Mesozoic Magmatic Rocks Research Articles

Evolution of deep crustal hot zones constrained by the diversity of Late Mesozoic magmatic rocks in SE China

The formation of widespread Late Mesozoic magmatic rocks (dominantly felsic rocks) in SE China needs a physical and chemical balance between crustal and mantle melting, which could be reconciled by complicated transcrustal magmatic systems in deep crustal hot zones with time. After the relative magmatic quiescence at ca. 210–180 Ma, magmatic activities were mainly developed in the interior of SE China with an areal distribution and subsequently concentrated in the present coastal area with an overall linear pattern after ca. 140 Ma except the Lower Yangtze region. The spatiotemporal change requires a unique crust-mantle geodynamic coupling with the evolution of deep crustal hot zones from intracontinental to continent marginal setting. Correspondingly, the magmatic rocks show a compositional change from bimodal distribution in Early and Middle Jurassic to unimodal and weakly bimodal pattern in Late Jurassic and Early Cretaceous. In general, most of the Late Mesozoic intermediate to felsic magmatic rocks show higher Mg# than typical rifting-related rocks at given SiO2, which requires the addition of water and possible mass transfer and mixing of underplated hydrous basaltic magma in the deep crustal hot zones. The occurrence of high-Sr/Y and La/Yb rocks and possibly thickened crust (ca. 40 km) before ca. 140 Ma, combined with the current thin crust thickness (ca. 31-33 km) as revealed by seismic data, indicates a widespread crustal thinning after ca. 140 Ma. The relatively thickened crust before ca. 140 Ma is beneficial for melting of underlying old crust that is composed of dominantly Neoproterozoic and Paleoproterozoic components, the formation of porphyritic Cu deposits, the subsequent magmatic differentiation and associated giant W, Sn, Nb, Ta and Li ore deposits. In contrast, the deep crustal hot zones progressively migrated to the present coastal area after ca. 140 Ma, driving the increasing amounts of intermediate rocks and intermediate-felsic volcanic rocks. In general, the diversity and compositional change of Late Mesozoic magmatic rocks and associated ore deposits in SE China may be controlled by the evolution of deep crustal hot zones in different tectonic settings in response to the subduction of Paleo-Pacific Plate.

Petrogenesis of Early Cretaceous granitoids and mafic microgranular enclaves from the giant Tonglushan Cu–Au–Fe skarn orefield, Eastern China

Late Mesozoic high-K calc-alkaline and high Sr/Y granitoids related to the skarn–porphyry–stratabound Cu–Au polymetallic deposits are widespread in the Middle–Lower Yangtze River Belt (MLYRB), Eastern China. However, the role of mantle-derived magmas in the petrogenesis of these granitoids remains controversial. In this study, we present new zircon U–Pb–Hf isotopes, and whole-rock geochemical and Sr–Nd isotopes data for quartz monzodiorites (porphyry), their hosting mafic microgranular enclaves (MMEs) and associated dykes from Tonglushan orefield in the MLYRB. Field relationships and geochronological data show that these intrusive rocks cluster around 141–139 Ma, indicating a short duration of magmatism, rather than a prolonged magmatism (~30 My) as previously thought. Crystallization age of the MMEs is identical to that of their quartz monzodiorite host. Petrographical and geochemical observations suggest that the MMEs represent mafic melts that injected into the quartz monzodioritic magmas, and magma mixing and (previously) fractional crystallization were involved in producing their compositional diversity. All the Tonglushan granitoids and MMEs show trace element geochemical and Sr–Nd–Hf isotope compositions similar to the coeval K-rich mafic rocks in the MLYRB. This indicates that these granitoids and MMEs were mainly sourced from the enriched subcontinental lithospheric mantle (SCLM) that metasomatized by slab derived fluids/melts, and had experienced significant fractional crystallization. The high oxygen fugacity and water content of the early Tonglushan quartz monzodiorite (porphyry) indicate melting of metasomatized SCLM and subsequently fractional crystallization and recharging/mixing in magmatic system have the potential to Cu–Au polymetallic mineralization. This study also highlights that the SCLM beneath the eastern Yangtze plate is one of the likely sources for Late Mesozoic magmatic rocks in the MLYRB.

