Changqian Ma

A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China

Three groups of Mesozoic shoshonitic or high-K calc-alkaline intrusive rocks are identified in Dabieshan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic zone, east-central China and they are related to: (I) slab breakoff; (II) magmatic underplating; and (III) doming. Group-I, the slab breakoff-type, consists of late Triassic (∼210 Ma) mafic monzodiorites. It has moderate to high Sr, and low Rb and Ba abundances, and moderate light rare earth element (LREE)/heavy rare earth element (HREE) and K/Rb ratios. Group-II, the underplating-type, consists mainly of middle Jurassic–early Cretaceous (160–120 Ma) hornblende quartz monzonitic, biotite monzogranitic, and syenogranitic plutons, characterized by relatively high LREE/HREE and K/Rb ratios, and by a large range in concentration of Sr and Ba, coupled with much smaller range in Rb. Group-III, the doming-type, is represented by Cretaceous (125–95 Ma) granitic stocks and granitic porphyry. Compared with group-II, it has high Rb, Y and HREE abundances, low Sr and Ba abundances and low LREE/HREE and K/Rb ratios. All groups have similar Nd and Sr isotopic compositions. Among the three groups, post-collisional granitoid magmatism (group-II) with ages of 160 to 120 Ma, post-dating the HP and UHP metamorphism at 245 to 220 Ma, is the most abundant in the Dabieshan area. The post-collisional granitoid plutons were initially emplaced at different levels ranging from mid-crust to near-surface. This study shows that the whole-rock chemistry of the granitoids vary systematically with crystallization pressures. For example, K2O, normative Or, Rb and Zr show the strongest increase with decreasing pressure, whereas Ba, Nb, Nd, Yb, MnO, and normative An decrease upward in the Dabie Block. It is suggested that ascent of differentiated, buoyant liquids, combined with fractionation paired with assimilation (AFC), is responsible for the vertical variation. Geological, geochemical and petrological data indicate that group-I could have been generated by partial melting of enriched subcontinental lithosphere mantle due to slab breakoff. Group-II rocks could have been produced mainly from crustal assimilation/melting and fractional crystallization of mantle-derived magmas, whereas group-III magma could have derived from anatexis of the Dabie complex and was highly evolved in a hot doming setting. The late Triassic-early Jurassic slab breakoff may be responsible for the exhumation of UHP rocks through the mantle. The voluminous granitic emplacement together with an episode of rapid denudation suggests that magmatic underplating and inflation could have played a role in the Middle Jurassic–Early Cretaceous rapid exhumation of Dabieshan.

Petrogenesis and tectonic significance of the Late Permian–Middle Triassic calc-alkaline granites in the Balong region, eastern Kunlun Orogen, China

Numerous calc-alkaline granitoid intrusions in the eastern Kunlun Orogen provide a valuable opportunity to constrain the evolution of the orogen. The age and genesis of these intrusions, however, remain poorly understood. The granitoid intrusions near the Balong region, eastern Kunlun Orogen, consist of granodiorite, diorite and syenogranite. The granodiorite contains crystallized segregations, abundant mafic microgranular enclaves (MMEs) and small quartz diorite stocks. In situ zircon U–Pb dating reveals that the granodiorites and quartz diorites were emplaced between 263 and 241 Ma, whereas the syenogranite was produced at c . 231 Ma. The granodiorite and quartz diorite have a calc-alkaline affinity and are metaluminous and Na-rich, with slightly enriched Sr–Nd isotope compositions. The granodiorite is characterized by fractionated REE patterns, whereas the quartz diorite displays a relatively flat REE pattern. The MMEs are consistent with the granodiorite in terms of incompatible elements and Sr–Nd isotope composition. Compared to the granodiorite and diorite, the syenogranite has higher SiO 2 , K, Rb, Th and Sr contents and a lower Rb/Sr ratio. The results presented here, when combined with regional geological data, indicate that the granodiorite and quartz diorite were derived from dehydration melting of mafic lower crustal rocks during the N-directed subduction of the Anyemaqen ocean lithosphere in Late Permian–Middle Triassic times, whereas the syenogranite was produced at a higher crustal level in a syn-collisional setting compared to the granodiorite.

