Underplating Of Mantle-derived Magmas Research Articles

Triassic lithospheric modification of the northern North China Craton: Evidences from the composite Kalaqin Batholith and ultramafic-mafic Heilihe Intrusive Complex in Inner Mongolia

Whole rock major and trace elements and Sr-Nd-Hf isotopes and zircon in situ UPb ages and HfO isotopes have been determined for the Kalaqin granitic batholith and Heilihe Intrusive Complex (HIC) on the northern margin of the North China Craton (NCC), to investigate their magmatic sources and petrogenesis and further to reveal the lithospheric evolution of the craton. Zircon UPb dating, combined with the field observations and petrographic features, shows that at least 5 episodes of granites can be identified in the Kalaqin Batholith, i.e., 246 Ma monzogranite in the north, 234–227 Ma medium-grained monzogranite in the southwest, 224–221 Ma syenogranites in the north, 211 Ma granites in the south-central part and 206 Ma coarse-grained to megacrystic monzogranites intruding into the early batholith, and 238–231 Ma for the HIC ultramafic to mafic rocks. The 246 Ma granites have high SiO2 contents, negative whole rock εNd(t) and εHf(t) values, and negative zircon εHf(t) values and high δ18O values (up to 6.55‰), with ancient Nd and Hf model ages, indicating that they were derived from an ancient continental crustal source. However, the 234–221 Ma granites have positive whole rock εNd(t) and εHf(t) values and zircon εHf(t) values, indicating that they were derived from a relatively young crustal source. The granites of later episodes own some transitional isotopic feature and are suggested to be derived from a mixed source. The 238–231 Ma HIC rocks have high MgO contents, primitive mantle-like whole rock εNd(t) and εHf(t) values and zircon εHf(t) values and slightly higher zircon δ18O values than mantle. Zircon HfO isotopic constraints have further pointed to a mixed origin by depleted mantle and enriched lithospheric mantle for the HIC rocks. Calculation of the two-pyroxene thermobarometer yields crystallizing conditions of gabbronorites at around 1080 °C and 5 kbar, suggesting the emplacement of hot mantle-derived magmas into mid-crustal depth. All of the observations show that the lower continental crust beneath the northern margin of the NCC had been modified from ancient to relatively young by mantle-derived magma underplating during 246–221 Ma, although no solid evidences supporting the direct magmatic linkage were found between the batholith and the HIC. It is suggested that the lithosphere of the northern NCC had been modified and became weakened during 246–221 Ma, which makes the NCC easy to be destructed during late Mesozoic.

Crust–mantle interaction during subduction zone processes: Insight from late Mesozoic I-type granites in eastern Guangdong, SE China

Zircon U–Pb dating and in-situ Hf–O isotopic compositions, combined with whole-rock geochemical and Sr–Nd isotope data, are systematically presented to constrain the timing and petrogenesis of the late Mesozoic granitoids in eastern Guangdong Province, SE China. Zircon U–Pb chronology shows that two intrusions were emplaced from the Middle Jurassic to Early Cretaceous (166–144 Ma). All the granitoids are classified as calc-alkaline I-type granites with highly diverse elemental and isotopic compositions. The systematic variations in zircon Hf–O isotopes show that the granites can be categorized into three groups. The Donggang granitic porphyries (166 Ma) have the highest zircon δ18O values of 6.3–8.2‰ (average of 7.7‰) but the lowest εHf(t) values of −8.0 to −1.4, with the oldest Hf model ages of 1.30–1.70 Ga. The Liuzhoushan monzogranites (144 Ma) have intermediate zircon δ18O values of 6.0–6.9‰ (average of 6.4‰) and εHf(t) values of −5.0 to 0.3, with Hf model ages of 1.17–1.51 Ga. The late Early Cretaceous granitoids have the lowest δ18O values of 4.9–6.6‰ (average of 5.5‰) but the highest εHf(t) values of −3.2 to 3.3, with the youngest Hf model ages of 0.95–1.36 Ga. Zircon saturation temperatures of the Donggang, Liuzhoushan, and Xinwei (Sanrao) intrusions are calculated to be 642 °C, 722 °C, and 748 °C, respectively. Together with the variations in Sr–Nd isotopes, it appears that these granitoids were derived from the mixing of different proportions of mantle-derived components with Paleoproterozoic basement crust-derived magmas. The underplating of mantle-derived magmas into the continental crust, which supplied both heat and material contribution, was likely responsible for the generation of the most late Mesozoic I-type granites in eastern Guangdong. From Middle Jurassic to late Early Cretaceous, the increasing εHf(t) and εNd(t) values as well as the decreasing δ18O values and TDM2 ages indicate a progressive role of the crust–mantle interaction, corresponding to a gradually enhanced extensional tectonic environment. The northeast (NE)-ward younging trend and the enhanced crust–mantle interaction along coastal SE China were probably induced by the changing direction of the subducted plate from oblique to parallel to the continental margin.

Metamorphic Age Comparison and Its Implications between the Zuoquan and Zanhuang Complexes in the Central North China Craton, Based on LA-ICP-MS Zircon U–Pb Dating