Multi-element geochemical data mining: Implications for block boundaries and deposit distributions in South China

To explore the macroscopic characteristics of geological block boundaries, Mesozoic magmatic rocks, and various ore deposit distributions in South China, we processed large-scale stream sediment geochemical data covering the entire South China region. Two unsupervised data mining algorithms, cluster analysis and correlation analysis, were used to identify the geochemical distribution patterns. The combined geochemical patterns of nine major elements show two gradient lines that divide the South China region into three blocks. These gradient lines reveal the eastern and western boundaries of the intracontinental orogenic belts between the Yangtze and Cathaysia blocks. We suggest that the Nanning-Pingxiang segment is located on the southern end of the Yangtze-Cathaysia block boundary, and the Jishou-Kaili segment is a part of the boundary between the Yangtze block and the eastern orogenic belt. In the Mesozoic, the distribution of magmatic rocks migrated from the inland to the southeast coast. Meanwhile, the mantle-derived compatible elements were gradually depleted, whereas the crust-derived incompatible elements were gradually enriched. The migration and enrichment of geochemical elements provide material sources for the formation of various ore deposits. High-temperature metallogenic deposits related to Mesozoic magmatic activity are widely developed in eastern South China, while low-temperature metallogenic, sedimentary, and metamorphic deposits are more developed in inland South China. The regional geochemical patterns indicate the distributions of deposits to varying degrees.

Chemical Composition of the Mesozoic Alkaline Rocks of the Medvedev Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

The study shows chemical composition of the Mesozoic igneous rocks of the Medvedev massif. The massif is a part of the Central Aldan ore region. It is spatially located in the central part of the Nimnyr Block and is a part of the Leglier ore cluster. The massif is a multiphase structure of the most productive stage of the territory development – the Mesozoic tectonic-magmatic activation of the Aldan-Stanovoy shield. According to the materials of previous researchers, the rocks of the massif are represented by the augite hornblende and hornblende syenite porphyry. As a result of field work, we found that the massif has a zonal structure and is represented by three phases of intrusion. The rocks of the selected phases differ in their structural and compositional characteristics. The main objective of the work was to establish compositional characteristics by studying the chemical compositions of potential gold mineralization rocks of the Medvedev massif. In the course of studies and interpretation of chemical analysis of the massif Mesozoic magmatic rocks, we have identified the following characteristics. According to various classification data the following groups of rocks in Medvedev massif are identified – quartz syenites (Phase I), quartz monzodiorites of shoshonite series (Phase II) and monzonites of high-potassium calc-alkaline series (Phase III). The total alkali content (Na2O+K2O)>5% in them corresponds to alkaline rocks. All rocks are characterized by high Al2O3%>14 and low TiO2<1%. The potassium type of alkalinity Na2O/K2O<1 is typical for quartz syenites of the massif, while for quartz monzodiorites this indicator is intermediate, and in monzonites of the massif this parameter Na2O/K2O>1 corresponds to rocks of the alkaline potassium-sodium series. The massif rocks characteristic feature is the decrease of rocks alumina (al’) values from quartz syenites to quartz monzodiorites followed by the increase to monzonites, while the value of the rocks AG coefficient is significantly reduced in the series. Based on these facts, it was suggested that there are two sources of the mantle and mantle-crustal nature. Also, based on the study of the material composition of the Medvedev massif magmatic rocks and the similarity of the geological development with the Mesozoic magmatism of the Samolazovsky and Ryabinovy deposits, it was concluded that gold manifestations can be associated with such multiphase massifs within the Leglier ore cluster.

Refining the spatio-temporal distributions of Mesozoic granitoids and volcanic rocks in SE China

Combining published age data and geological maps, we provide most updated information about the spatio-temporal distributions of Mesozoic granitoids and volcanic rocks in SE China. The outcrop areas of the Mesozoic felsic magmatic rocks with an interval of 10 Myrs are estimated, allowing us to put forward by far the finest evolution pattern of the spatio-temporal distributions of the Mesozoic magmatic rocks, which were not available before. The Early Mesozoic (Indosinian) felsic magmatic rocks originally occurred sporadically in the South China block, and then showed a NE-trending distribution followed by an outward (in particularly northeastward) migration. Subsequently, there was a relative magmatic quiescence from 210 Ma to 180 Ma which was followed by a tectonic transition from Tethys regime to Paleo-Pacific regime at 180–170 Ma (probably very close to 170 Ma). The Late Mesozoic (Yanshanian) magmatic activities also showed an outward expansion and migration. The 170–150 Ma magmatic activities migrated from present inland to the northeastern and coastal areas of SE China, with outcrop area increasing gradually. While the 150–120 Ma felsic magmatic activities showed dominantly a northeastward migration, with outcrop area decreasing gradually. Till the period 120–80 Ma, the felsic magmatic activities migrated to the present southeast coastal areas with increasing intensity of volcanic activity. Based on the detailed spatio-temporal evolution, we suggest that the formation of Mesozoic granitoids and volcanic rocks in SE China was related to multiple stages of subduction and roll-back of oceanic slab on the southeast margin of the South China Block.