Discovery of the Indosinian aluminum A-type granite in Zhejiang Province and its geological significance

Zircon LA-ICP-MS U-Pb dating of the Jingju syenogranites in the southwestern part of Zhejiang Province shows that these rocks were crystallized in the Late Triassic at 215±2 Ma, rather than in the Cretaceous as previously proposed. The Jingju syenogranites are characterized by relatively high K2O and FeO*, and low MgO. They have high concentrations of large ion lithophile elements (LIL) and LREE, such as K, Th, La, and Ce, but are depleted in high field strength elements (HFSE) such as Nb, Ta, and Ti. Their 104Ga/Al ratios and (Zr+Nb+Ce+Y) contents are also high. These characteristics are similar to those of A-type granites. The syenogranites have high ISr (0.7179–0.7203), low ɛNd(t) (from −14.2 to −13.2), and relatively old T2DM ages, similar to those of the ancient metamorphic basement in the Cathaysia Block. It is suggested that the Jingju syenogranites were formed by partial melting of the Cathaysia basement rocks during tectonic extension. This identification of Indosinian A-type granite in Jingju has significant implications for understanding the early Mesozoic tectonic evolution of South China.

Reworking of old continental lithosphere: an important crustal evolution mechanism in orogenic belts, as evidenced by Triassic I-type granitoids in the East Kunlun orogen, Northern Tibetan Plateau

There is a strong genetic relationship between the petrogenesis of I-type granitoids and the evolution of continental crust in orogenic belts. This study of I-type granitoids in the East Kunlun orogen, Northern Tibetan Plateau, shows that reworking of old continental lithosphere is an important key to this genetic relationship. The East Kunlun has numerous Triassic granitic plutons that are related to subduction of the Palaeo-Tethyan ocean and terrane collision in the early Mesozoic. U–Pb analysis of zircons from these Triassic granitoids indicates that the granitic magmatism lasted from 249 to 223 Ma. Based on elemental and isotopic compositions and their petrogenesis, the magmatism can be divided into three groups. (1) Group 1 consists of quartz diorites and granodiorites (241–249 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit typical subduction-related chemical characteristics. They were derived from lower crust mainly composed of Precambrian metabasaltic basement rocks with different degrees of involvement of mantle material. (2) Group 2 consists of granitic porphyries and syenogranites (231–238 Ma), which are high Rb/Sr, metaluminous to weakly peraluminous high-K alkali-calcic I-type granitoids, showing characteristics of typical pure crustal-derived granitoids. They were derived from partial melting of a Mesoproterozoic metagreywacke source in the lower crust. (3) Group 3 consists of porphyry granodiorites (c. 223 Ma), which are metaluminous high-K calc-alkaline I-type granitoids and exhibit the typical geochemical characteristics of adakites (e.g. high La/Yb and Sr/Y ratios and low Y and Yb contents). Their high K2O and low Mg# with evolved Sr–Nd–Hf isotopic compositions indicate that they were most probably derived from thickened mafic lower continental crust, which underwent partial melting induced by underplated hot mafic magma. Combining the present work with previous studies, we propose that the subduction of the Palaeo-Tethyan ocean lasted from 278 to 241 Ma, and the collision between the Bayan Har terrane and the East Kunlun occurred at 231–238 Ma, whereas the group 3 granitoids most probably formed in a post-collisional setting. Overall, all the studied I-type granitoids were derived from partial melting of old continental lower crust with minor addition of lithospheric mantle material; thus reworking of old continental lithosphere is an important mechanism for the evolution of orogenic crust. Supplementary materials: Analytical methods, zircon U–Pb data, geochemical data, and Sr–Nd–Hf isotope data for the granitoids are available at www.geolsoc.org.uk/SUP18758.

