ma. The quartz diorites display intermediate SiO2, high Mg# values (62–66) and evolved S-Nd-Hf isotope compositions, indicating an origin from magma mixing process. The MMEs display mineral compositions similar to those of the host rocks, only with more mafic minerals. The The Ren’ai and Daerzang plutons were part of the early Indosinian magmatic belt mainly distributed in the western and middle parts of the northern WQO. In comparison, late Indosinian magmatic rocks are mainly exposed in the eastern WQO. The asymmetric distribution of Early Mesozoic magmatism in the WQO might have been caused by a specific subduction pattern of the A’nimaque-Mianlue oceanic crust. The western and middle WQO experienced a period of flat or low angle subduction in the early stage, followed by slab rollback of the oceanic crust, which accounted for the flare-up of early Indosinian magmatism in the northern WQO. However, the distribution of late Indosinian magmatism might have been caused by asymmetric crustal delamination in the WQO.

Zircon U–Pb Geochronology and Petrogenesis of Early–Midlle Permian Arc–Related Volcanic Rocks in Central Jilin: Implications for the Tectonic Evolution of the Eastern Segment of Central Asian Orogenic Belt

AbstractThis paper presents age and geochemical data of a recently identified Late Paleozoic volcanic sequence in central Jilin Province, with aims to discuss the petrogenesis and to constrain the tectonic evolution of the Central Asian Orogenic Belt in this area. Firstly, the volcanic rocks have zircon U‐Pb ages of 290–270 Ma. Secondly, they are characterized by (a) ranging in composition from the low‐K tholeiite series to high‐K calc‐alkaline series; (b) enrichment in light rare earth elements and depletion of heavy rare earth elements, with negative Eu anomalies; and (c) negative Nb, Ta, and Ti anomalies. Finally, the volcanic rocks yield ∊Hf(t) values of +7.1 to +17. These data suggest that the central Jilin volcanic rocks were possibly derived from predominant partial melting of a depleted lithospheric mantle that might have been modified by subducted slab–derived fluids. Combined with previous studies, the Late Paleozoic–Early Mesozoic magmatism in Central Jilin can be divided into two stages: (a) a volcanic arc stage (290–270 Ma) represented by low‐K to high–K, tholeiite to calc–alkaline plutons and (b) a syn–collisional stage (260–240 Ma) represented by high‐K calc–alkaline I‐type granites. Furthermore, the timing and the tectonic setting of the above magmatic rocks show that the arc was probably produced by the northward subduction of the Paleo‐Asian Ocean and that the final closure of the Paleo‐Asian Ocean occurred prior to the Early Triassic.

A tectonic transition from closure of the Paleo-Asian Οcean to subduction of the Paleo-Pacific Plate: Insights from early Mesozoic igneous rocks in eastern Jilin Province, NE China

The timing of tectonic transition from the Paleo-Asian oceanic regime to the Paleo-Pacific regime in the easternmost Central Asian Orogenic Belt remains controversial. To constrain the event, we present new zircon U-Pb ages, Hf isotopic compositions, and whole-rock major- and trace-element data for early Mesozoic igneous rocks from eastern Jilin Province, NE China. These new ages, together with existing age data, indicate that early Mesozoic magmatism in the Kaiyuan–Yanji area can be subdivided into four stages: Early Triassic (ca 250 Ma), Early–Middle Triassic (249–235 Ma), Late Triassic (229–208 Ma) and latest Triassic to Early Jurassic (207–175 Ma). The Early Triassic igneous rocks constitute a bimodal igneous rock association that formed in an extensional environment caused by break-off of the subducting slab during final closure of the Paleo-Asian Ocean. The Early–Middle Triassic magmatism is dominated by adakitic granitoids whose primary magmas were derived from partial melting of thickened mafic lower crust, suggesting that the North China Craton and Songliao–Xilinhot Block were still undergoing collisional orogenesis at 249–235 Ma. The Late Triassic igneous rocks comprise a typical bimodal association of rocks in an E–W-trending belt, formed in a post-orogenic extensional environment related to final closure of the Paleo-Asian Ocean. The latest Triassic to Early Jurassic igneous rocks have calc-alkaline characteristics, were emplaced as a NE–SW-trending belt parallel to the eastern margin of Eurasia, and formed in an active continental margin setting associated with initial subduction of the Paleo-Pacific Plate. We conclude that the tectonic transition between the Paleo-Asian and the Paleo-Pacific regimes took place during the latest Triassic.