The Trans-North China Orogen (TNCO) is well-known as an early Precambrian orogenic belt in the North China Craton (NCC). Three meaningful groups of metamorphic ages have been reported in the metamorphic complexes throughout the TNCO, including c. 1.85 Ga, c. 1.95 Ga, and c. 2.5 Ga. The spatial distributions and relationships of these ages provide notable insight into the formation timing and tectonic evolution of the NCC basement. The Zuoquan and Zanhuang complexes are exposed in the south–central TNCO and are adjacent to the Eastern Block. In order to place new constraints on the timing of two phases of metamorphism that occurred in the complexes, combined U–Pb and rare earth element analyses were performed on zircons from different types of metamorphic rocks. Uranium–Pb zircon dating in this study shows that two groups of metamorphic ages of 1.88–1.85 Ga and 2.48–2.46 Ga were commonly recorded by metamorphic rocks in the Zanhuang and Zuoquan complexes, respectively. Our previous geochronological studies showed that metamorphic ages of c. 2.51 Ga and c. 1.90 Ga were locally recorded in the Zanhuang and Zuoquan complexes, respectively. These data indicate that metamorphic rocks in the two complexes underwent at least two phases of metamorphism, i.e., 2.51–2.46 Ga (Phase I) and 1.90–1.85 Ga (Phase II). In combination with previous studies regarding reaction microstructures, metamorphic pressure–temperature paths, and geochronology, the Phase II metamorphic ages are interpreted to be linked to the collision between the Western and Eastern Blocks along the TNCO between 1.97 Ga and 1.80 Ga, whereas the Phase I metamorphic ages, as a result of an earlier and extensive tectono-thermal event that occurred in the Eastern and Western Blocks of the NCC, were related to underplating of mantle-derived magma. It is inferred that the rocks with c. 2.51–2.46 Ga metamorphic ages in the two complexes formed in the Eastern Block and underwent regional metamorphism during that period, and then were tectonically involved in the TNCO and experienced c. 1.90–1.85 Ga metamorphism. Metamorphic peaks occurred at different crustal levels in the orogen, resulting in distinct metamorphic ages and peak conditions preserved by metamorphic rocks in the two complexes.

Zircon U-Pb Ages and Hf Isotopes of Neoproterozoic Meta-Igneous Rocks in the Liansandao Area, Northern Sulu Orogen, Eastern China, and the Tectonic Implications

The Sulu Orogen preserves the Neoproterozoic tectonic-magmatic events, corresponding to the breaking up of the Rodinia supercontinent. The ages and petrogenesis of meta-igneous rocks in the Liansandao area in the northern Sulu Orogen are not well-constrained. This study reports zircon U-Pb ages and Hf isotopes of these rocks from the Liansandao area. Three meta-igneous rock samples give similar weighted mean 206Pb/238U ages of 744±11, 767±12, and 762±15 Ma, respectively, indicating the Neoproterozoic crystallization ages. These rocks formed coevally with the Wulian and Yangkou intrusions that located along the Yantai-Qingdao-Wulian fault zone. The Neoproterozoic ages indicate that the meta-igneous rocks from the Liansandao area have affinity to the Yangtze Block. The three samples have eHf(t)Hf(t) values of -7.2–-10.5, -6.0–-17.5, and -6.8–-12.0, respectively. These negative eHf(t) values indicate a primarily crustal source. However, the widely various eHf(t) values that are higher than the continental crust, suggesting magma mixing between mantle-derived materials and the continental crust or source heterogeneity. Combined with the Hf model ages and geochemical characteristics, the monzodiorite (sample LSD-2) is most likely to be mantle-derived magma then interacted with ancient continental crust, and the granitic protolith (samples LSD-1 and LSD-3) in the Liansandao area might derive from the re-melting of a Paleoproterozoic continental crust at ~750 Ma, resulting from the upwelling and underplating of mantle-derived magma formed in an extensional setting due to the break- up of the Rodinia supercontinent.

Geochemistry and zircon U–Pb–Hf isotopes of the Mante Aobao granite porphyry at East Ujimqin Banner, Inner Mongolia: implications for petrogenesis and tectonic setting

AbstractWe present detailed petrography, geochemistry and zircon U–Pb–Hf isotopes of the Mante Aobao granite porphyry in East Ujimqin Banner, Inner Mongolia, with the aim of determining its age and petrogenesis, important for understanding the early Palaeozoic tectonic evolution of the Xing’an–Mongolian Orogenic Belt. The Mante Aobao granite porphyry consists of plagioclase, quartz and minor biotite, but without amphibole. Zircon U–Pb analyses yield ages of 450 ± 1 Ma and 445 ± 2 Ma for the granite porphyry, indicating that it formed during Late Ordovician time. The granite porphyry is metaluminous to slightly peraluminous (aluminous saturation index A/CNK = 0.98–1.11) with high SiO2, K2O and Na2O concentrations and differentiation index (DI = 85–90). Chondrite-normalized rare earth element (REE) patterns display enrichment of light REEs (LREEs) with high ratios of (La/Yb)N and negative Eu anomalies. In the mantle-normalized multi-element variation diagrams, all samples are characterized by depletions of high-field-strength elements (HFSEs; Nb, Ta and Ti) and enrichments of large-ion lithophiles (LILEs; Rb, Th, U and K). These geochemical features indicate that the granite porphyry is a highly fractionated I-type granite and formed in a subduction-related setting. Zircon grains have positive εHf(t) values of +9.2 to +11.2, and TDM2(Hf) ages of 691–821 Ma, suggesting that the granite porphyry was generated by partial melting of Neoproterozoic juvenile crust with involvement of fractional crystallization during magmatic evolution. It is likely that underplating of mantle-derived magmas during Late Ordovician time provided the necessary heat to partially melt this juvenile crust. Combined with the regional geological data, we infer that the Mante Aobao granite porphyry was emplaced in an active continental margin setting that was probably related to the northwards subduction of the Paleo-Asian Plate beneath the South Mongolian Terrane along the Sonid Zuoqi–Xilinhot axis.

Geochemistry and Geochronology of the Gebunongba Iron Polymetallic Deposit in the Gangdese Belt, Tibet

The Gebunongba iron polymetallic deposit is a typical skarn deposit located at the westernmost end of the discovered iron polymetallic deposits in the Gangdese metallogenic belt. Major and trace elements of the monzonite granite related to mineralization show that these rocks are peraluminous (ACNK=1.15-1.21) and are rich in Si (76.63 wt.%-76.93 wt.% SiO2), K (K2O/Na2O ratio of 1.80-2.30), LREE and LILEs (Rb, Ba, Th, U, Pb), but is depleted in high field strength elements (Nb, Ta, Ti and HREE). In addition, these rocks show obviously negative Eu anomalies (δEu=0.48-0.55). The LA-ICPMS U-Pb age of zircons in monzonite granite is 59.72±0.55 Ma (MSWD=0.79), whereas the 40Ar/39Ar age of muscovite in iron ores is 59.22±0.61 Ma (MSWD=16.20). This indicates that the deposit formed at the syn-collision stage of Lhasa-India terrane is later than the northward subduction of the Yajiang crust. The monzonite granite has been probably derived from the partial melting of ancient lower crustal materials, which is probably resulted from the underplating of mantle-derived magmas. It is favorable for the formation of iron polymetallic deposit. Iron polymetallic mineralization is prevalent in Gangdese metallogenic belt at syn-collision stage. Therefore, syn-collision stage is an important mineralization stage for iron polymetallic deposits. The results of this study proved that iron polymetallic mineralization still took place in the western segment of Gangdese metallogenic belt and provided basis for further prospecting the deposits of the same type.