Petrogenesis and tectonic implications of A-type granites in the Dabie orogenic belt, China: geochronological and geochemical constraints

The Dabie orogenic belt is characterized by the presence of large volumes of intrusive and volcanic rocks that formed in Late Mesozoic times. Most of the intrusive bodies are I-type granites but it is still unclear whether there are contemporary A-type granites. Here, we report the first unambiguous discovery of A-type granite from Baiyashan in the North Dabie tectonic belt. The crystallization age of the body has been fixed as 120.4 ± 1.2 Ma using U–Pb analysis of zircons by LA-ICPMS. The Baiyashan granite is enriched in Si, K, Na, Rb and REE, has elevated FeO tot /(FeO tot + MgO) and Ga/Al ratios, and is depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. The REE composition shows highly fractionated patterns with (La/Yb) N = 6.95–16.68 and Eu*/Eu = 0.33–0.59. Its crystallization age, field relationships, petrographic and geochemical data show beyond doubt that the Baiyashan granite is an Early Cretaceous A-type granite. Sr–Nd isotope systematics are characterized by a high I Sr of 0.708–0.714 and a low ɛ Nd of −7.5 to −19.4, with T DM2 = 1.5–2.5 Ga, and these data indicate that the magmas were dominantly sourced from partial melting of middle to lower crustal intermediate-felsic igneous rocks and mingling with mafic to intermediate magmas, during rift-related magmatism associated with subduction of the Palaeo-Pacific Plate beneath Eastern China in Early Cretaceous times.

Geochronology and Petrogenesis of Triassic High-K Calc-Alkaline Granodiorites in the East Kunlun Orogen, West China: Juvenile Lower Crustal Melting during Post-Collisional Extension

This study reports zircon U-Pb and Hf isotopes and whole-rock elemental data for granodiorites from the East Kunlun orogen. The zircon U-Pb dating defines their crystallization age of 235 Ma. The rocks are characterized by high-K calc-alkaline, magnesian and metaluminous with (K2O+Na2O)=6.38 wt.%–7.01 wt.%, Mg#=42–50 [Mg#=100×molar Mg/(Mg+FeOT)], A/CNK=0.92–0.98, coupled with high εHf(t) values from -0.65 to -1.80. The rocks were derived from partial melting of a juvenile mafic crustal source within normal crust thickness. The juvenile lower crust was generated by mixing lithospheric mantle-derived melt (55%–60%) and supracrustal melt (40%–45%) during the seafloor subduction. Together with available data from the East Kunlun, it is proposed that the studied Middle Triassic granodiorites were formed in post-collisional extension setting, in which melting of the juvenile lower crust in response to the basaltic magma underplating resulted in the production of high-K granodioritic melts.

Genesis of highly fractionated I-type granites from Fengshun complex: Implications to tectonic evolutions of South China

The South China Block is characterized by the large-scale emplacement of felsic magmas and giant ore deposits during the Yanshanian. We present zircon Hf isotopic compositions, whole-rock major and trace element compositions of the Fengshun complex, located in eastern Guangdong Province, South China. The Fengshun complex is a multi-stage magmatic intrusion. It is composed of two main units, i.e., the Mantoushan (MTS) syeno-monzogranites, alkali feldspar granites and the Hulutian (HLT) alkali feldspar granites. LA-ICPMS zircon dating shows that the complex emplaced in 166–161 and 139±2 Ma, respectively. Geochemically, the MTS granites show relatively various geochemical compositions with low REE contents (87.76×10-6–249.71×10-6), Rb/Sr ratios (1.19–58.93), pronounced Eu negative anomaly (0.01–0.37) and low Nb/Ta ratios (2.40–6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35×10-6–282.17×10-6), Rb/Sr ratios (2.05–10.30) and relatively high Nb/Ta ratios (4.45–13.00). The isotopic data of the MTS granites display relatively enriched values, with ISr varying from 0.708 2 to 0.709 7, εNd(t) from -7.8 to -6.9 and εHf(t) from -7.4 to -3.2, in comparison with those of the HLT which are ISr=0.703 05–0.704 77, εNd(t)=-5–-3.4 and εHf(t)=-0.7–1.8). The two-stage model ages of the MTS granites (T2DM(Nd)=1.51–1.59 Ga and T2DM(Hf)=1.26–1.48 Ga) are also higher than those of the HLT granites (T2DM(Nd)=1.21–1.34 Ga and T2DM(Hf)=0.96–1.10 Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-Proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, southeastern China. We propose that the MTS granites were mainly formed in Paleo-Tethyan post-orogenic extensional tectonic setting whereas the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending.