Geodynamics of Late Paleozoic to Early Mesozoic Magmatism in South China: Insights from the Genesis of the Late Permian S-Type Granites in the Yunkai Massif

AbstractLate Paleozoic granites have rarely been reported in South China. We present the first detailed zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating an...

Opening and closure history of the Mudanjiang Ocean in the eastern Central Asian Orogenic Belt: Geochronological and geochemical constraints from early Mesozoic intrusive rocks

The tectonic evolution of the ancient Mudanjiang Ocean within the Central Asian Orogenic Belt (CAOB), is strongly debated. The ocean played an important role in the amalgamation of the Songnen and Jiamusi massifs; however, the timings of its opening and closure have remained ambiguous until now. In this study, we analyzed early Mesozoic intrusive rocks from the eastern Songnen and western Jiamusi massifs in the eastern CAOB. The new zircon UPb ages, Hf isotope data, and whole-rock major and trace element data are used to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon UPb dating indicates that early Mesozoic magmatism in the eastern Songnen Massif occurred in three stages: Early to Middle Triassic (ca. 250 Ma), Late Triassic (ca. 211 Ma), and Early Jurassic (ca. 190 Ma). The Triassic intrusive rocks typically consist of bimodal rock suites, which include gabbros, hornblende gabbros, and granitoids. The compositional information indicates an extensional environment that was probably related to the final closure of the Paleo-Asian Ocean. We integrated the results with observations from Triassic A-type granitoids and coeval sedimentary formations in the eastern Songnen Massif, as well as depositional ages of metasedimentary rocks from Heilongjiang Complex. We conclude that the opening of the Mudanjiang Ocean took place in the Early to Middle Triassic. The Early Jurassic intrusive rocks are bimodal and include olivine gabbros, hornblendites, hornblende gabbros, gabbro diorites, and granitoids. The bimodal rock suite indicates a back-arc style extensional environment. This setting formed in relation to westward subduction of the Paleo-Pacific plate beneath the Eurasia during the Early Jurassic. Following subduction, the closure of the Mudanjiang Ocean and subsequent amalgamation of the Songnen and Jiamusi massifs happened during the late Early Jurassic to Middle Jurassic. This sequence of events is further supported by ages of metamorphism and deformation acquired from the Heilongjiang Complex. Based on these observations, we conclude that the Mudanjiang Ocean existed between the Middle Triassic and Early Jurassic, making it rather short-lived.

Geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Late Permian–Early Triassic granitoids in Eastern Kunlun Orogen, Northwest China: petrogenesis and implications for geodynamic setting

ABSTRACT The Eastern Kunlun Orogenic Belt (EKOB) in Northwest China, is an ideal area for understanding the tectonic evolution of the Palaeo-Tethys Ocean. The EKOB is marked by widespread Palaeozoic to Early Mesozoic magmatism. However, the Late Permian–Early Mesozoic tectonic evolution of the Palaeo-Tethys Ocean tectonic regime is poorly understood. Here, we report new zircon LA-ICP-MS U–Pb dating and geochemical analyses of Late Permian–Early Triassic intrusive rocks in the Xiangyanggouxi and Dageledong areas in the EKOB, Northwest China, to discuss their petrogenesis and geodynamic setting and reconstruct the tectonic evolution of Palaeo-Tethys oceanic tectonic regime. Zircon U-Pb dating results indicate that these granitoids crystallized during ca. 256–248 Ma, i.e. the Late Permian to Early Triassic. These granitoids are mainly comprised of granodiorites and monzonitic granites that possess high concentrations of SiO2 (66.81–75.98 wt.%), K2O+Na2O (6.21–7.92 wt.%), and low contents of MgO, CaO, Cr, Ni, and Mg# (<0.44). They are classified as medium- to high-K calc-alkaline and metaluminous I-type granitoids. These granitoids are rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs) with arc affinity. In addition, combined with the εHf(t) values of −2.4 to +2.4 with two-stage model ages ranging from 1409 Ma to 1027 Ma, and the εNd(t) values from −3.73 to −2.95, it is suggested that the primary magma of these granitoids was derived from the partial melting of the Late Meso–Proterozoic mafic lower crust. These findings, combined with spatio-temporal distributions of regional magmatism, reveal that the Late Permian and Early Triassic granitoids formed in a continental magmatic arc setting related to the northward subduction of the Palaeo-Tethys Ocean plate beneath the EKOB-Qaidam terrane.