Neoarchean crust-mantle interactions in the Yishui Terrane, south-eastern margin of the North China Craton: Constraints from geochemistry and zircon U-Pb-Hf isotopes of metavolcanic rocks and high-K granitoids

The Neoarchean Yishui Terrane (YST) is situated in the east of Western Shandong Province (WSP), south-eastern margin of the North China Craton (NCC). The metavolcanic rocks of the YST are fine-grained hornblende plagioclase gneisses (Group #1) and fine-grained amphibolites (Group #2) in the Yangzhuangzhen area and fine- to medium-grained amphibolites (Group #3) in the Leigushan area. The high-K granitoids associated with Groups #1 and 2 are dominated by fine- to medium-grained monzogranitic gneisses. Zircon LA-ICP-MS U-Pb dating reveals that the magmatic precursors of Groups #1 and #2 were formed at 2641 Ma and the magmatic precursors of concomitant monzogranitic gneisses were emplaced from 2615 to 2575 Ma, whereas Group #3 represents a later 2500 Ma volcanic eruption, and all these metamorphic volcanic rocks and monzogranitic gneisses were subjected to subsequent 2470–2460 Ma metamorphism.The metamorphic volcanic rock samples in Group #1 exhibit the chemical compositions of calc-alkaline andesites, showing fractionated chondrite-normalized REE patterns ((La/Yb)N = 10.48–19.30) and negative Nb, Ta and Ti anomalies ((Nb/La)PM = 0.13–0.22), which are akin to those of typical high-Mg andesites (HMAs) in the subduction-related settings. The magmatic precursors of the Group #1 samples were derived from partial melting of a fluid- or melt-metasomatized depleted mantle wedge at deep levels in the upper mantle. Samples in Group #2 show calc-alkaline chemical compositions with less fractionated chondrite-normalized REE patterns ((La/Yb)N = 2.24–3.34) and negative Nb, Ta and Ti anomalies ((Nb/La)PM = 0.47–0.76), which are consistent with those of the volcanic rocks in the Aleutian island arc. The magmatic precursors of Group #2 were generated by partial melting of a fluid-metasomatized depleted mantle wedge at shallow levels in the upper mantle. The monzogranitic gneisses exhibit high SiO2 and K2O contents with high-K calc-alkaline affinities and peraluminous characteristics. Based on their distinct HREE contents and chondrite-normalized REE patterns, these granitoid samples are subdivided into low-Yb monzogranitic gneisses (LYMGs) and high-Yb monzogranitic gneisses (HYMGs). The LYMG magma was derived from partial melting of a mixed source of juvenile two-mica pelites and minor basic-intermediate igneous rocks at lower crustal levels with pyroxene + amphibole + garnet as the main residual phases, and the HYMG magma was derived from partial melting of multi-sourced juvenile two-mica pelites at middle to lower crustal levels with pyroxene + amphibole and subordinate plagioclase and garnet as the main residual phases. In addition, Group #3 resembles tholeiitic back-arc basalts in the Okinawa Trough and displays flat chondrite-normalized REE patterns ((La/Yb)N = 1.22–2.08) and slightly negative Nb and Ta anomalies ((Nb/La)PM = 0.35–0.59). This group was most likely derived from partial melting of a depleted mantle source that had been modified by the addition of subducted slab-derived fluids at shallow levels in the upper mantle. These metavolcanic rocks and concomitant high-K granitoids record important Neoarchean crust-mantle interactions involving the first modification and partial melting of the lithospheric mantle induced by oceanic crust subduction; then, upwelling and underplating of mantle-derived magmas triggered partial melting of the middle to lower crust and mixing between crust- and mantle-derived magmas. These processes imply that Neoarchean crust-mantle interaction played a crucial role in the evolution of the southeastern margin of the NCC.Available whole-rock Sm-Nd and zircon Lu-Hf isotopic data from metamorphic volcanic rocks and plutonic granitoids from this study and previous studies reveal that YST experienced three crucial juvenile crustal growth events from ~2.78–2.69 Ga, ~2.64–2.56 Ga and ~2.54–2.50 Ga.

Petrogenesis of late Neoarchean high-K granitoids in the Western Shandong terrane, North China Craton, and their implications for crust-mantle interactions

Late Neoarchean plutonic rocks in the eastern range of the Western Shandong (WSD) terrane, North China Craton, consist of fine-grained monzogranites (Group #1), fine-grained monzogranitic gneisses (Group #2), coarse-grained granodioritic gneisses (Group #3) and coarse-grained gabbro-diorites (Group #4). The first three groups of granitoids exhibit high SiO2 and K2O contents and plot in the quartz monzonite, granodiorite and granite fields in the TAS classification diagram and the high-K calc-alkaline series field in the SiO2 and K2O diagram. These granitoids show meta- to peraluminous characteristics in the A/NK versus A/CNK plot. In contrast, the gabbro and diorite samples display low SiO2 and K2O contents. LA-ICP-MS zircon U-Pb isotopic dating reveals that the high-K granitoids (Groups #1, 2 and 3) were emplaced at ∼2511–2497 Ma and that the gabbro-diorites were emplaced at ∼2511 Ma. All of these samples generally underwent regional metamorphism at ∼2475–2451 Ma. Detailed petrogenetic and geochemical studies suggest that the fine-grained monzogranites (Group #1) were derived from the hydrous partial melting of juvenile sediments at mid-crustal levels, while the fine-grained monzogranitic gneisses (Group #2) were most likely derived from the partial melting of metagreywackes at lower to mid-crustal levels. The gabbro-diorites (Group #4) were derived from the partial melting of a fluid-metasomatized mantle source at relatively deeper levels of the upper mantle. The granodioritic gneisses (Group #3) were generated from magma mixing between melts derived from the partial melting of metagreywackes (∼90%) and melts derived from a metasomatized depleted mantle source (∼10%) with minor ancient basement material additions.These findings, combined with previously obtained zircon U-Pb dating and Lu-Hf isotope data, suggest that crust-mantle interaction constituted the essential mechanism for the formation of high-K granitoids in the WSD terrane. The upwelling and underplating of mantle-derived magmas (Group #4) not only provided sufficient heat for melting crustal materials at various levels (Group #1 and #2) but also mixed mantle materials with crustal-derived melts, thereby generating the magmatic precursors of the granodioritic gneisses (Group #3).