Exotic origin of Pingtan Island in the Pingtan-Dongshan Metamorphic Belt (SE China): Zircon U-Pb age and Hf isotope evidences

Cathaysia Block in South China had accreted tremendous amounts of Mesozoic magmatic rocks. Recent zircon UPb age results revealed existence of some Early Jurassic gneissic granites (200–190 Ma) in the Pingtan-Dongshan Metamorphic Belt (PDMB), coastal SE China which corresponds one stage of magmatic quiescence in the rest part of East Cathaysia. To better understand Mesozoic thermal evolution of the PDMB, UPb geochronology and Hf isotope composition of zircon separates (LA-ICPMS) and inclusions (NanoSIMS, age only) are examined for two granitic rocks and one meta-sandstone in Pingtan area (northern PDMB) and three granites in Matsu Islands further north. For Pingtan rocks, ages of discrete zircons give two Early Jurassic ages (196 Ma and 173 Ma) and prossess in dominance positive ɛHf(t) values (−0.2 to 7.5). On the other hand, zircon inclusions within rock-forming minerals yield a wide age span from Permo-Triassic to Early Cretaceous in six clusters, namely 270–206 Ma, 191–170 Ma, 168–151 Ma, 146–132 Ma, 126–119 Ma and 109–100 Ma that basically coincide the major Mesozoic thermal episodes in Cathaysia except the second cluster. For Matsu granites, discrete zircons exhibit Late Jurassic (162–158 Ma) and Early Cretaceous (137 Ma) ages with ɛHf(t) values of −9.1 to −1.6 and −4.9 to −0.8, respectively. Nevertheless, zircon inclusions show ages corresponding to Pingtan samples in the last four clusters. With reference to Mesozoic magmatic rock units in the neighboring Southeast China Magmatic Belt (SCMB), our data suggest irrelevance of Pingtan samples to the Cathaysia and linkage of off-shore Matsu granites to the PDMB. Noting that there are similarities of zircon ages and ɛHf(t) values between the granite-schist associations in Tananao Metamorphic Belt (TMB) of eastern Taiwan and Pingtan island of northern PDMB, these Early Jurassic granites were probably originated from a same source. Three consistent Early Cretaceous age clusters of zircon inclusions over the studied samples match two well-known magma prevailing episodes in the coastal SE China, with one tectonic episode (128–119 Ma) in the PDMB that is often invisible from the zircon separates remains unexplainable. In response to a plausible tectonic evolution, we support the earlier proposition for a now-concealed microcontinent that had impinged on Cathaysia around 125 Ma, but further suggest that it left Pingtan Island behind as a remnant fragment when drifted away at later time.

Early Cretaceous Xiuyan adakitic granitoids in the Liaodong Peninsula, eastern China: petrogenesis and implications for lithospheric thinning of the North China Craton