Early Mesozoic Magmatism Within the Tibetan Plateau: Implications for the Paleo‐Tethyan Tectonic Evolution and Continental Amalgamation

AbstractNumerous early Mesozoic magmatic rocks within the Tibetan Plateau ultimately have a geodynamic controll caused by the Paleo‐Tethyan tectonic evolution and the assembly of Pangea. Based on their temporal variation and the timing of closure (235 ± 10 Ma) of the Paleo‐Tethys Ocean, they can be broadly divided into two groups by zircon U‐Pb geochronology: an Early to Middle Triassic (252–235 Ma) group and a Late Triassic to Early Jurassic (ca. 235–190 Ma) group. Early magmatic rocks are mainly found in the middle northern Tibetan Plateau, whereas late magmatic rocks are more widespread in the whole Tibetan Plateau. Nd and Hf isotope mapping suggests that the eastern segment of the Qiangtang is underlain by old (late Archean to early Paleoproterozoic) basement, while Southern Lhasa is underlain by juvenile lower crust, which yields Neoproterozoic to early Paleozoic model ages. Early Mesozoic tectonomagmatism was likely caused by episodic southward opening of multi‐Tethys oceans, punctuated by episodes of subduction and collision. The Middle to Late Permian tectonomagmatic history of Paleo‐Tethys was dominated by continental arc‐related systems. During the Early to Middle Triassic, the tectonomagmatic history was associated with the ongoing closure of the Paleo‐Tethys Ocean and a tectonic transition from subduction to collision. Late Triassic tectonomagmatism formed in a post‐collisional setting following the closure of the Paleo‐Tethys Ocean and the ultimate assembly of Pangea. Early Mesozoic magmatism within the Tibetan Plateau thus provides crucial information on the history of oceanic‐continental evolution and helps constrain the tectonic transition from subduction/accretion through to collision/postcollision for East Paleo‐Tethys.

Early Mesozoic Alkaline Magmatism in the Western Framing of the Mongol–Okhotsk Belt: Age and Structural Position

A zonal igneous areal appeared at the western end of the Mongol–Okhotsk Belt in the Early Mesozoic. Its central part is comprised of the Khentei–Daurian giant batholith, which is framed on its north and west by riftlike structures with manifestations of bimodal and alkaline granite magmatism. Geochronological studies of the Choyr and Gal-Shara massifs of peralkaline granites, belonging to the North Gobi (southern) border of the batholith, are carried out. The ages of the former and latter are 215 and 213 Ma, respectively. It is shown that associations involving alkaline igneous rocks are fairly well developed in the North Gobi zone and controlled by NE-trending faults and grabens. Thus, it is established that the zonal magmatic area was formed under a specific regime. In its central part, which corresponds to the zone of the Mongol–Okhotsk suture, anatexis and batholith-formation processes took place, while the periphery was involved in rifting. The data obtained indicate the closure of the western part of the Mongol–Okhotsk trough before the formation of the zonal magmatic areal. The structure of the magmatic areal was determined by the mechanisms of the plume–lithosphere interaction between the collisional suture (anatectic melting zone) and its less tectonically strained framing (rifting zones).

Zircon U–Pb geochronology, major-trace elements and Sr–Nd isotope geochemistry of Mashhad granodiorites (NE Iran) and their mafic microgranular enclaves: evidence for magma mixing and mingling

ABSTRACT Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO2 = 67.5–69.4 wt%, high K2O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd(t) (−3 to −2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO2 (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd(t) (−2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.