Petrogenesis and tectonic implications of early Paleozoic granitoids in East Kunlun belt: Evidences from geochronology, geochemistry and isotopes

The East Kunlun Orogenic Belt (EKOB) provides an important link to reconstruct the evolution of the Proto-Tethys and Paleo-Tethys realm. The EKOB is marked by widespread Early Paleozoic magmatism. Here we report the petrology, bulk geochemistry, zircon U–Pb dating and, Lu–Hf and Sr–Nd isotopic data of the Early Paleozoic granitic rocks in Zhiyu area of the southern EKOB. Based on the zircon U–Pb dating, these granitoids, consisting of diorite, granodiorite and monzogranite, were formed during 450–430 Ma the Late Ordovician to Middle Silurian. The diorite and granodiorite are high Sr/Y ratio as adakitic affinities, and the monzogranite belongs to highly fractionated I-type. Their (87Sr/86Sr)i values range from 0.7059 to 0.7085, εNd(t) values from –1.6 to –6.0 and the zircon εHf(t) values show large variations from +9.1 to –8.6 with Hf model ages (TDM2) about 848 Ma and 1970 Ma. The large variations of whole-rock Nd and zircon Hf isotopes demonstrate strong isotopic heterogeneity of the source regions which probably resulted from multi-phase underplating of mantle-derived magmas. Geochemical and isotopic studies proved that the diorite and granodiorite had been derived from partial melting of heterogeneous crustal source with variable contributions from ancient continental crust and juvenile components, and the monzogranites were representing fractional crystallization and crustal contamination for arc magma. The Early Paleozoic adakitic rocks and high-K calc-alkaline granitoids in the southern EKOB were likely emplaced in a continental marginal arc setting possibly linked to the southwards subduction of the Paleo Kunlun Ocean and the magma generation is linked to partial melting of thickened continental crust induced by underplating of mantle-derived magmas.

Crust–mantle interaction during Early Jurassic subduction of Neo-Tethyan oceanic slab: Evidence from the Dongga gabbro–granite complex in the southern Lhasa subterrane, Tibet

Gabbro–granite complexes that form in active continental margins provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas and offer important insights into crust–mantle interaction. In this paper, we present detailed petrographic, mineralogical, whole-rock geochemical, Sr–Nd isotopic, zircon U–Pb, and Hf isotopic data for the Dongga gabbro–granite complex of the southern Lhasa subterrane, with the aim of elucidating the petrogenesis of these rocks and gaining insights into the processes of crust–mantle interaction associated with the northward subduction of the Neo-Tethyan oceanic slab. The Dongga complex consists predominantly of tonalite and granodiorite with abundant mafic microgranular enclaves (MMEs) and lesser amounts of hornblende gabbro. LA–ICP–MS zircon U–Pb dating yields crystallization ages of 178.0±1.5Ma and 192.3±1.5Ma for the hornblende gabbros, and 178.1±1.7Ma and 178.8±1.8Ma for the tonalites and MMEs, respectively. Geochemically, the gabbroic rocks have low SiO2 and Na2O+K2O concentrations and high Mg# values. These gabbroic rocks are enriched in large ion lithophile elements (LILEs; e.g., Ba, K, Pb, and Sr) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P, Zr, and Hf). The tonalites are subalkaline and metaluminous and are also characterized by an enrichment of LILEs and LREEs and a depletion of HFSEs. The MMEs are silica poor and alkali rich and have broadly similar trace element signatures to those of the host tonalites. The gabbroic rocks, tonalites, and MMEs have similar initial 87Sr/86Sr ratios (ISr) of 0.7035–0.7036 and positive εNd(t) values of +5.3 to +6.8. However, zircon εHf(t) values from the hornblende gabbros (+14.0 to +18.2) are somewhat higher than those of the tonalites (+11.4 to +15.0) but are similar to the MMEs (+14.4 to +18.2). The combined petrological, geochemical, and isotopic features of the Dongga gabbro–granite complex suggest that the southern Lhasa subterrane was located in an active continental margin setting during the Early Jurassic. The gabbroic rocks were generated by the hydrous partial melting of a depleted mantle wedge metasomatized by aqueous fluids released from the subducted Neo-Tethyan oceanic slab. The tonalites and MMEs were produced by the mixing of mafic magmas related to the gabbroic rocks and felsic magmas derived from the remelting of the newly formed juvenile lower crust. The underplating of mantle-derived magmas and the resulting crustal melting and magma mixing occurred during the Early Jurassic subduction of the Neo-Tethyan oceanic slab. The mantle-derived magmas not only provided substantial heat to melt the lower crust but also mixed with the crust-derived magmas to form granitoids and MMEs.