Mesozoic magmatic rocks are widely distributed in the North China Craton (NCC) and are crucial to understanding the timing, location, and geodynamic mechanisms of lithospheric thinning of the NCC. In this study, we report geochronological, petrogeochemical, and Lu–Hf isotopic data for adakitic granitoids from different parts of Xiuyan pluton in the Liaodong Peninsula, aiming to constrain their magma sources, petrogenesis, and tectonic implications. The adakites are metaluminous to weakly peraluminous and are classified as high-K calc-alkaline I-type granite with Early Cretaceous zircon U–Pb ages of 129–126 Ma. They exhibit adakite-like geochemical characteristics, such as high Sr content and low Yb and Y contents, coupled with high Sr/Y and no pronounced Eu anomalies. They are enriched in Rb, U, and light rare-earth elements and are depleted in Ta, Nb, P, and Ti. The adakites from the eastern part of the pluton have low εHf(t) values (–8.5 to –4.0) with old TDM2ages (1.57–1.31 Ga), indicating they were derived from the lower crust containing juvenile mantle-derived materials. In contrast, adakites from the northern part of the pluton have lower εHf(t) values (–19.7 to –16.6) with older TDM2ages (2.21–2.03 Ga), indicating that they were derived mainly from an ancient crust. Our results show that both adakitic magmas were derived from partial melting of delaminated lower crust. Their relatively high MgO and Ni contents and Mg# values indicate that the melts interacted with mantle peridotites. The lower crust delamination beneath the Liaodong Peninsula resulted from paleo-Pacific plate subduction during the Early Cretaceous, which resulted in thinning of Mesozoic crust in the Xiuyan area.

Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.

Late Early-Cretaceous Magma Mixing in the Langqi Island, Fujian Province, China: Evidences from Petrology, Geochemistry and Zircon Geochronology

The mafic enclaves from Mesozoic intermediate-acid magmatic rocks, widely developed along Fujian coast, are considered to be the results of large-scale crust-mantle interaction and magma mixing. In this paper, petrography, mineralogy, and geochemistry of granites and mafic microgranular enclaves (MMEs) in Langqi Island are studied to provide new information for tracing crust-mantle interaction. The zircon U-Pb dating results show that the Langqi rocks were formed at ~101 Ma, which are metaluminous, enriched in silica and high-K calc-alkaline I-type granites. The enclaves have a typical magmatic structure, which is characterized by magma mixing between high-temperature basic magma and low-temperature acidic magma through injecting. The enclaves and host granites show a tendency to mixed major and trace elements, displaying a clear-cut contact relationship, which is indicative of coeval magmatism. The genesis of Langqi rocks is related to the extensional setting caused by the subduction of Paleo-Pacific Plate, and they are the results of mixing of subduction-related metasomatized mantle-derived mafic and induced crustal-melted granitic magma originating from partial melting of the crustal material.

Late paleozoic to early mesozoic paleo-tethys tectonic evolution of central NE Tibetan Plateau: Insights from the Zhiduo mafic-ultramafic complex

Recent research has identified abundant Early Permian to Late Triassic ultramafic-mafic rocks along the west Jinsha to Garzê-Litang suture, central NE Tibetan Plateau. However, the petrogenesis and tectonic setting of these rocks are hotly debated. This paper presents a combination of zircon UPb ages, major and trace element concentrations, and SrNd isotopic data for mafic rocks from the Zhiduo Complex in the west Jinsha suture. Three geochemical subgroups are recognized among these rocks; metagabbros showing arc-type affinity (Group 1; 286–268 Ma); isotropic gabbros with an oceanic-island basalt-like signature (Group 2; 252 ± 2 Ma); and pillow/massive basalts (Group 3; ca. 235 Ma) having back-arc basin basalt affinity. Group 1 rocks exhibit moderate-high MgO (5.98–8.07 wt%) and Al2O3 (13.75–17.40 wt%) contents and are characterized by subduction-type trace element patterns and high initial 87Sr/86Sr ratios (0.71155–0.71261) and low εNd(t) values (−7.0 to −6.0), indicating that they originated from a spinel lithospheric mantle metasomatized by slab-derived fluids. Group 2 rocks have high contents of TiO2 (2.28–4.96 wt%), total Fe2O3 (11.67–14.80 wt%), Nb (18.3–22.8 ppm), and Ta (1.52–1.81 ppm) and more radiogenic Nd isotopes [εNd(t) = +0.3 to +0.9]. These data show that they were derived from a spinel + garnet mantle source enriched by plume-type components. Group 3 rocks possess high MgO (7.29–9.29 wt%), Cr (427–488 ppm), and Ni (215–238 ppm) contents but low Nb (5.00–5.60 ppm) and Ta (0.36–0.84 ppm), coupled with low Sr (initial 87Sr/86Sr = 0.70523–0.70567) and high Nd isotopes [εNd(t) = +3.1 to +3.5]. These values suggest that they were sourced from a garnet + spinel mantle affected by slight plume contamination. Combined with previous structural and geochemical studies, the ultramafic-mafic rocks of the Zhiduo Complex and arc magmatic rocks from nearby regions indicate an integrated Early Permian to Late Triassic oceanic basin–arc–back-arc system developed in the central NE Tibetan Plateau. Thus, the west Jinsha to Garzê-Litang represents a mature Paleo-Tethys Ocean rather than small back-arc basins. Southward subduction and associated back-arc spreading led to the genesis of widespread late Paleozoic to early Mesozoic magmatic rocks in the hinterland and along the northern margin of the North Qiangtang Block.