Final Closure of the Paleo‐Asian Ocean and Onset of Subduction of Paleo‐Pacific Ocean: Constraints From Early Mesozoic Magmatism in Central Southern Jilin Province, NE China

AbstractWhen the Paleo‐Asian Ocean closed and the Paleo‐Pacific subduction started in NE China remains controversial. To constrain these key issues, we carried out zircon U‐Pb‐Hf isotopic and whole‐rock geochemical studies of Early Mesozoic igneous rocks in central southern Jilin Province, NE China. Early Mesozoic igneous rocks in region were formed at three stages: the middle Triassic (246 Ma), the late Triassic (219–220 Ma), and the early Jurassic (172–194 Ma). Middle Triassic trachyandesites in southern Jilin Province were solely generated through partial melting of ancient lower crust of the North China Craton (NCC), implying ongoing southward subduction of Paleo‐Asian Ocean Plate beneath the NCC at this time. Late Triassic trachyandesites in southern Jilin Province were derived from the magma mixing/mingling of the NCC lithospheric mantle and juvenile eastern Central Asian Orogenic Belt crustal sources. Late Triassic hornblende gabbros in central Jilin Province were derived from depleted lithospheric mantle previously metasomatized by subduction‐related fluids. Coeval adakitic granodiorites were likely produced through the partial melting of thickened juvenile lower crust. Spatially, these rocks occurred on both sides of the Solonker‐Xra‐Moron‐Changchun Suture Zone and record N–S directed extension accompanied by the final closure of the Paleo‐Asian Ocean. Early Jurassic andesites were produced through mixing/mingling between partial melts derived from eastern Central Asian Orogenic Belt lithospheric mantle and ancient NCC crust. In contrast, Early Jurassic felsic rocks, including A‐type granites, were derived from the partial melting of Neoproterozoic accreted lower crust, jointly implying an extensional setting as the result of the initial subduction from Paleo‐Pacific Plate at this time.

Geochronology and geochemistry of early Mesozoic magmatism in the northeastern North China Craton: Implications for tectonic evolution

This paper reports geochronological, geochemical, zircon U–Pb and Hf–O isotopic data of the Late Triassic and Early Jurassic intrusive rocks in the northeastern North China Craton (NCC), with the aim of reconstructing the tectonic evolution and constraining the spatial–temporal extent of multiple tectonic regimes during the early Mesozoic. Zircon U–Pb ages indicate that the early Mesozoic magmatism in the northeastern NCC can be subdivided into two stages: Late Triassic (221–219 Ma) and Early Jurassic (180–177 Ma). Late Triassic magmatism produced mainly granodiorite and monzogranite, which occur as a NE–SW-trending belt parallel to the Sulu–Jingji Belt. Geochemically, they are classified as high-K calc-alkaline and metaluminous to weakly peraluminous granitoids, and are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high-field-strength elements (HFSEs; e.g., Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating an affinity to adakite. Combined with their εHf(t) values (−17.9 to −3.2) and two-stage model ages (2387–1459 Ma), we conclude that the Late Triassic granitoid magma in the northeastern NCC was derived from partial melting of the thickened lower crust of the NCC and was related to deep subduction and collision between the NCC and the Yangtze Craton (YC). The Early Jurassic magmatism is composed mainly of monzogranites, which are classified as metaluminous, high-K calc-alkaline, and I-type granite. Their εHf(t) values and two-stage model ages are −16.7 to −4.2 and 2282–1491 Ma, respectively. Compared with the Late Triassic granitoids, the Early Jurassic granitoids have relatively high HREE contents, similar to calc-alkaline igneous rocks in an active continental margin setting. These Early Jurassic granitoids, together with the coeval calc-alkaline volcanic rocks and gabbro–diorite–granodiorite association in the northeastern (NE) Asian continental margin, comprise a NNE–SSW-trending belt parallel to the NE Asian continental margin, indicative of the onset of Paleo-Pacific Plate subduction beneath Eurasia.