Re–Os isotope geochronology of the Shangbao pyrite–flourite deposit in southeastern Hunan, South China: Evidence for multiple mineralization events and the role of crust–mantle interaction in polymetallic deposits

In South China, both crustal reworking and crust–mantle interaction were important geological processes during the Paleozoic and Mesozoic eras. However, the relationships between these two processes and metal mineralization are still unknown. Here we report rhenium and osmium isotopic data for pyrite grains from a pyrite deposit associated with a granite intrusion in the Shangbao area, southeastern Hunan Province (South China). Two pyrite samples, both containing many euhedral pyrite grains, were collected from the same locality, but the samples yield distinct ages. Six euhedral pyrite grains from one sample yield an isochronal age of 279 ± 12 Ma, with an initial 187Os/188Os ratio of 0.39 ± 0.71, and Re and Os concentrations of 0.12–63.5 ppb and 2.14–185 ppt, respectively. This Early Permian age is in good agreement with the age of the strata that host the pyrite deposit. Five euhedral pyrite grains from the other sample yield an isochronal age of 75.2 ± 4.3 Ma, with an initial 187Os/188Os ratio of 0.141 ± 0.030 and Re and Os concentrations of 0.15–0.43 ppb and 1.0–39.9 ppt, respectively. If one pyrite grain with the highest 187Re/188Os and 187Os/188Os ratios is excluded, other four pyrite grains give an isochronal age of 85 ± 13 Ma. The Late Cretaceous age (75–85 Ma) is consistent with the zircon U–Pb age of the Shangbao granites (80.1 ± 0.3 Ma) to within uncertainties. Considering also the relatively lower radiogenic initial 187Os/188Os ratio of this sample, we suggest that the later stage pyrite ore was probably formed through crystallization from the magmatic hydrothermal fluids. Combined with other geological and associated magmatic data, we propose a skarn-related fluid–ore interaction process to explain the second stage of metallogenesis in the Shangbao pyrite deposit. The Early Permian pyrite ore was deposited in a brine basin with evaporites during the Early Permian. Later magmatic hydrothermal fluids originating from the Shangbao granites, which included mantle components, interacted with the strata and the Early Permian pyrite ore during the Late Cretaceous and precipitated a later stage pyrite ore. During the Late Mesozoic, the roll-back of subducted Paleo-Pacific plate caused lithospheric extension in South China, triggering the upwelling and partial melting of the asthenosphere. The resulting underplating of mantle-derived magmas provided a vast amount of heat and materials for the formation of the granites and polymetallic deposits in South China. Given that the multiple mineralization events were spatially and temporally associated with the Paleozoic–Mesozoic magmatism, the Re–Os isotopic dating of euhedral pyrite grains has been shown to be a viable method for unveiling the evolutionary history of ore-deposits. Skarn development caused by granite and mafic dike emplacement resulting from crust–mantle interaction explains the occurrence of two mineralization episodes at the same locality.

Geochemical and zircon U–Pb geochronological study of the Yangshan A-type granite: Insights into the geological evolution in south Anhui, eastern Jiangnan Orogen

The Early Cretaceous Yangshan granite is an A-type granitic intrusion that was emplaced along the eastern Jiangnan Orogen in southern Anhui Province, South China. The Yangshan intrusion mainly consists of syenite porphyry (127.0±0.6Ma) and alkali-feldspar granite porphyry (126.0±1.0Ma). As a part of Qingyang–Jiuhuashan complex intrusion, the Yangshan A-type granites have lower MgO, CaO, Co, Sr, and higher Rb, Nb, Th and HREE contents, with enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and slightly negative Eu anomalies. However, the syenite porphyry and the alkali-feldspar granite porphyry differ in terms of zircon εHf(t) values: small variations in the syenite porphyry from −5.5 to −3.7, corresponding to Hf model ages (tDMC) between 1.42Ga and 1.53Ga, and large variations in the alkali-feldspar granite porphyry from −6.4 to +4.4, yielding tDMC of 0.90–1.59Ga. While both rocks also have similar εNd(t) values ranging from −7.02 to −5.47, corresponding Nd model ages (TDMC) are 1.37–1.49Ga, falling within the Hf model ages. We take these features to indicate that the Qingyang I-type granites, which are isotopically similar, and Yangshan A-type granites were originated from partial melting of Mesoproterozoic–Neoproterozoic crust, with minor juvenile crust input for the alkali-feldspar granite porphyry, followed by fractional crystallization. In combination with previous studies, we propose that the Cretaceous A-type granitic rocks formed between 135Ma and 122Ma, implying an important Mesozoic extensional event in eastern Jiangnan Orogen, which facilitated underplating of mantle-derived magma and crustal heating. This may have occurred in a back-arc extension in response to the drift of subduction direction of the Paleo-Pacific plate, which started as early as ~135Ma ago.

Petrogenesis of two types of Late Triassic granite from the Guandimiao Complex, southern Hunan Province, China

Two types of Late Triassic granite are found in the Guandimiao Complex of the South China Block (SCB). Here, we present new LA–ICP-MS zircon U–Pb ages as well as geochemical and Sr–Nd–Pb–Hf isotopic data in order to elucidate the genesis of these granites. The Guandimiao Complex, located in southern Hunan Province, consists dominantly of the Shizhuqiao two-mica alkali feldspar granite and the Jingtou hornblende-bearing biotite monzogranite. The latter contains abundant microgranular enclaves. Zircon U–Pb isotopic analyses show that the microgranular enclaves and the two types of granite were all emplaced during the Late Triassic (226–220Ma). The Shizhuqiao peraluminous granite has high (87Sr/86Sr)i ratios (0.72173–0.72485), enriched εNd(t) and εHf(t) values (–9.6 to –9.4 and –10.5 to –5.5, respectively), and Pb isotopic compositions similar to those of the metamorphic basement of the Cathaysia Block (part of the SCB), implying derivation from the crust. The granite’s low molar CaO/(MgO+FeOT) ratios and high molar Al2O3/(MgO+FeOT) ratios indicate a metasedimentary source. The Jingtou metaluminous granite exhibits εHf(t) values (–10.0 to –5.6) that are similar to those of the Shizhuqiao granite, but it has lower (87Sr/86Sr)i ratios (0.71326–0.71454) and higher εNd(t) values (–7.2 to –6.6). Its high ratios of molar CaO/(MgO+FeOT) and low ratios of molar Al2O3/(MgO+FeOT) suggest an amphibolitic source. The microgranular enclaves contain acicular apatite and are more mafic than their hosts. The combined textural, geochemical, and isotopic data indicate that the enclaves in the Jingtou granite originated from a more mafic crust-derived melt that was injected into the host felsic melt. The geochemical signatures indicate that the microgranular enclaves and the two types of coeval granite that constitute the Guandimiao Complex were derived from different source rocks. The Late Triassic granites in the SCB were emplaced in an extensional post-orogenic setting and related to underplating of mantle-derived magma.