Petrogenesis and tectonic implications of the Mesozoic granitoid intrusions in the eastern Liaoning Peninsula, NE China

Mesozoic magmatic rocks are widely distributed in the eastern region of the North China Craton (NCC), providing important information for understanding the Mesozoic tectonic regimes in this area. This paper reports newly obtained geological, geochemical, and zircon U-Pb-Lu-Hf isotope data for the Xiaoheishan-Daheishan intrusion (XDI) and the Yanghezhen-Dayingzhen intrusion (YDI) newly identified from Precambrian metamorphic basement range in the eastern Liaoning Peninsula, and provides a case study demonstrating that the petrogenesis of granitoid rocks was closely related to the tectonic regimes. The Middle-Late Jurassic XDI granitoids commonly have the adakite-like geochemical characteristics and their primary magma was derived from the partial melting of mafic rocks and Neoarchean tonalites in the thickened lower crust. The Early Cretaceous YDI granitoid rocks generally show diversified geochemical characteristics and their primary magma was generated by the partial melting of Paleoproterozoic metagreywackes and Neoarchean tonalites in the lower crust with minor addition of mantle-derived magma. These differences indicate that the granitoid magmatism in the eastern Liaoning Peninsula has occurred a significant variation during the Jurassic to Cretaceous. The variation of late Mesozoic granitoid magmatism may be responses to the regional tectonic transition from compression to extension in the eastern Liaoning Peninsula, which is induced by the geodynamic regime of the Paleo-Pacific Plate from low-angle subduction to slab rollback.

Geochronology and geochemistry of the Late Mesozoic magmatic rocks in the Chizhou Area, Lower Yangtze River Metallogenic Belt: Constraints on petrogenesis and tectonic setting

Late Mesozoic magmatism and its associated polymetallic mineralization are widespread in the Chizhou area, forming one of the ore clusters in the Lower Yangtze River Metallogenic Belt (LYRMB). The mineralization in the Chizhou area, however, is not as same as the other ore clusters (e.g., Tongling, Edong, and Jiurui) in age, source, and mechanism. In this study, we used zircon UPb dating, LuHf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes to investigate six ore-bearing intrusions in the Chizhou area. These intrusions comprise high-K calc-alkaline series pyroxene diorite, quartz diorite (porphyry), and granodiorite (porphyry). LA-ICP-MS zircon UPb dating results indicate that these intrusions formed between 150 and 141 Ma. The mafic intrusion, the Xiaodingchong pyroxene diorite, has low SiO2 and MgO contents, arc-like trace element characteristics, as well as enriched Sr-Nd-Hf isotopic compositions and high radiogenic Pb isotopes, indicating that it was derived from an enriched lithospheric mantle source. The acid intrusion, the Pailou granite, has high SiO2, enriched Sr-Nd-Hf isotopic compositions, continental crust-like geochemical characteristics, and adakitic affinities, such as high Sr, low Y, and low Yb, suggesting that the Pailou granite was derived from partial melting of the Meso-Neoproterozoic thickened accretionary arc crust. The remaining intermediate-acid ore-bearing intrusions were produced by magma mixing of these two end members. The metallogenic deposits in the Chizhou area were mainly controlled by two different magma sources: an enriched mantle and the accreted Meso-Neoproterozoic crust. Furthermore, the low Ti-in-zircon temperatures and the variable amounts of inherited zircons indicate that the Late Mesozoic Chizhou ore-bearing intrusions were formed in hydrous conditions, which is consistent with the tectonic evolution of the low-angle paleo-Pacific plate subduction with slab foundering and roll-back.

Zircon U–Pb geochronology and Sr–Nd–Hf–O isotope geochemistry of Late Jurassic granodiorites in the southern Qiangtang block, Tibet: Remelting of ancient mafic lower crust in an arc setting?