Petrogenetic and tectonic significance of Permian calc-alkaline lamprophyres, East Kunlun orogenic belt, Northern Qinghai-Tibet Plateau

We present new geochronological, mineralogical, geochemical, and isotopic data for recently recognized lamprophyre dikes in the East Kunlun orogenic belt of NW China. Based on euhedral amphibole phenocrysts and fine-grained, plagioclase-bearing groundmass with accessory magnetite, apatite, and titanite, these dikes are classified as spessartites. Plagioclase in these rocks is Ca-rich with An ranging from 45 to 82. Most of the amphibole phenocrysts are magnesiohastingsite or ferropargasite, with systematic ‘‘normal’ zoning in which Al2O3, CaO, and Mg# decrease from core to rim. The dikes have moderate Mg# (43–49) and high Al2O3 (17.5–18.0 wt.%), FeOtotal (7.4–8.4 wt.%), and CaO (5.9–7.4 wt.%). Based on low total alkalis (Na2O + K2O = 4.2%–5.0 wt.%), most samples plot in the low-K, calc-alkaline field. They are enriched in large-ion lithophile elements (e.g. K, Rb, Sr, and Ba) and light rare-earth elements, but are depleted in high-field-strength elements (e.g. Ta, Nb, P, and Ti), and have enriched Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i = 0.70883–0.71045, εNd(t) = –5.51–5.72, εHf(t) = –4.42–0.38). Zircon U–Pb geochronology indicates that the dikes were emplaced at 253 ± 2.5 Ma and are unrelated to their granite host, which has an age of 443 ± 1.7 Ma. The geochemical and isotopic data suggest derivation from an enriched lithospheric mantle source that had been metasomatized by subduction-related fluids. Low degrees of partial melting of a phlogopite-bearing, spinel peridotite, followed by fractional crystallization of olivine, amphibole, and Ti-bearing minerals, can account for the observed geochemical features of the dikes. Trace element geochemistry and regional geology suggest that the East Kunlun lamprophryes formed in a subduction-related setting.

An Early Mesozoic transcontinental palaeoriver in South China: evidence from detrital zircon U–Pb geochronology and Hf isotopes

Detrital zircon geochronology reveals that Late Triassic–Early Jurassic fluvial sandstones from the major basins of the South China Craton have similar age patterns and define four populations at 2.6–2.4 Ga, 2.0–1.7 Ga, 850–700 Ma and 480–210 Ma. The late Palaeoproterozoic group is predominant in all of the five samples, and yielded remarkable age peaks at c. 1.85 Ga. These zircons have ϵHf(t) values between −22.5 and +3.6, suggesting derivation from reworked Archaean crust and minor juvenile crustal additions in the late Palaeoproterozoic. These characteristics differ from those of the Yangtze Block but correlate well with those of samples from the eastern Cathaysia Block. Palaeocurrent analysis of the Early Mesozoic sandstones shows predominant west- and NW-directed palaeoflows, supporting derivation of the sediments from the Cathaysia Block. The remarkable similarities in provenance signatures and spatial changes of lithofacies of the Triassic–Jurassic around the South China Craton delineate an east–west-trending sedimentary zone extending from Korea to West China. Accumulation of these sediments was probably related to the development of an active continental margin produced by westward subduction of the Palaeo-Pacific Plate. A c. 2000 km long westerly draining transcontinental palaeoriver probably had existed in the Early Mesozoic and fed the basins in Korea, South China and West China. Supplementary material: Sensitive high-resolution ion microprobe and laser ablation–inductively coupled plasma mass spectrometry zircon U–Th–Pb and Hf isotope data are available at www.geolsoc.org.uk/SUP18514.

40Ar–39Ar age and geochemistry of subduction-related mafic dikes in northern Tibet, China: petrogenesis and tectonic implications

The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende 40Ar–39Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO2 = 46.55–55.75%, MgO = 2.80–7.38%, Mg# = 36–61, and (Na2O + K2O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their ISr and ϵNd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.