Early paleozoic granodioritic plutons in the Shedong W–Mo ore district, Guangxi, southern China: Products of re-melting of middle Proterozoic crust due to magma underplating

The Shedong W–Mo ore district in the south–central Dayaoshan Uplift of Guangxi, southern China hosts the Baoshan and Pingtoubei deposits, both of which occur in granodioritic plutons. Zircon U–Pb dating of granodiorites and its mafic microgranular enclaves (MMEs) in the Baoshan deposit yielded ages of 439.8±3.2 and 441.1±2.2Ma, respectively. Granodiorites have moderate SiO2 (54.5–63.0wt.%) and high Al2O3 (15.4–17.8wt.%) contents, wide variations in major element ratios, significant rare earth element fractionation, and small negative Eu anomalies. They are rich in Th, U, Zr, and Hf, and depleted in Ba, Nb, and Ti. Their initial 87Sr/86Sr, εNd(t), and εHf(t) values are in the range of 0.7086–0.7091, −5.2 to −6.6 and −6.3 to +1.6, respectively. Rounded or lenticular MMEs have relatively low silica and high mafic components, depletion in Eu, Sr, and Zr, and marked negative Eu anomalies. Rb/Sr and Nb/Ta ratios, and εNd(t) and εHf(t) values of the MMEs are higher than those of host granodiorites, indicating a different magmatic source. Zircon U–Pb dating of the unexposed granodiorite porphyry in the Pingtoubei deposit yielded an age of 440.0±1.7Ma. The granodiorite porphyries have high SiO2 and low K2O, FeOT, and MgO contents, with similar trace element features to the granodiorites at the Baoshan deposit, although the former has small negative Eu anomalies. Its initial 87Sr/86Sr values range from 0.7162 to 0.7173, εNd(t) values from −8.7 to −12.3, and εHf(t) values from −7.8 to +1.3, indicative of a crustal source. Nd and Hf two-stage model ages of the granodiorites, MMEs, and granodiorite porphyries have a narrow range between 1.3 and 2.2Ga. We propose that the granodiorites and MMEs at the Baoshan deposit were produced through re-melting of middle Proterozoic crust as a result of underplating of mantle-derived magmas in a transitional compression-to-extension tectonic setting. Mantle-derived magmas provided the heat and material for the formation of the granodiorites and MMEs.

Geochronology and geochemistry of the major host rock of the Dong’an gold deposit, Lesser Khingan Range: Implications for petrogenesis and metallogenic setting during the Early–Middle Jurassic in northeast China

The Dong’an gold deposit is a large-sized epithermal gold deposit recently discovered in the Lesser Khingan Range, NE China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of a medium–coarse grained alkali-feldspar granite, the major host rock of the Dong’an gold deposit. The LA–ICP–MS zircon U–Pb dating of the medium–coarse grained alkali-feldspar granite yields an early Jurassic age of 176.3±1.1Ma (MSWD=0.62). The whole-rock geochemical data indicate that the samples are felsic, ferroan, alkali-calcic and peraluminous with relatively high alkali (K2O+Na2O) content. They are enriched in LREEs and LILEs (e.g., Rb, Ba, K), but are depleted in HFSEs (e.g., Nb, Ta, P, Ti), especially in P and Ti, showing characteristics of volcanic arc magmas and similarities with the Early–Middle Jurassic granitic rocks in Xing’an Mongolian orogenic belt. Meanwhile, the negative Eu, Nb, Ta, Ti, and P anomalies are consistent with fractional crystallization of plagioclase, Ti-bearing phases (rutile, ilmenite, titanite, etc.) and apatite during magma evolution. The samples have low Nb/Ta ratios (8.65–14.91) and low Mg# values (18–36), which are indicative of crustal derived magmas and no interaction between source magmas and the mantle. In-situ Hf isotopic analyses of the zircons from the medium–coarse grained alkali-feldspar granite yield εHf(t) values of +3.38–+5.68 and two-stage model ages (TDM2) of 772–900 Ma, indicating the magmas formed this intrusion were generated by partial melting of Neoproterozoic basaltic materials in the young lower crust, and the magma source could be derived from a depleted mantle. The medium–coarse grained alkali-feldspar granite most likely formed in the late stage of Toarcian subduction of the Pacific plate, which can be identified on the tectonic setting discrimination diagrams, and the formation of this intrusion was associated with underplating of mantle-derived magmas, which provided heat for crustal partial melting. Similar to the medium–coarse grained alkali-feldspar granite, large amounts of granitic rocks and a series of nonferrous metal hydrothermal deposits (Mo, Cu, Au) formed in northeast China as results of magmatic activities triggered by subduction of the Pacific plate during the Early–Middle Jurassic.