A Late Jurassic–Late Cretaceous magmatic belt extends for >800 km along the southern margin of the southern Qiangtang Block (SQB) in central Tibet. However, the Mesozoic tectonic setting of these igneous rocks remains uncertain. Here, we report new in situ zircon U–Pb ages and Hf–O isotopes as well as whole-rock major- and trace-element compositions and Sr–Nd isotopes for the Jiacuo granodiorites and dikes in the SQB. Zircon laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating indicates that these rocks were emplaced during the Late Jurassic (ca. 153–151 Ma). All of the rocks have high SiO2 (64.7–68.4 wt%) and K2O (4.2–5.2 wt%) contents and are characterized by enrichment in light rare-earth elements ((La/Yb)N = 16–19) and large-ion lithophile elements, and depletion in high-field-strength elements. The granodiorites and dikes have variable zircon Hf–O isotope values (εHf(t) = −15.7 to −10.1, δ18O = +5.32‰ to +7.88‰), but uniform and high whole-rock initial 87Sr/86Sr isotopic (0.7079 to 0.7080) and low εNd(t) (−7.9 to −7.6) values. We suggest that these granodiorites were originated by partial melting of ancient mafic lower crust of the SQB. Combining our new data with the temporal and spatial distributions of Mesozoic magmatic rocks in the SQB, we propose that Late Jurassic magmatism in the SQB was resulted from northward subduction of the Bangong–Nujiang Tethyan oceanic slab during this period.

Petrogenesis of late Mesozoic granitoids from the southern segment of the Tan–Lu Fault, eastern China: implications for the tectonic affinity of the Zhangbaling Uplift

ABSTRACT Late Mesozoic magmatic rocks of the Zhangbaling Uplift (ZBU) in eastern China, which comprises the northern Zhangbaling area and the southern Feidong area, can provide insights into the deep crustal processes and tectonic affinity of the massif. These rocks are dominated by granitoids with formation ages of 132–120 Ma and a peak at 129 Ma. The Shankouling dyke (120.4 ± 1.1 Ma) and the Zhuyuanzhang quartz monzodiorite (129.0 ± 2.2 Ma) crop out sporadically in the Feidong area. These rocks show adakitic signatures of high Al2O3 and Sr contents, high Sr/Y, (La/Yb)N, and (Dy/Yb)N ratios, and low Y and YbN contents. They have enriched (87Sr/86Sr)i ratios, negative εNd(t) and εHf(t) values, and unradiogenic Pb isotopic compositions that resemble those of coeval high Sr/Y granitoids in the Dabie Orogen. These ZBU rocks can be divided into two groups: low-Mg granitoids with more enriched Sr–Nd isotopic signatures and low Mg# values (<50), and high-Mg granitoids with weakly enriched Sr–Nd isotopic signatures and high Mg# values (mostly >50). Combining their geochemical features with previous results of experimental melts, we conclude that the low-Mg granitoids were derived from fluid-absent partial melting of in situ thickened lower continental crust, resulted from Triassic continental subduction and collision in this area, whereas the high-Mg granitoids were derived from delaminated eclogitic lower crust followed by interaction with mantle peridotites during magma ascent. Isotopic compositions show that these granitoids could have been derived from ancient crust of the Yangtze Block via re-melting of Neoarchean and Paleoproterozoic lower continental crust. The great depth of the Tan–Lu Fault Zone enabled delamination of the thickened eclogitic mountain root beneath the ZBU, which resulted in the formation and emplacement of the high-Mg rocks, followed by uplift of the Zhangbaling massif associated with rollback of the subducting Paleo-Pacific (Izanagi) Plate.

Tectonic mechanism and evolution of eastern China during the Early Cretaceous: a view from magmatism in the middle to Southern Tan-Lu fault zone

ABSTRACT Mesozoic magmatic rocks are widely distributed in and around the Tan-Lu fault zone (TLF) and show great significance related to the destruction processes of the lithosphere and the evolution of magma sources. This study aims to decipher the petrogenesis of the Early Cretaceous magmatism along the TLF, in order to address the tectonic mechanism for the North China Craton (NCC) lithospheric destruction. This contribution combines investigations of geochemistry, SHRIMP zircon U-Pb ages and O-Hf isotopes of the intermediate to felsic rocks from middle-southern TLF with previous geochronological and geochemical characteristics of other rocks from the study area. Four different rocks are identified: high Mg# adakitic rocks (HMA), low Mg# adakitic rocks (LMA), normal intermediate-felsic rocks (NIF) and A-type granitoids. The emplacement time of the LMA rocks in the Dabie area (143 Ma) were earlier than in the Sulu area (130 Ma). The HMA rocks were formed at the same time (~135 Ma) in the above two areas but the magmatism lasted longer in Jiaodong area, which was near the subduction zone of the Paleo-pacific plate. The HMA rocks are concluded to be derived from the melting of a delaminated thickened lower crust and including reaction with overlying mantle. After 135 Ma, the magmatism in this area exhibited an eastward-younging trend, consistent with plate retreat in the Early Cretaceous. Plutons that were emplaced during 135–125 Ma have higher Ba/Th, Ba/La and Ce/Pb, suggesting magma source for these rocks was metasomatized by hydrous fluids from the subducted plate. At ~125 Ma, A-type granitoids began to be formed. The TLF acted as a favourable channel for transporting mantle material and fluids which made the lithosphere in this area more likely to be destroyed and thinned.

Late Mesozoic magmatism in the Jiaodong Peninsula, East China: Implications for crust–mantle interactions and lithospheric thinning of the eastern North China Craton

A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula, East China, containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions, was chosen to investigate the lithospheric evolution of the eastern North China Craton (NCC). Zircon U–Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic (~145 ​Ma) and late Early Cretaceous (112–107 ​Ma), respectively; high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at ~139 ​Ma and ~118 ​Ma, and 125–145 ​Ma and 115–120 ​Ma, respectively. The geochemical data, including whole-rock major and trace element compositions and Sr–Nd–Pb isotopes, imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab. Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC, while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts. Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous. We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic. Slab rollback of the plate from ~160 ​Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.

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.

Petrogenesis of Jurassic Xietongmen intrusive rocks at the southern margin of the Lhasa terrane: implications for intra-oceanic arc evolution

The Gangdese batholith, Tibet, records the opening and closing of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. The Mesozoic magmatic rocks play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. This study focuses on Jurassic intrusive rocks in the Xietongmen area of the southern margin of the Lhasa terrane adjacent to the Yarlung–Tsangpo suture. Zircon U–Pb dating yielded Middle Jurassic dates for ca 170 Ma hornblende gabbro and ca 173 Ma granodiorite intrusions. All of the samples are medium- to high-K calc-alkaline, and the majority are metaluminous and enriched in the large ion lithophile elements and depleted in the high-field-strength elements. This indicates the magma was generated in a subduction-related tectonic setting. The intrusive rocks have high and positive εHf(t) values (hornblende gabbro: 13.3–18.7; granodiorite: 14.2–17.6) that yield Hf model ages younger than 312 Ma. These new data, combined with the results of previous research, suggest that the Jurassic igneous rocks were derived from a metasomatised region of an asthenospheric mantle wedge. Extremely depleted Sr–Nd–Pb–Hf isotope compositions are similar to the Yarlung ophiolite and igneous rocks within other intra-oceanic island arcs. Together with the existence of sandstone that is identified as the product of the oceanic island arc environment, we suggest formation in an intra-oceanic island arc.The new zircon U–Pb dating has yielded Middle Jurassic ages for the ca 170 Ma hornblende gabbro and ca 173 Ma granodiorite phases of the Xietongmen intrusion.Jurassic igneous rocks formed from a metasomatised asthenospheric mantle wedge by northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane.Late Triassic–Jurassic igneous rocks, which are characterised by highly depleted isotopic compositions within the Southern Lhasa sub-terrane, record residual intra-oceanic island arcs in the eastern Tethyan belt.