The Late Cretaceous I- and A-type granite association of southeast China: Implications for the origin and evolution of post-collisional extensional magmatism

We present new geochronological, mineralogical, and geochemical data for granitic plutons that crop out within the Zhoushan archipelago, northeastern coastal Zhejiang Province, in order to constrain their origin, and the genetic relationship between the I- and A-type granites. These granites can be divided into two groups: (1) the northern I-type Putuoshan (PTS) and Dadong'ao (DDA) plutons; and (2) the southern A-type Daqingshan (DQS), Taohuadao (THD), and Xiazhidao (XZD) plutons. Zircon LA–ICP–MS U–Pb dating yielded ages of 98–96Ma for the northern I-type plutons and 89–86Ma for the southern A-type plutons. All of these granites are highly siliceous, K-rich, and have similar total alkali and total rare earth element (REE) abundances. However, there are also geochemical differences between the I-type and the A-type granites. The northern I-type alkali-feldspar granites are high-K calc-alkaline, metaluminous to mildly peraluminous, contain low concentrations of the high field strength elements (HFSE; e.g., Nb, Ta, Zr, and Hf), and have low Ga/Al ratios (2.04–2.44). In contrast, the southern A-type granites are peralkaline and F-rich, and have lower CaO and Al2O3 concentrations, and higher Fe2O3T and HFSE concentrations and Ga/Al ratios (3.25–3.86). Meanwhile, they have slightly higher heavy REE (HREE) concentrations, and are more depleted in Ba, Sr, P, Ti, and Eu than the northern I-type granites. Both the I- and A-type granites have homogeneous whole-rock Nd and highly variable zircon Hf isotopic compositions. Of note, the southern peralkaline A-type granites appear to have more radiogenic Nd and Hf isotope compositions than the northern I-type granites. The present data, together with the results of a previous study on mafic enclaves within the PTS pluton, suggest that the northern I-type alkali-feldspar granites were generated by mixing of mantle-derived material with crustal-derived magmas that formed by dehydration melting of mica-bearing basaltic rocks, leaving a granulite residue in the lower crust. Both the I- and A-type granites were emplaced during post-collisional extensional tectonism associated with rollback of the steeply subducting paleo-Pacific Plate. An increase of the dip angle of the subducted palaeo-Pacific plate between the Early and Late Cretaceous resulted in enhanced extension, further thinning the lithosphere and causing more intensive underplating of mantle-derived magmas. This generated high temperatures and caused the partial melting of the residual granulite material, producing relatively anhydrous F-bearing felsic melts. The mixing of these unusual crustal melts with more voluminous mantle-derived mafic magmas generated the parental magmas that eventually formed the peralkaline A-type granites.

Geochronological, geochemical and Nd–Hf isotopic constraints on the petrogenesis of Late Cretaceous A-type granites from the southeastern coast of Fujian Province, South China

We present comprehensive petrological, geochronological, major and trace element, and Nd–Hf isotopic data for the Baishishan, Jingangshan, and Wushan granitic plutons on the southeastern coast of Fujian Province, South China, with the aims of elucidating their origin and gaining new insights into the petrogenesis of aluminous A-type granites. Zircon U–Pb ages obtained by laser ablation–inductively coupled plasma–mass spectrometry show that the three investigated plutons were emplaced at 92–86Ma, indicating that they were generated during a Late Cretaceous magmatic event. The granites from the three plutons are composed mainly of perthite, quartz, plagioclase, and minor biotite; they have high SiO2 contents, and low CaO, MnO, Fe2O3tot, and MgO contents, and show a metaluminous to slightly peraluminous signature. The granites are enriched in some large ion lithophile elements (e.g., Rb, Th, and U) and high field strength elements (e.g., Nb and Ta) with elevated Ga/Al ratios, and spidergrams show strong depletions in Ba, Sr, Ti, and P. Chondrite-normalized REE patterns show relative enrichments in light rare earth elements, flat heavy rare earth element profiles, and strongly negative Eu anomalies. These mineralogical and geochemical characteristics suggest that all three plutons can be classified as aluminous A-type granites. The plutons exhibit nearly identical whole-rock Nd and zircon Hf isotopic compositions, and yield Mesoproterozoic two-stage model ages (1.4–1.1Ga) for both Nd and Hf isotopes. Based on a synthesis of the geochemical and isotopic data and petrogenetic modelling, we suggest that these A-type granitic rocks were most likely formed by variable degrees of fractional crystallization of magmas produced by the partial melting of a tonalitic to granodioritic source, with plagioclase-rich residual phases in the middle to lower crust, and emplaced at shallow crustal levels along the Changle–Nan’ao Fault. Our data on the Baishishan, Jingangshan, and Wushan granites, coupled with previous studies of Cretaceous magmatism in coastal areas of SE China, indicate that a gradually intensifying regional extension took place in the region during the late Yanshanian (Cretaceous), which peaked during the Late Cretaceous in response to the roll-back of the Palaeo-Pacific plate. Consequently, we conclude that the generation of A-type granites was related to intensive tectonic extension along the Changle–Nan’ao Fault and the induced underplating of mantle-derived magma.

Origin of Triassic granites in central Hunan Province, South China: constraints from zircon U–Pb ages and Hf and O isotopes

Triassic granites crop out extensively in central Hunan Province, South China. Representative granites include the Baimashan, Weishan, and Ziyunshan plutons. Lithologically, these granites mainly comprise biotite monzogranite, two-mica granites, hornblende-biotite granite, hornblende-biotite monzogranite, and garnet-muscovite granite. These granites have in situ zircon secondary ion mass spectrum U–Pb ages between 223.2 ± 3.3 and 209.3 ± 4.0 Ma, indicating that they likely formed predominantly in the Late Triassic. These granitic plutons have similar zircon Hf and O isotopic compositions, with εHf(t) values of −0.8 to −9.0, two-stage depleted mantle model ages (TDM2) of 1.81–1.31 Ga, and weighted mean of δ18OZrc values of 8.53 ± 0.58‰ to 9.12 ± 0.28‰. Combined with U–Pb dating and Hf isotopic data, the elevated and variable δ18O and εHf(t) values of the individual granites indicate that these Triassic granites were likely produced by partial melting of upper Paleoproterozoic to lower Mesoproterozoic metasedimentary rocks and are S-type granites. The variable proportions of inherited zircons in certain samples with U–Pb ages of 627–992 Ma indicate the involvement of lower–middle Neoproterozoic crustal materials during magma crystallization through wall-rock contamination, which resulted in the wide range of isotopic compositions. Underplating of mantle-derived magma may have provided the thermal energy for partial melting of the upper Paleoproterozoic and lower Mesoproterozoic basements, thereby generating these late Triassic granites. The lack of positive εHf(t) values and high δ18OZrc values indicate that the contribution of mantle-derived magmas to these granites may be insignificant.