Provenance of the East Guangdong Basin and Yong’an Basin in southeast China: Response to the Mesozoic tectonic regime transformation

The East Guangdong Basin and Yong’an Basin, located in the south-eastern South China Block, were influenced by both the Tethyan tectonic domain and the Paleo-Pacific tectonic domain, and the records of magmatic activity, tectonic deformation, and basin sedimentation in these areas preserve important geological information about the Mesozoic tectonic setting of the South China Block. In this study, detrital zircon U-Pb dating was conducted on the Late Triassic to Middle Jurassic sequences in the East Guangdong Basin and Yong’an Basin to study the sediment provenances, constrain the timing of tectonic regime transformation and discuss the mechanism driving this transformation in South China. The results show that the detrital zircon U-Pb ages from Late Triassic-Early Jurassic samples from the East Guangdong Basin form six major age groups of 2300–2600 Ma, 1800–2000 Ma, 650–1050 Ma, 410–480 Ma, 350–250 Ma, and 250–210 Ma, while the samples from the Yong’an Basin form two groups of 650–1050 Ma and 410–480 Ma. The detrital zircon U-Pb ages from the Middle Jurassic samples from the two basins mainly form three age groups of 1650–2100 Ma, 250–200 Ma, and 180–160 Ma. Provenance analysis suggests that the Late Triassic and Early Jurassic detritus in the East Guangdong Basin was derived from the south-western or southern margin of the Yangtze Block, Hainan Island, the Yunkai Massif, and the Nanling tectonic belt in the South China Block and that the detritus in the Yong’an Basin was sourced from the southern Wuyi Massif to the north of the basin. In contrast, the Middle Jurassic detrital zircons were mainly sourced from uplifted regions in north-eastern Fujian and south-western Zhejiang and were derived from early Mesozoic magmatic and metamorphic rocks, which are widely distributed in the South China Block. The variations in the main age groups of the detrital zircons in the study areas and the corresponding provenance areas combined with tectonic information, paleo-current data, and magmatic activities indicate that the tectonic regime of the South China Block experienced a transition from the Tethyan tectonic domain to the Paleo-Pacific tectonic domain in the early Mesozoic and the transition occurred between the Early Jurassic and completed at 165 ± 5 Ma. The driving force for the transformation was the westward subduction of the Paleo-Pacific Plate beneath the South China Block, resulting in Middle-Late Jurassic magmatism and extrusion and uplift of the eastern part of the South China Block.

Subduction and rollback of the paleo-Pacific plate during the Early Cretaceous: Insights from the Jiangmiao gabbroic intrusion in the Lower Yangtze River Belt, central-eastern China

The dynamics of Mesozoic tectonism recorded by the Lower Yangtze River Belt (LYRB), located in central-eastern China, are controversial. Between 148 and 123 Ma, three major and continuous stages of magmatism occurred in the LYRB: 1) high-K calc-alkaline intrusions, 2) andesitic shoshonitic volcanism, and 3) A-type granitic intrusions. The Jiangmiao gabbroic intrusion, which is located at the eastern end of the LYRB, is composed of gabbro and diorite and was emplaced at ~124 Ma (Dong et al., 2017; Sun et al., 2017a), simultaneous with the A-type granites of the third stage of Mesozoic magmatism in the LYRB. Petrography and mineral chemistry indicate that the Jiangmiao intrusion was formed by the intense crystal fractionation of an oxidized (ΔNNO ca. +1.0) and hydrous (3.5–4.5 wt% H2O) basaltic magma that produced the evolved dioritic magma and gabbroic cumulate. The accumulation of magnetite, olivine, clinopyroxene, and plagioclase resulted in abnormally high Fe2O3 (12.44–16.30 wt%) and low SiO2 (40.75–45.88 wt%) contents in the gabbro, as well as the enrichment of TiO2, Sr, Cr, Ni, Co, and V in the gabbro compared with the diorite. Intense crystal fractionation coupled with the fluid transport led to the enrichment of highly fluid-mobile elements (Rb, Th, U, and Pb) in the evolved diorite but depletion of these elements in the gabbro. The mineral chemistry of clinopyroxene and whole-rock Sr−Nd−Pb isotopic compositions indicate that the primitive basaltic magma originated from the partial melting of an enriched lithospheric mantle source in a subduction environment, but more asthenosphere mantle components were incorporated into the magma source compared with the earlier Mesozoic magmatic rocks in the LYRB. The coevality of the Jiangmiao gabbroic intrusion and the A-type granites supports the tectonic model of Mesozoic rollback of the westward-subducting paleo-Pacific plate. The Jiangmiao gabbroic intrusion likely represents the edge of the back-rolling slab at ~124 Ma, whereas the simultaneous A-type granites in the middle of the LYRB were formed as a result of intraplate extension generated by rollback.