U–Pb dating, geochemistry, and Sr–Nd isotopic composition of a granodiorite porphyry from the Jiadanggen Cu–(Mo) deposit in the Eastern Kunlun metallogenic belt, Qinghai Province, China

The Jiadanggen porphyry Cu–(Mo) deposit is newly discovered and located in the Eastern Kunlun metallogenic belt of Qinghai Province, China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of the mineralization causative granodiorite porphyry. The new data indicate that the granodiorite porphyry is characterized by high SiO2 (68.21–70.41wt.%) and Al2O3, relatively high K2O, low Na2O, and low MgO and CaO concentrations, and is high-K calc-alkaline and peraluminous. The granodiorite porphyry has low Mg# (38–46) values that are indicative of no interaction between the magmas and the mantle. The samples that we have examined have low Nb/Ta (9.17–10.3) and Rb/Sr (0.28–0.39) ratios, which are indicative of crustal-derived magmas. Source region discrimination diagrams indicate that the magmas that formed the granodiorite porphyry were derived from melting of a mixed amphibolite source in the lower crust. The samples have ISr values of 0.70954–0.70979, εNd(t) values of −8.3 to −7.9, and t2DM ages ranging from 1644 to 1677Ma. These indicate that the magmas that formed this intrusion were generated by melting of Mesoproterozoic lower crustal material. Higher K(Rb) contents of the samples indicate that the magma source is high potassium basaltic material in the lower crust, which could be derived from an enriched mantle source. LA-ICP-MS zircon U–Pb dating of the granodiorite porphyry yields a late Indosinian age (concordia age of 227±1Ma; MSWD=0.31), which is close to the molybdenite Re–Os isochron age (227.2±1.9Ma), indicating further the close relationship between the granodiorite porphyry and the Cu–(Mo) mineralization. These samples are LREE and LILE (e.g., Rb, K, Ba, and Sr) enriched, and HFSE (e.g., Nb, Ta, P, and Ti) depleted, especially in P and Ti, similar to the characteristics of volcanic arc magmas. This intrusion most likely formed during the later stage of Indosinian deep subduction of oceanic slab. This was associated with underplating of mantle-derived magmas, which provided heat for crustal melting. Similar to the Jiadanggen granodiorite porphyry, Indosinian hypabyssal intermediate-felsic intrusive rocks, formed under subduction tectonism or a transitional regime from subduction to syn-collision, make up the most important targets for porphyry Cu(Mo) deposits in the Eastern Kunlun metallogenic belt.

AMPHIBOLITES OF THE CENTRAL ZONE: NEW SHRIMP U-PB AGES AND IMPLICATIONS FOR THE EVOLUTION OF THE DAMARA OROGEN, NAMIBIA

Research Article| June 01, 2014 AMPHIBOLITES OF THE CENTRAL ZONE: NEW SHRIMP U-PB AGES AND IMPLICATIONS FOR THE EVOLUTION OF THE DAMARA OROGEN, NAMIBIA L. LONGRIDGE; L. LONGRIDGE School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa VM Investment Company, 24 Fricker Rd, Illovo, Johannesburg, South Africa, e-mail: lukelongridge@gmail.com Search for other works by this author on: GSW Google Scholar J.A. KINNAIRD; J.A. KINNAIRD School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za Search for other works by this author on: GSW Google Scholar R.L. GIBSON; R.L. GIBSON School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za Search for other works by this author on: GSW Google Scholar R.A. ARMSTRONG R.A. ARMSTRONG PRISE, Australian National University, Research School of Earth Sciences, Building 61, Mills Road, Acton, 0200, Australia, e-mail: richard.armstrong@anu.edu.au Search for other works by this author on: GSW Google Scholar Author and Article Information L. LONGRIDGE School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa VM Investment Company, 24 Fricker Rd, Illovo, Johannesburg, South Africa, e-mail: lukelongridge@gmail.com J.A. KINNAIRD School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za R.L. GIBSON School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za R.A. ARMSTRONG PRISE, Australian National University, Research School of Earth Sciences, Building 61, Mills Road, Acton, 0200, Australia, e-mail: richard.armstrong@anu.edu.au Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2014 Geological Society of South Africa South African Journal of Geology (2014) 117 (1): 67–86. https://doi.org/10.2113/gssajg.117.1.67 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation L. LONGRIDGE, J.A. KINNAIRD, R.L. GIBSON, R.A. ARMSTRONG; AMPHIBOLITES OF THE CENTRAL ZONE: NEW SHRIMP U-PB AGES AND IMPLICATIONS FOR THE EVOLUTION OF THE DAMARA OROGEN, NAMIBIA. South African Journal of Geology 2014;; 117 (1): 67–86. doi: https://doi.org/10.2113/gssajg.117.1.67 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract New SHRIMP U-Pb zircon and titanite data are presented for two amphibolites from the Central Zone of the Damara Orogen, Namibia. These data (the first ages for amphibolites in the Central Zone) show that amphibolites are derived from both Palaeoproterozoic (2026.9 ± 2.3 Ma – sample ACAM-1) and Pan-African (557.2 ± 7.4 Ma – sample LKR021) precursors emplaced into the Palaeoproterozoic Abbabis Complex basement in the Central Zone. The younger amphibolites are interpreted as part of a mafic to dioritic phase of magmatism in the Central Zone (the Goas Suite), and major- and trace-element geochemistry from a suite of amphibolites from the southwestern Central Zone confirms this relationship. This phase of mafic to dioritic magmatism is contemporaneous with the onset of collision between the Congo and Kalahari Cratons. It is suggested that delamination of dense mafic lower crust and underplating of mantle-derived magmas following crustal thickening may have led to partial melting of pre-Pan-African mafic lower crust, generating this mafic to dioritic suite of rocks. Metamorphic zircon overgrowths in sample LKR021 give an age of 520 ± 6.9 Ma, consistent with recent estimates for the age of peak metamorphism in the southwestern Central Zone. Titanites from LKR021 give an age of 493.4 ± 6.4 Ma, and this is interpreted as a cooling age for the Central Zone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.